Identification and characterization of alpha 2D-adrenergic receptors in bovine pineal gland

The vertebrate mechanisms for generating circadian rhythms, regulating the synthesis of melatonin, and modulating the activities of pineal indole-synthesizing enzymes differ significantly among species. The synthesis of melatonin in mammalian pineal glands is stimulated by beta 1-adrenergic receptor agonists and is potentiated by alpha 1-adrenergic receptor agonists, whereas in the avian pineal gland alpha 2-adrenergic receptor agonists inhibit its synthesis. By using [3H]rauwolscine, a selective alpha 2-adrenergic receptor antagonist, we have identified for the first time alpha 2-adrenergic receptor sites in a mammalian pineal gland. [3H] Rauwolscine bound in a saturable manner to a single class of receptors, with an equilibrium dissociation constant (KD) of 1.4 nM and a density (Bmax) of 71 fmol/mg of protein, in crude synaptic membrane preparations from bovine pineal gland. Competition studies carried out with various adrenergic antagonists supported the conclusion that [3H]rauwolscine binding sites were alpha 2-adrenergic receptors. The bovine pineal alpha 2-adrenergic receptor appears to represent a pharmacological subtype distinct from the three currently proposed subtypes, i.e., alpha 2A found in a human colonic adenocarcinoma cell line (HT29 cell), alpha 2B found in rat lung, and alpha 2C found in an opossum kidney cell line (OK cell). However, the pharmacologic profile of the pineal alpha 2 receptor resembles that found in the rat submaxillary gland. We suggest that the bovine pineal receptor may represent a fourth pharmacological subtype, which would be designated as alpha 2D.