Synaptic Physiology PDF

Synaptic Physiology 1 Neurons communicate with other cells at synapses • Where found? Neurons communicating with… – other neurons (alter activity of postsynaptic neuron) – skeletal muscle (cause muscle contraction) – cardiac muscle (alter activity—rate, contraction strength—of heart) – smooth mus...

Synaptic Physiology 1 Neurons communicate with other cells at synapses • Where found? Neurons communicating with… – other neurons (alter activity of postsynaptic neuron) – skeletal muscle (cause muscle contraction) – cardiac muscle (alter activity—rate, contraction strength—of heart) – smooth muscle (increase/decrease contraction) – glands, either... • endocrine (release of hormones into blood) • exocrine (release of enzymes, etc., into gut) 2 So-called electrical synapses • Common in invertebrates, rare in vertebrates • Neuron-neuron connections in specific brain centers • Mediated via gap junctions – “channels” spanning membranes of 2 cells – permit exchange of small molecules • ions (Na+, K+, etc.) = electrotonic current flow • metabolites (glucose, amino acids, etc.) – molecular-weight cutoff: ~1000 Da • things bigger (e.g., proteins) can’t get through – cell Ca++,  cell pH, closes gap junctions • isolates a damaged cell 3 4 An aside…Syncytial tissues • Bunches of cells connected by gap junctions called a syncytium (or syncytial tissue) • Cells share metabolites, electrotonic current, and thus all work together • Common examples include – epithelia (e.g., layer of tissue lining gut, glands) – heart – smooth muscle (e.g., motility of gut) 5 Back to electrical synapses... • Advantage: cell-to-cell communication extremely rapid • Disadvantage: size problem when small neuron synapses with large neuron small neuron large neuron current flow time mV time mV 6 Poor “safety factor” for communication • Small neurons produce small electrotonic currents • Currents insufficient to depolarize a largediameter postsynaptic cell: communication ineffective • Electrical synapses only effective if… – connections between similar size neurons – connections of large neuron to smaller neuron 7 Vast majority of synapses: chemical synapses • Types: a) – neuron-neuron – neuron-skel. muscle (“endplate”) b) – neuron-heart or smooth muscle – neuron-glands (epithelial cells) – Also… sensory cells-neurons pre post pre post pre No physical connection between pre- and postsynaptic cells! 8 Simplest example: neuron-neuron synapse presynaptic axon mitochondrion synaptic cleft 30-50 nm gap nerve terminal subsynaptic membrane membrane vesicles postsynaptic membrane 9 Sequence of events: transmission at chemical synapse (1) AP propagates down presynaptic axon (2) Vesicles move to presynaptic membrane of terminal (3) Vesicles fuse with presynaptic membrane, a process called exocytosis (4) Vesicles contain chemical substance called transmitter (or neurotransmitter); transmitter released into cleft (5) Increase in transmitter concentration in cleft (6) Transmitter diffuses across cleft to subsynaptic membrane (7) Transmitter binds to protein receptor on subsynaptic membrane (8) Receptor linked to ion channel, which opens (9) gNa+ gK produces EPSP gK and/or gCl produces IPSP 10 Details: presynaptic vesicles • Vesicles 30-50 nm in diameter • Each contains hundreds to thousands of transmitter molecules • Minimum amount of transmitter released is one vesicle – Called a quantum of transmitter – NOT one molecule! 11 Details: exocytosis • Common in many cellular processes • Like coalescing of soap bubbles presynaptic membrane • Opposite of exocytosis is endocytosis (takes up substances into cell). Two types: – Pinocytosis (“cell sipping”): uptake of molecules – Phagocytosis (“cell eating”): uptake of entire cells, bacteria, etc. 12 What controls synaptic release? • Recall… AP arrives at terminal, which depolarizes presynaptic membrane • Voltage-dependent Ca++ channels open: – Inward Ca++ current – Rise in nerve-terminal [Ca++] – Activation of various secondary messengers – Alter cytoskeletal proteins – Vesicle migration to membrane and exocytosis • Critical dependence on extracellular [Ca++]! 13 Dependence on extracellular divalent cation concentrations • Hypocalcemia (low extracellular [Ca++]) – Causes  Ca++ influx into terminal – Result:  synaptic release • Hypermagnesemia (high extracellular [Mg++]) – Competitive inhibitor of Ca++ influx – Result:  synaptic release • Low extracellular Ca++ and/or Mg++ – Causes hyperexcitably in axons (easier to fire APs), via screening of surface charges – Independent effect on AP 14 Diffuse double layer: Normocalcemic conditions Na+ concentration  Neutral lipid (e.g.,PE) Neg. charge (e.g.,PS) Na+ Ca++ in x (nm) out Diffuse double layer: Hypocalcemic conditions Na+ concentration  Neutral lipid (e.g.,PE) Neg. charge (e.g.,PS) Na+ Ca++ in x (nm) out What stops exocytosis of transmitter? • Presynaptic terminal… – repolarizes following AP closes Ca++ channels – also, Ca++ channels spontaneously inactivate (close) • Decreased influx of Ca++ • Ca++ levels decrease in terminal, due to… – Ca++ pump, a Ca-ATPase (primary active transport) – Na+/Ca++ exchange process (secondary active transport) • Result: exocytosis and transmitter release halts 17 Chemical synapses exhibit a delay • Called synaptic delay – ~ 0.5 ms (time between arrival of AP at terminal to change in postsynaptic-membrane potential) • Is it the time for diffusion of transmitter across cleft? – No! Proof… (note: 50 nm = 5106 cm)  5  10 cm  x 6  1  10 s  0.001 ms t  -5 2 2 D 2  10 cm / s 2 6 2 • 0.5-ms synaptic delay caused by – time required for Ca++ to signal start of exocytosis – time required for exocytosis itself 18 What determines EPSP or IPSP? • Postsynaptic channel selectivity determines result... – gNa+ gK produces EPSP – gK and/or gCl produces IPSP • Not dependent on transmitter itself – Transmitter can be excitatory at one synapse – Transmitter can be inhibitory at another synapse • Analogy… consider a door – Channel pore = doorway (can be large or small) – Postsynaptic receptor = lock on the door – Transmitter = key to the lock 19 Not all chemical synapses involve opening of postsynaptic ion channels! • Transmitter binding to postsynaptic receptor can result in no postsynaptic membrane-potential change, but can… – activate cyclases that increase postsynaptic cAMP or cGMP (intracellular messengers) – activate protein kinases (that phosphorylate other proteins thereby changing their activity) – cause hydrolysis of membrane lipids (releasing IP3 and DAG, other intracellular messengers) – release Ca++ from intracellular stores • More about this later in the course 20 General characteristics of synaptic transmission • Postsynaptic membrane... – has receptors to multiple different transmitters – some are excitatory, some are inhibitory (e.g., “dual innervation”) – Notable exception: innervation of skeletal muscle (EPP) solely excitatory • Presynaptic cells – only release one principal type of transmitter 21 Some neurotransmitters… Acetylcholine (ACh) • Acetate + choline • Neurons that release ACh termed 3 H3C CH + + AcetylCoA N cholinergic HO CH3 • Transmitter at all skeletal-muscle Choline NMJ’s (excitatory, produce EPP) • Numerous other synapses mostly O 3 H3C CH + outside CNS (e.g., inhibitory at heart) N H3C O CH3 • Two principal types of receptors Acetylcholine (ACh) – Nicotinic (NMJ, other places) – Muscarinic (glands, heart, etc.) 22 Transmitters derived from tyrosine • Dopamine – from dopaminergic neurons • Norepinephrine (= noradrenalin) – common outside CNS – from adrenergic neurons – two main classes of receptors:  and  (e.g., -receptors in heart cause in heart rate) • Epinephrine (= adrenalin) – not “true” neurotransmitter; acts mostly as a circulating hormone (released by medulla of adrenal gland) Tyrosine (amino acid) Dihydroxyphenylalanine (dopa) HO HO H2 C C H2 NH2 Dopamine OH HO NH2 C H C H2 HO Norepinephrine (= Noradrenalin) OH HO H N C CH3 H C H2 HO Epinephrine (= Adrenalin) 23 Misc. other transmitters… • Serotonin (derived from amino-acid tryptophan = 5-HT = 5-hydroxytryptamine) • Glutamate (an amino acid) – MSG = monosodium glutamate (flavor enhancer) • -aminobutyric acid (GABA) – major inhibitory transmitter in spinal cord – disease tetanus caused by blocking of these synapses • And other neurotransmitters … • And there are neuromodulators: substances released by neurons that acts as an autocrine or paracrine that alter (slowly) neurotransmitter release (e.g., quantal content) 24 What causes postsynaptic repolarization?   TR • Reversible binding to receptor via reaction: T  R   • decrease in [T] in cleft: – causes left-shift of reaction reversible reaction – [TR] (T bound to R) decreases – postsynaptic channel closes – postsynaptic Vm repolarizes • Any process that causes  in cleft [T] will aid in terminating postsynaptic event (e.g., EPSP, IPSP, EPP) 25 Three mechanisms to decrease [TR] • (1) T diffuses out of cleft, diluting into interstitial fluid • (2) Presynaptic terminal sucks up T for reuse – mechanism at adrenergic synapses – active transport process for uptake of norepinephrine – norepi. repackaged into new synaptic vesicles • (3) Destroy (hydrolyze) T into substances that do NOT act as transmitters – mechanism at cholinergic neurons via cleft enzyme acetylcholinesterase (or “cholinesterase,” or “ACh-ase”) – Reaction: ACh  acetate + choline – Note: choline not wasted; taken up into presynaptic terminal via active-transport process 26 Neuropharmacology Drugs/toxins that affect synaptic release Botulinum toxin • Most lethal toxin known (LD50 1.3 ng/kg) • From Clostridium botulinum (anaerobic bacterium) – Latin for sausage = botulus (“sausage poisoning”) – Killed by NaNO3 (used to cure meats like bacon, pastrami) • Permanently poisons synapse by preventing vesicle release of ACh from motor neurons; causes flaccid paralysis • Therapeutic uses (weaken muscle for 3-4 months): – Cosmetic (removes wrinkles) (Why need for re-treatment every few months?) – Treatment of spasms and dystonias (tremors, strabismus, achalasia) – Excessive sweating (hyperhidrosis) Tetanus toxin • Almost as lethal as botulinum (LD50 1 ng/kg) • From Clostridinum tetani (anaerobic bacterium) – In soil worldwide; requires anaerobic conditions for growth (e.g., puncture wound) • Permanently poisons GABA neurons by preventing vesicle release; causes loss of inhibition of -motor neurons – Uncontrolled muscle contraction leading to death via asphyxiation • Therapeutic uses: none • Prevent with immunization (booster required every 10 years) Black-widow spider venom • Lactrodectus spiders produce -Latrotoxin (LTX) • Causes insertion of Ca++ channels in nerve terminal resulting in inward Ca++ current – Early response: uncontrolled dumping of synaptic vesicles – Late response: depletion of synaptic vesicles • Latrodectism = illness related to spider bite – Symptoms: pain, cramps, sweating, muscle cramps – Rarely fatal – Antivenom (from horse) available if needed • Not used clinically (but used as research tool to deplete presynaptic terminal vesicles) Hemicholinium • Blocks uptake of choline into presynaptic nerve terminal of cholinergic neurons (after prior release of ACh) • Blocks synthesis of ACh from choline + acetyl-CoA (choline acetyltransferase) • Reduces quantal content of synaptic vesicles – E.g., reduces EPP amplitude • Not used clinically, but used as a research tool (e.g., showed that choline uptake is the rate-limiting step in presynaptic ACh synthesis). Neuropharmacology Non-depolarizing paralytics Curare (d-tubocurarine) • South American arrow/dart poison: paralyzes by binding to nicotinic ACh receptor, preventing EPP – Causes death via axphyxiation • Not toxic if ingested p.o. (great for hunting) • Analogs (e.g., pancuronium) used clinically – Blocks withdrawal reflex during surgery – Administered after anesthesia and patient on artificial respirator • The “second drug” used in lethal injections of convicted criminals Atropine • Blocks muscarinic-type ACh receptor affecting heart, glands, iris of eye (and others) – Causes increased heart rate, dry mouth, dilation of pupil, etc. • From belladonna (“beautiful woman”) plant – Used when courting during medieval times • Named for Atropos (Greek mythology), the oldest of the “Three Fates” who “cut the thread of life.” (Roman equivalent is Morta.) • Clinical use: “antidote” for poisoning by cholinesterase inhibitors – Carried in a self-injector by troops in danger of poisoning by nerve agents (chemical warfare) Neuropharmacology Depolarizing paralytics Succinylcholine (suxamethonium chloride) • Binds to nicotinic ACh receptor and opens postsynaptic channel – Produces sustained and large ampitude EPPs that depolarizes muscle fiber – Not hydrolyzed by acetylcholinesterase – Sustained depolarization causes closure of inactivation (h) gate, preventing opening of Na+ channel, thus paralyzing muscle • Used clinically, especially emergency intubation for trauma – Relatively short acting (several minutes) – Hydrolyzed by butyrylcholinesterase (non-specific liverderived cholinesterase) Neuropharmacology Cholinesterase inhibitors Cholinesterase inhibitor as a class • Inhibits acetylcholinesterase (ACh-ase) at cholinergic synapses preventing hydrolysis of ACh – Causes huge amplitude and prolonged EPPs (skeletal muscle) effectively paralyzing muscle – Can stop the heart (due to increased ACh concentration) • Atropine is antidotal, but only at muscarinic (e.g., heart) • Uses and specific names: – Clinical: treatment of myasthenia gravis by neostigmine or physostigmine (reversible drugs) – Industrial/agriculture: organophosphate insecticides like malathion (You’ll see poisoning every Spring in the ER.) – Chemical warfare: nerve agents (nerve gasses) like tabun, sarin and VX (poorly reversible and persistent)