GABA-ACTIVATED CHLORIDE CURRENTS OF POSTNATAL MOUSE RETINAL GANGLION-CELLS ARE BLOCKED BY ACETYLCHOLINE AND ACETYLCARNITINE - HOW SPECIFIC ARE ION CHANNELS IN IMMATURE NEURONS

The goal of this study was to clarify pharmacological properties of GABA(A) receptors in cells of the mouse retinal ganglion cell layer in situ. Spontaneous synaptic currents and responses to exogenous GABA were recorded from individual neurons in retinal whole mounts (postnatal days 1-3) or retinal stripe preparations (postnatal days 4-6). Drugs were applied by a fast local superfusion system. Current responses were measured with the patch-clamp technique in the whole-cell configuration. All cells responded to exogenous GABA (average EC(50) and Hill coefficient: 16.7 mu M and 0.95 respectively) and generated GABAergic synaptic currents in response to elevated KCl. GABA-induced currents of retinal ganglion cells were blocked by bicuculline, picrotoxin and Zn2+, as well as strychnine, and increased by pentobarbital, clonazepam and 3 alpha-hydroxy-5 alpha-pregnan-20-one. In some retinal ganglion cells GABA caused an increase in the frequency of spontaneous synaptic currents, which points to a partially depolarizing action of this traditionally inhibitory neurotransmitter in the neural retina. Our major observation is that acetylcholine and acetylcarnitine blocked or reduced GABAergic inhibitory postsynaptic currents and responses to exogenous GABA. This effect was seen in only a fraction of retinal ganglion cells and occurred in both the undesensitized and the desensitized state of the GABA(A) receptor. The block was voltage-independent and persisted during coapplication with the nicotinic and muscarinic acetylcholine receptor antagonists D-tubocurarine and atropine. In contrast to GABA-activated Cl- currents, glycine-activated Cl- currents remained unaffected by acetylcholine and acetylcarnitine. Acetylcarnitine had no effect on voltage-activated Ca2+ channel currents and glutamate-activated currents. Similar results were obtained in a dissociated cell culture preparation from the neonatal rat superior colliculus. In these cells acetylcholine induced a rightward shift in the dose-response curve for GABA. Taken together, these results indicate that acetylcholine and acetylcarnitine can act directly at the GABA(A) binding site and thereby reduce the action of GABA in the immature retina.