The presence of delta and mu-, but not kappa or ORL 1 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 (ORL 1) receptors, respectively. The saturation experiments on bovine pinealocyte membranes for [ 3H]DPDPE binding provided B max and K d values of 553±24 fmol/mg protein and 1.3±0.6 nM; and for [ 3H]DAMGO binding provided B max and K d values 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 ORL 1 receptors 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 ORL 1 receptors are present on the pinealocytes, and the majority of opioid receptors found in the bovine pineal gland are δ (possibly, both δ 1 and δ 2) 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.