Antipyretic doses of α-MSH do not alter afebrile body temperature in the cold

Targeting Melanocortin Receptors as a Novel Strategy to Control Inflammation

Adrenocorticotropic hormone and alpha-, beta-, and gamma-melanocyte-stimulating hormones, collectively called melanocortin peptides, exert multiple effects upon the host. These effects range from modulation of fever and inflammation to control of food intake, autonomic functions, and exocrine secretions. Recognition and cloning of five melanocortin receptors (MCRs) has greatly improved understanding of peptide-target cell interactions. Preclinical investigations indicate that activation of certain MCR subtypes, primarily MC1R and MC3R, could be a novel strategy to control inflammatory disorders. As a consequence of reduced translocation of the nuclear factor kappaB to the nucleus, MCR activation causes a collective reduction of the major molecules involved in the inflammatory process. Therefore, anti-inflammatory influences are broad and are not restricted to a specific mediator. Short half-life and lack of selectivity could be an obstacle to the use of the natural melanocortins. However, design and synthesis of new MCR ligands with selective chemical properties are already in progress. This review examines how marshaling MCR could control inflammation.

Antipyretic properties of centrally administered alpha-MSH fragments in the rabbit

alpha-Melanocyte stimulating hormone (alpha-MSH 1-13) has marked antipyretic effects when administered centrally or peripherally in small doses. A C-terminal fragment, alpha-MSH (11-13), contains an antipyretic message sequence of alpha-MSH; however, the lesser potency of this fragment relative to that of the entire molecule suggests that other amino acids of the alpha-MSH sequence are essential for the full antipyretic effect. Graded doses of alpha-MSH (11-13) (Ac LysProVal NH2), alpha-MSH (10-13) (Ac GlyLysProVal NH2), and alpha-MSH (8-13) (Ac ArgTrpGlyLysProVal NH2), were injected into the cerebral ventricles of rabbits made febrile by IV administration of crude interleukin-1. All three fragments reduced fever in a dose-related manner. The (8-13) sequence was much more effective than the other two fragments, and the (10-13) portion was less effective than the (11-13) tripeptide. None of the fragments was as potent as alpha-MSH (1-13). The results confirm that an antipyretic message resides within alpha-MSH (11-13) and sequential addition of amino acids to alpha-MSH (11-13) can both enhance and reduce the potency of the fragment.

Effects of massive doses of alpha-MSH on thermoregulation in the rabbit

alpha-MSH-related compounds may prove to be clinically useful antipyretics since the parent peptide is extremely potent in reducing fever, it is effective when given orally, and it neither stimulates corticosteroid activity nor has marked melanotropic effects in man. To determine whether or in what doses alpha-MSH might cause harmful side-effects, we injected doses greatly exceeding those required to reduce fever into a lateral cerebral ventricle of afebrile rabbits. One hundred to seven hundred and fifty micrograms alpha-MSH caused large and prolonged reductions in body temperature and the dose-response relation was bell-shaped for both magnitude and duration. These doses caused no apparent injury to the animals. One mg alpha-MSH elicited hyperthermic responses that were variable in magnitude and duration. Animals that had previously received large doses of alpha-MSH (greater than or equal to 100 micrograms) did not develop hyperthermia, even when given 2 mg, indicating an acquired tolerance to this hyperthermic action of alpha-MSH. All animals, tolerant or previously uninjected, showed symptoms with doses greater than or equal to 1 mg alpha-MSH that included: increased salivation, agitation, ataxia, respiratory distress, and death (in 30% of the animals); those that recovered from these large doses resumed outwardly healthy appearance and behavior. Although alpha-MSH is toxic when given centrally in large doses, the 5000-fold difference between antipyretic and toxic doses indicates a wide safety margin should this peptide be used clinically as an antipyretic drug.

Antipyretic activity of a potent alpha-MSH analog

[Nle4,D-Phe7]-alpha-MSH has exceptional potency in certain biological assays of alpha-MSH activity such as skin darkening in frogs. However, this analog was equipotent to alpha-MSH in induction of grooming in the rat and had opposite effects on the performance of a visual discrimination task. These results led to the suggestion that distinct differences may exist between the melanocyte and CNS receptors for alpha-MSH. We determined the antipyretic and hypothermic potencies of centrally and peripherally administered [Nle4,D-Phe7]-alpha-MSH, relative to those of alpha-MSH, in the rabbit. Central injections of 40 and 80 ng of [Nle4,D-Phe7]-alpha-MSH caused hypothermia in afebrile rabbits, whereas 20 and 10 ng, which had no effect on afebrile body temperature, caused greater than 40% reduction in leukocytic pyrogen-induced fever. These results indicate that this analog is approximately 10 times more potent in reducing fever than alpha-MSH, making it the most potent antipyretic substance yet described. In contrast, IV administration of 16 micrograms of the analog, an extremely large dose relative to established antipyretic doses of alpha-MSH, elicited weak, variable responses. Since this analog is said to be resistant to degradation by serum enzymes, the contrast between the effects of central and peripheral administration may reflect a limited ability of the analog to cross the blood brain barrier when given IV. Our results do not suggest any distinct differences between the melanocyte receptors for alpha-MSH and those involved with CNS control of temperature. The marked central potency of [Nle4,D-Phe7]-alpha-MSH could result from an increased duration of action and/or a greater affinity for central receptor sites relative to alpha-MSH.

Effect of alpha-MSH 11-13 (lysine-proline-valine) on fever in the rabbit

In previous experiments, alpha-MSH (1-13) and ACTH (1-24), which contains the alpha-MSH 1-13 amino acid sequence, were found to reduce fever after central and peripheral administration of low, non-hypothermic doses. Shorter molecules, including alpha-MSH 1-10, had no effect. The idea that the 11-13 amino acid sequence is important to the effect of the parent molecule was tested by giving lysine-proline-valine both centrally and peripherally to rabbits made febrile by IV administration of leukocytic pyrogen. The tripeptide reduced fever after both central (0.5-2.0 mg) and peripheral (2-200 mg) administration. It appears that the 11-13 sequence is part of the message sequence of alpha-MSH with regard to antipyretic activity. However, the lower potency relative to that of the parent molecule suggests that other portions of the molecule are essential to full expression of the antipyretic effect.