NEUROTRANSMITTER RECEPTORS OF STARBURST AMACRINE CELLS IN RABBIT RETINAL SLICES

The receptor pharmacology of cholinergic (''starburst'') amacrine cell s was studied in a newly developed rabbit retinal slice preparation wi th whole-cell patch clamp. Displaced starburst cells were labeled with the fluorescent dye 4,6-diamidino-2-phenylindole (DAPI), and their de ndritic morphology was identified in the slice with Lucifer yellow. Un der conditions in which synaptic transmission was blocked by Cd2+, sta rburst cells responded vigorously to the bath-applied neurotransmitter s GABA, glycine, and glutamate. The response to GABA consisted of an i nward current and an increase in noise, which could be mimicked by the GABA(A) agonists muscimol and trans-4-aminocrotonic acid (TACA), but not by the GABA(B) agonist baclofen or the GABA(C) agonist cis-4-amino crotonic acid (CACA). The GABA-evoked currents were reversibly inhibit ed by bicuculline and picrotoxin and had a reversal potential close to the chloride equilibrium potential, Noise analysis of GABA-activated whole-cell currents yielded elementary conductance estimates of 12.5 p S. Glycine (30-200 mu M) also activated a Cl- conductance in starburst cells, which could be completely blocked by strychnine. The non-NMDA agonists kainate (KA, 30-100 mu M) and lpha-amino-3-hydroxy-5-methylis oxazole-4-propionic acid (AMPA, 60 mu M) evoked robust responses, whic h were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQ X), and which reversed near the equilibrium potential for cations. NMD A coapplied with glycine in salines free of Cd2+ and Mg2+ elicited sma ll but detectable responses. The I/V relation of the NMDA-evoked respo nse showed a characteristic ''J''-shaped region in a saline containing 1 mM Mg2+ and 0 Cd2+, indicating that NMDA receptors were present dir ectly on starburst cells. This was consistent with our finding that wh ole-cell currents evoked by KA and NMDA had different noise characteri stics. These results place new constraints on models of starburst cell function and suggest that GABA-mediated inhibition of the starburst c ell itself may play an important role in directional selectivity in th e retina.