Autoradiographic Localization of Dopaminergic and Noradrenergic Receptors in the Bovine Pineal Gland

Dopamine and norepinephrine are involved in regulation of melatonin synthesis in the pineal gland. In bovine pineal gland, D1- and D2-dopaminergic and alpha 1-adrenergic receptors have been characterized pharmacologically in several laboratories, while beta 1-adrenergic receptors have been studied using physiological technique. The current study presents a quantitative autoradiographic analysis of these four dopaminergic and noradrenergic receptors in bovine pineal gland. The density order of the receptors is D1 greater than alpha 1 greater than D2 greater than or equal to beta 1. The Bmax of dopamine D1 receptors is about 5 to 6 times higher than the Bmax for alpha 1-adrenergic receptors and about 20 times higher than the Bmax values for beta 1-adrenergic and D2-dopaminergic receptors. Dopamine D1 receptors are significantly denser in the pineal cortex than in the medulla. Both dopamine receptors are more concentrated in the distal area than in the proximal area (close to the habenula), whereas both noradrenergic receptors are homogeneously distributed along the longitudinal axis. Only D1-dopaminergic receptors display a heterogeneous distribution between the superior and the inferior areas, being denser in the inferior area. The observation of a much higher concentration of D1-dopaminergic receptors relative to the other receptors suggests an important role for dopamine in the regulation of bovine pineal physiology.