Selective melanocortin MC4 receptor agonists reverse haemorrhagic shock and prevent multiple organ damage
2007

In circulatory shock, melanocortins have life-saving effects likely to be mediated by MC4 receptors. To gain direct insight into the role of melanocortin MC4 receptors in haemorrhagic shock, we investigated the effects of two novel selective MC4 receptor agonists.

Experimental approach]Severe haemorrhagic shock was produced in rats under general anaesthesia. Rats were then treated with either the non-selective agonist [Nle4, D-Phe7]?-melanocyte-stimulating hormone (NDP-?-MSH) or with the selective MC4 agonists RO27-3225 and PG-931. Cardiovascular and respiratory functions were continuously monitored for 2 h; survival rate was recorded up to 24 h. Free radicals in blood were measured using electron spin resonance spectrometry; tissue damage was evaluated histologically 25 min or 24 h after treatment.

Key results]All shocked rats treated with saline died within 30-35 min. Treatment with NDP-?-MSH, RO27-3225 and PG-931 produced a dose-dependent (13-108 nmol kg-1 i.v.) restoration of cardiovascular and respiratory functions, and improved survival. The three melanocortin agonists also markedly reduced circulating free radicals relative to saline-treated shocked rats. All these effects were prevented by i.p. pretreatment with the selective MC4 receptor antagonist HS024. Moreover, treatment with RO27-3225 prevented morphological and immunocytochemical changes in heart, lung, liver, and kidney, at both early (25 min) and late (24 h) intervals.

Conclusions and Implications]Stimulation of MC4 receptors reversed haemorrhagic shock, reduced multiple organ damage and improved survival. Our findings suggest that selective MC4 receptor agonists could have a protective role against multiple organ failure following circulatory shock.

Keywords]Melanocortin peptides and in particular the adrenocorticotropin/?-melanocyte-stimulating hormone (ACTH/?-MSH) sequences, as well as shorter fragments, have a life-saving effect in animals and humans in conditions of circulatory shock. These neuropeptides, including ACTH-(1–24), ?-MSH and synthetic analogs, are likewise protective in other severe hypoxic conditions, such as prolonged respiratory arrest, myocardial ischaemia and ischaemic stroke, as well as in experimental heart transplantation.

Circulatory shock, including haemorrhagic shock, initiates an inflammatory cascade with production of cytokines and recruitment of neutrophils. The multiple organ injury that often occurs after haemorrhagic shock is believed to be caused by ischaemia (during haemorrhage) and reperfusion (during resuscitation) of target organs. Moreover, the pathophysiologic processes involved in multiple organ injury are greatly influenced by reactive oxygen species (ROS), released in large amounts during haemorrhagic shock.

Recent research indicates that during shock due to endotoxin, haemorrhage or splanchnic artery occlusion, the central nervous system (CNS) modulates the systemic inflammatory response through the rapid activation of efferent vagal nerve fibers (the ‘brain cholinergic anti-inflammatory pathway'). It has been demonstrated that the antishock effect of melanocortins, which is independent of effects on the adrenal gland, is due to the activation of the brain cholinergic anti-inflammatory pathway and is associated with a marked reduction in the plasma levels of inflammatory mediators including free radicals. Indirect evidence from our studies suggests that the cholinergic anti-inflammatory pathway is activated by stimulation of melanocortin MC4 receptors within the CNS.

To gain direct insight into the role of MC4 receptors in haemorrhagic shock-induced cardiovascular and respiratory failure and organ injury, we determined the effects of two novel melanocortin agonists, highly selective at MC4 receptors, in a rat model of severe haemorrhagic shock.

The present research on haemorrhage-shocked rats shows that the non selective melanocortin agonist NDP-?-MSH, which activates the MC1, MC3, MC4 and MC5 receptor subtypes, and the selective MC4 receptor agonists RO27-3225 and PG-931, produced a rapid and dose-dependent restoration of cardiovascular and respiratory functions and increased survival. This antishock effect occurred at nanomolar doses and was mediated by melanocortin MC4 receptors. Indeed, the selective MC4 receptor antagonist HS024 totally prevented effects of NDP-?-MSH, RO27-3225 and PG-931.

The fact that the MC receptor antagonist HS024 has an approximately 65-fold higher affinity for the MC4 receptors relative to MC1 receptors argues against an involvement of MC1 receptors in the here reported protective effects of melanocortins.

In conclusion, the present data give direct evidence for the first time of the antishock and life-saving effects of melanocortin MC4 receptor agonists. Moreover, our data suggest for the first time that selective MC4 agonists, in addition to their effects on systemic inflammatory responses and free radical discharge, could also prevent or significantly reduce late organ damage following a shock condition. In fact, whereas shock either resolves rapidly or is fatal, serious abnormalities in multiple organs occur hours to days after the onset of shock. Using the present experimental model of haemorrhagic shock, which rapidly (within 30–35min) progresses to irreversibility and death, we have been able to detect only initial signs of organ damage in shocked control rats; further, a proportion of melanocortin-treated shocked rats were still alive 24h after shock and did not show appreciable signs of organ damage. Therefore, investigations designed at determining the early pathophysiological molecular mechanisms of organ damage following haemorrhagic shock ? and the molecular targets of selective MC4 agonists ? may help to better assess the protective effect of melanocortins against multiple organ failure. Such findings could be of clinical relevance for the management of circulatory shock.