Astrocytes in sensory circumventricular organs of the rat brain express functional binding sites for endothelin

Abstract

Sensory circumventricular organs bordering the anterior third cerebral ventricle, the subfornical organ and the organum vasculosum laminae terminalis, lack blood–brain barrier characteristics and are therefore accessible to circulating peptides like endothelins. Astrocytes of the rat subfornical organ and organum vasculosum laminae terminalis additionally showed immunocytochemical localization of endothelin-1/endothelin-3-like peptides, possibly acting as circumventricular organ-intrinsic modulators. Employing [ 125I]endothelin-1 as radioligand, quantitative autoradiography demonstrated specific binding sites throughout the rat organum vasculosum laminae terminalis and subfornical organ, and competitive displacement studies revealed expression of both ET A and ET B receptor subtypes for either circumventricular organ. ET B receptor binding prevailed for the whole brain and ET A receptors could be labelled in the peripheral vascular system. To characterize endothelin-specific receptors in astrocytes of both circumventricular organs, alterations in the intracellular calcium concentration due to endothelin-1/endothelin-3 stimulation were studied in primary culture of subfornical organ and organum vasculosum laminae terminalis cells obtained from early postnatal rat pups. Endothelin-1 and endothelin-3 induced Ca 2+ transients in 9–13% of either subfornical organ or organum vasculosum laminae terminalis astrocytes, respectively, and some glial cells (subfornical organ: 2%, organum vasculosum laminae terminalis: 5%) responded to both endothelin analogues. The antagonistic action of BQ123 specific for ET A receptors (74% of all astrocytes tested), and the pronounced responsiveness to the ET B receptor agonist [4Ala]ET-1 (subfornical organ: 27%, organum vasculosum laminae terminalis: 35%) demonstrated glial expression of both endothelin receptor subtypes. Agonist-induced elevations in the intracellular calcium concentration proved to be independent of extracellular Ca 2+. In summary, the results indicate that endothelin(s) interact(s) with circumventricular organ astrocytes. Competitive receptor binding techniques using brain tissue sections as well as a fura-2 loaded primary cell culture system of the subfornical organ and organum vasculosum laminae terminalis demonstrate glial expression of functional ET A and ET B receptors, with calcium as intracellular messenger emerging primarily from intracellular stores. Endothelin(s) of both circulating and circumventricular organ-intrinsic origin may afferently transfer information important for cardiovascular homeostasis to circumventricular organs serving as “windows to the brain”.