The intracellularly recorded responses of goldfish Mauthner neurones to iontophoretically applied pulses of amino acids have been analysed: their time courses have been compared with each other, and with those predicted from diffusion theory. The rise time of the response to GABA is slower than that to glycine or L glutamate. The possibility that the responses are influenced by some desensitizing process was investigated by applying a second (test) drug pulse during the response to a prior conditioning one. In the case of glycine and of L glutamate there was no attenuation of the response to a second pulse at any time. With GABA, however, the second response was reduced during the period of the conditioning response; the reduction was progressively less marked the later the test pulse occurred. A similar effect with GABA was seen when glycine was used as the test pulse. The responses to long maintained drug pulses also indicated that for GABA, but not for glycine or glutamate, there seems to be some desensitizing process present. Calculated time courses of responses to brief pulses of glycine and L glutamate (based upon diffusion theory) differed somewhat from the observed curves, largely during the falling phase. However, when the calculations were based upon second order reactions (two molecules of drug per receptor) the diffusion model gave results very like the observed ones.
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