Impaired formation of stimulus-response, but not action-outcome, associations in rats with methamphetamine-induced neurotoxicity

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behaviour

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational and cognitive processes involved in regulating the pursuit and consumption of food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal disrupted their ability to exert flexible goal-directed control over reward seeking. Specifically, morphine-withdrawn rats were impaired in using current reward value to select actions both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case, rats were only impaired in using reward value to select actions in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Neither Amphetamine nor Sub-Anesthetic Ketamine Treatment during Adolescence Impairs Devaluation in Rats Tested during Adulthood

BACKGROUND

Much of the existing animal literature on the devaluation task suggests that prior repeated exposure to drugs of abuse during adulthood can impair goal-directed action, but the literature on human drug users is mixed. Also, the initiation of drug use often occurs during adolescence, but examinations of the effects of drug exposure during adolescence on behavior in the devaluation task are lacking.

METHODS

We examined whether repeated exposure during adolescence to amphetamine (3 mg/kg injections every-other day from post-natal day 27-45) or ketamine (twice daily 30 mg/kg injections from post-natal day 35-44) would impair behavior in a devaluation test when tested drug-free in adulthood. Rats were trained to press a left lever with a steady cue-light above it for one reinforcer and a right lever with a flashing cue-light above it for a different reinforcer. We tested whether any impairments in goal-directed action could be overcome by compensation between strategies by giving rats information based on lever-location and cue-lights during the test that was either congruent (allowing compensation) or incongruent (preventing compensation between strategies) with the configurations during training.

RESULTS

Our results provided no evidence for impairment of goal-directed action during adulthood after adolescent amphetamine or ketamine exposure.

CONCLUSIONS

We discuss possible reasons for this discrepancy with the prior literature, including (1) the age of exposure and (2) the pattern in the previous literature that most previous demonstrations of drug exposure impairing devaluation in laboratory animals may be attributed to either drug-associated cues present in the testing environment and/or accelerated habit learning in tasks that predispose laboratory animals towards habit formation with extended training (with training procedures that should resist the formation of habits in the current experiment). However, additional research is needed to examine the effects of these factors, as well a potential role for the particular doses and washout periods to determine the cause of our finding of no devaluation impairment after drug exposure.

Effects of methamphetamine-induced neurotoxicity on striatal long-term potentiation

RATIONALE

Methamphetamine (METH) exposure is associated with damage to central monoamine systems, particularly dopamine signaling. Rodent models of such damage have revealed a decrease in the amplitude of phasic dopamine signals and significant striatal dysfunction, including changes in the molecular, system, and behavioral functions of the striatum. Dopamine signaling through D1 receptors promotes corticostriatal long-term potentiation (LTP), a critical substrate of these striatal functions.

OBJECTIVES

Therefore, the purpose of this study was to determine if METH-induced dopamine neurotoxicity would impair D1 receptor-dependent striatal LTP in mice.

METHODS

Mice were treated with a METH binge regimen (4 × 10 mg/kg d,l-methamphetamine, s.c.) that recapitulates all of the known METH-induced neurotoxic effects observed in humans, including dopamine toxicity. Three weeks later, acute brain slices containing either the dorsomedial striatum (DMS) or dorsolateral striatum (DLS) were prepared, and plasticity was assessed using white matter, high-frequency stimulation (HFS), and striatal extracellular electrophysiology.

RESULTS

Under these conditions, LTP was induced in brain slices containing the DMS from saline-pretreated mice, but not mice with METH-induced neurotoxicity. Furthermore, the LTP observed in DMS slices from saline-pretreated mice was blocked by the dopamine D1 receptor antagonist SCH23390, indicating that this LTP is dopamine D1 receptor-dependent. Finally, acute in vivo treatment of METH-pretreated mice with bupropion (50 mg/kg, i.p.) promoted LTP in DMS slices.

CONCLUSIONS

Together, these studies demonstrate that METH-induced neurotoxicity impairs dopamine D1 receptor-dependent LTP within the DMS and that the FDA-approved drug bupropion restores induction of striatal LTP in mice with METH-induced dopamine neurotoxicity.

Habit, choice, and addiction

Addiction was suggested to emerge from the progressive dominance of habits over goal-directed behaviors. However, it is generally assumed that habits do not persist in choice settings. Therefore, it is unclear how drug habits may persist in real-world scenarios where this factor predominates. Here, we discuss the poor translational validity of the habit construct, which impedes our ability to determine its role in addiction. New evidence of habitual behavior in a drug choice setting are then described and discussed. Interestingly, habitual preference did not promote drug choice but instead favored abstinence. Here, we propose several clues to reconcile these unexpected results with the habit theory of addiction, and we highlight the need in experimental research to face the complexity of drug addicts' decision-making environments by investigating drug habits in the context of choice and in the presence of cues. On a theoretical level, we need to consider more complex frameworks, taking into account continuous interactions between goal-directed and habitual systems, and alternative decision-making models more representative of real-world conditions.

Habitual Behavior Is Goal-Driven

We address the relation between goal-driven and habitual behaviors. Whereas in recent years the two have been juxtaposed, we suggest that habitual behavior is in fact goal-driven. To support this view, we show that habitual behavior is sensitive to changes in goal properties (reward contingencies), namely goal value and its expectancy of attainment. Whereas adjustment to these properties may be slower for habitual (or overlearned) than for nonhabitual behavior, this is likely due to the routinized (or automatic) nature of such behavior, characterized as it is by reduced attention to its consequences. Furthermore, we show that habitual behavior’s prolonged persistence despite its manifest detachment from the original goal likely stems from its attachment to a different goal. Thus, there is no need to postulate purposeless behavior. The view that habitual behavior is goal-driven offers an integrative account of a considerable body of evidence and is consistent with a functional account of psychological processes.

Addiction is driven by excessive goal-directed drug choice under negative affect: translational critique of habit and compulsion theory

Drug addiction may be a goal-directed choice driven by excessive drug value in negative affective states, a habit driven by strong stimulus-response associations, or a compulsion driven by insensitivity to costs imposed on drug seeking. Laboratory animal and human evidence for these three theories is evaluated. Excessive goal theory is supported by dependence severity being associated with greater drug choice/economic demand. Drug choice is demonstrably goal-directed (driven by the expected value of the drug) and can be augmented by stress/negative mood induction and withdrawal-effects amplified in those with psychiatric symptoms and drug use coping motives. Furthermore, psychiatric symptoms confer risk of dependence, and coping motives mediate this risk. Habit theory of addiction has weaker support. Habitual behaviour seen in drug-exposed animals often does not occur in complex decision scenarios, or where responding is rewarded, so habit is unlikely to explain most human addictive behaviour where these conditions apply. Furthermore, most human studies have not found greater propensity to habitual behaviour in drug users or as a function of dependence severity, and the minority that have can be explained by task disengagement producing impaired explicit contingency knowledge. Compulsion theory of addiction also has weak support. The persistence of punished drug seeking in animals is better explained by greater drug value (evinced by the association with economic demand) than by insensitivity to costs. Furthermore, human studies have provided weak evidence that propensity to discount cost imposed on drug seeking is associated with dependence severity. These data suggest that human addiction is primarily driven by excessive goal-directed drug choice under negative affect, and less by habit or compulsion. Addiction is pathological because negative states powerfully increase expected drug value acutely outweighing abstinence goals.

Lesions of the Patch Compartment of Dorsolateral Striatum Disrupt Stimulus-Response Learning

The striatum mediates habit formation and reward association. The striatum can be divided into the patch and matrix compartment, which are two distinct regions that sub-serve different aspects of behavior. The patch compartment may mediate reward-related behaviors, while the matrix compartment may mediate adaptive motor functions. Previous studies indicate that enhanced relative activation of the patch versus matrix compartment is associated with inflexible behaviors, such as stereotypy. Habitual behaviors are also inflexible in nature, but whether enhanced activation of the patch compartment contributes to habitual behavior is not known. The goal of the current study was to examine the role of patch compartment in the development of habit formation. We used dermorphin-saporin to ablate neurons of the patch compartment in the dorsolateral striatum prior to training animals to self-administer sucrose on a random interval schedule of reinforcement. Our data showed that patch compartment lesions in the dorsolateral striatum reduced the reinstatement of sucrose self-administration after sucrose devaluation, indicating that destruction of this region prevented the development of habitual behavior. Additionally, in animals with patch compartment lesions in the DLS that did not develop habitual behavior, activation of the dorsolateral striatum and sensorimotor cortex was diminished, while activity in the dorsomedial striatum and prefrontal cortex was increased, suggesting less engagement of regions that mediate habitual behaviors and heightened engagement of regions that mediate goal-directed behaviors occurs with reduced habit formation. These data indicate that the dorsolateral patch compartment may mediate habit formation by altering information flow through basal ganglia circuits.

Behavioral and neural mechanisms underlying habitual and compulsive drug seeking

Addiction is characterized by compulsive drug use despite negative consequences. Here we review studies that indicate that compulsive drug use, and in particular punishment resistance in animal models of addiction, is related to impaired cortical control over habitual behavior. In humans and animals, instrumental behavior is supported by goal-directed and habitual systems that rely on distinct corticostriatal networks. Chronic exposure to addictive drugs or stress has been shown to bias instrumental response strategies toward habit learning, and impair prefrontal cortical (PFC) control over responding. Moreover, recent work has implicated prelimbic PFC hypofunction in the punishment resistance that has been observed in a subset of animals with an extended history of cocaine self-administration. This may be related to a broader role for prelimbic PFC in mediating adaptive responding and behavioral flexibility, including exerting goal-directed control over behavior. We hypothesize that impaired cortical control and reduced flexibility between habitual and goal-directed systems may be critically involved in the development of maladaptive, compulsive drug use.

The impact of drugs of abuse on executive function: characterizing long-term changes in neural correlates following chronic drug exposure and withdrawal in rats

Addiction has long been characterized by diminished executive function, control, and impulsivity management. In particular, these deficits often manifest themselves as impairments in reversal learning, delay discounting, and response inhibition. Understanding the neurobiological substrates of these behavioral deficits is of paramount importance to our understanding of addiction. Within the cycle of addiction, periods during and after withdrawal represent a particularly difficult point of intervention in that the negative physical symptoms associated with drug removal and drug craving increase the likelihood that the patient will relapse and return to drug use in order to abate these symptoms. Moreover, it is often during this time that drug induced deficits in executive function hinder the ability of the patient to refrain from drug use. Thus, it is necessary to understand the physiological and behavioral changes associated with withdrawal and drug craving-largely manifesting as deficits in executive control-to develop more effective treatment strategies. In this review, we address the long-term impact that drugs of abuse have on the behavioral and neural correlates that give rise to executive control as measured by reversal learning, delay discounting, and stop-signal tasks, focusing particularly on our work using rats as a model system.

Pre-trial cocaine biases choice toward cocaine through suppression of the nondrug option

Being under the influence during choice between drug and nondrug options can have a dramatic effect on choice outcomes. When rats face a choice between cocaine and sweet water and are not under the influence, they prefer sweet water. In contrast, when they are under the influence of cocaine, this causes them to shift their choice to cocaine nearly exclusively. Here we sought to characterize the behavioral mechanisms underlying the influence of cocaine on choice. In theory, rats under the influence of cocaine should be in a mixed motivational state, at least temporarily, with both their motivation for cocaine and their motivation for the nondrug option suppressed by the drug satiating and anorexic effects of cocaine, respectively. For this mixed state to shift choice to cocaine, the satiated motivation for cocaine should recover before the suppressed motivation for the preferred nondrug option. The goal of the present study was to test this prediction in rats that expressed a preference for sweet water after extended access to cocaine self-administration. We measured their choice and response latencies to each option after pre-trial, passive administration of cocaine to estimate the duration of its drug satiating and anorexic effects. As expected, pre-trial cocaine caused most rats to shift their choice to cocaine. Though this shift was not simply due to a longer latency to respond for sweet water than for cocaine after pre-trial cocaine, it nevertheless occurred while rats' motivation for the nondrug option was still partially suppressed. Thus, cocaine seems to bias choice toward more cocaine mainly via suppression of the nondrug option.

A Single High Dose of Methamphetamine Reduces Monoamines and Impairs Egocentric and Allocentric Learning and Memory in Adult Male Rats

Methamphetamine (MA) alters dopamine markers and cognitive function in heavy users. In rodents, there are MA dosing regimens that induce concordant effects using repeated administration at spaced intervals. These regimens are effective but complicate experiments designed to disentangle the effects of the drug on different brain regions in relation to their cognitive effects because of treatment spacing. In an effort to simplify the model, we tested whether a single dose of MA could induce the same monoamine and cognitive effects as multiple, spaced dosing without affecting survival. Adult male Sprague-Dawley rats were treated with 40 mg/kg MA subcutaneously once and tested starting 2 weeks later. MA-treated rats showed deficits in egocentric navigation in Cincinnati water maze, in spatial navigation in the Morris water maze, and in choosing a consistent problem-solving strategy in the Star water maze when given the option to show a preference. MA-treated rats had persistent dopamine and serotonin reductions in the neostriatum and nucleus accumbens, and serotonin reductions in the hippocampus of the same magnitude as in repetitive treatment models. The data demonstrate that a single dose recapitulates the neurocognitive and monoamine effects of multiple-dose regimens, thereby simplifying the model of MA-induced neurotoxicity.

Prior Cocaine Self-Administration Increases Response-Outcome Encoding That Is Divorced from Actions Selected in Dorsal Lateral Striatum

Dorsal lateral striatum (DLS) is a highly associative structure that encodes relationships among environmental stimuli, behavioral responses, and predicted outcomes. DLS is known to be disrupted after chronic drug abuse; however, it remains unclear what neural signals in DLS are altered. Current theory suggests that drug use enhances stimulus-response processing at the expense of response-outcome encoding, but this has mostly been tested in simple behavioral tasks. Here, we investigated what neural correlates in DLS are affected by previous cocaine exposure as rats performed a complex reward-guided decision-making task in which predicted reward value was independently manipulated by changing the delay to or size of reward associated with a response direction across a series of trial blocks. After cocaine self-administration, rats exhibited stronger biases toward higher-value reward and firing in DLS more strongly represented action-outcome contingencies independent from actions subsequently taken rather than outcomes predicted by selected actions (chosen-outcome contingencies) and associations between stimuli and actions (stimulus-response contingencies). These results suggest that cocaine self-administration strengthens action-outcome encoding in rats (as opposed to chosen-outcome or stimulus-response encoding), which abnormally biases behavior toward valued reward when there is a choice between two options during reward-guided decision-making.SIGNIFICANCE STATEMENT Current theories suggest that the impaired decision-making observed in individuals who chronically abuse drugs reflects a decrease in goal-directed behaviors and an increase in habitual behaviors governed by neural representations of response-outcome (R-O) and stimulus-response associations, respectively. We examined the impact that prior cocaine self-administration had on firing in dorsal lateral striatum (DLS), a brain area known to be involved in habit formation and affected by drugs of abuse, during performance of a complex reward-guided decision-making task. Surprisingly, we found that previous cocaine exposure enhanced R-O associations in DLS. This suggests that there may be more complex consequences of drug abuse than current theories have explored, especially when examining brain and behavior in the context of a complex two-choice decision-making task.

Subchronic anesthetic ketamine injections in rats impair choice reversal learning, but have no effect on reinforcer devaluation

Previous exposure to a variety of drugs of abuse has been shown to cause long-term impairments in reversal learning and reinforcer devaluation tasks. However, there is mixed evidence in the literature for a long-term effect of ketamine exposure on reversal learning and the long-term effect of ketamine exposure on devaluation is not known. We determined whether repeated injections of an anesthetic dose of ketamine would lead to impairments in choice reversal learning after discrimination learning or impairments in reinforcer devaluation. In two experiments, rats received three injections once-daily of ketamine (100 mg/kg, intraperitoneally) or saline and then began behavioral training 19 days later so that the key reversal learning and devaluation tests would occur about 1 month after the final ketamine injection. This ketamine exposure regimen did not impair learning in our discrimination task, but led to an increase in perseverative errors in reversal learning. However, the same ketamine exposure regimen (or injections of a lower 50 mg/kg dose) had no effect on behavior in the devaluation task. The behavioral patterns observed suggest possible neural mechanisms for the effects of ketamine, but future neurobiological investigations will be needed to isolate these mechanisms.

Chronic methamphetamine self-administration disrupts cortical control of cognition

Methamphetamine (meth) is one of the most abused substances worldwide. Chronic use has been associated with repeated relapse episodes that may be exacerbated by cognitive impairments during drug abstinence. Growing evidence demonstrates that meth compromises prefrontal cortex activity, resulting in persisting attentional and memory impairments. After summarizing recent studies of meth-induced cognitive dysfunction using a translationally relevant model of self-administered meth, this review emphasizes the cortical brain changes contributing to cognitive dysregulation during abstinence. Finally, we propose the use of cognitive enhancers during abstinence that may promote a drug-free state by reversing cortical dysfunction linked with prolonged meth abuse.

Relative Timing Between Kappa Opioid Receptor Activation and Cocaine Determines the Impact on Reward and Dopamine Release

Negative affective states can increase the rewarding value of drugs of abuse and promote drug taking. Chronic cocaine exposure increases levels of the neuropeptide dynorphin, an endogenous ligand at kappa opioid receptors (KOR) that suppresses dopamine release in the nucleus accumbens (NAc) and elicits negative affective states upon drug withdrawal. However, there is evidence that the effects of KOR activation on affective state are biphasic: immediate aversive effects are followed by delayed increases in reward. The impact of KOR-induced affective states on reward-related effects of cocaine over time is not known. We hypothesize that the initial aversive effects of KOR activation increase, whereas the delayed rewarding effects decrease, the net effects of cocaine on reward and dopamine release. We treated rats with cocaine at various times (15 min to 48 h) after administration of the selective KOR agonist salvinorin A (salvA). Using intracranial self-stimulation and fast scan cyclic voltammetry, we found that cocaine-induced increases in brain stimulation reward and evoked dopamine release in the NAc core were potentiated when cocaine was administered within 1 h of salvA, but attenuated when administered 24 h after salvA. Quantitative real-time PCR was used to show that KOR and prodynorphin mRNA levels were decreased in the NAc, whereas tyrosine hydroxylase and dopamine transporter mRNA levels and tissue dopamine content were increased in the ventral tegmental area 24 h post-salvA. These findings raise the possibility that KOR activation-as occurs upon withdrawal from chronic cocaine-modulates vulnerability to cocaine in a time-dependent manner.

A Serotonin Circuit Acts as an Environmental Sensor to Mediate Midline Axon Crossing through EphrinB2

Modulation of connectivity formation in the developing brain in response to external stimuli is poorly understood. Here, we show that the raphe nucleus and its serotonergic projections regulate pathfinding of commissural axons in zebrafish. We found that the raphe neurons extend projections toward midline-crossing axons and that when serotonergic signaling is blocked by pharmacological inhibition or by raphe neuron ablation, commissural pathfinding is disrupted. We demonstrate that the serotonin receptor htr2a is expressed on these commissural axons and that genetic knock-down of htr2a disrupts crossing. We further show that knock-down of htr2a or ablation of the raphe neurons increases ephrinB2a protein levels in commissural axons. An ephrinB2a mutant can rescue midline crossing when serotonergic signaling is blocked. Furthermore, we found that regulation of serotonin expression in the raphe neurons is modulated in response to the developmental environment. Hypoxia causes the raphe to decrease serotonin levels, leading to a reduction in midline crossing. Increasing serotonin in the setting of hypoxia restored midline crossing. Our findings demonstrate an instructive role for serotonin in axon guidance acting through ephrinB2a and reveal a novel mechanism for developmental interpretation of the environmental milieu in the generation of mature neural circuitry.

SIGNIFICANCE STATEMENT

We show here that serotonin has a novel role in regulating connectivity in response to the developmental environment. We demonstrate that serotonergic projections from raphe neurons regulate pathfinding of crossing axons. The neurons modulate their serotonin levels, and thus alter crossing, in response to the developmental environment including hypoxia. The findings suggest that modification of the serotonergic system by early exposures may contribute to permanent CNS connectivity alterations. This has important ramifications because of the association between premature birth and accompanying hypoxia, and increased risk of autism and evidence associating in utero exposure to some antidepressants and neurodevelopmental disorders. Finally, this work demonstrates that the vertebrate CNS can modulate its connectivity in response to the external environment.

A Closer Look at the Effects of Repeated Cocaine Exposure on Adaptive Decision-Making under Conditions That Promote Goal-Directed Control

It has been proposed that compulsive drug seeking reflects an underlying dysregulation in adaptive behavior that favors habitual (automatic and inflexible) over goal-directed (deliberative and highly flexible) action selection. Rodent studies have established that repeated exposure to cocaine or amphetamine facilitates the development of habits, producing behavior that becomes unusually insensitive to a reduction in the value of its outcome. The current study more directly investigated the effects of cocaine pre-exposure on goal-directed learning and action selection using an approach that discourages habitual performance. After undergoing a 15-day series of cocaine (15 or 30 mg/kg, i.p.) or saline injections and a drug withdrawal period, rats were trained to perform two different lever-press actions for distinct reward options. During a subsequent outcome devaluation test, both cocaine- and saline-treated rats showed a robust bias in their choice between the two actions, preferring whichever action had been trained with the reward that retained its value. Thus, it appears that the tendency for repeated cocaine exposure to promote habit formation does not extend to a more complex behavioral scenario that encourages goal-directed control. To further explore this issue, we assessed how prior cocaine treatment would affect the rats' ability to learn about a selective reduction in the predictive relationship between one of the two actions and its outcome, which is another fundamental feature of goal-directed behavior. Interestingly, we found that cocaine-treated rats showed enhanced, rather than diminished, sensitivity to this action-outcome contingency degradation manipulation. Given their mutual dependence on striatal dopamine signaling, we suggest that cocaine's effects on habit formation and contingency learning may stem from a common adaptation in this neurochemical system.

Methamphetamine-induced neurotoxicity disrupts pharmacologically evoked dopamine transients in the dorsomedial and dorsolateral striatum

Phasic dopamine (DA) signaling, during which burst firing by DA neurons generates short-lived elevations in extracellular DA in terminal fields called DA transients, is implicated in reinforcement learning. Disrupted phasic DA signaling is proposed to link DA depletions and cognitive-behavioral impairment in methamphetamine (METH)-induced neurotoxicity. Here, we further investigated this disruption by assessing effects of METH pretreatment on DA transients elicited by a drug cocktail of raclopride, a D2 DA receptor antagonist, and nomifensine, an inhibitor of the dopamine transporter (DAT). One advantage of this approach is that pharmacological activation provides a large, high-quality data set of transients elicited by endogenous burst firing of DA neurons for analysis of regional differences and neurotoxicity. These pharmacologically evoked DA transients were measured in the dorsomedial (DM) and dorsolateral (DL) striatum of urethane-anesthetized rats by fast-scan cyclic voltammetry. Electrically evoked DA levels were also recorded to quantify DA release and uptake, and DAT binding was determined by means of autoradiography to index DA denervation. Pharmacologically evoked DA transients in intact animals exhibited a greater amplitude and frequency and shorter duration in the DM compared to the DL striatum, despite similar pre- and post-drug assessments of DA release and uptake in both sub-regions as determined from the electrically evoked DA signals. METH pretreatment reduced transient activity. The most prominent effect of METH pretreatment on transients across striatal sub-region was decreased amplitude, which mirrored decreased DAT binding and was accompanied by decreased DA release. Overall, these results identify marked intrastriatal differences in the activity of DA transients that appear independent of presynaptic mechanisms for DA release and uptake and further support disrupted phasic DA signaling mediated by decreased DA release in rats with METH-induced neurotoxicity.

Changes in neural circuitry regulating response-reversal learning and Arc-mediated consolidation of learning in rats with methamphetamine-induced partial monoamine loss

Methamphetamine (METH)-induced neurotoxicity results in long-lasting depletions of monoamines and changes in basal ganglia function. We previously reported that rats with METH-induced neurotoxicity no longer engage dorsomedial striatum during a response-reversal learning task, as their performance is insensitive to acute disruption of dorsomedial striatal function by local infusion of an N-methyl-D-aspartate receptor antagonist or an antisense oligonucleotide against the activity-regulated cytoskeleton-associated (Arc) gene. However, METH-pretreated rats perform the task as well as controls. Therefore, we hypothesized that the neural circuitry involved in the learning had changed in METH-pretreated rats. To test this hypothesis, rats were pretreated with a neurotoxic regimen of METH or with saline. After 3-5 weeks, rats were trained on the reversal-learning task and in situ hybridization for Arc was performed. A significant correlation between Arc expression and performance on the task was found in nucleus accumbens shell of METH-, but not saline-, pretreated rats. Consistent with the idea that the correlation between Arc expression in a brain region and behavioral performance implicates that brain region in the learning, infusion of an antisense oligonucleotide against Arc into the shell impaired consolidation of reversal learning in METH-, but not saline-, pretreated rats. These findings provide novel evidence suggesting that METH-induced neurotoxicity leads to a shift from dorsal to ventral striatal involvement in the reversal-learning task. Such reorganization of neural circuitry underlying learning and memory processes may contribute to impaired cognitive function in individuals with METH-induced neurotoxicity or others with striatal dopamine loss, such as patients with Parkinson's disease.

Methamphetamine-induced neurotoxicity disrupts naturally occurring phasic dopamine signaling

Methamphetamine (METH) is a highly addictive drug that is also neurotoxic to central dopamine (DA) systems. Although striatal DA depletions induced by METH are associated with behavioral and cognitive impairments, the link between these phenomena remains poorly understood. Previous work in both METH-pretreated animals and the 6-hydroxydopamine model of Parkinson's disease suggests that a disruption of phasic DA signaling, which is important for learning and goal-directed behavior, may be such a link. However, previous studies used electrical stimulation to elicit phasic-like DA responses and were also performed under anesthesia, which alters DA neuron activity and presynaptic function. Here we investigated the consequences of METH-induced DA terminal loss on both electrically evoked phasic-like DA signals and so-called 'spontaneous' phasic DA transients measured by voltammetry in awake rats. Not ostensibly attributable to discrete stimuli, these subsecond DA changes may play a role in enhancing reward-cue associations. METH pretreatment reduced tissue DA content in the dorsomedial striatum and nucleus accumbens by ~55%. Analysis of phasic-like DA responses elicited by reinforcing stimulation revealed that METH pretreatment decreased their amplitude and underlying mechanisms for release and uptake to a similar degree as DA content in both striatal subregions. Most importantly, characteristics of DA transients were altered by METH-induced DA terminal loss, with amplitude and frequency decreased and duration increased. These results demonstrate for the first time that denervation of DA neurons alters naturally occurring DA transients and are consistent with diminished phasic DA signaling as a plausible mechanism linking METH-induced striatal DA depletions and cognitive deficits.

Phasic-like stimulation of the medial forebrain bundle augments striatal gene expression despite methamphetamine-induced partial dopamine denervation

Methamphetamine-induced partial dopamine depletions are associated with impaired basal ganglia function, including decreased preprotachykinin mRNA expression and impaired transcriptional activation of activity-regulated, cytoskeleton-associated (Arc) gene in striatum. Recent work implicates deficits in phasic dopamine signaling as a potential mechanism linking methamphetamine-induced dopamine loss to impaired basal ganglia function. This study thus sought to establish a causal link between phasic dopamine transmission and altered basal ganglia function by determining whether the deficits in striatal neuron gene expression could be restored by increasing phasic dopamine release. Three weeks after pretreatment with saline or a neurotoxic regimen of methamphetamine, rats underwent phasic- or tonic-like stimulation of ascending dopamine neurons. Striatal gene expression was examined using in situ hybridization histochemistry. Phasic-like, but not tonic-like, stimulation induced immediate-early genes Arc and zif268 in both groups, despite the partial striatal dopamine denervation in methamphetamine-pretreated rats, with the Arc expression occurring in presumed striatonigral efferent neurons. Phasic-like stimulation also restored preprotachykinin mRNA expression. These results suggest that disruption of phasic dopamine signaling likely underlies methamphetamine-induced impairments in basal ganglia function, and that restoring phasic dopamine signaling may be a viable approach to manage long-term consequences of methamphetamine-induced dopamine loss on basal ganglia functions.

Perseverative behavior in rats with methamphetamine-induced neurotoxicity

Methamphetamine induces monoamine depletions thought to contribute to cognitive and behavioral dysfunctions. Previously, we reported that methamphetamine-induced neurotoxicity is associated with impaired formation of stimulus-response associations. Additionally, subjective observations suggested that behavioral flexibility might be affected. Thus, the present study examined whether methamphetamine neurotoxicity induces perseverative behavior. Rats were pretreated with (±)-methamphetamine (4 × 10 mg/kg, 2-hr intervals) or saline. Three weeks later, rats were trained to press a lever on one side of an operant chamber and then retrieve the reinforcer from a magazine on the opposite side until they reached criterion (>50 reinforcers/30-min). After four consecutive sessions performing the task at criterion, rats were sacrificed and brains removed for monoamine determinations. Methamphetamine-pretreated rats had ∼50% loss of striatal dopamine and prefrontal serotonin. Methamphetamine- and saline-pretreated rats were not different in the number of sessions required to reach criterion or in the total numbers of lever presses and/or head entries made across the four consecutive sessions at criterion-level performance. However, methamphetamine-pretreated rats earned fewer reinforcers, because they made extra lever-presses and head entries when they should have been retrieving the reinforcer or returning to the lever. Latencies for methamphetamine-pretreated rats to switch between the two behaviors also were significantly slower than latencies for controls. Interestingly, the degree of additional lever-presses negatively correlated with serotonin-transporter binding in the prefrontal cortex, even in saline-pretreated controls. These data suggest that methamphetamine-induced partial monoamine toxicity is associated with perseveration and that the degree of perseveration may depend on serotonin innervation of the frontal cortex.

Disruption of subcellular Arc/Arg 3.1 mRNA expression in striatal efferent neurons following partial monoamine loss induced by methamphetamine

The immediate-early gene Arc (activity-regulated cytoskeleton-associated protein) is provocative in the context of neuroplasticity because of its experience-dependent regulation and mRNA transport to and translation at activated synapses. Normal rats have more preproenkephalin-negative (ppe-neg; presumed striatonigral) neurons with cytoplasmic Arc mRNA than ppe-positive (ppe-pos; striatopallidal) neurons, despite equivalent numbers of these neurons showing novelty-induced transcriptional activation of Arc. Furthermore, rats with partial monoamine loss induced by methamphetamine (METH) show impaired Arc mRNA expression in both ppe-neg and ppe-pos neurons relative to normal animals following response-reversal learning. In this study, Arc expression induced by exposure to a novel environment was used to assess transcriptional activation and cytoplasmic localization of Arc mRNA in striatal efferent neuron subpopulations subsequent to METH-induced neurotoxicity. Partial monoamine depletion significantly altered Arc expression. Specifically, basal Arc expression was elevated, but novelty-induced transcriptional activation was abolished. Without novelty-induced Arc transcription, METH-pre-treated rats also had fewer neurons with cytoplasmic Arc mRNA expression, with the effect being greater for ppe-neg neurons. Thus, METH-induced neurotoxicity substantially alters striatal efferent neuron function at the level of Arc transcription, suggesting a long-term shift in basal ganglia neuroplasticity processes subsequent to METH-induced neurotoxicity. Such changes potentially underlie striatally based learning deficits associated with METH-induced neurotoxicity.