Role of the anterior thalamic nucleus in the motor hyperactivity induced by systemic MK-801 administration in rats

Non-competitive N-methyl-D-aspartate receptor (NMDA-R) antagonists have been extensively used in rodents to model psychotic symptoms of schizophrenia. Although the motor syndrome induced by acute and systemic administration of low doses of dizocilpine (MK-801) has been extensively characterized, its neurobiological basis is not fully understood. NMDA-R antagonists can disinhibit excitatory inputs in certain brain areas, but the precise circuitry is not fully known. We examined the involvement of the anterior thalamic nucleus (ATN) in hyperlocomotion and other related behaviors (stereotypies, ataxia signs) induced after acute systemic administration of MK-801. Since GABAergic neurons of the reticular thalamic nucleus (RTN) exert the main inhibitory control on thalamic projection neurons, we hypothesized that systemically injected MK-801 might block NMDA-R on RTN GABAergic neurons. This effect would subsequently result in disinhibition of GABAergic inputs onto ATN projections to cortical motor areas, thereby inducing behavioral effects. We evaluated the behavioral syndrome induced by the systemic administration MK-801 (0.2 mg/kg) in control rats and in rats subjected to a bilateral stereotaxic infusion of the GABA(A) agonist muscimol (0.2 μl of 2.5 and 5.0 mM; 0.5-1 nmol per application, respectively) into the ATN. As previously reported, MK-801-induced hyperlocomotion in parallel with disorganized movements (e.g. not guided by normal exploration) slight ataxia signs and stereotypies. All responses were antagonized by pre-infusion of muscimol but not saline into the ATN. According to our results we suggest that the ATN plays a role on hyperlocomotion evoked by MK-801 and could involve a thalamic GABAergic disinhibition mechanism.