Selective 6OHDA-induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats

Sex differences in novelty- and psychostimulant-induced behaviors of C57BL/6 mice

RATIONALE

Women are more sensitive than men to psychostimulants and progress from initial use to drug addiction more quickly. The mouse has been an under-utilized model to study sex differences in psychostimulant action. Mice could serve as an ideal genetically tractable model for mechanistic studies into sex and hormone effects on psychostimulant behavior.

OBJECTIVES

The objective of this study was to characterize psychostimulant effects in male and female mice with a combination of automated data collection and behavioral observation.

METHODS

Male and female C57BL/6 mice (Charles River) were given a single dose or sequential ascending binge doses of D-amphetamine (AMPH) or cocaine (COC). Behavior was assessed in open field chambers using both automated photobeam interruptions and behavioral observations. Brain psychostimulant concentrations were determined at the time of maximum behavioral stimulation.

RESULTS

Psychostimulants induced behavioral activation in mice including both increased locomotion as detected with an automated system and a sequence of behaviors progressing from stereotyped sniffing at low doses to patterned locomotion and rearing at high doses. Females exhibited more patterned locomotion and a shift towards higher behavior scores after either psychostimulant despite having lower AMPH and equivalent COC brain levels as males.

CONCLUSIONS

Female C57BL/6 mice exhibit enhanced psychostimulant-induced behavior compared to males, similar to reports in rats. The combination of automated behavioral measures and behavioral observation was essential for verifying the existence of these differences. These results indicate the importance of testing both sexes when characterizing genetically manipulated mice to control for potential sex-specific effects.

In-vivo measurement of LDOPA uptake, dopamine reserve and turnover in the rat brain using [18F]FDOPA PET

Longitudinal measurements of dopamine (DA) uptake and turnover in transgenic rodents may be critical when developing disease-modifying therapies for Parkinson's disease (PD). We demonstrate methodology for such measurements using [(18)F]fluoro-3,4-dihydroxyphenyl-L-alanine ([(18)F]FDOPA) positron emission tomography (PET). The method was applied to 6-hydroxydopamine lesioned rats, providing the first PET-derived estimates of DA turnover for this species. Control (n=4) and unilaterally lesioned (n=11) rats were imaged multiple times. Kinetic modeling was performed using extended Patlak, incorporating a k(loss) term for metabolite washout, and modified Logan methods. Dopaminergic terminal loss was measured via [(11)C]-(+)-dihydrotetrabenazine (DTBZ) PET. Clear striatal [(18)F]FDOPA uptake was observed. In the lesioned striatum the effective DA turnover increased, shown by a reduced effective distribution volume ratio (EDVR) for [(18)F]FDOPA. Effective distribution volume ratio correlated (r>0.9) with the [(11)C]DTBZ binding potential (BP(ND)). The uptake and trapping rate (k(ref)) decreased after lesioning, but relatively less so than [(11)C]DTBZ BP(ND). For normal controls, striatal estimates were k(ref)=0.037±0.005 per minute, EDVR=1.07±0.22 and k(loss)=0.024±0.003 per minute (30 minutes turnover half-time), with repeatability (coefficient of variation) ≤11%. [(18)F]fluoro-3,4-dihydroxyphenyl-L-alanine PET enables measurements of DA turnover in the rat, which is useful for developing novel therapies for PD.

Elevated Expression of Serotonin 5-HT(2A) Receptors in the Rat Ventral Tegmental Area Enhances Vulnerability to the Behavioral Effects of Cocaine

The dopamine mesocorticoaccumbens pathway which originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens and prefrontal cortex is a circuit important in mediating the actions of psychostimulants. The function of this circuit is modulated by the actions of serotonin (5-HT) at 5-HT(2A) receptors (5-HT(2A)R) localized to the VTA. In the present study, we tested the hypothesis that virally mediated overexpression of 5-HT(2A)R in the VTA would increase cocaine-evoked locomotor activity in the absence of alterations in basal locomotor activity. A plasmid containing the gene for the 5-HT(2A)R linked to a synthetic marker peptide (Flag) was created and the construct was packaged in an adeno-associated virus vector (rAAV-5-HT(2A)R-Flag). This viral vector (2 μl; 10(9-10) transducing units/ml) was unilaterally infused into the VTA of male rats, while control animals received an intra-VTA infusion of Ringer's solution. Virus-pretreated rats exhibited normal spontaneous locomotor activity measured in a modified open-field apparatus at 7, 14, and 21 days following infusion. After an injection of cocaine (15 mg/kg, ip), both horizontal hyperactivity and rearing were significantly enhanced in virus-treated rats (p < 0.05). Immunohistochemical analysis confirmed expression of Flag and overexpression of the 5-HT(2A)R protein. These data indicate that the vulnerability of adult male rats to hyperactivity induced by cocaine is enhanced following increased levels of expression of the 5-HT(2A)R in the VTA and suggest that the 5-HT(2A)R receptor in the VTA plays a role in regulation of responsiveness to cocaine.

Repeated exposure to MDMA and amphetamine: sensitization, cross-sensitization, and response to dopamine D₁- and D₂-like agonists

RATIONALE

Acute exposure to (±) 3, 4-methylenedioxymethamphetamine (MDMA) produces hyperlocomotion that is preferentially expressed in the periphery of closed chambers. Following repeated administration, however, a sensitized hyperlocomotor response is preferentially expressed in the center of an activity box, so that the response resembles the more generalized activity that is produced by D-amphetamine (AMPH).

OBJECTIVES

The present study was designed to determine whether common neuroadaptations underlie the acute and sensitized responses to MDMA and AMPH.

METHODS

Rats were pretreated with five daily injections of MDMA (10.0 mg/kg), AMPH (2.0 mg/kg), or saline. Following a 2-day drug-free period, dose-response curves for hyperactivity produced by MDMA (2.5-10.0 mg/kg), AMPH (0.5-2.0 mg/kg), SKF-81297 (1.0-2.0 mg/kg), or quinpirole (0.25-1.0 mg/kg) were obtained.

RESULTS

Effects of MDMA and AMPH were increased by pretreatment with both drugs. The sensitized response following MDMA exposure was preferentially expressed in the center compartment, but, following AMPH pretreatment, the sensitized response was observed in both compartments. Cross-sensitization was unidirectional; AMPH pretreatment failed to sensitize to the effects of MDMA, but MDMA pretreatment sensitized to the effects of AMPH. MDMA and AMPH pretreatment produced marginal increases in the effects of SKF-81297. The response to quinpirole was, however, greater following MDMA, but not AMPH, pretreatment.

CONCLUSIONS

These data suggest that repeated MDMA exposure produces sensitization via a unique neurochemical effect.

A novel mechanism of cocaine to enhance dopamine d2-like receptor mediated neurochemical and behavioral effects. An in vivo and in vitro study

Recent in vitro results suggest that cocaine may exert direct and/or indirect allosteric enhancing actions at dopamine (DA) D(2) receptors (D(2)Rs). In the present paper we tested the hypothesis that cocaine in vivo can enhance the effects of the D(2)-likeR agonist quinpirole in rats by using microdialysis and pharmacological behavioral studies. Furthermore, in vitro D(2)-likeR binding characteristics and Gα(i/o)-protein coupling, in the absence and in the presence of cocaine, have been investigated in rat striatal membranes. Intra-nucleus accumbens perfusion of the D(2)-likeR agonist quinpirole (10 μM) reduced local dialysate glutamate levels, whereas cocaine (10 and 100 nM) was ineffective. At a low concentration (100 nM), cocaine significantly enhanced quinpirole-induced reduction of accumbal extracellular glutamate levels. The behavioral experiments showed that cocaine (0.625 mg/kg), but not the DA uptake blocker GBR 12783 (1.25 mg/kg), enhanced quinpirole (1 mg/kg)-induced hyperlocomotion. Finally, cocaine (100 nM), but not GBR 12783 (200 nM), produced a small, but significant increase in the efficacy of DA to stimulate binding of GTPγS to striatal D(2)-likeRs, whereas the D(2)-likeR binding characteristics were unchanged in striatal membranes by cocaine in the nM range. The significant increase in the maximal response to DA-stimulated GTPγS binding to D(2)-likeRs by 100 nM cocaine remained in the presence of GBR 12783. The observed cocaine-induced enhancement of the Gα(i/o)-protein coupling of D(2)Rs may be in part because of allosteric direct and/or indirect enhancing effects of cocaine at these receptors. These novel actions of cocaine may have relevance for understanding the actions of cocaine upon accumbal DA, and/or glutamate transmission and thus its rewarding as well as relapsing effects.

CyPPA, a Positive SK3/SK2 Modulator, Reduces Activity of Dopaminergic Neurons, Inhibits Dopamine Release, and Counteracts Hyperdopaminergic Behaviors Induced by Methylphenidate

Dopamine (DA) containing midbrain neurons play critical roles in several psychiatric and neurological diseases, including schizophrenia and attention deficit hyperactivity disorder, and the substantia nigra pars compacta neurons selectively degenerate in Parkinson’s disease. Pharmacological modulation of DA receptors and transporters are well established approaches for treatment of DA-related disorders. Direct modulation of the DA system by influencing the discharge pattern of these autonomously firing neurons has yet to be exploited as a potential therapeutic strategy. Small conductance Ca²⁺-activated K⁺ channels (SK channels), in particular the SK3 subtype, are important in the physiology of DA neurons, and agents modifying SK channel activity could potentially affect DA signaling and DA-related behaviors. Here we show that cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA), a subtype-selective positive modulator of SK channels (SK3 > SK2 > > > SK1, IK), decreased spontaneous firing rate, increased the duration of the apamin-sensitive afterhyperpolarization, and caused an activity-dependent inhibition of current-evoked action potentials in DA neurons from both mouse and rat midbrain slices. Using an immunocytochemically and pharmacologically validated DA release assay employing cultured DA neurons from rats, we show that CyPPA repressed DA release in a concentration-dependent manner with a maximal effect equal to the D2 receptor agonist quinpirole. In vivo studies revealed that systemic administration of CyPPA attenuated methylphenidate-induced hyperactivity and stereotypic behaviors in mice. Taken together, the data accentuate the important role played by SK3 channels in the physiology of DA neurons, and indicate that their facilitation by CyPPA profoundly influences physiological as well as pharmacologically induced hyperdopaminergic behavior.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

CM156, a high affinity sigma ligand, attenuates the stimulant and neurotoxic effects of methamphetamine in mice

Methamphetamine (METH) is a highly addictive psychostimulant drug of abuse. Low and high dose administration of METH leads to locomotor stimulation, and dopaminergic and serotonergic neurotoxicity, respectively. The behavioral stimulant and neurotoxic effects of METH can contribute to addiction and other neuropsychiatric disorders, thus necessitating the identification of potential pharmacotherapeutics against these effects produced by METH. METH binds to σ receptors at physiologically relevant concentrations. Also, σ receptors are present on and can modulate dopaminergic and serotonergic neurons. Therefore, σ receptors provide a viable target for the development of pharmacotherapeutics against the adverse effects of METH. In the present study, CM156, a σ receptor ligand with high affinity and selectivity for σ receptors over 80 other non-σ binding sites, was evaluated against METH-induced stimulant, hyperthermic, and neurotoxic effects. Pretreatment of male, Swiss Webster mice with CM156 dose dependently attenuated the locomotor stimulation, hyperthermia, striatal dopamine and serotonin depletions, and striatal dopamine and serotonin transporter reductions produced by METH, without significant effects of CM156 on its own. These results demonstrate the ability of a highly selective σ ligand to mitigate the effects of METH.

A mobility gradient in the organization of vertebrate movement: The perception of movement through symbolic language

Ordinary language can prevent us from seeing the organization of whole-animal movement. This may be why the search for behavioral homologies has not been as fruitful as the founders of ethology had hoped. The Eshkol-Wachman (EW) movement notational system can reveal shared movement patterns that are undetectable in the kinds of informal verbal descriptions of the same behaviors that are in current use. Rules of organization that are common to locomotor development, agonistic and exploratory behavior, scent marking, play, and dopaminergic drug-induced stereotypies in a variety of vertebrates suggest that behavior progresses along a “mobility gradient” from immobility to increasing complexity and unpredictability. A progression in the opposite direction, with decreasing spatial complexity and increased stereotypy, occurs under the influence of the nonselective dopaminergic drugs apomorphine and amphetamine and partly also the selective dopamine agonist quinpirole. The behaviors associated with the mobility gradient appear to be mediated by a family of basal ganglia-thalamocortical circuits and their descending output stations. Because the small number of rules underlying the mobility gradient account for a large variety of behaviors, they may be related to the specific functional demands on these neurological systems. The EW system and the mobility gradient model should prove useful to ethologists and neurobiologists.

Attenuated methamphetamine induced neurotoxicity by modafinil administration in mice

Methamphetamine (METH) is a highly addictive drug that might induce neurotoxicity. Clinical trials have reported that modafinil, a wake-promoting agent used to treat sleep disorders, may have some efficacy for the treatment of psychostimulant addiction. In this study we tested possible neuroprotective effects of modafinil after toxic METH administration in mice. We evaluated the effect of modafinil (two injections of either 90 or 180 mg/kg) and METH binge (3 × 7 mg/kg i.p. injections, 3-h apart) coadministration on DA striatal content, TH immunoreactivity in striatal areas and spontaneous locomotor activity. We also investigated acute locomotor activity and stereotypy profile in mice treated with a single METH dose (2 and 7 mg/kg) pretreated with modafinil (90 and 180 mg/kg). We found that mice treated with a METH binge showed a marked decrease in DA and dopaminergic metabolites as well as lower levels of TH immunoreactivity in the dorsal striatum. Pretreatment with modafinil (both 90 and 180 mg/kg) attenuated these effects but did not prevent METH induced decrease in locomotion. We also found that groups that received the combination of both modafinil and single dose METH showed a decrease in total distance traveled in an open field compared with METH groups. We observed an increment in the time mice expended doing stereotypic movements (continuous sniffing) in the group that received the combination of both METH and modafinil (i.e., decreasing locomotion). Our results suggest a possible protective role of modafinil against METH acute striatal toxicity.

Enhanced effects of amphetamine but reduced effects of the hallucinogen, 5-MeO-DMT, on locomotor activity in 5-HT(1A) receptor knockout mice: implications for schizophrenia

Serotonin-1A (5-HT(1A)) receptors may play a role in schizophrenia and the effects of certain antipsychotic drugs. However, the mechanism of interaction of 5-HT(1A) receptors with brain systems involved in schizophrenia, remains unclear. Here we show that 5-HT(1A) receptor knockout mice display enhanced locomotor hyperactivity to acute treatment with amphetamine, a widely used animal model of hyperdopaminergic mechanisms in psychosis. In contrast, the effect of MK-801 on locomotor activity, modeling NMDA receptor hypoactivity, was unchanged in the knockouts. The effect of the hallucinogen 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) was markedly reduced in 5-HT(1A) receptor knockout mice. There were no changes in apomorphine-induced disruption of PPI, a model of sensory gating deficits seen in schizophrenia. Similarly, there were no major changes in density of dopamine transporters (DAT) or dopamine D(1) or D(2) receptors which could explain the behavioural changes observed in 5-HT(1A) receptor knockout mice. These results extend our insight into the possible role of these receptors in aspects of schizophrenia. As also suggested by previous studies using agonist and antagonist drugs, 5-HT(1A) receptors may play an important role in hallucinations and to modulate dopaminergic activity in the brain.

Genetically expressed HIV-1 viral proteins attenuate nicotine-induced behavioral sensitization and alter mesocorticolimbic ERK and CREB signaling in rats

The prevalence of tobacco smoking in HIV-1 positive individuals is 3-fold greater than that in the HIV-1 negative population; however, whether HIV-1 viral proteins and nicotine together produce molecular changes in mesolimbic structures that mediate psychomotor behavior has not been studied. This study determined whether HIV-1 viral proteins changed nicotine-induced behavioral sensitization in HIV-1 transgenic (HIV-1Tg) rats. Further, we examined cAMP response element binding protein (CREB) and extracellular regulated kinase (ERK1/2) signaling in the prefrontal cortex (PFC), nucleus accumbens (NAc) and ventral tegmental area (VTA). HIV-1Tg rats exhibited a transient decrease of activity during habituation, but showed attenuated nicotine (0.35mg/kg, s.c.)-induced behavioral sensitization compared to Fisher 344 (F344) rats. The basal levels of phosphorylated CREB and ERK2 were lower in the PFC of HIV-1Tg rats, but not in the NAc and VTA, relative to the controls. In the nicotine-treated groups, the levels of phosphorylated CREB and ERK2 in the PFC were increased in HIV-1Tg rats, but decreased in F344 animals. Moreover, repeated nicotine administration reduced phosphorylated ERK2 in the VTA of HIV-1Tg rats and in the NAc of F344 rats, but had no effect on phosphorylated CREB, indicating a region-specific change of intracellular signaling. These results demonstrate that HIV-1 viral proteins produce differences in basal and nicotine-induced alterations in CREB and ERK signaling that may contribute to the alteration in psychomotor sensitization. Thus, HIV-1 positive smokers are possibly more vulnerable to alterations in CREB and ERK signaling and this has implications for motivated behavior, including tobacco smoking, in HIV-1 positive individuals who self-administer nicotine.

Blockade of mGLUR5 receptors differentially alters amphetamine-induced enhancement of locomotor activity and of brain stimulation reward

This study was aimed at determining the role of mGLUR5 glutamate receptors on amphetamine-induced enhancement of locomotion and of brain stimulation reward (BSR). The effect of different doses of the mGLUR5 antagonist, MPEP (0, 1, 3 and 9 mg/kg, i.p.), was assessed on reward induced by electrical stimulation of the lateral hypothalamus, and on the enhancement of reward by amphetamine (1 mg/kg, i.p.) in adult male Long Evans rats. The effect of a single dose of MPEP (0 and 9 mg/kg) on amphetamine-induced increase in locomotor activity was also assessed. Systemic injection of MPEP alone did not alter reward threshold and maximum rate of responding. Amphetamine produced a 25-30% decrease in reward threshold, an effect not altered by the highest dose of MPEP. At this dose, MPEP produced a weak inhibition of spontaneous locomotion and a significant attenuation of the enhanced locomotor activity induced by amphetamine. These findings show that mGLUR5 glutamate receptors are unlikely to constitute important elements of the reward-relevant pathway, and do not intervene in the enhancement effect of amphetamine. They also show, however, that these glutamate receptors play a key role in amphetamine-induced increased locomotor activity, providing additional evidence for a dissociation between the substrates that mediate these two behaviours.

Drug-sensitive reward in crayfish: an invertebrate model system for the study of SEEKING, reward, addiction, and withdrawal

In mammals, rewarding properties of drugs depend on their capacity to activate appetitive motivational states. With the underlying mechanisms strongly conserved in evolution, invertebrates have recently emerged as a powerful new model in addiction research. In crayfish natural reward has proven surprisingly sensitive to human drugs of abuse, opening an unlikely avenue of research into the basic biological mechanisms of drug addiction. In a series of studies we first examined the presence of natural reward systems in crayfish, then characterized its sensitivity to a wide range of human drugs of abuse. A conditioned place preference (CPP) paradigm was used to demonstrate that crayfish seek out those environments that had previously been paired with the psychostimulants cocaine and amphetamine, and the opioid morphine. The administration of amphetamine exerted its effects at a number of sites, including the stimulation of circuits for active exploratory behaviors (i.e., SEEKING). A further study examined morphine-induced reward, extinction and reinstatement in crayfish. Repeated intra-circulatory infusions of morphine served as a reward when paired with distinct visual or tactile cues. Morphine-induced CPP was extinguished after repeated saline injections. Following this extinction phase, morphine-experienced crayfish were once again challenged with the drug. The priming injections of morphine reinstated CPP at all tested doses, suggesting that morphine-induced CPP is unrelenting. In an exploration of drug-associated behavioral sensitization in crayfish we concurrently mapped measures of locomotion and rewarding properties of morphine. Single and repeated intra-circulatory infusions of morphine resulted in persistent locomotory sensitization, even 5 days following the infusion. Moreover, a single dose of morphine was sufficient to induce long-term behavioral sensitization. CPP for morphine and context-dependent cues could not be disrupted over a drug free period of 5 days. This work demonstrates that crayfish offer a comparative and complementary approach in addiction research. Serving as an invertebrate animal model for the exposure to mammalian drugs of abuse, modularly organized and experimentally accessible nervous systems render crayfish uniquely suited for studying (1) the basic biological mechanisms of drug effects, (2) to explore how the appetitive/seeking disposition is implemented in a simple neural system, and (3) how such a disposition is related to the rewarding action of drugs of abuse. This work aimed to contribute an evolutionary, comparative context to our understanding of a key component in learning, and of natural reward as an important life-sustaining process.

Nucleus accumbens lesions modulate the effects of methylphenidate

The psychostimulant methylphenidate (MPD, Ritalin) is the prescribed drug of choice for treatment of ADHD. In recent years, the diagnosis rate of ADHD has increased dramatically, as have the number of MPD prescriptions. Repeated exposure to psychostimulants produces behavioral sensitization in rats, an experimental indicator of a drug's potential liability. In studies on cocaine and amphetamine, this effect has been reported to involve the nucleus accumbens (NAc), one of the nuclei belonging to the motive circuit. The aim of this study was to investigate the role of the NAc on the expression of behavioral sensitization as a response to MPD exposure. In the present study, 20 male Sprague-Dawley rats were divided randomly into three groups: an intact control group, a sham-operated group, and a NAc bilateral electrical lesion group. Locomotor activity was assessed for the first 2h following 2.5mg/kg MPD injection, using open field monitoring systems. Recordings were made during 6 days of continuous MPD administration, and then upon re-challenge with the same dose following 3 days of washout. Acute MPD exposure elicited an increase in locomotor activity in all three groups. However, the NAc lesion group exhibited significantly increased locomotor activity in comparison to sham and control groups. Chronic MPD did not elicit sensitization in the NAc lesion group, while both sham and control groups did exhibit behavioral sensitization to repetitive MPD administration. These findings suggest that the NAc plays a significant role in eliciting locomotor activity as an acute effect of MPD, and in the expression of sensitization due to chronic MPD exposure.

Modeling the positive symptoms of schizophrenia in genetically modified mice: pharmacology and methodology aspects

In recent years, there have been huge advances in the use of genetically modified mice to study pathophysiological mechanisms involved in schizophrenia. This has allowed rapid progress in our understanding of the role of several proposed gene mechanisms in schizophrenia, and yet this research has also revealed how much still remains unresolved. Behavioral studies in genetically modified mice are reviewed with special emphasis on modeling psychotic-like behavior. I will particularly focus on observations on locomotor hyperactivity and disruptions of prepulse inhibition (PPI). Recommendations are included to address pharmacological and methodological aspects in future studies. Mouse models of dopaminergic and glutamatergic dysfunction are then discussed, reflecting the most important and widely studied neurotransmitter systems in schizophrenia. Subsequently, psychosis-like behavior in mice with modifications in the most widely studied schizophrenia susceptibility genes is reviewed. Taken together, the available studies reveal a wealth of available data which have already provided crucial new insight and mechanistic clues which could lead to new treatments or even prevention strategies for schizophrenia.

Dopamine, schizophrenia, mania, and depression: Toward a unified hypothesis of cortico-striatopallido-thalamic function

Considerable evidence from preclinical and clinical investigations implicates disturbances of brain dopamine (DA) function in the pathophysiology of several psychiatric and neurologic disorders. We describe a neural model that may help organize theseindependent experimental observations. Cortical regions classically associated with the limbic system interact with infracortical structures, including the nucleus accumbens, ventral pallidum, and dorsomedial nucleus of the thalamus. In our model, overactivity in forebrain DA systems results in the loss of lateral inhibitory interactions in the nucleus accumbens, causing disinhibition of pallidothalamic efferents; this in turn causes rapid changes and a loss of focused corticothalamic activity in cortical regions controlling cognitive and emotional processes. These effects might be manifested clinically by some symptoms of psychoses. Underactivity of forebrain DA results in excess lateral inhibition in the nucleus accumbens, causing tonic inhibition of pallidothalamic efferents; this perpetuates tonic corticothalamic activity and prevents the initiation of new activity in other critical cortical regions. These effects might be manifested clinically by some symptoms of depression. This model parallels existing explanations for the etiology of several movement disorders, and may lead to testable inferences regarding the neural substrates of specific psychopathologies.

Behavioural phenotype of APPC100.V717F transgenic mice over-expressing a mutant Abeta-bearing fragment is associated with reduced NMDA receptor density

The aim of this study was to characterize APPC100.V717F transgenic (TgC100.V717F) mice which over-express a mutant C100 fragment of the amyloid precursor protein. The mice were compared to TgC100 wild type mice (TgC100.WT) and non-transgenic controls at 4-9 and 16-22 months of age. TgC100.V717F mice showed behavioural hyperactivity, particularly at a younger age, as shown by increased numbers of elevated plus maze arm entries and Y-maze arm entries, enhanced baseline locomotor activity in the open field, and enhanced amphetamine-induced hyperlocomotion. This hyperactivity was less pronounced in TgC100.WT which only displayed significant differences to non-transgenic controls at a younger age for the number of Y-maze arm entries and baseline locomotor activity in the open field. In addition, TgC100.V717F mice, but not TgC100.WT, demonstrated cognitive deficits, as shown by reduced spontaneous alternation in the Y-maze and markedly reduced retention in a passive avoidance test. At an older age, TgC100.V717F mice showed enhanced startle and increased immobility time in the forced swim test. In the TgC100.V717F mice, but not TgC100.WT, the behavioural changes were paralleled by a significant reduction in the expression of hippocampal NMDA receptor subunits types 1 and 2A. Concomitantly, we detected axonal disruption and apoptosis in the hippocampus of TgC100.V717F mice. In conclusion, these data demonstrate that the mutant C100 fragment is an effector of biochemical and both cognitive and non-cognitive behaviours. These transgenic mice may be a model for the psychotic features associated with early Alzheimer's disease.

Neurocircuitry of addiction

Drug addiction is a chronically relapsing disorder that has been characterized by (1) compulsion to seek and take the drug, (2) loss of control in limiting intake, and (3) emergence of a negative emotional state (eg, dysphoria, anxiety, irritability) reflecting a motivational withdrawal syndrome when access to the drug is prevented. Drug addiction has been conceptualized as a disorder that involves elements of both impulsivity and compulsivity that yield a composite addiction cycle composed of three stages: 'binge/intoxication', 'withdrawal/negative affect', and 'preoccupation/anticipation' (craving). Animal and human imaging studies have revealed discrete circuits that mediate the three stages of the addiction cycle with key elements of the ventral tegmental area and ventral striatum as a focal point for the binge/intoxication stage, a key role for the extended amygdala in the withdrawal/negative affect stage, and a key role in the preoccupation/anticipation stage for a widely distributed network involving the orbitofrontal cortex-dorsal striatum, prefrontal cortex, basolateral amygdala, hippocampus, and insula involved in craving and the cingulate gyrus, dorsolateral prefrontal, and inferior frontal cortices in disrupted inhibitory control. The transition to addiction involves neuroplasticity in all of these structures that may begin with changes in the mesolimbic dopamine system and a cascade of neuroadaptations from the ventral striatum to dorsal striatum and orbitofrontal cortex and eventually dysregulation of the prefrontal cortex, cingulate gyrus, and extended amygdala. The delineation of the neurocircuitry of the evolving stages of the addiction syndrome forms a heuristic basis for the search for the molecular, genetic, and neuropharmacological neuroadaptations that are key to vulnerability for developing and maintaining addiction.

Environmental novelty and illumination modify ethanol-induced open-field behavioral effects in mice

Both spontaneous and drug-induced animal behaviors can be modified by exposure to novel stimuli or different levels of environmental illumination. However, research into how these factors specifically impact ethanol (ETH)-induced behavioral effects is currently lacking. We aimed to investigate the effects of these two factors, considered separately or in conjunction, on ETH-induced acute hyperlocomotor effect and its sensitization in adult male Swiss mice. Mice were placed in a novel or familiar open-field under normal light (200 lx) or low light (9 lx) immediately after receiving an ip injection of either 1.8 g/kg ETH or saline (SAL). After 7 days, all animals received an ip challenge injection of 1.8 g/kg ETH, and were placed in the open-field under the same light conditions described above. Novelty increased central locomotion and decreased grooming, while low light increased grooming. Acute ETH administration increased both total and peripheral locomotion and these effects were potentiated by low light. Both low light and novelty were able to facilitate ETH-induced locomotor sensitization, which was detected by the central locomotion parameter. However, there was no synergism between the effects of these two modulating factors on ETH-induced behavioral sensitization. We conclude that both the acute behavioral effects of ETH and behavioral sensitization induced by previous administration of this drug can be critically modified by environmental factors. In addition, our study stresses the importance of using different behavioral parameters to evaluate the interaction between environmental factors and ETH effects.

Behavioral, thermal and neurochemical effects of acute and chronic 3,4-methylenedioxymethamphetamine ("Ecstasy") self-administration

3,4-Methylenedioxymethamphetamine (MDMA) is a popular methamphetamine derivative associated with young adults and all-night dance parties. However, the enduring effects of MDMA at voluntary intake levels have not been extensively investigated. In this study, MDMA-influenced behaviors and core temperatures were assessed over the course of 20 daily MDMA self-administration sessions in rats. In vivo microdialysis techniques were used in a subsequent MDMA challenge test session to determine extracellular nucleus accumbens dopamine (NAcc DA) and serotonin (5-HT) levels in MDMA-experienced and naïve animals before and after a self-administered MDMA injection (3.0mg/kg, i.v.). During self-administration sessions, gradual and significant increases in MDMA intake and MDMA-stimulated locomotor activity were observed across sessions. Core temperature significantly decreased during initial MDMA sessions, but was unaltered by the last 10 sessions. In the MDMA challenge test, MDMA-naïve rats showed significantly higher NAcc 5-HT responses compared to MDMA-experienced rats, though MDMA experience did not affect the magnitude of NAcc DA response. The overall findings suggest that changes in MDMA-induced responses over the course of increasing levels of drug exposure may reflect the development of tolerance to a number of MDMA effects.

Self-administration of cocaine and nicotine mixtures by rhesus monkeys

RATIONALE

The concurrent use of cocaine and nicotine is associated with increases in their relative rates of intake. While this increase could be due to a high reinforcing effect of the drug combination, higher rates of intake could also be explained by a decrease in the drugs' relative reinforcing effects.

OBJECTIVES

To determine if nicotine could modulate cocaine's reinforcing effects, the current study compared the reinforcing potency and strength of cocaine to cocaine mixed with various concentrations of nicotine.

METHOD

Five rhesus monkeys were allowed to self-administer cocaine (25-400 microg/kg/inj), nicotine (12-50 microg/kg/inj), or combinations of the two under a progressive ratio schedule of reinforcement.

RESULTS

Nicotine alone did not function as a reinforcer. Cocaine injections increased in a dose-dependent manner when taken alone and when taken as a mixture with nicotine. Furthermore, adding nicotine to cocaine shifted the cocaine dose-response function to the left in four of the five monkeys. Analysis of the ED(50) values for cocaine and the mixtures indicated that some mixtures of cocaine and nicotine were more potent than cocaine alone. There were no differences in maximum injections between cocaine or any of the mixtures of cocaine and nicotine.

CONCLUSION

These results suggest that nicotine, under certain conditions, can increase cocaine's potency as a reinforcer without affecting its maximum reinforcing strength.

Serotonergic lesions of the dorsal hippocampus differentially modulate locomotor hyperactivity induced by drugs of abuse in rats: implications for schizophrenia

RATIONALE

Psychotomimetic drug-induced locomotor hyperactivity is a widely used animal model of psychotic states, such as in schizophrenia. We previously found that serotonergic lesions of the dorsal, but not ventral, hippocampus in rats result in enhanced phencyclidine-induced locomotor hyperactivity.

OBJECTIVES

The objective of this study was to investigate the effect of serotonin depletion in the dorsal and ventral hippocampus on hyperlocomotion induced by ketamine, cocaine, 3,4-methylenedioxymethampethamine (MDMA), methamphetamine, and D: -amphetamine.

MATERIALS AND METHODS

Male Sprague-Dawley rats were bilaterally microinjected with vehicle or the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the dorsal or ventral hippocampus using a stereotaxic approach. Separate cohorts of rats were used for each drug of abuse; each rat received saline and a low, medium, and high dose of the drug in a random-sequence, repeated-measures protocol. Locomotor hyperactivity following treatment was measured using automated photocell cages.

RESULTS

Similar to phencyclidine, 5,7-DHT-induced lesions of the dorsal hippocampus enhanced ketamine-induced hyperlocomotion at all doses. They also reduced methamphetamine-induced hyperlocomotion at the high dose only and caused a minor, biphasic modulation of responses to cocaine. Locomotor responses to D: -amphetamine and MDMA were unchanged by lesions of the dorsal hippocampus. Serotonergic lesions of the ventral hippocampus did not significantly alter locomotor hyperactivity induced by any of the drugs investigated.

CONCLUSIONS

These findings further implicate a role for serotonin in the dorsal hippocampus in modulating the behavioral effects of dissociative anesthetics, such as ketamine, with more subtle effects on psychostimulant drugs of abuse. The dorsal hippocampus may be a site of serotonergic dysfunction in aspects of schizophrenia.

Ketamine-induced deficit of auditory gating in the hippocampus of rats is alleviated by medial septal inactivation and antipsychotic drugs

RATIONALE

Gating of sensory responses is impaired in schizophrenic patients and animal models of schizophrenia. Ketamine, an N-methyl-D-aspartate receptor antagonist, is known to induce schizophrenic-like symptoms including sensory gating deficits in humans.

OBJECTIVE

This study aims to investigate the mechanisms underlying ketamine's effect on gating of auditory evoked potentials in the hippocampus of freely moving rats.

METHODS

Gating was measured by the ratio of the test-click response (T) to the conditioning-click response (C), or T/C, with T and C measured as peak amplitudes.

RESULTS

Ketamine (1, 3, or 6 mg/kg s.c.) injection dose-dependently increased T/C ratio as compared to saline injection (s.c.). T/C ratio was 0.48 +/- 0.05 after saline injection and 0.73 +/- 0.17 after ketamine (3 mg/kg s.c.) injection. The increase in T/C ratio after ketamine was blocked by prior inactivation of the medial septum with GABA(A) receptor agonist muscimol or by systemic administration of antipsychotic drugs, including chlorpromazine (5 mg/kg i.p.), haloperidol (1 mg/kg i.p.), or clozapine (7.5 mg/kg i.p.). Infusion of muscimol into the medial septum or injection of an antipsychotic drug alone did not affect the T/C ratio. However, rats with selective lesion of the septohippocampal cholinergic neurons by 192 IgG-saporin showed a significantly higher T/C ratio (0.86 +/- 0.10) than sham lesion rats (0.26 +/- 0.07), and ketamine did not further increase T/C ratio in rats with septohippocampal cholinergic neuron lesion.

CONCLUSIONS

Ketamine's disruption of hippocampal auditory gating was normalized by inactivation of the medial septum; in addition, septal cholinergic neurons participate in normal auditory gating.

Human methamphetamine pharmacokinetics simulated in the rat: behavioral and neurochemical effects of a 72-h binge

Bingeing is one pattern of high-dose methamphetamine (METH) abuse, which involves continuous drug taking over several days and can result in psychotic behaviors for which the brain pathology remains poorly defined. A corresponding animal model of this type of METH exposure may provide novel insights into the neurochemical and behavioral sequelae associated with this condition. Accordingly, to simulate the pharmacokinetic profile of a human METH binge exposure in rats, we used a computer-controlled, intravenous METH procedure (dynamic infusion, DI) to overcome species differences in METH pharmacokinetics and to replicate the human 12-h plasma METH half-life. Animals were treated over 13 weeks with escalating METH doses, using DI, and then exposed to a binge in which drug was administered every 3 h for 72 h. Throughout the binge, behavioral effects included unabated intense oral stereotypies in the absence of locomotion and in the absence of sleep. Decrements in regional brain dopamine, norepinephrine, and serotonin levels, measured at 1 and 10 h after the last injection of the binge, had, with the exception of caudate-putamen dopamine and frontal cortex serotonin, recovered by 48 h. At 10 h after the last injection of the binge, [(3)H]ligand binding to dopamine and vesicular monoamine transporters in caudate-putamen were reduced by 35 and 13%, respectively. In a separate METH binge-treated cohort, post-binge behavioral alterations were apparent in an attenuated locomotor response to a METH challenge infusion at 24 h after the last injection of the binge. Collectively, the changes we characterized during and after a METH binge suggest that for human beings under similar exposure conditions, multiple time-dependent neurochemical deficits contribute to their behavioral profiles.