INHIBITORY EFFECTS EVOKED FROM THE ANTERIOR HYPOTHALAMUS ARE SELECTIVE FOR THE NOCICEPTIVE RESPONSES OF DORSAL HORN NEURONES WITH HIGH AND LOW THRESHOLD INPUTS. Workman, B.J. and Lumb, B.M. Department of Physiology, School of Medical Sciences, University Walk, Bristol, BS8 1TD, U.K.
APStracts 4:0057N, 1997.
ABSTRACT
The aim of the present study was to examine the selectivity of descending conrtrol of nociceptive information in the spinal dorsal horn following neuronal activation at 'pressor' sites in the anterior hypothalamus. Extracellular single unit activity was recorded from eleven dorsal horn neurones in the lower lumbar spinal cord of anaesthetized rats. Neurones selected for investigation were those that responded to noxious (pinch and radiant heat >46oC) and non-noxious (prod, stroke and/or brush) stimulation within their cutaneous receptive fields on the ipsilateral hind paw. These are referred to as Class 2 neurones. Micropipettes were inserted stereotaxically into the anterior hypothalamus at sites where injection of the excitatory amino acid L-homocysteic acid (L-HCA) evoked increases in arterial blood pressure. The effects of microinjection of L-HCA at 'pressor' sites in the anterior hypothalamus were then tested on the responses of Class 2 neurones to noxious and non-noxious stimulation of their excitatory receptive fields. The high threshold (pinch and/or radiant heat) responses of 7/7 Class 2 neurones tested were inhibited by an average of 66.3% + 8.8% (mean + S.E.M.) by neuronal activation at hypothalamic 'pressor' sites. The low threshold (prod) responses of 10/10 Class 2 neurones tested were not inhibited by neuronal activation at hypothalamic 'pressor' sites; in 6 of these cells the response to low intensity stimulation was increased by between 4 and 20%. Control injections of the inhibitory amino acid GABA at the same hypothalamic 'pressor' sites had no significant effects on arterial blood pressure or neuronal activity. With regard to sensory processing in the spinal cord, these data suggest that descending inhibitory control that originates from neurones in 'pressor' regions of the anterior hypothalamus is highly selective for nociceptive inputs to Class 2 neurones. 

Received 18 November 1996; accepted in final form 5 February 1997.
APS Manuscript Number J910-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 20 February 1997