Brief early handling increases morphine dependence in adult rats

Impacts of maternal separation stress on ethanol-related responses, anxiety- and depressive-like behaviors in adolescent mice

The present work evaluated the consequences of chronic maternal separation (MS), an animal model of early-life stress, on ethanol intake and striatal Fos expression induced by ethanol consumption. Furthermore, we analyzed MS impacts on anxiety- and depressive-like behaviors and on locomotor and plasma corticosterone responses to intraperitoneal treatment with ethanol in adolescent mice. For that, male and female C57BL/6J mice were exposed or not to MS stress, for 3 h per day, from postnatal day (PND) 1 to 14, and submitted to behavioral tests from PND 28. In Experiment 1, MS and control groups of mice were submitted to an involuntary ethanol intake protocol, and striatal Fos expression following ethanol exposure was analyzed. In Experiment 2, mice behavior was assessed in elevated plus-maze, sucrose splash, saccharin preference, and open field tests. Locomotor and plasma corticosterone responses induced by a systemic dose of ethanol (1.75 g/kg) were also evaluated. Our results demonstrated that MS increased ethanol intake only in an acute manner and did not impact ethanol-induced Fos expression in the dorsal striatum and nucleus accumbens (NAc) core and shell subregions. MS did not change the parameters analyzed during elevated plus-maze, sucrose splash, preference for saccharin, and open field tests. MS did not affect locomotor activity following ethanol injection nor plasma corticosterone response to the drug. Thus, our data showed that MS transiently increased ethanol intake. However, early-life stress did not impact Fos, locomotor, or plasma corticosterone responses to the drug. In addition, MS did not affect anxiety- and depressive-like behaviors in adolescent mice.

Cannabidiol treatment prevents drug reinstatement and the molecular alterations evoked by amphetamine on receptors and enzymes from dopaminergic and endocannabinoid systems in rats

In psychostimulant drug addiction, relapse is the most concerning outcome to be managed, considering there is no approved treatment for this neuropsychiatric condition. Here, we investigated the effects of the CBD treatment on the relapse behavior triggered by stress, after being submitted to the amphetamine (AMPH)-induced conditioned place preference (CPP) in rats. To elucidate the mechanisms of action underlying the CBD treatment, we evaluated the neuroadaptations on dopaminergic and endocannabinoid targets in the ventral striatum (VS) and ventral tegmental area (VTA) of the brain. Animals received d,l-AMPH (4 mg/kg, i.p.) or vehicle in the CPP paradigm for 8 days. Following the first CPP test, animals were treated with CBD (10 mg/kg, i.p.) or its vehicle for 5 days and subsequently submitted to forced swim stress protocol to induce AMPH-CPP relapse. Behavioral findings showed that CBD treatment prevented AMPH-reinstatement, also exerting anxiolytic activity. At the molecular level, in the VTA, CBD restored the CB1R levels decreased by AMPH-exposure, increased NAPE-PLD, and decreased FAAH levels. In the VS, the increase of D1R and D2R, as well as the decrease of DAT levels induced by AMPH were restored by CBD treatment. The current outcomes evidence a substantial preventive action of the CBD on the AMPH-reinstatement evoked by stress, also involving neuroadaptations in both dopaminergic and endocannabinoid systems in brain areas closely involved in the addiction. Although further studies are needed, these findings support the therapeutic potential of CBD in AMPH-relapse prevention.

Early life stress and substance use disorders: The critical role of adolescent substance use

Early life stress (ELS) is a well-established risk factor for many psychiatric and medical disorders, including substance use disorders (SUDs). The relationship between ELS and SUDs is complex and there are likely multiple pathways from ELS to adverse substance use outcomes. The association between ELS and substance use emerges in adolescence. Adolescence is a critical period in development during which substance exposure markedly increases risk for SUDs. Therefore, this review focuses on the literature supporting the hypothesis that ELS increases risk for the development of SUDs through its influence on adolescent substance use. We discuss studies substantiating the role of ELS in adolescent substance use and explore how internalizing and externalizing psychopathology may be antecedents of substance use in adolescence. We examine clinical work suggesting ELS sculpts the Hypothalamic-Pituitary-Adrenal (HPA) Axis and developing brain-particularly subcortical brain regions that underlie stress response, mesocorticolimbic brain systems associated with reward sensitivity, and prefrontal regions that underlie executive control-in a way that increases risk for adolescent substance use and SUDs. We further explore how substance use during adolescence alters structure and function of these same systems, and how brain changes following ELS and adolescent substance use may independently, additively, or interactively contribute to risk for addiction. We conclude by discussing how the current literature can inform interventions aimed at reducing risk for SUDs in individuals with a history of ELS.

Cannabidiol prevents amphetamine relapse and modulates D1- and D2-receptor levels in mesocorticolimbic brain areas of rats

Amphetamine (AMPH) is an addictive psychostimulant highly used worldwide and its consumption is related to neurotoxic effects. Currently, there is no pharmacotherapy approved for treating AMPH or other psychostimulant drug addiction. Different studies have shown promising properties of cannabidiol (CBD) for treating many neurological and psychiatric diseases, and recently, CBD is being considered a potential strategy for the treatment of drug addiction disorders. Thus, we investigated possible CBD beneficial effects on relapse symptoms following AMPH re-exposure considering drug relapse is the most difficult clinical factor to control during addiction treatment. Rats received d,l-AMPH (4 mg/kg, i.p.) or vehicle in the conditioned place preference (CPP) paradigm (8 days), when each experimental group was re-assigned to receive CBD at two different doses (5 or 10 mg/kg, i.p) or control, for 5 days. Subsequently, animals were re-exposed to AMPH-CPP (4 mg/kg, i.p.) for 3 additional days to assess relapse behavior. Besides locomotor and anxiety-like behaviors, dopaminergic molecular parameters were quantified in both prefrontal cortex and ventral striatum. Regarding molecular levels, CBD modulated at basal levels the dopaminergic targets (D1R, D2R, DAT, and TH) in the assessed brain areas, preventing AMPH relapse and decreasing anxiety-like behavior per se and in AMPH-CPP animals. The current findings give evidence about CBD-induced AMPH-relapse prevention, which may be linked to dopaminergic mesocorticolimbic system modulation. Although future and clinical studies are needed, our outcomes show that CBD may be a useful alternative to prevent AMPH relapse.

Involvement of the endogenous opioid system in the beneficial influence of physical exercise on amphetamine-induced addiction parameters

Psychostimulant drugs addiction is a chronic public health problem and individuals remain susceptible to relapses increasing public expenses even after withdrawal and treatment. Our research group has focused on finding new therapies to be employed in drug addiction treatment, suggesting the physical exercise as a promising tool. This way, it is necessary to know the mechanisms involved in the beneficial influences of physical exercise observing the pathway that could be explored in drug addiction treatment. Male Wistar rats were conditioned with amphetamine (AMPH) following the conditioned place preference (CPP) protocol and subsequently submitted to swimming for 5 weeks (1 h per day, 5 days per week). Half of the animals were injected with Naloxone (0.3 mg/mL/kg body weight, i.p.) 5 min prior each physical exercise day. After AMPH-CPP re-exposure, our outcomes showed that physical exercise, in addition to minimizing the relapse behavior in the CPP, it increased D1R, D2R and DAT in the Ventral Tegmental Area (VTA), but not in the Nucleus accumbens (NAc). Interestingly, while naloxone inhibited the partial beneficial influence of the exercise on drug-relapse behavior, exercise-induced changes in the dopaminergic system were not observed in the group administered with naloxone as well. Based on these evidences, besides reinforcing the beneficial influence of the physical exercise on AMPH-induced drug addiction, we propose the involvement of endogenous opioid system activation, not as a single one, but as a possible mechanism of action resulting from the physical activity practice, thus characterizing an important therapeutic approach, which may contribute to drug withdrawal consequently preventing relapse.

Early life stress and the propensity to develop addictive behaviors

There is a vast literature on effects of early life manipulations in rodents much of which is aimed at investigating the long-term consequences related to emotion and cognition in adulthood. Less is known about how these manipulations affect responses reflective of alcohol (AUD) and substance (SUD) use disorders. The purpose of this paper is to review the literature of studies that employed early life manipulations and assessed behavioral responses to psychoactive substances, specifically alcohol, opiates, and stimulants, in rodents. While the findings with alcohol are more limited and mixed, studies with opiates and stimulants show strong support for the ability of these manipulations to enhance behavioral responsivity to these substances in line with epidemiological data. Some outcomes show sex differences. The mechanisms that influence these enduring changes may reflect epigenetic alterations. Several studies support a role for altered DNA methylation (and other epigenetic mechanisms) as biological responses to early environmental insults. The chemical changes induced by DNA methylation affect transcriptional activity of DNA and thus can have a long-term impact on the individual's phenotype. Such effects are particularly robust when they occur during sensitive periods of brain development (e.g., first postnatal weeks in rodents). We review this emerging literature as it relates to the known neurobiology of AUDs and SUDs and suggest new avenues of research. Such findings will have implications for the treatment and prevention of AUDs and SUDs and could provide insight into factors that support resiliency.

Omega-3 decreases D1 and D2 receptors expression in the prefrontal cortex and prevents amphetamine-induced conditioned place preference in rats

Amphetamine (AMPH) abuse is a serious public health problem due to the high addictive potential of this drug, whose use is related to severe brain neurotoxicity and memory impairments. So far, therapies for psychostimulant addiction have had limited efficacy. Omega-3 polyunsaturated fatty acids (n-3 PUFA) have shown beneficial influences on the prevention and treatment of several diseases that affect the central nervous system. Here, we assessed the influence of fish oil (FO), which is rich in n-3 PUFA, on withdrawal and relapse symptoms following re-exposure to AMPH. Male Wistar rats received d,l-AMPH or vehicle in the conditioned place preference (CPP) paradigm for 14 days. Then, half of each experimental group was treated with FO (3 g/kg, p.o.) for 14 days. Subsequently, animals were re-exposed to AMPH-CPP for three additional days, in order to assess relapse behavior. Our findings have evidenced that FO prevented relapse induced by AMPH reconditioning. While FO prevented AMPH-induced oxidative damages in the prefrontal cortex, molecular assays allowed us to observe that it was also able to modulate dopaminergic cascade markers (DAT, TH, VMAT-2, D1R and D2R) in the same brain area, thus preventing AMPH-induced molecular changes. To the most of our knowledge, this is the first study to show a natural alternative tool which is able to prevent psychostimulant relapse following drug withdrawal. This non-invasive and healthy nutraceutical may be considered as an adjuvant treatment in detoxification clinics.

Social Factors in Ethanol Sensitization

Behavioral sensitization is a neuroadaptive process characterized by an increase in a particular behavior after repeated exposure to drugs or other stimuli, such as stress. Sensitization can also be extended to neurochemical and neuroendocrine sensitization. Several factors can influence sensitization to the effects of ethanol. For instance, stress is an important component in addiction that can strengthen ethanol-induced behaviors. In animal models, stressful situations can be induced by alterations in social aspects of the animal's environment, such as maternal separation, social conflicts, and housing conditions. Social conflict models involve acute, chronic or intermittent interaction of an animal to a conspecific and can occur at any stage of life, including preweaning, adolescence or adulthood. These events can influence ethanol-induced behavioral sensitization in different ways, such as increases in locomotion, drug reward, and drug-taking behaviors. On the other hand, environmental enrichment can produce a protective phenotype against drug-related behaviors. In this chapter, we discuss findings regarding consequences of social stress and environmental enrichment on sensitization to ethanol.

m-Trifluoromethyl-diphenyldiselenide as a pharmacological tool to treat preference symptoms related to AMPH-induced dependence in rats

Amphetamine (AMPH) abuse is a world concern and a serious public health problem. Repeated administration of high doses of AMPH induces neuropsychiatric consequences, including addiction, reward and psychosis, whose pharmacological treatment has shown limited effectiveness. The m-trifluoromethyl-diphenyldiselenide [(m-CF3-PhSe)2] has been documented as a promising pharmacological agent in different animal models related to oxidative damage. In this study, we examined the influence of (m-CF3-PhSe)2 on withdrawal following re-exposure to AMPH. Wistar rats received d,l-AMPH or saline in the conditioned place preference (CPP) paradigm for 8days. Then, half of each initial (AMPH or saline) experimental group was treated with (m-CF3-PhSe)2 or vehicle, resulting in four final groups: i) Saline/vehicle; ii) (m-CF3-PhSe)2/saline; iii) AMPH/vehicle; and iv) AMPH/(m-CF3-PhSe)2. After fourteen days of (m-CF3-PhSe)2 treatment, animals were re-exposed to AMPH or vehicle in the CPP paradigm for three more days in order to assess drug re-conditioning and memory/locomotor activity, performed 24h after AMPH re-exposure in the CPP and the Y maze, respectively. Subsequently, ex-vivo assays were carried out in samples of the prefrontal cortex (PFC) of the animals. The (m-CF3-PhSe)2 treatment was able to prevent AMPH-induced re-conditioning symptoms in rats. Behavioral observations in the Y maze task showed no significant changes. AMPH exposure was able to increase 5-HT uptake as well as oxidative damage in the PFC, whereas (m-CF3-PhSe)2 treatment exerted a preventative effect against these alterations. The current findings suggest that (m-CF3-PhSe)2 might be considered a promising therapeutic tool for AMPH-induced addiction.

Toxicological aspects of trans fat consumption over two sequential generations of rats: Oxidative damage and preference for amphetamine

Chronic consumption of processed food causes structural changes in membrane phospholipids, affecting brain neurotransmission. Here we evaluated noxious influences of dietary fats over two generations of rats on amphetamine (AMPH)-conditioned place preference (CPP). Female rats received soybean oil (SO, rich in n-6 fatty acids (FA)), fish oil (FO, rich in n-3 FA) and hydrogenated vegetable fat (HVF, rich in trans fatty acids (TFA)) for two successive generations. Male pups from the 2nd generation were maintained on the same supplementation until 41 days of age, when they were conditioned with AMPH in CPP. While the FO group showed higher incorporation of n-3 polyunsaturated-FA (PUFA) in cortex/hippocampus, the HVF group showed TFA incorporation in these same brain areas. The SO and HVF groups showed AMPH-preference and anxiety-like symptoms during abstinence. Higher levels of protein carbonyl (PC) and lower levels of non-protein thiols (NPSH) were observed in cortex/hippocampus of the HVF group, indicating antioxidant defense system impairment. In contrast, the FO group showed no drug-preference and lower PC levels in cortex. Cortical PC was positively correlated with n-6/n-3 PUFA ratio, locomotion and anxiety-like behavior, and hippocampal PC was positively correlated with AMPH-preference, reinforcing connections between oxidative damage and AMPH-induced preference/abstinence behaviors. As brain incorporation of trans and n-6 PUFA modifies its physiological functions, it may facilitate drug addiction.

The impact of maternal separation on the number of tyrosine hydroxylase-expressing midbrain neurons during different stages of ontogenesis

Early life stressors have life-long functional and anatomical consequences. Though many neurotransmitters are involved in the functional impact of early life stress, dopamine seems to be important because of its roles in motor control, adaptation to stressful conditions, mood, cognition, attention and reward. Thus, in the present study, we investigated the way that early life stress, in the form of maternal separation (MS), affects the populations of tyrosine hydroxylase-immunoreactive (TH-IR) dopaminergic neurons in rat midbrain structures during ontogenesis. We included in the study the sub-regions of the substantia nigra (SN) and the ventral tegmental area (VTA). In both the control and MS rats, we found that the estimated total number of TH-expressing neurons fluctuated during ontogenesis. Moreover, MS influenced the number of TH-IR cells, especially in the SN pars reticulata (SNr) and VTA. Shortly after the termination of MS, on postnatal day (PND) 15, a decrease in the estimated total number of TH-IR neurons was observed in the SNr and VTA (in both males and females). On PND 35, MS caused a transient increase in the number of TH-IR cells only in the SNr of female rats. On PND 70, MS affected the number of TH-IR neurons in the VTA of females; specifically, an increase in the number of these cells was observed. Additionally, MS did not alter TH-IR cell sizes or the total levels of TH (measured by Western blot analysis) in the SN and VTA for all stages of ontogenesis in both males and females. The results from the study herein indicate that early life stress has enduring effects on the populations of midbrain TH-expressing dopaminergic neurons (especially in female rats), which are critically important for dopamine-regulated brain function throughout ontogenesis.

The role of early life stress as a predictor for alcohol and drug dependence

RATIONALE

Genetic and environmental influences on the development of alcohol and drug dependence are equally important. Exposure to early life stress, that is unfortunately common in the general population, has been shown to predict a wide range of psychopathology, including addiction.

OBJECTIVE

This review will look at the characteristics of early life stress that may be specific predictors for adolescent and adult alcohol and drug dependence and will focus on studies in humans, non-human primates and rodents.

RESULTS

Experiencing maltreatment and cumulative stressful life events prior to puberty and particularly in the first few years of life is associated with early onset of problem drinking in adolescence and alcohol and drug dependence in early adulthood. Early life stress can result in permanent neurohormonal and hypothalamic-pituitary-adrenal axis changes, morphological changes in the brain, and gene expression changes in the mesolimbic dopamine reward pathway, all of which are implicated in the development of addiction. However, a large proportion of children who have experienced even severe early life stress do not develop psychopathology indicating that mediating factors such as gene-environment interactions and family and peer relationships are important for resilience.

CONCLUSIONS

There appears to be a direct pathway from chronic stress exposure in pre-pubertal children via adolescent problem drinking to alcohol and drug dependence in early adulthood. However, this route can be moderated by genetic and environmental factors. The role that gene-environment interactions play in the risk-resilience balance is being increasingly recognized.

The influence of GABRA2, childhood trauma, and their interaction on alcohol, heroin, and cocaine dependence

BACKGROUND

The GABRA2 gene has been implicated in addiction. Early life stress has been shown to alter GABRA2 expression in adult rodents. We hypothesized that childhood trauma, GABRA2 variation, and their interaction would influence addiction vulnerability.

METHODS

African-American men were recruited for this study: 577 patients with lifetime DSM-IV single and comorbid diagnoses of alcohol, cocaine, and heroin dependence, and 255 control subjects. The Childhood Trauma Questionnaire (CTQ) was administered. Ten GABRA2 haplotype-tagging single-nucleotide polymorphisms (SNPs) were genotyped.

RESULTS

We found that exposure to childhood trauma predicted substance dependence (p < .0001). Polysubstance dependence was associated with the highest CTQ scores (p < .0001). The African Americans had four common haplotypes (frequency: .11-.30) within the distal haplotype block: two that correspond to the Caucasian and Asian yin-yang haplotypes, and two not found in other ethnic groups. One of the unique haplotypes predicted heroin addiction, whereas the other haplotype was more common in control subjects and seemed to confer resilience to addiction after exposure to severe childhood trauma. The yin-yang haplotypes had no effects. Moreover, the intron 2 SNP rs11503014, not located in any haplotype block and potentially implicated in exon splicing, was independently associated with addiction, specifically heroin addiction (p < .005). Childhood trauma interacted with rs11503014 variation to influence addiction vulnerability, particularly to cocaine (p < .005).

CONCLUSIONS

Our results suggest that at least in African-American men, childhood trauma, GABRA2 variation, and their interaction play a role in risk-resilience for substance dependence.

Chronic social stress, hedonism and vulnerability to obesity: lessons from rodents

Obesity is a current health pandemia. Determinants of this pathology are rather complex and include genetic, developmental and environmental factors only partially disclosed. Stress related neuroendocrine dysregulation and overconsumption of high palatable high caloric food and are likely to contribute to this modern health threats. Despite the evidence that psychosocial stress is one of the main sources of stress in humans and may play an important role in the development of the stress disorders, including obesity and metabolic syndrome, animal models focusing on the relationship between chronic stress and energy homeostasis are scattered and most of them encompasses physical rather than psychosocial stress. Aim of the present paper is to review rodent studies on the effect of psychosocial stress throughout life on body weight and food intake regulation. In the second part of the review special focus will be given on the mechanisms linking stress and the reward system.

The impact of postnatal environment on opioid peptides in young and adult male Wistar rats

Early environmental influences can change the neuronal development and thereby affect behavior in adult life. The aim in the present study was to thoroughly examine the impact of early environmental factors on endogenous opioids by using a rodent maternal separation (MS) model. The endogenous opioid peptide system is not fully developed at birth, and short- and/or long-term alterations may occur in these neural networks in animals exposed to manipulation of the postnatal environment. Rat pups were subjected to one of five rearing conditions; 15 min (MS15) litter (l) or individual (i), 360 min (MS360) l or i daily MS, or housed under normal animal facility rearing (AFR) conditions during postnatal days 1-21. Measurements of immunoreactive (ir) Met-enkephalin-Arg6Phe7 (MEAP) and dynorphin B (DYNB) peptide levels in the pituitary gland and in a number of brain areas, were performed at three and 10 weeks of age, respectively. MS-induced changes were more pronounced in ir MEAP levels, especially in individually separated rats at three weeks of age and in litter-separated rats at 10 weeks of age. The enkephalin and dynorphin systems have different developmental patterns, dynorphin appearing earlier, which may point at a more sensitive enkephalin system during the early postnatal weeks. The results provide evidence that opioid peptides are sensitive for early environmental factors and show that the separation conditions are critical and also result in changes manifesting at different time points. MS-induced effects were observed in areas related to stress, drug reward and dependence mechanisms. By describing effects on opioid peptides, the study addresses the possible role of a deranged endogenous opioid system in the previously described behavioral consequences of MS.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Maternal deprivation and handling modify the effect of the dopamine D3 receptor agonist, BP 897 on morphine-conditioned place preference in rats

RATIONALE

Maternal deprivation and handling can lead to a vulnerability to opiate dependence. However, the involvement of the dopamine D3 receptors has not been investigated.

OBJECTIVES

This study analysed the effects of a selective partial D3 receptor agonist, BP 897, on morphine-conditioned place preference (CPP) in deprived and handled rats.

MATERIALS AND METHODS

The effects of BP 897 were studied on the expression and the extinction of morphine CPP. Quantitative autoradiography of D2, D3 receptors and immunoautoradiography of dopamine transporter were performed in some saline- and morphine-treated rats 24 h after the place preference test.

RESULTS

Morphine (5 mg/kg) induced a more prolonged morphine CPP in deprived and handled rats than in control animals. BP 897 (0.5 or 2 mg/kg) enhanced the expression of morphine conditioning in control rats. Same doses did not change morphine conditioning in deprived rats. BP 897 (2 mg/kg) suppressed morphine CPP in handled rats. An increase in basal D2 receptor density in the mesencephalon of handled rats, which was suppressed after morphine CPP, was observed. A decrease in D2 receptor levels in morphine-treated deprived rats occurred in the nucleus accumbens.

CONCLUSIONS

This study shows that maternal deprivation and handling induced a prolonged morphine CPP, and different changes of D2/D3 receptor functioning revealed after morphine CPP. Early manipulations of infant-mother relationships may have different consequences on the balance of opioidergic and dopaminergic neurotransmission and may be of interest to reveal pharmacological properties of dopamine receptor partial agonists or antagonists potentially useful for therapeutic applications.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Mild postnatal manipulation reduces proenkephalin mRNA in the striatum in developing mice and increases morphine conditioned place preference in adulthood

Stressful events during certain neonatal periods may increase the vulnerability of an individual to develop psychopathology and/or drug dependence later in life. Therefore, in the present study, we assessed activity levels, emotionality, sensitivity to the effects of morphine, as well as expression of proenkephalin and prodynorphin in several brain regions in 35 and 90-day-old male mice, subjected to postnatal manipulation consisting in brief exposures to clean bedding (CB). In comparison with controls, CB mice showed reduced emotionality expressed as percentage of time in open arms of the elevated plus maze both at 35 days of life and in adulthood. Increased nociceptive threshold was also present in both time points measured. Conversely, higher locomotor activity was recorded in 35 days of life but not in adulthood. Analysis of film autoradiograms revealed no changes in prodynorphin mRNA level, but statistically significant decrease in the level of proenkephalin mRNA in striatum in young CB mice in comparison with young controls; no difference was observed between adult CB and control animals. CB adult mice also showed hypersensitivity to the rewarding effect of morphine in comparison with controls in the place preference test. In conclusion, our results revealed that in the critical period of development the effects of manipulation were evident, not only on behavioral responses but also on the neurochemical markers considered in the present research. Postnatal manipulation could induce changes in the dynamic neuronal processes occurring during development with long-term behavioral effects.

Maternal deprivation specifically enhances vulnerability to opiate dependence

Maternal deprivation has been shown to increase vulnerability to morphine dependence and to disturb the enkephalinergic system in adulthood. To study whether or not this vulnerability to opiates is a specific feature, we examined oral self-administration behaviour of various reinforcing substances. Experiments were performed with morphine (25 mg/l), ethanol (10%), amphetamine (25 mg/l) and cocaine (100 mg/l). Drugs were available in a continuous free choice paradigm during 3 months. Cocaine and ethanol consumption and preference were similar in both deprived and control rats. Deprived rats greatly increased their morphine consumption and 78% of them showed a progressive decrease in morphine aversion. Only a slight, but significant, increase in oral amphetamine consumption was observed in deprived rats when compared with control rats. The difference in amphetamine self-administration in control and deprived rats cannot be explained by a modification of dopamine transporter expression measured by immunoautoradiography. Altogether, we conclude that maternal deprivation worsens inherent susceptibility to dependence, specifically for opiates, and therefore represents a highly valuable model to study environmentally triggered interindividual vulnerability to opiate addiction.

Maternal separation alters drug intake patterns in adulthood in rats

Maternal separation/handling (MS/H) is an animal model of early life stress that causes profound neurochemical and behavioral alterations in pups that persist into adulthood. Many recent studies have used the MS/H model to study changes in drug effects in adulthood that are linked to behavioral treatments and stressors in the perinatal period. The drug effects focused on in this review are the reinforcing properties of the abused drugs, cocaine and alcohol. A striking finding is that variations in maternal separation and handling cause changes in ethanol and cocaine self-administration. Further, these changes indicate that various manipulations in the perinatal period can have long lasting effects of interest to biochemical pharmacologists. This article will review recent studies on ethanol and cocaine self-administration using the MS/H model and the neurochemical alterations that may play a role in the effects of MS/H on ethanol and cocaine self-administration. Studying the MS/H model can provide important clues into the vulnerability to drug abuse and perhaps identify a crucial window of opportunity for therapeutic intervention.