Enhancement of recurrent inhibition of the spinal monosynaptic reflex by preceding stimulation of the medullary raphé in rats.

Recurrent inhibition of the spinal monosynaptic reflex (MSR) elicited by conditioning stimulation of the ventral root in anesthetized rats was weaker than both the recurrent inhibition of the disynaptic reflex and the inhibition of the MSR elicited by conditioning stimulation of the adjacent dorsal root. Among these 3 inhibitions, the recurrent inhibition of the MSR was enhanced to a markedly greater extent by a preceding stimulation of the medullary raphé nucleus than were the other inhibitions. The magnitude of the enhancement of the recurrent inhibition of MSR also was much greater when the medullary stimulation was delivered 20 ms prior to the ventral root activation, as compared with a 30-ms interval. Recurrent inhibition of the MSR was enhanced by intravenous injection of lysergic acid diethylamide (LSD); however, the enhanced effect on recurrent inhibition elicited by stimulation of the raphé nucleus was not attenuated by the drug. These results suggest that there is a non-serotonergic, descending pathway which is capable of modulating motor output solely by means of recurrent inhibition of the MSR.