Transient and sustained inhibitory synaptic inputs to on-centre, off-centre, and on-off
ganglion cells in the mudpuppy retina were studied using intracellular recording in the superfused eye-cup preparation. When chemical transmission was blocked with 4 mM-Co2+, application of either
glycine or
gamma-aminobutyric acid (
GABA) caused a hyperpolarization and conductance increase in all
ganglion cells. For both
amino acids, the responses were dose dependent in the range 0.05-10 mM, with a half-maximal response at about 0.7 mM.
Glycine and
GABA sensitivities were very similar in all three types of
ganglion cells. The response to applied
glycine was selectively antagonized by 10(-5) M-
strychnine and the response to applied
GABA was selectively antagonized by 10(-5) M-
picrotoxin. In all
ganglion cells, 10(-5) M-
strychnine eliminated the transient inhibitory events which occur at the onset and termination of a light stimulus. The block of transient inhibition was associated with a relative depolarization of membrane potential and decrease in conductance at these times.
Strychnine had no effect on membrane potential or conductance in darkness or during sustained inhibitory responses to light.
Picrotoxin (10(-5) M) did not block transient inhibitory events in any
ganglion cells, but did affect other components of their responses. The results suggest that in all three classes of
ganglion cells transient inhibition, but not sustained inhibition, may be mediated by
glycine or a closely related substance.