Purpose: Repeated, intermittent administration of the psychotropic NMDA antagonist phencyclidine (PCP) to laboratory animals causes impairment in cognitive and executive functions, modeling important sequelae of schizophrenia; these effects are thought to be due to a dysregulation of neurotransmission within the prefrontal cortex. Atypical antipsychotic drugs have been reported to have measurable, if incomplete, effects on cognitive dysfunction in this model, and these effects may be due to their ability to normalize a subset of the physiological deficits occurring within the prefrontal cortex. Asenapine is an atypical antipsychotic approved in the US for the treatment of schizophrenia and for the treatment, as monotherapy or adjunctive therapy to lithium or valproate, of acute manic or mixed episodes associated bipolar I disorder. To understand its cognitive and neurochemical actions more fully, we explored the effects of short- and long-term dosing with asenapine on measures of cognitive and motor function in normal monkeys and in those previously exposed for 2 weeks to PCP; we further studied the impact of treatment with asenapine on dopamine and serotonin turnover in discrete brain regions from the same cohort. Methods: Monkeys were trained to perform reversal learning and object retrieval procedures before twice daily administration of PCP (0.3 mg/kg intra-muscular) or saline for 14 days. Tests confirmed cognitive deficits in PCP-exposed animals before beginning twice daily administration of saline (control) or asenapine (50, 100, or 150 μg/kg, intra-muscular). Dopamine and serotonin turnover were assessed in 15 specific brain regions by high-pressure liquid chromatography measures of the ratio of parent amine to its major metabolite. Results: On average, PCP-treated monkeys made twice as many errors in the reversal task as did control monkeys. Asenapine facilitated reversal learning performance in PCP-exposed monkeys, with improvements at trend level after 1 week of administration and reaching significance after 2-4 weeks of dosing. In week 4, the improvement with asenapine 150 μg/kg (p = 0.01) rendered the performance of PCP-exposed monkeys indistinguishable from that of normal monkeys without compromising fine motor function. Asenapine administration (150 μg/kg twice daily) produced an increase in dopamine and serotonin turnover in most brain regions of control monkeys and asenapine (50-150 μg/kg) increased dopamine and serotonin turnover in several brain regions of subchronic PCP-treated monkeys. No significant changes in the steady-state levels of dopamine or serotonin were observed in any brain region except for the central amygdala, in which a significant depletion of dopamine was observed in PCP-treated control monkeys; asenapine treatment reversed this dopamine depletion. A significant decrease in serotonin utilization was observed in the orbitofrontal cortex and nucleus accumbens in PCP monkeys, which may underlie poor reversal learning. In the same brain regions, dopamine utilization was not affected. Asenapine ameliorated this serotonin deficit in a dose-related manner that matched its efficacy for reversing the cognitive deficit. Conclusions: In this model of cognitive dysfunction, asenapine produced substantial gains in executive functions that were maintained with long-term administration. The cognition-enhancing effects of asenapine and the neurochemical changes in serotonin and dopamine turnover seen in this study are hypothesized to be primarily related to its potent serotonergic and noradrenergic receptor binding properties, and support the potential for asenapine to reduce cognitive dysfunction in patients with schizophrenia and bipolar disorder. This article is part of a Special Issue entitled 'Schizophrenia'.
Bibliographical noteFunding Information:
JDJ and RHR were recipients of grant funding from Pfizer Central Research and Organon Research Laboratories (formerly Schering-Plough, now Merck, Whitehouse Station, NJ, USA). JDJ is a consultant to Pfizer. MS and HM are employees of MSD, Lanarkshire, UK. EW was an employee of Pfizer, Ann Arbor, MI, USA. Merck/MSD currently distributes and has a commercial interest in asenapine. Co-authors who were employees at sponsoring organizations participated in and approved the experimental design of these experiments and the final version of this manuscript.
We gratefully acknowledge the staff of the St. Kitts Biomedical Research Foundation for expert assistance in the care and treatment of the animals. These studies were funded, in part, by National Institutes of Health grants MH-57483 , MH-57483S (to RHR), and RR-20750 , MH-77248 , DA-22539 (to JDJ), and Organon Research Laboratories, Ltd . (formerly Schering-Plough, now Merck, Whitehouse Station, NJ, USA), and Pfizer Central Research . Editorial support was provided by Complete Healthcare Communications Inc and was funded by Schering-Plough Corporation, now Merck (Whitehouse Station, NJ, USA. We thank the sponsors for help in formatting and copy-editing this manuscript and preparing it for submission to the special issue on schizophrenia in Neuropharmacology.
- Cognitive function