Agonist-Induced Regulation of Adrenoceptor Subtypes in Cerebral Cortical Slices

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Abstract
Cerebral cortical slices from rat brain were incubated at 37.degree. C for 2 h in the presence of isoproterenol, noradrenaline or adrenaline, and binding affinities and densities of adrenoceptor subtypes were subsequently examined in homogenized tissue. The density of .alpha.2- and total .beta.-adrenoceptors was estimated using the radioligands [3H]rauwolscine and [3H]dihydroalprenolol (DHA), respectively. The percentages of .beta.1-and .beta.2-adrenoceptors were defined by inhibiting the binding of [3H]DHA with the .beta.1-selective antagonist metoprolol. Exposure of slices to noradrenaline and adrenaline significantly decreased the maximal number of binding sites (Bmax) of .alpha.2-adrenoceptors (48 and 37% respectively) without significantly affecting affinity; isoproterenol had no effect. Exposure to isoproterenol, noradrenaline, and adrenaline significantly decreased the Bmax of .beta.-adrenoceptors (by 60, 34, and 24%, respectively) but did not affect the affinity. Isoproterenol and adrenaline significantly decreased the density of .beta.1-adrenoceptors by 75 and 24% and .beta.2-adrenoceptors by 23 and 28%, respectively. Noradrenaline significantly decreased the density of .beta.1-adrenoceptors by 42% without affecting the number of .beta.2-adrenoceptors. These findings indicate that subtypes of adrenoceptors in rat cerebral cortex are differentially regulated by adrenergic agonists.