GABAergic expression is differentially expressed in subregions of the prefrontal cortex in rats sensitized to methamphetamine

Purpose: Inhibitory gamma-aminobutyric acid (GABA)-mediated neurotransmission plays an important role in the regulation of the prefrontal cortex (PFC), with increasing evidence suggesting that dysfunctional inhibitory control of the PFC may underlie the executive deficits reported in psychotic disorders. Methamphetamine (‘ice’) is a commonly used psychostimulant that can induce an acute psychosis and cognitive deficits that are indistinguishable from schizophrenia, suggesting that chronic methamphetamine exposure may alter the GABAergic profile of the prefrontal cortex similarly to that reported in psychotic disorders. Using label-free shotgun proteomics, we have previously found that GAD₆₇ and GAT₁ proteins are significantly downregulated in the PFC following sensitization to methamphetamine. We have also found several changes to GABAergic mRNA expression in the whole PFC of rats following chronic methamphetamine exposure. As the PFC is a heterogeneous area of multiple brain regions, the aim of the present study was to investigate changes to GABAergic mRNA expression in subregions of the prefrontal cortex, specifically the orbitofrontal (OFC), infralimbic (IFC) and prelimbic (PRL) cortices, of rats sensitized to methamphetamine. Methods: Male Sprague Dawley rats (n = 12) underwent repeated methamphetamine (1 ml/kg intraperitoneal (i.p.) days 1 & 7; 5 mg/kg i.p. days 2−6) or saline (1 ml/kg i.p.) injections for 7 days. Following 14 days of withdrawal, rats were challenged with an acute methamphetamine injection (1 mg/kg i.p.). Sixty minutes post challenge, rats were euthanized, their brains removed and frozen at −80ºC. The orbitofrontal, infralimbic and prelimbic cortices were dissected out on dry ice. RNA was isolated from tissue samples and quantitative PCR (qPCR) was used to compare the relative expression of GABAergic genes using specifically designed primer pairs against a housekeeping gene, GAPDH. Results: Methamphetamine challenge resulted in significant sensitized locomotor response in methamphetamine pre-treated animals when compared to saline controls, p<0.001. QPCR revealed that metabotropic GABAB2 mRNA expression was 1.72- fold upregulated in the IF (p<0.05) while GAD67, GAT-1 and GAT-3 mRNA expression were 1.53-fold (p < 0.0001), 1.40- fold (p<0.005) and 1.80-fold (p<0.005) upregulated in the PRL, respectively. Ionotropic GABAA receptor subunits α1 (1.55-fold, p<0.0001), α3 (1.19-fold, p<0.005), α5 (1.33-fold, p<0.0001) and β2 (1.21-fold, p < 0.0001), were specifically upregulated in the OFC. Conclusions: Alterations to GABAergic mRNA expression following sensitization to methamphetamine are biologically dissociated between the orbitofrontal, infralimbic and prelimbic cortices,suggesting that GABAergic gene expression is significantly altered following chronic methamphetamine exposure in a brain-region and GABA-specific manner. These changes may lead to profound consequences on central inhibitory mechanisms of localised regions of the PFC as a way of maintaining an appropriate degree of neuronal excitability. Our findings suggest that alterations to the GABAergic system in the prefrontal cortex may represent a common biological substrate that underpins the cognitive and behavioural dysfunction commonly seen in psychotic disorders.2 page(s