Withdrawal of repeated cocaine decreases autoradiographic [3H]mazindol-labelling of dopamine transporter in rat nucleus accumbens

REVIEW ■ : Long-lasting Effects of Recreational Drugs of Abuse on the Central Nervous System

Although a wealth of knowledge exists regarding the acute pharmacological effects of recreational drugs on the CNS, much less is known about the long-term toxic effects of recreational drugs on the CNS. Recent findings in nonhuman primates treated with amphetamine analogs, such as ±3,4-methylenedioxymetham phetamine (MDMA), indicate that these drugs can produce long-lasting, probably permanent, changes in brain serotonin innervation. Similarly, animals treated with phencyclidine (PCP) and related drugs develop neurodegenerative changes in selected brain regions. It seems clear, then, that some psychoactive drugs have the potential to produce persistent changes in CNS structure and, possibly, function. The goal of this article is to summarize current knowledge regarding the long-term effects of several popular recreational drugs, including MDMA and related amphetamine analogs, cocaine, marijuana, alcohol, PCP, lysergic acid (LSD), and opiates. Gaps in the current knowledge base are identified, and areas ripe for future research efforts are suggested. NEUROSCIENTIST 3:399-411, 1997

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Dopamine transporter down-regulation following repeated cocaine: implications for 3,4-methylenedioxymethamphetamine-induced acute effects and long-term neurotoxicity in mice

BACKGROUND AND PURPOSE

3,4-Methylenedioxymethamphetamine (MDMA) and cocaine are two widely abused psychostimulant drugs targeting the dopamine transporter (DAT). DAT availability regulates dopamine neurotransmission and uptake of MDMA-derived neurotoxic metabolites. We aimed to determine the effect of cocaine pre-exposure on the acute and long-term effects of MDMA in mice.

EXPERIMENTAL APPROACH

Mice received a course of cocaine (20 mg*kg(-1), x2 for 3 days) followed by MDMA (20 mg*kg(-1), x2, 3 h apart). Locomotor activity, extracellular dopamine levels and dopaminergic neurotoxicity were determined. Furthermore, following the course of cocaine, DAT density in striatal plasma membrane and endosome fractions was measured.

KEY RESULTS

Four days after the course of cocaine, challenge with MDMA attenuated the MDMA-induced striatal dopaminergic neurotoxicity. Co-administration of the protein kinase C (PKC) inhibitor NPC 15437 prevented cocaine protection. At the same time, after the course of cocaine, DAT density was reduced in the plasma membrane and increased in the endosome fraction, and this effect was prevented by NPC 15437. The course of cocaine potentiated the MDMA-induced increase in extracellular dopamine and locomotor activity, following challenge 4 days later, compared with those pretreated with saline.

CONCLUSIONS AND IMPLICATIONS

Repeated cocaine treatment followed by withdrawal protected against MDMA-induced dopaminergic neurotoxicity by internalizing DAT via a mechanism which may involve PKC. Furthermore, repeated cocaine followed by withdrawal induced behavioural and neurochemical sensitization to MDMA, measures which could be indicative of increased rewarding effects of MDMA.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

Dopamine transporter levels in cocaine dependent subjects

Cocaine use is a significant problem in the US and it is well established that cocaine binds to the dopamine transporter (DAT) in the brain. This study was designed to determine if the DAT levels measured by 99mTc TRODAT SPECT (single photon emission computed tomography) brain scans are altered in cocaine dependent subjects and to explore clinical correlates of such alterations. SPECT brain scans were acquired on 21 cocaine dependent subjects and 21 healthy matched controls. There were significantly higher DAT levels in cocaine dependent subjects compared to controls for the anterior putamen (p=0.003; Cohen's d effect size=0.98), posterior putamen (p<0.001; effect size=1.32), and caudate (p=0.003; effect size=0.97). DAT levels in these regions were 10%, 17%, and 8% higher in the cocaine dependent subjects compared to controls. DAT levels were unrelated to craving, severity of cocaine use, or duration of cocaine use, but DAT levels in the caudate and anterior putamen were significantly (p<0.05) negatively correlated with days since last use of cocaine.

Decrease of D2 receptor binding but increase in D2-stimulated G-protein activation, dopamine transporter binding and behavioural sensitization in brains of mice treated with a chronic escalating dose 'binge' cocaine administration paradigm

Understanding the neurobiology of the transition from initial drug use to excessive drug use has been a challenge in drug addiction. We examined the effect of chronic 'binge' escalating dose cocaine administration, which mimics human compulsive drug use, on behavioural responses and the dopaminergic system of mice and compared it with a chronic steady dose (3 x 15 mg/kg/day) 'binge' cocaine administration paradigm. Male C57BL/6J mice were injected with saline or cocaine in an escalating dose paradigm for 14 days. Locomotor and stereotypy activity were measured and quantitative autoradiographic mapping of D(1) and D(2) receptors, dopamine transporters and D(2)-stimulated [(35)S]GTPgammaS binding was performed in the brains of mice treated with this escalating and steady dose paradigm. An initial sensitization to the locomotor effects of cocaine followed by a dose-dependent increase in the duration of the locomotor effect of cocaine was observed in the escalating but not the steady dose paradigm. Sensitization to the stereotypy effect of cocaine and an increase in cocaine-induced stereotypy score was observed from 3 x 20 to 3 x 25 mg/kg/day cocaine. There was a significant decrease in D(2) receptor density, but an increase in D(2)-stimulated G-protein activity and dopamine transporter density in the striatum of cocaine-treated mice, which was not observed in our steady dose paradigm. Our results document that chronic 'binge' escalating dose cocaine treatment triggers profound behavioural and neurochemical changes in the dopaminergic system, which might underlie the transition from drug use to compulsive drug use associated with addiction, which is a process of escalation.

Twenty years of dopamine research: individual differences in the response of accumbal dopamine to environmental and pharmacological challenges

Individual differences in the dopaminergic system of the nucleus accumbens of rats have extensively been reported. These individual differences have frequently been used to explain individual differences in response to environmental and pharmacological challenges. Remarkably, only little attention is paid to the factors that underlie these individual differences. This review gives an overview of the studies that have been performed in our institute during the last 20 years to investigate individual differences in accumbal dopamine release. Data are summarised demonstrating that individual differences in accumbal dopamine release are due to individual differences in: the functional reactivity of the noradrenergic system, the accumbal concentration of vesicular monoamine transporters and tyrosine hydroxylase as well as in the quantal size of the presynaptic pools of dopamine. Our data are embedded in the available literature to create a model that illustrates the putative hardware giving rise to the individual-specific release of accumbal dopamine. An important role is contributed to individual differences in the reactivity of the: hypothalamic-pituitary-adrenal axes, the reactivity of second messenger systems as well in the aminergic reactivity of the accumbens shell and core. The consequences of the individual-specific make-up and reactivity of the nucleus accumbens on the regulation of behaviour and the response to drugs of abuse will also be discussed. Apart from agents that interact with dopaminergic receptors, re-uptake or breakdown, noradrenergic agents as well as agents that interact with vesicular monoamine transporters or tyrosine hydroxylase are suggested to have therapeutic effects in subjects that are suffering from diseases in which the dopaminergic system is disturbed.

Preprodynorphin mediates locomotion and D2 dopamine and mu-opioid receptor changes induced by chronic 'binge' cocaine administration

Evidence suggests that the kappa-opioid receptor (KOP-r) system plays an important role in cocaine addiction. Indeed, cocaine induces endogenous KOP activity, which is a mechanism that opposes alterations in behaviour and brain function resulting from repeated cocaine use. In this study, we have examined the influence of deletion of preprodynorphin (ppDYN) on cocaine-induced behavioural effects and on hypothalamic-pituitary-adrenal axis activity. Furthermore, we have measured mu-opioid receptor (MOP-r) agonist-stimulated [(35)S]GTPgammaS, dopamine D(1), D(2) receptor and dopamine transporter (DAT) binding. Male wild-type (WT) and ppDYN knockout (KO) mice were injected with saline or cocaine (45 mg/kg/day) in a 'binge' administration paradigm for 14 days. Chronic cocaine produced an enhancement of locomotor sensitisation in KO. No genotype effect was found on stereotypy behaviour. Cocaine-enhanced MOP-r activation in WT but not in KO. There was an overall decrease in D(2) receptor binding in cocaine-treated KO but not in WT mice. No changes were observed in D(1) and DAT binding. Cocaine increased plasma corticosterone levels in WT but not in KO. The data confirms that the endogenous KOP system inhibits dopamine neurotransmission and that ppDYN may mediate the enhancement of MOP-r activity and the activation of the hypothalamic-pituitary-adrenal axis after chronic cocaine treatment.

Neurotoxins and neurotoxicity mechanisms. an overview

Neurotoxins represent unique chemical tools, providing a means to 1) gain insight into cellular mechanisms of apopotosis and necrosis, 2) achieve a morphological template for studies otherwise unattainable, 3) specifically produce a singular phenotype of denervation, and 4) provide the starting point to delve into processes and mechanisms of nerve regeneration and sprouting. There are many other notable uses of neurotoxins in neuroscience research, and ever more being discovered each year. The objective of this review paper is to highlight the broad areas of neuroscience in which neurotoxins and neurotoxicity mechanism come into play. This shifts the focus away from neurotoxins per se, and onto the major problems under study today. Neurotoxins broadly defined are used to explore neurodegenerative disorders, psychiatric disorders and substance use disorders. Neurotoxic mechanisms relating to protein aggregates are indigenous to Alzheimer disease, Parkinson's disease. NeuroAIDS is a disorder in which microglia and macrophages have enormous import. The gap between the immune system and nervous system has been bridged, as neuroinflammation is now considered to be part of the neurodegenerative process. Related mechanisms now arise in the process of neurogenesis. Accordingly, the entire spectrum of neuroscience is within the purview of neurotoxins and neurotoxicity mechanisms. Highlights on discoveries in the areas noted, and on selective neurotoxins, are included, mainly from the past 2 to 3 years.

Effects of subchronic methamphetamine exposure on basal dopamine and stress-induced dopamine release in the nucleus accumbens shell of rats

RATIONALE

Subchronic administration of stimulants reduces basal dopamine (DA) concentrations and blocks stress-induced DA release in the nucleus accumbens (NA) of rats during withdrawal. However, no studies have attempted to relate early withdrawal from chronic drug exposure to stress reactivity and changes in DA transmission.

OBJECTIVES

The effects of subchronic low-dose methamphetamine (METH) administration on regional changes in dopamine transporter (DAT) and norepinephrine transporter (NET) immunoreactivity and function during early withdrawal were examined. The effects of subchronic METH on stress responsivity measured by DA release in the nucleus accumbens shell (NA SHELL) and core (NA CORE) during acute restraint stress were also examined.

METHODS

Male rats received single injections of METH (2.0 mg/kg i.p.) or saline (SAL) for 10 days and then were killed 24 h after the last injection. DAT and NET protein in NA, striatum (STR), medial prefrontal cortex (mPFC), and hippocampus were assayed by Western blot analysis. Experiment 2 measured basal extracellular DA concentrations and restraint-stress-induced DA release in vivo in the NA SHELL and CORE of SAL- and METH-pretreated rats after 24-h withdrawal. Experiment 3 examined the in vivo regulation of extracellular DA in the NA SHELL and/or CORE after local administration of GBR12909 (50 microM) or nisoxetine (100 microM; NA SHELL).

RESULTS

Subchronic METH increased DAT but not NET immunoreactivity in the NA compared to the STR and mPFC. METH reduced basal extracellular DA and blocked restraint-stress-induced DA release in the NA SHELL. DA uptake blockade increased extracellular DA more in the NA SHELL of METH rats, whereas NE uptake blockade increased basal DA concentrations to a similar extent in METH and SAL rats.

CONCLUSIONS

These results suggest that subchronic METH exposure selectively increases NA DAT and consequently reduces basal and stress-induced DA release in the NA SHELL during early withdrawal.

Acute and chronic cocaine-induced potentiation of synaptic strength in the ventral tegmental area: electrophysiological and behavioral correlates in individual rats

The initiation of the psychostimulant sensitization process depends on the mesolimbic system, which projects from the ventral tegmental area (VTA) to the nucleus accumbens. Although such initiation is primarily dependent on glutamatergic activity in VTA neurons, the exact role VTA excitatory synapses play in this process is poorly understood. Here, we examine the effects of repeated in vivo injections of cocaine on the magnitude and duration of the increase in strength at VTA excitatory synapses reported previously to be elicited by a single in vivo exposure to cocaine (Ungless et al., 2001; Saal et al., 2003). We also compare the synaptic modifications induced by cocaine with its effects on locomotor activity. Surprisingly, repeated cocaine exposure potentiated the ratio of AMPA receptor-mediated to NMDA receptor-mediated EPSCs to a similar extent and duration as a single in vivo cocaine exposure. In naive animals, the magnitude of the cocaine-induced locomotor activity after a single injection of cocaine correlated with the magnitude of the accompanying synaptic enhancement. This correlation was lost on the seventh day of repeated cocaine administration, as well as when a challenge injection was given 10 d after the cessation of repeated cocaine administration. These results suggest that the cocaine-induced synaptic plasticity at VTA excitatory synapses is transient, and its duration depends on the last exposure to cocaine. Furthermore, chronic cocaine exposure disrupts the normal, presumably adaptive relationship between synaptic enhancement in the VTA and behavior.

Behavioral sensitization and long-term neurochemical alterations associated with the fungicide triadimefon

Triadimefon (TDF), a widely used triazole fungicide, blocks reuptake of the neurotransmitter dopamine (DA), similarly to cocaine. Preliminary studies show that intermittent intraperitoneal injections of TDF increase ambulatory and vertical activity across repeated injections [Neurotoxicology (in press)] leading to the hypothesis tested here, that exposure to TDF may influence the development and expression of behavioral sensitization, a model of psychostimulant-induced psychosis. Exposure of adult male C57BL/6 mice to 75 mg/kg i.p. TDF (TDF75) twice a week for 7 weeks increased vertical activity at each injection. Following a 2-week withdrawal period, a TDF challenge to test for expression of behavioral sensitization revealed further increases in vertical activity levels relative to all other conditions. TDF induction/expression of behavioral sensitization was associated with long-term, perhaps permanent modulation of dopaminergic function that included increases in striatal dihydroxyphenylacetic acid (DOPAC) and DA turnover, increases in medial prefrontal cortex (mPFC) dopamine transporter (DAT) binding, as well as decreases in DA D1 and increases in DA D2 and DAT receptor binding that appeared to target the nucleus accumbens shell (NAs) subregion. Thus, TDF exposure may serve as an environmental risk factor for DA system dysfunctions.

Long-lasting behavioral sensitization to psychostimulants following p-chloroamphetamine-induced neurotoxicity in mice

Amphetamine analogs such as p-chloroamphetamine (PCA) cause serotonergic and dopaminergic neurotoxicity. The behavioral consequences and the responsiveness to psychostimulants following the neurotoxic insult are unclear. The present study was undertaken to investigate the outcome of neurotoxic and non-neurotoxic PCA pre-treatments on the sensitivity of Swiss Webster mice to the psychomotor stimulating effects of PCA, 3,4-methylenedioxymethamphetamine (MDMA) and cocaine. PCA (15 mg/kg x 2; i.p.) caused 37-70% depletion of dopaminergic and serotonergic markers in mouse brain. Saline and PCA (15 mg/kg x 2) mice were challenged on days 5, 12, 40 and 74 with one of the following drugs: PCA (5 mg/kg), MDMA (10 mg/kg) and cocaine (20 mg/kg). The PCA pre-exposed mice showed marked locomotor sensitization from days 5-74 to all three drugs tested. The time course of the sensitized response coincided with the time course of the neurotoxic insult as determined by reduced densities of striatal dopamine transporter and frontal cortex serotonin transporter binding sites. A single injection of PCA (5 mg/kg) caused neither neurotoxicity nor sensitization to the locomotor stimulating effects of PCA, MDMA and cocaine. Repeated administration of a low non-neurotoxic dose of PCA (5 mg/kg/day; 6 days) caused a transient locomotor sensitization to PCA that dissipated after one month. Results of the present study suggest that PCA-induced serotonergic and dopaminergic neurotoxicity coincides with long-lasting locomotor sensitization to psychostimulants. These findings may be relevant to the psychopathology of amphetamines-induced neurotoxicity.

The role of the dopamine transporter in cocaine abuse

There have been many studies aimed at understanding the role that the dopamine transporter plays in cocaine abuse. Most studies suggest that inhibition of dopamine uptake by cocaine is the primary mechanism by which its behavioral effects are produced. Because of the strong relationship between binding to the dopamine transporter and the behavioral effects of cocaine, the dopamine transporter has on occasion been referred to as the cocaine binding site. Chronic studies using cocaine or selective inhibitors of dopamine, norepinephrine, or serotonin uptake suggest that while a selective dopamine uptake inhibitor can produce sensitization to cocaine, the long-lasting sensitized response to a cocaine challenge observed in cocaine-pretreated rats is due to cocaine's action on a system other than, or in addition to, dopamine. Thus, while dopamine appears to be important for the behavioral effects of cocaine, it appears that neurochemical systems other than dopamine likely play a role in the behavioral effects of chronic cocaine.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Exercise induces behavioral recovery and attenuates neurochemical deficits in rodent models of Parkinson's disease

Exercise is thought to improve motor function and emotional well-being in patients with Parkinson's disease (PD). However, it is not clear if the improvements are due to neurochemical alterations within the affected nigrostriatal region or result from a more general effect of exercise on affect and motivation. In this study we show that motorized treadmill running improves the neurochemical and behavioral outcomes in two rodent models of PD: the unilateral 6-hydroxydopamine (6-OHDA) rat model and bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model in aged C57bl mice. Exposure to the dopamine (DA) toxins 6-OHDA or MPTP resulted in permanent behavioral and neurochemical loss. In contrast, when lesioned animals were exposed to treadmill activity two times a day for the first 10 days post-lesion they displayed no behavioral deficits across testing days and had significant sparing of striatal DA, its metabolites, tyrosine hydroxylase, vesicular monoamine transporter, and DA transporter levels compared to lesion sedentary animals. These results demonstrate that exercise following nigrostriatal damage ameliorates related motor symptoms and neurochemical deficits in rodent models of PD.

The significance of increase in striatal D(2) receptors in epileptic EL mice

The present study systematically and quantitatively analyzed the immunohistochemical distribution of various substances involved in synthesis, binding, and transport of dopamine in the forebrain of epileptic mice (EL mouse strain) using a brain mapping analyzer. A reduction in serum calcium levels decreases calcium/calmodulin-dependent-dopamine synthesis in the brain and subsequently increases susceptibility to epileptic convulsions and induces abnormal behavior in EL mice. The immunohistochemical levels of D(2) receptors in the medial area of the neostriatum were significantly higher in EL mice than in ddY mice (mother strain of EL mice), while there were no differences in the levels of tyrosine hydroxylase, calcium/calmodulin-dependent protein kinase II, calmodulin, D(1) receptors, and dopamine transporters. Together with our previous findings, the results suggest that the decrease in serum calcium levels and subsequent decrease in brain dopamine synthesis comprise the primary physiologic disorder in EL mice, and convulsions or increased D(2) receptors are secondarily-induced phenomena to improve or compensate for the principal disorder.

Methamphetamine-induced deficits of brain monoaminergic neuronal markers: distal axotomy or neuronal plasticity

We examined the effects of methamphetamine (METH) on monoaminergic (i.e. dopamine and serotonin) axonal markers and glial cell activation in the rat brain. Our findings indicate that the loss of dopamine transporters (DAT), serotonin transporters (5-HTT), vesicular monoamine transporter type-2 (VMAT-2) and glial cell activation induced by METH in the striatum and in the central gray are consistent with a degenerative process. Our novel finding of METH effects on monoaminergic neurons in the central gray may have important implications on METH-induced hyperthermia. In other brain regions examined, DAT and 5-HTT deficits after METH administration were present in the absence of lasting changes in VMAT-2 levels or glial cell activation. Brain regions exhibiting protracted deficits in DAT and/or 5-HTT and VMAT-2 levels also expressed increased levels of [(3)H]-R-PK11195 binding to peripheral benzodiazepine receptors, a quantitative marker of glial cell activation. Immunohistochemical assessment of microglia and astrocytes confirmed the PBR results. Microglia activation was more pronounced than astrocytosis in affected regions in most METH-exposed brains with the exception of a small number of rats that were most severely affected by METH based on loss of body weight. In these rats, both microglia and astrocytes were highly activated and expressed a distinct regional pattern suggestive of widespread brain injury. The reason for the pattern of glial cell activation in this group of rats is not currently known but it may be associated with METH-induced hyperthermia. In summary, our findings suggest two neurotoxic endpoints in the brain of METH-exposed animals. Brain regions exhibiting DAT and 5-HTT deficits that co-localize with decreased VMAT-2 levels and glial cell activation may represent monoaminergic terminal degeneration. However, the DAT and 5-HTT deficits in brain regions lacking a deficit in VMAT-2 and glial cell activation may reflect drug-induced modulation of these plasma membrane proteins.

Tolerance and sensitization to the locomotor-activating effects of cocaine are mediated via independent mechanisms

Tolerance or sensitization to the locomotor-activating effects of cocaine occurs depending upon the treatment regimen that is used. When cocaine is injected on a daily basis, sensitization occurs, whereas continuously infused cocaine leads to tolerance. Male Sprague-Dawley rats were treated for 7 days with continuous cocaine (50 mg/kg/day) via subcutaneously implanted osmotic minipumps, after which the pumps were removed. Locomotor activity was measured for 1 h each day. Some rats were challenged with an injection of cocaine (7.5, 15 or 30 mg/kg) either 2 or 9 days after pump removal. Two days after the pumps were removed (Day 10), there were no significant differences between cocaine- or saline-treated rats in the amount of locomotor activity produced by the challenge injections. However, cocaine-treated rats challenged with cocaine 9 days after pumps were removed (Day 17) exhibited significant tolerance, as evidenced by a shift downward of the cocaine curve, as compared to saline controls. When the rats were injected again on the next day (Day 18), the activity levels of both groups increased, as compared to the effects observed on Day 17. Thus, although the cocaine-treated rats were still tolerant compared to the saline-treated rats, they were sensitized compared to their previous response to a challenge injection. These findings indicate that tolerance and sensitization to the locomotor-activating effects of cocaine can exist simultaneously, which suggests that they are mediated by separate mechanisms.

L-DOPA does not cause neurotoxicity in VMAT2 heterozygote knockout mice

One of the most useful treatments of Parkinson's disease (PD) is dihydroxyphenylalanine (L-DOPA) administration. However, L-DOPA has been suggested to be toxic to dopamine (DA) neurons and perhaps contribute to the progression of the disease. Sequestration of DA and dopaminergic neurotoxins into vesicles by the vesicular monoamine transporter 2 (VMAT2) is a key factor in preventing cellular damage. Mice with reduced expression of VMAT2 (VMAT2 heterozygote knockout mice; VMAT2 (+/-)) are more sensitive to the neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and methamphetamine. In this study, we subjected VMAT2 (+/-) mice to subchronic administration of L-DOPA to determine if it was toxic in this model. VMAT2 wild-type (VMAT2 (+/+)) and VMAT2 (+/-) mice were given i.p. injections of L-DOPA:carbidopa (50:5 mg/kg) three times a day for 28 days. Biochemical analysis revealed a significant increase in striatal DA levels in both groups of mice treated with L-DOPA. L-DOPA treatment significantly decreased DAT levels in VMAT2 (+/+) mice, but not in VMAT2 (+/-) mice. VMAT2 protein levels, an index of terminal integrity and the number of tyrosine hydroxylase (TH)-positive nigral cells remained unchanged after L-DOPA treatment. These data indicate that in an animal model that displays increased susceptibility to dopaminergic injury, a subchronic administration of L-DOPA does not induce toxicity.

Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions

Despite an abundance of studies on mechanisms of behavioral sensitization, considerable uncertainty exists as to whether alterations in dopamine neurotransmission underlie the exacerbated behavioral effects of cocaine observed during the early stages of abstinence. One of the factors contributing to the uncertainty and controversy may be the limitations in utilized measurement techniques (mostly conventional microdialysis). The techniques of quantitative microdialysis under transient conditions and rotating disk electrode voltammetry were used to characterize basal dopamine dynamics as well as time-related changes in extracellular dopamine concentrations and dopamine uptake that occur in response to an acute drug challenge in control animals and animals with previous history of cocaine. Basal extracellular dopamine concentrations were unaltered on abstinence day 3 from repeated cocaine administration (5 days, 20 mg/kg, i.p.). The extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly lower in cocaine-sensitized animals relative to controls. These two facts, taken together, suggest that basal dopamine release is depressed in cocaine-sensitized animals on abstinence day 3. At the same time, a cocaine challenge decreased the extraction fraction and increased the extracellular dopamine concentration in both experimental groups. The magnitude of the increase in extracellular dopamine concentration was greater in cocaine-sensitized animals, while the ability of cocaine to decrease the extraction fraction was unaltered, suggesting that the increase in extracellular dopamine concentration reflects an increase in drug-evoked dopamine release. Moreover, cocaine-pretreated rats demonstrated greater depolarization-induced dopamine release and the ability of dopamine D(2) receptor agonist, quinpirole, to inhibit release was decreased in these animals. These data demonstrate that a cocaine treatment regimen resulting in behavioral sensitization is associated with a reduction in basal dopamine release, an enhancement in both cocaine and K(+)-evoked dopamine release, and a subsensitivity of dopamine D(2) autoreceptors that regulate dopamine release in the nucleus accumbens.

Forced nonuse in unilateral parkinsonian rats exacerbates injury

Diagnosis of Parkinson's disease (PD) is based on the presentation of clinical symptoms such as bradykinesia, resting tremor, and rigidity. However, one feature of PD that often begins years before diagnosis is decreased physical activity. We hypothesized that this depressed activity is not only a symptom of the early dopaminergic loss but also a catalyst in the degenerative process. Two experiments were performed to test this hypothesis. First, rats were exposed to a mild dose of 6-hydroxydopamine unilaterally into the nigrostriatal dopamine (DA) projections, which would normally result in an approximately 20% DA loss and no detectable behavioral asymmetries. A subset of these lesioned animals then had a cast applied for 7 d to the contralateral forelimb. After the cast was removed, these animals displayed long-term behavioral asymmetry and exacerbation of neurochemical loss (approximately 60% depletion). Second, a group of animals received a high dose of 6-hydroxydopamine that normally would yield a severe loss of nigrostriatal terminals (approximately 90% loss) and chronic sensorimotor deficits. During the first 7 d after neurotoxin exposure, a subset of these animals were forced to rely on the contralateral forelimb, a procedure we have previously reported to protect DA terminals and behavioral function. Some of these rats then had the use of their "recovered" forelimb restricted during the second or third week after lesioning. This precipitated a severe and chronic loss of DA terminals and functional deficits. These results suggest decreased physical activity not only is a symptom of PD but also may act to potentiate the underlying degeneration.

Cocaine increases dopamine uptake and cell surface expression of dopamine transporters

In HEK 293 cells expressing the human dopamine transporter (DAT), a 10-min incubation with 10 microM cocaine followed by extensive washing resulted in a 30% increase in [3H]dopamine (DA) uptake as well as an increase in cell surface DAT in biotinylation experiments. Consistent with this novel regulation, [3H]DA uptake into synaptosomes prepared from the nucleus accumbens of rats sacrificed 30 min after a single cocaine injection (30 mg/kg) was significantly increased compared to controls (56% increase in V(max), no change in K(m)). In addition, DA clearance in the striatum of anesthetized rats was increased after local application of a low (3 pmol) but not high (65 pmol) dose of cocaine, presumably as a result of mobilization of DAT to the cell surface. Cocaine-induced increases in cell surface expression of DAT and associated changes in DA clearance represent a novel mechanism that may play a role in its addictive properties.

Is methamphetamine abuse a risk factor in parkinsonism?

Parkinsons disease (PD) is a neurodegenerative disorder with increased incidence in individuals beyond 50 years of age. The etiology of PD is currently not known, but it appears that environmental factors may play an important role. The molecular basis of PD is the nearly complete loss of the neurotransmitter dopamine (DA) in the basal ganglia (caudate/putamen). The decrease in dopamine levels is the result of degeneration of dopamine-containing neurons in the substantia nigra. This biochemical deficit in the nigrostriatal pathway leads to the emergence of motor impairments typical of PD. Methamphetamine (METH) is a psychostimulant drug with increasing use in certain segments of the population in the United States and worldwide. In experimental animal models and human studies, METH administration has been shown to decrease markers of dopaminergic neuron terminal integrity in the basal ganglia. A long-standing question has been whether the reductions in dopaminergic markers induced by METH constitute degenerative changes or reflect drug-induced modulation. Resolving this question is important because the irreversible loss of dopaminergic function may increase the likelihood of Parkinsonism with advancing age.

Kappa opioid agonists alter dopamine markers and cocaine-stimulated locomotor activity

To better understand the influence of kappa-opioid agonists on the effects of cocaine, rats were treated with daily injections of the selective kappa-opioid agonist U-69593 or bremazocine. In combination with 10 mg/kg cocaine, both compounds, at a dose of 0.32 mg/kg, greatly diminished locomotor activity, and these effects were maintained over a period of 5 days. In addition, the response to a challenge injection of 10 mg/kg cocaine several days after the end of kappa-Opioid agonist treatment with or without cocaine was markedly reduced. When naltrexone was given in combination with U-69593, it blocked the reduction in cocaine-induced locomotor activity after U-69593 treatment alone. However, a single injection of either kappa-opioid agonist alone had no effect on cocaine-induced locomotion several days later (i.e. no long-term effects), suggesting that multiple injections of the kappa-opioid agonist are needed to reduce the locomotor activating effects of cocaine other than acutely. In addition, treatment with the kappa-opioid agonist U-69593 (0.32 mg/kg) over a 5-day period decreased dopamine transporter densities in the caudate putamen, and this was also blocked by co-administration of naltrexone.

Progression of changes in dopamine transporter binding site density as a result of cocaine self-administration in rhesus monkeys

The present study examined the time course of alterations in levels of dopamine transporter (DAT) binding sites that accompany cocaine self-administration using quantitative in vitro receptor autoradiography with [(3)H]WIN 35,428. The density of dopamine transporter binding sites in the striatum of rhesus monkeys with 5 d, 3.3 months, or 1.5 years of cocaine self-administration experience was compared with DAT levels in cocaine-naive control monkeys. Animals in the long-term (1.5 years) exposure group self-administered cocaine at 0.03 mg/kg per injection, whereas the initial (5 d) and chronic (3.3 months) treatment groups were each divided into lower dose (0.03 mg/kg per injection) and higher dose (0.3 mg/kg per injection) groups. Initial cocaine exposure led to moderate decreases in [(3)H]WIN 35,428 binding sites, with significant changes in the dorsolateral caudate (-25%) and central putamen (-19%) at the lower dose. Longer exposure, in contrast, resulted in elevated levels of striatal binding sites. The increases were most pronounced in the ventral striatum at the level of the nucleus accumbens shell. At the lower dose of the chronic phase, for example, significant increases of 21-42% were measured at the caudal level of the ventral caudate, ventral putamen, olfactory tubercle, and accumbens core and shell. Systematic variation of cocaine dose and drug exposure time demonstrated the importance of these factors in determining the intensity of increased DAT levels. With self-administration of higher doses especially, increases were more intense and included dorsal portions of the striatum so that every region at the caudal level exhibited a significant increase in DAT binding sites (20-54%). The similarity of these findings to previous studies in human cocaine addicts strongly suggest that the increased density of dopamine transporters observed in studies of human drug abusers are the result of the neurobiological effects of cocaine, ruling out confounds such as polydrug abuse, preexisting differences in DAT levels, or comorbid psychiatric conditions.

Autoradiographic evidence that prolonged withdrawal from intermittent cocaine reduces mu-opioid receptor expression in limbic regions of the rat brain

Numerous reports support evidence that dopaminergic mesolimbic pathways interact with opioid systems to influence the reinforcing properties of cocaine. Withdrawal from chronic administration of cocaine in rats causes an upregulation of mesocorticolimbic mu-opioid receptors during early stages, but information about prolonged cocaine abstinence is lacking. We addressed this issue by treating rats with cocaine or saline (control) intermittently (1 mg/kg, i.v., every 12 min for 2 h daily) for 10 days followed by a 10- or 20-day withdrawal period. The animals were then decapitated and the brains removed for quantitative in vitro autoradiographic analysis of 14 brain regions with (125)I-DAMGO. A separate group of animals received two consecutive cycles of the 10-day cocaine/10-day withdrawal regimen. Only the group that participated in the two consecutive cycles showed a significant effect of treatment: downregulation of mu-opiate receptors in limbic cortical layer 3 (17% lower than saline-treated controls, P = 0.03), the core of the nucleus accumbens (16% decrease, P = 0.05), and the nucleus of the diagonal band (18% decrease, P = 0.05). The mu-receptor may manifest, as do other neural markers (e.g., dopamine transporter, dopamine efflux), a biphasic temporal pattern with upregulation during early phases of cocaine withdrawal but a downregulation at later times.

Dopamine transporter synthesis and degradation rate in rat striatum and nucleus accumbens using RTI-76

Intracerebroventricular injections of the irreversible dopamine transporter (DAT) inhibitor, RTI-76 [3beta-(3-p-chlorophenyl) tropan-2beta-carboxylic acid p-isothiocyanatophenylethyl ester hydrochloride], decreased DAT binding in both the striatum and nucleus accumbens as measured by both [3H]GBR12935 and by [3H]WIN35,428. This decrease was dose-related, with 100 nmol RTI-76 producing approximately a 50% decrease in both regions. The maximal inhibition of DAT binding was observed 24 h after RTI-76 injection, and binding was fully restored 7 days after injection. The DAT protein half-life determined under these conditions was about 2 days. [3H]Nisoxetine binding at norepinephrine transporters in the cortex was not altered by RTI-76 administration at any time point or dose examined.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

Estrous cyclicity and behavioral sensitization in female rats following repeated intravenous cocaine administration

Repeated intermittent administration of cocaine is well known to produce behavioral sensitization in male animals. The present studies explored whether intact adult female rats maintained normal estrous patterns in response to repeated IV cocaine administration and whether behavioral sensitization occurred with this route of administration. Adult female Sprague-Dawley rats (N = 48) were surgically implanted with an intravenous access port. Animals received 3.0 mg/kg IV cocaine once/day for 14 days. Daily vaginal lavages indicated that female rats continued to cycle normally throughout the experiment. Estimates of statistical power for detecting alterations in estrous cycle length ranged from 0.61-0.95 for small (0.1) to large (0.4) effect sizes. Moreover, no cocaine-treated animals displayed persistent vaginal estrus or were acyclic and cocaine treatment did not decrease body weight. Immediately after the cocaine injection, animals were placed in IR photocell activity chambers for 60 min. Female rats displayed a significant 75% increase in locomotor activity across the 14-day time course of IV cocaine injections. These data indicate that 3.0 mg/kg of IV cocaine does not interfere with normal estrous cyclicity, and that behavioral sensitization occurs in female rats following repeated daily IV cocaine dosing. Collectively, these data suggest that the IV cocaine-dosing model may be particularly useful in exploring the gender-dependent effects of cocaine using intact female rats.

Cocaine upregulates the dopamine transporter in fetal rhesus monkey brain

Cocaine is a highly addictive drug that binds to the dopamine transporter (DAT), inhibits the reuptake of dopamine, and initiates multiple actions within midbrain dopaminergic systems. Using the rhesus monkey, we have investigated the consequences of in utero cocaine exposure on the expression of DAT in the fetal brain. By using the selective DAT ligand [125I]RTI-121 and tyrosine hydroxylase (TH) immunocytochemistry, we found that DAT binding sites are highly developed by day 70 of gestation and show a distribution pattern similar to TH. The rank order of specific 3beta-(4-[125I]iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([125I]RTI-121) binding densities was substantia nigra-ventral tegmental area > putamen > caudate > lateral hypothalamus > accumbens > linear/interfascicular nuclei >/= globus pallidus > prefrontal cortex. Furthermore, we observed that DAT mRNA was differentially expressed within fetal midbrain dopamine neurons with the highest levels detected in the ventral tier of the substantia nigra pars compacta, and the lowest levels in the ventral tegmental area and the linear/interfascicular nuclei. In utero cocaine exposure between days 22 and 70 significantly increased DAT mRNA expression, and the density of [125I]RTI-121 binding sites within midbrain dopamine neurons in the 70-d-old fetus. This increased DAT expression is accompanied by other presynaptic and postsynaptic neuronal changes, which collectively suggest that midbrain dopamine neurons are hypoactive after prolonged cocaine exposure, a state that may be a contributing factor in the development of attention deficit disorders observed in subjects exposed prenatally to cocaine.

Localization of the delta-opioid receptor and dopamine transporter in the nucleus accumbens shell: implications for opiate and psychostimulant cross-sensitization

Opiate- and psychostimulant-induced modulation of dopamine transmission in the nucleus accumbens shell (AcbSh) is thought to play a key role in their potent reinforcing and locomotor effects. To investigate the cellular basis for potential functional interactions involving opiates active at the delta-opioid receptor (DOR) and psychostimulants that bind selectively to the dopamine transporter (DAT), we examined the electron microscopic localization of their respective antisera in rat AcbSh. DOR immunoperoxidase labeling was seen primarily, and DAT immunogold particles exclusively, in axon terminals. In these terminals, DOR immunoreactivity was prominently associated with discrete segments of the plasma membrane and the membranes of nearby small synaptic and large dense core vesicles. DAT immunogold particles were almost exclusively distributed along nonsynaptic axonal plasma membranes. Thirty-nine percent DOR-labeled profiles (221/566) either apposed DAT-immunoreactive terminals or also contained DAT. Of these 221 DOR-labeled profiles, 13% were axon terminals containing DAT and 15% were dendritic spines apposed to DAT-immunoreactive terminals. In contrast, 70% were morphologically heterogeneous axon terminals and small axons apposed to DAT-immunoreactive terminals. Our results indicate that DOR agonists in the AcbSh can directly modulate the release of dopamine, as well as postsynaptic responses in spiny neurons that receive dopaminergic input, but act principally to control the presynaptic secretion of other neurotransmitters whose release may influence or be influenced by extracellular dopamine. Thus, while opiates and psychostimulants mainly have differential sites of action, cross-sensitization of their addictive properties may occur through common neuronal targets.

Long-term alterations in benzodiazepine, muscarinic and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor density following continuous cocaine administration

Animal models of clinical phenomena, such as stimulant-induced psychosis have focused primarily on persisting alterations that develop in brain after chronic stimulant administration. The present study utilized autoradiographic measures to examine changes in the density of benzodiazepine ([3H] flunitrazepam), muscarinic ([3H] quinuclidinyl benzilate), and non-NMDA glutamatergic (3H alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid: AMPA) receptor binding in rats 21 days following two exposures to cocaine administered continuously for 5 days via subcutaneous pellets. A marked, selective increase in [3H] flunitrazepam binding in both the lateral and medial habenula nucleus was observed. Reduced [3H] quinuclidinyl benzilate binding was observed in various brain areas, including large decreases in the anterior cingulate cortex and ventral thalamus. A reduction in [3H]AMPA binding was observed in the ventral striatum and was suggested in the nucleus accumbens. [3H] Flunitrazepam binding was also examined 12 hr following a single 5 day cocaine exposure to determine if the long-term habenular changes were evident at acute withdrawal. No alterations in [3H] flunitrazepam binding were observed in the habenula or any other structure analyzed at this time point. The relation of these results to persisting alterations in mesocorticolimbic pathways and previous findings of cocaine-induced degeneration in lateral habenula circuitry is discussed.

Functional-anatomical implications of the nucleus accumbens core and shell subterritories

The nucleus accumbens, a major part of the ventral striatum, comprises numerous subterritories and compartments, of which the core and shell appear to be dominant. Shell exhibits greater chemical neuroanatomical diversity than core and is rather directly connected to it by a robust, feed-forward, striatopallido-thalamocortico-striatal pathway. Shell and extended amygdala share afferents, but the two are distinguished by their outputs, strongly toward cortex for shell and descendent toward brain stem effector sites for extended amygdala. Shell responds independently to stimulation by excitatory amino acids and dopamine, which are more mutually permissive in the core. Accordingly, the shell responds to a broad variety of physiological and pharmacological stimuli, including psychomotor and opioid drugs. Whereas locomotion and oro-facial movements are elicitable from the shell, lesions and blockade of EAA transmission in the core reduce locomotion. It is hypothesized that core-shell has a feed-forward functional organization.

Functional heterogeneity in dopamine release and in the expression of Fos-like proteins within the rat striatal complex

The dorsolateral striatum, and the core and shell of the nucleus accumbens are three major anatomical regions of the striatal complex. The shell is considered as a part of the extended amygdala, and is involved in the control of motivation and reward. The core and the striatum are considered central to sensory motor integration. In this study we compared the responses of these three regions to mild stress and drugs of abuse by measuring extracellular dopamine (DA) concentrations and Fos-like immunoreactivity (Fos-LI). The results are summarrized as follows. (i) In unchallenged conditions, extracellular DA concentrations were highest in the dorsolateral striatum and lowest in the core, whereas Fos-LI was highest in the shell and lowest in the dorsolateral striatum. (ii) After challenges that increase DA by depolarizing DAergic neurons (injection stress or 2 mg/kg morphine), the shell presented the largest increase in DA levels and Fos-LI. (iii) After the administration of a DA-uptake blocker (15 mg/kg cocaine), the percentage increase in DA was still largest in the shell. However, the absolute increase in DA and Fos-LI in the shell and the dorsolateral striatum were similar. (iv) After a full D1 agonist (SKF82958), Fos-LI was highest in the shell and lowest in the dorsolateral striatum. In conclusion, the nucleus accumbens shell seems to be the area of the striatal complex most functionally reactive to stress and drugs of abuse. However, the dorsolateral striatum and the core appear functionally distinct, as for most of the parameters studied these two regions differed.

Comparison between dopamine transporter affinity and self-administration potency of local anesthetics in rhesus monkeys

Local anesthetics bind to dopamine transporters and inhibit dopamine uptake in rodent brain. Additionally, local anesthetics are self-administered in rhesus monkeys. The present study determined binding affinities of cocaine and five local anesthetics at dopamine transporters in rhesus monkey brain, and compared binding affinities to published self-administration potencies in rhesus monkeys. The affinity order at dopamine transporters was cocaine > dimethocaine > tetracaine > procaine > or = chloroprocaine > lidocaine. The correlation between dopamine transporter affinities and self-administration potencies was significant. Binding affinities were also determined at sodium (Na2+) channels in rhesus monkey brain. There was not a significant correlation between Na2+ channel affinities and self-administration potencies Local anesthetics with high dopamine transporter and low Na2+ channel affinities were self-administered, whereas those with either high or low affinity at both sites were not consistently self-administered. These data suggest that affinity at dopamine transporters is related to the reinforcing effects of local anesthetics in rhesus monkeys, and Na2+ channel effects may interfere with the reinforcing effect of these drugs.

Long-term changes in brain following continuous phencyclidine administration: an autoradiographic study using flunitrazepam, ketanserin, mazindol, quinuclidinyl benzilate, piperidyl-3,4-3H(N)-TCP, and AMPA receptor ligands

Phencyclidine induces a model psychosis which can persist for prolonged periods and presents a strong drug model of schizophrenia. When given continuously for several days to rats, phencyclidine and other N-methyl-D-aspartate (NMDA) antagonists induce neural degeneration in a variety of limbic structures, including retrosplenial cortex, hippocampus, septohippocampal projections, and piriform cortex. In an attempt to further clarify the mechanisms underlying these degeneration patterns, autoradiographic studies using a variety of receptor ligands were conducted in animals 21 days after an identical dosage of the continuous phencyclidine administration employed in the previous degeneration studies. The results indicated enduring alterations in a number of receptors: these included decreased piperidyl-3,4-3H(N)-TCP (TCP), flunitrazepam, and mazindol binding in many of the limbic regions in which degeneration has been reported previously. Quinuclidinyl benzilate and (AMPA) binding were decreased in anterior cingulate and piriform cortex, and in accumbens and striatum. Piperidyl-3,4-3H(N)-TCP binding was decreased in most hippocampal regions. Many of these long-term alterations would not have been predicted by prior studies of the neurotoxic effects of continuous phencyclidine, and these results do not suggest a unitary source for the neurotoxicity. Whereas retrosplenial cortex, the structure which degenerates earliest, showed minimal alterations, some of the most consistent, long term alterations were in structures which evidence no immediate signs of neural degeneration, such as anterior cingulate cortex and caudate nucleus. In these structures, some of the receptor changes appeared to develop gradually (they were not present immediately after cessation of drug administration), and thus were perhaps due to changed input from regions evidencing neurotoxicity. Some of these findings, particularly in anterior cingulate, may have implications for models of schizophrenia.

Effects of antipsychotics, vitamin E, and MK-801 on dopamine dynamics in the rat brain following discontinuation of cocaine

Cocaine, 10 mg/kg, I.P., twice daily, was given to rats for 1 week. At 1 and 4 weeks following discontinuation of cocaine, the initial rate of 3,4-dihydroxyphenylacetic acid (DOPAC) formation was assessed. The initial rate of DOPAC formation was found to be decreased in the frontal and cingulate cortices at 1 week, but was only decreased in the frontal cortex at 4 weeks. When administered in conjunction with cocaine, haloperidol, clozapine, and vitamin E, but not MK-801, were found to prevent cocaine's effects. In addition to the potential value these findings have for further understanding cocaine abuse, it is proposed that the alteration in dopamine metabolism produced by cocaine, and the ability of haloperidol, clozapine and vitamin E to decrease cocaine's effects, model some biochemical aspects of schizophrenia.

Postnatal development of dopamine and serotonin transporters in rat caudate-putamen and nucleus accumbens septi

Density of dopamine transporter (DAT) and serotonin transporter (5-HTT) membrane proteins in the caudate-putamen (CPu) and nucleus accumbens (NAc) of rat brain was assessed at seven ages at postnatal days (PD) 7-60, by in vitro quantitative autoradiography. Binding of [3H]GBR-12935 (to DAT) and [3H]paroxetine (to 5-HTT) increased steadily and very similarly, from low levels at PD-7 to maximal levels, to 6-7-fold higher density at PD-60 in both regions. These findings indicate that DAT and 5-HTT follow a synchronized course of development in rat CPu and NAc. In contrast to reported elimination of excessive receptors in CPu and NAc during maturation, there was no evidence of pruning of DAT or 5-HTT in these regions of rat forebrain.

Striatal [125I]RTI-55 binding sites in cocaine-abusing humans

1. Previous experiments in this laboratory found that striatal [3H]WIN 35428 binding was increased in post mortem specimens from human cocaine users (Little et al, 1993a). Although structurally similar, preliminary studies have suggested that [3H]WIN 35428 and the related cocaine congener [125I]RTI-55 differ in some respects pharmacologically. 2. The present experiments tested the hypothesis that striatal [125I]RTI-55 binding would be increased, as was [3H]WIN 35428 binding, in post mortem specimens from cocaine users compared to matched controls. 3. However, computer-generated parameters derived from saturation experiments found only trends toward increased Bmax and decreased affinity (increased KD) in the cocaine users. The magnitude of the increases were notably smaller than the statistically significant increases previously found in high affinity [3H]WIN 35428 binding in these same subjects. 4. Evidence from the present and earlier experiments suggests that cocaine exposure may induce conformational changes in the dopamine transporter.

Effects of pharmacologic increases in brain GABA levels on cocaine-induced changes in extracellular dopamine

Cocaine-induced increases in extracellular dopamine (DA) concentrations were measured using in vivo microdialysis techniques in the nucleus accumbens (NACC) of freely moving rats. In control animals, cocaine increased extracellular DA concentrations approximately 482% 60 min following administration, returning to baseline values 200 min later. When administered 2 h following an acute dose of gamma-vinyl-GABA (GVG, Vigabatrin), cocaine-induced increases in extracellular DA were reduced to approximately 365% of baseline values. Chronic GVG administration further dose-dependently attenuated the effects of cocaine but did not alter the rate of increase or the rate of return to baseline values. These results indicate that GVG, a drug that increases brain GABA concentrations, is effective in attenuating the effect of cocaine on NACC DA. Taken with our earlier findings, these results support the targeting of brain GABAergic systems as a potentially effective pharmacologic treatment strategy for cocaine addiction.

Lack of effect of chronic clorgyline or selegiline on dopamine and serotonin transporters in rat caudate-putamen or nucleus accumbens septi

Rats were injected intraperitoneally thrice weekly for 4 weeks with doses of selective inhibitors of monoamine oxidase type A (clorgyline, 1 mg/kg) or B ((-)-selegiline, 10 mg/kg), or saline. Both treatments produced sustained elevations of concentrations of dopamine and serotonin, and decreased their deaminated metabolites in forebrain tissue. Nevertheless, no change in binding of [3H]GBR-12935 to the dopamine transporter or of [3H]paroxetine to the serotonin transporter in caudate-putamen or nucleus accumbens septi was found with quantitative autoradiography. These results support the impression that transporter proteins for these monoamines are not regulated by increased ligand abundance.

Cocaine abusers do not show loss of dopamine transporters with age

Cocaine blocks dopamine transporters (DAT) and this effect is crucial to its reinforcing properties. To assess the effects of chronic cocaine on DAT we evaluated 20 current cocaine abusers and 20 age matched controls using PET and [C-11]cocaine as a DAT ligand. Though there were no differences in DAT availability between groups, current cocaine abusers (and 12 detoxified cocaine abusers studied previously) did not show the typical age-related decline in DAT seen in controls. Though further studies are required to rule out sampling effects and to control for confounding variables (i.e. smoking), one could speculate that chronic DAT blockade by cocaine has a protective effect on the loss of DAT with age.

The dopamine transporter: comparative ultrastructure of dopaminergic axons in limbic and motor compartments of the nucleus accumbens

The dopamine transporter (DAT) regulates extracellular dopamine concentrations, transports neurotoxins, and acts as a substrate for cocaine reinforcement. These functions are known to differ in the limbic-associated shell and motor-associated core compartments of the nucleus accumbens (NAc). Previous studies have shown differential expression of DAT in the NAc shell and core but were limited in resolution to the regional level. Thus, it is not known whether there are differences in the amount, subcellular localization, or plasmalemmal targeting of DAT within individual dopaminergic axons in the two regions. We used high-resolution electron microscopic immunocytochemistry to investigate these possibilities. We show that in both the shell and core, DAT immunogold labeling is present in tyrosine hydroxylase-immunoreactive varicose axons that form symmetric synapses. Within these labeled axons, most DAT gold particles are located on extrasynaptic plasma membranes, but some are associated with intracellular membranes. Dopaminergic axons in the shell contain lower mean densities of both total DAT gold particles (per square micron) and plasmalemmal DAT gold particles (per micron) than those in the core. Within labeled axons in the NAc shell and core, however, there are no detectable differences in the subcellullar distribution of DAT or the percentage of total DAT gold particles that are located on plasma membranes. These studies are the first to examine and compare the subcellular localization of DAT in the NAc shell and core. As a result, they identify intrinsic, cell-specific differences in the expression of DAT within dopaminergic axons in these functionally distinct striatal compartments.

Expression of dopamine transporter and vesicular monoamine transporter 2 mRNAs in rat midbrain after repeated amphetamine administration

The dopamine transporter (DAT) in pre-synaptic membranes and the vesicular monoamine transporter 2 (VMAT2) in membranes of synaptic vesicles are involved in mediating the acute effects of amphetamine on dopamine transmission. Therefore, using a quantitative method of in situ hybridization and computerized image analysis, the expression of DAT and VMAT2 mRNAs was examined in rats treated for 5 days with amphetamine and killed 3 or 14 days after the last injection. We examined ventral tegmental area (VTA), substantia nigra (SN) and the transitional zone between VTA and SN. Each of these regions was further subdivided into rostral, intermediate and caudal portions. In control rats, autoradiographs revealed a gradient of both DAT and VMAT2 mRNA levels, decreasing gradually from rostral to caudal rat midbrain. After 3 days of withdrawal, a significant increase in DAT mRNA levels was found in rostral portions of VTA (117.9 + 5.8% of control group), SN (116.5 + 4.5%) and the transitional zone (119.6 + 5.6%) and in the intermediate portion of SN (113.5 + 4.3%). VMAT2 mRNA was significantly increased only in rostral and intermediate portions of the transitional zone (120.9 + 4.8 and 113.6 + 4.1%). After 14 days of withdrawal, there was a trend towards increased DAT mRNA levels in intermediate-caudal portions of midbrain, but a statistically significant increase was observed only in the intermediate portion of VTA (120.2 + 7.9%). No changes in VMAT2 mRNA levels were found. Thus, repeated amphetamine administration exerts modest and regionally selective effects on DAT and VMAT2 mRNA expression in subpopulations of midbrain dopamine neurons.