Coronaridine congeners modulate mitochondrial α3β4 nicotinic acetylcholine receptors with different potency and through distinct intra-mitochondrial pathways.
In contrast to plasma membrane-expressed nicotinic acetylcholine receptors (nAChRs), mitochondrial nAChRs function in an ion-independent manner by triggering intra-mitochondrial kinases that regulate the release of cytochrome c (Cyt c), an important step in cellular apoptosis. The aim of this study is to determine the structural requirements for mitochondrial α3β4 nAChR activation by measuring the modulatory effects of two noncompetitive antagonists of these receptors, ()-catharanthine and (±)-18-methoxycoronaridine (±)-18-MC, on Cyt c release from wild-type and α7-- mice mitochondria. The sandwich ELISA results indicated that α3β4 nAChRs are present in liver mitochondria in higher amounts compared to that in brain mitochondria and that these receptors are up-regulated in α7-- mice. Correspondingly, (±)-18-MC decreased Cyt c release from liver mitochondria of wild-type mice and from brain and liver mitochondria of α7-- mice. The effect in wild-type mice mitochondria was mediated mainly by the Src-dependent pathway, regulating the apoptogenic activity of reactive oxygen species, while in α7-- mice mitochondria, (±)-18-MC strongly affected the calcium-calmodulin kinase II-dependent pathway. In contrast, ()-catharanthine was much less potent than (±)-18-MC and triggered several signaling pathways, suggesting the involvement of multiple nAChR subtypes. These results show for the first time that noncompetitive antagonists can induce mitochondrial α3β4 nAChR signaling, giving a more comprehensive understanding on the function of intracellular nAChR subtypes.