Activation of supraoptic magnocellular neurons by gamma 2-melanocyte stimulating hormone (gamma 2-MSH)

Activation and endocytic internalization of melanocortin 3 receptor in neuronal cells

Melanocortins play a central role in autonomic modulation of metabolism by acting through a family of highly homologous G protein-coupled receptors. Studies with gene knockout mice have implicated neural melanocortin receptors, MC3R and MC4R, in the etiology of obesity, insulin resistance, and salt-sensitive hypertension. In an attempt to better understand the mechanisms of function of these receptors, we expressed MC3R and MC4R in neuronal cells and demonstrated their co-localization to several membrane regions. We now show that in cultured neuronal cells, MC3R localizes to lipid rafts and undergoes endocytic internalization upon activation by gamma-MSH through a protein kinase-sensitive pathway. The appearance of the internalized receptor in lysosomes suggests that it is subsequently degraded. The expression of protein kinase A regulatory subunits and of c-Jun and c-Fos was analyzed by either immunoblotting or real-time PCR. No discernable changes were observed in the expression levels of these protein kinase A and protein kinase C responsive genes. Immunohistochemical studies showed a robust expression of MC3R protein in brain nuclei with relevance to cardiovascular function and fluid homeostasis further supporting the notion that the physiological effects of melanocortins on the cardiovascular system arise from effects on the central nervous system.

Central receptors mediating the cardiovascular actions of melanocyte stimulating hormones

OBJECTIVE

Alpha and gamma-melanocyte stimulating hormones (MSH) are peptides that possess potent hypertensinogenic actions when injected intravenously or intracerebroventricularly. We sought to define the central receptor(s) mediating these cardiovascular actions.

METHODS

We gave bolus injections of synthetic alpha or gamma-MSH intravenously or intracerebroventricularly to anesthetized wild-type (Mc3r+/+, Mc4r+/+) mice and mice with targeted disruption of the gamma-MSH receptor (Mc3r-/-) or the melanocortin 4 receptor (Mc4r-/-).

RESULTS

Gamma-MSH injected intravenously increased mean arterial pressure (MAP) and heart rate (HR) dose-dependently, with the effect being evident at 10 mol/kg; the maximum increase, at 10 mol/kg, was 38 mmHg in both strains from similar control MAP. Parallel increases in HR also occurred. Injection of the sodium channel blocker, benzamil, 4 microg/kg intracerebroventricularly, before intravenous gamma-MSH completely prevented the increases in MAP and HR in both strains. Injection of 2 x 10 mol/g body weight alpha-MSH intravenously had no effect on MAP or HR in Mc4r wild-type or -/- mice. However, the same dose given intracerebroventricularly to wild-type mice increased MAP from 76 +/- 4 to 95 +/- 5 mmHg at 10 min (P < 0.01) and HR from 416 +/- 15 to 480 +/- 15 beats/min (P < 0.01). In Mc4r-/- mice, the intracerebroventricular administration of the peptide did not alter these variables, in contrast to the results in wild-type mice.

CONCLUSION

Both MSH peptides exert their hypertensinogenic effects through central sites of action, which probably reflect the activation of sympathetic outflow. The actions of intracerebroventricular alpha-MSH appear to be mediated via Mc4r, whereas those of gamma-MSH are independent of its receptor Mc3r, but reflect the activation of a sodium channel in the central nervous system. These results help to reconcile the hypertensive action of gamma-MSH injections with the hypertension observed in states of gamma-MSH deficiency.

Gamma(2)-melanocyte-stimulating hormone suppression of systemic inflammatory responses to endotoxin is associated with modulation of central autonomic and neuroendocrine activities

Central autonomic and neuroendocrine activities are important components of the host response to bacterial inflammation. We demonstrate that intravenous infusion of gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH), a potent autonomic regulating peptide, prevents lipopolysaccharide (LPS)-induced hypotension and tachycardia, and modulates the ACTH response to endotoxin. In the hypothalamic paraventricular nucleus, a major neuroendocrine and autonomic center, gamma(2)-MSH inhibits LPS-induced increases in CRF mRNA levels, but does not suppress LPS-augmented arginine vasopressin heteronuclear RNA expression. In the locus coeruleus, a brainstem noradrenergic center, gamma(2)-MSH inhibits LPS-induced increases in tyrosine hydroxylase mRNA levels. Gamma(2)-MSH inhibits LPS-induced IL-1beta gene expression in the brain, pituitary and thymus, and prevents increases in plasma NO levels. These findings reveal that gamma(2)-MSH attenuates systemic inflammatory responses to endotoxin and suggest that modulation of central autonomic and neuroendocrine activities by gamma(2)-MSH contributes to its anti-inflammatory effects.

Dietary sodium intake modulates pituitary proopiomelanocortin mRNA abundance

The pituitary prohormone proopiomelanocortin gives rise to melanocortins of alpha, beta, and gamma primary structure in addition to corticotropin. Melanocortins have a variety of actions in mammals, and each is natriuretic. In particular, gamma-melanocyte-stimulating hormone has been shown to mediate reflex natriuresis after acute unilateral nephrectomy. We examined whether this peptide could play a role in longer term adjustments in sodium balance by measuring plasma gamma-melanocyte-stimulating hormone and corticotropin concentrations, as well as pituitary proopiomelanocortin mRNA abundance, in Sprague-Dawley rats ingesting either a low (0.07% NaCl) or high (7.5% NaCl) sodium diet. One week after the high sodium diet, plasma gamma-melanocyte-stimulating hormone concentration was double the value seen in rats on the low sodium diet (158 +/- 5 [SE] versus 76 +/- 9 fmol/mL, P < .001), a change that was accompanied by a fivefold increase in plasma atrial natriuretic peptide concentration but no change in plasma corticotropin. Whole pituitary proopiomelanocortin mRNA abundance, measured with a probe to exon 3 of the rat proopiomelanocortin gene, was significantly increased after 1 week of the high sodium diet compared with the low sodium diet and increased further at 2 and 3 weeks. This increase occurred primarily in the neurointermediate lobe as demonstrated by in situ hybridization; the content of gamma-melanocyte-stimulating hormone immunoreactivity was also increased in this lobe, but not the anterior lobe, after 1 week of the high sodium diet. These results demonstrate that high dietary sodium intake increases neurointermediate lobe proopiomelanocortin mRNA abundance compared with a very low sodium diet and also suggest that proopiomelanocortin is preferentially processed into gamma-melanocyte-stimulating hormone rather than corticotropin. These observations consequently raise the possibility of a role for this peptide hormone system in the adjustments to a high salt diet.