Met-enkephalin (met-Enk) is an opioid peptide that acts via three main subtypes of receptors referred to as μ (mu)-, δ (delta)- and ζ (zeta)-receptor. While the first two receptor subtypes mediate the classic opioid effects of met-Enk, ζ-receptors are reported to be involved in the non-opioid actions of the peptide, i.e. the inhibitory effect on the cell growth. Despite the fact that met-Enk is known to regulate the function of the hypothalamic-pituitary-adrenal axis acting on both its central and peripheral branches, none is known on the effects of met-Enk on adrenal growth. Hence, we have investigated the effects of met-Enk and its receptor agonists and antagonists on cell proliferation in three different models of rat adrenal growth: i) immature adrenal cortex, ii) regenerating adrenal cortex and iii) primary cultures of adrenocortical cells. In in vivo experiments, rats were given subcutaneous injections of 1 nmol/100 g of the peptides 28, 16 and 4 h before the sacrifice, and proliferative activity was assessed by counting the number of metaphase-arrested cells (after vincristine administration). In in vitro studies, cultured adrenocortical cells were exposed for 48 h to the peptides at a concentration of 10-6 M, and proliferative activity was measured by the EZ4U method. The blockade of μ- and δ-receptors raised proliferative activity in immature adrenals and decreased it in regenerating glands, and the effects were reversed by μ- and δ-receptor agonists. Naltrexone-induced blockade of all met-Enk receptor subtypes decreased proliferative activity in immature adrenal and raised it in regenerating glands. The exposure to either μ- or δ-receptor agonists and antagonists evoked doubtful or no effects on the proliferative activity of cultured adrenocortical cells. In contrast, met-Enk exerted a marked antiproliferogenic effect that was reversed by naltrexone. Taken together, these findings allow us to draw the following conclusions: i) μ- and δ-receptor activation inhibits the growth of immature adrenals, stimulates adrenal regeneration, and does not affect prolifer-ation of cultured adrenocortical cells; ii) ζ-receptors mediate the growth inhibitory effect of met-Enk on both regenerating adrenals and cultured adrenocortical cells, but unexpectedly their activation stimulates the growth of immature gland; and iii) the effects of μ- and δ-receptor activation in in vivo experiments are probably mediated by extra-adrenal indirect mechanisms.

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