The influence of naloxone on the C fiber response of dorsal horn neurons and their inhibitory control by raphe magnus stimulation.

In intact rats anesthetized with chloralose, the effects of naloxone were studied on the responses of spinal cord dorsal horn neurons to C fiber stimulation and upon the inhibition induced on these responses by stimulation of the nucleus raphé magnus (NRM). (1) A mean 44% facilitatory effect on responses to C fibers was observed for 12/19 units. (2) A mean 30% reduction of the inhibitory effects of NRM was found for 14/29 units. (3) However there is no clear relationship between these facilitatory effects and the diminution of the efficiency of NRM stimulation. These results demonstrate a facilitatory effect of naloxone upon the transmission of noxious messages at the spinal level and confirm that opiate endogenous substances are implicated in the inhibitory mechanisms activated by stimulation of NRM.