Interaction of endokinin A/B and (Mpa(6))-γ2-MSH-6-12 in pain regulation in mice.

The present study focused on the interactive effects of (Mpa(6))-γ2-MSH-6-12 (Mpa, spinal level) and endokinin A/B (EKA/B, supraspinal level) on pain regulation in mice. EKA/B (30 pmol) only weakened 100 pmol Mpa-induced hyperalgesia at 5 min, but could enhance it during 20-30 min. However, EKA/B (100 pmol) antagonized all dose levels of Mpa significantly at 5 min and blocked them completely at 10 min. EKA/B (3 nmol) co-injected with Mpa presented marked analgesia at 5 min and enduring hyperalgesia within 20-60 min. To investigate the underlying mechanisms between Mpa and EKA/B, SR140333B and SR142801 (NK1 and NK3 receptor antagonists, respectively) were utilized. SR140333B had no influence on Mpa, while SR142801 potentiated it during 20-30 min. Whereas, SR140333B and SR142801 could block the co-administration of Mpa and EKA/B (30 pmol) separately at 5 min and 30 min. These phenomena might attribute to that these two antagonists promoted the antagonism of EKA/B (30 pmol) at the early stage, while antagonized EKA/B preferentially in the latter period. SR140333B weakened the analgesia of EKA/B (3 nmol), but produced no effect on Mpa. However, SR140333B failed to affect the co-injection of Mpa and EKA/B, which implied that EKA/B cooperated with Mpa prior to SR140333B. These results could potentially help to better understand the interaction of NK and MrgC receptors in pain regulation in mice.

Role of gamma melanocyte-stimulating hormone-renal melanocortin 3 receptor system in blood pressure regulation in salt-resistant and salt-sensitive rats.

Melanocortin 3 receptor (MC3-R) has high affinity and specificity to gamma melanocyte-stimulating hormone (gammaMSH), a natriuretic peptide involved in regulation of blood pressure (BP) and sodium excretion. Recent studies showing increased MC3-R expression and elevated plasma gammaMSH in normal rats fed a high-salt diet support the role of this system in sodium homeostasis. We hypothesized that dysregulation of MC3-R response to dietary salt may contribute to salt retention and BP elevation in salt-sensitive hypertension. We examined renal MC3-R expression, plasma gammaMSH concentration, and response to MC3-R agonist and antagonist in Dahl salt-sensitive (DSS) and Dahl salt-resistant (DSR) rats fed high-salt (8%) or low-salt (0.07%) diets for 3 weeks. Consumption of high-salt diet significantly increased BP in the DSS but not the DSR group. High-salt diet led to a 5-fold increase in plasma gammaMSH and a 2-fold increase in renal MC3-R in DSR rats. Plasma gammaMSH and renal MC3-R abundance in DSS rats were maximally elevated on low-salt diet and remained unchanged on high-salt diet. Administration of MC3-R agonist melanotan II significantly lowered BP and raised fractional Na excretion in the DSR but not the DSS rats consuming high-salt diet. In contrast, MC3-R antagonist SHU9119 significantly raised BP and lowered fractional Na excretion in both groups. Thus, the data suggest that gammaMSH-renal MC3-R pathway is activated and appears to be biologically functional in the DSS rats.

Evidence for a noradrenergic mechanism causing hypertension and abnormal glucose metabolism in rats with relative deficiency of gamma-melanocyte-stimulating hormone.

A close association between salt-sensitive hypertension and insulin resistance has been recognized for more than two decades, although the mechanism(s) underlying this relationship have not been elucidated. Recent data in mice with genetic disruption of the gamma-melanocyte-stimulating hormone (gamma-MSH) system suggest that this system plays a role in the pathophysiological relationship between hypertension and altered glucose metabolism during ingestion of a high-sodium diet (8% NaCl, HSD). We tested the hypothesis that these two consequences of interrupted gamma-MSH signalling were the result of sympathetic activation by studying rats treated with the dopaminergic agonist bromocriptine (5 mg kg(-1) i.p., daily for 1 week; Bromo) to cause relative gamma-MSH deficiency. Bromo-treated rats fed the HSD developed hypertension and also exhibited fasting hyperglycaemia (P < 0.005) and hyperinsulinaemia (P < 0.025). Furthermore, Bromo-treated rats on the HSD had impaired glucose tolerance and blunted insulin-mediated glucose disposal. Intravenous infusion of gamma(2)-MSH, or of the alpha-adrenergic receptor antagonist phentolamine, to Bromo-HSD rats lowered both mean arterial pressure (MAP) and blood glucose to normal after 15 min (P < 0.001 versus control), but had no effect in rats receiving vehicle and fed the HSD; gamma(2)-MSH infusion also reduced the elevated plasma noradrenaline to control levels in parallel with the reductions in MAP and blood glucose concentration. Infusion of hydralazine to Bromo-HSD rats lowered MAP but had only a trivial effect on blood glucose. We conclude that rats with relative gamma-MSH deficiency develop abnormal glucose metabolism, with features of insulin resistance, in association with hypertension when ingesting the HSD. Elevated plasma noradrenaline concentration in Bromo-HSD rats is normalized by gamma(2)-MSH infusion, suggesting that an adrenergic mechanism may link the salt-sensitive hypertension and the impaired glucose metabolism of relative gamma-MSH deficiency.

Further evidence that melanocortins prevent myocardial reperfusion injury by activating melanocortin MC3 receptors.

In rats subjected to myocardial ischemia/reperfusion, melanocortin peptides, including gamma(1)-melanocyte-stimulating hormone (gamma(1)-MSH), are able to exert a protective effect by stimulating brain melanocortin MC(3) receptors. A non-melanocortin receptor belonging to a group of receptors for Phe-Met-Arg-Phe-NH(2) (FMRFamide)-like peptides may be involved in some of the cardiovascular effects of the gamma-MSHs. FMRFamide-like peptides and gamma(1)-/gamma(2)-MSH share, among other things, the C-terminal Arg-Phe sequence, which seems to be essential for cardiovascular effects in normal animals. So we aimed to further investigate which receptor and which structure are involved in the protective effects of melanocortins in anesthetized rats subjected to myocardial ischemia by ligature of the left anterior descending coronary artery (5 min), followed by reperfusion. In saline-treated rats, reperfusion induced, within a few seconds, a high incidence of ventricular tachycardia and ventricular fibrillation, and a high percentage of death within the 5 min of observation period. Reperfusion was associated with a massive increase in free radical blood levels and with an abrupt and marked fall in systemic arterial pressure. The i.v. treatment (162 nmol/kg) during the ischemic period with the adrenocorticotropin fragment 1-24 [ACTH-(1-24): the reference protective melanocortin which binds all melanocortin receptors], as well as with both the melanocortin MC(3) receptor agonists gamma(2)-MSH and [D-Trp(8)]gamma(2)-MSH, reduced the incidence of ventricular tachycardia, ventricular fibrillation and death, the increase in free radical blood levels and the fall in arterial pressure. On the contrary, gamma(2)-MSH-(6-12) (a fragment unable to bind melanocortin receptors) was ineffective. Such protective effect was prevented by the melanocortin MC(3)/MC(4) receptor antagonist SHU 9119. In normal (i.e., not subjected to myocardial ischemia/reperfusion) rats, the same i.v. dose (162 nmol/kg) of gamma(2)-MSH, [D-Trp(8)]gamma(2)-MSH and gamma(2)-MSH-(6-12) provoked a prompt and transient increase in arterial pressure; on the other hand, ACTH-(1-24), which lacks the C-terminal Arg-Phe sequence, decreased arterial pressure, but only at higher doses. Heart rate of normal rats was not affected by any of the assayed peptides. The present data confirm and extend our previous findings that melanocortins prevent myocardial reperfusion injury by activating melanocortin MC(3) receptors. Moreover, they further support the notion that, in normal rats, cardiovascular effects of gamma-MSHs are mediated by receptors for FMRFamide-like peptides, for whose activation, but not for that of melanocortin MC(3) receptors, the C-terminal Arg-Phe structure being relevant.

Suppression of gamma-melanocyte-stimulating hormone secretion is accompanied by salt-sensitive hypertension in the rat.

Gamma-melanocyte-stimulating hormone (gamma-MSH) is a natriuretic peptide derived from proopiomelanocortin (POMC) in the pituitary neurointermediate lobe (NIL); its plasma concentration in rats doubles after ingestion of a high (HSD; 8% NaCl) compared with a low sodium diet (LSD; 0.07%). Because NIL function is regulated through dopaminergic pathways, we asked whether dopaminergic stimulation with bromocriptine (5 mg/kg IP daily for 1 week) or inhibition with haloperidol (5 mg/kg IP for 1 week) alters the gamma-MSH response to a HSD. In vehicle-treated rats, plasma gamma-MSH and NIL gamma-MSH content on the HSD were both markedly elevated over values in rats on the LSD (P<0.001); no difference in mean arterial pressure (MAP) occurred. In haloperidol-treated rats on the LSD, both plasma gamma-MSH and NIL gamma-MSH content were greater than in vehicle-treated rats (P<0.05) and did not increase further on the HSD; MAP was also no different. In bromocriptine-treated rats, neither plasma gamma-MSH nor NIL gamma-MSH content increased on the HSD versus LSD, and MAP was markedly elevated on the HSD (132+/-3 versus 106+/-3 mm Hg, P<0.001). Intravenous infusion of gamma-MSH (0.4 pmol/min) to bromocriptine-treated rats on the HSD restored plasma gamma-MSH concentration to a level appropriate for the HSD and lowered MAP from 131+/-6 to 108+/-5 mm Hg (P<0.01). These results demonstrate that the increases in NIL content and plasma concentration of gamma-MSH normally occurring during ingestion of the HSD are prevented by dopaminergic suppression of NIL function. This results in deficiency of gamma-MSH on the HSD and is accompanied by elevated blood pressure, which is corrected by infusion of the peptide. gamma-MSH may be an important component in the normal response to a HSD; interruption of this response leads to salt-sensitive hypertension.

Redundancy of a functional melanocortin 1 receptor in the anti-inflammatory actions of melanocortin peptides: studies in the recessive yellow (e/e) mouse suggest an important role for melanocortin 3 receptor.

The issue of which melanocortin receptor (MC-R) is responsible for the anti-inflammatory effects of melanocortin peptides is still a matter of debate. Here we have addressed this aspect using a dual pharmacological and genetic approach, taking advantage of the recent characterization of more selective agonists/antagonists at MC1 and MC3-R as well as of the existence of a naturally defective MC1-R mouse strain, the recessive yellow (e/e) mouse. RT-PCR and ultrastructural analyses showed the presence of MC3-R mRNA and protein in peritoneal macrophages (M phi) collected from recessive yellow (e/e) mice and wild-type mice. This receptor was functional as Mphi incubation (30 min) with melanocortin peptides led to accumulation of cAMP, an effect abrogated by the MC3/4-R antagonist SHU9119, but not by the selective MC4-R antagonist HS024. In vitro M phi activation, determined as release of the CXC chemokine KC and IL-1 beta, was inhibited by the more selective MC3-R agonist gamma(2)-melanocyte stimulating hormone but not by the selective MC1-R agonist MS05. Systemic treatment of mice with a panel of melanocortin peptides inhibited IL-1 beta release and PMN accumulation elicited by urate crystals in the murine peritoneal cavity. MS05 failed to inhibit any of the inflammatory parameters either in wild-type or recessive yellow (e/e) mice. SHU9119 prevented the inhibitory actions of gamma(2)-melanocyte stimulating hormone both in vitro and in vivo while HS024 was inactive in vivo. In conclusion, agonism at MC3-R expressed on peritoneal M phi leads to inhibition of experimental nonimmune peritonitis in both wild-type and recessive yellow (e/e) mice.