Neurotransmitter receptors of starburst amacrine cells in rabbit retinal slices

Abstract

The receptor pharmacology of cholinergic ("starburst") amacrine cells was studied in a newly developed rabbit retinal slice preparation with whole-cell patch clamp. Displaced starburst cells were labeled with the fluorescent dye 4,6-diamidino-2-phenylindole (DAPI), and their dendritic morphology was identified in the slice with Lucifer yellow. Under conditions in which synaptic transmission was blocked by Cd2+, starburst cells responded vigorously to the bath-applied neurotransmitters GABA, glycine, and glutamate. The response to GABA consisted of an inward current and an increase in noise, which could be mimicked by the GABAA agonists muscimol and trans-4-aminocrotonic acid (TACA), but not by the GABAB agonist baclofen or the GABAC agonist cis-4-aminocrotonic acid (CACA). The GABA-evoked currents were reversibly inhibited 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 pS. Glycine (30-200 microM) also activated a Cl- conductance in starburst cells, which could be completely blocked by strychnine. The non-NMDA agonists kainate (KA, 30-100 microM) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 60 microM) evoked robust responses, which were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and which reversed near the equilibrium potential for cations. NMDA coapplied with glycine in salines free of Cd2+ and Mg2+ elicited small but detectable responses. The I/V relation of the NMDA-evoked response showed a characteristic "J"-shaped region in a saline containing 1 mM Mg2+ and 0 Cd2+, indicating that NMDA receptors were present directly on starburst cells. This was consistent with our finding that whole-cell currents evoked by KA and NMDA had different noise characteristics. These results place new constraints on models of starburst cell function and suggest that GABA-mediated inhibition of the starburst cell itself may play an important role in directional selectivity in the retina.