In addition, there is a marked variation in basal [35S]GTPγS binding across different brain regions. For example, in the A/A mice brain, the basal [35S]GTPγS binding ranged from 1127 nCi/g in the VTA to 5028 nCi/g in the superficial layers of the superior colliculus. Our findings are consistent with those of Sim, Selley, Dworkin et al. (1996). GDP is required to reduce basal [35S]GTPγS binding (Traynor and Nahorski, 1995) and the concentration required varies among cells and tissues (Huang, Xu, Yoon et al., 2007). It is conceivable that brain regions may require different GDP concentrations to achieve similar basal [35S]GTPγS binding. However, we had to use one concentration of GDP (2 mM) in the experiment for all brain regions, which may contribute to varying levels of basal [35S]GTPγS binding. Thus, the net DAMGO-stimulated [35S]GTPγS binding as % of basal binding is consequently variable across brain regions and is quite low in some regions, which are consistent with the finding of (Wang et al., 2011) and Sim et al. (1996).
