The bifunctional role of pro-opiomelanocortin derivatives in the mediobasal hypothalamus of the rat.

In the present study, a polyclonal antibody against pro-opiomelanocortin derivatives was characterized biochemically. Its immunoreactivity with structures of the arcuate nucleus and the median eminence was investigated by means of the immunogold method and compared with its reaction on adenohypophyseal cells with and without pre-adsorption with pro-opiomelanocortin derivatives. The antiserum detects ACTH and its fragments, in particular alpha-MSH, and beta-endorphin. In the adenohypophysis gold particles are exclusively located on small secretory granules situated in the periphery of branched cells. In the perikarya of the arcuate nucleus gold particles are observed on terminal vesicles abutting from the cis-face of the Golgi apparatus, on granules in its direct vicinity and on small dense core vesicles preferentially located in the cell periphery. Immunoreactive gold-labeled fiber profiles are found in a sub- or intra-ependymal position as well as in the nuclear neuropil proper. Here axodendritic and axosomatic synapses are observed. In both situations the gold particles are mostly restricted to the small dense core vesicles and do not decorate the synaptic vesicles. In the median eminence gold labeled fibers are detected in all layers. The labeled fibers can be closely apposed to tanycytic processes, without, however, forming special contact differentiations. In direction to the perivascular layer of the external zone the labeled profiles are more frequently arranged in groups intermingled with unlabeled fibers. The axons decorated with gold particles can be freely exposed to the perivascular space or are found as single processes in close vicinity to the capillary wall. Subsequent to preincubation of the native antiserum with ACTH1-39 and ACTH18-39 (= CLIP) neither adenohypophyseal cells nor perikarya and fibers in the arcuate nucleus nor axons in the median eminence are decorated with gold particles. Preincubation of the native antiserum with alpha-MSH or beta-endorphin does not change the immunoreaction with the small, peripherally situated granules in the branched adenohypophyseal cells. In neurons of the arcuate nucleus and in fibers of the median eminence, however, the immunoreaction is completely extinguished when the antibody is pre-incubated with alpha-MSH, whereas subsequent to preincubation with beta-endorphin only the amounts of labeled structures are reduced.(ABSTRACT TRUNCATED AT 400 WORDS)