Physiological and pharmacological basis of GABA and glycine action on neurons of mudpuppy retina. III. Amacrine-mediated inhibitory influences on ganglion cell receptive-field organization: A model

Intracellular recordings were carried out in the perfused retina-eyecup preparation of the mudpuppy. Most ganglion cell recordings showed some type of inhibitory input. The antagonists, picrotoxin, bicuculline, and strychnine were added to the bathing medium in concentrations demonstrated to have a selective blocking action on exogenous γ-aminobutyric acid (GABA) or glycine (Gly) application. Since the antagonists do not block presynaptic excitatory drive, the use of antagonists provided insight into GABA- and glycinergic inhibitory pathways at the ganglion cell level. Additional evaluation of inhibitory input into ganglion cells was obtained by intracellular current injection studies and also by exposing the cell for a brief period to a chloride-free environment. This treatment reversed hyperpolarizing inhibitory postsynaptic potentials (IPSPs) to depolarizing responses and proved valuable for separating inhibition from disfacilitation. Most cells that could be classified as on, off, or on-off on the basis of excitatory inputs, demonstrated prominent IPSPs at both light onset and offset. In most of these cells, the pattern of inhibition was predominately or exclusively glycine- or GABAnergic. However, a few examples of mixed GABA- and glycinergic on-off inhibition were found. On some occasions a hybrid form of on-off inhibition was observed in which the off IPSP was glycinergic while the on IPSP was GABAnergic. Furthermore, some on cells were observed with only an on GABAnergic inhibitory input. Off cells with a predominantly off inhibitory input were also occasionally encountered, but were not studied pharmacologically. The pharmacological, current-injection, and ion-substitution experiments of this study suggest the existence of four different inhibitory amacrine cells that utilize GABA or glycine: an on-type, which releases GABA; an off-type, which releases glycine; and two on-off types, one of which releases GABA, while the other utilizes glycine. The findings of this study suggest a new model of the inner retina in which inhibitory and excitatory inputs are more completely specified. It appears that GABA- and glycinergic mechanisms directed at the ganglion cell level play a major role in defining receptive-field organization.