Chronic Amphetamine Research Articles

Neural Substrates of Amphetamine-Induced Behavioral Sensitization: Unconditioned (Zero Context) and Conditioned (Switch versus Same Context) Components in c-fos Overexpression

The neural substrates of the unconditioned and conditioned components of amphetamine (AMPH)-induced behavioral sensitization remain unknown. The present study examines the brain activation of rats in response to an AMPH challenge with augmented locomotion in groups receiving chronic AMPH under chloral hydrate anesthetization (i.e., the ‘zero context’) or when tested in the ‘same context’ as a chronic treatment, or when tested in a ‘different context’. The neural activations of the three groups reveal fairly consistent patterns: (a) The substantia nigra is activated in the same context condition and the pure AMPH effect (i.e., the zero context with the unconditioned component), but not in the switch context condition. (b) The ventral pallidum showed Fos expression in the switch context and the same context, but not in the zero context condition. (c) The other nuclei, including the medial prefrontal cortex, nucleus accumbens, caudate putamen, medial thalamus, hippocampus, amygdala, and ventral tegmental area, are activated in all contextual conditions and the pure AMPH effect (the zero context). The context exerts definable effects on the mesocorticolimbic dopamine system on AMPH-induced behavioral sensitization. (d) The ventral pallidum and the substantia nigra activations dissociate the unconditioned component from the conditioned component in behavioral sensitization. Further studies are needed to determine how these two nuclei mediate the effect in terms of primary and conditioned rewards.

Influence of chronic amphetamine treatment and acute withdrawal on serotonin synthesis and clearance mechanisms in the rat ventral hippocampus

Amphetamine withdrawal in both humans and rats is associated with increased anxiety states, which are thought to contribute to drug relapse. Serotonin in the ventral hippocampus mediates affective behaviors, and reduced serotonin levels in this region are observed in rat models of high anxiety, including during withdrawal from chronic amphetamine. This goal of this study was to understand the mechanisms by which reduced ventral hippocampus serotonergic neurotransmission occurs during amphetamine withdrawal. Serotonin synthesis (assessed by accumulation of serotonin precursor as a measure of the capacity of in vivo tryptophan hydroxylase activity), expression of serotonergic transporters, and in vivo serotonergic clearance using in vivo microdialysis were assessed in the ventral hippocampus in adult male Sprague Dawley rats at 24 h withdrawal from chronic amphetamine. Overall, results showed that diminished extracellular serotonin at 24 h withdrawal from chronic amphetamine was not accompanied by a change in capacity for serotonin synthesis (in vivo tryptophan hydroxylase activity), or serotonin transporter expression or function in the ventral hippocampus, but instead was associated with increased expression and function of organic cation transporters (low-affinity, high-capacity serotonin transporters). These findings suggest that 24 h withdrawal from chronic amphetamine reduces the availability of extracellular serotonin in the ventral hippocampus by increasing organic cation transporter-mediated serotonin clearance, which may represent a future pharmacological target for reversing anxiety states during drug withdrawal.

MRI reveals differential effects of amphetamine exposure on neuroglia in vivo

How amphetamine affects the neuroglia in living brains is not well understood. In an effort to elucidate this effect, we investigated neuroglia in response to amphetamine exposure using antisense (AS) or sense (S) phosphorothioate-modified oligodeoxynucleotide (sODN) sequences that correspond to glial fibrillary acidic protein (GFAP) mRNA (AS-gfap or S-gfap, respectively) expression. The control is a random-sequence sODN (Ran). Using cyanine 5.5-superparamagnetic iron oxide nanoparticle (Cy5.5-SPION) labeling and fluorescent microscopy, we demonstrated that living neural progenitor cells (PC-12.1), as well as the cells in fresh brain slices and intact brains of male C57BL6 mice, exhibited universal uptake of all of the sODNs but rapidly excluded all sODN-Ran and most S-gfap. Moreover, transmission electron microscopy revealed electron-dense nanoparticles only in the neuroglia of normal or transgenic mice [B6;DBA-Tg(Fos-tTA, Fos-EGFP*)1MmayTg(tetO-lacZ,tTA*)1Mmay/J] that had been administered AS-gfap or Cy5.5-SPION-gfap. Subtraction R2* maps from mice with acute and chronic amphetamine exposure demonstrated, validated by postmortem immunohistochemistry, a reduction in striatal neuroglia, with gliogenesis in the subventricular zone and the somatosensory cortex in vivo. The sensitivity of our unique gene transcript targeted MRI was illustrated by a positive linear correlation (r(2)=1.0) between in vivo MRI signal changes and GFAP mRNA copy numbers determined by ex vivo quantitative RT-PCR. The study provides direct evidence for targeting neuroglia by antisense DNA-based SPION-gfap that enables in vivo MRI of inaccessible tissue with PCR sensitivity. The results enable us to conclude that amphetamine induces toxicity to neuroglia in vivo, which may cause remodeling or reconnectivity of neuroglia.

Maternal immune activation in late gestation enhances locomotor response to acute but not chronic amphetamine treatment in male mice offspring: Role of the D1 receptor

Exposure to elevated levels of maternal cytokines can lead to functional abnormalities of the dopaminergic system in the adult offspring, including enhanced amphetamine (AMPH)-induced locomotion. Therefore, it seems reasonable to consider that offspring of challenged mothers would behave differently in models of addictive behavior, such as behavioral sensitization. Thus, we sought to evaluate the effects of prenatal exposure to lipopolysaccharide (LPS) on the locomotor response to acute and chronic AMPH treatment in male mice offspring. For this purpose, LPS (Escherichia coli 0127:B8; 120μg/kg) was administered intraperitoneally to pregnant Swiss mice on gestational day 17. At adulthood, male offspring were studied under one of the following conditions: (1) locomotor response to acute AMPH treatment (2.5 or 5.0mg/kg) in an open field test; (2) behavioral sensitization paradigm, which consists of a daily injection of AMPH (1.0mg/kg) for 10 days and observation of locomotion in the open field on days 1, 5, 10 (development phase), 15 and 17 (expression phase). The LPS stimulated offspring showed enhancement of the locomotor-stimulant effect after an acute AMPH challenge in comparison to baseline and saline pre-treated mice. They also showed development of behavioral sensitization earlier than the saline pre-treated group, although no changes between saline and LPS pre-treated groups were observed on development or expression of locomotor behavioral sensitization to AMPH. Furthermore, there was up-regulation of D1 receptor protein level within striatum in the LPS-stimulated offspring which was strongly correlated with increased grooming behavior. Taken together, our results indicate that motor and dopaminergic alterations caused by maternal immune activation are restricted to the acute AMPH challenge, mostly due to up-regulation of the D1 receptor within the mesolimbic and nigrostriatal pathways, but no locomotor differences were observed for behavioral sensitization to AMPH.

Methamphetamine-Evoked Depression of GABAB Receptor Signaling in GABA Neurons of the VTA

Psychostimulants induce neuroadaptations in excitatory and fast inhibitory transmission in the ventral tegmental area (VTA). Mechanisms underlying drug-evoked synaptic plasticity of slow inhibitory transmission mediated by GABA(B) receptors and G protein-gated inwardly rectifying potassium (GIRK/Kir(3)) channels, however, are poorly understood. Here, we show that 1 day after methamphetamine (METH) or cocaine exposure both synaptically evoked and baclofen-activated GABA(B)R-GIRK currents were significantly depressed in VTA GABA neurons and remained depressed for 7 days. Presynaptic inhibition mediated by GABA(B)Rs on GABA terminals was also weakened. Quantitative immunoelectron microscopy revealed internalization of GABA(B1) and GIRK2, which occurred coincident with dephosphorylation of serine 783 (S783) in GABA(B2), a site implicated in regulating GABA(B)R surface expression. Inhibition of protein phosphatases recovered GABA(B)R-GIRK currents in VTA GABA neurons of METH-injected mice. This psychostimulant-evoked impairment in GABA(B)R signaling removes an intrinsic brake on GABA neuron spiking, which may augment GABA transmission in the mesocorticolimbic system.

Epitope‐tagged dopamine transporter knock‐in mice reveal rapid endocytic trafficking and filopodia targeting of the transporter in dopaminergic axons

The plasma membrane dopamine (DA) transporter (DAT) is essential for reuptake of extracellular DA. DAT function in heterologous cells is regulated by subcellular targeting, endocytosis, and intracellular trafficking, but the mechanisms regulating neuronal DAT remain poorly understood. Hence, we generated a knock-in mouse expressing a hemagglutinin (HA)-epitope-tagged DAT to study endogenous transporter trafficking. Introduction of the HA tag into the second extracellular loop of mouse DAT did not perturb its expression level, distribution pattern, or substrate uptake kinetics. Live-cell fluorescence microscopy imaging using fluorescently labeled HA-specific antibody and a quantitative HA-antibody endocytosis assay demonstrated that in axons HA-DAT was primarily located in the plasma membrane and internalized mostly in growth cones and varicosities, where synaptic vesicle markers were also concentrated. Formation of varicosities was frequently preceded or accompanied by highly dynamic filopodia-like membrane protrusions. Remarkably, HA-DAT often concentrated at the tips of these filopodia. This pool of HA-DATs exhibited low lateral membrane mobility. Thus, DAT-containing filopodia may be involved in synaptogenesis in developing DA neurons. Treatment of neurons with amphetamine increased mobility of filopodial HA-DAT and accelerated HA-DAT endocytosis in axons, suggesting that chronic amphetamine may interfere with DA synapse development. Interestingly, phorbol esters did not accelerate endocytosis of axonal DAT.

Acute and chronic ethanol differentially modify the emotional significance of a novel environment: implications for addiction

Using open-field behaviour as an experimental paradigm, we demonstrated a complex interaction between the rewarding/stimulating effects and the anxiogenic/stressful effects of both novelty and acute or chronic amphetamine in mice. As a consequence of this interaction, acute amphetamine-induced hyperlocomotion was inhibited, whereas the expression of its sensitization was facilitated in a novel environment. In the present study, we aimed to investigate the interactions between exposure to a novel environment and the acute and chronic effects of ethanol (Eth), a drug of abuse known to produce anxiolytic-like behaviour in mice. Previously habituated and non-habituated male Swiss mice (3 months old) were tested in an open field after receiving an acute injection of Eth or following repeated treatment with Eth. Acute Eth administration increased locomotion with a greater magnitude in mice exposed to the apparatus for the first time, and this was thought to be related to the attenuation of the stressful effects of novelty produced by the anxiolytic-like effect of acute Eth, leading to a subsequent prevalence of its stimulant effects. However, locomotor sensitization produced by repeated Eth administration was expressed only in the previously explored environment. This result might be related to the well-known tolerance of Eth-induced anxiolytic-like behaviour following repeated treatment, which would restore the anxiogenic effect of novelty. Our data suggest that a complex and plastic interaction between the emotional and motivational properties of novelty and drugs of abuse can critically modify the behavioural expression of addiction-related mechanisms.

Effect of chronic amphetamine administration on the behaviour of rats in the open field apparatus: reversal of post-withdrawal depression by two antidepressants.

Effect of chronic amphetamine administration on the behaviour of rats in the open field apparatus: reversal of post-withdrawal depression by two antidepressants.

Serotonergic neurotransmission in the ventral hippocampus is enhanced by corticosterone and altered by chronic amphetamine treatment

The ventral hippocampus modulates anxiety-like behavior in rats, and serotonergic transmission within the hippocampus facilitates adaptation to stress. Chronic amphetamine treatment results in anxiety-like behavior in rats and reduced monoamine concentrations in the ventral hippocampus. Since reduced hippocampal serotonergic transmission in response to stress is observed in rats that display high anxiety-like behavior, anxiety states in amphetamine-treated rats may be associated with reduced stress-related serotonergic transmission in the hippocampus. Therefore, using in vivo microdialysis in anesthetized rats, we investigated the effect of corticosterone infused locally into the ventral hippocampus on serotonergic transmission, and the effect of chronic amphetamine pretreatment on corticosteroid receptor protein expression and the corticosterone-induced serotonergic response. Extracellular serotonin in the ventral hippocampus was increased by corticosterone in drug naive rats, and this corticosterone-induced serotonin augmentation was blocked by the glucocorticoid receptor antagonist mifepristone. Furthermore, chronic pretreatment with amphetamine abolished the serotonin response to physiologically relevant corticosterone levels and reduced glucocorticoid receptor protein expression. Together, our results suggest that chronic amphetamine exposure reduces serotonergic neurotransmission, in part via alterations to glucocorticoid receptor-facilitation of serotonin release in the rat ventral hippocampus. Reduced serotonergic activity in the ventral hippocampus may contribute to altered stress responses and adaptive coping following repeated drug exposure.

Chronic amphetamine transforms the emotional significance of a novel but not a familiar environment: implications for addiction

Both drug-induced locomotor sensitization and reactivity to novelty in rodents have been related to drug-craving mechanisms in humans. We investigated whether the exposure to a completely novel environment would modulate the expression of locomotor sensitization induced by repeated administration of amphetamine (Amp) in mice. In addition to locomotion, different open-field behavioural parameters were used to evaluate the possible involvement of anxiogenic-like effects induced by Amp, novelty or a combination of the two. In order to avoid misinterpretations due to different locomotor baseline conditions, we used an open-field illumination condition in which previous exposure to the apparatus did not modify locomotion (although it reliably increased grooming behaviour). Acute Amp administration increased locomotion in mice previously habituated to the open field (Hab) but not in mice exposed to the apparatus for the first time (Nov). This absence of Amp-induced locomotor activation in Nov mice may be related to higher anxiety-like levels, because these animals displayed longer freezing duration. However, only Nov mice developed locomotor sensitization. Because Amp challenge in Amp pre-treated Nov mice did not induce an increase in freezing behaviour, the locomotor sensitization in Nov mice might be related to the tolerance of Amp-induced anxiogenic-like behaviour in novel environments. Repeated Amp administration increased motivation to explore the environment in Nov mice in that these animals presented a within-session locomotion-habituation deficit. Our data suggest that a complex and plastic interaction between the anxiogenic and motivational properties of both novelty and Amp can critically modify the behavioural expression of craving-related mechanisms.

Melatonin attenuates the amphetamine-induced decrease in vesicular monoamine transporter-2 expression in postnatal rat striatum

The vesicular monoamine transporter-2 (VMAT-2) is responsible for packaging intraneuronal dopamine into synaptic vesicles in preparation for synaptic release and is a critical regulator of cytoplasmic dopamine levels and dopaminergic function. It has long been recognized that VMAT-2 is also a critical mediator of amphetamine-induced dopamine release. Amphetamine-induced lesions during development have the potential to produce numerous permanent abnormalities in neural circuitry and function. Therefore, in the present study, we investigated the effects of amphetamine on the levels of VMAT-2, α-synuclein and phosphorylated tyrosine hydroxylase in the striatum of neonatal rats. We found that chronic amphetamine administration in postnatal rats produces dopaminergic deficits in the striatum, including decreases in the levels of VMAT-2 and phosphorylated tyrosine hydroxylase. In addition, an increase in α-synuclein expression was observed in the striatum of postnatal rats following chronic amphetamine treatment. Furthermore, we identified a role of (10mg/kg) melatonin, a methoxyindole released from the pineal gland, in attenuating the detrimental effects of amphetamine on dopaminergic neurons.

The Dopamine D1-D2 Receptor Heteromer Localizes in Dynorphin/Enkephalin Neurons: INCREASED HIGH AFFINITY STATE FOLLOWING AMPHETAMINE AND IN SCHIZOPHRENIA

The distribution and function of neurons coexpressing the dopamine D1 and D2 receptors in the basal ganglia and mesolimbic system are unknown. We found a subset of medium spiny neurons coexpressing D1 and D2 receptors in varying densities throughout the basal ganglia, with the highest incidence in nucleus accumbens and globus pallidus and the lowest incidence in caudate putamen. These receptors formed D1-D2 receptor heteromers that were localized to cell bodies and presynaptic terminals. In rats, selective activation of D1-D2 heteromers increased grooming behavior and attenuated AMPA receptor GluR1 phosphorylation by calcium/calmodulin kinase IIα in nucleus accumbens, implying a role in reward pathways. D1-D2 heteromer sensitivity and functional activity was up-regulated in rat striatum by chronic amphetamine treatment and in globus pallidus from schizophrenia patients, indicating that the dopamine D1-D2 heteromer may contribute to psychopathologies of drug abuse, schizophrenia, or other disorders involving elevated dopamine transmission.

Chronic amphetamine treatment enhances corticotropin-releasing factor-induced serotonin release in the amygdala

Amphetamine use is associated with dysphoric states, including heightened anxiety, that emerge within 24h of withdrawal from the drug. Corticotropin-releasing factor increases serotonin release in the central nucleus of the amygdala, and this neurochemical circuitry may play a role in mediating fear and anxiety states. We have previously shown that chronic amphetamine treatment increases corticotropin-releasing factor receptor type-2 levels in the serotonergic dorsal raphe nucleus of the rat. Therefore, we hypothesized that chronic amphetamine treatment would enhance the amygdalar serotonergic response to corticotropin-releasing factor infused into the dorsal raphe nucleus. Male rats were injected once-daily with d-amphetamine (2.5mg/kg i.p., or saline) for two weeks. Serotonin release within the central nucleus of the amygdala in response to intra-raphe infusion of corticotropin-releasing factor (100ng) was measured 24h after the last treatment in urethane-anesthetized (1.8mg/kg, i.p.) rats using in vivo microdialysis. Rats pretreated with amphetamine showed significantly enhanced serotonin release in the central nucleus of the amygdala in response to corticotropin-releasing factor infusion when compared to saline pretreated rats. Furthermore, this enhanced response was blocked by the corticotropin-releasing factor type-2 receptor antagonist antisauvagine-30 (2μg) infused into the dorsal raphe nucleus. These results suggest increased sensitivity to corticotropin-releasing factor as mediated by type-2 receptors following chronic amphetamine treatment, which may underlie dysphoric states observed during amphetamine withdrawal.

Withdrawal from chronic exposure to amphetamine, but not nicotine, leads to an immediate and enduring deficit in motivated behavior without affecting social interaction in rats

Psychostimulant withdrawal leads to depressive symptoms, such as anhedonia and social dysfunction. We determined the effects of withdrawal from chronic exposure to nicotine (9 mg/kg/day salt, 28 days) or amphetamine (10 mg/kg/day salt, 7 days) on the motivated response for a sucrose reward and on social interaction in rats. Both nicotine and amphetamine exposure increased the motivated response for sucrose. However, only spontaneous amphetamine withdrawal led to an immediate and persistent decrease in motivated behavior, which was not correlated with body weight loss. Social interaction was not affected during withdrawal from either drug. These results indicate that withdrawal from chronic amphetamine exposure leads to an immediate and enduring anhedonic state.

Alterations in subcellular expression of acid-sensing ion channels in the rat forebrain following chronic amphetamine administration

Acid-sensing ion channels (ASICs) are densely expressed in broad areas of mammalian brains and actively modulate synaptic transmission and a variety of neuronal activities. To explore whether ASICs are linked to addictive properties of drugs of abuse, we investigated the effect of the psychostimulant amphetamine on subcellular ASIC expression in the rat forebrain in vivo. Repeated administration of amphetamine (once daily for 7 days, 1.25 mg/kg for days 1/7, 4 mg/kg for days 2–6) induced typical behavioral sensitization. At a 14-day withdrawal period, ASIC1 protein levels were increased in the defined surface and intracellular compartments in the striatum (both caudate putamen and nucleus accumbens) in amphetamine-treated rats relative to saline-treated rats as detected by a surface protein cross-linking assay. ASIC2 proteins, however, remained stable in the striatum. In the medial prefrontal cortex, repeated amphetamine administration had no effect on ASIC1 expression in either the surface or the intracellular pool. However, amphetamine selectively reduced the surface expression of ASIC2 in this region. These data identify ASICs as a sensitive target to repeated stimulant exposure. The region- and compartment-specific regulation of ASIC1 and ASIC2 expression may constitute a key synaptic adaptation in reward circuits critical for psychomotor plasticity.

Withdrawal from chronic amphetamine produces persistent anxiety-like behavior but temporally-limited reductions in monoamines and neurogenesis in the adult rat dentate gyrus

Acute amphetamine administration activates monoaminergic pathways and increases systemic corticosterone, both of which influence anxiety states and adult dentate gyrus neurogenesis. Chronic amphetamine increases anxiety states in rats when measured at 24 h and at 2 weeks of withdrawal. However, the effects of chronic amphetamine exposure and withdrawal on long term anxiety-like behavior and adult neurogenesis in the dentate gyrus are unknown. Adult male rats were administered amphetamine (2.5 mg/kg, ip.) daily for two weeks. Anxiety-like behaviors were increased markedly in amphetamine-treated rats following four weeks of withdrawal from amphetamine. Plasma corticosterone level was unaltered by amphetamine treatment or withdrawal. However, norepinephrine and serotonin concentrations were selectively reduced in the dentate gyrus 20 h following amphetamine treatment. This effect did not persist through the four week withdrawal period. In separate experiments, rats received bromodeoxyuridine to label cells in S-phase, prior to or immediately following amphetamine treatment. Newly generated cells were quantified to measure extent of progenitor cell proliferation and neurogenesis following treatment or withdrawal. Progenitor cell proliferation and neurogenesis were not significantly affected by amphetamine exposure when measured 20 h following the last amphetamine treatment. However, neurogenesis in the dentate gyrus was reduced after four weeks of withdrawal when compared to saline-pretreated rats. Overall, our findings indicate that withdrawal from chronic amphetamine leads to persistent anxiety-like behavior which may be maintained by reduced neurogenesis in the dentate gyrus at this protracted withdrawal time point. However, neurogenesis is unaffected at earlier withdrawal time points where anxiety states emerge, suggesting different mechanisms may underlie the emergence of anxiety states during amphetamine withdrawal.

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.

Genome-wide Analysis of Chromatin Regulation by Cocaine Reveals a Role for Sirtuins

Changes in gene expression contribute to the long-lasting regulation of the brain's reward circuitry seen in drug addiction; however, the specific genes regulated and the transcriptional mechanisms underlying such regulation remain poorly understood. Here, we used chromatin immunoprecipitation coupled with promoter microarray analysis to characterize genome-wide chromatin changes in the mouse nucleus accumbens, a crucial brain reward region, after repeated cocaine administration. Our findings reveal several interesting principles of gene regulation by cocaine and of the role of DeltaFosB and CREB, two prominent cocaine-induced transcription factors, in this brain region. The findings also provide comprehensive insight into the molecular pathways regulated by cocaine-including a new role for sirtuins (Sirt1 and Sirt2)-which are induced in the nucleus accumbens by cocaine and, in turn, dramatically enhance the behavioral effects of the drug.

Stability of surface NMDA receptors controls synaptic and behavioral adaptations to amphetamine

Plastic changes in glutamatergic synapses that lead to enduring drug craving and addiction are poorly understood. By focusing on the turnover and trafficking of NMDA receptors, we found that chronic exposure to the psychostimulant amphetamine induces selective downregulation of NMDA receptor NR2B subunits in the confined surface membrane pool of rat striatal neurons at synaptic sites. Remarkably, this downregulation is a long-lived event and results from the destabilization of surface-expressed NR2B due to accelerated ubiquitination and degradation of crucial NR2B-anchoring proteins by the ubiquitin-proteasome system. The biochemical loss of synaptic NR2B further translates to the significant modulation of synaptic plasticity in the form of long-term depression at cortico-accumbal glutamatergic synapses. Behaviorally, genetic disruption of NR2B induces, whereas restoration of NR2B loss prevents, behavioral sensitization to amphetamine. Our data identify NR2B as a key regulator in the remodeling of excitatory synapses and persistent psychomotor plasticity in response to amphetamine.

Hematological, immunological and neurochemical effects of chronic amphetamine treatment in male rats

In the present study, we have analyzed the effect of chronic amphetamine sulfate (AMPH) treatment on haematological, immunological and neurochemical parameters in the male rat. AMPH increased the total peripheral leukocyte count, and altered its differential counts, decreasing lymphocytes and increasing neutrophils. Flow cytometry study showed that the decline in circulating lymphocytes was caused by the loss of a particular lymphocyte subset, B-cell, which reduced both in percentage and in absolute number by 50%. T-cell population increased by 15% but not in absolute number, however there was no difference in either CD4+ or CD8+ T lymphocyte subsets between experimental groups. Neurochemically, AMPH reduced norepinephrine (NE) and serotonin (5-HT) contents in the hypothalamus and increased dopamine (DA) content in the striatum. Chronic AMPH increased in a dose-dependent manner serum corticosterone levels, had no effect on circulating catecholamines, reduced adrenal weights, and did not affect spleen weights although reduced their cellularities. These results show that chronic AMPH have important effects on immune function, particularly on humoral immune response because it reduced the circulating B cell population by half. In addition, AMPH plays an important role in the redistribution and trafficking of leukocytes, and both effects seem to be mediated by sympathetic innervation of the lymphoid organs.