a-MSH in rat brain: occurrence within and outside of fl-endorphin neurons

After the discovery of the two enkephalins 14,z° and fl-endorphin (fl-END) 5,6,xt, 12,ts,2s, these peptides and related substances were the focus of a large number of immunohistochemical and lesion studies1-4,7,9,1a,21-24,~9,ax,a2, a4-as. These studies indicated the separateness of the enkephalin and fl-END systems in rat central nervous systema,27, 34. The enkephalin systems are composed of many lightly stained cells and fine fibers localized throughout the neuraxisg,la,29, a2,35, fl-END neurons can be visualized only in the arcuate nucleus with some cells spreading laterally out toward the lateral hypothalamus2,a,23,25,al,34, a6-~s. It appears that the single fl-END cell group distributes its axons and fibers throughout the central nervous system, with very long projection pathways through hypothalamus, dorsal thalamus, periaqueductal gray finally ending in the region of the locus coeruleus. Immunohistochemical studies using antisera against fl-lipotropin (fl-LPH) reveal an identical distribution2a,a6, as. In the original immunohistochemical study by Moon et al. 2°, fl-LPH was localized in pituitary corticotrophs and all the cells of the intermediate lobe. Subsequent visualization studies in pituitary using fl-END antibody 4 further confirmed that fl-END and fl-LPH are located in the same cells as ACTH. Pelletier et al. 2~ in elegant electron microscopic studies demonstrated that fl-LPH and ACTH were found in the same granules of pituitary corticotrophs and intermediate lobe cells. Since fl-LPH and fl-END had been shown to exist in rat brain2,3,17,~a,25,at,34,36, a8, it seemed reasonable to attempt to localize ACTH-like immunoreactivity in rat brain as well. Immunochemical and immunocytochemical studies using antisera against various portions of ACTH2,16,21,24,81,~4,37 revealed the existence of ACTH-like material in brain, which appeared to survive hypophysectomy. In subsequent studies it was possible to demonstrate that fl-END, fl-LPH and ACTH immunoreactivity were found within the same hypothalamic neurons~,~l,2a,al, 34. Thus, it appears likely that the 31K precursorS,19, 26 for pituitary ACTH, fl-LPH and fl-END is present in rat hypothalamic neurons as well.