Effects of steroids on beta-adrenergic binding sites in sheep pineal glands

As an initial step in investigations of putative differences between central nervous system light-sensitive mechanisms in seasonally shedding and non-shedding breeds of sheep, some beta-adrenoceptor characteristics of Merino sheep pineal glands were determined, using [3H]dihydroalprenolol as the labelled ligand. Overall, a dissociation constant of 17.2 +/- 2.6 nmoles/l and a daytime beta-receptor density of 1.6 +/- 0.3 pmoles/mg were determined at 37 degrees. The binding sites exhibited stereospecificity, saturability and apparent homogeneity. 17 beta-Estradiol and progesterone implants that provided hormone concentrations in the physiological range had no significant effect on pineal beta-receptors in male sheep castrated shortly after birth. Dexamethasone injections, on the other hand, in doses sufficient to loosen the attachment of wool fibres to the skin, resulted in decreased pineal beta-receptor density and increased receptor affinity for dihydroalprenolol. This effect was apparently not mediated by altered plasma catecholamine concentrations, since the glucocorticoid treatment did not affect jugular venous noradrenaline, adrenaline or dopamine levels. The possible involvement of glucocorticoids in the regulation of wool growth could thus have a central neuronal component, medicated via action on pineal beta-adrenoceptors in sheep; however, the existence of the putative gonadal steroid feedback on beta-adrenoceptor-mediated pineal function remains to be demonstrated in this species.