Receptor for adenosine. The activity of this receptor is mediated by G proteins which inhibits adenylyl cyclase (PubMed:1323836). May play a role during reproduction (PubMed:1647979). Testis, particularly in spermatocytes and spermatids but not in spermatogonia. Low levels in the brain. Expressed during spermiogenesis. Phosphorylation on Thr-318 and Thr-319 may be crucial for rapid desensitization. Phosphorylation on Thr-318 may be necessary for phosphorylation on Thr-319 to occur. Belongs to the G-protein coupled receptor 1 family.