Subnuclear localization of FOS-like immunoreactivity in the parabrachial nucleus after orofacial nociceptive stimulation of the awake rat.

We used FOS-like immunohistochemistry to detect neuronal activity in the pontine parabrachial nucleus after injection of formalin into the lower lip of the awake rat and compared the labeling pattern with that seen after formalin injection into the hindpaw. One hour after a formalin injection into the lip, many FOS-immunoreactive cells were seen in the parabrachial nucleus, preferentially on the side ipsilateral to the injection site. Detailed anatomical analysis revealed that FOS-immunoreactive neurons were localized predominantly to three regions of the parabrachial nucleus: the external lateral, the external medial, and the Kölliker-Fuse subnuclei, with sparser labeling present in the dorsal and superior lateral subnuclei and in the medial parabrachial nucleus. In contrast, a formalin injection into the hindpaw resulted in dense FOS-labeling in the superior, dorsal, and central lateral subnuclei, with sparse to moderate labeling in the Kölliker-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 noxious orofacial stimulation corresponds to the termination pattern in the parabrachial nucleus of fibers that originate from neurons in the marginal zone of the trigeminal dorsal horn and is different from that seen after nociceptive stimulation of other body parts. Considering the differences in efferent connections of parabrachial subnuclei, the present findings imply that noxious information from the orofacial region to the parabrachial nucleus has other functional roles than noxious information from the trunk and limbs. Such roles may include the integration of somatosensory and gustatory information, which has been suggested to be of importance for feeding behavior.