Coexistence of corticotropin-releasing factor and enkephalin in the paraventricular nucleus of the rat.

Corticotropin-releasing factor and enkephalin-containing neurons are found in the paraventricular nucleus of the rat hypothalamus. Their immunocytochemical distribution suggests that a subpopulation of neurons of the paraventricular nucleus might contain both peptides. The present study describes the coexistence of corticotropin-releasing factor and enkephalin in parvicellular neurons of the paraventricular nucleus. Immunocytochemical labeling of peptides was combined with in situ hybridization of mRNA to visualize peptide and mRNA labeling in the same tissue section. This resulted in several observations: (1) neurons labeling for the peptide corticotropin-releasing factor also contain the mRNA to synthesize corticotropin-releasing factor, (2) neurons labeling for the peptide enkephalin also contain the mRNA to synthesize the peptide enkephalin, (3) a subpopulation of corticotropin-releasing factor neurons contains the mRNA to synthesize enkephalin, and (4) a subpopulation of enkephalin neurons contains the mRNA to synthesize corticotropin-releasing factor. A large percentage of enkephalin immunoreactive neurons contains corticotropin-releasing factor mRNA, whereas a smaller percentage of corticotropin-releasing factor immunoreactive neurons contains enkephalin mRNA. These double-labeled neurons are present throughout the rostral-caudal extent of the paraventricular nucleus; the majority of these neurons is present in the medial parvicellular paraventricular nucleus.