Increased motivation to eat in opiate-withdrawn mice

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 Specifically, morphine-withdrawn rats displayed insensitivity to reward devaluation both when relying on prior action-outcome learning and when given direct feedback about the consequences of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.

Long-term effects of THC exposure on reward learning and motivated behavior in adolescent and adult male rats

RATIONALE

The endocannabinoid system makes critical contributions to reward processing, motivation, and behavioral control. Repeated exposure to THC or other cannabinoid drugs can cause persistent adaptions in the endocannabinoid system and associated neural circuitry. It remains unclear how such treatments affect the way rewards are processed and pursued.

OBJECTIVE AND METHODS

We examined if repeated THC exposure (5 mg/kg/day for 14 days) during adolescence or adulthood led to long-term changes in rats' capacity to flexibly encode and use action-outcome associations for goal-directed decision making. Effects on hedonic feeding and progressive ratio responding were also assessed.

RESULTS

THC exposure had no effect on rats' ability to flexibly select actions following reward devaluation. However, instrumental contingency degradation learning, which involves avoiding an action that is unnecessary for reward delivery, was augmented in rats with a history of adult but not adolescent THC exposure. THC-exposed rats also displayed more vigorous instrumental behavior in this study, suggesting a motivational enhancement. A separate experiment found that while THC exposure had no effect on hedonic feeding behavior, it increased rats' willingness to work for food on a progressive ratio schedule, an effect that was more pronounced when THC was administered to adults. Adolescent and adult THC exposure had opposing effects on the CB1 receptor dependence of progressive ratio performance, decreasing and increasing sensitivity to rimonabant-induced behavioral suppression, respectively.

CONCLUSIONS

Our findings reveal that exposure to a translationally relevant THC exposure regimen induces long-lasting, age-dependent alterations in cognitive and motivational processes that regulate the pursuit of rewards.

Gut microbiota suppress feeding induced by palatable foods

Feeding behaviors depend on intrinsic and extrinsic factors including genetics, food palatability, and the environment.^1,2,3,4,5 The gut microbiota is a major environmental contributor to host physiology and impacts feeding behavior.^{6,7,8,9,10,11,12} Here, we explored the hypothesis that gut bacteria influence behavioral responses to palatable foods and reveal that antibiotic depletion (ABX) of the gut microbiota in mice results in overconsumption of several palatable foods with conserved effects on feeding dynamics. Gut microbiota restoration via fecal transplant into ABX mice is sufficient to rescue overconsumption of high-sucrose pellets. Operant conditioning tests found that ABX mice exhibit intensified motivation to pursue high-sucrose rewards. Accordingly, neuronal activity in mesolimbic brain regions, which have been linked with motivation and reward-seeking behavior,³ was elevated in ABX mice after consumption of high-sucrose pellets. Differential antibiotic treatment and functional microbiota transplants identified specific gut bacterial taxa from the family S24-7 and the genus Lactobacillus whose abundances associate with suppression of high-sucrose pellet consumption. Indeed, colonization of mice with S24-7 and Lactobacillus johnsonii was sufficient to reduce overconsumption of high-sucrose pellets in an antibiotic-induced model of binge eating. These results demonstrate that extrinsic influences from the gut microbiota can suppress the behavioral response toward palatable foods in mice.

Morphine reduces the interest for natural rewards

RATIONALE

Alongside a pathological, excessive, motivation for substances of abuse, substance use disorder (SUD) patients often show a dramatic loss of interest for naturally rewarding activities, such as positive peer social interaction and food intake. Yet, pre-clinical evidence of the latter SUD features remains scarce and inconsistent.

OBJECTIVES

In the current study, we investigated the effect of non-rewarding and rewarding doses of morphine upon social behaviour, motivation for and intake of palatable food, in male and female C57BL/6J mice.

METHODS

First, the rewarding effects of two relatively low morphine doses (1.25 and 2.5 mg/kg) were assessed using a newly established single substance administration/conditioning trial conditioned place preference (CPP) paradigm. Then, morphine (1.25 and 2.5 mg/kg) effects upon social behaviour, motivation for and intake of palatable food were examined by the three-chamber (3-CH), an operant behaviour and a palatable food preference test, respectively.

RESULTS

Morphine (2.5 mg/kg) induced CPP in both male and female mice, whereas morphine (1.25 mg/kg) induced CPP only in female mice. Both morphine doses (1.25 and 2.5 mg/kg) reduced sociability, motivation for and intake of palatable food in male and female mice, independently of cognitive function or locomotor activity.

CONCLUSIONS

Female mice were more sensitive than male mice to the rewarding effects of morphine. Moreover, both a non-rewarding and a rewarding dose of morphine impaired the interest for naturally rewarding activities, indicating that brain reward systems might be more sensitive to the deleterious than to the rewarding effects of substances of abuse.

The CRF1 receptor mediates social behavior deficits induced by opiate withdrawal

Poor sociability and aggressive behavior are key clinical features of opioid use disorders. The corticotropin-releasing factor (CRF) system may mediate behavioral effects of substances of abuse but its implication in substance-induced social behavior deficits and outward-directed hostility remains largely unknown. CRF signaling is mediated by two receptor types, termed CRF₁ and CRF₂ . The present study aimed at understanding the role for the CRF₁ receptor in social and aggressive behavior induced by withdrawal from repeated opiate administration. Thus, wild-type (CRF₁ +/+), CRF₁ receptor heterozygous (CRF₁ +/-), and null mutant (CRF₁ -/-) female and male mice were treated with saline or escalating doses of morphine (20-100 mg/kg, i.p.) during six consecutive days and tested in the three-chamber task for sociability (i.e., preference for an unfamiliar same-sex conspecific vs. an object) 7 days after the last administration. Moreover, aggressive biting behavior toward the unfamiliar conspecific was assessed during the three-chamber test. Opiate withdrawal disrupted sociability in CRF₁ +/+ and CRF₁ +/-, but not in CRF₁ -/-, female mice, without affecting aggressive biting behavior in any genotype. In contrast, opiate withdrawal did not affect sociability but increased aggressive biting behavior in male mice, independently of CRF₁ receptor-deficiency. Nevertheless, in opiate-withdrawn CRF₁ +/+, but not CRF₁ +/- and CRF₁ -/-, male mice, sociability directly correlated with aggressive biting behavior, suggesting a role for the CRF₁ receptor in hostility-linked social approach. These findings demonstrate the implication of the CRF₁ receptor in social behavior deficits associated with repeated opiate administration and withdrawal, revealing a new potential target for the treatment of opioid use disorders.

Long-lasting pseudo-social aggressive behavior in opiate-withdrawn mice

Poor sociability and aggressive behavior are major clinical features of opiate use disorders that may contribute to the establishment and maintenance of these harmful diseases. The present study investigated the long-term effects of chronic morphine administration and withdrawal upon social and aggressive behavior as well as the interrelationship between these two behaviors. Thus, social behavior was measured in C57BL/6J male mice 7, 21, 35 and 49 days after cessation of escalating morphine doses (20-100 mg/kg, i.p.) administered during 6 consecutive days, using the three-chamber task for sociability (i.e., preference for an unfamiliar conspecific vs. an object) and social novelty preference (i.e., preference for a novel vs. a familiar conspecific). Moreover, aggressive biting behavior towards an unfamiliar conspecific was assessed throughout the three-chamber tests. Opiate withdrawal increased both social approach and aggressive biting behavior. Moreover, in morphine-withdrawn, but not in control, mice social approach and aggressive behavior followed a similar time-course and positively correlated one with each other, suggesting that social interest was mainly driven by aggressiveness. Aggressive biting behavior was still elevated 49 days after the last morphine administration, revealing that opiate withdrawal is followed by long-lasting aggressiveness. Throughout, opiate withdrawal did not affect social novelty preference, ruling out a role for olfactory or social discrimination dysfunction in the elevated social approach and aggressive behavior. The present findings of very long-lasting aggressive behavior and aggression-driven social approach in opiate-withdrawn mice might help understanding the behavioral and brain underpinnings of poor sociability and aggressiveness commonly observed in opiate use disorders patients.

The effects of acute stress on consummatory and motivational responses for sucrose in rats after long-term withdrawal from morphine

BACKGROUND

Negative affective states, e.g., anhedonia, may be linked to the long-lasting motivational processes associated with relapse. Here, this study investigated whether, and how, anhedonic states are influenced by stressful events that contribute to craving and relapse.

METHODS

All male rats were pretreated with a binge-like morphine paradigm for five days. After 12 to 16 days of withdrawal, rats were subjected to a one-hour free consumption test or three operant tasks with increasing cost/benefit ratio, i.e., fixed ratio 1 (FR1), progressive ratio (PR), and PR-punishment procedure of reinforcement, with sucrose solutions of three concentrations (4%, 15%, and 60%) as rewards. The consumption and operant responses under FR1 and PR procedures were measured following exposure to acute foot-shock stress (intermittent foot shock, 0.5 mA × 0.5 s × 10 min; mean intershock interval, 40 s), and the operant responses for 60% sucrose solution under PR-punishment procedure was measured following a forced-swim stress (5 minutes).

RESULT

Foot-shock stress increased water consumption in a subpopulation of rats and decreased consumption of sucrose solutions, while it did not influence the operant responses for sucrose solutions under either FR1 or PR procedure. The forced-swim stress reduced operant responses for 60% sucrose solution under PR-punishment procedure, but did not influence responding for 60% sucrose solution under PR procedure. In addition, the forced-swim stress also elevated anxiety level of rats in an open area test.

CONCLUSIONS

Acute stress induced hedonic but not motivational deficit for sucrose reward in protracted drug-abstinent animals. Additional negative emotional states besides anhedonia were evoked by acute stress.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

MnSOD mediated by HSV vectors in the periaqueductal gray suppresses morphine withdrawal in rats

Morphine appears to be the most active metabolite of heroin; therefore, the effects of morphine are important in understanding the ramifications of heroin abuse. Opioid physical dependence (withdrawal response) may have very long-lasting effects on the motivation for reward, including the incubation of cue-induced drug-seeking behavior. However, the exact mechanisms of morphine withdrawal (MW) are not clear yet, and its treatment remains elusive. Periaqueductal gray (PAG) is one of the important sites in the pathogenesis of MW. Here, we used recombinant herpes simplex virus (HSV) vectors that encode the sod2 gene expressing manganese superoxide dismutase (MnSOD) to evaluate its therapeutic potential in MW. Microinjection of HSV vectors expressing MnSOD into the PAG reduced the MW syndrome. MnSOD vectors suppressed the upregulated mitochondrial superoxide, and endoplasmic reticulum stress markers (glucose-related protein 78 (GRP78) and activating transcription factor 6 alpha (ATF6α)) in the PAG induced by MW. Immunostaining showed that mitochondrial superoxide, GRP78 and ATF6α were colocalized with neuronal nuclei (a neuronal-specific marker), suggesting that they are located in the neurons in the PAG. These results suggest that overexpression of MnSOD by HSV vectors may relieve opioid dependence. This study may provide a novel therapeutic approach to morphine physical withdrawal response.

The consummatory and motivational behaviors for natural rewards following long-term withdrawal from morphine: no anhedonia but persistent maladaptive behaviors for high-value rewards

RATIONALE

The negative affective state, e.g., anhedonia, emerges after abstinence from abused drugs may be linked to the motivational processes of drug craving and relapse. Although anhedonia diminishes over time with drug abstinence, it is not yet rather explicit whether anhedonia exists or not following protracted withdrawal.

OBJECTIVES

The behavioral responses to natural rewards were examined after 2 to 3 weeks withdrawal from morphine. Male rats were pretreated with either a binge-like morphine paradigm or daily saline injection for 5 days. The consummatory and motivational behaviors for three natural rewards (sucrose solutions 4, 15, and 60%, social stimulus: male rat, and sexual stimulus: estrous female rat) were examined under varied testing conditions.

RESULTS

The morphine-withdrawn rats significantly increased their intake of 15% sucrose solution during the 1-h consumption test and their operant responding for 15% sucrose solution under a progressive ratio (PR) schedule of reinforcement. When obtaining a reinforcer was associated with a 0.5 mA foot shock under a PR-punishment schedule, the morphine-withdrawn rats showed a higher performance for 60% sucrose solution. Meanwhile, the morphine-withdrawn rats displayed a higher motivation to sexual stimulus during the free-approach test and more approaching behaviors towards sexual stimulus in a conflict-based approach test (concurrent presence of reward and aversive stimulus).

CONCLUSIONS

No anhedonia-like behavior but sensitized behaviors for natural rewards were found after long-term morphine withdrawal. Notably, the morphine-withdrawn rats displayed persistent motivated behaviors for high-value rewards (60% sucrose and sexual stimulus) in the conflict tests suggesting impairments in inhibitory control in morphine-treated rats.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Phosphorylation of the N-methyl-d-aspartate receptor is increased in the nucleus accumbens during both acute and extended morphine withdrawal

Opioid withdrawal causes a dysphoric state that can lead to complications in pain patients and can propagate use in drug abusers and addicts. Opioid withdrawal changes the activity of neurons in the nucleus accumbens, an area rich in both opioid-binding mu opioid receptors and glutamate-binding NMDA receptors. Because the accumbens is an area important for reward and aversion, plastic changes in this area during withdrawal could alter future behaviors in animals. We discovered an increase in phosphorylation of serine 897 in the NR1 subunit of the NMDA receptor (pNR1) during acute morphine withdrawal. This serine can be phosphorylated by protein kinase A (PKA) and dephosphorylated by calcineurin. We next demonstrated that this increased pNR1 change is associated with an increase in NR1 surface expression. NR1 surface expression and pNR1 levels during acute withdrawal were both reduced by the NMDA receptor antagonist MK-801 (dizocilpine hydrogen maleate) and the PKA inhibitor H-89(N-[2-[[3-(4-bromophenyl)-2-propenyl]amino]ethyl]-5-isoquinolinesulfonamide dihydrochloride hydrate). We also found that pNR1 levels remained high after an extended morphine withdrawal period of 2 months, correlated with reward-seeking behavior for palatable food, and were associated with a decrease in accumbal calcineurin levels. These data suggest that NR1 phosphorylation changes during the acute withdrawal phase can be long lasting and may reflect a permanent change in NMDA receptors in the accumbens. These altered NMDA receptors in the accumbens could play a role in long-lasting behaviors associated with reward and opioid use.

CRF2 Receptor Deficiency Eliminates the Long-Lasting Vulnerability of Motivational States Induced by Opiate Withdrawal

Vulnerability to stressful life events is a hallmark of drug dependence that may persist long after cessation of drug intake and dramatically fuel key clinical features, such as deregulated up-shifted motivational states and craving. However, to date, no effective therapy is available for reducing vulnerability to stressful events in former drug users and drug-dependent patients, mostly because of poor knowledge of the mechanisms underlying it. In this study, we report that genetic inactivation of the stress-responsive corticotropin-releasing factor receptor-2 (CRF2-/-) completely eliminates the reemergence of increased nonrewarded nose-pokes, reflecting up-shifted motivational states, triggered by ethological environmental stressors long after cessation of morphine administration in mice. Accordingly, CRF2 receptor deficiency completely abolishes the increase in biomarkers of synthesis of major brain motivational substrates, such as ventral tegmental area (VTA) dopamine (DA) and amygdala γ-aminobutyric acid (GABA) systems, associated with the stress-induced reemergence of up-shifted motivational states long after opiate withdrawal. Nevertheless, neither CRF2 receptor deficiency nor long-term opiate withdrawal affects amygdala CRF or hypothalamus CRF expression, indicating preserved brain stress-coping systems. Moreover, CRF2 receptor deficiency does not influence the locomotor or the anxiety-like effect of long-term opiate withdrawal. Thus, the present results reveal an essential and specific role for the CRF2 receptor in the stress-induced reemergence of up-shifted motivational states and related alterations in brain motivational systems long after opiate withdrawal. These findings suggest new strategies for the treatment of the severe and long-lasting vulnerability that inexorably follows drug withdrawal and hinder drug abstinence.

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.

Lighting the darkness of addiction: can phototherapy enhance contingency-management-based treatment of substance-related and addictive disorders?

Maladaptive patterns of substance use are serious social problems. Both pharmacological and nonpharmacological treatments are available, but nondrug options may be preferable because they avoid the expense and adverse side effects of psychotropic medication. Contingency management (CM) and nondrug social and recreational activities (NDSRAs) are based on operant conditioning principles and seek to decrease substance use by means of nondrug rewards. However, their efficacy may be hindered where brain reward circuitry is dysfunctional. Research shows that substance abuse biases neural reward systems in favor of drug-induced highs, while disrupting circadian-based rhythms. Circadian systems also have been found to influence human reward pathways. Possibly, a bidirectional relationship exists between circadian disturbance and substance abuse effects. If so, repair of abnormal circadian rhythms might help normalize reward response in substance abusers, with positive effects on CM or NDSRA treatment outcomes. Phototherapy has been effective in repairing circadian rhythms in persons with seasonal affective disorder and other chronobiological conditions. This article proposes that it similarly may repair circadian response in substance abusers, thereby normalizing brain reward systems. By doing so, it would enhance the efficacy of CM and NDSRA therapies and may also help prevent relapse. Given its low cost and ease of administration, phototherapy seems a promising avenue to pursue.

CRF₂ receptor-deficiency reduces recognition memory deficits and vulnerability to stress induced by cocaine withdrawal

Psychostimulant drug abuse, dependence and withdrawal are associated with cognitive dysfunction and impact stress-sensitive systems. The corticotropin-releasing factor (CRF) system orchestrates stress responses via CRF1 and CRF2 receptors and is implicated in substance use disorders. However, CRF2 role in psychostimulant drug-induced cognitive dysfunction remains to be elucidated. In the present study, wild-type and CRF2-/- mice are injected with cocaine and memory assessed by the novel object recognition (NOR) task throughout relatively long periods of drug withdrawal. Following recovery from the drug-induced memory deficits, the mice are stressed prior to the NOR task and brain gene expression evaluated by in situ hybridization. Cocaine impairs NOR memory in wild-type and CRF2-/- mice. However, following cocaine withdrawal NOR memory deficits last less time in CRF2-/- than in wild-type mice. Furthermore, a relatively mild stressor induces the re-emergence of NOR deficits in long-term cocaine-withdrawn wild-type but not CRF2-/- mice. Cocaine-withdrawn mice show a genotype-independent higher c-fos expression in the NOR memory-relevant perirhinal cortex than drug-naïve mice. However neither genotype nor drug withdrawal affect the expression of tyrosine hydroxylase in the ventral tegmental area or the locus coeruleus and CRF in the central nucleus of the amygdala or the paraventricular nucleus of the hypothalamus, brain regions implicated in stress and drug responses. These data indicate a new role for the CRF2 receptor in cognitive deficits induced by cocaine withdrawal, both as regards to their duration and their re-induction by stress. Interestingly, prototypical brain stress systems other than CRF do not appear to be involved.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

CRF1 receptor-deficiency induces anxiety-like vulnerability to cocaine

RATIONALE

The intake of psychostimulant drugs may induce cognitive dysfunction and negative affective-like states, and is associated with increased activity of stress-responsive systems. The corticotropin-releasing factor (CRF) system mediates neuroendocrine, behavioural and autonomic responses to stressors, and might be implicated in substance-related disorders. CRF signalling is mediated by two receptor types, named CRF1 and CRF2.

OBJECTIVES

The present study aims to elucidate the role for the CRF1 receptor in cognitive dysfunction and anxiety-like states induced by cocaine.

RESULTS

The genetic inactivation of the CRF1 receptor (CRF1+/- and CRF1-/-) does not influence recognition memory in drug-naïve mice, as assessed by the novel object recognition (NOR) test. Moreover, the chronic administration of escalating doses of cocaine (5-20 mg/kg, i.p.) induces NOR deficits, which are unaffected by CRF1 receptor-deficiency. However, the same drug regimen reveals an anxiety-like vulnerability to cocaine in CRF1-/- but not in wild-type or CRF1+/- mice, as assessed by the elevated plus maze test.

CONCLUSIONS

The present findings indicate dissociation of cognitive dysfunction and anxiety-like states induced by cocaine. Moreover, they unravel a novel mechanism of vulnerability to psychostimulant drugs.

Delay discounting of oral morphine and sweetened juice rewards in dependent and non-dependent rats

RATIONALE

Opioid-dependent humans are reported to show accelerated delay discounting of opioid rewards when compared to monetary rewards. It has been suggested that this may reflect a difference in discounting of consumable and non-consumable goods not specific to dependent individuals. Here, we evaluate the discounting of similar morphine and non-morphine oral rewards in dependent and non-dependent rats

METHODS

We first tested the analgesic and rewarding effects of our morphine solution. In a second experiment, we assigned rats randomly to either dependent or non-dependent groups that, 30 min after daily testing, received 30 mg/kg subcutaneous dose of morphine, or saline, respectively. Delay discounting of drug-free reward was examined prior to initiation of the dosing regimen. We tested discounting of the morphine reward in half the rats and retested the discounting of the drug-free reward in the other half. All tests were run 22.5 h after the daily maintenance dose.

RESULTS

Rats preferred the morphine cocktail to the drug-free solution and consumed enough to induce significant analgesia. The control quinine solution did not produce these effects. Dependent rats discounted morphine rewards more rapidly than before dependence and when compared to discounting drug-free rewards. In non-dependent rats both reward types were discounted similarly.

CONCLUSIONS

These results show that morphine dependence increases impulsiveness specifically towards a drug reward while morphine experience without dependence does not.

Increased impulsive choice for saccharin during PCP withdrawal in female monkeys: influence of menstrual cycle phase

BACKGROUND

In previous studies with male and female rhesus monkeys, withdrawal of access to oral phencyclidine (PCP) self-administration reduced responding for food under a high fixed-ratio (FR) schedule more in males than females, and with a delay discounting (DD) task with saccharin (SACC) as the reinforcer impulsive choice for SACC increased during PCP withdrawal more in males than females.

OBJECTIVES

The goal of the present study was to examine the effect of PCP (0.25 or 0.5 mg/ml) withdrawal on impulsive choice for SACC in females during the follicular and luteal phases of the menstrual cycle.

MATERIALS AND METHODS

In component 1, PCP and water were available from two drinking spouts for 1.5 h sessions under concurrent FR 16 schedules. In component 2, a SACC solution was available for 45 min under a DD schedule. Monkeys had a choice of one immediate SACC delivery (0.6 ml) or six delayed SACC deliveries, and the delay was increased by 1 s after a response on the delayed lever and decreased by 1 s after a response on the immediate lever. There was then a 10-day water substitution phase, or PCP withdrawal, that occurred during the mid-follicular phase (days 7-11) or the late luteal phase (days 24-28) of the menstrual cycle. Access to PCP and concurrent water was then restored, and the PCP withdrawal procedure was repeated over several follicular and luteal menstrual phases.

RESULTS

PCP deliveries were higher during the luteal (vs follicular) phase. Impulsive choice was greater during the luteal (vs follicular) phase during withdrawal of the higher PCP concentration.

CONCLUSIONS

PCP withdrawal was associated with elevated impulsive choice for SACC, especially in the luteal (vs follicular) phase of the menstrual cycle in female monkeys.

The corticotropin-releasing factor receptor-2 mediates the motivational effect of opiate withdrawal

Altered motivational processes are key features of drug dependence and withdrawal, yet their neural mechanisms remain largely unknown. The present study shows that genetic disruption of the corticotropin-releasing factor receptor-2 (CRF₂-/-) does not impair motivation for palatable food in drug-naïve mice. However, CRF₂ receptor-deficiency effectively reduces the increase in palatable food-driven motivation induced by opiate withdrawal. Indeed, both in male and female wild-type mice, withdrawal from escalating morphine doses (20-100 mg/kg) induces a dramatic and relatively long-lasting (6 days) increase in palatable food-driven operant behavior under a progressive ratio (PR) schedule of reinforcement. In contrast, either male or female morphine-withdrawn CRF₂-/- mice show smaller and shorter (2 days) increases in motivation than wild-type mice. Nevertheless, CRF₂ receptor-deficiency does not impair the ability to discriminate reinforced behavior prior to, during the partial opiate withdrawal periods occurring between morphine injections and following drug discontinuation, indicating preserved cognitive function. Moreover, CRF₂ receptor-deficiency does not affect the ambulatory or body weight effects of intermittent morphine injections and withdrawal. These results provide initial evidence of a gender-independent and specific role for the CRF₂ receptor in the motivational effects of opiate withdrawal.

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.