The region of the pontine parabrachial nucleus is a major target of dehydration-sensitive CRH neurons in the rat lateral hypothalamic area.

Neurons in a restricted part of the lateral hypothalamic area (LHA) show increased expression of corticotropin-releasing hormone (CRH) mRNA as a consequence of cellular dehydration. In the present study, we have investigated the organization of their efferent projections by using anterograde and retrograde tracing techniques. Additionally, we have compared the distribution of CRH mRNA-containing neurons after cellular dehydration and intraventricular (i.c.v.) colchicine injections. Our results show that cellular dehydration activates a more restricted neuronal population than does i.c.v. colchicine. Iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) were placed in the LHA of animals drinking hypertonic saline and their proximity to activated CRH neurons determined by in situ hybridization for CRH mRNA. Although labelled fibers from these injections were seen throughout the brain, the region of the parabrachial nucleus and nucleus of the solitary tract (NTS) were most conspicuous in also having CRH immunoreactive fibers. Injections of Fluoro-Gold placed in these two structures were used to confirm these findings in dehydrated animals. Significant numbers of neurons containing both Fluoro-Gold and CRH mRNA were seen in the lateral hypothalamus after injections in the lateral and medial parts of the parabrachial nucleus; far fewer were seen after injections in the NTS. These results strongly suggest that the CRH neurons in the LHA activated by cellular dehydration provide an input to the region of the parabrachial nucleus. The altered biochemical composition of this pathway may well be able to modify sensory and motor patterns both during and after dehydration.