Identification of β-adrenergic receptors in rat hypothalamus

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Abstract
(-)-[3H]Dihydroalprenolol ((-)-[3H]DHA) binding in the rat hypothalamus appears to possess all the characteristics expected of physiologically relevant .beta.-adrenergic receptors. Binding of (-)-[3H]DHA to the hypothalamic sites was rapid (k1 = 1.3 .times. 107 min-1 M-1) and rapidly reversible. Binding was saturable at low concentrations of ligand (.apprx. 50-100 nM). The dissociation constant (Kd) of (-)-[3H]DHA binding determined by equilibrium analysis was 19 nM. Binding displayed .beta.-adrenergic specificity. .beta.-Adrenergic agonists inhibited binding in the following order of potency: (-)-isoproterenol .simeq. (-)-epinephrine > (-)-norepinephrine. Specific .beta.-adrenergic antagonists (-)-propranol and (-)-alprenolol inhibited binding at low concentrations (Kd = 25-50 nM), whereas the .alpha.-antagonist phentolamine inhibited binding at very high concentrations (Kd = 42 .mu.M). Interactions of agonists and antagonists with the sites showed stereoselectivity. The (-)-isomers of all .beta.-adrenergic agents tested were more potent than their respective (+)-isomers. Specific receptor sites for .beta.-adrenergic catecholamines apparently are present in the rat hypothalamus. [Hypothalamic hormone secretion is discussed.].