Serotonergic depression of spinal monosynaptic transmission is mediated by 5-HT 1B receptors

Abstract

In the spinal cord, various subtypes of serotonin (5-hydroxytryptamine; 5-HT) receptors are involved in the modulation of motor output. Although the excitatory role of 5-HT 2 receptors is known, the receptor subtypes mediating the inhibitory effect of 5-HT on monosynaptic reflex transmission remain unclear. In this study, segmental spinal reflexes were recorded to examine the receptor subtypes underlying 5-HT-mediated inhibition of monosynaptic reflex transmission in spinalized rats. Under conditions of monoamine oxidase blockade with clorgyline, the 5-HT precursor l-5-hydroxytryptophan depressed the monosynaptic reflex. 3-Hydroxybenzylhydrazine dihydrochloride (NSD-1015), a centrally active decarboxylase inhibitor, abolished this inhibition, confirming that the depression of the monosynaptic reflex by l-5-hydroxytryptophan was due to 5-HT. In the presence of GR127935 or isamoltane, which show high affinity for 5-HT 1B receptors, l-5-hydroxytryptophan did not suppress the monosynaptic reflex, whereas 5-HT 1A, 5-HT 1D, 5-HT 2 and 5-HT 7 receptor antagonists did not alter the inhibitory effect of l-5-hydroxytryptophan. These results suggest that serotonergic depression of monosynaptic reflex transmission is mediated by 5-HT 1B receptors.