Brain stem involvement in the effects of chlorpromazine on the monosynaptic reflex of the rat lumbar spinal cord.

The brain stem areas involved in the inhibitory effect of chlorpromazine (CPZ) on the monosynaptic reflex (MSR) in the lumbar spinal cord of rats were investigated following brain stem transection and focal thermo-lesioning. Transection of the medulla oblongata markedly reduced the effect of CPZ, while transection in the mid-brain or more anterior level did not alter the effect of CPZ. With regard to focal lesions at various sites of the brain stem, lesioning of the medial portion of the medulla oblongata most effectively attenuated the MSR inhibition by CPZ and also attenuated the MSR inhibition induced by phenoxybenzamine. Bilateral lesioning of the locus coeruleus did not attenuate the MSR inhibition by CPZ. These results suggest that the pathway involved in the inhibitory effect of CPZ on the MSR originates in the pons and passes through the medial portion of the medulla oblongata, and that the coerulo-spinal pathway is not the major pathway involved in the effect of CPZ.