The alpha2-adrenergic receptor system in the hypothalamus of the Pekin duck

Summary In the present study, we have employed the monoradioiodinated ₂-agonist clonidine ([¹²⁵I]-CLO) to characterize duck hypothalamic ₂-adrenoceptors and to localize ₂-specific binding sites in the duck brain. To validate the ₂-specificity of [¹²⁵I]-CLO using an enriched duck hypothalamic membrane fraction, a radioreceptor assay was established by altering the membrane protein concentration, time, temperature and ionic milieu of incubation, and in the presence or absence of protease inhibitors. Competitive displacement studies revealed the following sequence of potency to displace [¹²⁵I]-CLO: yohimbine>(-)-epinephrine>clonidine> (-)-norepinephrine>phentolamine>(-)-phenylephrine>(-)-isoproterenol>prazosin. The non-hydrolyzable guanosine 5-triphosphate analog guanylylimidodiphosphate markedly inhibited [¹²⁵I]-CLO binding suggestive of G-protein involvement. With regard to the histological distribution, diencephalic structures, such as the habenula and the nucleus reticularis of the thalamus, were densely labeled by [¹²⁵I]-CLO. In the hypothalamus, ₂-adrenoceptors were detected in the antidiuretic hormone-synthesizing nucleus paraventricularis, the nucleus praeopticus medialis, the nucleus anterior medialis hypothalami, the nucleus magnocellularis praeopticus, the nucleus commissurae pallii, the nucleus inferior hypothalami and the regio lateralis hypothalami. Circumventricular organs, such as the plexus choroidei, organum subfornicale, organum paraventriculare and the corpus pineale, were endowed with ₂-specific binding sites, as were the cell layers of the tectum opticum. In addition, telencephalic structures revealed high receptor densities. The presence of well characterized ₂-specific adrenoceptors in hypothalamic structures of the duck brain including associated telencephalic regions supports physiological investigations with regard to functional ₂-adrenergic modulation of antidiuretic hormone release in the duck.