Serotonin involvement in the blockade of bulbospinal inhibition of the spinal monosynaptic reflex

Bulbospinal inhibition of the extensor quadriceps monosynaptic reflex (MSR) was antagonized by the serotonin precursor, 5-hydroxytryptophan (5-HTP, 75 mg/kg), in unanesthetized, mid-collicular, decerebrate cats. Fluoxetine HCl (Lilly 110140, 0.25 - 6 mg/kg), a specific serotonin neuronal uptake blocker, also blocked this inhibition as well as bulbospinal inhibition of the flexor posterior biceps-semi-tendinosus MSR. The serotonin antagonist, cyproheptadine HCl (5 mg/kg), partially reversed the above blocking actions of 5-HTP and fluoxetine and enhanced bulbospinal inhibition when administered alone in doses of 2.5-5 mg/kg. Imipramine HCl (0.125 - 4 mg/kg) was more potent in antagonizing bulbospinal inhibition of the dorsal root-ventral root MSR when administered intra-arterially to the spinal cord than when injected intra-arterially to the brain stem or intravenously, indicating that the spinal cord is the site of imipramine's action. These results support our earlier proposal that a 5-HT system antagonizes bulbospinal inhibition of the MSR. They also indicate that the 5-HT system is tonically active and exerts its blocking action in the spinal cord.