Sex chromosome complement influences vulnerability to cocaine in mice (original) (raw)

. Author manuscript; available in PMC: 2021 Sep 1.

Abstract

Women acquire cocaine habits faster and are more motivated to obtain drug than men. In general, female rodents acquire intravenous cocaine self-administration (SA) faster and show greater locomotor responses to cocaine than males. Sex differences are attributed to differences in circulating estradiol. We used the four core genotype (FCG) mouse to ask whether sex chromosome complement influences vulnerability to cocaine’s reinforcing and/or locomotor-activating effects. The FCG cross produces ovary-bearing mice with XX or XY genotypes (XXF, XYF) and testes-bearing mice with XX or XY genotypes (XXM, XYM). A greater percentage of gonadal females acquired cocaine SA via infusions into jugular catheters as compared with XYM mice, but XXM mice were not significantly different than any other group. Discrimination of the active versus inactive nose poke holes and cocaine intake were in general greater in gonadal females than in gonadal males. Progressive ratio tests for motivation revealed an interaction between sex chromosomes and gonads: XYM mice were more motivated to self-administer cocaine taking more infusions than mice in any other group. Locomotor responses to cocaine exposure revealed effects of sex chromosomes. After acute exposure, activity was greater in XX than in XY mice and the reverse was true for behavioral sensitization. Mice with XY genotypes displayed more activity than XX mice when given cocaine after a 10-day drug-free period. Our data demonstrate that sex chromosome complement alone and/or interacting with gonadal status can modify cocaine’s reinforcing and locomotor-activating effects. These data should inform current studies of sex differences in drug use.

Keywords: mice, cocaine, self-administration, sex chromosomes, locomotor activity, sex differences, addiction

Introduction

Women are more motivated to take cocaine and develop cocaine use disorder more quickly after initial use than men (DeVito et al., 2014). Many studies in rats report faster acquisition of intravenous cocaine self-administration (SA) in ovary-intact females as compared with testes-intact males (Becker and Koob, 2016). Sex differences in vulnerability to cocaine are also present in mice; however, fewer studies have been published. In general, in gonadally intact mice, females acquire cocaine SA faster than males, take more cocaine, and are more motivated to obtain infusions of cocaine under a progressive ratio (PR) schedule of reinforcement (Castro-Zavala et al., 2020; Engeln et al., 2019; Martini et al., 2014; but see Griffin et al., 2007). Sex differences in both rats and mice have also been reported for the behavioral activating effects of cocaine, measured by locomotor activity. Typically gonad intact females show greater locomotor activity after cocaine than their male counterparts (Bowman and Kuhn, 1996; Thomsen and Caine, 2011; Van Swearingen et al., 2013).

For both the reinforcing and locomotor effects of cocaine, sex differences are generally attributed to differences in circulating sex hormones, particularly estradiol (Sell et al., 2000). In rats, gonadectomy attenuates the reinforcing effects of cocaine to equally low levels in both sexes (Caine et al., 2004). Hormone replacement studies in females show that estradiol treatment following ovariectomy restores rates of acquisition of cocaine SA to levels in gonad-intact female (Festa and Quinones-Jenab, 2004; Hu and Becker, 2008; Lynch et al., 2001). Likewise, sensitization to the locomotor effects of cocaine are reduced by ovariectomy and enhanced by estradiol in ovariectomized rats (LaRese et al., 2019; Martinez et al., 2016; Peterson et al., 2016; Souza et al., 2014). Interestingly, estradiol has little or no effect in gonad intact or castrated male rats when cocaine SA, acute locomotor activity, or sensitization in response to repeated administrations of cocaine are measured (Cummings et al., 2014; Jackson et al., 2006). These findings show that while estradiol modifies cocaine-related behaviors in females, it fails to do so in males suggesting that other, unknown factors, contribute to well-documented sex differences. One potential factor that we explore here is sex chromosome complement. Although not yet investigated in the context of cocaine addiction, sex chromosome complement is known to impact the reward pathway and responses to other drugs of abuse, such as alcohol (Barker et al., 2010).

Using the four core genotype mouse model we tested the influence of sex chromosome complement on cocaine-related behaviors. The four types of offspring in this model are ovary-intact females with XX chromosomes (XXF), ovary-intact females with XY chromosomes (XYF), testes-intact males with XY chromosomes (XYM), and testes-intact males with XX chromosomes (XXM; De Vries et al., 2002). The FCG mice have been used to examine the role of sex chromosome complement in rewarding behaviors. For example, XX mice (with or without gonads) were faster at food-reinforced instrumental habit formation than XY mice, regardless of gonadal phenotype (Quinn et al., 2007). In contrast, when alcohol was used as the reward, chromosomal XX mice remained sensitive to outcome devaluation, but XY mice did not, suggesting that XY mice were faster to show habit formation (Barker et al., 2010). This result was noted in both gonadectomized and gonad intact mice. In gonadectomized FCG mice tested for motivation to obtain a highly palatable diet, mice with the XY sex chromosome complement, as compared to those with two X chromosomes, were more sensitive to reward (Seu et al., 2014). The findings reviewed above indicate that the role of sex chromosome complement in behavior depends on the particular behavior and that sex chromosomes interact with gonadal hormones to influence rewarding behaviors. Here, we assessed the role of sex chromosome complements in cocaine-related behaviors using gonad-intact mice. Estradiol and testosterone levels in the FCG mice are typical for their gonadal sex (Arnold and Chen, 2009; Corre et al., 2016; Gatewood et al., 2006) making it possible, within each gonadal sex, to disassociate the actions of gonadal hormones from the effects of sex chromosomes. We chose this paradigm because it is more translational than gonad-less mice, however to completely unravel interactions between the two factors we are now using gonadectomized mice with equivalent hormone treatments.

Material and Methods

Animals and general husbandry

For all experiments we used four core genotype (FCG) mice fully backcrossed to C57BL/6J. The mice were produced by 25 breeding pairs. The FCG mice are produced by crossing normal XX females with males bearing a spontaneous mutation in the testes determining gene, Sry, which has been rescued by an autosomal insertion (on Chromosome 3) of a Sry transgene (designated as XY−Sry; Itoh et al., 2015; Mahadevaiah et al., 1998). Genotyping for the Y chromosome was conducted as described in the literature (De Vries et al., 2002). The colony was bred and maintained at North Carolina State University. Mice were housed in groups of 3 to 5 animals (of the same gonadal sex) with food (Envigo Teklad2020, Madison, WI, USA) and water ad libitum, room lights were on a reverse L:D cycle of 12:12 (lights off at 0800h). Mice were tested starting between 70-85 days of age. All animal care and procedures were approved by the NCSU animal care and use committee and in accordance with AAALAC standards.

Experiment 1

One week before jugular catheter implantation surgery, animals used for cocaine SA procedure (XXF n=17; XYF n=16; XXM n=17; XYM n=20) were moved to individual home cages fitted with vertical metal plates. These plates allowed them to habituate rapidly to the test apparatus (Miczek and de Almeida, 2001). The plates appear as a solid wall with a middle hole for access to their water bottle spout. In their home cage no other holes were present, but in the test apparatus, the home cage plate was replaced with one that contained the active and inactive nose poke holes. During the SA procedure, two mice in the XXF, XYF and XXM groups and one mouse in XYM group lost catheter patency and were removed from all statistical analyses of data. The final numbers per group that completed acquisition testing were: XXF n = 15; XYF n = 14; XXM n = 15; XYM n = 19.

Experiment 2

Another FCG cohort was used for Experiment 2 to examine cocaine-induced locomotor activity. For the acute component, locomotor activity was assessed in response to an intraperitoneal (IP) injection of saline (n = 14/group) or a low (5 mg/kg; n = 8 for XXF and XYF and n = 6 for XXM and XYM), moderate (10 mg/kg; n = 8/group), or high dose (20 mg/kg; n = 8/group) of cocaine. For behavioral sensitization, a subset of these mice (saline and 20 mg/kg dose) continued to receive either saline or cocaine for 4 more days and then a final injection after a 10-day drug-free period. This created two groups: mice treated with 20 mg/kg cocaine each day (n = 8 per group), and mice that received saline on all test days except the final one when 20 mg/kg cocaine was given (n = 8 per group). All mice in this experiment were group housed (by gonadal sex) for the duration of the experiment except during the locomotor test sessions as described below.

Drug

Cocaine-HCl was obtained from the National Institute of Drug Abuse and dissolved in sterile 0.9% physiological saline. Cocaine was administered via intravenous infusions for the SA procedure and via IP injections for the locomotor activity procedure. For SA, infusion durations were adjusted daily based on individual body weight (2 s/27.5 g) to maintain a consistent cocaine dose (0.3, 0.6 or 1.0 mg/kg). For locomotor behavior, cocaine-HCl (5, 10 or 20 mg/kg) was dissolved in saline and administered in a volume of 0.1 ml/10 g body weight.

Procedures

Experiment 1. Jugular catheterization and maintenance

Catheterization methods were similar to those described in the literature (Kmiotek et al., 2012; Thomsen and Caine, 2007). Catheters made from silicon tubing (Access Technologies; 0.2mm ID x 0.4mm OD) were connected under the skin to a 26 GA catheter (PlasticsOne catalog #315BM) protruding from the back. Mice were allowed 3–5 days to recover from surgery. Prior to SA testing, catheters were flushed daily with heparinized saline (30 USP units/ml) and antibiotic (ticarcillin, 67 mg/ml). Thereafter, catheters were flushed before and after each daily testing session with heparinized saline. Patency of intravenous catheters was evaluated every 7-8 days, and when SA behavior appeared to deviate dramatically from that observed on the previous day of the same cocaine dose. Patency was checked by infusion of 20 μl of Brevital Sodium (Methohexital Sodium, JHP Pharmaceuticals). If prominent signs of anesthesia were not apparent within 3-s of the infusion, the catheter was assumed to be clogged and the mouse was removed from the experiment.

Acquisition of Cocaine SA

The SA apparatus consisted of a metal plate equipped with two lit nose poke holes, which was inserted vertically into the home cage during testing (Miczek and de Almeida, 2001). Each home cage was placed within a ventilated sound-attenuating box (ENV-018M, Med Associates, St. Albans, VT, USA). A spring-supported arm holding a liquid swivel (Instech Laboratories, Plymouth Meeting, PA) was affixed to the top of the panel, and a house light illuminated the cage. One of the nose poke holes was identified as the “active” hole by an internal yellow light. The active hole (left versus right) was counterbalanced across subjects and remained constant throughout the experiment. Each daily session started with the house light on, the presentation of active hole cue light, and a priming infusion of the drug. Each reinforcement was followed by a 30-s time-out to prevent overdose. During the time-out, the house and cue lights were off; nose poking in both holes was recorded throughout the test.

Mice were tested under a fixed-ratio (FR1) schedule with ascending doses of cocaine (0.3, 0.6 and 1 mg/kg/infusion) each available for four consecutive daily sessions (starting at 1000 h). This dose-ascending dose procedure was selected in order to maximize individual differences in vulnerability to cocaine reinforcing effects, which are more apparent at low doses, while also maximizing the likelihood of eventual acquisition, which is increased at higher doses. Daily sessions terminated after the maximum number of reinforcements were delivered (20 mg/kg/session; 66 reinforcements for 0.3 mg/kg; 33 reinforcements for 0.6 mg/kg and 20 reinforcements for 1 mg/kg) or after 2-h, whichever occurred first. Acquisition was defined as two consecutive sessions with an average intake of 10 mg/kg/day and with 70% or more of responding occurring in the active nose poke hole. One mouse reached criteria on the last day of the test series, and we extended its SA testing for an additional day (for a total of 13 days).

Progressive Ratio Test

After mice achieved the acquisition criteria, differences in the reinforcing effects of cocaine were evaluated using a PR schedule in which the response requirement to earn an infusion escalated according to the following series: 1, 2, 4, 6, 9, 12, 16, 20, 25, 30, 36, 42, 49, 56, 64, 72, 81, 90, 100, 110, 121, 132, 144, 156, 169, 182, 196, 210, 225, 240. Progressive ratio sessions lasted for 4-h or until the individual failed to respond for 1-hour. The final ratio completed, or the breakpoint, are both sensitive measures of reinforcing efficacy or motivation for the drug (Arnold and Roberts, 1997). Sessions were conducted daily for 3 consecutive days.

Experiment 2. Spontaneous Activity

Tests were similar to those used by others (Balda et al., 2009). Mice were handled for 2-4 mins daily starting 3 days before the study. Prior to injection mice were habituated to the open field boxes (60cm x 60cm x 45cm) for 30 min. Mice then received an acute IP injection of saline or cocaine (5, 10 or 20 mg/kg) and locomotor activity was recorded for 1 h. Mice in the behavioral sensitization study continued to receive either saline or the 20 mg/kg dose of cocaine for 5 consecutive days in the open field with a test recorded on the fifth day. These mice underwent a final recorded test session after a 10 day drug-free period wherein, after habituation to the field, both the saline and the cocaine (20 mg/kg) groups received a 20 mg/kg injection of cocaine. Activity data (in meters moved in the horizontal plane) were analyzed using Noldus EthoVision XT (Leesburg, VA). All tests were performed during the dark phase of the light/dark cycle.

Data analysis

For acquisition, dependent measures were the percentage of mice per group that acquire SA, the difference between nose pokes in the active and inactive holes and cocaine intake. Rate of acquisition and percentage of mice acquiring SA were compared between the four groups (XXF, XYF, XXM, XYM) using a Kaplan–Meier survival analysis and the Log-rank (Mantel–Cox) statistic (Aarde et al., 2015; Burke and Miczek, 2015; Campbell et al., 2002; Lynch, 2009; Sanchez et al., 2015). Since this analysis only allows for a single factor, acquisition was also assessed using multi-factor ANOVA to which factor(s) chromosomal sex versus gonadal sex contributed to group differences. For cocaine intake and the discrimination ratio of active versus inactive responses over the acquisition period we used repeated measure two-way ANOVA. For the PR data, the dependent measure was the average number of infusions was calculated across the three sessions and group differences were examined using a two-way ANOVA. Cocaine intake was calculated for both all the subjects and also for only those mice that acquired during the 12-day acquisition period. Post-hoc comparisons were made using Bonferroni-corrected comparisons.

Locomotor activity induced by an acute injection of cocaine (0, 5, 10, or 20 mg/kg) was analyzed with a three-way ANOVA (chromosomal sex x gonadal sex x dose). Behavioral sensitization following repeated exposure and a 10-day injection free period was examined by comparing the difference in locomotor activity on day 1 of administration versus day 15 using a three-way repeated measures ANOVA (chromosomal sex x gonadal sex x treatment).

All the analyses were conducted using the statistical package Statistical Package for the Social Sciences (SPSS) 25 for Windows (SPSS, Chicago, IL, USA). Differences were considered significant if the probability of error was less than 5 %.

Results

Cocaine SA

Gonadal sex affects acquisition

Acquisition of SA was achieved by 93% of XYF, 80% of XXF and 53% of the XXM group, and only 42% of the mice in XYM group. The differences were significant based on the survival distribution (χ2(3)=8.86, p<0.03; Fig. 1). Subsequent pairwise comparisons revealed that significantly fewer XYM acquired SA as compared with either group of gonadal females (versus XXF, χ2=4.43, p<0.04; versus XYF, χ2=6.93, p<0.01). The XXM group was statistically similar to all other groups.

Fig. 1. Acquisition of cocaine self-administration.

Fig. 1.

Percent of each group reaching cocaine self-administration acquisition criteria. Mice were trained using a Fixed Ratio of 1 with escalating doses of cocaine (0.3, 0.6 and 1 mg/kg/infusion). Symbols in grey represent gonadal females and black are gonadal males. Data from XX genotypes are shown in circles and XY are in squares. *Significant difference between the XYM group as compared with both groups of gonadal females (p < 0.05). N's per group XXF = 15, XYF = 14, XXM = 15, XYM = 19.

Both sex chromosomes and gonadal sex affect discrimination between active and inactive nose poke holes

Analysis of the discrimination ratio between active and inactive nose poke holes revealed an overall effect of gonadal sex, female mice showed better discrimination as compared to males. A repeated measures ANOVA of the discrimination ratio showed an effect of day (Greenhouse-Geisser correction F(7.35,426.46)=17.36, η2=0.23, p<0.001), an interaction between day and gonadal sex (Greenhouse-Geisser correction F(7.35,426.46)=2.09, η2=0.035, p<0.045), and an interaction between day, gonadal sex, and chromosomal sex (Greenhouse-Geisser correction F(7.35,426.46)=2.30, η2=0.038, p<0.025). A significant effect of gonadal sex was present on the first and final days of SA (F1,64)=3.93, 11.58 and η2=0.061, 0.167 respectively, p<0.05 at least ), at both times, gonadal females had better discrimination as compared to gonadal males. On the third day at the medium cocaine dose we noted a significant interaction between sex chromosomes and gonadal sex (F1,64)=5.73, η2=0.087, p<0.02) which was caused by a lower discrimination ratio in XYM mice as compared to XXM (p<0.012) and XYF mice (p<0.02, Fig. 2).

Fig. 2. Discrimination ratio between active and inactive nose poke holes.

Fig. 2.

Difference between active and inactive nose pokes (Mean + SEM) during acquisition training. Grey symbols represent gonadal females and black symbols show data from gonadal males. The XY genotype groups are in squares with dashed lines, and the XX are in circles with solid lines. * Significant difference between gonadal females and gonadal males (p < 0.05). ** Significant difference between the XYM group and both the XXM and XYF groups (p < 0.02 at least). N's per group XXF = 15, XYF = 14, XXM = 15, XYM = 19.

Intake is greater in gonadal females than gonadal males

When all mice were included in the analysis, gonadal females infused significantly more drug than gonadal males (Fig. 3A). Repeated measures ANOVA for cocaine intake revealed that the amount infused was significantly affected by gonadal sex and day (F(11, 649)=2.17, η2=0.036, p=0.014) with gonadal females taking more drug than gonadal males. There was a significant three-way interaction of day by sex chromosome by gonadal sex (F(11, 649)=2.93, η2=0.047 p<0.001). A two-way ANOVA on data from the final session on each cocaine dose, showed that while all groups obtained the same amount of cocaine at the lowest dose (0.3 mg/kg), a gonadal sex effect was present at the medium and high doses of cocaine. In both cases gonadal females took more cocaine than gonadal males (0.6 mg/kg, F(1,63)=4,87,η2=0.076, p<0.05; 1 mg/kg, F(1,63)=6.21, η2=0.095, p<0.05). However when only the mice that acquired were included in this analysis we noted that the amount infused was significantly affected by test day (Greenhouse-Geisser correction F(3.88, 143.65)=90.26, η2=0.709 p<0.001, Fig. 3B). There was a significant three-way interaction of day by sex chromosome by gonadal sex (Greenhouse-Geisser correction F(3.88, 143.65)=2.70, η2=0.068 p<0.035), no paired comparisons were significant.

Fig. 3. Cocaine intake over self-administration.

Fig. 3.

Mean (+SEM) cocaine intake (mg/kg). Grey symbols represent gonadal females and black symbols show data from gonadal males. The XY genotype groups are in squares and the XX are in circles. A. All mice in the SA study are included. #Significant difference based on gonadal sex, intake of cocaine in gonadal females is greater than in gonadal males (p < 0.05). *XYF intake significantly more than XYM mice on days 8 and 12. N's per group XXF = 15, XYF = 14, XXM = 15, XYM = 19. B. No significant effects were found for sex chromosomes or gonadal sex in mice that acquired SA. N's per group XXF = 10, XYF = 11, XXM = 7, XYM = 6.

Effects of gonadal and chromosomal sex on motivation for cocaine

The mice that achieved the acquisition criteria, were further tested for motivation to obtain cocaine using a PR schedule. Sex chromosome complement (F(1,30)= 12.98, η2=0.302, p<0.001) and gonadal sex (F(1,30)=5.78, η2=0.162, p<0.025) significantly affected motivation to obtain cocaine. We also found an interaction between these variables (F(1,30)=6.11, η2=0.169, p<0.02; Fig. 4). The effects were caused by the XYM group which was significantly different than all other groups (p<0.05).

Fig. 4. Progressive ratio data for mice that acquired self-administration.

Fig. 4.

Individual scores for each mouse in the progressive ratio test. Average number of infusions recorded and ratio reached (breakpoints) after 3 daily consecutive PR cocaine self-administration sessions. Grey symbols represent gonadal females and black symbols show data from gonadal males. The XY genotype mice are in squares and the XX are in circles. *XYM group is significantly different from all other groups (p < 0.01). N's per group XXF = 10, XYF = 11, XXM = 7, XYM = 6.

Cocaine-induced activity and behavioral sensitization

Sex chromosome complement affects cocaine-induced locomotor activity

Sex chromosome complement produced differences in acute responses to cocaine with XX mice of both gonadal sexes displaying more locomotor activity overall than XY mice (F(1,148)=12.97, η2=0.089, p<0.001). There was an effect of dose (F(3,148)=125.93, η2=0.741, p<0.001) as anticipated, more activity was elicited proportional to the dose of cocaine. An interaction between chromosomal sex and dose was noted (F(3,148)=3.42, η2=0.072, p<0.019). At the lowest dose of cocaine (5 mg/kg) XXM mice were significantly more active than XYM mice (F(1,24)=6.61, η2=0.216, p<0.05; Fig. 5). At the middle dose XXF mice were more active than XY mice, both gonadal males and females (F(1,28)=6.04, η2=0.178, p<0.05). No significant differences were present with the highest dose of cocaine.

Fig. 5. Activity after the initial exposure to cocaine.

Fig. 5.

Mean (+SEM) horizontal distance traveled (in meters, m) after an initial acute injection of saline or cocaine. # Significant effect of sex chromosome complement, XX mice move a greater distance than XY mice (p < 0.001). *Significantly different from XYM given the same cocaine dose (p < 0.05). +Significant difference between XXF mice and XY mice (of both gonadal sexes) given the same dose (p < 0.05). N's per group: saline n = 14 for each group; cocaine dose of 5 mg/kg; n = 8 for XXF and XYF groups and n = 6 for XXM and XYM groups; for cocaine doses of 10 and 20 mg/kg; n = 8 for each group.

Sex chromosomes affect behavioral sensitization

Two groups of mice were tested, one that received saline, and another that received 20 mg/kg cocaine, for 5 consecutive days, 10 days later, both received cocaine injection (20 mg/kg). A repeated measures ANOVA found a significant effect of day (F(1,56)=180.58, η2=0.763 p<0.001) and an interaction of day by treatment (F(1,56)=26.54, η2=0.322 p<0.001, Fig. 6). In addition there was an interaction between day, sex chromosomes, and treatment (F(1,56)=4.82, η2=0.079 p<0.035). Comparing the differences in locomotor responses between the final and first day within each group, no effects of sex chromosome or interactions with this factor and others were found for the mice that received saline on Day 1 and a single challenge of cocaine on Day 15. In the group of mice that received cocaine from Day 1 through 5 and a challenge of cocaine on Day 15, the differences in locomotor response were attributed to sex chromosomes (F(1,32)=4.16, η2=0.129 p<0.05). This was due to more locomotor activity on Day 15 as compared to Day 1 in mice with XY genotypes than in XX mice.

Fig. 6. Behavioral sensitivity: Activity differences between the first and final injections.

Fig. 6.

Mean (+SEM) difference in horizontal distance traveled (in meters, m) between the final and first acute injection. One group of mice received saline for 5 days and after 10 days without disturbance an injection of cocaine (Saline+Cocaine 20 mg/kg, white histograms). A second group received 20mg/kg cocaine for 5 days and a cocaine challenge after 10-days without injections (Cocaine 20mg/kg, black histograms). * In Cocaine 20mg/kg mice XY individuals had more activity as compared with XX mice, on the final day as compared to the first day of treatment (p < 0.05). N's per group = 8/each treatment and genotype.

Discussion

This is the first demonstration that sex chromosome complement influences some aspects of vulnerability and behavioral responsivity to cocaine. Since the physiological mechanisms that underlie sex differences in human drug use are largely unknown, we suggest sex chromosome complement is involved in vulnerability and other aspects of cocaine use. Since we assessed several behaviors our data reveal several patterns, which we will discuss in detail. For behaviors related to acquisition, the major factor contributing to sex differences was gonadal sex. For both cocaine-induced activity behaviors, there were sex chromosome effects as we hypothesized. Finally, and perhaps the most interesting outcome, is one suggesting interactions between gonadal and chromosomal sex. These effects were best illustrated by the PR data in which the XYM group was more motivated to obtain cocaine than any other group.

The most straightforward effect we can attribute to gonadal sex only is cocaine intake. Cocaine intake over the 12-day acquisition period was higher in gonadal females than in gonadal males. This is confirmation of data from rats in which gonadal females take more cocaine during SA than males (Lynch and Carroll, 1999; Swalve et al., 2016). However, when only the mice that acquired are examined there were no differences in intake. Our result suggests that estradiol may be responsible for this sex difference as it is in rats. In gonadectomized rats the sex difference in intake is noted (Hu et al., 2004) but is only facilitated by estradiol in females (Perry et al., 2013). These gonadal sex effects indirectly confirm that there is nothing exceptional about neonatal hormone levels, puberty onset, estrous cycles, or adult estradiol or testosterone levels in FCG mice (Corre et al., 2016; Gatewood et al., 2006; Itoh et al., 2015).

Acquisition was greatest in gonadal females, while XYM had the lowest rates. Strikingly, while the vast majority of XYF (93%) mice, but only 43% of mice in the XYM group, acquired under any dose. Thus, an XY genotype facilitated vulnerability in mice with ovaries but delayed or prevented acquisition in testes-bearing mice. Somewhat similarly, discrimination between the active and the inactive nose-poke holes gonadal females had better discrimination than gonadal males both early in acquisition and at the end. However at the middle cocaine dose, XYF and XXM mice discriminated better than XYM mice. This suggests that at some doses of cocaine there are interactions between chromosomal and gonadal sex. The effects of cocaine dose are also noted in cocaine-induced activity.

One alternate explanation for the acquisition data worth considering is that these effects are produced by genotypic differences in learning abilities in different genotypes of FCG mice. The radial arm maze (RAM) test revealed a robust sex difference (M>F) in both working and reference memory in rats as well as mice (Jonasson, 2005). In gonad-intact FCG mice tested in the RAM, males of both sex chromosome genotypes learned the task faster than females (Corre et al., 2016). In an active avoidance task conducted with the FCG mice, gonadal females escaped shock faster than gonadal males and no differences in shock sensitivity were noted (McPhie-Lalmansingh et al., 2008). These data do not support a sex chromosome effect on learning. Moreover, sex chromosome complement also affected acute activity responses to cocaine which are not learned behaviors.

Another possible explanation is that in rodents, positive correlations between cocaine acquisition and anxiety have been reported (Anderson et al., 2018; Bavley and Rajadhyaksha 2019). There are two reports on anxiety-like behavior in the FCG mice. In a standard elevated plus maze test no sex chromosome or gonadal sex effects were found (McPhie-Lalmansingh et al., 2008). Using a more extensive battery of anxiety tests, and the FCG mouse in a different genetic background (MF1) time in the dark half of the light/dark box was greater in XY than XX mice (Kopsida et al., 2013). However, in three other anxiety tests no sex chromosome effects were found. This second study proves weak support for higher anxiety in XY then XX mice, and we might predict anxious mice would acquire SA faster than non-anxious mice (Anderson et al., 2018; Bavley and Rajadhyaksha 2019). Yet, the reverse was noted in our data set. Maybe this explains the greater percentage of XYF that acquire, although this is not statistically different from XXF.

An interaction between gonadal and chromosomal sex is illustrated best by the PR data. Motivation to obtain cocaine was only tested in the subset of mice that acquired. The majority of the XYM group failed to acquire thus this was the smallest group (n=6) tested, yet, these six animals were highly motivated to perform for cocaine. This result is different than one might expect based on reports in both mice and rats (Algallal et al., 2019; Lynch, 2008; Martini et al., 2014; Nicolas et al., 2019). But exceptions are noted, for example, C57BL/6J male mice were more motivated than females in a PR test when a more demanding PR2 schedule was used (Griffin et al., 2007).

This finding is the first, to our knowledge, to show a dissociation between acquisition of SA and motivation for the drug. Both of these measures are believed to reflect vulnerability to its reinforcing effects. It is possible that since only a minority of the XYM were tested for PR they reflect a vulnerable sub-population. We asked if some aspect of litter size, sex ratio, or maternal history could have produced this population. Reviewing our colony notes did not reveal obvious differences between the XYM mice that did and did not acquire. Further studies examining molecular differences, for example gene expression patterns in response to cocaine, in these sub-populations will provide very useful information.

As we noted here for acquisition, in another rewarding behavior, male sexual behavior, FCG males with XX genotypes displayed more copulatory behaviors (ejaculations and thrusts per mounts) than XY males (Bonthuis et al., 2012). Using a second mouse model with sex chromosome aneuploidy the same pattern was noted in XXY versus XY males. In a food-reinforced task XX mice with either gonad type were faster to acquire habit formation than XY mice, but following more extensive training both genotypes acquired the instrumental habit (Quinn et al., 2007). Examination of neurotransmitters in the prefrontal cortex of chronical stressed FCG mice revealed that stress increased expression of a number of dopamine, GABA and glutamate related genes and receptors, but only in XX individuals (Barko et al., 2019).

Sex differences in locomotor responses to cocaine have been described in mice and rats and typically females are more responsive both to acute and sensitized administration (Festa and Quinones-Jenab, 2004; Rowson et al., 2018; Zhou et al., 2016). This sex difference has also generally been attributed to estradiol (Cummings et al., 2014; LaRese et al., 2019; Peterson et al., 2016) but exceptions are found (no sex difference in C57BL/6J mice: Griffin and Middaugh, 2006; Zombeck et al., 2010). Here we found sex chromosome effects for both acute cocaine injections and sensitization. Overall, acute cocaine enhanced activity to a larger degree in XX than in XY mice. Yet in the sensitization data set the reverse was noted (XY>XX). In addition interactions between cocaine dose and sex chromosome complements warrant more investigation.

There are several mechanisms that could promote gonadal and chromosomal sex interactions. One straightforward interaction would be if any of the Y-chromosome genes (other than Sry) or X-chromosome genes (particularly those that escape X-inactivation) had direct protein-protein interactions with one of the gonadal hormone receptors. These physical interactions might lead to more or less transcription of target genes. Evidence of this sort is available in cancer-related studies of prostate cell lines. For example, an in vitro study with prostate cells reported that protein produced by the Y-chromosome gene, histone lysine specific demethylase 5D physically interacts with androgen receptor (AR) in the cell nucleus (Xie et al., 2017). Another possibility is that genes on the X chromosome that escape X-inactivation could interact with steroid receptors. It is important to recall that in women about 25% of the genes on the silenced X are not inactivated (Tukiainen et al., 2017). This number is smaller in mice but nearly all X-inactivation escaping genes on the mouse X are redundant with humans (Ma et al., 2018). One of these is the lysine demethylase gene, Kdm6a, which resides on the X-chromosome and escapes X-inactivation, thus it is over-expressed in XX individuals. One study described a physical interaction between the AR and KDM6A in a prostate cancer cell-line (Grasso et al., 2012). Indirect evidence of gonadal and chromosomal sex interactions come from studies of calbindin and growth hormone using a combination of FCG mice, ER alpha knockout mice and gonadectomy with or without estradiol replacement (Abel et al., 2011; Addison and Rissman, 2012; Bonthuis et al., 2011; Bonthuis and Rissman, 2013). Results demonstrated that sex differences were influenced by estradiol, its receptor and/or sex chromosome complement. Thus, there is precedence for interactions between sex chromosomes, steroids and their receptors but more direct work is needed.

This line of research is in its infancy but given the current drug crisis it is important to pinpoint mechanisms underlying drug vulnerability. Here we show, when differences in gonadal hormone levels are maximal, sex chromosome complement and gonadal sex interact to change motivation for cocaine. Interactions between these two factors is implied by the acquisition data since XXM mice are not different from XYM or gonadal females. We also see interactions in the PR results, as XYM mice differ from the other three groups. Moving forward, we hypothesize that either an XX sex chromosome complement and/or high estradiol levels facilitate vulnerability to cocaine. Work is ongoing in our laboratory to unravel these two factors and their roles in these important behaviors.

Highlights:

Acknowledgements

This work was funded by R21 DA042599 and R01 DA048638 (EFR and WJL) and a pilot grant from NIEHS funded under award P30ES025128 (EFR and MM).

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Declarations of interest: none

References

  1. Aarde SM, Huang PK, Dickerson TJ, Taffe MA, 2015. Binge-like acquisition of 3,4-methylenedioxypyrovalerone (MDPV) self-administration and wheel activity in rats. Psychopharmacology (Berl) 232, 1867–1877. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Abel JM, Witt DM, Rissman EF, 2011. Sex differences in the cerebellum and frontal cortex: roles of estrogen receptor alpha and sex chromosome genes. Neuroendocrinology 93, 230–240. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Addison ML, Rissman EF, 2012. Sexual dimorphism of growth hormone in the hypothalamus: regulation by estradiol. Endocrinology 153, 1898–1907. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Algallal H, Allain F, Ndiaye NA, Samaha AN, 2019. Sex differences in cocaine self-administration behaviour under long access versus intermittent access conditions. Addiction biology, e12809. [DOI] [PubMed] [Google Scholar]
  5. Anderson EM, Larson EB, Guzman D, Wissman AM, Neve RL, Nestler EJ, Self DW, 2018. Overexpression of the Histone Dimethyltransferase G9a in Nucleus Accumbens Shell Increases Cocaine Self-Administration, Stress-Induced Reinstatement, and Anxiety. J Neurosci. 38, 803–813. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Arnold AP, Chen X, 2009. What does the "four core genotypes" mouse model tell us about sex differences in the brain and other tissues? Front Neuroendocrinol 30, 1–9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Arnold JM, Roberts DC, 1997. A critique of fixed and progressive ratio schedules used to examine the neural substrates of drug reinforcement. Pharmacol Biochem Behav 57, 441–447. [DOI] [PubMed] [Google Scholar]
  8. Balda MA, Anderson KL, Itzhak Y, 2009. Development and persistence of long-lasting behavioral sensitization to cocaine in female mice: role of the nNOS gene. Neuropharmacology 56, 709–715. [DOI] [PubMed] [Google Scholar]
  9. Barker JM, Torregrossa MM, Arnold AP, Taylor JR, 2010. Dissociation of genetic and hormonal influences on sex differences in alcoholism-related behaviors. The Journal of neuroscience 30, 9140–9144. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Barko K, Paden W, Cahill KM, Seney ML, Logan RW, 2019. Sex-Specific Effects of Stress on Mood-Related Gene Expression. Mol Neuropsychiatry 5, 162–175. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Bavley CC, Rajadhyaksha AM, 2019. Anxiety, the Chicken or the Egg of Addiction: Targeting G9a for the Treatment of Comorbid Anxiety and Cocaine Addiction. Neuropsychopharmacology. 44, 1345–1346. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Becker JB, Koob GF, 2016. Sex Differences in Animal Models: Focus on Addiction. Pharmacol Rev 68, 242–263. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Bonthuis PJ, Cox KH, Rissman EF, 2012. X-chromosome dosage affects male sexual behavior. Horm Behav 61, 565–572. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Bonthuis PJ, Patteson JK, Rissman EF, 2011. Acquisition of sexual receptivity: roles of chromatin acetylation, estrogen receptor-alpha, and ovarian hormones. Endocrinology 152, 3172–3181. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Bonthuis PJ, Rissman EF, 2013. Neural growth hormone implicated in body weight sex differences. Endocrinology 154, 3826–3835. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Bowman BP, Kuhn CM, 1996. Age-related differences in the chronic and acute response to cocaine in the rat. Dev Psychobiol 29, 597–611. [DOI] [PubMed] [Google Scholar]
  17. Burke AR, Miczek KA, 2015. Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior. Psychopharmacology (Berl) 232, 3067–3079. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Caine SB, Bowen CA, Yu G, Zuzga D, Negus SS, Mello NK, 2004. Effect of gonadectomy and gonadal hormone replacement on cocaine self-administration in female and male rats. Neuropsychopharmacology 29, 929–942. [DOI] [PubMed] [Google Scholar]
  19. Campbell UC, Morgan AD, Carroll ME, 2002. Sex differences in the effects of baclofen on the acquisition of intravenous cocaine self-administration in rats. Drug Alcohol Depend 66, 61–69. [DOI] [PubMed] [Google Scholar]
  20. Castro-Zavala A, Martin-Sanchez A, Valverde O, 2020. Sex differences in the vulnerability to cocaine's addictive effects after early-life stress in mice. Eur Neuropsychopharmacol 32, 12–24. [DOI] [PubMed] [Google Scholar]
  21. Corre C, Friedel M, Vousden DA, Metcalf A, Spring S, Qiu LR, Lerch JP, Palmert MR, 2016. Separate effects of sex hormones and sex chromosomes on brain structure and function revealed by high-resolution magnetic resonance imaging and spatial navigation assessment of the Four Core Genotype mouse model. Brain Struct Funct 221, 997–1016. [DOI] [PubMed] [Google Scholar]
  22. Cummings JA, Jagannathan L, Jackson LR, Becker JB, 2014. Sex differences in the effects of estradiol in the nucleus accumbens and striatum on the response to cocaine: neurochemistry and behavior. Drug Alcohol Depend 135, 22–28. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. De Vries GJ, Rissman EF, Simerly RB, Yang LY, Scordalakes EM, Auger CJ, Swain A, Lovell-Badge R, Burgoyne PS, Arnold AP, 2002. A model system for study of sex chromosome effects on sexually dimorphic neural and behavioral traits. The Journal of neuroscience 22, 9005–9014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. DeVito EE, Herman AI, Waters AJ, Valentine GW, Sofuoglu M, 2014. Subjective, physiological, and cognitive responses to intravenous nicotine: effects of sex and menstrual cycle phase. Neuropsychopharmacology 39, 1431–1440. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Engeln M, Mitra S, Chandra R, Gyawali U, Fox ME, Dietz DM, Lobo MK, 2019. Sex-Specific Role for Egr3 in Nucleus Accumbens D2-Medium Spiny Neurons Following Long-Term Abstinence From Cocaine Self-administration. Biol Psychiatry. [DOI] [PMC free article] [PubMed]
  26. Festa ED, Quinones-Jenab V, 2004. Gonadal hormones provide the biological basis for sex differences in behavioral responses to cocaine. Horm Behav 46, 509–519. [DOI] [PubMed] [Google Scholar]
  27. Gatewood JD, Wills A, Shetty S, Xu J, Arnold AP, Burgoyne PS, Rissman EF, 2006. Sex chromosome complement and gonadal sex influence aggressive and parental behaviors in mice. The Journal of neuroscience 26, 2335–2342. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Grasso CS, Wu YM, Robinson DR, Cao X, Dhanasekaran SM, Khan AP, Quist MJ, Jing X, Lonigro RJ, Brenner JC, Asangani IA, Ateeq B, Chun SY, Siddiqui J, Sam L, Anstett M, Mehra R, Prensner JR, Palanisamy N, Ryslik GA, Vandin F, Raphael BJ, Kunju LP, Rhodes DR, Pienta KJ, Chinnaiyan AM, Tomlins SA, 2012. The mutational landscape of lethal castration-resistant prostate cancer. Nature 487, 239–243. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Griffin WC 3rd, Middaugh LD, 2006. The influence of sex on extracellular dopamine and locomotor activity in C57BL/6J mice before and after acute cocaine challenge. Synapse 59, 74–81. [DOI] [PubMed] [Google Scholar]
  30. Griffin WC 3rd, Randall PK, Middaugh LD, 2007. Intravenous cocaine self-administration: individual differences in male and female C57BL/6J mice. Pharmacol Biochem Behav 87, 267–279. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Hu M, Becker JB, 2008. Acquisition of cocaine self-administration in ovariectomized female rats: effect of estradiol dose or chronic estradiol administration. Drug Alcohol Depend 94, 56–62. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Hu M, Crombag HS, Robinson TE, Becker JB, 2004. Biological basis of sex differences in the propensity to self-administer cocaine. Neuropsychopharmacology 29, 81–85. [DOI] [PubMed] [Google Scholar]
  33. Itoh Y, Mackie R, Kampf K, Domadia S, Brown JD, O'Neill R, Arnold AP, 2015. Four core genotypes mouse model: localization of the Sry transgene and bioassay for testicular hormone levels. BMC Res Notes 8, 69. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Jackson LR, Robinson TE, Becker JB, 2006. Sex differences and hormonal influences on acquisition of cocaine self-administration in rats. Neuropsychopharmacology 31, 129–138. [DOI] [PubMed] [Google Scholar]
  35. Jonasson Z, 2005. Meta-analysis of sex differences in rodent models of learning and memory: a review of behavioral and biological data. Neurosci Biobehav Rev 28, 811–825. [DOI] [PubMed] [Google Scholar]
  36. Kmiotek EK, Baimel C, Gill KJ, 2012. Methods for intravenous self administration in a mouse model. J Vis Exp, e3739. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Kopsida E, Lynn PM, Humby T, Wilkinson LS, Davies W, 2013. Dissociable effects of Sry and sex chromosome complement on activity, feeding and anxiety-related behaviours in mice. PloS one 8,e73699. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. LaRese TP, Rheaume BA, Abraham R, Eipper BA, Mains RE, 2019. Sex-Specific Gene Expression in the Mouse Nucleus Accumbens Before and After Cocaine Exposure. J Endocr Soc 3, 468–487. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Loyd DR, Murphy AZ, 2014. The neuroanatomy of sexual dimorphism in opioid analgesia. Exp Neurol 259, 57–63. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Lynch WJ, 2008. Acquisition and maintenance of cocaine self-administration in adolescent rats: effects of sex and gonadal hormones. Psychopharmacology (Berl) 197, 237–246. [DOI] [PubMed] [Google Scholar]
  41. Lynch WJ, 2009. Sex and ovarian hormones influence vulnerability and motivation for nicotine during adolescence in rats. Pharmacol Biochem Behav 94, 43–50. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Lynch WJ, Carroll ME, 1999. Sex differences in the acquisition of intravenously self-administered cocaine and heroin in rats. Psychopharmacology (Berl) 144, 77–82. [DOI] [PubMed] [Google Scholar]
  43. Lynch WJ, Roth ME, Mickelberg JL, Carroll ME, 2001. Role of estrogen in the acquisition of intravenously self-administered cocaine in female rats. Pharmacol Biochem Behav 68, 641–646. [DOI] [PubMed] [Google Scholar]
  44. Ma W, Bonora G, Berletch JB, Deng X, Noble WS, Disteche CM, 2018. X-Chromosome Inactivation and Escape from X Inactivation in Mouse. Methods Mol Biol 1861, 205–219. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Mahadevaiah SK, Odorisio T, Elliott DJ, Rattigan A, Szot M, Laval SH, Washburn LL, McCarrey JR, Cattanach BM, Lovell-Badge R, Burgoyne PS, 1998. Mouse homologues of the human AZF candidate gene RBM are expressed in spermatogonia and spermatids, and map to a Y chromosome deletion interval associated with a high incidence of sperm abnormalities. Hum Mol Genet 7, 715–727. [DOI] [PubMed] [Google Scholar]
  46. Martinez LA, Gross KS, Himmler BT, Emmitt NL, Peterson BM, Zlebnik NE, Foster Olive M, Carroll ME, Meisel RL, Mermelstein PG, 2016. Estradiol Facilitation of Cocaine Self-Administration in Female Rats Requires Activation of mGluR5. eNeuro 3. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Martini M, Pinto AX, Valverde O, 2014. Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice. Psychopharmacology (Berl) 231, 2647–2659. [DOI] [PubMed] [Google Scholar]
  48. McPhie-Lalmansingh AA, Tejada LD, Weaver JL, Rissman EF, 2008. Sex chromosome complement affects social interactions in mice. Horm Behav 54, 565–570. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Miczek KA, de Almeida RM, 2001. Oral drug self-administration in the home cage of mice: alcohol-heightened aggression and inhibition by the 5-HT1B agonist anpirtoline. Psychopharmacology (Berl) 157, 421–429. [DOI] [PubMed] [Google Scholar]
  50. Nicolas C, Russell TI, Pierce AF, Maldera S, Holley A, You ZB, McCarthy MM, Shaham Y, Ikemoto S, 2019. Incubation of Cocaine Craving After Intermittent-Access Self-administration: Sex Differences and Estrous Cycle. Biol Psychiatry 85, 915–924. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Perry AN, Westenbroek C, Becker JB, 2013. Impact of pubertal and adult estradiol treatments on cocaine self-administration. Horm Behav 64, 573–578. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Peterson BM, Martinez LA, Meisel RL, Mermelstein PG, 2016. Estradiol impacts the endocannabinoid system in female rats to influence behavioral and structural responses to cocaine. Neuropharmacology 110, 118–124. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Quinn JJ, Hitchcott PK, Umeda EA, Arnold AP, Taylor JR, 2007. Sex chromosome complement regulates habit formation. Nature neuroscience 10, 1398–1400. [DOI] [PubMed] [Google Scholar]
  54. Rowson SA, Foster SL, Weinshenker D, Neigh GN, 2018. Locomotor sensitization to cocaine in adolescent and adult female Wistar rats. Behav Brain Res 349, 158–162. [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. Sanchez V, Lycas MD, Lynch WJ, Brunzell DH, 2015. Wheel running exercise attenuates vulnerability to self-administer nicotine in rats. Drug Alcohol Depend 156, 193–198. [DOI] [PMC free article] [PubMed] [Google Scholar]
  56. Sell SL, Scalzitti JM, Thomas ML, Cunningham KA, 2000. Influence of ovarian hormones and estrous cycle on the behavioral response to cocaine in female rats. J Pharmacol Exp Ther 293, 879–886. [PubMed] [Google Scholar]
  57. Seu E, Groman SM, Arnold AP, Jentsch JD, 2014. Sex chromosome complement influences operant responding for a palatable food in mice. Genes Brain Behav 13, 527–534. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Souza MF, Couto-Pereira NS, Freese L, Costa PA, Caletti G, Bisognin KM, Nin MS, Gomez R, Barros HM, 2014. Behavioral effects of endogenous or exogenous estradiol and progesterone on cocaine sensitization in female rats. Braz J Med Biol Res 47, 505–514. [DOI] [PMC free article] [PubMed] [Google Scholar]
  59. Swalve N, Smethells JR, Carroll ME, 2016. Sex differences in the acquisition and maintenance of cocaine and nicotine self-administration in rats. Psychopharmacology (Berl) 233, 1005–1013. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Thomsen M, Caine SB, 2007. Intravenous drug self-administration in mice: practical considerations. Behav Genet 37, 101–118. [DOI] [PubMed] [Google Scholar]
  61. Thomsen M, Caine SB, 2011. Psychomotor stimulant effects of cocaine in rats and 15 mouse strains. Exp Clin Psychopharmacol 19, 321–341. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Tukiainen T, Villani AC, Yen A, Rivas MA, Marshall JL, Satija R, Aguirre M, Gauthier L, Fleharty M, Kirby A, Cummings BB, Castel SE, Karczewski KJ, Aguet F, Byrnes A, Consortium GT, Laboratory DA, Coordinating Center - Analysis Working, G., Statistical Methods groups- Analysis Working, G., Enhancing, G.g., Fund, N.I.H.C., Nih/Nci, Nih/Nhgri, Nih/Nimh, Nih/Nida, Biospecimen Collection Source Site, N., Biospecimen Collection Source Site, R., Biospecimen Core Resource, V., Brain Bank Repository-University of Miami Brain Endowment, B., Leidos Biomedical-Project, M., Study, E., Genome Browser Data, I., Visualization, E.B.I., Genome Browser Data, I., Visualization-Ucsc Genomics Institute, U.o.C.S.C., Lappalainen T, Regev A, Ardlie KG, Hacohen N, MacArthur DG, 2017. Landscape of X chromosome inactivation across human tissues. Nature 550, 244–248. [DOI] [PMC free article] [PubMed] [Google Scholar]
  63. Van Swearingen AE, Walker QD, Kuhn CM, 2013. Sex differences in novelty- and psychostimulant-induced behaviors of C57BL/6 mice. Psychopharmacology (Berl) 225, 707–718. [DOI] [PMC free article] [PubMed] [Google Scholar]
  64. Xie G, Liu X, Zhang Y, Li W, Liu S, Chen Z, Xu B, Yang J, He L, Zhang Z, Jin T, Yi X, Sun L, Shang Y, Liang J, 2017. UTX promotes hormonally responsive breast carcinogenesis through feed-forward transcription regulation with estrogen receptor. Oncogene 36, 5497–5511. [DOI] [PubMed] [Google Scholar]
  65. Zhou L, Sun WL, Weierstall K, Minerly AC, Weiner J, Jenab S, Quinones-Jenab V, 2016. Sex differences in behavioral and PKA cascade responses to repeated cocaine administration. Psychopharmacology (Berl) 233, 3527–3536. [DOI] [PubMed] [Google Scholar]
  66. Zombeck JA, Swearingen SP, Rhodes JS, 2010. Acute locomotor responses to cocaine in adolescents vs. adults from four divergent inbred mouse strains. Genes Brain Behav 9, 892–898. [DOI] [PMC free article] [PubMed] [Google Scholar]