Measuring the incentive value of escalating doses of heroin in heroin-dependent Fischer rats during acute spontaneous withdrawal

The effect of chronic stress on the immunogenicity and immunoprotection of the M6-TT vaccine in female mice

Active vaccination is an effective therapeutic option to reduce the reinforcing effects of opioids. Several studies showed that chronic stress affects the immune system decreasing the efficiency of some vaccines. Heroin withdrawal is a stressor and it is a stage in which the patient who abuses heroin is vulnerable to stress affects the immune response and consequently its immunoprotective capacity, then, the objective was to determine the effect of heroin-withdrawal and heroin-withdrawal plus immobilization, on the immune (immunogenicity) and protective response (behavioral response) of morphine-6-hemisuccinate-tetanus toxoid (M6-TT) vaccine in animals of two inbred mice strains with different sensitivity to drug-opioid and stress. Female BALB/c and C57Bl/6 inbred mice were immunized with the M6-TT. A solid-phase antibody-capture ELISA was used to monitor antibody titer responses after each booster dose in vaccinated animals. During the vaccination period, the animals were subjected to two different stress conditions: drug-withdrawal (DW) and immobilization (IMM). The study used tail-flick testing to evaluate the heroin-induced antinociceptive effects. Additionally, heroin-induced locomotor activity was evaluated. Stress decreased the heroin-specific antibody titer generated by the M6-TT vaccine in the two inbred mouse strains evaluated. In the two stress conditions, the antibody titer was not able to decrease the heroin-induced antinociceptive effects and locomotor activity. These findings suggest that stress decreases the production of antibodies and the immunoprotective capacity of the M6-TT vaccine. This observation is important to determine the efficacy of active vaccination as a potential therapy for patients with opioid drug use disorder, since these patients during drug-withdrawal present stress disorders, which could affect the efficacy of therapy such as active vaccination.

Activation of GLP-1 receptors attenuates oxycodone taking and seeking without compromising the antinociceptive effects of oxycodone in rats

Despite the effectiveness of current medications to treat opioid use disorder, there is still a high rate of relapse following detoxification. Thus, there is critical need for innovative studies aimed at identifying novel neurobiological mechanisms that could be targeted to treat opioid use disorder. A growing body of preclinical evidence indicates that glucagon-like peptide-1 (GLP-1) receptor agonists reduce drug reinforcement. However, the efficacy of GLP-1 receptor agonists in attenuating opioid-mediated behaviors has not been thoroughly investigated. Using recently established models of opioid-taking and -seeking behaviors, we showed that systemic administration of the GLP-1 receptor agonist exendin-4 reduced oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats. We also identified behaviorally selective doses of exendin-4 that reduced opioid-taking and -seeking behaviors and did not produce adverse feeding effects in oxycodone-experienced rats. To identify a central site of action, we showed that systemic exendin-4 penetrated the brain and bound putative GLP-1 receptors on dopamine D1 receptor- and dopamine D2 receptor-expressing medium spiny neurons in the nucleus accumbens shell. Consistent with our systemic studies, infusions of exendin-4 directly into the accumbens shell attenuated oxycodone self-administration and the reinstatement of oxycodone-seeking behavior without affecting ad libitum food intake. Finally, exendin-4 did not alter the analgesic effects of oxycodone, suggesting that activation of GLP-1 receptors attenuated opioid reinforcement without reducing the thermal antinociceptive effects of oxycodone. Taken together, these findings suggest that GLP-1 receptors could serve as potential molecular targets for pharmacotherapies aimed at reducing opioid use disorder.

Sex differences in the expression of morphine withdrawal symptoms and associated activity in the tail of the ventral tegmental area

Recent studies, in male rodents, have begun to elucidate a role for the GABAergic neurons in the tail of the ventral tegmental area (tVTA) in morphine withdrawal. To date, the mechanisms underlying morphine withdrawal have been studied almost exclusively in male animals. As a result, there is a considerable gap in our current understanding of the processes underlying sex differences in morphine withdrawal behaviors and its effects on cellular activity in the tVTA in females. The purpose of the present study was to investigate the influence of sex on the expression and duration of spontaneous somatic morphine withdrawal syndrome, and to characterize the relationship between spontaneous somatic withdrawal symptoms and cellular activation (measured as phosphorylated CREB; pCREB), in the GABAergic tVTA in male and female rats. Morphine-dependent adult male and female Long Evans rats underwent 72 h of spontaneous withdrawal, and somatic withdrawal symptoms were assessed every 12 h. Male morphine-dependent rats expressed more severe symptoms during the early phases of withdrawal compared to females. Although, females demonstrated lower overall symptom severity, their symptoms persisted for a longer period of time, thus demonstrating higher withdrawal-symptom severity than males during late withdrawal. pCREB activity in the tVTA was elevated in morphine-withdrawn rats and was positively correlated with the severity of withdrawal symptoms. These results demonstrate sex differences in the timing of the expression of somatic withdrawal. Our data add to the growing body of evidence demonstrating a role for the tVTA in morphine withdrawal and begin to establish a sex-dependent behavioral and molecular profile within this brain region.

Effects of heroin on rat prosocial behavior

Opioid use disorders are characterized in part by impairments in social functioning. Previous research indicates that laboratory rats, which are frequently used as animal models of addiction-related behaviors, are capable of prosocial behavior. For example, under normal conditions, when a 'free' rat is placed in the vicinity of rat trapped in a plastic restrainer, the rat will release or 'rescue' the other rat from confinement. The present study was conducted to determine the effects of heroin on prosocial behavior in rats. For 2 weeks, rats were given the opportunity to rescue their cagemate from confinement, and the occurrence of and latency to free the confined rat was recorded. After baseline rescuing behavior was established, rats were randomly selected to self-administer heroin (0.06 mg/kg/infusion i.v.) or sucrose pellets (orally) for 14 days. Next, rats were retested for rescuing behavior once daily for 3 days, during which they were provided with a choice between freeing the trapped cagemate and continuing to self-administer their respective reinforcer. Our results indicate that rats self-administering sucrose continued to rescue their cagemate, whereas heroin rats chose to self-administer heroin and not rescue their cagemate. These findings suggest that rats with a history of heroin self-administration show deficits in prosocial behavior, consistent with specific diagnostic criteria for opioid use disorder. Behavioral paradigms providing a choice between engaging in prosocial behavior and continuing drug use may be useful in modeling and investigating the neural basis of social functioning deficits in opioid addiction.

Expression of brain-derived neurotrophic factors, neurotrophin-3, and neurotrophin-4 in the nucleus accumbens during heroin dependency and withdrawal

Neurotrophins, brain-derived neurotrophic factors (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), have been implicated in the modulation of heroin dependency. This study was designed to explore the expression alterations of BDNF, NT-3, and NT-4 in the context of heroin dependence and withdrawal in the rat nucleus accumbens (NAc). Heroin dependence was induced by a progressive intraperitoneal treatment of heroin. The results showed that the expression levels of BDNF and NT-4 were significantly decreased in the NAc of rats with heroin addiction in comparison with the control group, whereas there was a significant increase in BDNF and NT-4 expressions in the groups of rats with both naloxone-induced and spontaneous withdrawal. Moreover, NT-3 expression was markedly increased in the NAc of rats with heroin addiction and spontaneous withdrawal in comparison with the control group, but decreased in the NAc of rats with naloxone-induced withdrawal. These results indicated that chronic administration of heroin results in the alterations of BDNF, NT-3, and NT-4 expressions in the rat NAc. BDNF, NT-3, and NT-4 may play a critical role in the development of heroin dependency and withdrawal.

Adolescent oxycodone self administration alters subsequent oxycodone-induced conditioned place preference and anti-nociceptive effect in C57BL/6J mice in adulthood

Adolescent and young adult abuse of short-acting MOP-r agonists such as oxycodone is a pressing public health issue. Few preclinical studies have examined how adolescent exposure to oxycodone impacts its effects in the transition to adulthood.

OBJECTIVE

To determine in mice how chronic adolescent oxycodone self-administration (SA) affects subsequent oxycodone-induced conditioned place preference (CPP), locomotor activity, and anti-nociception once mice reach early adulthood.

METHODS

Adolescent C57BL/6J male mice (4 weeks old, n = 6-11) and adult mice (10 weeks old, n = 6-10) were surgically implanted with indwelling jugular catheters. Mice then acquired oxycodone self-administration (14 consecutive 2-hr daily sessions; 0.25 mg/kg/infusion) followed by a 14-day drug-free (withdrawal) period in home cage. After the 14-day drug-free period, mice underwent a 10-day oxycodone CPP procedure (0, 1, 3, 10 mg/kg i.p.) or were tested for acute oxycodone-induced antinociception in the hot plate assay (3.35, 5, 7.5 mg/kg i.p.).

RESULTS

Mice that self-administered oxycodone during adolescence exhibited greater oxycodone-induced CPP (at the 3 mg/kg dose) than their yoked saline controls and mice that self-administered oxycodone during adulthood. Oxycodone dose-dependently increased locomotor activity, but sensitization developed only to the 3 mg/kg in the mice that underwent oxycodone self-administration as adolescents. Mice that self-administered oxycodone as adolescents decreased in the anti-nociceptive effects of oxycodone in one dose (5 mg/kg), whereas animals that self-administered oxycodone as adults did not show this effect.

CONCLUSION

Chronic adolescent oxycodone self-administration led to increased oxycodone-induced CPP (primarily 1 and 3 mg/kg, i.p.) and reduced antinociceptive effect of oxycodone (5 mg/kg, i.p.) in adulthood.

Cerebellar neuronal apoptosis in heroin-addicted rats and its molecular mechanism

BACKGROUND

The overall objective of this study was to investigate neuronal apoptosis and expression of apoptosis related proteins (c-jun, cytc and Bax) in the cerebellum of rates with heroin addiction.

MATERIAL/METHODS

40 adult male Sprague-Dawley rats which weighing 200-220 g were randomly divided into 5 groups (n = 8 per group): control group, 10-day heroin-addicted group, 20-day heroin-addicted group, 30-day heroin-addicted group and 40-day heroin-addicted group. Rats in the control group were treated with normal saline. Rats in the addiction groups (20 d, 30 d, 40 d) were all given subcutaneous injection with heroin for 15 days to induce heroin addiction. After injected with heroin for 15 days, rats were treated with naloxone at a dose of 5 mg/kg to induce abstinence for 30 mins to examine the addiction of rats. They were then continued to be treated with heroin for another 10 days, 20 days, 30 days, and 40 days respectively to establish heroin-addicted models. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) was employed to identify apoptotic cells [6]. Immunohistochemistry and Western blot assay were also used in the study to examine the protein expressions of c-jun, cytc and Bax in the cerebellum.

RESULTS

Compared with the control group, the proportion of apoptotic neurons increased significantly in the heroin addiction groups (10 d, 20 d, 30 d, 40 d) (P < 0.05), also accompanied by markedly increased expressions of c-jun, cytc and Bax (P < 0.05) depending on doses of heroin in the cerebellum. Thus, the significant differences were observed in heroin addiction groups (10 d, 20 d,30 d, 40 d) and control group (P < 0.05).

CONCLUSION

Long-term use of heroin may induce neuronal apoptosis in the cerebellum by raising the expressions of pro-apoptotic c-jun, cytc and Bax, which might be one of mechanisms underlying the heroin-induced cerebellum neuronal damage.

Self administration of oxycodone by adolescent and adult mice affects striatal neurotransmitter receptor gene expression

Illicit use of prescription opioid analgesics (e.g., oxycodone) in adolescence is a pressing public health issue. Our goal was to determine whether oxycodone self administration differentially affects striatal neurotransmitter receptor gene expression in the dorsal striatum of adolescent compared to adult C57BL/6J mice. Groups of adolescent mice (4 weeks old, n=12) and of adult mice (11 weeks old, n=11) underwent surgery during which a catheter was implanted into their jugular veins. After recovering from surgery, mice self administered oxycodone (0.25 mg/kg/infusion) 2 h/day for 14 consecutive days or served as yoked saline controls. Mice were sacrificed within 1h after the last self-administration session and the dorsal striatum was isolated for mRNA analysis. Gene expression was analyzed with real time PCR using a commercially available neurotransmitter receptor PCR array containing 84 genes. We found that adolescent mice self administered less oxycodone than adult mice over the 14 days. Monoamine oxidase A (Maoa) and neuropeptide Y receptor 5 mRNA levels were lower in adolescent mice than in adult mice without oxycodone exposure. Oxycodone self administration increased Maoa mRNA levels compared to controls in both age groups. There was a positive correlation of the amount of oxycodone self administered in the last session or across 14 sessions with Maoa mRNA levels. Gastrin-releasing peptide receptor mRNA showed a significant Drug × Age interaction, with point-wise significance. More genes in the dorsal striatum of adolescents (19) changed in response to oxycodone self administration compared to controls than in adult (4) mice. Overall, this study demonstrates that repeated oxycodone self administration alters neurotransmitter receptors gene expression in the dorsal striatum of adolescent and adult mice.

Suppression of hypothalamic-pituitary-adrenal axis by acute heroin challenge in rats during acute and chronic withdrawal from chronic heroin administration

It is known that heroin dependence and withdrawal are associated with changes in the hypothalamic-pituitary-adrenal (HPA) axis. The objective of these studies in rats was to systematically investigate the level of HPA activity and response to a heroin challenge at two time points during heroin withdrawal, and to characterize the expression of associated stress-related genes 30 min after each heroin challenge. Rats received chronic (10-day) intermittent escalating-dose heroin administration (3 × 2.5 mg/kg/day on day 1; 3 × 20 mg/kg/day by day 10). Hormonal and neurochemical assessments were performed in acute (12 h after last heroin injection) and chronic (10 days after the last injection) withdrawal. Both plasma ACTH and corticosterone levels were elevated during acute withdrawal, and heroin challenge at 20 mg/kg (the last dose of chronic escalation) at this time point attenuated this HPA hyperactivity. During chronic withdrawal, HPA hormonal levels returned to baseline, but heroin challenge at 5 mg/kg decreased ACTH levels. In contrast, this dose of heroin challenge stimulated the HPA axis in heroin naïve rats. In the anterior pituitary, pro-opiomelanocortin (POMC) mRNA levels were increased during acute withdrawal and retuned to control levels after chronic withdrawal. In the medial hypothalamus, however, the POMC mRNA levels were decreased during acute withdrawal, and increased after chronic withdrawal. Our results suggest a long-lasting change in HPA abnormal responsivity during chronic heroin withdrawal.

Effects of post-training heroin and d-amphetamine on consolidation of win-stay learning and fear conditioning

It has been proposed that the reinforcing properties of drugs of abuse are due, in part, to their ability to enhance memory consolidation. To test this hypothesis, heroin (0.03-3 mg/kg, SC) and d-amphetamine (0.5-2 mg/kg, SC) were administered to male Sprague-Dawley rats immediately or 4 h after training on win-stay and fear conditioning tasks. On the win-stay, immediate post-training administration of lower doses of heroin and d-amphetamine enhanced acquisition, and probe tests further revealed that these drugs enhanced different aspects of learning. Higher doses had no effect or impaired performance, particularly when administered repeatedly. On fear conditioning, the memory-enhancing effects of immediate post-training administration of lower heroin and d-amphetamine doses were revealed only when a single tone-shock pairing procedure was employed. Therefore, under appropriate experimental conditions, mildly stimulatory doses of heroin and d-amphetamine enhanced the acquisition of tasks thought to involve different types of learning. These results support the hypothesis that one of the ways in which drugs of abuse such as opiates and psychomotor stimulants reinforce behavior is by enhancing memory consolidation processes.

Proopiomelanocortin (POMC) expression and conditioned place aversion during protracted withdrawal from chronic intermittent escalating-dose heroin in POMC-EGFP promoter transgenic mice

In heroin-dependent individuals, the drive to avoid or ameliorate the negative affective/emotional state associated with the discontinuation of heroin contributes to the chronic relapsing nature of the disease. Here, we investigate changes in proopiomelanocortin (POMC) expression at three time points across an extended period of heroin withdrawal in a clinically relevant rodent model of addiction using conditioned place aversion (CPA) in POMC-EGFP (POMC-enhanced green fluorescent protein) bacterial artificial chromosome (BAC) transgenic mice. Neurons expressing POMC-EGFP were found in the medial nucleus of the amygdala (MeA), basomedial amygdala (BMA) and dentate gyrus of hippocampus (DG), as well as the arcuate nucleus of hypothalamus (ARC). Heroin-treated mice displayed robust CPA after acute spontaneous withdrawal (12h), which persisted across the extended (14days) withdrawal period. After 12-h withdrawal, heroin-treated mice showed lower signal intensity of POMC-EGFP-positive cells in the ARC, higher levels of POMC mRNA in the amygdala but lower levels in the hippocampus than saline controls. After 7-d withdrawal, heroin-treated mice showed fewer POMC-EGFP-positive cells in the MeA and lower POMC mRNA in the amygdala than saline controls. After extended (14days) withdrawal, heroin-treated mice showed more POMC-EGFP-positive cells in BMA and DG, increased intensity of POMC-EGFP signal in DG, and higher POMC mRNA levels in the hippocampus compared to controls. Our results show dynamic changes in POMC in hypothalamic and extra-hypothalamic regions that may contribute to the negative affective/emotional state of heroin withdrawal shown by CPA from acute to extended periods of heroin withdrawal.

Tolerance and sensitization to chronic escalating dose heroin following extended withdrawal in Fischer rats: possible role of mu-opioid receptors

RATIONALE/OBJECTIVES

Heroin addiction is characterized by recurrent cycles of drug use, abstinence, and relapse. It is likely that neurobiological changes during chronic heroin exposure persist across withdrawal and impact behavioral responses to re-exposure. We hypothesized that, after extended withdrawal, heroin-withdrawn rats would express behavioral tolerance and/or sensitization in response to heroin re-exposure and that these responses might be associated with altered mu-opioid receptor (MOPr) activity.

METHODS

Male Fischer rats were exposed chronically to escalating doses of heroin (7.5-75 mg/kg/day), experienced acute spontaneous withdrawal and extended (10-day) abstinence, and were re-exposed chronically to heroin. Homecage behaviors and locomotor activity in response to heroin, as well as somatic withdrawal signs, were recorded. Separate groups of rats were sacrificed after extended abstinence and MOPr expression and G-protein coupling were analyzed using [(3)H]DAMGO and [(35)S]GTPγS assays.

RESULTS

The depth of behavioral stupor was lower during the initial days of heroin re-exposure compared to the initial days of the first exposure period. Behavioral responses (e.g., stereotypy) and locomotion were elevated in response to heroin re-exposure at low doses. Rats conditioned for heroin place preference during the chronic re-exposure period expressed heroin preference during acute withdrawal; this preference was stronger than rats conditioned during chronic heroin exposure that followed chronic saline and injection-free periods. Extended withdrawal was associated with increased MOPr expression in the caudate-putamen and frontal and cingulate cortices. No changes in G-protein coupling were identified.

CONCLUSIONS

Aspects of tolerance/sensitization to heroin are present even after extended abstinence and may be associated with altered MOPr density.

The effects of acute and chronic steady state methadone on memory retrieval in rats

RATIONALE

Although widely prescribed to treat opioid addiction, little is known about the possible side effects of methadone on memory functions.

OBJECTIVES

The aim of this study is to compare the effects of acute and chronic methadone on memory retrieval in rats and to explore the selectivity of possible deficits.

METHODS

Administration of acute (0, 1.25, 2.5, and 5 mg/kg SC) and chronic steady state methadone (0, 10, 30, and 55 mg/kg/day SC by osmotic mini-pump) was tested on recall of three different types of information: stimulus-reward (10-arm parallel maze), stimulus-response (8-arm radial maze), and stimulus-stimulus (Barnes maze). Acute and steady state methadone doses were also compared on tests of locomotor activity and reactivity to aversive stimuli (i.e., swimming and acoustic startle).

RESULTS

In the stimulus-reward task, acute methadone impaired performance as a result of severe depression of locomotion. This motor deficit, however, was modulated by the motivational valence of environmental stimulation. In fact, acute methadone did not eliminate forced swimming behavior. In the stimulus-response and stimulus-stimulus tasks, accuracy was impaired independently of direct motor deficits, but rats were hyper-reactive to aversive stimulation and, in fact, 5 mg/kg enhanced acoustic startle. Importantly, chronic steady state methadone did not affect accuracy of memory retrieval, did not depress motor or swimming activity, and did not change startle reactivity.

CONCLUSION

Only acute methadone impaired accuracy and/or performance on three tests of memory retrieval. These findings in rats suggest that memory deficits reported in methadone-maintained individuals may not be directly attributable to methadone.

Dose escalation and dose preference in extended-access heroin self-administration in Lewis and Fischer rats

RATIONALE

A genetic component may be involved in different stages of the progression of drug addiction. Heroin users escalate unit doses and frequency of self-administration events over time. Rats that self-administer drugs of abuse over extended sessions escalate the amount of drug infused over days.

OBJECTIVES

Using a recently developed model of extended-access self-administration allowing for subject-controlled dose escalation of the unit dose, thus potentially escalating the unit dose and number of infusions, we compared for the first time two genetically different inbred rat strains, Fischer and Lewis.

METHODS

Extended (18 h/day) self-administration lasted for 14 days. Rats had access to two active levers associated with two different unit doses of heroin. If a rat showed preference for the higher unit dose, then the available doses were escalated in the following session. Four heroin unit doses were available (20, 50, 125, 250 μg/kg per infusion).

RESULTS

Fischer rats did not escalate the unit dose of heroin self-administered; daily amount of heroin administered remained low, with a mean daily intake of 1.27 ± 0.22 mg/kg per session. In marked contrast, Lewis rats escalated the total daily amount of heroin self-administered from 3.94 ± 0.82 mg/kg on day 1 to 8.95 ± 2.2 mg/kg on day 14; almost half of the subjects preferred a higher heroin dose than Fischer rats.

CONCLUSION

These data are consistent with the hypothesis that Lewis rats are prone to opiate taking and escalation, and are in agreement with our previous data obtained with cocaine.