Inhibitory effect of phencyclidine on rat spinal reflexes.

1. The effects of phencyclidine (PCP) on spinal reflexes and the excitability of motoneuron somata were studied in rats. 2. PCP decreased both the monosynaptic reflex (MSR) and the polysynaptic reflex (PSR) in spinal rats as well as in intact rats. 3. The effects of PCP were not antagonized by monoaminergic and cholinergic antagonists. 4. A low dose of PCP antagonized the PSR augmentation but not the MSR augmentation caused by N-methyl-D-aspartate (NMDA). 5. In the excitability test, PCP decreased not only the monosynaptic response but also the excitability of motoneuron somata. 6. These results suggest that the inhibitory effects of PCP on spinal reflexes are not due to mediation of the monoaminergic and cholinergic systems, but partly due to its blockade of NMDA-type receptors and also to a hyperpolarizing action or a membrane-stabilizing action on the motoneuron soma.