Photoperiod Impacts Nucleus Accumbens Dopamine Dynamics

Abstract Circadian photoperiod, or day length, changes with the seasons and influences behavior to allow animals to adapt to their environment. Photoperiod is also associated with seasonal rhythms of affective state, as evidenced by seasonality of several neuropsychiatric disorders. Interestingly, seasonality tends to be more prevalent in women for affective disorders such as major depressive disorder and bipolar disorder (BD). However, the underlying neurobiological processes contributing to sex-linked seasonality of affective behaviors are largely unknown. Mesolimbic dopamine input to the nucleus accumbens (NAc) contributes to the regulation of affective state and behaviors. Additionally, sex differences in the mesolimbic dopamine pathway are well established. Therefore, we hypothesize that photoperiod may drive differential modulation of NAc dopamine in males and females. Here, we used fast-scan cyclic voltammetry (FSCV) to explore whether photoperiod can modulate subsecond dopamine signaling dynamics in the NAc core of male and female mice raised in seasonally relevant photoperiods. We found that photoperiod modulates dopamine signaling in the NAc core, and that this effect is sex-specific to females. Both release and uptake of dopamine were enhanced in the NAc core of female mice raised in long, summer-like photoperiods, whereas we did not find photoperiodic effects on NAc core dopamine in males. These findings uncover a potential neural circuit basis for sex-linked seasonality in affective behaviors.


Introduction
Circadian photoperiod, or day length, is an environmental signal that synchronizes daily biological rhythms to local time and induces seasonal changes in behavior and physiology (Øverland et al., 2019;Magnusson, 2000;Zhang et al., 2021;Green et al., 2015;Siemann et al., 2021). While seasons in the temperate zones are marked by a variety of environmental changes, photoperiod is the most reliable environmental cue that encodes seasons and allows organisms to predict and adapt to seasonal changes to their environment. The amplitude of seasonal change in photoperiod varies by latitude, as does the prevalence of seasonal mood disorders (Øverland et al., 2019;Rosen et al., 1990;Magnusson, 2000;Siemann et al., 2021;Zhang et al., 2021). Bright light therapy treats seasonal mood disorders by increasing the duration of light a person experiences in a 24-h day, effectively lengthening their photoperiod (Melrose, 2015). Identifying photoperiod-sensitive systems within brain regions relevant to affective behaviors will provide insight the regulation of affective and motivated behaviors.
Dopamine is a neuromodulator that plays a critical role in regulating affective state and state-dependent behaviors (Mogenson et al., 1980;Nestler and Carlezon, 2006;Floresco, 2015;Turner et al., 2018). More specifically, dopamine input to the nucleus accumbens (NAc) from the ventral tegmental area (VTA) has been implicated in the pathophysiology of mood disorders (Nestler and Carlezon, 2006;Floresco, 2015). Evidence suggests photoperiod can modulate dopamine in other brain regions as measured by tissue content (Carlsson et al., 1980;Goda et al., 2015), dopamine turnover (Itzhacki et al., 2018), transporter availability (Neumeister et al., 2001), and presynaptic dopamine synthesis (Eisenberg et al., 2010). To date, studies have not examined photoperiod's effect on the dopamine system at the synaptic level.
Dopamine signaling in the NAc is tightly and dynamically regulated through both VTA neuron activity and intra-accumbal mechanisms at dopamine axon terminals (Sulzer et al., 2016;Nolan et al., 2020). VTA dopamine neurons display two functionally distinct firing modes. Shifts from low frequency tonic firing to phasic burst firing in these neurons encode information such as reward-prediction error and incentive salience (Ljungberg et al., 1992;Schultz, 1998Schultz, , 2007Tsai et al., 2009;Cohen et al., 2012;Kutlu et al., 2021). Dopamine release in the NAc is also regulated through local mechanisms including inhibition via presynaptic dopamine D2 autoreceptors as well as facilitation via presynaptic nicotinic acetylcholine receptors (Shin et al., 2017;Yorgason et al., 2017). Once released, clearance of dopamine from the synaptic space primarily occurs via re-uptake into dopaminergic terminals through the dopamine transporter (DAT; Giros et al., 1996;, where it is then repackaged into vesicles for future release by vesicular monoamine transporter 2 (VMAT2; Erickson et al., 1992;Nolan et al., 2020). DAT activity shapes the kinetics of dopamine signaling and can influence dopamine release through the recycling of transmitter (Egaña et al., 2009;Condon et al., 2019). Furthermore, changes to NAc DAT function by inhibition, reversal, or genetic manipulation results in changes to goal-directed behaviors Calipari et al., 2013Calipari et al., , 2017Sørensen et al., 2021), demonstrating how subsecond dopamine signaling dynamics powerfully influence behavior.
Sex differences in the function of the mesolimbic dopamine pathway are well established (Q.D. Walker et al., 2000;Dluzen and McDermott, 2008;Becker, 2009;Yoest et al., 2014;Calipari et al., 2017;Zachry et al., 2021), making it critically important to consider sex of the animal when assessing photoperiod effects on this system. VTA dopamine neuron activity, NAc dopamine release and uptake, DAT expression, and VMAT2 activity are all greater in females compared with males (Q.D. Walker et al., 2000;Dluzen and McDermott, 2008). Interestingly, several neuropsychiatric disorders that involve the dopamine system display seasonal patterns where seasonality is typically more prevalent in women (Magnusson, 2000;Kim et al., 2011;Geoffroy et al., 2014;Otte et al., 2016).
Here, we assess the effect of seasonal photoperiods on NAc dopamine dynamics in mice using fast-scan cyclic voltammetry (FSCV). We found a sex-specific effect wherein photoperiod modulates dopamine release and uptake in the NAc of female mice. Specifically, female mice raised in long summer-like photoperiods exhibit greater dopamine release, thus uncovering a potential synaptic basis that provides insight to sex-linked seasonality of affective behaviors.

Animals
We used male and female C3Hf 1/1 mice for all experiments. C3Hf 1/1 mice endogenously synthesize melatonin and lack the retinal degeneration alleles of the parent C3H strain and are therefore responsive to photoperiod manipulation (Ebihara et al., 1986;Goto et al., 1989;Tosini and Menaker, 1998;Contreras-Alcantara et al., 2012). Mice were bred and maintained in groups of two to five per cage under photoperiods we indicate here as Short (8 h of light 16 h of darkness), Equinox (12 h of light and 12 h of darkness), or Long (16 h of light and 8 h of darkness). Animals were continuously maintained in these photoperiod conditions from embryonic day 0 (E0) until they were used for experiments between postnatal day 50 (P50) and P90. FSCV experiments were conducted in NAc slices from Short, Equinox, and Long photoperiod mice. All tissues were isolated within a 2-h window of the middle of the light-phase of each light cycle. Lights off denoted zeitgeber time (ZT) 12; therefore, Short animals were used at ZT8, Equinox animals were used at ZT6, and Long animals were used at ZT4. Experiments were performed in accordance with Institutional Animal Care and Use Committee and National Institutes of Health guidelines.

Fast-Scan cyclic voltammetry
Ex vivo FSCV was used to assess dopamine release and uptake dynamics in the NAc core of Short, Equinox, and Long photoperiod animals (n = 4-7 animals per sex and photoperiod group, using 1 brain slice per animal). Mice were humanely killed and the brain was rapidly removed. Using a Leica Vibratome, 250-mm-thick sagittal sections containing the NAc core were collected from whole brain tissue in oxygenated (95% O 2 ; 5% CO 2 ) artificial CSF (aCSF) containing (in mM): 126 NaCl, 2.5 KCl, 1.2 NaH 2 PO 4 , 2.4 CaCl 2 , 1.2 MgCl 2 , 25 NaHCO 3 , 11 glucose, and 0.4 L-ascorbic acid, and pH adjusted to 7.4. Slices were transferred to a chamber containing oxygenated aCSF. All experiments were performed using a Scientifica SliceScope Pro System in 32°C aCSF with a flow rate of 2 ml/min. The carbon fiber microelectrode (100À200 mm in length, 7-mm radius) and bipolar stimulating electrode were placed in close proximity in the NAc core. A single electrical pulse (750mA, 4 ms, monophasic) was applied to the tissue every 4 min to evoke dopamine release. We also evoked dopamine release using a train of five pulses at 5, 10, or 20 Hz (750 mA, 4 ms, monophasic). Extracellular dopamine was recorded by applying a triangular waveform (À0.4 to 11.2 to À0.4 V vs Ag/AgCl, 400 V/s).
For pharmacological experiments using DAT inhibitor, GBR 12783 (Tocris Bioscience/Bio-Techne), peak evoked dopamine release was collected in aCSF until a stable baseline was established (three collections with ,10% variability) using a single electrical pulse every 4 min. GBR 12783 was bath applied to the slice at 300 nM, 1 mM, and 3 mM concentrations. We collected peak evoked dopamine release at each concentration until stable responding was achieved (three collections with ,10% variability) before moving onto the next highest concentration.

Voltammetry data analysis
Demon voltammetry and analysis software was used to analyze all FSCV data (Yorgason et al., 2011). Data collected using single electrical pulse were modeled using Michaelis-Menten kinetics using the kinetic analysis tool. By using Michaelis-Menten kinetic analysis, we derived parameters such as the peak dopamine concentration released following stimulation (amplitude of signal) and maximal rate of dopamine uptake by DAT (V max ). As described previously (Wu et al., 2001;Yorgason et al., 2011;Calipari et al., 2017), K m from the Michaelis-Menten equation represents the affinity for dopamine binding to DAT. According to standard FSCV analysis methods (Yorgason et al., 2011), and based on previous research on the affinity of dopamine for the DAT (Wu et al., 2001), we set the K m parameter to 160 nM for each animal and allowed V max to vary to determine baseline V max . For experiments using GBR 12783, we determined a baseline V max for each animal once stable baseline responding was achieved in aCSF as described above. For subsequent GBR 12783 concentrations, we held the baseline V max constant and allowed K m to vary to determine apparent K m . Apparent K m is the Michaelis-Menten constant as observed under conditions, such as competitive inhibition, that would impede determining its true value. Measuring apparent K m allows determination of the sensitivity of DAT to pharmacological competitive inhibition by GBR 12783. Raw data were calibrated in each data file before Michaelis-Menten modeling. Recording electrodes were calibrated by recording the peak current (nA) to a known concentration of dopamine (3 mM) via flow-injection system, and we used these values to convert current to dopamine concentration.

Statistical analysis
Prism 9 (GraphPad Software Inc.) was used for all statistical analyses. For all analyses, a was set as 0.05, with p values , a indicating a statistically significant difference. Statistical significance was determined by two-way ANOVA. All post hoc analysis was performed using Tukey's multiple comparison tests and standard error of the mean was used for all experiments. Power analyses were performed with preliminary data during the acquisition of each new data set. The sample size obtained from each power analysis calculation was then compared with sample sizes reported in the literature for similar experiments. Errors bars depicted in figures represent SEM.

Results
Long, summer-like photoperiod increases evoked dopamine release in the NAc core We measured electrically evoked DA release from the NAc core in ex vivo slices from both male and female mice raised in Short (8L:16D), Equinox (12L:12D), or Long (16L:8D) photoperiods ( Fig. 1A-C). The representative cyclic voltammogram ( Fig. 1D), shows the characteristic redox signature of dopamine. Electrical stimulation evoked clear and distinct transient dopamine release and uptake events that were time-locked to stimulation with current peaks at the oxidation (0.6 V) and reduction (À0.2 V) voltages for catecholamines ( Fig. 1E). Peak dopamine release concentration was calculated from the peak height of the FSCV response using calibrated electrodes, with sample sizes as follows: Short (N = 9), Equinox (N = 12), and Long (N = 10) where N is number of animals. To assess whether photoperiod differentially affected dopamine release from tonic and phasic firing modes of dopamine neurons projecting to the NAc core, we used a range of stimulation frequencies from single pulse to 20 Hz. Two-way ANOVA revealed that increasing stimulation frequency increased dopamine release in all photoperiod groups, as expected (Sulzer et al., 2016;Nolan et al., 2020;F (3,115) = 13.18, p , 0.0001; Fig. 1F). Photoperiod significantly affected the magnitude of evoked dopamine release (F (2,115) = 10.43, p , 0.0001). Specifically, Tukey's post hoc multiple comparisons test showed that dopamine release was elevated in NAc core from Long photoperiod mice compared with NAc core from Equinox photoperiod mice in the 10-Hz (p = 0.018) and 20-Hz (p = 0.046) conditions. Taken together, our results indicate that Long summer-like photoperiods can enhance evoked dopamine release in the NAc core.
Effects of long, summer-like photoperiod on dopamine release and uptake rate are female specific As there are known sex differences in the regulation of mesolimbic dopamine (Q.D. Walker et al., 2000;Dluzen and McDermott, 2008;Becker, 2009;Yoest et al., 2014;Calipari et al., 2017;Zachry et al., 2021), we investigated a potential interaction of sex and photoperiod on NAc core dopamine terminal function by analyzing data from male and female mice separately ( Fig. 2A,B). Sample sizes were as follows: Short (males N = 4, females N = 5), Equinox (males N = 5, females N = 7), and Long (males N = 6, females = 4) where N is number of animals. Using a twoway ANOVA, we found significant effect of stimulation frequency per se in both males (F (3,48) = 5.667, p = 0.002) and females (F (3,55) = 10.26, p , 0.0001). Strikingly, we found that Long photoperiod enhanced dopamine release only in females (F (2,55) = 26.69, p , 0.0001; Fig. 2C), while there was no significant effect of photoperiod on NAc core dopamine release in male mice (F (2,48) = 0.7842, p = 0.4622; Fig.  2D). Tukey's multiple comparison's post hoc tests showed significant differences in NAc core dopamine release between Long and Equinox photoperiod females at the single pulse (p = 0.029), 5-Hz (p = 0.002), 10-Hz (p = 0.0002), and 20-Hz (p = 0.001) stimulation frequencies, as well as significant differences between Short and Long photoperiod females at the 5-Hz (p = 0.031), 10-Hz (p = 0.003), and 20-Hz (p = 0.016) stimulation frequencies. Thus, the overall increase in NAc core dopamine release in Long photoperiod described above in Figure 1 reflects sex-specific effects of photoperiod on dopamine release in females. It is also important to note that DA release in NAc core slices from Females in the Long photoperiod group exhibit enhanced dopamine release at all stimulation frequencies tested, indicating that the photoperiod effect we observed is not dependent on stimulation frequency.
To assess possible synaptic mechanisms through which photoperiod exerts its effects on NAc core dopamine, we analyzed concentration of single pulse evoked DA release in females and males (Fig. 2E) and for the maximal rate of dopamine uptake (V max ; Fig. 2F). Two-way ANOVA revealed a significant main effect of photoperiod on dopamine release (F (2,27) = 3.424, p = 0.047). Tukey's post hoc multiple comparisons test showed that NAc core from Long photoperiod females had elevated dopamine release compared with those from Equinox photoperiod females (p = 0.02). We also found a significant main effect of photoperiod on dopamine uptake (F (2,27) = 6.029, p = 0.007) as well as a significant interaction of sex and photoperiod (F (2,27) = 4.334, p = 0.023) using two-way ANOVA. Maximal dopamine uptake in the NAc core was enhanced in Long photoperiod females compared with both Short (p = 0.004) and Equinox (p = 0.001) photoperiod groups as tested by Tukey's post hoc multiple comparisons test. We did not observe any significant differences in dopamine release or uptake in the males as tested by Tukey's post hoc multiple comparisons test. Taken together, our results indicate photoperiod has female-specific effects on both release and uptake of NAc core dopamine.

Long photoperiod enhances DAT inhibition in females
As photoperiod may exert its effects on NAc core dopamine release and uptake in females through modulation of DAT, we examined a role for DAT using a selective DAT inhibitor (GBR 12783). We measured peak dopamine release and uptake and derived changes to apparent DAT K m across doses of GBR 12783 and photoperiods (Fig. 3). Sample sizes were as follows: Short (males N = 4, females N = 4), Equinox (males N = 4, females N = 5), and Long (males N = 5, females N = 6) where N is number of animals. Consistent with Figure 2E, we again confirmed with twoway ANOVA the significant effect of photoperiod on evoked  3E). In addition, in NAc core from female mice the apparent K m of DAT in response to uptake inhibition showed main effects of photoperiod (F (2,48) = 16.21, p , 0.0001) and of GBR 12783 dose (F (3,48) = 12.27, p , 0.0001), as well as a significant interaction of photoperiod and dose (F (6,48) = 4.585, p = 0.0009; Fig. 3C) as tested by two-way ANOVA. We found that the action of GBR 12783 on DAT uptake inhibition was enhanced in slices from Long photoperiod females at 1 mM compared with Short (p = 0.019) and Equinox females (p = 0.017) and at 3 mM compared with Short (p , 0.0001) and Equinox females (p , 0.0001) using Tukey's post hoc multiple comparisons test. Thus, pharmacological inhibition of DAT by GBR 12783 was more sensitive in the NAc core of Long photoperiod female mice. We did not find a significant effect of photoperiod on pharmacological DAT inhibition in males (F (2,40) = 0.3393, p = 0.7143; Fig. 3F). Our results showing that GBR 12783 was more potent in Long photoperiod females demonstrates sex-specific photoperiod effects on DAT function in NAc core dopaminergic terminals.

Discussion
Here, we have laid the groundwork for the broader question of how environmental signals can program or shape the neural circuits that promote affective behavior. We have shown that photoperiod, one of the most pervasive and reliable environmental signals, impacts mesolimbic dopamine signaling, a key component of the neural circuitry that contributes to affective state. Our results, for the first time, demonstrate that photoperiod fine-tunes the subsecond signaling dynamics of neuromodulatory synapses.
Research Article: New Research enhanced dopamine release and uptake. This female-specific photoperiod effect was present at all stimulation frequencies we tested. This indicates that the enhancement of dopamine release in Long photoperiod females was not dependent on stimulation frequency, suggesting that Longsummer like photoperiod enhances dopamine release in both tonic and phasic firing conditions. There are several possible mechanisms through which this could occur. Cholinergic interneurons (CINs) in the NAc can enhance and even directly elicit dopamine release from terminals in the NAc through presynaptic nicotinic acetylcholine receptors (Yorgason et al., 2017). There is evidence that NAc CIN activity is diurnal and drives diurnal rhythms of NAc dopamine release (Stowe et al., 2022), so it is possible that CIN firing rate may be responsive to photoperiod. Another possible mechanism through which photoperiod could affect dopamine release is through regulation of D2 dopamine autoreceptors. Previous work has shown that the ratio of D2 to D1 dopamine receptor expression in the striatum shifts to increase D2 and decrease D1 expression in a mutant clock gene mouse model (Spencer et al., 2012), which warrants further investigation as to whether photoperiod may also regulate the D2 receptor to exert its effects on dopamine release.
We also found that dopamine uptake was enhanced in the Long-photoperiod females. Because DAT activity can Research Article: New Research influence dopamine release through the recycling of transmitter (Egaña et al., 2009;Condon et al., 2019), it is also possible that the enhanced DAT activity in Long photoperiod females also enhances dopamine release. Using the DAT inhibitor, GBR 12783, we found that Long photoperiod modulated the sensitivity of DAT to pharmacological inhibition in females, consistent with our findings that female baseline DAT activity is also different by photoperiod. If photoperiod's effect on baseline DAT function drives the differences we see in dopamine release, one would expect that inhibition of DAT should decrease dopamine release. However, we found that inhibiting DAT using GBR 12783 did not have a significant effect on dopamine release at any of the concentrations we tested. It has been shown that GBR 12783 actually enhances dopamine release in striatal synaptosomes (Bonnet and Costentin, 1986), potentially occluding any decreases in dopamine release. Taken together, further studies are necessary to determine the mechanism through which dopamine release is modulated by photoperiod.
Previous studies have shown that increased DAT expression in genetic and experience-dependent models in rodents does not affect the potency of DAT inhibitors (Calipari et al., 2013(Calipari et al., , 2014. Rather, functional modifications to DAT, such as phosphorylation at threonine 53, enhance baseline dopamine uptake and potency of DAT inhibitors (Morón et al., 2003;Foster et al., 2012;Calipari et al., 2017). Thus, because we found photoperioddependent differences in DAT inhibitor potency in females, we suggest it is likely that photoperiod acts on dopamine signaling at least in part through increased DAT function through regulation of DAT phosphorylation.
The sex-specific effects of photoperiod we have observed here warrant further investigation of the mechanisms through which photoperiod affects males and females differently. There are at least three major biological phenomena that can underlie sexually dimorphic phenotypes: sex hormone action in the adult organism, sex hormone action during development, and genetic influence of sex chromosomes (Becker et al., 2005;McCarthy and Arnold, 2011). Estrous cycle-dependent and sex-hormone regulation of the adult mesolimbic dopamine system is well established (Becker, 2009;Yoest et al., 2014;Calipari et al., 2017;Becker and Chartoff, 2019), and decades of literature have examined how photoperiod affects sex hormone regulation with respect to seasonal mating behaviors (Ortavant et al., 1988;Luboshitzky and Lavie, 1999;Walton et al., 2011;Borniger and Nelson, 2017). Furthermore, studies have shown a link between photoperiod, melatonin, and sex hormones in humans (Reiter, 1998;Luboshitzky and Lavie, 1999;Kuzmenko et al., 2021). It is possible that photoperiod influences the estrous-cycle to exert female-specific effects on synaptic dopamine dynamics, as previous work has shown that females in estrous show enhanced dopamine release and uptake (Calipari et al., 2017) similar to the females raised in Long photoperiod. Photoperiod has also been linked to regulation of puberty onset and sex hormones during the peripubertal period (Ebling and Foster, 1989;Adam and Robinson, 1994;Ebling, 2010;Bohlen et al., 2018), which is a critical developmental period that affects the mesolimbic dopamine system (Becker, 2009;Kuhn et al., 2010; as well as reward-related behaviors (Yoest et al., 2014;Harden et al., 2018;Becker and Chartoff, 2019). Future research using multiple developmental timepoints may reveal whether female-specific photoperiod modulation of dopamine signaling depends on a critical period of development. To our knowledge, studies have not examined how sex chromosomes influence photoperiodism in mammals. However, sex chromosomes, independent of gonadal phenotype, have been directly implicated in sex differences in the dopamine system (Carruth et al., 2002;Dewing et al., 2006), and have even been shown to play a role in regulating reward-related behavior (Barker et al., 2010). Future studies using the four core genotype model should examine the mechanisms through which sex-specific effects of photoperiod could emerge across development either by sex chromosomes or through gonadal hormones.
We were intrigued to find that dopamine release and uptake in the NAc core of male mice were not significantly influenced by photoperiod, especially since previous work using males have shown photoperiod can impact the striatal dopamine system by several measures. Findings from Goda et al. (2015) showed that longer photoperiod increased tissue dopamine content in the striatum of male chipmunks. Additionally, dopamine turnover was blunted in the NAc of male mice raised in winter-like short photoperiod (Itzhacki et al., 2018). In humans, striatal presynaptic dopamine synthesis was found to be enhanced in fall/winter months in both men and women (Eisenberg et al., 2010). Extracellular dopamine concentration has been shown to have an inverse relationship with evoked synaptic dopamine release , and dopamine synthesis rate can paradoxically change in the opposite direction from overall dopamine tissue content . As previously mentioned, our study is the first to examine photoperiod's effect on the functional subsecond signaling of dopamine in the NAc core. As such, it is possible that photoperiod may indeed affect measures of dopamine in males as assessed by the aforementioned studies independently from our measures of synaptic dopamine dynamics. Further studies are needed to understand the effect of photoperiod on the male dopamine system and how it may differ from females to produce seasonal changes in behavior.
Another important distinction between previous photoperiod studies and our own is our use of the melatonincompetent C3Hf 1/1 strain. The commonly used C57/BL6J strain (C57), does not produce physiological levels of endogenous melatonin because of genetic mutations in the melatonin synthesis pathway (Ebihara et al., 1986;Goto et al., 1989). Melatonin is a hormone that is released from the pineal gland at the onset of darkness and is inhibited by the onset of light, effectively encoding photoperiod. Therefore, melatonin is considered a main biological signal through which seasonal light information is transduced (Brzezinski, 1997;Hardeland et al., 2006). Interestingly, Green et al. (2015) showed that photoperiodic influence of affective behaviors in C3Hf 1/1 mice depends on melatonin signaling through the melatonin MT1 receptor (Green et al., 2015). The species-specific effect of photoperiod on striatal dopamine content reported by Goda et al. (2015) could therefore be explained by the fact that chipmunks endogenously produce melatonin while C57 mice do not. The only commonly used lab strain of mice that produce endogenous melatonin is the C3H strain (Goto et al., 1989). This strain is homozygous for a mutation in the rd gene that results in rod/cone degeneration rendering them visually impaired (Tosini and Menaker, 1998), but as they retain their circadian photosensitive ganglion cells, they still entrain to circadian photoperiods (Panda et al., 2003). The C3Hf 1/1 strain has had the retinal degeneration mutation bred out (Contreras-Alcantara et al., 2012) so that mice retain normal vision for behavioral experiments, are sensitive to photoperiodic light information, and are melatonin competent.
Several studies have shown that nearly all aspects of dopamine transmission, including synaptic dopamine release, DAT activity, expression of dopamine receptors, dopamine synthesis and dopamine-dependent behaviors show diurnal rhythms (Naber et al., 1980;Schade et al., 1995;Akhisaroglu et al., 2005;Sleipness et al., 2007;Ferris et al., 2014;Stowe et al., 2022). Experiments in the present study were conducted during the mid-light phase of each photoperiod where synaptic dopamine release and DAT function are predicted to be at their peak . Using a single time point, we cannot determine whether photoperiod changes the amplitude, period, or waveform of the known diurnal rhythm of NAc dopamine dynamics to result in the differences we report here, and additional time point studies around the clock will be needed.
Previous work from our group used the same mouse model and photoperiod conditions to test how seasonally relevant photoperiods impact affective behaviors (Green et al., 2015). Mice raised in the Long summer-like photoperiod showed less anxiodepressive-like behaviors than mice raised in Short or Equinox photoperiods, as measured by forced swim test and elevated zero maze (Green et al., 2015). The effect of photoperiod on synaptic dopamine dynamics that we report here may influence dopamine-dependent affective behaviors beyond the anxiodepressive behaviors examined by Green et al. (2015). NAc core dopamine is highly involved in reinforcement learning (Tsai et al., 2009;Floresco, 2015;Kutlu et al., 2021), and our data predicts that photoperiod could drive differences in female reinforcement behaviors such as reward responding, acquisition, and motivation through modulation of NAc core dopamine. Stowe et al., 2022 demonstrated that the diurnal rhythm of sign-tracking behavior is synchronous with diurnal rhythms of phasic/tonic ratio of electrically evoked NAc core dopamine. Sign-tracking is an associative learning behavior dependent on NAc dopamine that is linked to addiction-related behaviors (Robinson et al., 2014). Based on our findings, we would predict that females raised in Long photoperiod would exhibit more sign-tracking behavior than males or females raised in other photoperiods. Such studies would be especially pertinent as evidence suggests that substance use disorders show seasonality as well (Sandyk and Kanofsky, 1992;Sher, 2004;Morales-Muñoz et al., 2017; Barbosa-Méndez and Salazar-Juárez, 2020).
Cocaine-related behaviors in rodents have also been shown to be circadian (Baird and Gauvin, 2000;Akhisaroglu et al., 2004;McClung, 2007;Falcón and McClung, 2009;Parekh and McClung, 2016;Depoy et al., 2017;Depoy et al., 2021), and disrupting circadian rhythm via mutations in the clock gene, Npas2, has female-specific effects on vulnerability to substance use (Depoy et al., 2017;Depoy et al., 2021). Npas2 mutant females, especially in the dark phase, showed increased cocaine self-administration, faster acquisition of cocaine self-administration, higher break point ratio, and increased extinction responding (Depoy et al., 2017;Depoy et al., 2021). These effects were abolished with ovariectomy of Npas2 mutant females, suggesting that ovarian hormones mediate these female-specific, circadian-dependent effects (Depoy et al., 2017;Depoy et al., 2021). Because they have more robust dopamine signaling, it is possible that females raised in Long photoperiods may be more vulnerable to increased substance use in response to circadian disruption. In fact, photoperiod has been shown to impact cocaine reinstatement behaviors, where switching to shorter photoperiods decreased cocaine-induced reinstatement, while switching to longer photoperiods had no significant effects (Sorg et al., 2011). However, this study used exclusively male rats, so future studies using females may reveal differential effects of photoperiod on cocaine reinstatement and other substance use behaviors. The mesolimbic dopamine system is implicated in the pathophysiology of many neuropsychiatric disorders, including major depressive disorder (MDD; Nestler and Carlezon, 2006;Øverland et al., 2019), seasonal affective disorder (SAD; Rosen et al., 1990;Praschak-Rieder and Willeit, 2012), and bipolar disorder (BD; Cousins et al., 2009). Multiple studies have shown that seasonality of depressive symptoms in MDD is more prevalent in women (Otte et al., 2016), and that there is a higher prevalence of SAD in women than in men (Magnusson, 2000). Additionally, more women exhibit seasonality in their BD symptoms than men with BD (Arnold, 2003;Geoffroy et al., 2014). The female sex-specific effects of photoperiod on NAc dopamine dynamics shown here, along with evidence of female-specific circadian effects on dopamine-dependent behaviors (Depoy et al., 2017;Depoy et al., 2021), indicate the possibility that the female dopamine system is more sensitive to circadian light information than males. Thus, females may experience more fluctuation or seasonal changes to the dopamine system contributing to female-specific seasonality of certain neuropsychiatric disorders. These data open the door to future investigation of the mechanisms of sex differences in seasonality of neuropsychiatric disorders.
Taken together, our study demonstrates that photoperiod impacts the subsecond release and uptake dynamics of synaptic mesolimbic dopamine, revealing a novel process by which photoperiod impacts neural systems involved in reward and affective behaviors. These findings identify a potential role for the dopamine system in mediating sex differences in seasonality on a synaptic basis, and an animal model in which to further explore potential mechanisms of sex differences in neuropsychiatric disorders.