Non-competitive NMDA receptor (NMDA-R) antagonists like ketamine, dizocilpine (MK-801) and phencyclidine (PCP) are used as pharmacological models of schizophrenia and they are also being studied as antidepressant drugs [1]. In rodents, NMDA-R antagonists induce a behavioral syndrome including hyperlocomotion, ataxia signs and stereotypies, as well as other behavioral responses like disorganization in the locomotor pattern and a decrease in the exploratory activity [2]. Previous studies proposed that PCP activates thalamo-cortical circuits after the blockade of NMDA-R in reticular thalamic GABAergic neurons [3]. Since the GluN2C subunit of NMDA-R is densely expressed in thalamus and cerebellum [4], our purpose was to study the involvement of the GluN2C subunit in the behavioral syndrome induced by ketamine, MK-801 and PCP under the working hypothesis that those effects would partly depend on the blockade of thalamo-cortical NMDA-R containing the GluN2C receptor subunit.
Male adult (25-35g) wild-type (WT) and GluN2C receptor subunit knockout (KOGluN2C) mice backcrossed onto a C57BL/6J genetic background were used. Ketamine hydrochloride (Ketolar®, Pfizer), dizocilpine maleate (Sigma-Aldrich) and phencyclidine hydrochloride (Sigma-Aldrich) were dissolved in saline (PCP solution was pH adjusted to 6–6.5 with sodium bicarbonate). Ketamine (10 mg/kg, i.p.), MK-801 (0.10 and 0.25 mg/kg, i.p.) and PCP (5 mg/kg, s.c.) were injected immediately, 30 min and 15 min before behavioral testing, respectively. Mice were placed in a dimly lighted open field cage where activity was recorded with a video camera during 30 min or 2 h. Distance moved (cm) was calculated by the videotrack View Point software (France). Behavioral signs such as number of rearings, number of falls, circling and hind-limb abduction were observed and scored by an experimenter blind to mice genotype and treatment. Statistical analyses were carried out using two-way ANOVA followed by Newman-Keuls post-hoc comparisons. Data are expressed as mean ± SEM and the level of significance was set at p<0,05.
The administration of ketamine and MK-801 significantly enhanced locomotor activity and decreased exploratory behavior (number of rearings) similarly in both genotypes (p<0.01). However, KOGluN2C mice treated with PCP showed increased hyperlocomotion and number of rearings compared to WT mice (p<0.01 and p<0.05, respectively). Moreover, the number of falls induced by all three non-competitive NMDA-R antagonists was markedly and significantly reduced in mice lacking the GluN2C subunit (p<0.01).
Overall, these results show that the GluN2C subunit appears to be strongly involved in the stereotyped behavior induced by non-competitive NMDA-R antagonists. KOGluN2C mice showed less motor impairment, as suggested by the decrease in the number of falls and the increased exploratory behavior, which may account for the genotype differences in the locomotor activity observed in PCP-treated mice. Further, the dramatic difference in the number of falls between WT and KOGluN2C mice suggests the involvement of the cerebellum, a key brain structure for the control of motor coordination.
