The presence of delta and mu-, but not kappa or ORL<inf>1</inf>receptors in bovine pinealocytes

Abstract

Physicians have noted since antiquity that their patients complained of less pain and required fewer analgesics at night-time. In humans, the circulating levels of melatonin, a pineal substance with analgesic and hypnotic properties, exhibit a pronounced circadian rhythm with serum levels being high at night and low during day-time. Moreover, pinealectomy abolishes the analgesic effects of melatonin, and naloxone disrupts the day-night rhythm of nociception. In this study, we have attempted to identify and characterize the nature and types of opioid receptor in bovine pinealocyte membranes, using a radioligand binding technique with the selective radioligands [3H]DAMGO, [3H]DPDPE, [3H]U69593 and [3H]orphanin-FQ (OFQ) for identifying mu (μ)-, delta (δ)-, kappa (κ)- and opioid receptor-like (ORL1) receptors, respectively. The saturation experiments on bovine pinealocyte membranes for [3H]DPDPE binding provided Bmaxand Kdvalues of 553±24 fmol/mg protein and 1.3±0.6 nM; and for [3H]DAMGO binding provided Bmaxand Kdvalues of 6.3±1.3 fmol/mg protein and 1.2±0.4 nM, respectively. On the other hand, the specific radioligands ([3H]U69593 and [3H]OFQ) binding of κ and ORL1receptors were undetectable in bovine pinealocyte membranes. Furthermore, competitive experiments with opioid agonist and antagonist and related compounds confirmed the presence of μ- and δ-opioid binding sites in bovine pinealocyte membranes. These results indicate that neither κ nor ORL1receptors are present on the pinealocytes, and the majority of opioid receptors found in the bovine pineal gland are δ (possibly, both δ1and δ2) types, with a minority being μ type, and that both are primarily located on the bovine pinealocyte membranes. These opioid receptors, by stimulating the activity of N-acetyltransferase, enhance the synthesis of melatonin. © 2002 Elsevier Science B.V. All rights reserved.