Immunological characterization of endorphins, adrenocorticotropin, and melanotropins in frog hypothalamus

Abstract

To gain more information about the nature and regulation of opiomelanocorticotropic peptides in the frog diencephalon, radioimmunological determinations of α- and β-MSH, ACTH, β- and γ-endorphins have been performed in hypothalamic extracts. Sephadex G-50 gel filtration revealed a single peak of α-MSH-like immunoreactivity (α-MSH-LI) comigrating with synthetis α-MSH. Two peaks of ACTH-LI were observed; the major one eluting slightly before human ACTH. Using a porcine β-endorphin antiserum which exhibited 45% cross-reaction (on a molar basis) with ovine β-LPH, one major peak of β-endorphin-LI and two additional components were observed in the elution profile; none of these peaks coeluted with synthetic porcine β-endorphin. No significant β-MSH or γ-endorphin-LI was detected. To investigate a possible role of glucocorticoids on the level of opiomelanocorticotropic peptides in frog hypothalamus, 60 male frogs were treated with dexamethasone (300 μg/day) during 8 days. Dexamethasone treatment did not modify the chromatographic distribution and the total amount of α-MSH-LI and ACTH-LI in hypothalamic extracts. A slight (15%) increase in β-endorphin-LI was even observed after 8 days of dexamethasone treatment. From these results it is concluded that three classes of opiomelanocorticotropic peptides are present in the frog hypothalamus in the following order of concentration: β-endorphin-LI > α-MSH ⪢ ACTH. In addition, circulating glucocorticoids which significantly reduce the concentration of opiomelanocorticotropic hormones in the distal lobe of the frog pituitary (S. Jégou, M. C. Tonon, F. Leboulenger, C. Delarue, J. Côté, G. Pelletier, and H. Vaudry (1981a). In “Adv. Physiol. Sci.” E. Stark, G. B. Makara, Zs. Acs, and E. Endröczi, eds., Vol. 13, pp. 129–133. Pergamon, Budapest.) do not modify the amount of these peptides in the hypothalamus.