IMMUNOCYTOCHEMICAL AND IN-VITRO AUTORADIOGRAPHIC EVIDENCE FOR A DIRECT SOMATOSTATINERGIC MODULATION OF THE ENKEPHALINERGIC HYPOTHALAMOSEPTAL TRACT OF THE GUINEA-PIG

The present study was undertaken to determine whether the enkephaliner gic hypothalamoseptal tract originating in the magnocellular dorsal nu cleus in the guinea-pig brain is under the influence of somatostatin. In the first step, double immunocytochemical labeling of enkephalinerg ic cells and somatostatinergic fibers was combined at the light and el ectron microscopic levels in the magnocellular dorsal nucleus. As a se cond step, an in vitro radioautography was used to determine whether s omatostatin receptors are present in the same area. A close relationsh ip between somatostatin nerve endings and enkephalin perikarya was obs erved at both the light and electron microscopic levels. Contacts were more numerous in the ventral part of the magnocellular dorsal nucleus . Whenever synaptic images were clearly observable; they appeared symm etrical. In the same area, a moderate concentration of G-protein-coupl ed somatostatin binding sites was also visualized. These results sugge st that somatostatin has a regulator role on the enkephalinergic hypot halamoseptal tract, directly at the level of the magnocellular dorsal nucleus.