In anesthetized cats, recordings were obtained from single lumbar dorsal horn neurons and from primary afferent fibers of the posterior tibial nerve excited by controlled noxious radiant heating of glabrous hindpaw skin. Electrical stimulation in four brain stem regions (periaqueductal gray and lateral reticular formation in the midbrain, raphe and reticular formation in the medulla) during noxious skin heating markedly reduced the nociceptive excitation of the dorsal horn neurons. In contrast, such brain stem stimulation had small and variable effects upon the noxious heat-evoked activity in the primary afferent fibers; both increases and decreases were observed. The brain stimulation also produced transient changes in blood pressure, suggesting that circulatory effects may underlie the mechanism of nociceptor modulation. It is concluded that brain stem stimulation can modulate cutaneous nociceptor activity, but that this modulatory effect on nociceptor inflow is too small and inconsistent to explain the marked descending inhibition of the nociceptive excitation of dorsal horn neurons.
aDivision of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, Australia
bDrug Safety and Evaluation Branch, Therapeutic Goods Administration, Canberra, Australia
cInstitute for Neuroscience, University of Delaware, Newark, DE 19716, USA
dDepartment of Physiology, Wuhan Medical College, Wuhan, China
eII. Physiologisches Institut der Universität Heidelberg, Abteilung Zentralnervensystem, Im Neuenheimer Feld 326, D-69120 Heidelberg, Federal Republic of Germany
*Corresponding author. Tel.: +49 6221 544050; fax: +49 6221 546364.
Received July 3, 1996; revised version received January 8, 1997; accepted January 15, 1997.