L. Daws¹; N. L. Baganz Strode¹; J. L. Munn¹; J. M. Boyce-Rustay²; A. Holmes²
1. Physiology, UTHSCSA, San Antonio, TX, USA.
2. Behavioral Science and Genetics, NIAAA, Bethesda, MD, USA.

The serotonin (5-HT) transporter (SERT; 5-HTT) provides the primary mechanism for terminating 5-HT neurotransmission and there is evidence that it is under regulatory control of hormones released in response to stress. Stress is linked to alcoholism and there is growing evidence that genetically driven SERT expression may be an important predisposing factor for stress and/or alcohol-related disorders. For example, humans and non-human primates with a low-functioning form of the SERT exhibit altered sensitivity to the intoxicating effects of ethanol and an increased propensity to consume ethanol. Moreover, recent studies indicate that the effects of SERT gene-deficiency may be further modified by exposure to stress. Here we examine the relationship between chronic stress, SERT function and sensitivity to ethanol in vivo mouse models.

Male C57BL/6J mice were stressed (daily 10 min forced swim for 14 days) or were unhandled. Twenty four hours following the final stress session, mice were either prepared for chronoamperometry to measure 5-HT clearance in CA3 region of hippocampus, or tested in behavioral assays. The loss of righting reflex was used as a measure of the sedative/hypnotic effect of ethanol. Parallel studies using SERT mutant mice (C57BL/6J background), were carried out in non-stressed, unhandled mice. SERT heterozygotes express 50% fewer SERTs than wild-type litter mates and SERT KOs lack the SERT. Quantitative autoradiography was used to assess total SERT expression in all mice.

Stressed non-mutant C57BL/6J mice mirrored SERT KO mice in that 5-HT clearance was drastically reduced relative to control mice. Total SERT expression was not different between stressed and non-stressed mice. Thus, it appears that the reduced rate of 5-HT clearance in stressed mice may be attributed to either reduced plasma membrane expression and/or reduced intrinsic activity of the SERT. Relative to wild-type littermates, SERT KO mice displayed increased sensitivity to the sedative/hypnotic effects of ethanol, while the ability of ethanol to retard serotonin clearance was significantly potentiated in SERT heterozygote and KO mice. The effects of chronic swim stress on behavioral responses to ethanol in non-mutant C57BL/6J mice were similar to those of SERT KO; stress significantly increased sensitivity to the sedative/hypnotic effect of ethanol. Altered metabolism of ethanol could not account for these differences. By contrast, the effect of ethanol on 5-HT clearance was not different between stressed and non-stessed mice. Interestingly, the ability of the selective 5-HT uptake inhibitor, fluvoxamine, to inhibit 5-HT clearance was augmented in stressed mice, compared to non-stressed mice. This result parallels our previously published finding where SERT heterozygote mice also showed an exaggerated response to fluvoxamine. Fluvoxamine did not alter 5-HT clearance in SERT KO mice.

These data suggest that reduced SERT function, as a consequence of either genetic or environmental manipulation leads to an enhanced response to the 5-HT uptake inhibiting effect of fluvoxamine and enhanced behavioral responses to ethanol. In contrast, adaptive changes that occur in response to a genetic reduction in SERT expression appear requisite for the enhanced ability of ethanol to inhibit 5-HT clearance. This latter finding is highly intriguing given growing evidence that a low-functioning variant in the promoter region of the human 5-HTT gene is positively associated with risk for various neuropsychiatric diseases, chiefly mood disorders and alcoholism. Support Contributed By: RO1-MH64489 & NARSAD (LCD) and NIAAA-IRP (JMB-R, AH).