Changes in progressive ratio responding for intravenous cocaine throughout the reproductive process in female rats

Sex differences and sex-specific regulation of motivated behavior by Melanin-concentrating hormone: a short review

Recent preclinical research exploring how neuropeptide transmitter systems regulate motivated behavior reveal the increasing importance of sex as a critical biological variable. Neuropeptide systems and their central circuits both contribute to sex differences in a range of motivated behaviors and regulate sex-specific behaviors. In this short review, we explore the current research of how sex as a biological variable influences several distinct motivated behaviors that are modulated by the melanin-concentrating hormone (MCH) neuropeptide system. First, we review how MCH regulates feeding behavior within the context of energy homeostasis differently between male and female rodents. Then, we focus on MCH's role in lactation as a sex-specific process within the context of energy homeostasis. Next, we discuss the sex-specific effects of MCH on maternal behavior. Finally, we summarize the role of MCH in drug-motivated behaviors. While these topics are traditionally investigated from different scientific perspectives, in this short review we discuss how these behaviors share commonalities within the larger context of motivated behaviors, and that sex differences discovered in one area of research may impact our understanding in another. Overall, our review highlights the need for further research into how sex differences in energy regulation associated with reproduction and parental care contribute to regulating motivated behaviors.

The Integration of the Maternal Care with Sleep During the Postpartum Period

Our entire life occurs in a constant alternation between wakefulness and sleep. The impossibility of living without sleep implies that any behavior must adapt to the need for sleep, and maternal behavior does not escape from this determination. Additionally, maternal behavior in mammals is a highly motivated behavior, essential for the survival of the offspring. Thus, the mother has to adapt her physiology of sleep to the constant demands of the pups, where each species will have different strategies to merge these two physiological needs. However, all studied female mammals will experience sleep disturbances at some point of the postpartum period.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

Introduction

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures.

Methods

In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-h sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests.

Results

In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase.

Discussion

These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Effects of sex and estrous cycle on intravenous oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures. In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-hr sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests. In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase. These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats.

Sex/Gender Differences in the Time-Course for the Development of Substance Use Disorder: A Focus on the Telescoping Effect

Sex/gender effects have been demonstrated for multiple aspects of addiction, with one of the most commonly cited examples being the "telescoping effect" where women meet criteria and/or seek treatment of substance use disorder (SUD) after fewer years of drug use as compared with men. This phenomenon has been reported for multiple drug classes including opioids, psychostimulants, alcohol, and cannabis, as well as nonpharmacological addictions, such as gambling. However, there are some inconsistent reports that show either no difference between men and women or opposite effects and a faster course to addiction in men than women. Thus, the goals of this review are to evaluate evidence for and against the telescoping effect in women and to determine the conditions/populations for which the telescoping effect is most relevant. We also discuss evidence from preclinical studies, which strongly support the validity of the telescoping effect and show that female animals develop addiction-like features (e.g., compulsive drug use, an enhanced motivation for the drug, and enhanced drug-craving/vulnerability to relapse) more readily than male animals. We also discuss biologic factors that may contribute to the telescoping effect, such as ovarian hormones, and its neurobiological basis focusing on the mesolimbic dopamine reward pathway and the corticomesolimbic glutamatergic pathway considering the critical roles these pathways play in the rewarding/reinforcing effects of addictive drugs and SUD. We conclude with future research directions, including intervention strategies to prevent the development of SUD in women. SIGNIFICANCE STATEMENT: One of the most widely cited gender/sex differences in substance use disorder (SUD) is the "telescoping effect," which reflects an accelerated course in women versus men for the development and/or seeking treatment for SUD. This review evaluates evidence for and against a telescoping effect drawing upon data from both clinical and preclinical studies. We also discuss the contribution of biological factors and underlying neurobiological mechanisms and highlight potential targets to prevent the development of SUD in women.

Hormonal milieu drives economic demand for cocaine in female rats

There are substantial sex differences in drug abuse, and a key feature of cocaine addiction is pathologically high motivation for drug. We investigated the role of ovarian hormones on cocaine demand in female rats using a within-session threshold behavioral economics (BE) procedure, which allows us to compare motivation for drug across hormonal states and sex while controlling for differences in dose and intake. This approach quantifies demand elasticity (α) and free consumption (Q₀, consumption at null effort) to determine motivation for cocaine. Overall, female rats showed greater motivation for cocaine compared to males. However, this difference was cycle phase-dependent - motivation for cocaine when females were in proestrus was lower compared to the same animals across cycle phases, and overall similar to that of males. Hormonal cycle phase accounted for 70% of the within-subject variance in demand elasticity, obscuring other individual differences in female demand. High serum progesterone (P4; e.g., in proestrus) predicted decreased cocaine motivation (high demand elasticity), whereas serum estradiol (E2) correlated to greater intake at null effort (Q₀). However, individual differences were revealed across OVX females, who displayed a range of demand elasticity, as seen in males. E2 replacement in OVX females increased motivation for cocaine, whereas P4 replacement decreased motivation. We also found that as few as 4 weeks of cocaine self-administration accelerated estropause in female rats as young as 12 weeks old. By 13 weeks of self-administration, proestrus epochs were no longer observed, and cocaine demand was potentiated by persistent estrus in all females. Thus, P4 signaling is a key modulator of cocaine demand in females that may underlie previously observed sex differences in addiction phenotypes.

The effects of strain and estrous cycle on heroin- and sugar-maintained responding in female rats

Heroin intake decreases during the proestrus phase of the estrous cycle in female, Long-Evans rats. The purpose of this study was to (1) determine if proestrus-associated decreases in heroin intake extend across rat strains and (2) determine if proestrus-associated decreases in responding extend to a nondrug reinforcer. Female rats were implanted with intravenous catheters and trained to self-administer heroin. Estrous cycle was tracked daily for the duration of the study. During testing, Lewis, Sprague Dawley, and Long-Evans rats self-administered low (0.0025 mg/kg) and high (0.0075 mg /kg) doses of heroin and then self-administered sugar on fixed ratio (FR1) schedules of reinforcement. Heroin intake decreased significantly during proestrus in all three rat strains under at least one dose condition; however, sugar intake did not decrease during proestrus in any strain. These data suggest that responding maintained by heroin, but not a nondrug reinforcer, significantly decreases during proestrus in female rats and that these effects are consistent across rat strain.

Behavioral economics of cocaine self-administration in male and female rats

There are sex differences in the development of cocaine addiction. For example, the time that it takes for women from initial use to addiction is significantly shorter than for men. Thus, understanding why females are more vulnerable to cocaine addiction will provide insights into sex differences in the mechanisms underlying cocaine addiction. This study aimed to determine how cocaine demand intensity and elasticity might differ between sexes. In addition, the impact of estrous cycle and cocaine intake on demand was investigated. Male and female rats were trained to self-administer 0.125 mg of cocaine intravenously under a chained schedule in daily 2-h sessions for 2 weeks, and then, the cocaine demand function was determined with a modified within-session threshold procedure. Following the test, the rats began to self-administer a higher dose of cocaine (0.25 mg) to increase the cocaine intake. The demand function was then similarly determined in the same rats after 2 weeks of cocaine self-administration of the higher dose. No sex differences were found in either demand intensity or elasticity. Neither did the level of cocaine intake have an impact on demand. The demand elasticity, but not intensity, was significantly lower during proestrus/estrus compared with diestrus. These data suggest that the faster transition to cocaine addiction in women cannot be explained by sex differences in the demand for cocaine and such a demand may change during different phases of estrus cycle.

Assessment of conditioned fear extinction in male and female adolescent rats

Pavlovian fear conditioning and extinction have been widely studied across many species to understand emotional learning and memory. Importantly, it is becoming clear that these processes are affected by sex and age. In adult rodents and humans, sex differences are evident in extinction, with estradiol playing a significant role. In adolescence, an extinction deficit has been reported in rodents and humans. However, the influence of sex on extinction during adolescence is unknown. This is surprising, since adolescence coincides with the onset of hormone cycling, and therefore it might be expected that hormones fluctuations exert a more profound effect at this time. Therefore, we examined Pavlovian fear conditioning and extinction in adolescent male and female rats. In experiment 1, 35-day-old male and female rats were exposed to 6 pairings of a conditioned stimulus (CS, a tone) with an aversive unconditioned stimulus (US, a footshock). The next day they were extinguished in a contextually distinct chamber, via 60 presentations of the CS without the US. Extinction recall was tested 24 hours later in the extinction context. Estrous phase was monitored by cytology on vaginal smears taken 1 hour after each behavioral session. In experiment 2, male and female rats were given sham surgery or gonadectomy at 21 days of age. They were then trained and tested as for experiment 1. We observed that females in proestrus or met/diestrus during extinction showed delayed extinction and impaired extinction recall the next day compared to males. Ovariectomy enhanced extinction for female rats, but orchidectomy delayed extinction for males. Plasma analyses showed that met/di/proestrus phases were associated with high estradiol levels. These findings suggest that high plasma estradiol levels impair extinction for adolescent females. These results contradict what is reported in adult animals, suggesting that hormonal influences on extinction are dependent on age. Given that impaired extinction is widely used as a model to understand resistance to exposure-based therapies, our findings have important implications for understanding mental health treatments in adolescents.

Adaptations in reward-related behaviors and mesolimbic dopamine function during motherhood and the postpartum period

Initiation and maintenance of maternal behavior is driven by a complex interaction between the physiology of parturition and offspring stimulation, causing functional changes in maternal brain and behavior. Maternal behaviors are among the most robust and rewarding motivated behaviors. Mesolimbic dopamine (DA) system alterations during pregnancy and the postpartum enable enhanced reward-related responses to offspring stimuli. Here, we review behavioral evidence demonstrating postpartum rodents exhibit a bias towards pups and pup-related stimuli in reward-related tasks. Next, we provide an overview of normative adaptations in the mesolimbic DA system induced by parturition and the postpartum, which likely mediate shifts in offspring valence. We also discuss a causal link between dopaminergic dysfunction and disrupted maternal behaviors, which are recapitulated in postpartum depression (PPD) and relevant rodent models. In sum, mesolimbic DA system activation drives infant-seeking behavior and strengthens the mother-infant bond, potentially representing a therapeutic target for reward-related deficits in PPD.

Smoking-related symptomatology in pregnant smokers during ad libitum smoking and following overnight smoking abstinence

Objective

Current literature suggests there may be a relationship between sex hormones, which dramatically increase during pregnancy, and nicotine use behaviors. We hypothesized that higher progesterone and progesterone:estradiol ratio (P/E₂) would be associated with less smoking-related symptomatology (SRS), better mood and fewer cigarettes smoked per day (CPD) during ad libitum smoking and following overnight abstinence in pregnant women. Associations between SRS, mood, smoking behavior and sex hormones were estimated using multiple linear regression with adjustment for CPD and pregnancy trimester.

Results

There were 35 second trimester and 42 third trimester participants. Participants mean age was 26.2 (SD: 4.1), they smoked 11.3 CPD (SD: 4.4) and the mean nicotine dependence score was 4.94 (SD: 1.98). There were no statistically significant associations between progesterone levels, estradiol levels, or the P/E₂ ratio and SRS or mood measures during ad libitum smoking or following overnight abstinence in this sample of pregnant women. Similarly, there were no associations between sex hormone levels and number of CPD smoked during the ad libitum period. Contrary to our hypothesis, we found no significant associations between sex hormones and SRS, mood or smoking behavior in this sample of pregnant women.

Trial registration ClinicalTrials.gov (NCT01811225), December 6, 2012

Cues play a critical role in estrous cycle-dependent enhancement of cocaine reinforcement

While preclinical work has aimed to outline the neural mechanisms of drug addiction, it has overwhelmingly focused on male subjects. There has been a push in recent years to incorporate females into existing addiction models; however, males and females often have different behavioral strategies, making it important to not only include females, but to develop models that assess the factors that comprise female drug addiction. Traditional self-administration models often include light or tone cues that serve as discriminative stimuli and/or consequent stimuli, making it nearly impossible to disentangle the effects of cue learning, the cues themselves, and acute effects of psychostimulant drugs. To disentangle the interaction between drug-associated cues and the consummatory and appetitive responding driven by cocaine, we have developed a new behavioral procedure that combines Pavlovian-instrumental transfer with behavioral economic analysis. This task can be completed within a single session, allowing for studies looking at estrous cycle stage-dependent effects in intact cycling females, something that has been difficult in the past. In this study, we found no differences in self-administration across the estrous cycle in the absence of cues; however, when cues were introduced, the cues that acquired value during estrus-but not during diestrus or in males-increased motivation. Cues paired during estrus also increased c-fos expression to a greater extent in striatal regions, an effect that may underlie the observed increases in seeking induced by these cues, even weeks later. Together, these data suggest that fundamental differences in the motivational properties of psychostimulant drugs between males and females are complex and are driven primarily by the interaction between drug-associated stimuli and drug effects.

Exogenous progesterone for cannabis withdrawal in women: Feasibility trial of a novel multimodal methodology

BACKGROUND

Sex differences in cannabis use disorder (CUD) and its treatment have been identified. Women report more severe withdrawal and have shown worse treatment outcomes. Ovarian hormones are implicated in these differences and research suggests that exogenous progesterone may be an effective pharmacotherapy.

METHODS

The current randomized, placebo-controlled, feasibility trial tested a novel multimodal methodology for administering exogenous progesterone during acute cannabis withdrawal. Eight heavy cannabis using women received micronized progesterone (200 mg bid) (n = 3) or matching placebo (n = 5) during the early follicular phase of their menstrual cycle over a 5-day study period while abstaining from cannabis. Laboratory visits (days 1 and 5) included biological and self-report assessments, while home-based procedures (days 2-4) included ambulatory assessments, video data capture and tele-drug testing, and biological assessments. Primary outcomes were medication adherence and salivary hormone levels, and the exploratory outcome was cannabis withdrawal severity.

RESULTS

Medication adherence rates were high as assessed via self-report (100.0%) and video data capture (98.0%). Salivary progesterone levels differed between groups over time (p < 0.027) and the progesterone group achieved levels within the normal range during the luteal phase in healthy adults. All tele-drug tests were negative confirming cannabis abstinence and there was an indication (p = 0.07) of reduced cannabis craving among participants receiving progesterone.

CONCLUSION

More effective and sex-based treatments for cannabis use disorder are needed. The current study provides a novel multimodal methodology with low participant burden for investigating new medications for cannabis withdrawal. Clinical trials of progesterone for cannabis withdrawal may be warranted.

Palatable Food Affects HPA Axis Responsivity and Forebrain Neurocircuitry in an Estrous Cycle-specific Manner in Female Rats

Eating palatable foods can provide stress relief, but the mechanisms by which this occurs are unclear. We previously characterized a limited sucrose intake (LSI) paradigm in which twice-daily access to a small amount of 30% sucrose (vs. water as a control) reduces hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress and alters neuronal activation in stress-regulatory brain regions in male rats. However, women may be more prone to 'comfort feeding' behaviors than men, and stress-related eating may vary across the menstrual cycle. This suggests that LSI effects may be sex- and estrous cycle-dependent. The present study therefore investigated the effects of LSI on HPA axis stress responsivity, as well as markers of neuronal activation/plasticity in stress- and reward-related neurocircuitry in female rats across the estrous cycle. We found that LSI reduced post-restraint stress plasma ACTH in female rats specifically during proestrus/estrus (P/E). LSI also increased basal (non-stress) FosB/deltaFosB- and pCREB-immunolabeling in the basolateral amygdala (BLA) and central amygdala specifically during P/E. Finally, Bayesian network modeling of the FosB/deltaFosB and pCREB expression data identified a neurocircuit that includes the BLA, nucleus accumbens, prefrontal cortex, and bed nucleus of the stria terminalis as likely being modified by LSI during P/E. When considered in the context of our prior results, the present findings suggest that palatable food reduces stress responses in female rats similar to males, but in an estrous cycle-dependent manner. Further, the BLA may contribute to the LSI effects in both sexes, whereas the involvement of other brain regions appears to be sex-dependent.

Compulsive sucrose- and cocaine-seeking behaviors in male and female Wistar rats

RATIONALE

Compulsive cocaine use is a key feature of cocaine addiction and understanding the factors that promote the development of such a behavior will provide important insights into the mechanism of cocaine addiction and is essential for the clinical management of the disorder.

OBJECTIVES

This study aimed to determine how the preexisting compulsive reward-seeking behavior is related to the development of compulsive cocaine-seeking behavior in male and female rats and the potential impact of the reward value and estrous cycle on such behaviors.

METHODS

Adult male and female Wistar rats were first trained to self-administer sucrose pellets under a chained schedule, and then, the intensity-response effects of footshock punishment on sucrose SA reinforced by different values of sucrose were measured. Subsequently, the same rats went on to self-administer intravenous cocaine and the punishment intensity-response effects on cocaine SA reinforced by different doses of cocaine were similarly determined. For the female rats, the measurements were made during different phases of the estrous cycle.

RESULTS

The rats showed a wide range of levels of the compulsive behaviors despite the similar training history. Surprisingly, the compulsive sucrose-seeking behavior did not predict the compulsive cocaine-seeking behavior in either sex. Increasing cocaine dose significantly increased the compulsive cocaine-seeking behavior in the female but not male rats. Estrous cycle did not have impact on the compulsive behaviors.

CONCLUSION

Preexisting differences in compulsive sucrose-seeking behavior do not predict compulsive cocaine-seeking behavior. Compulsive cocaine-seeking behavior is influenced by cocaine dose but not estrous cycle in the female rats.

The effects of social contact on cocaine intake in female rats

BACKGROUND

Studies conducted in male rats report that social contact can either facilitate or inhibit drug intake depending on the behavior of social partners. The purpose of the present study was to: (1) examine the effects of social contact on cocaine intake in female rats, (2) examine the behavioral mechanisms by which social contact influences cocaine intake, and (3) examine whether the estrous cycle moderates the effects of social contact on cocaine intake.

METHODS

Female rats were assigned to either isolated or pair-housed conditions in which a social partner either had access to cocaine (cocaine partner) or did not have access to cocaine (abstinent partner). Pair-housed rats were tested in custom-built operant conditioning chambers that allowed both rats to be tested simultaneously in the same chamber.

RESULTS

Rats housed with a cocaine partner self-administered more cocaine than isolated rats and rats housed with an abstinent partner. A behavioral economic analysis indicated that these differences were driven by a greater intensity of cocaine demand (i.e., greater intake at lower unit prices) in rats housed with a cocaine partner. Multivariate modeling revealed that the estrous cycle did not moderate the effects of social contact on cocaine intake.

CONCLUSIONS

These findings indicate that: (1) social contact influences cocaine self-administration in females in a manner similar to that reported in males, (2) these effects are due to differences in the effects of social contact on the intensity of cocaine demand, and (3) these effects are consistent across all phases of the estrous cycle.

Place preferences associated with pups or cocaine change the expression of D2R, V1aR and OTR in the NAcc and MeA and the levels of plasma AVP, OT, T and E2 in mandarin vole fathers

Drug abuse often has negative impacts on parenting behavior. The dopamine (DA), arginine vasopressin (AVP) and oxytocin (OT) systems are involved in paternal behavior and drug-induced behaviors. Mandarin voles (Microtus mandarinus) are socially monogamous rodents with high levels of paternal behavior. The aims of this study were to examine the protein expression levels of the DA 2-type receptor (D2R), AVP receptor 1A(V1aR) and OT receptor (OTR) in the nucleus accumbens (NAcc) and medial amygdala (MeA) as well as the plasma hormone responses after mandarin vole fathers were conditioned with their pups or cocaine. Our experimental models are based on the conditioned place preference (CPP) paradigm. We observed CPP in response to either pup- or cocaine-associated cues in the mandarin vole fathers. Fathers that were conditioned to either pups or cocaine had a lower expression of D2R and V1aR in the NAcc than did controls. Fathers that were conditioned to pups had higher levels of OTR expression in the MeA and higher plasma levels of AVP, OT, estradiol (E2), and lower plasma levels of testosterone (T) than did controls. Fathers that were conditioned to cocaine exhibited lower levels of plasma AVP and T. These results indicate that the reward effects of pup and cocaine are both mediated by D2R, V1aR and OTR in the NAcc and MeA and that there are subtle differences between the pup and cocaine reward mechanisms that are associated with altered plasma AVP, OT, T and E2.

Social context has differential effects on acquisition of nicotine self-administration in male and female rats

RATIONAL

Smoking typically begins during adolescence or early adulthood in a social context, yet the role of social context in animal models is poorly understood.

OBJECTIVES

The present study examined the effect of social context on acquisition of nicotine self-administration.

METHODS

Sixty-day-old male and female Sprague-Dawley rats were trained to press a lever for nicotine (0.015 mg/kg, IV) or saline infusions (males only) on a fixed ratio (FR1) schedule of reinforcement across nine sessions in duplex chambers that were conjoined with either a solid wall or a wall containing wire mesh creating a social context between rat dyads (social visual, auditory, and olfactory cues). In a subsequent experiment, sex differences and dose-dependent effects of nicotine [0 (saline), 0.015 or 0.03 mg/kg, IV] were directly compared in rats trained in the isolated or social context on a schedule progressing from FR1 to FR3. These rats were given 20 sessions followed by 3 extinction sessions.

RESULTS

We consistently found transient social facilitation of low-dose nicotine self-administration in males during the first session. However, across training overall, we found social suppression of nicotine intake that was most prominent in females during later sessions.

CONCLUSIONS

Collectively, these findings suggest that at the age of transition from adolescence to adulthood, a social context enhances the initial reinforcing effects of nicotine in males, but protects against nicotine intake during later sessions especially in females. These findings highlight the importance of sex and social context in studying neural mechanisms involved in initiation of nicotine use.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Cocaine Shifts the Estrus Cycle Out of Phase and Caffeine Restores It

Background: Sex differences in cocaine abuse are well established. Females have a higher sensitivity and thus higher vulnerability to cocaine abuse compared to males. There are many studies showing that sensitivity to cocaine reward varies during the estrus cycle. Methods: Vaginal smears were examined through a DIFF staining kit and viewed through a microscope to determine the estrus cycle stage. Smears were taken immediately before and after cocaine and/or caffeine injections. Furthermore, we suggest a new tool to analyze the estrus cycle by using electrical resistance of the vaginal mucosa. Results: In the present study, we discovered that cocaine directly induced changes in the estrus cycle. Interestingly, caffeine did not affect the estrus cycle and nor did the combination of cocaine and caffeine. We observed that caffeine blocked the cocaine-induced estrus cycle changes using conventional exfoliate cytology. Therefore, caffeine may have neuroprotective properties on the changes induced by cocaine. Conclusion: These phase changes in the estrus cycle may be the underlying cause of sex differences in cocaine addiction that can be blocked by caffeine. Thus, we propose a valuable insight into sex differences in cocaine abuse and reveal a possible treatment with antagonizing the adenosine system.

Emotion and mood adaptations in the peripartum female:complementary contributions of GABA and oxytocin

Peripartum hormones and sensory cues from young modify the maternal brain in ways that can render females either at risk for, or resilient to, elevated anxiety and depression. The neurochemical systems underlying these aspects of maternal emotional and mood states include the inhibitory neurotransmitter GABA and the neuropeptide oxytocin (OXT). Data from laboratory rodents indicate that increased activity at the GABA(A) receptor contributes to the postpartum suppression of anxiety-related behaviour that is mediated by physical contact with offspring, whereas dysregulation in GABAergic signalling results in deficits in maternal care, as well as anxiety- and depression-like behaviours during the postpartum period. Similarly, activation of the brain OXT system accompanied by increased OXT release within numerous brain sites in response to reproductive stimuli also reduces postpartum anxiety- and depression-like behaviours. Studies of peripartum women are consistent with these findings in rodents. Given the similar consequences of elevated central GABA and OXT activity on maternal anxiety and depression, balanced and partly reciprocal interactions between these two systems may be essential for their effects on maternal emotional and mood states, in addition to other aspects of postpartum behaviour and physiology.

Cocaine-associated odor cue re-exposure increases blood oxygenation level dependent signal in memory and reward regions of the maternal rat brain

BACKGROUND

Cue triggered relapse during the postpartum period can negatively impact maternal care. Given the high reward value of pups in maternal rats, we designed an fMRI experiment to test whether offspring presence reduces the neural response to a cocaine associated olfactory cue.

METHODS

Cocaine conditioned place preference was carried out before pregnancy in the presence of two distinct odors that were paired with cocaine or saline (+Cue and -Cue). The BOLD response to +Cue and -Cue was measured in dams on postpartum days 2-4. Odor cues were delivered to dams in the absence and then the presence of pups.

RESULTS

Our data indicate that several limbic and cognitive regions of the maternal rat brain show a greater BOLD signal response to a +Cue versus -Cue. These include dorsal striatum, prelimbic cortex, parietal cortex, habenula, bed nucleus of stria terminalis, lateral septum and the mediodorsal and the anterior thalamic nucleus. Of the aforementioned brain regions, only the parietal cortex of cocaine treated dams showed a significant modulatory effect of pup presence. In this area of the cortex, cocaine exposed maternal rats showed a greater BOLD activation in response to the +Cue in the presence than in the absence of pups.

CONCLUSIONS

Specific regions of the cocaine exposed maternal rat brain are strongly reactive to drug associated cues. The regions implicated in cue reactivity have been previously reported in clinical imaging work, and previous work supports their role in various motivational and cognitive functions.

The development of a preference for cocaine over food identifies individual rats with addiction-like behaviors

RATIONALE

Cocaine dependence is characterized by compulsive drug taking that supercedes other recreational, occupational or social pursuits. We hypothesized that rats vulnerable to addiction could be identified within the larger population based on their preference for cocaine over palatable food rewards.

OBJECTIVES

To validate the choice self-administration paradigm as a preclinical model of addiction, we examined changes in motivation for cocaine and recidivism to drug seeking in cocaine-preferring and pellet-preferring rats. We also examined behavior in males and females to identify sex differences in this "addicted" phenotype.

METHODS

Preferences were identified during self-administration on a fixed-ratio schedule with cocaine-only, pellet-only and choice sessions. Motivation for each reward was probed early and late during self-administration using a progressive-ratio schedule. Reinstatement of cocaine- and pellet-seeking was examined following exposure to their cues and non-contingent delivery of each reward.

RESULTS

Cocaine preferring rats increased their drug intake at the expense of pellets, displayed increased motivation for cocaine, attenuated motivation for pellets and greater cocaine and cue-induced reinstatement of drug seeking. Females were more likely to develop cocaine preferences and recidivism of cocaine- and pellet-seeking was sexually dimorphic.

CONCLUSIONS

The choice self-administration paradigm is a valid preclinical model of addiction. The unbiased selection criteria also revealed sex-specific vulnerability factors that could be differentiated from generalized sex differences in behavior, which has implications for the neurobiology of addiction and effective treatments in each sex.

Effects of menstrual cycle phase on cocaine self-administration in rhesus macaques

Epidemiological findings suggest that men and women vary in their pattern of cocaine use resulting in differences in cocaine dependence and relapse rates. Preclinical laboratory studies have demonstrated that female rodents are indeed more sensitive to cocaine's reinforcing effects than males, with estrous cycle stage as a key determinant of this effect. The current study sought to extend these findings to normally cycling female rhesus macaques, a species that shares a nearly identical menstrual cycle to humans. Dose-dependent intravenous cocaine self-administration (0.0125, 0.0250, and 0.0500 mg/kg/infusion) using a progressive-ratio schedule of reinforcement was determined across the menstrual cycle. The menstrual cycle was divided into 5 discrete phases - menses, follicular, periovulatory, luteal, and late luteal phases - verified by the onset of menses and plasma levels of estradiol and progesterone. Dependent variables including number of infusions self-administered per session, progressive ratio breakpoint, and cocaine intake were analyzed according to cocaine dose and menstrual cycle phase. Analysis of plasma hormone levels verified phase-dependent fluctuations of estradiol and progesterone, with estrogen levels peaking during the periovulatory phase, and progesterone peaking during the luteal phase. Progressive ratio breakpoint, infusions self-administered, and cocaine intake did not consistently vary based on menstrual cycle phase. These findings demonstrate that under the current experimental parameters, the reinforcing effects of cocaine did not vary across the menstrual cycle in a systematic fashion in normally cycling rhesus macaques.

Accumbal dopamine function in postpartum rats that were raised without their mothers

Postpartum rats that had been previously raised in an artificial rearing (AR) apparatus, without their mothers or siblings during the preweaning period, show altered maternal responses towards their own offspring in adulthood. In mother-reared (MR) rats, nucleus accumbens (NAC) dopamine (DA) responses to pups evoke a robust sustained rise during the postpartum period and following treatment with estrogen/progesterone parturient-like hormones (Afonso et al., 2009). These MR females had siblings that received AR rearing with varying amounts of preweaning tactile stimulation (ARmin; ARmax). The present study examined NACshell DA responses to pup and food stimuli in these AR rats, and statistically compared them to their MR siblings. Microdialysis samples were collected from adult (90 days postnatal) AR females in different parity states (cycling vs. postpartum, Exp. 1), or after ovariectomy with different hormone treatments (sham vs. hormone, Exp. 2. After basal sample collection, pup and then food stimuli were individually presented to the females in the dialysis chamber. As with their MR siblings, basal DA concentrations were lower and pup-evoked DA responses greater in hormonally-primed AR females than in non-primed AR controls. Compared to their postpartum MR sisters (Exp. 1), AR rats had increased basal DA levels, reduced pup related DA elevations, and disrupted maternal behavior. The postpartum AR impairment in pup-evoked DA was reversed by additional pre-weaning tactile stimulation. Exogenous hormones (Exp. 2) eliminated AR impairments on pup-evoked DA responses. Although MR and AR siblings had comparable DA responses to food stimuli, upon reanalyzing MR data it was found that only postpartum dams had DA responses to pups greater than to food. These data suggest that that the hormonally induced suppression of basal DA levels may reflect saliency of pups which was greater in MR than in AR dams. Preweaning tactile stimulation could partially reverse these effects only in naturally cycling or parturient animals.

Effects of progesterone and testosterone on cocaine self-administration and cocaine discrimination by female rhesus monkeys

The neuroactive steroid hormone progesterone attenuates cocaine's abuse-related effects in women and in rodents under some conditions, but the effects of testosterone are unknown. We compared the acute effects of progesterone (0.1, 0.2, and 0.3 mg/kg, intramuscularly (i.m.)), testosterone (0.001, 0.003, and 0.01 mg/kg, i.m.), and placebo on cocaine self-administration and cocaine discrimination dose-effect curves in female rhesus monkeys. Cocaine self-administration (0.03 mg/kg per inj.) was maintained on a fixed ratio 30 schedule of reinforcement, and monkeys had unlimited access to cocaine for 2 h each day. Cocaine doses were administered in an irregular order during each dose-effect curve determination, and the same dose order was used in each subject in all treatment conditions. Blood samples for hormone analysis were collected at the end of each test session. Banana-flavored food pellets (1 g) were also available in three 1-h daily sessions. In drug discrimination studies, the effects of pretreatment with progesterone (0.032-0.32 mg/kg, i.m.) and testosterone (0.001-0.01 mg/kg, i.m.) on the discriminative stimulus effects of cocaine (0.18 mg/kg, i.m.) were examined. Progesterone and testosterone did not alter cocaine discrimination, and did not substitute for cocaine. In contrast, progesterone and testosterone each significantly decreased cocaine self-administration, and produced a downward and rightward shift in the cocaine self-administration dose-effect curve. These findings are concordant with clinical reports that progesterone administration may decrease ratings of positive subjective effects of cocaine in women, and suggest the possible value of neuroactive steroid hormones for the treatment of cocaine abuse and reduction of risk for relapse.

The effects of oral micronized progesterone on smoked cocaine self-administration in women

There are currently no FDA-approved pharmacotherapies for cocaine abuse. Converging preclinical and clinical evidence indicates that progesterone may have potential as a treatment for cocaine-abusing women, who represent a growing portion of cocaine users. We have previously shown that oral progesterone reduced the positive subjective effects of cocaine in female cocaine users during the follicular phase of the menstrual cycle, when endogenous progesterone levels were low. To extend these findings, the present study assessed the effects of oral progesterone (150 mg BID) administered during the follicular phase on smoked cocaine self-administration in women relative to the normal follicular and luteal phases. Healthy, non-treatment seeking female cocaine smokers (N=10) underwent three 4-day inpatient stays, during: 1) a normal follicular phase; 2) a normal luteal phase; and 3) a follicular phase when oral progesterone was administered. During each stay, participants completed 4 self-administration sessions in which they first smoked a "sample" dose of cocaine (0, 12, 25 or 50 mg) and then had 5 opportunities at 14-minute intervals to self-administer that dose at a cost of $5 per dose. Expected cocaine dose effects on self-administration, subjective effects, and cardiovascular effects were observed. However, there was no effect of oral progesterone administration or menstrual cycle phase on cocaine self-administration. Thus, oral progesterone was not effective in reducing cocaine use in women under the current conditions. However, based on previous literature, further research assessing the role of oral progesterone for the treatment of cocaine dependence in women is warranted.

Does the response to cocaine differ as a function of sex or hormonal status in human and non-human primates?

Stimulant abuse continues to be a growing problem among women. Over the last 10-15 years, an increasing number of studies have focused on factors that may be implicated in stimulant abuse in women as compared to men, including the role of hormonal fluctuations across the menstrual cycle. Numerous preclinical studies have documented that female rodents are more sensitive than male rodents to the behavioral effects of stimulant administration and the hormone estradiol is involved in the enhanced response to stimulants observed in females. In contrast, fewer studies have been conducted in humans and non-human primates addressing the role of sex and gonadal hormones on the effects of cocaine. This review paper presents a recent update on data collected in our Human Cocaine Challenge Laboratory and our Non-human Primate Laboratory, including analysis of cocaine pharmacokinetics, sex differences, the menstrual cycle, and the role of progesterone in modulating the response to cocaine. Our studies indicate that there is minimal evidence that the response to intranasal cocaine varies across the menstrual cycle or between men and women. In contrast, the response to smoked cocaine is greater in the follicular phase than the luteal phase and differences between men and women generally only emerge when men are compared to women in the luteal phase. In terms of potential hormonal mechanisms for these differences, the hormone progesterone attenuates the subjective response to cocaine. With respect to cocaine self-administration, there are minimal changes across the menstrual cycle in both humans and non-human primates. Thus, there is converging evidence across a range of species that the behavioral effects of cocaine (1) differ between males and females, (2) differ in relation to hormonal fluctuations, (3) can be attenuated by progesterone (at least in females), and (4) do not appear to be related to differences in cocaine pharmacokinetics.

The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats

Converging evidence suggests that the motivation to seek cocaine during the postpartum period is significantly impacted by the competing incentives of offspring, a stimulus unique to this life stage. In the present study, the functional role of the medial preoptic area (mPOA), a critical site involved in maternal responsiveness, on processing incentive value of pup-associated cues and influencing response allocation for pup- over cocaine-associated environments was investigated using a concurrent pup/cocaine choice conditioned place preference (CPP) paradigm. Early postpartum females with bilateral guide cannulae aimed into the mPOA or into anatomical control sites were conditioned, from postpartum days (PPD) 4 to 7, to associate different uniquely featured environments with pups or cocaine. CPP was tested on PPD8 following intra-mPOA infusions of either 2% bupivacaine or saline vehicle. In two additional experiments, the effects of intra-mPOA infusions of bupivacaine on expression of conditioned responding induced by environments associated with either pups or cocaine were examined separately. Transient inactivation of the mPOA selectively blocked the conditioned preferences for pup-associated environments, significantly contrasting the robust pup-CPP found in non-surgical and intra-mPOA vehicle-treated females. In contrast, mPOA inactivation failed to alter cocaine-CPP in postpartum females. When given a choice between environments associated with pups or cocaine, transient functional inactivation of the mPOA altered choice behavior, biasing the preference of females toward cocaine-associated environments, such that almost all preferred cocaine- and none the pup-associated option. The anatomical specificity was revealed when inactivation of adjacent regions to the mPOA did not affect CPP responses for pups. The findings support a critical role for the mPOA in mediating pup-seeking behavior, and further suggest that the competing properties of pups over alternative incentives, including drugs of abuse, rely on mPOA integrity to provide relevant pup-related information to the circuitry underlying the choice behavior between pups and alternative stimuli.

Neural mechanisms of reproduction in females as a predisposing factor for drug addiction

There is an increasing awareness that adolescent females differ from males in their response to drugs of abuse and consequently in their vulnerability to addiction. One possible component of this vulnerability to drug addiction is the neurobiological impact that reproductive physiology and behaviors have on the mesolimbic dopamine system, a key neural pathway mediating drug addiction. In this review, we examine animal models that address the impact of ovarian cyclicity, sexual affiliation, sexual behavior, and maternal care on the long-term plasticity of the mesolimbic dopamine system. The thesis is that this plasticity in synaptic neurotransmission stemming from an individual's normal life history contributes to the pathological impact of drugs of abuse on the neurobiology of this system. Hormones released during reproductive cycles have only transient effects on these dopamine systems, whereas reproductive behaviors produce a persistent sensitization of dopamine release and post-synaptic neuronal responsiveness. Puberty itself may not represent a neurobiological risk factor for drug abuse, but attendant behavioral experiences may have a negative impact on females engaging in drug use.

Hormones that increase maternal responsiveness affect accumbal dopaminergic responses to pup- and food-stimuli in the female rat

The present study investigated hormonal mediation of maternal behavior and accumbal dopamine (DA) responses to pup-stimuli, as measured in microdialysis samples collected from the nucleus accumbens shell of female rats in non-homecage environment. In Experiment 1, samples were collected before and after continuous homecage pup experience from either intact postpartum or cycling females. In Experiment 2, samples were collected before and after responding maternally in homecage from ovariectomized females given either parturient-like hormone or sham treatments. After baseline sample collection in the dialysis chamber, pup and food stimuli were individually presented to females. Upon sampling completion, all animals were placed back into their homecage with donor pups for several days, and then the sample collection procedure was repeated. Prior to stimulus presentation, postpartum and hormone-treated females had decreased basal DA release compared to their controls. In response to pup stimuli, only postpartum and hormone-treated females had increased DA release compared to basal release (both sampling days). In response to food stimuli, all females had increased DA responses from basal; although there were group differences on the initial day of sampling. Findings suggest that hormones associated with inducing maternal behavior in the postpartum rat play a significant role in modifying accumbal dopaminergic responses on first exposure to pup stimuli in the rat. However, the postpartum experience provides further modifications to this brain region to promote DA responses to pup stimuli.

Attenuation of cocaine-seeking by progesterone treatment in female rats

Clinical research suggests that gender differences exist in cocaine dependence. Similarly, preclinical studies have shown that female rats exhibit higher response rates during cocaine self-administration, early extinction, and cocaine-primed reinstatement of drug-seeking. These effects are also estrous cycle dependent and inversely related to plasma progesterone, in that proestrus females (high progesterone) exhibit less cocaine-seeking, while estrous females (low progesterone) show the greatest cocaine-seeking. Based on these findings, we hypothesized that progesterone would attenuate cocaine-seeking behavior in intact, freely cycling animals. The role of the estrous cycle on cocaine-seeking behavior during early (first acquisition day) versus late (last maintenance day) cocaine self-administration was also examined. Female, Sprague-Dawley rats self-administered cocaine (0.5 mg/kg/infusion, IV) along a FR1 schedule, followed by daily extinction sessions in the absence of cocaine reinforcement. Once responding was extinguished, rats received an injection of cocaine (10 mg/kg, IP) immediately prior to reinstatement testing. Progesterone (2 mg/kg, SC) or vehicle was administered 20 and 2h prior to the first day of extinction (early cocaine withdrawal) and the reinstatement trials. To determine estrous cycle phase, we assessed vaginal cytology prior to the first acquisition and last maintenance days of cocaine self-administration, the first day of extinction training, and each reinstatement test. During early and late cocaine self-administration, proestrus and estrous females exhibited the greatest levels of active lever responding, respectively. A significant increase in responding also occurred during cocaine-primed reinstatement for estrous versus nonestrous females, an effect that was selectively attenuated by progesterone. However, progesterone was not effective at reducing cocaine-primed reinstatement for females in other phases of the estrous cycle, nor was it effective at reducing cocaine-seeking during early withdrawal. Taken together, these results suggest that progesterone may be a useful therapeutic for preventing relapse in abstinent female cocaine users, especially when the likelihood of relapse is greatest.

Novelty-induced locomotion is positively associated with cocaine ingestion in adolescent rats; anxiety is correlated in adults

The present studies assessed the roles of sex, age, novelty-seeking and plus-maze behavior on cocaine drinking in rats. Cocaine/saccharin solution was available in three daily, 5-hour sessions then a saccharin-only solution was also available in following sessions. In the one-bottle drinking phase, early and late adolescent males, post-natal day 28 (PN28) and PN42, consumed more cocaine/saccharin solution than young adults (PN65), but females did not exhibit significant age differences. Adolescents of both sexes consumed more cocaine/saccharin than adults during choice drinking. Saccharin availability in the two-bottle trials decreased cocaine/saccharin consumption in PN28 and PN65 rats. After a drug-free period, cocaine-stimulated locomotion was lower in cocaine/saccharin drinking than saccharin-only males, indicating tolerance. We tested the hypothesis that individual differences in pre-screened behavioral traits would correlate with cocaine/saccharin consumption in PN28 and PN65 male rats. High locomotor responses to novelty were associated with greater cocaine/saccharin drinking in adults in one-bottle sessions. In the subsequent choice drinking phase, correlations were age-specific. Adolescents with high novelty-induced locomotion and adults that spent less time on open arms of the elevated plus-maze drank more cocaine/saccharin. Thus, behavioral phenotypes correlated with individual differences in cocaine/saccharin consumption in an age-related manner.

Incentive salience of cocaine across the postpartum period of the female rat

RATIONALE-OBJECTIVES: Our prior conditioned place preference (CPP) work demonstrates that late (day16) postpartum female rats consistently prefer cocaine- over pup-associated chambers, whereas far fewer early postpartum (day8) females prefer the cocaine-associated chamber. The present study examines early and late postpartum females' preference for a cocaine-associated chamber when contrasted with a chamber associated with saline (rather than pups).

MATERIALS AND METHODS

Postpartum females were tested for conditioned preference for chambers associated with cocaine (10 mg/kg subcutaneous (SC) or 0.5, 5, 10, or 20 mg/kg intraperitoneal (IP) injections) versus saline; preferences of virgin female and male rats for select cocaine stimuli (10mg/kg SC or IP) were also tested. Locomotion was recorded during CPP conditioning and testing.

RESULTS

Early and late postpartum females expressed strikingly similar preference for the cocaine-associated chamber across all administration routes and doses. IP cocaine produced an orderly, inverted U-shaped dose-preference curve, with preference peaking at the 5 mg/kg dose (83% of females). While many postpartum females preferred 10mg/kg cocaine administered either SC or IP, both virgin females and males expressed strong aversion to SC cocaine and, while virgin females strongly preferred IP cocaine, males remained relatively indifferent. Across 10mg/kg IP cocaine-conditioning sessions, locomotor sensitization occurred exclusively in cocaine- but not saline-preferring postpartum females. Locomotor rate was lower in preferred versus nonpreferred chambers at CPP test.

CONCLUSIONS

Early and late postpartum females may be equally and uniquely susceptible to sampling and/or abuse of modestly salient doses of cocaine (10mg/kg SC; 5mg/kg IP) compared to virgin females and/or males.

Protracted time-dependent increases in cocaine-seeking behavior during cocaine withdrawal in female relative to male rats

RATIONALE

Female rats display higher sensitivity to cocaine relative to males under a variety of conditions. Time-dependent increases in cocaine-seeking behavior (as measured by nonreinforced operant responses) during cocaine withdrawal have been reported in male, but not female, rats.

OBJECTIVES

The present study determines sex and estrous cycle influences on time-dependent changes in cocaine-seeking behavior.

MATERIALS AND METHODS

Male and female Sprague-Dawley rats were reinforced for "active lever" responses by a cocaine infusion (0.50 mg/kg/infusion, i.v., fixed ratio schedule of reinforcement, FR1) followed by a 20-s time-out when reinforcement was not delivered. Infusions were paired with a light + tone conditioned stimulus. Next, rats underwent cocaine withdrawal for 1, 14, 60, or 180 days before testing cocaine-seeking behavior. Each rat was tested for extinction of operant responding, conditioned-cued reinstatement, and cocaine-primed (10 mg/kg, i.p.) reinstatement.

RESULTS

Both males and females displayed a time-dependent increase in cocaine-seeking behavior (active lever presses) under extinction of operant responding and conditioned-cued reinstatement conditions after 60 days of cocaine withdrawal. Moreover, cocaine-seeking behavior during extinction of operant responding in females, but not males, remained elevated at 180 days of cocaine withdrawal. Furthermore, females tested during estrus exhibited higher cocaine-seeking behavior under both extinction of operant responding and cocaine-primed reinstatement conditions relative to other rats independent of the duration of cocaine withdrawal.

CONCLUSIONS

The effects of reproductive cycle and withdrawal duration on cocaine-seeking behavior are additive and time-dependent increases in cocaine-seeking behavior are more enduring in females than in male rats.

Acquisition and maintenance of cocaine self-administration in adolescent rats: effects of sex and gonadal hormones

RATIONALE

Previous work has shown that adult female rats are more sensitive than adult male rats to the reinforcing effects of cocaine, an effect that appears to be due, at least in part, to ovarian hormones.

OBJECTIVE

In this study, we examine sex differences in cocaine self-administration during adolescence, a period of marked hormonal change.

MATERIALS AND METHODS

Adolescent male and female Sprague-Dawley rats were trained to self-administer cocaine (0.75 mg/kg per infusion) under a fixed ratio 1 schedule (i.e., each response was reinforced by an infusion of cocaine) beginning on postnatal day 30. After acquisition, responding was assessed under a progressive-ratio schedule until postnatal day 50 with blood sampling occurring before the first five sessions to determine the relationship between gonadal hormones (i.e., estradiol, progesterone, and testosterone) and motivation for cocaine. Estrous cycle phase was monitored throughout the study. Separate groups of adolescent male and female rats were compared on the acquisition of and progressive-ratio responding for sucrose reinforcement.

RESULTS

Females acquired cocaine self-administration more readily than did males, and a greater percentage of females acquired self-administration. Under progressive-ratio testing conditions, adolescent females responded at higher levels than adolescent males to obtain cocaine infusions, and in females, responding was positively associated with levels of estradiol and greatest during estrus. No sex differences were observed for sucrose reinforcement.

CONCLUSION

These findings suggest that sex differences are relevant during adolescence with evidence implicating circulating estradiol level as a factor that contributes to the enhanced sensitivity in females to the reinforcing effects of cocaine.

Effects of estradiol on cocaine self-administration and cocaine discrimination by female rhesus monkeys

The ovarian steroid hormone, estradiol, enhances the reinforcing and locomotor activating effects of cocaine in rodents under some conditions. The present study evaluated the acute effects of estradiol benzoate (E(2)beta) on cocaine self-administration and cocaine discrimination in female rhesus monkeys. Cocaine self-administration (0.10 mg/kg/inj., i.v.) was maintained on a fixed-ratio (FR) 30 schedule of reinforcement, and monkeys had access to cocaine during one 2-h session each day. E(2)beta in a cyclodextrin vehicle (0.00001-0.01 mg/kg, i.m.) was administered 30 min before test sessions conducted twice each week. Cocaine doses were administered in an irregular order during each dose-effect curve determination (0.001-0.3 mg/kg/inj.). Blood samples were collected after test sessions to determine 17beta-estradiol levels. Banana-flavored food pellets were available on an FR 30 schedule in three 1-h sessions each day. Five monkeys were trained to discriminate cocaine (0.18 mg/kg, i.m.) from saline in a two-key food-reinforced procedure, and the effects of pretreatment with E(2)beta in cyclodextrin and in sesame oil were studied. Acute administration of E(2)beta did not consistently alter the cocaine self-administration or drug discrimination dose-effect curves in comparison to saline control treatment. Females also did not self-administer E(2)beta (0.00001-0.10 mg/kg, i.v.) above saline levels. Finally, E(2)beta (0.0001-0.01 mg/kg, i.m.) did not substitute for cocaine in monkeys trained to discriminate cocaine from saline. Taken together, these data suggest that over the dose range studied, estradiol administration does not consistently alter the abuse-related effects of cocaine in female rhesus monkeys.

Effects of pregnancy on nicotine self-administration and nicotine pharmacokinetics in rats

RATIONALE

Because of the adverse effects of smoking during pregnancy, understanding the factors that influence maternal smoking may help in developing better treatments to help women quit smoking during pregnancy. Animal models could be useful for this purpose.

OBJECTIVE

The purpose of the present study was to begin the development of an animal model of smoking during pregnancy by initially characterizing nicotine self-administration (NSA) in pregnant rats. Another purpose was to begin to explore the effects of pregnancy on nicotine pharmacokinetics in rats.

MATERIALS AND METHODS

In experiment 1, female rats self-administering nicotine during 23-h sessions were examined throughout gestation and lactation. In experiment 2, locomotor activity was measured during pregnancy to assess further potential motor effects of pregnancy. Experiments 3 and 4 compared the single-dose pharmacokinetics of nicotine in male, nonpregnant female, and pregnant females in the first and third trimester of pregnancy and the first week of lactation.

RESULTS

NSA decreased over the course of pregnancy with NSA significantly lower in the third trimester compared to nonpregnant controls. NSA remained suppressed for up to 10 days into lactation. Locomotor behavior was also significantly suppressed during the second and third trimesters and throughout lactation. Nicotine elimination was slower in pregnant females compared to nonpregnant females only in the third trimester.

CONCLUSIONS

NSA, locomotor behavior, and nicotine elimination in rats are decreased during late pregnancy. The present study is the first to characterize NSA during pregnancy in animals, providing a potential model of maternal smoking in humans.

Increasing the incentive salience of cocaine challenges preference for pup- over cocaine-associated stimuli during early postpartum: place preference and locomotor analyses in the lactating female rat

RATIONALE/OBJECTIVES

Prior studies using a dual-choice conditioned place preference (CPP) procedure revealed that postpartum female rats (dams) strongly prefer chambers associated with pups over those associated with subcutaneously administered cocaine almost exclusively during early but not late postpartum (Mattson et al. 2001). The present study examines whether early postpartum dams retain strong pup-associated chamber preference when contrasted with a cocaine stimulus of greater incentive salience (intraperitoneal [IP] injections with brief conditioning sessions). Locomotor rate was measured during conditioning (stimuli-present) and test (stimulus-absent) sessions.

MATERIALS AND METHODS

A three-chambered CPP apparatus was used to compare preferences for chambers associated with IP cocaine vs age-matched pups. Unconditioned stimuli were systematically assigned to the least-preferred chamber of separate groups of dams before conditioning. Control dams verified that unconditioned stimuli were necessary for CPP and stimulus-associated locomotion.

RESULTS

Compared with most late postpartum dams (60%), only 31% of early postpartum dams preferred the cocaine-associated chamber (P < 0.05). Substantially more dams preferred the pup-associated chamber during early postpartum (27%) than late postpartum (5%; P < 0.05). Locomotor sensitization emerged across cocaine-conditioning sessions in cocaine-preferring but not pup-preferring dams (P < 0.05). Locomotor rates were consistently lower in preferred vs nonpreferred chambers during test.

CONCLUSIONS

After increasing cocaine's incentive salience, more early postpartum dams prefer the cocaine-associated chamber than previously reported (Mattson et al. 2001). However, pup-associated chamber preference was still higher in early vs late postpartum. Pup- and cocaine-preferring dams expressed differences in the induction phase of locomotor sensitization across cocaine conditioning but expressed similar motoric patterns in their preferred chambers at test.

Sex and menstrual cycle effects on progressive ratio measures of cocaine self-administration in cynomolgus monkeys

Fluctuations in ovarian steroid hormones across the menstrual/estrous cycle influence the abuse-related effects of acute cocaine administration in women and chronic cocaine self-administration in rodents, but there have been no comparable studies in non-human primates. The interactions among sex, menstrual cycle phase, and cocaine self-administration (0.0032, 0.01, and 0.032 mg/kg/injection (inj)) under a progressive ratio schedule were investigated in four female and two male cynomolgus monkeys. Females were given unrestricted access to cocaine across 54 menstrual cycles, and males were studied over 23 pseudo-cycles of 30 days duration. Ovulatory cycles were defined by luteal phase elevations in progesterone and 44 cycles were ovulatory. During ovulatory menstrual cycles, females reached significantly higher progressive ratio break points than males at all three unit doses of cocaine (P<0.001). During anovulatory cycles, females also reached significantly higher break points than males for 0.032 mg/kg/inj cocaine (P<0.01). Progressive ratio break points for cocaine (0.01 and 0.032 mg/kg/inj) did not vary significantly as a function of ovarian steroid hormone levels during the follicular and the luteal phase of ovulatory menstrual cycles, or during anovulatory cycles. Progressive ratio break points for 0.0032 mg/kg/inj cocaine were significantly higher during the follicular phase than during the late luteal phase (P<0.05-0.001). There were no systematic changes in progressive ratio break points in male pseudo-cycles. Significant cocaine dose-related sex differences were observed, but no consistent changes in cocaine self-administration as a function of menstrual cycle phase, or levels of estradiol and progesterone, were detected in female cynomolgus monkeys.

Development of cocaine sensitization before pregnancy affects subsequent maternal retrieval of pups and prefrontal cortical activity during nursing

Pups are a highly rewarding stimulus for early postpartum rats. Our previous work supports this notion by showing that suckling activates the mesocorticolimbic system in mothers. In the present study, we tested whether development of behavioral sensitization to cocaine before pregnancy affects the neural response to pups during the early postpartum days (PD). Virgin rats were repeatedly administered cocaine for 14 days (15 mg kg(-1)) and withdrawn from treatment during breeding and pregnancy. The neural response to suckling was measured at PD 4-8 using blood-oxygen-level-dependent (BOLD) MRI or microdialysis. Our results show that BOLD activation in the medial prefrontal cortex (PFC), septum and auditory cortex was curtailed in cocaine-sensitized dams. No differences between cocaine sensitized and saline control dams were observed in the nucleus accumbens, olfactory structures, or in 48 additional major brain regions that were analyzed. Baseline, but not pup-stimulated, dopamine (DA) levels in the medial PFC were lower in cocaine-sensitized dams than in controls. When tested for maternal behaviors, cocaine-sensitized dams showed significantly faster retrieval of pups without changes in other maternal behaviors such as grouping, crouching and defending the nest. Taken together, the present findings suggest that maternal motivation to retrieve pups was enhanced by repeated cocaine exposure and withdrawal, a result reminiscent of 'cross-sensitization' between the drug and a natural reward. Changes in retrieval behavior in cocaine-sensitized mothers might be associated with a hypo-responsive medial PFC.

Interactions among ovarian hormones and time of testing on behavioral sensitization and cocaine self-administration

Ovarian hormones play a role in the use of drugs of abuse in women. In female rats estradiol has been shown to enhance acquisition of cocaine self-administration and behavioral sensitization induced by repeated cocaine treatment. Experiments were conducted to determine the effects of estradiol and/or progesterone on cocaine self-administration and behavioral sensitization to cocaine (10mg/kg; in animals with unilateral 6-hydroxydopamine lesions). Five groups of ovariectomized females were tested: (1) oil vehicle; (2) estradiol (E); (3) progesterone (P); (4) estradiol and progesterone given concurrently (EPC); (5) estradiol and progesterone given sequentially (EPS: 3 days of estradiol, 1 day progesterone, 1 day oil). All animals were tested during the dark phase of the light:dark cycle at ZT1600 and ZT2000-2100. Behavioral sensitization results: there was substantial conditioned turning throughout the habituation periods, and all animals exhibited behavioral sensitization with repeated cocaine treatment. Multivariate analysis indicated a significant effect of hormone treatment, time of day and day of testing. When individual groups were compared, however, only at ZT1600 did the E-treated and the EPS-treated animals show a trend (p<0.06) for greater behavioral sensitization to cocaine relative to the oil-treated animals. Self-administration results: all groups showed rapid acquisition of cocaine self-administration at 0.3 mg/kg/infusion, so we did not see an effect of ovarian hormones on acquisition, or a difference between groups tested at ZT1600 versus ZT2100 (p<0.05). There was, however, enhanced total intake of cocaine at 0.75 mg/kg/infusion in the E and the EPS groups. Concurrent administration of progesterone with estradiol counteracted the effect of estradiol on cocaine intake at 0.75 mg/kg/infusion, while progesterone alone did not enhance cocaine self-administration.

Influence of sex, estrous cycle, and drug-onset age on cocaine self-administration in rats (Rattus norvegicus)

The influence of sex, phase of the estrous cycle, and age of drug onset on cocaine self-administration was examined. Adult male, adult female, and adolescent male rats (Rattus norvegicus) were evaluated using low fixed-ratio (FR) schedules of drug delivery with a single fixed cocaine unit dose or a range of cocaine unit doses with a single FR schedule. Sex differences in adults were observed for mg/kg consumption of the 3.0-mg/kg unit dose, with consumption being significantly less in estrus females than in males. Over the estrous cycle, mg/kg consumption of this unit dose was significantly less during estrus than during metestrus-diestrus. Differences due to age of drug onset were also observed, with mg/kg consumption of the 3.0-mg/kg unit dose being significantly less in adolescent males than adult males or adult females during metestrus-diestrus. In contrast, these various groups did not have significantly different mg/kg intakes of cocaine unit doses <3.0 mg/kg, nor did they significantly differ in the rates and patterns of responding and number of infusions earned as a function of FR schedule or unit dose of cocaine available. The role of sex, estrus cycle, and drug-onset age on cocaine self-administration appears to be minimal under these experimental conditions. Experimental conditions that favor no sex or age differences in cocaine intake (1.0-mg/kg unit dose and low FR) may be useful for evaluating potential sex or age differences in the consequences of cocaine self-administration more reliably, as cocaine intake would not be an uncontrolled factor.

Exogenous progesterone attenuates the subjective effects of smoked cocaine in women, but not in men

In a previous study, we showed that the positive subjective effects of cocaine were higher during the follicular phase compared to the luteal phase of the menstrual cycle. The purpose of the present study was to determine if exogenously administered progesterone during the follicular phase in females would attenuate the response to cocaine compared to the normal follicular phase, thus making the response to cocaine similar to the luteal phase. To address the role of sex differences, males were also administered exogenous progesterone during one inpatient stay. In all, 11 female and 10 male non-treatment-seeking cocaine smokers participated. Females had three inpatient stays: one during a normal follicular phase, one during a normal luteal phase, and one during a follicular phase when exogenous progesterone was administered. Males had two inpatient stays: one when exogenous progesterone was administered and the other when placebo was administered. During each inpatient admission, there were four smoked cocaine administration sessions: participants were administered six doses of cocaine (0, 6, 12, or 25 mg cocaine base) at 14 min intervals. Smoked cocaine increased heart rate, blood pressure and several subjective effects such as 'good drug effect' and 'drug quality' cluster scores. Administration of progesterone during the follicular phase in women attenuated the positive subjective effects of cocaine, whereas only minimal changes were observed in men. These results indicate that progesterone modulates the response to cocaine in women and suggests that fluctuations in endogenous progesterone levels account for some of the sex differences observed in humans.

Sex differences and hormonal influences on acquisition of cocaine self-administration in rats

Men and women differ in their response to cocaine, and a woman's response varies with the menstrual cycle. For example, women have greater subjective responses to cocaine in the follicular phase of the menstrual cycle when estradiol is predominant, than they do during the luteal phase when both estradiol and progesterone are elevated. Similarly, female rats show significantly more cocaine-induced locomotor behavior and cocaine self-administration during behavioral estrus, shortly after estradiol peaks, than during other stages of the cycle, and estradiol administration to ovariectomized (OVX) females enhances the acquisition of cocaine self-administration. The purpose of this study was to expand upon these findings by studying the effects of progesterone administration to females, and estradiol administration to males, on acquisition of cocaine self-administration. We report here that there are both sex differences in and effects of circulating ovarian hormones on acquisition of cocaine self-administration. We demonstrate that although estradiol administration enhances acquisition of cocaine self-administration in OVX female rats, concurrent administration of progesterone with estradiol inhibits this effect of estradiol. In a separate experiment, we demonstrate that estradiol administration does not enhance acquisition of cocaine self-administration in castrated male rats. We conclude that (1) there is a sex difference in the effects of estradiol on cocaine self-administration: it facilitates acquisition in female, but not male rats; and that (2) in females concurrent progesterone treatment counteracts the facilitory effect of estradiol on cocaine self-administration.

Sex differences in the escalation of intravenous cocaine intake following long- or short-access to cocaine self-administration

Preclinical data have indicated that extended access to cocaine self-administration (e.g., 6-12 h/day) facilitates an escalation in daily cocaine intake that is not seen when rats are given shorter (e.g., 1-2 h/day) access to cocaine for self-administration. Data from studies with rats have shown that females self-administer more cocaine than males during all phases of drug abuse (e.g., acquisition, maintenance, and reinstatement). The purpose of this study was to examine potential differences between males and females in the escalation of intravenous cocaine intake following a differential access (e.g., 1 vs. 6 h) period of cocaine self-administration. Four groups of rats were compared: (1) long-access (LgA; 6 h) females; (2) LgA males; (3) short-access (ShA; 1 h) females; and (4) ShA males. Animals were given LgA or ShA to intravenous cocaine (0.5 mg/kg/infusion) self-administration under an Fr 1 schedule for 21 days. Subsequently, access conditions were made equal (3 h) across groups, and dose-response curves for cocaine were compared. Results revealed that the LgA groups' dose-response curves were significantly elevated above those of ShA groups. Additionally, the dose-response curve of LgA female rats was significantly elevated above that of LgA male rats. These results suggest that female rats are more sensitive than male rats to factors that contribute to the escalation of cocaine intake (e.g., extended access conditions).

Testosterone self-administration in female hamsters

Abuse of anabolic-androgenic steroids (AAS) is a growing public health concern. In addition to their anabolic effects, steroids are also reinforcing as demonstrated by testosterone self-administration in male hamsters. However, steroid use in women lags behind that in men. Are androgens also rewarding in females? We determined if female hamsters voluntarily consume testosterone by intracerebroventricular (i.c.v.) self-administration in an operant chamber. Twelve ovary-intact female hamsters self-administering testosterone (1.0 microg/microl) i.c.v. for 19.1 +/- 2.3 days developed a significant preference (P < 0.05) for the active nose-poke (31.5 +/- 6.1 nose-pokes/4 h) over the inactive nose-poke (12.5 +/- 1.1 nose-pokes/4 h). Operant behavior in females was similar to that reported previously for male hamsters. Estrous cycles became irregular 9.6 +/- 2.3 days after the start of self-administration. Regular cycles resumed 13.7 +/- 2.6 days after testosterone was discontinued. To determine the effect of ovarian steroids on androgen self-administration, females were ovariectomized (OVX) and allowed to self-administer testosterone for 10.8 +/- 0.5 days. Afterwards, estrogen was replaced, and self-administration continued for an additional 9.7 +/- 0.6 days. OVX females maintained their preference for the active (23.9 +/- 7.0 nose-pokes/4 h) over the inactive nose-poke (12.6 +/- 3.4 nose-pokes/4 h, P < 0.05), and estrogen had no effect on responding for androgen (active: 25.8 +/- 6.5 nose-pokes; inactive: 8.2 +/- 2.0 nose-pokes/4 h, P < 0.05). Estrous female hamsters did not show a significant preference for stimulus males or females when mating was blocked, and testosterone self-administration did not alter partner preference. However, activity in the preference chamber predicted subsequent androgen intake (R(2) = 0.66, P < 0.05). These findings are consistent with the idea that anabolic steroids have inhibitory effects on female reproduction. Moreover, they suggest that sex differences in androgen reward do not underlie sex differences in AAS abuse in humans.

Pharmacokinetics of intravenous cocaine across the menstrual cycle in rhesus monkeys

Several studies in rodents suggest that there are sex differences in response to cocaine that are related to fluctuations in the ovarian hormones of females. Female rhesus monkeys have menstrual cycles that are remarkably similar to human menstrual cycles in both duration and hormonal variations. Therefore, data obtained in monkeys should be an ideal model for assessing the effects of cocaine across the menstrual cycle in humans. The present study assessed the acute effects of intravenous cocaine (0, 0.25, 0.50, and 1.00 mg/kg) in five female rhesus monkeys during four phases of the menstrual cycle: menses, midfollicular, periovulatory, and midluteal. To reduce the effects of stress that can occur from sedation, all animals were trained to enter primate chairs so that repeated blood samples could be obtained in awake animals. Hormone levels for estradiol and progesterone were measured each session before cocaine administration. Cocaine and cocaine metabolite plasma levels were measured at 5, 15, 30, 45, 60, and 90 min after cocaine administration. Similarly, levels of luteinizing hormone (LH) were measured before, 15, 30, 45, 60, and 90 min after cocaine administration. Within 5 min of cocaine administration, cocaine plasma levels peaked and dose-dependent behavioral changes (ie increased motor activity, mydriasis, and refusal of treats) were observed. These effects typically resolved in 15-30 min. There were few differences in the pharmacokinetic profile of cocaine across the menstrual cycle. However, the cocaine metabolites, BZE and EME, did vary across the menstrual cycle, with both being increased in the luteal phase, particularly following the highest dose of cocaine. In addition, unlike previous studies, cocaine did not produce consistent increases in LH levels. Rather, the change in LH levels depended on menstrual cycle phase and cocaine dose. In summary, there is little evidence that the pharmacokinetics of cocaine vary as a function of menstrual cycle phase.

Preferences for cocaine- or pup-associated chambers differentiates otherwise behaviorally identical postpartum maternal rats

RATIONALE AND OBJECTIVES

Our previous work uncovered a differential preference of maternal female rats for cues associated with pups versus cues associated with cocaine at three different postpartum time points. Our current study examines the preference for these cues in conjunction with an assessment of the capacity to express the maternal behavior at one of these time points. We examined dams at day 10 postpartum using a procedure that included two additional controls, and a complete assessment of the expression of maternal behavior and locomotor activity.

METHODS

A conditioned place-preference procedure was used to determine the preference for cocaine- or pup-associated cues. The two controls were (1) a preconditioning test to verify no initial chamber preference and (2) a separate control group of postpartum day-10 dams exposed to chambers and cues but not to unconditioned stimuli. The expression of maternal behavior was determined by measurement of maternal nest building, retrieval of pups to the nest, grooming, crouching over pups, nursing, and maternal aggression. Locomotor activity was measured with an automated apparatus.

RESULTS

Dams conditioned with cocaine or pups showed a preference for either the cocaine-associated chamber or the pup-associated chamber, confirming the existence of two similar-sized preference groups at this time point. Regardless of preference, dams had equal and robust expression of maternal behavior and similar locomotor capacity. The pre-conditioning test showed no initial chamber preferences and did not alter the conditioned preference response. The use of unconditioned stimuli in the place-preference conditioning procedure was effective and necessary for the preference response.

CONCLUSION

Our current study has revealed that differences in the motivational state of the maternal dam emerge even while the expression of maternal behavior is constant and substantial. The data suggest that the difference in preference is a very specific appetitive response that is not linked to expression of maternal behavior or locomotor capacity.