MODULATION OF CUTANEOUS NOCICEPTOR ACTIVITY BY ELECTRICAL-STIMULATIONIN THE BRAIN-STEM DOES NOT INHIBIT THE NOCICEPTIVE EXCITATION OF DORSAL HORN NEURONS

In anesthetized cats, recordings were obtained from single lumbar dors al horn neurons and from primary afferent fibers of the posterior tibi al nerve excited by controlled noxious radiant heating of glabrous hin dpaw skin. Electrical stimulation in four brain stem regions (periaque ductal gray and lateral reticular formation in the midbrain, raphe and reticular formation in the medulla) during noxious skin hearing marke dly reduced the nociceptive excitation of the dorsal horn neurons. In contrast, such brain stem stimulation had small and variable effects u pon the noxious heat-evoked activity in the primary afferent fibers; b oth increases and decreases were observed. The brain stimulation also produced transient changes in blood pressure, suggesting that circulat ory effects may underlie the mechanism of nociceptor modulation, It is concluded that brain stem stimulation can modulate cutaneous nocicept or activity, but that this modulatory effect on nociceptor inflow is t oo small and inconsistent to explain the marked descending inhibition of the nociceptive excitation of dorsal horn neurons. (C) 1997 Interna tional Association for the Study of Pain.