Supraspinal monoaminergic involvement in the blockade of recurrent inhibition of the monosynaptic reflex

Imipramine HCl (5 mg/kg i.v.) and pargyline HC1 (30 mg/kg i.v.) antagonized recurrent inhibition of the dorsal root-ventral root monosynaptic reflex in unanaesthetized decerebrate cats. Recurrent inhibition of the extensor quadriceps monosynaptic reflex, but not of the flexor posterior biceps-semitendinosus monosynaptic reflex, was blocked by imipramine. Imipramine failed to block recurrent inhibition in animals pretreated with either dl-p-chlorophenylalanine (300mg/kg i.p. for three consecutive days) or dl-α-methyl-p-tyrosine methyl ester HCl (125 |mg/kg i.p. 16 and 4 hr prior to recording). Application of the reversible spinal cord “cold block” technique increased recurrent inhibition of the quadriceps monosynaptic reflex but completely reversed the blocking action of imipramine on this inhibition. The results of this study suggest that: (l)a descending tonic inhibitory system impinges on the recurrent inhibitory pathway of the quadriceps monosynaptic reflex; and (2) a supraspinal monoaminergic system involving both 5-hydroxytryptamine and noradrenaline antagonizes recurrent inhibition of the monosynaptic reflex. These two inhibitory systems may be the same.