D-amphetamine-induced behavioral sensitization: effect of lesioning dopaminergic terminals in the medial prefrontal cortex, the amygdala and the entorhinal cortex

Behavioral sensitization to psychostimulants and opioids: What is known in rodents and what still needs to be explored in humans?

Repeated exposure to substances of abuse results in an increase in some behavioral responses. This phenomenon, called behavioral sensitization (BS), is well described in preclinical models. However, its existence in humans is still a matter of debate. After a review of preclinical evidence of BS and its mechanisms in animal models, we reviewed the evidence supporting the existence of BS in humans, despite the limited research available in this regard. We focused our review on opioids and psychostimulants, since they share the ability to promote addictive behaviors. Further, they induce BS despite their distinct sedative and stimulant properties. Moreover, we proposed future research perspectives in this review to address the remaining unsolved questions, especially regarding BS in humans using a harm reduction approach.

Neuron-specific cilia loss differentially alters locomotor responses to amphetamine in mice

The neural mechanisms that underlie responses to drugs of abuse are complex, and impacted by a number of neuromodulatory peptides. Within the past 10 years it has been discovered that several of the receptors for neuromodulators are enriched in the primary cilia of neurons. Primary cilia are microtubule-based organelles that project from the surface of nearly all mammalian cells, including neurons. Despite what we know about cilia, our understanding of how cilia regulate neuronal function and behavior is still limited. The primary objective of this study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to amphetamine. To test the consequences of cilia loss on amphetamine-induced locomotor activity we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice. Cilia loss had no effect on baseline locomotion in either mouse strain. In mice lacking cilia on dopaminergic neurons, locomotor activity compared to wild- type mice was reduced in both sexes in response to acute administration of 3.0 mg/kg amphetamine. In contrast, changes in the locomotor response to amphetamine in mice lacking cilia on GAD2-GABAergic neurons were primarily driven by reductions in locomotor activity in males. Following repeated amphetamine administration (1.0 mg kg^-1  day^-1 over 5 days), mice lacking cilia on GAD2-GABAergic neurons exhibited enhanced sensitization of the locomotor stimulant response to the drug, whereas mice lacking cilia on dopaminergic neurons did not differ from wild-type controls. These results indicate that cilia play neuron-specific roles in both acute and neuroplastic responses to psychostimulant drugs of abuse.

Influence of dopamine transmission in the medial prefrontal cortex and dorsal striatum on the emission of 50-kHz ultrasonic vocalizations in rats treated with amphetamine: Effects on drug-stimulated and conditioned calls

Rat ultrasonic vocalizations (USVs) of 50 kHz are increasingly being evaluated as a behavioral marker of the affective properties of drugs. Studies in amphetamine-treated rats have shown that activation of dopamine transmission in the nucleus accumbens (NAc) initiates the emission of 50-kHz USVs, but little is known on how dopamine transmission in other brain regions modulates the effects of drugs on calling behavior. To clarify this issue, we evaluated 50-kHz USV emissions in rats subjected to dopaminergic denervation of either the medial prefrontal cortex (mPFC) or the dorsal striatum (DS) and treated with amphetamine. Rats received amphetamine (1 mg/kg, i.p. × 5) on alternate days in a test cage; 7 days later, they were re-exposed to the test cage, to measure calling behavior that may reflect drug conditioning, and then challenged with amphetamine (1 mg/kg, i.p.). The numbers of total and categorized 50-kHz USVs emitted were evaluated, along with immunofluorescence for Zif-268 in the NAc. Dopamine-denervated and sham-operated rats displayed comparable patterns of calling behavior during amphetamine treatment and after amphetamine challenge. Conversely, rats that were dopamine-denervated in the mPFC, but not DS, emitted low numbers of 50-kHz USVs on test cage re-exposure. Finally, dopamine-denervated rats displayed a less marked increase in Zif-268-positive neurons in the NAc shell after amphetamine challenge, compared with sham-operated rats. These results may be relevant to identify the neuronal circuits that modulate 50-kHz USV emissions in rats treated with amphetamine, as well as the interplay between calling behavior and affective properties of drugs.

Effects of Ethanol Exposure During Adolescence or Adulthood on Locomotor Sensitization and Dopamine Levels in the Reward System

Behavioral sensitization is a process of neuroadaptation characterized by a gradual increase in motor behaviors. The major neural substrates involved in the behavioral sensitization lie on the dopaminergic mesocorticolimbic pathway, which is still under development during adolescence. To investigate age-differences in ethanol behavioral sensitization and dopamine levels in distinct brain regions of the reward system, adolescent and adult mice were repeatedly pretreated with saline or ethanol (2.0 g/kg i.p.) during 15 consecutive days and challenged with saline or ethanol 5 days after pretreatment. Dopamine and its metabolites were measured in tissue samples of the prefrontal cortex (PFC), nucleus accumbens (NAc) and striatum by HPLC analysis. While repeated ethanol administration resulted in the development of locomotor sensitization in both adult and adolescent mice, only the adults expressed sensitization to a subsequent ethanol challenge injection. Neurochemical results showed reduced dopamine levels in adolescents compared to adults. Specifically, mice pretreated with ethanol during adolescence displayed lower dopamine levels in the PFC compared to the respective adult group in response to an ethanol challenge injection, and preadolescent mice exhibited lower dopamine levels in the NAc following an acute ethanol treatment compared to adults. These findings suggest that adolescent mice are not only less sensitive to the expression of ethanol-induced sensitization than adults, but also show lower dopamine content after ethanol exposition in the PFC and NAc.

D1 receptor-mediated inhibition of medial prefrontal cortex neurons is disrupted in adult rats exposed to amphetamine in adolescence

Amphetamine (AMPH) exposure leads to changes in behavior and dopamine receptor function in the prefrontal cortex (PFC). Since dopamine plays an important role in regulating GABAergic transmission in the PFC, we investigated if AMPH exposure induces long-lasting changes in dopamine's ability to modulate inhibitory transmission in the PFC as well as whether the effects of AMPH differed depending on the age of exposure. Male Sprague-Dawley rats were given saline or 3 mg/kg AMPH (i.p.) repeatedly during adolescence or adulthood and following a withdrawal period of up to 5 weeks (Experiment 1) or up to 14 weeks (Experiment 2), they were sacrificed for in vitro whole-cell recordings in layer V/VI of the medial PFC. We found that in brain slices from either adolescent- or adult-exposed rats, there was an attenuation of dopamine-induced increases in inhibitory synaptic currents in pyramidal cells. These effects did not depend on age of exposure, were mediated at least partially by a reduced sensitivity of D1 receptors in AMPH-treated rats, and were associated with an enhanced behavioral response to the drug in a separate group of rats given an AMPH challenge following the longest withdrawal period. Together, these data reveal a prolonged effect of AMPH exposure on medial PFC function that persisted for up to 14 weeks in adolescent-exposed animals. These long-lasting neurophysiological changes may be a contributing mechanism to the behavioral consequences that have been observed in those with a history of amphetamine abuse.

MK-801-induced behavioural sensitisation alters dopamine release and turnover in rat prefrontal cortex

RATIONALE

Repeated exposure to psychostimulants that either increase dopamine (DA) release or target N-methyl-D-aspartate (NMDA) receptors can induce behavioural sensitisation, a phenomenon that may be important for the processes of addiction and even psychosis. A critical component of behavioural sensitisation is an increase in DA release within mesocorticolimbic circuits. In particular, sensitisation to amphetamine leads to increased DA release within well-known sub-cortical brain regions and also regulatory regions such as prefrontal cortex (PFC). However, it is unknown how DA release within the PFC of animals is altered by sensitisation to NMDA receptor antagonists.

OBJECTIVES

The aims of the present study were twofold, firstly to examine whether a single dose of dizocilpine maleate (MK-801) could induce long-term behavioural sensitisation and secondly to examine DA release in the PFC of sensitised rats.

MATERIALS AND METHODS

Behavioural sensitisation was assessed by measuring locomotion after drug exposure. DA release in the PFC was measured using freely moving microdialysis.

RESULTS

We show that a single dose of MK-801 can induce sensitisation to subsequent MK-801 exposure in a high percentage of rats (66 %). Furthermore, rats sensitised to MK-801 have altered DA release and turnover in the PFC compared with non-sensitised rats.

CONCLUSION

Schizophrenia patients have been postulated to have 'endogenous sensitisation' to psychostimulants. MK-801-induced sensitised rats, in particular when compared with non-sensitised rats, provide a useful model for studying PFC dysfunction in schizophrenia.

Prefrontal glutamate levels differentiate early phase schizophrenia and methamphetamine addiction: a (1)H MRS study at 3Tesla

Acute symptoms of methamphetamine-induced psychosis are similar to those of primary schizophrenia. Understanding similarities or differences in the biological substrate of these psychoses could lead to early differentiation of these two clinical conditions resulting in more efficient treatment strategies. Proton magnetic resonance spectroscopy was acquired from the medial prefrontal cortex in 29 unmedicated patients with first episode of psychosis (FEP), 29 abstinent methamphetamine-addicted people (METH) and 45 healthy controls (HCs) (age range 17.3 to 29.9years old). The METH group displayed robust reductions in concentration levels of glutamate (Glu) relative to FEP (Cohen's d=1.20) and HC (d=0.87). The METH group also displayed reduced levels of N-acetylaspartate (NAA) relative to FEP (d=0.53) and HC (d=0.76). The HC group displayed a positive association between levels of Glu and NAA, r(45)=0.52, p<0.001, while the two clinical groups failed to show this normal association. This suggests that the cellular metabolism is altered in both conditions. These data support the assumption that cellular abnormalities differ between primary schizophrenia and methamphetamine addiction despite the overlap in clinical presentation.

Clozapine pre-treatment has a protracted hypolocomotor effect on the induction and expression of amphetamine sensitization

Amphetamine locomotor sensitization is an animal model for the study of addiction and schizophrenia. The antipsychotic clozapine blocks the hyperlocomotion induced by an acute injection of amphetamine, but its effect on locomotor sensitization after repeated amphetamine administration remains unknown. In the present study we investigate the effect of repeated administration of clozapine on the induction and expression of amphetamine locomotor sensitization. We propose that repeated administration of clozapine blocks the induction and expression of amphetamine sensitization. Male Sprague-Dawley rats were classified according to their locomotor response to an acute saline injection in high responder saline (HRS) or low responder saline (LRS). Rats from both groups were injected once daily with amphetamine for 5 consecutive days. Horizontal locomotor activity was measured during 40 min. Four days after the last injection, an acute dose of amphetamine was administered to assess the expression of sensitization. Clozapine was injected once daily for 4 consecutive days before (pre-treatment) or after (treatment) induction of sensitization. Pre-treatment with clozapine significantly decreases both acute amphetamine-induced hyperlocomotion and the induction and expression of amphetamine sensitization only in LRS rats, showing a protracted hypolocomotor effect. On the other hand, clozapine treatment had no effect over locomotor response on the expression of amphetamine sensitization in either LRS or HRS rats. These data suggest that clozapine effect on amphetamine locomotor response depends on individual differences. Also, our results suggest that clozapine pre-treatment attenuates the neuroplasticity underlying amphetamine sensitization, but clozapine treatment is unable to reverse these changes once amphetamine sensitization has been induced.

Effects of lithium and valproate on oxidative stress and behavioral changes induced by administration of m-AMPH

In the last years our research group has studied and validated the animal model of mania induced by dextroamphetamine (d-AMPH). Considering the lack of animal models of mania reported in the literature; this study evaluated the possibilities to validate the animal model induced by methamphetamine (m-AMPH). Then, we evaluated the effects of lithium (Li), valproate (VPA) on the behavior and parameters of oxidative damage in rat brain after administration of m-AMPH. In the prevention treatment, Wistar rats were pretreated with Li, VPA or saline (Sal) for 14 days, and then, between days 8 and 14, rats were treated with m-AMPH (1, 0.5 or 0.25 mg/kg) or Sal. In the reversal treatment, rats were first given m-AMPH (0.25 mg/kg) or Sal. Locomotor behavior was assessed using the open-field task and parameters of oxidative damage were measured in brain structures. Our results show that the hyperactivity was prevented and reverted by Li and VPA only when m-AMPH was administered in the dose of 0.25mg/kg. In addition, the m-AMPH in all doses administrated induced oxidative damage in both structures tested in two models. Li and VPA reversed and prevented this impairment, however in a way dependent of cerebral area, the dose of m-AMPH and technique.

Opiate sensitization induces FosB/ΔFosB expression in prefrontal cortical, striatal and amygdala brain regions

Sensitization to the effects of drugs of abuse and associated stimuli contributes to drug craving, compulsive drug use, and relapse in addiction. Repeated opiate exposure produces behavioral sensitization that is hypothesized to result from neural plasticity in specific limbic, striatal and cortical systems. ΔFosB and FosB are members of the Fos family of transcription factors that are implicated in neural plasticity in addiction. This study examined the effects of intermittent morphine treatment, associated with motor sensitization, on FosB/ΔFosB levels using quantitative immunohistochemistry. Motor sensitization was tested in C57BL/6 mice that received six intermittent pre-treatments (on days 1, 3, 5, 8, 10, 12) with either subcutaneous morphine (10 mg/kg) or saline followed by a challenge injection of morphine or saline on day 16. Mice receiving repeated morphine injections demonstrated significant increases in locomotor activity on days 8, 10, and 12 of treatment (vs. day 1), consistent with development of locomotor sensitization. A morphine challenge on day 16 significantly increased locomotor activity of saline pre-treated mice and produced even larger increases in motor activity in the morphine pre-treated mice, consistent with the expression of opiate sensitization. Intermittent morphine pre-treatment on these six pre-treatment days produced a significant induction of FosB/ΔFosB, measured on day 16, in multiple brain regions including prelimbic (PL) and infralimbic (IL) cortex, nucleus accumbens (NAc) core, dorsomedial caudate-putamen (CPU), basolateral amygdala (BLA) and central nucleus of the amygdala (CNA) but not in a motor cortex control region. Opiate induced sensitization may develop via Fos/ΔFosB plasticity in motivational pathways (NAc), motor outputs (CPU), and associative learning (PL, IL, BLA) and stress pathways (CNA).

Sensitized activation of Fos and brain-derived neurotrophic factor in the medial prefrontal cortex and ventral tegmental area accompanies behavioral sensitization to amphetamine

Behavioral sensitization, or augmented locomotor response to successive drug exposures, results from neuroadaptive changes contributing to addiction. Both the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA) influence behavioral sensitization and display increased immediate-early gene and BDNF expression after psychostimulant administration. Here we investigate whether mPFC neurons innervating the VTA exhibit altered Fos or BDNF expression during long-term sensitization to amphetamine. Male Sprague-Dawley rats underwent unilateral intra-VTA infusion of the retrograde tracer Fluorogold (FG), followed by 5 daily injections of either amphetamine (2.5 mg/kg, i.p.) or saline vehicle. Four weeks later, rats were challenged with amphetamine (1.0 mg/kg, i.p.) or saline (1.0 mL/kg, i.p.). Repeated amphetamine treatment produced locomotor sensitization upon drug challenge. Two hours later, rats were euthanized, and mPFC sections were double-immunolabeled for either Fos-FG or Fos-BDNF. Tissue from the VTA was also double-immunolabeled for Fos-BDNF. Amphetamine challenge increased Fos and BDNF expression in the mPFC regardless of prior drug experience, and further augmented mPFC BDNF expression in sensitized rats. Similarly, more Fos-FG and Fos-BDNF double-labeling was observed in the mPFC of sensitized rats compared to drug-naïve rats after amphetamine challenge. Repeated amphetamine treatment also increased VTA BDNF, while both acute and repeated amphetamine treatment increased Fos and Fos-BDNF co-labeling, an effect enhanced in sensitized rats. These findings point to a role of cortico-tegmental BDNF in long-term amphetamine sensitization.

Vulnerability to nicotine abstinence-related social anxiety-like behavior: molecular correlates in neuropeptide Y, Y2 receptor and corticotropin releasing factor

An outbred rat model of the novelty-seeking phenotype is used to study nicotine vulnerability, where experimentally naïve rats were phenotype screened as high or low responders (HRs or LRs, ranking in the upper or lower one-third of the population respectively) based on locomotor activity displayed in a novel environment. Following nicotine training and abstinence, HR animals pre-trained with nicotine showed expression of locomotor sensitization to nicotine challenge along with enhanced social anxiety-like behavior in the social interaction test compared to saline pre-trained controls. HR rats also showed a downregulation in neuropeptide Y (NPY) mRNA levels in the medial nucleus of amygdala and the CA1 field of the hippocampus, an upregulation in Y2 mRNA levels in the CA3 field of the hippocampus, and an upregulation in the corticotropin releasing factor (CRF) mRNA levels in the central nucleus of the amygdala. These findings implicate dysregulations in the NPY-CRF systems in the HR hippocampus and amygdala associated with the emergence of social anxiety-like behavior, and a novel Y2R-mediated pathway in nicotine relapse.

Impact of metabotropic glutamate 2/3 receptor stimulation on activated dopamine release and locomotion

RATIONALE

Activation of metabotropic glutamate (mGlu) 2/3 receptors may provide a novel strategy for treating schizophrenia. This effect is thought to be mediated through dopamine-independent mechanisms because mGlu2/3-receptor agonists have no considerable affinity for dopamine receptors. These agonists, however, reduce amphetamine-induced hyperlocomotion suggesting that they influence dopamine neurotransmission.

OBJECTIVE

We evaluated whether the inhibitory effect of mGlu2/3-receptor activation on amphetamine-induced hyperlocomotion correlates with attenuated dopamine release. We also assessed whether mGlu 2/3 receptor activation has inhibitory effects on activity-dependent vesicular release of dopamine in behaving animals.

METHODS

Microdialysis was used to measure extracellular levels of dopamine in the dorsal striatum (DStr) and nucleus accumbens (NAc) of freely moving rats. The effect of the mGlu2/3-receptor agonist LY354740 on dopamine release and locomotion elicited by amphetamine, electrical stimulation of the ventral tegmental area, or L-dopa was assessed.

RESULTS

We find that the inhibitory effect of mGlu2/3 activation on amphetamine-induced hyperlocomotion correlates with an attenuated increase in dopamine release in the NAc and DStr. However, when dopamine levels were increased by electrical stimulation of dopamine neurons or by administration of the dopamine precursor L-dopa, activation of mGlu2/3 receptors had no effect on dopamine release or on behavior.

CONCLUSIONS

Activation of mGlu2/3 receptors attenuates amphetamine-induced dopamine release through a mechanism that does not affect activity dependent vesicular release, reuptake or synthesis of dopamine.

Amphetamine exposure selectively enhances hippocampus-dependent spatial learning and attenuates amygdala-dependent cue learning

Behaviorally sensitizing regimen of amphetamine (AMPH) exposure has diverse effects on learning, memory, and cognition that are likely to be a consequence of long-term neural adaptations occurring in the cortico-limbic-striatal circuitry. In particular, altered dopamine signaling in the nucleus accumbens and medial prefrontal cortex has been implicated to underlie AMPH-induced changes in behavior. This study sought to test the hypothesis that repeated AMPH exposure disrupts the regulation of limbic information processing and the balance of competing limbic control over appetitive behavior. Mice received seven intraperitoneal injections of D-AMPH (2.5 mg/kg or 5 mg/kg) or vehicle solution (saline) and were trained in (1) a simultaneous conditioned cue and place preference task using a six-arm radial maze, found to depend on the integrity of the hippocampus (HPC) and basolateral amygdala (BLA), respectively and (2) a conditional BLA-dependent cue, and HPC-dependent place learning task using an elevated T-maze. In both tasks, the vehicle pretreatment group initially acquired cue learning, followed by the emergence of significant place/spatial learning. In contrast, pretreatment with repeated AMPH caused marked deviations from normal acquisition patterns of place and cue conditioning, significantly facilitating HPC-dependent place conditioning in the first task while attenuating BLA-dependent cue conditioning in both tasks. These findings provide the first demonstration of an aberrant regulation of HPC- and BLA-dependent learning as a result of AMPH exposure, highlighting the importance of the meso-coticolimbic dopamine system in maintaining the balance of limbic control over appetitive behavior.

Adaptations in medial prefrontal cortex function associated with amphetamine-induced behavioral sensitization

Neuroadaptations in the prefrontal cortex (PFC) are hypothesized to play an important role in the behavioral changes associated with repeated psychostimulant exposure, but there are few published studies that measure neuronal activity during the development and expression of sensitization. To address this, we recorded single neuron activity in the medial PFC (mPFC) of male rats that were exposed for 5 days to saline or amphetamine (AMPH; 1.0 mg/kg i.p.) and then given saline or AMPH challenges following a three-day withdrawal. We found that rats exposed to AMPH developed locomotor sensitization to the drug that emerged on the fifth treatment session and became statistically significant at AMPH challenge. This was associated with no change in baseline (i.e., pre-injection) activity of mPFC neurons across the treatment or challenge sessions. Following the first AMPH injection, mPFC neurons responded primarily with reductions in firing, with the overall pattern and magnitude of responses remaining largely similar following repeated treatment. The exception was in the minority of cells that respond to AMPH with increases in firing rate. In this population, the magnitude of excitations peaked during the fifth AMPH exposure and was still relatively elevated at the AMPH challenge. Furthermore, these units increased firing during a saline challenge that was given to assess associative conditioning. These results suggest that AMPH-induced adaptations in mPFC function are not as apparent as AMPH-induced adaptations in behavior. When mPFC adaptations do occur, they appear limited to the population of neurons that increase their firing in response to AMPH.

Bilateral six-hydroxydopamine administration to PFC prevents the expression of behavioral sensitization to methylphenidate

Psychostimulants like amphetamine and methylphenidate (MPD) are used to treat attention deficit hyperactivity disorder (ADHD), which is marked by developmentally inappropriate inattention, hyperactivity, and impulsivity. Neuropsychological analyses indicate that ADHD patients are impaired on tasks of behavioral inhibition, reward reversal, and working memory, which are functions of the prefrontal cortex (PFC) and are modulated by the mesocortical dopamine (DA) system. Non-specific electrical lesioning of PFC eliminated the expression of behavioral sensitization elicited by chronic MPD administration. Behavioral sensitization is the progressive augmentation of locomotor activity as a result of repetitive (chronic) exposure to the drug. It is believed that the sensitization to chronic drug treatment is caused due to an increase in DA in the mesocorticolimbic DA system, which includes the PFC. Therefore, this study investigated the role of PFC DA in mediating the behavioral sensitization to repeated administration of MPD in adult male Sprague-Dawley rats. On experimental day (ED) 1, the behavior was recorded post-saline injection. On ED 2, the rats were divided into three groups--control, sham and bilateral 6-OHDA treated group; and the sham and 6-OHDA treated groups underwent respective surgeries. After 5 days of rest following surgery, the post-surgery baseline was recorded on ED 8 following a saline injection. All three groups received 2.5 mg/kg MPD for 6 days (from ED 9 to ED 14), followed by a 3-day washout period (ED 15 to ED 18). On ED 19, a rechallenge injection of 2.5 mg/kg MPD was given and locomotor activity was recorded. It was found that the 6-OHDA lesion group failed to exhibit behavioral sensitization to MPD. The involvement of the dopaminergic afferents of PFC in behavioral sensitization to MPD is discussed.

A functional role for the dopamine D3 receptor in the induction and expression of behavioural sensitization to ethanol in mice

RATIONALE

We previously reported that mice lacking dopamine D3 receptors (D3R) were resistant to behavioural sensitization to ethanol (EtOH). Since knockout mice have permanent receptor inactivation, we investigated how temporary pharmacological blockade and activation of D3Rs affected the induction or expression of EtOH sensitization.

MATERIALS AND METHOD

In induction studies, DBA/2 mice received 0, 10 or 75 mg/kg, subcutaneous (s.c.) of the D3R antagonist, U99194A ,before each of seven EtOH (2.2 g/kg, intraperitoneal) or saline injections. Locomotor activity (LMA) was assessed in activity chambers. In expression studies, mice received seven injections of EtOH or saline, followed 14 days later with U99194A or vehicle immediately before a test dose of EtOH (1.8 g/kg). In separate experiments, the effects of the D3R agonist PD128907 (0.01 mg/kg, s.c.) were similarly examined during and after EtOH sensitization.

RESULTS

Chronic co-administration of low-dose U99194A blocked the induction of EtOH sensitization, while acute U99194A had no effect in mice that were already sensitized. Chronic co-administration of PD128907 resulted in decreased LMA but this effect was also seen in saline-treated mice, suggesting a simple subtractive effect. In previously sensitized mice, acute PD128907 significantly attenuated the expression of EtOH sensitization, indicating an enhanced response to the drug.

CONCLUSIONS

Results suggest a modulatory role for the D3R in behavioural sensitization to EtOH, where D3R blockade is associated with induction and D3R stimulation is associated with expression of EtOH sensitization. A model is suggested to account for these complementary functions of the D3R at different stages of EtOH sensitization.

Blockade of D1 dopamine receptors in the medial prefrontal cortex attenuates amphetamine- and methamphetamine-induced locomotor activity in the rat

The medial prefrontal cortex (mPFC) is a component of the mesolimbic dopamine (DA) system involved in psychostimulant-induced hyperactivity and previous studies have shown that altering DA transmission or D2 receptors within the mPFC can decrease this stimulant effect. The goal of this study was to investigate a potential modulatory role for D1 receptors in the mPFC in amphetamine (AMPH)- and methamphetamine (METH)-induced hyperactivity. Locomotor activity in an open-field arena was measured in male, Sprague-Dawley rats given an intra-mPFC infusion of vehicle or the D1 receptor antagonist SCH 23390 (0.25 or 1.0 microg) prior to systemic (i.p.) injection of saline, AMPH (1 mg/kg), or METH (1 mg/kg). We found that SCH 23390 produced a dose-dependent decrease in AMPH- and METH-induced locomotion and rearing but had no significant effect on spontaneous behavior that occurred following systemic saline injections. Because SCH 23390 has been shown to have agonist-like properties at 5-HT(2C) receptors, a follow-up experiment was performed to determine if this contributed to the attenuation of METH-induced activity that we observed. Rats were given intra-mPFC infusions of both SCH 23390 (1.0 microg) and the 5-HT(2C) antagonist RS 102221 (0.25 microg) prior to METH (1 mg/kg, i.p.). The addition of the 5-HT(2C) antagonist failed to alter SCH 23390-induced decreases in METH-induced locomotion and rearing; infusion of RS 102221 alone had no significant effects on locomotion and produced a non-significant decrease in rearing. The results of these studies suggest that D1 activation in the mPFC plays a significant role in AMPH- and METH-induced hyperactivity.

Psychostimulant treatment for ADHD is modulated by prefrontal cortex manipulation

The psychostimulant amphetamine (Amph) is widely used treatments for attention-deficit hyperactivity disorder (ADHD). Chronic intermittent exposure to psychostimulants induces behavioral sensitization. The objective of this study was to investigate the role of prefrontal cortex (PFC) in the acute and chronic effect of Amph using the open-field assay. Male Sprague-Dawley rats were assigned randomly to three groups, (1) an intact control group (2) a PFC sham-operated group, and (3) a PFC lesion group. All the three groups showed increases in locomotor activity after acute amphetamine injection (P<0.05), and activity levels were especially augmented in PFC lesion group. Following chronic amphetamine, the control group and sham-operated group exhibited behavioral sensitization (P<0.05). However, the PFC lesion group failed to exhibit behavioral sensitization and the pattern of locomotion was altered, which indicated that the nature of behavioral sensitization was changed. The results suggest that PFC lesion enhance the acute effects of amphetamine on locomotor activity and is required for development of behavior sensitization.

EEG ALPHA SENSITIZATION IN INDIVIDUALIZED HOMEOPATHIC TREATMENT OF FIBROMYALGIA

Fibromyalgia (FM) patients show evidence of sensitizability in pain pathways and electroencephalographic (EEG) alterations. One proposed mechanism for the claimed effects of homeopathy, a form of complementary medicine used for FM, is time-dependent sensitization (TDS, progressive amplification) of host responses. This study examined possible sensitization-related changes in EEG relative alpha magnitude during a clinical trial of homeopathy in FM. A 4-month randomized, placebo-controlled double-blind trial of daily orally administered individualized homeopathy in physician-confirmed FM, with an additional 2-month optional crossover phase, included three laboratory sessions, at baseline, 3 and 6 months (N = 48, age 49.2 +/- 9.8 years, 94% women). Nineteen leads of EEG relative alpha magnitude at rest and during olfactory administration of treatment and control solutions were evaluated in each session. After 3 months, the active treatment group significantly increased, while the placebo group decreased, in global alpha-1 and alpha-2 during bottle sniffs over sessions. At 6 months, the subset of active patients who stayed on active continued to increase, while the active-switch subgroup reversed direction in alpha magnitude. Groups did not differ in resting alpha. Consistent with the TDS hypothesis, sniff alpha-1 and alpha-2 increases at 6 months versus baseline correlated with total amount of time on active remedy over all subjects (r = 0.45, p = .003), not with dose changes or clinical outcomes in the active group. The findings suggest initiation of TDS in relative EEG alpha magnitude by daily oral administration of active homeopathic medicines versus placebo, with laboratory elicitation by temporolimbic olfactory stimulation or sniffing.

Environmental modulation of ethanol-induced locomotor activity: Correlation with neuronal activity in distinct brain regions of adolescent and adult Swiss mice

Drug abuse is a concerning health problem in adults and has been recognized as a major problem in adolescents. Induction of immediate-early genes (IEG), such as c-Fos or Egr-1, is used to identify brain areas that become activated in response to various stimuli, including addictive drugs. It is known that the environment can alter the response to drugs of abuse. Accordingly, environmental cues may trigger drug-seeking behavior when the drug is repeatedly administered in a given environment. The goal of this study was first to examine for age differences in context-dependent sensitization and then evaluate IEG expression in different brain regions. For this, groups of mice received i.p. ethanol (2.0 g/kg) or saline in the test apparatus, while other groups received the solutions in the home cage, for 15 days. One week after this treatment phase, mice were challenged with ethanol injection. Acutely, ethanol increased both locomotor activity and IEG expression in different brain regions, indistinctly, in adolescent and adult mice. However, adults exhibited a typical context-dependent behavioral sensitization following repeated ethanol treatment, while adolescent mice presented gradually smaller locomotion across treatment, when ethanol was administered in a paired regimen with environment. Conversely, ethanol-treated adolescents expressed context-independent behavioral sensitization. Overall, repeated ethanol administration desensitized IEG expression in both adolescent and adult mice, but this effect was greatest in the nucleus accumbens and prefrontal cortex of adolescents treated in the context-dependent paradigm. These results suggest developmental differences in the sensitivity to the conditioned and unconditioned locomotor effects of ethanol.

Morphine-induced behavioral sensitization increased the mRNA expression of NMDA receptor subunits in the rat amygdala

This study was designed to evaluate the effect of repeated morphine treatment on rat behavioral responses. In the genetic section, the mRNA expression of NMDA receptor subunits (NR1 and NR2A) was measured in certain areas of the male rat brain (striatum, prefrontal cortex, hippocampus, hypothalamus and amygdala). In the behavioral section, the effect of repeated morphine treatment on animal models such as locomotion, oral stereotypy, and state-dependent memory in a passive avoidance test was evaluated in the presence or absence of MK801 (NMDA receptor antagonist). Our results showed that chronic morphine treatment, followed by a 7-day (but not 24-hour) washout period, potentiated the effect of test doses of morphine, which is referred to as behavioral sensitization. Meanwhile, pretreatment of animals with MK801 (0.1 and 0.25 mg/kg), 30 min before a test dose of morphine (5 mg/kg), failed to attenuate the locomotion and oral stereotypy in the behavioral sensitization state. Interestingly, a higher dose of MK801 (0.25 mg/kg) decreased memory retrieval induced by morphine (2.5 mg/kg) in state-dependent memory. This effect may be due to the intrinsic motor enhancer property of higher doses of MK801, rather than the blockade of NMDA receptors. It can be concluded that MK801 does not affect morphine-induced behavioral sensitization in the expression phase. In the genetic section of the study, results of quantitative real-time RT-PCR clearly indicated that morphine sensitization increased the expression of NMDA receptor subunits mRNA in the amygdala (NR1 by 104% and NR2A by 85%), while the other areas of the brain were unaffected. Maenwhile, no change in the mRNA levels was observed in non-sensitized animals (chronic morphine treatment followed by a 24-hour washout period). In summary, the present study indicates that repeated morphine treatment followed by long-term (7-day washout) induces behavioral sensitization and causes a delayed increase in mRNA levels of NMDA receptor subunits in the rat amygdala. Meanwhile, it has previously been reported that the amygdala is involved in behavioral sensitization. Thus, it can be concluded that the increase in NMDA receptor expression is associated with morphine-induced behavioral sensitization.

Endogenous nociceptin (orphanin FQ) suppresses basal hedonic state and acute reward responses to methamphetamine and ethanol, but facilitates chronic responses

The opioid peptide nociceptin (orphanin FQ) suppresses drug reward, drug self-administration, and impedes some of the processes believed to underlie the transition to addiction. As virtually all previous studies have used administration of nociceptin receptor agonists to evaluate the role of nociceptin on addiction-like behavior, the current study used a pharmacological (nociceptin receptor antagonist) and genetic (nociceptin receptor knockout mice) approach to elucidate the role of endogenous nociceptin. The nociceptin receptor antagonist UFP-101 induced a modest place preference, and enhanced the conditioned place preference induced by methamphetamine. In agreement with this, nociceptin receptor knockout mice had slightly enhanced methamphetamine and ethanol conditioned place preferences compared to wild-type mice. This effect did not appear to depend on differences in learning ability, as nociceptin receptor knockout mice had slightly weaker-conditioned place aversions to lithium chloride, the kappa-opioid receptor agonist, U50488H, and the general opiate antagonist, naloxone. The development of behavioral sensitization to methamphetamine was lower in nociceptin receptor knockout mice, and attenuated by UFP-101 administration to wild-type mice. Additionally, ethanol consumption and preference in a two-bottle choice test was lower in nociceptin receptor knockout mice, though ethanol-stimulated locomotion was stronger. Whereas the rewarding effect of methamphetamine and ethanol following chronic treatment, as measured by place conditioning, strengthened in wild-type mice, this effect was absent in nociceptin receptor knockout mice. These results suggest that endogenous N/OFQ suppresses basal and drug-stimulated increases in hedonic state, and plays either a permissive or facilitatory role in the development of addiction.

Biological treatments for amfetamine dependence : recent progress

Amfetamine abuse has grown into a worldwide epidemic. Methamfetamine, a derivative of amfetamine made from readily accessible chemicals, has plagued the US since the 1960s, with an alarming recent surge in the numbers of those meeting the criteria for amfetamine abuse and dependence. We review this problem using a computerised literature search (PubMed 1964-2007) to summarise knowledge from animal and human studies about treatments for amfetamine dependence, while exploring the potential of pharmacogenetics to help uncover new treatment targets. Several promising therapeutic targets have come from animal models of reward, drug-taking behaviour and withdrawal. Although preclinical and selected clinical results have been promising, clinical studies have yielded inconsistent results. To improve these outcomes, pharmacogenetic studies may be used to identify candidate alleles that predict therapeutic response. Exciting preclinical findings and a steady progression of clinical results offer hope for the development of a treatment for amfetamine dependence.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.

Activation of afferents to the ventral tegmental area in response to acute amphetamine: a double-labelling study

The ventral tegmental area (VTA), primary source of the mesocorticolimbic dopaminergic system, is regarded as a critical site for initiation of behavioural sensitization to psychostimulants. The present study was undertaken to identify the neural pathways converging on the VTA that are potentially implicated in this process. Rats were sensitized by a single exposure to amphetamine (5 mg/kg, s.c.). The distribution of VTA-projecting neurons activated by amphetamine was examined by combining retrograde transport of the cholera toxin beta subunit (CTb), injected into the VTA, with immunodetection of Fos. The quantitative analysis of CTb-Fos double labelling demonstrates that amphetamine induced a rapid activation of Fos in a large number of brain areas projecting to the VTA. More than half of the CTb-Fos double-labelled neurons were located in the prefrontal cortex, lateral preoptic area-lateral hypothalamus, pontomesencephalic tegmentum, dorsal raphe nucleus, ventral pallidum and nucleus accumbens. In addition, scattered CTb-Fos double-labelled cells were observed in many other VTA afferent structures, such as claustrum, lateral septum, diagonal band-magnocellular preoptic nucleus, deep mesencephalic nucleus, oral part of pontine reticular nucleus and dorsomedial tegmental area. This suggests that systemic amphetamine activates a wide population of neurons projecting to the VTA that may be important for the modulation of neurobehavioural plasticity produced by this psychostimulant.

Lesions of the medial prefrontal cortex prevent the induction but not expression of morphine-induced behavioral sensitization in mice

Repetitive exposure to morphine induces behavioral sensitization, which is supposed to involve in the process of addiction to drugs. As the underlying neuropharmacological mechanisms and anatomical substrates are considerably different between different drugs and different phases of behavioral sensitization, this study was designed to investigate the roles of the medial prefrontal cortex in the induction and expression of morphine-induced behavioral sensitization in mice. In experiments 1 and 2, mice were dosed with morphine (10 mg/kg, i.p.) once daily for 7 continuous days after induction of sham or kainic acid lesions of the medial prefrontal cortex. Locomotor activity was measured on days 1 and 7 to test acute morphine-induced hyperactivity and the induction of behavioral sensitization. In experiment 3, mice were subjected to surgery after behavioral sensitization was achieved. The mice were challenged with morphine 7 days later to evaluate the expression of behavioral sensitization. The results showed that lesions of the medial prefrontal cortex (mPFC) blocked acute morphine-induced hyperactivity and the induction of behavioral sensitization, but the lesions had no effect on the expression of behavioral sensitization. These results provide the first direct evidence that the mPFC may be involved in the induction, but not the expression, of morphine-induced behavioral sensitization.

Role of the ERK pathway in psychostimulant-induced locomotor sensitization

BACKGROUND

Repeated exposure to psychostimulants results in a progressive and long-lasting facilitation of the locomotor response that is thought to have implications for addiction. Psychostimulants and other drugs of abuse activate in specific brain areas extracellular signal-regulated kinase (ERK), an essential component of a signaling pathway involved in synaptic plasticity and long-term effects of drugs of abuse. Here we have investigated the role of ERK activation in the behavioral sensitization induced by repeated administration of psychostimulants in mice, using SL327, a brain-penetrating selective inhibitor of MAP-kinase/ERK kinase (MEK), the enzyme that selectively activates ERK.

RESULTS

A dose of SL327 (30 mg/kg) that reduced the number of activated ERK-positive neurons by 62 to 89% in various brain areas, had virtually no effect on the spontaneous locomotor activity or the acute hyperlocomotion induced by cocaine or D-amphetamine. Pre-treatment with SL327 (30 mg/kg) prior to each drug administration prevented the locomotor sensitization induced by repeated injections of D-amphetamine or cocaine. The SL327 pre-treatment abolished also conditioned locomotor response of mice placed in the context previously paired with cocaine or D-amphetamine. In contrast, SL327 did not alter the expression of sensitized response to D-amphetamine or cocaine.

CONCLUSION

Altogether these results show that ERK has a minor contribution to the acute locomotor effects of psychostimulants or to the expression of sensitized responses, whereas it is crucial for the acquisition of locomotor sensitization and psychostimulant-conditioned locomotor response. This study supports the important role of the ERK pathway in long-lasting behavioral alterations induced by drugs of abuse.

α2A-Adrenoceptors regulate d-amphetamine-induced hyperactivity and behavioural sensitization in mice

Stimulants, such as d-amphetamine, enhance the release of dopamine in the central nervous system (CNS) and induce locomotor activation in mice. When amphetamine is administered repeatedly, the locomotor activation is progressively increased. This behavioural sensitization may be associated with the development of drug craving, addiction and dependence. Also noradrenergic mechanisms participate in the mediation of the effects of psychostimulants. In this study we show that mice lacking the alpha(2)-adrenoceptor subtype A (alpha(2A)-AR knock-out (KO) on C57Bl/6J background) are supersensitive to the acute locomotor effects of d-amphetamine (5 mg/kg) in a novel environment compared to wild-type (WT) control mice. When both genotypes were treated repeatedly with d-amphetamine (2 mg/kg) they developed locomotor hyperactivation (sensitization), but its amplitude was lower in alpha(2A)-AR KO mice. Development of hyperactivation was reduced in both genotypes by pretreatment with the selective alpha(2)-adrenoceptor antagonist, atipamezole (1 mg/kg). Acute atipamezole also attenuated the expression of d-amphetamine-induced behavioural sensitization especially in WT mice. Interestingly, alpha(2A)-AR KO mice failed to exhibit persistent sensitization after 2 weeks of abstinence from repeated d-amphetamine. Rewarding properties of d-amphetamine, measured by conditioned place preference, were similar in both genotypes. These findings indicate that d-amphetamine-induced acute and sensitized locomotor effects are controlled by alpha(2)-adrenoceptors. Drugs antagonizing the alpha(2A)-adrenoceptor subtype may provide a novel approach for reducing drug sensitization and motor complications caused by dopaminergic agents.

Methylphenidate and amphetamine modulate differently the NMDA and AMPA glutamatergic transmission of dopaminergic neurons in the ventral tegmental area

Behavioral and neurochemical studies suggest that the induction of behavioral sensitization to psychostimulants involves transient changes at the synapses of the ventral tegmental area's dopaminergic neurons (VTA-DA). Differences in the behavioral response to amphetamine (Amph) and methylphenidate (MPD) were observed. In an attempt to understand these behavioral differences at the neuronal level, the dose-response characteristics of these two psychostimulants on electrophysiologically identified VTA-DA neurons at the glutamatergic synapse were investigated. Miniature excitatory post-synaptic currents (mEPSCs) and electrically induced EPSCs were recorded from horizontal midbrain slices of rats that had been pretreated intraperitoneally (i.p.) with saline (control), Amph (2.5, 5.0, 10.0 or 20.0 mg/kg), or MPD (2.5, 5.0, 10.0 or 20.0 mg/kg) 24 h before the recording. Perfusion of Amph through the bath (2.5, 5.0, 10.0 or 20.0 microM) increased the frequency (p<0.01) and the amplitude (p<0.05) of mEPSCs in dose-response characteristics, while MPD perfusion through the bath (2.5, 5.0, 10.0, or 20.0 microM) increased only the frequency (p<0.05) of the mEPSC. Both psychostimulants increased the prefrontal cortex's (PFC) glutamatergic EPSC in the VTA-DA neurons. However, only the higher doses of MPD induced significant effects (p<0.05) on the N-methyl-D-aspartate (NMDA) receptor-mediated EPSC but had no effects on the EPSC mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptors. Contrarily, Amph increased both kinds of mediated EPSC, but mainly the EPSC mediated by AMPA/kainate receptors (p<0.01). These electrophysiological differences could represent the underlying mechanism responsible for the differences of behavioral effects, such as behavioral sensitization, elicited by MPD and Amph.

Enhancement of central dopaminergic activity in the kainate model of temporal lobe epilepsy: implication for the mechanism of epileptic psychosis

There is an increased incidence of schizophrenia-like psychosis in temporal lobe epilepsy (TLE), and several risk factors have been implicated, including the duration of epilepsy and temporal lobe neuropathology. To investigate the biological mechanism of epileptic psychosis, we examined alterations of central dopaminergic systems in the kainate model of TLE. In adult rats, kainate was microinjected into the left amygdala to induce status epilepticus. An indirect dopamine agonist methamphetamine (MAP, 2 mg/kg, i.p.) was administered before and 1 month after the kainate treatment. MAP-induced locomotor activity was significantly enhanced in the kainate group compared with the baseline (pre-kainate) level, which was antagonized by pretreatment with haloperidol. The enhancement of locomotor activity in the kainate group was significantly correlated with the density of hippocampal CA1 neurons. Although the basal extracellular dopamine concentration was significantly lower in the striatum in the kainate group than in the control group (5.5 vs 39.2 fmol/20-min sample), the maximal concentration following MAP administration did not differ between the two groups. These results clearly demonstrate that hypersensitivity of the dopamine systems develops in the chronic phase of the kainate-induced TLE model, which may be responsible for the mechanism of epileptic psychosis.

Long-term behavioral and neuronal cross-sensitization to amphetamine induced by repeated brief social defeat stress: Fos in the ventral tegmental area and amygdala

Repeated exposure to stress induces cross-sensitization to psychostimulants. The present study assessed functional neural activation during social defeat stress-induced sensitization to a subsequent amphetamine challenge. Social defeat stress was induced in intruder rats during short confrontations with an aggressive resident rat once every third day during the course of 10 days. Rats received d-amphetamine injections (1 mg/kg, i.p.) 17 or 70 days after the first social defeat stress exposure. Amphetamine administration induced a significantly higher frequency of locomotor activity in stressed animals than in handled control rats, which was still evident 2 months after the last social stress exposure. Immunohistochemistry for Fos-like proteins was used to detect activated neural profiles in the striatum, nucleus accumbens (NAc), prefrontal cortex, amygdala, and ventral tegmental area (VTA). Repeated social defeat stress significantly increased Fos-like immunoreactive (Fos-LI) labeling 17 days after the start of stress exposure in the prelimbic and infralimbic cortical regions, NAc shell and core, medial, central and basolateral amygdala, and VTA, which probably represented the expression of chronic Fos-related antigens. Amphetamine augmented stress-induced Fos-LI labeling 17 days after the first stress episode in the dorsal striatum, NAc core, and medial amygdala, reflecting a cross-sensitization of Fos response. Amphetamine challenge 70 days after social stress exposures revealed sensitized Fos-LI labeling in the VTA and the amygdala. These data suggest that episodes of repeated social stress induce a long-lasting neural change that leads to an augmented functional activation in the VTA and amygdala, which might represent a neurobiological substrate for long-lasting cross-sensitization of repeated social defeat stress with psychostimulant drugs.

Sensitization of psychomotor stimulation and conditioned reward in mice: differential modulation by contextual learning

Incentive motivation theory ascribes a critical role to reward-associated stimuli in the generation and maintenance of goal-directed behavior. Repeated psychomotor stimulant treatment, in addition to producing sensitization to the psychomotor-activating effects, can enhance the incentive salience of reward-associated cues and increase their ability to influence behavior. In the present study, we sought to investigate this incentive sensitization effect further by developing a model of conditioned reinforcement (CR) in the mouse and investigating the effects of a sensitizing treatment regimen of amphetamine on CR. Furthermore, we assessed the role of contextual stimuli in amphetamine-induced potentiation of CR. We found that mice responded selectively on a lever resulting in the presentation of a cue previously associated with 30% condensed milk solution, indicating that the cue had attained rewarding properties. Prior treatment with amphetamine (4 x 0.5 mg/kg i.p.) resulted in psychomotor sensitization and enhanced subsequent responding for the CR. Furthermore, this enhancement of responding for the cue occurred independent of the drug-paired context, whereas the sensitized locomotor response was only observed when mice were tested in the same environment as that in which they had received previous amphetamine. These results demonstrate that the CR paradigm previously developed in the rat can be successfully adapted for use in the mouse, and suggest that behavioral sensitization to amphetamine increases the rewarding properties (incentive salience) of reward-paired cues, independent of the drug-paired context.

Dopamine in the medial prefrontal cortex controls genotype-dependent effects of amphetamine on mesoaccumbens dopamine release and locomotion

Mice of background DBA/2J are hyporesponsive to the behavioral effects of D-amphetamine in comparison with the widely exploited murine background C57BL/6J. In view of the important role of dopamine (DA) release in the nucleus accumbens (NAc) regarding the behavioral effects of psychostimulants, we tested the hypothesis of an inverse relationship between mesocortical and mesoaccumbens DA functioning in the two backgrounds. Systemic D-amphetamine induces a sustained increase in DA release in the medial prefrontal cortex (mpFC) accompanied by a poor increase in the NAc in mice of the low-responsive DBA/2J background, as shown by intracerebral microdialysis in freely moving animals. The opposite occurs in C57BL/6J mice, which show low prefrontal cortical DA outflow accompanied by high accumbal extracellular DA. Moreover, the DBA/2J background showed lower locomotor activity than C57BL/6J mice following D-amphetamine challenge. Selective DA depletion in the mpFC of DBA/2J mice produced a clear-cut increase in D-amphetamine-induced DA outflow in the NAc as well as locomotor activity that reached levels similar to those observed in C57BL/6J mice. Finally, local infusion of D-amphetamine by reverse microdialysis produced a similar increase in extracellular DA in both the mpFC and the NAc of DBA/2J mice. This finding points to similar transporter-related mechanisms in the two brain areas and supports the hypothesis that low accumbal DA release induced by systemic D-amphetamine in the DBA/2J background is determined by the inhibitory action of prefrontal cortical DA. The present results indicate that genotype-dependent susceptibility to addictive properties of D-amphetamine involves unbalanced DA transmission in the mesocorticolimbic system.

Mediation of amphetamine-induced long-term depression of synaptic transmission by CB1 cannabinoid receptors in the rat amygdala

The amygdala is thought to mediate memory consolidation of amphetamine-induced conditioned place preference, a behavioral paradigm that requires memory for an association between environmental cues and the affective state produced by the drug treatment. Here we show that amphetamine induces long-term synaptic depression (LTD) in the amygdala. Amphetamine LTD is not affected by dopamine, serotonin 1A, and norepinephrine alpha2 receptor antagonists but is blocked by the cannabinoid CB1 receptor antagonist AM251. It is mimicked by the CB1 agonist WIN55212-2 and facilitated and partially occluded by endocannabinoid uptake inhibitor AM404. Both amphetamine and WIN55212-2 LTDs are associated with an increase in the ratio of paired-pulse facilitation and a decrease in the frequency but not the amplitude of miniature EPSCs. They are also sensitive to block by P/Q type calcium channel blocker and occluded by each other, indicating that these two forms of synaptic plasticity share a common underlying mechanism. Loading postsynaptic neuron with calcium chelator blocked amphetamine LTD in some but not all neurons tested. However, in the presence of AM404, amphetamine LTD was present in all neurons recorded. These results suggest that amphetamine-induced endocannabinoid release depends on a rise in intracellular calcium and the incomplete block of LTD in some neurons may be attributable to the spillover of endocannabinoid from nearby cells. The finding that endocannabinoids underlie the synaptic actions of amphetamine may open a new avenue for the treatment of psychostimulants addiction.

Alteration in behavioral sensitivity to amphetamine after treatment with oxotremorine

Our earlier experiment revealed that rats pretreated once with an anticholinesterase develop hyposensitivity to amphetamine (AMPH). One of the likely causes of this effect might be a transient hyperexcitation of the central muscarinic receptors. It has appeared, however, that rats pretreated with oxotremorine (OX), a muscarinic agonist, show an augmented behavioral response to AMPH weeks later. The present experiments were performed in order to obtain more information on the relationship between the OX-induced sensitization to AMPH and the OX dose and dosing regime (single or repeated), and to find out whether the environment associated with the acute effects of OX could affect the response to AMPH. In experiment 1, adult male rats were given a single i.p. injection of OX in home cages at a moderate (0.5 mg/kg) or high (1.0 mg/kg) dose. In experiment 2, the rats received eight 1.0 mg/kg doses of OX in the course of three days. After each injection, some animals returned to their home cages, and some were placed in the test cages for 30 min. In both experiments, the response to AMPH was assessed on day 21 after the treatment. The obtained results indicate that: (i) a single i.p. exposure to OX results in an increase of the rat's behavioral sensitivity to AMPH but the moderate dose is more effective in inducing this effect; (ii) repeated exposure to OX at high doses, in a regime enabling development of tolerance to the acute OX effects, does not alter the rat sensitivity to AMPH, and (iii) expression of the AMPH response is suppressed in environment which has been associated with acute effects of OX.

Differential regulation by stimulants of neocortical expression of mrt1, arc, and homer1a mRNA in the rats treated with repeated methamphetamine

The present work was conducted to obtain clues for the possible roles of a novel stimulant-inducible gene mrt1 (methamphetamine-responsive transcript 1) encoding a PDZ-PX protein in stimulant-induced behavioral sensitization. In the young adult rats, repeated daily treatment with methamphetamine (4 mg/kg, intraperitoneally, once a day) for 5 days caused an enhanced behavioral response to methamphetamine: behavioral sensitization. The 5-day intermittent administration of MAP upregulated the basal expression of mrt1 transcripts and eliminated the increasing effects of a challenge dose of MAP (1.6 mg/kg, i.p.) or cocaine (30 mg/kg, i.p.) on mrt1 expression on day 14 of withdrawal in the neocortex that has been considered to be composed of a neuron circuit implicated in the sensitization phenomenon. In contrast, the basal expression of other stimulant-inducible and plasticity-related genes arc and homer1a and the ability of MAP or cocaine challenge to augment the amounts of their transcripts were not affected by the repeated MAP regimen in the cortical area. These findings suggest the differential regulation by stimulant of neocortical mrt1, arc, and homer1a expression in the behaviorally sensitized animals and supports the view that stimulant induction of mrt1 may be involved in the early molecular signalings for stimulant sensitization.

Prenatal amphetamine exposure alters behavioral reactivity to amphetamine in rats

In utero exposure to psychostimulants produces neurobehavioral alterations in the offspring of laboratory animals. Most amphetamine-related behavioral changes have been related to changes in the monoamine transmission levels, where monoamines may act as developmental regulatory substances for maturation of neuronal population. This study investigates the effect of prenatal-amphetamine exposure on the offspring's behavioral responses under amphetamine conditioning settings. Pregnant female rats were injected (subcutaneous) with amphetamine or saline during the pregnancy [gestation day (GD) 8 until parturition day]. The prenatal amphetamine exposure resulted in significantly decreased birth weights. The offspring from the saline group displayed a significantly lower number of stereotyped behaviors across the four challenge doses of amphetamine injections. Offspring from the amphetamine-treated prenatal group displayed significantly increased average startle amplitude compared to the controlled offspring. Moreover, offspring from amphetamine-treated prenatal group showed significantly less inhibition for the prepulse startle trials compared to those of the offspring from saline group. These results, taken together, indicate that the prenatally exposed rats displayed a significantly different profile of behavioral reactivity upon amphetamine challenges.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Amphetamine withdrawal modulates FosB expression in mesolimbic dopaminergic target nuclei: effects of different schedules of administration

Different patterns of psychostimulant intake can elicit widely varying behavioral and neurochemical consequences. Accordingly, rats were studied during withdrawal from either of two schedules of amphetamine administration, one consisting of 6 days of low-dose (1.5 mg/kg, i.p.) daily intermittent (INT) amphetamine (AMPH) injections, and the other of 6 days of moderately high-dose (1-5 mg/kg, i.p.) escalating (ESC) AMPH injections, for the effects of these treatments on numbers of FosB-positive nuclei and monoamine utilization in dopaminergic target areas. Withdrawal from AMPH pretreatment according to the ESC schedule markedly increased FosB expression in the nucleus accumbens shell and basolateral amygdala. In contrast, withdrawal from INT-AMPH administration did not increase FosB expression in any of the regions examined. Post-mortem neurochemical analyses of these same brain regions did not reveal effects of withdrawal from either INT or ESC administration of AMPH. These results suggest that withdrawal from a moderately high-dose AMPH regimen modifies patterns of gene expression in mesocorticolimbic dopaminergic target nuclei without significantly affecting basal monoamine levels. The strength of these effects in the nucleus accumbens shell and basolateral nucleus of the amygdala are consistent with behavioral and clinical data indicating the importance of these areas in the neuroadaptive changes which characterize addiction and withdrawal states.