Adenosine A 2B receptor down-regulates metabotropic glutamate receptor 5 in astrocytes during postnatal development.

Metabotropic glutamate receptor 5 (mGluR5) in astrocytes is a key molecule for controlling synapse remodeling. Although mGluR5 is abundant in neonatal astrocytes, its level is gradually down-regulated during development and is almost absent in the adult. However, in several pathological conditions, mGluR5 re-emerges in adult astrocytes and contributes to disease pathogenesis by forming uncontrolled synapses. Thus, controlling mGluR5 expression in astrocyte is critical for several diseases, but the mechanism that regulates mGluR5 expression remains unknown. Here, we show that adenosine triphosphate (ATP)/adenosine-mediated signals down-regulate mGluR5 in astrocytes. First, in situ Ca ²⁺ imaging of astrocytes in acute cerebral slices from post-natal day (P)7-P28 mice showed that Ca ²⁺ responses evoked by (S)-3,5-dihydroxyphenylglycine (DHPG), a mGluR5 agonist, decreased during development, whereas those evoked by ATP or its metabolite, adenosine, increased. Second, ATP and adenosine suppressed expression of the mGluR5 gene, Grm5, in cultured astrocytes. Third, the decrease in the DHPG-evoked Ca ²⁺ responses was associated with down-regulation of Grm5. Interestingly, among several adenosine (P1) receptor and ATP (P2) receptor genes, only the adenosine A _2B receptor gene, Adora2b, was up-regulated in the course of development. Indeed, we observed that down-regulation of Grm5 was suppressed in Adora2b knockout astrocytes at P14 and in situ Ca ²⁺ imaging from Adora2b knockout mice indicated that the A _2B receptor inhibits mGluR5 expression in astrocytes. Furthermore, deletion of A _2B receptor increased the number of excitatory synapse in developmental stage. Taken together, the A _2B receptor is critical for down-regulation of mGluR5 in astrocytes, which would contribute to terminate excess synaptogenesis during development.
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