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Lecture 6

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Rutsuko Ito

Lecture 6 Synaptic organization of the hippocampus Lecture outline Hippocampus: 1. Together with the amygdala, the hippocampus is the central component of the limbic system Hippocampus and cortico-striatal loops: 2. The hippocampus is a major component of the ‘affective’ cortico-striatal loop, providing limbic information to both the ventral striatum and medial prefrontal cortex Coronal plane of the hippocampus: H.M. – the most studied person in neuroscience? 3. The most famous case of amnesia a. Absence seizures started at 10 years old i. Short term seizures characterized by sudden onset b. Generalized (whole body) seizures started at 16 c. Medication was not controlling seizures d. At 27 decided to have surgery to remove his medial temporal lobe bilaterally 4. Result of surgery: a. Global amnesia i. Anterograde amnesia: 1. Unable to form new declarative long term memories a. Episodic b. Semantic ii. Retrograde amnesia 1. Unable to retrieve any declarative memories from the 11 years before surgery b. Other aspects of memory and cognition are preserved i. Short term working memory ii. Procedural memory iii. Language iv. Visuospatial perception v. Attention vi. Etc Medial temporal lobe circuitry: 1. Hippocampus is part of the medial temporal lobe a. Perirhinal / postrhinal cortex i. Reciprocal connections with cortical sensory areas b. Entorhinal cortex i. Reciprocal connections with 1. Frontal cortex 2. Perirhinal / postrhinal cortex ii. Connections with 1. Hippocampus proper c. Hippocampus proper i. Subdivision into 1. Mossy fiber 2. Schaffer’s collateral 3. Subiculum a. These subdivisions are connected to each other ii. Connections with 1. Layers III / IV / V of entorhinal cortex 2. Frtonal cortex Functioning of hippocampus proper: 1. Spatial learning and memory / navigation a. Length of time spent as taxi driver correlates with increased volume of right posterior hippocampus b. Place cells in the hippocampus of rodents i. Firing of place cells in accordance of specific environmental spatial areas 2. Context learning and retrieval a. Rats without a hippocampus do not show freezing (stopping of activity) to a cue that has previously paired with shock b. PET study – context (e.g. spatial scene or a room) anxiety in humans is associated with activation in the right hippocampus and ventral pallidum 3. Working memory a. fMRI study – hippocampal activation during the maintenance phase of working memory task b. Hippocampal lesions impair spatial working memory task using radial maze i. HPC-lesioned rats show increased re-entries into already visited arms of the radial maze 4. Higher order perception of spatial information a. Rodents and humans 5. Novelty detection a. Rodents and humans 6. Timing 7. Anxiety a. Rodents 8. Behavioral inhibition / control of impulsivity a. Rodents Hippocampus proper: 1. Well defined laminar structure with clear layers made up of rows of pyramidal cells a. Septotemporal axis b. Transverse axis Hippocampal network: Transverse pathways: 1. Generally unidirection information flow a. Entorhinal cortex  dentate gyrus (DG) and CA3 pyramidal neurons via the perforant path (PP) b. DG  CA3 neurons via mossy fibers (MF) c. CA3  CA1 neurons via schaffer collateral pathway (SC) d. CA3  CA1 neurons in contralateral HPC via associational commissural pathway (AC) e. CA1  Subiculum  entorhinal cortex Principal neurons: 1. Dentate gyrus a. Granule cell i. Unipolar ii. Dendrites emanate upwards 2. CA3 / CA1 / Subiculum a. Pyramidal neurons i. Top down: 1. Tuft 2. Apical dendrites 3. Soma 4. Basal dendrites Dendritic length of principal neurons: 1. CA1 a. Pyramidal cell layer i. Stratum pyramidale b. Stratum radiatum c. Stratum oriens 2. DG area a. Granule cell layer b. Molecular layer c. Polymorphy cell layer (Hilus) Synaptic inputs to pyramidal neurons: 1. Tuft: a. Excitatory inputs from extrinsic sources i. E.g. 1. Entorhinal cortex 2. Thalamus 2. Apical and some basal dendrites a. Excitatory inputs from local sources i. E.g. 1. CA3 b. Apical areas 3. Basal dendrites a. Inhibitory (GABAergic) inputs from local interneurons b. Perisomatic areas Characteristic intracellular response to stimulation 1. A typical synaptic response recorded intracellularly in a CA1 pyramidal neuron during schaffer collateral stimulation a. The EPSP is followed by a prolonged IPSP mediated by feed-forward inhibition from interneurons that stimulate postsynaptic GABAa and GABAb receptors Intrinsic electrophysiological properties of hippocampal neurons 1. CA1 a. Prolonged firing i. With pronounced spike frequency adaptation ii. Slow after hyperpolarization 2. CA3 a. Brief higher frequency bursts of AP i. Gives rise to ‘theta’ rhythm associated with behavioral exploration and spatial learning Extracellular responses in the hippocampus: 1. Field potentials (using an electrode to measure readings in the extracellular regions of a cell) recordings from the hippocampal CA1 during stimulation of the schaffer collaterals a. Time course of field potentials often matches the time course of the underlying synaptic currents b. Relatively large and reliable population EPSPs can be recorded from the hippocampus due to the fact that the neurons are in the same orientation and receive synaptic inputs in the same area 2. Field potential inflections (positive or negative) are opposite in direction to intracellular recordings a. Fiber volley: Brief negative transient firing that results from presynaptic activity as current flows into the dendrites i. Field potential measures: 1. Current flows into the cell, away from the neuron  results in a negative reading b. pEPSP: Summed activity across a population of neurons i. Current moving into dendrites during pEPSP will exit the neurons near the soma and a positive potential can be recorded from the stratum pyramidale at this point (negative potential at the stratum radiatum) 1. Why does it flow out near the soma of stratum pyramidale? c. pSpike: A negative potential resulting from inward current during postsynaptic AP Perforant pathways: 1. Perforant path axons bifurcate and synapse repeatedly on granule cell dendrites within the dentate gyrus 2. The PP has two parts a. Medial perforant path b. Lateral perforant path 3. The two paths terminate on distinct regions within the granule cell dendritic field (molecular layer) and CA3 dendritic field a. In both regions, the MPP terminates in a layer more proximal to the pyramidal cell soma 4. Distinct populations of CA1 receive LPP and MPP inputs 5. Functional dissociation of the MPP and LPP can be demonstrated using selective pharmacological deactivation of the pathways a. AP5 blocks NMDAR mediated LTP (LTP  unit of learning) in the MPP but not LPP b. Naloxone blocks micro-opiate receptor-mediated LTP in LPP but not MPP Spatial vs non-spatial perforant pathways: 1. AP5 blocks MPP LTP 2. Naloxone blocks LPP LTP 3. Study: a. Dependent measure:
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