Organic Cation Transporter 3 Upregulation in Serotonin Transporter Deficient Mice Potentiates Ethanol‐Induced Serotonin Clearance Impairments
Authors: Gilman T Lee , Horton Rebecca E , Baganz Nicole L , Alvarado Steven R , Daws Lynette C 
Published: November 2023

Altered serotonin (5‐HT) signaling is hypothesized to contribute to alcoholism and alcohol abuse vulnerability. Genetic reductions in serotonin transporter (SERT) function, resulting in attenuated clearance of extracellular 5‐HT, are associated with increased likelihood of alcohol use disorders. Indeed, we have shown that reduction (+/−) or complete loss (−/−) of SERT function potentiates the ethanol‐mediated impairment of extracellular 5‐HT clearance in mice. Likewise, co‐administration of a selective serotonin reuptake inhibitor (SSRI) exacerbated the 5‐HT clearance impairment elicited by ethanol in wildtype (SERT+/+) mice (Daws et al., 2006 J Neurosci 25(24):6431–6438). These data suggest that ethanol is acting at a site other than SERT to impair 5‐HT clearance. We have previously discovered that increased expression of organic cation transporter 3 (OCT3) in SERT‐deficient mice serves as a compensatory 5‐HT uptake mechanism (Baganz et al., 2008 PNAS 105(48):18976–18981). The objective of the present study was to determine if ethanol is acting at OCT3 to inhibit 5‐HT clearance in wildtype and SERT‐deficient mice. To evaluate this, we used high‐speed chronoamperometry to detect 5‐HT uptake in the hippocampus (CA3 region) or striatum of wildtype and SERT‐deficient mice. Ethanol was applied locally in conjunction with blockers of OCT3 (corticosterone), SERT (fluvoxamine), norepinephrine transporter (NET; reboxetine), or dopamine transporter (DAT; GBR 12909; striatum only). Application of the OCT3 blocker did not increase the impaired hippocampal 5‐HT uptake resulting from ethanol in any SERT genotype, indicating that ethanol does indeed act via OCT3 to impair 5‐HT uptake. In hippocampus, co‐infusion of a SERT inhibitor with ethanol potentiated the impairment in 5‐HT uptake only in wildtype and SERT+/− mice, whereas co‐infusion of a NET inhibitor selectively exacerbated the reduced 5‐HT uptake resulting from ethanol in SERT−/− mice. In striatum, ethanol application resulted in greater 5‐HT uptake impairment in SERT−/− mice compared to wildtype mice. This was potentiated by a DAT blocker in all three genotypes, but remained unaffected by SERT, NET, and OCT3 blockers, indicative of the promiscuous uptake of 5‐HT by DAT. Together, these findings provide evidence that OCT3 is a target for ethanol, whereas SERT, NET, and DAT are not. In addition, ethanol blockade of OCT‐mediated 5‐HT uptake is pronounced when SERT function is genetically or pharmacologically reduced and other compensatory transporters (e.g., NET, DAT) are inactivated. The present data therefore highlight the importance of considering genetic SERT status and compensatory transporter function (i.e., OCT3, NET, and DAT) in individuals with alcohol use disorders, and when evaluating SSRI efficacy, adherence, and treatment outcomes.Support or Funding InformationNIH R01 1MH64489 and R01 MH093320 to LCD. TLG is also supported by NIH T32 DA031115‐06 to Dr. Charles P. France.