In vivo labelling of pituitary dopamine D-2 receptors in the male rat using [3H]-raclopride

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Abstract
The substituted benzamide drug [3H]-raclopride (Köhler et al., 1985) was used to label dopamine D-2 receptors within the individual lobes of the pituitary gland as well as in the brain of male rats in vivo. The in vivo [3H]-raclopride binding was found to be saturable, reversible and of high specificity. Between 5–30% of the binding was non-specific at saturating concentrations dependent upon the lobe of the pituitary gland as well as of the brain region (e.g., caudate nucleus and olfactory tubercle) studied. Saturation analyses revealed Bmax-values of 12.9±1.6 and 2.2±0.9pmol·g−1 wet weight in the intermediate and anterior lobes, respectively with respective KD values of 6.5±4.6 and 7.3±2.4 nmol·kg−1. Quantitative autoradiographic studies using a single concentration of [3H]-raclopride showed a similar relationship with regard to binding densities in the different lobes, and showed, in addition, that the posterior lobe contained the lowest number of specific [3H]-raclopride binding sites. The binding capacities and affinities of binding were 12.9±1.7 and 9.2±2.8 respectively in the caudate nucleus and 6.1±0.7 and 9.3±2.7 respectively in the olfactory tubercle. The pharmacological analysis revealed that (S)sulpiride, remoxipride and raclopride were 10 to 125 times more potent than their corresponding isomers [(R)sulpridie, FLA 731(−), and FLB 472, respectively] in blocking the in vivo [3H]raclopride binding in the pituitary gland as well as in brain. The in vivo potency of different D-2 antagonists in preventing the [3H]-raclopride binding in the anterior and intermediate lobes was: spiperone > domperidone > raclopride > (S)sulpiride > remoxipride. The D-1 selective antagonist SCH 23390 did not block the in vivo binding of [3H]-raclopride neither in the pituitary lobes nor in the brain. In agreement with these findings the D-2 agonists N,N-propylnorapomorphine and quinpirole (LY 171555) but not the D-1 agonist SKF 38393-A blocked the specific in vivo [3H]-raclopride binding in the pituitary gland as well as in the brain. Comparisons between the relative potencies of different drugs in blocking pituitary and brain D-2 receptors in vivo showed that some drugs, including sulpiride and domperidone, were more potent in the pituitary gland than in the brain, while remoxipride and raclopride were equipotent in the two areas. The D-2 agonists tested appeared to be slightly more potent in the brain than in the pituitary gland.

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