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Notes: Summary Dopamine receptors subtypes were studied in homogenates from rat brain areas, mainly the corpus striatum, using the two highly selective ligands 3H-apomorphine and 3H-domperidone. The clearly biphasic inhibition of the specific binding of these two ligands by some agents allowed us to define four distinct classes of binding site. 3H-apomorphine labels two classes of site displaying a large difference in affinity for domperidone, i.e. class I sites well recognized (IC50=5 nM) and class II sites poorly recognized (IC50=10 μM). 3H-domperidone also labels two distinet classes of site displaying a large difference in affinity for apomorphine and dopamine, i.e. class III sites well recognized by these agents (IC50=5 and 35 nM, respectively) and class IV sites poorly recognized (IC50=790 nM and 14 μM, respectively). The two classes I and III represent a single pharmacological class of dopaminergic receptors (labelled by either 3H-apomorphine or 3H-domperidone) as indicated by 1) their almost identical pharmacological specificities (high correlation between K d or K i values for a variety of dopaminergic agonists and antagonists); 2) their similar capacity in striatum as well as in other brain regions; 3) the identical decrease in capacity following kainate lesions; 4) their similar sensitivity to GTP and thermal denaturation. Because the pharmacological specificity of these sites excludes the possibility that they represent the recognition sites of the dopamine-sensitive adenylate cyclase, i.e. D-1 receptors, we propose to term them D-2 receptors. Class II and IV sites also differ from D-1 receptors as shown by drug specificity and the effect of kainate. We propose to term class II sites D-3 receptors and class IV sites D-4 receptors. D-2 receptors are characterised by a high affinity for both dopamine receptor agonists and antagonists (K i and K d values in the nM range). They are localised post-synaptically to dopaminergic terminals in the striatum as indicated by 1) their decreased number (−60%) following kainate lesions of intrinsic neurones, and 2) their increased number (+40%) after 6-OHDA-induced degeneration of dopaminergic neurones. The capacity of D-2 receptors is decreased by 80% in the presence of 25 μM GTP. The binding of ligands to D-2 receptors preicubated at 45°C decreases with a half-life of 10 min. D-2 receptors may mediate behavioral actions of apomorphine in low dosage which are easily antagonised by neuroleptics. D-3 receptors appear to be, at least in part, autoreceptors: their number decreases in striatum after 6-OHDA lesions (−30%) and is not modified following kainate lesions. They are characterised by a high affinity (K i in the nM range) for dopaminergic agonists (except for bromocriptine) contrasting with a rather low affinity for antagonists. The pharmacologically homogeneous class of D-3 receptor appears heterogeneous regarding both localisation and regulation by GTP. D-4 receptors are partly localised on intrastriatal neurones (−17% after kainate lesions, +17% following 6-OHDA lesions). However, the small change after kainate-induced lesions suggests that a significant fraction of D-4 receptors is localised on terminals from extrinsic neurones. D-4 receptors are characterised by a high affinity for dopamine receptor antagonists (K i in the nM range) contrasting with a relatively low affinity for agonists. The number of D-4 receptors increases after either GTP or heat denaturation, a change which probably corresponds to the decrease in D-2 receptors. D-4 receptors may mediate typical behavioral actions of apomorphine in moderate dosage.

Notes: Summary The labelling of rat cerebral cortex α2-adrenoceptors with [3H]-yohimbine ([3H]-YOH) was investigated. At 25° C, binding equilibrium was reached in about 10 min and dissociation occurred with a half time of about 1 min. Saturation experiments gave an equilibrium K D value of 10.13±1.95 nM and a maximum number of sites of 254±22 fmol/mg protein. The [3H]-YOH binding sites exhibited α2-adrenergic receptor specificity; the order of potency for the antagonists was rauwolscine 〉 yohimbine ≫ prazosin 〉 corynanthine. For the agonists, the order was: oxymetazoline 〉 clonidine 〉 (−)-adrenaline 〉 (−)-noradrenaline ≫ (−)-phenylephrine. Agonists exhibited shallow curves in inhibiting [3H]-YOH binding, with pseudo-Hill coefficients (nH) of less than 1.0. These curves were shifted to lower overall affinity and steepened in the presence of 100 μM GTP. Antagonist competition curves were also shallow but GTP had no significant effect. Divalent cations at millimolar concentrations decreased the [3H]-YOH binding: IC50 values were about 6.0, 6.8 and 0.3 mM for Ca2+, Mg2+ and Mn2+ respectively. The maximal number of [3H]-YOH binding sites in the cortex was close to that labelled by the agonist [3H]-paraaminoclonidine ([3H]-PAC). The regional distribution of these sites in the brain, examined at a single concentration of [3H]-YOH and [3H]-PAC, showed a similar pattern except in the striatum. Taken together, the results indicate that like [3H]-PAC, [3H]-YOH labels α2-adrenoceptors in rat brain cortex. They also show that [3H]-YOH is a useful tool for the study of the high and low affinity sites.