The influence of NMDA and GABA(A) receptors and glutamic acid decarboxylase (GAD) activity on attention

Attention dysfunction is the hallmark of cognitive deficits associated with major psychiatric illnesses including schizophrenia. Cognitive deficits of schizophrenia have been attributed to reduced function of the N-methyl-d-aspartate (NMDA) receptor or reduced expression of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase-67, which presumably leads to attenuated neurotransmission at GABA(A) receptors. The present study used a rodent model to compare the inhibition of NMDA and GABA(A) receptors, and GAD activity on attention. We tested the impact of inhibiting these proteins brain wide or in the anterior cingulate cortex (ACC), a prefrontal cortex region critical for attentional processing. Rats were trained on the three choice serial reaction time task (3-CSRT), an attention test. The impact of systemic or intra-ACC injection of drugs on performance was measured in well-trained rats. Reducing GABA(A) receptor function within the ACC with the direct antagonist SR95531 (1 or 3 ng/side) or brain wide using systemic injection of the benzodiazepine inverse agonist FG7142 (5 mg/kg) impaired accuracy and increased omissions. Systemic or intra-ACC inhibition of NMDA receptors using MK-801 (at 3 mg/kg or 3 mu g, respectively) also impaired performance. Inhibition of GAD with 3-mercaptopropionic acid, even at high doses, had no effect on 3-CSRT accuracy or omissions when administered systemically or within the ACC. These data demonstrate that, while tonic stimulation of NMDA and GABA(A) receptors within the ACC are critical for attentional performance, reduction in GAD activity may have little functional significance and is not indicative of reduced GABA neurotransmission.