We have used in situ hybridization to determine the regional distribution of mRNA encoding the neurotensin/neuromedin N (NT/N) precursor in the forebrain of the adult male rat. Cells containing NT/N mRNA are widely distributed in the forebrain. These areas include the septum, bed nucleus of the stria terminalis, preoptic area, hypothalamus, amygdala, accumbens nucleus, caudate-putamen, and piriform and retrosplenial cortex. In general, the regional distribution of NT/N mRNA corresponds to the previously determined distribution of neurotensin-immunoreactive cell bodies; however, several notable exceptions were observed.The most striking difference occurs specifically in the CA1 region of the hippocampus, where intense labeling is associated with the pyramidal cell layer despite the reported absence of neurotensin-immunoreactive cells in this region. Analysis of microdissected tissue by S1 nuclease protection assay confirmed the abundance of authentic NT/N mRNA in CAL. A second major discrepancy between NT/N mRNA abundance and neurotensin-immunoreactivity occurs in the intensely labeled subiculum, a region that contains only scattered neurotensin-immunoreactive cells in the adult. These results suggest that, in specific regions of the forebrain, NT/N precursor is processed to yield products other than neurotensin. In addition, these results provide an anatomical basis for studying the physiological regulation of NT/N mRNA levels in the forebrain.Neurotensin (NT), a tridecapeptide with widespread distribution in the central nervous system and peripheral tissues of mammals, is one of a family of structurally related peptides with members manifesting diverse biological activities (1). The structure of the rat gene encoding NT has recently been determined (2), and the general features of the predicted NT precursor for the rat are the same as those of the predicted precursors for the dog (3) and cow (2). In each species, a precursor protein of 169-170 amino acid residues encodes both NT and neuromedin N, a peptide similar in structure to NT (4). A comparison of the sequences of the complete neurotensin/neuromedin N (NT/N) precursor from rat, dog, and cow reveals a high degree of conservation. This finding is consistent with the idea that the precursor may be processed to yield biologically active peptides that are as yet unknown.There is increasing evidence that NT functions as a neuroregulatory substance in the mammalian central nervous system (1, 5). Effects produced by NT administration in the central nervous system include hypothermia, hypotension, and antinociception. NT has also been implicated in the central regulation of anterior pituitary function (6, 7). NTimmunoreactive cell bodies are most numerous in the forebrain, with widespread distribution in limbic regions such as the septum, preoptic area, hypothalamus, and amygdala (8).Other forebrain regions, such as the dorsal subiculum, contain many NT-immunoreactive cell bodies during early stages of development, but the number of neurons with detectable...