The action of synaptic blocking agents on the chloride-dependent ON-OFF responses to central illumination of depolarizing bipolar cells was investigated in salamander retinal slices by means of whole cell recordings. Strychnine sulfate (lO-5O micromoles)and bicuculline-methobromide (50-500 micromoles) did not have any effect. Picrotoxin (50-100 micromoles) transiently decreased the current amplitude to 40-50%, but within approximately one minute the current increased again to stabilize at 60-70% of control amplitude. Except for the incomplete block by picrotoxin, these results appear to fit a recent description of a third class of GABA receptor.
