| Abstract: | In rats with prenigral decerebration, the effect was studied of electrical stimulation of the periaqueductal grey matter (PAG) on the activity recorded from axons ascending in the spinal cord. These axons were activated by electrical stimulation of afferent Aβ, Aδ and C fibres in the ipsilateral sural nerve.Stimulation of the PAG with trains of impulses by itself evoked ascending activity, but strongly depressed the impulse transmission from C fibres to neurones with ascending axons. It exerted a weaker effect on impulse transmission from Aδ fibres and had no effect on impulse transmission from Aβ fibres to neurones with ascending axons. Intravenous naloxone, 1 mg/kg, did not diminish the depressant effect of PAG stimulation.Intravenous morphine depressed the activation of ascending axons from afferent C fibres (0.5 mg/kg) more markedly than that from afferent Aδ fibres (2 mg/kg), but did not modify the depression of ascending activity produced by PAG stimulation. Naloxone antagonized the depressant effect of morphine.The results indicate that PAG stimulation inhibits ascending activity evoked by noxious stimuli by a mechanism which does not necessarily involve endogenous opiates. |
