Amphetamine locomotor sensitization is accompanied with an enhanced high K⁺-stimulated Dopamine release in the rat medial prefrontal cortex

Susceptibility to express amphetamine locomotor sensitization correlates with dorsolateral striatum bursting activity and GABAergic synapses in the globus pallidus

Repeated psychostimulant administration results in behavioral sensitization, a process that is relevant in the early phases of drug addiction. Critically, behavioral sensitization is not observed in all subjects. Evidence shows that differential neuronal activity in the dorsolateral striatum (DLS) accompanies the expression of amphetamine (AMPH) locomotor sensitization. However, whether individual differences in DLS activity previous to AMPH administration can predict the expression of locomotor sensitization has not been assessed. Here, we examined DLS neuronal activity before and after repeated AMPH administration and related it to the susceptibility of rats to sensitize. For that, single-unit recordings on DLS medium spiny neurons (MSNs) were carried out in freely moving male Sprague Dawley rats during repeated AMPH administration. We also examined differences in neurostructure that could accompany sensitization. We quantified the density of the inhibitory postsynaptic marker gephyrin (Geph) in the entopeduncular nucleus (EP) and globus pallidus (GP). A higher burst firing and a lower percentage of correlation between MSNs post-Saline firing rate vs. locomotion predicted the expression of locomotor sensitization. Moreover, during the AMPH challenge, we observed that burst firing decreased in sensitized rats, in contrast to non-sensitized rats in which burst firing was maintained. Finally, a higher Geph density on GP but not EP was observed in non-sensitized rats after AMPH challenge. These results indicate that initial differences in DLS burst firing might underlie the susceptibility to express locomotor sensitization and suggest that the potentiation of dorsal striatum indirect pathway could be considered a protective mechanism to locomotor sensitization.

Electrical Stimulation Evokes Rotational Behavior In Tandem with Exocytotic-like Increases in Dopamine Measured by In Vivo Intracerebral Microdialysis

BACKGROUND

Electrical Stimulation is a traditional tool in neuroscience and is commonly used in vivo to evoke behavior and in vitro to study neural mechanisms. In vivo intracerebral microdialysis, also a traditional technique, is used to assay neurotransmitter release. However, the combination of these techniques is highly limited to studies using anesthetized animals; therefore, evoking and measuring exocytotic neurotransmitter release in awake models is lacking. Combining these techniques in an awake animal preparation is presented here with evidence to support the mechanistic action of electrical stimulation in vivo.

NEW METHODS

This report presents converging evidence to validate the combination of intracerebral electrical stimulation with microdialysis as a novel procedure to study exocytotic-like dopamine release in behaving animals.

RESULTS

It is shown that electrical stimulation of the medial forebrain bundle can be used to evoke frequency- and intensity-dependent exocytotic-like dopamine overflow and rotational behavior that are sensitive to Na+ channel blockade and Ca++ availability.

COMPARISON WITH EXISTING METHODS

Studies using modern techniques to evoke neurotransmitter release, combined with in vivo intracerebral microdialysis, and measured behavioral output are scarce. In contrast, commonly used pharmacological methods often are less precise and inefficient to evoke exocytotic dopamine release and behavior. Here we demonstrate, the combination of in vivo intracerebral microdialysis with electrical stimulation as a simple approach to simultaneously assess physiologically relevant neurotransmitter 'release' and behavior.

CONCLUSIONS

Research that aims to understand how dopamine neurotransmission is altered in behavioral disorders can utilize this innovative combination of electrical stimulation with in vivo intracerebral microdialysis.

The therapeutic potential of exercise for neuropsychiatric diseases: A review

Exercise is known to have a myriad of health benefits. There is much to be learned from the effects of exercise and its potential for prevention, attenuation and treatment of multiple neuropsychiatric diseases and behavioral disorders. Furthermore, recent data and research on exercise benefits with respect to major health crises, such as, that of opioid and general substance use disorders, make it very important to better understand and review the mechanisms of exercise and how it could be utilized for effective treatments or adjunct treatments for these diseases. In addition, mechanisms, epigenetics and sex differences are examined and discussed in terms of future research implications.

Crosstalk Between Kappa Opioid and Dopamine Systems in Compulsive Behaviors

The strength of goal-oriented behaviors is regulated by midbrain dopamine neurons. Dysfunctions of dopaminergic circuits are observed in drug addiction and obsessive-compulsive disorder. Compulsive behavior is a feature that both disorders share, which is associated to a heightened dopamine neurotransmission. The activity of midbrain dopamine neurons is principally regulated by the homeostatic action of dopamine through D2 receptors (D2R) that decrease the firing of neurons as well as dopamine synthesis and release. Dopamine transmission is also regulated by heterologous neurotransmitter systems such as the kappa opioid system, among others. Much of our current knowledge of the kappa opioid system and its influence on dopamine transmission comes from preclinical animal models of brain diseases. In 1988, using cerebral microdialysis, it was shown that the acute activation of the Kappa Opioid Receptors (KOR) decreases synaptic levels of dopamine in the striatum. This inhibitory effect of KOR opposes to the facilitating influence of drugs of abuse on dopamine release, leading to the proposition of the use of KOR agonists as pharmacological therapy for compulsive drug intake. Surprisingly, 30 years later, KOR antagonists are instead proposed to treat drug addiction. What may have happened during these years that generated this drastic change of paradigm? The collected evidence suggested that the effect of KOR on synaptic dopamine levels is complex, depending on the frequency of KOR activation and timing with other incoming stimuli to dopamine neurons, as well as sex and species differences. Conversely to its acute effect, chronic KOR activation seems to facilitate dopamine neurotransmission and dopamine-mediated behaviors. The opposing actions exerted by acute versus chronic KOR activation have been associated with an initial aversive and a delayed rewarding effect, during the exposure to drugs of abuse. Compulsive behaviors induced by repeated activation of D2R are also potentiated by the sustained co-activation of KOR, which correlates with decreased synaptic levels of dopamine and sensitized D2R. Thus, the time-dependent activation of KOR impacts directly on dopamine levels affecting the tuning of motivated behaviors. This review analyzes the contribution of the kappa opioid system to the dopaminergic correlates of compulsive behaviors.

Individual Differences in Amphetamine Locomotor Sensitization are Accompanied with Changes in Dopamine Release and Firing Pattern in the Dorsolateral Striatum of Rats

Not all the people that consume drugs of abuse develop addiction. In this sense, just a percentage of rats express locomotor sensitization after repeated psychostimulant exposure. Neurochemical evidence has shown that locomotor sensitization is associated with changes in dorsolateral striatum (DLS) activity. However, it is unknown if individual differences observed in locomotor sensitization are related to differential neuro-adaptations in DLS activity. In this study, we measured basal dopamine (DA) levels and single unit activity in the DLS of anesthetized rats, after repeated amphetamine (AMPH) administration. Rats were treated with AMPH 1.0 mg/kg ip or saline ip for 5 days. Following 5 days of withdrawal, a challenge dose of AMPH 1.0 mg/kg ip was injected. In-vivo microdialysis experiments and single unit recording were carried out twenty-four hours after the last AMPH injection. Sensitized rats showed increased basal DA levels and baseline firing rate of medium spiny neurons (MSNs) compared to non-sensitized rats. The local variation index (Lv) was used to measure the firing pattern of MSNs. In saline rats, a bursty firing pattern was observed in MSNs. A decrease in MSNs baseline Lv accompanies the expression of AMPH locomotor sensitization. Moreover, a decrease in Lv after an acute AMPH 1.0 mg/kg injection was only observed in saline and sensitized rats. Our results show individual differences in DLS basal DA levels and firing pattern after repeated AMPH administration, suggesting that an hyperfunction of nigrostriatal pathway, accompanied by a decrease in DLS MSNs firing irregularity underlies the expression of AMPH locomotor sensitization.

LiCl Pretreatment Ameliorates Adolescent Methamphetamine Exposure-Induced Long-Term Alterations in Behavior and Hippocampal Ultrastructure in Adulthood in Mice

BACKGROUND

Adolescent methamphetamine exposure causes a broad range of neurobiological deficits in adulthood. Glycogen synthase kinase-3β is involved in various cognitive and behavioral processes associated with methamphetamine exposure. This study aims to investigate the protective effects of the glycogen synthase kinase-3β inhibitor lithium chloride on adolescent methamphetamine exposure-induced long-term alterations in emotion, cognition, behavior, and molecule and hippocampal ultrastructure in adulthood.

METHODS

A behavioral test battery was used to investigate the protective effects of lithium chloride on adolescent methamphetamine exposure-induced long-term emotional, cognitive, and behavioral impairments in mice. Western blotting and immunohistochemistry were used to detect glycogen synthase kinase-3β activity levels in the medial prefrontal cortex and dorsal hippocampus. Electron microscopy was used to analyze changes in synaptic ultrastructure in the dorsal hippocampus. Locomotor sensitization with a methamphetamine (1 mg/kg) challenge was examined 80 days after adolescent methamphetamine exposure.

RESULTS

Adolescent methamphetamine exposure induced long-term alterations in locomotor activity, novel spatial exploration, and social recognition memory; increases in glycogen synthase kinase-3β activity in dorsal hippocampus; and decreases in excitatory synapse density and postsynaptic density thickness in CA1. These changes were ameliorated by lithium chloride pretreatment. Adolescent methamphetamine exposure-induced working memory deficits in Y-maze spontaneous alternation test and anxiety-like behavior in elevated-plus maze test spontaneously recovered after long-term methamphetamine abstinence. No significant locomotor sensitization was observed after long-term methamphetamine abstinence.

CONCLUSIONS

Hyperactive glycogen synthase kinase-3β contributes to adolescent chronic methamphetamine exposure-induced behavioral and hippocampal impairments in adulthood. Our results suggest glycogen synthase kinase-3β may be a potential target for the treatment of deficits in adulthood associated with adolescent methamphetamine abuse.

Involvement of dopamine D2 receptor in a single methamphetamine-induced behavioral sensitization in C57BL/6J mice

A single exposure to drugs of abuse is sufficient to induce behavioral sensitization, which is a form of long-lasting neuroplasticity. Dopamine D2 receptors are the main receptor for antipsychotic drugs, but little is known about their role in a single methamphetamine-induced behavioral sensitization. In the present study, we examined whether typical antipsychotic haloperidol and atypical antipsychotic risperidone, both targeting dopamine D2 receptor, could prevent the methamphetamine sensitization when they were given at the different phase of behavioral sensitization. A single methamphetamine exposure induced robust and reliable behavioral sensitization to the lower challenge dose of methamphetamine after 7 days of drug-free period. At doses that did not affect general locomotion, haloperidol and risperidone not only significantly attenuated methamphetamine induced hyperlocomotion, but also completely prevented the development of behavioral sensitization to methamphetamine challenge when they were pretreated before the first exposure to methamphetamine. When haloperidol and risperidone were given in the early period of transfer (2 h after the first methamphetamine exposure), they also dose-dependently attenuated the transfer to expression of methamphetamine sensitization from the hyperlocomotion. These data suggest that dopamine D2 receptors play an important role in methamphetamine sensitization, especially in protecting against the development and transfer in the earlier labile period after the methamphetamine exposure. Therefore, clinically approved dopamine D2 receptor antagonists may be useful in the treatment of methamphetamine addiction.

Amphetamine treatment affects the extra-hypothalamic vasopressinergic system in a sex- and nucleus-dependent manner

The lateral septum (LS), a brain structure implicated in addictive behaviours, regulates the activation of dopaminergic neurones in the ventral tegmental area. Vasopressinergic projections from the extended amygdala to the LS, which are sexually dimorphic, could be responsible for the vulnerability to addiction in a sex-dependent manner. The present study aimed to investigate the modulatory effects of amphetamine (AMPH) on the expression of vasopressin (AVP) in the vasopressinergic extra-hypothalamic system in sensitised male and female rats. Adult male and female Sprague-Dawley rats underwent an AMPH-locomotor sensitisation protocol. Acute AMPH increased AVP mRNA expression in the medial amygdala (MeA), whereas AMPH-induced sensitisation increased AVP mRNA expression in the bed nucleus of the stria terminalis (BNST) only in females. Interestingly, the increase in AVP expression in BNST was higher in oestrus females compared to dioestrus females and acute AMPH resulted in a decrease in AVP levels in the LS, only in males. Thus, there are complex and region-specific interactions between AMPH and the extra-hypothalamic vasopressinergic system in the brain, underlying possible alterations in different behaviours caused by acute and chronic AMPH exposure.

Reduced dopamine and glutamate neurotransmission in the nucleus accumbens of quinpirole-sensitized rats hints at inhibitory D2 autoreceptor function

Dopamine from the ventral tegmental area and glutamate from several brain nuclei converge in the nucleus accumbens (NAc) to drive motivated behaviors. Repeated activation of D2 receptors with quinpirole (QNP) induces locomotor sensitization and compulsive behaviors, but the mechanisms are unknown. In this study, in vivo microdialysis and fast scan cyclic voltammetry in adult anesthetized rats were used to investigate the effect of repeated QNP on dopamine and glutamate neurotransmission within the NAc. Following eight injections of QNP, a significant decrease in phasic and tonic dopamine release was observed in rats that displayed locomotor sensitization. Either a systemic injection or the infusion of QNP into the NAc decreased dopamine release, and the extent of this effect was similar in QNP-sensitized and control rats, indicating that inhibitory D2 autoreceptor function is maintained despite repeated activation of D2 receptors and decreased dopamine extracellular levels. Basal extracellular levels of glutamate in the NAc were also significantly lower in QNP-treated rats than in controls. Moreover, the increase in NAc glutamate release induced by direct stimulation of medial prefrontal cortex was significantly lower in QNP-sensitized rats. Together, these results indicate that repeated activation of D2 receptors disconnects NAc from medial prefrontal cortex and ventral tegmental area. Repeated administration of the dopamine D2 receptor agonist quinpirole (QNP) induces locomotor sensitization. We found that the NAc of QNP-sensitized rats has reduced glutamate levels coming from prefrontal cortex together with a decreased phasic and tonic dopamine neurotransmission but a conserved presynaptic D2 receptor function. We suggest that locomotor sensitization is because of increased affinity state of D2 post-synaptic receptors.

MK-801-induced behavioural sensitisation alters dopamine release and turnover in rat prefrontal cortex

RATIONALE

Repeated exposure to psychostimulants that either increase dopamine (DA) release or target N-methyl-D-aspartate (NMDA) receptors can induce behavioural sensitisation, a phenomenon that may be important for the processes of addiction and even psychosis. A critical component of behavioural sensitisation is an increase in DA release within mesocorticolimbic circuits. In particular, sensitisation to amphetamine leads to increased DA release within well-known sub-cortical brain regions and also regulatory regions such as prefrontal cortex (PFC). However, it is unknown how DA release within the PFC of animals is altered by sensitisation to NMDA receptor antagonists.

OBJECTIVES

The aims of the present study were twofold, firstly to examine whether a single dose of dizocilpine maleate (MK-801) could induce long-term behavioural sensitisation and secondly to examine DA release in the PFC of sensitised rats.

MATERIALS AND METHODS

Behavioural sensitisation was assessed by measuring locomotion after drug exposure. DA release in the PFC was measured using freely moving microdialysis.

RESULTS

We show that a single dose of MK-801 can induce sensitisation to subsequent MK-801 exposure in a high percentage of rats (66 %). Furthermore, rats sensitised to MK-801 have altered DA release and turnover in the PFC compared with non-sensitised rats.

CONCLUSION

Schizophrenia patients have been postulated to have 'endogenous sensitisation' to psychostimulants. MK-801-induced sensitised rats, in particular when compared with non-sensitised rats, provide a useful model for studying PFC dysfunction in schizophrenia.

Withdrawal from chronic amphetamine reduces dopamine transmission in the rat lateral septum

The lateral septum (LS) is a brain nucleus implicated in the addictive process. This study investigated whether withdrawal from chronic amphetamine (AMPH) induces alterations in dopamine (DA) transmission within the LS. Male Sprague-Dawley rats were injected with AMPH (2.5 mg/kg i.p.) or saline during 14 days and thereafter subjected to 24 hr or 14 days of withdrawal. After these withdrawal periods, we measured DA extracellular levels by in vivo microdialysis, DA tissue levels, and tyrosine hydroxylase (TH) and vesicular monoamine transporter-2 (VMAT2) expression in the LS. Our results showed a significant decrease in K(+) -induced release of DA in the LS of AMPH-treated rats, 14 days after withdrawal compared with saline-treated rats. There were no significant differences in DA tissue content and TH expression. Interestingly, there was a decrease of LS VMAT2 expression in AMPH-treated rats, 14 days after withdrawal compared with saline-treated rats. This is the first neurochemical evidence showing that withdrawal from repeated AMPH administration decreases K(+) -induced DA release in the rat LS. Our results suggest that this decrease in DA releasability could be due to a decrease in DA vesicular uptake. The possibility that these neurochemical changes are associated with AMPH abstinence syndrome should be further explored.

Clozapine pre-treatment has a protracted hypolocomotor effect on the induction and expression of amphetamine sensitization

Amphetamine locomotor sensitization is an animal model for the study of addiction and schizophrenia. The antipsychotic clozapine blocks the hyperlocomotion induced by an acute injection of amphetamine, but its effect on locomotor sensitization after repeated amphetamine administration remains unknown. In the present study we investigate the effect of repeated administration of clozapine on the induction and expression of amphetamine locomotor sensitization. We propose that repeated administration of clozapine blocks the induction and expression of amphetamine sensitization. Male Sprague-Dawley rats were classified according to their locomotor response to an acute saline injection in high responder saline (HRS) or low responder saline (LRS). Rats from both groups were injected once daily with amphetamine for 5 consecutive days. Horizontal locomotor activity was measured during 40 min. Four days after the last injection, an acute dose of amphetamine was administered to assess the expression of sensitization. Clozapine was injected once daily for 4 consecutive days before (pre-treatment) or after (treatment) induction of sensitization. Pre-treatment with clozapine significantly decreases both acute amphetamine-induced hyperlocomotion and the induction and expression of amphetamine sensitization only in LRS rats, showing a protracted hypolocomotor effect. On the other hand, clozapine treatment had no effect over locomotor response on the expression of amphetamine sensitization in either LRS or HRS rats. These data suggest that clozapine effect on amphetamine locomotor response depends on individual differences. Also, our results suggest that clozapine pre-treatment attenuates the neuroplasticity underlying amphetamine sensitization, but clozapine treatment is unable to reverse these changes once amphetamine sensitization has been induced.

Exercise as a novel treatment for drug addiction: a neurobiological and stage-dependent hypothesis

Physical activity, and specifically exercise, has been suggested as a potential treatment for drug addiction. In this review, we discuss clinical and preclinical evidence for the efficacy of exercise at different phases of the addiction process. Potential neurobiological mechanisms are also discussed focusing on interactions with dopaminergic and glutamatergic signaling and chromatin remodeling in the reward pathway. While exercise generally produces an efficacious response, certain exercise conditions may be either ineffective or lead to detrimental effects depending on the level/type/timing of exercise exposure, the stage of addiction, the drug involved, and the subject population. During drug use initiation and withdrawal, its efficacy may be related to its ability to facilitate dopaminergic transmission, and once addiction develops, its efficacy may be related to its ability to normalize glutamatergic and dopaminergic signaling and reverse drug-induced changes in chromatin via epigenetic interactions with brain-derived neurotrophic factor (BDNF) in the reward pathway. We conclude with future directions, including the development of exercise-based interventions alone or as an adjunct to other strategies for treating drug addiction.