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

We used FOS-like immunohistochemistry to detect neuronal activity in t he pontine parabrachial nucleus after injection of formalin into the l ower lip of the awake rat and compared the labeling pattern with that seen after formalin injection into the hindpaw. One hour after a forma lin injection into the lip, many FOS-immunoreactive cells were seen in the parabrachial nucleus, preferentially on the side ipsilateral to t he injection site. Detailed anatomical analysis revealed that FOS-immu noreactive neurons were localized predominantly to three regions of th e parabrachial nucleus: the external lateral, the external medial, and the Kolliker-Fuse subnuclei, with sparser labeling present in the dor sal and superior lateral subnuclei and in the medial parabrachial nucl eus. In contrast, a formalin injection into the hindpaw resulted in de nse FOS-labeling in the superior, dorsal, and central lateral subnucle i, with sparse to moderate labeling in the Kolliker-Fuse nucleus, and sparse labeling in the external lateral and external medial subnuclei, as described previously (Hermanson and Blomqvist, J. Comp. Neurol., [ 1996] 368:45-56). The distribution of FOS-labeled neurons after noxiou s orofacial stimulation corresponds to the termination pattern in the parabrachial nucleus of fibers that originate from neurons in the marg inal zone of the trigeminal dorsal horn and is different from that see n after nociceptive stimulation of other body parts. Considering the d ifferences in efferent connections of parabrachial subnuclei, the pres ent findings imply that noxious information from the orofacial region to the parabrachial nucleus has other functional roles than noxious in formation from the trunk and limbs. Such roles may include the integra tion of somatosensory and gustatory information, which has been sugges ted to be of importance for feeding behavior. (C) 1997 Wiley-Liss, Inc .