Dopamine (DA) is a neurotransmitter implicated in different physiological functions and the dopamine transporter (DAT) plays a key role in regulating DA homeostasis in the brain. Bile acids (BAs) are molecules derived from cholesterol participating in dietary fat absorption. Several studies report the presence of BAs in the brain where act as regulators of neuronal function. Recently, in mice, it has been discovered that a bile diversion surgery that increases circulating BAs affects DA dynamics in the nucleus accumbens and reduces reward-related behavior induced by cocaine. Feeding obeticholic acid (OCA), an FDA-approved semi-synthetic bile acid, to mice induced the same effects of bariatric surgery. This study investigates the interactions of the OCA with DAT and mechanistically defines the regulation of DAT activity via two-electrode voltage clamp approach on Xenopus laevis oocytes heterologously expressing murine DAT and through molecular modeling. Electrophysiological data show that the OCA acts directly on mDAT inducing a small fast-inward Na+ dependent current. OCA also inhibits the DAT-mediated Li+ leak current, supporting the hypothesis of direct binding. Dose-response experiments in the presence of OCA resulted in unaltered DAT Imax and K0.5. Perfusing a different bile acid, lithocholic acid (LCA), induces similar effects on mDAT. Docking simulations were performed to identify the potential binding sites of OCA to DAT. The residue D421, which is physiologically involved in coordinating the binding of the Na+ ion, may contribute to the action of OCA on DAT. Moreover, the binding to the residues R445 and D436 may stabilize the inward-facing open state by preventing the reformation of the salt bridges required for substrate transport. These preliminary results indicate a novel and undocumented interaction between DAT (and potentially other SLC6) and BAs, raising new questions about the role of these molecules in brain function and in associated behaviors.
Bile acids directly act on the dopamine transporter mediated currents
Tiziana Romanazzi;Elena Bossi
2021-01-01
Abstract
Dopamine (DA) is a neurotransmitter implicated in different physiological functions and the dopamine transporter (DAT) plays a key role in regulating DA homeostasis in the brain. Bile acids (BAs) are molecules derived from cholesterol participating in dietary fat absorption. Several studies report the presence of BAs in the brain where act as regulators of neuronal function. Recently, in mice, it has been discovered that a bile diversion surgery that increases circulating BAs affects DA dynamics in the nucleus accumbens and reduces reward-related behavior induced by cocaine. Feeding obeticholic acid (OCA), an FDA-approved semi-synthetic bile acid, to mice induced the same effects of bariatric surgery. This study investigates the interactions of the OCA with DAT and mechanistically defines the regulation of DAT activity via two-electrode voltage clamp approach on Xenopus laevis oocytes heterologously expressing murine DAT and through molecular modeling. Electrophysiological data show that the OCA acts directly on mDAT inducing a small fast-inward Na+ dependent current. OCA also inhibits the DAT-mediated Li+ leak current, supporting the hypothesis of direct binding. Dose-response experiments in the presence of OCA resulted in unaltered DAT Imax and K0.5. Perfusing a different bile acid, lithocholic acid (LCA), induces similar effects on mDAT. Docking simulations were performed to identify the potential binding sites of OCA to DAT. The residue D421, which is physiologically involved in coordinating the binding of the Na+ ion, may contribute to the action of OCA on DAT. Moreover, the binding to the residues R445 and D436 may stabilize the inward-facing open state by preventing the reformation of the salt bridges required for substrate transport. These preliminary results indicate a novel and undocumented interaction between DAT (and potentially other SLC6) and BAs, raising new questions about the role of these molecules in brain function and in associated behaviors.
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