Demonstration of high- and low-affinity β-adrenergic receptors in slide-mounted sections of rat and human brain

Abstract

Studies using isolated cell membranes have shown that β-adrenergic receptors exist in two interconvertible conformations, one with high affinity for agonists and the other with low affinity. Guanine nucleotides and sodium shift high-affinity receptor sites into low-affinity sites. We sought to demonstrate affinity states of the β-adrenergic receptor in slide-mounted sections of rat and human postmortem brain, and to determine using quantitative receptor autoradiography the regional distribution in the brain of high-affinity receptors relative to the total population of β-adrenergic receptors. The β-adrenergic agonist isoproterenol inhibited the binding of 125I-iodopindolol to slide-mounted sections of rat forebrain and human premotor cortex in a biphasic manner. Approximately 80% of the β-receptor sites in both rat and human brain showed high-affinity for isoproterenol (7–14 nM). Treatment with 0.1 mM non-hydrolyzable guanine nucleotide 5′-guanylylimidodiphosphate and 25 mM NaCl abolished high-affinity binding in rat brain sections and reduced it in human brain sections. These findings were confirmed by membrane studies using similar tissue samples. For autoradiographic studies, 17 nM isoproterenol displaced 71% of the high-affinity sites without affecting low-affinity β-adrenergic receptors. Digital subtraction was used to selectively visualize β-receptors in the high-affinity conformation. The proportion of β-adrenergic receptors in the high-affinity conformation differed significantly across rat brain regions and across human cortical layers. We conclude that β-adrenergic affinity states, presumably reflecting the interaction of the receptor with a G-protein, occur in slide-mounted sections of rat brain and persist in human postmortem material. Quantitative receptor autoradiographic analysis of receptor affinity states may provide a better index of receptor function than determination of binding site density alone. This work was previously presented in abstract form 3.