Contrasting effects of d-amphetamine on extracellular glutamate in different areas of the rat cortex

Ash ES*, Urch K and Stanford SC. Dept Pharmacology, University College London, London, UK

d -Amphetamine (d-AMP) causes impulse-independent release of monoamines. There are also reports that it increases extracellular glutamate (GLU; ‘efflux’) in the rat CNS (e.g., Xue et al., 1996). In our experiments, using in vivo microdialysis to investigate this action, the effects of d-AMP on GLU efflux seemed to differ in distinct regions of the rat cerebral cortex. Here, we have compared, in detail, the effects of d‑AMP on GLU efflux in the anterior cingulate cortex (ACC) and the adjacent prefrontal cortex (PFC). We have also investigated mechanisms that could explain any differences. Microdialysis probes were implanted under halothane anaesthesia into the ACC and/or PFC of male Sprague Dawley rats (250-300 g). After 24 h, probes were infused at 2 μl min-1 and samples collected every 20 min. Once stable (basal) efflux was established, d-AMP was given systemically (3 mg kg-1 i.p.) or infused (1, 10 & 100 μM; 80 min each) via the probe. In a second batch of rats, we investigated the effect of dopamine (DA) infusion (0.05, 0.5, 5 & 50 μM) on GLU efflux in the PFC and ACC. Finally, the D2-like receptor antagonist, haloperidol, was administered (at a dose that blocks these receptors in vivo: 0.1 mg kg-1 i.p. (Devoto et al., 2004)), 2 h before infusion of d-AMP in the PFC (10 - 100 μM). Raw data were analysed by split-plot ANOVA. Basal GLU efflux did not differ in the ACC and PFC (10.2±1.1 and 12.7±1.4 pmol 20 min-1). The GLU response to d-AMP depended on route of drug administration and brain region (F12,162=1.76; P<0.04). In the ACC, systemic d-AMP caused a 2-fold increase in GLU efflux (F1,8=7.3; P<0.02). In contrast, in the PFC, there was no change. Local infusion of d-AMP (100 μM) caused a 2-fold increase in PFC GLU (F1,9=10.4; P<0.01). In contrast, there was no increase in the ACC. The GLU response to local infusion of DA depended on brain region, also (F1,110=6.176; P<0.014). In the PFC, it was increased approximately 50% by DA (> 0.5 μM; F 1,12=6.686; P<0.024), but there was no change in the ACC. Finally, haloperidol did not prevent the GLU response to d-AMP in the PFC. Systemic d-AMP increased GLU efflux in the ACC but not the PFC. Conversely, local d-AMP increased efflux in the PFC but not the ACC. These findings suggest that d-AMP does not target DA or GLU neurones in the ACC. Alternatively, any increase in GLU efflux could be masked by action(s) at more remote sites. Actions upstream of the ACC mediate the GLU response to systemic d-AMP. Because local infusion of DA increased GLU efflux in the PFC, this neurotransmitter could mediate the GLU response to d-AMP in this region. However, the GLU response to local infusion of d-AMP in the PFC was not prevented by pretreatment with the D2-like receptor antagonist, haloperidol. We are currently investigating the effects of a D1-like receptor antagonist on the GLU response to d-AMP. To conclude, d-AMP increases GLU efflux in the PFC through an action that could involve DA but is not mediated by D2-like receptors. However, in the adjacent ACC, the effects of d-AMP are apparent only when remote targets are recruited.

Devoto, P. et al. (2004) J. Neurochem. 88, 1003-9.
Xue, C.J. et al. (1996) J. Neurochem. 67 , 352-63.

*MRC Research Scholar. This work was funded by the University of London Central Research Fund and RenaSci Consultancy Ltd.