Announcements –Midterm room assignments Thursday –Midterm conflict policy posted later tonight –Some practice questions from previous midterms will be posted –TA office hours Mon 10-12, Wed 12-1 S161 or S526

Last lecture Vesicle cycle Spontaneous and evoked release Discrete units – quanta Evidence for vesicles = quanta Today Role of depolarization What does Calcium do in the nerve terminal? Some molecules involved in Vesicle fusion Excitation and Inhibition

What is the role of nerve terminal depolarization?

Postsynaptic neuron Postsynaptic response Presynaptic response Depolarizing current Increasing Postsynaptic response with increasing Presynaptic depolarization Squid Giant Synapse with tetrodotoxin  blocks Na+ channels  no AP Presynaptic neuron These are postsynaptic potentials

Effectiveness of Presynaptic Depolarization depends on Ca++ Presynaptic Response (mV) Postsynaptic response (mV) mM Ca++ 11 mM Ca++ 2 mM Ca++

Presynaptic neuron Postsynaptic neuron Postsynaptic response Presynaptic response Depolarizing current Calcium signal corresponds to postsynaptic response Role of Ca++ Photo-tube Filled with aequorin

Relationship between Ca++ and synaptic transmission med zero low high Ca++ concentration (mM) Synaptic Potential amplitude (mV) Ca++ concentration

Summary 1.Neurotransmitter release can cause synaptic potentials 2.Spontaneous events correspond to smallest nerve stimulated events 3.The quantal unit 4.AP depolarization necessary to admit Ca++ to presynaptic nerve terminal

What does Ca++ do to cause fusion? Triggers the final fusion of the vesicle membrane with the presynaptic membrane –Likely that Ca++ binds a calcium sensing protein called synaptotagmin Protein-protein interactions are required –SNARE proteins likely the minimal proteins required for fusion

Wild-type Syntaxin mutant Synaptobrevin mutant Use Fruit Fly mutations to reduce the amount of SNARE proteins Measure synaptic transmission at different [Ca]

Botulinum toxin (BoTox) Tetanus toxin –Enzymes that cleave SNARE proteins  Blocks neurotransmitter release

Excitation & Inhibition Excitatory synapse makes it more likely a cell will fire AP Inhibitory synapse makes it less likely a cell will fire AP

‘Likely’ depends on: 1.Neurotransmitter receptor 2.The permeable ions

A. Excitor B. Inhibitor Record voltage

Simple case: Vm Threshold Depolarizing  excitatory EPSP hyperpolarizing  inhibitory IPSP Vm Threshold B A A+B=smaller

How to get hyperpolarizing potential? Neurotransmitter receptor is permeable to an ion whose E ion is more negative than resting membrane potential usually Cl- or K+

mV +60 mV 0 mV Hyperpolarizing Synaptic Potential K+

More complex case: Vm Depolarizing  excitatory Depolarizing  Threshold B A Vm A+B=smaller inhibitory Why???

Reversal Potential Membrane potential at which there is no net synaptic current

eg. Frog NMJ Control resting membrane potential Current source stimulus Measuring Reversal Potential Reversal potential Record membrane potential Stimulate nerve

Many neurotransmitter receptors are permeable to more than one ion –Non-selective The reversal potential depends on the equilibrium potential and permeability of each ion –It will usually be between the equilibrium potential of the permeable ions

eg. Acetylcholine channel Permeable to both K+ and Na+ For Frog muscle: E K = -90 mV E Na = +60 mV

Vm=E rev E rev >Vm>E K Vm  E K Na+ Vm  E Na K+ E K = -90 mV Neurotransmitter receptor -90 E Na = +60 mV Reversal potential

How can depolarizing potential be inhibitory? Excitatory synapses have a reversal potential more positive than threshold Inhibitory synapses have a reversal potential more negative than threshold

How can depolarizing potential be inhibitory? Vm Threshold B A E rev Example: Cl - permeable receptor in a cell whose V thresh >E Cl - > Vm

Inhibition Channels of inhibitory synapses ‘short- circuit’ excitatory synapses Because neurotransmitter channels will drive the membrane potential toward their reversal potential

Summary Evidence that depolarization is important Evidence that depolarization regulates presynaptic intracellular [Ca] SNARE proteins and Synaptotagmin Excitation and Inhibition –Importance of synaptic reversal potential