Indoleamine-mediated reciprocal modulation of on-centre and off-centre ganglion cell activity in the retina of the cat.

The effects of applying the indoleamines serotonin (5-hydroxytryptamine) and 5-methoxy-N,N-dimethyltryptamine (MDT) ionophoretically onto cat retinal ganglion cells were studied in the optically intact eye of the cat. Serotonin suppressed both the spontaneous activity and light-evoked discharge of on-centre ganglion cells, regardless of the visual stimulus used; on the other hand, it enhanced the activity of off-centre ganglion cells. MDT produced the opposite effects, i.e. it enhanced the activity of on-centre ganglion cells but suppressed that of off-centre ganglion cells. Much of the effect of serotonin on light-evoked discharge can be attributed to an alteration of maintained discharge. There was no major difference in the way brisk-sustained (X) and brisk-transient (Y) cells responded to the application of serotonin and MDT, although the effects of the indoleamines on brisk-transient (Y) cells were generally weaker than on brisk-sustained (X) cells. In view of the fact that the action of serotonin is similar to the action of a gain control system, a possible contribution of indoleamine-accumulating neurones to gain control in the cat retina is discussed.