Programming of Dopaminergic Neurons by Early Exposure to Sex Hormones: Effects on Morphine-Induced Accumbens Dopamine Release, Reward, and Locomotor Behavior in Male and Female Rats

Disruption of positive- and negative-feature morphine interoceptive occasion setters by dopamine receptor agonism and antagonism in male and female rats

RATIONALE

Internally perceived stimuli evoked by morphine administration can form Pavlovian associations such that they can function as occasion setters (OSs) for externally perceived reward cues in rats, coming to modulate reward-seeking behaviour. Though much research has investigated mechanisms underlying opioid-related reinforcement and analgesia, neurotransmitter systems involved in the functioning of opioids as Pavlovian interoceptive discriminative stimuli remain to be disentangled despite documented differences in the development of tolerance to analgesic versus discriminative stimulus effects.

OBJECTIVES

Dopamine has been implicated in many opioid-related behaviours, so we aimed to investigate the role of this neurotransmitter in expression of morphine occasion setting.

METHODS

Male and female rats were assigned to positive- (FP) or negative-feature (FN) groups and received an injection of morphine or saline before each training session. A 15-s white noise conditioned stimulus (CS) was presented 8 times during every training session; offset of this stimulus was followed by 4-s access to liquid sucrose on morphine, but not saline, sessions for FP rats. FN rats learned the reverse contingency. Following stable discrimination, rats began generalization testing for expression of morphine-guided sucrose seeking after systemic pretreatment with different doses of the non-selective dopamine receptor antagonist, flupenthixol, and the non-selective dopamine receptor agonist, apomorphine, combined with training doses of morphine or saline in a Latin-square design.

RESULTS

The morphine discrimination was acquired under both FP and FN contingencies by males and females. Neither flupenthixol nor apomorphine at any dose substituted for morphine, but both apomorphine and flupenthixol disrupted expression of the morphine OS. This inhibition was specific to sucrose seeking during CS presentations rather than during the period before CS onset and, in the case of apomorphine more so than flupenthixol, to trials on which access to sucrose was anticipated.

CONCLUSIONS

Our findings lend support to a mechanism of occasion setting involving gating of CS-induced dopamine release rather than by direct dopaminergic modulation by the morphine stimulus.

Early-life exposure to sex hormones promotes voluntary ethanol intake in adulthood. A vulnerability factor to drug addiction

While there is extensive research on alcohol dependence, the factors that make an individual vulnerable to developing alcoholism haven't been explored much. In this study, we aim to investigate how neonatal exposure to sex hormones affects alcohol intake and the regulation of the mesolimbic pathway in adulthood. The study aimed to investigate the impact of neonatal exposure to a single dose of testosterone propionate (TP) or estradiol valerate (EV) on ethanol consumption in adult rats. The rats were subjected to a two-bottle free-choice paradigm, and the content of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens (NAcc) was measured using HPLC-ED. The expression of critical DA-related proteins in the mesolimbic pathway was evaluated through RT-qPCR and western blot analysis. Supraphysiological neonatal exposure to EV or TP resulted in increased ethanol intake over four weeks in adulthood. In addition, the DA and DOPAC content was reduced and increased in the NAcc of EV and TP-treated rats, and β-endorphin content in the hypothalamus decreased in EV-treated rats. The VTA μ receptor and DA type 2 form short receptor (D2S) expression were significantly reduced in EV and TP male rats. Finally, in an extended 6-week protocol, the increase in ethanol consumption induced by EV was mitigated during the initial two hours post-naloxone injection. Neonatal exposure to sex hormones is a detrimental stimulus for the brain, which can facilitate the development of addictive behaviors, including alcohol use disorder.

Generalization of a positive-feature interoceptive morphine occasion setter across the rat estrous cycle

INTRODUCTION

Interoceptive stimuli elicited by drug administration acquire conditioned modulatory properties of the induction of conditioned appetitive behaviours by exteroceptive cues. This effect may be modeled using a drug discrimination task in which the drug stimulus is trained as a positive-feature (FP) occasion setter (OS) that disambiguates the relation between an exteroceptive light conditioned stimulus (CS) and a sucrose unconditioned stimulus (US). We previously reported that females are less sensitive to generalization of a FP morphine OS than males, so we investigated the role of endogenous ovarian hormones in this difference.

METHODS

Male and female rats received intermixed injections of 3.2 mg/kg morphine or saline before each daily training session. Training consisted of 8 presentations of the CS, each followed by access to sucrose on morphine, but not saline sessions. Following acquisiton, rats were tested for generalization of the morphine stimulus to 0, 1.0, 3.2, and 5.4 mg/kg morphine. Female rats were monitored for estrous cyclicity using vaginal cytology throughout the study.

RESULTS

Both sexes acquired stable drug discrimination. A gradient of generalization was measured across morphine doses and this behaviour did not differ by sex, nor did it differ across the estrous cycle in females.

CONCLUSIONS

Morphine generalization is independent of fluctuations in levels of sex and endogenous gonadal hormones in females under these experimental conditions.

Estradiol and Mu opioid-mediated reward: The role of estrogen receptors in opioid use

Opioid use and opioid use disorder are characterized by sex and gender differences, and some of these differences may be mediated by differences in the hormonal milieu within and across individuals. This review focuses on the role of ovarian hormones, and particularly estradiol, on the endogenous mu opioid receptor system. There is an abundance of data indicating that estradiol influences the activity of endogenous mu opioid peptides, the activation of mu opioid receptors, and the internalization and desensitization of mu opioid receptors. These effects have functional consequences on behaviors mediated by endogenous mu opioid receptor activity and on sensitivity to mu opioid agonists and antagonists. Recent behavioral data suggest these consequences extend to mu opioid reward, and preclinical studies report that estradiol decreases self-administration of mu opioid receptor agonists across a range of experimental conditions. Data collected in human laboratory studies suggest that estradiol may have functionally similar effects in clinical populations, and thus estrogen receptors may be a potential target in the development of novel therapeutics. This review summarizes data from cellular assays to clinical trials to explore how estradiol influences mu opioid receptor activity, as well as potential ways in which estrogen receptors may be targeted to address the problems of opioid use.

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.

Early-Life Exposure to Non-Absorbable Broad-Spectrum Antibiotics Affects the Dopamine Mesocorticolimbic Pathway of Adult Rats in a Sex-Dependent Manner

Gut microbiota with a stable, rich, and diverse composition is associated with adequate postnatal brain development. Colonization of the infant’s gut begins at birth when parturition exposes the newborn to a set of maternal bacteria, increasing richness and diversity until one to two first years of age when a microbiota composition is stable until old age. Conversely, alterations in gut microbiota by diet, stress, infection, and antibiotic exposure have been associated with several pathologies, including metabolic and neuropsychiatric diseases such as obesity, anxiety, depression, and drug addiction, among others. However, the consequences of early-life exposure to antibiotics (ELEA) on the dopamine (DA) mesocorticolimbic circuit are poorly studied. In this context, we administered oral non-absorbable broad-spectrum antibiotics to pregnant Sprague-Dawley dams during the perinatal period (from embryonic day 18 until postnatal day 7) and investigated their adult offspring (postnatal day 60) to assess methylphenidate-induced conditioned place preference (CPP) and locomotor activity, DA release, DA and 3,4-dihydroxyphenylacetic acid (DOPAC) content in ventral tegmental area (VTA), and expression of key proteins within the mesocorticolimbic system. Our results show that ELEA affect the rats conduct by increasing drug-seeking behavior and locomotor activity induced by methylphenidate of males and females, respectively, while reducing dopamine striatal release and VTA content of DOPAC in females. In addition, antibiotics increased protein levels of DA type 1 receptor in prefrontal cortex and VTA of female rats, and tyrosine hydroxylase in VTA of adult male and female rats. Altogether, these results suggest that ELEA alters the development of the microbiota-gut-brain axis affecting the reward system and the response to abuse drugs in adulthood.

Effect of Testosterone on the Extinction Period of Morphine-induced CPP in Male Rats

Background: Sex-related differences in the incidence, prevalence, symptoms, and side effects of drug use, especially narcotics, have been previously shown in animal models and human studies. Objectives: In the presents study, the effects of different doses of testosterone on morphine extinction period were investigated in a rat model. Methods: Forty mature male Wistar rats were randomly allocated to four categories (10 in each group), including control (received intramuscular injection of vehicle) and testosterone (received intramuscular injection of testosterone at 1, 2.5 and 5 mg/kg) during the extinction period. Conditioned place preference (CPP) test was done to assess the psychological phenomena of drug craving and relapse. The CPP score was calculated in four stages, including the baseline (preconditioning), expression (postconditioning), extinction, and reinstatement. Results: Our results demonstrated that testosterone (1, 2.5 and 5 mg/kg) significantly extended morphine extinction duration compared to the control group. Conclution: It has been shown that dopamine neurotransmission in mesocorticolimbic system is affected by testosterone through androgen receptors in adolescence and alteration in testosterone level could affect drug use vulnerability. It seems that normalization of testosterone levels reduces the symptoms of opioid withdrawal syndrome and have important clinical implication for clinicians to understand the effects of testosterone dysregulation on the extinction and withdrawal periods.

Effects of Early Life Exposure to Sex Hormones on Neurochemical and Behavioral Responses to Psychostimulants in Adulthood: Implications in Drug Addiction

Early life exposure to sex hormones affects several brain areas involved in regulating locomotor and motivation behaviors. Our group has shown that neonatal exposure to testosterone propionate (TP) or estradiol valerate (EV) affected the brain dopamine (DA) system in adulthood. Here, we studied the long-lasting effects of neonatal exposure to sex hormones on behavioral and neurochemical responses to amphetamine (AMPH) and methylphenidate (MPD). Our results show that AMPH-induced locomotor activity was higher in female than male control rats. The conditioned place preference (CPP) to AMPH was only observed in EV male rats. In EV female rats, AMPH did not increase locomotor activity, but MPD-induced CPP was observed in control, EV and TP female rats. Using in vivo brain microdialysis, we observed that AMPH-induced extracellular DA levels were lower in nucleus accumbens (NAcc) of EV and TP female rats than control rats. In addition, MPD did not increase NAcc extracellular DA levels in EV rats. Using in vivo fast-scan cyclic voltammetry in striatum, MPD-induced DA reuptake was higher in EV than control rats. In summary, our results show that early life exposure to sex hormones modulates mesolimbic and nigrostriatal DA neurons producing opposite neurochemical effects induced by psychostimulant drugs in NAcc or striatum.

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

Enhanced heroin self-administration and distinct dopamine adaptations in female rats

Increasing evidence suggests that females are more vulnerable to the harmful effects of drugs of abuse, including opioids. Additionally, rates of heroin-related deaths substantially increased in females from 1999 to 2017 [1], underscoring the need to evaluate sex differences in heroin vulnerability. Moreover, the neurobiological substrates underlying sexually dimorphic responding to heroin are not fully defined. Thus, we evaluated male and female Long Evans rats on acquisition, dose-responsiveness, and seeking for heroin self-administration (SA) as well as using a long access model to assess escalation of intake at low and high doses of heroin, 0.025 and 0.1 mg/kg/inf, respectively. We paired this with ex vivo fast-scan cyclic voltammetry (FSCV) in the medial nucleus accumbens (NAc) shell and quantification of mu-opioid receptor (MOR) protein in the ventral tegmental area (VTA) and NAc. While males and females had similar heroin SA acquisition rates, females displayed increased responding and intake across doses, seeking for heroin, and escalation on long access. However, we found that males and females had similar expression levels of MORs in the VTA and NAc, regardless of heroin exposure. FSCV results revealed that heroin exposure did not change single-pulse elicited dopamine release, but caused an increase in dopamine transporter activity in both males and females compared to their naïve counterparts. Phasic-like stimulations elicited robust increases in dopamine release in heroin-exposed females compared to heroin-naïve females, with no differences seen in males. Together, our results suggest that differential adaptations of dopamine terminals may underlie the increased heroin SA behaviors seen in females.

Neonatal programming with sex hormones: Effect on expression of dopamine D1 receptor and neurotransmitters release in nucleus accumbens in adult male and female rats

Steroid sex hormones produce physiological effects in reproductive and non-reproductive tissues, such as the brain. In the brain, sex hormones receptors are expressed in cortical, limbic and midbrain areas modulating memory, arousal, fear and motivation between other behaviors. One neurotransmitters system regulated by sex hormones is dopamine (DA), where during adulthood, sex hormones promote neurophysiological and behavioral effects on DA systems such as tuberoinfundibular (prolactin secretion), nigrostriatal (motor circuit regulation) and mesocorticolimbic (driving of motivated behavior). However, the long-term effects induced by neonatal exposure to sex hormones on DA release induced by D1 receptor activation and its expression in nucleus accumbens (NAcc) have not been fully studied. To answer this question, neurochemical, cellular and molecular techniques were used. The data show sex differences in NAcc DA extracellular levels induced by D1 receptor activation and protein content of this receptor in male and female control rats. In addition, neonatal programming with a single dose of TP increases the NAcc protein content of D1 receptors of adult male and female rats. Our results show new evidence related with sex differences that could explain the dependence to drug of abuse in males and females, which may be associated with increased reinforcing effects of drugs of abuse.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Chronic modafinil administration to preadolescent rats impairs social play behavior and dopaminergic system

Some clinical trials are investigating modafinil (Mod) as a treatment for attentional deficit and hyperactivity disorder (ADHD) in children and adolescents. Mod increases dopamine (DA) levels in the reward system by blocking dopamine transporter (DAT). Social interactions are rewarding behaviors and evidence reveals the importance of reward circuitry in social interactions. Chronic psychostimulant treatments alter DA neurotransmission and associated behaviors. The aim of this work was to evaluate the effects of chronic Mod treatment during preadolescence on social play behavior, locomotor activity, and DA in nucleus accumbens (NAc). Preadolescent male Sprague-Dawley rats were injected with Mod (75 mg/kg i.p.) or vehicle for 14 days (PND22 to PND35). After that, we measured social play behavior, content and DA release in NAc by HPLC coupled to electrochemical detection, protein levels of DA type 2 receptor (D₂) by Western blot and DA kinetic by fast-scan cyclic voltammetry (FSCV) in NAc. Regarding social play, the total number of pinning events decreased in the Mod group compared with the vehicle. The K⁺-stimulated DA release in NAc was significantly lower in Mod-treated rats compared with vehicle group. Also, Mod increases locomotor activity at the first injection, but this effect is almost completely lost at day 14 of Mod treatment. Chronic Mod treatment during preadolescence in rats impairs dopaminergic neurotransmission in NAc and decreases the capacity of rats to perceive rewarding effects of social play. Importantly, as Mod is being evaluated to treat ADHD in children and adolescents, potential effects on social behavior should be considered since this kind of behavior in this particular stage is crucial for neurodevelopment.

Sex Differences in Animal Models of Opioid Reward

PURPOSE OF REVIEW

This review aims to discuss sex differences observed in preclinical rodent models of opioid reward.

RECENT FINDINGS

Utilizing a variety of methodological approaches and drug regimens, no clear consensus has emerged regarding the effects of opiates between males and females. This is quite dissimilar to work examining psychostimulants, where female animals reliably exhibit stronger behavioral responses.

SUMMARY

With opioid research quickly expanding to determine the neural underpinnings of opioid addiction, further research is essential to determine the conditions wherein sex differences may occur and how they may relate to the human condition.

Sexual Differentiation and Substance Use: A Mini-Review

The organizational/activational hypothesis suggests that gonadal steroid hormones like testosterone (T) and estradiol (E2) are important at 2 different times during the lifespan when they perform 2 different functions. First steroids "organize" brain structures early in life and during puberty, and in adults these same hormones "activate" sexually dimorphic behaviors. This hypothesis has been tested and proven valid for a large number of behaviors (learning, memory, social, and sexual behaviors). Sex differences in drug addiction are well established both for humans and animal models. Previous research in this field has focused primarily on cocaine self-administration by rats. Traditionally, observed sex differences have been explained by the sex-specific concentrations of gonadal hormones present at the time of the drug-related behavior. Studies with gonadectomized rodents establishes an activational role for E2 that facilitates vulnerability in females, and when E2 is combined with progesterone, addiction is attenuated. Literature on organizational actions of steroids is sparse but predicts that T, after it is aromatized to E2, changes aspects of the neural reward system. Here we summarize these data and propose that sex chromosome complement also plays a role in determining sex-specific drug-taking behavior. Future research is needed to disentangle the effects of hormones and sex chromosome complement, and we propose the four core genotype mouse model as an effective tool for answering these questions.

Early postnatal allopregnanolone levels alteration and adult behavioral disruption in rats: Implication for drug abuse

Several studies have highlighted the role that early postnatal levels of allopregnanolone play in the development of the CNS and adult behavior. Changes in allopregnanolone levels related to stress have been observed during early postnatal periods, and perinatal stress has been linked to neuropsychiatric disorders. The alteration of early postnatal allopregnanolone levels in the first weeks of life has been proven to affect adult behaviors, such as anxiety-related behaviors and the processing of sensory inputs. This review focuses on the first studies about the possible relationship between the early postnatal allopregnanolone levels and the vulnerability to abuse of drugs such as alcohol in adulthood, given that (1) changes in neonatal allopregnanolone levels affect novelty exploration and novelty seeking has been linked to vulnerability to drug abuse; (2) early postnatal administration of progesterone, the main allopregnanolone precursor, affects the maturation of dopaminergic meso-striatal systems, which have been related to novelty seeking and drug abuse; and (3) alcohol consumption increases plasma and brain allopregnanolone levels in animals and humans. Manipulating neonatal allopregnanolone by administering finasteride, an inhibitor of the 5α-reductase enzyme that participates in allopregnanolone synthesis, increases alcohol consumption and decreases the locomotor stimulant effects of low alcohol doses. At a molecular level, finasteride decreases dopamine and serotonin in ventral striatum and dopamine release in nucleus accumbens. Preliminary results suggest that serotonin 5HT3 receptors could also be affected. Although an in-depth study is necessary, evidence suggests that there is a relation between early postnatal allopregnanolone and vulnerability to drug use/abuse.

•

Early postnatal AlloP levels alteration affects brain maturation and adult behavior.

•

Early stress interacts to AlloP influencing neuropsychiatric disorders vulnerability.

•

Fluctuations in neonatal AlloP levels play a role in alcohol abuse vulnerability.

•

Neonatal finasteride induces novelty-seeking profile and increases ethanol intake.