Central administration of 1-isoproterenol in vivo induces a preferential regulation of β₂-adrenoceptors in the central nervous system of the rat

1-Isoproterenol has equal affinity for β₁- and β₂-adrenoceptors and is a full agonist at both subtypes. However, when infused in vivo into the rat brain, it has been shown to induce a preferential reduction of central β₂-adrenoceptors. To investigate this phenomenon further, in the present study rats were infused centrally with higher doses of 1-isoproterenol (15 or 45 μg/h). Furthermore, isoproterenol was infused into rats lesioned neonatally with 6-hydroxydopamine (6-OHDA). Subtypes of β-adrenoceptors were measured by quantitative autoradiography of the binding of [¹²⁵I]iodopindolol ([¹²⁵I]IPIN). In sham lesioned rats, infusions of isoproterenol at both doses caused comparable reductions in the density of [¹²⁵I]IPIN binding sites in many brain regions. The binding to β²-adrenoceptors was decreased in a larger number of brain areas than the binding to β₁-adrenoceptors and the magnitude of the reduction was greater for β₂- than for β₂-adrenoceptors. However, isoproterenol at these doses did produce greater effects on the β₁-subtype than those found previously with a lower dose. Treatment with 6-OHDA induced significant increases in the binding of [¹²⁵I]IPIN to both β₁- and β₂-adrenoceptors in cerebral cortical and hippocampal areas, indicating that endogenous norepinephrine may regulate both subtypes in these regions Even in the 6-OHDA-lesioned rats, the binding of [¹²⁵I]IPIN to β₂-adrenoceptors was reduced to a greater extent that the binding to β₁-adrenoceptors. Thus, these studies demonstrate that the non-selective β-adrenergic agonist isoproterenol induces a preferential regulation of β₂-adrenoceptors, even at relatively high doses and in norepinephrine-depleted animals.