Histological and ultrastructural evidence that D-amphetamine causes degeneration in neostriatum and frontal cortex of rats

Patterns of methamphetamine abuse and their consequences

The abuse of methamphetamine (METH) continues to increase throughout all age groups in different regions of the United States. "Ice," the popularized jargon for (+) methamphetamine hydrochloride, is the predominant drug form that is now consumed. "Ice" is effectively absorbed after either smoking or snorting and it is this rapid influx of drug that produces effects similar to those after intravenous administration. The intensity of METH actions in the central and peripheral nervous system shows tolerance after chronic administration, indicating that neuroadaptations have occurred. Thus, the physiological processes and corresponding biochemical mechanisms that regulate neuronal function have been changed by METH exposure. These biological alterations contribute to the craving and dependence associated with METH abuse and the withdrawal syndrome upon abstinence. However, these changes in behavior may also result from METH-induced neurotoxicity. This article reviews aspects of METH pharmacokinetics and related molecular pharmacodynamics that represent METH pharmacology and then relates those actions to their potential to produce neurotoxicity in humans.

L-ephedrine-induced neurodegeneration in the parietal cortex and thalamus of the rat is dependent on hyperthermia and can be altered by the process of in vivo brain microdialysis

Multiple doses of the dietary supplement L-ephedrine can cause severe hyperthermia and modest dopamine depletions in the rat brain. Since D-amphetamine treatment can result in neurodegeneration, the potential of L-ephedrine to produce similar types of degeneration was investigated. Adult male rats, some implanted in the caudate/putamen (CPu) for microdialysis, were given four doses of 25 mg/kg L-ephedrine or 5 mg/kg D-amphetamine (2 h between doses) at an ambient temperature of 23 degrees C. L-ephedrine-induced degeneration in the forebrain was dependent on the degree of hyperthermia. Layer IV of the parietal cortex was the most sensitive to L-ephedrine treatment with peak body temperatures of at most 40.0 degrees C necessary to produce degeneration. Extensive neurodegeneration in the parietal cortex after L-ephedrine treatment was as pronounced as that previously described for D-amphetamine treatment and also occurred in the intralaminar, ventromedial and ventrolateral thalamic nuclei in rats with severe hyperthermia (peak body temperatures>41.0 degrees C). The neurodegeneration induced by L-ephedrine may have resulted in part from excitotoxic mechanisms involving the indirect pathways of the basal ganglia and related areas. No differences were observed between microdialysis and non-implanted rats with respect to degree of tyrosine hydroxylase (TH) loss in the CPu after either D-amphetamine or L-ephedrine treatment. However, neurodegeneration resulting from D-amphetamine and L-ephedrine was reduced in the microdialysis animals in the hemisphere ipsilateral to the probe, which raises concerns when using the technique of in vivo microdialysis to evaluate neurodegeneration. The results of this study, in conjunction with human clinical evaluation of ephedrine neurotoxicity, indicate that regionally specific damage may occur in the cortex of some humans exposed to ephedrine in the absence of stroke or hemorrhage.

Increased anxiety and impaired memory in rats 3 months after administration of 3,4-methylenedioxymethamphetamine ("ecstasy")

Male Wistar rats were administered either (a) a high dose regime of 3,4-methylenedioxymethamphetamine (MDMA) (4 x 5 mg/kg, i.p. over 4 h on each of 2 consecutive days), (b) a moderate dose regime of MDMA (1 x 5 mg/kg on each of 2 consecutive days), (c) D-amphetamine (4 x 1 mg/kg over 4 h on each of 2 days), or (d) vehicle injections. The high MDMA dose regime and the amphetamine treatment both produced acute hyperactivity and hyperthermia. Twelve weeks later, all rats were tested in the drug-free state on a battery of anxiety tests (elevated plus maze, emergence and social interaction tests). A further 2 weeks later they were tested on a novel object recognition memory task. Rats previously given the neurotoxic dose of MDMA showed greater anxiety-like behaviour on all three anxiety tests relative to both controls and D-amphetamine-treated rats. Rats given the moderate MDMA dose regime also showed increased anxiety-like behaviour on all three tests, although to a lesser extent than rats in the high dose group. In the object recognition task, rats given the high MDMA dose regime showed impaired memory relative to all other groups when tested at a 15-min delay but not at a 60-min delay. Rats previously exposed to amphetamine did not differ from saline controls in the anxiety or memory tests. These data suggest that moderate to heavy MDMA exposure over 48 h may lead to increased anxiety and memory impairment 3 months later, possibly through a neurotoxic effect on brain serotonin systems.

Methamphetamine neurotoxicity: necrotic and apoptotic mechanisms and relevance to human abuse and treatment

Research into methamphetamine-induced neurotoxicity has experienced a resurgence in recent years. This is due to (1) greater understanding of the mechanisms underlying methamphetamine neurotoxicity, (2) its usefulness as a model for Parkinson's disease and (3) an increased abuse of the substance, especially in the American Mid-West and Japan. It is suggested that the commonly used experimental one-day methamphetamine dosing regimen better models the acute overdose pathologies seen in humans, whereas chronic models are needed to accurately model human long-term abuse. Further, we suggest that these two dosing regimens will result in quite different neurochemical, neuropathological and behavioral outcomes. The relative importance of the dopamine transporter and vesicular monoamine transporter knockout is discussed and insights into oxidative mechanisms are described from observations of nNOS knockout and SOD overexpression. This review not only describes the neuropathologies associated with methamphetamine in rodents, non-human primates and human abusers, but also focuses on the more recent literature associated with reactive oxygen and nitrogen species and their contribution to neuronal death via necrosis and/or apoptosis. The effect of methamphetamine on the mitochondrial membrane potential and electron transport chain and subsequent apoptotic cascades are also emphasized. Finally, we describe potential treatments for methamphetamine abusers with reference to the time after withdrawal. We suggest that potential treatments can be divided into three categories; (1) the prevention of neurotoxicity if recidivism occurs, (2) amelioration of apoptotic cascades that may occur even in the withdrawal period and (3) treatment of the atypical depression associated with withdrawal.

Methamphetamine causes differential regulation of pro-death and anti-death Bcl-2 genes in the mouse neocortex

Bcl-2, an inner mitochondrial membrane protein, inhibits apoptotic neuronal cell death. Expression of Bcl-2 inhibits cell death by decreasing the net cellular generation of reactive oxygen species. Studies by different investigators have provided unimpeachable evidence of a role for oxygen-based free radicals in methamphetamine (METH) -induced neurotoxicity. In addition, studies from our laboratory have shown that immortalized rat neuronal cells that overexpress Bcl-2 are protected against METH-induced apoptosis in vitro. Moreover, the amphetamines can cause differential changes in the expression of Bcl-X splice variants in primary cortical cell cultures. These observations suggested that METH might also cause perturbations of Bcl-2-related genes when administered to rodents. Thus, the present study was conducted to determine whether the use of METH might indeed be associated with transcriptional and translational changes in the expression of Bcl-2-related genes in the mouse brain. Here we report that a toxic regimen of METH did cause significant increases in the pro-death Bcl-2 family genes BAD, BAX, and BID. Concomitantly, there were significant decreases in the anti-death genes Bcl-2 and Bcl-XL. These results thus support the notion that injections of toxic doses of METH trigger the activation of the programmed death pathway in the mammalian brain.

Adaptative response of antioxidant enzymes in different areas of rat brain after repeated d-amphetamine administration

d-Amphetamine has been shown to be a potential brain neurotoxic agent, particularly to dopaminergic neurones. Reactive oxygen species indirectly generated by this drug have been indicated as an important factor in the appearance of neuronal damage but little is known about the adaptations of brain antioxidant systems to its chronic administration. In this study, the activities of several antioxidant enzymes in different areas of rat brain were measured after repeated administration of d-amphetamine sulphate (sc, 20 mg/kg/day, for 14 days), namely glutathione-S-transferase (GST), glutathione peroxidase (GPx), glutathione reductase (GRed), catalase, and superoxide dismutase (SOD). When compared to a pair-fed control group, d-amphetamine treatment enhanced the activity of GST in hypothalamus to 167%, GPx in striatum to 127%, in nucleus accumbens to 192%, and in medial prefrontal cortex to 139%, GRed in hypothalamus to 139%, as well as catalase in medial prefrontal cortex to 153%. However, the same comparison revealed a decrease in the activity of GRed in medial pre-frontal cortex by 35%. Food restriction itself reduced GRed activity by 49% and enhanced catalase activity to 271% in nucleus accumbens. The modifications observed for the measured antioxidant enzymes reveal that oxidative stress probably plays a role in the deleterious effects of this drug in CNS and that, in general, the brain areas studied underwent adaptations which provided protection against the continuous administration of the drug.

Methamphetamine and HIV-1: potential interactions and the use of the FIV/cat model

The interaction of methamphetamine with human immunodeficiency virus (HIV), the aetiologic agent of Acquired Immune Deficiency Syndrome (AIDS), has not been thoroughly investigated. However, increasingly, a larger proportion of HIV infected individuals acquire the virus through methamphetamine use or are exposed to this drug during their disease course. In certain populations, there is a convergence of methamphetamine use and HIV-1 infection; yet our understanding of the potential effects that simultaneous exposure to these two agents have on disease progression is extremely limited. Studying the interactions between methamphetamine and lentivirus in people is difficult. To thoroughly understand methamphetamine's effects on lentivirus disease progression, an animal model that is both clinically relevant and easily manipulated is essential. In this report, we identified potential problems with methamphetamine abuse in individuals with a concurrent HIV-1 infection, described the Feline Immunodeficiency Virus (FIV)/cat model for HIV-1, and reported our early findings using this modelling system to study the interaction of methamphetamine and lentivirus infections.

Role of corticostriatal and nigrostriatal inputs in malonate-induced striatal toxicity

The striatal neuronal loss evident following cellular metabolic compromise may be dependent upon the presence of glutamate and dopamine within the striatum. In order to investigate the relative roles of corticostriatal and nigrostriatal projections in malonate-induced neuronal loss, the extent of toxicity was quantified in animals with cortical lesions to deplete the striatum of glutamate, nigrostriatal lesions to deplete the striatum of dopamine, or both. We found that malonate-induced striatal toxicity was significantly reduced following lesions of either the glutamatergic or dopaminergic afferents to the striatum. The extent of attenuation following the loss of both inputs within the same animal was similar to that seen following lesions of either alone. These data suggest that malonate-induced toxicity in the striatum depends upon the integrity of interactive influences from both glutamatergic and dopaminergic afferents.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

Effect of amphetamine on the expression of the metabotropic glutamate receptor 5 mRNA in developing rat brain

Mechanisms underlying the acute effects of amphetamine (AMP) were examined by monitoring the expression of metabotropic glutamate receptor 5 (mGluR5) and specific 3H-glutamate binding in the developing rat brain. Each of the postnatal day (P) 4, P21 and P60 rats received one intraperitoneal injection of AMP, 5 mg/kg or saline and were sacrificed one hour later. In situ hybridization analysis revealed that the AMP treatment raised the levels of the mGluR5 mRNA by 9-28% in the neurons of the layer 5 of motor and somatosensory cortices, whereas reduced the levels by 12-28% in the layer 5 of perirhinal cortex and the ventromedial part of caudate-putamen of the 3 ages. In the layer 2/3 neurons of cingular cortex, an 18% higher and 14% and 22% lower than control levels of the mRNA were detected in the P4 and in the P21 and P60 rats injected with AMP. Moreover, the levels of mGluR5 mRNA in the hippocampi and dentate gyri were elevated by AMP to 110-151% of controls in the rats of 3 ages. Reversible 3H-glutamate binding assay showed an increase of 25% and a 12% decrease in the binding levels in the cortices of AMP-treated P4 and P21 rats. The AMP administration also produced a 27% reduction and 62% elevation in the binding of the hippocampi of P4 and P60 rats. The results reveal age- and region-dependent changes in the expression of the glutamate receptors induced by AMP and may indicate differential plastic capability of the neurons to the drug perturbation.

Age-dependent differential responses of monoaminergic systems to high doses of methamphetamine

Abuse of methamphetamine (METH) by adolescents is a major public health issue in the U.S.A. Because of the neurotoxic potential of METH, we examined the response of CNS monoaminergic systems in young (adolescent) animals [postnatal day (PND) 40] to high-dose treatments (10 mg/kg, four injections, 2-h intervals) of this drug and contrasted these effects to those seen in older (young adult) rats (PND 90). Consistent with previous reports, we observed that PND 40 animals did not manifest the long-term (7-day) deficits in extrapyramidal dopamine (DA) parameters observed in PND 90 rats. In contrast, METH-induced rapid (1-h) reduction in the activity of striatal DA transporters occurred in both age groups. In addition, both persistent (7-day) and rapid (1-h) deficits in serotonergic systems (measured as reductions in tryptophan hydroxylase activity) were observed in PND 40 and 90 rats. Age-related differences in METH-induced hyperthermia did not appear to be a principal cause for our observations; however, age-dependent pharmacokinetics of this drug might have contributed to the differential METH monoaminergic responses by PND 40 and 90 animals.

Recovery from methamphetamine induced long-term nigrostriatal dopaminergic deficits without substantia nigra cell loss

After administration of methamphetamine (METH) (2x2 mg/kg, 6 h apart) to vervet monkeys, long term but reversible dopaminergic deficits were observed in both in vivo and post-mortem studies. Longitudinal studies using positron emission tomography (PET) with the dopamine transporter (DAT)-binding ligand, [11C]WIN 35,428 (WIN), were used to show decreases in striatal WIN binding of 80% at 1 week and only 10% at 1.5 years. A post-mortem characterization of other METH subjects at 1 month showed extensive decreases in immunoreactivity (IR) profiles of tyrosine hydroxylase (TH), DAT and vesicular monoamine transporter-2 (VMAT) in the striatum, medial forebrain bundle and the ventral midbrain dopamine (VMD) cell region. These IR deficits were not associated with a loss of VMD cell number when assessed at 1.5 years by stereological methods. Further, at 1.5 years, IR profiles of METH subjects throughout the nigrostriatal dopamine system appeared similar to controls although some regional deficits persisted. Collectively, the magnitude and extent of the dopaminergic deficits, and the subsequent recovery were not suggestive of extensive axonal degeneration followed by regeneration. Alternatively, this apparent reversibility of the METH-induced neuroadaptations may be related primarily to long-term decreases in expression of VMD-related proteins that recover over time.

D-amphetamine-induced depletion of energy and dopamine in the rat striatum is attenuated by nicotinamide pretreatment

The present study examined the effects of nicotinamide on the D-amphetamine (AMPH)-induced dopamine (DA) depletion and energy metabolism change in the rat striatum. In chronic studies, co-administration of AMPH with desipramine, a drug that retards the metabolism of AMPH, (10 mg/kg, intraperitoneal [i.p.], respectively) caused a significant decrease of striatal DA content measured 7 days later. Pretreatment with nicotinamide (500 mg/kg, i.p.), the precursor molecule for the electron carrier molecule nicotinamide adenine dinucleotide (NAD), attenuated this effect of AMPH, whereas itself exerted no long-term effect on striatal DA content. In acute studies, a decrease in striatal adenosine triphospate/adenosine diphosphate (ATP/ADP) ratio was found 3 h after co-injection of AMPH and desipramine. However, nicotinamide pretreatment blocked the reduced striatal ATP/ADP ratio and resulted in a striking increase in striatal NAD content in AMPH-treated rats. Furthermore, nicotinamide was noted to increase striatal ATP/ADP ratio and NAD content in saline-treated rats. These findings suggest that nicotinamide protects against AMPH-induced DAergic neurotoxicity in the striatum of rats via energy supplement.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

An integrated view of pathophysiological models of schizophrenia

Pathophysiological processes that underlie the profound neuropsychiatric disturbances in schizophrenia are poorly understood. However, the clinical course of the disease, and a number of clinical and basic science observations, provide direction for formulating pathophysiological models that could be empirically tested. For example, repeated psychostimulant administration to healthy subjects can induce psychotic symptoms, and acute stimulant challenge in schizophrenia patients can precipitate psychosis. Also, NMDA antagonists induce positive, negative, and cognitive schizophrenic-like symptoms in healthy volunteers and precipitate thought disorder and delusions in schizophrenia patients. These human studies provide support for the dopamine and NMDA receptor hypofunction hypotheses of schizophrenia. Well-documented effects of NMDA antagonists on dopamine systems provide a basis to integrate the dopamine and NMDA receptor hypofunction hypotheses. Furthermore, it has become apparent that prominent actions of antipsychotic drugs, especially those with 'atypical' properties, involve antagonism of behavioral, electrophysiological and brain metabolic effects produced by administration of NMDA receptor antagonists. A confluence of clinical and basic science data suggests that an early developmental insult, potentially involving reduced NMDA receptor function, could facilitate sensitization of dopamine systems, leading to the formal onset of schizophrenia in late adolescence and early adulthood. Although clearly speculative, this conceptual model is consistent with existing evidence and suggests lines of future experimental investigation.

High-dose methamphetamine treatment alters presynaptic GABA and glutamate immunoreactivity

The goal of this study was to determine if high-dose methamphetamine treatment altered presynaptic immunoreactivity for the amino acid neurotransmitters GABA and glutamate within the basal ganglia. Methamphetamine (15 mg/kg every 6 h, four doses) treatment in rats resulted in severe hyperthermia and a long-lasting (four weeks) depletion of striatal dopamine content (>80%). Severe dopamine loss correlated with a decrease in the density of presynaptic immunolabeling for GABA one week post-drug, and an increase after four weeks. Although no changes were seen in presynaptic striatal glutamate immunoreactivity, there was a significant increase in the percentage of glutamate-immuno-positive terminals associated with perforated postsynaptic densities. Rats given the same dose of methamphetamine but prevented from becoming hyperthermic showed less severe dopamine depletions and a lack of ultrastructural or immunocytochemical changes. In addition, induction of hyperthermia in the absence of drug decreased immunolabeling within mitochondria, but had no effect on dopamine content, morphology or nerve terminal immunoreactivity. Altered presynaptic GABA immunolabeling and terminal size were found in both the striatum and globus pallidus, suggesting that dynamic changes occur in the striatopallidal pathway following methamphetamine-induced dopamine loss. In addition, ultrastructural changes in glutamate-positive synapses which have been correlated with increased synaptic activity were found. These results are similar to changes in GABA and glutamate synapses that follow nigrostriatal dopamine loss in 6-hydroxydopamine-lesioned animals and in Parkinson's disease, and provide the first direct evidence that methamphetamine-induced dopamine loss alters the GABAergic striatopallidal pathway. Exposure to either methamphetamine or prolonged hyperpyrexia decreased mitochondrial Immunoreactivity, indicating that hyperthermia may contribute to methamphetamine toxicity by affecting energy stores.

Methamphetamine neurotoxicity: dissociation of striatal dopamine terminal damage from parietal cortical cell body injury

Methamphetamine (m-AMPH) administration injures both striatal dopaminergic terminals and certain nonmonoaminergic cortical neurons. Fluoro-Jade histochemistry was used to label cortical cells injured by m-AMPH in order to identify factors that contribute to the cortical cell body damage. Rats given four injections of m-AMPH (4 mg/kg) at 2-h intervals showed hyperthermia (mean = 40.0 +/- 0.10 degrees C) and increased behavioral activation relative to animals given saline (SAL). Three days later, m-AMPH-treated animals showed indices of injury to striatal DA terminals (depletion of tyrosine hydroxylase immunoreactivity) and parietal cortical cell bodies (appearance of Fluoro-Jade stained cells). Pretreatment with a dopamine (DA) D1, D2, or N-methyl-D-aspartate (NMDA) receptor antagonist, or administration of m-AMPH in a 4 degrees C environment, prevented or attenuated m-AMPH-induced hyperthermia, behavioral activation, and injury to striatal DA terminals and parietal cortical cell bodies. Animals pretreated with a DA transport inhibitor prior to m-AMPH showed hyperthermia, behavioral activation, and parietal cortical cell body injury, but they did not show striatal DA terminal injury. Pretreatment with a 5HT transport inhibitor failed to prevent m-AMPH-induced damage to striatal DA terminals or parietal cortical cell bodies. Animals given four injections of SAL in a 37 degrees C environment became hyperthermic, but showed no injury to striatal DA terminals or cortical cell bodies. The ability of the DA transport inhibitor to block m-AMPH-induced striatal DA damage, but not cortical injury, and the inability of hyperthermia alone to cause the cortical cell body injury suggests that m-AMPH-induced behavioral activation and hyperthermia may both be necessary for the subsequent parietal cortical cell body damage.

Characterizing cortical neuron injury with Fluoro-Jade labeling after a neurotoxic regimen of methamphetamine

We used Fluoro-Jade, a recently-developed fluorescent indicator of neuronal damage, to identify neurons injured 1-21 days after repeated injections of methamphetamine (m-AMPH) or saline. The m-AMPH-treated rats showed Fluoro-Jade positive neurons in parietal cortex (layers III and IV) and had less striatal tyrosine hydroxylase immunoreactivity than did saline-injected controls. Fluoro-Jade positive neurons were greatest in number 3 days post-treatment; some fluorescent neurons displayed bud-like surface protrusions. These observations support the hypothesis that certain neocortical neurons degenerate after m-AMPH.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Neuronal degeneration in rat forebrain resulting from D-amphetamine-induced convulsions is dependent on seizure severity and age

Neuronal damage and degeneration in the rat forebrain was characterized by B4 isolectin and Fluoro-Jade labeling techniques after 4 doses of 15 mg/kg amphetamine i.p. in 70- and 180-day-old Sprague-Dawley rats. In amphetamine-dosed rats some seizure activity occurred in all rats exhibiting pronounced hyperthermia but the degree of seizure activity varied greatly between individual rats. Over 90% of the rats in both age groups that showed behavioral signs of limbic seizures had somatic degeneration in the taenia tecta within 3 days of amphetamine exposure. Degenerating small star-shaped cells were seen in the septum and hippocampus in 70-day-old rats having extensive seizure activity. Although somatic degeneration only sporadically occurred in the piriform cortex of the younger rats, extensive B4 isolectin binding to activated microglia was observed in this area. In older rats prominent somatic degeneration was seen in the piriform cortex and orbital and insular areas of the frontal cortex of rats having seizures. Damage to the basal ganglia and related areas, including the thalamus, parietal cortex and dorsal medial striatum, occurred in rats with pronounced hyperthermia but only correlated with seizures in older rats. In the more severe cases of thalamic damage the highest density of neurodegeneration was localized perivascularly. Thus, amphetamine can produce notable damage to the limbic system when seizures occur and to the basal ganglia and related areas when hyperthermia occurs but the neurotoxicity profiles in these areas are age-dependent and not produced solely by hyperthermia. Further studies to determine whether neuronal damage is the result of or the cause of amphetamine-induced seizures are necessary.

Glutamatergic involvement in psychomotor stimulant action

The sympathomimetic psychomotor stimulants, including cocaine, amphetamines, and the phenylethylamine amphetamine-like derivatives, exert actions in mammalian systems that implicate involvement of the excitatory neurotransmitter, glutamate and its receptors. Despite evidence that psychomotor stimulants do not directly stimulate glutamate receptors, blockade of acute lethal, convulsive, circulatory, thermoregulatory, locomotor and stereotypical responses, as well as interference with slowly developing behavioral sensitization and brain monoaminergic neurotoxicities, can be achieved by receptor antagonists at both N-methyl-D-aspartate and AMPA/kainate glutamate receptor subtypes. Alterations in glutamatergic neurobiology, including elevations in extracellular glutamate levels, changes in glutamate receptor properties and glutamatergic neuronal degeneration, have also been attributed to psychomotor stimulant administration. Blockade of glutamate receptors offers therapeutic options in management of psychomotor stimulant toxicity.

Methamphetamine-induced alterations in dopamine transporter function

Repeated methamphetamine (METH) administration has been shown to produce differing neurochemical as well as behavioral effects in rats. This study was designed to examine the effects of acute and chronic METH exposure on uptake and release of [3H]dopamine (DA) in cultured midbrain dopamine neurons to determine if persistent neuronal adaptations ensue. In addition, we have assessed DA D2 receptor function to determine if chronic METH alters this receptor. Fetal midbrain cultures were exposed to METH (1, 10 microM) for 5 days and dopaminergic function examined 1 or 7 days after drug removal. The ability of METH to release [3H]DA was compared to other releasing agents as well as several potent uptake inhibitors. Chronic exposure to a release-promoting concentration of METH resulted in either no change or a reduction in [3H]DA release upon subsequent METH challenge. Pretreatment with METH was also found to cause a decrease in the Bmax for [3H]raclopride binding, suggesting that persistently elevated DA levels cause a downregulation of DA D2 receptors. Examination of transporter kinetics utilizing initial velocity of uptake revealed that METH treatment caused a significant decrease in affinity (K(m)) for the substrate (DA), while not altering the maximal velocity of uptake (Vmax). Binding studies with [125I]RTI-55 revealed that there was no alteration in either the Bmax or Kd for this ligand, suggesting that the changes induced by METH treatment are due to alterations in K(m) and not in the number of DA transport sites. The results from these studies indicate that METH treatment produces a modification in transporter function which may be associated with both the altered uptake and release of [3H]DA. These changes have broad implications for the regulation of transporter activity not only because of the relevance to pre-synaptic mechanisms controlling neurotransmission, but also to the importance of the neuronal adaptation that occurs in response to chronic METH exposure.

Persistent structural modifications in nucleus accumbens and prefrontal cortex neurons produced by previous experience with amphetamine

Experience-dependent changes in behavior are thought to involve structural modifications in the nervous system, especially alterations in patterns of synaptic connectivity. Repeated experience with drugs of abuse can result in very long-lasting changes in behavior, including a persistent hypersensitivity (sensitization) to their psychomotor activating and rewarding effects. It was hypothesized, therefore, that repeated treatment with the psychomotor stimulant drug amphetamine, which produces robust sensitization, would produce structural adaptations in brain regions that mediate its psychomotor activating and rewarding effects. Consistent with this hypothesis, it was found that amphetamine treatment altered the morphology of neurons in the nucleus accumbens and prefrontal cortex. Exposure to amphetamine produced a long-lasting (>1 month) increase in the length of dendrites, in the density of dendritic spines, and in the number of branched spines on the major output cells of the nucleus accumbens, the medium spiny neurons, as indicated by analysis of Golgi-stained material. Amphetamine treatment produced similar effects on the apical (but not basilar) dendrites of layer III pyramidal neurons in the prefrontal cortex. The ability of amphetamine to alter patterns of synaptic connectivity in these structures may contribute to some of the long-term behavioral consequences of repeated amphetamine use, including amphetamine psychosis and addiction.

Methamphetamine alters presynaptic glutamate immunoreactivity in the caudate nucleus and motor cortex

It has been suggested that methamphetamine (METH)-induced neurotoxicity requires the activation of both dopamine (DA) and glutamate (GLU) systems. To investigate the possibility that METH-induced increases in extracellular GLU, as measured by in vivo microdialysis [Nash and Yamamoto (1992) Brain Res., 581:237-243], arise from neuronal stores, postembedding immunogold electron microscopy was used to measure the density of presynaptic GLU immunoreactivity within the striatum, the shell of the nucleus accumbens, and the motor cortex. Rats were treated with METH (5 mg/kg), or an equivalent volume of saline (SAL), every 2 h for a total of four injections. No ultrastructural evidence of terminal degeneration was observed. Significant decreases in the density of nerve terminal GLU immunolabeling occurred 12 h following METH administration within the primary motor cortex and the ventrolateral caudate/putamen, and a trend towards depletion was seen within the dorsolateral caudate/putamen. Although GLU immunolabeling within the shell of the nucleus accumbens was unaffected, DA content was decreased in all regions examined 1 week following METH treatment. The lack of degeneration, coupled with a partial recovery of DA levels, suggests that moderate doses of METH may inhibit DA biosynthesis without widespread terminal loss. Furthermore, METH administration results in a decrease in presynaptic GLU that correlates both temporally and anatomically with delayed GLU overflow, suggesting that neuronally derived GLU may play a role in METH-induced neurotoxicity. However, there does appear to be a dissociation between DA loss and altered GLU immunocytochemistry within the nucleus accumbens.

Methamphetamine exposure can produce neuronal degeneration in mouse hippocampal remnants

Neuronal cell death in hippocampal remnants was seen after methamphetamine (METH) exposure. Two techniques (Fluoro-Jade labeling and argyrophylia) showed that neuronal degeneration occurred in the indusium griseum, tenia tecta and fasciola cinerea within 5 days post-METH exposure in 70% of the mice. Neurodegeneration also occasionally occurred in the piriform cortex, hippocampus and frontal/parietal cortex. This cell death, unlike striatal neurotoxicity, was not dependent on magnitude of hyperthermia occurring but did correlate with behavioral seizure activity during METH exposure. Excitotoxic mechanisms may be underlying the neuronal degeneration since co-administration of phenobarbital blocked cell death.

Pharmacological aspects of human and canine narcolepsy

Narcolepsy-cataplexy is a disabling neurological disorder that affects 1/2000 individuals. The main clinical features of narcolepsy, excessive daytime sleepiness and symptoms of abnormal REM sleep (cataplexy, sleep paralysis, hypnagogic hallucinations) are currently treated using amphetamine-like compounds or modafinil and antidepressants. Pharmacological research in the area is facilitated greatly by the existence of a canine model of the disorder. The mode of action of these compounds involves presynaptic activation of adrenergic transmission for the anticataplectic effects of antidepressant compounds and presynaptic activation of dopaminergic transmission for the EEG arousal effects of amphetamine-like stimulants. The mode of action of modafmil is still uncertain, and other neurochemical systems may offer interesting avenues for therapeutic development. Pharmacological and physiological studies using the canine model have identified primary neurochemical and neuroanatomical systems that underlie the expression of abnormal REM sleep and excessive sleepiness in narcolepsy. These involve mostly the pontine and basal forebrain cholinergic, the pontine adrenergic and the mesolimbic and mesocortical dopaminergic systems. These studies confirm a continuing need for basic research in both human and canine narcolepsy, and new treatments that act directly at the level of the primary defect in narcolepsy might be forthcoming.

Striatal and cortical NMDA receptors are altered by a neurotoxic regimen of methamphetamine

Methamphetamine (m-AMPH) treatment produces long-lasting damage to striatal and cortical monoaminergic terminals and may also injure nonmonoaminergic cortical neurons. Evidence suggests that both dopamine (DA) and glutamate (GLU) play crucial roles in producing this damage. We used quantitative autoradiography to examine [3H]mazindol ([3H]MAZ) binding to striatal DA transporters and [3H]GLU binding to N-methyl-D-aspartate (NMDA) receptors in the striatum and cortex 1 week and 1 month after a neurotoxic regimen of m-AMPH. Rats received m-AMPH (4 mg/kg) or saline (SAL) (1 ml/kg) in four s.c. injections separated by 2 h intervals. One week after m-AMPH, the ventral and lateral sectors of the striatum showed the greatest decreases in both [3H]MAZ and [3H]GLU binding, while the nucleus accumbens (NA) showed no significant decreases. One month after m-AMPH, striatal [3H]MAZ binding was still significantly decreased, while NMDA receptor binding had recovered. Surprisingly, the parietal cortex showed a m-AMPH-induced increase in NMDA receptor binding in layers II/III and IV 1 week after m-AMPH and only in layers II/III 1 month after m-AMPH. The prefrontal cortex showed no m-AMPH-induced changes in NMDA receptor binding at either time point. This is the first demonstration that a regimen of m-AMPH that results in long-lasting damage to DA terminals can alter forebrain NMDA receptor binding. Thus, repeated m-AMPH treatments may produce changes in glutamatergic transmission in selected striatal and cortical regions.

6-[18F]fluoro-L-DOPA-PET studies show partial reversibility of long-term effects of chronic amphetamine in monkeys

The acute and long-term effects of chronic amphetamine administration on the striatal dopamine system in monkeys were assessed with 6-[18F]fluoro-L-DOPA (FDOPA) and positron emission tomography (PET). Vervet monkeys (Cerecopithecus aethiops) were administered amphetamine doses, i.m., that increased from 4 mg/kg/d to 18 mg/kg/d over a 10 day period. Post-amphetamine FDOPA-PET scans at 1-2, 3-4, and 6 week time points in individual subjects showed persistent decrements in dopamine synthesis capacity as reflected by FDOPA influx rate constant (Ki) values being approximately 30% that of pre-drug assessment. In other animals that were administered the same drug regimen, biochemical analysis of striatal regions at 1-2 weeks post-drug indicated that dopamine concentrations were decreased by approximately 95% throughout caudate and putamen regions, while the homovanillic acid/dopamine level ratio was increased 3-10-fold. Post-drug FDOPA-PET Ki values remained consistently low up to 6 weeks; however, at the 5-6 month time point, relative increases in FDOPA-Ki values (approximately 53% of pre-drug values) were observed for all subjects, indicative of partial recovery of striatal dopamine synthesis capacity. These results demonstrate that FDOPA-PET can reveal temporal activity changes within the striatal dopamine system of individual subjects. The apparent, partial reversibility of amphetamine's neurotoxic effects suggests a plasticity of dopaminergic function that may include regeneration of dopaminergic terminals and compensatory increases in residual dopamine synthesis rates. The persistence of the partial decrement in dopamine synthesis capacity, however, may indicate a long term component of amphetamine's toxic effects.

Effect of haloperidol and its metabolites on dopamine and noradrenaline uptake in rat brain slices

The effects of haloperidol and its metabolites on dopamine (DA) and noradrenaline (NA) uptake were investigated. Both direct uptake of [3H]DA and [3H]NA into the rat striatal and hippocampus slices and binding of a specific DA uptake inhibitor [3H]GBR-12935 were employed in the present study. Haloperidol pyridinium (HP+), haloperidol 1,2,3,6-tetrahydropyridine (HTP), 4-(4-chlorophenyl)-1,2,3,6-tetrahydropyridine (CPTP) and reduced haloperidol (RHAL) are potent inhibitors of DA uptake. HTP N-oxide (HTPNO) exhibits a relatively weak effect on DA uptake. Other metabolites of haloperidol, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP) and haloperidol N-oxide (HNO), as well as haloperidol itself possess negligible inhibitory effect on DA uptake. HP+ has been shown to be an amine releaser. It is possible that HP+ may induce amphetamine-like neurotoxicity. The effects of the metabolites of haloperidol on [3H]NA uptake are similar to those on [3H]DA uptake. HP+ appears to be different from MPP+, which is a more potent [3H]NA uptake blocker than on [3H]DA uptake. Although haloperidol exhibits no DA uptake inhibitory effect, it has a high affinity for the [3H]GBR-12935 binding site. The possible pharmacological implications such inhibitory effects on amine uptake are discussed.

Protective effects of MK-801 on methamphetamine-induced depletion of dopaminergic and serotonergic terminals and striatal astrocytic response: an immunohistochemical study

It has been shown previously that methamphetamine induces dopaminergic nerve terminal degeneration, serotonin depletion and striatal reactive astrogliosis, and that the noncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801 can block methamphetamine (MA)-induced depletion of dopamine and serotonin and reduction in activity of their synthetic enzymes. In this study, immunohistochemistry was used to evaluate the effect of MK-801 on methamphetamine-induced neuropathological alterations of dopaminergic and serotonergic terminals and striatal astrocytic responses. Adult male rats were treated with methamphetamine (4 injections of 10 mg/kg at 2 hour intervals) in conjunction with MK-801 which was administered 15 min before each methamphetamine administration at doses of 1 mg/kg or 2 mg/kg. Brains were examined three days following treatment. MK-801 administration prevented methamphetamine-induced depletion of 5-hydroxytryptophan (5-HT) terminals in the forebrain and depletion of tyrosine hydroxylase-positive dopaminergic terminals and astrocytic response in the neostriatum in most animals. These results support the concept that excitatory amino acids acting through an NMDA receptor are involved in methamphetamine-induced neuronal damage on dopaminergic and serotonergic terminal fields. A minor depletion of TH-positive terminals and astrogliosis in the neostriatum was seen in three of nine MA-MK-801-treated animals. This indicates that the protective effects of MK-801 on MA-induced dopaminergic terminal degeneration varies among animals with complete protection in most animals and partial protection in the others using the present doses and dosing regimen.

General considerations in assessing neurotoxicity using neuroanatomical methods

Neurotoxicology is a major focus of scientists and policy makers. Neurotoxicological investigations provide vital information needed by regulatory scientists to protect public health and can also elucidate fundamental mechanisms governing nervous system function and enhance our understanding of neurodegenerative diseases as well. A definition of neurotoxicity, developed by the Interagency Committee on Neurotoxicology, includes both permanent and reversible adverse effects on the nervous system. There are a number of factors that can greatly affect the outcome of any study designed to assess neurotoxicity, such as the choice of animal species, dose and dosage regimen, and route of administration. In considering neuroanatomical methodologies for assessing neurotoxicity, it is important to evaluate each technique for such factors as: limits of detection (sensitivity and signal to noise ratio), ability to be quantified, sampling problems, what is being measured and what can interfere with this measurement. Other questions relating to the strengths and weaknesses of neuroanatomical techniques that should be addressed include: is the technique difficult to perform? Is it reproducible? Which elements of the nervous system are best evaluated? Does the technique reveal the neuronal circuitry involved in the neurotoxic effect? Is successful application of the technique dependent on timing factors? Clearly, there are many factors that can influence the assessment of neurotoxicity so that it is best to base this assessment on converging data based on complementary techniques.

Electron microscopic evidence for neurotoxicity in the basal ganglia

The dopaminergic projection from the substantia nigra to the neostriatum is vulnerable to several neurotoxins including 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), amphetamine, and 5-hydroxydopamine. We have treated rats or mice with these agents and examined various regions of their brains with a combination of Fink-Heimer, immunohistochemical, serial-section electron microscopic, and three-dimensional reconstruction methods. In addition to degenerating or swollen axons, we found darkened glial processes and some damage to postsynaptic cells and dendrites. The particular effects observed critically depend on experimental variables such as dose, time, species and strain and raise questions about the correlation of light and electron microscopic results. These studies provide the basis for a discussion of the advantages and disadvantages of an ultrastructural examination of the effects of neurotoxins.

The effects of amfonelic acid, a dopamine uptake inhibitor, on methamphetamine-induced dopaminergic terminal degeneration and astrocytic response in rat striatum

Administration of methamphetamine (MA) induces degeneration of dopaminergic nerve terminals and astrogliosis, such as hypertrophy and an increase in apparent number, in the neostriatum. In this experiment adult rats were treated with MA (10 mg/kg, i.p.) 4 times in one day at 2 h intervals. Amfonelic acid (AFA), a dopamine reuptake inhibitor, was administered (20 mg/kg, i.p.) at the same time the last MA dose was given. Three days later, dopaminergic terminals and astrocytes were examined immunohistochemically and the contents of striatal dopamine and its metabolites were analyzed by HPLC. The results showed that MA-induced the typical depletion of dopaminergic terminals, reduction of dopamine content and astrogliosis in the neostriatum. AFA treatment completely prevented the effects of MA on the dopaminergic system, both morphologically and biochemically. However, the reaction of astrocytes remained in the region where the most severe depletion of dopaminergic terminals was seen in MA treated animals (ventral-lateral portion of neostriatum). The results support the concept that the dopamine transporter is involved in MA-induced dopaminergic nerve terminal degeneration. The results also indicate that blocking the dopamine transporter cannot completely prevent the reaction of astrocytes in the neostriatum, which indicates that the astrocytic reaction can be induced by factors other than degeneration of dopaminergic terminals in this region. Based on these and other data, it is hypothesized that MA may cause degeneration of corticostriatal glutamate pathways and this effect may be responsible for the astrogliosis in MA-AFA treated animals.

Stimulant-induced psychosis, the dopamine theory of schizophrenia, and the habenula

While one of the original underpinnings of the dopamine theory of schizophrenia was the paranoid psychosis which often develops during the binges or speed runs of chronic amphetamine addicts (and, more recently, in cocaine addicts), neurochemical studies of such drug abusers or from animals given continuous stimulants in an effort to model stimulant psychoses have not played a major role in the further evolution of this theory. One clear persisting alteration produced by continuous amphetamine is a neurotoxicity to dopaminergic innervations in caudate. Yet continuous cocaine administration apparently does not induce a similar neurotoxicity and this makes this effect a poor candidate for an underpinning of stimulant psychoses. However, it has recently been found that both continuous amphetamine and cocaine induce a strong pattern of degeneration which is highly confined to the lateral habenula and its principal output pathway, fasciculus retroflexus. This finding has led to a reconsideration of the role of these structures in psychoses. The habenula, as the chief relay nucleus of the descending dorsal diencephalic system (consisting of stria medullaris, habenula and fasciculus retroflexus), is an important link between limbic and striatal forebrain and lower diencephalic and mesencephalic centers. Studies of glucose utilization have consistently shown the habenula to be highly sensitive to dopamine agonists and antagonists. Lesions of habenula produce a wide variety of behavioral alterations. The dorsal diencephalic system has major and predominantly inhibitory connections onto dopamine-containing cells and it mediates part of the negative feedback from dopamine receptors onto dopamine cell bodies. It represents one of the major inputs in brain to the raphe nuclei and has anatomical and functional connections to modulate important functions such as sensory gating through thalamus, pain gating through central gray and raphe and motor stereotypies and reward mechanisms through substantia nigra and the ventral tegmental area. It is argued that alterations in these pathways are ideal candidates for producing the behaviors which occur during psychosis and that future considerations of the circuitry underlying psychoses need to include this highly important but relatively neglected system.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.

Continuous amphetamine and cocaine have similar neurotoxic effects in lateral habenular nucleus and fasciculus retroflexus

Both amphetamine and cocaine lead to an intake pattern in chronic addicts in which the drug is taken repeatedly over prolonged periods. While continuously administered amphetamines, designed to mimic this intake pattern, have a neurotoxic effect on caudate dopamine terminals, several studies have failed to find similar effects following continuous cocaine. In this study, these findings in striatum were replicated in rats using silver staining for degenerating neurons. But it was further found that either amphetamine or cocaine given continuously over a 3- to 5-day period induce a highly specific pattern of axonal degeneration extending from the lateral habenular nucleus along the fasciculus retroflexus towards the ventral tegmentum. This finding supports a rich literature on the involvement of these same pathways in the actions of dopamine agonists, reward mechanisms, and the integration of limbic, extrapyramidal, and midbrain centers.

The effects of four days of continuous striatal microdialysis on indices of dopamine and serotonin neurotransmission in rats

The effects of 4 days of continuous microdialysis with a small-diameter concentric-style probe on indices of striatal dopamine (DA) and serotonin neurotransmission were assessed. It was found that over 4 days of dialysis, there was a marked time-dependent decrease in the basal concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in dialysate and in amphetamine-stimulated DA release. In contrast, there was no decrease in basal DA or in the ability of cocaine to elevate the concentration of DA in dialysate over the same period of time. There were only very modest changes in dialysate levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), relative to the marked changes in DA metabolites. It is suggested that 4 days of continuous dialysis does not result in a non-specific decrease in diffusibility of these compounds into the dialysis probe, but that the changes are more likely due to probe-induced damage to the nigrostriatal DA system. It is also suggested that a "stable" basal concentration of DA in dialysate is an especially poor indicator of the integrity of the dopaminergic input to the striatum. The implications of these findings for within-subjects design microdialysis experiments are discussed.