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Kappa‐opioid receptors (KOR) control dopamine (DA) levels in the striatum and contribute significantly to the progression of drug addiction. Repeated exposure to psychostimulants has been associated with up‐regulated KOR activity and increased DA levels in dorsal striatum. However, it has not been tested if both processes are linked. In this work, we studied if a mechanism mediated by KOR is contributing to the increase in DA levels in the dorsolateral striatum (DLS) after amphetamine (AMPH) sensitization. The AMPH sensitization was assessed after single or repeated once‐a‐day AMPH injections (1 mg/kg). Only repeated AMPH exposure produced a significant locomotor sensitization. No‐net flux microdialysis was used to assess basal DA dialysate, DA extracellular concentration (Cext), and DA uptake in DLS of anesthetized rats. The role of KOR on DA dynamics in DLS was evaluated by local perfusion (250 μM) and systemic administration (10 mg/kg) of the KOR antagonist nor‐binaltorphimine. A significant decrease in DA Cext is observed in the DLS after an AMPH challenge in rats exposed to a single dose of AMPH. The decrease in DA Cext was associated with both a decreased basal DA dialysate and an increased DA uptake. Conversely, the expression of AMPH sensitization was accompanied by a significant increase in DA Cext associated with an increased basal DA dialysate and an attenuation in DA uptake. Both local and systemic administration of nor‐binaltorphimine reversed changes in DLS after AMPH pre‐treatment. These findings indicate that endogenous KOR system tunes DLS DA dynamics during the progression to AMPH sensitization. We proposed that dynorphin contributes to dopamine (DA) dynamics during progression to amphetamine (AMPH) sensitization modifying DA transporter activity. A significant decrease in DA extracellular levels is observed in the dorsolateral striatum (DLS) after a single dose of AMPH. The decrease in DA extracellular is associated with an increased DA uptake. Conversely, the expression of AMPH sensitization is accompanied by a significant increase in DA extracellular levels associated with attenuation in DA uptake. The administration of nor‐BNI reverses the changes in DLS after AMPH pre‐treatment tending to normalize DA extracellular levels modifying DA uptake activity. [ABSTRACT FROM AUTHOR]

Rationale: Amphetamine enhances dopamine (DA) transmission and induces psychotic states or exacerbates psychosis in at-risk individuals. Amphetamine sensitization of the DA system has been proposed as a rodent model of schizophrenia-like symptoms. In humans, excessive nonphysiologic drinking or primary polydipsia is significantly associated with a diagnosis of schizophrenia. In rodents, nonphysiologic drinking can be induced by intermittent presentation of food in the presence of a drinking spout to a hungry animal; this phenomenon is termed, 'schedule-induced polydipsia' (SIP). Objective: This study aims to determine the effects of amphetamine sensitization on SIP. Methods: We injected rats with amphetamine (1.5 mg/kg) daily for 5 days. Following 4 weeks of withdrawal, animals were food restricted and exposed to the SIP protocol (noncontingent fixed-time 1-min food schedule) for daily 2-h sessions for 24 days. Results: Results showed that previously amphetamine-injected animals drank more in the SIP protocol and drank more than controls when the intermittent food presentation schedule was removed. Conclusions: These findings suggest that hyperdopaminergia associated with schizophrenia may contribute to the development of polydipsia in this population. Whether animals that develop SIP have DA dysfunction or aberrant activity of other circuits that modulate DA activity has yet to be clearly defined. [ABSTRACT FROM AUTHOR]

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

The neural substrates of the unconditioned and conditioned components of amphetamine (AMPH)-induced behavioral sensitization remain unknown. The present study examines the brain activation of rats in response to an AMPH challenge with augmented locomotion in groups receiving chronic AMPH under chloral hydrate anesthetization (i.e., the 'zero context') or when tested in the 'same context' as a chronic treatment, or when tested in a 'different context'. The neural activations of the three groups reveal fairly consistent patterns: (a) The substantia nigra is activated in the same context condition and the pure AMPH effect (i.e., the zero context with the unconditioned component), but not in the switch context condition. (b) The ventral pallidum showed Fos expression in the switch context and the same context, but not in the zero context condition. (c) The other nuclei, including the medial prefrontal cortex, nucleus accumbens, caudate putamen, medial thalamus, hippocampus, amygdala, and ventral tegmental area, are activated in all contextual conditions and the pure AMPH effect (the zero context). The context exerts definable effects on the mesocorticolimbic dopamine system on AMPH-induced behavioral sensitization. (d) The ventral pallidum and the substantia nigra activations dissociate the unconditioned component from the conditioned component in behavioral sensitization. Further studies are needed to determine how these two nuclei mediate the effect in terms of primary and conditioned rewards. Copyright © 2012 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Aberrant dopamine-mediated behaviours are a hallmark of a number of psychiatric disorders, including substance use disorders and schizophrenia. It has been demonstrated recently that rodent models of these diseases display enhanced dopamine neuron activity throughout the ventral tegmental area (VTA). It is known, however, that the VTA is not a homogeneous structure, and that the dopamine neuron population provides discrete, topographical innervation of nucleus accumbens subregions. In addition, these ventromedial and ventrolateral dopamine systems are known to subserve complementary but distinct aspects of goal-directed behaviour. Using in-vivo extracellular recordings of identified dopamine neurons in chloral hydrate-anaesthetized rats, we examined the level of dopamine neuron population activity across the mediolateral extent of the VTA following amphetamine sensitization or gestational methylazoxymethanol acetate (MAM) treatment, a verified rodent model of schizophrenia. Here we demonstrate that both models display an augmented medial VTA-ventromedial striatal dopamine system function that correlates with the augmented locomotor response to amphetamine observed in both models. In contrast, only MAM-treated rats exhibit an increase in VTA-ventrolateral striatal dopamine system function. This latter finding is consistent with human imaging studies in schizophrenia patients. In summary, we demonstrate that, although a number of disorders involving a hyperdopaminergic state demonstrate an increase in dopamine neuron population activity, there is divergence in the exact populations of neurons affected. This distinction probably underlies the observed differences in disease symptomatology. [ABSTRACT FROM AUTHOR]

Rationale: Psychosis susceptibility is mediated in part by the dopaminergic neurotransmitter system. In humans, individual differences in vulnerability for psychosis are reflected in differential sensitivity for psychostimulants such as amphetamine. We hypothesize that the same genes and pathways underlying behavioral sensitization in mice are also involved in the vulnerability to psychosis. Objectives: The aim of the current study was to investigate which genes and pathways may contribute to behavioral sensitization in different dopaminergic output areas in the mouse brain. Methods: We took advantage of the naturally occurring difference in psychostimulant sensitivity in DBA/2 mice and selected animals displaying extremes in behavioral sensitization to amphetamine. Subsequently, the dopamine output areas, prefrontal cortex, nucleus accumbens, and cornu ammonis 1 (CA1) area of the hippocampus, were isolated by laser microdissection and subjected to DNA microarray analysis 1 h after a challenge dose of amphetamine. Results: A large number of genes with differential expression between high and low responders were identified, with no overlap between brain regions. Validation of these gene expression changes with real-time quantitative polymerase chain reaction demonstrated that the most robust and reproducible effects on gene expression were in the CA1 region of the hippocampus. Interestingly, many of the validated genes in CA1 are members of the cAMP response element (CRE) family and targets of the glucocorticoid receptor (GR) and myocyte enhancer factor 2 (Mef2) transcription factors. Conclusion: We hypothesize that CRE, Mef2, and GR signaling form a transcription regulating network, which underlies differential amphetamine sensitivity, and therefore, may play an important role in susceptibility to psychosis. [ABSTRACT FROM AUTHOR]

Amphetamine (AMPH) sensitization in the nonhuman primate induces persistent aberrant behaviors reminiscent of the hallmark symptoms of schizophrenia, including hallucinatory-like behaviors, psychomotor depression, and profound cognitive impairment. The present study examined whether AMPH sensitization induces similarly long-lasting morphologic alterations in prefrontal cortical pyramidal neurons. Three to 3½ years postsensitization, sensitized, and AMPH-naïve control monkeys were killed. Blocks of prefrontal cortex were Golgi-impregnated for elucidation of pyramidal dendritic morphology in layers II/superficial III (II/IIIs), deep III, and V/VI. In AMPH-sensitized animals as compared to AMPH-naïve controls, pyramidal dendrites in layer II/IIIs exhibited reduced overall dendritic branching and reduced peak spine density (22%) on the apical trunk. Across all layers, the distance from soma to peak spine density along the apical trunk was decreased (126.38±7.65 μm in AMPH-sensitized compared to 162.98±7.26 μm in AMPH-naïve controls), and basilar dendritic length was reduced (32%). These findings indicate that chronic dopamine dysregulation, consequent to AMPH sensitization, results in enduring, atrophic changes in prefrontal pyramidal dendrites that resemble the pathologic alterations described in patients with schizophrenia and may contribute to the persistence of schizophrenia-like behavioral changes and cognitive dysfunction associated with sensitization. These findings may also provide key insights into the etiologic origin of the pronounced behavioral disturbances and cognitive dysfunction associated with schizophrenia.Neuropsychopharmacology (2007) 32, 919–931. doi:10.1038/sj.npp.1301179; published online 16 August 2006 [ABSTRACT FROM AUTHOR]

Neurons in ventral pallidum fire to reward and its predictive cues. We tested mesolimbic activation effects on neural reward coding. Rats learned that a Pavlovian conditioned stimulus (CS+1 tone) predicted a second conditioned stimulus (CS+2 feeder click) followed by an unconditioned stimulus (UCS sucrose reward). Some rats were sensitized to amphetamine after training. Electrophysiological activity of ventral pallidal neurons to stimuli was later recorded under the influence of vehicle or acute amphetamine injection. Both sensitization and acute amphetamine increased ventral pallidum firing at CS+2 (population code and rate code). There were no changes at CS+1 and minimal changes to UCS. With a new ‘ Profile Analysis’, we show that mesolimbic activation by sensitization/amphetamine incrementally shifted neuronal firing profiles away from prediction signal coding (maximal at CS+1) and toward incentive coding (maximal at CS+2), without changing hedonic impact coding (maximal at UCS). This pattern suggests mesolimbic activation specifically amplifies a motivational transform of CS+ predictive information into incentive salience coded by ventral pallidal neurons. Our results support incentive-sensitization predictions and suggest why cues temporally proximal to drug presentation may precipitate cue-triggered relapse in human addicts. [ABSTRACT FROM AUTHOR]

Despite the beneficial effects of prototypical glutamatergic receptor antagonists in animal models, the pharmacological attempts by the use of such agents have met with very limited clinical success because these compounds produce adverse side effects and possess an intrinsic neurotoxicity at neuroprotective and therapeutic concentrations. Interestingly, nitrous oxide and xenon, which are anesthetic gases with a remarkably safe clinical profile, have been shown to be effective inhibitors of the NMDA receptor. We briefly review accumulating evidence that nitrous oxide and xenon at subanesthetic concentrations may have potentially neuroprotective and therapeutic properties, with a particular focus on their beneficial effects on ischemia-induced neuronal death and amphetamine-induced sensitization. Nitrous oxide at 75-vol% and xenon up to 70-vol% reduce ischemia-induced neuronal death induced by occlusion of the middle cerebral artery in rodents, and decrease NMDA-induced Ca2+ influx in neuronal cell cultures, a critical event involved in excitotoxicity. Nitrous oxide at 75-vol% and xenon at 50-vol% further reduced amphetamine-induced loco- motor sensitization in rodents. However, at a higher concentration of 75-vol%, xenon shows potentially neurotoxic properties and adverse side effects. Because both agents are rapidly eliminated from the body, it is plausible that their administration at appropriate subanesthetic neuroprotective and therapeutic concentrations may not be associated, in contrast with prototypical NMDA receptor antagonists, with adverse side effects and potentially neurotoxicity. Finally, the possible therapeutic implications in humans are discussed. [ABSTRACT FROM AUTHOR]

Circling behavior was measured in female rats treated with amphetamine (1.25 mg/kg d-amphetamine sulfate, IP) on 3 consecutive days. On days 4 and 5 saline was substituted for amphetamine. While the amphetamine-induced response was attenuated by both a D-1 (SCH 23390) and a D-2 (metoclopramide) antagonist, sensitization to amphetamine was attenuated by the D-1 antagonist only. In addition, expression of a placebo-like response observed on day 4 (characterized previously as a Pavlovian conditioned response) was attenuated by both the D-1 and D-2 antagonists without disrupting extinction of the response. [ABSTRACT FROM AUTHOR]