As an alternative to chronic
As an alternative to chronic receptor blockade, we have been targeting adenosine kinase (ADK) – an astrocyte-based enzyme that catalyses the phosphorylation of adenosine, as a means to modify gedatolisib adenosinergic signalling (Boison, 2006, Etherington et al., 2009). Up-regulating ADK facilitates the clearance of intracellular adenosine by driving adenosine influx into astrocytes via bi-directional nucleoside transporters, and thus reduces extracellular adenosine levels (Boison et al., 2010). This has been achieved by replacing the endogenous Adk gene with an ubiquitin-driven loxP-flanked transgene [-Tg(UbiAdk), henceforth ‘Adk-tg’], which led to a 2.2-fold elevation in ADK activity and brain-wide reduction in adenosinergic tone (Fedele et al., 2005). Adk-tg mice were associated with severe performance deficits in the Morris water maze tests of spatial memory and Pavlovian conditioned freezing (Yee et al., 2007). The outcomes are therefore opposite to the pro-cognitive effects associated with AR blockade by caffeine (Yonkov, 1984, Cestari and Castellano, 1996, Kopf et al., 1999, Prediger and Takahashi, 2005, Prediger et al., 2005, Costa et al., 2008, Capek and Guenther, 2009, but also see Zimmerberg et al., 1991, Kant, 1993) and selective genetic inactivation of striatal A2ARs (Wei et al., 2011). Instead, they lend support to the hypothesis that physiological adenosine concentration is necessary for the homeostatic maintenance of neural network stability, including glutamatergic and dopaminergic networks, such that related behavioural outputs would be severely disturbed by the inhibition of adenosinergic neuromodulation (Yee et al., 2007, Boison et al., 2012). The opposite outcomes between global ADK over-expression (Yee et al., 2007) and striatal-specific A2AR disruption (Wei et al., 2011) regarding working memory performance suggest that the reduction in extra-striatal adenosinergic tone in the cortex and hippocampus might contribute to the behavioural deficits seen in Adk-tg mice. Within the hippocampus, adenosine modulates the function of ionotropic N-methyl-d-aspartate receptor (NMDARs) and metabotropic glutamate receptor 5 (mGluR5) via A1R and A2AR, respectively (Ribeiro and Sebastião, 2009). A substantial reduction of adenosinergic tone in the hippocampus may disrupt normal hippocampal function, including spatial learning. In parallel, the enhanced motor response to MK-801 (a non-competitive NMDAR blocker) seen in Adk-tg mice (Yee et al., 2007) is also suggestive of cortical/hippocampal NMDAR functional deficiency (Carlsson, 1993, Takahata and Moghaddam, 2003) – an effect not consistently associated with caffeine (see Table 1 of Boison et al., 2012) or reproducible by striatal-specific A2AR disruption (Wei et al., unpublished data). Hence, the homeostatic modulation by cortical and hippocampal adenosine may be uniquely critical in this respect. Here, we focused on the contribution of cortical/hippocampal adenosine reduction to the prominent behavioural phenotypes previously demonstrated in Adk-tg mice (Yee et al., 2007), namely, spatial memory and reaction to NMDAR blockade, and evaluated the extent to which they might be modifiable by cortical/hippocampal-specific interventions. To this end, we made use of the fact that the Adk transgene in the Adk-tg mice was floxed and therefore excisable by Cre recombinase. By restricting Cre expression to the telencephalon using the Emx-1 promoter (Iwasato et al., 2004), we generated mice (denoted as ‘fb-Adk-def’) that lacked the Adk transgene specifically in the cortex and hippocampus while it remained overexpressed in the rest of the brain (i.e., similar to the original Adk-tg mice). Thus, fb-Adk-def mice were characterized by increased adenosine levels in the cortex and decreased adenosine levels in the striatum (Shen et al., 2011). With wild type (WT) mice providing the necessary control for gauging the direction of the phenotype relative to normal behaviour, the comparison between fb-Adk-def and Adk-tg mice allowed us to effectively compare opposite changes in cortical/hippocampal adenosine against the same background of adenosine deficiency outside the telencephalon. The regional molecular dissection achieved in the present study is therefore instructive in clarifying the direction and specificity of cognitive modulation maintained by cortical and hippocampal adenosinergic activity. This approach would be particularly relevant to models of disease, such as schizophrenia, that are hypothesized to be associated with a global adenosinergic hypofunction (e.g., Lara et al., 2006).