LECTURE 9: INTEGRATION OF SYNAPTIC INPUTS (Ionotropic Receptors) REQUIRED READING: Kandel text, Chapter 12 At neuromuscular synapse, single axonal action potential generates a muscle action potential. The large arborized endplate contains 500,000 acetylcholine receptors generating 500 nA I EPSP sufficient to depolarize muscle past threshold. Individual neuron-to-neuron synapses are much smaller and do not generate sufficient I EPSP to trigger action potential in postsynaptic cell. Neuronal excitation requires near-simultaneous inputs from multiple excitatory synapses. E.g., a motor neuron will need excitatory inputs to give EPSP beyond threshold. Neurons also have synapses which mediate inhibitory postsynaptic potentials (IPSPs). IPSPs oppose depolarization generated by EPSPs. Neurons continuously integrate inhibitory and excitatory synaptic inputs to determine whether to fire action potentials and with what frequency.

THE IPSP DETECTED IN MOTOR NEURON BY INPUT FROM INTERNEURON

TWO FUNCTIONS OF IPSPs I. IPSPs counteract EPSPs to reduce or abolish neural firing triggered by excitatory synaptic inputs. II. IPSPs can interfere with the rhythmic spontaneous firing of neurons. The pattern of inhibitory synaptic inputs “sculpts” the spontaneous periodic firing.

EXCITATORY AND INHIBITORY SYNAPSES HAVE DIFFERENT MORPHOLOGIES Axo-axonic synapses do not directly generate postsynaptic currents These synapses mediate short- and long-term signaling events that modulate how much neurotransmitter is released by an action potential reaching its terminus.

MOST EXCITATORY SYNAPSES ELICIT EPSP WITH REVERSAL POTENTIAL OF 0 mV NEUROTRANSMITTE R IONOTROPIC RECEPTOR ION PERMEABILITY GLUTAMATE AMPA GluRNa +, K + GLUTAMATE Kainate GluRNa +, K + GLUTAMATE NMDA GluRNa +, K +, Ca ++ ACETYLCHOLINENicotinic AChRNa +, K + ATP ATP ReceptorNa +, K +, Ca ++ SEROTONIN5-HT 3 ReceptorNa +, K + Excitatory reversal potential, E EPSP, is near 0 mV, due to permeability of receptor to both sodium and potassium

NMDA AND NON-NMDA RECEPTORS FUNCTION DIFFERENTLY NMDA receptors open only when depolarization precedes glutamate binding. Depolarization releases Mg +2 blocking particle from ligand-binding site. NMDA receptors only open with prolonged presynaptic activity. Calcium entry through NMDARs induces signaling processes that can modify synaptic behavior both short- and long-term

NMDA RECEPTORS CONDUCT LATE CURRENT AFTER DEPOLARIZATION NMDA receptors open only when depolarization precedes glutamate binding. Depolarization release Mg +2 blocking particle from ligand-binding site. NMDA receptors only open with prolonged presynaptic activity. Calcium entry through NMDARs induces signaling processes that can modify synaptic behavior both short- and long-term Single Channel Recordings in V-Clamp Whole Cell Recordings in V-Clamp

MOST INHIBITORY SYNAPSES ELICIT IPSP WITH REVERSAL POTENTIAL OF -60 mV NEUROTRANSMITTE R IONOTROPIC RECEPTOR ION PERMEABILITY GABA GABA A Receptor Cl - Glycine Glycine Receptor Cl -

IPSP ACTS TO SHORT-CIRCUIT EPSP CURRENT AND BLOCK DEPOLARIZATION TWO WAYS TO THINK OF HOW IPSP CURRENTS INHIBIT EXCITATION I. Goldman’s equation shows that membrane potential is driven to a level determined by the weighted sum of each ionic Nernst potential weight by the relative permeability of each ion. Increasing Cl- or K+ permeability reduces the effect of excitatory Na+ current II. Inhibitory channels gate ions (usually Cl-) with Nernst (reversal) potential of -60 to -70 mV. Since this is about the same potential as that of leak channels, we can consider inhibitor channels as increasing the leak conductance. Since at the peak of an EPSP, I EPSP (in) = I leak (out), Ohm’s law says  V EPSP = I EPSP (in) / g leak. The larger the leak conductance the smaller the depolarization induced by excitatory inward currents. P K E K + P Na E Na + P Cl E Cl P K + P Na + P Cl V m =

INTEGRATION OF MULTIPLE SYNAPTIC INPUTS DETERMINED BY CELL ARCHITECTURE, ACTIVE DENDRITIC CURRENTS, AND LEAK CURRENTS Time constant of an EPSP determined by leak conductance. If leak conductance is low, EPSP persists well after I EPSP current ends (long time constant). A second I EPSP can induce further depolarization than did the first. This is called TEMPORAL SUMMATION If leak conductance is high, EPSP is finished before a second I EPSP, so there is no temporal summation

INTEGRATION OF MULTIPLE SYNAPTIC INPUTS DETERMINED BY CELL ARCHITECTURE, ACTIVE DENDRITIC CURRENTS, AND LEAK CURRENTS Length constant of an EPSP determined by ratio of axial conductance to leak conductance; I.e., by the cable properties of the dendrite The greater the ratio of g dendrite to g leak, the less an EPSP diminishes over distance; I.e., the bigger the length constant EPSP with bigger length constant can more readily undergo spatial summation with the EPSP at another synapse

INTEGRATION OF MULTIPLE SYNAPTIC INPUTS DETERMINED BY CELL ARCHITECTURE, ACTIVE DENDRITIC CURRENTS, AND LEAK CURRENTS Axosomatic inhibitory synapse exerts a more powerful inhibitory effect on excitation than does an axodendritic inhibitory synapse. Axosomatic inhibitory currents are shunts preventing dendritic EPSPs from propagating past to reach the trigger zone.

INTEGRATION OF MULTIPLE SYNAPTIC INPUTS DETERMINED BY CELL ARCHITECTURE, ACTIVE DENDRITIC CURRENTS, AND LEAK CURRENTS In large neurons with long, extensively arborized dendrites, currents from dendritic voltage-gated calcium channels (VGCCs) can boost distant dendritic EPSPs towards the soma. The density of VGCCs in proximal dendritic trunk and soma are much lower, so active propagation does not proceed across soma to sodium channel trigger zone. Temporal and spatial summation of excitatory inputs are still required to induce the axonal action potential. EPSP in DISTAL DENDRITE CALCIUM ACTION POTENTIAL DOWN DENDRITE SUBTHRESHOLD DEPOLARIZATION in PROXIMAL DENDRITE

SUBUNIT STRUCTURES OF LIGAND GATED IONOTROPIC RECEPTORS

IMPERMEABILITY OF AMPA RECEPTORS TO CALCIUM GENERATED BY RNA EDITING

NEXT LECTURE: Metabotropic Receptors READING: KANDEL text, Chapter 13