Inhibitory effect of midbrain raphe stimulation on the maintenance of an active avoidance reflex

Performance of an active avoidance (bench-jumping) reflex has been studied in rats during stimulation of the midbrain raphe nuclei. Raphe stimulation (10 cps, 0.2 msec, 2.5-5.0 V) inhibited the performance of the reflex. A serotonin receptor blocker (methysergide, 2.0 mg/kg IP) increased the reflex performance in non-stimulated animals and prevented the action of raphe stimulation. The data indicate that the cerebral serotoninergic system might have an inhibitory control over the performance of conditioned avoidance reflex.