Different affective response to opioid withdrawal in adolescent and adult mice

Cannabidiol Protects against the Reinstatement of Oxycodone-Induced Conditioned Place Preference in Adolescent Male but Not Female Rats: The Role of MOR and CB1R

Cannabidiol (CBD), a phytocannabinoid, appeared to satisfy several criteria for a safe approach to preventing drug-taking behavior, including opioids. However, most successful preclinical and clinical results come from studies in adult males. We examined whether systemic injections of CBD (10 mg/kg, i.p.) during extinction of oxycodone (OXY, 3 mg/kg, i.p.) induced conditioned place preference (CPP) could attenuate the reinstatement of CPP brought about by OXY (1.5 mg/kg, i.p.) priming in adolescent rats of both sexes, and whether this effect is sex dependent. Accordingly, a priming dose of OXY produced reinstatement of the previously extinguished CPP in males and females. In both sexes, this effect was linked to locomotor sensitization that was blunted by CBD pretreatments. However, CBD was able to prevent the reinstatement of OXY-induced CPP only in adolescent males and this outcome was associated with an increased cannabinoid 1 receptor (CB1R) and a decreased mu opioid receptor (MOR) expression in the prefrontal cortex (PFC). The reinstatement of CCP in females was associated with a decreased MOR expression, but no changes were detected in CB1R in the hippocampus (HIP). Moreover, CBD administration during extinction significantly potentialized the reduced MOR expression in the PFC of males and showed a tendency to potentiate the reduced MOR in the HIP of females. Additionally, CBD reversed OXY-induced deficits of recognition memory only in males. These results suggest that CBD could reduce reinstatement to OXY seeking after a period of abstinence in adolescent male but not female rats. However, more investigation is required.

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behaviour

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational and cognitive processes involved in regulating the pursuit and consumption of food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal disrupted their ability to exert flexible goal-directed control over reward seeking. Specifically, morphine-withdrawn rats were impaired in using current reward value to select actions both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case, rats were only impaired in using reward value to select actions in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Experimental Evidence on Age-related Differential Outcomes Associated With Substance Abuse

Growing evidence indicates that adolescent substance abuse is now an alarming concern that imposes a considerable socio-economic burden on societies. On the other hand, numerous studies have shown that due to specific neurophysiological features, the brain is more vulnerable to the adverse effects of psychoactive drugs at an early age. Unfortunately, these negative effects are not limited to the period of drug use, but can persistently affect the brain's responsiveness to future exposures to the same or other types of drug. For researchers to develop pharmacological strategies for managing substance abuse disorders, they need to gain a deep understanding of the differences in behavioral outcomes associated with each type of drug across different age groups. The present study was conducted to review the experimental evidence revealing the mentioned differential effects with an emphasis on common drugs of abuse, including cocaine, nicotine, cannabis, and opioids. Although the cellular mechanisms underlying age-related effects have not been exclusively addressed for each drug, the most recent results are presented and discussed. Future studies are required to focus on these mechanisms and reveal how molecular changes during brain development can result in differential responses to drugs at the behavioral level.

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behavior

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational, and cognitive processes involved in regulating the pursuit and consumption of natural food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically-driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened emotional reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal also disrupted their ability to exert flexible goal-directed control over their reward-seeking behavior. Specifically, morphine-withdrawn rats displayed insensitivity to reward devaluation both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case insensitivity to reward devaluation was only observed in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Development of anxiety-like behaviors during adolescence: Persistent effects of adolescent morphine exposure in male rats

Epidemiological studies show the prevalence of opioid use, misuse and abuse in adolescents, which imposes social and economic accountability worldwide. Chronic opioid exposure, especially in adolescents, may have lasting effects on emotional behaviors that persist into adulthood. The current experiments were therefore designed to study the effects of sustained opioid exposure during adolescence on anxiety-like behaviors. Adolescent male Wistar rats underwent increasing doses of morphine for 10 days (PNDs 31-40). After that the open field test (OFT) and elevated plus maze (EPM) test were performed over a 4-week postmorphine treatment from adolescence to adulthood. Moreover, the weight of the animals was measured at these time points. We found that chronic adolescent morphine exposure reduces the weight gain during the period of morphine treatment and 4 weeks after that. It had no significant effect on the locomotor activity in the animals. Moreover, anxiolytic-like behavior was observed in the rats exposed to morphine during adolescence evaluated by OFT and EPM test. Thus, long-term exposure to morphine during adolescence has the profound potential of altering the anxiety-like behavior profile in the period from adolescence to adulthood. The maturation of the nervous system can be affected by drug abuse during the developmental window of adolescence and these effects may lead to behaviorally stable alterations.

The Extract of Corydalis yanhusuo Prevents Morphine Tolerance and Dependence

The opioid epidemic was triggered by an overprescription of opioid analgesics. In the treatment of chronic pain, repeated opioid administrations are required which ultimately lead to tolerance, physical dependence, and addiction. A possible way to overcome this conundrum consists of a co-medication that maintains the analgesic benefits of opioids while preventing their adverse liabilities. YHS, the extract of the plant Corydalis yanhusuo, has been used as analgesic in traditional Chinese medicine for centuries. More recently, it has been shown to promote analgesia in animal models of acute, inflammatory, and neuropathic pain. It acts, at least in part, by inhibiting the dopamine D2 receptor, suggesting that it may be advantageous to manage addiction. We first show that, in animals, YHS can increase the efficacy of morphine antinociceptive and, as such, decrease the need of the opioid. We then show that YHS, when coadministered with morphine, inhibits morphine tolerance, dependence, and addiction. Finally, we show that, in animals treated for several days with morphine, YHS can reverse morphine dependence and addiction. Together, these data indicate that YHS may be useful as a co-medication in morphine therapies to limit adverse morphine effects. Because YHS is readily available and safe, it may have an immediate positive impact to curb the opioid epidemic.

The self-serving benefits of being a good host: A role for our micro-inhabitants in shaping opioids' function

Opioids are highly efficacious in their ability to relieve pain, but they are liable for abuse, dependence, and addiction. Risk factors to develop opioid use disorders (OUD) include chronic stress, socio-environment, and preexisting major depressive disorders (MDD) and posttraumatic stress disorders (PTSD). Additionally, opioids reduce gut motility, induce loss of gut barrier function, and alter the composition of the trillions of microbes hosted in the gastrointestinal tract, known as the gut microbiota. The microbiota are significant contributors to the reciprocal communication between the central nervous system (CNS) and the gut, termed the gut-brain axis. They have strong influences on their host behaviors, including the ability to cope with stress, sociability, affect, mood, and anxiety. Thus, they are implicated in the etiology of MDD and PTSD. Here we review the latest studies demonstrating that intestinal flora can, directly and indirectly, by affecting sociability levels, responses to stress, and mental state, alter the responses to opioids. It suggests that microbiota can potentially be used to increase the resilience to develop analgesic tolerance and OUD.

Review of addiction risk potential associated with adolescent opioid use

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

The Negative Affect of Protracted Opioid Abstinence: Progress and Perspectives From Rodent Models

Opioid use disorder (OUD) is characterized by the development of a negative emotional state that develops after a history of long-term exposure to opioids. OUD represents a true challenge for treatment and relapse prevention. Human research has amply documented emotional disruption in individuals with an opioid substance use disorder, at both behavioral and brain activity levels; however, brain mechanisms underlying this particular facet of OUD are only partially understood. Animal research has been instrumental in elucidating genes and circuits that adapt to long-term opioid use or are modified by acute withdrawal, but research on long-term consequences of opioid exposure and their relevance to the negative affect of OUD remains scarce. In this article, we review the literature with a focus on two questions: 1) Do we have behavioral models in rodents, and what do they tell us? and 2) What do we know about the neuronal populations involved? Behavioral rodent models have successfully recapitulated behavioral signs of the OUD-related negative affect, and several neurotransmitter systems were identified (i.e., serotonin, dynorphin, corticotropin-releasing factor, oxytocin). Circuit mechanisms driving the negative mood of prolonged abstinence likely involve the 5 main reward-aversion brain centers (i.e., nucleus accumbens, bed nucleus of the stria terminalis, amygdala, habenula, and raphe nucleus), all of which express mu opioid receptors and directly respond to opioids. Future work will identify the nature of these mu opioid receptor-expressing neurons throughout reward-aversion networks, characterize their adapted phenotype in opioid abstinent animals, and hopefully position these primary events in the broader picture of mu opioid receptor-associated brain aversion networks.

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.

Sex-specific enhancement of palatability-driven feeding in adolescent rats

It has been hypothesized that brain development during adolescence perturbs reward processing in a way that may ultimately contribute to the risky decision making associated with this stage of life, particularly in young males. To investigate potential reward dysfunction during adolescence, Experiment 1 examined palatable fluid intake in rats as a function of age and sex. During a series of twice-weekly test sessions, non-food-deprived rats were given the opportunity to voluntarily consume a highly palatable sweetened condensed milk (SCM) solution. We found that adolescent male, but not female, rats exhibited a pronounced, transient increase in SCM intake (normalized by body weight) that was centered around puberty. Additionally, adult females consumed more SCM than adult males and adolescent females. Using a well-established analytical framework to parse the influences of reward palatability and satiety on the temporal structure of feeding behavior, we found that palatability-driven intake at the outset of the meal was significantly elevated in adolescent males, relative to the other groups. Furthermore, although we found that there were some group differences in the onset of satiety, they were unlikely to contribute to differences in intake. Experiment 2 confirmed that adolescent male rats exhibit elevated palatable fluid consumption, relative to adult males, even when a non-caloric saccharin solution was used as the taste stimulus, demonstrating that these results were unlikely to be related to age-related differences in metabolic need. These findings suggest that elevated palatable food intake during adolescence is sex specific and driven by a fundamental change in reward processing. As adolescent risk taking has been hypothesized as a potential result of hypersensitivity to and overvaluation of appetitive stimuli, individual differences in reward palatability may factor into individual differences in adolescent risky decision making.

Kappa opioid receptor antagonism and chronic antidepressant treatment have beneficial activities on social interactions and grooming deficits during heroin abstinence

Addiction is a chronic brain disorder that progressively invades all aspects of personal life. Accordingly, addiction to opiates severely impairs interpersonal relationships, and the resulting social isolation strongly contributes to the severity and chronicity of the disease. Uncovering new therapeutic strategies that address this aspect of addiction is therefore of great clinical relevance. We recently established a mouse model of heroin addiction in which, following chronic heroin exposure, 'abstinent' mice progressively develop a strong and long-lasting social avoidance phenotype. Here, we explored and compared the efficacy of two pharmacological interventions in this mouse model. Because clinical studies indicate some efficacy of antidepressants on emotional dysfunction associated with addiction, we first used a chronic 4-week treatment with the serotonergic antidepressant fluoxetine, as a reference. In addition, considering prodepressant effects recently associated with kappa opioid receptor signaling, we also investigated the kappa opioid receptor antagonist norbinaltorphimine (norBNI). Finally, we assessed whether fluoxetine and norBNI could reverse abstinence-induced social avoidance after it has established. Altogether, our results show that two interspaced norBNI administrations are sufficient both to prevent and to reverse social impairment in heroin abstinent animals. Therefore, kappa opioid receptor antagonism may represent a useful approach to alleviate social dysfunction in addicted individuals.

Long-lasting effects of adolescent oxycodone exposure on reward-related behavior and gene expression in mice

RATIONALE

Prescription opioid abuse and transition to heroin use are growing problems in the USA. However, the long-term consequences of adolescent prescription opioid abuse on subsequent drug use and affective-like behavior are unknown.

OBJECTIVES

This study aims to determine if adolescent exposure to oxycodone alters the rewarding effects of morphine, anxiety-like behavior, and reward-related gene expression later in adulthood.

METHODS

Adolescent male C57Bl/6 mice were exposed to oxycodone (3 mg/kg/day) via osmotic minipumps for 28 days. Following a 28-day withdrawal period, mice were tested in morphine-conditioned place preference paradigm (CPP), morphine sensitization, open field, marble burying, and forced swim (FST) tests. To determine if effects were specific to adolescent exposure, adult mice were exposed to oxycodone for 28 days and underwent 28 days of withdrawal prior to the same behavioral testing schedule. Expression of reward-related genes including dopamine receptor 1 (D1) and dopamine transporter (DAT) in the nucleus accumbens (NAc) and ventral tegmental area (VTA) was examined.

RESULTS

Adolescent oxycodone exposure significantly increased (300 %) response to morphine CPP during adulthood and significantly reduced D1 expression (30 %) in the NAc and DAT expression (75 %) in the VTA. Adult oxycodone exposure did not affect subsequent responses to morphine CPP. Oxycodone exposure did not affect the development of morphine sensitization or affective-like behaviors. Corticosterone response to a stressor (FST) was significantly reduced (65 %) in mice exposed to oxycodone during adolescence but not adulthood.

CONCLUSIONS

Adolescent oxycodone exposure enhances rewarding effects of morphine in adulthood with no effect on other affective-like behaviors.

Morphine-induced conditioned place preference and effects of morphine pre-exposure in adolescent and adult male C57BL/6J mice

RATIONALE

Given the increasing abuse of prescription opioids, particularly in adolescents, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the conditioned rewarding effects of morphine, without (experiment 1) and with morphine pre-exposure (experiment 2), in adolescent and adult male mice.

METHODS

Experiment 1: On each of three consecutive days, one of the two conditioning sessions was preceded by an injection of a particular dose of morphine (0.1, 0.32, 1, 3.2, 10, 32, or 100 mg/kg, intraperitoneal) and the other by saline; place preference was tested on day 4. Experiment 2: Mice received once daily injections of saline or a particular dose of morphine (17.8 or 56 mg/kg) for 4 days, and 3 days later, place conditioning with morphine (0.32, 1, 3.2, or 10 mg/kg) began.

RESULTS

In both experiments, morphine induced conditioned place preference along similar inverted U-shaped dose-response curves in adolescent and adult mice, with maximal effects between 0.32 and 10 mg/kg. Morphine pre-exposure did not sensitize morphine-induced conditioned place preference; instead, tolerance occurred, but only in adults. Adolescents were more sensitive than adults to morphine-induced locomotor stimulation. Response to novelty predicted the locomotor stimulating effects of morphine in adolescents, but not its rewarding effects.

CONCLUSIONS

The rewarding effects of morphine were similar in adolescent and adult mice but showed differential tolerance after morphine pre-exposure. Adolescents were more sensitive than adults to the acute locomotor stimulating effects of morphine, consistent with dopamine systems involved in locomotor activity being overactive during adolescence.

Effects of age, but not sex, on elevated startle during withdrawal from acute morphine in adolescent and adult rats

Investigations into animal models of drug withdrawal have largely found that emotional signs of withdrawal (e.g. anxiety, anhedonia, and aversion) in adolescents are experienced earlier and less severely than in their adult counterparts. The majority of these reports have examined withdrawal from ethanol or nicotine. To expand our knowledge about the emotional withdrawal state in adolescent rats, we used potentiation of the acoustic startle reflex after an acute dose of morphine (10 mg/kg, subcutaneously) as a measure of opiate withdrawal. Startle was measured at four time points after morphine injection (2, 3, 4, and 5 h) in 28-day-old and 90-day-old male and female rats. The results of this experiment revealed that peak potentiation of the startle reflex occurred at 3 h in the adolescent rats and at 5 h in the adult rats, and that the magnitude of withdrawal was larger in the adults. No sex differences were observed. Overall, these results affirm that, similar to withdrawal from ethanol and nicotine, opiate withdrawal signs are less severe in adolescent than in adult rats.

Dissociation of heroin-induced emotional dysfunction from psychomotor activation and physical dependence among inbred mouse strains

RATIONALE

Opiate addiction is a brain disorder emerging through repeated intoxication and withdrawal episodes. Epidemiological studies also indicate that chronic exposure to opiates may lead in susceptible individuals to the emergence of depressive symptoms, strongly contributing to the severity and chronicity of addiction. We recently established a mouse model of heroin abstinence, characterized by the development of depressive-like behaviors following chronic heroin exposure.

OBJECTIVES

While genetic factors regulating immediate behavioral responses to opiates have been largely investigated, little is known about their contribution to long-term emotional regulation during abstinence. Here, we compared locomotor stimulation and physical dependence induced by heroin exposure, as well as emotional dysfunction following abstinence, across mice strains with distinct genetic backgrounds.

METHODS

Mice from three inbred strains (C57BL/6J, Balb/cByJ, and 129S2/SvPas) were exposed to an escalating chronic heroin regimen (10-50 mg/kg). Independent cohorts were used to assess drug-induced locomotor activity during chronic treatment, naloxone-precipitated withdrawal at the end of chronic treatment, and emotional-like responses after a 4-week abstinence period.

RESULTS

Distinct behavioral profiles were observed across strains during heroin treatment, with no physical dependence and low locomotor stimulation in 129S2/SvPas. In addition, different behavioral impairments developed during abstinence across the three strains, with increased despair-like behavior in 129S2/SvPas and Balb/cByJ, and low sociability in 129S2/SvPas and C57BL/6J.

CONCLUSIONS

Our results indicate that depressive-like behaviors emerge during heroin abstinence, whatever the severity of immediate behavioral responses to the drug. In addition, inbred mouse strains will allow studying several aspects of mood-related deficits associated with addiction.

Distinct mu, delta, and kappa opioid receptor mechanisms underlie low sociability and depressive-like behaviors during heroin abstinence

Addiction is a chronic disorder involving recurring intoxication, withdrawal, and craving episodes. Escaping this vicious cycle requires maintenance of abstinence for extended periods of time and is a true challenge for addicted individuals. The emergence of depressive symptoms, including social withdrawal, is considered a main cause for relapse, but underlying mechanisms are poorly understood. Here we establish a mouse model of protracted abstinence to heroin, a major abused opiate, where both emotional and working memory deficits unfold. We show that delta and kappa opioid receptor (DOR and KOR, respectively) knockout mice develop either stronger or reduced emotional disruption during heroin abstinence, establishing DOR and KOR activities as protective and vulnerability factors, respectively, that regulate the severity of abstinence. Further, we found that chronic treatment with the antidepressant drug fluoxetine prevents emergence of low sociability, with no impact on the working memory deficit, implicating serotonergic mechanisms predominantly in emotional aspects of abstinence symptoms. Finally, targeting the main serotonergic brain structure, we show that gene knockout of mu opioid receptors (MORs) in the dorsal raphe nucleus (DRN) before heroin exposure abolishes the development of social withdrawal. This is the first result demonstrating that intermittent chronic MOR activation at the level of DRN represents an essential mechanism contributing to low sociability during protracted heroin abstinence. Altogether, our findings reveal crucial and distinct roles for all three opioid receptors in the development of emotional alterations that follow a history of heroin exposure and open the way towards understanding opioid system-mediated serotonin homeostasis in heroin abuse.

Effects of repeated exposure to morphine in adolescent and adult male C57BL/6J mice: age-dependent differences in locomotor stimulation, sensitization, and body weight loss

RATIONALE

Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the motor stimulating and ataxic effects of repeatedly administered morphine in adolescent, late adolescent, and adult mice.

METHODS

Mice were treated with saline or morphine (10-100 mg/kg, i.p.) once per day for 4 days, and morphine (3.2-56 mg/kg)-induced locomotion was assessed 3 days or 5 weeks later. Different mice were treated repeatedly with morphine and ataxia was measured.

RESULTS

Acute administration of morphine increased locomotion more in adolescents than in adults. Repeated morphine enhanced morphine-induced locomotion, assessed 3 days later, to a similar extent in each age group (minimum effective dose 17.8 mg/kg). This sensitization was still evident 5 weeks later when the adolescents had become adult, but was smaller and occurred at a higher dose (56 mg/kg). In animals treated repeatedly with morphine as adults, sensitization was no longer apparent 5 weeks later. Intermittent morphine was at least 10-fold less potent to produce body weight loss in adolescents than in adults. Repeated morphine did not alter morphine-induced ataxia at any age.

CONCLUSIONS

Compared with adults, adolescents were more sensitive to the acute locomotor stimulating effects of morphine and to its long-lasting locomotor sensitizing effects, consistent with overactivity of dopamine systems during adolescence. In contrast, adolescents were less sensitive than adults to body weight loss induced by intermittent morphine, an effect indicative of morphine withdrawal in adult rodents.

The multiple facets of opioid receptor function: implications for addiction

Addiction is characterized by altered reward processing, disrupted emotional responses and poor decision-making. Beyond a central role in drug reward, increasing evidence indicate that opioid receptors are broadly involved in all these processes. Recent studies establish the mu opioid receptor as a main player in social reward, which attracts increasing attention in psychiatric research. There is growing interest in blocking the kappa opioid receptor to prevent relapse, and alleviate the negative affect of withdrawal. The delta opioid receptor emerges as a potent mood enhancer, whose involvement in addiction is less clear. All three opioid receptors are likely implicated in addiction-depression comorbidity, and understanding of their roles in cognitive deficits associated to drug abuse is only beginning.

A role for the prefrontal cortex in heroin-seeking after forced abstinence by adult male rats but not adolescents

Adolescent drug abuse is hypothesized to increase the risk of drug addiction. Yet male rats that self-administer heroin as adolescents show attenuated drug-seeking after abstinence, compared with adults. Here we explore a role for neural activity in the medial prefrontal cortex (mPFC) in age-dependent heroin-seeking. Adolescent (35-day-old at start; adolescent-onset) and adult (86-day-old at start) male rats acquired lever-pressing maintained by heroin using a fixed ratio one reinforcement schedule (0.05 and 0.025 mg/kg per infusion). Following 12 days of forced abstinence, rats were tested for heroin-seeking over 1 h by measuring the number of lever presses on the active lever. Unbiased stereology was then used to estimate the number of Fos-ir(+) and Fos-ir(-) neurons in prelimbic and infralimbic mPFC. As before, adolescents and adults self-administered similar amounts of heroin, but subsequent heroin-seeking was attenuated in the younger rats. Similarly, the adolescent-onset group failed to show significant neural activation in the prelimbic or infralimbic mPFC during the heroin-seeking test, whereas the adult-onset heroin self-administration group showed two to six times more Fos-ir(+) neurons than their saline counterparts in both mPFC subregions. Finally, the overall number of neurons in the infralimbic cortex was greater in rats from the adolescent-onset groups than adults. The mPFC may thus have a key role in some age-dependent effects of heroin self-administration.

Attenuated effects of experimenter-administered heroin in adolescent vs. adult male rats: physical withdrawal and locomotor sensitization

OBJECTIVES

Early onset of heroin use during adolescence might increase chances of later drug addiction. Prior work from our laboratory suggests, however, that adolescent male rats are actually less sensitive than adults to some enduring effects of heroin self-administration. In the present study, we tested two likely correlates of sensitivity to behavioral reinforcement in rats: physical withdrawal and locomotor sensitization.

METHODS

Adolescent (35 days old at start) and adult (79 days old) male Sprague-Dawley rats were administered escalating doses of heroin, increasing from 1.0 to 8.0 mg/kg (i.p.) every 12 h, across 13 days. Somatic signs of spontaneous withdrawal were scored 12 and 24 h after the last injection, and then every 24 h for 5 days; locomotion was recorded concurrently. Challenge injections of heroin (1 mg/kg i.p.) were given at four points: as the first of the escalating doses (day 1), at days 7 and 13 during the escalating regimen, and after 12 days of forced abstinence. Body mass and food intake were measured throughout experimentation.

RESULTS

A heroin withdrawal syndrome was not observed among adolescents as it was among adults, including somatic signs as well as reduced locomotion, body mass, and food intake. On the other hand, heroin-induced locomotor sensitization did not differ across ages.

CONCLUSION

Reduced withdrawal is consistent with the attenuated reinforcing effects of heroin among adolescent male rats that we reported previously. Thus, it is possible that adolescent rats could reveal important neuroprotective factors for use in treatment of heroin dependence.

A history of chronic morphine exposure during adolescence increases despair-like behaviour and strain-dependently promotes sociability in abstinent adult mice

A crucial issue in treating opiate addiction, a chronic relapsing disorder, is to maintain a drug-free abstinent state. Prolonged abstinence associates with mood disorders, strongly contributing to relapse. In particular, substance use disorders occurring during adolescence predispose to depression later in adulthood. Using our established mouse model of opiate abstinence, we characterized emotional consequences into adulthood of morphine exposure during adolescence. Our results indicate that morphine treatment in adolescent mice has no effect on anxiety-like behaviours in adult mice, after abstinence. In contrast, morphine treatment during adolescence increases behavioural despair in adult mice. We also show that morphine exposure strain-dependently enhances sociability in adult mice. Additional research will be required to understand where and how morphine acts during brain maturation to affect emotional and social behaviours into adulthood.

Opioid receptors: distinct roles in mood disorders

The roles of opioid receptors in pain and addiction have been extensively studied, but their function in mood disorders has received less attention. Accumulating evidence from animal research reveals that mu, delta and kappa opioid receptors (MORs, DORs and KORs, respectively) exert highly distinct controls over mood-related processes. DOR agonists and KOR antagonists have promising antidepressant potential, whereas the risk-benefit ratio of currently available MOR agonists as antidepressants remains difficult to evaluate, in addition to their inherent abuse liability. To date, both human and animal studies have mainly examined MORs in the etiology of depressive disorders, and future studies will address DOR and KOR function in established and emerging neurobiological aspects of depression, including neurogenesis, neurodevelopment, and social behaviors.

Morphine alters the locomotor responses to a D2/D3 dopamine receptor agonist differentially in adolescent and adult mice

The D2-like dopamine receptors mediate the emotional/aversive state during morphine withdrawal. Given age-dependent differences in the affective responses to withdrawal, this study examined whether the response to dopamine receptor agonists is altered differentially across ages following morphine administration. Adolescent and adult mice were injected with morphine (twice daily, 10-40 mg/kg, s.c.) or saline for 6 days. Subsequently, they were examined for their locomotor response to quinpirole, a D2/D3 receptor agonist, and SKF 38393, a D1 receptor agonist. Quinpirole dose-dependently reduced locomotion in drug-naïve animals. Initial suppression was also observed in morphine-treated animals, but was followed by enhanced locomotion. Notably, this enhanced locomotion was markedly greater in adolescents than adults. Quinpirole-induced hypo-locomotion is thought to be mediated by the presynaptic D2Short receptors, whereas its activating effect is mediated by postsynaptic D2Long/D3 receptors. This suggests that following morphine administration, the postsynaptic, but not the presynaptic, dopaminergic signaling is differentially modulated across ages. This locomotor supersensitivity was not observed for SKF 38393, a D1 dopamine receptor agonist. The D2/D3 receptors are involved in the pathophysiology of many mental illnesses. Thus, this study offers a potential explanation for the increased psychiatric disorder co-morbidities when drug use begins during adolescence.

Social influences on morphine sensitization in adolescent rats

Given that social influences are among the strongest predictors of adolescents' drug use, this study examines the effects of social interactions on morphine sensitization in both adolescent and adult rats. Rats treated with morphine (twice daily, 6 days, 2.5-10 mg/kg, subcutaneously, s.c.) or saline were group-housed in two different conditions. Thus, four experimental groups were examined for each age group: (1) morphine-treated rats housed physically and visually separate from saline-injected rats ('morphine only'); (2) morphine-treated rats housed together with saline-injected rats ('morphine cage-mates'); (3) saline-injected rats housed together with morphine-treated rats ('saline cage-mates'); and (4) saline-injected rats housed physically and visually separate from morphine-treated rats ('saline only'). Starting 9 days following the last morphine injection, rats were individually examined once daily for 5 consecutive days for their locomotor response to 2.5 mg/kg of morphine. For both age groups, there were no significant differences in morphine-induced hyper-locomotion between saline cage-mates and saline only rats. Morphine only rats exhibited morphine locomotor sensitization as compared to both the saline only and saline cage-mates rats. Notably, a significant difference was observed between the adolescent morphine cage-mates and morphine only rats. The adolescent morphine cage-mates did not exhibit the enhanced locomotor response as compared to the saline only and saline cage-mate rats. A trend of reduced morphine locomotor sensitization was observed in the adult morphine cage-mates as compared to morphine only but it did not reach statistical significance. Thus, this study demonstrates social influences on morphine sensitization which are more prevalent in adolescents as compared to adults.

Morphine-induced motor stimulation, motor incoordination, and hypothermia in adolescent and adult mice

RATIONALE

Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

OBJECTIVES

This study compared the motor stimulating, ataxic, and hypothermic effects of morphine in adolescent, late adolescent, and adult mice. Plasma and brain levels of morphine were assessed to examine possible pharmacokinetic differences among the age groups.

METHODS

Locomotion was measured as occlusions of horizontal infrared light beams, ataxia as failing the horizontal wire test, body temperature by rectal probe, and morphine levels by HPLC-UV.

RESULTS

Morphine (3.2-56 mg/kg, i.p.) increased locomotion along an inverted U-shaped dose-response curve in adolescent, late adolescent, and adult male C57BL/6J mice. Its potency to stimulate locomotion was similar in all age groups. However, maximal stimulation was higher in adolescents than in late adolescents, and higher in late adolescents than in adults. In contrast, adolescents showed less ataxia than adults when given morphine (5.6-100 mg/kg, i.p.). The hypothermic effects of morphine did not differ among the age groups. Morphine levels, which peaked in plasma at 15 min and in brain at 45 min after i.p. injection, did not show age-related differences.

CONCLUSIONS

The finding that adolescents are not generally more sensitive to morphine than adults, but differ in their sensitivity to effects involving nigrostriatal/mesolimbic dopamine systems, is consistent with evidence of overactivity of these dopamine systems during adolescence relative to adulthood. The age-related differences observed here are unlikely due to pharmacokinetic factors.

Heroin self-administration and reinstatement of heroin-seeking in adolescent vs. adult male rats

RATIONALE

Heroin abuse is prevalent among teenagers, and early onset drug use might predict long-term drug dependence. However, adolescent sensitivity to drug reinforcement has not been explored thoroughly in animal models.

OBJECTIVES

This study aimed to compare intravenous (i.v.) self-administration of heroin, as well as extinction and reinstatement of heroin-seeking, in adolescent vs. adult male rats.

METHODS

Adolescent (35 days old at start) and adult (86 days old at start) male Sprague-Dawley rats spontaneously acquired lever pressing maintained by i.v. heroin infusions. In experiment 1, self-administration was tested on a fixed ratio 1 schedule of reinforcement (0.05 and 0.025 mg/kg per infusion), followed by within-session extinction and reinstatement tests after 1 or 12 days of abstinence. In experiment 2, self-administration was tested on a progressive ratio schedule (0.0125-0.1 mg/kg per infusion), followed 12 days later by a single test of extinction responding in the presence of cues.

RESULTS

In experiment 1, adolescent rats self-administered more heroin than adults. After 1 or 12 days of abstinence, adolescents exhibited less heroin-seeking than adults, although levels of heroin-seeking increased over abstinence period for both age groups. In experiment 2, adolescents and adults reached the same maximal response ratio (breakpoint), although adolescents earned more infusions when response requirements were low. For extinction responding in the presence of cues, heroin-seeking was similar across ages.

CONCLUSIONS

Lower levels of heroin-seeking suggest that younger rats are less sensitive than adults to some residual effects of heroin intake.

Age-dependent and strain-dependent influences of morphine on mouse social investigation behavior

Opioid-coded neural circuits play a substantial role in how individuals respond to drugs of abuse. Most individuals begin using such drugs during adolescence and within a social context. Several studies indicate that adolescent mice exhibit a heightened sensitivity to the effects of morphine, a prototypical opiate drug, but it is unclear whether these developmental differences are related to aspects of motivated behavior. Moreover, exposure to opioids within the rodent brain can alter the expression of social behavior, yet little is known about whether this relationship changes as a function of development or genetic variation. In this study, we conducted a series of experiments to characterize the relationship between genetic background, adolescent development and morphine-induced changes in mouse social investigation (SI). At two time points during adolescent development [postnatal days (PD) 25 and 45], social interactions of test mice of the gregarious C57BL/6J (B6) strain were more tolerant to the suppressive effects of morphine [effective dose 50 (ED50)=0.97 mg/kg and 2.17 mg/kg morphine, respectively] than test mice from the less social BALB/cJ (BALB) strain (ED50=0.61 mg/kg and 0.91 mg/kg morphine, respectively). By contrast, this strain-dependent difference was not evident among adult mice on PD 90 (ED50=1.07 mg/kg and 1.41 mg/kg morphine for BALB and B6 mice, respectively). An additional experiment showed that the ability of morphine to alter social responsiveness was not directly related to drug-induced changes in locomotor behavior. Finally, administration of morphine to stimulus mice on PD 25 reduced social investigation of test mice only when individuals were from the B6 genetic background. Overall, these results indicate that alterations in endogenous opioid systems are related to changes in SI that occur during adolescence, and that morphine administration may mimic rewarding aspects of social encounter.

Social influences on plasma testosterone levels in morphine withdrawn adolescent mice and their drug-naïve cage-mates

Opioid administration in males results in opioid-induced androgen deficiency which persists throughout the treatment. In adults, this quickly reverses once opioid administration is suspended. However, less is known about the duration of the effect following drug discontinuation in adolescents. Given the significant implications to sexual maturation in adolescent males, this study examined plasma testosterone levels in both morphine withdrawn mice and their drug-naïve (saline-injected) cage-mates as compared to drug-naïve mice housed physically and visually separate from the morphine-treated mice ('saline only'). Consistent with the literature, plasma testosterone levels in morphine withdrawn adults were reduced on withdrawal day 1 (WD1) and returned to baseline levels by WD9. No significant effects were observed in their saline cage-mates. In the adolescents, no significant differences were observed on WD1 between the morphine withdrawn mice, their saline cage-mates, and the saline only mice - all of which had significantly lower plasma testosterone levels than adults. By WD9, testosterone levels in the saline only adolescent mice had reached adult levels. Notably, plasma testosterone levels were reduced in both the morphine withdrawn adolescent mice and their saline cage-mates, as compared to saline only mice. The effect was not a drug effect per se, given that reduced plasma testosterone levels were not observed in individually housed morphine withdrawn mice. Moreover, our results also suggest that these social effects are not solely explained by stress. These results have numerous implications to the short term and long term health of both adolescents requiring pain management and of adolescent drug addicts.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Sex differences in affective response to opioid withdrawal during adolescence

Drug withdrawal is suggested to play a role in precipitating mood disorders in individuals with familial predisposition. Age-related differences in affective responses to withdrawal might explain the increased risk of mental illnesses when drug use begins during adolescence. Recently we observed that, in contrast to adult male mice, adolescent males exhibited a decrease in immobility in the forced swim test on the third day of withdrawal, as compared with controls. Thus, the present study examined forced swim test behaviors of adolescent female mice during opioid withdrawal. Similar to the male study, adolescent female mice were injected with two morphine regimens which differed in dosage. Three and nine days following discontinuation of morphine administration, forced swim test immobility time and locomotion were evaluated. In contrast to males, which exhibited a decrease in immobility, no significant differences in immobility were observed in female adolescents undergoing withdrawal as compared with saline-injected controls. This sex difference in forced swim test behaviors was not due to changes in overall motor activity, since differences in locomotion were not observed in either male or female adolescent mice. Thus, this study demonstrates sex differences in forced swim test behavior during opioid withdrawal. Forced swim test behaviors are classically used to evaluate mood in rodents, thus this study suggests that opioid withdrawal might affect mood differentially across sexes.

Social influences on morphine sensitization in adolescent females

We recently observed that social interactions influence morphine responsiveness in adolescent males. Given sex-related differences in both social interactions and responses to morphine, the present study examines social influences on morphine sensitization in adolescent female mice. Four experimental groups were examined: (1) morphine-treated mice (twice daily, 10-40 mg/kg, s.c.) housed physically and visually separated from saline-treated mice ('morphine only'), (2) morphine-treated mice housed together with saline-treated mice ('morphine cage-mates (of saline)'), (3) saline-treated mice housed together with morphine-treated mice ('saline cage-mates (of morphine)'), and (4) saline-treated mice housed physically and visually separated from morphine-treated mice ('saline only'). Following the treatment period, mice were tested individually for their locomotor response to 20 mg/kg morphine (s.c.). There were no significant differences in morphine-induced hyper-locomotion between saline only and saline cage-mates (of morphine) female adolescent mice. Notably, morphine only mice exhibited significantly greater morphine sensitization as compared to morphine cage-mates (of saline). Thus, this study demonstrates social influences on morphine sensitization in adolescent females. Drug use during early adolescence is a key predictor of later drug abuse and dependence during adulthood. Thus, understanding the specific vulnerabilities to drug use in this age group may represent a first step in helping develop more effective treatment programs.

Socially induced morphine pseudosensitization in adolescent mice

Given that social influences are among the strongest predictors of adolescents' drug use, this study examined the effect of social interaction on morphine-induced hyperlocomotion in both adolescent and adult mice. Three experimental groups of adolescent and adult male mice were examined (i) morphine-treated mice (twice daily, 10-40 mg/kg, subcutaneous), (ii) saline-injected mice housed together with the morphine-treated mice ('saline cage-mates'), and (iii) saline-injected mice housed physically and visually separated from the morphine-treated mice ('saline alone'). After the treatment period, mice were tested individually for their locomotor response to 10 mg/kg morphine (subcutaneous). Adolescent saline cage-mates, though administered morphine for the very first time, exhibited an enhanced hyperlocomotion response similar to the locomotor sensitization response exhibited by the morphine-treated mice. This was not observed in adults. In adults, there were no significant differences in morphine-induced hyperlocomotion between saline alone and saline cage-mates. As expected, morphine-treated adults and adolescents both exhibited locomotor sensitization. These results show a vulnerability to social influences in adolescent mice, which does not exist in adult mice.