Effect of Noradrenergic Lesions on Subtypes of α2‐Adrenoceptors in Rat Brain

The binding of [3H]rauwolscine to alpha 2A- (also referred to as alpha 2D-) and alpha 2C-adrenoceptors in homogenates of rat cerebral cortex was measured by exploiting the selectivity of oxymetazoline for alpha 2A-adrenoceptors. Inhibition of [3H]rauwolscine binding by oxymetazoline was modeled best assuming binding to two sites (p < 0.001). Competition curves for oxymetazoline were shifted rightward by the addition of GTP (250 microM) but were still fit best by a two-site model (p < 0.001). A concentration of oxymetazoline was calculated that would optimally antagonize [3H]rauwolscine binding (with GTP present) to oxymetazoline-sensitive alpha 2A-adrenoceptors, minimally inhibiting binding to alpha 2C-adrenoceptors. Subsequently, [3H]rauwolscine binding to alpha 2A- and alpha 2C-adrenoceptors in cortex was examined 3 weeks after destruction of noradrenergic terminals. Binding to alpha 2C-adrenoceptors was increased significantly after treatment with 6-hydroxydopamine, (6-OHDA) compared with vehicle-treated controls, whereas binding to alpha 2A-adrenoceptors was unchanged. Pretreatment of rats with desipramine before 6-hydroxydopamine, to protect noradrenergic neurons, resulted in no changes in binding to either alpha 2A- or alpha 2C-adrenoceptors. Thus, alpha 2C-adrenoceptors are regulated by changes in synaptic availability of norepinephrine. alpha 2A-Adrenoceptors are either not regulated by synaptic norepinephrine or are located both post- and presynaptically so that up-regulation of postsynaptic alpha 2A-adrenoceptors is offset by a loss of presynaptic alpha 2A-adrenoceptors.