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The effects of the GABAB (gamma-amino butyric acidB) receptor agonist, baclofen and its antagonist, phaclofen on physiologically identified retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized cats. These results were compared with the effects of the GABAA receptor agonist, muscimol and its antagonist, bicuculline. Baclofen inhibited the total visually driven firing of both ON- and OFF-cells more effectively upon OFF- than ON-cells; this action was weaker and slower than that of muscimol. Whilst bicuculline raised the firing level of only ON-cells in the area centralis together with all peripheral cells, phaclofen raised that of all OFF-cells. Paradoxically, in OFF-cells, baclofen enhanced the driven transient component and suppressed the sustained component, whilst phaclofen raised the sustained component. Thus, GABAB receptors activated by tonically released GABA may modulate the sustained and transient excitatory inputs to OFF-cells.

The non N-methyl-D-aspartate (NMDA) receptor antagonists dinitroquinoxaline-2,3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), both inhibit the visually driven response of sustained ganglion cells in the cat retina in vivo. In contrast to these findings, the potent NMDA receptor antagonist 3-[+/- )-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) has no effect. Thus, the endogenous excitatory amino acid released onto these cells on visual stimulation acts at non-NMDA receptors.