The distribution of substance P in the habenulo-interpeduncular system of the frog shown by an immunohistochemical method.

The distribution of substance P(SP)-like immunoreactivity was studied in the habenulo-interpeduncular (HAB-ITP) complex of the frog Rana esculenta by means of the immunohistochemical peroxidase-antiperoxidase (PAP) method. The HAB of the frog were subdivided into dorsal and ventral nuclei, corresponding to the medial and lateral HAB of mammals respectively. In addition, the dorsal HAB of the frog were morphologically asymmetric, since only the left has a lateral and a medial subnucleus. SP-like immunoreactive cells were found in the right dorsal HAB, in the lateral subnucleus of the left dorsal HAB and in a nucleus of scattered cells located rostrodorsally to the ITP. However, the medial subnucleus of the left dorsal HAB and a portion of the right dorsal HAB facing the 3rd ventricle, did not show SP immunoreactivity. Immunoreactive fibers were found in the medial subnucleus of the left dorsal HAB, in the two fasciculi retroflexes and in the ITP. Moreover, immunoreactive terminals were observed in the ventral HAB of both sides, in the ITP and on the blood vessels which cross the ITP. It is assumed that a portion of fibers connecting the various structures of the HAB-ITP complex belongs to the SP peptidergic system as suggested for mammals. However, in contrast to the mammals, the results on the frog are in favour of an histochemical asymmetry of the SP-like immunoreactivity, since a different distribution of this peptide was found between the right and left side of the brain. Such histochemical asymmetry was less pronounced than the morphological asymmetry and its functional meaning is unknown.