SEROTONIN MODULATES INDUCED SYNAPTIC ACTIVITY IN THE OPTIC TECTUM OF THE FROG

The extent to which retinal signals are modulated at central sites is unknown. We sought to determine the effects of serotonin, a neurotrans mitter present in the retinorecipient layers of the frog tectum, on re tinotectal transmission. Acute electrical stimulation delivered to the retinorecipient layer of optic tectum brain slices was used to model the activation of tectal neurons by visual inputs. This stimulation ev oked either a monosynaptic or a polysynaptic current response in patch -clamped tectal neurons. External application of serotonin blocked bot h of these induced currents as did 5-carbotryptamine (5-CT), a nonsele ctive agonist of 5-HT1 receptors. Alpha-methylserotonin, a nonselectiv e agonist of 5-HT2 receptors, also blocked polysynaptic responses but was less effective than either serotonin or 5-CT in blocking monosynap tic ones. Lateral synaptic interactions between tectal cells, modeled by acute electrical stimulation in the main cellular layer of the tect um, were also blocked by serotonin, 5-CT or alpha-methylserotonin. The presented data suggest that endogenous serotonin may strongly affect visual signal processing by modulating synaptic transmission between b oth the retina and the tectum as well as between tectal neurons. This modulation is likely to be due, at least in part, to a demonstrated ou tward current induced by serotonin in a subpopulation of tectal cells. (C) 1998 Elsevier Science B.V.