A melanocyte-stimulating hormone-releasing factor in hypothalamic extracts

Inhibition by L-prolyl-L-leucyl-glycinamide (PLG) of alpha-melanocyte stimulating hormone release from hypothalamic slices

Release of alpha-MSH from male rat hypothalamic slices was studied using a sensitive bioassay (1-2 pg). Addition of 60 mM KCl to superfusion medium resulted in a twofold increase in alpha-MSH release compared to spontaneous release. Both spontaneous and potassium-induced release were inhibited in a dose-response manner by the tripeptide Pro-Leu-Gly-NH2 (PLG, or MIF-1); 0.04 microgram to 1 microgram PLG inhibited the alpha-MSH release but the lowest dose demonstrated a greater inhibitory effect; high concentrations of PLG, on the other hand, did not modify either spontaneous or potassium-evoked alpha-MSH release from the slices. Contrarily, DA did not modify either spontaneous or potassium- induced alpha-MSH release at any of the doses tested. These findings demonstrate that the inhibitory behavior of PLG and DA in the central nervous system (CNS) differs from their behavior towards alpha-MSH release in the pituitary. This suggests differences in the regulation of alpha-MSH release from the pituitary and the CNS.

Assay of melanotropic peptides in an in vitro mammalian system

A reproducible and sensitive assay for melanotropic agents is described employing mouse melanoma cells in culture and measuring tyrosinase activity in terms of production of tritiated water from L-(ring-3,5-3H)-tyrosine. Molar concentrations of peptides inducing one-half maximal stimulation of tyrosinase activity were: beta-MSH, 1 +/- 2 x 10(-9); alpha-MSH and Beta h-LPH, 1 +/- 2 x 10(-8); ACTHp, 1 +/- 2 x 10(-7). Beta p 9-18-MSH and melanotropin potentiating factor, beta s 88-91-LPH exhibited no activity at concentrations as high as 10(-5)M.

Biosynthesis, processing, and control of release of melanotropic peptides in the neurointermediate lobe of Xenopus laevis

The neurointermediate lobes of dark-adapted toads Xenopus laevis were incubated for 30 min in [3H]arginine and then "chased" for various time periods. By use of this pulse-chase paradigm there were detected 10 trichloroacetic acid (TCA)-precipitable peptides separated on acid-urea polyacrylamide gels and one TCA-soluble peptide separated by high-voltage electrophoresis (pH 4.9) with melanotropic activity. Each of these peptides had a different degree of melanocyte stimulating hormone (MSH) activity as revealed by the Anolis skin bioassay. Three of these TCA-precipitable peptides comigrated with ACTH, beta-lipotrophin, and alpha-MSH on acid-urea gels. Evidence suggesting a precursor-product mode of biosynthesis of the melanotropic peptides is presented. 7 of the 10 TCA-precipitable peptides and the one TCA-soluble peptide with melanotropic activity were released into the medium. The half-time of release of the TCA-precipitable peptides was about 2 h, whereas the half-time of TCA-soluble peptide release was about 30 min. The release of these peptides was inhibited by 5 X 10(-5) M dopamine. Dopamine inhibition of release did not appear to affect the biosynthesis of the melanotropic peptides, but did appear to enhance the degradation of the newly synthesized TCA-soluble peptide in the tissue. White adaptation of the toads greatly decreased the biosynthesis of all of the TCA-precipitable melanotropic peptides.

Dopamine synthesis: stimulation by a hypothalamic factor

The effect of treatment with the factor that inhibits the release of melanocyte stimulating hormone (MSH) identified as 1-prolyl-1-leucylglycinamide (MIF) on brain catecholamine synthesis was examined in normal and hypophysectomized rats. The tripeptide induced a dose-related increase in striatal dopamine synthesis in slices obtained from treated normal animals but not in hypophysectomized animals. Hypothalamic norepinephrine synthesis was unaltered by MIF treatment in normal as well as in hypophysectomized rats. In addition, dopamine and norepinephrine syntheses were depressed in untreated hypophysectomized animals, as compared to normal controls. These results constitute the first direct demonstration of a central neurochemical effect of a hypothalamic factor.

Hypothalamic regulatory hormones

The molecular structures of several polypeptides isolated from hypothalamic tissue have been established and the synthesis of these compounds has been achieved. These polypeptides selectively stimulate or inhibit the release of anterior pituitary hormones and melanocyte-stimulating hormone. Various studies indicate their important physiological role and support the concept that some of these polypeptides are hormones. Some synthetic hypothalamic hormones and their derivatives may find important clinical and veterinary applications.

Regulation of formation and proposed structure of the factor inhibiting the release of melanocyte-stimulating hormone

Microsomal preparations from the stalk median eminence of female rats are shown to contain an enzymic activity that is responsible for the formation of MSH-release-inhibiting factor (MSH-R-IF). The amount of this activity remains constant throughout the estrous cycle. The corresponding mitochondrial preparations from the stalk median eminence contain another enzymic principle, estrous cycle-dependent, which competes with the enzyme present in the microsomal preparation for the same "substrate", and can thereby prevent the formation of MSH-R-IF. Several neurohypophyseal hormones, analogs, and peptide intermediates have been tested for their intrinsic MSH-R-IF activity and for their ability to be transformed into MSH-R-IF by incubation with microsomal preparations of stalk median eminence from male rats; it is concluded that the enzyme responsible for the formation of MSH-R-IF is an exopeptidase and that the release-inhibiting factor itself is a tripeptide. Oxytocin is converted by the incubation to (L)-prolyl-(L)-leucylglycinamide; nanogram amounts of this tripeptide inhibit the release of MSH from the pituitary both in vivo and in vitro.

The effect of an exteroceptive stimulus on the concentration of melanocyte-stimulating hormone in the pituitary of lactating rats

1. A study was made of the effect of an exteroceptive stimulus, produced by a team of lactating rats and of litters while suckling, on the pituitary melanocyte-stimulating hormone (MSH) activity of a group of lactating rats.2. After being isolated for 9 hr from their litter, one group of lactating rats was killed after being allowed to nurse for 30 min, another group was subjected to the exteroceptive stimulus for 15 min and then killed. Control rats were killed after 9 hr of isolation from the litter without being subjected to either suckling or the exteroceptive stimulus.3. The group of rats which had suckled their young showed a pituitary MSH activity equal to 35 +/- 2.29%, and the group exposed to the exteroceptive stimulus an activity equal to 59.5 +/- 3.16% of that in the control animals.4. When deaf rats were used, only the suckled mother showed a decrease of the pituitary MSH activity which was similar to that obtained with the normal suckled mother, indicating that the exteroceptive stimulus depended on hearing for its effect.5. A light dose of sodium pentobarbitone was sufficient to block the effect of suckling and the auditory stimuli on the pituitary MSH activity.6. The role of the central nervous system in this phenomenon and the possibility that MSH is implicated in the process of lactation are discussed.