SUBNUCLEAR LOCALIZATION OF FOS-LIKE IMMUNOREACTIVITY IN THE RAT PARABRACHIAL NUCLEUS AFTER NOCICEPTIVE STIMULATION

The effect of noxious stimulation on the expression of FOS-like immuno reactivity (FOS-LI) in neurons of the parabrachial nucleus (PB) was st udied in awake, freely moving rats. In one series of experiments, the rats were subjected to noxious mechanical stimulation (pinch) of eithe r the nape of the neck or the base of the tail for 20 seconds every 5 minutes for 90 minutes, and then they were killed by transcardial perf usion after 45-210 minutes. Control animals received innocuous mechani cal stimulation (brush) of the tail. Noxious stimuli resulted in FOS-L I in neurons in the dorsal part of the lateral PB, with heavy labeling in the superior lateral (PBsl) and the dorsal lateral (PBdl) subnucle i. FOS-LI was also elicited in the central lateral subnucleus (PBcl) a nd, although much more sparsely, in the external lateral subnucleus an d the Kalliker-Fuse nucleus. Tail and neck stimulation resulted in sim ilar labeling patterns, but more neurons, particularly in PBsl, expres sed FOS-LI after pinch of the tail than of the neck. In another series of experiments, rats received injection of 5% formalin into one hindp aw. After 75-90 minutes, FOS-LI was seen in the same parts of PB as af ter noxious mechanical stimulation. The heaviest labeling was seen on the side contralateral to the injection side, with statistically signi ficant (P < 0.05) side differences present in PBsl and PBdl. In a thir d series of experiments, rats were hemisected at low cervical-upper th oracic segments, allowed 2 weeks to recover, and then given formalin i njections in both hindpaws. Significantly more neurons were FOS-labele d in PBdl, PBsl, and PBcl on the side contralateral to the hemisection than on the ipsilateral side. These observations are discussed in rel ation to the organization of the spinal afferent input and the efferen t connections of PB. It is concluded that the FOS-LI expression in PBd l and PBsl and probably also in PBcl, to a large extent, is evoked by the ascending spinal nociceptive input to PB. Because these subnuclei project to several hypothalamic regions, it is suggested that neurons in PB that express FOS after noxious mechanical and chemical stimulati on primarily are involved in autonomic and homeostatic responses to be havioral situations that involve tissue-damaging stimuli. (C) 1996 Wil ey-Liss, Inc.