REGIONAL DIFFERENCES OF THE INHIBITION OF GABA(B) LIGAND-BINDING BY THE GTP ANALOG GPP(NH)P

In order to determine whether the interactions between GABA(B) recepto rs and G-proteins differ in several brain areas, we have used the redu ction in high-affinity GABA(B) binding by the GTP analogue Gpp(NH)p as an internal assay marker for G-protein linkage to GABA(B) receptors. The results indicate that Gpp(NH)p inhibits the binding of the GABA(B) receptor agonist [H-3]CGP 27492 (80 to 95%) in a biphasic manner betw een 0.1 nM and 1 mM. The IC50 for high-affinity sites is significantly higher in cerebellum (70 nM, 53% of binding sites) than in cortex, hi ppocampus, corpus striatum and thalamus (15-30 nM, 63-73% of binding s ites). The IC(50)s of the low-affinity sites in hippocampus and cortex (170 mu M and 210 FLM, respectively) were significantly higher than t he IC(50)s in cerebellum, thalamus and corpus striatum (18-39 mu M). A ll these binding sites are sensitive to pertussis toxin (PTX; 7-15 mu g/mg protein), implicating that they are linked either to G(i) or to G (0) proteins. The two binding sites observed (high affinity, nM and lo w affinity, mu M for Gpp(NH)p) and the regional dependence in affinity of these sites may originate either from different GABA(B) receptor s ubtypes, different G-proteins or different coupling mechanisms between G-proteins and GABA(B) receptors. Whereas the PTX site of G-protein l inked to GABA(B) receptors changes with age [24], the GTP binding site does not differ between peripubertal rats (5-6 weeks) and adults rats (10-12 weeks).