α-MSH Release Research Articles

Neuropeptide Y does not inhibit the release of alpha-MSH from the pars intermedia of the rat adenohypophysis.

Neuropeptide Y in concentrations from 10(-8) to 10(-6) M inhibits the release of alpha-MSH from the frog (Rana pipiens) pituitary in a reversible, sustained, and concentration-related manner. However, it does not inhibit the release of alpha-MSH from the rat pars intermedia. Thus, while neuropeptide Y may play a role in the control of alpha-MSH release in amphibia, it appears not to be a regulatory peptide for the mammalian pars intermedia.

The regulation of the corticomelanotropic cell activity in aves. III—Effect of various peptides on the release of MSH from dispersed, perfused duck pituitary cells. Cosecretion of acth with MSH

1. 1. The melanotropin-releasing activity of arginine-vasopressin (AVP), arginine-vasotocin (AVT), oxitocin (OT), mesotocin (MT) and corticotropin-releasing factor (CRF) was studied in the duck using dispersed, perfused pituitary cells and a specific α-MSH RIA. 2. 2. Log dose-response curves were obtained for all the peptides ranging from 5 to 100 ng ml . All peptides behaved as partial agonists compared to duck median eminence extracts (DME). 3. 3. AVT and MT displayed an α-MSH releasing capacity of 60% relative to DME whereas all other peptides behaved as weak agonists with less than 15% capacity relative to DME. 4. 4. AVT and CRF when perfused together acted synergistically on α-MSH release yielding a dose response line whose slope approximated that of DME. 5. 5. ACTH was cosecreted together with α-MSH in all situations studied with an ACTH to α-MSH molar ratio of about 10. 6. 6. It is concluded that CRF and neurohypophyseal peptides may be physiological stimulators of both α-MSH and ACTH release in aves.

Hypothalamic α-melanocyte-stimulating hormone (α-MSH) is not under dopaminergic control

A possible dopaminergic regulation of hypothalamic proopiomelanocortin (POMC)-containing neurons has been investigated in rats by means of in vivo and in vitro approaches. Acute or 3-weeks chronic in vivo treatments with the dopaminergic agonists apomorphine (1 mg/kg; s.c.) and 2-Br-α-ergocriptine (2.5 mg/kg; s.c.) or the dopaminergic antagonist haloperidol (0.15–3 mg/kg; i.p.) had no significant effect on the concentration of α-melanocyte-stimulating hormone (α-MSH) in two hypothalamic regions: arcuate nucleus (AN) and dorsolateral area (DLH). In the same way, chronic administration of the dopaminergic agonists or antagonist did not induce chromatographic analysis revealed that acetic acid extracts of AN and DLH both contained two major forms of α-MSH-like peptides: deacetylated α-MSH and authentic α-MSH. The ratio between these two forms was not altered after acute haloperidol treatment (3 mg/kg, i.p.). The possible effect of dopamine on the release of hypothalamic α-MSH was studied in vitro using perifused rat hypothalamic slices. Infusion of dopamine (10 −7−;10 −5M) or its antagonist haloperidol (10 −5M) had no effect on spontaneous α-MSH release from hypothalamic tissue. In addition, none of these drugs had any effect on potassium (50 mM)-induced α-MSH release. It is concluded that dopaminergic neurons are not involved in the regulation of synthesis, post-translational processing (acetylation) or release of hypothalamic α-MSH.

The effects of nitrous oxide on the secretory activity of pro-opiomelanocortin peptides from basal hypothalamic cells attached to cytodex beads in a superfusion in vitro system

Dispersed cells from adult rat basal hypothalami, attached to Cytodex-3 microcarrier beads, were placed in a column and superfused with aerated high glucose media or media enriched with variable concentrations of nitrous oxide with oxygen. β-Endorphin and α-MSH content was measured in the effluent collected during superfusion and demonstrated a near constant baseline release. Nitrous oxide, 60% ( P < 0.025) and 80% ( P < 0.02), caused significant increases in release of β-endorphin. Potassium chloride (50 mM) caused a significant increase in release ( P < 0.007) of β-endorphin whereas saline and 30% nitrous oxide did not. Neither nitrous oxide-enriched media nor potassium chloride had any statistically significant effect on α-MSH release. The increase in β-endorphin secretory activity during exposure to nitrous oxide demonstrates that nitrous oxide may have a stimulatory effect on central pro-opiomelanocortin neurons.

α-Melanocyte-stimulating hormone (α-MSH) release from perifused rat hypothalamic slices

A perifusion system was developed to investigate the control of α-melanocyte-stimulating hormone (α-MSH) release from rat brain. Hypothalamic slices were perifused with Krebs-Ringer bicarbonate (KRB) medium supplemented with glucose, bacitracin and bovine serum albumine. Fractions were set apart every 3 min and α-MSH levels were measured by means of a specific and sensitive radioimmunoassay method. Hypothalamic tissue in normal KRB medium released α-MSH at a constant rate corresponding to 0.1% of the total hypothalamic content per 3 min. The basal release was not altered by Ca 2+ omission in the medium or addition of the sodium channel blocker tetrodotoxine (TTX). Depolarizing agents such as potassium (50 mM) and veratridine (50 μM), which is known to increase Na + conductance, significantly stimulated α-MSH release in a Ca 2+-dependent manner. When Na +-channels were blocked by TTX (0.5 μM) the stimulatory effect of veratridine was completely abolished whereas the K +-evoked release was unaffected. These findings suggest that: (1) voltage-dependent sodium channels are present on α-MSH hypothalamic neurons; (2) depolarization by K + induces a marked stimulation of α-MSH release; (3) K +- and veratridine-evoked releases are calcium-dependent. Altogether, these data provide evidence for a neurotransmitter or neuromodulator role for α-MSH in rat hypothalamus.

VI. The benzodiazepine agonist clonazepam potentiates the effects of γ-aminobutyric acid on α-MSH release from neurointermediate lobes in vitro.

The action of the central-type benzodiazepine-receptor agonist clonazepam on α-MSH release has been studied in vitro using perifused frog neurointermediate lobes. High concentrations of clonazepam (3.16 × 10 −5 and 10 −4 M) caused an inhibition of α-MSH release and this effect was reversed by the central-type benzodiazepine-receptor antagonist Ro 15-1788. High doses of GABA (10 −5 and 10 −4 M) induced a biphasic effect on pars intermedia cells: a brief stimulation followed by a sustained inhibition of α-MSH secretion. Administration of clonazepam (10 −5 M) in the presence of various concentrations of GABA (10 −6 to 10 −4 M) led to a potentiation of both stimulatory and inhibitory phases of α-MSH secretion induced by GABA. Ro 15-1788 completely abolished the potentiating effect of clonazepam. Thus our results indicate that endogenous benzodiazepine receptors may modulate the effects of GABA on α-MSH secretion.

V. Melanotropin release inhibiting activity of neuropeptide Y: Structure-activity relationships

We have recently shown that the release of α-MSH by the intermediate lobe of the frog pituitary is inhibited by neuropeptide Y (NPY). Using the perifusion technique, we have compared in the present study, the α-MSH release inhibiting activities of NPY, various NPY short chain analogues and two other members of the pancreatic polypeptide family, peptide YY (PYY) and avian pancreatic polypeptide (APP). The order of biological potency was NPY > NPY[2–36] > NPY[16–36] > NPY[25–36] > NPY[1–15]. Among the two pancreatic polypeptides tested, PYY appeared to be almost as potent as NPY while APP was 6 times less active than NPY. Neither NPY[1–15] nor NPY[16–36] could antagonize the inhibitory effect of NPY on α-MSH release. The structure-activity relationship study suggests that the bioactive determinant of NPY is located in the C-terminal part of the molecule.

III. Regulation of cyclic-AMP synthesis in amphibian melanotrope cells through catecholamine and GABA receptors

Catecholamines and GABA are neurotransmitters involved in the regulation of release of pro-opiomelanocortin (POMC) derived peptides from the neurointermediate lobe of Xenopus laevis. The present study concerns the relation of these neurotransmitters to the adenylate cyclase system of the melanotrope cell. During in vitro incubation of isolated melanotrope cells it was found that dopamine, adrenaline and LY 171555 induced inhibition of forskolin-stimulated cAMP production and concomitantly inhibited MSH release. Activation of the GABAb receptors by baclofen also induced inhibition of cAMP production and αMSH secretion. Activation of the GABAa receptors evoked stimulation of cAMP production, while αMSH release was slightly inhibited, indicating that the GABAa mechanism may prove to be complex. A dual regulation through two subtypes of this receptor might be involved, one stimulating release through the adenylate cyclase system, while the other would inhibit secretion.

II. Atrial natriuretic factor (ANF) stimulates the release of α-MSH from frog neuro- intermediate lobes in vitro. Interaction with dopamine, GABA and neuropeptide Y.

The action of atrial natriuretic factor (ANF) on α-MSH release from frog neurointermediate lobe was studied in vitro using a perifusion technique. Synthetic ANF Arg101Tyr126, at concentrations ranging from 10 −7 to 10 −5M, caused a dose-related stimulation of α-MSH release. In addition, dopamine, GABA and NPY, three neuroendocrine factors which inhibit α-MSH secretion totally suppressed the action of ANF on α-MSH production. The neural lobe of the amphibian pituitary contains numerous ANF immunoreactive fibers, and this regulatory peptide may diffuse from nerves terminating in the pars nervosa to the pars intermedia. Thus, our results suggest that ANF of hypothalamo-neurohypophysial origin may be involved in the multineuronal regulation of amphibian melanotrophs.

Biphasic effect of thyrotropin-releasing factor (TRH) on α-melanotropin secretion from frog intermediate lobe in vitro

The kinetics of a-MSH secretion induced by prolonged TRH infusion were studied using perfused frog neurointermediate lobe (NIL). During a 2 h administration of TRH (10 −8 M), the secretion rate of α-MSH displayed two phases. During the first phase, secretion of α-MSH increased rapidly reaching a maximum within 20 min and then, despite continued TRH infusion, this secretion slowly declined. The second phase was characterized as plateau of elevated release (relative to basal secretion); within this second phase there was often a small peak of released α-MSH occurring at about 100 min. Exposure of NIL to another TRH (10 −8 M) pulse 90 min later induced a normal stimulation of α-MSH secretion, thus demonstrating the viability of tissue in perifusion. Continuous infusion of cycloheximide (10 −5 M) during a 5 h period totally inhibited the biosynthetic activity of NIL but did not influence TRH-induced α-MSH secretion. In particular, cycloheximide had no effect on the second phase of the response to prolonged infusion of TRH. Similarly, during continuous infusion of the monovalent carboxylic ionophore monensin (10 −6 M), the biphasic response to prolonged infusion of TRH (10 −8 M) was still observed. Administration of a short pulse of TRH (10 −7 M) during the declining part of the first phase or during the second phase of prolonged TRH (10 −8 M) infusion induced a significant enhancement of α-MSH stimulation. From these results we conclude that (1) prolonged TRH infusion causes α-MSH release in a biphasic manner; (2) attenuation of the secretory response to continuous TRH administration does not result from exhaustion of the releasable pool of α-MSH; (3) the biphasic response depends neither on de novo biosynthesis nor on intracellular transit of the prohormone from the endoplasmic reticulum to the Golgi complex. Thus, we propose that prolonged exposure of frog NIL to TRH may cause transconformational changes of membrane TRH receptors from high to low affinity states.

Melanin concentrating hormone inhibits the release of αMSH from teleost pituitary glands

A radioimmunoassay was developed for salmonid melanin concentrating hormone (MCH) and used to measure immunoreactive (ir)MCH in the hypothalamus and pituitary of trout ( Salmo gairdneri) and eels, ( Anguilla anguilla) maintained under different regimes of background color. In trout, 95% of the total irMCH was located in the pituitary gland. The amount of MCH in both pituitary and hypothalamus was increased when white-adapted trout were transferred to a black background. In eels, a similar change of background led to an accumulation of MCH in the pituitary but not in the hypothalamus. The results suggest that MCH is released from the neurohypophysis in association with physiological color change. Neurointermediate lobes of trout and eels released both irαMSH and irMCH when they were cultured in vitro. The release of αMSH was significantly enhanced when endogenous MCH was immunoabsorbed by MCH antiserum added to the culture medium. The results indicate that MCH can induce pallor in fish not only by its peripheral effect on the melanophores but also by an inhibitory action on the release of αMSH from the pituitary.

In vitro study of frog ( Rana ridibunda Pallas) neurointermediate lobe secretion by use of a simplified perifusion system : IV. Interaction between dopamine and thyrotropin-releasing hormone on α-melanocyte-stimulating hormone secretion

The interaction between dopamine and TRH on α-melanocyte-stimulating hormone (MSH) release from the intermediate lobe of amphibian pituitary has been studied in vitro using the perifusion model. Dopamine (10 −10 to 10 −6 M) was responsible for a dose-related inhibition of α-MSH secretion. The inhibitory effect of dopamine (10 −8 and 3.16 × 10 −8 M) was completely abolished in the presence of haloperidol (10 −5 and 10 −6 M, respectively). It has been previously established that, in amphibians, TRH stimulated α-MSH release in vitro and that the action of TRH was not mediated via an inhibition of the release of endogenous dopamine (M. C. Tonon, P. Leroux, M. E. Stoeckel, S. Jégou, G. Pelletier, and H. Vaudry 1986, Endocrinology 112, 133–141). In the present study we demonstrate that TRH (10 −7 M) reverses the inhibitory effect of dopamine (for concentrations ranging from 3.16 × 10 −8 to 10 −6 M) on α-MSH secretion and that the effects of TRH and dopamine are additive. Thus, these results indicate that the intracellular events associated with TRH-induced stimulation and dopamine-induced inhibition of α-MSH release are not linked together.

GABA-ergic control of α-Melanocyte-Stimulating Hormone (α-MSH) release by frog neurointermediate lobe in vitro

Measurement of glutamate decarboxylase (GAD) activity in the intermediate lobe of the frog pituitary and brain showed that neurointermediate lobe extracts represented 12% of the GAD activity detected in the whole brain. No significant activity was measured in distal lobe extracts. Immunocytochemical studies revealed GAD-containing fibers among the parenchyma! cells of the pars intermedia. The localization of GAD-like material in the intermediate lobe of the frog pituitary suggested a possible role of γ-aminobutyric acid (GABA) in the regulation of melanotropic cell secretion. Administration of GAB A (10 −6 to 10 −4 M), to perifused neurointermediate lobes caused a brief stimulation of alpha-melanocyte stimulating hormone (α-MSH) release followed by an inhibition. Picrotoxin (10 −4 M), a Cl − channel blocker, abolished only the stimulatory effect of GAB A (10 −4 M), whereas bicuculline (10 −4 M), a specific antagonist of GABA A receptors, totally inhibited the effects of GABA (both stimulatory and inhibitory phases). Bicuculline induced by itself a slight stimulation of α-MSH release, suggesting that GABA-ergic nerve fibers present in the intermediate lobe are functionally active in vitro. The GABA A agonist muscimol (10 −7 to 10 −4 M) mimicked the biphasic effect of GABA on α-MSH release. Administration of baclofen, a specific GABA BB agonist (10 −7 to 10 −4 M) induced a dose-dependent inhibition of α-MSH secretion. In contrast to GABA or muscimol, baclofen did not cause any stimulatory effect whatever the dose. Taken together these result suggested that GABA A and GABa b receptors were present on frog melanotrophs. Since bicuculline totally inhibited GABA effects (stimulation and inhibition) on α-MSH release, it appears however that the effect of GABA is mainly achieved through activation of G AB A A receptors.

Neuropeptide Y in the intermediate lobe of the frog pituitary acts as an α-MSH-release inhibiting factor

The presence of neuropeptide tyrosine (NPY) in the intermediate lobe of the frog pituitary was demonstrated using indirect immunofluorescence, the immunogold technique and a specific radioimmunoassay combined with high pressure liquid chromatography (HPLC). A high density of NPY-containing fibers, was found among the parenchymal cells of the intermediate lobe. These fibers originated from the ventral infundibular nucleus, travelled via the median eminence to the pars intermedia. At the electron microscopic level, NPY-like material was found exclusively in nerve fibers where the product of the immunoreaction was associated to dense-core vesicles. High concentrations of NPY-like peptide were found in neurointermediate lobe extracts. After Sephadex G-50 gel filtration the major peak of immunoreactive material appeared to co-elute with synthetic porcine NPY. Conversely, HPLC analysis revealed that the NPY-like peptide of the frog pituitary had a retention time shorter than the porcine NPY. The localization of NPY-like material in the pars intermedia suggested a possible role of NPY in the regulation of melanotropic cell secretion. In fact, graded concentrations of synthetic NPY induced a dose-dependent inhibition of alpha-melanotropin (α-MSH) release in vitro . The lack of effect of a dopaminergic antagonist on NPY-induced α-MSH release inhibition demonstrated that the local dopaminergic system could not account for the NPY action. These results indicate that NPY located in the hypothalamo-hypophyseal system of the frog may act as a melanotropin-release inhibiting factor.

Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and α-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro

The ability of corticoliberin (CRF), urotensin I, sauvagine, arginine-vasopressin (AVP), and mesotocin to stimulate ACTH release by frog anterior pituitary cells and α-melanotropin (MSH) by frog neurointermediate lobe was studied in vitro using a perifusion technique. CRF and AVP were found to be potent stimulators of ACTH secretion, whereas urotensin I and sauvagine were totally inactive. In opposition to recent findings in the rat, CRF did not modify α-MSH secretion by the frog neurointermediate lobe. Mesotocin, which is present in the parenchymal cells of the frog pars intermedia, had no effect on α-MSH release in vitro. No potentiation of CRF-induced ACTH release was observed when anterior pituitary cells were incubated with a combination of AVP and CRF. Together with the recent elucidation of a CRF-like molecule in the frog diencephalon, these results suggest that, in Amphibia, CRF and AVP exert their stimulatory action specifically on distal lobe corticotrophs.

Regulation of biosynthesis and release of pars intermedia peptides in Rana ridibunda: Dopamine affects both acetylation and release of α-MSH

Dopamine is likely an important physiological melanotropin release-inhibiting factor in amphibians. This study concerns the effects of dopamine on the biosynthesis and release of peptides from the pars intermedia of the frog Rana ridibunda. Our results show that this secretagogue has no immediate effects on either the rate of biosynthesis of pro-opiomelanocortin nor on the direction of processing of this prohormone. Pulse-chase experiments revealed that dopamine inhibited the release of all newly synthesized POMC-related peptides in a dose-dependent manner. For each dopamine concentration tested, the degree of inhibition exerted on the release of the various newly synthesized peptides by a given concentration of secretagogue was approximately the same, with the exception of that found for the α-MSH related peptides. Analysis of the release of these melanotropins was complex because dopamine not only inhibited their release but also, either directly or indirectly, inhibited the acetylation reaction which converts des-Nα-acetyl α-MSH to α-MSH. Dopamine was shown to be less potent in inhibiting the release of des-Nα-acetyl α-MSH than inhibiting release of the acetylated form of the peptide. In amphibians a preferential release of the less-bioactive non-acetylated form of MSH under inhibitory conditions induced by dopamine may be of physiological importance.

The effect of λ-type endorphins on α-MSH release in the rat

Neuroleptic drugs increase the level of alpha-melanotropin (alpha-MSH) in the blood of the rat. We have investigated whether neuroleptic-like peptides, the gamma-type endorphins, also affect alpha-MSH release. A structure-activity study revealed that (des-enkephalin)-gamma-endorphin (DE gamma E, beta-LPH-(66-77), beta-endorphin-(6-17)) is able to increase plasma alpha-MSH levels after intracerebroventricular injection, while the longer gamma-type endorphins, i.e. gamma E (beta-LPH-(61-77)), beta-endorphin-(1-17)), and DT gamma E (beta-LPH-(62-77), beta-endorphin-(2-17)) were without effect in the dosage used. A dose-response study revealed a more or less bell-shaped relationship for the effect of DE gamma E on plasma alpha-MSH levels. The effect of DE gamma E could not be counteracted by apomorphine or naloxone. The observations indicate that DE gamma E increases plasma alpha-MSH levels in a way distinct from that of haloperidol and the opiate peptide beta-endorphin. On the other hand, a time-course of plasma alpha-MSH levels after DE gamma E administration resembled the one which has been seen after haloperidol injection. From experiments performed on pituitary neurointermediate lobes incubated in vitro, it seems not likely that DE gamma E acts directly on the dopamine receptors of the pituitary in affecting alpha-MSH release. In conclusion, it appears that DE gamma E affects alpha-MSH levels in plasma in a way distinct from that of the neuroleptic drug haloperidol and of the opiate-peptide beta-endorphin.

Agonist and antagonist effects of 3-PPP enantiomers on functional dopamine autoreceptors and postsynaptic dopamine receptors in vitro

In contrast to racemic 3-PPP (3-(3-hydroxyphenyl)-N-n-propylpiperidine), (+)-3-PPP appeared to inhibit the electrically evoked release of both [ 3H]dopamine (DA) and [ 14C]acetylcholine (ACh) from superfused rat neostriatal slices, although it was considerably less potent in this respect that the DA receptor agonists apomorphine, TL-99 (6,7-dihydroxy-N,N-dimethyl-2-aminotetralin) and LY 141865. At concentrations higher than 1 μM both of the 3-PPP enantiomers increased the spontaneous efflux of 3H but not that of 14C (+)3-PPP also inhibited the cholera toxin-stimulated release of immunoreactive α-MSH from dispersed intermediate lobe cells of the rat pituitary gland. The inhibitory effects of (+)3-PPP on both transmitter and α-MSH release were antagonized by the selective D-2 receptor antagonist (—)-sulpiride. Neither [ 3H]DA nor [ 14C]ACh release were inhibited by (—)3-PPP but, in contrast, the release-inhibiting effect of the selective D-2 receptor agonist LY 141865 as well as that of (+)3-PPP were antagonized by (—)3-PPP, although less effectively than by (—)sulpiride. The inhibitory effect of LY 141865 on α-MSH release from intermediate lobe cells was also antagonized by (—)3-PPP. The data indicate that (+)3-PPP is a weak agonist and (—)3-PPP a weak antagonist at D-2 receptors and that neither of the 3-PPP enantiomers interacts selectively with DA autoreceptors mediating presynaptic modulation of striatal DA release.

Two dopamine receptors: Biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and α-MSH from the pituitary gland, as well as of acetylcholine, DA and β-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

α-MSH and coat color changes in the mouse

The effect of α-MSH on coat color was examined in viable yellow mice (C3H/He-A∗ vy). These mice normally grow a coat of darkly pigmented hair at puberty. This darkening effect was also evident in hair that grew in a region that had been plucked at 13 days of age. Administration of α-MSH increased the darkness of this hair and the hair which grew naturally in an unplucked area. However, the natural coat darkening that occurred at puberty was not associated with an increase in plasma immunoreactive α-MSH levels. Moreover, although bromocryptine, a dopamine agonist that inhibits α-MSH release from the pituitary reduced the darkness of the coat that grew after plucking the reduction in coat darkening was unrelated to changes in plasma α-MSH. Nevertheless, this effect of bromocryptine was reversed when α-MSH was administered together with the drug. Apomorphine had no effect on coat darkening and produced only a slight decrease in plasma α-MSH. Melatonin reduced coat darkening slightly but, like apomorphine, had little effect on plasma α-MSH concentrations. Although α-MSH may have a physiological role in coat darkening in the C3H/He-A∗ vy mouse at puberty the response seems to be unrelated to an increase in circulating α-MSH. Thus, other factors, such as changes in melanocyte sensitivity to α-MSH or inhibitory mechanisms that prevent coat darkening during prepubertal and adult life may be involved in regulation of coat color in the viable yellow mouse.