[Distribution of the enkephalin neuron system in the rat thalamus; an immunohistochemical analysis]

Met-and Leu-enkephalin immunoreactive structures in the rat thalamus were investigated by indirect immunofluorescent method. Both structures showed similar distributions. Distribution: A large number of enkephalin-like immunoreactive (ENKI) terminals were observed in the antero-ventral nucleus (AV), the region between the parataenial nucleus, the antero-medial thalamic nucleus and ventral part of the ventral nucleus. ENKI cells were seen in the medial thalamic nucleus and ventral nucleus of the lateral geniculate body. Furthermore, ENKI cells are seen at the pars parvicellular of the posterior commissure and the lateral posterior thalamic nucleus detected only following pretreatment with colchicine. Fiber connection: The origin of ENKI terminals in the AV was examined by means of experimental manipulations. Injections of HRP (horseradish peroxidase) into the AV resulted in labeling of the MM (medial mammillary nucleus pars medialis) and MMC (medial mammillary nucleus pars centralis) but not the PM (premammillary nucleus). Destruction of the MM, where a number of ENKI neurons were seen, resulted in a disappearance of ENKI terminals in the ipsilateral AV. These findings suggest that MM is a major source of ENKI terminals in the AV.