Further pharmacological characterization of [ 3H]idazoxan binding sites in rat brain: evidence for predominant labeling of α2-adrenergic receptors

Abstract

In addition to binding to α 2-adrenergic receptors, the antagonist [ 3H]idazoxan has been reported to bind to non-adrenergic sites in a number of tissues and species. In the present study, the pharmacological nature of [ 3H]idazoxan binding sites in rat brain slices has been examined using radioligand binding and autoradiographic technniques. In Na 2KHPO 4 buffer, four drugs with high affinity for α 2-adrenergic binding sites were potent inhibitors of [ 3H]idazoxan binding, with the rank order of potency being RX821002 > phentolamine > yohimbine > (-)epinephrine. Non-linear regression analysis resolved all competition curves into two components, with the high affinity site representing the majority of total [ 3H]idazoxan binding. In autoradiographic studies performed in Na 2KHPO 4 buffer, all α 2-selective ligands displaced >/75% of total [ 3H]idazoxan binding to most brain regions. These findings indicate that the major component of [ 3H]idazoxan binding was to sites that are α 2-adrenergic in nature. [ 3H]Idazoxan binding was also examined in glycylglycine buffer. In contrast to binding in Na 2KHPO 4 buffer, the proportion of low affinity sites was significantly increased in glycylcine buffer. Autoradiographic studies confirmed these findings. These pharmacological data are consistent with our previously reported conclusions that, under appropriate assay conditions. [ 3H]idazoxan predominantly labels α 2-adrenergic binding sites in rat brain. These sites are widely distributed and have pharmacological characteristics consistent with those previously reported for α 2A-adrenergic receptors.