Characterization of a new radioiodinated probe for the alpha sub(2C) adrenoceptor in the mouse brain

[ super(125)I]17 alpha -hydroxy-20 alpha -yohimban-16 beta -(N-4-hydroxyphenethyl)carboxami de or [ super(125)I]rauwolscine-OHPC, a new radioiodinated probe derived from rauwolscine was synthesized and its binding characteristics investigated on sections of the mouse caudate putamen. [ super(125)I]rauwolscine-OHPC binding was saturable and revealed interaction with a single class of binding sites (K sub(D) = 0.171 nM, B sub(max) = 3082 pCi/mg of tissue). The kinetically derived affinity was in close agreement with the affinity evaluated by saturation experiments: k sub(-1)/k sub(+1) (0.0403 min super(-1)/114 10 super(6) M super(-1)min super(-1)) = 0.35 nM. Competition studies revealed interaction with one single class of binding sites for each of the twelve compounds tested. The rank of potency suggested an interaction with alpha sub(2) adrenoceptors (atipamezole greater than or equal to RX 821002 > yohimbine > (-)epinephrine). Moreover, the good affinity of [ super(125)I] rauwolscine-OHPC binding sites for spiroxatrine, yohimbine, WB 4101, the relatively good affinity for prazosin (K sub(i) = 37.4 nM) and the affinity ratio prazosin/oxymetazoline (37.4/43.4 = 0.86) were consistent with an alpha sub(2C) selective labelling of [ super(125)I]rauwolscine-OHPC. The distribution of [ super(125)I]rauwolscine-OHPC binding sites in mouse brain was characterized by autoradiography. The density of binding sites was high in the islands of Calleja, accumbens nucleus, caudate putamen and olfactory tubercles, moderate in the hippocampus, amygdala and anterodorsal nucleus of the thalamus. These findings demonstrated that [ super(125)I]rauwolscine-OHPC is a useful radioiodinated probe to label alpha sub(2C) adrenoceptors in mouse brain.