Modified in vitro assay for melanocyte-stimulating hormones

Isolation, structure, biological characterization, and synthesis of beta-[Tyr9]melanotropin-(9-18) decapeptide from pig hypothalami

A decapeptide with the amino acid sequence H-Tyr-Phe-Arg-Trp-Gly-Ser-Pro-Pro-Lys-Asp-OH was isolated from acid extracts of porcine hypothalami, structurally and biologically characterized, and synthesized. Except for the NH2-terminal tyrosine, this decapeptide corresponds to the amino acid sequences 9-18 of porcine beta-melanotropin (beta-MSH) and 49-58 of porcine beta-lipotropin (beta-LPH); it also has a tetrapeptide sequence of amino acids (Phe-Arg-Trp-Gly) common to the 7-10 sequences in corticotropin (ACTH) and alpha-MSH. beta-MSH, beta-LPH, alphaMSH, and ACTH from various species all have a histidine residue in the position immediately preceding the common sequence, and the occurrence of a natural peptide with the tyrosine residue in the corresponding site has not been previously reported. This suggests that the beta-[Tyr9]MSH-(9-18) decapeptide might be a fragment of a still larger precursor (prohormone) possibly related to beta-LPH.

Disapearance and excretion of labeled alpha-MSH in man

Despite the considerable evidence for the CNS actions of alpha-melanocyte-stimulating hormone (alpha-MSH) in man, little attention has been given to its half-time disappearance from plasma and urinary excretion in normal individuals. In the first experiment, a healthy man was given 15 muCi of tritiated (3H)-alpha-MSH as a rapid IV injection. A plot of the disappearance time in plasma was characteristic of a multiexponential curve, the linear components of which were resolved by the subtraction method and half-time disappearance calculated directly from the slope of the regression line. The half-time disappearance was 1 min for the first component and 25 min for the second component. After the IV adminisration of 50 muCi of 125I-alpha-MSH in the second experiment, the two components showed halftime disappearances of 1 min and 4.8 min respectively. These times were not changed by precipitation of the plasma with 10% trichloroacetic acid. Thirty-eight percent and 42% of the label appeared in the urine 4 hr after the injection of either 3H-alpha-MSH or 125I-alpha-MSH. The results suggest that the persistence of high levels of alpha-MSH in the blood after injection in man may be too short to fully explain the CNS effects of alpha-MSH.

Biodegradation of alpha-MSH and derived peptides by rat brain extracts, and by rat and human serum

The peptides alpha-MSH and MSH/ACTH 4-10 were degraded by rat brain extracts and serum to yield free amino acids among the end-products. Breakdown of these two peptides was double that of a related synthetic hexapeptide Met (0)-Glu-His-Phe-D-Lys-Phe. No significant breakdown of the hexapeptide occurred after incubation with human serum; it also had almost negligible pigmentary effects in vivo and in vitro when compared to alpha-MSH. The patterns of amino acid release indicate possible endopeptidase cleavage at Phe-Arg in alpha-MSH followed by secondary exopeptidase action to release free amino acids. For the hexapeptide, the primary cleavage point occurred at the -His3-Phe4 bond. The stability of this analog in human sera, coupled with its lower rate of degradation in the CNS, may contribute to its more potent behavioral actions in vivo.

Distribution of 3H-alpha-MSH in rat brain

Radioactive synthetic alpha-MSH, prepared by selective tritiation of its dibromo-L-tyrosine-derivative, was injected into the carotid artery of intact, hypophysectomized or pinealectomized rats. More radioactivity was found in the occipital cortex, cerebellum, and pons-medulla than in most other brain parts of rats decapitated 15 sec and, to a less significant extent, 30 min after injection. Part of this radioactivity behaved like alpha-MSH in several identification procedures. After administration of 3H-tyrosine, the least radioactivity was consistently found in the pons-medulla. Large amounts of radioactivity after 3H-alpha-MSH were found in the pituitary and, particularly in the hypophysectomized rats, in the pineal. The half-time disappearance of the first component of radioactivity from the blood of each group of rats injected with tritiated-alpha-MSH was no more than 2.5 minutes, a time considerably shorter than the behavioral and EEG effects of this pituitary hormone on visual attention.

Alpha-MSH and MIF-2 effects on serotonin levels and accumulation in various rat brain areas

Levels as well as accumulation of serotonin (5-HT) were measured in various brain regions of the rat after administration of alpha-melanocyte-stimulating hormone (MSH) and Pro-Leu-Gly-NH2 (MIF-I). The method used in determining the serotonin measured both 5-OH-tryptamine (5-HT) and 5-methoxytryptamine (5-MT). No statistically significant changes in levels or accumulation of serotonin after pargyline injection were found when unoperated control rats were treated with either MSH or MIF-I. Similar treatment of hypophysectomized rats indicated that both peptides significantly (p less than 0.05) lowered serotonin accumulation only in the area of the frontal cortex; a similar but smaller, not statistically significant, decrease was seen in the hypothalamus and hippocampus of the hypophysectomized rat. Since only hypophysectomized rats were affected, no correlation between the behavioral effects of these peptides (which has been found to occur in both unoperated and hypophysectomized rats) and the biochemical changes could be made.