Abstract

[³H]Dihydroalprenolol ([³H]DHA) binding sites in membrane preparations of rat and monkey brain appear to involve beta adrenergic receptors. [³H]DHA binding is saturable, with high affinity and an apparent dissociation constant of about 1 nM. Determination of the dissociation constant by kinetic studies measuring rate constants for association and dissociation provided K_D values similar to those obtained in equilibrium experiments. [³H]DHA binding is stereospecific for adrenergic agonists and antagonists. The relative potencies of numerous drugs in competing for [³H]DHA binding sites parallel their pharmacological activity at beta receptors, and suggest that the receptors are of the beta₁ type. Subcellular fractionation studies show an enrichment of [³H]DHA binding sites in "synaptic membrane" fractions. Regional distribution studies reveal fairly low densities of binding sites in the hypothalamus, which contains the highest norepinephrine concentration, suggesting that norepinephrine receptors in the hypothalamus are predominantly of the alpha variety. The relatively high levels of [³H]DHA binding in the cerebral cortex and cerebellum suggest that synaptic actions of norepinephrine in these regions may involve beta receptors.