Formalin induced FOS-like immunoreactive neurons in the trigeminal spinal caudal subnucleus project to contralateral parabrachial nucleus in the rat

Abstract

By combining the retrograde-labeling method of injecting Fluoro-Gold (FG) into the parabrachial nucleus (PB) and the immunocytochemical staining of the FOS-like immunoreactive neurons (FLNs) in the trigeminal spinal caudal subnucleus (TSCS) induced by s.c. formalin injection into the perioral region in the rat, it was demonstrated that there are FLNs, FG-labeled neurons and neurons containing both FOS-like immunoreactivity and FG fluorescence in the TSCS. The three kinds of labeled neurons are distributed mainly in laminae I, II and V of the TSCS and there are also some neurons containing both FOS-like immunoreactivity and FG distributed in the adjacent ventrolateral reticular formation. The retrograde-labeling of FG- and double-labeled neurons showed contralateral predominance. In addition, we found that there are retrogradely labeled neurons in bilateral nuclei of the solitary tract with a contralateral predominance. The results suggest that FOS-like immunoreactivity might serve as an indicator for the nociceptive response after formalin injection into the trigeminal region and that the PB might be an important relay station for the further processing of the nociceptive information relayed from the trigeminal afferents. As the PB is known as a relay structure for visceral sensory pathway, it is proposed that there might be viscero-somatic convergence in this nucleus.