Ontogenesis of the alpha2-adrenergic receptor system in the hypothalamo-limbic system of the Pekin duck.

The development of the central nervous alpha2-adrenergic system in the duck was studied by semiquantitative autoradiography at the ontogenetic stages embryonic days 20 (E20) and 27 (E27) and postnatal days 3 (P3) and 14 (P14) by using the monoradioiodinated alpha2-agonist clonidine ([125I]CLO) as radioligand. All structures endowed with alpha2-adrenoceptors in the adult animal were specifically labeled with [125I]CLO by E20. A detailed analysis of the binding capacity for [125I]CLO was performed for parts of several functional systems: hypothalamic structures (nucleus inferior hypothalami, nucleus magnocellularis preopticus, nucleus paraventricularis), limbic system (habenula, nucleus septalis lateralis, nucleus striae terminalis), circumventricular organs (organum pineale, organum subfornicale, plexus choroidei ventriculi tertii and ventriculi lateralis), visual system (hyperstriatum accessorium, nucleus reticularis superior, tectum opticum), and associative cortex (hyperstriatum ventrale). Except for the nucleus inferior hypothalami and the plexus choroideus ventriculi lateralis, all structures showed a perinatal (E27-P3) maximum of alpha2-adrenoceptor-binding capacity with a subsequent decline to values of prehatching stages. This uniform expression pattern of alpha2-adrenoceptors indicates that the days around hatching are a critical period for the development of the adrenergic system in the brain of the duck.