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

Influence of Age and Genetic Background on Ethanol Intake and Behavioral Response Following Ethanol Consumption and During Abstinence in a Model of Alcohol Abuse

Genetic background and age at first exposure have been identified as critical variables that contribute to individual vulnerability to drug addiction. Evidence shows that genetic factors may account for 40–70% of the variance in liability to addiction. Alcohol consumption by young people, especially in the form of binge-drinking, is becoming an alarming phenomenon predictive of future problems with drinking. Thus, the literature indicates the need to better understand the influence of age and genetic background on the development of alcohol dependence. To this aim, the inbred rat strains Lewis (LEW, addiction prone) and Fischer 344 (F344, addiction resistant) were used as a model of genetic vulnerability to addiction and compared with the outbred strain Sprague-Dawley (SD) in a two-bottle choice paradigm as a model of alcohol abuse. During a 9-week period, adolescent and adult male rats of the three strains were intermittently exposed to ethanol (20%) and water during three 24-h sessions/week. Adult and adolescent SD and LEW rats escalated their alcohol intake over time reaching at stable levels, while F344 rats did not escalate their intake, regardless of age at drinking onset. Among adolescents, only F344 rats consumed a higher total amount of ethanol than adults, although only SD and LEW rats escalated their intake. Adult LEW rats, albeit having a lower ethanol consumption as compared to SD rats but greater than F344, showed a more compulsive intake, consuming higher amounts of ethanol during the first hour of exposure, reaching a higher degree of ethanol preference when start drinking as adolescents. Behavioral analysis during the first hour of ethanol consumption revealed significant strain differences, among which noticeable the lack of sedative effect in the LEW strain, at variance with F344 and SD strains, and highest indices of withdrawal (most notable jumping) in LEW rats during the first hour of abstinence days. The present results underscore the importance of individual genetic background and early onset of alcohol use in the progression toward abuse and development of alcohol addiction.

Neuroadaptations and TGF-β signaling: emerging role in models of neuropsychiatric disorders

Neuropsychiatric diseases are manifested by maladaptive behavioral plasticity. Despite the greater understanding of the neuroplasticity underlying behavioral adaptations, pinpointing precise cellular mediators has remained elusive. This has stymied the development of pharmacological interventions to combat these disorders both at the level of progression and relapse. With increased knowledge on the putative role of the transforming growth factor (TGF- β) family of proteins in mediating diverse neuroadaptations, the influence of TGF-β signaling in regulating maladaptive cellular and behavioral plasticity underlying neuropsychiatric disorders is being increasingly elucidated. The current review is focused on what is currently known about the TGF-β signaling in the central nervous system in mediating cellular and behavioral plasticity related to neuropsychiatric manifestations.

Nalmefene, a mu opioid receptor antagonist/kappa opioid receptor partial agonist, potentiates cocaine motivation but not intake with extended access self-administration in adult male mice

The mu opioid receptor antagonist/kappa opioid receptor (KOR) partial agonist nalmefene (NMF), a close structural analog of naltrexone (NTX), has been shown to reduce cocaine reward in preclinical models. Given the greater KOR potency and improved bioavailability compared to NTX, NMF may be a promising pharmacotherapeutic for cocaine use disorder (CUD). Here we examine the effects of NMF pretreatment on chronic daily extended access (4h) cocaine intravenous self-administration (IVSA) in adult male C57Bl/6J mice.

METHODS

separate groups of mice had daily 4h cocaine IVSA sessions (0.25 or 0.5 mg/kg/inf, FR1) for 14 days. Starting on day 8, mice were pretreated with NMF (0, 1, or 10 mg/kg) 30m before each session. A separate group of mice acquired cocaine IVSA [seven days FR1 then four FR3 of 4h daily sessions (0.5 mg/kg/inf)] prior to a single progressive ratio 3 session to examine the effect of 1 mg/kg NMF on cocaine motivation.

RESULTS

No significant effect of NMF pretreatment on cocaine intake was observed. Acute pretreatment of 1 mg/kg NMF significantly potentiated cocaine motivation as measured by progressive ratio breakpoint.

CONCLUSIONS

NMF did not significantly attenuate cocaine intake and increased motivation for cocaine suggesting that NMF may not be suitable for non-abstinent CUD patients. Further research is needed with KOR selective partial or full agonists to determine their effect on cocaine reinforcement.

Persistence of Operant Responding for Food After Prior Cocaine Exposure in Fischer 344 But Not Lewis Rats

BACKGROUND AND OBJECTIVES

Chronic cocaine exposure has differential neural effects in Fischer 344 (F344) vs Lewis inbred rats that may explain strain-dependent differences during acquisition vs maintenance of cocaine self-administration. We assessed whether prior cocaine exposure alters operant responding for food across various phases (acquisition, maintenance, extinction, spontaneous recovery, reinitiation) in these strains.

METHODS

Lewis and F344 rats (N = 12) were administered three cocaine (15 mg/kg) or saline injections at hourly intervals for 3 consecutive days. Beginning the next day for 24 days, rats had access to operant chambers in which one lever depression resulted in the delivery of a food pellet. Then, four extinction sessions were conducted in which food was no longer available, but other stimulus conditions remained the same. After a 2-day break, spontaneous recovery was assessed over four sessions. Food delivery was then restored for 3 days to test reinitiation followed by a progressive ratio session.

RESULTS

Lewis rats acquired the operant faster than F344 rats. F344 rats showed lower maintenance rates than Lewis rats but higher spontaneous recovery responding. Cocaine exposure caused persistence of responding during extinction in F344 but not Lewis rats. All groups reinitiated responding when food was available again and did not differ in final ratios completed under the progressive ratio schedule.

DISCUSSION AND CONCLUSIONS

That prior cocaine exposure led to persistence of responding in F344 rats during extinction may reflect heightened contextual conditioning that interferes with the ability to extinguish responding.

SCIENTIFIC SIGNIFICANCE

Results have implications for the genetic contribution to relapse-like behaviors. (Am J Addict 2021;00:00-00).

Effects of ultrasound-guided erector spinae plane block on postoperative analgesia and plasma cytokine levels after uniportal VATS: a prospective randomized controlled trial

PURPOSE

Although uniportal video-assisted thoracoscopic surgery (VATS) has been widely used, the associated postoperative pain is still severe. Currently, a variety of regional anesthesia methods have been used to relieve postoperative pain. In our study, we wanted to evaluate the effectiveness of ultrasound-guided erector spinae plane block (ESPB) as a postoperative analgesia after uniportal VATS.

METHODS

Eighty patients scheduled to undergo uniportal VATS were randomly divided into Group ESP and Group C. In Group ESP, the patients underwent ultrasound-guided ESPB under general anesthesia before surgery, while Group C was set as blank control group without ESPB. The primary outcome was the sufentanil dose within 24 h after surgery. The secondary outcomes mainly included postoperative pain scores at 2, 4, 8, and 24 h evaluated using a numeric rating scale (NRS), intraoperative opioid dosage, levels of inflammatory cytokines, including tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-10 (IL-10) in the plasma, side effect profile, and length of postoperative hospital stay.

RESULTS

Postoperative sufentanil consumption (32.5 ± 6.3 μg vs. 42.8 ± 7.6 μg, P < 0.001) was significantly lower in Group ESP than in Group C. Intraoperative sufentanil consumption was significantly lower in Group ESP than in Group C (P < 0.001). The postoperative NRS score and levels of inflammatory cytokines were significantly lower in Group ESP than in Group C (P < 0.05).

CONCLUSIONS

Ultrasound-guided ESPB decreased the consumption of sufentanil both postoperatively and intraoperatively for patients undergoing uniportal VATS and appeared to be an effective treatment option.

Role of genetic background in the effects of adolescent nicotine exposure on mesolimbic dopamine transmission

Smoking during adolescence may increase the likelihood to develop nicotine dependence and to abuse other drugs such as cocaine. Despite great efforts to understand underlying neurobiological mechanisms of this progression, less attention has been paid to the role of genetic factors. Here, we investigated the influence of both genetic background and age at first nicotine exposure in the long-lasting effects on mesolimbic dopamine transmission including the increased cocaine-rewarding effect. Mid-adolescent and adult rats of inbred strains Lewis (addiction prone) and Fischer 344 (addiction resistant) were administered nicotine (0.4 mg/kg) or vehicle once daily for 5 days. Changes in dopamine transmission were investigated by in vivo microdialysis and electrophysiology after 30 days of withdrawal, whereas changes in cocaine-rewarding effect were assessed via conditioned place preference paradigm. Nicotine pre-exposure differentially changed mesolimbic dopamine transmission depending on strain and age of pre-exposure. A potentiation of dopamine response to nicotine was observed in nucleus accumbens (NAc) core of both strains and age groups, whereas dopamine response in NAc shell was enhanced exclusively in Lewis rats exposed to nicotine during adolescence. A similar response was observed following cocaine challenge at adulthood. Changes in VTA dopamine cell population and activity were observed only in adolescent nicotine-pretreated Lewis rats, which also showed an increased cocaine-rewarding effect at adulthood. These results highlight the influence of genetic background in the long-lasting effects of nicotine exposure and suggest that exposure during adolescence might increase nicotine and cocaine-rewarding properties in genetically vulnerable individuals, thereby facilitating progression toward dependence.

Opposite regulation of conditioned place preference and intravenous drug self-administration in rodent models: Motivational and non-motivational examples

Although developed from a common antecedent, conditioned place preference (CPP) and intravenous drug self-administration (SA) represent different behavioral paradigms, each with strong face validity. The field has treated results from these studies largely interchangeably; however, there is considerable evidence of opposite modulation of CPP vs. SA. This review outlines four manipulations that differentially affect CPP and SA based on alterations of motivation. These examples are contrasted with one example of differential CPP and SA results that can be explained by simple parallel shifts in dose-response functions. The final two examples have yet to be classified as motivation-based or parallel shifts. Important aspects, including motivation, volitional control of drug administration, reward, and the role of cues are discussed. One major conclusion of this paper is that explanations for apparent discrepancies between CPP and SA require full dose effect functions and assessment of PR breakpoints. Overall, this manuscript offers a more nuanced insight into how CPP and SA can be used to study different aspects of substance use disorders.

Effects of Kappa opioid receptor blockade by LY2444296 HCl, a selective short-acting antagonist, during chronic extended access cocaine self-administration and re-exposure in rat

RATIONALE

Cocaine addiction is a chronic brain disease characterized by compulsive drug intake and dysregulation of brain reward systems. Few preclinical studies have modeled the natural longitudinal course of cocaine addiction. Extended access self-administration protocols are powerful tools for modeling the advanced stages of addiction; however, few studies have duration of drug access longer than 12 h/session, potentially limiting their construct validity. Identification of changes in cocaine intake patterns during the development of addictive-like states may allow better treatments for vulnerable subjects. The kappa opioid receptor (KOPr) system has been implicated in the neurobiological regulation of addictive states as well as mood and stress disorders, with selective KOPr antagonists proposed as possible pharmacotherapeutic agents. Chronic cocaine exposure increases the expression of KOPr and its endogenous agonists, the dynorphins, in several brain areas in rodents.

OBJECTIVES

To examine the behavioral pattern of intake during chronic (14 days) 18 h intravenous cocaine self-administration (0.5 mg/kg/infusion) and the effect of a novel short-acting KOPr antagonist LY2444296 HCl (3 mg/kg) administered during sessions 8 to 14 of chronic 18 h/day cocaine self-administration and prior to a single re-exposure session after 2 cocaine-free withdrawal days.

RESULTS

Both daily and hourly cocaine intake patterns changed over 14 days of 18 h self-administration. LY pretreatment affected the pattern of self-administration across the second week of extended access cocaine self-administration and prevented the increase in cocaine intake during re-exposure.

CONCLUSIONS

Overall, the KOPr antagonist attenuated escalated cocaine consumption in a rat model of extended access cocaine self-administration.

Region-specific changes in markers of neuroplasticity revealed in HIV-1 transgenic rats by low-dose methamphetamine

Methamphetamine abuse co-occurring with HIV infection presents neuropathology in brain regions that mediate reward and motivation. A neuronal signaling cascade altered acutely by meth and some HIV-1 proteins is the mitogen-activated protein kinase (MAPK) pathway. It remains unknown if chronic co-exposure to meth and HIV-1 proteins converge on MAPK in vivo. To make this determination, we studied young adult Fischer 344 HIV-1 transgenic (Tg) and non-Tg rats that self-administered meth (0.02-0.04 mg/kg/0.05 ml iv infusion, 2 h/day for 21 days) and their saline-yoked controls. One day following the operant task, rats were killed. Brain regions involved in reward-motivation [i.e., nucleus accumbens (NA) and ventral pallidum (VP)], were assayed for a MAPK cascade protein, extracellular signal-regulated kinase (ERK), and a downstream transcription factor, ΔFosB. In the NA, activated (phosphorylated; p) ERK-to-ERK ratio (pERK/ERK) was increased in meth-exposed Tg rats versus saline Tg controls, and versus meth non-Tg rats. ΔFosB was increased in meth Tg rats versus saline and meth non-Tg rats. Assessment of two targets of ΔFosB-regulated transcription revealed (1) increased dopamine D1 receptor (D1R) immunoreactivity in the NA shell of Tg-meth rats versus saline Tg controls, but (2) no changes in the AMPA receptor subunit, GluA2. No changes related to genotype or meth occurred for ERK, ΔFosB or D1R protein in the VP. Results reveal a region-specific activation of ERK, and increases in ΔFosB and D1R expression induced by HIV-1 proteins and meth. Such effects may contribute to the neuronal and behavioral pathology associated with meth/HIV comorbidity.

Colon dysregulation in methamphetamine self-administering HIV-1 transgenic rats

The integrity and function of the gut is impaired in HIV-infected individuals, and gut pathogenesis may play a role in several HIV-associated disorders. Methamphetamine is a popular illicit drug abused by HIV-infected individuals. However, the effect of methamphetamine on the gut and its potential to exacerbate HIV-associated gut pathology is not known. To shed light on this scenario, we evaluated colon barrier pathology in a rat model of the human comorbid condition. Intestinal barrier integrity and permeability were assessed in drug-naïve Fischer 344 HIV-1 transgenic (Tg) and non-Tg rats, and in Tg and non-Tg rats instrumented with jugular cannulae trained to self-administer methamphetamine or serving as saline-yoked controls. Intestinal permeability was determined by measuring the urine content of orally gavaged sugars. Intestinal barrier integrity was evaluated by immunoblotting or immunofluorescence of colon claudin-1 and zonula occludens-1 (ZO-1), two major tight junction proteins that regulate gut epithelial paracellular permeability. Both non-Tg and Tg rats self-administered moderate amounts of methamphetamine. These amounts were sufficient to increase colon permeability, reduce protein level of claudin-1, and reduce claudin-1 and ZO-1 immunofluorescence in Tg rats relative to non-Tg rats. Methamphetamine decreased tight junction immunofluorescence in non-Tg rats, with a similar, but non-significant trend observed in Tg rats. However, the effect of methamphetamine on tight junction proteins was subthreshold to gut leakiness. These findings reveal that both HIV-1 proteins and methamphetamine alter colon barrier integrity, and indicate that the gut may be a pathogenic site for these insults.

"Effects of the novel relatively short-acting kappa opioid receptor antagonist LY2444296 in behaviors observed after chronic extended-access cocaine self-administration in rats"

RATIONALE

The recruitment of the stress circuitry contributes to a shift from positive to negative reinforcement mechanisms sustaining long-term cocaine addiction. The kappa opioid receptor (KOPr) signaling is upregulated by stress and chronic cocaine exposure. While KOPr agonists induce anhedonia and dysphoria, KOPr antagonists display antidepressant and anxiolytic properties. Most of the knowledge on KOPr antagonism is based on drugs with unusual pharmacokinetic and pharmacodynamic properties, complicating interpretation of results. Here we characterized in vivo behavioral and neuroendocrine effects of the novel relatively short-acting KOPr antagonist LY2444296. To date, no study has investigated whether systemic KOPr blockade reduced anxiety-like and depressive-like behaviors in animals previously exposed to chronic extended access cocaine self-administration.

OBJECTIVES

We tested the effect of LY2444296 in blocking KOPr-mediated aversive and neuroendocrine effects. Then, we tested acute systemic LY2444296 in reducing anxiety- and depression-like behaviors, as well as releasing the stress hormone corticosterone (CORT), observed after chronic extended access (18 h/day for 14 days) cocaine self-administration.

RESULTS

LY2444296 blocked U69,593-induced place aversion and -reduced motor activity as well as U69,593-induced release of serum CORT, confirming its major site of action, without exerting an effect per se. Acute systemic administration of LY2444296 reduced anxiety-like and depressive-like behaviors, as well as CORT release, in rats tested after chronic extended access cocaine self-administration, but not in cocaine-naïve rats.

CONCLUSIONS

Results suggest that acute blockade of KOPr by a relatively short-acting antagonist produces therapeutic-like effects selectively in rats with a history of chronic extended access cocaine self-administration.

Activin A is increased in the nucleus accumbens following a cocaine binge

Drug addiction is a long-lasting disease characterized by compulsive drug intake mediated in part by neuronal and biological adaptations in key brain areas, such as the nucleus accumbens (NAc). While we previously demonstrated involvement of the activin 2a receptor in drug taking, the role of its ligand, activin A, in cocaine relapse is unknown. Activin A levels in the NAc were assessed via ELISA and immunohistochemistry (in neurons, astrocytes, and microglia) following a cocaine binge paradigm. Cocaine exposure significantly increased the levels of activin A in the NAc of animals that had self-administered cocaine prior to the 14-day withdrawal compared with levels in saline controls. This was accompanied by an increase in the proportion of IBA1+ microglia in the NAc that were immunopositive for activin A. In contrast, the proportions of NeuN+ neurons and GFAP+ astrocytes that were immunopositive for activin A remained unaltered. In conclusion, these data suggest that increased secretion of activin A, particularly from microglia, in the NAc represents a novel potential target for the treatment of cocaine relapse.

The Affective Properties of Synthetic Cathinones: Role of Reward and Aversion in Their Abuse

The drug class known as synthetic cathinones has gained significant attention in the last few years as a result of increased use and abuse. These compounds have been shown to possess reinforcing efficacy in that they are abused in human populations and are self-administered in animal models. The present chapter outlines the affective properties of synthetic cathinones that are thought to impact drug self-administration in general and presents research confirming that these drugs have both rewarding and aversive effects in standalone and concurrent assessments. The implications of these affective properties for the overall abuse potential of these compounds are discussed along with directions for future research.

Fischer 344 and Lewis Rat Strains as a Model of Genetic Vulnerability to Drug Addiction

Today it is well acknowledged that both nature and nurture play important roles in the genesis of psychopathologies, including drug addiction. Increasing evidence suggests that genetic factors contribute for at least 40–60% of the variation in liability to drug dependence. Human genetic studies suggest that multiple genes of small effect, rather than single genes, contribute to the genesis of behavioral psychopathologies. Therefore, the use of inbred rat strains might provide a valuable tool to identify differences, linked to genotype, important in liability to addiction and related disorders. In this regard, Lewis and Fischer 344 inbred rats have been proposed as a model of genetic vulnerability to drug addiction, given their innate differences in sensitivity to the reinforcing and rewarding effects of drugs of abuse, as well their different responsiveness to stressful stimuli. This review will provide evidence in support of this model for the study of the genetic influence on addiction vulnerability, with particular emphasis on differences in mesolimbic dopamine (DA) transmission, rewarding and emotional function. It will be highlighted that Lewis and Fischer 344 rats differ not only in several indices of DA transmission and adaptive changes following repeated drug exposure, but also in hypothalamic-pituitary-adrenal (HPA) axis responsiveness, influencing not only the ability of the individual to cope with stressful events, but also interfering with rewarding and motivational processes, given the influence of corticosteroids on dopamine neuron functionality. Further differences between the two strains, as impulsivity or anxiousness, might contribute to their different proneness to addiction, and likely these features might be linked to their different DA neurotransmission plasticity. Although differences in other neurotransmitter systems might deserve further investigation, results from the reviewed studies might open new vistas in understanding aberrant deviations in reward and motivational functions.

Strain and cocaine-induced differential opioid gene expression may predispose Lewis but not Fischer rats to escalate cocaine self-administration

The aim of the present study was to investigate alterations in gene expression of opioid system components induced by extended access (18 h) cocaine self-administration and to determine the impact of genetic background in the vulnerability to escalate cocaine intake. Comparing two inbred rat strains, we previously reported that Lewis rats progressively escalated cocaine consumption compared to Fischer rats, in a new translational model of intravenous cocaine self-administration, which included 14 sessions of 18-h operant sessions in which rats were allowed to select the cocaine unit dose to self-administer. We compare here Fischer and Lewis rats in the gene expression of endogenous opioid peptides (Pomc, Penk, Pdyn) and cognate receptors (Oprm, Oprk and Oprd) in reward-related brain regions, after exposure to either cocaine self-administration or yoked-saline, in the aforementioned translational paradigm. We performed a correlation analysis between the mRNA level, found in the Dorsal Striatum (DS), Nucleus accumbens (NAcc) shell and core respectively, and individual cocaine intake. Our findings show that the gene expression of all the aforementioned opioid genes exhibit strain-dependent differences in the DS, in absence of cocaine exposure. Also, different strain-specific cocaine-induced mRNA expression of Oprm and Oprk was found in DS. Only few differences were found in the ventral parts of the striatum. Moreover, gene expression level of Pdyn, Penk, Oprk, and Oprm in the DS was significantly correlated with cocaine intake only in Fischer rats. Overall, these data shed light on potential genetic differences which may predispose of subjects to initiate and escalate cocaine consumption.

Persistent increases in rat hypothalamic POMC gene expression following chronic withdrawal from chronic "binge" pattern escalating-dose, but not steady-dose, cocaine

Recent research suggests an involvement of pro-opiomelanocortin (POMC) gene products (e.g., beta-endorphin) in modulating cocaine-induced reward and addiction-like behaviors in rodents. In this study, we investigated whether chronic "binge" cocaine and its withdrawal altered POMC gene expression in the brain of rats. Male Fischer rats were treated with two different chronic (14-day) "binge" pattern cocaine administration regimens (three injections at 1-h intervals, i.p.): steady-dose (45mg/kg/day) and escalating-dose (90mg/kg on the last day). Although there was no POMC mRNA alteration after chronic steady-dose cocaine, a significant decrease in POMC mRNA levels in the hypothalamus was found after chronic escalating-dose cocaine. In contrast, after acute (1-day) withdrawal from chronic "binge" escalating-dose regimen, but not steady-dose regimen, there were increased hypothalamic POMC mRNA levels that persisted into 14days of protracted withdrawal. To study the role of the endogenous opioid systems in the cocaine withdrawal effects, we administered a single naloxone injection (1mg/kg) that caused elevated POMC mRNA levels observed 24h later in cocaine naïve rats, but it did not lead to further increases in cocaine-withdrawn rats. Our results suggest that during withdrawal from chronic "binge" escalating-dose cocaine: (1) there was a persistent increase in hypothalamic POMC gene expression; and (2) hyposensitivity of the POMC gene expression to naloxone indicates altered opioidergic tone at or above the hypothalamic level.

Strain dependence of adolescent Cannabis influence on heroin reward and mesolimbic dopamine transmission in adult Lewis and Fischer 344 rats

Adolescent Cannabis exposure has been hypothesized to act as a gateway to opiate abuse. In order to investigate the role of genetic background in cannabinoid-opiate interactions, we studied the effect of Δ(9) -tetrahydrocannabinol (THC) exposure of adolescent Lewis and Fischer 344 rats on the responsiveness of accumbens shell and core dopamine (DA), as monitored by microdialysis, to THC and heroin at adulthood. Heroin reward and reinstatement by heroin priming were studied by conditioned place preference (CPP) and cognitive and emotional functions by object recognition, Y maze and elevated plus maze paradigms. THC stimulated shell DA in Lewis but not in Fischer 344 rats. Adolescent THC exposure potentiated DA stimulant effects of heroin in the shell and core of Lewis and only in the core of Fischer 344 rats. Control Lewis rats developed stronger CPP to heroin and resistance to extinction compared with Fischer 344 strain. In Lewis rats, THC exposure did not affect heroin CPP but potentiated the effect of heroin priming. In Fischer 344 rats, THC exposure increased heroin CPP and made it resistant to extinction. Lewis rats showed seeking reactions during extinction and hedonic reactions in response to heroin priming. Moreover, adolescent THC exposure affected emotional function only in Lewis rats. These observations suggest that long-term effects of Cannabis exposure on heroin addictive liability and emotionality are dependent on individual genetic background.

Effects of handling and vehicle injections on adrenocorticotropic and corticosterone concentrations in Sprague-Dawley compared with Lewis rats

The hypothalamic-pituitary-adrenal (HPA) axis is a key factor in the trajectory of the addiction-like cycle (a pattern of behavior characterized by escalating drug use, withdrawal, and relapse) in preclinical and clinical studies. Concentrations of HPA hormones change in laboratory animals in response to standard experimental procedures, including handling and vehicle injections. We compared HPA activity in adult male Lewis (inbred) and Sprague-Dawley (outbred) rats, 2 common strains in rodent models of addiction, after different schedules of handling and saline injections, to explore the extent to which HPA responses differ by strain and whether interindividual differences underlie addiction vulnerability. The 4 treatment conditions were no, short, or long handling and saline injections. In handled groups, rats were handled for 1 to 2 min for 3 times daily and were euthanized after 7 d (short handling) or 14 d (long handling). The injection schedule in the saline injection group mimicked that in a model of binge-like cocaine exposure. Across all treatment groups, concentrations of adrenocorticotropic hormone were higher in Sprague-Dawley than in Lewis rats. In Sprague-Dawley rats, corticosterone concentrations decreased after continued handling but remained constant in Lewis rats. Interindividual variability in hormone levels was greater in Sprague-Dawley than Lewis rats, although corticosterone variability decreased after continued handling. Prolactin did not differ between groups of either Sprague-Dawley and Lewis rats before or after handling. This study underscores the importance of prolonged handling before experimenter-provided drug-administration paradigms and of strain-associated differences that may affect study outcomes.

3,4-Methylenedioxypyrovalerone (MDPV)-induced conditioned taste avoidance in the F344/N and LEW rat strains

The inbred Fischer (F344) and Lewis (LEW) rats, while originally developed as animal models for cancer and tissue transplantation research, have since been used to study genetic differences in a variety of physiological and behavioral endpoints. In this context, LEW rats show greater sensitivity to the aversive effects of cocaine as compared to F344 rats in a conditioned taste avoidance procedure. Like cocaine, 3,4-methylenedioxypyrovalerone (MDPV; "bath salts") acts as a dopamine transport blocker and possesses aversive properties, making it a good candidate for assessing whether the aforementioned strain differences with cocaine would generalize to drugs with similar biochemical action. Accordingly, male F344 and LEW rats were exposed to a novel saccharin solution followed by injections of one of four doses of MDPV in a taste avoidance procedure. Over the four saccharin/MDPV pairings during conditioning, core body temperatures were also assessed. Similar to previous research, MDPV induced robust dose-dependent taste avoidance, although no effect of strain was observed. MDPV also produced hyperthermia that was independent of strain and unrelated to the conditioned taste avoidance. These findings argue for a complex influence of multiple (and likely interacting) monoaminergic systems mediating MDPV-induced taste avoidance in the two strains and suggest different mechanisms of avoidance learning for cocaine and MDPV.

Acute binge pattern cocaine administration induces region-specific effects in D1-r- and D2-r-expressing cells in eGFP transgenic mice

Cocaine addiction is driven by genetic, neurologic and environmental components. The D1-like (D1 and D5) and D2-like (D2, D3 and D4) families of dopamine receptors play an important role in modulating the effects of cocaine administration on drug-seeking behavior. The advent of bacterial artificial chromosome-eGFP (enhanced green fluorescent protein) transgenic mice that express eGFP driven by the endogenous D1-receptor (D1-r) or D2-receptor (D2-r) promoters provides a unique opportunity to distinguish between these subpopulations of cells. In an effort to identify cocaine-induced alterations in D1-r- versus D2-r-expressing cells during the initial stages of addiction, we examined cells that expressed D1-rs in Drd1-eGFP mice, or D2-rs in Drd2-eGFP mice, after an acute, 1-day binge pattern of cocaine administration. We used multiphoton confocal microscopy and Visiopharm© software, to conduct unbiased stereological counts of D1-r-labeled or D2-r-labeled cells in various striatal regions. Mice were sacrificed at 30 min and 24-h post cocaine or saline administration. Compared to saline controls, Drd1-eGFP mice that received cocaine had a higher count of D1-r-labeled cells in the dorsolateral (DL) striatum, at the 30-min and 24-h time-points. No changes in the nucleus accumbens (NAc) core or shell were observed in Drd1-eGFP mice. Drd2-eGFP mice that received cocaine had fewer D2-r-labeled cells in the DL striatum and NAc core compared to saline controls. This effect was observed at the 30-min time-point but not at 24h. Drd2-eGFP mice that received cocaine also had fewer numbers of D2-r-labeled cells in the NAc core compared to saline controls, but no significant differences in the number of D2-r-labeled cells in the NAc shell. These results suggest that acute binge pattern cocaine administration may induce region-specific alterations in D1-r or D2-receptor gene expression, and may help elucidate the differential role of dopamine receptors in the initial stages of the addiction cycle.

The effects of cocaine self-administration on dendritic spine density in the rat hippocampus are dependent on genetic background

Chronic exposure to cocaine induces modifications to neurons in the brain regions involved in addiction. Hence, we evaluated cocaine-induced changes in the hippocampal CA1 field in Fischer 344 (F344) and Lewis (LEW) rats, 2 strains that have been widely used to study genetic predisposition to drug addiction, by combining intracellular Lucifer yellow injection with confocal microscopy reconstruction of labeled neurons. Specifically, we examined the effects of cocaine self-administration on the structure, size, and branching complexity of the apical dendrites of CA1 pyramidal neurons. In addition, we quantified spine density in the collaterals of the apical dendritic arbors of these neurons. We found differences between these strains in several morphological parameters. For example, CA1 apical dendrites were more branched and complex in LEW than in F344 rats, while the spine density in the collateral dendrites of the apical dendritic arbors was greater in F344 rats. Interestingly, cocaine self-administration in LEW rats augmented the spine density, an effect that was not observed in the F344 strain. These results reveal significant structural differences in CA1 pyramidal cells between these strains and indicate that cocaine self-administration has a distinct effect on neuron morphology in the hippocampus of rats with different genetic backgrounds.

Cocaine self-administration differentially modulates the expression of endogenous cannabinoid system-related proteins in the hippocampus of Lewis vs. Fischer 344 rats

The endocannabinoids anandamide and 2-arachidonyl glycerol (2-AG) are modulators of glutamate and γ-aminobutyric acid (GABA), two transmitters involved in cocaine addiction. However, little is known on the effects of cocaine on the enzymes that produce and degrade endocannabinoids. The present work addresses the effects of cocaine self-administration on the immunohistochemical expression of endocannabinoid signalling (ECS)-related proteins in the hippocampus. The study has been performed on two different strains of rats, Lewis (Lew) and Fischer 344 (F344), which are characterized for displaying a differential sensitivity to cocaine, thus making them suitable in the study of vulnerability to drug addiction. Both strains showed differences in the expression of ECS-related proteins in the hippocampus, i.e. Lew rats exhibited lower CB1 expression but higher CB2 expression than F344 rats. After setting similar cocaine self-administration, both strains showed clear differences in the expression of ECS-related proteins, which were differentially restricted to either the 2-AG or anandamide signalling pathways in a self-administration training/drug-dependent manner. The decreases observed in CB1 expression and N-acyl phosphatidylethanolamine phospholipase D:fatty acid amino hydrolase ratio after saline self-administration were enhanced only in cocaine self-administered Lew rats. CB2 expression increase and diacylglycerol lipase α:monoacylglycerol lipase ratio decrease detected after saline self-administration were blocked only in cocaine self-administered F344 rats. These findings indicate that cocaine may regulate hippocampal GABA/glutamate synapses by directly modulating endocannabinoid production/degradation enzymes and that these actions are strain-dependent. This differential response suggests that the endogenous cannabinoid system might contribute to genotype/strain differences on the sensitivity to self-administration training and cocaine addiction.

Adolescents are more vulnerable to cocaine addiction: behavioral and electrophysiological evidence

In humans, adolescence is a period of heightened propensity to develop cocaine addiction. It is unknown whether this is attributable to greater access and exposure to cocaine at this age, or whether the adolescent brain is particularly vulnerable to the addictive properties of cocaine. Here, we subjected male adolescent (P42) and adult (∼P88) rats to a wide range of cocaine self-administration procedures. In addition, to determine whether behavioral differences are associated with developmental differences in dopaminergic activity, we examined and manipulated the activity of dopamine neurons. Relative to adults, adolescent rats took cocaine more readily, were more sensitive to lower doses, showed greater escalation of cocaine intake, and were less susceptible to increases in price (i.e., were more "inelastic"). In parallel, adolescents also showed elevated activity of ventral tegmental area dopamine neurons, a feature known to be associated with increased self-administration behavior. Pharmacological manipulation of dopamine D2 receptor function with quinpirole (agonist) or eticlopride (antagonist), to alter dopamine neuron activity, eliminated age differences in cocaine self-administration. These data suggest a causal relationship between behavioral and electrophysiological determinants of cocaine addiction liability. In conclusion, adolescents show behavioral and electrophysiological traits of heightened addiction liability.

Chronic escalating cocaine exposure, abstinence/withdrawal, and chronic re-exposure: effects on striatal dopamine and opioid systems in C57BL/6J mice

Cocaine addiction is a chronic relapsing disease with periods of chronic escalating self-exposure, separated by periods of abstinence/withdrawal of varying duration. Few studies compare such cycles in preclinical models. This study models an "addiction-like cycle" in mice to determine neurochemical/molecular alterations that underlie the chronic, relapsing nature of this disease. Groups of male C57BL/6J mice received acute cocaine exposure (14-day saline/14-day withdrawal/13-day saline + 1-day cocaine), chronic cocaine exposure (14 day cocaine) or chronic re-exposure (14-day cocaine/14-day withdrawal/14-day cocaine). Escalating-dose binge cocaine (15-30 mg/kg/injection × 3/day, i.p. at hourly intervals) or saline (14-day saline) was administered, modeling initial exposure. In "re-exposure" groups, after a 14-day injection-free period (modeling abstinence/withdrawal), mice that had received cocaine were re-injected with 14-day escalating-dose binge cocaine, whereas controls received saline. Microdialysis was conducted on the 14th day of exposure or re-exposure to determine striatal dopamine content. Messenger RNA levels of preprodynorphin (Pdyn), dopamine D1 (Drd1) and D2 (Drd2) in the caudate putamen were determined by real-time PCR. Basal striatal dopamine levels were lower in mice after 14-day escalating exposure or re-exposure than in those in the acute cocaine group and controls. Pdyn mRNA levels were higher in the cocaine groups than in controls. Long-term adaptation was observed across the stages of this addiction-like cycle, in that the effects of cocaine on dopamine levels were increased after re-exposure compared to exposure. Changes in striatal dopaminergic responses across chronic escalating cocaine exposure and re-exposure are a central feature of the neurobiology of relapsing addictive states.

Effects of withdrawal from chronic escalating-dose binge cocaine on conditioned place preference to cocaine and striatal preproenkephalin mRNA in C57BL/6J mice

Relapse is a serious problem for the effective treatment of cocaine addiction.

RATIONALE

Examining cocaine re-exposure-induced behavioral and neurobiological alterations following chronic escalating-dose binge cocaine administration and withdrawal may provide insight into the neurobiological basis of cocaine relapse.

OBJECTIVES

Our goal was to determine how exposure to chronic escalating-dose cocaine affects development of subsequent cocaine-induced conditioned place preference (CPP) and changes in endogenous opioid systems.

METHODS

Mice were injected with either escalating-dose binge cocaine (15-30 mg/kg/injection × 3/day) or saline for 14-days and conditioned with 15 mg/kg of cocaine or saline (once per day for 10-days), starting either 1 or 14-days after the last day of binge injections.

RESULTS

Mice exposed to chronic escalating cocaine did not develop CPP to cocaine when conditioning commenced on the first day of withdrawal (CPP test on day 10 of withdrawal). By contrast, mice did develop CPP to cocaine when conditioning started on the 14th day of withdrawal (CPP test on day 24 of withdrawal). Furthermore, preproenkephalin (Penk) mRNA levels in caudate putamen were significantly higher in mice that received 14-day withdrawal from escalating-dose binge cocaine before the CPP procedure (tested 24 days post-binge) than those that received 1-day withdrawal (tested 10 days post-binge).

CONCLUSIONS

The rewarding effect of cocaine was blunted in early withdrawal from chronic escalating exposure, but recovered in more prolonged withdrawal. Time-dependent elevations in Penk mRNA levels may be part of the underlying mechanisms of this effect.

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.

Persistent increase in hypothalamic arginine vasopressin gene expression during protracted withdrawal from chronic escalating-dose cocaine in rodents

Arginine vasopressin (AVP) from the paraventricular nucleus (PVN) of hypothalamus has important roles in regulation of the hypothalamic-pituitary-adrenal (HPA) axis and stress-related behaviors during chronic stress. It is unknown, however, whether AVP in the PVN is involved in the modulation of HPA activity after chronic cocaine exposure. Here, we examined the gene expression alterations of AVP in the hypothalamus, and V1b receptor and pro-opiomelanocortin (POMC) in the anterior pituitary, as well as HPA hormonal changes, in Fischer rats after chronic cocaine and withdrawal, using two different chronic (14-day) 'binge' pattern administration regimens: steady-dose cocaine (SDC, 45 mg/kg/day) and escalating-dose cocaine (EDC, 45 up to 90 mg/kg/day). There was a significant (7-fold) plasma adrenocorticotropic hormone (ACTH) elevation after chronic EDC (but not SDC), coupled with increased V1b and POMC mRNA levels in the anterior pituitary. From acute (1-day) to protracted (14-day) withdrawal from chronic EDC (but not from SDC), we found persistent elevations of both plasma ACTH and corticosterone levels and AVP mRNA levels in the PVN. Selective V1b antagonist SSR149415 (5 mg/kg) attenuated acute withdrawal-induced HPA activation after EDC. To study potential roles of endogenous opioids in modulating the AVP gene, we administered naloxone (1 mg/kg); we found that opioid receptor antagonism increased AVP mRNA levels in cocaine-naive rats, but not in cocaine-withdrawn rats, suggesting less tonic opioid inhibition of PVN AVP neurons after chronic EDC. To assess the effects of cocaine withdrawal on sub-populations of PVN AVP neurons, we utilized AVP-enhanced green fluorescent protein (EGFP) promoter transgenic mice and found that acute withdrawal following chronic EDC increased the number of AVP-EGFP neurons in the parvocellular PVN (pPVN). These results suggest that during protracted withdrawal, enhanced pPVN AVP gene expression is associated with persistent elevations of basal HPA activity; a hyposensitivity of PVN AVP gene expression to naloxone is indicative of reduced opioidergic tone. Our studies indicate that the AVP and its V1b receptor system may be a potential therapeutic target for treating anxiety and depressive symptoms associated with cocaine addiction.

The science of making drug-addicted animals

Research involving animal models of drug addiction can be viewed as a sort of reverse psychiatry. Contrary to clinicians who seek to treat addicted people to become and remain abstinent, researchers seek to make drug-naïve animals addicted to a drug with known addictive properties in humans. The goals of this research are to better understand the neuroscience of drug addiction and, ultimately, to translate this knowledge into effective treatments for people with addiction. The present review will not cover the vast literature that has accumulated over the past 50 years on animal models of drug addiction. It is instead more modestly devoted to recent research spanning the past decade on drug self-administration-based models of addiction in the rat (the animal species most frequently used in the field), with a special focus on current efforts to model compulsive cocaine use as opposed to nonaddictive use. Surprisingly, it turns out that modeling compulsive cocaine use in rats is possible but more difficult than previously thought. In fact, it appears that resilience to cocaine addiction is the norm in rats. As in human cocaine users, only few individual rats would be vulnerable. This conclusion has several important implications for future research on the neuroscience of cocaine addiction and on preclinical medication development.

Sensitivity to rewarding or aversive effects of methamphetamine determines methamphetamine intake

Amphetamines have rewarding and aversive effects. Relative sensitivity to these effects may be a better predictor of vulnerability to addiction than sensitivity to one of these effects alone. We tested this hypothesis in a dose-response study in a second replicate set of mouse lines selectively bred for high vs. low methamphetamine (MA) drinking (MADR). Replicate 2 high (MAHDR-2) and low (MALDR-2) MA drinking mice were bred based on MA consumption in a two-bottle choice procedure and examined for novel tastant drinking. Sensitivities to the rewarding and aversive effects of several doses of MA (0.5, 2 and 4 mg/kg) were measured using a place conditioning procedure. After conditioning, mice were tested in a drug-free and then drug-present state for time spent in the saline- and MA-paired contexts. Similar to the first set of MADR lines, by the end of selection, MAHDR-2 mice consumed about 6 mg MA/kg/18 h, compared to nearly no MA in MALDR-2 mice, but had similar taste preference ratios. MAHDR-2 mice exhibited place preference in both the drug-free and drug-present tests, and no significant place aversion. In contrast, MALDR-2 mice exhibited no place preference or aversion during the drug-free test, but robust place aversion in the drug-present test. These data extend our preliminary findings from the first set of MADR lines and support the hypothesis that the combination of greater sensitivity to the rewarding effects of MA and insensitivity to the aversive effects of MA is genetically associated with heightened risk for MA consumption.