Adjunctive alpha2-adrenoceptor blockade enhances the antipsychotic-like effect of risperidone and facilitates cortical dopaminergic and glutamatergic, NMDA receptor-mediated transmission.

Compared to both first- and second-generation antipsychotic drugs (APDs), clozapine shows superior efficacy in treatment-resistant schizophrenia. In contrast to most APDs clozapine possesses high affinity for alpha2-adrenoceptors, and clinical and preclinical studies provide evidence that the alpha2-adrenoceptor antagonist idazoxan enhances the antipsychotic efficacy of typical D2 receptor antagonists as well as olanzapine. Risperidone has lower affinity for alpha2-adrenoceptors than clozapine but higher than most other APDs. Here we examined, in rats, the effects of adding idazoxan to risperidone on antipsychotic effect using the conditioned avoidance response (CAR) test, extrapyramidal side-effect (EPS) liability using the catalepsy test, brain dopamine efflux using in-vivo microdialysis in freely moving animals, cortical N-methyl-D-aspartate (NMDA) receptor-mediated transmission using intracellular electrophysiological recording in vitro, and ex-vivo autoradiography to assess the in-vivo alpha2A- and alpha2C-adrenoceptor occupancies by risperidone. The dose of risperidone needed for antipsychotic effect in the CAR test was approximately 0.4 mg/kg, which produced 11% and 17% in-vivo receptor occupancy at alpha2A- and alpha2C-adrenoceptors, respectively. Addition of idazoxan (1.5 mg/kg) to a low dose of risperidone (0.25 mg/kg) enhanced the suppression of CAR, but did not enhance catalepsy. Both cortical dopamine release and NMDA receptor-mediated responses were enhanced. These data propose that the therapeutic effect of risperidone in schizophrenia can be enhanced and its EPS liability reduced by adjunctive treatment with an alpha2-adrenoceptor antagonist, and generally support the notion that the potent alpha2-adrenoceptor antagonistic action of clozapine may be highly important for its unique efficacy in schizophrenia.

Asenapine, a novel psychopharmacologic agent: preclinical evidence for clinical effects in schizophrenia.

RATIONALE: Asenapine is a novel psychopharmacologic agent being developed for the treatment of schizophrenia and bipolar disorder. MATERIALS AND METHODS: The present study was undertaken to investigate the effects of asenapine using animal models predictive of antipsychotic efficacy (conditioned avoidance response [CAR]) and extrapyramidal side effects (EPS; catalepsy). In parallel, the effects of asenapine on regional dopamine output using in vivo microdialysis in freely moving rats, dopamine output in the core and shell subregions of nucleus accumbens (NAc) using in vivo voltammetry in anesthetized rats, and N-methyl-D: -aspartate (NMDA)-induced currents in pyramidal neurons of the medial prefrontal cortex (mPFC) using the electrophysiological technique intracellular recording in vitro were assessed. RESULTS: Asenapine (0.05-0.2 mg/kg, subcutaneous [s.c.]) induced a dose-dependent suppression of CAR (no escape failures recorded) and did not induce catalepsy. Asenapine (0.05-0.2 mg/kg, s.c.) increased dopamine efflux in both the mPFC and the NAc. Low-dose asenapine (0.01 mg/kg, intravenous [i.v.]) increased dopamine efflux preferentially in the shell compared to the core of NAc, whereas at a higher dose (0.05 mg/kg, i.v.), the difference disappeared. Finally, like clozapine (100 nM), but at a considerably lower concentration (5 nM), asenapine significantly potentiated the NMDA-induced responses in pyramidal cells of the mPFC. CONCLUSIONS: These preclinical data suggest that asenapine may exhibit highly potent antipsychotic activity with very low EPS liability. Its ability to increase both dopaminergic and glutamatergic activity in rat mPFC suggests that asenapine may possess an advantageous effect not only on positive symptoms in patients with schizophrenia, but also on negative and cognitive symptoms.

Galantamine enhances dopaminergic neurotransmission in vivo via allosteric potentiation of nicotinic acetylcholine receptors.

Clinical studies suggest that adjunct galantamine may improve negative and cognitive symptoms in schizophrenia. These symptoms may be related to impaired dopaminergic function in the prefrontal cortex. Indeed, galantamine has been shown to increase dopamine release in vitro. Galantamine is an allosteric modulator of nicotinic acetylcholine receptors (nAChRs) and, at higher doses, an acetylcholine esterase (AChE) inhibitor. We have previously shown that nicotine, through stimulation of nAChRs in the ventral tegmental area (VTA), activates midbrain dopamine neurons and, hence, potentiation of these receptors could be an additional mechanism by which galantamine can activate dopaminergic pathways. Therefore, the effects of galantamine (0.01-1.0 mg/kg s.c.) on dopamine cell firing were tested in anaesthetized rats. Already at a low dose, unlikely to result in significant AchE inhibition, galantamine increased firing activity of dopaminergic cells in the VTA. The effect of galantamine was prevented by the nAChR antagonist mecamylamine (1.0 mg/kg s.c.), but not the muscarinic receptor antagonist scopolamine (0.1 mg/kg s.c.), and it was not mimicked by the selective AChE inhibitor donepezil (1.0 mg/kg s.c.). Our data thus indicate that galantamine increases dopaminergic activity through allosteric potentiation of nAChRs. Galantamine's effect was also prevented by the alpha7 nAChR antagonist methyllycaconitine (6.0 mg/kg i.p.) as well as the N-methyl-D-aspartate antagonist CGP39551 (2.5 mg/kg s.c.), indicating a mechanism involving presynaptic facilitation of glutamate release. In parallel microdialysis experiments, galantamine was found to increase extracellular levels of dopamine in the medial prefrontal cortex. These results may have bearing on the enhancement of negative and cognitive symptoms in schizophrenia.

Noradrenaline reuptake inhibition enhances the antipsychotic-like effect of raclopride and potentiates D2-blockage-induced dopamine release in the medial prefrontal cortex of the rat.

We have previously observed that addition of an alpha(2)-adrenoceptor antagonist to a selective dopamine (DA) D(2) receptor antagonist enhances the antipsychotic-like effect of the D(2) blocker and also selectively increases DA output in the medial prefrontal cortex (mPFC) in rats. These data also correlate well with previous clinical trials showing augmentation by an equivalent drug combination in schizophrenia. Since the selective noradrenaline reuptake inhibitor reboxetine was found to cause similar effects on the mesolimbocortical DA system as alpha(2)-adrenoceptor antagonists, the present study was undertaken to explore whether also reboxetine might augment the effect of the DA D(2) receptor antagonist raclopride in the same preclinical model of antipsychotic activity, the conditioned avoidance response (CAR) test. We also investigated the effect of this combination in the catalepsy test for extrapyramidal side effect liability, as well as on DA output in the mPFC and the nucleus accumbens, respectively. Reboxetine (6 mg/kg, i.p.) significantly enhanced the suppressant effect of raclopride (0.1 mg/kg, s.c.) on CAR without affecting catalepsy. Administration of raclopride (0.1 mg/kg, s.c.) to rats pretreated with reboxetine (6 mg/kg, i.p.) resulted in a greatly enhanced effect on DA output in the mPFC but not in the nucleus accumbens when compared with raclopride alone. Consequently, these results suggest that noradrenaline reuptake inhibition may provide means of augmenting the efficacy of classical D(2)-antagonists in the treatment of schizophrenia, and, in principle, to generate an atypical antipsychotic drug profile.

Adjunctive galantamine, but not donepezil, enhances the antipsychotic-like effect of raclopride in rats.

Acetylcholine (ACh) esterase inhibitors like galantamine and donepezil have been tested as adjunct treatment in schizophrenia. Although ACh esterase inhibition might confer some antipsychotic activity, the role of allosteric potentiation of nicotinic ACh receptors (nAChRs), which is an additional mechanism of galantamine, remains elusive. Therefore, the potential antipsychotic-like effects of galantamine and donepezil, respectively, alone, and in combination with the dopamine D2/3 receptor antagonist, raclopride, were tested in the conditioned avoidance response (CAR) test and extrapyramidal side-effect liability was assessed with the catalepsy test. Neither galantamine nor donepezil alone suppressed CAR selectively. Galantamine, but not donepezil, enhanced the raclopride-induced suppression of CAR, predicting augmentation of antipsychotic activity. In contrast to donepezil, galantamine did not increase catalepsy, alone or combined with raclopride. These data suggest that allosteric potentiation of nAChRs may mediate the antipsychotic-like effect of adjunctive galantamine and provide support for the development of alpha7 nAChR-selective allosteric potentiators for schizophrenia.

Enhanced efficacy of both typical and atypical antipsychotic drugs by adjunctive alpha2 adrenoceptor blockade: experimental evidence.

Adjunctive treatment with the selective alpha2 adrenoceptor antagonist idazoxan augments the effect of conventional antipsychotics in treatment-resistant schizophrenics comparing favourably with clozapine. Clozapine has high affinity for alpha2 adrenoceptors. Previously, we found that adjunctive idazoxan treatment to the dopamine (DA) D2/3 antagonist raclopride enhanced raclopride-induced effects in an animal model of antipsychotic activity (conditioned avoidance response, CAR) and, similarly to clozapine, reversed the disruption of working memory induced by N-methyl-D-aspartate receptor blockade in rats with a concomitant increase in prefrontal DA efflux. To further investigate the significance of alpha2 adrenoceptor affinity for antipsychotic efficacy, we here investigated, in rats, the effects of adjunctive idazoxan treatment to low doses of a typical (haloperidol) and an atypical (olanzapine) antipsychotic drug, both lacking appreciable alpha2 adrenoceptor affinity, on (i) CAR; (ii) catalepsy; and (iii) DA output in the prefrontal cortex and the nucleus accumbens using microdialysis. Adjunctive treatment with idazoxan to haloperidol or olanzapine enhanced suppression of CAR to a level predicting sufficient antipsychotic activity, increased DA output preferentially in the prefrontal cortex, and reversed haloperidol-induced catalepsy. Our data confirm and extend our previous findings as well as clinical observations, and suggest that adjunctive alpha2 adrenoceptor blockade both typical and atypical antipsychotic drugs, lacking appreciable affinity for the alpha2 adrenoceptor, may contribute to a more advantageous therapeutical profile of these drugs in schizophrenia treatment, allowing for reduced DA D2 occupancy and reduction of unwanted side-effects.

Adjunctive treatment with mianserin enhances effects of raclopride on cortical dopamine output and, in parallel, its antipsychotic-like effect.

Clinical studies indicate that adjunctive treatment with the antidepressant drug mianserin, a 5-hydroxytryptamine (5-HT)(2A/C) receptor antagonist and an alpha(2)- and alpha(1)-adrenoceptor antagonist, may enhance the effect of conventional antipsychotic drugs in schizophrenia, in particular on negative symptoms such as withdrawal retardation, akathisia, and some aspects of cognitive impairment. Here, we have examined the effect of mianserin in combination with the selective dopamine (DA) D(2/3) receptor antagonist raclopride on conditioned avoidance response (CAR), a preclinical test of antipsychotic efficacy with high predictive validity; catalepsy, a preclinical test of extrapyramidal side effect liability; and DA output in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAC), respectively. Mianserin (5 mg/kg intraperitoneal) significantly enhanced the suppressant effect of a low dose of raclopride (0.1 mg/kg subcutaneous) on CAR without any increase in catalepsy. Administration of raclopride to rats pretreated with mianserin resulted in a large enhancement of DA output in the mPFC and, at the same time, a small but significant reduction in the raclopride-induced DA output in the NAC. These experimental results indicate that adjunctive treatment with mianserin to a typical D(2) antagonist generates an atypical antipsychotic profile.

Adjunctive treatment with mianserin enhances effects of raclopride on cortical dopamine output and, in parallel, its antipsychotic-like effect.

Clinical studies indicate that adjunctive treatment with the antidepressant drug mianserin, a 5-hydroxytryptamine (5-HT)(2A/C) receptor antagonist and an alpha(2)- and alpha(1)-adrenoceptor antagonist, may enhance the effect of conventional antipsychotic drugs in schizophrenia, in particular on negative symptoms such as withdrawal retardation, akathisia, and some aspects of cognitive impairment. Here, we have examined the effect of mianserin in combination with the selective dopamine (DA) D(2/3) receptor antagonist raclopride on conditioned avoidance response (CAR), a preclinical test of antipsychotic efficacy with high predictive validity; catalepsy, a preclinical test of extrapyramidal side effect liability; and DA output in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAC), respectively. Mianserin (5 mg/kg intraperitoneal) significantly enhanced the suppressant effect of a low dose of raclopride (0.1 mg/kg subcutaneous) on CAR without any increase in catalepsy. Administration of raclopride to rats pretreated with mianserin resulted in a large enhancement of DA output in the mPFC and, at the same time, a small but significant reduction in the raclopride-induced DA output in the NAC. These experimental results indicate that adjunctive treatment with mianserin to a typical D(2) antagonist generates an atypical antipsychotic profile.