PSYB64H3 Chapter Notes - Chapter 4: Tryptophan Hydroxylase, Aromatic L-Amino Acid Decarboxylase, Tyrosine Hydroxylase

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19 Nov 2012
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4 | Psychopharmacology
Neurotransmitters, Neuromodulators, Neurohormones
- neurotransmitters: chemical messenger that communicates across a synapse
- neuromodulators: chemical messenger that communicates w/ target cells more distant than synapse
by diffusing away from point of release
- neurohoromone: chemical messenger that communicates w/ target cells at great distance, often by
traveling through circulation
Identifying Neurotransmitters
- agree w/ additional criteria:
1) neurotransmitter must be synthesized w/in neuron
2) in response to arrival of action potential = substance released in sufficient quantities = produce
effect on postsynaptic cell
3) should be able to duplicate action of suspected neurotransmitter experimentally on postsynaptic
cell
4) a mechanism exists that ends interaction between neurotransmitter and postsynaptic cell
Types of Neurotransmitters
- figure 4.2, p.96 major categories of neurotransmitters, neuromodulators, and neurohormones
- table 4.1, p.97 features of Small-Molecule Transmitters and Neuropeptides
The Small-Molecule Transmitters
- meet most or all of preceding criteria specific for neurotransmitters and appear to play vital role in
neurotransmission:
acetylcholine
5 monoamines
amino acids
energy molecule adenosine triphosphate (ATP)
- acetylcholine (ACh): used at neuromuscular junction, in autonomic nervous system and central
nervous system
neurons that use this as major neurotransmitter are referred as cholinergic neurons
cholinergic neurons
use building blocks
o choline obtained from dietary fats
o acetyle coenzyme A (acetyl CoA) results from metabolic activites of mitochondria
enzyme choline acetyltransferase (ChAT)
o acts on two building blocks (or precursors) = produce acetylcholine
o useful marker for identifying cholinergic neurons
o only found in neurons that produce ACh
also produces acetylcholinesterase (AChE): enzyme that breaks down acetylcholine
o choline from breakdown = recaptured by presynaptic neuron and resynthesized into
more ACh
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- ACh is primary neurotransmitter at neuromuscular junction = synapse between neuron and muscle
fiber
essential to operation of autonomic nervous system
all preganglionic synapses use ACh as neurotransmitter
same w/ postganglionic synapses in parasympathetic division
- cholinergic neurons important in peripheral nervous system, widely distributed in brain
figure 4.3, p.97
major groups of cholinergic neurons located in: basal forebrain, septum, and brainstem
project to neocortex (sight and hearing in mammals), hippocampus, and amygdala
these areas especially likely to deteriorate because of Alzheimer’s
given memory loss associated w/ this disease, neurons appear to participate in learning and
memory
- many subtypes of cholinergic receptors found
two major subtypes
1) nicotinic receptors: postsynaptic receptor that responds to nicotine and ACh
2) muscarinic receptors: postsynaptic receptor that responds to both ACh and muscarine
o derived from hallucinogenic mushroom amanita muscaria
- difference between nicotinic and muscarinic receptors
mechanism of action
N = ionotropic receptors
M = metabotropic
location
N = neuromuscular junction (speed for muscular responses)
CNS and ANS contain both (M more common in CNS
- five monamines: one of major group of biogenic amine neurotransmitters
further divided into two subgroups
1) catecholamines (dopamine, norepinephrine, and epinephrine)
2) indoleamines (serotonin and melatonin)
all monamines subject to reuptake from synaptic gap following release
monamines not enased in vesicle w/in axon terminal nroken down by enzyme monoamine
oxidase (MAO)
- figure 4.4, p.98 catecholamines share common synthesis pathway
begins w/ amino acid tyrosine
neurons also contain enzyme tyrosine hydroxylase (TH)
when TH acts on tyrosine = L-dopa
production of dopamine enzyme dopa decarboxylase acts on L-dopa
dopamine converted to norepinephrine action of enzyme dopamine -hydroxylase (DBH)
takes place w/in vesicles
epinephrine reaction between norepinephrine and enzyme phenylethanolamine N-
methyltransferase (PNMT)
PNMT located in intracellular fluid of axon terminal of neurons that use epinephrine
Once norepinephrine synthesized in synaptic vesicles = released into intracellular fluid =
interact w/ [NMT = epinephrine
Epinephrine transported into vesicles
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