RESTRAINT REDUCES FORMALIN-TEST PAIN BUT THE EFFECT IS NOT INFLUENCEDBY LESIONS OF THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS

Previous research indicates that the paraventricular nucleus of the hy pothalamus (PVN) plays an important role in the development of stress- induced analgesia (SIA). Research implicating the PVN in SIA has gener ally employed the cold-water swim as the stressor and a phasic pain te st, such as the tail-flick test, as the pain model. The present study, using the formalin test for tonic pain, investigated the effect of PV N lesions on (1) tonic pain responses and (2) SIA caused by 30 min of restraint. Male Long-Evans rats were randomly assigned to one of four groups. Two groups received electrolytic lesions of the PVN and two ad ditional groups served as sham-operated controls. One group which rece ived PVN lesions and one group which was sham-operated were exposed to 30 min of restraint immediately prior to a 0.05-ml injection of 2.5% formalin into the planter sur face of one hindpaw. The remaining group s which either received PVN lesions or were sham-operated received the formalin injection without prior exposure to restraint. During the fi rst phase of the formalin response, PVN lesions did not alter duration of paw elevation scores, but significantly increased duration of paw licking scores. A 30-min period of restraint had no effect on duration of paw elevation scores, but significantly decreased duration of paw licking scores. PVN lesions did not alter the significant decrease in paw licking scores as a result of restraint. During the second phase o f the formalin response, PVN lesions did not alter either the duration of paw elevation scores or the duration of paw licking scores. A 30-m in period of restraint significantly decreased duration of paw elevati on scores, but had no effect on duration of paw licking scores. PVN le sions did not alter the significant decrease in paw elevation scores a s a result of restraint. The results indicate that PVN lesions increas e paw licking only during the first phase of the formalin response, wi th no other alterations in paw licking or duration of paw elevation. I n addition, a 30-min period of restraint can produce short-term and lo ng-term SIA for tonic pain. The shortterm SIA is reflected as a decrea se in paw licking, whereas the long-term SIA is reflected as a decreas e in paw elevation. In addition, PVN lesions failed to alter SIA durin g both phases of the formalin test. The differential effect of restrai nt on pain responses during the two phases of the formalin test and th e lack of effect of PVN lesions on SIA for tonic pain suggest that str ess engages multiple endogenous pain inhibitory systems. (C) 1996 Acad emic Press, Inc.