Repeated Amphetamine Exposure Research Articles

Angiotensin II involvement in the development and persistence of amphetamine-induced sensitization: Striatal dopamine reuptake implications.

Amphetamine (AMPH) exposure induces behavioural and neurochemical sensitization observed in rodents as hyperlocomotion and increased dopamine release in response to a subsequent dose. Brain Angiotensin II modulates dopaminergic neurotransmission through its AT1 receptors (AT1-R), positively regulating striatal dopamine synthesis and release. This work aims to evaluate the AT1-R role in the development and maintenance of AMPH-induced sensitization. Also, the AT1-R involvement in striatal dopamine reuptake was analysed. The sensitization protocol consisted of daily AMPH administration for 5days and tested 21 days after withdrawal. An AT1-R antagonist, candesartan, was administered before or after AMPH exposure to evaluate the participation of AT1-R in the development and maintenance of sensitization, respectively. Sensitization was evaluated by locomotor activity and c-Fos immunostaining. Changes in dopamine reuptake kinetics were evaluated 1day after AT1-R blockade withdrawal treatment, with or without the addition of AMPH in vitro. The social interaction test was performed as another behavioural output. Repeated AMPH exposure induced behavioural and neurochemical sensitization, which was prevented and reversed by candesartan. The AT1-R blockade increased the dopamine reuptake kinetics. Neither the AMPH administration nor the AT1-R blockade altered the performance of social interaction. Our results highlight the AT1-R's crucial role in AMPH sensitization. The enhancement of dopamine reuptake kinetics induced by the AT1-R blockade might attenuate the neuroadaptive changes that lead to AMPH sensitization and its self-perpetuation. Therefore, AT1-R is a prominent candidate as a target for pharmacological treatment of pathologies related to dopamine imbalance, including drug addiction and schizophrenia.

Sustained inhibitory transmission but dysfunctional dopamine D2 receptor signaling in dorsal striatal subregions following protracted abstinence from amphetamine

Behavioral sensitization to amphetamine is a complex phenomenon that engages several neurotransmitter systems and brain regions. While dysregulated signaling in the mesolimbic dopamine system repeatedly has been linked to behavioral sensitization, later research has implicated dorsal striatal circuits and GABAergic neurotransmission in contributing to behavioral transformation elicited by amphetamine. The aim of this study was thus to determine if repeated amphetamine exposure followed by abstinence would alter inhibitory neurotransmission in dorsal striatal subregions. To this end, male Wistar rats received amphetamine (2.0 mg/kg) in an intermittent manner for a total of five days. Behavioral sensitization to amphetamine was measured in locomotor-activity boxes, while neuroadaptations were recorded in the dorsolateral (DLS) and dorsomedial striatum (DMS) using ex vivo electrophysiology at different timepoints of amphetamine abstinence (2 weeks, 4–5 weeks, 10–11 weeks). Data show that repeated drug-exposure produces behavioral sensitization to the locomotor-stimulatory properties of amphetamine, which sustains for at least ten weeks. Electrophysiological recordings demonstrated a long-lasting suppression of evoked population spikes in both striatal subregions. Furthermore, following ten weeks of abstinence, the responsiveness to a dopamine D2 receptor agonist was significantly impaired in brain slices from rats previously receiving amphetamine. However, neither the frequency nor the amplitude of spontaneous inhibitory currents was affected by treatment at any of the time points analyzed. In conclusion, passive administration of amphetamine initiates long-lasting neuroadaptations in brain regions associated with goal-directed behavior and habitual performance, but these transformations do not appear to be driven by changes in GABAergic neurotransmission.

Differential and long-lasting changes in neurotransmission in the amygdala of male Wistar rats during extended amphetamine abstinence

Amphetamine addiction is associated with maladaptive actions that promotes continued use despite negative consequences, and a high risk of relapse even after protracted abstinence. Considering the role of the amygdala in regulating incentive motivation and reward-based behavior, the aim of this study was to assess neuroadaptations in subregions of the amygdala elicited by a brief period of discontinuous amphetamine exposure (2.0 mg/kg/day, 5 days) followed by abstinence (2 weeks, 1 month, 3 months) in male Wistar rats. Electrophysiological field potential recordings demonstrated that repeated amphetamine exposure significantly depressed evoked populations spikes in the basolateral amygdala (BLA). Evoked populations spikes were normalized after three months abstinence, but one challenge dose of amphetamine (0.5 mg/kg) was sufficient to reinstate synaptic depression in animals previously receiving amphetamine. In the central nucleus of the amygdala (CeA), amphetamine produced a long-lasting hyperexcitability that sustained even after three months abstinence. In the CeA, there were no significant differences between treatment groups following bath perfusion of the GABAA receptor antagonist bicuculline, indicating that amphetamine acts by reducing the inhibitory tone. Recordings performed in brain subregions interlinked with the amygdala, including medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens shell (nAc), revealed no significant neuroadaptations after two weeks abstinence. However, synaptic output was significantly depressed in the nAc after one- and three-month abstinence. In conclusion, the data presented here shows that five days of discontinuous exposure to amphetamine is sufficient to produce long-lasting neuroadaptations, which may contribute to compulsive drug taking and increase the risk for relapse.

Subregion-specific effects on striatal neurotransmission and dopamine-signaling by acute and repeated amphetamine exposure

Repeated administration of psychostimulants, such as amphetamine, is associated with a progressive increased sensitivity to some of the drug's effects, but tolerance towards others. We hypothesized that these adaptations in part could be linked to differential effects by amphetamine on dopaminergic signaling in striatal subregions. To test this theory, acute and long-lasting changes in dopaminergic neurotransmission were assessed in the nucleus accumbens (nAc) and the dorsomedial striatum (DMS) following amphetamine exposure in Wistar rats. By means of in vivo microdialysis, dopamine release induced by local administration of amphetamine was monitored in nAc and DMS of amphetamine naïve rats, and in rats subjected to five days of systemic amphetamine administration (2.0 mg/kg/day) followed by two weeks of withdrawal. In parallel, ex vivo electrophysiology was conducted to outline the effect of acute and repeated amphetamine exposure on striatal neurotransmission. The data shows that amphetamine increases dopamine in a concentration-dependent and subregion-specific manner. Furthermore, repeated administration of amphetamine followed by abstinence resulted in a selective decrease in baseline dopamine in the nAc, and a potentiation of the relative dopamine elevation after systemic amphetamine in the same area. Ex vivo electrophysiology demonstrated decreased excitatory neurotransmission in brain slices from amphetamine-treated animals, and a nAc selective shift in the responsiveness to the dopamine D2-receptor agonist quinpirole. These selective effects on dopamine signaling seen in striatal subregions after repeated drug exposure may partially explain why tolerance develops to the rewarding effects, but not towards the psychosis inducing properties of amphetamine.

An assessment of the functional effects of amphetamine-induced dendritic changes in the nucleus accumbens, medial prefrontal cortex, and hippocampus on different types of learning and memory function

The present experiments investigated the effects of repeated amphetamine exposure on neural networks mediating different forms of learning and memory. Different components of these networks were assessed using various functional assays. The hypothesis was that abnormal dendritic changes in nucleus accumbens, medial prefrontal cortex, and hippocampus mediated by repeated amphetamine exposure would produce impairments on forms of learning and memory dependent on neural circuits relying on these brain systems, and have little or no effect on other forms of learning not dependent on these networks. Surprisingly, the results showed that many of the dendritic changes normally found in the nucleus accumbens, prefrontal cortex, and hippocampus following repeated amphetamine exposure were reversed back to control levels following extensive multi-domain cognitive training. Learning and memory functions associated with different neural networks also appeared normal except in one case. A neural network that includes, but is not limited to, the basolateral amygdala and nucleus accumbens was dysfunctional in rats repeatedly exposed to amphetamine despite the reversal of the majority of dendritic changes in the nucleus accumbens following cognitive training. Importantly, an increase in spine density that normally occurs in these brain regions following repeated amphetamine exposure remained following extensive cognitive training, particularly in the nucleus accumbens.

On the relationship of first-episode psychosis to the amphetamine-sensitized state: a dopamine D2/3 receptor agonist radioligand study

Schizophrenia is characterized by increased behavioral and neurochemical responses to dopamine-releasing drugs. This prompted the hypothesis of psychosis as a state of “endogenous” sensitization of the dopamine system although the exact basis of dopaminergic disturbances and the possible role of prefrontal cortical regulation have remained uncertain. To show that patients with first-episode psychosis release more dopamine upon amphetamine-stimulation than healthy volunteers, and to reveal for the first time that prospective sensitization induced by repeated amphetamine exposure increases dopamine-release in stimulant-naïve healthy volunteers to levels observed in patients, we collected data on amphetamine-induced dopamine release using the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and positron emission tomography. Healthy volunteers (n = 28, 14 female) underwent a baseline and then a post-amphetamine scan before and after a mildly sensitizing regimen of repeated oral amphetamine. Unmedicated patients with first-episode psychosis (n = 21; 6 female) underwent a single pair of baseline and then post-amphetamine scans. Furthermore, T1 weighted magnetic resonance imaging of the prefrontal cortex was performed. Patients with first-episode psychosis showed larger release of dopamine compared to healthy volunteers. After sensitization of healthy volunteers their dopamine release was significantly amplified and no longer different from that seen in patients. Healthy volunteers showed a negative correlation between prefrontal cortical volume and dopamine release. There was no such relationship after sensitization or in patients. Our data in patients with untreated first-episode psychosis confirm the “endogenous sensitization” hypothesis and support the notion of impaired prefrontal control of the dopamine system in schizophrenia.

Omega-3 fatty acid deficiency impairs frontostriatal recruitment following repeated amphetamine treatment in rats: A 7 Tesla in vivo phMRI study

Although attention deficit hyperactivity disorder is associated with deficits in docosahexaenoic acid (DHA), an omega-3 fatty acid implicated in dopamine and glutamate synaptic plasticity, its role in neuroplastic brain changes that occur following repeated amphetamine (AMPH) treatment are not known. This study used pharmacological magnetic resonance imaging to investigate the impact of repeated AMPH exposure and alterations in brain DHA levels on AMPH-induced brain activation patterns. Male rats were fed a diet with no n-3 fatty acids (Deficient, DEF, n = 20), a diet fortified with preformed DHA (fish oil, FO, n = 20), or a control diet fortified with alpha-linolenic acid (n = 20) from P21 to P90. During adolescence (P40–60), one-half of each diet group received daily AMPH injections escalated weekly (0.5, 1.0, 2.5, 5.0 mg/kg/d) or drug vehicle. Following a 30-d abstinence period blood oxygen level dependent (BOLD) responses were determined in a 7 T Bruker Biospec system following an AMPH challenge (7.5 mg/kg, i.v). Postmortem erythrocyte and forebrain DHA composition were determined by gas chromatography. Compared with control rats, forebrain and erythrocyte DHA levels were significantly lower in DEF rats and significantly higher in FO rats. Across AMPH doses DEF rats exhibited greater locomotor activity compared to control and FO rats. In AMPH-naïve rats, the AMPH challenge increased BOLD activity in the substantia nigra and basal forebrain and no diet group differences were observed. In AMPH-pretreated control and FO rats, the AMPH challenge similarly increased BOLD activation in the bilateral caudate putamen, thalamus, and motor and cingulate cortices. In contrast, BOLD activation in AMPH-pretreated DEF rats was similar to AMPH-naïve DEF animals, and AMPH-pretreated DEF rats exhibited attenuated frontostriatal BOLD activation compared with AMPH-pretreated control and FO rats. These findings demonstrate that chronic escalating AMPH treatment induces enduring frontostriatal recruitment and that peri-adolescent deficits in brain DHA accrual impair this response.

Repeated exposure to amphetamine during adolescence alters inhibitory tone in the medial prefrontal cortex following drug re-exposure in adulthood

Behavioral sensitization following repeated amphetamine (AMPH) exposure is associated with changes in GABA function in the medial prefrontal cortex (mPFC). In rats exposed to AMPH during adolescence compared to adulthood, there are unique patterns of sensitization that may reflect age-dependent differences in drug effects on prefrontal GABAergic function. In the current study, we used a sensitizing regimen of repeated AMPH exposure in adolescent and adult rats to determine if a post-withdrawal AMPH challenge would alter inhibitory transmission in the mPFC in a manner that depends on age of exposure. Male Sprague-Dawley rats were treated with saline or 3mg/kg AMPH (i.p.) during adolescence [postnatal day (P) 27–P45] or adulthood (P85– P103) and were sacrificed either at similar ages in adulthood (∼P133; experiment 1) or after similar withdrawal times (3–4 weeks; experiment 2). Spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded in vitro from deep layer pyramidal cells in the mPFC using the whole-cell configuration. We found no effect of AMPH pre-exposure on baseline sIPSC frequency. Subsequent application of AMPH (25μM) produced a stable increase in sIPSC frequency in controls, suggesting that AMPH increases inhibitory tone in the mPFC. However, AMPH failed to increase sIPSCs in adolescent- or adult-exposed rats. In experiment 2, where withdrawal period was kept similar for both exposure groups, AMPH induced a suppression of sIPSC activity in adolescent-exposed rats. These results suggest that sensitizing treatment with AMPH during adolescence or adulthood dampens inhibitory influences on mPFC pyramidal cells, but potentially through different mechanisms.

A previous history of repeated amphetamine exposure modifies brain angiotensin II AT1 receptor functionality

Previous results from our laboratory showed that angiotensin II AT1 receptors (AT1-R) are involved in the neuroadaptative changes induced by amphetamine. The aim of the present work was to study functional and neurochemical responses to angiotensin II (ANG II) mediated by AT1-R activation in animals previously exposed to amphetamine. For this purpose male Wistar rats (250–320g) were treated with amphetamine (2.5mg/kg/day intraperitoneal) or saline for 5days and implanted with intracerebroventricular (i.c.v.) cannulae. Seven days after the last amphetamine administration the animals received ANG II (400pmol) i.c.v. One group was tested in a free choice paradigm for sodium (2% NaCl) and water intake and sacrificed for Fos immunoreactivity (Fos-IR) determinations. In a second group of rats, urine and plasma samples were collected for electrolytes and plasma renin activity determination and then they were sacrificed for Fos-IR determination in Oxytocinergic neurons (Fos-OT-IR). Results: Repeated amphetamine exposure (a) prevented the increase in sodium intake and Fos-IR cells in caudate-putamen and accumbens nucleus induced by ANG II i.c.v. (b) potentiated urinary sodium excretion and Fos-OT-IR in hypothalamus and (c) increased the inhibitory response in plasma renin activity, in response to ANG II i.c.v. Our results indicate a possible functional desensitisation of AT1-R in response to ANG II, induced by repeated amphetamine exposure. This functional AT1-R desensitisation allows to unmask the effects of ANG II i.c.v. mediated by oxytocin. We conclude that the long lasting changes in brain AT1-R functionality should be considered among the psychostimulant-induced neuroadaptations.

A High Fat Diet Reduces Dopamine Terminal Function and Increases Behavioral Sensitization to Amphetamine in C57BL/6 Mice

High fat (HF) diet-induced obesity has been shown to have a profound impact on neurobiological health, and has been associated with neurodegenerative and addictive disorders involving the mesolimbic dopamine system. Little is known as to how HF alters dopamine neurotransmission, so we used fast scan cyclic voltammetry and behavioral pharmacology to identify dopamine terminal changes that occur in the nucleus accumbens (NAc) of C57BL/6 mice after six-week access to a HF diet. Mice with unrestricted access to HF (60% kcals from fat) gained significantly more weight than chow fed controls (34.5 g and 25.9 g, respectively; p < 0.0001), and exhibited increased behavioral sensitivity to repeated amphetamine (AMPH) administration compared to chow fed mice (p < 0.0001). Baseline evoked dopamine release in the NAc was comparable between chow and HF (0.86 and 1.07 μM, respectively), and baseline Vmax was unaltered by HF (chow: 1.63 μM/s; HF: 1.74 μM/s). A history of AMPH exposure, however, significantly elevated baseline dopamine release in chow (+2.5 fold) but not HF (-0.87 fold) mice (p < 0.001), and HF attenuated baseline reuptake following repeated AMPH (+3.2 vs. +1.3 fold in Vmax for chow and HF, respectively; p < 0.01). HF diet increased the sensitivity of D2 type autoreceptors in the NAc relative to chow; however, HF mice were resistant to enhanced D2 sensitivity that occurred in chow mice after repeated AMPH exposure. These data show that a HF diet reduced the ability of AMPH to change dopamine terminal function. Increased behavioral sensitization to AMPH most likely results from blunted dopamine reuptake in HF mice, and suggests that increased D2 sensitivity is not sufficient to overcome the baseline dopaminergic changes that result from HF.

Oxytocin reverses amphetamine-induced deficits in social bonding: evidence for an interaction with nucleus accumbens dopamine.

Drug addiction has devastating consequences on social behaviors and can lead to the impairment of social bonding. Accumulating evidence indicates that alterations in oxytocin (OT) and dopamine (DA) neurotransmission within brain reward circuitry may be involved. We investigated this possibility, as well as the therapeutic potential of OT for drug-induced social deficits, using the prairie vole (Microtus ochrogaster)-a socially monogamous rodent that forms enduring pair bonds between adult mates. We demonstrate that repeated exposure to the commonly abused psychostimulant amphetamine (AMPH) inhibits the formation of partner preferences (an index of pair bonding) in female prairie voles. AMPH exposure also altered OT and DA neurotransmission in regions that mediate partner preference formation: it decreased OT and DA D2 receptor immunoreactivity in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAcc), respectively, and increased NAcc DA levels. Administration of OT directly into the mPFC of AMPH-exposed voles restored partner preferences, and altered NAcc DA levels, and this effect was dependent on OT receptor activation. Together, these data suggest that repeated AMPH exposure impairs pair bonding through an OT-mediated mechanism, and that OT and DA systems within brain reward circuitry may interact to mediate the complex relationship between drug abuse and social bonding. Further, these results provide empirical support for the idea that the central OT system may represent an important target for the treatment of social deficits in addiction.

The effects of mGluR2/3 activation on acute and repeated amphetamine-induced locomotor activity in differentially reared male rats.

Environmental stimuli play a key role in affecting the likelihood to abuse drugs. Environmental enrichment can reduce that likelihood. The neurotransmitter glutamate contributes to both drug reward and rearing-induced changes in the brain. The current study investigated the effects of the Group-2 metabotropic glutamate receptor (mGluR2/3) agonist, LY-379268 (0.5, 1.0 mg/kg), on acute and repeated amphetamine-induced locomotor activity in differentially reared male rats. Male Sprague-Dawley rats were randomly assigned to one of 3 environmental conditions postweaning: enriched (EC), isolated (IC), or standard (SC), where they reared for 30 days. The effect of LY-379268 on acute amphetamine-induced locomotor activity was assessed. Rats were injected with either LY-379268 (0.5, 1.0 mg/kg) or saline prior to an amphetamine (0.5 mg/kg) or saline challenge injection. Rats were also administered amphetamine (0.5 mg/kg) or saline injections prior to 5 locomotor sessions. Following a rest period of 14-15 days, the effects of repeated amphetamine exposure were evaluated using LY-379268 (0.5, 1.0 mg/kg) or saline injections 30 min prior to receiving amphetamine (0.5 mg/kg). Results showed that LY-379268 administration dose-dependently attenuated acute amphetamine-induced locomotor activity, with EC rats generally displaying less attenuation than IC or SC rats. After repeated amphetamine administrations, the ability of LY-379268 to attenuate the final expression of amphetamine-induced locomotor activity in differentially reared rats was dose-dependent. The differing effect of LY-379268 observed in EC rats suggests enrichment-induced glutamatergic alterations that may protect against sensitivity to psychostimulants.

Effects of withdrawal from repeated amphetamine exposure in peri-puberty on neuroplasticity-related genes in mice

Although extensive evidence demonstrates that repeated administration of amphetamine (AMPH) induces behavioral and neurochemical sensitization, the influence of the developmental timing of AMPH administration is unknown. This is an important issue to address because it could help clarify the influence of early drug exposure on neuronal plasticity and the involvement of dopaminergic sensitization in the etiopathology of neuropsychiatric disorders.Thus, we decided to investigate the molecular alterations induced by the administration of AMPH during adolescence, when repeated exposure to the psychostimulant may interfere with developmental neuroplasticity. We investigated the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and of two inducible-early genes (arc and cfos) that bridge neuronal activity with long-lasting functional alterations. We found that peri-pubertal treatment with AMPH induces long-lasting changes in the expression of bdnf and of activity-regulated genes in the hippocampus and in the prefrontal/frontal cortex, and leads to alterations of their short-term modulation in response to a subsequent acute AMPH challenge. These data suggest that AMPH exposure in peri-puberty may negatively affect the maturation of brain structures, such as the prefrontal cortex, which facilitate the development of dopamine sensitization and may contribute to dopamine-dependent behavioral dysfunctions and molecular alterations in adulthood.

Acetylcholine Encodes Long-Lasting Presynaptic Plasticity at Glutamatergic Synapses in the Dorsal Striatum after Repeated Amphetamine Exposure

Locomotion and cue-dependent behaviors are modified through corticostriatal signaling whereby short-term increases in dopamine availability can provoke persistent changes in glutamate release that contribute to neuropsychiatric disorders, including Parkinson's disease and drug dependence. We found that withdrawal of mice from repeated amphetamine treatment caused a chronic presynaptic depression (CPD) in glutamate release that was most pronounced in corticostriatal terminals with a low probability of release and lasted >50 d in treated mice. An amphetamine challenge reversed CPD via a dopamine D1-receptor-dependent paradoxical presynaptic potentiation (PPP) that increased corticostriatal activity in direct pathway medium spiny neurons. This PPP was correlated with locomotor responses after a drug challenge, suggesting that it may underlie the sensitization process. Experiments in brain slices and in vivo indicated that dopamine regulation of acetylcholine release from tonically active interneurons contributes to CPD, PPP, locomotor sensitization, and cognitive ability. Therefore, a chronic decrease in corticostriatal activity during withdrawal is regulated around a new physiological range by tonically active interneurons and returns to normal upon reexposure to amphetamine, suggesting that this paradoxical return of striatal activity to a more stable, normalized state may represent an additional source of drug motivation during abstinence.

Individual differences in the conditioned and unconditioned rat 50-kHz ultrasonic vocalizations elicited by repeated amphetamine exposure

Adult rats often produce 50-kHz ultrasonic vocalizations (USVs), particularly the frequency-modulated varieties, in appetitive situations. These calls are thought by some to reflect positive affective states and the reinforcing value of drugs such as amphetamine and cocaine. The objective of this study was to determine whether the number of unconditioned 50-kHz USVs elicited by amphetamine predicts the development and/or magnitude of drug-conditioned motivation. In three experiments, we recorded USVs before and after injections of 1mg/kg amphetamine (i.v. or i.p.) administered once per session. Rats were categorized as "high callers" or "low callers" according to individual differences in the number of 50-kHz USVs elicited by their first amphetamine injection. We examined the conditioned appetitive behavior and conditioned place preference (CPP) that emerged in high and low callers after repeated pairings of amphetamine with specific contexts. We also examined whether amphetamine-induced calling was affected by treatment within an unfamiliar (test chamber) versus familiar (home cage) context. Within an unfamiliar environment, the high callers consistently produced more amphetamine-induced 50-kHz USVs than the low callers. Compared to the low callers, high callers showed significantly greater amphetamine CPP as well as enhanced conditioned 50-kHz USVs and locomotor activity during anticipation of amphetamine. Individual differences were stable when amphetamine was administered in test chambers, but when it was administered in home cages, low callers showed an increase in 50-kHz calling that matched the high callers. These findings suggest that individual differences in drug-induced USVs can reveal environment-sensitive traits involved in drug-related appetitive motivation.

Age-dependent effects of repeated amphetamine exposure on working memory in rats

Cognitive dysfunction is a hallmark of chronic psychostimulant misuse. Adolescents may have heightened risk of developing drug-induced deficits because their brains are already undergoing widespread changes in anatomy and function as a normal part of development. To address this hypothesis, we performed two sets of experiments where adolescent and young adult rats were pre-exposed to saline or amphetamine (1 or 3mg/kg) and subsequently tested in a prefrontal cortex (PFC)-sensitive working memory task. A total of ten injections of AMPH or saline (in control rats) were given every other day over the course of 19 days. After rats reached adulthood (>90 days old), cognitive performance was assessed using operant-based delayed matching-to-position (DMTP) and delayed nonmatching-to-position (DNMTP) tasks. DNMTP was also assessed following challenges with amphetamine (0.3–1.25mg/kg), and ketamine (5.0–10mg/kg). In experiment one, we also measured the locomotor response following the first and tenth pre-exposure to amphetamine and after an amphetamine challenge given at the conclusion of operant testing. Compared to adult-exposed groups, adolescents were less sensitive to the psychomotor effects of amphetamine. However, they were more vulnerable to exposure-induced cognitive impairments. For example, adolescent-exposed rats displayed delay-dependent deficits in accuracy, increased sensitivity to proactive interference, and required more training to reach criterion. Drug challenges produced deficits in DNMTP performance, but these were not dependent on pre-exposure group. These studies demonstrate age of exposure-dependent effects of amphetamine on cognition in a PFC-sensitive task, suggesting a heightened sensitivity of adolescents to amphetamine-induced neuroplasticity.

Comparison of the long-term consequences of withdrawal from repeated amphetamine exposure in adolescence and adulthood on information processing and locomotor sensitization in mice

Repeated administration of the indirect dopamine receptor agonist amphetamine (AMPH) produces robust locomotor sensitization and additional behavioral abnormalities. Accumulating evidence suggests that the developmental timing of drug exposure can critically influence this effect. The present study compared the consequences of withdrawal from repeated AMPH exposure in adolescence and adulthood on information processing and locomotor sensitization in C57BL/6 mice. Animals were injected daily with AMPH (1 or 2.5mg/kg) or vehicle on 7 consecutive days starting either from postnatal day 35 to 42, or from postnatal day 70 to 77, following which they were given a 4 week withdrawal period before behavioral and pharmacological testing commenced. We found that withdrawal from the higher dose of AMPH (2.5mg/kg/day) given either in adolescence or adulthood similarly disrupted selective associative learning as measured by the latent inhibition paradigm. None of the AMPH withdrawal groups displayed alterations in sensorimotor gating in the form of prepulse inhibition. Withdrawal from adult AMPH exposure at both doses induced marked locomotor sensitization, whereas adolescent pre-treatment with the higher (2.5mg/kg/day) but not lower (1mg/kg/day) dose of AMPH potentiated the locomotor-enhancing effects of acute AMPH re-challenge. Our study suggests that withdrawal from repeated AMPH exposure in adolescence and adulthood has similar consequences on selective associative learning, but the two manipulations differ with respect to their efficacy to induce long-term locomotor sensitization to the drug. The latter finding supports the hypothesis that the precise developmental timing determines, at least in part, the impact on long-term dopamine-associated sensitization processes.

Sensitivity to the rewarding and locomotor effects of amphetamine following early treatment with methotrexate and cytarabine in adolescent mice

Stimulants such as amphetamine (AMPH) are popular medications for the cognitive late effects of childhood cancer treatment. While the reinforcing and stimulatory properties of AMPH are well documented, little is known about these effects in childhood cancer survivors. In this study, conditioned place preference (CPP) and locomotor activity were measured to investigate sensitivity to AMPH in adolescent (PND 35) mice following neonatal exposure to chemotherapeutic agents methotrexate (MTX) and cytarabine (Ara‐C). Mice were treated with saline, MTX (2.0 mg/kg), Ara‐C (20 mg/kg) or a combination of MTX and Ara‐C (2.0 mg/kg and+ 20 mg/kg) at PND 14, 15, and 16. At the low dose of AMPH (1 mg/kg), chemotherapy‐treated mice spent significantly more time on the AMPH‐paired side. MTX and Ara‐C treated mice showed higher activity levels compared to saline following each AMPH injection, but only the Ara‐C group displayed behavioral sensitization across repeated AMPH exposure. At the high dose of AMPH (4 mg/kg), only mice treated with Ara‐C spent significantly more time on the AMPH‐paired side. All groups showed a higher activity level following the first AMPH injection compared to baseline. Findings suggest that sensitivity to the rewarding and locomotor effects of AMPH is enhanced following early treatment with MTX and Ara‐C. Since these effects were observed at a lower AMPH dose compared to the saline‐treated group, it may be that the dose‐response curve for AMPH has shifted following early chemotherapeutic treatment. (Supported by CA129092 and T32 DA07237).

Impact of mGluR5 during amphetamine-induced hyperactivity and conditioned hyperactivity in differentially reared rats

3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP) is a metabotropic glutamate receptor 5 (mGluR5) antagonist that may alter drug sensitivity in differentially reared rats due to its involvement in the psychostimulant reward pathway and plasticity. The purpose of this study was to assess the effects of MTEP on acute amphetamine-induced hyperactivity, conditioned hyperactivity, and sensitization. Rats were reared in an enriched (EC), isolated (IC), or standard (SC) condition after which rats were either administered MTEP (1.0mg/kg, ip) or saline prior to an acute (0.5 or 1.0mg/kg, sc) or repeated (0.3mg/kg, sc) amphetamine exposure. Rats undergoing repeated amphetamine exposure were administered MTEP prior to conditioned hyperactivity and sensitization tests. EC and SC rats administered with MTEP prior to acute amphetamine demonstrated attenuated amphetamine-induced locomotor activity compared to controls, while IC rats administered MTEP following repeated amphetamine exposure demonstrated attenuated amphetamine-induced locomotor activity. Interestingly, MTEP treatment only altered conditioned hyperactivity in EC rats, as MTEP pretreatment resulted in conditioned hyperactivity in EC rats while conditioned hyperactivity was not observed in EC rats pretreated with saline. Glutamatergic pathways are altered during differential rearing, which differentially alters the role of mGluR5 in EC, IC, and SC rats when administered psychostimulant acutely versus repeatedly. These findings suggest that differential rearing alters glutamatergic function, which reduces sensitivity to psychostimulants.

Repeated amphetamine exposure disrupts dopaminergic modulation of amygdala-prefrontal circuitry and cognitive/emotional functioning.

Repeated exposure to psychostimulants such as amphetamine (AMPH) disrupts cognitive and behavioral processes mediated by the medial prefrontal cortical (mPFC) and basolateral amygdala (BLA). The present study investigated the effects of repeated AMPH exposure on the neuromodulatory actions of dopamine (DA) on BLA-mPFC circuitry and cognitive/emotional processing mediated by these circuits. Rats received five AMPH (2 mg/kg) or saline injections (controls) over 10 d, followed by 2-4 week drug washout. In vivo neurophysiological extracellular recordings in urethane-anesthetized rats were used to obtain data from mPFC neurons that were either inhibited or excited by BLA stimulation. In controls, acute AMPH attenuated BLA-evoked inhibitory or excitatory responses; these effects were mimicked by selective D(2) or D(1) agonists, respectively. However, in AMPH-treated rats, the ability of these dopaminergic manipulations to modulate BLA-driven decreases/increases in mPFC activity was abolished. Repeated AMPH also blunted the excitatory effects of ventral tegmental area stimulation on mPFC neural firing. Behavioral studies assessed the effect of repeated AMPH on decision making with conditioned punishment, a process mediated by BLA-mPFC circuitry and mesocortical DA. These treatments impaired the ability of rats to use conditioned aversive stimuli (footshock-associated cue) to guide the direction of instrumental responding. Collectively, these data suggest that repeated AMPH exposure can lead to persistent disruption of dopaminergic modulation of BLA-mPFC circuitry, which may underlie impairments in cognitive/emotional processing observed in stimulant abusers. Furthermore, they suggest that impairments in decision making guided by aversive stimuli observed in stimulant abusers may be the result of repeated drug exposure.