Synthetic peptide TPLVTLFK (octarphin) reduces the corticosterone production by rat adrenal cortex through nonopioid β-endorphin receptor.

The synthetic peptide octarphin (TPLVTLFK) corresponding to the sequence 12-19 of β-endorphin, a selective agonist of nonopioid β-endorphin receptor, was labeled with tritium to a specific activity of 29 Ci/mmol. [(3)H]Octarphin was found to bind to high-affinity naloxone-insensitive binding sites on membranes isolated from rat adrenal cortex (K(d) = 35.7 ± 2.3 nM, B(max) = 41.0 ± 3.6 pmol/mg protein). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but to α-endorphin, γ-endorphin, [Met(5) ]enkephalin, and [Leu(5) ]enkephalin as well. At the same time, the [(3) H]octarphin-specific binding with adrenal cortex membranes was inhibited by unlabeled β-endorphin (K(i) = 32.9 ± 3.8 nM). Octarphin at concentrations of 10(-9) -10(-6) M was found to inhibit the adenylate cyclase activity in adrenocortical membranes, whereas intranasal injection of octarphin at doses of 5 and 20 µg/rat was found to reduce the secretion of corticosterone from the adrenals to the bloodstream. Thus, octarphin decreases the adrenal cortex functional activity through the high affinity binding to nonopioid receptor of β-endorphin.