Influence of estrous cycle and estradiol on behavioral sensitization to cocaine in female rats

The psychedelic (-)-2,5-dimethoxy-4-iodoamphetamine [(-)-DOI] demonstrates efficacy in reducing cocaine reward and motivation in male rats

RATIONALE AND OBJECTIVES

Overdose fatalities involving cocaine continue to rise with over 5.3 million cocaine users reported in the United States in 2022. The abuse liability of cocaine is reliant upon inhibition of dopamine (DA) reuptake and consequent increase in DA efflux in meso-corticolimbic circuitry that controls reward and motivation. Cocaine also increases serotonin (5-HT) efflux which is integral in cocaine abuse. The 5-HT2A receptor (5-HT2AR) is a key regulator of meso-corticolimbic DA release and controls cellular mechanisms underlying cocaine effects. 5-HT2AR actions contribute importantly to psychedelic mechanisms of action, and the efficacy of these compounds in limiting cocaine intake is unknown. The present studies evaluated the efficacy of acute administration of a psychedelic to reduce cocaine intake using standard and advanced preclinical models of drug self-administration.

METHODS

Both a standard fixed ratio (FR) schedule and behavioral economics threshold procedure of cocaine intravenous self-administration were employed to evaluate the efficacy of the psychedelic 5-HT2AR agonist (-)-2,5-dimethoxy-4-iodoamphetamine [(-)-DOI] to decrease cocaine intake and motivation for cocaine in male rats. The 5-HT2AR-selective antagonist M100907 was utilized to explore the role of 5-HT2AR in the effects of (-)-DOI on cocaine intake.

RESULTS

We found that (-)-DOI dose-dependently reduced intake on the FR5 schedule of cocaine IVSA and left shifted the demand curve to evoke greater sensitivity to price increases in the behavioral economics paradigm. Pretreatment with M100907 abated the efficacy of (-)-DOI on cocaine intake in both paradigms.

CONCLUSION

(-)-DOI 'devalued' cocaine reward and motivation to take cocaine in a 5-HT2AR-dependent manner. As serotonergic psychedelics emerge as therapeutic candidates, investigations of 5-HT2AR-acting psychedelics in preclinical analyses of cocaine intake and relapse vulnerability during abstinence will be valuable as prelude to future clinical trials.

Sex and estradiol effects in the rodent dorsal striatum

17β-Estradiol (E2) is a sex hormone that acts on many brain regions to produce changes in neuronal activity and learning. A key brain region sensitive to E2 is the dorsal striatum (also called caudate-putamen), which controls motor behaviour, goal-directed learning and habit learning. In adult rodents, oestrogen receptors (ERs) in the dorsal striatum are localized to the plasma membrane and include ERα, ERβ and G protein-coupled ER (GPER). E2, either naturally produced or exogenously applied, may influence neuronal excitability, basal synaptic transmission and long-term synaptic potentiation. These effects may be due to direct action on signalling pathways or may be due to changes in dopamine availability. In particular, estradiol influences dopamine release, dopamine receptor expression and dopamine transporter expression. We review the cellular effects that E2 has in the dorsal striatum, distinguishing between exogenously applied E2 and the oestrous cycle, as well as its influence on dorsal striatal-dependent motor and learning behaviour.

Cocaine sensitization in male rats requires activation of estrogen receptors

Gonadal steroids play a modulatory role in cocaine use disorders, and are responsible for many sex differences observed in the behavioral response to cocaine. In females, it is well established that estradiol enhances the behavioral response to cocaine. In males, we have recently shown that testosterone enhances sensitization to cocaine but its mechanism of action remains to be elucidated. The current study investigated the contribution of DHT, a non-aromatizable androgen, and of estradiol, in regulating cocaine-induced sensitization in male rats. Gonadectomized (GDX) male rats treated with estradiol sensitized to repeated cocaine administration, while GDX rats treated with DHT did not, implicating estradiol in cocaine sensitization. Furthermore, intact male rats treated with the antiestrogen ICI 182,780 did not show sensitization to repeated cocaine. This study demonstrates the pivotal role of estradiol in cocaine-induced neuroplasticity and neuroadaptations in the rodent brain.

Cocaine-induced sensitization and glutamate plasticity in the nucleus accumbens core: effects of sex

BACKGROUND

The development and persistence of addiction is mediated in part by drug-induced alterations in nucleus accumbens (NAc) function. AMPA-type glutamate receptors (AMPARs) provide the main source of excitatory drive to the NAc and enhancements in transmission of calcium-permeable AMPARs (CP-AMPARs) mediate increased cue-triggered drug-seeking following prolonged withdrawal. Cocaine treatment regimens that result in psychomotor sensitization enhance subsequent drug-seeking and drug-taking behaviors. Furthermore, cocaine-induced locomotor sensitization followed by 14 days of withdrawal results in an increase in glutamatergic synaptic transmission. However, very few studies have examined cocaine-induced alterations in synaptic transmission of females or potential effects of experimenter-administered cocaine on NAc CP-AMPAR-mediated transmission in either sex.

METHODS

Male and female rats were given repeated systemic cocaine injections to induce psychomotor sensitization (15 mg/kg, i.p. 1 injection/day, 8 days). Controls received repeated saline (1 mL/kg, i.p). After 14-16 days of withdrawal brain slices were prepared and whole-cell patch-clamp approaches in the NAc core were used to measure spontaneous excitatory post-synaptic currents (sEPSC), paired pulse ratio, and CP-AMPAR transmission. Additional female rats from this same cohort were also given a challenge injection of cocaine at withdrawal day 14 to assess the expression of sensitization.

RESULTS

Repeated cocaine produced psychomotor sensitization in both sexes. In males this was accompanied by an increase in sEPSC frequency, but not amplitude, and there was no effect on the paired pulse ratio. Males treated with cocaine and saline had similar sensitivity to Naspm. In contrast, in females there were no significant differences between cocaine and saline groups on any measure, despite females showing robust psychomotor sensitization both during the induction and expression phase.

CONCLUSIONS

Overall, these data reveal striking sex differences in cocaine-induced NAc glutamate plasticity that accompany the induction of psychomotor sensitization. This suggests that the neural adaptations that contribute to sensitization vary by sex.

Intermittent cocaine self-administration has sex-specific effects on addiction-like behaviors in rats

Intermittent access (IntA) models of cocaine self-administration were developed to better model in rodents how cocaine is used by human drug users. Compared to traditional continuous access (ContA) models, IntA has been shown to enhance several pharmacological and behavioral effects of cocaine, but few studies have examined sex differences in IntA. Moreover, no one has examined the efficacy of cue extinction to reduce cocaine seeking in the IntA model, which has previously shown to be ineffective in other models that promote habit-like cocaine seeking. Therefore, rats were implanted with jugular vein catheters and dorsolateral striatum (DLS) cannulae and trained to self-administer cocaine paired with an audiovisual cue with ContA or IntA. In subsets of rats, we evaluated: the ability of Pavlovian cue extinction to reduce cue-induced drug seeking; motivation for cocaine using a progressive ratio procedure; punishment-resistant cocaine taking by pairing cocaine infusions with footshocks; and dependence of drug-seeking on DLS dopamine (a measure of habit-like behavior) with the dopamine antagonist cis-flupenthixol. Overall, cue extinction reduced cue-induced drug seeking after ContA or IntA. Compared to ContA, IntA resulted in increased motivation for cocaine exclusively in females, but IntA facilitated punished cocaine self-administration exclusively in males. After 10 days of IntA training, but not fewer, drug-seeking was dependent on DLS dopamine most notably in males. Our results suggest that IntA may be valuable for identifying sex differences in the early stages of drug use and provide a foundation for the investigation of the mechanisms involved.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor α and estrogen receptor β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

Estradiol and progesterone in female reward-learning, addiction, and therapeutic interventions

Sex steroid hormones like estradiol (E2) and progesterone (P4) guide the sexual organization and activation of the developing brain and control female reproductive behavior throughout the lifecycle; importantly, these hormones modulate functional activity of not just the endocrine system, but most of the nervous system including the brain reward system. The effects of E2 and P4 can be seen in the processing of and memory for rewarding stimuli and in the development of compulsive reward-seeking behaviors like those seen in substance use disorders. Women are at increased risk of developing substance use disorders; however, the origins of this sex difference are not well understood and therapeutic interventions targeting ovarian hormones have produced conflicting results. This article reviews the contribution of the E2 and P4 in females to functional modulation of the brain reward system, their possible roles in origins of addiction vulnerability, and the development and treatment of compulsive reward-seeking behaviors.

The influence of sex and reproductive cycle on cocaine-induced behavioral and neurobiological alterations: a review

This systematic review (SR) was aimed at answering two questions: (1) how sex and ovarian hormones alter behavior associated with cocaine use; (2) which possible neurobiological mechanisms explain behavioral differences. Three different researchers conducted a search in PUBMED for all kinds of articles published between the years of 1991 to 2021 on the theme "reproductive cycle and cocaine", "estrous cycle and cocaine", "menstrual cycle and cocaine", "fluctuation of ovarian hormones and cocaine", "estrogen and cocaine" and "progesterone and cocaine". Sixty original studies were identified and subdivided into experimental rodent studies and clinical trials. Experimental studies were characterized by author/year, species/strain, sex/number, age/weight, dose/route/time of administration, hormonal assessment, or administration. Clinical trials were characterized by author/year, sex/number, age, exclusion criterion, dose/route of administration/time of cocaine, and hormonal assessment. Results gathered showed that rodent females develop increased consumption, seeking behavior, craving, relapse, locomotion, increases in stress and anxiety, among other behavioral alterations during peaks of estrogen. These observations are related to the direct effects played by ovarian hormones (in particularly estradiol), in dopamine, but also in serotonin neurons, and in brain regions such as the tegmental area, the nucleus accumbens, the hypothalamus, the amygdala and the prefrontal cortex. Increased sensitization to cocaine presented by high estradiol females was linked to the activation of a CBR1-mediated mechanism and GABA-A-dependent suppression of inhibitory synaptic activity of the prelimbic prefrontal cortex. Estradiol facilitation of cocaine-increased locomotion and self-administration was shown to require the release of glutamate and the activation of metabotropic glutamate receptors subtype 5. Clinical studies also tend to point to a stimulatory effect of estradiol on cocaine sensitization and a neuroprotective effect of progesterone. In conclusion, the results of the present review indicate a need for further preclinical and clinical trials and neurobiological studies to better understand the relationship between sex and ovarian hormones on cocaine sensitization.

D2 receptor antagonism enhances cocaine-induced behavioral sensitization in female, but not male Japanese quail ( Coturnix japonica )

Sex differences in cocaine-induced behaviors are well established. In rodents, females show enhanced locomotion to cocaine over multiple trials compared with males, a behavioral response known as sensitization. Estradiol enhances cocaine-induced sensitization in female rats by agonizing dopaminergic activity within the brain. In female quail, cocaine does not increase locomotion regardless of increased estradiol. A higher D2:D1 dopamine receptor ratio in quail compared with rodents may explain this sex and species difference. The goal of the present work was to investigate the role of D2 receptors in cocaine-induced locomotion and sensitization in Japanese quail and to determine whether a greater D2 receptor availability contributed to the lack of cocaine-induced sensitization in female quail found in previous studies. Male and female quail were administered 0, 0.03, 0.05, or 0.07 mg/kg of eticlopride (Eti) followed by 10 mg/kg of cocaine or saline then immediately placed in open-field chambers. Distance traveled was recorded for 30 min daily for 7 days. In female quail, cocaine-induced sensitization was observed with 0.03 or 0.05 mg/kg Eti, but not in cocaine-only females. In male quail, cocaine-induced sensitization was observed similar to previous research. However, Eti did not enhance cocaine-induced locomotion or produce sensitization in male quail. The D2 receptor likely mediates cocaine's motor stimulating effects in quail. In females, this effect is more pronounced. Since high D2 availability is protective against stimulant abuse, Japanese quail may be a useful model for investigating the role of the D2 receptor in cocaine addiction, but further research is needed.

Sex differences in cocaine-associated memory: The interplay between CB1, mGluR5, and estradiol

We know surprisingly little about the sex differences in the neurobiology of cocaine addiction, except females are more susceptible to the rewarding effects of cocaine than their male counterparts. Only a handful of recent studies have examined the neurobiology of cocaine-induced conditioned place preference (CPP) memory among female rodents. We contribute to this emerging line of research by documenting sex differences in cocaine-associated memory and illustrating the underlying signaling pathways in five experiments. Rimonabant (Rim), a cannabinoid CB₁ antagonist and inverse agonist, exerted a facilitating effect for low-dose cocaine and an impairing effect for high-dose cocaine CPP memory in male mice, as in our previous study, but not in female mice. Nor did we observe the effect exist among CB₁ knockout male mice, which indicated that the CB₁ receptors played a mediating role. We also found that the metabotropic glutamate receptor 5 (mGluR5) was located in the same signaling pathway as CB₁ in male mice. To clarify the mechanisms behind the sex differences, we used ovariectomized (OVX) female mice with estradiol benzoate (EB) replacement. In the OVX female mice, we showed that Rim-alone and EB-alone, but not Rim-and-EB-combined, facilitated the low-dose cocaine CPP memory. Moreover, 4-hydroxytamoxifen (4-OHT), an estrogen receptor (ER) antagonist, blocked Rim's and EB's facilitating effect. Finally, 2-methyl-6-(phenylethynyl)pyridine (MPEP), an mGluR5 antagonist, partially blocked EB's facilitating effect. In sum, we identified sex-specific effects of Rim on cocaine-induced CPP memory and the respective signaling pathways: mGluR5-CB₁ for male mice and ER-mGluR5-CB₁ for female mice. These findings may have merits for the development of sex-specific treatment for cocaine addiction.

The effects of chronic estradiol treatment on opioid self-administration in intact female rats

Heroin intake decreases significantly during proestrus in normally cycling female rats, and this effect is mediated by endogenous estradiol but not endogenous progesterone. The purpose of this study was to determine whether chronic administration of exogenous estradiol decreases intake of the semi-synthetic opioid, heroin, and the fully synthetic opioid, remifentanil, in intact female rats. Normally cycling female rats were implanted with intravenous catheters and trained to self-administer heroin on a fixed ratio (FR1) schedule of reinforcement. Rats were treated chronically with daily administration of either a low dose of estradiol (0.5 mcg, sc), a high dose of estradiol (5.0 mcg, sc), or vehicle (peanut oil, sc). After two weeks of heroin self-administration training, dose-effect curves were determined for both heroin and remifentanil. Chronic administration of estradiol non-significantly decreased heroin intake and significantly decreased remifentanil intake. Estradiol-induced decreases in remifentanil intake were dose-dependent, characterized by large effect sizes, and greatest in rats treated with the high dose of estradiol. These data indicate that chronic estradiol administration decreases opioid intake in intact female rats with medium to large effect sizes across opioids. These findings suggest that estrogen-based pharmacotherapies may represent a novel treatment approach for women with opioid use disorder.

Estradiol and the control of feeding behavior

This review lays out the evidence for the role of E2 in homeostatic and hedonic feeding across several species. While significant effort has been expended on homeostatic feeding research, more studies for hedonic feeding need to be conducted (i.e. are there increases in meal size and enhanced motivation to natural food rewards). By identifying the underlying neural circuitry involved, one can better delineate the mechanisms by which E2 influences feeding behavior. By utilizing more selective neural targeting techniques, such as optogenetics, significant progress can be made toward this goal. Together, behavioral and physiological techniques will help us to better understand neural deficits that can increase the risk for obesity in the absence of E2 (menopause) and aid in developing therapeutic strategies.

Locomotor sensitization to cocaine in adolescent and adult female Wistar rats

Adolescent stress exposure is a risk factor for drug abuse, and sex differences contribute to psychostimulant responses. Although many studies have utilized the Wistar rat strain in adolescent stress paradigms, the impact of adolescent stress exposure on addiction-like outcomes has not been rigorously tested in female Wistar rats. In this study, locomotor sensitization was assessed in adolescent and adult female Wistar rats following either chronic stress during adolescence (CAS) or no stress (NS). Adolescent, but not adult, female Wistar rats developed locomotor sensitization to 15 mg/kg cocaine over 5 days of treatment, regardless of stress history. CAS reduced the initial locomotor response to novelty in both adolescent and adult rats compared to NS controls but had no effect on locomotor sensitization to cocaine in adolescents or adult female rats. These studies expand our understanding of age and adolescent stress on cocaine-induced behavioral plasticity in female Wistar rats.

PPARγ agonism attenuates cocaine cue reactivity

Cocaine use disorder is a chronic relapsing condition characterized by compulsive drug seeking and taking even after prolonged abstinence periods. Subsequent exposure to drug-associated cues can promote intense craving and lead to relapse in abstinent humans and rodent models. The responsiveness to these cocaine-related cues, or 'cue reactivity', can trigger relapse and cocaine-seeking behaviors; cue reactivity is measurable in cocaine-dependent humans as well as rodent models. Cue reactivity is thought to be predictive of cocaine craving and relapse. Here we report that PPARγ agonism during abstinence from cocaine self-administration reduced previously active lever pressing in Sprague Dawley rats during cue-reactivity tests, while administration of the PPARγ antagonist, GW9662, reversed this effect. PPARγ agonism also normalized nuclear ERK activity in the medial prefrontal cortex and hippocampus which was reversed with GW9662. Our results support the utility of PPARγ agonism as a relapse prevention strategy to maintain abstinence in the presence of cocaine-associated cues.

A rapid enhancement of locomotor sensitization to amphetamine by estradiol in female rats

Estradiol moderates the effects of drugs of abuse in both humans and rodents. Estradiol's enhancement of behavioral effects resulting from high (>2.5mg/kg) doses of amphetamine is established in rats; there is less evidence for the role of estradiol in locomotor effects elicited by lower doses, which are less aversive, increase incentive motivation, involve different neural mechanisms than higher doses, and often more readily reveal group differences than do higher doses. Further, the extent to which estradiol is required for the induction versus the expression of sensitization is unknown. To establish a protocol, we replicated the effects of estradiol on locomotor sensitization to amphetamine reported in a previous study that involved a high locomotor-activating dose (1.5mg/kg) of amphetamine, but with a lower dose. Ovariectomized female rats received 5μg of estradiol benzoate (EB) or OIL 30min before each of 5 treatments of 1.0mg/kg amphetamine or saline; all received a 0.5mg/kg challenge dose three days later. Compared with results for OIL, EB enhanced the locomotor-activating effects of repeated 1.0mg/kg amphetamine across treatment days. In contrast, on challenge day, there was no difference between EB-saline and EB-amphetamine to the lower dose (i.e., no sensitization). Experiments 2 and 3 involved a shorter induction (2days) and a lengthier withdrawal (9days) before the challenge test for the expression of sensitization to better differentiate the induction phase from the expression phase. In Expt2, EB-, and not OIL-, treated rats showed sensitization to 0.5mg/kg amphetamine; neither group showed sensitization to 1.5mg/kg amphetamine (ceiling effect?). In Expt3, rats were treated with EB either in both the induction and expression phases, in one of the phases only, or in neither phase. There was an effect of hormone treatment on challenge day and not on induction day; rats given EB on Challenge day showed sensitization to 0.5mg/kg amphetamine; OIL rats did not. The results suggest rapid effects of estradiol on amphetamine sensitization consistent with rapid effects of estradiol reported for other behaviours.

Sex differences in addiction

Women exhibit more rapid escalation from casual drug taking to addiction, exhibit a greater withdrawal response with abstinence, and tend to exhibit greater vulnerability than men in terms of treatment outcome. In rodents, short-term estradiol intake in female rats enhances acquisition and escalation of drug taking, motivation for drugs of abuse, and relapse-like behaviors. There is also a sex difference in the dopamine response in the nucleus accumbens. Ovariectomized female rats exhibit a smaller initial dopamine increase after cocaine treatment than castrated males. Estradiol treatment of ovariectomized female rats enhances stimulated dopamine release in the dorsolateral striatum, but not in the nucleus accumbens, resulting in a sex difference in the balance between these two dopaminergic projections. In the situation where drug-taking behavior becomes habitual, dopamine release has been reported to be enhanced in the dorsolateral striatum and attenuated in the nucleus accumbens. The sex difference in the balance between these neural systems is proposed to underlie sex differences in addiction.

Cocaine induces DNA damage in distinct brain areas of female rats under different hormonal conditions

We evaluated levels of neuronal DNA damage after acute or repeated cocaine treatment in different brain areas of female rats after ovariectomy or sham surgery. Rats in the control and acute groups were given saline i.p., whereas in the repeated group were given 15 mg/kg, i.p., cocaine for 8 days. After a 10 day washout period, the control group was given saline i.p., whereas rats in the acute and repeated groups were given a challenge dose of 15 mg/kg, i.p., cocaine. After behavioural assessment, rats were killed and the cerebellum, hippocampus, hypothalamus, prefrontal cortex and striatum were dissected for the Comet assay. Acute cocaine exposure induced DNA damage in all brain areas. This effect persisted after repeated administration, except in the hypothalamus, where repeated treatment did not cause increased DNA damage. Sexual hormones exhibited a neuroprotective effect, decreasing cocaine-induced DNA damage in cycling rats in all brain areas.

Future pharmacological treatments for substance use disorders

Substance use disorders represent a serious public health and social issue worldwide. Recent advances in our understanding of the neurobiological basis of the addictive processes have led to the development of a growing number of pharmacological agents to treat addictions. Despite this progress, there are no approved pharmacological treatments for cocaine, methamphetamine and cannabis addiction. Moving treatment development to the next stage will require novel ways of approaching substance use disorders. One such novel approach is to target individual vulnerabilities, such as cognitive function, sex differences and psychiatric comorbidities. This review provides a summary of promising pharmacotherapies for alcohol, opiate, stimulant and nicotine addictions. Many medications that target positive and negative reinforcement of drugs, as well as individual vulnerabilities to addiction, are in different phases of development. Clinical trials testing the efficacy of these medications for substance use disorder are warranted.

Behavioral effects of endogenous or exogenous estradiol and progesterone on cocaine sensitization in female rats

Cocaine sensitization is a marker for some facets of addiction, is greater in female rats, and may be influenced by their sex hormones. We compared the modulatory effects of endogenous or exogenous estradiol and progesterone on cocaine-induced behavioral sensitization in 106 female rats. Ovariectomized female rats received progesterone (0.5 mg/mL), estradiol (0.05 mg/mL), progesterone plus estradiol, or the oil vehicle. Sham-operated control females received oil. Control and acute subgroups received injections of saline, while the repeated group received cocaine (15 mg/kg, ip) for 8 days. After 10 days, the acute and repeated groups received a challenge dose of cocaine, after which locomotion and stereotypy were monitored. The estrous cycle phase was evaluated and blood was collected to verify hormone levels. Repeated cocaine treatment induced overall behavioral sensitization in female rats, with increased locomotion and stereotypies. In detailed analysis, ovariectomized rats showed no locomotor sensitization; however, the sensitization of stereotypies was maintained. Only females with endogenous estradiol and progesterone demonstrated increased locomotor activity after cocaine challenge. Estradiol replacement enhanced stereotyped behaviors after repeated cocaine administration. Cocaine sensitization of stereotyped behaviors in female rats was reduced after progesterone replacement, either alone or concomitant with estradiol. The behavioral responses (locomotion and stereotypy) to cocaine were affected differently, depending on whether the female hormones were of an endogenous or exogenous origin. Therefore, hormonal cycling appears to be an important factor in the sensitization of females. Although estradiol increases the risk of cocaine sensitization, progesterone warrants further study as a pharmacological treatment in the prevention of psychostimulant abuse.

Estrogen receptors mediate estradiol's effect on sensitization and CPP to cocaine in female rats: role of contextual cues

Preclinical studies show that estradiol enhances sensitization to cocaine in females by mechanisms not fully understood. These studies consistently show that ovariectomized (OVX) rats exhibit little or no sensitization to cocaine compared to OVX rats administered estradiol. In this study we varied the dose of cocaine (10, 15, and 30mg/kg), the length of cocaine treatment (from 5 to 10days) and the context of cocaine injections to determine if these factors play a role on estradiol's effects on cocaine sensitization. Because OVX rats are hormonally compromised, they are not representative of the natural state of the animal, and thus the physiological context of these studies remains unclear. To address this issue, we blocked ERs in gonadally intact females by icv administration of the antiestrogen ICI-182,780. Varying the dose or length of exposure to cocaine does not alter estradiol's effect on cocaine sensitization. In contrast, a highly context-dependent sensitization protocol results in robust sensitization even in OVX rats. Interestingly, using this protocol, sensitization in OVX rats diminished with time, suggesting that estradiol is necessary for the maintenance of cocaine sensitization. Blocking brain ERs with ICI completely abolishes the development and expression of cocaine sensitization in gonadally intact female rats, even when tested in a highly context-dependent sensitization protocol. Given these findings, we propose that activation of brain ERs is required for the development and maintenance of sensitization and CPP.

Analysis of kalirin-7 knockout mice reveals different effects in female mice

Estradiol treatment of ovariectomized rodents is known to affect the morphology of dendritic spines and produce behavioral and cognitive effects. Kalirin-7 (Kal7), a postsynaptic density (PSD)-localized Rho-guanine nucleotide exchange factor, is important for dendritic spine formation and stability. Male Kal7 knockout [Kal7(KO)] mice exhibit a number of abnormal behavioral and biochemical phenotypes. Given that chronic 17β-estradiol (E2) replacement of ovariectomized rats enhanced Kal7 expression in the hippocampus and primary hippocampal cultures, we assessed the behavioral and biochemical effects of chronic E2 treatment of ovariectomized female wild-type and Kal7(KO) mice. Both intact and ovariectomized Kal7(KO) female mice exhibited decreased anxiety-like behavior compared with the corresponding wild type in the elevated zero maze and were unaffected by E2 treatment. Chronic E2 decreased locomotor activity in the open field and enhanced performance in a passive-avoidance fear conditioning task, which were both unaffected by genotype. Kal7(KO) female mice engaged in significantly more object exploration, both familiar and novel, than did wild-type females. E2 enhanced the acute locomotor response to cocaine, with no significant effect of genotype. Similar to Kal7(KO) males, Kal7(KO) females had decreased levels of N-methyl-d-aspartate receptor 2B in hippocampal PSD fractions with no effect of E2 treatment. The differing behavioral effects of Kal7 ablation in female and male mice may offer insight into the molecular underpinnings of these differences.

Sex differences in behavioral and neural cross-sensitization and escalated cocaine taking as a result of episodic social defeat stress in rats

RATIONALE

Episodic social defeat stress results in cross-sensitization to cocaine, characterized by augmentation of locomotor activity, dopamine (DA) levels in the nucleus accumbens (NAc), and cocaine self-administration during a 24-h "binge" in male rats. However, females are more vulnerable than males at each phase of cocaine addiction, and while these sex differences have been replicated in rats, the role of social stress in females remains largely neglected.

OBJECTIVE

This study examined sex and estrous cycle differences in behavioral and dopaminergic cross-sensitization to cocaine, as well as cocaine taking in an unlimited-access self-administration "binge."

METHODS

Long-Evans rats underwent episodic social defeat and were assessed 10 days later for either (1) behavioral sensitization, as determined by locomotor activity in response to acute cocaine (10 mg/kg, i.p.), (2) neural sensitization, as determined by in vivo microdialysis of DA in the NAc shell in response to acute cocaine, or (3) intravenous self-administration of cocaine (0.3 mg/kg/infusion) in an unlimited-access "binge."

RESULTS

Social defeat stress resulted in behavioral and dopaminergic cross-sensitization in both sexes, but the effect was larger and longer lasting in stressed females. Furthermore, while stress engendered a longer "binge" in both sexes, females had a significantly longer "binge" duration than males.

CONCLUSIONS

These data suggest that socially stressed females exhibit a larger and longer lasting behavioral and neural cross-sensitization, as well as more dysregulated cocaine taking, than males possibly due to different alterations in the dopaminergic response in the NAc. Furthermore, estrogens appear to play a facilitatory role in both behavioral and dopaminergic sensitization.

Locomotor activity changes in female adolescent and adult rats during repeated treatment with a cannabinoid or club drug

Adolescents and young adults of both sexes are the primary consumers of "club" drugs; yet, most of the mechanistic preclinical research in this area has been performed in adult male rodents. The purpose of this study was to evaluate the acute and repeated effects of drugs that are commonly abused by adolescents in female adolescent and adult rats in a rodent model of behavioral sensitization. During two five-day periods separated by a two-day break, rats were injected daily with saline or with one of the following drugs: cocaine (7 or 15 mg/kg), ketamine (3 or 10 mg/kg), 3,4-methylenedioxymethamphetamine (MDMA) (3, 10, or 30 mg/kg), or Δ(9)-tetrahydrocannabinol (THC) (0.03, 0.1, 0.3 or 1 mg/kg) and their locomotor activity was measured. Cocaine increased activity across days in both age groups. Whereas ketamine produced progressive increases in activity with repeated administration in rats of both ages, MDMA increased, and then decreased, activity in the chronic dosing regimen in female adolescents only. Tolerance to the initial stimulatory effects of low doses of THC was observed at both ages. The results with THC are similar to those obtained for male rats tested under identical conditions in a previous study; however, in contrast with the present results in females, male adolescent rats in the previous study failed to develop behavioral sensitization to ketamine. Together, these results suggest that age and sex strongly influence the progressive adaptive changes that occur with repeated administration of some, but not all, of these commonly abused substances.

BOLD signal response to cocaine varies with sexual receptivity in female rats

Blood oxygen level dependent MRI was used to test whether cocaine-stimulated mesolimbic activity varied with sexual receptivity. Rats were randomly screened for lordotic responses and were then imaged for their responses to centrally administered cocaine. We observed that female rats expressing no lordosis showed a greater activation of mesolimbic and nigrostriatal structures than lordotic female rats. Our data suggest that the differential sensitivity to cocaine occurs not only as a result of hormonal changes of the estrous cycle, but also in association with changes in sexual receptivity.

Sex differences and effects of cocaine on excitatory synapses in the nucleus accumbens

Human and animal studies indicate that drugs of abuse affect males and females differently, but the mechanism(s) underlying sex differences are unknown. The nucleus accumbens (NAc) is central in the neural circuitry of addiction and medium spiny neurons (MSNs) in the NAc show drug-induced changes in morphology and physiology including increased dendritic spine density. We previously showed in drug-naïve rats that MSN dendritic spine density is higher in females than males. In this study, we investigated sex differences in the effects of cocaine on locomotor activity as well as MSN dendritic spine density and excitatory synaptic physiology in rats treated for 5 weeks followed by 17-21 days of abstinence. Females showed a greater locomotor response to cocaine and more robust behavioral sensitization than males. Spine density was also higher in females and, particularly in the core of the NAc, the magnitude of the cocaine-induced increase in spine density was greater in females. Interestingly, in cocaine-treated females but not males, cocaine-induced behavioral activation during treatment was correlated with spine density measured after treatment. Miniature EPSC (mEPSC) frequency in core MSNs also was higher in females, and increased with cocaine in both the core and shell of females more than males. We found no differences in mEPSC amplitude or paired-pulse ratio of evoked EPSCs, suggesting that sex differences and cocaine effects on mEPSC frequency reflect differences in excitatory synapse number per neuron rather than presynaptic release probability. These studies are the first to demonstrate structural and electrophysiological differences between males and females that may drive sex differences in addictive behavior.

Estradiol lowers intracranial self-stimulation thresholds and enhances cocaine facilitation of intracranial self-stimulation in rats

Women initiate cocaine use at a younger age and have more complications (e.g., higher rates of major or minor depression) related to cocaine use than men. It has been proposed that estrogens play an important role in these sex differences. The addictive potential of psychoactive drugs can be measured in rats via a rewarding intracranial self-stimulation (ICSS) procedure. The rate-independent method of ICSS allows researchers to assess the "pure" rewarding effect of cocaine without influence of nonspecific motor reactions. The present study aimed to estimate effects of estradiol and a combination of estradiol and cocaine on ICSS in ovariectomized female rats. 17-β-estradiol (5μg/animal/day, 2 days) produced a long-lasting gradual lowering of the thresholds for ICSS. The ability of estradiol to decrease thresholds for ICSS has never been shown previously. Combination of 17-β-estradiol and cocaine (5.0mg/kg, 5 days) produced a greater effect on ICSS thresholds than the effect of either compound alone. No tolerance or sensitization to cocaine developed during the study. Present findings suggest estradiol increases sensitivity of the brain reward system in rats, which may have an important implication in understanding sex differences in cocaine effects.

Estrogens of multiple classes and their role in mental health disease mechanisms

Gender and sex hormones can influence a variety of mental health states, including mood, cognitive development and function, and vulnerability to neurodegenerative diseases and brain damage. Functions of neuronal cells may be altered by estrogens depending upon the availability of different physiological estrogenic ligands; these ligands and their effects vary with life stages, the genetic or postgenetic regulation of receptor levels in specific tissues, or the intercession of competing nonphysiological ligands (either intentional or unintentional, beneficial to health or not). Here we review evidence for how different estrogens (physiological and environmental/dietary), acting via different estrogen receptor subtypes residing in alternative subcellular locations, influence brain functions and behavior. We also discuss the families of receptors and transporters for monoamine neurotransmitters and how they may interact with the estrogenic signaling pathways.

Sex differences and ovarian hormones in animal models of drug dependence

Increasing evidence indicates the presence of sex differences in many aspects of drug abuse. Most studies reveal that females exceed males during the initiation, escalation, extinction, and reinstatement (relapse) of drug-seeking behavior, but males are more sensitive than females to the aversive effects of drugs such as drug withdrawal. Findings from human and animal research indicate that circulating levels of ovarian steroid hormones account for these sex differences. Estrogen (E) facilitates drug-seeking behavior, while progesterone (P) and its metabolite, allopregnanalone (ALLO), counteract the effects of E and reduce drug seeking. Estrogen and P influence other behaviors that are affiliated with drug abuse such as drug-induced locomotor sensitization and conditioned place preference. The enhanced vulnerability to drug seeking in females vs. males is also additive with the other risk factors for drug abuse (e.g., adolescence, sweet preference, novelty reactivity, and impulsivity). Finally, treatment studies using behavioral or pharmacological interventions, including P and ALLO, also indicate that females show greater treatment effectiveness during several phases of the addiction process. The neurobiological basis of sex differences in drug abuse appears to be genetic and involves the influence of ovarian hormones and their metabolites, the hypothalamic pituitary adrenal (HPA) axis, dopamine (DA), and gamma-hydroxy-butyric acid (GABA). Overall, sex and hormonal status along with other biological risk factors account for a continuum of addiction-prone and -resistant animal models that are valuable for studying drug abuse prevention and treatment strategies.

Estradiol: a key biological substrate mediating the response to cocaine in female rats

A consistent finding in drug abuse research is that males and females show differences in their response to drugs of abuse. In women, increased plasma estradiol is associated with increased vulnerability to the psychostimulant and reinforcing effects of drugs of abuse. Our laboratory has focused on the role of estradiol in modulating the response to cocaine. We have seen that ovariectomy increases the locomotor response to a single cocaine injection, whereas estradiol exacerbates the locomotor response to repeated cocaine administration. Cocaine-induced sensitization of brain activity, as measured by fMRI, is also dependent on plasma estradiol. Moreover, we observed that although all ovariectomized rats show conditioned place preference to cocaine, it is more robust in ovariectomized rats with estradiol. Opioid receptors are enriched in brain regions associated with pleasure and reward. We find that in females, the effectiveness of kappa opioid agonists in decreasing the locomotor response to repeated cocaine varies with plasma estradiol. We also find that estradiol regulates the density of mu opioid receptors in brains areas associated with reward. These data hint that in females, estradiol modulates the behavioral effects of cocaine by regulating mu and kappa opioid signaling in mesocorticolimbic brain structures. Identifying the mechanisms that mediate differences in vulnerability to drugs of abuse may lead to effective therapeutic strategies for the treatment and prevention of addiction and relapse. We encourage health practitioners treating persons addicted to drugs to consider gender differences in response to particular pharmacotherapies, as well the sex steroid milieu of the patient.

Progesterone attenuates cocaine-induced responses

In this review, we summarize literature focused on how progesterone alters cocaine-induced psychomotor, reinforcement, and physiological responses. Clinical studies suggest that progesterone attenuates the subjective effects of cocaine. Similarly, preclinical studies have demonstrated that cocaine-induced reward and psychomotor responses are attenuated after progesterone administration. In rats progesterone also reduces the reinforcement effects of cocaine attenuates acquisition, escalation, reinstatement of cocaine self-administration, and cocaine-seeking behaviors. Progesterone also counteracts the facilitatory effects of estrogen on cocaine self-administration and psychomotor activation. These findings suggest that progesterone has a potential in clinical applications as a treatment for cocaine addiction. Constantly changing progesterone serum levels in female humans and rats affect the female's reinforcement responses to cocaine and may in part contribute to the known sex differences in cocaine responses.

Sexual differentiation of motivation: a novel mechanism?

Sex differences in motivation are apparent for the motivation to engage in sexual behavior, the motivation to take drugs of abuse, and the motivation to engage in parental behavior. In both males and females there is an increase in NAcc DA associated with motivated behaviors. Here it proposed that sex differences in the regulation of DA activity in the ascending mesolimbic projections may underlie sex differences in motivation. In particular, sex differences in the neuroendocrine regulation of this brain system play a role in the expression of sex differences in motivated behaviors. Here it is proposed that sexual differentiation of motivation is mediated, at least in part, by a novel mechanism in which ovarian hormones secreted at puberty in the female actively feminize the DA system.

Behavioral and neurobiological characteristics influencing social hierarchy formation in female cynomolgus monkeys

Socially housed monkeys have been used as a model to study human diseases. The present study examined behavioral, physiological and neurochemical measures as predictors of social rank in 16 experimentally naïve, individually housed female cynomolgus monkeys (Macaca fascicularis). The two behavioral measures examined were novel object reactivity (NOR), as determined by latency to touch an opaque acrylic box placed in the home cage, and locomotor activity assessed in a novel open-field apparatus. Serum cortisol concentrations were evaluated three times per week for four consecutive weeks, and stress reactivity was assessed on one occasion by evaluating the cortisol response to adrenocorticotropic hormone (ACTH) following dexamethasone suppression. Measures of serotonin (5-HT) function included whole blood 5-HT (WBS) concentrations, cerebrospinal fluid (CSF) concentrations of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) and brain 5-HT transporter (SERT) availability obtained using positron emission tomography (PET). After baseline measures were obtained, monkeys were assigned to four social groups of four monkeys per group. The two measures that correlated with eventual social rank were CSF 5-HIAA concentrations, which were significantly higher in the animals who eventually became subordinate, and latency to touch the novel object, which was significantly lower in eventual subordinate monkeys. Measures of 5-HT function did not change as a consequence of social rank. These data suggest that levels of central 5-HIAA and measures of novel object reactivity may be trait markers that influence eventual social rank in female macaques.

Estrogen regulates responses of dopamine neurons in the ventral tegmental area to cocaine

RATIONALE

Sex differences in cocaine abuse have been well documented. However, the underlying mechanism remains unclear.

OBJECTIVES

To explore the potential role of ovarian hormones in the regulation of dopamine (DA) neuron firing activity in ventral tegmental area (VTA) induced by acute cocaine in intact female or ovariectomized (OVX) rats.

RESULTS

The basal firing activity of VTA DA neurons was changed in a manner phase-locked to the estrous cycle: being highest in estrus and lowest in proestrus. Acute cocaine produced greater inhibition (P < 0.05) on the firing of VTA DA neurons during proestrus than during estrus. The inhibitory effect was completely blocked by OVX and restored by replacement of 17-beta-estradiol or, to a less extent, by replacement of progesterone. In addition, we also detected female hormone-associated changes in slow oscillation in VTA DA neurons. The results indicate that ovarian hormones, particularly estrogen, not only synergize with the inhibitory effect of cocaine on VTA DA neuron activity but also play an essential role in maintaining the sensitivity of DA neurons to cocaine-mediated inhibition on firing. Moreover, pretreatment of estrogen receptor (ER) antagonist raloxifene or a selective ERalpha antagonist Y134 largely attenuated the cocaine-inhibited DA neuron firing. We also found that cocaine-induced locomotor activity was estrous cycle dependent; 17-beta-estradiol but not progesterone replacement restored the cocaine-induced locomotor activity in OVX rats.

CONCLUSION

The present results demonstrated that ovarian hormones, particularly estrogen, produce profound effect on VTA DA neuron activity, which, in turn, may contribute to the sex differences in response to psychostimulants.

Estradiol-sertraline synergy in ovariectomized rats

This study investigated estradiol (E(2)) modulation of the antidepressant effects of a selective serotonin (5-HT) reuptake inhibitor (SSRI; sertraline) and a tricyclic antidepressant (imipramine) as measured by the forced swim test (FST) followed by assessment of gene and protein expression for the 5-HT transporter (SERT) and multiple 5-HT receptors. Female Sprague-Dawley rats were ovariectomized (OVX) and two-thirds of the rats received E(2) implants (OVE). 4 weeks later, implants were withdrawn in half of the OVE rats (OVW) to capture a time point when E(2) levels were rapidly declining. Rats in each hormone group were treated with vehicle, sertraline (10 mg/kg) or imipramine (10 mg/kg), 24, 5 and 1h before the FST. Immediately after the FST, midbrain, hippocampus and prefrontal cortex tissue was removed and frozen for analysis of gene expression via quantitative real-time PCR (midbrain tissue) and protein expression via Western blot (prefrontal cortex and hippocampal tissue). In the FST, sertraline decreased immobility and increased swimming in OVE rats, as well as increased swimming in OVW rats. In contrast, no sertraline effect was observed in OVX rats. Rats treated with imipramine showed increased climbing but no changes in immobility or swimming. No changes in protein expression were detected in any treatment group. However, in vehicle-treated rats, E(2) increased midbrain SERT mRNA expression, with no effect on midbrain mRNA for the 5-HT receptors. In sertraline-treated rats, E(2) decreased 5-HT(2A) receptor mRNA, and E(2)-withdrawal increased 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor mRNA. In imipramine-treated rats, E(2) (and E(2)-withdrawal) did not affect mRNA expression for any of the target genes. Thus, E(2) synergized behaviorally and neurochemically with an SSRI but not a tricyclic antidepressant.

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.

Sex differences in drug abuse

Sex differences are present for all of the phases of drug abuse (initiation, escalation of use, addiction, and relapse following abstinence). While there are some differences among specific classes of abused drugs, the general pattern of sex differences is the same for all drugs of abuse. Females begin regularly self-administering licit and illicit drugs of abuse at lower doses than do males, use escalates more rapidly to addiction, and females are at greater risk for relapse following abstinence. In this review, sex differences in drug abuse are discussed for humans and in animal models. The possible neuroendocrine mechanisms mediating these sex differences are discussed.

Altered levels of sex and stress steroid hormones assessed daily over a 28-day cycle in early abstinent cocaine-dependent females

RATIONALE

There is growing evidence of alterations in brain stress and reward circuits associated with cocaine dependence. Sex differences are also documented and sex steroid hormones have been linked to cocaine reinforcement.

OBJECTIVES

The current study therefore assessed daily fluctuations in stress and sex hormones in cocaine-dependent females compared with healthy females.

METHOD

Daily salivary samples of cortisol, progesterone, and estradiol were collected at waking across 28 days from 12 cocaine-dependent females receiving inpatient treatment and 10 healthy females. Participants also completed mood-rating scales each week corresponding to four phases of the menstrual cycle and cocaine craving was monitored in cocaine patients at each phase.

RESULTS

Cocaine-dependent females in their first month of abstinence demonstrated significantly higher levels of both cortisol and progesterone across the menstrual cycle and significantly lower estradiol/progesterone (E2/P) ratios compared to healthy controls. They also showed significantly increased negative mood compared with controls, but no variation in cocaine craving across the menstrual cycle.

CONCLUSIONS

Findings indicate altered stress and sex hormones suggestive of an overactive stress system during the first month of cocaine abstinence after chronic cocaine abuse. These increased levels of cortisol and progesterone could impact both abstinence-related symptoms such as negative mood and susceptibility to drug-seeking behavior in cocaine-dependent females.

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.

Plasma progesterone levels and cocaine-seeking in freely cycling female rats across the estrous cycle

Previous studies have reported sex and estrous cycle-dependent differences in the reinstatement of cocaine-seeking triggered by cocaine injections or drug-paired cues. However, the relationship between estradiol or progesterone levels and cocaine-seeking in a reinstatement model of relapse has not been explored. Thus, we examined changes in plasma hormone levels during cocaine-taking and -seeking behaviors in gonadally intact female rats. Rats self-administered cocaine (0.5mg/kg infusion) during daily 2-h sessions, followed by extinction. For reinstatement, cocaine (0, 5, or 10mg/kg, i.p.) was administered 30 min prior to testing. Vaginal smears and blood samples were collected prior to and during chronic cocaine self-administration, extinction, and reinstatement testing. Relative to non-estrous females, females in estrus showed greater responding during self-administration, extinction, and during cocaine-primed reinstatement. The highest progesterone levels were noted at the time of lowest cocaine-seeking (proestrus) and the lowest levels of progesterone occurred at the time of highest cocaine-seeking (estrus). In contrast, plasma estradiol levels did not show any clear pattern with cocaine-seeking. These data from an animal model of relapse supports recent clinical evidence that progesterone reduces subjective craving in cocaine-dependent women. Overall, these results suggest that progesterone administration may be a useful intervention for reducing the incidence of relapse.

Estrogen receptor beta, but not alpha, mediates estrogen's effect on cocaine-induced reinstatement of extinguished cocaine-seeking behavior in ovariectomized female rats

Preclinical and clinical studies indicate that females are more vulnerable to relapse than males, and the neurobiological effects of estrogen are thought to mediate, in part, the sex differences in cocaine-taking behavior. The goal of the present study was to investigate the involvement of estrogen receptor alpha (ERalpha) and beta (ERbeta) in estrogen-mediated increases in cocaine-induced reinstatement of extinguished cocaine-seeking behavior in ovariectomized (OVX) female rats. Rats were initially trained to self-administer cocaine (0.4 mg/kg/inf, i.v.) under a fixed-ratio 1 (FR 1) schedule of reinforcement during daily 2-h sessions. After a 10-day maintenance period, cocaine solutions were replaced with saline, and self-administration was extinguished over a 14-day period. OVX rats were then treated with either the mixed ERalpha/beta agonist estradiol benzoate (EB), the ERalpha-selective agonist, propyl-pyrazole-triol (PPT), the ERbeta-selective agonist, diarylpropionitrile (DPN), or a vehicle control (dimethyl sulfoxide, DMSO). Treatment lasted a total of 9 days, and during this time, rats were assessed for nonreinforced reinstatement of extinguished cocaine-seeking behavior after priming injections of saline or cocaine (5, 10, or 15 mg/kg, i.p.). OVX rats showed no differences in self-administration during maintenance or extinction. OVX rats treated with EB exhibited greater responding for cocaine during reinstatement compared to OVX+DMSO controls. Selective activation of ERbeta with DPN also increased cocaine-induced reinstatement responding, whereas selective activation of ERalpha with PPT did not affect cocaine-seeking behavior. These results indicate that estrogen influences the propensity for reinstatement of extinguished cocaine-seeking behavior, and that estrogen-mediated enhancement of cocaine-induced reinstatement responding involves the activation of ERbeta.

Estrogen-modulated frontal cortical CaMKII activity and behavioral supersensitization induced by prolonged cocaine treatment in female rats

RATIONALE

Females have been demonstrated repeatedly to be more sensitive to cocaine. The role of the frontal cortex (FCX) in mediating behavioral sensitization and the underlying signaling pathways are unclear.

OBJECTIVE

The study was designed to characterize the role of FCX calcium/calmodulin-dependent protein kinase II (CaMKII) activity in the behavioral supersensitization observed in female rats after prolonged cocaine exposure.

MATERIALS AND METHODS

Intact female rats that received cocaine for 9 days followed by 7 days of drug withdrawal constituted the model used for studying the mechanism of supersensitization.

RESULTS

This cocaine withdrawal treatment resulted in behavioral supersensitization in intact female rats as indicated by an enhanced behavioral response to cocaine challenge assessed on day 16 (7-day withdrawal) and compared to the response on day 9 of cocaine treatment. This treatment regimen did not lead to supersensitization in male or in ovariectomized (OVX) rats. Administration of estrogen to OVX rats restored behavioral supersensitivity to repeated cocaine. FCX CaMKII activity was significantly altered by cocaine in females, and this effect was related to estrogen's presence; cocaine-induced changes in striatal CaMKII activity were, however, less estrogen-sensitive. Furthermore, estrogen-modulated FCX CaMKII activity in cocaine-supersensitized rats was dependent on D(1) dopamine receptor activation.

CONCLUSION

Estrogen-modulated D(1) dopamine receptor activity mediates the effects of prolonged cocaine exposure on FCX CaMKII, and this, in turn, may contribute to the development of behavioral supersensitivity to repeated cocaine treatment in intact female rats.

Estradiol-induced conditioned place preference may require actions at estrogen receptors in the nucleus accumbens

Intrinsic rewarding effects of estradiol (E(2)) may underlie some of the sex differences that emerge postpuberty for the prevalence of drug use and behavioral responses to drugs, but the effects and mechanisms of E(2) for reward have not been well characterized. Conditioned place preference (CPP), as measured by the time spent on the nonpreferred/drug-associated side of the chamber, was utilized as a functional assay to investigate the effects and mechanisms of E(2) in the nucleus accumbens for reward. To determine whether intracellular estrogen receptors (ERs) are important for E(2)-induced CPP, rats were administered E(2) (10 microg; subcutaneously (s.c.)), which produced CPP in each experiment, and/or ER blockers, such as tamoxifen (Experiment 1), ICI 182,780 (Experiment 2), or antisense oligonucleotides targeted to ERs (Experiment 3). Experiment 1: E(2) significantly increased the time spent on the originally nonpreferred side of the chamber. Coadministration of tamoxifen (10 mg/kg; s.c.) attenuated effects of E(2) to produce a CPP, but tamoxifen alone, increased time spent on the nonpreferred side. Experiment 2: coadministration of ICI 182,780 (10 microg/microl) to the nucleus accumbens attenuated effects of E(2) to enhance CPP and did not produce a CPP when administered alone. Experiment 3: coadministration of s.c. E(2) with ER antisense oligonucleotides to the nucleus accumbens significantly decreased time spent on the nonpreferred side and expression of ERs in the nucleus accumbens compared to scrambled antisense oligonucleotides or saline vehicle administration. Thus, E(2)'s rewarding effects may involve actions at ERs in the nucleus accumbens.

Why are women from Venus and men from Mars when they abuse cocaine?

Both preclinical and clinical studies have shown sexually dimorphic patterns in behavioral responses to cocaine in all phases of the cocaine addiction process (induction, maintenance, and relapse). Thus, a clear picture is emerging which suggests that the biological basis of sex-specific differences in cocaine addiction resides in the disparate regulation of the CNS by male and female gonadal hormones. This review discusses the role that gonadal hormones play in these sexually dimorphic patterns of behavioral responses to cocaine.

Estrogen and progesterone affect cocaine pharmacokinetics in female rats

Several studies have reported sex differences in behavioral responses to cocaine whereby females display a greater degree of locomotor activity. Fluctuations in estrogen and progesterone during the estrous cycle have been postulated to underlie these behavioral differences. In this study, we tested the hypothesis that hormonal replacement (estrogen or progesterone) in ovariectomized rats affects cocaine pharmacokinetics. We found that estrogen replacement did not affect cocaine-induced locomotor activity, but progesterone attenuated locomotor counts in comparison with control groups receiving only sesame oil. Estrogen, however, decreased brain levels of cocaine and norcocaine 30 min after cocaine administration in comparison to the group-receiving vehicle at that time point. In addition, in progesterone-treated rats, levels of benzoylecgonine and ecgonine methylester were higher at 30 min post-administration than at 15 min. No changes were found in blood levels of the metabolites. These findings suggest that while progesterone has an impact on locomotor behavior, pharmacokinetic effects may have a limited role in mediating behavioral responses to cocaine.

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.

Effect of short- vs. long-term estrogen on reinstatement of cocaine-seeking behavior in female rats

Estrogen effects on cocaine-induced reinstatement of lever responding were examined in sham-operated, vehicle-treated (SH+VEH), ovariectomized (OVX+VEH), and OVX female Wistar rats with estrogen replacement (OVX+EB). The effect of long- (64+/-1.56 days) and short-term (9 days) EB treatment on reinstatement of cocaine-seeking behavior was compared in Experiment 1 and 2, respectively, in order to compare the effect of EB when it was present during the development vs. expression of reinstatement of cocaine-seeking behavior. Rats were trained to self-administer 0.4 mg/kg/inf cocaine. After the acquisition criteria were met, rats continued to respond for cocaine for 2 h/day for a 14-day maintenance period. Cocaine was then replaced with saline and the 21-day extinction period commenced. Subsequently, rats were tested for reinstatement of lever responding on the previously drug-paired lever after alternating daily injections of saline or cocaine. In both experiments, there were no differences between groups in self-administration behavior during training, maintenance, or extinction. In Experiment 1, SH+VEH and chronically treated OVX+EB rats had greater cocaine-induced reinstatement than OVX+VEH rats. In Experiment 2, short-term treated OVX+EB rats also showed enhanced cocaine-induced reinstatement compared to OVX+VEH rats. The results indicate that EB-mediated enhancement of cocaine-induced reinstatement is dependent on EB presence during the expression of reinstatement but not during the formation of stimulus-reward associations during the development of cocaine-reinforced behavior.

Gonadal hormones provide the biological basis for sex differences in behavioral responses to cocaine

Both clinical and rodent studies show sexually dimorphic patterns in the behavioral response to cocaine in all phases of the addiction process (induction, maintenance, and relapse). Clinical and rodent studies also indicate that hormonal fluctuations during the menstrual/estrous cycle modulate cocaine-induced subjective effects in women and locomotor activity in female rats. Evidence suggests that gonadal hormones underlie these observed differences and could be the biological basis of sex-specific differences in cocaine addiction. To study the effects of gonadal hormones on cocaine-induced activity, two approaches have been used. First, studies have examined the role of endogenous hormones through gonadectomy (GDX) and side-by-side comparisons with intact rats. Second, the individual contributions of testosterone, progesterone, and estrogen have been determined by hormone replacement in GDX rats. In this review, we discuss gonadal hormones as the biological basis for the behavioral responses to cocaine, and the clinical implications of these findings.

Effect of gonadectomy and gonadal hormone replacement on cocaine self-administration in female and male rats

Both sex and gonadal steroid hormones may influence the abuse-related behavioral effects of cocaine under some conditions, but there is considerable inconsistency in the literature. In the present study, rats were trained under a fixed ratio (FR) 5 schedule of food presentation and were then allowed to self-administer cocaine (1.0 mg/kg/injection) until behavior stabilized. Subsequently, complete dose-effect functions for cocaine self-administration (0.032-3.2 mg/kg/injection) were determined in female and male rats before and after gonadectomy, and in gonadectomized female and male rats before and during chronic treatment with estradiol or testosterone, respectively. Sex, gonadectomy, and gonadal hormones did not alter the shape or position of dose-effect functions for cocaine self-administration. These results suggest that sex, estrogen, and testosterone levels are not critical determinants of cocaine's reinforcing effects in rats under these conditions. This study differed from earlier studies in that complete dose-effect functions for cocaine were determined. These findings suggest that the behavioral training history, the unit dose of cocaine, and the schedule of reinforcement are important variables in studies of sex and gonadal hormone effects on cocaine self-administration.

Estrogen-dependent alterations in D2/D3-induced G protein activation in cocaine-sensitized female rats

Estrogen potentiates behavioral sensitization to cocaine in the female rat by mechanisms that remain undetermined. In this study, functional receptor autoradiography was used to investigate estrogen modulation of D2/D3 receptor-induced G protein activation in components of the reward pathway of female rats treated acutely and repeatedly with cocaine. Rats were ovariectomized and given an empty (OVX group) or estradiol benzoate-filled (OVX-EB group) implant. After a week, animals received a daily saline or cocaine injection (15 mg/kg, i.p.) for 5 days, and again on day 13. Animals were killed, and brains were removed and cryosectioned. D2/D3-stimulated [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding was assessed in the cingulate cortex area 2 (Cg2), striatum (STR), nucleus accumbens (NAc) and ventral tegmental area (VTA). OVX-EB rats showed more [35S]GTPgammaS binding in the Cg2 and lower binding in the VTA than OVX rats; in the VTA this effect was reversed by a single cocaine injection. Repeated cocaine administration had opposite effects in OVX and OVX-EB rats. [35S]GTPgammaS binding was decreased in the Cg2, NAc and STR of OVX-EB rats, and increased in OVX rats. The present results support the hypothesis that cocaine-induced changes in D2/D3 receptor activation are regulated by estrogen. These data suggest that changes in D2/D3 receptor function represent one mechanism by which estrogen regulates behavioral sensitization to cocaine.