A study of afferent projections to the rat interpeduncular nucleus.

Forebrain and brainstem afferents projecting to the interpeduncular nucleus (IPN) have been demonstrated in male rats by retrograde transport of fluorescent dye, "fast blue," microinjected in IPN, followed by intraventricular colchicine 48 hr prior to perfusion. The most intensely labeled cells projecting to IPN were concentrated throughout the entire rostrocaudal extent of the medial habenular nuclei. A small number of labeled medial habenular cells located dorsomedially also revealed SP immunofluorescence. Additional forebrain afferents originate from septal, hypothalamic and mammillary nuclei. Of brainstem afferents projecting to IPN, the most intensely labeled neurons were present in a circumscribed region overlying the dorsal aspect of the dorsal tegmental nucleus, an area described in the cat as the nucleus incertus [5], and which we now suggest is present in the rat. Many labeled cells in the medial aspect of this nucleus also revealed L-ENK immunofluorescence. Additional brainstem afferents include the raphe, dorsolateral tegmental nuclei and locus coeruleus. This study demonstrates both forebrain and brainstem afferents projecting to IPN and reveals an SP and L-ENK projection from the medial habenula and nucleus incertus, respectively.