Altered dopamine D2 receptor function in VTA DA cells in adenosine A2A knockout mice

R. Al-Hasani, S.M.O. Hourani, I. Kitchen and Y. Chen, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, UK.

Adenosine A2A receptors are highly expressed in the dopaminergic (DA) pathways in the brain. A2A receptor knockout mice exhibit altered addictive behaviours, which include reduced cocaine self–administration (Soria et al, 2005), abolition of cannabinoid reward (Soria et al, 2004) and increased signs of morphine withdrawal (Berrendero et al, 2003; Bailey et al, 2004). Adenosine A2A receptors are co-localized with dopamine D2 receptors in the striatum (Ferre et al, 1997), and direct interactions between the two may be an underlying mechanism for the involvement of adenosine A2A receptors in the reward pathways. The DA neurons in the ventral tegmental area (VTA) project and receive feedback to and from the nucleus accumbens, forming part of the mesocorticolimbic DA pathway responsible for the brain’s reward process.

We hypothesised that VTA DA cell firing and dopamine D2 receptor function may be altered in adenosine A2A receptor knockout mice and could account for altered addictive behaviours. Midbrain slices containing the VTA (300μM) were prepared from wildtype and adenosine A2A knockout mice and immersed in artificial cerebrospinal fluid. The VTA DA cells exhibit pacemaker-like regular firing and the firing of single VTA DA cells was recorded.

There was no observed difference in the basal firing rate between the wildtype (1.803 ± 0.1850 Hz, n=22) and adenosine A2A knockout mice (1.806 ± 0.2496 Hz, n=14). The dopamine D2 receptor agonist, quinpirole (0.01-30µM), caused a concentration-dependent decrease in DA cell firing rate in both genotypes. At high concentrations of quinpirole (1- 30μM) DA cell firing was completely inhibited in the wildtype mice, whereas in the adenosine A2A knockout mice quinpirole was unable to abolish the firing, even at 30µM. Overall the inhibition caused by quinpirole was significantly reduced in the adenosine A2A knockout mice from that in the wildtype mice (two-way ANOVA for factors of genotype and concentration, P< 0.01, n=7-8). The adenosine A2A agonist CGS 21680 (1nM-10µM) produced no significant response in either the wildtype or the adenosine A2A knockout mice (n=6). Furthermore, CGS 21680 (1µM) had no effect on the action of quinpirole (0.03µM).

These results show that there is a significant reduction in dopamine D2 receptor function in the VTA DA cells in the adenosine A2A knockout mice. The lack of a direct effect of CGS 21680 shows the absence of adenosine A2A receptors on VTA DA cells and hence ruling out direct dopamine D2 adenosine A2A receptor interactions in the VTA. The altered dopamine D2 receptor function may the result of an adaptive change in the mesolimbic pathway in the adenosine A2A knockout mice through a feedback mechanism.

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This work was supported by a MRC studentship.