Aquaporin‐4 Knockout Abolishes Apomorphine‐Induced Tardive Dyskinesia Following Chronic Treatment with Neuroleptics

Tardive dyskinesia (TD) is a side effect of long-term administration of typical neuroleptics such as haloperidol, and is characterized by excessive and involuntary movements of the face, mouth, tongue as well as other parts of the body [1]. In contrast, atypical antipsychotic drugs such as clozapine exhibit a reduced risk of TD [2]. However, the neurochemical mechanisms that underlie the differential effects between typical and atypical antipsychotic drugs on TD remain unclear [3]. Abnormalities in dopaminergic function, specifically, the supersensitivity of dopamine (DA) receptors, have been demonstrated to involved in the pathophysiology of TD [4]. Behavioral evidence of dopamine hypersensitivity is convinced in many different animal models. The up-regulation of D2 dopamine receptors was thought to be the primary mechanism responsible for the development of behavioral sensitivity [4]. Apomorphine (APO), a nonselective D1/D2 agonist, can induce stereotyped behavior and vacuous chewing movements (VCMs) following chronic treatment with neuroleptics and routinely has been used as an experimental model of TD [5]. Aquaporin-4 (AQP4), a predominant water channel in brain, is highly expressed in perivascular astrocyte endfeet and plays crucial roles in central nervous system [6]. AQP4 knockout increases DA synthesis and turnover in various brain regions. Currently, multiple lines of evidence suggest that AQP4 is involved in anomalous dopaminergic neurotransmission-induced diseases, such as Parkinson’s disease [7], addiction [8] and mood disorders. However, the implication of AQP4 in TD remains unclear. In the present study, AQP4 knockout mice (AQP4 / ) were applied to define the roles of AQP4 in TD. Twelve-week-old male AQP4 and AQP4 / mice were injected with haloperidol (2 mg/kg i.p.), clozapine (10 mg/kg i.p.), or saline once a day for 21 days. Locomotor activities were monitored after the final injections by using ZIL-2 activity meter (Chinese Academy of Medical Sciences). Mice were placed into activity monitor chambers (20 cm 9 20 cm) for 10 min, and then activities were measured at 10-min intervals. Subsequently, mice received an injection of APO (1 mg/kg, s.c.). Following this injection, the numbers of activities were recorded for the next 10-min intervals. As shown in Figure 1, both haloperidol and clozapine showed a significant inhibitory effect on locomotor activities in AQP4 and AQP4 / mice. The typical antipsychotic haloperidol was much more potent in inhibiting locomotor activity than the atypical antipsychotic clozapine in both genotypes of mice (Figure 1A). AQP4 knockout did not influence these behavioral differences in locomotor activity. AQP4 mice displayed marked hyperlocomotion after APO-stimulation. Notably, AQP4 knockout abolished APO-induced hyperlocomotion following both chronic neuroleptics and saline treatment (Figure 1B). The results indicate that AQP4 knockout may directly block psychomotor stimulant action of APO. Vacuous chewing movements induced by a long-term neuroleptic administration in rodent animals have been the most extensively used phenomenological animal model of TD[5]. VCMs were referred to as single mouth openings in the vertical plane not directed toward physical material. The numbers of VCMs were counted three days after the final injections. Mice were placed individually into clear observation cages (16 cm 9 30 cm 9 19 cm) without food for a 1-hour habituation period. VCMs were measured in 5-min intervals 5 min after the injection of APO (1 mg/kg, s.c.). We found AQP4 mice that stimulated by APO displayed a significant increase in VCMs in haloperidol pretreated group, but not in clozapine pretreated group. However, no change of VCMs in respond to APO was observed in AQP4 / mice following treated with haloperidol, clozapine or