P.1.026 Sub-chronic phencyclidine alters GABAergic gene expression in rat frontal cortex: focus on tonic inhibition and its relevance to cognition

Abstract

Introduction: Schizophrenia is a serious psychiatric disorder, characterised by 3 symptom domains: positive, negative and cognitive deficits. Sub-chronic administration of the psychotomimetic agent phencyclidine (PCP) has been shown to cause enduring deficits in a range of cognitive tests of relevance to schizophrenia symptomatology in the rat [1]. GABAergic dysfunction has been implicated in schizophrenia. In particular cognitive dysfunction may be linked to disturbances in GABAergic signalling which may be mimicked by the PCP model in animal studies [1]. Tonic GABAergic inhibition is mediated by extra/perisynaptically located GABAA receptors containing mainly a4, a5 and d subunits [2], as well as the GABA transporter GAT-1 and the GAD67 isoform of glutamate decarboxylase. The aim of this study was to investigate the effect of sub-chronic PCP administration on gene expression, with a particular focus on genes that regulate GABAergic tonic inhibition. Methods: Adult female Hooded-Lister rats received PCP (2mg/kg b.i.d i.p) or vehicle for 7 days, followed by a 7 day washout period before sacrifice. Brains were removed, dissected into regions according to Bregma coordinates (frontal cortex = +4.7mm to +3.2mm, striatum = +1.6mm to −0.3mm, hippocampus = −2.5mm to −3.8mm and cerebellum = −10.3mm to −12.7mm) and placed into Sigma-Aldrich RNAlaterTM solution on ice, in preparation for RNA extraction and analysis. Brain regions were homogenised in 1ml Trizol® reagent and RNA extracted using Invitrogen mini columns. After conversion to cDNA using a Qiagen reverse transcription kit, gene expression was quantified using SYBr Green qRT-PCR using a GAPDH normalising gene. The REST programme was used to analyse the data. Results: Sub-chronic PCP treatment resulted in significant (P< 0.001) reductions in the extrasynaptically located GABAA receptor subunits a4 and a5 in the frontal cortex and a significant (P< 0.01) up-regulation of d subunit in the cerebellum. Sub-chronic PCP induced a large and significant transcriptional downregulation (P< 0.001) of GAD1 and calretinin in frontal cortex. Significant reductions in GAT-1 (P< 0.05) and parvalbumin (P< 0.01) were also observed. A significant (P< 0.05) reduction in GFAP transcription was also observed in the frontal cortex. Conclusions: Sub-chronic PCP markedly reduced the transcription of GABAergic tonic inhibition genes in the frontal cortex. Large losses of parvalbumin/calretinin combined with a4 and a5 receptor subunits suggest a widespread loss of GABAergic tonic inhibition in the cortex. Lack of d subunit loss in the cortex suggests a specific population of GABAA receptors are being affected by sub-chronic PCP, most likely the a4/5+g2 group. The increase in d subunit mRNA expression in the cerebellum supports the suggestion of a brain region-specific effect of sub-chronic PCP, possibly through differential regulation of BDNF, which has been shown to be differentially regulated in different brain regions [3]. Reduction of GAT-1 and GFAP expression suggests a concurrent loss of astrocytic processes. The molecular mechanism by which PCP produces these expressional changes has yet to be elucidated. These preliminary findings suggest that a reduction in GABAergic tonic inhibition in the frontal cortex could play a role in the cognitive deficits produced in the sub-chronic PCP model.