Enkephalin-like immunoreactivity in the nucleus submedius of the cat and rat thalamus

In the present study, we employed the peroxidase-antiperoxidase immunocytochemical technique to study the presence and distribution of enkephalin in the nucleus submedius of both cats and rats at the light- and electron-microscopic levels. The enkephalin-like immunoreactive (ENK-LI) fibers were present in a concentrated, albeit limited, manner in the nucleus submedius of both species. These fibers were located close to the dorsal and caudal edge of the nucleus, and were confined to a small area that never exceeded 350 microns in the rostrocaudal or 250 microns in the dorsoventral direction. Mediolaterally, however, the fibers extended some 700 microns. No ENK-LI cell bodies were seen in the nucleus submedius, even in colchicine-treated animals. At the electron-microscopic level, the ENK-LI terminals were seen to synapse on dendrites. These data indicate a previously unsuspected role of enkephalin in synaptic transmission processes within the nucleus submedius, and provide additional support for the role of this nucleus in the processing of nociceptive information at medial thalamic levels.