Analysis of the habenulopetal enkephalinergic system in the rat brain: an immunohistochemical study

The enkephalinergic afferent system in the rat habenula was examined by the indirect immunofluorescence method with antibodies against leucine-enkephalin. Leucine-enkephalin-like immunoreactive (L-ENKI) fibers were observed in the dorsal portion of the medial habenular nucleus (MHb), the intermediate portion of the lateral habenular nucleus (LHb), and the border zone between the MHb and the LHb (BZHb). Knife-cut studies demonstrated that almost all the fibers were supplied via the stria medullaris. Also two discrete ENKergic afferents to the MHb and the LHb were found by several kinds of lesion studies: the MHb was ipsilaterally innervated from L-ENKI neurons in "septoperiventricular area," which is the junctional area between the ventral supracommissural septum and the rostral thalamic periventricular region. The LHb was ipsilaterally innervated from L-ENKI neurons in the rostral portion of the lateral hypothalamus. Our findings also suggested that the L-ENKI fibers in the BZHb are supplied via the stria medullaris with an ipsilateral predominance and that, at the most caudal level, they arise not only from the ipsilateral stria medullaris but also from the contralateral stria medullaris via the habenular commissure.