Effects of the NMDA antagonist ketamine on task-switching performance: evidence for specific impairments of executive control

In humans, the effects of subanesthetic doses of ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, substantially impair executive control functions. Here, we consider whether ketamine exposure can provide an animal model for the effects of ketamine on executive control. Two monkeys (Macaca mulatta) performed a cued task-switching paradigm. We studied their behavior before and after a range of ketamine doses. We found that ketamine slowed overall performance and decreased overall accuracy, strongly impaired the capacity to ignore task-irrelevant information and, to a lesser degree, decreased accuracy when a task switch was required. This pattern of results is very similar to that found in studies of schizophrenic patients performing task-switching paradigms or the Stroop task. We conclude that ketamine in monkeys provides a good animal model for exploring the relationship between the glutamate system, executive control, and the symptoms of schizophrenia.

New developments in the pharmacotherapy of schizophrenia

This review summarizes current key research strategies and the most prominently pursued new potential treatments for schizophrenia. First, new routes of administration for second generation antipsychotics are presented. These include rapidly dissolving tablets, drops and sirups as well as new intramuscular formulations. Newly available short acting and long acting (depot) antipsychotics complement oral antipsychotics so that the full spectrum of routes of administration is now available for second generation antipsychotics. Next to antipsychotic polypharmacy, in which two or more antipsychotics are combined, pharmacological add-on treatments, mainly with benzodiazepines, antidepressants and mood stabilizers enjoy increasing popularity. Most of this practice is driven by personal preferences, clinical experience and marketing rather than evidence based medicine. New pharmacological mechanisms currently utilized in advanced states of development include partial dopamine D2-receptor agonism, supplementation with glutamatergic agents, estrogen and omega-3-fatty acids. While the concept of partial D1-agonism has already led to the successful launch of a new antipsychotic, aripiprazole, the other attempts to improve therapeutic response in schizophrenia patients have so far provided equivocal results. It is argued that they may be helpful for certain subgroups or specific symptoms of schizophrenia patients. In conclusion, many exciting new pharmacological leads are currently pursued and this will very likely augment the options for treating patients with schizophrenia.

NMDA receptor activity and the transmission of sensory input into motor output in introverts and extraverts

Recent research suggests that individual differences in brain dopamine functioning may be related to the personality dimension of extraversion. The major goal of the present study was to answer the question of whether a pharmacologically induced change in glutamatergic NMDA receptor activity would also differentially affect the transmission of sensory input into motor out-put in introverts and extraverts. Therefore, in a double-blind within-subjects design, either 30 mg of the NMDA receptor antagonist memantine or placebo were administered to 48 healthy male volunteers before performing a choice reaction-time task. In introverts, memantine caused a pronounced increase in lift-off time (i.e., the time required to lift the finger from a home button) compared to that in extraverts, whereas movement time (i.e., the time required to move the finger from the home button to a response button) was decreased in both groups. The pattern of results suggests that extraversion-related differential sensitivity to pharmacologically induced changes in NMDA receptor activity is limited to functions that involve an interaction between the glutamatergic and dopaminergic systems.