Regional Differences in the Coupling of 5-Hydroxytryptamine-1A Receptors to G Proteins in the Rat Brain

Numerous data showed that 5-hydroxytryptamine-1A (5-HT1A) receptors couple to Gαo/αi proteins for signal transduction. However, the α subunit isoforms really involved in 5-HT1A receptor coupling in brain remain to be identified. Moreover, regional differences in the functional characteristics of brain 5-HT1A receptors have been evidenced repeatedly. Because such differences could be due to variations in G proteins interacting with the same receptor, relevant approaches were used for identifying α subunits physically coupled to 5-HT1A receptors in different regions of the rat brain. Using immunoaffinity chromatography coupled to Western blot detection, 5-HT1A receptors were found to interact equally with Gαo and Gαi3 in the cerebral cortex, mainly with Gαo and weakly with Gαi3 in the hippocampus and exclusively with Gαi3 in the anterior raphe area. In the hypothalamus, 5-HT1A receptors seemed to be coupled to the latter two G proteins plus Gαi1 and Gαz. Complementary experiments based on an antibody capture technique coupled to both classic radioactivity and scintillation proximity assay detections showed that hippocampal 5-HT1A receptor stimulation induced 5′-O-(3-[35S]thio)triphosphate binding to immunoprecipitates with Gαi3 and Gαo antisera. In the anterior raphe, such 5-HT1A receptor-mediated effect was obtained with Gαi3 antiserum only. These results demonstrated the existence of regional differences in the coupling of 5-HT1A receptors to G proteins in the rat brain. In the anterior raphe, 5-HT1A receptors seem to interact specifically with Gαi3, whereas in the hippocampus, they are mainly coupled to Gαo proteins. Such a disparity in G-protein coupling might explain regional differences in adaptive regulations of brain 5-HT1A receptors.