Different synaptic and subsynaptic localization of adenosine A2A receptors in the hippocampus and striatum of the rat

Adenosine A2A receptors are most abundant in the striatum where they control the striatopallidal pathway thus controlling locomotion. Extra-striatal A2A receptors are considerably less abundant but their blockade confers robust neuroprotection. We now have investigated if striatal and extra-striatal A2A receptors have a different neuronal location to understand their different functions. The binding density of the A2A antagonist, [3H]-7-(2-phenylethyl)-5-amino-2-(2-furyl)pyrazolo[4,3e][1,2,4]triazolo[1,5-c]pyrimidine ([3H]SCH 58261), was enriched in nerve terminals membranes (Bmax=103±12 fmol/mg protein) compared with total membranes (Bmax=29±4 fmol/mg protein) from the hippocampus, the same occurring with A2A receptor immunoreactivity. In contrast, there was no enrichment of [3H]SCH 58261 binding or A2A receptor immunoreactivity in synaptosomal compared with total membranes from the striatum. Further subcellular fractionation of hippocampal nerve terminals revealed that A2A receptor immunoreactivity was enriched in the active zone of presynaptic nerve terminals, whereas it was predominantly located in the postsynaptic density in the striatum, although a minority of striatal A2A receptors were located in the presynaptic active zone. These results indicate that A2A receptors in the striatum are not enriched in synapses in agreement with the preponderant role of A2A receptors in signal processing in striatopallidal neurons. In contrast, hippocampal A2A receptors are enriched in synapses, mainly in the active zone, in accordance with their role in controlling neurotransmitter release. This regional variation in the neuronal distribution of A2A receptors reinforces the care required to extrapolate our knowledge from striatal A2A receptors to other brain preparations.