Characterization of beta-adrenergic receptors in rat brain and pituitary using a new high-affinity ligand, [125I]iodocyanopindolol.

The binding of [125I]iodocyanopindolol (ICYP) to membrane preparations from rat cerebral cortex, hypothalamus and anterior pituitary gland was characterized in regard to specificity, density, and the proportion of beta-adrenergic receptor subtypes. By employing a mixture of ligands specific for alpha-adrenergic, serotoninergic and dopaminergic receptors, it was possible to eliminate most of the less-specific contributions to ICYP binding profiles, which resulted in narrowing the range of measured dissociation constants to 35-50 pM for all neural tissues studied. These values corresponded well with constant for the 'slow' component discernible in ICYP association with cerebral cortical membranes at 37 degrees C. The maximum binding values were 63, 29 and 5.6 fM/mg membrane protein in cortical, hypothalamic and anterior pituitary membrane fractions, respectively. Evaluation of the beta-adrenergic receptor subtypes using 4 selective competitors indicated an average 19% content of the beta 2-subtype in cortical membranes, while in hypothalamic membranes 47% of the receptors could be assigned to that subtype. In the anterior pituitary as well as in the cerebellum, the receptors were predominantly of beta 2-subtype. These findings are discussed in terms of possible physiological functions of beta-receptors in these tissues, including the regulation of the release of pituitary hormones.