Post-training cocaine exposure facilitates spatial memory consolidation in C57BL/6 mice

Chronic social defeat stress in prairie voles (Microtus ochrogaster): A preclinical model for the study of depression-related phenotypes

BACKGROUND

Stress-induced illnesses, like major depression, are among the leading causes of disability across the world. Consequently, there is a dire need for the validation of translationally-suited animal models incorporating social stress to uncover the etiology of depression. Prairie voles (Microtus ochrogaster) are more translationally relevant than many other rodent models as they display monogamous social and bi-parental behaviors. Therefore, we evaluated whether a novel social defeat stress (SDS) model in male prairie voles induces depression-relevant behavioral outcomes.

METHODS

Adult sexually-naïve male prairie voles experienced SDS bouts from a conspecific pair-bonded male aggressor, 10 min per day for 10 consecutive days. Non-stressed controls (same-sex siblings) were housed in similar conditions but never experienced physical stress. Twenty-four h later, voles were evaluated in social interaction, sucrose preference, and Morris water maze tests - behavioral endpoints validated to assess social withdrawal, anhedonia-related behavior, and spatial memory performance, respectively.

RESULTS

SDS-exposed voles displayed lower sociability and body weight, decreased preference for a sucrose solution, and impairment of spatial memory retrieval. Importantly, no differences in general locomotor activity were observed as a function of SDS exposure.

LIMITATIONS

This study does not include female voles in the experimental design.

CONCLUSIONS

We found that repeated SDS exposure, in male prairie voles, results in a depression-relevant phenotype resembling an anhedonia-like outcome (per reductions in sucrose preference) along with social withdrawal and spatial memory impairment - highlighting that the prairie vole is a valuable model with potential to study the neurobiology of social stress-induced depression-related outcomes.

Sex-Specific Alterations in Spatial Memory and Hippocampal AKT-mTOR Signaling in Adult Mice Pre-exposed to Ketamine and/or Psychological Stress During Adolescence

Background

Ketamine (KET) is administered to manage major depression in adolescent patients. However, the long-term effects of juvenile KET exposure on memory-related tasks have not been thoroughly assessed. We examined whether exposure to KET, psychological stress, or both results in long-lasting alterations in spatial memory in C57BL/6 mice. Furthermore, we evaluated how KET and/or psychological stress history influenced hippocampal protein kinase B-mechanistic target of rapamycin (AKT-mTOR)-related signaling.

Methods

On postnatal day 35, male and female mice underwent vicarious defeat stress (VDS), a form of psychological stress that reduces sociability in both sexes, with or without KET exposure (20 mg/kg/day, postnatal days 35-44). In adulthood (postnatal day 70), mice were assessed for spatial memory performance on a water maze task or euthanized for hippocampal tissue collection.

Results

Juvenile pre-exposure to KET or VDS individually increased the latency (seconds) to locate the escape platform in adult male, but not female, mice. However, juvenile history of concomitant KET and VDS prevented memory impairment. Furthermore, individual KET or VDS pre-exposure, unlike their combined history, decreased hippocampal AKT-mTOR signaling in adult male mice. Conversely, KET pre-exposure alone increased AKT-mTOR in the hippocampus of adult female mice. Lastly, rapamycin-induced decreases of mTOR in naïve adult female mice induced spatial memory retrieval deficits, mimicking adult male mice with a history of exposure to VDS or KET.

Conclusions

Our preclinical model shows how KET treatment for the management of adolescent psychological stress-induced sequelae does not impair spatial memory later in life. However, juvenile recreational KET misuse, like psychological stress history, results in long-term spatial memory deficits and hippocampal AKT-mTOR signaling changes in a sex-specific manner.

Role of dopamine D1 receptor in the modulation of memory consolidation by passive and self-administered heroin and associated conditioned stimuli

It has been proposed that opiates modulate memory consolidation, but recent work has indicated that this effect may be mediated by how the drug is experienced (i.e., passive injections vs. self-administration). Because the dopamine (DA) D1 receptor is involved in processing of learning signals and attribution of salience to events experienced by an organism, two studies in male Sprague-Dawley rats tested the effect of blocking this receptor on modulation of memory consolidation by passive and self-administered heroin, in addition to conditioned memory modulation by heroin-paired cues. Using the object location memory task, Study 1 employed SCH23390 (0, 0.05, 0.10 mg/kg, SC) to modulate enhancement of memory consolidation induced by post-training injections of heroin (1 mg/kg, SC) as well as by exposure to the environment paired with heroin injections (6 pairings, 1 h each, 1 mg/kg). Study 2 was conducted in rats that could self-administer heroin (0.05 mg/kg/infusion, IV) and tested whether SCH23390 (0 and 0.1 mg/kg, SC) could prevent memory modulation induced by a change in schedule of self-administration (from fixed to variable ratio). It was found that while repeated passive injections of heroin retained their enhancing effect on memory, when self-administered, heroin enhanced consolidation of object location memory only at the beginning of self-administration and after a change in schedule. Importantly, SCH23390 blocked memory modulation by heroin when passively administered and when the drug was self-administered on a novel schedule. SCH23390 also blocked conditioned memory modulation induced by post-training exposure to heroin-paired cues. Taken together, these results suggest that modulation of memory consolidation by unconditioned and conditioned opiate reinforcers involve a D1-dependent mechanism of salience attribution linked to the anticipation of drug effects.

Alprazolam exposure during adolescence induces long-lasting dysregulation in reward sensitivity to morphine and second messenger signaling in the VTA-NAc pathway

Increased use of benzodiazepines in adolescents have been reported, with alprazolam (ALP) being the most abused. Drug abuse during adolescence can induce changes with lasting consequences. This study investigated the neurobiological consequences of ALP exposure during adolescence in C57BL/6J male mice. Mice received ALP (0, 0.5, 1.0 mg/kg) once/daily (postnatal day 35-49). Changes in responsiveness to morphine (2.5, 5.0 mg/kg), using the conditioned place preference paradigm, were assessed 24-h and 1-month after ALP exposure. In a separate experiment, mice received ALP (0, 0.5 mg/kg) and then sacrificed 24-h or 1-month after treatment to assess levels of extracellular signal regulated kinase 1/2 (ERK1/2) gene expression, protein phosphorylation, and downstream targets (CREB, AKT) within the ventral tegmental area (VTA) and nucleus accumbens (NAc). ALP-pretreated mice developed a strong preference to the compartment(s) paired with a subthreshold dose (2.5 mg/kg) of MOR short-term, and this effect was also present in the 1-month group. Adolescent ALP exposure resulted in dysregulation of ERK-signaling within the VTA-NAc pathway 24-h and 1-month after ALP exposure. Results indicate ALP exposure during adolescence potentiates the rewarding properties of MOR and induces persistent changes in ERK-signaling within the VTA-NAc pathway, a brain circuit highly implicated in the regulation of both drug reward and mood- related behaviors.

Sex Differences in Cocaine Sensitization Vary by Mouse Strain

INTRODUCTION

Preclinical literature, frequently utilizing rats, suggests females display a more rapid advancement of substance abuse and a greater risk of relapse following drug abstinence. In clinical populations, it is less clear as to what extent biological sex is a defining variable in the acquisition and maintenance of substance use. Even without considering environmental experiences, genetic factors are presumed to critically influence the vulnerability to addiction. Genetically diverse mouse models provide a robust tool to examine the interactions between genetic background and sex differences in substance abuse.

METHODS

We explored mouse strain variability in male versus female behavioral sensitization to cocaine. Locomotor sensitization was observed following 5 consecutive days of subcutaneous cocaine across three genetically different mice strains: C57BL/6J, B6129SF2/J, and Diversity Outbred (DO/J).

RESULTS

Sex differences in cocaine locomotor sensitization were dependent on mouse strain. Specifically, we observed opposing sex differences in locomotor sensitization, with male C57BL/6J and female B6129SF2/J mice displaying heightened activity compared to their opposite sex counterparts. Conversely, no sex differences were observed in the DO/J mice. Acute cocaine administration resulted in locomotor differences across strains in male, but not female, mice. The magnitude of sensitization (or lack thereof) also varied by genetic background.

CONCLUSIONS

While sex differences in drug addiction may be observed, these effects can be mitigated, or even reversed, depending on genetic background. The clinical implications are that in the absence of understanding the genetic variables underlying vulnerability to addiction, sex provides little information regarding the predisposition of an individual to drug abuse.

Effect of early-life stress or fluoxetine exposure on later-life conditioned taste aversion learning in Sprague-Dawley rats

In rodents, early-life exposure to environmental stress or antidepressant medication treatment has been shown to induce similar long-term consequences on memory- and depression-related behavior in adulthood. To expand on this line of work, we evaluated how juvenile exposure to chronic variable stress (CVS) or the selective serotonin reuptake inhibitor fluoxetine (FLX) influences conditioned taste aversion (CTA) learning in adulthood. To do this, in Experiment 1, we examined how adolescent CVS alone (postnatal day [PND] 35-48), or with prenatal stress (PNS) history (PNS + CVS), influenced the acquisition and extinction of CTA in adult male Sprague Dawley rats. Specifically, at PND70+ (adulthood), rats were presented with 0.15 % saccharin followed by an intraperitoneal (i.p.) injection of lithium chloride (LiCl) to induce visceral malaise. A total of four saccharin (conditioned stimulus) and LiCl (unconditioned stimulus) pairings occurred across the CTA acquisition phase. Next, saccharin was presented without aversive consequences, and intake was measured across consecutive days of the extinction phase. No differences in body weight gain across the experimental days, rate of CTA acquisition, or extinction of CTA, were observed among the experimental groups (control, n = 7; CVS, n = 12; PNS + CVS, n = 9). In Experiment 2, we evaluated if early-life FLX exposure alters CTA learning in adulthood. Specifically, adolescent stress naïve male and female rats received FLX (0 or 20 mg/kg/i.p) once daily for 15 consecutive days (PND35-49). During antidepressant exposure, FLX decreased body weight gain in both male (n = 7) and female rats (n = 7), when compared to respective controls (male control, n = 8; female control, n = 8). However, juvenile FLX exposure decreased body weight-gain in adult male, but not female, rats. Lastly, adolescent FLX history had no effect on CTA acquisition or extinction in adulthood (PND70), in neither male nor female rats. Together, the data indicate that juvenile FLX exposure results in a long-term decrease of body weight-gain in a male-specific manner. Yet, independent of sex, neither early-life stress nor FLX exposure alters CTA learning in adulthood.

Adolescent fluoxetine exposure increases ERK-related signaling within the prefrontal cortex of adult male Sprague-Dawley rats

There has been a disproportionate increase in fluoxetine (FLX) prescription rates within the juvenile population. Thus, we evaluated how adolescent FLX exposure alters expression/phosphorylation of proteins from the extracellular signal regulated kinase (ERK)-1/2 cascade within the adult prefrontal cortex (PFC). Male Sprague-Dawley rats were exposed to FLX (20 mg/kg) for 15 consecutive days (postnatal-day [PD] 35-49). At PD70 (adulthood), we examined protein markers for ERK1/2, ribosomal S6 kinase (RSK), and mammalian target of rapamycin (mTOR). FLX-pretreatment decreased body weight, while increasing PFC phosphorylation of ERK1/2 and RSK, as well as total mTOR protein expression in adulthood. We provide first-line evidence that juvenile FLX-pretreatment induces long-term decreases in body weight-gain, along with neurobiological changes in the adult PFC - highlighting that early-life antidepressant exposure increases ERK-related signaling markers in later life.

GSK3β Activity in Reward Circuit Functioning and Addiction

Glycogen synthase kinase-3β (GSK3β), primarily described as a regulator of glycogen metabolism, is a molecular hub linking numerous signaling pathways and regulates many cellular processes like cytoskeletal rearrangement, cell migration, apoptosis, and proliferation. In neurons, the kinase is engaged in molecular events related to the strengthening and weakening of synapses, which is a subcellular manifestation of neuroplasticity. Dysregulation of GSK3β activity has been reported in many neuropsychiatric conditions, like schizophrenia, major depressive disorder, bipolar disorder, and Alzheimer’s disease. In this review, we describe the kinase action in reward circuit-related structures in health and disease. The effect of pharmaceuticals used in the treatment of addiction in the context of GSK3β activity is also discussed.

Persistent changes in exploration and hyperactivity coexist with cognitive impairment in mice withdrawn from chronic cocaine

Repeated cocaine exposure induces lasting neurobehavioral adaptations such as cognitive decline in animal models. However, persistent changes in spontaneous -unconditioned- motor and exploratory responses are scarcely reported. In this study, mice were administered with cocaine (20 mg/kg/day) or vehicle for 12 consecutive days. After 24 days of drug abstinence, a behavioral assessment was carried out in drug-free conditions and in unfamiliar environments (i.e. no cocaine-associated cues were presented). The cocaine-withdrawn mice showed cognitive deficits in spontaneous alternation behavior and place recognition memory. Importantly, they also displayed hyperlocomotion, increased rearing activity and altered exploratory patterns in different tasks. In the forced swimming test, they were more active (struggled/climbed more) when trying to escape from the water albeit showing normal immobility behavior. In conclusion, in addition to cognitive deficits, chronic cocaine in rodents may induce long-lasting alterations in exploratory activity and psychomotor activation that are triggered even in absence of drug-related stimuli.

Autophagy Induction and Accumulation of Phosphorylated Tau in the Hippocampus and Prefrontal Cortex of Adult C57BL/6 Mice Subjected to Adolescent Fluoxetine Treatment

BACKGROUND

Fluoxetine (FLX) represents the antidepressant of choice for the management of pediatric mood-related illnesses. Accumulating preclinical evidence suggests that ontogenic FLX exposure leads to deregulated affect-related phenotypes in adulthood. Mood-related symptomatology constitutes a risk-factor for various neurological disorders, including Alzheimer's disease (AD), making it possible for juvenile FLX history to exacerbate the development of neurodegenerative diseases.

OBJECTIVE

Because AD is characterized by the pathological accumulation of hyperphosphorylated tau, which can result from impaired function of protein degradation pathways, such as autophagy and the ubiquitin-proteasome system (UPS), we evaluated the long-term effects of adolescent FLX exposure on these pathways, using mice as a model system.

METHODS

We subjected C57BL/6 adolescent male mice to FLX (20 mg/kg/day) from postnatal day (PD) 35 to PD49. Twenty-one days after the last FLX injection (i.e., adulthood; PD70), mice were euthanized and, using immunoblotting analysis, we evaluated protein markers of autophagy (Beclin-1, LC3-II, p62) and the UPS (K48-pUb), as well as AD-associated forms of phosphorylated tau, within the hippocampus and prefrontal cortex.

RESULTS

Juvenile FLX pre-exposure mediated long-term changes in the expression of protein markers (increased LC3-II and decreased p62) that is consistent with autophagy activation, particularly in the prefrontal cortex. Furthermore, FLX history induced persistent accumulation of AD-associated variants of tau in both the hippocampus and prefrontal cortexConclusion: Adolescent FLX treatment may have enduring effects in the neuronal protein degradation machinery, which could adversely influence clearance of abnormal proteins, potentially predisposing individuals to developing AD in later life.

Adolescent fluoxetine treatment mediates a persistent anxiety-like outcome in female C57BL/6 mice that is ameliorated by fluoxetine re-exposure in adulthood

The objective of this study was to evaluate whether juvenile fluoxetine (FLX) exposure induces long-term changes in baseline responses to anxiety-inducing environments, and if so, whether its re-exposure in adulthood would ameliorate this anxiety-like phenotype. An additional goal was to assess the impact of adolescent FLX pretreatment, and its re-exposure in adulthood, on serotonin transporters (5-HTT) and brain-derived-neurotrophic-factor (BDNF)-related signaling markers (TrkB-ERK1/2-CREB-proBDNF-mBDNF) within the hippocampus and prefrontal cortex. To do this, female C57BL/6 mice were exposed to FLX in drinking water during postnatal-days (PD) 35–49. After a 21-day washout-period (PD70), mice were either euthanized (tissue collection) or evaluated on anxiety-related tests (open field, light/dark box, elevated plus-maze). Juvenile FLX history resulted in a persistent avoidance-like profile, along with decreases in BDNF-signaling markers, but not 5-HTTs or TrkB receptors, within both brain regions. Interestingly, FLX re-exposure in adulthood reversed the enduring FLX-induced anxiety-related responses across all behavioral tasks, while restoring ERK2-CREB-proBDNF markers to control levels and increasing mBDNF within the prefrontal cortex, but not the hippocampus. Collectively, these results indicate that adolescent FLX history mediates neurobehavioral adaptations that endure into adulthood, which are indicative of a generalized anxiety-like phenotype, and that this persistent effect is ameliorated by later-life FLX re-exposure, in a prefrontal cortex-specific manner.

Adolescent Fluoxetine Exposure Induces Persistent Gene Expression Changes in the Hippocampus of Adult Male C57BL/6 Mice

Mood-related disorders have a high prevalence among children and adolescents, posing a public health challenge, given their adverse impact on these young populations. Treatment with the selective serotonin reuptake inhibitor fluoxetine (FLX) is the first line of pharmacological intervention in pediatric patients suffering from affect-related illnesses. Although the use of this antidepressant has been deemed efficacious in the juvenile population, the enduring neurobiological consequences of adolescent FLX exposure are not well understood. Therefore, we explored for persistent molecular adaptations, in the adult hippocampus, as a function of adolescent FLX pretreatment. To do this, we administered FLX (20 mg/kg/day) to male C57BL/6 mice during adolescence (postnatal day [PD] 35-49). After a 21-day washout period (PD70), whole hippocampal tissue was dissected. We then used qPCR analysis to assess changes in the expression of genes associated with major intracellular signal transduction pathways, including the extracellular signal-regulated kinase (ERK), the phosphatidylinositide-3-kinase (PI3K)/AKT pathway, and the wingless (Wnt)-dishevelled-GSK3β signaling cascade. Our results show that FLX treatment results in long-term dysregulation of mRNA levels across numerous genes from the ERK, PI3K/AKT, and Wnt intracellular signaling pathways, along with increases of the transcription factors CREB, ΔFosB, and Zif268. Lastly, FLX treatment resulted in persistent increases of transcripts associated with cytoskeletal integrity (β-actin) and caspase activation (DIABLO), while decreasing genes associated with metabolism (fucose kinase) and overall neuronal activation (c-Fos). Collectively, these data indicate that adolescent FLX exposure mediates persistent alterations in hippocampal gene expression in adulthood, thus questioning the safety of early-life exposure to this antidepressant medication.

Effects of ecgonine methyl ester on cognition in scopolamine-impaired and aged rats

RATIONALE

Searches for antidotes to cocaine, and for cognition enhancers potentially applicable to Alzheimer's disease, have revealed a novel regulatory site on nicotinic acetylcholine receptors. In the presence of an agonist, inhibitors binding to this site changed the ion channel equilibrium from the open-channel form towards the closed form. Other, related, molecules could bind to the site without changing the equilibrium. These latter compounds were predicted to displace the inhibitors without affecting receptor function per se. These compounds alleviated the inhibition. One of them is ecgonine methyl ester (EME), which is generally described as inactive, but this work suggested a beneficial effect on cognition.

OBJECTIVE

This in vivo study tested for cognitive enhancement by EME in scopolamine-impaired, and aged, rats.

METHODS

Memory was the primary endpoint, but thigmotaxis became an important secondary endpoint in the light of observations made during the study. Impaired cognition was pharmacologically induced by scopolamine in young rats, and spontaneously present in aged rats. Learning ability before and after administration of EME was tested in Morris water maze protocols. Concentrations of EME in the brain and plasma were analyzed by gas chromatography-mass spectrometry.

RESULTS

A single dose of EME reversed scopolamine impairment, indicating involvement of acetylcholine receptors. Longer-term treatment improved cognition in aged rats, with enhanced rates of learning in the absence of an exogenous cognition-impairing compound. Impairment returned with a new challenge; the improvement could be re-established with continued dosing. EME also reversed thigmotaxis seen in aged rats; thigmotaxis is believed to indicate anxiety. The concentrations of EME in the brain proved adequate drug exposure.

CONCLUSIONS

Since other investigators have shown cognition impairment caused by cocaine in aged rats, this work shows that cocaine and EME have opposite effects in Morris water maze models. EME might induce cognitive enhancement and relief of anxiety in cocaine-impaired humans, and in other cognitive disorders.

Modulation of object memory consolidation by heroin and heroin-conditioned stimuli: Role of opioid and noradrenergic systems

There is recent evidence that cocaine, nicotine, and their conditioned stimuli have the ability to enhance memory consolidation. The present study compared the effects of post-training heroin and of a heroin contextual conditioned stimulus (CS+) on consolidation of object recognition memory and investigated the roles of opioid and beta-adrenergic receptors in heroin/CS+ memory modulation by co-administering the respective antagonists, naltrexone (NTX) and propranolol (PRO). Three experiments were performed in male Sprague-Dawley rats demonstrating that immediate, but not delayed, post-sample exposure to heroin (0.3, 1 mg/kg), or exposure (30 min) to a contextual CS+ paired with 1 mg/kg heroin (5 pairings, each 120 min), equally enhanced object memory. Importantly, while the memory enhancing effects of 1 mg/kg heroin and of the contextual CS+ were not altered by post-training co-administration of 3 mg/kg naltrexone, they were blocked by post-training co-administration of 10 mg/kg propranolol. Taken together, these data suggest that a context paired with heroin shares the memory enhancing effect of heroin itself and that these unconditioned and conditioned drug stimuli may modulate memory through the activation of beta-noradrenergic receptors.

Adolescent fluoxetine history impairs spatial memory in adult male, but not female, C57BL/6 mice

BACKGROUND

Epidemiological reports indicate that mood-related disorders are common in the adolescent population. The prevalence of juvenile major depressive disorder has resulted in a parallel increase in the prescription rates of fluoxetine (FLX) within this age group. Although such treatment can last for years, little is known about the enduring consequences of adolescent antidepressant exposure on memory-related performance.

METHODS

We exposed separate groups of adolescent (postnatal day [PD] 35) male and female C57BL/6 mice to FLX (20 mg/kg) for 15 consecutive days (PD35-49). Three weeks after FLX exposure (PD70), we assessed learning and memory performance on a single-day training object novelty recognition test, or a spatial memory task on the Morris water maze (MWM).

RESULTS

We found that FLX pretreatment did not influence performance on either the object novelty recognition task or the MWM, 24 h after training. Conversely, 48 h post spatial-training on the MWM, FLX pretreated male mice spent significantly less time on the quadrant of the missing platform during a standard probe trial. No differences in MWM performance were observed in the adult female mice pretreated with FLX.

LIMITATIONS

A limitation of this study is that normal adolescent mice (i.e., non-stressed) were evaluated for memory-related behavior three weeks after antidepressant exposure. Thus, it is possible that FLX pre-exposure in combination with animal models for the study of depression may yield different results.

CONCLUSION

Together, these results demonstrate enduring spatial memory-related deficiencies after pre-exposure to FLX during adolescence in male, but not female, C57BL/6 mice.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Behavioral Modulation by Spontaneous Activity of Dopamine Neurons

Dopamine modulates a variety of animal behaviors that range from sleep and learning to courtship and aggression. Besides its well-known phasic firing to natural reward, a substantial number of dopamine neurons (DANs) are known to exhibit ongoing intrinsic activity in the absence of an external stimulus. While accumulating evidence points at functional implications for these intrinsic "spontaneous activities" of DANs in cognitive processes, a causal link to behavior and its underlying mechanisms has yet to be elucidated. Recent physiological studies in the model organism Drosophila melanogaster have uncovered that DANs in the fly brain are also spontaneously active, and that this activity reflects the behavioral/internal states of the animal. Strikingly, genetic manipulation of basal DAN activity resulted in behavioral alterations in the fly, providing critical evidence that links spontaneous DAN activity to behavioral states. Furthermore, circuit-level analyses have started to reveal cellular and molecular mechanisms that mediate or regulate spontaneous DAN activity. Through reviewing recent findings in different animals with the major focus on flies, we will discuss potential roles of this physiological phenomenon in directing animal behaviors.

Lewis and Fischer 344 rats as a model for genetic differences in spatial learning and memory: Cocaine effects

Lewis (LEW) and Fischer 344 (F344) rats are considered a model of genetic vulnerability to drug addiction. We previously showed important differences in spatial learning and memory between them, but in contrast with previous experiments demonstrating cocaine-induced enhanced learning in Morris water maze (MWM) highly demanding tasks, the eight-arm radial maze (RAM) performance was not modified either in LEW or F344 rats after chronic cocaine treatment. In the present work, chronically cocaine-treated LEW and F344 adult rats have been evaluated in learning and memory performance using the Y-maze, two RAM protocols that differ in difficulty, and a reversal protocol that tests cognitive flexibility. After one of the RAM protocols, we quantified dendritic spine density in hippocampal CA1 neurons and compared it to animals treated with cocaine but not submitted to RAM. LEW cocaine treated rats showed a better performance in the Y maze than their saline counterparts, an effect that was not evident in the F344 strain. F344 rats significantly took more time to learn the RAM task and made a greater number of errors than LEW animals in both protocols tested, whereas cocaine treatment induced deleterious effects in learning and memory in the highly difficult protocol. Moreover, hippocampal spine density was cocaine-modulated in LEW animals whereas no effects were found in F344 rats. We propose that differences in addictive-like behavior between LEW and F344 rats could be related to differences in hippocampal learning and memory processes that could be on the basis of individual vulnerability to cocaine addiction.

Retrograde conditioning of place preference and motor activity with cocaine in mice

RATIONALE

In order to improve understanding of the nature of drug-associated memory, the current studies addressed whether conditioned place preference (CPP) could develop under conditions in which there was a delay between presentation of context and drug exposure (i.e., retrograde or trace conditioning).

OBJECTIVES

The objective was to assess development of CPP when cocaine or methamphetamine was injected simultaneously with exposure to a salient context (S+), or after delays differing in length.

METHODS

Dose response curves for conventional CPP were established using separate groups of Swiss-Webster mice injected with cocaine or methamphetamine just prior to S+ exposure. To assess the development of retrograde CPP, other groups received trace conditioning, where cocaine (15 mg/kg) or methamphetamine (0.5 mg/kg) was injected after a delay of 15, 60, 120, 180, 240, or 480 min following the end of the S+ session.

RESULTS

Mice receiving conventional CPP with cocaine or methamphetamine during S+ showed significant place preference. None of the groups receiving delayed methamphetamine showed significant CPP; however, CPP was evident in mice receiving cocaine after delays of up to 4 h following S+. In a separate study, delayed methamphetamine also did not result in significant place preference when presented in doses of 0.25 or 1 mg/kg.

CONCLUSIONS

These results suggest that psychostimulant drug taking may be broadly generalized to context through retrograde association with events in recent memory, a factor that may contribute to drug-seeking and relapse following abstinence.

Social defeat stress induces depression-like behavior and alters spine morphology in the hippocampus of adolescent male C57BL/6 mice

Social stress, including bullying during adolescence, is a risk factor for common psychopathologies such as depression. To investigate the neural mechanisms associated with juvenile social stress-induced mood-related endophenotypes, we examined the behavioral, morphological, and biochemical effects of the social defeat stress model of depression on hippocampal dendritic spines within the CA1 stratum radiatum. Adolescent (postnatal day 35) male C57BL/6 mice were subjected to defeat episodes for 10 consecutive days. Twenty-four h later, separate groups of mice were tested on the social interaction and tail suspension tests. Hippocampi were then dissected and Western blots were conducted to quantify protein levels for various markers important for synaptic plasticity including protein kinase M zeta (PKMζ), protein kinase C zeta (PKCζ), the dopamine-1 (D1) receptor, tyrosine hydroxylase (TH), and the dopamine transporter (DAT). Furthermore, we examined the presence of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor subunit GluA2 as well as colocalization with the post-synaptic density 95 (PSD95) protein, within different spine subtypes (filopodia, stubby, long-thin, mushroom) using an immunohistochemistry and Golgi-Cox staining technique. The results revealed that social defeat induced a depression-like behavioral profile, as inferred from decreased social interaction levels, increased immobility on the tail suspension test, and decreases in body weight. Whole hippocampal immunoblots revealed decreases in GluA2, with a concomitant increase in DAT and TH levels in the stressed group. Spine morphology analyses further showed that defeated mice displayed a significant decrease in stubby spines, and an increase in long-thin spines within the CA1 stratum radiatum. Further evaluation of GluA2/PSD95 containing-spines demonstrated a decrease of these markers within long-thin and mushroom spine types. Together, these results indicate that juvenile social stress induces GluA2- and dopamine-associated dysregulation in the hippocampus - a neurobiological mechanism potentially underlying the development of mood-related syndromes as a consequence of adolescent bullying.

A place for the hippocampus in the cocaine addiction circuit: Potential roles for adult hippocampal neurogenesis

Cocaine addiction is a chronic brain disease in which the drug seeking habits and profound cognitive, emotional and motivational alterations emerge from drug-induced neuroadaptations on a vulnerable brain. Therefore, a 'cocaine addiction brain circuit' has been described to explain this disorder. Studies in both cocaine patients and rodents reveal the hippocampus as a main node in the cocaine addiction circuit. The contribution of the hippocampus to cocaine craving and the associated memories is essential to understand the chronic relapsing nature of addiction, which is the main obstacle for the recovery. Interestingly, the hippocampus holds a particular form of plasticity that is rare in the adult brain: the ability to generate new functional neurons. There is an active scientific debate on the contributions of these new neurons to the addicted brain. This review focuses on the potential role(s) of adult hippocampal neurogenesis (AHN) in cocaine addiction. Although the current evidence primarily originates from animal research, these preclinical studies support AHN as a relevant component for the hippocampal effects of cocaine.

Memory Systems and the Addicted Brain

The view that anatomically distinct memory systems differentially contribute to the development of drug addiction and relapse has received extensive support. The present brief review revisits this hypothesis as it was originally proposed 20 years ago (1) and highlights several recent developments. Extensive research employing a variety of animal learning paradigms indicates that dissociable neural systems mediate distinct types of learning and memory. Each memory system potentially contributes unique components to the learned behavior supporting drug addiction and relapse. In particular, the shift from recreational drug use to compulsive drug abuse may reflect a neuroanatomical shift from cognitive control of behavior mediated by the hippocampus/dorsomedial striatum toward habitual control of behavior mediated by the dorsolateral striatum (DLS). In addition, stress/anxiety may constitute a cofactor that facilitates DLS-dependent memory, and this may serve as a neurobehavioral mechanism underlying the increased drug use and relapse in humans following stressful life events. Evidence supporting the multiple systems view of drug addiction comes predominantly from studies of learning and memory that have employed as reinforcers addictive substances often considered within the context of drug addiction research, including cocaine, alcohol, and amphetamines. In addition, recent evidence suggests that the memory systems approach may also be helpful for understanding topical sources of addiction that reflect emerging health concerns, including marijuana use, high-fat diet, and video game playing.

Fluoxetine exposure during adolescence increases preference for cocaine in adulthood

Currently, there is a high prevalence of antidepressant prescription rates within juvenile populations, yet little is known about the potential long-lasting consequences of such treatments, particularly on subsequent responses to drugs of abuse. To address this issue at the preclinical level, we examined whether adolescent exposure to fluoxetine (FLX), a selective serotonin reuptake inhibitor, results in changes to the sensitivity of the rewarding properties of cocaine in adulthood. Separate groups of male c57bl/6 mice were exposed to FLX (0 or 20 mg/kg) for 15 consecutive days either during adolescence (postnatal days [PD] 35–49) or adulthood (PD 65–79). Twenty-one days after FLX treatment, behavioral responsivity to cocaine (0, 2.5, 5, 10, or 20 mg/kg) conditioned place preference was assessed. Our data shows that mice pretreated with FLX during adolescence, but not during adulthood, display an enhanced dose-dependent preference to the environment paired with cocaine (5 or 10 mg/kg) when compared to age-matched saline pretreated controls. Taken together, our findings suggest that adolescent exposure to FLX increases sensitivity to the rewarding properties of cocaine, later in life.

Dopamine D1 and D5 receptors modulate spike timing-dependent plasticity at medial perforant path to dentate granule cell synapses

Although evidence suggests that DA modulates hippocampal function, the mechanisms underlying that dopaminergic modulation are largely unknown. Using perforated-patch electrophysiological techniques to maintain the intracellular milieu, we investigated how the activation of D1-type DA receptors regulates spike timing-dependent plasticity (STDP) of the medial perforant path (mPP) synapse onto dentate granule cells. When D1-type receptors were inhibited, a relatively mild STDP protocol induced LTP only within a very narrow timing window between presynaptic stimulation and postsynaptic response. The stimulus protocol produced timing-dependent LTP (tLTP) only when the presynaptic stimulation was followed 30 ms later by depolarization-induced postsynaptic action potentials. That is, the time between presynaptic stimulation and postsynaptic response was 30 ms (Δt = +30 ms). When D1-type receptors were activated, however, the same mild STDP protocol induced tLTP over a much broader timing window: tLTP was induced when -30 ms ≤ Δt ≤ +30 ms. The result indicated that D1-type receptor activation enabled synaptic potentiation even when postsynaptic activity preceded presynaptic stimulation within this Δt range. Results with null mice lacking the Kv4.2 potassium channel and with the potassium channel inhibitor, 4-aminopyridine, suggested that D1-type receptors enhanced tLTP induction by suppressing the transient IA-type K(+) current. Results obtained with antagonists and DA receptor knock-out mice indicated that endogenous activity of both D1 and D5 receptors modulated plasticity in the mPP. The DA D5 receptors appeared particularly important in regulating plasticity of the mPP onto the dentate granule cells.

Altered gene expression and spine density in nucleus accumbens of adolescent and adult male mice exposed to emotional and physical stress

Stressful early life experiences are implicated in lifelong health. However, little is known about the consequences of emotional stress (ES) or physical stress (PS) on neurobiology. Therefore, the following set of experiments was designed to assess changes in transcription and translation of key proteins within the nucleus accumbens (NAc). Male adolescent (postnatal day 35) or adult (8-week-old) mice were exposed to ES or PS using a witness social defeat paradigm. Then, 24 h after the last stress session, we measured levels of specific mRNAs and proteins within the NAc. Spine density was also assessed in separate groups of mice. Exposure to ES or PS disrupted extracellular signal-related kinase 2 (ERK2), reduced transcription of ΔFosB and had no effect on cAMP response element-binding protein (CREB) mRNA. Western blots revealed that exposure to ES or PS decreased ERK2 phosphorylation in adolescents, whereas the same stress regimen increased ERK2 phosphorylation in adults. Exposure to ES or PS had no effect on ΔFosB or CREB phosphorylation. ES and PS increased spine density in the NAc of adolescent exposed mice, but only exposure to PS increased spine density in adults. Together, these findings demonstrate that exposure to ES or PS is a potent stressor in adolescent and adult mice and can disturb the integrity of the NAc by altering transcription and translation of important signaling molecules in an age-dependent manner. Furthermore, exposure to ES and PS induces substantial synaptic plasticity of the NAc.

Drugs of abuse as memory modulators: a study of cocaine in rats

RATIONALE

It has been proposed that drugs of abuse reinforce behavior partly, or wholly, because they facilitate learning by enhancing memory consolidation. Cocaine can clearly serve as a reinforcer, but its effect on learning has not been fully characterized.

OBJECTIVES

To explore the effects of different regimens of pre- and post-training cocaine administration on win-stay and object learning.

METHODS

Cocaine naïve and cocaine pre-exposed (30 mg/kg/day, × 5 days followed by 7 days drug-free) male Sprague-Dawley rats received cocaine (0, 1, 2.5, 7.5, or 20 mg/kg, i.p.) immediately following training on a win-stay task in a radial maze or following the sample phase of an object learning task. Win-stay performance was also assessed in tests of extinction and after a set shift.

RESULTS

Post-training cocaine did not improve accuracy on the win-stay task and produced performance deficits at 20 mg/kg. These deficits were attenuated by prior cocaine exposure. There was indirect evidence of facilitated learning in extinction and set shift tests, but the effective dosage was different (2.5 and 7.5 mg/kg, respectively). Post-training cocaine produced dose-dependent improvements in object learning.

CONCLUSION

Post-training cocaine administration can facilitate learning, but this effect is highly dependent on the dose and the type of task employed.

Effects of psychotropic drugs on second messenger signaling and preference for nicotine in juvenile male mice

RATIONALE

A common treatment strategy for pediatric attention deficit/hyperactivity disorder (ADHD) and major depressive disorder (MDD) is combined methylphenidate (MPH) and fluoxetine (FLX). This has raised concerns because MPH + FLX treatment may have pharmacodynamic properties similar to cocaine, potentially increasing drug abuse liability.

OBJECTIVES

To examine the short- and long-term consequences of repeated vehicle, MPH, FLX, MPH + FLX, and cocaine treatment on gene expression in juvenile (postnatal days [PD] 20-34) and adult (PD 70-84) male mice. We further assessed whether juvenile drug treatment influenced subsequent sensitivity for nicotine in adulthood.

METHODS

Juvenile and adult C57BL/6J mice received vehicle, MPH, FLX, MPH + FLX, or cocaine twice-daily for 15 consecutive days. Mice were sacrificed 24 h or 2 months after the last drug injection to assess drug-induced effects on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway within the ventral tegmental area. Subsequent sensitivity for nicotine (0.05, 0.07, and 0.09 mg/kg) was measured using the place-conditioning paradigm (CPP) 24 h and 2 months after juvenile drug exposure.

RESULTS

MPH + FLX, or cocaine exposure in juvenile mice increased mRNA expression of ERK2 and its downstream targets (CREB, cFos, and Zif268), and increased protein phosphorylation of ERK2 and CREB 2 months after drug exposure. Similar mRNA findings were observed in the adult-treated mice. Findings on gene expression 24 h following drug treatment were variable. Juvenile drug exposure increased preference for nicotine when tested in adulthood.

CONCLUSIONS

Early-life MPH + FLX, or cocaine exposure similarly disrupts the ERK pathway, a signaling cascade implicated in motivation and mood regulation, and increases sensitivity for nicotine in adulthood.

D1/D5 receptors and histone deacetylation mediate the Gateway Effect of LTP in hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus is critical for spatial memory and is also thought to be involved in the formation of drug-related associative memory. Here, we attempt to test an aspect of the Gateway Hypothesis, by studying the effect of consecutive exposure to nicotine and cocaine on long-term synaptic potentiation (LTP) in the DG. We find that a single injection of cocaine does not alter LTP. However, pretreatment with nicotine followed by a single injection of cocaine causes a substantial enhancement of LTP. This priming effect of nicotine is unidirectional: There is no enhancement of LTP if cocaine is administrated prior to nicotine. The facilitation induced by nicotine and cocaine can be blocked by oral administration of the dopamine D1/D5 receptor antagonist (SKF 83566) and enhanced by the D1/D5 agonist (SKF 38393). Application of the histone deacetylation inhibitor suberoylanilide hydroxamic acid (SAHA) simulates the priming effect of nicotine on cocaine. By contrast, the priming effect of nicotine on cocaine is blocked in genetically modified mice that are haploinsufficient for the CREB-binding protein (CBP) and possess only one functional CBP allele and therefore exhibit a reduction in histone acetylation. These results demonstrate that the DG of the hippocampus is an important brain region contributing to the priming effect of nicotine on cocaine. Moreover, both activation of dopamine-D1 receptor/PKA signaling pathway and histone deacetylation/CBP mediated transcription are required for the nicotine priming effect in the DG.

Fluoxetine exposure during adolescence alters responses to aversive stimuli in adulthood

The mechanisms underlying the enduring neurobiological consequences of antidepressant exposure during adolescence are poorly understood. Here, we assessed the long-term effects of exposure to fluoxetine (FLX), a selective serotonin reuptake inhibitor, during adolescence on behavioral reactivity to emotion-eliciting stimuli. We administered FLX (10 mg/kg, bi-daily, for 15 d) to male adolescent [postnatal day 35 (P35) to P49] C57BL/6 mice. Three weeks after treatment (P70), reactivity to aversive stimuli (i.e., social defeat stress, forced swimming, and elevated plus maze) was assessed. We also examined the effects of FLX on the expression of extracellular signal-regulated kinase (ERK) 1/2-related signaling within the ventral tegmental area (VTA) of adolescent mice and Sprague Dawley rats. Adolescent FLX exposure suppressed depression-like behavior, as measured by the social interaction and forced swim tests, while enhancing anxiety-like responses in the elevated plus maze in adulthood. This complex behavioral profile was accompanied by decreases in ERK2 mRNA and protein phosphorylation within the VTA, while stress alone resulted in opposite neurobiological effects. Pharmacological (U0126) inhibition, as well as virus-mediated downregulation of ERK within the VTA mimicked the antidepressant-like profile observed after juvenile FLX treatment. Conversely, overexpression of ERK2 induced a depressive-like response, regardless of FLX pre-exposure. These findings demonstrate that exposure to FLX during adolescence modulates responsiveness to emotion-eliciting stimuli in adulthood, at least partially, via long-lasting adaptations in ERK-related signaling within the VTA. Our results further delineate the role ERK plays in regulating mood-related behaviors across the lifespan.

Effect of post-training administration of cocaine, diazepam and their combination on a win-stay task

According to the memory-enhancing hypothesis of addictive drugs, post-training administration of cocaine should enhance consolidation and thus facilitate learning. This hypothesis has not been tested in appetitive tasks reinforced by sucrose. The current study assessed the effect of post-training cocaine administration on the acquisition of a win-stay task, and modulation of this effect by co-administration of diazepam. Male Sprague-Dawley rats (n=63) were trained for 5 days on a win-stay task performed on an 8-arm radial maze, and were administered cocaine (0, 2.5, 7.5 or 20mg/kg), diazepam (1mg/kg), or cocaine (7.5mg/kg)+diazepam (1mg/kg) immediately following each training session. Post-training cocaine caused dose-dependent impairments that appeared linked to the development of cocaine-induced sucrose taste avoidance and/or cocaine-induced anxiety. When it was attempted to modify these learned side effects of cocaine by co-administration of diazepam, it was observed that the drug combination slowed task completion and reduced overall number of nose pokes. These findings suggest that post-training cocaine can alter behavior on appetitive tasks through learned motivational deficits rather than through a selective action on memory consolidation. The implications for the memory-enhancing hypothesis of addictive drugs are discussed.

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 represses protein phosphatase-1Cβ through DNA methylation and Methyl-CpG Binding Protein-2 recruitment in adult rat brain

Repeated cocaine exposure induces epigenetic factors such as DNA methyl-binding proteins, indicating that resulting changes in gene expression are mediated by alterations in brain DNA methylation. While the activity of protein phosphatase type-1 (PP1) is involved in cocaine effects and in brain plasticity, the expression of the PP1Cβ catalytic subunit gene was identified here as modulated by cocaine. Its expression was induced together with that of PP1Cγ in the brain of Methyl-CpG Binding Protein-2 (Mecp2) mutant mice, whereas PP1Cα expression was not affected, illustrating a different regulation of PP1C isoforms. Repeated cocaine administration was found to increase DNA methylation at the PP1Cβ gene together with its binding to Mecp2 in rat caudate putamen, establishing a link between two genes involved in cocaine-related effects and in learning and memory processes. Cocaine also increased DNMT3 expression, resulting in PP1Cβ repression that did not occur in the presence of DNMT inhibitor. Cocaine-induced PP1Cβ repression was observed in several brain structures, as evaluated by RT-qPCR, immunohistochemistry and Western blot, but did not occur after a single cocaine injection. Our data demonstrate that PP1Cβ is a direct MeCP2-target gene in vivo. They suggest that its repression may participate to behavioral adaptations triggered by the drug.

Distinct roles of methamphetamine in modulating spatial memory consolidation, retrieval, reconsolidation and the accompanying changes of ERK and CREB activation in hippocampus and prefrontal cortex

Drugs of abuse modulated learning and memory in humans yet the underlying mechanism remained unclear. The extracellular signal-regulated kinase (ERK) and the transcription factor cAMP response element-binding protein (CREB) were involved in neuroplastic changes associated with learning and memory. In the current study, we used a Morris water maze to examine the effect of methamphetamine (METH) on different processes of spatial memory in mice. We then investigated the status of ERK and CREB in the hippocampus and prefrontal cortex (PFC). We found that 1.0 mg/kg dose of METH facilitated spatial memory consolidation when it was injected immediately after the last learning trial. In contrast, the same dose of METH had no effect on spatial memory retrieval when it was injected 30 min before the test. Furthermore, 1.0 mg/kg dose of METH injected immediately after retrieval had no effect on spatial memory reconsolidation. Activation of both ERK and CREB in the hippocampus was found following memory consolidation but not after retrieval or reconsolidation in METH-treated mouse groups. In contrast, activation of both ERK and CREB in the PFC was found following memory retrieval but not other processes in METH-treated mouse groups. These results suggested that METH facilitated spatial memory consolidation but not retrieval or reconsolidation. Moreover, activation of the ERK and CREB signaling pathway in the hippocampus might be involved in METH-induced spatial memory changes.

Juvenile administration of concomitant methylphenidate and fluoxetine alters behavioral reactivity to reward- and mood-related stimuli and disrupts ventral tegmental area gene expression in adulthood

There is a rise in the concurrent use of methylphenidate (MPH) and fluoxetine (FLX) in pediatric populations. However, the long-term neurobiological consequences of combined MPH and FLX treatment (MPH + FLX) during juvenile periods are unknown. We administered saline (VEH), MPH, FLX, or MPH + FLX to juvenile Sprague Dawley male rats from postnatal day 20 to 34, and assessed their reactivity to reward- and mood-related stimuli 24 h or 2 months after drug exposure. We also assessed mRNA and protein levels within the ventral tegmental area (VTA) to determine the effect of MPH, FLX, or MPH + FLX on the extracellular signal-regulated protein kinase-1/2 (ERK) pathway--a signaling cascade implicated in motivation and mood regulation. MPH + FLX enhanced sensitivity to drug (i.e., cocaine) and sucrose rewards, as well as anxiety (i.e., elevated plus maze)- and stress (i.e., forced swimming)-eliciting situations when compared with VEH-treated rats. MPH + FLX exposure also increased mRNA of ERK2 and its downstream targets cAMP response element-binding protein (CREB), BDNF, c-Fos, early growth response protein-1 (Zif268), and mammalian target of rapamycin (mTOR), and also increased protein phosphorylation of ERK2, CREB, and mTOR 2 months after drug exposure when compared with VEH-treated rats. Using herpes simplex virus-mediated gene transfer to block ERK2 activity within the VTA, we rescued the MPH and FLX-induced behavioral deficits seen in the forced-swimming task 2 months after drug treatment. These results indicate that concurrent MPH + FLX exposure during preadolescence increases sensitivity to reward-related stimuli while simultaneously enhancing susceptibility to stressful situations, at least in part, due to long-lasting disruptions in ERK signaling within the VTA.