No basal or drug‐induced sex differences in striatal dopaminergic levels: a cluster and meta‐analysis of rat microdialysis studies

Sex differences in behavioural patterns of drug abuse and dependence have been hypothesized to be a consequence of sexual dimorphisms in brain pathways, particularly within the dopaminergic reward circuitry. Yet, how potential sex differences are manifested at a neurochemical level remains unclear. Here, we use a meta‐analysis approach to investigate whether animal studies robustly indicate a different regulation of striatal dopamine transmission in males and females. Data from 39 microdialysis experiments on female rats (n = 676) were extracted and statistically compared with data from 1523 male rats. All drugs of abuse, independent of their molecular mechanisms of action, notably increase extracellular dopamine concentrations in the nucleus accumbens (NAc) and caudate putamen (CPu). No significant sex differences in basal levels or in dopaminergic response to drugs of abuse were found. However, basal dopamine levels in CPu (but not NAc) were significantly altered by ovariectomy. In conclusion, there are no sex‐dependent differences in basal dopamine levels within the NAc and CPu. Previously reported sex differences in the CPu seem to be a result of ovariectomy and may only to a lesser, non‐significant degree be attributed to a sexual duality.

Sex matters. The 3rd millennium began with this important message from the US National Institute of Medicine (Wizemann and Pardue, 2001) encouraging translational research especially on sex differences in brain organization and function. To date, a large body of evidence exists that indicate sex differences in substance use disorders. While there is a higher prevalence of drug use and abuse as well as of substance-use disorders in men (Carvalho, et al., 2019), recent clinical and pre-clinical studies suggests sexual dimorphisms in the entire disease dynamic of drug abuse and dependence. In particular, females show an accelerated progression from casual drug intake to addiction. These observations stress the importance of sex-specific approaches for optimal prevention and treatment strategies for addiction (Lynch, 2006;Sanchis-Segura and Becker, 2016). Thereby, similarities in animal and human studies, for instance in how patterns of drug acquisition and relapse differ between the sexes, suggest a common biological basis of sex differences in vulnerability to drug abuse. Hormonal cycles and differences in neuronal processes in the brain may both contribute to such differences (Becker, et al., 2012;Becker, 2016) and pre-clinical studies were used to provide mechanistic insights on how sex differences in behavioural patterns may relate to the underlying biology.
To date, pre-clinical findings suggest that ovarian hormones critical affect the reinforcing effects of drugs of abuse in females. In particular, acute oestradiol administration in female animals have been shown to enhance acquisition, motivation and escalation of drug intake, and increase the probability of relapse-like behaviours (Becker and Cha, 1989;Becker, 1990;Becker and Rudick, 1999;Hu, et al., 2004). These hormonal effects have been hypothesized to impact on sex differences in dopaminergic neurotransmission in striatum (Walker, et al., 2006;Riccardi, et al., 2011;Bobzean, et al., 2014). Previous studies have shown a lower level of cocaine-induced increase in dopamine (DA) levels in ovariectomized female rats than castrated males (Hu, et al., 2004). Furthermore, oestradiol treatment in ovariectomized rats was demonstrated to enhance stimulated dopamine release (Castner, et al., 1993;Cummings, et al., 2014;Becker, 2016). However, the effects of oestradiol were mostly on caudate putamen (CPu), whereas DA levels in nucleus accumbens (NAc) remained unaffected by hormonal treatment (Cummings, et al., 2014). This discrepancy in how dopamine and hormonal systems interacts, have been discussed in light of the different functions of each brain region. While NAc is associated with positive reinforcement and rewarding aspects of drug use (Spanagel and Weiss, 1999), CPu is merely engaged in habit formation and compulsive drug taking (Belin and Everitt, 2008;Willuhn, et al., 2012). However, these studies were conducted in male rats. Indeed, the questions remains of to what degree any data obtained from male animals can be used to interpret findings from female animals. Indeed, all reported sex differences are based on within-study statistics with relatively small number of animals and a global comparison of male-female findings of different studies is still missing.
A large-scale between-study analysis allows us to evaluate the robustness of conclusions made by individual studies and often leads to the generation of new mechanistic hypotheses (Flather, et al., 1997). In this study, we use an established (Noori, et al., 2018) hypothesis-free, global approach to statistically compare two large bodies of evidence, namely findings from all publications reporting on dopamine concentrations in NAc and CPu of male and/or female rats. To this end, we performed systematic data mining, extraction and analysis of basal levels of dopamine, as well as relative changes of DA in response to acute administration of drugs of abuse. Thereby, our search included studies that used in vivo microdialysis to determine extracellular DA concentrations in either male, female rats or both. This strategy allows an explorative investigation since it includes data that were not primarily obtained to analyse sex differences. The entirety of data was consistently integrated, meta-analysed and statistical tests including analysis of variance, cluster and sensitivity analyses were conducted to compare the findings with respect to sex but also to shine a light on effect modifiers and secondary factors.

Methods
A previously reported (Noori, et al., 2012a;Hirth, et al., 2016;Fritze, et al., 2017;Noori, et al., 2018), we have established a robust pipeline for standardized data mining, database creation and integrative analysis of preclinical neuropsychopharmacology experiments. This approach allows accurate extraction of maximum amount of data with a minimized chance for missing critical information and was therefore applied here to characterize basal dopamine concentrations as well as overlapping and distinct features of striatal dopaminergic response to systemically administered drugs of abuse in female rats.

Study selection
A systematic screening of the original research articles published until 31.04.2019 was performed on the online portal of the National Library of Medicine (http://www.ncbi.nlm.nih.gov/pubmed/) based on the keywords rat (AND) microdialysis (OR) (female (OR) sex (OR) gender) (AND) (dopamine (OR) DA) (AND) (striatum (OR) nucleus accumbens (OR) caudate putamen (OR) reward) (AND) (alcohol (OR) ethanol (OR) (D,L)-amphetamine (OR) methamphetamine (OR) cocaine (OR) ketamine (OR) morphine (OR) nicotine (OR) phencyclidine (OR) PCP (OR) tetrahydrocannabinol (OR) THC). An identical set of keywords with the exception of (female (OR) sex (OR) gender) was used in our previous study (Noori, et al., 2018) to screen the same database for basal levels and drug-induced changes in dopamine concentrations in male animals. In order to consistently maximize the outreach of the search space within the constraints of the pubmed-database, we further screened the bibliographies of review and meta-analyses as well as identified articles for relevant references. The titles and abstracts of research articles were first screened by pairs of reviewers (LE and EM) independently. Subsequently, full text of any potentially eligible title and/or abstract were reviewed by both reviewers. Disagreements among reviewers were resolved through discussion.

Inclusion and exclusion criteria
The inclusion criteria were set strictly to achieve high level of consistency in article selection. In agreement with the guidelines for meta-analyses of pre-clinical studies (Vesterinen, et al., 2014), only original research articles were included. Meta-analyses, reviews, commentaries and other manuscript styles were excluded. However, the reference sections of such articles were screened for grey literature search, as described above. We focused our search on articles published in English language. Among these, studies were included only if they provided basal dopamine concentrations and/or relative changes in dopamine levels following acute administration of drugs of abuse with the striatal complex; that is NAc and CPu either numerically or in graphical manner. Measurements of other neurotransmitters or in other brain regions were excluded. Studies investigating chronic drug effects were only included if they either provided basal level prior to treatment or if they provided data for the acute drug effects as well. Only systematic drug administrations such as intraperitoneal or intravenous injections were included. Studies reporting local intracerebral injections of drugs into brain regions were excluded. Only microdialysis experiments in animals were included. In vitro microdialysis measurements were excluded. Furthermore, studies using animals other than outbred rats were excluded. Furthermore, animals that received genetic, behavioural or surgical manipulations such intracerebral lesions prior baseline measurements and/or drug treatments were excluded. In particular, animals that underwent social and environmental manipulations such as stress by isolation, prolonged food restriction or altered temperature were excluded. Since this data-mining procedure was focused on female animals and data from male animals were extracted in our previous study (Noori, et al., 2018), we excluded studies from the current literature selection that were reporting data only on male rats. In addition to healthy drug-naive animals data were also collected from animals that were ovariectomized or pretreated with hormones and analysed a posteriori to identify whether the manipulations would lead to any significant differences in dopaminergic overflow. We only included peer-reviewed publications. Data reported in conference abstracts and unpublished studies were excluded. If crucial information was missing, corresponding authors of eligible articles were contacted directly.

Data extraction
A previously reported (Noori, et al., 2012a;Hirth, et al., 2016;Fritze, et al., 2017;Noori, et al., 2018), three categories (i.e. biological, experimental procedure and outcome) of study variables and parameters were extracted using a structured template. The set of extracted biological parameters and variables is comprised of rat strain, state of consciousness, that is awake and freely moving or anaesthetized (anaesthetic agent and the dosage), age or weight, oestrous cycle, ovariectomy (if yes, also time after it), hormonal pre-treatment (type of drug, dose and route of administration), and number of animals used in each experiment. The second category, namely the experimental procedure parameters and variables contains data on sampling rate (min), perfusion rate (µl/min), length (mm), outer diameter and molecular cut-off (kDa) of microdialysis membranes, recovery time following probe implanation (hrs), calcium concentration in perfusate (mM) and dialysate matrix (e.g. Ringer solution), targeted brain region, neurochemical detection assay, route of drug administration, drug name and applied dose. The outcome variables were basal levels of dopamine, maximal drug dose effects (%) and the time Ti at which the maximum occurred; that is for a specific dose of the drug the absolute or relative changes of dopamine concentrations within a brain region were obtained. The drug effects were normalized to the basal levels if absolute values were provided, in order to obtain relative changes.

Anatomical nomenclature
A common issue of pre-clinical studies is the inconsistent use of anatomical nomenclature. While a few studies report accurate coordinates for probe placement, the designation of the targeted brain area often differs. In order to avoid false positives we unified in a cluster analysis the terminology using a coarse-grained nomenclature for brain regions (Noori, et al., 2012b;. Thereby, dorsal striatum, striatum, neostriatum and caudate were grouped as caudate putamen (CPu) and ventral striatum, nucleus accumbens shell/core were grouped as NAc.

Quality assessment
Several factor may influence the quality of the datasets and induce risks of bias. While a risk of bias assessment tool for animal intervention studies exists (Hooijmans, et al., 2014), it could not be utilized systematically in this study since numerous items related to performance and detection bias were not reported in the included studies. In addition to the items within the SYRCLE's (www. SYRCLE.nl) risk of bias tool (Hooijmans, et al., 2014), we identified at least two more factors that could potentially affect our dataset. In different countries, clinically approved drugs have different trade names that if used, instead of the INN or International Union of Pure and Applied Chemistry designations, lead to inconsistencies in datasets. However, this issue did not occur in this study. In addition, the technical performance and completeness of reports vary among studies. In order to address this issue, we conducted a series of sensitivity analyses to assess the effect of effect modifiers and missing data on the final outcome of the metaanalysis. In order to investigate publication bias, study distribution was compared with the funnel-shape distribution. Since most studies were located near the average, it is safe to assume that most studies were of high precision and no indication towards the existence of publication bias was found.

Outcomes and effect modifiers
The primary outcomes were baseline levels and normalized alterations in extracellular dopamine concentrations (peak%baseline value) within the regions of interest. The time-point of extremal response (i.e. peak) was a secondary outcome. Numerous factors such as rat strain (e.g. Sprague-Dawley, Wistar, Lister-hooded), age, use of anaesthesia (Muller, et al., 2011), pre-treatment with hormones such as oestradiol, ovariectomy (OVX), number of animals, applied dose of drugs, route of drug administration (e.g. intravenous, intraperitoneal), and technical parameters such as length, outer diameter and molecular cut-off of the membranes, flow rate and calcium concentrations within artificial cerebrospinal fluid or Ringer's solutions as well as the recovery time following implantation of the probes were considered as potential effect modifiers.

Meta-analysis
Dopamine concentrations in dialysate and percentage change in DA compared to baseline were the endpoints of our study. In general, each microdialysis experiment provides time-series of basal concentration and drug-induced changes (in minute scale) for a limited number of neurotransmitters and metabolites within one brain region. The baseline concentration depends on sampling time and perfusion rate and is therefore often reported in pg/µL or fmol/min units. Therefore, in the first step, all extracted basal levels were converted into nM (=fmol/µL) unit. To this end, if basal levels were provided in pg/µL then the value was divided by the molecular weight of dopamine 153.18 g/mol (https://pubchem.ncbi.nlm.nih. gov/compound/Dopamine) and multiplied with 1000. If data were provided in fmol/min, then the data were divided by perfusion rate (in µL/min). For averaging the dopaminergic response to drugs, we extract the peak response (x i ) of a transmitter to the drug normalized to its baseline value and the time of the peak. As previously reported (Noori, et al., 2018), we conducted weighted (by inverse variance of each data point) meta-analyses of the extracted basal concentrations and drug effects (%) using fixed effect model. The choice of fixed versus random effect models depend strongly on the question at hand. If one can assume that the different studies are reporting an estimate of the same effect then a fixed effect model is more appropriate (Vesterinen, et al., 2014). In this study, the metaanalysis was related consistently for each case to one effect. For instance, meta-analysis of basal values of dopamine in caudate putamen of female rats. Review Manager 5.3 (https://community.c ochrane.org/help/tools-and-software/revman-5) was used for the calculation of heterogeneity scores (I 2 = (Q À df)/Q 9 100%, where Q is the chi-squared statistic and df is its degrees of freedom) and tests for overall effects as well as generation of forest plots.

Sensitivity analysis
We used one-factor-at-a-time sensitivity analysis to evaluate the robustness of the weighted meta-analyses with respect to the abovementioned effect modifiers. To this end, the dataset was divided into subgroups associated with a specific parameter, for instance adult versus adolescent rats and analysis of variance (ANOVA) was conducted (p < 0.05) to identify significant differences because of study design and specific choice of effect modifiers. While most of the parameters were reported consistently among the studies, two effect modifiers did not allow further analyses. The molecular cutoff of the membrane (if provided, within the range of 6-20 kDa) was in only 32% of studies reported, which does not allow reliable statistical comparisons. The second parameter was the recovery time following probe implantation. While this is a critical factor in determining basal transmitter levels, it was not consistently provided in a quantitative manner. Instead, the majority of the studies indicate an 'overnight' recovery, which lacks the necessary accuracy for statistical testing.
Statistical analysis of sex-differences in baseline levels and druginduced concentrations One-way analysis of variance (a < 0.05) was used to statistically compare absolute or relative dopamine levels between NAc and CPu within each brain region with respect to sex. If multiple comparisons were conducted, then the level of significance was adjusted using Bonferroni-correction. For basal dopamine levels, data for male animals were obtained previously . For druginduced relative dopamine changes, data were extracted from syphad (www.syphad.com) database (Noori, et al., 2018). Since absolute values are used, a statistical outlier identification algorithm (isoutlier(A,'method') in MATLAB R2018a) was applied prior to conducting the F-test. By default, an outlier is calculated by the method 'median' as a value that is more than three scaled median absolute deviations (c 9 median (|A imedian (A)|) with c = À1/ (√2*erfcinv (3/2))) away from median. Since the choice of the method is not unique, 'quartiles' and 'grubbs' methods were applied and the findings of ANOVA were compared a posteriori. Since the outcomes did not differ, all results are reported based on the least conservative algorithm with minimal a priori assumption, namely 'quartiles'. All statistical analyses were conducted in non-blinded manner.

Cluster analysis
Drug effects on dopamine concentrations, dose of drugs, peak time and technical microdialysis parameters such as flow rate and calcium concentration of the perfusate, sampling time and length of the probe are considered as a function of continuous numerical variables. In turn, age, strain, state of consciousness, hormonal pre-treatment, ovariectomy, drug designation and route of administration were treated as discrete categorical variables. In order to discover patterns in mixed datasets derived from the literature, we applied a two-step clustering algorithm (https://www.ibm.com/sup port/knowledgecenter/bg/SSLVMB_24.0.0/spss/base/idh_twoste p_main.html) using IBM SPSS statistics software. This clustering technique allows clustering data with both continuous and categorical attributes and uses a distance measure derived from a probabilistic model. The distance between two clusters is equivalent to the decrease in log-likelihood function. In a first step, a k-means procedure was applied to pre-cluster the data. Subsequently, we conducted a modified hierarchical agglomerative clustering procedure combining the objects sequentially to form homogenous clusters. Furthermore, using Bayesian information criterion, the procedure indicates the importance of each variable (predictor) for the formation of a specific cluster. This information is reflected as the 'goodness of cluster' parameter associated to each variable, which takes values between 0 and 1, representing the least and most important variables in generating distinct clusters respectively.

Data distribution
Systematic literature search identified in the first step 389 original publications. Out of these, 196 studies were relevant for data mining and data from 39 original research articles on in vivo microdialysis in female rat brain (covering studies involving 676 animals) were finally selected for the metaanalysis ( Fig. 1; Table 1). No data could be found for ketamine, phencyclidine and tetrahydrocannabinol.
In general, the microdialysis experiments were conducted using comparable experimental parameters (Table 2). Moreover, 96% of studies were performed on adult animals and drugs were applied in 94% of cases intraperitoneal. Therefore, these variables were not considered as effect modifiers. While most studies used Sprague-Dawley rats (62%), sufficient data could be obtained from other strains such as Long-Evans (22%), and Holtzmann (11%) in order to statistically evaluate the impact of strain on dopaminergic response to drugs of abuse. Nonetheless, only 4% of all entries relate to experiments on Wistar rats, which are often used to assess the effect of drugs on neurochemical concentrations in male rats (Noori, et al., 2018). Therefore, special care needs to be applied in interpreting the overall findings of the present meta-analysis in light of previous investigations and our findings should be considered as relating mostly to Sprague-Dawley rats. Approximately one third of all experiments were conducted on ovariectomized rats, which provides sufficient data to analyse the effect of surgical pre-treatment on drug-induced changes in striatal dopamine concentrations.

Impact of ovariectomy on striatal dopamine overflow
Ovarian hormones have been suggested to enhance dopaminergic transmission with reward circuitry and thereby modulate the neurochemical response to psychostimulants (Becker, 1990;Castner, et al., 1993). These investigations often apply hormones such as oestradiol to OVX rats prior to treatment with psychostimulant and subsequently measure the dopamine overflow in striatal regions. However, it remains unclear whether the overiectomy procedure by itself alters basal dopamine level. The analysis of variance with respect to OVX-procedure suggests that basal concentrations of dopamine in caudate putamen but not nucleus accumbens (F 1,11 = 0.002, p = 0.96) are affected significantly by ovariectomy. Indeed, basal dopamine in caudate putamen of OVX-rats is lowered significantly (F 1,22 = 11.27, p < 0.01) to a level four times smaller than in non-operated female rats of comparable age.

Nicotine effects
Nicotine-induced changes in dopamine concentration were only reported within nucleus accumbens for systemic administration of the dose of 0.4 mg/kg in (n = 27) female rats (McCallum, et al., 2012;Eggan and McCallum, 2016;Eggan and McCallum, 2017). Thereby, nicotine increases dopamine levels by 169.44 AE 4.73 % in female animals. In average, no significant differences were found between DA

Amphetamine effects
Data of 232 female rats were provided from 12 original studies (Robinson, et al., 1988;Becker and Cha, 1989;Robinson and Camp, 1990;Robinson and Camp, 1991;Maisonneuve and Glick, 1992;Castner, et al., 1993;Glick, et al., 1993;Becker and Rudick, 1999;Shoblock, et al., 2003;Geiger, et al., 2009;McCallum, et al., 2012;Shams, et al., 2016). Two-step cluster analysis suggests that psychostimulants, particularly amphetamines, induce the strongest impact on dopamine levels in striatum. However, no dose-response relationship could be identified (0.5 mg/ kg: 920.83 AE 143.61% in CPu; 0.75 mg/kg: 400% in NAc and 350% in CPu; 1 mg/kg: 1625% in NAc; 1.25 mg/kg: Table 1 Outcome of data mining process. In total, less than 10% of studies identified by keyword search (S) were included for data analysis. K and S denote the number of included and total number of identified studies respectively. N represents the number of animals included in the meta-analysis k (S) n Dose range (mg/kg)  (Becker, 1990;Castner, et al., 1993), no significant differences were found in how dopamine transmission was enhanced in male and female caudate putamen (0.5 mg/kg: F 1,7 = 1.42, p = 0.27; 2 mg/kg: F 1,23 = 1.18, p = 0.29; 2.5 mg/kg: F 1,7 = 2.47, p = 0.16). Because of insufficient number of observations, a statistical comparison of male and female response to amphetamines within nucleus accumbens was not possible.
proper sample size, choice of statistical methods and data convergence may further influence the outcome of investigations and thus, conclusions made on sex differences. Our scoping review and meta-analysis of pre-clinical studies suggest that sex differences were observed entirely in experiments with relatively small sample size between male and mostly ovariectomized female rats. However, the collective of all published studies provides the critical mass that is necessary for conducting second-order advanced statistical analyses. By comparing the overall findings on female rats with overall findings for male rats in a hypothesis-free and systematic manner, we show that there are no sex differences in basal dopamine concentrations in nucleus accumbens and caudate putamen. Moreover, there are no differences in the magnitude of response to drugs of abuse in both regions between males and females. Interestingly, the most critical effect modifier was ovariectomy (OVX), which is a common surgical procedure in studies that addressed sex differences in light of hormonal changes. OVX leads to a significant reduction of basal dopamine levels in caudate putamen but not in nucleus accumbens, which may account for previously reported dopaminergic sex differences in CPu but not NAc. The importance of basal neurotransmitter levels or generally initial brain state on the magnitude of drug response is a largely ignored factor in psychopharmacology. The theory of dynamical systems, which implicitly underlies any temporal measurement, states that the qualitative behaviour of a process strongly depends on its initial state (Perko, 2008). Consequently, a procedure such as OVX that leads to a significant change in basal levels in Fig. 3 The outcome of meta-analysis and forest plot of basal dopamine levels (nM) in caudate putamen of 272 female rats. While in both brain regions the data show considerable heterogeneity I 2 = 98%, the pattern of data distribution among included studies for caudate putamen (Robinson and Camp, 1990;Maisonneuve, et al., 1991;Robinson and Camp, 1991;Maisonneuve and Glick, 1992;Blanchard, et al., 1993a;Blanchard, et al., 1993b;Blanchard and Glick, 1995;Pearl, et al., 1996;Becker and Rudick, 1999;Maisonneuve and Glick, 1999;Cummings, et al., 2014;Shams, et al., 2016) indicates that the year of publication is a critical factor. In particular, studies published before 1995 report significantly (p < 0.0001) different basal dopamine levels that are in average approximately eightfold higher than publications in the following years. comparison to non-ovariectomized females will affect all following pharmacological manipulations. Therefore, statistical tests of drug response in OVX and male rats are objectively not optimal to address sex differences, even when a hormonal pre-treatment was applied.
In spite of the absence of sex differences in striatal DA system, extracellular dopamine levels were notably enhanced by all drugs of abuse independent of the molecular modes of action and the dose of the drugs. Indeed, as previously reported (Noori, et al., 2018), dopaminergic system responds to approximately 260 clinically approved and experimental neuropsychiatric drugs, some of which having no direct interactions with DA transmission. These findings raise the question of dopamine possesses the necessary specificity to be considered as a reliable marker for drug effects.
Big data and meta-analyses are powerful tools for evidence-based medicine and have attracted increasing attention in recent years beyond clinical applications, namely in basic and pre-clinical research as well. Data derived from a meta-analysis are useful for textbook knowledge, provide better comparability of data given by the generalization of already existing data, should be seen within the framework of the 3R principle of animal experimentation, and assist investigators for power analysis for designing experiments. Generally, meta-analyses are highly sensitive to the outreach of their underlying literature search and their input spaces. Our study solely focused on studies that are indexed within the National Library of Medicine database. The inclusion of other databases such as EMBASE (https://www.embase.c om/) could have potentially led to the inclusion of studies that our search has missed and thus, the conclusions of our study must be interpreted within the context of the data that it included. In addition, the accuracy of the outcomes of a meta-analysis strongly depends on the quality of the experimental procedures and reporting of the studies that it integrates. Therefore, while big data and meta-analyses approaches converge existing data into new knowledge and hypotheses, an inherent limitation of our study is the quality of its underlying data. There is large variability in perfusion rates, exact positioning of microdialysis probe, age of the animals and even in how the results are reported that may affect the overall findings. One way to reduce the variability in study design, which applies to both experimental studies and systematic reviews, is to standardize the approaches and to state the 'research questions and analysis plan clearly prior to observing the research outcomes-a process that is called pre-registration' (Nosek, et al., 2018). Nonetheless, the unavoidably rough temporal resolution of microdialysis experiments (i.e. sampling times in tens of minutes) limits the interpretation of neurochemical changes with respect to neuronal dynamics. An issue that is inherent to the technique itself and indicates the urgent need for the development of rapid neurochemical monitoring methods (Schwerdt, et al., 2018).
In conclusion, our study suggests that there are no sex differences in striatal dopaminergic overflow, although an impact of ovariectomy on basal levels was found. However, our study does not exclude the possibility of sex differences in other spatiotemporal scales of consideration or other components of reward circuitry and more studies are needed to provide clarity on potential sexual dimorphism in the brain related to addiction.