| Research Abstract |
Retinal horizontal cells in the lower vertebrate are known to play a crucial role on formation of the center-surround antagonism in the receptive field as well as conversion from the trichromatic process to the opponent color process. To understand synaptic mechanisms which underlie these retinal functions, it is necessary to extensively examine the properties of ligand-gated ion channels expressed on the horizontal cells. In this study I made an effort to elucidate the GABA sensitivity of horizontal cells enzymatically dissociated from catfish (Ictalurus punctatus), by using the whole-cell voltage-clamp technique.
GABA-induced currents of the horizontal cells are composed of two components : a GABA_c receptor-mediated current and an electrogenic GABA transporter-mediated current.
1. The GABA_c receptor-mediated current was carried mainly by C1^-, the equilibrium potential of whose was measured at about-30mV.The current did not show a clear desensitization. Hill coefficient and EC_<50> w
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as 2 and 2 muM,respectively. The current was suppressed by picrotoxin and penicillin, but not by bicuculline. Neither pentobarbital nor diazepam potentiated the GABA-induced current. Baclofen and CGP35348 were ineffective. CACA and TACA evoked currents, as do GABA.
2. The electrogenic GABA transporter-mediated current was carried by Na^+ and C1^-. Hill coefficient and EC_<50> was 1 and 10 muM,respectively. The current was not reversed the polarity at any potential from -70 mV to+70mV.Nipecotic acid and its derivative, SKF89976A blocked the GABA-induced current.
Although the horizontal cells release GABA as their transmitter and then take GABA up via the GABA transporter, [GABA]_0 is calculated to be at the order of muM in the extracellular space around the horizontal cells from the present results and previous works (especially Kanner et al., 1987 &1988). GABA in the extracellular space could activate the GABA_c receptors to depolarize the horizontal cells, thereby accelerating GABA release. Less
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