Structure-activity analysis for the effects of gamma-MSH/ACTH-like peptides on cerebral hemodynamics in rats

In a previous structure-activity analysis we have shown that the gamma-melanocyte-stimulating hormones (gamma-MSHs) and structurally related adrenocorticotropic hormone (ACTH) fragments share an amino-acid sequence which is determinant for the effects of these peptides on peripheral hemodynamics, viz. a pressor and a tachycardiac response, in conscious rats. We now investigated whether these structural features are also important for the effects of these peptides on cerebral hemodynamics in urethane-anesthetized rats. After intracarotid and intravenous administration, the 'mother' peptides, Lys-gamma2-MSH and gamma2-MSH, and, with a 10-fold lower potency, ACTH-(4-10), caused a dose-dependent pressor and tachycardiac response, as well as an increase in extra- and intracranial blood flow and microcirculatory cerebrocortical blood flow. Removal of C-terminal amino acids resulted in gamma-MSH-fragments which were devoid of effects on peripheral and central hemodynamics. Fragments of gamma2-MSH which were shortened at the N-terminal side (gamma-MSH-(4-12) and gamma-MSH-(5-12)) were less potent than gamma2-MSH, but had an intrinsic activity similar to that of gamma2-MSH with respect to the pressor and tachycardiac effect. However, the potency and intrinsic activity of these shortened fragments on intracerebral hemodynamic parameters were the same as those of gamma2-MSH. This suggests that different mechanisms (e.g., site of action and/or melanocortin receptor subtype) are involved in the cerebral hemodynamic effects of the melanocortins and in their peripheral hemodynamic effects. Surprisingly, removal of an additional residue, His5, resulting in the fragment gamma-MSH-(6-12), led to full restoration of potency with respect to extracranial blood flow, blood pressure and heart rate. Neither the structurally related analog, [Nle4,D-Phe7]alpha-MSH (NDP-MSH), nor ACTH-(1-24) was able to induce a pressor effect or cerebral hemodynamic effects. In contrast, both compounds had a depressor effect. It is concluded that the C-terminal amino acids in the structure of gamma-MSH/ACTH-like peptides are essential for efficacy for the central hemodynamic effects, i.e., the increase in intracerebral (microcirculatory) blood flow. However, in contrast to what holds for the peripheral hemodynamic features, the N-terminal sequence has hardly any influence on potency or efficacy. The results with NDP-MSH and ACTH-(1-24) and the other fragments lead us to postulate that it is not one of the five known subtypes of melanocortin receptors which mediates the hemodynamic effects of the melanocortins, but an additional, still unidentified subtype. A clue for the elucidation of such a receptor might be found in the structural features of gamma-MSH-(6-12) that appear to be very important determinants for the effectiveness to alter peripheral and central hemodynamics.