Binding of (+/-)-3H-lofexidine to alpha-adrenoceptors in membranes from rat brain

The binding of (+/-)-3H-lofexidine (spec. act. 26.8 Ci/mmol) to rat brain membranes was studied. The specific binding of (+/-)-3H-lofexidine, defined as the excess over blanks containing excess (-)-noradrenaline was a saturable process. Scatchard analysis indicated a single class of binding sites (KD = 5.5 nM). The maximal number of binding sites amounted to 280 fmol/mg protein, comparable to the number occupied by 3H-clonidine and 3H-guanfacine. Specific binding reached equilibrium by about 5 min and dissociated in a biphasic manner. A rapid and a slow component of dissociation was found. The specific binding sites identified by (+/-)-3H-lofexidine in rat brain tissue are to be classified as alpha 2-adrenoceptors. Selective agonists as well as antagonists of alpha 1-adrenoceptors were weak displacers of the binding, whereas preferential stimulants as well as blocking drugs of alpha 2-adrenoceptors displayed high affinity for the (+/-)-3H-lofexidine sites. The relative affinity of various drugs for the sites labeled by (+/-)-3H-lofexidine perfectly corresponded with that for the specific binding sites identified by 3H-clonidine and 3H-guanfacine. This study shows the usefulness of (+/-)-3H-lofexidine as an additional radioligand for a direct characterization of alpha 2-adrenoceptors in brain tissue. It is concluded that (+/-)-3H-lofexidine labels the same population of alpha 2-adrenoceptors in rat brain as accessible to 3H-clonidine and 3H-guanfacine.