Interactions between visceral and cutaneous nociception in the rat. I. Noxious cutaneous stimuli inhibit visceral nociceptive neurons and reflexes

Abstract

1. Numerous studies have demonstrated that neural, behavioral, and reflex responses to a nociceptive test stimulus are inhibited by conditioning nociceptive stimuli; this inhibition has supraspinal, intraspinal, and segmental components. The general phenomenon is defined here as nocigenic inhibition. The present study of nocigenic inhibition documents that noxious cutaneous pinch and heat, used as conditioning stimuli, inhibit neuronal and reflex responses evoked by a noxious visceral stimulus, colorectal distension. 2. A total of 196 dorsal horn neurons were examined: 110 were short latency-abrupt (SL-A) neurons that were excited at short latency by colorectal distension and returned to baseline activity abruptly after termination of the distending stimulus, and 86 were short latency-sustained (SL-S) neurons that also were excited by colorectal distension at short latency, but demonstrated after-discharges for 4–240 s after termination of the distending stimulus. All SL-A and SL-S neurons studied had convergent cutaneous receptive fields. 3. The spontaneous activities of 100% of the 110 SL-A neurons tested were inhibited by greater than 25% by noxious pinch in sites distant from their convergent cutaneous receptive fields. In both anesthetized, intact, and spinalized rats, noxious conditioning pinch or noxious conditioning heat significantly reduced responses of SL-A neurons during noxious test colorectal distension. The magnitude of this nocigenic inhibition was graded with the intensity of the noxious conditioning stimulus. Noxious conditioning tail heating produced a parallel shift to the right in stimulus-response functions relating neuronal responses to the intensity of colorectal distension (20-100 mmHg).(ABSTRACT TRUNCATED AT 250 WORDS)