Organic Cation Transporter 3 is a Crucial Modulator of Serotonin Clearance in Basolateral Amygdala and Sex-Dependently Modulates Fear Behaviors

Abstract

<b>Abstract ID 29086</b> <b>Poster Board 265</b> Dysregulation of serotonergic neurotransmission has long been implicated in neuropsychiatric illness, including stress related disorders, therefore understanding the role serotonin (5-HT) plays in circuitry relevant to emotion could open new avenues for improving treatment of stress-related disorders. Considerable research has investigated the role of the high-affinity, low-capacity ‘uptake-1’ 5-HT transporter (SERT) in modulation of emotion-regulating circuitry. To date, pharmacological targeting of SERT has shown variable clinical efficacy, suggesting a role for other mechanisms in manifestation of disorders associated with serotonergic dysregulation. Evidence from our laboratory and others suggests that the corticosterone-sensitive, organic cation transporter 3 (OCT3), a low-affinity, high-capacity ‘uptake-2’ transporter of 5-HT, has a significant impact on serotonergic homeostasis and may play a major role in stress-related disorders. Here we used genetic and viral approaches to test the hypothesis that OCT3 is an important mechanism for 5-HT clearance in basolateral amygdala (BLA, a brain region essential to processing and consolidation of fear memory), and strongly modulates fear related behaviors in adult male and female mice. Oct3 floxed mice were crossed with either ePet-cre mice to knockout (KO) OCT3 from 5-HT neurons during development, or with Nestin-cre/ERT2 mice, to allow tamoxifen induced global depletion of OCT3 from neurons and glia. To assess involvement of OCT3 primarily on post-synaptic neurons in BLA, we bilaterally injected AAV5-EF1a-mCherry-IRES-WGA-Cre, or control virus, into BLA of OCT3 floxed mice. We characterized 5-HT clearance in BLA using <i>in&nbsp;vivo</i> high-speed chronoamperometry. Consistent with OCT3 being an important regulator of 5-HT homeostasis, we found 5-HT clearance in BLA to be impaired in mice of both sexes regardless of constitutive KO (OCT3 floxed x ePet1-cre) or conditional (tamoxifen-inducible) depletion. Preliminary data from mice injected with virus are showing similar trends. Mice constitutively lacking OCT3 on 5-HT neurons of either sex showed no differences in fear acquisition or memory. Mice with conditional OCT3 global depletion or virally induced OCT3 depletion in BLA, likewise showed no difference in fear acquisition compared to relevant control mice. However, these mice showed dramatic sex-dependent differences in fear memory. Tamoxifen-inducible OCT3 depletion resulted in poorer contextual fear memory in males, but increased contextual fear memory in females, such that freezing behavior remained at almost 100% across the test period. Preliminary data from mice with viral depletion of OCT3 in BLA reveal no consequence for fear acquisition and memory in males, whereas females show both increased contextual and cued fear memory. The lack of behavioral consequence for fear acquisition and memory in mice with constitutive loss of OCT3 from 5-HT neurons suggests compensation may be involved. Alternatively, OCT3 located on other neuronal types and/or glia may be more important for these behavioral effects. Together, these data support the hypothesis that OCT3 is an important mechanism for 5-HT clearance in BLA, and plays a critical, sex-dependent role in fear memory, though the underlying mechanism(s) remain to be elucidated. Supported by R01 MH093320 to LCD and GMT, NJC is supported by T32 DA031115.