Existence and function of opioid receptors on mammalian pinealocytes

Previous studies in our laboratories have identified a single population of opioid receptors in bovine pineal gland, which we have chosen to characterize further on pinealocytes isolated from the cow and rat pineal gland. The bovine pinealocytes isolated by trypsinization or mechanical manipulation revealed receptor density (B(max)) values of 206.95 ± 131.15 and 220.34 ± 11.80 fmol/mg protein, respectively, and dissociation equilibrium constant (K(d)) values of 1.93 ± 0.48 and 1.96 ± 0.21 nM, respectively. The rat pinealocytes cultured for 7 days exhibited a [³H]diprenorphine binding site of 56 fmol/10⁶ cells. Morphine (100 μM) enhanced the activity of N-acetyltransferase and the level of melatonin in rat pineal gland in culture incubated for 21 hr. The results of these studies suggest that opioidergic receptors exist on pinealocytes and they are involved in stimulating the activity of N-acetyltransferase and the synthesis of melatonin, thereby regulating the physiology of mammalian pineal gland.