Inhibition of the spinal transmission of nociceptive information by supraspinal stimulation in the cat.

In cats anaesthetized with alpha-chloralose or sodium pentobarbitone a study was made of the effects of supraspinal electrical stimulation on the excitation of dorsal horn neurones by noxious and non-noxious cutaneous stimuli. Stimulation near the dorsal raphe of the midbrain non-selectively reduced the responses of neurones to both noxious and non-noxious stimuli. Intravenous naloxone (0.3-0.6 mg/kg) had no effect on this inhibition. Electrical stimulation near the medullary raphe selectively reduced the excitation of dorsal horn neurones by noxious cutaneous stimuli. Excitation of neurones by deflection of hairs was unaffected. The inhibition of nociceptive responses outlasted the period of raphe stimulation by up to 6 min. Intravenous naloxone (0.6-1.0 mg/kg) also failed to affect this selective inhibition.