Characterization and measurement of [¹²⁵I]iodopindolol binding in individual rat pineal glands: existence of a 24-h rhythm in β-adrenergic receptor density

A simple procedure has been developed that permits measurement of β-receptors in membrane preparations from individual rat pineal glands using [¹²⁵I]iodopindolol ([¹²⁵I]PIN). [¹²⁵I]PIN binding to pineal membranes was stereospecific and saturable. Scatchard analysis of saturation isotherms yielded a K_d of 147.3 ± 54 pM and a B_max of ± 1.5 mol/pineal glands. Binding was linear suggesting that [¹²⁵I]PIN binds to a single population of pineal β-adrenergic receptors. This procedure was used to evaluate 24-h variations in density of pineal [¹²⁵I]PIN binding sites in male rats maintained in a 14:10 h light:dark cycle. Binding remained uniformly low during the daytime, increased slightly prior to lights off and peaked after 6 h of darkness decreasing abruptly 2 h later, before lights on. In animals maintained in light at night, the number of binding sites also increased, but did not exhibit the darkness-related decrease. The results demonstrate that β-adrenergic receptors defined via [¹²⁵I]PIN binding can be measured in tissue samples equivalent to less than one pineal gland. Moreover, the technique can be used in studies concerning the noradrenergic regulation of pineal function.