Inhibitors of ectonucleotidases have paradoxical effects on synaptic transmission in the mouse cortex

Extracellular adenosine plays prominent roles in the brain in both physiological and pathological conditions. Adenosine can be generated following the degradation of extracellular nucleotides by various types of ectonucleotidases. Several ectonucleotidases are present in the brain parenchyma: ecto-nucleotide triphosphate diphosphohydrolases 1 and 3 (NTPDase 1 and 3), ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP 1), ecto-5'-nucleotidase (eN), and tissue non-specific alkaline phosphatase (TNAP, whose function in the brain has received little attention). Here we examined, in a living brain preparation, the role of these ectonucleotidases in generating extracellular adenosine. We recorded local field potentials evoked by electrical stimulation of the lateral olfactory tract in the mouse piriform cortex in vitro. Variations in adenosine level were evaluated by measuring changes in presynaptic inhibition generated by adenosine A1 receptors (A1Rs) activation. A1R-mediated presynaptic inhibition was present endogenously and was enhanced by bath-applied AMP and ATP. We hypothesized that inhibiting ectonucleotidases would reduce extracellular adenosine concentration, which would result in a weakening of presynaptic inhibition. However, inhibiting TNAP had no effect in controlling endogenous adenosine action and no effect on presynaptic inhibition induced by bath-applied AMP. Furthermore, contrary to our expectation, inhibiting TNAP reinforced, rather than reduced, presynaptic inhibition induced by bath-applied ATP. Similarly, inhibition of NTPDase 1 and 3, NPP1, and eN induced stronger, rather than weaker, presynaptic inhibition, both in endogenous condition and with bath-applied ATP and AMP. Consequently, attempts to suppress the functions of extracellular adenosine by blocking its extracellular synthesis in living brain tissue could have functional impacts opposite to those anticipated.