Impairment of spatial learning following preweaning cocaine exposure in the adult rat

MDPV (3,4-methylenedioxypyrovalerone) administered to mice during development of the central nervous system produces persistent learning and memory impairments

BACKGROUND

Synthetic cathinones (SC) constitute the second most frequently abused class of new psychoactive substances. They serve as an alternative to classic psychostimulatory drugs of abuse, such as methamphetamine, cocaine, or 3,4-methylenedioxymethamphetamine (MDMA). Despite the worldwide prevalence of SC, little is known about their long-term impact on the central nervous system. Here, we examined the effects of repeated exposure of mice during infancy, to 3,4-methylenedioxypyrovalerone (MDPV), a SC potently enhancing dopaminergic neurotransmission, on learning and memory in young adult mice.

METHODS

All experiments were performed on C57BL/6J male and female mice. Animals were injected with MDPV (10 or 20 mg/kg) and BrdU (bromodeoxyuridine, 25 mg/kg) during postnatal days 11-20, which is a crucial period for the development of their hippocampus. At the age of 12 weeks, mice underwent an assessment of various types of memory using a battery of behavioral tests. Afterward, their brains were removed for detection of BrdU-positive cells in the dentate gyrus of the hippocampal formation with immunohistochemistry, and for measurement of the expression of synaptic proteins, such as synaptophysin and PSD95, in the hippocampus using Western blot.

RESULTS

Exposure to MDPV resulted in impairment of spatial working memory assessed with Y-maze spontaneous alternation test, and of object recognition memory. However, no deficits in hippocampus-dependent spatial learning and memory were found using the Morris water maze paradigm. Consistently, hippocampal neurogenesis and synaptogenesis were not interrupted. All observed MDPV effects were sex-independent.

CONCLUSIONS

MDPV administered repeatedly to mice during infancy causes learning and memory deficits that persist into adulthood but are not related to aberrant hippocampal development.

Differential effects of MDMA and cocaine on inhibitory avoidance and object recognition tests in rodents

INTRODUCTION

Drug addiction continues being a major public problem faced by modern societies with different social, health and legal consequences for the consumers. Consumption of psychostimulants, like cocaine or MDMA (known as ecstasy) are highly prevalent and cognitive and memory impairments have been related with the abuse of these drugs.

AIM

The aim of this work was to review the most important data of the literature in the last 10 years about the effects of cocaine and MDMA on inhibitory avoidance and object recognition tests in rodents.

DEVELOPMENT

The object recognition and the inhibitory avoidance tests are popular procedures used to assess different types of memory. We compare the effects of cocaine and MDMA administration in these tests, taking in consideration different factors such as the period of life development of the animals (prenatal, adolescence and adult age), the presence of polydrug consumption or the role of environmental variables. Brain structures involved in the effects of cocaine and MDMA on memory are also described.

CONCLUSIONS

Cocaine and MDMA induced similar impairing effects on the object recognition test during critical periods of lifetime or after abstinence of prolonged consumption in adulthood. Deficits of inhibitory avoidance memory are observed only in adult rodents exposed to MDMA. Psychostimulant abuse is a potential factor to induce memory impairments and could facilitate the development of future neurodegenerative disorders.

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.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

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.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Residual performance impairments in adult rats trained on an object discrimination task subsequent to cocaine administration during adolescence

The present study was conducted to determine whether cognitive impairments in adult rats treated with cocaine during adolescence demonstrated in previous investigations extend to tests of object discrimination learning. Accordingly, 30-day-old male Long-Evans rats were injected subcutaneously with either 10 or 20 mg/kg cocaine or received control injections of saline for 7-8 consecutive days. An extended abstinence period was then introduced (mean = 70.7 ± 9.8 days) before subjects, who were now young adults (mean = 106.3 ± 10.2 days old), were assessed for acquisition of a two-choice object discrimination task. Using a correctional learning procedure conducted in a water maze, subjects were trained (eight trials per day for 10 days) to approach one of two multi-dimentional 'junk' objects. Although all animals acquired the discrimination to a reasonable extent, cocaine-treated subjects exhibited lower percentages of correct choices over the course of training (10 mg/kg = 59.6 ± 7.2% and 20 mg/kg = 59.4 ± 4.9%) relative to the saline control group (67.5 ± 4.9%). Further analyses revealed that saline-treated subjects acquired proficient discrimination performance earlier during the course of training, achieving an approximate 72% performance rate after only 3 days of training. This was in contrast to the two cocaine-treated groups needing 7 days of training to achieve comparable levels of performance. In addition, saline-treated subjects required significantly fewer trials (20.8 ± 8.9) than either cocaine-treated group (10 mg/kg = 52.2 ± 11.9 and 20 mg/kg = 63.3 ± 8.7) to reach an 87.5% correct response criterion (i.e. 7-correct-out-of-8-consecutive-trials) and performed at a higher above-chance level (13.5%) than either cocaine-treated group (3.6% and 5.3% for the 10 and 20 mg/kg cocaine groups, respectively). These findings demonstrate the existence of cognitive impairments in adulthood subsequent to cocaine exposure during adolescence despite a prolonged drug-free interval. Speculation regarding the neurobiological basis for this effect, especially with regard to alterations to prefrontal circuitry, is provided.

Different chronic cocaine administration protocols induce changes on dentate gyrus plasticity and hippocampal dependent behavior

Hippocampus is a limbic structure that participates in learning and memory formation. Specifically the dentate gyrus has been described as a hippocampal subregion with high rates of plasticity and it is targeted by different psychoactive drugs modulating synaptic plasticity. Repeated cocaine administration induces sensitization to the locomotor effects and it is believed that sensitization involves the same mechanisms of drug seeking and relapse. Although, the mechanisms underlying sensitization is not fully understood. In this work we investigated the impact of repeated intraperitoneal administration of cocaine (15 or 20 mg/kg/day along 5 or 15 days respectively; and 15 mg/kg/day along 5 day followed by a challenge dose after three days of withdrawal) on the dentate gyrus synaptic plasticity, differentiating between sensitized and nonsensitized rats. Furthermore, we correlated changes on the hippocampal synaptic plasticity to memory retention. Our results revealed that the prevalence of cocaine sensitization (around 50%) was identical in all protocols used. The results found in the threshold to generate LTP were similar for all protocols used, being the threshold values cocaine-treated groups (sensitized and nonsensitized) significantly reduced compared to controls, observing the highest reduction in the sensitized group. Moreover, we observed a facilitated retention of recent memory formation only in sensitized animals the nonsensitized subjects remained at the control levels. In conclusion, sensitization to cocaine generates a high efficiency of hippocampal synaptic plasticity that may underlie the aberrant engagement of learning processes occurred during drug addiction.

Sex differences in the effects of cocaine abuse across the life span

Cocaine alters brain function from the early days of development throughout the entire life of an individual. Since the first preclinical research on cocaine sensitization was published, sex differences in response to the drug in adult rats have been noted. With the appearance of reports on "crack babies" during the 1980s, sex differences in response to prenatal (developmental) exposure have been identified in both clinical and preclinical reports. Cocaine administered during early development in the rat produces wide-spread alterations in function which depend on the timing of drug administration as well as the sex of the animal. In males, the response patterns following postnatal days (PND) 11-20 cocaine administration (equivalent to the late prenatal period in humans) are quite similar to those seen following prenatal exposure (equivalent to the first half of pregnancy in humans). There is a general decrease in dopaminergic (DA) markers and reactivity perhaps due to the uncoupling of the D1 receptor from its second messenger system. While similar changes in D1 uncoupling are seen in females, behavioral and metabolic responses to drug challenges generally show increases in DA responsivity (except adolescents) perhaps due to the activational effects of estrogen and/or decreases in serotonin (5-HT) mediated regulation of DA function. We have found that a significant factor in the hyper-responsivity of the female is the role of the testing environment and the responses to stress which can obscure underlying neurochemical dysregulation. Whether parallel factors are operational in adult males and females is currently under investigation.

Cocaine self-administration alters the relative effectiveness of multiple memory systems during extinction

Rats were trained to run a straight-alley maze for an oral cocaine or sucrose vehicle solution reward, followed by either response or latent extinction training procedures that engage neuroanatomically dissociable "habit" and "cognitive" memory systems, respectively. In the response extinction condition, rats performed a runway approach response to an empty fluid well. In the latent extinction condition, rats were placed at the empty fluid well without performing a runway approach response. Rats trained with the sucrose solution displayed normal extinction behavior in both conditions. In contrast, rats trained with the cocaine solution showed normal response extinction but impaired latent extinction. The selective impairment of latent extinction indicates that oral cocaine self-administration alters the relative effectiveness of multiple memory systems during subsequent extinction training.

Exploratory behavior in rats postnatally exposed to cocaine and housed in an enriched environment

Exposure to cocaine in early periods of postnatal life is usually associated with changes in development of neurotransmitter systems and structure of the central nervous system. Such changes are most likely correlated with behavioral alterations. Environmental enrichment conditions (EC) in early stages is a factor that affects structural and behavioral development. The purpose of this study is to examine the effects of EC on rats postnatally exposed to cocaine on exploratory behavior. Wistar rats were assigned to four groups-Group 1: pups exposed to cocaine hydrochloride (15 mg/kg body weight/day) s.c., in two daily doses, from postnatal day (PND) 1 to 28 and reared in EC; Group 2: pups exposed to cocaine as previously described and reared in a standard environmental conditions (SC); Group 3: pups saline-injected and reared in EC; and Group 4: pups saline-injected and reared in SC. On PND 21, 24, and 28, groups of four rats (to reduce anxiety) were placed for 10 minutes into an arena with several objects. The following exploratory behavioral categories were examined: object interaction, exploration, manipulation, approximation, and total time of object contact. Animals from Group 2 showed decreased object interaction and total contact on PND 21. Control offspring reared in EE showed decreases in exploratory behavior at all ages analyzed compared with the control SE group, while cocaine-exposed animals reared in EC showed decreased object interaction, object approximation, and total exploratory behavior. The results in this group suggest that EC improved information acquisition and memory processes in animals postnatally exposed to cocaine.

Cocaine self-administration improves performance in a highly demanding water maze task

RATIONALE

Long-term potentiation (LTP) is considered to be a cellular substrate of learning and memory. Indeed, the involvement of LTP-like mechanisms in spatial learning has consistently been demonstrated in the Morris water maze test. We have previously shown that hippocampal LTP in Lewis rats was modulated by cocaine self-administration, although the performance of cocaine-self-administered rats in the Morris water maze was not altered.

OBJECTIVE

Given that the ease of the task previously used could have masked any possible effects of the cocaine-induced LTP enhancement on spatial learning, a new and more difficult water maze task was devised to address this issue.

MATERIALS AND METHODS

Animals self-administered cocaine (1 mg/kg) or saline under a fixed ratio 1 schedule of reinforcement for 22 days. Spatial learning was assessed in a difficult water maze task (four sessions, two trials per session with a 90-min intertrial interval), and spatial memory was also evaluated 48 h after training (a 90-s test). Additionally, reversal learning and perseverance were also studied.

RESULTS

There was a reduced latency in finding the hidden platform during training, as well as improved memory of the platform location in cocaine-self-administered rats with respect to animals that self-administered saline. No differences were observed in reversal learning or perseverance between groups.

CONCLUSIONS

Our data suggest that cocaine self-administration facilitates learning and memory in the water maze test only when animals are submitted to highly demanding tasks, involving working memory or consolidation-like processes during the intertrial interval.

Visuospatial working memory in school-aged children exposed in utero to cocaine

OBJECTIVE

Among the neurocognitive impairments reported as associated with prenatal cocaine exposure, slower response time, and less efficient learning in school-aged children are common to findings from several laboratories. This study presents performance data on a spatial working memory task in 75 prenatally cocaine exposed (CE) and 55 nondrug-exposed (NDE) 8- to 10-year-old children.

METHODS

Children were administered a novel neuropsychological measure of immediate- and short-term memory for visuospatial information, the Groton Maze Learning Test (GMLT), a computer-based hidden maze learning test that consists of a "timed chase test" (a simple measure of visuomotor speed), eight learning trials followed by a delayed recall trial after an 8-minute delay and a reverse learning trial. Performance is expressed as correct moves per second and number of errors per trial.

RESULTS

Across all trials, the cocaine-exposed group showed significantly slower correct moves per second and made significantly more errors. There were no significant main effects for amounts of alcohol, tobacco, or marijuana exposure. After an 8-minute delay and compared to the eighth trial, cocaine-exposed children showed less consolidation in learning compared to nonexposed children. When asked to complete the maze in reverse, cocaine-exposed children showed a greater decrement in performance (decreased correct moves per second and increased errors) compared to the eighth learning trial.

CONCLUSIONS

Children exposed in utero to cocaine exhibit a possible impairment in procedural learning and diminished efficiency in creating and accessing an internal spatial map to master the hidden maze.

Cocaine in adolescent rats produces residual memory impairments that are reversible with time

Rats received injections (subcutaneous) of either 10 or 20 mg/kg cocaine on postnatal days 26-33, while lab chow-fed and pair-fed controls received saline. Spatial memory in a Morris water maze was assessed on four different occasions commencing 10 days postcocaine and ending approximately 12 months later. To determine whether there existed long-term changes in cholinergic processes, maze performance was evaluated following 1 mg/kg scopolamine challenge 4 months postcocaine. Subjects survived under standard laboratory housing conditions until they died. Results from the first assessment indicated a working memory deficit in the low-dose cocaine group and a long-term memory impairment in the high-dose cocaine group. These decrements neither were permanent nor were exacerbated by age-related processes in that cocaine-treated subjects performed at control levels on subsequent assessments. An exception to this was the results derived from the third assessment indicating that animals previously treated with 20 mg/kg cocaine were impaired when challenged with scopolamine. Examination of mortality rates revealed that cocaine-treated rats died significantly sooner than lab chow-fed control subjects. Taken together, these data indicate that cocaine during adolescence causes residual, but not permanent, deleterious effects on memory that may be mediated by alterations in cholinergic neurochemistry. More provocatively, the results showed that cocaine during adolescence shortened the lifespan of rats. This latter finding suggests that cocaine during adolescence may produce residual physiological effects that last well into adulthood.

Beneficial effects of the sigma1 receptor agonists igmesine and dehydroepiandrosterone against learning impairments in rats prenatally exposed to cocaine

In utero cocaine (IUC) exposure results in offspring rats in complex neurochemical and behavioral alterations, particularly affecting learning and memory processes. We examined here the impact of IUC exposure on memory functions in male and female offspring rats and report that selective sigma(1) (sigma(1)) receptor agonists are effective in reversing the deficits. Dams received a daily cocaine, 20 mg/kg ip, injection between gestational days E17 to E20. Learning was examined in offspring between day P30 and P41 using delayed alternation in the T-maze, water-maze learning and passive avoidance. Both male and female rats prenatally exposed to cocaine showed delayed alternation deficits and impairments of acquisition of a fixed platform position in the water maze, as shown by higher acquisition latencies and diminutions of time spent in the training quadrant during the probe test. The acquisition of a daily changing platform position also demonstrated impaired working memory. Finally, passive avoidance deficits were observed. Pretreatment with the synthetic sigma(1) agonist igmesine (0.1-1 mg/kg ip) or the neuroactive steroid dehydroepiandrosterone (DHEA 10-40 mg/kg ip) reversed the prenatal cocaine-induced learning deficits in offspring rats for each test. The sigma(1) antagonist BD1063 (1 mg/kg ip) failed to affect performances alone but blocked the igmesine and DHEA effects, confirming the involvement of the sigma(1) receptor. IUC exposure thus results in marked memory deficits, affecting spatial and nonspatial short- and long-term memories in juvenile male and female offspring rats. The activation of the sigma(1) neuromodulatory receptor allows a complete behavioral recovery of the memory functions in prenatally cocaine-exposed rats.

The effect of developmental exposure to the fungicide triadimefon on behavioral sensitization to triadimefon during adulthood

Triadimefon (TDF) is a triazole fungicide that acts as an indirect dopamine (DA) agonist by binding to the dopamine transporter (DAT) and increasing levels of synaptic DA. Studies in this laboratory have found that repeated dosing with TDF in adult mice leads to the development and robust expression of behavioral sensitization, a response mediated by dopaminergic and glutamatergic neurotransmitter systems, and causing long-term changes in dopaminergic function. Few studies have focused on the potential for TDF to be a developmental neurotoxicant. As such, the objective of the present study was to determine whether postnatal exposure to TDF would permanently alter DA systems and thereby influence TDF-induced expression of behavioral sensitization during adulthood. Male C57BL/6 mice were dosed intraperitoneally (i.p.) with 25 mg/kg TDF (TDF25), or oil (veh) from postnatal day (PND) 8 to 21. At 8-9 weeks of age, mice were split into four groups and treated with 75 mg/kg TDF (TDF75) or vehicle twice a week for a total of seven injections, with locomotor activity measured immediately after each injection. After a 2-week withdrawal period, mice were further split into eight groups, and challenged with TDF75 or vehicle to test for the expression of behavioral sensitization. Postnatal TDF exposure attenuated both the induction and expression of TDF-induced vertical but not horizontal sensitization in adults. Postnatal TDF exposure also produced long-term decreases in basal striatal dihydroxyphenylacetic acid (DOPAC) levels and nucleus accumbens shell DAT binding. These results indicate for the first time that TDF may be considered an environmental risk factor for developmental dopaminergic neurotoxicity.

An animal model with relevance to schizophrenia: sex-dependent cognitive deficits in osteogenic disorder-Shionogi rats induced by glutathione synthesis and dopamine uptake inhibition during development

Low glutathione levels have been observed in the prefrontal cortex and the cerebrospinal fluid of schizophrenic patients, possibly enhancing the cerebral susceptibility to oxidative stress. We used osteogenic disorder Shionogi mutant rats, which constitute an adequate model of the human redox regulation because both are unable to synthesize ascorbic acid. To study the long-term consequences of a glutathione deficit, we treated developing rats with L-buthionine-(S,R)-sulfoximine (BSO), an inhibitor of glutathione synthesis, and later investigated their behavior until adulthood. Moreover, some rats were treated with the dopamine uptake inhibitor GBR 12909 in order to elevate dopamine extracellular levels, thereby mimicking the dopamine hyperactivity proposed to be involved in schizophrenia. BSO and GBR 12909 alone or in combination minimally affected the development of spontaneous alternation or basic sensory and motor skills. A major effect of BSO alone or in combination with GBR 12909 was the induction of cataracts in both sexes, whereas GBR 12909 induced an elevation of body weight in females only. Sex and age-dependent effects of the treatments were observed in a test of object recognition. At postnatal day 65, whereas male rats treated with both BSO and GBR 12909 failed to discriminate between familiar and novel objects, females were not affected. At postnatal day 94, male object recognition capacity was diminished by BSO and GBR 12909 alone or in combination, whereas females were only affected by the combination of both drugs. Inhibition of brain glutathione synthesis and dopamine uptake in developing rats induce long-term cognitive deficits occurring in adulthood. Males are affected earlier and more intensively than females, at least concerning object recognition. The present study suggests that the low glutathione levels observed in schizophrenic patients may participate in the development of some of their cognitive deficits.