The effects of a single exposure to uncontrollable stress on the subsequent conditioned place preference responses to oxycodone, cocaine, and ethanol in rats

Stress- and drug-induced neuroimmune signaling as a therapeutic target for comorbid anxiety and substance use disorders

Stress and substance use disorders remain two of the most highly prevalent psychiatric conditions and are often comorbid. While individually these conditions have a debilitating impact on the patient and a high cost to society, the symptomology and treatment outcomes are further exacerbated when they occur together. As such, there are few effective treatment options for these patients, and recent investigation has sought to determine the neural processes underlying the co-occurrence of these disorders to identify novel treatment targets. One such mechanism that has been linked to stress- and addiction-related conditions is neuroimmune signaling. Increases in inflammatory factors across the brain have been heavily implicated in the etiology of these disorders, and this review seeks to determine the nature of this relationship. According to the "dual-hit" hypothesis, also referred to as neuroimmune priming, prior exposure to either stress or drugs of abuse can sensitize the neuroimmune system to be hyperresponsive when exposed to these insults in the future. This review completes an examination of the literature surrounding stress-induced increases in inflammation across clinical and preclinical studies along with a summarization of the evidence regarding drug-induced alterations in inflammatory factors. These changes in neuroimmune profiles are also discussed within the context of their impact on the neural circuitry responsible for stress responsiveness and addictive behaviors. Further, this review explores the connection between neuroimmune signaling and susceptibility to these conditions and highlights the anti-inflammatory pharmacotherapies that may be used for the treatment of stress and substance use disorders.

Transgenerational evidence of increases in dopamine D2 receptor sensitivity in rodents: Impact on sensorimotor gating, the behavioral response to nicotine and BDNF

BACKGROUND/AIMS

Neonatal quinpirole (NQ) treatment to rats increases dopamine D₂ (DAD₂) receptor sensitivity in adult animals. We investigated if increased DAD₂ sensitivity would be passed to the next (F1) generation, and if these animals demonstrated sensorimotor gating deficits and enhanced behavioral responses to nicotine.

METHODS

Male and female rats were intraperitoneal (IP) administered quinpirole (1 mg/kg) or saline (NS) from postnatal day (P)1-21. Animals were either behaviorally tested (F0) or raised to P60 and mated, creating F1 offspring.

RESULTS

Experiment 1 revealed that F1 generation animals that were the offspring of at least one NQ-treated founder increased yawning behavior, a DAD₂-mediated behavioral event, in response to acute quinpirole (0.1 mg/kg). F1 generation rats also demonstrated increased striatal β arrestin-2 and decreased phospho-AKT signaling, consistent with increased G-protein independent DAD₂ signaling, which was equal to F0 NQ-treated founders, although this was not observed in all groups. RNA-Seq analysis revealed significant gene expression changes in the F1 generation that were offspring of both NQ-treated founders compared to F0 NQ founders and controls, with enrichment in sensitivity to stress hormones and cell signaling pathways. In Experiment 2, all F1 generation offspring demonstrated sensorimotor gating deficits compared to controls, which were equivalent to F0 NQ-treated founders. In Experiment 3, all F1 generation animals demonstrated enhanced nicotine behavioral sensitization and nucleus accumbens (NAcc) brain-derived neurotrophic factor (BDNF) protein. Further, F1 generation rats demonstrated enhanced adolescent nicotine conditioned place preference equivalent to NQ-treated founders conditioned with nicotine.

CONCLUSIONS

This represents the first demonstration of transgenerational effects of increased DAD₂ sensitivity in a rodent model.

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Oxycodone, fentanyl, and morphine amplify established neuropathic pain in male rats

Opioids are widely prescribed for chronic pain, including neuropathic pain, despite growing evidence of long-term harm. Previous preclinical studies have documented exacerbation of nociceptive hypersensitivity, including that induced by peripheral nerve injury, by morphine. The present series of behavioral studies sought to replicate and extend our prior research, which demonstrated a multimonth exacerbation of nociceptive hypersensitivity by a 5-day course of morphine initiated 10 days after nerve injury. The current studies demonstrate that enduring exacerbation of nociceptive hypersensitivity is not restricted to morphine, but rather is also created by the clinically relevant opioids fentanyl and oxycodone when these are likewise administered for 5 days beginning 10 days after nerve injury. Furthermore, enduring exacerbation of nociceptive hypersensitivity is also observed when the same dosing regimen for either morphine, fentanyl, or oxycodone begins 1 month after nerve injury. Finally, a striking result from these studies is that no such exacerbation of nociceptive hypersensitivity occurs when either morphine, fentanyl, or oxycodone dosing begins at the time of nerve injury. These results extend our previous findings that morphine exacerbates nociceptive hypersensitivity to the clinically relevant opioids fentanyl and oxycodone when administered after the development of nociceptive hypersensitivity, while also providing possible clinically relevant insight into when these opioids can be safely administered and not exacerbate neuropathic pain.

Ventral tegmental area inactivation alters hormonal, metabolic, and locomotor responses to inescapable stress

Context: The involvement of unilateral and bilateral inhibition of the ventral tegmental area (VTA) in response to stress is not well understood. Objective: In the present study, the effect of unilateral and bilateral inhibition of the VTA on hormonal, metabolic, and locomotor responses to stress was assessed. Material and methods: Male rats seven days after cannulation into the VTA received electro foot-shock stress for seven consecutive days. Twenty minutes before induction of stress, 2% lidocaine hydrochloride or sterile saline (control) was injected either uni- or bi-laterally into the VTA. Results: Results showed that stress significantly increased serum corticosterone level, adrenal gland weight and anorexia, reduced weight gain, food-intake, and locomotor activity. However, bilateral inactivation of VTA prevented stress-induced these parameters changes. Conclusion: The present study demonstrated that the bilateral VTA blockade effectively relieves the symptoms of stress, while the unilateral VTA blockade does not significantly improve the changes caused by stress.

Oxycodone Exposure: A Magnetic Resonance Imaging Study in Response to Acute and Chronic Oxycodone Treatment in Rats

The present study was designed to use blood-oxygen-level dependent (BOLD) imaging to "fingerprint" the change in activity in response to oxycodone (OXY) in drug naïve rats before and after repeated exposure to OXY. It was hypothesized that repeated exposure to OXY would initiate adaptive changes in brain organization that would be reflected in an altered response to opioid exposure. Male rats exposed to OXY repeatedly showed conditioned place preference, evidence of drug-seeking behavior and putative neuroadaptation. As these studies were done on awake rats we discovered it was not possible to image rats continuously exposed to OXY due to motion artifact judged to be withdrawal while in the scanner. To circumvent this problem manganese-enhanced MRI (MEMRI) was used to map the distributed integrated activity pattern resulting from continuous OXY exposure. Rats were administered OXY (2.5 mg/kg, i.p.) during image acquisition and changes in BOLD signal intensity were recorded and the activation and deactivation of integrated neural circuits involved in olfaction and motivation were identified. Interestingly, the circuitry of the mesencephalic dopaminergic system showed little activity to the first exposure of OXY. In the MEMRI study, rats received OXY treatments (2.5 mg/kg, twice daily) for four consecutive days following intraventricular MnCl2. Under isoflurane anesthesia, T1-weighted images were acquired and subsequently analyzed showing activity in the forebrain limbic system, ventral striatum, accumbens, amygdala and hippocampus. These results show brain activity is markedly different when OXY is presented to drug naïve rats versus rats with prior, repeated exposure to drug.

Oral Binge-Like Ethanol Pre-Exposure During Juvenile/Adolescent Period Attenuates Ethanol-Induced Conditioned Place Aversion in Rats

Aims

To determine if oral ethanol self-administration produces a conditioned place preference (CPP) and to determine if ethanol pre-exposure conditions during the juvenile/adolescent period alter the conditioned effects of ethanol and subsequent ethanol self-administration.

Short summary

Modified conditioned place preference paradigm allowed rats to orally self-administer ethanol followed by short duration exposure to conditioning chambers. Ethanol produced a conditioned place aversion even though rats self-administered ethanol following the final conditioning test. Juvenile/adolescent pre-exposure to ethanol decreased the place aversion but did not produce place preference.

Methods

Juvenile/adolescent rats consumed sweetened 5% ethanol in the home-cage either during continuous access or intermittent access with water restriction that promoted binge-like consumption. A control group had water access during the 4-week period. Adult rats were conditioned using a modified CPP paradigm wherein rats were water-restricted overnight before being placed in operant chambers to respond for 5% ethanol for 7 min. Following the operant session, rats were placed in the conditioning chamber for 8 min. After the conditioning post-test, rats self-administered ethanol during daily operant sessions.

Results

Ethanol produced a conditioned place aversion in water access rats and the continuous access rats. Binge-like ethanol consumption induced by intermittent access with water restriction abolished the place aversion, but did not allow place preference to develop. After conditioning, continuous access rats self-administered ethanol above ~0.6 g/kg which was similar to rats with binge-like experience via intermittent access.

Conclusions

Results suggest that oral ethanol self-administration elicits aversive properties in this model even though ethanol continues to maintain self-administration. Pre-exposure to ethanol during the juvenile/adolescent period may produce tolerance to ethanol's aversive properties only when consumed in a binge-like manner with water restriction. More exploration is needed to understand how behavioral history can influence sensitivity to ethanol's rewarding and aversive properties and subsequent ethanol consumption or self-administration.

Comparison of chronic physical and emotional social defeat stress effects on mesocorticolimbic circuit activation and voluntary consumption of morphine

Chronic social defeat stress (CSDS) is a well-established rodent model of depression that induces persistent social avoidance. CSDS triggers molecular adaptations throughout the mesocorticolimbic reward circuit, including changes in the activity of dopamine neurons in the ventral tegmental area (VTA), that may also influence drug reward. One limitation of traditional, physical CSDS (PS) is that injury complicates the study of opiate drugs like morphine. Thus, we sought to characterize a variation of CSDS, termed emotional CSDS (ES), that eliminates this confound. We assessed the effect of PS and ES on mesocorticolimbic circuit activation, VTA gene expression, and morphine intake. We found that PS and ES similarly induced ΔFosB in the hippocampus, but only PS significantly increased ΔFosB expression in the prefrontal cortex and striatum. In contrast, cFos expression was similarly reduced by both PS and ES. Interestingly, we found that PS and ES similarly increased voluntary morphine consumption immediately following stress, despite differences in the magnitude of the depressive phenotype and striatal ΔFosB expression at this time point. Combined, these data suggest that both stress paradigms may be useful for investigation of stress-induced changes in drug behavior.

Brain nicotinic acetylcholine receptors are involved in stress-induced potentiation of nicotine reward in rats

The aim of the present study was to examine the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus (CA1 regions), the medial prefrontal cortex or the basolateral amygdala in the effect of acute or sub-chronic stress on nicotine-induced conditioned place preference. Our results indicated that subcutaneous administration of nicotine (0.2 mg/kg) induced significant conditioned place preference. Exposure to acute or sub-chronic elevated platform stress potentiated the response of an ineffective dose of nicotine. Pre-conditioning intra-CA1 (0.5-4 µg/rat) or intra-medial prefrontal cortex (0.2-0.3 µg/rat) microinjection of mecamylamine (a non-selective nicotinic acetylcholine receptor antagonist) reversed acute stress-induced potentiation of nicotine reward as measured in the conditioned place preference paradigm. By contrast, pre-conditioning intra-basolateral amygdala microinjection of mecamylamine (4 µg/rat) potentiated the effects of acute stress on nicotine reward. Our findings also showed that intra-CA1 or intra-medial prefrontal cortex, but not intra-basolateral amygdala, microinjection of mecamylamine (4 µg/rat) prevented the effect of sub-chronic stress on nicotine reward. These findings suggest that exposure to elevated platform stress potentiates the rewarding effect of nicotine which may be associated with the involvement of nicotinic acetylcholine receptors. It seems that there is a different contribution of the basolateral amygdala, the medial prefrontal cortex or the CA1 nicotinic acetylcholine receptors in stress-induced potentiation of nicotine-induced conditioned place preference.

Role of single prolonged stress in acquisition of alcohol conditioned place preference in rats

AIMS

Previous studies showed that exposure to certain types of stressors enhance the rewarding effects of many drugs of abuse, including alcohol; however, no systematic study has investigated the role of single prolonged stress (SPS) in acquisition of alcohol conditioned place preference (CPP). The purpose of this study was to examine whether SPS would facilitate the acquisition of alcohol CPP in rats.

MAIN METHODS

Male Sprague-Dawley rats were randomly assigned to either SPS exposure condition or no exposure condition. Freezing behavior and Elevated plus maze (EPM) were employed to evaluate PTSD-like symptoms induced by SPS. Further, using unbiased procedure, CPP conditioning was conducted with alcohol (2g/kg).

KEY FINDINGS

SPS significantly enhanced freezing behavior of rats, decreased percentages (%) of both time spent and number of entry into the open arms, and facilitated the acquisition of alcohol CPP without inhibiting rats' activity.

SIGNIFICANCE

Our findings suggest that SPS plays an important role in alcohol dependence, and CPP paradigm with SPS may be useful for exploring the rewarding mechanism of alcohol with regard to the interaction between alcohol and post-traumatic stress disorder (PTSD).

Stress, sex, and addiction: potential roles of corticotropin-releasing factor, oxytocin, and arginine-vasopressin

Stress sensitivity and sex are predictive factors for the development of neuropsychiatric disorders. Life stresses are not only risk factors for the development of addiction but also are triggers for relapse to drug use. Therefore, it is imperative to elucidate the molecular mechanisms underlying the interactions between stress and drug abuse, as an understanding of this may help in the development of novel and more effective therapeutic approaches to block the clinical manifestations of drug addiction. The development and clinical course of addiction-related disorders do appear to involve neuroadaptations within neurocircuitries that modulate stress responses and are influenced by several neuropeptides. These include corticotropin-releasing factor, the prototypic member of this class, as well as oxytocin and arginine-vasopressin that play important roles in affiliative behaviors. Interestingly, these peptides function to balance emotional behavior, with sexual dimorphism in the oxytocin/arginine-vasopressin systems, a fact that might play an important role in the differential responses of women and men to stressful stimuli and the specific sex-based prevalence of certain addictive disorders. Thus, this review aims to summarize (i) the contribution of sex differences to the function of dopamine systems, and (ii) the behavioral, neurochemical, and anatomical changes in brain stress systems.

The heritability of oxycodone reward and concomitant phenotypes in a LG/J × SM/J mouse advanced intercross line

The rewarding property of opioids likely contributes to their abuse potential. Therefore, determining the genetic basis of opioid reward could aid in understanding the neurobiological mechanisms of opioid addiction, provided that it is a heritable trait. Here, we characterized the rewarding property of the widely abused prescription opioid oxycodone (OXY) in the conditioned place preference (CPP) assay using LG/J and SM/J parental inbred mouse strains and 17 parent-offspring families of a LG/J × SM/J F47 /F48 advanced intercross line (AIL). Following OXY training (5 mg/kg, i.p.), SM/J mice and AIL mice, but not LG/J mice, showed an increase in preference for the OXY-paired side, suggesting a genetic basis for OXY-CPP. SM/J mice showed greater locomotor activity than LG/J mice in response to both saline and OXY. LG/J, SM/J, and AIL mice all exhibited robust OXY-induced locomotor sensitization. Narrow-sense heritability (h(2) ) estimates of the phenotypes using linear regression and maximum likelihood estimation showed good agreement (r = 0.91). OXY-CPP was clearly not a heritable trait whereas drug-free- and OXY-induced locomotor activity and sensitization were significantly and sometimes highly heritable (h(2)  = 0.30-0.84). Interestingly, the number of transitions between the saline- and OXY-paired sides emerged as a reliably heritable trait following OXY training (h(2)  = 0.46-0.66) and could represent a genetic component of drug-seeking behavior. Thus, although OXY-CPP does not appear to be amenable to genome-wide quantitative trait locus mapping, this protocol will be useful for mapping other traits potentially relevant to opioid abuse.

Involvements of stress hormones in the restraint-induced conditioned place preference

The conditioned place preference (CPP) paradigm is widely used when examining the reinforcing effects of drugs. Some previous studies have shown that an acute stressor, such as restraint could also induce CPP. Although the modulating effects of stress hormones on various forms of learning are well known, the finding that a stressor has a potentially direct role in the reinforcement mechanism is novel. This study focused on the function of stress hormones in restraint-induced CPP in Wistar rats administered agonist or antagonist of 2 critical stress hormones prior to conditioning. Results showed that peripheral applications of corticosterone (CORT, 1, 3, 5, and 10 mg/kg, subcutaneously) failed to induce CPP. Furthermore, a glucocorticoid (GC) antagonist (mifepristone, 10, 40, or 100 mg/kg, sc) failed to block the restraint-induced CPP. Intracerebroventricular injection of a selective corticotropin-releasing factor receptor 1 (CRFR1) antagonist antalarmin (1 μg/5 μl), on the contrary, completely blocked the restraint-induced CPP. We concluded that CRFR1 plays an essential role in the neural mechanism of restraint-induced CPP. Negative feedback of CORT from peripheral sources may not be involved in this phenomenon.

Increased anxiety, voluntary alcohol consumption and ethanol-induced place preference in mice following chronic psychosocial stress

Stress exposure is known to be a risk factor for alcohol use and anxiety disorders. Comorbid chronic stress and alcohol dependence may lead to a complicated and potentially severe treatment profile. To gain an understanding of the interaction between chronic psychosocial stress and drug exposure, we studied the effects of concomitant chronic stress exposure on alcohol reward using two-bottle choice and ethanol-conditioned place preference (CPP). The study consisted of exposure of the chronic subordinate colony (CSC) mice "intruders" to an aggressive "resident" mouse for 19 consecutive days. Control mice were single housed (SHC). Ethanol consumption using two-bottle choice paradigm and ethanol CPP acquisition was assessed at the end of this time period. As expected, CSC exposure increased anxiety-like behavior and reduced weight gain as compared to SHC controls. Importantly, in the two-bottle choice procedure, CSC mice showed higher alcohol intake than SHC. When testing their response to ethanol-induced CPP, CSC mice achieved higher preference for the ethanol-paired chamber. In fact, CSC exposure increased ethanol-CPP acquisition. Taken together, these data demonstrate the long-term consequences of chronic psychosocial stress on alcohol intake in male mice, suggesting chronic stress as a risk factor for developing alcohol consumption and/or anxiety disorders.

Evidence for conditioned place preference to a moderate dose of ethanol in adult male Sprague-Dawley rats

The present series of experiments examined affective properties of a moderate dose of ethanol using the conditioned place preference (CPP) paradigm in ethanol-naïve, adult male Sprague-Dawley rats. The apparatus and the procedure used were both unbiased. In Experiment 1, rats were given four 30 min conditioning sessions with 1.5 g/kg ethanol (i.p.) or an equivalent volume of saline on the paired side. Animals were found to demonstrate CPP to the ethanol-paired side, an unexpected finding at this relatively high dose in rats. To replicate this finding, and to examine the possibility of non-associative conditioning, an unpaired control group was included in Experiment 2. Once again, rats showed a CPP to the side paired with ethanol relative to either control group. Given that testing in an unfamiliar environment typically results in elevated levels of anxiety and that animals in Experiments 1 and 2 were not exposed to the apparatus prior to conditioning, Experiment 3 was conducted to examine the potential role of context unfamiliarity for induction of ethanol CPP in this test situation by varying whether animals were exposed to the apparatus prior to conditioning. In this study, pre-exposure to the CPP apparatus was found to eliminate the CPP to ethanol observed in rats who were not familiarized with the apparatus. Collectively, these studies demonstrate that ethanol-naïve rats can find ethanol reinforcing as indexed by the CPP test, and provide some evidence for the conditions under which this uncommon finding is observed.

Effect of yohimbine stress on reacquisition of oxycodone seeking in rats

RATIONALE

Stress, a powerful precipitant of drug seeking during abstinence, may also accelerate the return to pathological patterns of intake after initial instances of drug reuse.

OBJECTIVE

To explore the effect of stress on a learning process underlying relapse, this study assessed the effect of yohimbine on reacquisition of oxycodone seeking.

METHODS

One hundred thirty-two male Sprague-Dawley rats underwent place conditioning with oxycodone (2 mg/kg, SC; ×6 days), extinction (vehicle × 6 days), and reconditioning with 0, 0.25, 2, or 5 mg/kg oxycodone (2 days). Yohimbine (0, 2.5, or 5 mg/kg, IP) was administered 30 min prior to reconditioning.

RESULTS

Pretreatment with 2.5 mg/kg yohimbine increased, while 5 mg/kg yohimbine decreased, reacquisition of oxycodone-induced place preference. A follow-up study (n = 30) further indicated that the effect of yohimbine was specific to reacquisition.

CONCLUSION

The observation that yohimbine can enhance reacquisition of oxycodone seeking supports the hypothesis that stress can facilitate learning processes involved in the unfolding of relapse.

Effects of acute stress on acquisition of nicotine conditioned place preference in adolescent rats: a role for corticotropin-releasing factor 1 receptors

RATIONALE

Studies indicate that adolescence is a time of increased sensitivity to the rewarding effects of nicotine, and that stress is associated with an increased risk for smoking initiation in this age group. It is possible that stress leads to increased nicotine use in adolescence by augmenting its rewarding properties. Corticotropin-releasing factor type 1 receptors (CRF-R1) mediate physiological and behavioral stress responses. They may also mediate stress-induced potentiation of activity in multiple neural substrates implicated in nicotine reward.

OBJECTIVES

The aim of the present study was to determine the effect of acute stressor exposure on single trial nicotine conditioned place preference (CPP) in adolescent male rats using a biased CPP procedure and the role of CRF-R1 in this effect.

RESULTS

A single episode of intermittent footshock administered 24 h before the start of place conditioning dose-dependently facilitated acquisition of CPP to nicotine (0.2, 0.4, and 0.6 mg/kg). Pretreatment with CP-154,526 (20 mg/kg), a selective CRF-R1 antagonist, 30 min before footshock exposure significantly attenuated the effect of prior stress to facilitate nicotine CPP acquisition. CP-154,526 pretreatment had no effect in animals conditioned with a nicotine dose that produced CPP under non-stress conditions, suggesting a specific role for CRF-R1 following stress.

CONCLUSIONS

Taken together, the results suggest that during adolescence, nicotine reward is enhanced by recent stressor exposure in a manner that involves signaling at CRF-R1. Information from studies such as this may be used to inform efforts to prevent and treat adolescent nicotine dependence.

The Varied Uses of Conditioned Place Preference in Behavioral Neuroscience Research: An Investigation of Alcohol Administration in Model Organisms

Place conditioning procedures have been used to study human addiction to alcohol for the past several years. This experimental resource has been utilized successfully due to the fact that investigators can carefully manipulate the experimental design in order to explore specific hypotheses. Only three choices exist regarding animal response to place conditioning: aversion, preference, or no change. This review provides an in-depth analysis of five variables commonly adjusted or changed in place conditioning experiments with ethanol. These include: apparatus design, administration methods, choice of model organism, age of model organism, and model paradigms. It is suggested that the two-chamber design, the intragastric administration, the mouse model, the adolescent age group, and the pre-exposure to stress paradigm are the best current options available in place conditioning experiments with ethanol. The basis for evaluation used throughout this review is that investigators should adjust the variables employed in place conditioning experiments in a manner that most accurately represents and models complex human addiction to alcohol.

Molecular and genetic substrates linking stress and addiction

Drug addiction is one of the top three health concerns in the United States in terms of economic and health care costs. Despite this, there are very few effective treatment options available. Therefore, understanding the causes and molecular mechanisms underlying the transition from casual drug use to compulsive drug addiction could aid in the development of treatment options. Studies in humans and animal models indicate that stress can lead to both vulnerability to develop addiction, and increased drug taking and relapse in addicted individuals. Exposure to stress or drugs of abuse results in long-term adaptations in the brain that are likely to involve persistent alterations in gene expression or activation of transcription factors, such as the cAMP Response Element Binding (CREB) protein. The signaling pathways controlled by CREB have been strongly implicated in drug addiction and stress. Many potential CREB target genes have been identified based on the presence of a CRE element in promoter DNA sequences. These include, but are not limited to CRF, BDNF, and dynorphin. These genes have been associated with initiation or reinstatement of drug reward and are altered in one direction or the other following stress. While many reviews have examined the interactions between stress and addiction, the goal of this review was to focus on specific molecules that play key roles in both stress and addiction and are therefore posed to mediate the interaction between the two. Focus on these molecules could provide us with new targets for pharmacological treatments for addiction.

Stress-induced potentiation of cocaine reward: a role for CRF R1 and CREB

Both clinical and preclinical research have shown that stress can potentiate drug use; however, the underlying mechanisms of this interaction are unknown. Previously, we have shown that a single exposure to forced swim (FS) reinstates extinguished conditioned place preference (CPP) to cocaine and that cAMP response element binding protein (CREB) is necessary for this response. CREB can be activated by corticotropin releasing factor (CRF) receptor type 1 (CRF(R1)) binding, which mediates neuroendocrine and behavioral responses to stress as well as to drugs of abuse. The present experiments investigate whether changes in cocaine reward elicited by previous exposure to stress are mediated by CREB and/or CRF(R1). Chronic exposure to FS in advance of conditioning enhances cocaine CPP in wild-type mice, but this is blocked in CREB-deficient mice. In addition, pretreatment with the CRF(R1) antagonist, antalarmin, before FS exposure blocks this stress-induced enhancement of cocaine CPP. Furthermore, FS-induced increase in phosphorylated CREB (pCREB), specifically in the lateral septum (LS) and nucleus accumbens (NAc) is also blocked by antalarmin. Taken together, these studies suggest that both CREB and CRF(R1) activation are necessary for stress-induced potentiation of drug reward.

The medial prefrontal cortex regulates the differential expression of morphine-conditioned place preference following a single exposure to controllable or uncontrollable stress

Experiential factors, such as stress, are major determinants of vulnerability to drug addiction and relapse. The behavioral controllability of the stressor is a major determinant of how exposure to a stressor impacts addictive processes. Recent evidence suggests that controllable stressors, such as escapable shock (ES), activate ventral regions of the medial prefrontal cortex (mPFCv), whereas physically identical, but uncontrollable stress (inescapable shock, IS) does not. This activation is critical to the blunting effect that control has on neurochemical and behavioral sequelae of stress. Our laboratory has previously shown that IS, but not ES, potentiates morphine-conditioned place preference (CPP). However, the role of the mPFCv in this phenomenon is unknown. The present experiments investigated the role of the mPFCv during ES and IS in determining the effects of the stressor on subsequent morphine-CPP. Intra-mPFCv microinjection of the GABA(A) agonist muscimol 1 h before ES led ES to potentiate morphine-CPP, as does IS. Conversely, the potentiation of morphine-CPP normally observed in IS rats was blocked by intra-mPFCv microinjection of the GABA(A) antagonist picrotoxin 1 h before IS. These results suggest that during stress, activation of the mPFCv prevents subsequent potentiation of morphine-CPP, whereas inactivation of the mPFCv during stress does not. Thus, activation of the mPFCv during a stress experience is both necessary and sufficient to block the impact of stress on morphine-CPP, and control over stress blunts stress-induced potentiation of morphine effects by activating the mPFCv.

Stress-induced, glucocorticoid-dependent strengthening of glutamatergic synaptic transmission in midbrain dopamine neurons

Stress facilitates development of addictive behaviors in part by stress-induced increase in the strength of glutamatergic synapses at dopamine (DA) neurons within the ventral tegmental area (VTA). Here, we further demonstrate that this stress-induced synaptic adaptation is glucocorticoid-dependent and is progressively developed. Activation of glucocorticoid receptors (GRs) either by in vivo injection of dexamethasone (Dex) or incubation of the VTA slice with Dex potentiate the synaptic strength of glutamatergic synapses at VTA DA neurons, whereas preventing the activation of GRs by Ru486 abolishes this effect. These results suggest that the VTA GRs play a critical role in stress-induced cellular adaptations.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Long-term neuroendocrine and behavioural effects of a single exposure to stress in adult animals

There is now considerable evidence for long-lasting sequels of stress. A single exposure to high intensity predominantly emotional stressors such as immobilisation in wooden-boards (IMO) induces long-term (days to weeks) desensitization of the hypothalamic-pituitary-adrenal (HPA) response to the same (homotypic) stressor, whereas the response to novel (heterotypic) stressors was enhanced. In addition, long-lasting changes in behaviour have been described after a single exposure to brief or more prolonged sessions of shocks, predator, predator odour, underwater stress or a combination of three stressors on 1 day. The most consistent changes are reduced entries into the open arms of the elevated plus-maze and enhanced acoustic startle response, both reflecting enhanced anxiety. However, it is unclear whether there is any relationship between the intensity of the stressors, as evaluated by the main physiological indexes of stress (e.g. HPA axis), the putative traumatic experience they represent and their long-term behavioural consequences. This is particularly critical when trying to model post-traumatic stress disorders (PTSD), which demands a great effort to validate such putative models.

Stress and Rodent Models of Drug Addiction: Role of VTA-Accumbens-PFC-Amygdala Circuit

Stress can trigger, intensify, and prolong drug consumption, as well as reinstate previously extinguished drug-taking behavior by directly impacting a neural circuit often referred to as a reward pathways. Animal models of drug abuse have been used to understand these neural circuits mediating stress-induced drug intake and relapse through examination of cellular and subcellular molecular mechanisms. Several types of intermittent stressors have been shown to induce cross-sensitization to psychomotor stimulants, enhance conditioned place preference under most conditions, increase self-administration of cocaine and amphetamine and induce reinstatement of heroin and cocaine seeking via activation of the mesocorticolimbic dopamine system.

Role of stress in acquisition of alcohol-conditioned place preference in adolescent and adult mice

BACKGROUND

Both clinical evidence and findings from animal models demonstrate that there are differences between adolescents and adults in alcohol dependence. As stress plays a critical role in processes of alcohol addiction, we tested whether stress is involved in alcohol vulnerability differently during adolescence and adulthood in mice.

METHODS

To determine whether age differences exist in the acquisition of alcohol-conditioned place preference (CPP) in mice, adolescent and adult mice were trained for CPP with different doses of alcohol (0, 0.5, 1, and 2 g/kg, i.p.). To explore the effects of stress (footshock) on acquisition of alcohol CPP in mice of different ages, adolescent and adult mice underwent acute (1 day) or chronic (1 week) stress before CPP training. Acquisition of CPP was examined after the CPP training.

RESULTS

Under nonstress conditions, adult mice acquired alcohol CPP when trained with 2 g/kg alcohol, while adolescent mice did not acquire alcohol CPP. After chronic but not acute stress exposure, adolescent mice acquired significant CPP trained with 2 g/kg alcohol that did not produce CPP under nonstress conditions. However, stress did not have significant effect on acquisition of CPP in adult mice trained CPP with 1 g/kg alcohol.

CONCLUSIONS

These results indicate that there is an age difference in acquiring alcohol CPP and adolescent mice are more sensitive than adults to stress.

The effects of a single session of inescapable tailshock on the subsequent locomotor response to brief footshock and cocaine administration in rats

RATIONALE

We have previously shown that exposure to a single session of inescapable (IS), but not escapable (ES), tailshock can sensitize the subsequent conditioned place preference and locomotor responses to opioids, but not other drug classes. However, prior work suggests that IS might sensitize nonopioid drug responding if the drug were to be preceded by a mild stressor.

OBJECTIVES

In the following experiments, we examined the effects of IS and ES on the subsequent locomotor response to brief footshock and/or cocaine administration.

METHODS

First, we measured the locomotor response to cocaine (0, 1, 5, 10 mg/kg, intraperitoneally) 48 h after a single session of IS in adult, male Sprague-Dawley rats. Then, this procedure was repeated with 10 mg/kg cocaine, except that half of the rats received two footshocks immediately before drug administration. Finally, we manipulated the escapability of the initial stressor, as rats received either ES or yoked IS 48 h prior to footshock and cocaine administration.

RESULTS

IS did not affect the subsequent locomotor response to cocaine, but did enhance this response when cocaine administration was immediately preceded by two footshocks. The footshocks alone were without effect. This sensitizing effect was dependent on the escapability of the initial stressor, as ES did not alter the locomotor response to footshock and cocaine administration.

CONCLUSIONS

These results indicate that acute exposure to IS, but not ES, can sensitize the locomotor response to cocaine 48 h later, but only when cocaine administration is immediately preceded by a brief stressor.