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.
1 A study was made in cats anaesthetized with barbiturate or a-chloralose, of the excitation of dorsal horn neurones by impulses in unmyelinated (C) primary afferent fibres of the tibial nerve. 2 Block of conduction in the first lumbar segment by cooling produced large increases in the number of action potentials evoked by C fibre afferents in neurones of more caudal segments. 3 Morphine (0.3 to 1.0 mg/kg) reduced the excitation of neurones by C fibre afferents and also reduced the increase produced by blocking conduction in the spinal cord. Naloxone (0.1 to 0.3 mg/kg) reversed these effects of morphine.4 This decrease in descending inhibition supports findings in the decerebrate cat but gives no support to the hypothesis that an important component of morphine analgesia is an activation of descending inhibitory pathways.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.