Sustained synaptic input to ganglion cells of mudpuppy retina

Abstract

Intracellular responses were recorded from on-center and off-center ganglion cells in isolated eyecups of the mudpuppy, N. maculosus. Current-voltage relations were measured in darkness, during illumination of the receptive field center, and after chemically mediated synaptic inputs were blocked by 4 mM-Co chloride. In on-center cells the membrane potential in darkness was -56 .+-. 6 mV (mean .+-. SD). Addition of Co2+ resulted in an average depolarization of 10 mV and an average decrease in conductance of 2.1 nS. In darkness on-center cells may be tonically inhibited by synaptic input which increases conductance and may have a reversal potential more negative than the dark membrane potential. In off-center cells the membrane potential in darkness was -46 .+-. 5 mV. Addition of Co2+ caused an average hyperpolarization of 6 mV and an average decrease in conductance of 1.5 nS. In darkness off-center cells may receive a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. In on-center cells light causes a sustained depolarization. This response involves an increase in a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. In off-center cells, light causes a sustained hyperpolarization. This response involves an increase in a sustained inhibitory input which increases conductance and has a reversal potential more negative than the dark membrane potential. The depolarizing off-response of off-center cells is associated with an increase in an excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. This response may be due to a temporary increase in the excitatory input which is tonically active in darkness or may reflect an additional excitatory input. Apparently, in both on- and off-center ganglion cells the balance of sustained excitatory and inhibitory synaptic inputs determines the resting potential in darkness. Center illumination alters the balance of these inputs, by increasing one and decreasing the other, to produce the characteristic sustained light responses. The possible presynaptic sources of the sustained excitatory and inhibitory inputs are discussed.