Mice with reduced DAT levels recreate seasonal-induced switching between states in bipolar disorder

Abstract

Developing novel therapeutics for bipolar disorder (BD) has been hampered by limited mechanistic knowledge how sufferers switch between mania and depression-how the same brain can switch between extreme states-described as the "holy grail" of BD research. Strong evidence implicates seasonally-induced switching between states, with mania associated with summer-onset, depression with winter-onset. Determining mechanisms of and sensitivity to such switching is required. C57BL/6J and dopamine transporter hypomorphic (DAT-HY 50% expression) mice performed a battery of psychiatry-relevant behavioral tasks following 2-week housing in chambers under seasonally relevant photoperiod extremes. Summer-like and winter-like photoperiod exposure induced mania-relevant and depression-relevant behaviors respectively in mice. This behavioral switch paralleled neurotransmitter switching from dopamine to somatostatin in hypothalamic neurons (receiving direct input from the photoperiod-processing center, the suprachiasmatic nucleus). Mice with reduced DAT expression exhibited hypersensitivity to these summer-like and winter-like photoperiods, including more extreme mania-relevant (including reward sensitivity during reinforcement learning), and depression-relevant (including punishment-sensitivity and loss-sensitivity during reinforcement learning) behaviors. DAT mRNA levels switched in wildtype littermate mice across photoperiods, an effect not replicated in DAT hypomorphic mice. This inability to adjust DAT levels to match photoperiod-induced neurotransmitter switching as a homeostatic control likely contributes to the susceptibility of DAT hypormophic mice to these switching photoperiods. These data reveal the potential contribution of photoperiod-induced neuroplasticity within an identified circuit of the hypothalamus, linked with reduced DAT function, underlying switching between states in BD. Further investigations of the circuit will likely identify novel therapeutic targets to block switching between states.

Fig. 1

Winter- and summer-like photoperiods drive depression- and mania-relevant behaviors and neurochemistry. C57BL/6J mice are awake during dark photoperiods and so were housed in short-active (SA; 19L:5D), vs. normal active (NA; 12L:12D), vs. long-active (LA; 19D:5L) photoperiod conditions for 2 weeks (schematic on top) prior to testing. SA-housed mice exhibited depression-relevant behavior relative to controls (NA) as indicated by increased immobility (despair-like behavior), in the forced swim test (FST; a). In contrast, mice housed in summer-like (LA; 5L:19D) photoperiod conditions for 2 weeks exhibited mania-like behavior as indicated by increased open arm entries (risk-prone behavior) in the elevated plus maze (EPM; b). This change in behavior was accompanied by photoperiod-induced regulation of the number of TH+ (c), and SST+ (d), neurons in the paraventricular nucleus (PVN) of these mice without altering the number of CRH+ neurons (e), compared to NA controls. Representative images of TH and SST immunoreactivity within the PVN from each photoperiod condition are presented. Data presented as individual data points plus mean ± S.E.M. Asterisk denotes p < 0.05, **p < 0.01 compared to NA controls

Fig. 2

Mice with reduced dopamine transporter (DAT) expression exhibit a hypersensitivity to the effects of winter-like and summer-like photoperiods effect on depression-relevant and mania-relevant behaviors. DAT hypomorphic (HY) and wildtype (WT) littermate mice exhibit increased depression-relevant behavior when housed in short-active (SA; 19 L:5D) photoperiod conditions for 2 weeks, relative to controls (NA; 12 L:12D) as indicated by increased immobility (despair-like behavior), in the forced swim test (FST; a). This effect was greater in DAT HY compared with WT littermate mice. Mice housed in long-active (LA; 5 L:19D) photoperiod conditions for 2 weeks exhibit mania-like behavior as indicated by increased open arm entries (risk-prone behavior) in the elevated plus maze (EPM; b). Again this effect was exaggerated in DAT HY compared with WT littermate mice. Although neither photoperiod altered effortful motivation (breakpoint) as measured in the progressive ratio breakpoint test (PRBS; c), LA photoperiod condition increased reward sensitivity in DAT HY but not WT littermate mice as measured by a faster latency to collect rewards (d). Data presented as individual data points plus mean ± S.E.M. Asterisk denotes p < 0.05 as indicated

Fig. 3

Mice with reduced dopamine transporter (DAT) expression exhibit a hypersensitivity to the effects of winter-like and summer-like photoperiods effect on depression-relevant and mania-relevant behavior from the probabilistic learning task (PLT). The PLT assesses learning within a single session (a). Rodents select from two options (designated target and non-target) that provide reward (milkshake) and punishment (timeout) feedback based on an 80/20 or 20/80 feedback ratio (respectively). Overall accuracy (target:non-target choices) can be measured. Importantly for mania-relevant and depression-relevant research however, reward-based (win-stay: repeating a choice after a reward, reward latency: time to collect a reward), and punishment-based (lose-shift: shifting choice after a punishment, punish perseveration: repeating responses during a punishment when all apertures are unresponsive), decision-making can also be quantified. DAT hypomorphic (HY) mice housed for 2 weeks in a short-active (SA; 19 L:5D) photoperiod exhibited impaired accuracy compared with those housed in a normal-active (NA; 12 L:12D) photoperiod (b). Photoperiod condition did not affect wildtype (WT) littermate mice. The SA-induced deficits in accuracy in DAT-HY mice was likely a result of increased punish sensitivity behavior as measured by increased lose-shift (c), and punish perseveration (e) in these mice relative to NA-housed mice. Although long-active (LA; 5 L;19D) photoperiod lengths did not affect accuracy, mice housed in the LA photoperiod exhibited increased reward-sensitive behavior as measured by increased win-stay behavior irrespective of genotype (d), with DAT-HY mice again selectively exhibiting faster reward collection latencies (f), compared with the NA photoperiod condition. Data presented as individual data points plus mean ± S.E.M. Asterisk denotes p < 0.05 as indicated

Fig. 4

Winter-like and summer-like photoperiods regulate the number of TH+ and SST+ neurons in the paraventricular nuclei (PVN) of mice with reduced dopamine transporter (DAT HY) and their wildtype (WT) littermates. Short-active (SA; 19 L;5D shown in red; a), and long-active (LA; 5 L;19D; shown in yellow; b) photoperiods differentially affected expression on the number of TH+ neurons in the PVN, irrespective of genotype compared with normal active (NA shown in green; 12:12) photoperiod exposure. SA photoperiod exposure reduced TH, while LA photoperiod exposure elevated TH expression. Similarly, SA (c) and LA (d) photoperiods also differentially affected the number of SST+ neurons, elevating and reducing numbers respectively, in both WT and HY mice. Representative images of TH and SST immunoreactivity within the PVN from each photoperiod condition in DAT HY mice are presented. Data presented as mean ± S.E.M. *p < 0.05, **p < 0.01, ***p < 0.001 compared with NA (normal active, 12 L:12D), photoperiod control

Fig. 5

Altered photoperiod does not affect dopamine transporter (DAT) mRNA levels in the hypothalamus of DAT-Hypomorphic (HY) mice, hence the hypersensitivity of DAT-HY mice to altering photoperiod may arise from lack of homeostatic control. Altering photoperiod lengths affected DAT mRNA levels in wildtype (WT), but not DAT-HY mice (a). Average effect sizes for immunohistochemical and depression-relevant and mania-relevant behaviors were calculated for WT and DAT HY mice in each photoperiod (short-active; SA vs. long-active; LA) relative to NA controls. Hence, WT mice exhibited significant but modest changes in behavior (red line), tyrosine hydroxylase (TH; blue line), and somatostatin (SST; green line) expression as a result of changing photoperiods, with DAT mRNA (yellow line) changing to maintain homeostatic control (b). In contrast, DAT HY mice exhibited similar changes in TH and SST expression but stronger changes in behavior, likely as a result of inability to exert DAT homeostatic control (b). The increased effect sizes of photoperiod on behavior in DAT HY vs. WT mice are indicated in orange, with depression-relevant behaviors during SA, and mania-relevant behaviors during LA photoperiods