The organization of the efferent projections of the parabrachial nucleus of the forebrain in the rat: a retrograde fluorescent double-labeling study.

The organization of the efferent projections of the parabrachial nucleus (PBN) to the forebrain has been investigated in the rat by means of combined injections of two fluorescent retrograde tracers: red fluorescent Evans Blue and a blue fluorescent mixture of 4',6'-diamidino-2-phenylindol 2 HCl and primuline. First, the distributions of retrogradely labeled neurons in the PBN after bilateral injections of tracers in the central nucleus of the amygdala (CNA) was examined. The CNA on one side of the brain was injected with one of the tracers and the CNA on the opposite side of the brain was injected with the other tracer. Next, the distributions of labeled neurons were examined after bilateral ventral medial thalamus (VMT) injections. Finally, the retrograde labeling of the PBN was studied after combined ipsilateral injections of one tracer in the CNa and the other tracer in the VMT. After the various injections, characteristic distributions of populations of labeled neurons within the PBN were seen. Double-labeled neurons were present only after bilateral VMT injections. From this it was concluded that the PBN projections to the VMT in the rat are bilateral. Based on the relative distributions of populations primarily taste information to the VMT and mainly visceral information to the CNA. This transfer of information to the forebrain is discussed.