Comparative topography of the immunoreactive alpha-melanocyte-stimulating hormone neuronal systems in the brains of horses and rats.

The present study describes and compares the topography of alpha-melanocyte-stimulating hormone (alpha-MSH)-immunoreactive neurons in the forebrain of rats and horses. Two groups of immunoreactive cells were present in both species. These groups were distinguished by their cross-immunoreactivity and topography. The topography of cell bodies with cross-immunoreactivity for peptides derived from pro-opiomelanocortin (POMC) including adrenocorticotropin, 16K, alpha-MSH and beta-endorphin was similar in both species. However, the topography of perikarya which stained only for alpha-MSH, and not for other POMC-derived neuropeptides, was subject to pronounced species differences. In rats, perikarya which stained only for alpha-MSH (alpha 2-system) have a specific distribution in the dorsal and lateral hypothalamic regions from the level of the midoptic chiasm to the premamillary region of the posterior hypothalamus. At anterior levels of the rat brain, the cells are bipolar and oriented horizontally. Posteriorly, all alpha 2-group perikarya are stellate in rats. In the equine species, alpha 2-group cell bodies occupy a shorter anterioposterior field extending from the posterior level of the paraventricular nucleus to the supramamillary nucleus. Stellate cells are mixed with bipolar cells at all levels of the alpha 2-field in the equine hypothalamus. Further, in contrast to the continuous cell group found in rats, the alpha 2-group divides into perifornical and periventricular subpopulations at posterior levels of the equine hypothalamus. The results from this study suggest that the topography of the arcuate opiocortin cell pool, which contains peptides derived from POMC, is similar in phylogenetically diverse mammalian species whereas the topography of the cell pool staining only for alpha-MSH is subject to species differences. In both species, the major terminus of alpha 2-neurons appears to be the cerebral cortex and hippocampus. It has been proposed that alpha-MSH is one of several neuropeptides concerned with memory and behavior. Thus, species differences in the topography of the alpha 2-pool may reflect evolutionary changes in the importance of this peptide to these physiological processes.