5-HYDROXYTRYPTAMINE MODIFIES NEURONAL RESPONSES TO GLUTAMATE IN THE RED NUCLEUS OF THE RAT

The effects of 5-hydroxytryptamine (5-HT) on the responses of red nucl eus (RN) neurones to glutamate (glu) and its agonists were studied usi ng a microiontophoretic technique in anaesthetised rats. Extracellular unitary recordings of RN neuronal activity showed that 5-HT applicati on induced a significant and reversible depression of glu-evoked excit ations in 85% of the RN units tested. This effect was independent of t he action of the amine on background firing, which appeared enhanced i n the majority of cases but was either depressed or uninfluenced in ot her cases. Microiontophoretic 5-HT also depressed the excitatory respo nses evoked in RN neurones by electrical stimulation of sensorimotor c ortex. Methysergide application, which prevented the enhancing effects of 5-HT on the background firing, was scarcely effective in antagonis ing the depression of glu responses. In contrast, the serotonergic eff ects on the glu responses were reduced by the iontophoretically applie d antagonist of 5-HT1A receptors, NAN-190. Microiontophoretic 5-HT was also able to influence the neuronal responses evoked by glu agonists quisqualate (quis) and N-methyl-D-aspartate (NMDA), acting on non-NMDA and NMDA receptors respectively. In fact 5-HT depressed quis-evoked e xcitations and induced mixed effects on NMDA responses, which were red uced in 45%, enhanced in 34% and unmodified in 21% of the units tested . These results suggest that 5-HT is able to modulate the motor glutam atergic input to RN by acting mostly on non-NMDA receptors. The modula tion of non-NMDA and NMDA receptors by 5-HT in the RN appears signific ant and its functional meaning is discussed.