[Octarphin--Nonopioid Peptide of the Opioid Origin]

The data on the properties and mechanism of action of the peptide octarphin (TPLVTLFK, the fragment 12-19 of β-endorphin)--a selective agonist of nonopioid (insensitive to the action of the opioid antagonist naloxone) β-endorphin receptor found on n immune cells (peritoneal macrophages, T and B lymphocytes of spleen and blood), endocrine (adrenal cortex, hypothalamus), cardiovascular (cardiomyocytes) systems are analyzed and systematized. Binding to the receptor octarphin increases increases the mitogen-induced pro- liferation of human and mouse T and B lymphocytes in vitro, activates murine peritoneal macrophages in vitro and in vivo, stimulates growth of human T-lymphoblast cell lines Jurkat and MT-4, inhibits adenylate cyclase activity of rat adrenal cortex membranes and suppresses the secretion of glucocorticoids from the adrenal gland into the blood. It was shown that in a concentration range of 1-1000 nM the peptide increases the activity of inducible NO-synthase (iNOS), and the content of NO and cGMP in lipopolysaccharide-activated murine peritoneal macrophages. Taking into account that NO acts as a primary activator of soluble guanylate cyclase (sGC), it can be assumed that the activating effect of octarphin on macrophages is realized in the following way: increase in th iNOS expression --> increase in the NO production --> increase in the sGC activity --> increase in intracellular levels of cGMP.

Approaches to the rational design of selective melanocortin receptor antagonists

INTRODUCTION

When establishing the physiological roles of specific receptors in normal and disease states, it is critical to have selective antagonist ligands for each receptor in a receptor system with several subtypes. The melanocortin receptors have five subtypes referred to as the melanocortin 1 receptor, melanocortin 2 receptor, melanocortin 3 receptor, melanocortin 4 receptor and melanocortin 5 receptor, and they are of critical importance for many aspects of human health and disease.

AREAS COVERED

This article reviews the current efforts to design selective antagonistic ligands for the five human melanocortin receptors summarizing the currently published orthosteric and allosteric antagonists for each of these receptors.

EXPERT OPINION

Though there has been progress, there are still few drugs available that address the many significant biological activities and diseases that are associated with these receptors, which is possibly due to the lack of receptor selectivity that these designed ligands are currently showing. The authors believe that further studies into the antagonists' 3D conformational and topographical properties in addition to future mutagenesis studies will provide greater insight into these ligands which could play a role in the treatment of various diseases in the future.