Desensitization of adenosine A(1) receptors in rat immature cortical neurons.
Adenosine plays an important neuroprotective role in brain, usually mediated by the activation of adenosine A? receptors. Prolonged activation of a G-protein-coupled receptor generally leads to the partial loss of the responsiveness of receptor-mediated transduction pathways (desensitization). Rat immature cortical neurons were treated with 100 nM?N?-phenylisopropyladenosine (R-PIA), a selective A? receptor agonist, and the effect on adenosine A? receptor/adenylyl cyclase pathway was studied. Incubation with R-PIA for 6, 12, 24 and 48 h elicited a time-dependent decrease in adenosine A? receptors in plasma membranes (92, 58, 43 and 26% of control, respectively), which was associated with variations in microsomal fraction (21, 56, 124 and 233% of control, respectively), suggesting the internalization and down-regulation of adenosine A? receptors. Moreover, real-time PCR assays showed a significant increase in mRNA levels coding adenosine A? receptor after the longest treatment period (48 h). In addition, ?Gi??? protein levels detected in microsomes and mRNA levels coding ?Gi? protein were increased after 48 h of treatment with R-PIA, suggesting the synthesis of new ?Gi? proteins. Finally, adenylyl cyclase inhibition elicited by 2-Chloro-N6-cyclopentyladenosine (CPA), a selective adenosine A? receptor agonist, was significantly reduced after 12, 24 and 48h of treatment (37, 24 and 23%, respectively) as compared to controls (54%), suggesting the desensitization of adenosine A? receptor/adenylyl cyclase pathway. These results suggest that adenosine A? receptors desensitize slowly after prolonged receptor activation in immature cortical neurons, showing mechanisms of desensitization similar to those described not only in fetal but also in adult rat brain.