Melanocyte-stimulating Hormone Release-inhibiting Research Articles

Melanocyte‐Stimulating Hormone Release‐Inhibiting Factor‐1 (MIF‐1) Can Be Formed from Tyr‐MIF‐1 in Brain Mitochondria but Not in Brain Homogenate

Two samples of the peptide tyrosine-melanocyte-stimulating hormone release-inhibiting factor-1 (Tyr-MIF-1; Tyr-Pro-Leu-Gly-NH2) were tritiated on different amino acids (Tyr or Pro) and incubated together at 37 degrees C with fractions of rat brain. The amount of intact tetrapeptide remaining was determined by HPLC. By 3 min, most of the Tyr-MIF-1 was degraded. Because similar amounts of [3H]Pro and [3H]Tyr appeared after incubation of the Tyr-MIF-1 peptides in brain homogenate, even as early as 30 s, examination of only this crude preparation would misleadingly indicate that Tyr-MIF-1 is not a precursor of melanocyte-stimulating hormone release-inhibiting factor-1 (MIF-1; Pro-Leu-Gly-NH2) in brain tissue. However, incubation of the mitochondrial fractions of brain under the same conditions resulted in more than three times as much [3H]Tyr being formed as [3H]Pro, with accompanying accumulation of MIF-1. Addition of excess MIF-1 to the mitochondrial fraction completely suppressed the formation of MIF-1 and more than doubled the amount of Tyr-MIF-1 remaining intact. When Tyr-MIF-1 tritiated only on the Tyr was added to the mitochondrial fraction, the main peaks of radioactivity appeared only at the positions of Tyr and Tyr-MIF-1, not at the position of Tyr-Pro. The results indicate that Tyr-MIF-1 can serve as a precursor of MIF-1 in brain mitochondria, an effect not evident when crude brain homogenate is used.

Multi-Independent Actions of Peptides in the Brain: LHRH, MIF-1 and CRF

SYNOPSIS. The effects on the central nervous system of four hypothalamic peptides, luteinizing hormone-releasing hormone (LHRH), melanocyte stimulating hormone releaseinhibiting factor-1 (MIF-1), the structurally related Tyr-MIF-1, and corticotropin releasing factor (CRF) are reviewed. Attention is focused on those functions, particularly behavioral actions, that are not dependent upon the pituitary effects of the peptide. For LHRH, growing evidence indicates that thestructural requirements that are important for its behavioral effects are different from those essential for gonadotropin release. Actions of LHRH analogs at the pituitary often do not correlate with the behavioral effects and C-terminal fragments that are inactive at the pituitary can facilitate lordosis. The same portion of the decapeptide shows species differences in the naturally occurring form of the molecule, raising the possibility that these structural differences could be behaviorally relevant. MIF-1 appears to exert many of its effects on the CNS by antagonizing the effects of opiate substances or by potentiating the effects of dopamine. Neither of these actions appears to involve classical agonist- or antagonist-like binding to the opiate or dopamine receptor, but MIF-1 can increase the affinity of agonist binding to the dopamine receptor. Tyr-MIF-1, like MIF-1, can antagonize some opiate-induced effects but differs from MIF-1 in other tests including in vitro binding. CRF shows several behavioral effects that are independent of its ability to release the pituitary hormones ACTH, beta endorphin and MSH or are different from the effects seen after administration of those hormones.

Behavioral effects of melanocyte stimulating hormone release-inhibiting factor-1 (MIF-1)

Consideration of the isolation, structure, localization, and behavioral effects of melanocyte stimulating hormone release inhibiting factor (MIF-1) is followed by a review of its opiate antagonistic and clinical effects. Evidence pertaining to various hypotheses offered in explanation of these behavioral effects is examined and evaluated. It is concluded that MIF-1 affects behavior in many instances with possible antagonistic effects as well as clinical possibilities.

Identification of MSH release-inhibiting elements in the neurointermediate lobe of the rat

Neurointermediate lobes of rats comprise elements which, when excited in vitro, bring about an inhibition of the release of melanocyte stimulating hormone (MSH). Superfusion of neurointermediate lobes of intact donor rats with medium containing 45 mM K + induced a stimulation of the release of oxytocin, arginine-vasopressin and dopamine (DA) and inhibited the release of MSH. Fluorescence histochemical observations and the results of release studies indicate that electrothermic lesions in the mediobasal hypothalamus induced a more rapid degeneration of dopaminergic than of peptidergic terminals in the neurointermediate lobe. Dopaminergic nerve terminals and the stimulated release of DA had vanished completely on the second day after these lesions, which coincided with the disappearance of K +-induced inhibition of MSH release. Frontal hypothalamic deafferentations resulted in disappearance of peptidergic nerve terminals as evidence by the development of diabetes insipidus and the strong decline of depolarization-induced release of oxytocin and vasopressin from neurointermediate lobes in vitro. In contrast, the dopaminergic plexus was left intact, as was the K +-induced stimulation of DA release and inhibition of MSH release. We conclude that the K +-induced inhibition of MSH release is mediated by DA rather than by neuropeptides from terminals in the neurointermediate lobe. The results are in agreement with the proposed MSH release-inhibiting role of the dopaminergic tuberoinfundibular neurones.

Tripeptide analogues of melanocyte-stimulating hormone release-inhibiting hormone (Pro-Leu-Gly-NH2) as inhibitors of oxotremorine-induced tremor.

Fourteen di- and tripeptide analogues of MIF, Pro-Leu-Gly-NH2, have been synthesized and assayed for inhibition of oxotremorine-induced tremor. Replacement of Pro by HCO-Pro or cyclopentanecarboxylic acid gave inactive analogues, while some peptides of the general structure less than Glu-Leu-Gly-NR1R2 were highly active. Thus, R1 = C3H8 and R2 = H gave 4 times the activity of MIF, R1 = I-C3H8 and R2 = H gave 13 times the activity of MIF, and R1 = R2 = CH3 gave 29 times the activity of MIF. cyclo(-Pro-Leu-), Pro-Lys-Gly-NH2, and Pro-Arg-Gly-NH2 had no activity. Apparently, small modifications in the structure of MIF can yield highly active analogues with potential clinical value, e.g., in the treatment of Parkinson's disease or mental depression.

Pineal Vasotocin: Release into Cat Cerebrospinal Fluid by Melanocyte-Stimulating Hormone Release-Inhibiting Factor

The intracarotid injection of both synthetic melanocyte-stimulating hormone release-inhibiting factor (MIF) and purified MIF prepared from bovine hypothalami induces arginine vasotocin release into cerebrospinal fluid of cats and significantly decreases the pineal arginine vasotocin content at 5 minutes after the injection. The present results demonstrate an extrapituitary endocrine effect of synthetic and purified bovine MIF.

Correlation between the X-ray structure of melanostatin and its conformation in solution.

THE establishment of correlations between solid-state (for example, X-ray) conformations and conformations in solution (monitored by spectroscopic methods such as nuclear magnetic resonance (NMR)) is essential for the understanding of structure–function relationships in biologically active molecules. The relative functional importance of static and dynamic structures is of particular interest for peptide hormones which reach their site(s) of action on the receptor via a fluid medium. We describe here a comparative study of melanostatin (Pro–Leu–Gly–NH2) which acts as a melanocyte stimulating hormone release-inhibiting factor1.

Peptidase inactivation of hypothalamic releasing hormones.

With the structural characterization of the hypothalamic hormones, luteinizing hormone-releasing hormone (LH-RH), thyrotrophin-releasing (TRH), melanocyte-stimulating hormone release-inhibiting hormine (MIH), and growth hormone release-inhibiting hormone, (GH-RIH or somatostatin), it has been possible to investigate their enzymic inactivation by peptidases which are present at various sites in the body. Enzymes may play an important part in the control of polypeptide hormone levels and the peptidases acting on these four hypothalamic hormones may regulate the amount of TRH, LH-RH, MIH and somatostatin released from the hypothalamus, or their action at the level of the pituitary and their removal from the circulation. By studying the peptidase enzymes, further information may be obtained on the physiological mechanisms controlling the secretion and actions of hypothalamic hormones, as well as on the design of analogues with increased or competitive activity.

Conformational Mobility of the Pyrrolidine Ring of Proline in Peptides and Peptide Hormones as Manifest in Carbon 13 Spin-Lattice Relaxation Times

Abstract The spin-lattice relaxation times (T1) of carbon 13 in natural abundance were determined for proline, N-acetyl-prolineamide, glycylproline, cyclo(triprolyl), and a series of proline-containing peptide hormones. The data are interpreted in terms of rapid interconversion of proline between various ring-puckered forms. The nature of the puckering depends upon the type of group attached to proline. In proline, the β, γ, and δ carbon atoms are appreciably more mobile than the α carbon atom, suggesting rapid interconversion between a number of ring-puckered forms. In melanocyte-stimulating hormone release-inhibiting hormone (Pro-Leu-Gly-NH2) and its dimethylamido analog, the γ carbon atom has the greatest mobility, suggesting a rapid endo-exo interconversion at this position. In Gly-Pro and in acetyl-Pro-NH2, the cis and trans conformers of proline have very similar T1 values, indicating very little dependence of the dynamic proline ring conformation on the conformation at the peptide bond; in both isomers, the proline ring interconverts rapidly between half-chair conformers puckered at Cβ and Cγ. In thyrotropin-releasing hormone (lGlu-His-Pro-NH2), the proline ring conformation is similar to that in Gly-Pro, and independent of the state of ionization of the histidine residue. Oxytocin [see PDF for equation] , lysine-vasopressin [see PDF for equation] , [Ile5]angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), and luteinizing hormone-releasing hormone ( lGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) all have proline in the trans conformation about the peptide bond, and the δ carbon atom has a mobility comparable to that of the α carbon atom. The latter reflects severe steric constraints at Cδ due to the neighboring residues. In oxytocin and angiotensin II, the β carbon atom is considerably more mobile than Cα or Cδ. In luteinizing hormonereleasing hormone, the proline ring apparently undergoes rapid interconversion between half-chair forms puckered at Cβ and Cγ, whereas in lysine-vasopressin, the mobility of Cγ is greatest.

Determination by radioimmunoassay of the luteinising hormone-releasing hormone (LHRH) content of the hypothalamus of the rat and some lower vertebrates

Synthetic luteinising hormone-releasing hormone (LHRH) was bound to albumin and used as antigen to produce an antiserum in the guinea pig. A radioimmunoassay was developed using this antiserum which, at a dilution of 1:12,500, was capable of binding 50% of added 125I-labeled LHRH. The antiserum was specific for LHRH, no cross-reactivity being found with synthetic thyrotropin-releasing hormone, synthetic melanocyte-stimulating hormone release-inhibiting hormone, rat luteinising hormone, arginine vasopressin, or oxytocin. Extracts of the hypothalamus of the rat and Xenopus laevis were found to contain a substance immunologically identical to synthetic LHRH. However, the extracts of the hypothalamus of the dogfish, Scyliorhinus canicula, and of the goldfish, Carassius auratus, did not appear to contain a substance immunologically related to LHRH. The pars distalis and the neurointermediate lobe of the goldfish pituitary were also extracted and assayed and no immunological LHRH activity was found. It is concluded that present evidence suggests that the LHRH of vertebrates older in origin than the amphibia may have an LHRH of different chemical structure to that of synthetic LHRH. Such α- structural difference may remove immunological cross reaction but still allow synthetic LHRH to exert biological activity in these lower vertebrates.

Inactivation of Melanocyte-stimulating Hormone Release-inhibiting Factor by a Manganese-stimulated Bovine Brain Aminopeptidase

Abstract A Mn++-stimulated aminopeptidase was separated from homogenates of bovine cerebrum and was observed to inactivate melanocyte-stimulating hormone release-inhibiting factor (Pro-Leu-Gly-NH2) in a manner similar to leucine aminopeptidase and arylamidase, by liberating both proline and leucine from the amino end of the tripeptide amide. The brain enzyme has physical and enzymic properties distinctly different from rat brain arylamidase. The Mn++-stimulated aminopeptidase, in common with leucine aminopeptidase, does not hydrolyze thyrotropin-releasing factor, luteinizing hormone-releasing hormone, Lys-bradykinin, bradykinin, (Asp1Ile5) angiotensin I, oxytocin, or Arg-vasotocin. Two Mn++-inhibited bovine brain aminopeptidases, which cleave Leu-Gly-Gly in common with the Mn++-stimulated aminopeptidase, were inactive toward all the peptide hormones tested, including melanocyte-stimulating hormone release-inhibiting factor. Bovine cerebrum homogenates hydrolyzed the linear peptide hormones but did not act upon the cyclic ones, oxytocin and Arg-vasotocin. These results support the suggestion that brain peptidases capable of acting upon the peptide hormones exhibit a very high degree of substrate specificity.

Oxotremorine antagonism by a hypothalamic hormone, melanocyte-stimulating hormone release-inhibiting factor (MIF).

SummaryMIF was demonstrated to antagonize the central and peripheral effects of oxotremorine in normal as well as in hypophysectomized mice. Its effectiveness in hypophysectomized mice confirms our earlier report that MIF exerts actions upon the CNS which are independent of its MSH release-inhibiting activity.The authors thank Dr. E. Kimura and Mr. P. Young for the anticholinergic studies in the isolated guinea pig ileum.