Dual actions of some amino acids on spike discharges in the carp retina

The effects of some amino acids (Glu, Gly and GABA), applied in 3 different manners (electrophoretically, in the superfusate and by pressure-microinjection), were investigated on spontaneous and light-induced spike discharges in the isolated carp retina. When applied electrophoretically or by pressure-microinjection in the inner plexiform layer (IPL), the agents acted directly on spike-generating units. Electrophoretic application of Glu at IPL consistently increased while Gly and GABA always decreased spike discharges regardless of the light-induced response patterns, when the tangential distance between the recording and injection electrodes was 25--100 micron. Increasing the distance up to 400 micron diminished the effects, but did not invert them. When added to the superfusate, the amino acids produced a dual action (two different sequential effects); Glu (5 mM) initially decreased and increased spike discharges, while Gly and GABA (5 mM) produced opposite effects. Gly and GABA tended to suppress selectively off-discharges (of ON--OFF units and certain OFF-center units), leaving on-discharges (of ON--OFF units and certain ON-center units) unaffected. The amino acids produced different effects on some units, when applied by pressure-microinjection into OPL or IPL. When injected in OPL Glu suppressed, while in IPL it activated spike discharges, whereas Gly and GABA caused opposite changes to those observed with Glu. Therefore, the action of the agents when pressure-microinjected in OPL is equivalent to the initial action of the agents applied in the superfusate. The dual actions of the agents are assumed to be mediated by bipolar cells, resulting in disfacilitation (Glu) or in disinhibition (Gly or GABA) of spike-generating units.