Gender differences in kappa-opioid modulation of cocaine-induced behavior and NMDA-evoked dopamine release

Kappa Opioid Receptors Negatively Regulate Real Time Spontaneous Dopamine Signals by Reducing Release and Increasing Uptake

The role of the dynorphin/kappa opioid receptor (KOR) system in dopamine (DA) regulation has been extensively investigated. KOR activation reduces extracellular DA concentrations and increases DA transporter (DAT) activity and trafficking to the membrane. To explore KOR influences on real-time DA fluctuations, we used the photosensor dLight1.2 with fiber photometry in the nucleus accumbens (NAc) core of freely moving male and female C57BL/6 mice. First, we established that the rise and fall of spontaneous DA signals were due to DA release and reuptake, respectively. Then mice were systemically administered the KOR agonist U50,488H (U50), with or without pretreatment with the KOR antagonist aticaprant (ATIC). U50 reduced both the amplitude and width of spontaneous signals in males, but only reduced width in females. Further, the slope of the correlation between amplitude and width was increased in both sexes, suggesting that DA uptake rates were increased. U50 also reduced the frequency of signals in both males and females. All effects of KOR activation were stronger in males. Overall, KORs exerted significant inhibitory control over spontaneous DA signaling, acting through at least three mechanisms - inhibiting DA release, promoting DAT-mediated uptake, and reducing the frequency of signals.

The Bright Side of Psychedelics: Latest Advances and Challenges in Neuropharmacology

The need to identify effective therapies for the treatment of psychiatric disorders is a particularly important issue in modern societies. In addition, difficulties in finding new drugs have led pharmacologists to review and re-evaluate some past molecules, including psychedelics. For several years there has been growing interest among psychotherapists in psilocybin or lysergic acid diethylamide for the treatment of obsessive-compulsive disorder, of depression, or of post-traumatic stress disorder, although results are not always clear and definitive. In fact, the mechanisms of action of psychedelics are not yet fully understood and some molecular aspects have yet to be well defined. Thus, this review aims to summarize the ethnobotanical uses of the best-known psychedelic plants and the pharmacological mechanisms of the main active ingredients they contain. Furthermore, an up-to-date overview of structural and computational studies performed to evaluate the affinity and binding modes to biologically relevant receptors of ibogaine, mescaline, N,N-dimethyltryptamine, psilocin, and lysergic acid diethylamide is presented. Finally, the most recent clinical studies evaluating the efficacy of psychedelic molecules in some psychiatric disorders are discussed and compared with drugs already used in therapy.

Cocaine Modulates the Neuronal Endosomal System and Extracellular Vesicles in a Sex-Dependent Manner

In multiple neurodevelopmental and neurodegenerative disorders, endosomal changes correlate with changes in exosomes. We examined this linkage in the brain of mice that received cocaine injections for two weeks starting at 2.5 months of age. Cocaine caused a decrease in the number of both neuronal early and late endosomes and exosomes in the brains of male but not female mice. The response to cocaine in ovariectomized females mirrored male, demonstrating that these sex-differences in response to cocaine are driven by hormonal differences. Moreover, cocaine increased the amount of α-synuclein per exosome in the brain of females but did not affect exosomal α-synuclein content in the brain of males, a sex-difference eliminated by ovariectomy. Enhanced packaging of α-synuclein into female brain exosomes with the potential for propagation of pathology throughout the brain suggests a mechanism for the different response of females to chronic cocaine exposure as compared to males.

Cocaine Induces Sex-Associated Changes in Lipid Profiles of Brain Extracellular Vesicles

Cocaine is a highly addictive stimulant with diverse effects on physiology. Recent studies indicate the involvement of extracellular vesicles (EVs) secreted by neural cells in the cocaine addiction process. It is hypothesized that cocaine affects secretion levels of EVs and their cargos, resulting in modulation of synaptic transmission and plasticity related to addiction physiology and pathology. Lipids present in EVs are important for EV formation and for intercellular lipid exchange that may trigger physiological and pathological responses, including neuroplasticity, neurotoxicity, and neuroinflammation. Specific lipids are highly enriched in EVs compared to parent cells, and recent studies suggest the involvement of various lipids in drug-induced synaptic plasticity during the development and maintenance of addiction processes. Therefore, we examined interstitial small EVs isolated from the brain of mice treated with either saline or cocaine, focusing on the effects of cocaine on the lipid composition of EVs. We demonstrate that 12 days of noncontingent repeated cocaine (10 mg/kg) injections to mice, which induce locomotor sensitization, cause lipid composition changes in brain EVs of male mice as compared with saline-injected controls. The most prominent change is the elevation of GD1a ganglioside in brain EVs of males. However, cocaine does not affect the EV lipid profiles of the brain in female mice. Understanding the relationship between lipid composition in EVs and vulnerability to cocaine addiction may provide insight into novel targets for therapies for addiction.

Neurobiological mechanisms of early life adversity, blunted stress reactivity and risk for addiction

Blunted stress reactivity resulting from early exposure to stress during childhood and adolescence may increase vulnerability to addiction. Early life adversity (ELA) affects brain structure and function and results in blunted stress axis reactivity. In this review, we focus on the underlying neurobiological mechanisms associated with a blunted response to stress, ELA, and risk for addictive disorders. ELA and blunted reactivity are accompanied by unstable mood regulation, impulsive behaviors, and reduced cognitive function. Neuroimaging studies reveal cortical and subcortical changes in persons exposed to ELA and those who have a genetic disposition for addiction. We propose a model in which blunted stress reactivity may be a marker of risk for addiction through an altered motivational and behavioral reactivity to stress that contribute to disinhibited behavioral reactivity and impulsivity leading in turn to increased vulnerability for substance use. Evidence supporting this hypothesis in the context of substance use initiation, maintenance, and risk for relapse is presented. The effects of ELA on persons at risk for addiction may lead to early experimentation with drugs of abuse. Early adoption of drug intake may alter neuroregulation in such vulnerable persons leading to a permanent dysregulation of motivational responses consistent with dependence. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

The Impact of Gender on Interferon-Associated Depression and Anxiety

Population studies indicate women have higher prevalences of depression and anxiety than men. Interferon (IFN) is a biologic agent that can induce or exacerbate depression and/or anxiety. Whether women are more likely to experience these side effects of IFN during treatment remains to be determined. The aim of this study was to document levels of depression and anxiety in female and male patients before and during IFN-based treatment. This was a prospective open-label study in which depression was measured by Beck Depression Inventory (BDI) and anxiety by Hospital Anxiety and Depression Scale (HADS). Before treatment, the prevalence of depression was higher in females (3/13 [23%]) than males (1/25 [4%]), but the difference did not reach statistical significance (P = 0.12). Initial BDI scores were also higher in females but not significantly (P = 0.07). During treatment, BDI scores increased to a similar extent in both genders. A similar percentage of nondepressed patients at baseline developed depression (females: 50% versus males: 35%, P = 0.45). Before treatment, anxiety was significantly more common in females (7/13 [54%]) than males (3/25 [12%]) (P = 0.016) and median HADS scores were higher in females (P = 0.03). During treatment, increases in HADS scores were similar in the 2 genders. A similar percentage of patients without anxiety at baseline developed anxiety on treatment (females: 50% versus males: 23%, P = 0.31). The frequency and extent of IFN-induced/exacerbated depression and anxiety are not gender dependent.

A ketogenic diet diminishes behavioral responses to cocaine in young adult male and female rats

Ketogenic diets (KDs) are high fat, low carbohydrate formulations traditionally used to treat epilepsy; more recently, KDs have shown promise for a wide range of other neurological disorders. Drug addiction studies suggest that repeated exposure to drugs of abuse, including cocaine, results in a suite of neurobiological changes that includes neuroinflammation, decreased glucose metabolism, and disordered neurotransmission. Given that KDs positively regulate these factors, we addressed whether administration of a KD has potential as a novel therapy for drug addiction. In this study, male and female Sprague-Dawley rats were placed on a KD or a control diet (CD), beginning at five weeks of age and continuing through the end of behavioral testing. Three weeks after initiation of dietary treatments, rats received daily i.p. injections of cocaine (15 mg/kg) or saline vehicle for one week, were drug free for a subsequent week, and then all animals received a final challenge injection of 15 mg/kg cocaine. In the absence of cocaine injections, stereotyped locomotor responses were minimal and were unaffected by dietary treatment. In contrast, both males and females fed a KD exhibited decreased cocaine-induced stereotyped responses as compared to CD-fed rats. The sensitization of ambulatory responses was also disrupted in KD-fed rats. These results suggest that KDs directly impact dopamine-mediated behaviors, and hence may hold potential as a therapy for drug addiction.

Females are less sensitive than males to the motivational- and dopamine-suppressing effects of kappa opioid receptor activation

The neuropeptide dynorphin (DYN) activates kappa opioid receptors (KORs) in the brain to produce depressive-like states and decrease motivation. KOR-mediated suppression of dopamine release in the nucleus accumbens (NAc) is considered one underlying mechanism. We previously showed that, regardless of estrous cycle stage, female rats are less sensitive than males to KOR agonist-mediated decreases in motivation to respond for brain stimulation reward, measured with intracranial self-stimulation (ICSS). However, the explicit roles of KORs, circulating gonadal hormones, and their interaction with dopamine signaling in motivated behavior are not known. As such, we measured the effects of the KOR agonist U50,488 on ICSS stimulation thresholds before and after gonadectomy (or sham surgery). We found that ovariectomized females remained less sensitive than sham or castrated males to KOR-mediated decreases in brain stimulation reward, indicating that circulating gonadal hormones do not play a role. We used qRT-PCR to examine whether sex differences in gene expression in limbic brain regions are associated with behavioral sex differences. We found no sex differences in Pdyn or Oprk1 mRNA in the NAc and ventral tegmental area (VTA), but tyrosine hydroxylase (Th) mRNA was significantly higher in female compared to male VTA. To further explore sex-differences in KOR-mediated suppression of dopamine, we used fast scan cyclic voltammetry (FSCV) and demonstrated that U50,488 was less effective in suppressing evoked NAc dopamine release in females compared to males. These data raise the possibility that females are protected from KOR-mediated decreases in motivation by an increased capacity to produce and release dopamine.

Stress and Addiction: When a Robust Stress Response Indicates Resiliency

OBJECTIVE

Stress reactivity research has traditionally focused on the idea that exaggerated responses to stress may have adverse effects on health. Accumulating evidence suggests that attenuated responses to stress and delayed recovery may also be problematic.

METHODS

This review focuses on the role of the stress response of the hypothalamic-pituitary-adrenocortical axis, the endogenous opioid system, and the cardiovascular system in hypertension, pain perception, and addictive behaviors. Results from multiple methods of assessment and stress paradigms conducted in our laboratory over the past two decades are integrated with research from other investigators and with existing theories.

RESULTS

Research indicates that exaggerated biological and physiological responses to stress and attenuated pain perception are associated with hypertension and risk for cardiovascular diseases. This research complements work linking reduced stress responses with enhanced pain sensitivity and discomfort. Multiple studies have also demonstrated that an attenuated stress response is linked to exacerbation of withdrawal symptoms and relapse in nicotine addiction. Evidence indicates important moderators (i.e., sex, personality traits, and early life adversity) and hypothalamic-pituitary-adrenocortical- and endogenous opioid system-related mechanisms in the altered response to stress. I integrate these findings in a conceptual model emphasizing that robust stress responses in the context of addiction and relapse should be considered as a marker of resiliency.

CONCLUSIONS

A blunted stress response may indicate long-term physiological dysregulation that could usher harmful consequences for cardiovascular disease, pain perception, and addictive disorders. The impact of dysregulation is influenced by multiple individual and situational factors that should be considered in evaluating the clinical significance of stress response dysregulation.

Dynorphin/Kappa Opioid Receptor Signaling in Preclinical Models of Alcohol, Drug, and Food Addiction

The dynorphin/kappa opioid receptor (KOR) system is implicated in the "dark side" of addiction, in which stress exacerbates maladaptive responses to drug and alcohol exposure. For example, acute stress and acute ethanol exposure result in an elevation in dynorphin, the KOR endogenous ligand. Activation of KORs results in modulation of several neurotransmitters; however, this chapter will focus on its regulatory effects on dopamine in mesolimbic areas. Specifically, KOR activation has an inhibitory effect on dopamine release, thereby influencing reward processing. Repeated stimulation of KORs, for example, via chronic drug and/or stress exposure, results in increased function of the dynorphin/KOR system. This augmentation in KOR function shifts the homeostatic balance in favor of an overall reduction in dopamine signaling via either by reducing dopamine release or by increasing dopamine transporter function. This chapter examines the effects of chronic ethanol exposure on KOR function and the downstream effects on dopamine transmission. Additionally, the impact of chronic cocaine exposure and its effects on KOR function will be explored. Further, KORs may also be involved in driving excessive consumption of food, contributing to the risk of developing obesity. While some studies have shown that KOR agonists reduce drug intake, other studies have shown that antagonists reduce addiction-like behaviors, demonstrating therapeutic potential. For example, KOR inhibition reduces ethanol intake in dependent animals, motivation to self-administer cocaine in chronic stress-exposed animals, and food consumption in obese animals. This chapter will delve into the mechanisms by which modulation of the dynorphin/KOR system may be therapeutic.

The long-term effects of stress and kappa opioid receptor activation on conditioned place aversion in male and female California mice

Psychosocial stress leads to the activation of kappa opioid receptors (KORs), which induce dysphoria and facilitate depression-like behaviors. However, less is known about the long-term effects of stress and KORs in females. We examined the long-term effects of social defeat stress on the aversive properties of KOR activation in male and female California mice (Peromyscus californicus) using a conditioned place aversion paradigm. Female California mice naïve to social defeat, formed a place aversion following treatment with 2.5mg/kg of the KOR agonist U50,488, but females exposed to defeat did not form a place aversion to this dose. This supports the finding by others that social defeat weakens the aversive properties of KOR agonists. In contrast, both control and stressed males formed an aversion to 10mg/kg of U50,488. We also examined EGR1 immunoreactivity, an indirect marker of neuronal activity, in the nucleus accumbens (NAc) and found that stress and treatment with 10mg/kg of U50,488 increased EGR1 immunoreactivity in the NAc core in females but reduced activation in males. The effects of stress and U50,488 on EGR1 were specific to the NAc, as we found no differences in the bed nucleus of the stria terminalis. In summary, our data indicate important sex differences in the long-term effects of stress and indicate the need for further study of the molecular mechanisms mediating the behavioral effects of KOR in both males and females.

III. Historical significance of Shields’ 1975 essay: A brief commentary on four major contributions

This article argues that Shields’ work demonstrated that it is impossible to practice value-free science. And, despite the efforts of many feminist psychologists who have argued that the question of sex differences is someone else’s question, biological theories about the differences between women and men are still popular and influential today. This paper will call attention to four areas of scholarship produced by second-wave feminist psychologists who were inspired by Shields’ work: (1) rediscovery of the work of first-wave feminist psychologists, (2) discussion of the impossibility of value-free research on sex differences, (3) introduction of new categories of analysis such as “gender” and reframing research based on these new categories, and (4) addition of more value-laden categories to sex such as race, social class, and sexuality and using intersectionality theory to design new avenues of research.

Sex and menstrual cycle effects on chronic oral cocaine self-administration in rhesus monkeys: Effects of a nondrug alternative reward

BACKGROUND

In previous studies, female monkeys self-administered more oral phencyclidine (PCP) than males, and PCP intake differed by phase of menstrual cycle.

OBJECTIVES

The purpose of this study was to examine sex and hormonal influences on oral cocaine self-administration in male and female rhesus monkeys in the follicular vs. luteal phases of the menstrual cycle, with concurrent access to an alternative nondrug reward, saccharin (SACC) vs. water.

MATERIALS AND METHODS

Concurrent access to cocaine (0.2, 0.4, and 0.8 mg/ml) and SACC or water was available from two drinking spouts under concurrent fixed-ratio (FR) 2, 4, and 8 schedules during daily 3-h sessions.

RESULTS

Cocaine deliveries were similar in males and females in the females' luteal phase, but cocaine deliveries were higher in females during the follicular phase than the luteal phase and compared to males. When SACC was available, cocaine deliveries were reduced in females in the follicular phase of the cycle, and cocaine intake (mg/kg) was reduced in males and in females' follicular and luteal phases.

CONCLUSIONS

Access to concurrent SACC (vs. water) reduced cocaine intake (mg/kg) in males and in females during both menstrual phases, and the magnitude of the reduction in cocaine intake was greatest during the females' follicular phase. Thus, a nondrug alternative reward, SACC, is a viable alternative treatment for reducing cocaine's rewarding effects on male and female monkeys, and reductions in cocaine seeking were optimal in the females' luteal phase.

Sex Differences in Behavioral Dyscontrol: Role in Drug Addiction and Novel Treatments

The purpose of this review is to discuss recent findings related to sex differences in behavioral dyscontrol that lead to drug addiction, and clinical implications for humans are discussed. This review includes research conducted in animals and humans that reveals fundamental aspects of behavioral dyscontrol. The importance of sex differences in aspects of behavioral dyscontrol, such as impulsivity and compulsivity, is discussed as major determinants of drug addiction. Behavioral dyscontrol during adolescence is also an important consideration, as this is the time of onset for drug addiction. These vulnerability factors additively increase drug-abuse vulnerability, and they are integral aspects of addiction that covary and interact with sex differences. Sex differences in treatments for drug addiction are also reviewed in terms of their ability to modify the behavioral dyscontrol that underlies addictive behavior. Customized treatments to reduce behavioral dyscontrol are discussed, such as (1) using natural consequences such as non-drug rewards (e.g., exercise) to maintain abstinence, or using punishment as a consequence for drug use, (2) targeting factors that underlie behavioral dyscontrol, such as impulsivity or anxiety, by repurposing medications to relieve these underlying conditions, and (3) combining two or more novel behavioral or pharmacological treatments to produce additive reductions in drug seeking. Recent published work has indicated that factors contributing to behavioral dyscontrol are an important target for advancing our knowledge on the etiology of drug abuse, intervening with the drug addiction process and developing novel treatments.

Sex Differences in Kappa Opioid Receptor Function and Their Potential Impact on Addiction

Behavioral, biological, and social sequelae that lead to drug addiction differ between men and women. Our efforts to understand addiction on a mechanistic level must include studies in both males and females. Stress, anxiety, and depression are tightly linked to addiction, and whether they precede or result from compulsive drug use depends on many factors, including biological sex. The neuropeptide dynorphin (DYN), an endogenous ligand at kappa opioid receptors (KORs), is necessary for stress-induced aversive states and is upregulated in the brain after chronic exposure to drugs of abuse. KOR agonists produce signs of anxiety, fear, and depression in laboratory animals and humans, findings that have led to the hypothesis that drug withdrawal-induced DYN release is instrumental in negative reinforcement processes that drive addiction. However, these studies were almost exclusively conducted in males. Only recently is evidence available that there are sex differences in the effects of KOR activation on affective state. This review focuses on sex differences in DYN and KOR systems and how these might contribute to sex differences in addictive behavior. Much of what is known about how biological sex influences KOR systems is from research on pain systems. The basic molecular and genetic mechanisms that have been discovered to underlie sex differences in KOR function in pain systems may apply to sex differences in KOR function in reward systems. Our goals are to discuss the current state of knowledge on how biological sex contributes to KOR function in the context of pain, mood, and addiction and to explore potential mechanisms for sex differences in KOR function. We will highlight evidence that the function of DYN-KOR systems is influenced in a sex-dependent manner by: polymorphisms in the prodynorphin (pDYN) gene, genetic linkage with the melanocortin-1 receptor (MC1R), heterodimerization of KORs and mu opioid receptors (MORs), and gonadal hormones. Finally, we identify several gaps in our understanding of “if” and “how” DYN and KORs modulate addictive behavior in a sex-dependent manner. Future work may address these gaps by building on the mechanistic studies outlined in this review. Ultimately this will enable the development of novel and effective addiction treatments tailored to either males or females.

Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice

RATIONALE

Several findings on sex differences in cocaine response suggest a role for hormonal milieu in modulating the subjective effects of cocaine. Nitric oxide (NO) has been involved in the neurochemical, hormonal, and behavioral changes related to stress and anxiety. Within the brain, the anteroventral subdivision of the medial amygdala (MeAV) is an important area involved in processing emotional responses such as anxiety and a high density of NO-producing neurons is observed in this area.

OBJECTIVES

In this study, we hypothesize the possibility of sex/hormonal differences in response to cocaine and that these differences may reflect a change in the MeAV nitrergic system. We have examined cocaine's acute effects on nicotinamide adenine dinucleotide phosphate diaphorase (nadph-d) expression, as well as its effect on motor activity and anxiety in male and estrus and diestrus females.

RESULTS

Our results show that acute cocaine administration produces an increase in both anxiety behaviors and nadph-d expression in the MeAV. Male and diestrus female mice were more susceptible to these effects of cocaine than estrus female mice in which no differences were detected. In addition, we examined individual differences in male and female mice responding to intravenous cocaine reinforcement in a self-administration paradigm. Female mice acquired cocaine self-administration at a faster rate than males and showed a higher motivation to self-administer cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Our data suggest a complex interaction between hormonal milieu and the behavioral and reinforcing effects of cocaine.

Baseline Comorbidities Enhance the Risk of Treatment-Induced Depression in HCV-Infected Men: A Pilot Study

BACKGROUND

Hepatitis C virus (HCV) infection is associated with clinical depression,a condition that is aggravated on interferon-based therapy. In HCV infection, men often appear more resilient to depression than women. However, men are subject to depression in diseases that tend to be comorbid in HCV-infected.

AIM

This study examined whether HCV-infected men with baseline comorbidities were more or less susceptible to depression prior to and on treatment.

METHODS

Patients with chronic HCV infection preparing to begin treatment participated (n = 37). The presence of baseline comorbidities was determined by pretreatment medication regimes. Depression was measured by the Beck Depression Inventory prior to and following 2, 4, 8, and 12 weeks of interferon therapy.

RESULTS

At baseline, cohorts with (n = 16) and without (n = 21) comorbidities had equivocal demographics and infection characteristics. Comorbidities did not associate with baseline depression. However, on treatment, men with baseline comorbidities demonstrated an elevated risk for the onset of de novo depression (odds ratio = 19.25; confidence interval = 1.41, 582.14; p = .008). This was not observed for women. Baseline comorbidities did not alter the need for treatment discontinuations or the ability to achieve a sustained viral response.

CONCLUSION

The results of this study suggest that baseline comorbidities render men more susceptible to interferon treatment-induced depression.

Cocaine self-administration punished by i.v. histamine in rat models of high and low drug abuse vulnerability: effects of saccharin preference, impulsivity, and sex

A key feature of substance use disorders is continued drug consumption despite aversive consequences. This has been modeled in the animal laboratory by pairing drug self-administration with electric shock, thereby punishing drug intake (Deroche-Gamonet et al. 2004). In the present experiments, we examined the effects of punishment on i.v. cocaine self-administration by adding histamine to the cocaine solution with three different animal models of high and low vulnerability to drug abuse: rats selectively bred for high (HiS) and low (LoS) saccharin consumption, rats selected for high (HiI) and low (LoI) impulsivity, and sex differences. Animals were allowed to self-administer cocaine (0.4 mg/kg/infusion) to establish a baseline of operant responding. Histamine (4.0mg/kg/infusion) was then added directly into the cocaine solution and its consequent effects on self-administration were compared to baseline. The histamine+cocaine solution was then replaced with a cocaine-only solution, and the rats' operant responding was again compared to baseline. Concurrent histamine exposure was effective in reducing cocaine consumption in all groups of rats; however, LoS and female rats took longer to return to baseline levels of cocaine consumption after histamine was removed compared to HiS and male rats. These data suggest that the reduction of drug self-administration by aversive consequences may differ in groups that vary in drug use vulnerability . Such results may inform pharmacological strategies that enhance the negative aspects of drug consumption.

Long-term ethanol effects on acute stress responses: modulation by dynorphin

The brain stress-response system is critically involved in the addiction process, stimulating drug consumption and the relapse to drug taking in abstinent addicts. At the same time, its functioning is affected by chronic drug exposure. Here, we have investigated the role of the endogenous opioid peptide dynorphin as a modulator of effects of long-term ethanol consumption on the brain stress-response system. Using the two-bottle choice paradigm, we demonstrate an enhanced ethanol preference in male dynorphin knockout mice. Exposure to mild foot shock increased ethanol consumption in wild-type control littermates, but not in dynorphin-deficient animals. Blood adrenocorticotropic hormone levels determined 5 minutes after the shock were not affected by the genotype. We also determined the neuronal reactivity after foot shock exposure using c-Fos immunoreactivity in limbic structures. This was strongly influenced by both genotype and chronic ethanol consumption. Long-term alcohol exposure elevated the foot shock-induced c-Fos expression in the basolateral amygdala in wild-type animals, but had the opposite effect in dynorphin-deficient mice. An altered c-Fos reactivity was also found in the periventricular nucleus, the thalamus and the hippocampus of dynorphin knockouts. Together these data suggest that dynorphin plays an important role in the modulation of the brain stress-response systems after chronic ethanol exposure.

Sex differences in novelty- and psychostimulant-induced behaviors of C57BL/6 mice

RATIONALE

Women are more sensitive than men to psychostimulants and progress from initial use to drug addiction more quickly. The mouse has been an under-utilized model to study sex differences in psychostimulant action. Mice could serve as an ideal genetically tractable model for mechanistic studies into sex and hormone effects on psychostimulant behavior.

OBJECTIVES

The objective of this study was to characterize psychostimulant effects in male and female mice with a combination of automated data collection and behavioral observation.

METHODS

Male and female C57BL/6 mice (Charles River) were given a single dose or sequential ascending binge doses of D-amphetamine (AMPH) or cocaine (COC). Behavior was assessed in open field chambers using both automated photobeam interruptions and behavioral observations. Brain psychostimulant concentrations were determined at the time of maximum behavioral stimulation.

RESULTS

Psychostimulants induced behavioral activation in mice including both increased locomotion as detected with an automated system and a sequence of behaviors progressing from stereotyped sniffing at low doses to patterned locomotion and rearing at high doses. Females exhibited more patterned locomotion and a shift towards higher behavior scores after either psychostimulant despite having lower AMPH and equivalent COC brain levels as males.

CONCLUSIONS

Female C57BL/6 mice exhibit enhanced psychostimulant-induced behavior compared to males, similar to reports in rats. The combination of automated behavioral measures and behavioral observation was essential for verifying the existence of these differences. These results indicate the importance of testing both sexes when characterizing genetically manipulated mice to control for potential sex-specific effects.

Reinstatement of methamphetamine seeking in male and female rats treated with modafinil and allopregnanolone

BACKGROUND

Sex differences in methamphetamine (METH) use (females>males) have been demonstrated in clinical and preclinical studies. This experiment investigated the effect of sex on the reinstatement of METH-seeking behavior in rats and determined whether pharmacological interventions for METH-seeking vary by sex. Treatment drugs were modafinil (MOD), an analeptic, and allopregnanolone (ALLO), a neuroactive steroid and progesterone metabolite.

METHOD

Male and female rats were trained to self-administer i.v. infusions of METH (0.05 mg/kg/infusion). Next, rats self-administered METH for a 10-day maintenance period. METH was then replaced with saline, and rats extinguished lever-pressing behavior over 18 days. A multi-component reinstatement procedure followed whereby priming injections of METH (1mg/kg) were administered at the start of each daily session, preceded 30 min by MOD (128 mg/kg, i.p.), ALLO (15 mg/kg, s.c.), or vehicle treatment. MOD was also administered at the onset of the session to determine if it would induce the reinstatement of METH-seeking behavior.

RESULTS

Female rats had greater METH-induced reinstatement responding compared to male rats following control treatment injections. MOD (compared to the DMSO control) attenuated METH-seeking behavior in male and female rats; however, ALLO only reduced METH-primed responding in females. MOD alone did not induce the reinstatement of METH-seeking behavior.

CONCLUSIONS

These results support previous findings that females are more susceptible to stimulant abuse compared to males, and ALLO effectively reduced METH-primed reinstatement in females. Further, results illustrate the utility of MOD as a potential agent for prevention of relapse to METH use in both males and females.

Sex difference in κ-opioid receptor (KOPR)-mediated behaviors, brain region KOPR level and KOPR-mediated guanosine 5'-O-(3-[35S]thiotriphosphate) binding in the guinea pig

We examined whether sex differences in κ-opioid receptor (KOPR) pharmacology exist in guinea pigs, which are more similar to humans in the expression level and distribution of KOPR in the brain than rats and mice. The KOPR agonist trans-(±)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)benzeneacetamide methanesulfonate (U50,488H) produced a dose-dependent increase in abnormal postures and immobility with more effects in males than females. Males also showed more U50,488H-induced antinociception in the paw pressure test than females. Pretreatment with the KOPR antagonist norbinaltorphimine blocked U50,488H-induced abnormal body postures and antinociception. In contrast, inhibition of cocaine-induced hyperambulation by U50,488H was more effective in females than males. Thus, sex differences in the effects of U50,488H are endpoint-dependent. We then examined whether sex differences in KOPR levels and KOPR-mediated G protein activation in brain regions may contribute to the observed differences using quantitative in vitro autoradiography of [(3)H](5a,7a,8b)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)1-oxaspiro(4,5)dec-8-yl)benzeacetamide ([(3)H]U69,593) binding to the KOPR and U50,488H-stimulated guanosine 5'-O-(3-[(35)S]thiotriphosphate ([(35)S]GTPγS) binding. Compared with females, males exhibited more [(3)H]U69,593 binding in the deep layers of somatosensory and insular cortices, claustrum, endopiriform nucleus, periaqueductal gray, and substantial nigra. Concomitantly, U50,488H-stimulated [(35)S]GTPγS binding was greater in males than females in the superficial and deep layers of somatosensory and insular cortices, caudate putamen, claustrum, medial geniculate nucleus, and cerebellum. In contrast, compared with males, females showed more U50,488H-stimulated [(35)S]GTPγS binding in the dentate gyrus and a trend of higher [(35)S]GTPγS binding in the hypothalamus. These data demonstrate that males and females differ in KOPR expression and KOPR-mediated G protein activation in distinct brain regions, which may contribute to the observed sex differences in KOPR-mediated pharmacology.

Alcohol dependence, disinhibited behavior and variation in the prodynorphin gene

The results of the current analyses present preliminary evidence of an association between putatively functional variation in the prodynorphin (PDYN) gene and a dimensional measure of disinhibited behavior. A 68bp sequence in the core promoter region of the PDYN gene was genotyped in a community sample of 1021 adults aged 30-54. Participants were interviewed for lifetime history of DSM-IV alcohol dependence and completed two self-report measures of sensation seeking and impulsiveness. Fifteen percent (n=151) of the sample met DSM-IV criteria for alcohol dependence and while results did not support an association between the PDYN polymorphism and the diagnosis of alcohol dependence, we did observe an association between the "low" expressing L allele of the PDYN gene and a preference for engaging in disinhibited behavior. Additionally, people who had both a history of alcohol dependence and higher scores on this Disinhibited Behavior scale were most likely to carry an L allele. These results indicate that variation in the PDYN gene is associated with a dimensional trait or intermediate phenotype that reflects a preference for heavy drinking and engaging in related risky behaviors (e.g., drug use, sexual activity).

Sex differences in kappa opioid pharmacology

In recent years it has become apparent that sex is a major factor involved in modulating the pharmacological effects of exogenous opioids. The kappa opioid receptor (KOPR) system is a potential therapeutic target for pain, mood disorders and addiction. In humans mixed KOPR/MOPR ligands have been found to produce greater analgesia in women than men. In contrast, in animals, selective KOPR agonists have been found to produce greater antinociceptive effects in males than females. Collectively, the studies indicate that the direction and magnitude of sex differences of KOPR-mediated antinociception/analgesia are dependent on species, strain, ligand and pain model examined. Of interest, and less studied, is whether sex differences in other KOPR-mediated effects exist. In the studies conducted thus far, greater effects of KOPR agonists in males have been found in neuroprotection against stroke and suppression of food intake behavior. On the other hand, greater effects of KOPR agonists were found in females in mediation of prolactin release. In modulation of drugs of abuse, sex differences in KOPR effects were observed but appear to be dependent on the drug examined. The mechanism(s) underlying sex differences in KOPR-mediated effects may be mediated by sex chromosomes, gonadal hormonal influence on organization (circuitry) and/or acute hormonal influence on KOPR expression, distribution and localization. In light of the diverse pharmacology of KOPR we discuss the need for future studies characterizing the sexual dimorphism of KOPR neural circuitry and in examining other behaviors and processes that are modulated by the KOPR.

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.

Cocaine-induced behavioral sensitization in preweanling and adult rats: effects of a single drug-environment pairing

RATIONALE

Adult rats typically exhibit more robust behavioral sensitization than do preweanling rats. A possible explanation for this age-dependent difference is that environmental context may have relatively less impact on the psychostimulant-induced behaviors of preweanling rats.

OBJECTIVE

The purpose of this study was to assess the importance of environmental context for the development of cocaine-induced sensitization in preweanling and adult rats.

MATERIALS AND METHODS

On postnatal day (PD) 19 or PD 79, rats in the context-dependent condition were injected with 30 mg/kg cocaine immediately before being placed in a novel test chamber for 30 min. The same rats were then injected with saline 30 min after being returned to the home cage. Rats in the context-independent condition were injected with saline before being placed in the novel chamber and cocaine in the home age. Control rats were injected with saline at both time points. One day later, adult and preweanling rats were challenged with saline or 10 mg/kg cocaine (experiment 1), or preweanling rats were challenged with 5, 20, or 30 mg/kg cocaine (experiment 2). After being injected, rats were placed in the test chamber, and behavior was measured for 60 min.

RESULTS

Adult rats showed context-dependent locomotor sensitization and conditioned activity, with females exhibiting more locomotor activity than males. Preweanling rats did not exhibit conditioned activity, but they showed robust context-dependent and context-independent sensitization when challenged with 10-30 mg/kg cocaine.

CONCLUSIONS

Context did not influence the expression of behavioral sensitization in preweanling rats, suggesting that deficits in associative or memory processes may be responsible for age-dependent differences in behavioral sensitization and conditioned activity.

Assessment of opiate modulation of pain and nociceptive responding in young adults with a parental history of hypertension

This double blind, placebo-controlled study examined the effects of an opiate antagonist, naltrexone, on nociceptive flexion reflex (NFR) thresholds and subjective pain in individuals with and without a parental history of hypertension. Using a repeated measures design, NFR threshold was repeatedly assessed on two testing days after administration of either placebo or naltrexone. Immediately after NFR threshold was determined, participants rated the level of pain experienced during the preceding NFR assessment, and at the end of each session participants' electrocutaneous pain threshold was assessed. Two primary findings were obtained. First, individuals with a parental history of hypertension exhibited attenuated pain sensitivity. Second, endogenous opioid blockade was associated with increased pain ratings in women but with increased pain threshold in men. In sum, the present study did not support a direct involvement of the endogenous opioid system in the attenuated pain sensitivity observed in individuals at increased risk for hypertension.

Confirmation of the association between a polymorphism in the promoter region of the prodynorphin gene and cocaine dependence

The endogenous opioid system has been shown to have a role in the biological processes involved in addiction to numerous drugs of abuse including cocaine. It has recently been reported that the variable nucleotide tandem repeat (VNTR) polymorphism in the 5' promoter region of the prodynorphin gene, which encodes the precursor for three endogenous opioid peptides, is associated with the cocaine dependent phenotype. In order to confirm this finding, we genotyped the prodynorphin promoter polymorphism in cocaine dependent (n = 167) and control (n = 88) individuals of African descent. The results from this experiment indicate a statistically significant (chi2 = 5.64, OR = 1.59, P = 0.018) association between the prodynorphin promoter VNTR polymorphism and the cocaine dependent phenotype. In contrast to previous work showing increased risk conferred by one or two copies of the prodynorphin VNTR, the genotyping results from this study indicate that persons with three or four copies of this polymorphism are more likely to become cocaine dependent. This disparity suggests that the prodynorphin promoter VNTR may not be the functional polymorphism associating with the cocaine dependent phenotype. It is possible that different alleles of the prodynorphin promoter VNTR in the independent populations used for this and the previous study may be in linkage disequilibrium with a yet to be identified functional polymorphism in this gene.

Characterization of conditioned place preference to cocaine in congenic dopamine transporter knockout female mice

RATIONALE

The dopamine transporter (DAT) is thought to play a major role in the rewarding effects of cocaine. Therefore, it is surprising that cocaine reveals conditioned effects in DAT knockout (DAT-KO) mice.

OBJECTIVES

To examine these findings further, we obtained complete dose-effect curves for DAT-KO and DAT wild-type (DAT-WT) mice in a cocaine conditioned place preference (CPP) procedure.

METHODS

Congenic C57BL6 background female DAT-KO and DAT-WT mice were conditioned in a three-compartment place preference apparatus. Conditioning consisted of three 30-min sessions with cocaine (2.5, 5.0, 10.0, 20.0, or 40.0 mg/kg) and three 30-min sessions with saline. The distribution of time in each choice compartment was determined after each pair of conditioning sessions (one cocaine and one saline session).

RESULTS

DAT-WT mice revealed CPP over a wide range of cocaine doses (5.0-40 mg/kg), whereas DAT-KO mice revealed CPP over a more restricted range of doses, with consistent CPP only occurring with 10 mg/kg of cocaine.

CONCLUSIONS

CPP for cocaine develops in both DAT-KO and DAT-WT mice; however, the dose range at which CPP develops is much more restricted in DAT-KO mice than in DAT-WT mice. These observations corroborate the significant role of DAT inhibition in cocaine's conditioned effects.

Sex and estrogen influence drug abuse

Evidence is accumulating that the etiology, epidemiology, consequences and mechanisms that underlie drug abuse are different in males and females. In this review, we present examples of sex differences in all phases of drug abuse, including acquisition, steady-state maintenance, escalation, dysregulation, withdrawal, relapse and treatment. Most reported findings are based on laboratory research in animals, but there are corroborating reports from human clinical and epidemiological studies. In all phases of drug abuse, females seem to be more sensitive to the rewarding effects of drugs than males, and estrogen is a major factor that underlies these sex differences.

Differential effects of bremazocine on oral phencyclidine (PCP) self-administration in male and female rhesus monkeys

Sex differences exist in many phases of drug abuse, but few studies have focused on sex differences in drug abuse treatment. In this study, the effects of bremazocine, a kappa-opioid receptor agonist, were compared in age-matched male and female rhesus monkeys self-administering orally delivered phencyclidine (PCP). Bremazocine (0.00032. 0.001, and 0.0025 mg/kg, intramuscular) was administered for 5 consecutive days. 15 min prior to daily 3-hr sessions when PCP (0.25 mg/ml) and water were available under concurrent fixed-ratio schedules. Bremazocine dose-dependently decreased PCP-maintained responding and consumption (mg/kg) in males and females, and these measures were suppressed at a lower bremazocine dose in females than in males. The percentage reduction in PCP-maintained responding and intake (mg/kg) was significantly greater in females than it was in males at the low and middle doses of bremazocine, suggesting that females may be more responsive to kappa agonist treatment than males.

Sex differences in pain and hypothalamic-pituitary-adrenocortical responses to opioid blockade

OBJECTIVE

Sex differences in pain sensitivity and stress reactivity have been well documented. Little is known about the role of the endogenous opioid system in these differences. This study was conducted to compare adrenocortical, pain sensitivity, and blood pressure responses to opioid blockade using naltrexone in men and women.

METHODS

Twenty-six participants completed 2 sessions during which placebo or 50 mg of naltrexone was administered, using a double-blind, counterbalanced design. Thermal pain threshold and heat tolerance were assessed. Participants also rated pain during a 90-second cold pressor test (CPT) and completed the McGill Pain Questionnaire (MPQ) after each pain challenge. Blood and saliva samples and cardiovascular and mood measures were obtained throughout the sessions.

RESULTS

Plasma cortisol, adrenocorticotropin, beta endorphin, prolactin, and salivary cortisol levels increased similarly in men and women after naltrexone administration compared with placebo. Women reported more pain during both pain procedures and had lower thermal pain tolerance. In response to naltrexone, women exhibited reduced blood pressure responses and reduced MPQ pain ratings after CPT. No effects of naltrexone on these measures were found in men.

CONCLUSIONS

Although men and women exhibited similar hormonal responses to opioid receptor blockade, women reported less pain and showed smaller blood pressure responses during CPT. Results suggest differential effects of the endogenous opioid system on pain perception and blood pressure in men and women.

Effects of sex and estrogen on behavioral sensitization to cocaine in rats

Estrogen rapidly enhances dopamine (DA) activity in the striatum and nucleus accumbens as well as behavioral responses to psychomotor stimulants in female rats but not males. This experiment was conducted to investigate the role of pulsatile estrogen treatment on and sex differences in the development and expression of sensitization of cocaine-induced rotational behavior in rats with unilateral striatal DA denervation. Four groups were tested: ovariectomized (OVX) females treated with 5 microg of estradiol benzoate (OVX+E), OVX females, castrated (CAST) males, and intact males. Animals received estrogen or vehicle 30 min before cocaine (0, 5, 10, or 20 mg/kg, i.p.) on 4 consecutive days, followed by 3 d without treatment for 3 weeks. At the conclusion of the experiment, animals were withdrawn from hormone and/or cocaine for 10 d, and all groups underwent a challenge test with 10 mg/kg cocaine. We report here that OVX+E females exhibit significantly greater sensitization of rotational behavior with a faster rate of sensitization than the three other groups. There is also a sex difference independent of gonadal hormones: OVX females exhibit a greater magnitude of sensitization of rotational behavior than do CAST males at 20 mg/kg cocaine. Furthermore, on the challenge test, OVX+E animals tested without estrogen treatment continue to exhibit greater rotational behavior than do all other groups. Thus, estrogen enhances sensitization to cocaine, there are sex differences in behavioral sensitization, and sensitization that develops under conditions with estrogen persists even when estrogen levels are low.

Estrogen and opioids interact to modulate the locomotor response to cocaine in the female rat

Estrogen is known to modulate the behavioral response to cocaine; however the mechanisms by which this is accomplished is unknown. In this study we examine one possible candidate, the endogenous opioid system. Adult Sprague-Dawley rats were ovariectomized (OVX), half received Silastic implants with estradiol benzoate (OVX-EB), the other half received empty implants (OVX). After 1 week, spontaneous locomotor and stereotyped activity was measured for 60 min using an automated system. On day 2, locomotor activity was recorded for 30 min. Rats were injected with saline (SAL) or naloxone (NAL) (2 mg/kg, i.p.) and activity measured for the next 20 min. Each of these groups were further subdivided, one that received a saline injection (SAL) and another that received a cocaine injection (COC) (15 mg/kg, i.p.). Locomotor and stereotyped activities were recorded for 60 min. This resulted in the following injection groups: SAL-SAL, NAL-SAL, SAL-COC and NAL-COC. During habituation, OVX rats displayed an overall higher level of activity than OVX-EB rats. Similar to what is observed in males, naloxone significantly reduced locomotion and stereotyped behavior but only in OVX rats. Estrogen administration to OVX rats abolished the effect of naloxone. Surprisingly, when naloxone was administered prior to cocaine, an increase in cocaine-induced locomotor and stereotyped activity was observed, but only in OVX-EB rats. These results indicate that opioid modulation of cocaine-induced locomotor and stereotype activity in the female differs from that reported in the male. In addition in the female, the effect of opioids on cocaine-induced locomotor behavior is dependent on plasma levels of estrogen.

Mesolimbic gender differences in peptide CART mRNA expression: effects of cocaine

The putative neurotransmitter peptide CART has been suggested to be involved in the actions of psychostimulants. We analyzed the CART mRNA expression in mesolimbic brain areas of male and ovariectomized 17beta-estradiol- (30 microg) and vehicle-treated female rats. A gender difference was noted in the accumbens shell during basal conditions; male rats expressed higher levels of CART mRNA than both female groups. Following binge cocaine injections (3 x 15 mg/kg), elevated levels were found in the central amygdala of male but not female rats. In the medial accumbens shell CART mRNA was elevated after cocaine, but only in the non-treated females. The present results support a role of mesolimbic CART in psychostimulant drug action.