Role of Dopamine 2 Receptor in Impaired Drug-Cue Extinction in Adolescent Rats

Cocaine reward and reinstatement in adolescent versus adult rodents

Adolescence is a critical juncture when initiation of drug use intersects with profound developmental changes in the brain. Adolescent drug use increases the risk to develop substance use disorders (SUDs) later in life, but the mechanisms that confer this vulnerability are not understood. SUDs are defined by cycles of use, abstinence, and relapse. Intense craving during drug-free periods is often triggered by cues and environmental contexts associated with previous use. In contrast to our understanding of stimuli that elicit craving and relapse in adults, the behavioral processes that occur during periods of abstinence and relapse in adolescents are poorly understood. The current mini-review will summarize findings from preclinical rodent studies that used cocaine conditioned place preference and operant cocaine self-administration to examine subsequent effects on reward, relapse and incubation of craving.

Influence of reconsolidation in maintenance of cocaine-associated contextual memories formed during adolescence or adulthood

Adolescents are at increased risk to develop substance use disorders and suffer from relapse throughout life. Targeted weakening of drug-associated memories has been shown to reduce relapse-like behavior in adult rats, however this process has been understudied in adolescents. We aimed to examine whether adolescent-formed, cocaine-associated memories could be manipulated via reconsolidation mechanisms. To accomplish this objective, we used an abbreviated operant cocaine self-administration paradigm (ABRV Coc-SA). Adult and adolescent rats received jugular catheterization surgery followed by ABRV Coc-SA in a distinct context for 2 h, 2×/day over 5 days. Extinction training (EXT) occurred in a second context for 2 h, 2×/day over 4 days. To retrieve cocaine-context memories, rats were exposed to the cocaine-paired context for 15 min, followed by subcutaneous injection of vehicle or the protein synthesis inhibitor cycloheximide (2.5 mg/kg). Two additional EXT sessions were conducted before a 2 h reinstatement test in the cocaine-paired context to assess cocaine-seeking behavior. We find that both adult and adolescent cocaine-exposed rats show similar levels of cocaine-seeking behavior regardless of post-reactivation treatment. Our results suggest that systemic treatment with the protein synthesis inhibitor cycloheximide does not impair reconsolidation of cocaine-context memories and subsequent relapse during adulthood or adolescence.

Context-drug-associations and reinstatement of drug-seeking behavior in male rats: Adolescent and adult time-dependent effects

RATIONALE

Drug use during adolescence results in a life-long risk to develop substance-use disorders. Adolescent rats are sensitive to different drug-associated cues, compared to adults; however, the contribution of adolescent-formed context-drug-associations to elicit relapse-like behavior is underexplored.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations to elicit time-dependent increases in cocaine-seeking behavior. This objective was accomplished using an abbreviated (ABRV) operant cocaine self-administration (Coc-SA), Extinction (EXT) paradigm, with cocaine-seeking tests occurring 1 day after training (T1, early relapse) or following 15 days of abstinence (T15, late relapse).

METHODS

Adolescent and adult rats received ABRV Coc-SA in a distinct context (2 hr, 2x/day over 5 days) then EXT in a second context (2 hr, 2x/day over 4 days). Adolescent or adult cocaine-exposed rats were then tested (2 hr, non-rewarded) in either the previous EXT or Coc-paired contexts during early or late relapse.

RESULTS & CONCLUSIONS

As previously reported, both adolescent and adult cocaine-exposed rats displayed similar magnitudes of cocaine intake and lever presses during Coc-SA, EXT, and early relapse. Independent analysis of adolescent and adult groups revealed differences in lever responding, specifically rats with cocaine exposure during adolescence showed time-dependent increases in lever responding during late relapse. These data suggest that cocaine-context associations formed during adolescence can elicit craving during adulthood and that these age-specific differences in contextual sensitivity may not be immediately observed at early relapse periods.

DOPAMAP, high-resolution images of dopamine 1 and 2 receptor expression in developing and adult mouse brains

The dopaminergic system undergoes major reorganization during development, a period especially vulnerable to mental disorders. Forebrain neurons expressing dopamine 1 and 2 receptors (D1R and D2R, respectively) play a key role in this system. However, neuroanatomical information about the typical development of these neurons is sparse and scattered across publications investigating one or a few brain regions. We here present a public online collection of microscopic images of immunohistochemically stained serial sections from male and female mice at five stages of development (postnatal day 17 (P17), P25, P35, P49, and adult), showing the distribution of D1R and D2R expressing neurons across the forebrain. All images from adult brains are registered to the Allen Mouse brain Common Coordinate Framework, while images from P17-P35 age groups are registered to spatially modified atlas versions matching the morphology of young brains. This online resource provides microscopic visualization of the developing dopaminergic system in mice, which is suitable as a benchmark reference for performing new experiments and building computational models of the brain.

Measurement(s)	mRNA expression
Technology Type(s)	transgenic Mouse • immunohistochemistry staining method
Sample Characteristic - Organism	Mus musculus

The enduring behavioral and neurobiological effects of a flavor cue paired with alcohol drinking during adolescence on the incentive properties of the flavor cue in adulthood in female alcohol-preferring (P) rats

BACKGROUND

Alcohol use disorders (AUDs) affect 15 million people nationwide, 4% of which are adolescents (ages 12-17) and adolescents who binge drink significantly increase their likelihood of suffering from an AUD in adulthood. Research shows that cues (i.e. flavors) paired with alcohol (EtOH) produce significant cue-induced alcohol craving and contribute to relapse in adolescent and adult populations. However, there is a lack of research focused on how cues that accompany EtOH drinking during adolescence, affect EtOH craving later in life. The current study sought to examine the sex- and developmental-dependent effects of adolescent exposure to flavor cues associated with EtOH on operant-lick behavior and cue-induced dopamine (DA) levels within the nucleus accumbens shell (AcbSh; reward structure) in adulthood.

METHODS

Adolescent alcohol-preferring (P) rats were randomly assigned to one of 4 groups and received 24 hr. access to three bottles on their home cage: Paired: 0.1% blueberry flavor extract (BB) + 15% v/v EtOH and 2 water bottles; Unpaired: 0.1% BB, 15% v/v EtOH, and water; 15% EtOH alone, and 2 water bottles; BB alone and 2 water bottles. Home cage fluid consumption was measured for 2-weeks. On the third week bottles were removed and all animals underwent 9 days of operant training using an operant sipper paradigm. This consisted of two sipper spouts connected to the computer by a lickometer, which registered tongue contacts with the sipper tube (Paired: BB+EtOH or water; Unpaired BB or EtOH; EtOH alone: EtOH or water; BB alone: BB or water). When the fixed ratio (FR) requirement for number of licks/tongue contacts was met, a liquid delivery solenoid dispensed 0.05 ml of fluid into the sipper tube. Following the final operant session all rats remained in their home-cage for approximately 40 days until adulthood at which point they were returned to the operant chambers and tested for appetitive and consummatory behavior in response to the flavor cue (all rats: BB or water; NO EtOH). Two weeks after the final operant session all rats underwent microdialysis testing to examine cue-induced DA levels in the AcbSh.

RESULTS

Data indicated that animals in the paired group exhibited a significantly greater level of licking at the BB sipper and a significantly greater level of DA release in response to the flavor cue compared to the other groups.

CONCLUSIONS

Overall, the data suggest that cues paired with EtOH during adolescence may produce persistent changes to the behavioral and neurobiological mechanisms that contribute to an increased risk of developing an AUD later in life.

Effects of the GluN2B antagonist, Ro 25-6981, on extinction consolidation following adolescent- or adult-onset methamphetamine self-administration in male and female rats

Previous work suggests adolescent rats have deficient extinction consolidation relative to adults. Although the mechanisms underlying this age difference are currently unknown, studies in adult rats have implicated GluN2B-containing N-methyl-d-aspartate (NMDA) receptor function in extinction consolidation of drug-associated memory. Importantly, GluN2B neurotransmission emerges during adolescent development, and drugs of abuse during adolescence may delay the development of extinction consolidation by disrupting the ontogeny of GluN2B function. Here, we trained Sprague-Dawley rats of both sexes to self-administer methamphetamine [METH, 0.1 mg/kg/infusion intravenous (i.v.)] beginning during adolescence [postnatal (P) day 41] or adulthood (P91). Rats were given short access (2 h) to self-administer METH in seven daily sessions followed by 14 sessions with long access (6 h). Subsequently, rats underwent four daily 30-minute extinction sessions with immediate postsession injections of either a GluN2B antagonist [Ro25-6981; 6 mg/kg, intraperitoneal (i.p.)] or a vehicle solution. After four daily 2-h extinction sessions, a priming injection (1 mg/kg METH, i.p.) was given prior to a final 2-h reinstatement session. During LgA, adolescent-onset rats earn more METH than adult-onset rats and display greater drug-loading behavior. Rats reduced their drug-seeking behavior across the extinction sessions, with no significant group differences. Rats reinstated drug-seeking following the METH-priming injection, with females displaying greater reinstatement than males. These results do not support our a priori hypothesis that adolescent-onset METH use disrupts the ontogeny of GluN2B transmission and contributes to age-of-onset differences in extinction of METH-seeking. However, our findings suggest that age-of-onset contributes to excessive METH-taking, while sex confers vulnerability to relapse to METH-seeking.

Insular cortex dopamine 1 and 2 receptors in methamphetamine conditioned place preference and aversion: Age and sex differences

Rodent studies have proposed that adolescent susceptibility to substance use is at least partly due to adolescents experiencing reduced aversive effects of drugs compared to adults. We thus investigated methamphetamine (meth) conditioned place preference/aversion (CPP/CPA) in adolescent and adult mice in both sexes using a high dose of meth (3 mg/kg) or saline as controls. Mice tagged with green-fluorescent protein (GFP) at Drd1a or Drd2 were used so that dopamine receptor 1 (D1) and 2 (D2) expression within the insular cortex (insula) could be quantified. There are sex differences in how the density of D1+ and D2+ cells in the insula changes across adolescence that may be related to drug-seeking behaviors. Immunohistochemistry followed by stereology were used to quantify the density of cells with c-Fos and/or GFP in the insula. Unexpectedly, mice showed huge variability in behaviors including CPA, CPP, or no preference or aversion. Females were less likely to show CPP compared to males, but no age differences in behavior were observed. Conditioning with meth increased the number of D2 + cells co-labelled with c-Fos in adults but not in adolescents. D1:D2 ratio also sex- and age-dependently changed due to meth compared to saline. These findings suggest that reduced aversion to meth is unlikely an explanation for adolescent vulnerability to meth use. Sex- and age-specific expressions of insula D1 and D2 are changed by meth injections, which has implications for subsequent meth use.

Cocaine-Induced Changes in Tonic Dopamine Concentrations Measured Using Multiple-Cyclic Square Wave Voltammetry in vivo

For over 40 years, in vivo microdialysis techniques have been at the forefront in measuring the effects of illicit substances on brain tonic extracellular levels of dopamine that underlie many aspects of drug addiction. However, the size of microdialysis probes and sampling rate may limit this technique’s ability to provide an accurate assessment of drug effects in microneural environments. A novel electrochemical method known as multiple-cyclic square wave voltammetry (M-CSWV), was recently developed to measure second-to-second changes in tonic dopamine levels at microelectrodes, providing spatiotemporal resolution superior to microdialysis. Here, we utilized M-CSWV and fast-scan cyclic voltammetry (FSCV) to measure changes in tonic or phasic dopamine release in the nucleus accumbens core (NAcc) after acute cocaine administration. Carbon-fiber microelectrodes (CFM) and stimulating electrodes were implanted into the NAcc and medial forebrain bundle (MFB) of urethane anesthetized (1.5 g/kg i.p.) Sprague-Dawley rats, respectively. Using FSCV, depths of each electrode were optimized by determining maximal MFB electrical stimulation-evoked phasic dopamine release. Changes in phasic responses were measured after a single dose of intravenous saline or cocaine hydrochloride (3 mg/kg; n = 4). In a separate group, changes in tonic dopamine levels were measured using M-CSWV after intravenous saline and after cocaine hydrochloride (3 mg/kg; n = 5). Both the phasic and tonic dopamine responses in the NAcc were augmented by the injection of cocaine compared to saline control. The phasic and tonic levels changed by approximately x2.4 and x1.9, respectively. These increases were largely consistent with previous studies using FSCV and microdialysis. However, the minimal disruption/disturbance of neuronal tissue by the CFM may explain why the baseline tonic dopamine values (134 ± 32 nM) measured by M-CSWV were found to be 10-fold higher when compared to conventional microdialysis. In this study, we demonstrated phasic dopamine dynamics in the NAcc with acute cocaine administration. M-CSWV was able to record rapid changes in tonic levels of dopamine, which cannot be achieved with other current voltammetric techniques. Taken together, M-CSWV has the potential to provide an unprecedented level of physiologic insight into dopamine signaling, both in vitro and in vivo, which will significantly enhance our understanding of neurochemical mechanisms underlying psychiatric conditions.

BEHAVIORAL AND NEUROBIOLOGICAL MECHANISMS OF PAVLOVIAN AND INSTRUMENTAL EXTINCTION LEARNING

This article reviews the behavioral neuroscience of extinction, the phenomenon in which a behavior that has been acquired through Pavlovian or instrumental (operant) learning decreases in strength when the outcome that reinforced it is removed. Behavioral research indicates that neither Pavlovian nor operant extinction depends substantially on erasure of the original learning but instead depends on new inhibitory learning that is primarily expressed in the context in which it is learned, as exemplified by the renewal effect. Although the nature of the inhibition may differ in Pavlovian and operant extinction, in either case the decline in responding may depend on both generalization decrement and the correction of prediction error. At the neural level, Pavlovian extinction requires a tripartite neural circuit involving the amygdala, prefrontal cortex, and hippocampus. Synaptic plasticity in the amygdala is essential for extinction learning, and prefrontal cortical inhibition of amygdala neurons encoding fear memories is involved in extinction retrieval. Hippocampal-prefrontal circuits mediate fear relapse phenomena, including renewal. Instrumental extinction involves distinct ensembles in corticostriatal, striatopallidal, and striatohypothalamic circuits as well as their thalamic returns for inhibitory (extinction) and excitatory (renewal and other relapse phenomena) control over operant responding. The field has made significant progress in recent decades, although a fully integrated biobehavioral understanding still awaits.

Associative processes in addiction relapse models: A review of their Pavlovian and instrumental mechanisms, history, and terminology

Animal models of relapse to drug-seeking have borrowed heavily from associative learning approaches. In studies of relapse-like behaviour, animals learn to self-administer drugs then receive a period of extinction during which they learn to inhibit the operant response. Several triggers can produce a recovery of responding which form the basis of a variety of models. These include the passage of time (spontaneous recovery), drug availability (rapid reacquisition), extinction of an alternative response (resurgence), context change (renewal), drug priming, stress, and cues (reinstatement). In most cases, the behavioural processes driving extinction and recovery in operant drug self-administration studies are similar to those in the Pavlovian and behavioural literature, such as context effects. However, reinstatement in addiction studies have several differences with Pavlovian reinstatement, which have emerged over several decades, in experimental procedures, associative mechanisms, and terminology. Interestingly, in cue-induced reinstatement, drug-paired cues that are present during acquisition are omitted during lever extinction. The unextinguished drug-paired cue may limit the model’s translational relevance to cue exposure therapy and renders its underlying associative mechanisms ambiguous. We review major behavioural theories that explain recovery phenomena, with a particular focus on cue-induced reinstatement because it is a widely used model in addiction. We argue that cue-induced reinstatement may be explained by a combination of behavioural processes, including reacquisition of conditioned reinforcement and Pavlovian to Instrumental Transfer. While there are important differences between addiction studies and the behavioural literature in terminology and procedures, it is clear that understanding associative learning processes is essential for studying relapse.

Exploration of Epigenetic State Hyperdopaminergia (Surfeit) and Genetic Trait Hypodopaminergia (Deficit) During Adolescent Brain Development

Background

The risk for all addictive drug and non-drug behaviors, especially, in the unmyelinated Prefrontal Cortex (PFC) of adolescents, is important and complex. Many animal and human studies show the epigenetic impact on the developing brain in adolescents, compared to adults. Some reveal an underlying hyperdopaminergia that seems to set our youth up for risky behaviors by inducing high quanta pre-synaptic dopamine release at reward site neurons. In addition, altered reward gene expression in adolescents caused epigenetically by social defeat, like bullying, can continue into adulthood. In contrast, there is also evidence that epigenetic events can elicit adolescent hypodopaminergia. This complexity suggests that neuroscience cannot make a definitive claim that all adolescents carry a hyperdopaminergia trait.

Objective

The primary issue involves the question of whether there exists a mixed hypo or hyper-dopaminergia in this population.

Method

Genetic Addiction Risk Score (GARS®) testing was carried out of 24 Caucasians of ages 12-19, derived from families with RDS.

Results

We have found that adolescents from this cohort, derived from RDS parents, displayed a high risk for any addictive behavior (a hypodopaminergia), especially, drug-seeking (95%) and alcohol-seeking (64%).

Conclusion

The adolescents in our study, although more work is required, show a hypodopaminergic trait, derived from a family with Reward Deficiency Syndrome (RDS). Certainly, in future studies, we will analyze GARS in non-RDS Caucasians between the ages of 12-19. The suggestion is first to identify risk alleles with the GARS test and, then, use well-researched precision, pro-dopamine neutraceutical regulation. This "two-hit" approach might prevent tragic fatalities among adolescents, in the face of the American opioid/psychostimulant epidemic.

Role of adolescent-formed, context-drug-associations on reinstatement of drug-seeking behavior in rats

RATIONALE

Drug use during adolescence results in a lifelong risk to develop substance-use disorders. Adolescent rats are less reactive to cocaine-associated cues compared with adults; however, the contribution of adolescent-formed, context-drug-associations to elicit relapse-like behavior is underexplored. Although it is known that social isolation can impact drug-seeking behavior, the effects of housing conditions on context-induced, cocaine-seeking during adolescence vs adulthood are unknown.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations under different housing conditions (pair vs single) on cocaine-seeking behavior during adolescence or adulthood. This objective was accomplished using operant cocaine self-administration (Coc-SA) under a standard, non-abbreviated (Non-ABRV) or modified abbreviated (ABRV) paradigm.

METHODS

In experiment 1, adolescent and adult rats received Non-ABRV Coc-SA in a distinct context (2 h, 1×/day, 10 days), and extinction training (EXT) in a second context (1 h, 1×/day, 8 days) with reinstatement test (TEST) during adulthood in the cocaine-paired context. In experiments 2 and 3, rats received all behavioral phases during adolescence or adulthood: ABRV Coc-SA (2 h, 2×/day, 5 days), EXT (1 h, 4×/day, 2 days) with TEST in a cocaine-paired or novel, unpaired context. All experiments included pair and single-housing conditions.

RESULTS AND CONCLUSIONS

Age at cocaine exposure did not influence behavior in Non-ABRV or ABRV paradigms. Under Non-ABRV conditions, adolescent and adult single-housed rats had higher seeking behavior than pair housed. These data suggest that social isolation influences context-induced, cocaine-seeking regardless of age at drug exposure and provides a condensed, ABRV paradigm to investigate context-induced, cocaine-seeking behavior during adolescence.

AMPed-up adolescents: The role of age in the abuse of amphetamines and its consequences on cognition and prefrontal cortex development

Adolescent use of amphetamine and its closely related, methylated version methamphetamine, is alarmingly high in those who use drugs for nonmedical purposes. This raises serious concerns about the potential for this drug use to have a long-lasting, detrimental impact on the normal development of the brain and behavior that is ongoing during adolescence. In this review, we explore recent findings from both human and laboratory animal studies that investigate the consequences of amphetamine and methamphetamine exposure during this stage of life. We highlight studies that assess sex differences in adolescence, as well as those that are designed specifically to address the potential unique effects of adolescent exposure by including groups at other life stages (typically young adulthood). We consider epidemiological studies on age and sex as vulnerability factors for developing problems with the use of amphetamines, as well as human and animal laboratory studies that tap into age differences in use, its short-term effects on behavior, and the long-lasting consequences of this exposure on cognition. We also focus on studies of drug effects in the prefrontal cortex, which is known to be critically important for cognition and is among the later maturing brain regions. Finally, we discuss important issues that should be addressed in future studies so that the field can further our understanding of the mechanisms underlying adolescent use of amphetamines and its outcomes on the developing brain and behavior.

Extinction and drug-induced reinstatement of cocaine seeking following self-administration or conditioned place preference in adolescent and adult rats

Adolescence marks a particularly vulnerable period to developing substance use disorders, and people who start using drugs in adolescence are more likely to relapse. A limited number of studies have investigated age difference in relapse following re-exposure to the drug after a period of abstinence. Using a cocaine self-administration paradigm, we showed no age difference in acquisition or extinction of self-administration. Interestingly, adolescent rats displayed impaired cocaine-primed reinstatement of cocaine seeking. Using the same dose as that self-administered in the first experiment, we then investigated age differences in acquisition and extinction of conditioned place preference, as well as locomotor sensitization. While there were no differences in locomotor activity or acquisition of preference, adolescents failed to extinguish their preference, even when the number of extinction sessions was doubled from what adults received. Taken together, these results suggest that while cocaine has similar rewarding and reinforcing effects regardless of age, adolescents may attribute stronger salience to the drug-associated context. In addition, re-exposure to cocaine itself may not be a strong relapse trigger in adolescence. Overall, these findings suggest that we should focus more on alleviating drug-context salience compared to re-exposure to substance in order to reduce relapse of drug seeking in adolescents.

Assessment of conditioned fear extinction in male and female adolescent rats

Pavlovian fear conditioning and extinction have been widely studied across many species to understand emotional learning and memory. Importantly, it is becoming clear that these processes are affected by sex and age. In adult rodents and humans, sex differences are evident in extinction, with estradiol playing a significant role. In adolescence, an extinction deficit has been reported in rodents and humans. However, the influence of sex on extinction during adolescence is unknown. This is surprising, since adolescence coincides with the onset of hormone cycling, and therefore it might be expected that hormones fluctuations exert a more profound effect at this time. Therefore, we examined Pavlovian fear conditioning and extinction in adolescent male and female rats. In experiment 1, 35-day-old male and female rats were exposed to 6 pairings of a conditioned stimulus (CS, a tone) with an aversive unconditioned stimulus (US, a footshock). The next day they were extinguished in a contextually distinct chamber, via 60 presentations of the CS without the US. Extinction recall was tested 24 hours later in the extinction context. Estrous phase was monitored by cytology on vaginal smears taken 1 hour after each behavioral session. In experiment 2, male and female rats were given sham surgery or gonadectomy at 21 days of age. They were then trained and tested as for experiment 1. We observed that females in proestrus or met/diestrus during extinction showed delayed extinction and impaired extinction recall the next day compared to males. Ovariectomy enhanced extinction for female rats, but orchidectomy delayed extinction for males. Plasma analyses showed that met/di/proestrus phases were associated with high estradiol levels. These findings suggest that high plasma estradiol levels impair extinction for adolescent females. These results contradict what is reported in adult animals, suggesting that hormonal influences on extinction are dependent on age. Given that impaired extinction is widely used as a model to understand resistance to exposure-based therapies, our findings have important implications for understanding mental health treatments in adolescents.

Effects of Methamphetamine Exposure on Fear Learning and Memory in Adult and Adolescent Rats

Methamphetamine (meth) use is often comorbid with anxiety disorders, with both conditions predominant during adolescence. Conditioned fear extinction is the most widely used model to study the fear learning and regulation that are relevant for anxiety disorders. The present study investigates how meth binge injections or meth self-administration affect subsequent fear conditioning, extinction and retrieval in adult and adolescent rats. In experiment 1, postnatal day 35 (P35-adolescent) and P70 (adult) rats were intraperitoneally injected with increasing doses of meth across 9 days. At P50 or P85, they underwent fear conditioning followed by extinction and test. In experiments 2a-c, P35 or P70 rats self-administered meth for 11 days then received fear conditioning at P50 or P85, followed by extinction and test. We observed that meth binge exposure caused a significant disruption of extinction retrieval in adult but not adolescent rats. Interestingly, meth self-administration in adolescence or adulthood disrupted acquisition of conditioned freezing in adulthood. Meth self-administration in adolescence did not affect conditioned freezing in adolescence. These results suggest that intraperitoneal injections of high doses of meth and meth self-administration have dissociated effects on fear conditioning and extinction during adulthood, while adolescent fear conditioning and extinction are unaffected.

The effects of Pavlovian cue extinction and ceftriaxone on cocaine relapse after abstinence

BACKGROUND

Cocaine use disorder is a significant public health problem and currently no medications are FDA-approved to reduce cocaine relapse. Drug-associated cues are reported to elicit craving and cocaine-seeking in humans. Repeated, non-reinforced presentations of drug-associated cues (cue extinction) have been proposed to reduce the ability of such cues to prompt drug-seeking. In rodent models of cocaine relapse, cue extinction reduces cocaine relapse when such extinction occurs in the same context as cocaine self-administration, which is not akin to the manner in which treatment would occur in humans. Here we sought to determine whether cue extinction outside of the cocaine self-administration context would reduce relapse in the drug context. We also hypothesized that ceftriaxone, an antibiotic consistently shown to attenuate cocaine relapse in rats, would enhance the relapse-preventing effects of cue extinction.

METHODS

Rats self-administered intravenous cocaine for 12 days followed by 20-21 days of abstinence. Immediately preceding the relapse test, rats either underwent 6 single daily cue extinction sessions (1 h/day) outside the self-administration context or no extinction with daily handling. Rats also received vehicle or ceftriaxone (200 mg/kg IP) on those six days.

RESULTS

Ceftriaxone attenuated cued relapse relative to vehicle-treated rats, but there was no additive effect of cue extinction on cocaine-seeking. Cue extinction alone did not attenuate relapse.

CONCLUSIONS

Thus, in agreement with work in humans, when cue extinction is conducted outside the drug-associated context it does not reduce the risk of relapse alone. Ceftriaxone remains a strong possibility for medication to reduce cocaine relapse in humans.

Eternal sunshine of the neuromodulated mind: Altering fear memories through neuromodulation

Anxiety disorders pose one of the greatest threats to mental health. Modern treatment methods exist but are hindered by relapse, toxicity, and low efficacy. The use of neuromodulation to treat anxiety disorders has shown promising results, yet its underpinning mechanisms remain poorly understood. In this review, we make the case for further development of neuromodulation techniques to alter fear memories, with particular regard to future clinical applications in treating anxiety disorders. We start by briefly summarizing the neural circuitry of fear while identifying the pros and cons of possible neuromodulation targets. We then highlight recent advances in neuromodulation techniques that have been used to alter fear memories. Next, we apply a novel network-based approach to elucidate possible mechanisms of neuromodulation which may disrupt the consolidation of fear memory. Finally, we emphasize the need for more systematic neuromodulation studies on animal models and the developing brain. Overall, we aim to provide an integrated framework for future action, identifying key research priorities that must be addressed before effective neuromodulation-based treatments can be developed for practical use.

Role of prefrontal cortex in the extinction of drug memories

It has been recognized that drug addiction engages aberrant process of learning and memory, and substantial studies have focused on developing effective treatment to erase the enduring drug memories to reduce the propensity to relapse. Extinction, a behavioral intervention exposing the individuals to the drug-associated cues repeatedly, can weaken the craving and relapse induced by drug-associated cues, but its clinic efficacy is limited. A clear understanding of the neuronal circuitry and molecular mechanism underlying extinction of drug memory will facilitate the successful use of extinction therapy in clinic. As a key component of mesolimbic system, medial prefrontal cortex (mPFC) has received particular attention largely in that PFC stands at the core of neural circuits for memory extinction and manipulating mPFC influences extinction of drug memories and subsequent relapse. Here, we review the recent advances in both animal models of drug abuse and human addicted patients toward the understanding of the mechanistic link between mPFC and drug memory, with particular emphasis on how mPFC contributes to the extinction of drug memory at levels ranging from neuronal architecture, synaptic plasticity to molecular signaling and epigenetic regulation, and discuss the clinic relevance of manipulating the extinction process of drug memory to prevent craving and relapse through enhancing mPFC function.

Developmental perspectives on methamphetamine abuse: Exploring adolescent vulnerabilities on brain and behavior

Most people that experience illicit drugs do so for the first time during adolescence, and methamphetamine (meth) is no exception. Therefore, research into the effects of meth should highlight the adolescent period. Despite this, the vast majority of current literature has mainly focused on meth exposure during adulthood. In this review, we first describe existing literature that compares the behavioral effects of meth where exposure occurs in adolescence compared to adulthood. Given that there are actually very few such studies, we also look at what is known about neural effects of meth in the adult brain, and relate these to normal neural development occurring during the adolescent period to establish how meth may target maturing regions and related neurochemistry. What emerges overall is that adolescents appear to be more vulnerable to the rewarding and reinforcing effects of meth, and that meth indeed has effects on areas that are in flux during adolescence. However, there is some evidence for a paradoxical resistance to the neurotoxic effects during this period. We highlight the need for further age-related research to better understand, treat, and prevent meth use disorders and addiction in general.

Emerging role for the medial prefrontal cortex in alcohol-seeking behaviors

The medial prefrontal cortex (mPFC) plays an important role in high-order executive processes and sends highly organized projections to sub-cortical regions controlling mood, motivation and impulsivity. Recent preclinical and clinical studies have demonstrated alcohol-induced effects on the activity and composition of the PFC which are implicated in associative learning processes and may disrupt executive control over impulsivity, leading to an inability to self-limit alcohol intake. Animal studies have begun to dissect the role of the mPFC circuitry in alcohol-seeking behavior and withdrawal, and have identified a key role for projections to sub-cortical sites including the extended amygdala and the nucleus accumbens (NAc). Importantly, these studies have highlighted that alcohol can have contrasting effects on the mPFC compared to other addictive substances and also produce differential effects on the structure and activity of the mPFC following short-term versus long-term consumption. Because of these differences, how the mPFC influences the initial aspects of alcohol-seeking behavior and how we can better understand the long-term effects of alcohol use on the activity and connectivity of the mPFC need to be considered. Given the lack of preclinical data from long-term drinking models, an increased focus should be directed towards identifying how long-term alcohol use changes the mPFC, in order to provide new insights into the mechanisms underlying the transition to dependence.

Extinction of Conditioned Fear in Adolescents and Adults: A Human fMRI Study

Little is known about the neural correlates of fear learning in adolescents, a population at increased risk for anxiety disorders. Healthy adolescents (mean age 16.26) and adults (mean age 29.85) completed a fear learning paradigm across two stages during functional magnetic resonance imaging (fMRI). Stage 1 involved conditioning and extinction, and stage 2 involved extinction recall, re-conditioning, followed by re-extinction. During extinction recall, we observed a higher skin conductance response to the CS+ relative to CS− in adolescents compared to adults, which was accompanied by a reduction in dorsolateral prefrontal cortex (dlPFC) activity. Relative to adults, adolescents also had significantly reduced activation in the ventromedial PFC, dlPFC, posterior cingulate cortex (PCC), and temporoparietal junction (TPJ) during extinction recall compared to late extinction. Age differences in PCC activation between late extinction and late conditioning were also observed. These results show for the first time that healthy adolescent humans show different behavioral responses, and dampened PFC activity during short-term extinction recall compared to healthy adults. We also identify the PCC and TPJ as novel regions that may be associated with impaired extinction in adolescents. Also, while adults showed significant correlations between differential SCR and BOLD activity in some brain regions during late extinction and recall, adolescents did not show any significant correlations. This study highlights adolescent-specific neural correlates of extinction, which may explain the peak in prevalence of anxiety disorders during adolescence.

Divergent prefrontal dopaminergic mechanisms mediate drug- and fear-associated cue extinction during adolescence versus adulthood

Cue-associated learning is vital to guiding behaviour for survival. Adolescence represents a key developmental stage for perturbations in cue-related learning, including a characteristic deficit in cue extinction learning. The present review summarizes evidence from animal and human literature that cue extinction is critically mediated by prefrontal dopamine, a system that undergoes dramatic reorganization during adolescence. We propose that extinction learning and memory is governed by a developmentally dynamic balance of dopamine receptors in the prefrontal cortex, which changes across adolescence into adulthood. This is contrary to the previous idea that extinction deficits during adolescence reflect inefficiency in the same neural circuitry as adults. This leads to proposal of the novel theory that cue extinction involves divergent prefrontal dopaminergic mechanisms depending on the age of extinction.

Prefrontal-Amygdala Connectivity and State Anxiety during Fear Extinction Recall in Adolescents

While deficits in fear extinction recall have been suggested to underlie vulnerability to anxiety disorders in adolescents, the neurobiology of these deficits remain underexplored. Here we investigate the functional connectivity (FC) of the ventromedial prefrontal cortex (vmPFC) and dorsolateral PFC (dlPFC) underlying extinction recall in healthy adolescents, and assess associations between FC and state/trait anxiety. Adolescents (17) and adults (14, for comparison) completed a fear-learning paradigm involving extinction and extinction recall during a functional magnetic resonance imaging session, in which skin conductance response (SCR) was recorded. Psychophysiological interaction analyses revealed that during extinction recall there was significant negative connectivity between the vmPFC and amygdala in adults, but not adolescents. vmPFC-amygdala connectivity was positively correlated with SCR. Adolescents showed significant negative FC between the dlPFC and the left and right hippocampus, and the amygdala, which was positively correlated with state anxiety. Recall was also associated with negative connectivity between the dlPFC and thalamus, posterior cingulate cortex, fusiform gyrus, and pallidum in adolescents. These results demonstrate that fear extinction recall in healthy adolescents is associated with FC between prefrontal and limbic brain regions, and suggest that alterations in connectivity may be associated with vulnerability to anxiety in adolescence.

Late Reduction of Cocaine Cravings in a Randomized, Double-Blind Trial of Aripiprazole vs Perphenazine in Schizophrenia and Comorbid Cocaine Dependence

PURPOSE

Co-occurring schizophrenia spectrum disorder and International Statistical Classification of Diseases, 10th Revision cocaine dependence present a particularly destructive constellation that is often difficult to treat. Both conditions raise dopamine transmission effects in the brain. Traditional neuroleptics block dopamine receptors, whereas aripiprazole modulates dopamine activity as an agonist/antagonist. We tested whether dopamine modulation is superior to dopamine blocking in dual-diagnosis patients.

METHODS

In a randomized, double-blind, comparison design, cocaine-dependent schizophrenic subjects actively using cocaine received either aripiprazole or perphenazine in an 8-week trial. Primary outcome targeted cocaine-free urine sample proportions, whereas cocaine craving scores were a secondary variable.

RESULTS

Subjects (N = 44) randomized (n = 22 per group) did not differ at baseline. The proportion of cocaine-free urine samples did not differ by medication group. Contrasting weeks 3 to 5 vs 6 to 8 revealed significant late reductions in craving with aripiprazole. On the respective 5-point subscales, craving intensity decreased by 1.53 ± 0.43 (P < 0.0005) points, craving frequency by 1.4 ± 0.40 (P > 0.0004) points, and craving duration by 1.76 ± 0.44 (P > 0.0001) points.

CONCLUSIONS

A drug effect of aripiprazole on craving items appeared at week 6 of treatment, on average, and was not seen before that length of drug exposure. The data suggest that dopamine modulation reduces cocaine cravings but requires an acclimation period. To understand the mechanism of action better, a trial of depot aripiprazole may be useful. Clinically, a reduction in craving potentially offers a clearer focus for ongoing behavioral treatment. It may also offer a longer-term treatment effect with respect to the severity of relapse.

Aripiprazole Facilitates Extinction of Conditioned Fear in Adolescent Rats

Anxiety disorders are the most common type of mental disorder during adolescence, which is at least partly due to the resistance to extinction exhibited at this age. The dopaminergic system is known to be dysregulated during adolescence; therefore, we aimed to facilitate extinction in adolescent rats using the dopamine receptor 2 partial agonist aripiprazole (Abilify™), and examine the behavioral and neural outcomes. Adolescent rats were conditioned to fear a tone. The next day, rats received extinction 30 min after a systemic injection of either 5 mg/kg aripiprazole or vehicle, and then were tested the following day. For the immunohistochemistry experiment, naïve and "no extinction" conditions were added and rats were perfused either on the extinction day or test day. To assess the activation of neurons receiving dopaminergic input, c-Fos, and dopamine- and cAMP-regulated neuronal phosphoprotein (DARPP-32) labeled neurons were quantified in the amygdala and the medial prefrontal cortex (mPFC). Systemic treatment with aripiprazole at the time of extinction significantly reduced freezing at test the next day. This effect was not observed in rats that were fear conditioned but did not receive any extinction. Aripiprazole's facilitation of extinction was accompanied by increased activation of neurons in the mPFC. Taken together, aripiprazole represents a novel pharmacological adjunct to exposure therapy worthy of further examination. The effect of aripiprazole is related to enhanced activation of mPFC neurons receiving dopaminergic innervation.

Prefrontal Dopaminergic Mechanisms of Extinction in Adolescence Compared to Adulthood in Rats

Adolescents with anxiety disorders attain poorer outcomes following extinction-based treatment compared to adults. Extinction deficit during adolescence has been identified to involve immaturity in the medial prefrontal cortex (mPFC). Findings from adult rodents suggest extinction involves dopamine signaling in the mPFC. This system changes dramatically during adolescence, but its role in adolescent extinction is unknown. Therefore, we investigated the role of prefrontal dopamine in extinction using Pavlovian fear conditioning in adolescent and adult rats. Using quantitative PCR (qPCR) analyses, we measured changes in dopamine receptor gene expression in the mPFC before and after extinction. We then enhanced dopamine 1 receptor (D1R) or dopamine 2 receptor (D2R) signaling in the infralimbic cortex (IL) of the mPFC using agonists at the time of extinction. Adolescent rats displayed a deficit in extinction retention compared to adults. Extinction induced a reduction in D1R compared to D2R gene expression in adolescent rats, whereas an increase of D1R compared to D2R gene expression was observed in adult rats. Acutely enhancing IL D1R signaling using SKF-81297 had no effect on extinction at either age. In contrast, acutely enhancing IL D2R signaling with quinpirole significantly enhanced long-term extinction in adolescents, and impaired within-session extinction in adults. Our results suggest a dissociated role for prefrontal dopamine in fear extinction during adolescence compared to adulthood. Findings highlight the dopamine system as a potential pharmacological target to improve extinction-based treatments for adolescents.

Extinction of conditioned cues attenuates incubation of cocaine craving in adolescent and adult rats

Relapse to drug use is often precipitated by exposure to drug associated cues that evoke craving. Cue-induced drug craving has been observed in both animals and humans to increase over the first few weeks of abstinence and remain high over extended periods, a phenomenon known as 'incubation of craving'. As adolescence represents a period of vulnerability to developing drug addiction, potentially due to persistent reactivity to drug associated cues, we first compared incubation of cocaine craving in adolescent and adult rats. Adolescent (P35) and adult (P70) rats were trained to lever press to obtain intravenous cocaine, with each drug delivery accompanied by a light cue that served as the conditioned stimulus (CS). Following acquisition of stable responding, rats were tested for cue-induced cocaine-seeking after either 1 or 30days of abstinence. Additional groups of rats were also tested after 30days of abstinence, however these rats were subjected to a cue extinction session 1week into the abstinence period. Rats were injected with aripiprazole, a dopamine 2 receptor (D2R)-like partial agonist, or vehicle, 30min prior to cue extinction. We found that adolescent and adult rats acquired and maintained a similar level of cocaine self-administration, and rats of both ages exhibited a higher level of cue-induced cocaine-seeking if they were tested after 30days of abstinence compared to 1day. Incubation of cocaine craving was significantly reduced to 1day levels in both adults and adolescents that received cue extinction training. Administration of aripiprazole prior to cue extinction did not further reduce cue-induced drug-seeking. These results indicate that cue extinction training during abstinence may effectively reduce cue-induced relapse at a time when cue-induced drug craving is usually high.