Abstract:
Many G protein-coupled receptors have been shown to be sensitive to the presence of
sodium ions (Na+). Using radioligand competition binding assays, we have examined and
compared the effects of sodium ions on the binding affinities of a number of structurally
diverse ligands at human dopamine D2 and dopamine D3 receptor subtypes, which are
important therapeutic targets for the treatment of psychotic disorders. At both receptors, the
binding affinities of the antagonists/inverse agonists SB-277011-A, L,741,626, GR 103691
and U 99194 were higher in the presence of sodium ions compared to those measured in
the presence of the organic cation, N-methyl-D-glucamine, used to control for ionic strength.
Conversely, the affinities of spiperone and (+)-butaclamol were unaffected by the presence
of sodium ions. Interestingly, the binding of the antagonist/inverse agonist clozapine was
affected by changes in ionic strength of the buffer used rather than the presence of specific
cations. Similar sensitivities to sodium ions were seen at both receptors, suggesting parallel
effects of sodium ion interactions on receptor conformation. However, no clear correlation
between ligand characteristics, such as subtype selectivity, and sodium ion sensitivity were
observed. Therefore, the properties which determine this sensitivity remain unclear. However
these findings do highlight the importance of careful consideration of assay buffer composition
for in vitro assays and when comparing data from different studies, and may
indicate a further level of control for ligand binding in vivo.
