Synaptic transmission Module 725 Lecture 2

Aim nWhy do we need synapses? nTo know about chemical synapses u diversity u flexibility u mechanisms

Overview nNeurotransmitters nSynaptic structure nTransmitter release nTransmitter breakdown/uptake nIonotropic receptors

Neurotransmitters nAcetylcholine namino-acids u glutamate u GABA npeptides u FMRFamide nNO (see next week)

Synaptic structure nNeuromuscular junction nCNS synapses nCommon features u transmitter stored in vesicles u receptors on post- synaptic membrane u cell-cell signalling to organise synapse

Neuromuscular junction

CNS synapses

Overview of physiology nSimulation at ent/nervous_system/cell_neuro/synapses/ release.html

Transmitter release ncalcium-dependent nvesicle cycling - or kiss and run

Calcium domains nsquid giant synapse nuse n-aequorin-J as Ca indicator

Calcium-dependency nCalcium entry very near vesicle! nmicro or even nano-domains nsynaptotagmin as Ca sensor ?

Vesicle cycling nConventional view nneed recycling because no evidence of increase in membrane area u e.g. capacitance measurements

Vesicle proteins nsynaptobrevin & SNAP-25 anchor vesicle membrane to plasma membrane nsyntaxin helps in conformational change nsynaptotagmin as Ca sensor

Synaptotagmin ncalcium sensor u arginine in C 2 A and C 2 B domains

Exocytosis nfree energy barriers need to be overcome during fusion

Endocytosis nClathrin coats “empty” vesicles Movie from

Kiss and run

testing… testing... ncapacitance measurements- measures surface area nGFP-derivative called synaptophlorin reports pH (vesicles very acid) nFM dye which fluoresces only in membrane

Summary so far ntransmitter put into vesicles nvesicle release is Ca-dependent nmajor protein players include u synaptotagmin u SNARE u clathrin

Breakdown or uptake? nesterase (ACh, peptides) ntransport - mostly into glia (amino acids)

Re-uptake n12 membrane spanning regions nco-transport using Na gradient

Uptake inhibitors nmajor drugs u cocaine - block serotonin & dopamine reuptake u Prozac - selective serotonin reuptake inhibitors serotonin cocaine prozac

Summary so far ntransmitter put into vesicles nvesicle release is Ca-dependent ntransmitter recycled u after breakdown u direct pump for reuptake

Ionotropic receptors nIonotropic receptors have an integral channel which opens when receptor binds nMetabotropic receptors activate a second messenger internally nImportant ionotropic receptors include u nicotinic ACH receptor u glutamate (NMDA, AMPA, Kainate) u GABA

Nicotinic ACh receptor 2 , one each of  ,  ACh binds to  - need 2 ACh to open channel

Nicotinic ACh receptor nhighly conserved residues binding ACh

Glutamate receptor 2 , one each of  ,  glu binds to  - need 2 glu to open channel nNMDA receptors also need glycine

Multiple glu receptors nNamed after agonists

NMDA receptors (I) nblocked by Mg, except at depolarised voltages nneed glycine as cofactor squirt a-a and see cell current

NMDA receptors (II) nmay provide slow component to synapse stimulate presynaptic neuron and see cell current

AMPA & Kainate receptors nMay both be used at same synapse nstimulate presynaptic cell and record voltage u APV blocks NMDA, GYKI blocks AMPA LY blocks kainate

GABA A receptor

GABA A - drug action nBenzodiazepines e.g. Valium (=diazepam) u hypnotic, anxiolytic, anticonvulsant, myorelaxant and amnesic n Barbiturates e.g. pentobarbital u sedative action

GABA A - diazepam nlonger openings in bursts control DZ

Summary ntransmitter put into vesicles nvesicle release is Ca-dependent ntransmitter recycled u after breakdown u direct pump for reuptake nionotropic receptors u great diversity u homologous subunits nSynaptic transmission major drug target