EFFECT OF MUSCLE DENERVATION ON THE EXPRESSION OF SUBSTANCE-P IN THE VENTRAL RAPHE-SPINAL PATHWAY OF THE RAT

The medullary raphe nuclei, wherein serotonin (5-HT) coexists with sub stance P (SP) and thyrotropin-releasing hormone (TRH, innervate lower motor neurons in the spinal cord ventral horn by means of the ventral raphe-spinal pathway. Destruction of the ventral raphe-spinal pathway is associated with deficient recovery of denervated muscle, indicating that it may exert a trophic effect upon lower motor neurons. To deter mine whether SP could be a trophic factor for lower motor neurons with in the ventral raphe-spinal pathway, the effect of muscle denervation with botulinum toxin type A on SP-encoding beta-preprotachykinin mRNA in the rat medullary raphe was examined by in situ hybridization histo chemistry. Silver grain density over hybridized medullary raphe neuron s was increased by up to 11%, although the number of hybridized neuron s did not change in denervated as compared to control rats. Increased SP gene expression in the medullary raphe in response to motor unit le sioning suggests that raphe-spinal SP may be trophic to lower motor ne urons.