Effects of Dihydropyridine Calcium Channel Ligands on Rat Brain γ-Aminobutyric Acid_B Receptors

This study shows that low nanomolar concentrations of the calcium channel antagonist nifedipine displaced [³H]baclofen labeling of γ-aminobutyric acid_B (GABA_B) receptors, whereas similar concentrations of two calcium channel agonists stimulated this GA-BA_B receptor labeling. Neither effect was likely to be due to dihydropyridine (DHP) binding to baclofen recognition sites, because the inhibitory ligand nifedipine primarily affected apparent receptor density rather than affinity. Although these results could reflect the coupling of GABA_B receptors with calcium channels, they do not rule out other, possibly more direct interactions between GABA_B receptors and DHP binding sites. These DHP effects occur at much lower concentrations and display other significant differences from previously reported effects of DHPs on other transmitter receptors.