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Notes: Summary The in vivo accumulation of 3H-N-propyl norapomorphine in mouse striatum and tuberculum olfactorium and its inhibition by a series of classical neuroleptics and discriminant benzamide derivatives previously identified in behavioural and radioligand experiments has been studied. The ID50 values in the two brain areas did not significantly differ with any studied compound. In addition the regional distribution of a discriminant compound related to sulpiride and administered in tritiated form to rats was rather homogeneous. These data do not indicate a preferential accumulation of these compounds in limbic as opposed to striatal areas.

Notes: Summary In order to document the hypothesis that anti-psychotics may interact with more than one class of cerebral dopamine receptors, the relative potencies of a series of compounds were compared in three behavioral tests and in binding studies with two radioligands. Apomorphine (0.6 mg/kg) simultaneously clicited in rat two kinds of facial stereotypies (sniffing and licking) and a stereotyped climbing behavior, allowing to compare in the same animals the relative potencies of various antipsychotics against the three behaviors. Only some substituted benzamides (Sulpiride, LUR 2366 and DAN 2163) antagonised at significantly lower dosages climbing than sniffing (or licking). The possibility that this discriminant potency might be related to a distinct affinity for two classes of dopamine receptors was investigated by binding studies on striatal membranes with 3H-apomorphine and 3H-domperidone. From lesion and subcellular fractionation studies, two classes of binding sites both labeled with 3H-domperidone but distinguished by apomorphine i.e. D-2 sites with nM affinity and D-4 sites with μM affinity for the dopamine agonist (according to the nomenclature of Sokoloff et al. 1980b) appear to be differently localised in striatum. Thus D-2 sites, whose number decreases after kainate lesion, are not significantly modified following cortical lesions and preferentially sediment with heavy primary subcellular fractions. In contrast D-4 sites, less affected by kainate lesions, are significantly decreased following cortical lesions (−30%) and preferentially sediment with the light subcellular fractions. In addition the apparently heterogenous recognition of total 3H-domperidone binding sites (i.e. the sum of D-2 and D-4 sites) by dopamine and apomorphine persists in the presence of guanosine-5′-triphosphate (pseudo-Hill coefficient of 0.60 instead of 0.55). This suggests that D-2 and D-4 sites cannot be considered as two discrete states of the same receptor strictly convertible one into the other by guanylnucleotides. Whereas most dopamine antagonists inhibited D-2 and D-4 site binding with similar affinities, the three benzamide derivatives with the largest selectivity in behavioral tests displayed 2–3-fold higher affinity for D-4 than for D-2 sites and the ratios of ID50 values of the whole series of antagonists against sniffing (or licking) and climbing behaviors were correlated (P〈0.01) with the ratios of K i values regarding D-2 and D-4 site binding. Also, sulpiride and LUR 2366 unlike haloperidol and metoclopramide, inhibited the total specific 3H-domperidone binding in a biphasic manner. However, the distinction by sulpiride and LUR 2366 of low and high affinity sites did not superimpose that of D-2 and D-4 sites, as distinguished by agonists. In this test the relative proportion of low affinity sites was 2-fold higher than that of D-2 sites and K i values for high affinity sites were lower than that for D-4 sites. Also the heterogeneity of 3H-domperidone sites regarding affinity of LUR 2366 persisted in the presence of low concentrations of apomorphine. Hence low affinity sites for discriminant benzamide derivatives may exist in two forms, distinguished by agonists and possibly interconvertible by GTP. Thus the hypothesis that two classes of central dopamine receptor can be distinguished by some substituted benzamides, but perhaps display no great difference in affinity of agonists in their physiological state, fits partic-ularly well with behavioral data.