Acetylcholine may be an excitatory transmitter mediating visual excitation of 'transient' cells with the periphery effect in the cat retina: iontophoretic studies in vivo.

The effects of iontophoretically-applied acetylcholine and its antagonists on the response of retinal ganglion cells at the receptive field centre were studied in the optically intact eye of anaesthetised cats. Acetylcholine enhanced visually-driven excitation of all 'transient' cells with the property of a periphery effect, but failed to excite those weak 'transient' cells without a periphery effect. It also mimicked visual stimulation when applied iontophoretically on spontaneously firing 'transient' cells with a periphery effect and a pulse of acetylcholine produced a transient response. Visually-driven excitation at the receptive field centre and the periphery effect, as well as the acetylcholine-induced excitation in these 'transient' cells, were blocked by dihydro-beta-erythroidine but not by atropine applied by iontophoresis. Neither acetylcholine nor dihydro-beta-erythroidine, on the other hand, produced any significant effect on the visually-driven firing of 'sustained' cells. Acetylcholine, however, suppressed the spontaneous firing of 'sustained-on' cells with a relatively high rate of background discharge, whereas it enhanced that of 'sustained-off' cells with relatively low rates of background discharge, and such effects were again blocked by dihydro-beta-erythroidine. It is suggested that (1), visual excitation of 'transient' cells with the periphery effect is mediated by a cholinergic system and the receptor is nicotinic rather than muscarinic and (2), the spontaneous firing rate of 'sustained' cells may be regulated also by a cholinergic system.