Naloxone prevents the analgesic action of alpha-MSH in mice

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.

The role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception

The melanocortins are a family of bioactive peptides derived from proopiomelanocortin. Those peptides, included among hormones and comprising ACTH, alpha-MSH, beta-MSH and gamma-MSH, are best known mainly for their physiological effects, such as the control of skin pigmentation by alpha-MSH, and ACTH effects on pigmentation and steroidogenesis. Melanocortins are released in various sites in the central nervous system and in peripheral tissues, and participate in the regulation of multiple physiological functions. They are involved in grooming behavior, food intake and thermoregulation processes, and can also modulate the response of the immune system in inflammatory states. Research of the past decade provided evidence that melanocortins could elicit their diverse biological effects by binding to a distinct family of G protein-coupled receptors with seven transmembrane domains. To date, five melanocortin receptor genes have been cloned and characterized. Those receptors differ in their tissue distribution and in their ability to recognize various melanocortins. These advances have opened up new horizons for exploring the significance of melanocortins, their ligands and their receptors for a variety of important physiological functions. We reviewed the origin of MSH peptides, the function and distribution of melanocortin receptors and their endogenous and exogenous ligands and the role of melanocortins and their receptors in inflammatory processes, nerve regeneration and nociception. Moreover, we analyzed their interaction with opioid peptides and finally, we discussed the postulated role of the melanocortin system in pain transmission at the spinal cord level.

The melanocortin-1 receptor gene mediates female-specific mechanisms of analgesia in mice and humans

Sex specificity of neural mechanisms modulating nociceptive information has been demonstrated in rodents, and these qualitative sex differences appear to be relevant to analgesia from kappa-opioid receptor agonists, a drug class reported to be clinically effective only in women. Via quantitative trait locus mapping followed by a candidate gene strategy using both mutant mice and pharmacological tools, we now demonstrate that the melanocortin-1 receptor (Mc1r) gene mediates kappa-opioid analgesia in female mice only. This finding suggested that individuals with variants of the human MC1R gene, associated in our species with red hair and fair skin, might also display altered kappa-opioid analgesia. We found that women with two variant MC1R alleles displayed significantly greater analgesia from the kappa-opioid, pentazocine, than all other groups. This study demonstrates an unexpected role for the MC1R gene, verifies that pain modulation in the two sexes involves neurochemically distinct substrates, and represents an example of a direct translation of a pharmacogenetic finding from mouse to human.

Neuropathic pain: a possible role for the melanocortin system?

In humans, damage to the nervous system can lead to a pain state referred to as neuropathic pain. Here, we give a short overview of the clinical picture and classification of neuropathic pain and highlight some of the currently known pathophysiological mechanisms involved, with special emphasis on neuropeptide plasticity. In this context, we discuss a specific group of neuropeptides, the melanocortins. These peptides have been demonstrated to play a role in nociception and to functionally interact with the opiate system. Recently, we demonstrated that spinal melanocortin receptors are upregulated in a rat model of neuropathic pain and that blockade of the melanocortin MC(4) receptor has anti-allodynic effects in this condition, suggesting that the melanocortin system plays a role in neuropathic pain. A natural agonist of melanocortin receptors is alpha-melanocyte-stimulating hormone (alpha-MSH), derived from the precursor molecule pro-opiomelanocortin (POMC). Cleavage of this precursor also yields beta-endorphin, which is co-released with alpha-MSH in nociception-associated areas of the spinal cord. We hypothesise that melanocortin receptor blockade attenuates a tonic influence of alpha-MSH on nociception, thus allowing the analgesic effects of beta-endorphin to develop, resulting in the alleviation of allodynia. In this way, treatment with melanocortin receptor antagonists might enhance opioid efficacy in neuropathic pain, which would be of great benefit in clinical practice.

The gamma(2)-MSH peptide mediates a central analgesic effect via a GABA-ergic mechanism that is independent from activation of melanocortin receptors

Using the latency for tail-flick after thermal stimulation we have assessed the effects of alpha-, gamma(1)- and gamma(2)-MSH on nociceptive threshold in the mice. Intracisternal injections of gamma(2)-MSH induced a distinct analgesia, while gamma(1)-MSH in the same doses gave only a minor analgesia. Intracisternal alpha-MSH instead gave a short-term hyperalgesia. The effect of gamma(2)-MSH was not blocked by any of the MC(4)/MC(3)receptor antagonist HS014, naloxone or by the prior intracisternal administrations of gamma(1)-MSH. However, the gamma(2)-MSH analgesic response was completely attenuated by treating animals with the GABA(A)antagonist bicuculline. The gamma(2)-MSH analgesic effect was moreover additive to the analgesia afforded by muscimol and ethanol, but not to that afforded by diazepam. In addition both gamma(1)- and gamma(2)-MSH induced moderate catalepsy, but could at the same time attenuate haloperidol induced catalepsia. We conclude that gamma(2)-MSH mediates a central analgesic effect via GABA-receptor dependent pathway that is distinct from melanocortic- and opioid-receptors. Moreover, the mechanism for gamma(2)-MSH's analgesic effect appears to be distinct from that causing moderate catalepsia by gamma-MSH's.

Peripheral analgesic activities of peptides related to alpha-melanocyte stimulating hormone and interleukin-1 beta 193-195

1. The hyperalgesic effects of interleukin-1 beta (IL-1 beta) and prostaglandin E2 (PGE2) were measured in rats. 2. Hyperalgesic responses to IL-1 beta were inhibited in a dose-dependent manner by alpha-melanocyte stimulating hormone (alpha-MSH)-related peptides with the following order of potency: [N1(4),D-Phe7]alpha-MSH greater than alpha-MSH greater than Lys-D-Pro-Val greater than Lys-Pro-Val greater than Lys-D-Pro-Thr greater than D-Lys-Pro-Thr. 3. Hyperalgesic responses to PGE2 were not inhibited by Lys-D-Pro-Thr and D-Lys-Pro-Thr but were inhibited in a dose-dependent manner by the other peptides with the same order of potency as against IL-1 beta. 4. The potencies of [N1(4), D-Phe7]alpha-MSH and alpha-MSH were greatly diminished by deletion of their C-terminal tripeptide, Lys11-Pro-Val13. 5. Nor-binaltorphimine (Nor-BNI) largely reversed the analgesic effects of alpha-MSH, [N1(4), D-Phe7]alpha-MSH, Lys-Pro-Val and Lys-D-Pro-Val indicating that kappa-opioid receptors mediated the analgesic activity of these peptides. 6. Nor-BNI did not antagonize the inhibition by Lys-D-Pro-Thr and D-Lys-Pro-Thr of IL-1 beta evoked hyperalgesia indicating that these peptides were not acting via kappa-opioid receptors.

Influence of centrally administered peptides on ear withdrawal from heat in the rabbit

Certain neuropeptides previously linked to stress and implicated in CNS control of analgesia/algesia were tested using a recently developed analgesiometric model, the rabbit ear-withdrawal test. The latency to ear withdrawal increased in a dose-related manner after beta-endorphin was injected intracerebroventricularly (IVC). Intermediate doses (0.5 and 1.0 micrograms) of adrenocorticotropic hormone (ACTH) caused hyperalgesia as indicated by decreases in latency. Corticotropin-releasing factor (CRF, 0.5 and 1.0 micrograms) also caused significant hyperalgesia late in the testing period. alpha-Melanocyte stimulating hormone (alpha-MSH, 0.25-2.0 micrograms), a molecule that shares the first 13 amino acid sequence with ACTH, and somatostatin (0.25-2.0 micrograms), caused no significant change in latency. However, 1.0 microgram doses of each peptide antagonized the analgesic effect of beta-endorphin (1.0 microgram) in the following order of potency: ACTH = alpha-MSH greater than CRF greater than somatostatin. The results support the idea that CNS peptides that are released during stress can exert opposing actions on acute pain, even though they may cause little effect alone.