The pharmacology of the spinal monosynaptic reflex following conditioning of a cutaneous and muscle nerve.

Conditioning of the spinal monosynaptic reflex from cutaneous or muscle afferents results in facilitation followed by inhibition. We administered a series of agents which have been shown to alter the monosynaptic reflex by blocking specific inhibitory pathways. We found that administration of mecamylamine and atropine, agents which affect recurrent inhibition had no effect on the facilitation or inhibition of the MSR by conditioning the sural or medial gastrocnemius nerve. Similarly, bicuculline, an agent which blocks pre-synaptic inhibition, had no effect on the conditioning of the MSR. However, strychnine, a glycine antagonist which blocks post-synaptic inhibition, did alter both the facilitation and inhibition of the MSR by conditioning pulses. Strychnine enhanced the facilitation and partially blocked the inhibition of the MSR by both sural and medial gastrocnemius conditioning. These data show that the flexor reflex afferents and the larger diameter afferents alter the MSR solely through glycine-mediated post-synaptic inhibition.