1. ¹Department of Biology, Psychiatry CEDD, GlaxoSmithKline Medicines Research Centre, Verona, Italy
2. ²Laboratory Animal Science, Psychiatry CEDD, GlaxoSmithKline Medicines Research Centre, Verona, Italy

Correspondence: A Gozzi, Neuroimaging, Psychiatry CEDD, GlaxoSmithKline Research Centre, Fleming 4, Verona 37100, Italy. Tel: + 39 0458219233; Fax: +39 0458218073; E-mail: alessandro.2.gozzi@gsk.com

Received 11 May 2007; Revised 18 July 2007; Accepted 23 July 2007; Published online 5 September 2007.

Abstract

Acute administration of NMDA receptor (NMDAR) antagonists such as phencyclidine (PCP) or ketamine induces symptoms that closely resemble those of schizophrenia in humans, a finding that has led to the hypothesis that a decreased NMDAR function may be a predisposing or even causative factor in schizophrenia. However, the precise neuropharmacological mechanisms underlying these effects remain to be fully elucidated. Here, we applied pharmacological MRI (phMRI) to examine the brain circuitry underlying the psychotomimetic action of PCP in the anesthetized rat, and investigated how these functional changes are modulated by drugs that possess distinct pharmacological mechanisms. Acute administration of PCP (0.5 mg/kg i.v.) produced robust and sustained positive relative cerebral blood volume (rCBV) changes in discrete cortico-limbo-thalamic regions. Pretreatment with the selective D₂ dopamine antagonist raclopride (0.3 mg/kg i.p.) did not significantly affect the rCBV response to PCP, while the atypical antipsychotic clozapine (5 mg/kg i.p.) produced region-dependent effects, with complete suppression of the rCBV response in the thalamus, and weaker attenuation of the response in cortical and hippocampal structures. The response to PCP was strongly suppressed in all regions by pretreatment with two drugs that can inhibit aberrant glutamatergic activity: the anticonvulsant lamotrigine (10 mg/kg i.p.) and the mGluR2/3 agonist LY354740 (10 mg/kg i.p.). Taken together, our findings corroborate the pivotal role of dysfunctional glutamatergic neurotransmission in the functional response elicited by PCP, while the lack of effect of raclopride argues against a primary role of dopamine D₂ receptor activation in this process. Finally, the thalamic effect of clozapine could be key to elucidating the functional basis of its pharmacological action.