Autoradiographic study of 14C-sulpiride in monkey

Effects of typical and atypical antipsychotic drugs on two-way active avoidance. Relationship to DA receptor blocking profile

The dose-dependent and time-dependent effects of the novel antipsychotic compound remoxipride, as well as the reference compounds chlorpromazine, clozapine, haloperidol, pimozide and sulpiride upon the retention of two-way active avoidance (conditioned avoidance responses, CARs) were studied in male rats. The dose-dependent effects of remoxipride as well as haloperidol and chlorpromazine on the acquisition of CARs were also studied. The acquisition and retention of CARs were tested in shuttleboxes using a 1.0-mA shock intensity and a 10-stone signal (1000 Hz). All the compounds studied, including remoxipride, caused a dose-dependent impairment of acquisition and retention of CARs. The effect of remoxipride on CAR acquisition correlated with remoxipride's effectiveness to block the hyperactivity induced by the dopamine (DA) agonist apomorphine. Unlike chlorpromazine and haloperidol, the potency of remoxipride and clozapine for antagonising CAR retention was found at dose levels much lower than those producing cataleptic effects or blocking apomorphine-induced stereotypies. Based on the DA receptor blocking profile and the relative effectiveness to block CAR it is concluded that the mechanism(s) by which clozapine and remoxipride affect CAR differ from typical neuroleptic drugs. This difference may reflect an action upon different subtypes of functionally coupled DA D2 receptors.

Regional distribution and in vivo binding of the atypical antipsychotic drug remoxipride. A biochemical and autoradiographic analysis in the rat brain

The regional brain distribution and binding of the antipsychotic benzamide drug remoxipride was studied in the male rat. After i.v. injections of 3H-remoxipride (1 mumol.kg-1) more than 85% of the radioactivity was identified as authentic remoxipride in brain by using reversed-phase liquid chromatography. Autoradiographic and spectroscopic analysis showed that 3H-remoxipride was distributed relatively even in different brain areas, with exception of the following structures, which showed highest drug concentrations: the choroid plexus, septum, medial part of the caudate nucleus, different areas of the thalamus and hypothalamus situated close to the cerebral ventricles. A closer analysis of the autoradiograms showed a gradient of radioactivity extending from the cerebral ventricles to the deeper parts of the brain at 30 minutes after injections. After 60 minutes radioactivity was detected throughout all forebrain dopamine receptive areas. These findings suggest that remoxipride enters the cerebrospinal fluid (CSF) via the vascular bed of the choroid plexus and that it enters the brain interstitial fluid from the CSF. In the caudate nucleus, nucleus accumbens, olfactory tubercle and olfactory bulb 30-40% of the radioactivity was reduced by pretreatment with the dopamine D-2 selective drug raclopride. In addition, small, but significant, reductions (10-15%) of 3H-remoxipride derived radioactivity was found in the neocortex, hippocampus and the cerebellum, suggesting that remoxipride interacts with a D-2 receptor also in these cortical structures. Taken together, these studies show that after i.v. injections, 3H-remoxipride enters the brain primarily in unmetabolized form when given in doses that affect DA receptor mediated behaviours, that it distributes to most areas throughout the neuraxis and that it binds to D-2 receptors in different parts of the basal ganglia, neocortex, hippocampus and cerebellum.

Biochemical pharmacology of the atypical neuroleptic remoxipride

In vitro receptor ligand binding studies in the rat showed that remoxipride displaced different radioligands at the dopamine D2, but not the D1 receptor. Remoxipride did not block dopamine-stimulated adenylate cyclase activity in vitro suggesting that it did not directly interact with the dopamine D1 receptor. Like other antipsychotic compounds, it increased dopamine turnover in the dopamine-rich areas of the brain. It showed no affinity for a wide range of neurotransmitter receptors, with the exception of the opiate sigma receptor. The affinity of remoxipride for the D2 receptor was low in vitro, while in vivo, the affinity was relatively high. Remoxipride was far more potent in preventing [3H]raclopride-binding than [3H]spiperone-binding to the D2 receptor in vivo. When the D2 receptor was labelled with [3H]spiperone, remoxipride was shown to exert a preferential blockade of this binding in extrastriatal areas of the brain (for example, olfactory tubercle, septum, substantia nigra) in vivo. After the injection of high doses of remoxipride most if not all drug in the brain was identified as authentic remoxipride. After injection of [3H]remoxipride in smaller and larger doses, radioactivity was detected in all areas of brain examined, including cerebellum and neocortex. Most of the remoxipride-derived radioactivity was found in the choroid plexus and circumventricular organs, while smaller amounts were recovered in the striatum, olfactory tubercle, and substantia nigra. Taken together, these findings suggest that remoxipride acts at both the central D2 and sigma receptors and that its affinity for the D2 receptor is relatively low in vitro. A regional preference for D2 receptors can be observed in vivo depending upon the radioligand used.

In vivo labelling of rat brain dopamine D-2 receptors. Stereoselective blockade by the D-2 antagonist raclopride and its enantiomer of 3H-spiperone, 3H-N,N-propylnorapomorphine and 3H-raclopride binding in the rat brain

The stereospecific blockade by raclopride and FLB472 (the R enantiomer of raclopride) of the specific in vivo binding of [3H]-spiperone, [3H]-N,N-propylnorapomorphine (NPA) and [3H]-raclopride was studied in seven brain regions (e.g., caudate nucleus, olfactory tubercle, septum, hippocampus, frontal cortex, substantia nigra, pituitary gland) of the male albino rat. The binding of all three ligands was dose-dependently blocked by raclopride and FLB472. The blockade by FLB472 occurred at doses 50-100 times higher than that obtained by raclopride. The maximal blockade by raclopride of [3H]-spiperone binding differed between brain areas. Thus, the largest blockade was obtained in the substantia nigra (95%), septum (90%), caudate nucleus (60%) and olfactory tubercle (60%), while the blockade of [3H]-spiperone binding in the frontal cortex and pituitary gland did not exceed 30% and 50%, respectively. In contrast to [3H]-spiperone, the in vivo binding of [3H]-NPA and [3H]-raclopride was prevented by 90-100% in all brain areas examined. Taken together, the present findings indicate that the in vivo binding of three radioactive ligands to a central dopamine D-2 receptor can be stereoselectively blocked by the enantiomers of raclopride. The findings suggest that, under in vivo conditions, [3H]-raclopride and [3H]-NPA may label a closely related receptor site. However only some of the [3H]-spiperone binding sites may be identical to the [3H]-raclopride binding sites. The findings indicate furthermore that the relative overlap of D-2 sites shared by [3H]-spiperone and [3H]-raclopride may vary between brain regions.