Plasma membrane monoamine transporter (PMAT, Slc29a4) transports monoamine neurotransmitters, including dopamine and serotonin, faster than more studied monoamine transporters, e.g., dopamine transporter (DAT), or serotonin transporter (SERT), but with ~400–600-fold less affinity. A considerable challenge in understanding PMAT’s monoamine clearance contributions is that no current drugs selectively inhibit PMAT. To advance knowledge about PMAT’s monoamine uptake role, and to circumvent this present challenge, we investigated how drugs that selectively block DAT/SERT influence behavioral readouts in PMAT wildtype, heterozygote, and knockout mice of both sexes. Drugs typically used as antidepressants (escitalopram, bupropion) were administered acutely for readouts in tail suspension and locomotor tests. Drugs with psychostimulant properties (cocaine, D-amphetamine) were administered repeatedly to assess initial locomotor responses plus psychostimulant-induced locomotor sensitization. Though we hypothesized that PMAT-deficient mice would exhibit augmented responses to antidepressant and psychostimulant drugs due to constitutively attenuated monoamine uptake, we instead observed sex-selective responses to antidepressant drugs in opposing directions, and subtle sex-specific reductions in psychostimulant-induced locomotor sensitization. These results suggest that PMAT functions differently across sexes, and support hypotheses that PMAT’s monoamine clearance contribution emerges when frontline transporters (e.g., DAT, SERT) are absent, saturated, and/or blocked. Thus, known human polymorphisms that reduce PMAT function could be worth investigating as contributors to varied antidepressant and psychostimulant responses.

Despite the robustness of DRD4 polymorphism associations with brain‐based behavioral characteristics in candidate gene research, investigations have minimally explored associations between these polymorphisms and emotional responses. In particular, the prevalent single nucleotide polymorphism (SNP) ‐521C/T (rs1800955) in the promoter region of DRD4 remains unexplored relative to emotions. Here, two independent samples were evaluated using different emotion elicitation tasks involving social stimuli: Study 1 (N = 120) evoked positive and negative emotional responses to validated film clips; Study 2 (N = 122) utilized Cyberball to simulate social rejection and acceptance. Across studies, C/C individuals self‐reported higher mean positive affect scores using Likert scales versus T carrier individuals, selectively when presented with neutral or negative (but not positive) social stimuli. The consistent findings across these two studies supports a functional consequence of this DRD4 SNP on emotion processing during changing social contexts. Continued investigation will help clarify if a C/C genotype enhances positive emotions under negative circumstances, or if the presence of the T allele reduces positive emotions, and how rs1800955 behavioral associations might generalize across different demographics. Future studies could also reveal if this SNP interacts with other changing environmental conditions to affect emotional responses, such as social limitations during the COVID‐19 pandemic.

A hallmark symptom of many anxiety disorders, and multiple neuropsychiatric disorders more broadly, is generalization of fearful responses to non-fearful stimuli. Anxiety disorders are often comorbid with cardiovascular diseases. One established, and modifiable, risk factor for cardiovascular diseases is salt intake. Yet, investigations into how excess salt consumption affects anxiety-relevant behaviors remains little explored. Moreover, no studies have yet assessed how high salt intake influences generalization of fear. Here, we used adult C57BL/6J mice of both sexes to evaluate the influence of two or six weeks of high salt consumption (4.0% NaCl), compared to controls (0.4% NaCl), on contextual fear acquisition, expression, and generalization. Further, we measured osmotic and physiological stress by quantifying serum osmolality and corticosterone levels, respectively. Consuming excess salt did not influence contextual fear acquisition nor discrimination between the context used for training and a novel, neutral context when training occurred 48 prior to testing. However, when a four week delay between training and testing was employed to induce natural fear generalization processes, we found that high salt intake selectively increases contextual fear generalization in females, but the same diet reduces contextual fear generalization in males. These sex-specific effects were independent of any changes in serum osmolality nor corticosterone levels, suggesting the behavioral shifts are a consequence of more subtle, neurophysiologic changes. This is the first evidence of salt consumption influencing contextual fear generalization, and adds information about sex-specific effects of salt that are largely missing from current literature.

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The plasma membrane monoamine transporter (PMAT, Slc29a4) is a polyspecific cation transporter that, in the brain, predominantly takes up monoamine neurotransmitters like dopamine and serotonin. PMAT function is hypothesized to emerge when other monoamine transporter function is impaired, such as under conditions of heightened monoamine neurotransmitter release in response to stressors. In humans, common genetic polymorphisms can reduce PMAT function, but these have exclusively been studied in the context of metformin treatment response under diabetic conditions. Nothing is currently known about how reduced PMAT function affects fear processing, nor any other aspects of emotional learning or memory. Our lab sought to begin examining the effects of PMAT deficiency on fear processing by using mice with constitutive reductions in PMAT function (i.e., heterozygotes) and comparing these to wildtype mice. Specifically, we evaluated both cued and contextual fear processing. Based on evidence that chronic pharmacological blockade of serotonin transporter‐mediated monoamine uptake decreased both cued and contextual fear expression, we hypothesized heterozygote mice would display lower levels of both cued and contextual fear relative to wildtypes. Surprisingly, we only observed a trend (p=0.08) specifically in male heterozygotes for attenuated cued fear expression. Otherwise, heterozygotes of both sexes exhibited cued fear acquisition, extinction retention, and renewal that was indistinguishable from wildtypes, as was their contextual fear processing. Four weeks after fear processing behavior tests concluded, mice were subjected to a brief swim stress, and blood was collected for stress hormone quantification. These data are currently being analyzed, and we hypothesize that heterotypic stressor exposure will unmask the behavioral sequelae of reduced PMAT function. Our current findings are consistent with previous reports of sex‐selective effects of PMAT deficiency on behavioral responses to stressor exposure. Continued investigation into how reduced PMAT function affects emotion processing and stress responsivity could have translational implications for people with functional PMAT polymorphisms.