Comparative immunocytochemical localization of enkephalin and somatostatin in the median eminence, hypothalamus and adjacent areas of the guinea-pig brain.

Because of the presence of an enkephalin-related substance in the somatostatin terminals of the median eminence, an investigation was carried out on the staining for enkephalin of the somatostatin-containing perikarya localized in the hypothalamic periventricular nucleus. Only an occasional and weak enkephalin immunoreactivity was detected in these perikarya, making the common synthesis of the two peptides unlikely. Moreover, our results demonstrate an enkephalin staining in other fibers of the median eminence clearly distinct from the somatostatin ones. Enkephalin was demonstrated in perikarya distributed in the septum, preoptic area, dorsal and ventromedial hypothalamus, all regions known to possibly project into the median eminence. In none of these sites was enkephalin found in association with somatostatin. The present morphological results support the hypothesis that enkephalins may exert their GH-releasing effect at the hypothalamic level and suggest that enkephalins might directly modulate the somatostatin release into the blood of the portal plexus.