Further studies on the action of 5-hydroxytryptamine in the dorsal lateral geniculate nucleus of the cat.

The possibility that 5-HT has a presynaptic inhibitory influence on the optic nerve input to the LGNd has been examined. We have compared the effect of iontophoretic application of 5-HT on the excitatory responses of LGNd neurons to visual stimulation and iontophoretic pulses of ACh and DLH. In contrast to previous studies the results suggest that 5-HT has only postsynaptic depressant effects and we found no evidence for a selective inhibition of the optic nerve input. It is proposed that the difference between the present result and those previously reported may reflect the method of stimulation of the optic nerve input. The weaker excitatory responses produced with electrical stimulation and diffuse light flashes of the earlier studies may be more susceptible to the postsynaptic depressant actions of 5-HT than the powerful visually evoked responses of the present study. Our data are only consistent with a postsynaptic depressant role for 5-HT in the LGNd.