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Dirk De Clercq

Lecture 1: Neurocytology  Neurons are very active in protein synthesis and packaging. They also have complex shapes and dynamic polarization (where different parts of the neuron perform different functions).  Primary receptive surfaces are the cell body (soma) and dendrites. The soma is where transcription/translation occur, and has Nissl bodies (rough ER) and lots of Golgi. The soma may receive synapses.  Dendritic fields of different neurons can vary dramatically, though they’re usually pretty small, less than 100 microns. Their patterns results from genetic and environmental input, and varies with what they need to sample.  Dendritic spines have a head and neck, and they receive excitatory input. They are dynamic structures, coated with actin and myosin to help this. Big spines are most stable. All neurons have spines in development. Their purpose is to provide specificity for the kinds of synapses formed. Spine density changes, decreasing with age for example.  Spines are somewhat isolated from the parent dendrite, both electrically and chemically. They can “remember” or reflect what they were like a few seconds ago, and play a role in memory and learning.  At the base of a spine is a ribosomal rosette, which makes proteins unique to the spine.  The axon hillock and initial segment of the axon (which is never myelinated) integrate passive signals and can produce an AP due to a high density of VG Na channels. Most axons in the CNS send signals to 1000s of other cells via collateral branches.  Each neuron gives off 1 axon.  Variability in synaptic NT and NT receptors allows for the complexity of the brain. Glycoprotein in the cleft help guide NTs to receptors. The post-synaptic cell has a high density of transmembrane proteins. Many are ionotropic (ion channel; short and fast) or metabotropic (GPCR; long and slow) NT receptors.  VG Ca channels and Ca influx lead to fusion of synaptic vesicles. The closer a synapse is to the axon hillock, the bigger its effect. Generally, excitatory stimuli are on dendrites and inhibitory stimuli are on the cell body. Axo-axonic synapses are very powerful and also rare; they are often inhibitory. Most synapses are axo-dendritic or axo-spinous, though many are axo-somatic.  Inhibitory axo-somatic synapses tend to occur at the cell body because the driving force for Cl into the cell is near zero at -70 mV. So, it must be nearer to the axon hillock to be effective.  Dendro-dendritic synapses occur between dendrites, with one having NT vesicles, etc. It does not require an AP to get synaptic transmission.  Electrical synapses also occur at gap junctions. Connexin proteins form connexons (pores). These are too simple to be used too much.  All transcription, translation, packing and processing occurs in the soma. Most neurons won’t divide, so have lots of heterochromatin. They are extremely ribosome dense for lots of protein production. Free ribosomes are also in the dendrites, and cluster at the base of spines to make proteins concentrated there. Rough ER forms Nissl bodies. Robust Golgi and mitochondria are also present.  The axon hillock has some kind of gate (mechanism unknown) that prohibits ribosomes from entering the axon. Protein for the axon is made in the cell body and delivered by axoplasmic transport. 1  The neuronal cytoskeleton contributes to neuronal shape and transport in axons/dendrites. Microtubules (α,β tubulin, MAPs) primarily determine morphology. Neurofilaments (intermediate filaments, especially the heavy variety due to charged side bridges) determine the diameter of the axon (more of them makes it bigger, cuz they’re constantly spaced).  Axoplasmic transport: microtubules form with their + ends at the axon terminal. Thus, + end directed motors like kinesins act for anterograde transport. And, - end directed motors like dyneins act for retrograde transport.  Ion distributions (esp. via the Na/K pump) help maintain the resting potential. Signaling is mediated by changes in ion permeability. K , Na , Cout Cain K ,ina , in , Cin out out out Na channels are excitatory, K or Cl channels are inhibitory  E ionoltage at which there is no net flow of the ion across the membrane.  Driving force = Electrical gradient + Chemical gradient  Channels are selective for charge, but can be variable in ion specificity (most cation channels exclude Ca though).  Myelination increases the membrane resistance, which increases the length constant. Greater diameter decreases internal resistance, which also increases the length constant.  The length constant (distance before a depolarization will decrease in magnitude by a factor of e – or decrease by 2/3) is influenced by membrane resistance and internal resistance.  The time constant (the time it takes to get to reach a level of 63% V max for the influx of a given number of ions) is a function of resistance and capacitance. τ = RC, where R = internal resistance. Membranes are very thin, so neurons have very high capacitance. C = Q/V. Na ions that flow in must first discharge the capacitance (negatives on the inside of the axon) before than can build up voltage, and that takes time. Decreasing the capacitance, by myelination for example, will decrease the time constant and make you turn those charges into voltage faster.  So myelination both increases the length constant (by increasing membrane resistance) and decreases the time constant (by decreasing capacitance) to speed up conduction.  Synaptic potentials are integrated to decide if an action potential should be fired. Lecture 2: Membrane Potential  Na =in5 mM, Na = 150 out E = +64 Na  K = 155 mM, K = 5 mM, E = -86 in out K  Ca in.0001 mM, Ca = 1 mMout = +116 Ca  Cl = 5 mM, Cl = 110 mM, E = -78 in ou Cl  Nernst Equation: E = 60iong ([ion] /[ion] out in  Current flow through ion channels: I = g (V – E ) ion ion m ion  Electrical signals may be conducted passively/decrementally, or actively/regeneratively. Active propagation only happens when voltage gated channels are present. -x/λ  Passive spread: A depolarization of ∆V falls o0f at a distance x as: ∆V = ∆V (e ).xλ is 0 the length constant, and is defined as: λ = √(r /r).m i  riis internal resistance of a dendrite. R is ihe specific resistance of dendroplasm. ri = R/iπr ).  rmis the membrane resistance for a dendrite, with R being tme specific resistance of membrane in general. r = R m(2πr).m 2  As the radius goes up, the internal resistance shrinks faster than the membrane resistance. So bigger axons have relatively less internal resistance, and will have a longer length constant and conduct farther.  Capacitance is directly proportional to area, and it is inversely proportional to thickness. Think of it as how many charges it’ll take to produce a certain voltage, but more charges will go to the membrane (and not count) if it is broad or thin.  Membranes act like capacitors in addition to resistors. Once you first open a channel, the ions go to charge the membrane like a capacitor as well as run along the membrane. When the capacitor is fully charged, then they just proceed through the membrane or dendroplasm.  For a resistor and capacitor in parallel (like a membrane), the voltage change produced by a -t/τ constant current at time t is given by: Vt= V max– e ). τ is the time constant, a measure of how long it takes to charge the membrane up to 63% of it’s maximum potential. It is measured as: τ = r m m where c im the capacitance per unit length. It affects how long it takes voltages to be propagated along a dendrite.  All of this basically has consequences for the temporal and spatial summation that must occur at the axon hillock. Signals must be strong enough at that distance to meet the threshold, which is influenced by the diameter of their dendrites, and they must arrive at the same time to sum appropriately.  The axon hillock must be depolarized to ~ -55 mV to open VG Na channels and trigger an AP. This represents the point at which enough VG Na channels open to make the net current (Na vs. K) inward and initiate a positive feedback loop of Na influx.  VG Na channels start as deactivated, they activate (open), inactivate (close and can’t open yet), then return to a deactivated state. The maximum rate for this is about 1 millisecond, so you max out at around 1000 firings per second for a neuron.  Inactivation and VG K channels return the cell to resting potential. These channels remain open and are responsible for the After Hyperpolarization stage.  Hodgkin and Huxley figured out lots of what was going on with channels by doing experiments with squid giant axons. They used voltage clamps, which set a neuron at a constant voltage, and can measure the currents necessary to maintain that. Depolarization first caused negative current (Na in), then positive current (K out).  Removing Na from the medium eliminated the negative current of Na influx. Looking at the difference of: complete curve – (complete curve w/out Na) = curve for effects of Na.  Finally, they added TEA to block K, and saw just the negative current of Na influx. They thus showed that Na and K were causing these changes.  With axons of higher caliber, the resulting increased length constant allows the wave of depolarization to spread farther ahead and conduct the AP faster. Myelin decreases capacitance and increases r mo decrease the time constant and allow the axon to charge faster. Lecture 3: Synaptic Transmission  Katz knew APs are associated with Na influx and K efflux, so wanted to know if they were responsible for NT release. They placed pre and post-synaptic electrodes, using signals from the post as a measure of NT release. Depolarize pre cell, and even if you block Na with TTX and K with TEA, still get post-synaptic potential. So Na and K not responsible for NT release, though it is apparently voltage dependent. 3  More Katz experiments: Block all Na and K channels. Removing external Ca or blocking VG Ca channels during depolarization inhibits NT release. Monitoring extent of Ca influx with varying levels of depolarization showed it to coincide with levels of NT release. And, injection of Ca can evoke or augment NT release. Ca chelators blocked NT release. Overall, this basically proved that Ca was responsible.  After influx, calcium may interact directly with synaptic or vesicular proteins, or indirectly via Ca-calmodulin dependent protein kinase, or PKC. Synaptotagmin appears to play an important role. Synaptotagmin is bound to synaptic vesicles.  Katz also noted that NT release is quantal. Spontaneous mini-synaptic potential occur while recording, and post-synaptic potentials were found to be approximately integer multiples of these spontaneous ones. This results from NT being stored in vesicles of uniform size, and that fusion releases a rather consistent quantity of NT molecules.  In the post-synaptic cell, ACh opens a channel permeable to Na and K. To determine the reversal potential for a channel permeable to two ions, you use a weighted avg. of the Nernst equation: E revg /(Na+ Na)]*EK+ [gNa(g + K )]Na K K  If Na and K were equally permeable, it would just be the average of their individual equilibrium potentials (-11mV), but it’s actually closer to 0 mV, meaning Na is a little more permeable than K.  Something is excitatory if it increases the chance of firing an AP, so it must have a reversal or equilibrium potential above the threshold. A channel is inhibitory if its rev/eq’m potential is below the threshold. So, excitatory ones have to be permeable to Na or Ca, whereas inhibitory ones must be permeable to K or Cl.  1 NT can have multiple effects due to the existence of multiple receptors. So there’s divergence of signaling even at just 1 synapse.  GABA binds to: GABA ReceAtor, permeable to Cl GABA ReBeptor, permeable to K  Glutamate binds to: AMPA Receptor, permeable to Na/K NMDA Receptor, permeable to Na/K/Ca mGluR, a GPCR  NT actions can also change w/ development or hormonal changes. For example, neonates have much higher [Cl] inside cells than adults. This causes is EClo be about -5 mV, so it’s not inhibitory like the Clof -65 in adults. This may contribute to the frequency of seizures in neonates. Their levels become normal within about a week. Lecture 4: Myelination  Most knowledge of myelination comes from studies of the PNS.  Glial cells include: myelin forming cells (Schwann cells in PNS and Oligodendrocytes in CNS), astrocytes (CNS) and microglia (CNS). Schwann cells do jobs of all three cell types in the PNS.  Glial cell features: Neurons can’t function without them; they serve for support and maintenance. They have distinct shapes, functions, and markers. Glial cells are also very specialized in terms of organelles and structure for their specialized functions. Some glial cells can re-enter the cell cycle, which is important for de-myelinating diseases. Their special roles provide distinct targets for disease. 4  Schwann cells are the universal glial cells of the PNS. Individual cells about a millimeter apart along the axon. Osmium stains lipids dark and is good for seeing myelin. Between myelinated axons on this type of view are unmyelinated axons.  Myelinating Schwann cells wrap around a myelinated axon. Non-myelinating Schwann cells still ensheath unmyelinated fibers, may also act as “satellite cells” that wrap around sensory neruons’ bodies at DRG, or may surround the neurons of the enteric nervous system. Satellite cells cover the whole nerve cell body and processes. Their job is basically to separate the axon/myelin from the outside world by means of the basal lamina (outermost layer in both myelinating and non-myelinating Schwann cells, but ONLY in the PNS). This is much like astrocytes in the CNS. They have regular, low level renewal and proliferation.  A myelinating Schwann cell has a 1:1 ratio with its axon (disease may upset this), and induces its axon to grow bigger. The axon signals to the Schwann cell that it should be myelinated. These Schwann cells only demyelinate and proliferate in disease. Myelin has unique proteins and glycolipids, and forms up to 150 layers around the nerve.  In the compact myelin layers, cytoplasmic faces of the Schwann cell membrane fuse to form major dense lines, and extracellular faces form intraperiod lines. These alternate.  With disease or damage, the Schwann cell re-enters the cell cycle. Daughter cells then compete to remyelinate the original axon (see Guillain-Barre). If the axon had been damaged, they significantly aid axon regeneration.  A non-myelinating Schwann cell (aka ensheathing Schwann cell) wraps around (not a bunch of times) and supports multiple axons. The distribution of myelin around nerves is homogeneous, not nodes/internodes. There’s a low level of proliferation going on there. They also have other specialized functions and membrane components. In the axons with these Schwann cells, propagation is pretty slow and ion channels are evenly distributed.  Nodes of Ranvier are between Schwann cells, densely packed with Na and K channels that renew the AP. Where the myelin is, it is the internode. In between closer to the node is paranodal, and closer to the internode is a juxtaparanodal region. Schwann cell membrane attaches at paranodal regions, near the junction of nodes and internodes. Microvilli of the Schwann cell cover the nodes of Ranvier. Axon communicates with the Schwann cell nucleus via the Schmidt Lanterman incisures.  Compact myelin is held around axon by protein-protein interactions. Tight junctions and gap junctions occur in regions of non-compact myelin. Axoglial contacts (where axon and Schwann cell interface) occur at nodes, internodes (abaxonal membrane), and paranodal junctions. Axonal and Schwann cell proteins important for these interactions are known for each region (node, paranode, juxtaparanode, and internode).  In normal development, Schwann cells wrap around multiple axons. As they proliferate, they develop a 1:1 relationship with axons destined to become myelinated. They have reciprocal interactions. Axon instructs Schwann cell to myelinate, and Schwann cell instructs axon to get bigger. Unmyelinating Schwann cells will remain associated with multiple axons.  Myelinating Schwann cell development: Transcription factor SOX-10 helps convert neural crest cells into Schwann cell precursors. This is turned into immature Schwann cells by NRG1 (neuregulin 1, released by axons). Later, NRG1 dictates myelination patterns. A myelinating Schwann cell normally remains myelinating for the rest of adult life, it can revert in disease states.  The membrane that is in contact with axon wraps around it and forms the multi-layered myelin. A mesaxon also forms, suspending the axon from the myelin. 5  Having basal lamina in PNS allows damaged nerves to degenerate, but then regrow into the myelin and basal lamina that remain. This is absent in the CNS, which makes it harder to repair nerve damage there.  Demyelinating diseases: o Guillain-Barre Syndrome – very common. Acute autoimmune attack on myelinated peripheral nerves. You get weakness, loss of reflex, high spinal fluid production. Can be paralytic or fatal, mostly due to immune attack on myelination, and demyelination due to similarity of myelin to a pathogen. This can repair on its own once attack subsides. Treat w/ Ig or other techniques to slow autoimmune attack. o Charcot-Marie-Tooth disease: distal wasting of limbs, high arches and hammer toes (because extensors are in leg, whereas flexors are in foot…and the longer nerve path to the flexors makes them more susceptible to damage), claw hand, etc. Genetic disease leads to demyelination, in which Schwann cell divides and both daughters try to myelinate at the same time. Looks like onion bulbs and produces nerve hypertrophy.  Oligodendrocytes: myelin forming cells of the CNS. When one of these demyelinates, they don’t survive. They can act as satellite cells around neurons. There are lots of oligo precursors around the CNS. The myelin in CNS is different from PNS, and they have different diseases associated with them.  1 oligodendrocyte can myelinate multiple axons, requiring tremendous surface area. A consequence is that they’re susceptible to injury.  Some oligodendrocytes may act as satellite cells around neurons.  Oligodendrocytes are much less effective at proliferation and de/remyelination after demyelinating injuries than Schwann cells.  Multiple sclerosis: autoimmune attacks where you repeatedly demyelinate different portions of the CNS, but then seem to get better.  Neurons depend on neurotropic factors from supporting cells. Axons will degenerate if periods of demyelination are prolonged or severe. Lecture 5: Astrocytes  Virchow recognized the existence of neuroglia as something like glue. Dieters saw that it consisted of cells. Golgi suspected that processes were continuous and they played a role in nutrition. Cajal coined the term astrocyte, distinguished between protoplasmic (grey matter) ones and fibrous (white matter) ones, and came up with the idea that they insulate/ensheath nerves. Rio Hortega distinguished microglia and oligodendrocytes.  Astrocytes are the most numerous of the glial cells. In development they secrete growth factors, guide neuronal migration and enhance synapse formation. They also provide neurons with energy, remove accumulating ions and NTs, and support neuronal activity. Astrocytes also detect the activity of surrounding neurons with VG ion channels and NT receptors.  Cultured astrocytes are quite different from ones in situ.  Astrocytes are distributed throughout the gray matter with contact spacing and little overlap in their domains. They express Glial Fibrillary Acidic Protein (GFAP), an intermediate filament, in fibers of their primary processes; it is a marker for cell injury and is upregulated in that situation. These give it the star-like appearance. GFAP seems to be important for 6 structural support of the brain. Astrocytes can also be identified with S100β, glutamine synthetase, a glutamate transporter, and aquaporin 4.  Alexander disease is a fatal disorder of macrocephaly, seizures and retardation. With this disease, Rosenthal fibers are present. They are inclusions, within astrocytes, of GFAP and stress proteins. This disease is a gain of function mutation in GFAP.  Where processes of astrocytes contact capillaries, they expand and form “end feet” that completely cover the capillary wall. They have GLUT-1 channels and take up glucose from the blood and store it as glycogen, before delivering it to neurons as lactate.  Vasogenic edema is caused by a breakdown of the blood brain barrier. Cellular edema may occur during ischemia, hypoxia or brain trauma, and is associated with swollen astrocytes. The cells take up extra ions and NTs, then because they have aquaporin 4 highly enriched at their end feet, they take up water. Astrocytes are key for brain water homestasis.  GFAP is only in the main processes of the astrocyte, which actually has a very complex morphology. They have lots of processes which may be very thin and form isolated microdomains. Protoplasmic astrocytes fill up a lot of the space in the neuropil (gray matter)  Astrocyte processes ensheath synapses and have lots of glutamate transporters to remove the NT from the extracellular space. Without this, neurons would have tonic activation and would die of excitotoxicity. These glutamate transporters may run in reverse during ischemia due to high extracellular K and low extracellular Na. This could lead to neuronal death via excitotoxicity, but it’s not clear if astrocytes are the culprit.  Decreased expression of glutamate transporters in astrocytes is associated with ALS. It may accelerate the death of motor neurons by allowing excitotoxic damage to occur.  Astrocytes can’t fire APs. They have high conductance K channels that give it a very negative resting potential (-90mV), as well as a low membrane resistance. These channels are concentrated at the astrocyte processes (end feet). This may allow the bodies of astrocytes to pick up K released from active neurons and dump it out of the end feet to redistribute, return it to circulation or otherwise prevent its accumulation. This is the K siphoning or buffering hypothesis.  Astrocytes form a syncytium, and injection of markers shows them to diffuse throughout populations of these cells. They are connected by gap junctions formed by connexin 43. This provides a path for ions and metabolites to move between cells, though the ultimate function is not understood. May be involved in ion buffering, metabolic homeostasis, regulating proliferation, or astrocyte-astrocyte signaling (signal transfer or response amplification). The gap junctions can be modified by phosphorylating connexins. Often, 2 different connexins form the two sides of a gap junction and provide directed movement of components.  Astrocytes have ion channels and NT receptors similar to neurons. It allows for neuron to astrocyte signaling, which may help attract or maintain astrocytes around synapses. The presence of glutamate causes oscillations in levels of intracellular calcium. Calcium waves can spread among astrocytes over long distances via ATP mediated ATP release (with the help of PLC). ATP acts as a NT in the brain, so contributes to the spread of Ca waves as well as activate neurons. This calcium activity in neurons can increase the excitability of surrounding neurons by releasing ATP or glutamate.  This calcium signaling may play a role in synchronizing groups of neurons by releasing glutamate. Calcium may also help control the dilation of brain microcirculation by 7 controlling release of lipid metabolites (Ca may activate PLA2, and form arachidonic acid which induces smooth muscle constriction).  Astrocytes are throughout the CNS, but are absent from the retina and cerebellum. There, astroglial cells with similar properties take their place. In the cerebellum, these are Bergmann glial cells, in the retina they are Muller glial cells.  Brain disease or trauma often results in reactive astrocytosis, in which astrocytes undergo hypertrophy and hyperplasia and increased GFAP expression. This can lead to astroglial scars.  Astrocytomas are diffuse cancers. Because the cells migrate so much, they tend to produce multifocal tumors, so can rarely be surgically resected. Glioblastoma is most common, and results in increased intracranial pressure. These cancers secrete angiogenic factors like VEGF and are associated with vascular proliferation. Lecture 6: Development 1  Each neuron undergoes: proliferation, migration and aggregation (neurons of 1 type aggregate and segregate themselves, forming nuclei and layers), differentiation (chemical and morphological), axon growth, synapse formation and regressive events.  Proliferation: In gastrulation, the mesoderm and endoderm invaginate at the blastopore lip (aka Henson’s node); this is the tissue that leads to production of a nervous system. Transplanting that tissue right after gastrulation gives a brain, a little later and it gives a spinal cord.  Ectodermal cells individually will form neurons and glia as a default program. BMP-4 is produced by lateral mesoderm and endoderm and induces cells to become epithelial. The notochord secretes noggin, chordin and follistatin to counteract BMP-4 and push cells toward a neural fate. These secretions from the notochord also promote proliferation, leading to neurulation. Neural plate forms, then the neural folds and groove, followed by neural tube and neural crest.  Neural crest will produce PNS cells, while neural tube will produce CNS cells (except microglia which migrate into the CNS from the blood).  If neural crest cells migrate dorsally, they become melanocytes. If they go ventrally, different pathways lead to production of DRG/sympathetic/enteric/etc neurons, supporting cells, or adrenal chromaffin cells.  Neuroblasts in the neural tube attach to both lumenal and outer surfaces of the tube. Their nuclei wander up and down during S and G phases, but detach from the outer surface and remain at the lumenal surface for mitosis. Divisions may be symmetric (both re-enter cell cycle early in development, or both exit late in development) of asymmetric (one re-enters, the other migrates away). Asymmetric division is common as initial elements of neural structure are laid down.  Progenitors tend to produce neurons first and then glia later, with exceptions such as radial glial cells that play a supporting role, then can differentiate into neurons or glia much later. Glioblasts quickly become either precursor cells for oligodendrocytes (OPCs) or astrocytes (APCs). Astroctyes produce trophic factors (PDGF and NT-3) that allow OPCs to continue to divide.  Most neurons cease to proliferate at around 25 weeks, with a couple exceptions like the hippocampal formation (for spatial and declarative memory). 8  Proliferation occurs in the ventricular zone, adjacent to the lumen. Outside of this is an intermediate/mantle zone with post-mitotic cells, and the outer-most layer is the marginal zone, with processes of cells from the ventricular zone.  Migration: Radial glial cells are attached to the lumenal and outer surface of the neural tube throughout development. Post-mitotic cells migrate away from proliferative zones along radial glial cells.  The spinal cord has an outside-in pattern of cell layering in the intermediate/mantle zone, in which the oldest cells are on the outside. Cells cluster dorsally into the alar plate and ventrally into the basal plate. Between the two plates is a groove called the sulcus limitans (division between dorsal, somatosensory horn and the ventral, motor horn).  Neurons in the spinal cord may become motor neurons or various types of interneurons. Ventral cells become motor neurons or interneurons that coordinate movement. Dorsal cells become interneurons associated with sensory information. This is determined by Sonic Hedgehog, secreted from the notochord and floorplate.  SHH has various receptors that turn gene clusters on or off, and respond with varying sensitivity to [SHH]. These genes generally produce transcription factors. As [SHH] varies from dorsal to ventral, so do its effects on developing neurons.  The marginal zone of the spinal cord is outside the intermediate/mantle zone (cell bodies). Axons to and from the brain project through the marginal zone, so the white matter develops outside the grey matter in the spinal cord. Each part of the marginal zone is invaded by specific types of axons. Likewise, each part of the grey matter contains specific types of cell bodies.  The ventral horn, for example, has motor neurons and interneurons. Motor neurons to the arms or legs form separate clusters than those of the trunk. This type of aggregation of similar types of neurons is likely due to expression of cadherins, CAMs that display homotypic binding (the timing of this expression is important).  In the cerebral cortex, the original site of proliferation is the ventricular zone. Later the subventricular zone (just superficial to the ventricular zone) begins to generate cells. These make excitatory and supporting cells, which migrate radially to their final destinations. Inhibitory neurons arise from the lateral and medial ganglionic eminences, which also give rise to the basal ganglia. These follow a tangential migratory path.  Excitatory neurons express glutamate, inhibitory neurons GABA. The neurons of the basal ganglia are also GABAergic.  The first neurons in the cerebral cortex form the “preplate” in the intermediate zone. These preplate cells include the outer Cajal-Retzius cells and inner subplate neurons. Neurons born later will migrate into the space between these two populations of cells, and they will populate the space termed the “cortical plate.”  Neurons of the cerebral cortex attach themselves to a nearby radial glia cell, and will follow it to a specified place in the cortical plate. As cells are produced, the oldest layers remain deeper and closer to the lumen, while younger cells migrate beyond them. This forms an inside-out pattern of migration, with layer 6 closest to the lumen (produced first) on to layer 2, outside of which is layer 1 of Cajal-Retzius cells. These CR cells in layer 1 soon die.  Neurons along a radial line are also products of the same progenitor, made by asymmetric division. This is also the case in the retina, so cells along a particular information path are 9 daughters of the same mother. The retina is an outgrowth of the diencephalon, with ectoderm forming the lens and cornea.  Axons that enter and exit the cerebral cortex will be found below layer 6, not in the marginal zone. So, unlike the spinal cord, grey matter is on the outside with white matter on the inside.  Cajal-Retzius cells secrete reelin, which interacts with ECM and allows later neurons to migrate past earlier ones. Without this protein, the layers of the cortex will be produced with the wrong layering pattern, or lissencephaly (smooth brain surface) may be produced. Other defects can lead to ectopias, or neurons present in the white matter – can be caused by a DISC1 mutation, which underlies some forms of schizophrenia.  The brain itself is regionalized into the: Prosencephalon (tel- and di-), Mesencephalon, and Rhombencephalon (met- and myel-). The rhombencephalon is broken down into segments called rhombomeres. Alternating expression of repulsive ephrins and ephrin receptors produces this. Lecture 7: Development 2  Notochord stimulates neural induction in the ectoderm cells overlying it. The anterior end of the resulting neural tube forms the forebrain and midbrain, while the posterior part forms the hindbrain and spinal cord.  At different dorsal/ventral levels you get motor neurons (ventral), commissural neurons for pain and temperature (middle), etc. Dorsal/ventral patterning is mediated by diffusible signals from the notochord and roofplate.  The notochord produces SHH, which induces ventral (motor) fates at high concentration and interneuron fates at medium concentrations. It is not the only cue for the ventral spinal cord though, as SHH-/- and Gli3-/- should revert it to a wild-type state, but do so incompletely.  SHH acts as a morphgen, a diffusible factor that forms a gradient. Neurons have different receptors that affect transcription factors, all with varying sensitivities. By having some inducing and some repressing ones, you get a variety or responses to different concentrations. By having some of them repress each other, you can produce sharp boundaries.  SHH signaling absent: Patched inhibits Smoothened. Gli3 is free to cleave itself into the Gli3 Repressor and turn off genes.  SHH signaling present: SHH inhibits the inhibitory molecule Patched, effectively freeing Smoothened to act at the cell membrane. Active Smoothened prevents Gli3 cleavage. With no Gli3 Repressor around, target genes are on.  Dorsal fates are regulated by BMPs, Wnt, etc. produced in the roofplate and overlying ectoderm. They may be involved in ventral patterning as well. Just as with SHH, Math1, Ngn1 and Mash1 affect transcription factors and each other to produce different dorsal fates  Cells acquire a fate before leaving the cell cycle.  In cerebral cortex development, there is an intermediate zone in between the cortical and ventricular zones. It is simply the zone through which migrating cells move en route to the cortical zone. While all of this stuff is happening, the anterior part of the neural tube is regionalized into its major subdivisions. Recall that glial cells are generated after neurons.  We said that proliferating cells move up and down in the ventricular zone before dividing. They actually are farthest from the ventricle when they undergo S phase, then migrate down during G2 to undergo M phase at the ventricular surface. 10  In symmetric divisions, cells divide along a vertical cleavage plane; in asymmetric divisions they go along a horizontal cleavage plane.  Experiments show that about 80% of progenitor cells only give rise to neurons, and 2% only give rise to glia. The rest can produce both types of cells. About 8% of progenitors retain some capacity to divide and act as stem cells.  Radial glial cells not only act as a scaffold, but also as progenitors. They can divide to produce 1 neuron and another radial glia cell. In that case, the radial glia cell likely expresses Notch, and the progenitor that migrates away probably expresses Delta. The radial glia cell eventually seems to transform into an astrocyte precursor. Radial glia cells can become oligodendrocytes, neurons, astrocytes, or astrocytes with some stem cell properties.  Notch/Delta signaling plays a role in determining whether or not a cell will remain a progenitor or differentiate. Notch is a receptor that, when activated by ligands like Delta, suppresses bHLH transcription factors that normally promote neuronal, differentiated fates. So activated Notch keeps a cells as a progenitor. Cells with lots of Delta differentiate.  Notch activates CBF1 (or RBP-J), which activate HES genes, which encode bHLH proteins that inhibit the proneuronal bHLH factors. A constitutively active Notch maintains radial glia cells.  For the laminar fates in the cerebral cortex, it appears as though environmental cues can influence what a cell differentiates into. If you take a cell from an animal when it is generating layer 6 and put transplant it into an animal generating layers 2/3, the cell adopts the 2/3 fate. If cell has undergone its final division before transplantation, it remains type 6. Specification appears to occur during the final cell division cycle.  Radial glial cells act as guides and progenitors, but they are only present during development. Excitatory cells migrate radially along these. Inhibitory cells migrate tangentially from ventral ganglionic eminences. The genes Dlx1 and Dlx2 are necessary for this process of tangential migration.  Reelin mutants have a failure of proper radial migration. They produce a cortex with an outside-in pattern, though neurons still somehow establish decent connectivity.  Lis1 is mutated in lissencephaly, a condition where the brain is smooth and without gyri or sulci. Male patients with a DCX mutation have lissencephaly also, whereas females show a condition called “double cortex,” because DCX is on the X-chromosome and women are mosaics in which some cells migrate properly while some do not.  Lis1 and DCX are downstream in the Reelin signaling cascade. Lecture 8: Development 3  The retinotectal system produces an inverted topographic map of the visual field on the tectum. Anterior retinal axons go to the posterior tectum and vice versa. A chemoaffinity model (molecular identification of axon targets) was suggested by rotating frogs’ eyes and seeing that they re-innervated the original targets in the tectum, rather than compensating for the orientation. This suggested a gradient of markers and fixed cues guiding the axons.  Extending axons are selective in choosing their pathways. They join, or fasciculate with, certain existing neuronal pathways. Ablating the pathways prevents neurons from extending properly, and they don’t just join other axons. They stall where they should’ve found a path to follow. 11  Commissural axons extend from the dorsal to ventral spinal cord then cross the floor plate and run anteriorly. Multiple repulsive/attractive cues guide them. They may mediate chemoattraction/repulsion or contact attraction/repulsion. These are critical to axon (and dendrite) guidance.  Netrins (secreted) bind UNC-5s or DCCs. Semaphorins (secreted or TM) bind plexins, neuropilins or complexes of both. Slits (secreted) bind Robos. Ephrins A or B (membrane bound) bind EphAs/Bs.  Netrin-1 is concentrated ventrally in the spinal cord and guides commissural neurons to the midline. It acts as an attractive cue in vitro as well. The response to Netrin-1 depends on the receptors and intracellular signaling. With just DCC, it’s attractive. With DCC and UNC-5 present it’s repulsive. With just DCC and also PKA inhibited, it’s repulsive.  Slit and Robo allow commissural axons to cross the CNS midline. Axons are drawn ventrally to the midline by Netrin-1 and DCC. Robo3 is expressed, which prevents Robo1 and Robo2 from repelling it away from the midline. They cross the midline and downregulate Robo3. Then the Robo1/2-slit interaction repels them from the midline to keep them on the other side; this complex also inhibits the attractive forces of Netrin-1 – DCC.  Patients with horizontal gaze palsy and progressive scoliosis (HGPPS) have Robo3 mutations and can’t look left and right since neuronal connections can’t cross the hindbrain midline.  Morphogens (BMPs, Wnt) and growth factors can also play roles as neuronal guidance cues. Contributing to ventral movement of commissural axons before crossing, dorsal BMP repels them and SHH attracts them. After crossing the midline, high rostral concentrations of attractive Wnt interact with the Fz3 receptor to draw the axon rostrally. Repulsive Wnts that are concentrated rostraly repel cortical spinal tract axons caudally.  Sema3A binds neuropilin-1 and constrains axon growth in peripheral neuron projections. Semaphorins are potent repulsive signals, acting via neuropilins or plexins.  CAMs play a role in pathfinding as contact mediated cues. Many important ones contain Ig domains, and may be repulsive or attractive. The FasII protein is one such CAM that causes neurons expressing it to fasciculate together and form bundles. Other Ig superfamily members can cause repulsive cues upon homotypic binding, Dscam for example. This can be important for preventing dendritic branches of 1 neuron from fasciculating with each other, allowing the cells to form diffuse dendritic arborizations. Alternative splicing creates lots of different Dscams in different neurons so separate neurons don’t interact this way.  Attractive and repulsive cues from CAMs and Semaphorins may counter each other to maintain an appropriate balance of attraction/repulsion. When you lose the repulsion of Semas or gain function of adhesive CAMs, axons tend not to branch out of nerve tracts enough to innervate everything they should. If you lose both, you restore the balance of attraction and repulsion and get more appropriate motor innervation, for example.  Axonal growth cones have filopodia projecting from lamellipodia. The growth cone is constantly in flux and samples the environment to respond to directional cues. It has complex cytoskeletal dynamics.  Actin monomers are added to the barbed ends of filaments, which are present at the leading edge of the growth cone. Larger MT filaments are bundled in the axon, but individual MTs may expand into growth cones with elongation occurring at their + ends. Intermediate filaments are in the axon, but rare in growth cones. 12  Filopodia extend and retract by regulating actin polymerization/depolymerization and actin retrograde flow (via myosin). Repulsive/attractive cues mediate this. Elongation of actin filaments is associated with monomers binding ATP and delayed ATP hydrolysis. Regulation occurs via lots of different actin binding proteins.  Redistribution of MTs occurs by polymerization as well as movement and bending.  Actin can anchor to ECM stuff and prevent retrograde flow, thus facilitating protrusion of lamellae.  Site selection: Actin accumulates at sites where a guidance cue is contacted, and is depleted elsewhere. MTs extend toward the contact site, though the growth cone remains spread in other directions.  Site stabilization: MTs stabilize and form a bundle into the growth cone, and the rest of the growth cone kind of collapses around that area of contact w/ the guidance cue.  Inhibit actin polymerization via cytochalasin B, and you prevent filopodial protrusions. Inhibit myosin, and you block retrograde flow (protrusion continues). Block MT dynamic instability via vinblastine and you block directed outgrowth, but not filopodial protrusions.  Actin dynamics are regulated by Rho family GTPases (Rho, Rac, and CDC42). These are modulated by Rho GAPs and GEFs. Through various effector proteins, Rho limits axon growth and Rac/CDC42 promote axon growth and extension. Ex: Rho activates ROCK, which activates non-muscle myosin to retract actin fibers. Rho can also use phosphoinositide production to enhance actin depolymerization via actin binding proteins like gelsolin. And, CDC42 activates N-WASP that promotes actin polymerization and axon extension.  CDC42 appears to act earlier than Rho and Rac. All three of these respond to attractive/repulsive guidance cues to help guide the extending axon. Ex: Slit repels via a CDC42 GAP. Sema4A repels via a Rho GEF. Ephrin repels via a Rho GEF and Rac GAP.  Slit signaling inactivates CLASP, which stabilizes MTs and prevents growth cone extension. NGF signaling prevents APC degradation, and APC promotes MT assembly and neuronal expansion.  In the periphery, neurons can regenerate via Wallerian degeneration. Schwann cells provide permissive factors and macrophages remove myelin and debris. Not the case in the CNS, where myelin debris remains and inhibitory factors disrupt axon extension.  Shown that you can sever the optic nerve, add a sciatic nerve graft, and in this permissive path axons will regrow.  Myelin is a poor substrate for axon extension, and failure of CNS regeneration correlates with myelin production. Add antibodies to myelin (the IN-1 protein) and axons can extend some. If you do that you also see lots of collateral sprouting of axons in the brainstem and spinal cord. So, IN-1 seems to be a major inhibitor following injury. Collateral sprouting may also be a therapeutic strategy.  NOGO mimics inhibitory effects of CNS myelin. Some evidence that NOGO KOs show neuronal expansion after lesion. NOGO Receptor (NgR) triggers signaling cascades, so antagonizing the NOGO/NgR interaction is a potential way of promoting neuronal regeneration.  MAG and OMgp (components of myelin) also bind to NgR, and use p75 as a signal transducer. This activates Rho and promotes inhibition. 13  MAG is a repellant in adults, but early on it acts as an attractant. It is observed that peripheral lesions promote central regeneration of DRG axons, so these somehow must overcome MAG inhibition. High cAMP seems to make it attractive.  Approaches to promoting neuronal regeneration: Block multiple myelin inhibitors and repulsive cues. Overcome repulsive components of glial scars (CSPGs, semaphorins). Add neuronal growth factors. Lecture 9: Development 4  Cell death is common in the development of the nervous system. It appears to be strategy to match neurons with the appropriate targets, correct errors in connectivity, create sexually dimorphic regions, and remove cells of transient functions.  Neurons seem to die most often when they innervate their target tissues. The size of the target tissue determines how many neurons survive.  The neurotrophic hypothesis: neuronal survival depends on acquisition of target derived neurotrophic factors released from targets in limiting quantities. Neurons then compete for these, and those that get them survive. Other sources of trophic support include ECM, glia, cells in the pathway around a neuron, afferent nerves, hormones, etc.  Non-neuronal (glial) cells also die during nervous development, presumably to match the number of glial cells to the number of axons. Glial cells and neurons seem to provide reciprocal neurotrophic support for each other.  In apoptosis, cells condense chromatin, shrink, form apoptotic bodies. Necrosis involves mitochondrial dysfunction, lysis, etc.  Apoptosis: Bcl-2 family members include cell death promoters (Bak, Bax) and inhibitors (Bcl-2) that either promote or inhibit the release of cytochrome c and smac/DIABLO from the mitochondria. If released, cyt. c forms a complex with Apaf-1, which cleaves procaspase 9 and activates a caspase cascade.  There are lots of families of neuronal growth factors, including neurotrophins (NGF), FGF, IGF, EGF, PDGF, TGF and cytokines. NGF is a prototypical neurotrophic growth factor. It functions as a target-derived factor for peripheral neurons. This was shown by NGF rescuing neurons that otherwise die, more NGF leading to more sympathetic neurons, anti-NGF ab depletes peripheral neurons, neuronal cultures require NGF, death can be prevented by NGF even after axotomoy, NGF is expressed in nerve target tissues.  NGF and neurotrophins are about 120 AAs and function as homodimers. They are expressed in target tissues, glia and neurons.  Receptors for NGF is TrkA, receptor for BDNF and NT-4/5 is TrkB, and receptor for NT-3 is TrkC. TrkB and TrkC are widely expressed in the brain, but Trk A is only in a few types of neurons. Truncated versions of TrkB and TrkC are present on astrocytes.  Neurotrophin binding to a Trk receptor triggers autophosphorylation of tyrosine resides to increase catalytic activity and create docking sites. NGF eventually activates Bcl-2 that prevents apoptosis. Many other growth factors act via RTKs, and many downstream signal molecules are aberrantly expressed in cellular transformation. Since NGF is target derived, it must undergo retrograde transport to the cell body. 14  Synaptogenesis involves presynaptic differentiation, postsynaptic differentiation, and synapse elimination. NMJs are used as the model system to study this. Don’t worry about presynaptic stuff.  In synaptogenesis, the growth cone extends and contacts the muscle. Before the axon arrives, the post-synaptic cell has a diffuse distribution of ACh receptors. The motor neuron releases Agrin, which causes clustering of receptors. Additionally, extrasynaptic nuclei have AChR expression repressed, and subsynaptic nuclei increase AChR expression.  The signaling involved with this includes the motor neuron releasing Agrin for receptor clustering. The Agrin binds an RTK called MUSK, which triggers Rapsyn signaling to affect AChR distributions.  The motor neuron reaches the myotube and results in poly-innervation which is eventually cut back to provide mono-innervation by synapse elimination. This refines the diffuse pattern of synaptic connections in the brain. This may be the most efficient way to get the vast, specific connections required.  Synapse elimination is thought to occur by active synapses sending signals that not only protect themselves, but promote the elimination of adjacent synapses. If one axon is active and another isn’t, the active one persists. If both have equal activity, both persist. Lecture 10: Monoamine Systems I  Many drugs target ACh and Nepi. In early studies of nerve transmission, people thought it might be an electrical signal, but this didn’t account for both excitation and inhibition.  People observed that drugs could mimic autonomic stimulation. Muscarine mimics vagal slowing of the heart. Atropine blocks effects of both vagal stimulation and muscarine, suggesting that muscarine shares a common mechanism with endogenous vagal signals.  Loewi collected perfusate from a heart under vagal stimulation and applied it to a normal heart to produce vagal effects, confirming the existence of a transmitter. He then biochemically isolated it and identified it as ACh.  The minimal criteria to show something’s a NT include: It should mimic nerve stimulation, needs to be present endogenously, and its antagonists must block nerve transmission. Basically, it must be present, necessary and sufficient.  Biosynthesis: Acetyl-CoA + Choline → Acetylcholine, via enzyme Choline Acetyltransferase.  ACh is released into synapses, acts on receptors, then is degraded by acetylcholinesterase (into acetic acid and choline). High affinity choline uptake channels on the pre-synaptic nerve sequester the choline, then it converts it to ACh, use an ion gradient to concentrate it in vesicles so it’s ready for subsequent release.  AChE inhibitors block degradation to potentiate cholinergic transmission. This can be used to treat myasthenia gravis, where you have Ab to the ACh receptors and don’t respond well to ACh stimulation. Potent, irreversible AChEs like Sarin are used as nerve gasses. This can be fatal by interfering with diaphragm respiratory function.  Drugs can also specifically target certain receptor subtypes. Nicotinic receptors act on skeletal muscle. Muscarinic receptors act on heart and smooth muscle. Curare is a selective antagonist of nicotinic receptors, paralyzing the diaphragm and skeletal muscle. Atropine blocks only muscarinic receptors, which can reverse bradycardia. 15  Types of receptors. Nicotinic receptors are ligand-gated Na+/Ca++ channels that depolarize skeletal muscle. Muscarinic receptors come in at least 5 subtypes, and developing subtype selective drugs is important in pharmacology. Muscarinic receptors all respond to ACh, but they are coupled to different G proteins and signal cascades. M2 receptors in the heart activate G-proteins that are directly attached to K+ channels and open them to slow the heart rate.  ACh mediates parasympathetic actions, and NEpi was discovered to be the transmitter of the sympathetic nervous system. The closely related Epi is released from the adrenal medulla.  Biosynthesis: Tyrosine hydroxylase is the rate limiting step, and is selectively expressed in catecholamine neurons. Aromatic AA Decaryboxylase is widely expressed. Vesicular Monoamine Transporters (VMAT) sequester dopamine in vesicles, these are blocked by reserpine to decrease NEpi secretion and treat hypertension. Dopamine β-hydroxylase is in synaptic vesicles, where the final product of NEpi is made. In the adrenal medulla, corticosteroids regulate the transcription of PNMT to control Epi levels.  Tyrosine (Tyrosine Hydroxylase)→ L-DOPA (Aromatic AA Decarboxylase)→ Dopamine (Dopamine β-hydroxylase)→ NEpi (PNMT)→ Epi.  Inactivation of NEpi occurs by reuptake into nerve terminals by high affinity uptake channels. Cocaine blocks this process. Degradation isn’t the primary mechanism of inactivation, but once in the nerve NEpi can be degraded by Monoamine oxidase or catechol- O-methyltransferase.  All NEpi effects are mediated by GPCRs. The receptors are divided into α and β subtypes. α receptors inhibit adenylyl cyclase and contribute to smooth muscle contraction, so alpha antagonists can be anti-hypertensive. β adrenergic receptors stimulate adenylyl cyclase and increase heart rate, as well as bronchodilate. There are subtypes of both types of receptors. Selective agents for beta receptors treat tachycardia and bronchoconstriction. Lecture 11: Periepheral Autonmic Nervous System  The sympathetic nervous system innervates all tissues except the constrictor pupillae. The parasympathetic nervous system only innervates glands and thoracic/abdominal viscera. These two systems usually have opposite effects on dually-innervated tissues, such as symp increasing mucous secretion and psymp increasing serous (enzymatic) secretion from salivary glands. There may be some basal activity of each, but to alter the balance you generally excite one and inhibit the other. Reciprocal coordinate accomplished by CNS.  The heart is a good example. Psymp innervates the pacemaker cells via the vagus (ACh on muscarinic receptors). They have a rapid hyperpolarizing effect, mediated by a G-protein coupled to a K+ channel. The ACh also decreases Na+/CA++ permeability. Symp innervates both the pacemaker and myocardium (NEpi on adrenergic β1 receptors). Symp nerves go through the inferior cervical ganglion, but its NEpi acts slower because it acts via GPCR signaling cascades to increase Na+/Ca++ conductance.  Postganglionic nerves in both systems receive ACh from preganglionic nerves at nicotinic receptors. Postganglionic nerves in symp then release NEpi, and psymp ACh.  Cholinergic receptors: How can ACh slow the heart muscle and increase gastric motility? In the gut, M3 muscarinic receptors lead to ACh increasing Na+/Ca++ conductance to increase muscle contraction. In the heart, M2 muscarinic receptors increase K+ conductance and decrease Na+/Ca++ conduction. Nicotinic receptors at the autonomic ganglia cause depolarization and firing of postganglionic neurons (as well as stimulating the adrenal 16 medulla cells to secrete catecholamines…adrenal medulla basically acts like a post- ganglionic sympathetic neuron).  Adrenergic receptors: Alpha receptors act via PLC, DAG/IP3, and Ca++. They cause vascular smooth muscle to contract, as well as playing a role in negative feedback on nerve terminals secreting NEpi. The beta receptors (β1) on the heart’s pacemaker speed it up through adenyl cyclase to open Ca++ and Na+ channels; they also increase myocardial contractility. Beta receptors also relax bronchial, vascular and intestinal smooth muscle.  Sympathetic ganglia area site of lots of processing. Each preganglionic neuron may synapse on hundreds of postganglionic ones (divergence), since a limited number of preganglionic neurons in the spinal cord have to affect almost every tissue in the body. Parasympathetic nerves show much less divergence.  Each postganglionic nerve may receive input from up to ten preganglionic neurons. This convergence may provide the multiple inputs necessary to excite a postganglionic neuron. This may play a role in the patterning of sympathetic responses.  Experiements suggest that the organization of synapses in ganglia may be changed over a period of days, and may be considerably remodeled over time (ex: metabolic changes through different seasons).  Visceral afferents (non-somatic sensory info from tissues to the brain) are largely stretch receptors or chemoreceptors. Baroreceptors in the carotid sinus, for example, have very little smooth muscle and stretch with changes in pressure. The receptor nerves accurately represent the arterial pulse pressure, and convey the info via the glossopharyngeal nerve to the nucleus solitarius of the medulla.  Spinal and ganglionic systems are important for autonomic regulation, but organization of autonomic output really takes place in the brain. Visceral and somatic sensory info from the spine and cranial nerves converge on the brain. The brain plays an important role in stimulating autonomic preganglionic neurons, but also strongly inhibits spinal autonomic reflexes from stimulating these preganglionic neurons. Through this inhibition, supraspinal systems prevent inappropriate behavior in spinal autonomic systems.  Spinal cord injury not only removes the normal descending control of autonomics, but also can remove the inhibition of spinal autonomic reflexes that can be life-threatening. This is known as “autonomic dysreflexia.” With no supraspinal inhibition, sensory input like transabdominal pressure (from bladder fullness or even just cloth on the skin) can elicit a huge increase in arterial pressure. This can put you in danger of stroke. Lecture 12: Monoamine Systems II  In addition to ACh and NEpi that act peripherally, monoamines are critical to CNS function. These are affected in many neuropsych disorders and by many drugs.  Dopamine (DA) was known to be an intermediate in the NEpi biosynthetic pathway: Tyrosine (Tyrosine Hydroxylase)→ L-DOPA (Aromatic AA Decarboxylase)→ Dopamine  Vesicles in the neuron have an H+ ATPase to build a gradient, which is used a Vesicular Monoamine Transporter (VMAT) to exchange for DA. The drug reserpine inhibits the VMAT.  They saw that DA levels were decreased in Parkinson’s and present at higher concentrations than NEpi in some places, suggesting that it’s a NT. There are separate DA uptake channels, as well as DA GPCRs which made it clear. A minor set of neurons also appear to use Epi as a NT. DA, Epi, and NEpi = catecholamines, all appear to act as NTs. 17  Serotonin was first discovered in the serum as something regulating vascular tone. They found that LSD blocked serotonin’s vascular actions, and since LSD also caused hallucinations, it was suggested that: serotonin might be a NT in the brain, LSD elicits its psychoactive properties by affecting serotonin there, and since LSD mimics psychotic disorders serotonin may play a role in the pathophysiology of such disorders.  Serotonin (aka 5-hydroxytryptamine, or 5-HT) is also an intermediate in the biosynthetic pathway of melatonin, generated in the pineal gland. Like DA and NEpi, it’s mostly inactivated by reuptake. SSRIs are the mostly commonly prescribed antidepressants, and increase 5-HT tone.  Serotonin biosynthesis: Tryptophan (Tryptophan Hydroxylase)→ 5-Hydroxytryptophan (Aromatic AA Decarboxylase)→ Serotonin. Serotonin is also sequestered into vesicles by VMAT, and VMAT inhibitors for treating hypertension were found to decrease serotonin and increase the incidence of depression.  There are 14 different serotonin receptors, and rather than simply using SSRIs to increase [serotonin], altering these could have more specific effects.  Histochemical staining allows the detection of central monoamine systems and their architecture:  NEpi cell bodies are located in the locus coeruleus, in the dorsal pons under the 4 ventricle. Axons go from here to emanate throughout the forebrain.  5-HT cell bodies are in clusters along either side of the raphe of the brainstem, especially in dorsal and median raphe clusters. These also project throughout the forebrain.  DA cell bodes are located in the ventral midbrain in the substantia nigra. Loss of neurons here is associated with Parkinson’s, since they innervate the basal ganglia, a motor relay. DA neurons are also located on the midline medial to the substantia nigra in the ventral tegmental area (VTA). These VTA DA neurons innervate the forebrain and limbic system (regions of emotional and cognitive processing). It’s not as widespread as serotonin or NEpi.  For NEpi, serotonin, and DA, this organization allows a few neurons to exert widespread influence.  Monoamines have such diverse actions because each NT has multiple receptor types, and each receptor can be coupled to different downstream signaling systems in different ecll types. Monoamines often produce inhibitory effects by opening K+ channels, to which their receptors are directly coupled via a G-protein.  Monoamines can also act by modulation. Often monoamines alone have little effect on a resting target neuron, but will greatly amplify any excitatory stimulus that comes in. Normally, excitatory inputs trigger negative feedback mechanisms that open K+ channels. Monoamines can block these to accentuate an excitatory signal when it occurs.  The locus coerulues (NEpi) is very sensitive to stimulation of any sensory modality. It supposedly alerts you to increase vigilance in response to potentially threatening stimuli. Drugs that induce LC firing produce anxiety (this is seen in opiate withdrawal).  The demonstration that potency of drugs for anti-psychotic purposes parallels their affinity for DA receptors has linked DA to psychotic disorders. These drugs (haldol) block DA receptors and produce Parkinson’s-like side effects due to actions in the substantia nigra pathway, but blocking DA in the VTA may produce their anti-psychotic effects. Lecture 13/14: Amino Acid and Gaseous NTs 18  The amines (NEpi, Epi, DA, 5-HT, and ACh) are only active at about 1% of synapses each, and maybe 5% for ACh. GABA acts at 20-40%, glycine at 25%, and glutamate at 50% of brain synapses. GABA and glycine are the primary inhibitory NTs, and glutamate is the primary excitatory NT. NO and CO are on
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