Systemic cocaine challenge after chronic cocaine treatment reveals sensitization of extracellular dopamine levels in nucleus accumbens but direct cocaine perfusion into nucleus accumbens does not: implications for the neural locus of cocaine sensitization

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.

Cross-sensitization between cocaine and acute restraint stress is associated with sensitized dopamine but not glutamate release in the nucleus accumbens

Repeated administration of psychostimulant drugs or stress can elicit a sensitized response to the stimulating and reinforcing properties of the drug. Here we explore the mechanisms in the nucleus accumbens (NAc) whereby an acute restraint stress augments the acute locomotor response to cocaine. This was accomplished by a combination of behavioral pharmacology, microdialysis measures of extracellular dopamine and glutamate, and Western blotting for GluR1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor (AMPAR). A single exposure to restraint stress 3 weeks before testing revealed that enduring locomotor sensitization to cocaine was paralleled by an increase in extracellular dopamine in the core, but not the shell subcompartment, of the NAc. Wistar rats pre-exposed to acute stress showed increased basal levels of glutamate in the core, but the increase in glutamate by acute cocaine was blunted. The alterations in extracellular glutamate seem to be relevant, as blocking AMPAR by 6-cyano-7-nitroquinoxaline-2,3-dione microinjection into the core prevented both the behavioral cross-sensitization and the augmented increase in cocaine-induced extracellular dopamine. Further implicating glutamate, the locomotor response to AMPAR stimulation in the core was potentiated, but not in the shell of pre-stressed animals, and this was accompanied by an increase in NAc GluR1 surface expression. This study provides evidence that the long-term expression of restraint stress-induced behavioral cross-sensitization to cocaine recapitulates some mechanisms thought to underpin the sensitization induced by daily cocaine administration, and shows that long-term neurobiological changes induced in the NAc by acute stress are consequential in the expression of cross-sensitization to cocaine.

Age-dependent effects of kappa-opioid receptor stimulation on cocaine-induced stereotyped behaviors and dopamine overflow in the caudate-putamen: an in vivo microdialysis study

kappa-Opioid receptor stimulation attenuates psychostimulant-induced increases in extracellular dopamine in the caudate-putamen (CPu) and nucleus accumbens of adult rats, while reducing cocaine-induced locomotor activity and stereotyped behaviors. Because kappa-opioid receptor agonists (e.g., U50,488 or U69,593) often affect the behavior of preweanling rats in a paradoxical manner, the purpose of the present study was to determine whether kappa-opioid receptor stimulation differentially affects dopaminergic functioning in the CPu depending on age. In vivo microdialysis was used to determine whether U50,488 (5 mg/kg) attenuates cocaine-induced dopamine overflow in the dorsal CPu on postnatal day (PD) 17 and PD 85. In the microinjection experiment, cocaine-induced stereotyped behaviors were assessed in adult and preweanling rats after bilateral infusions of vehicle or U50,488 (1.6 or 6.4 microg per side) into the CPu. Results showed that U50,488 attenuated the cocaine-induced increases in CPu dopamine overflow on PD 85, while the same dose of U50,488 did not alter dopamine dialysate levels on PD 17. Cocaine also increased stereotyped behaviors (repetitive motor movements, behavioral intensity scores, and discrete behaviors) at both ages, but adult rats appeared to exhibit more intense stereotypic responses than the younger animals. Consistent with the microdialysis findings, bilateral infusions of U50,488 into the dorsal CPu decreased the cocaine-induced stereotypies of adult rats, while leaving the behaviors of preweanling rats unaffected. These results suggest that the neural mechanisms underlying kappa-opioid/dopamine interactions in the CPu are not fully mature during the preweanling period. This lack of functional maturity may explain why kappa-opioid receptor agonists frequently induce different behavioral effects in young and adult rats.

Microdialysis and the neurochemistry of addiction

Drug addiction is a process beginning with the initial exposure to a drug of abuse, and leading, in some individuals, to chronic habitual use, and high rates of relapse. Microdialysis allows researchers to monitor the neurochemical changes that occur in the brain after the initial exposure to a drug, and the neurochemical changes that occur with repeated exposure. These changes in the brain are often referred to as drug-induced neuroplasticity, and the aim of this article is to review studies that have utilized microdialysis to increase our understanding of the neuroplasticity that occurs in the process of addiction. We will review how several neurotransmitter systems, including glutamate, GABA, the monoamines, and others, are altered after chronic drug exposure, and how microdialysis can be used to determine if putative treatments for addiction can reverse the drug-induced neuroplasticity in these systems. We will also briefly discuss our recent research using a known change in GABA neurotransmission that occurs during reinstatement of drug-seeking to screen for possible novel treatments to prevent relapse. Overall, microdialysis in combination with other behavioral and pharmacological techniques has greatly increased our understanding of addiction-related neuroplasticity, and provides a means for discovering new ways to prevent these changes and treat addiction.

Perspectives on current directions in the neurobiology of addiction disorders relevant to genetic risk factors

There is a significant heritability of drug addiction disorders, but potential genes that may underlie such vulnerability have not been clearly identified. Common neurobiological candidates for drug abuse include genes related to dopamine, opioid neuropeptide, and glutamate transmission that play important roles in drug reward and inhibitory control. This article provides an overview of genetic polymorphisms linked to these neurobiological systems, particularly in relation to psychostimulant- and opioid-addiction vulnerability.

A slow-onset, long-duration indanamine monoamine reuptake inhibitor as a potential maintenance pharmacotherapy for psychostimulant abuse: effects in laboratory rat models relating to addiction

Slow-onset, long-lasting dopamine reuptake blockers with reduced abuse potential have been suggested as maintenance therapies for cocaine addiction. We have synthesized a series of 3-(3',4'-dichlorophenyl)-1-indanamine monoamine reuptake inhibitors as candidates for such maintenance pharmacotherapy. The initial lead compound, the N,N-dimethyl analogue 30,640 was then subjected to testing in addiction-relevant animal models. Compound 30,640 (2 mg/kg i.p.) produced a pronounced slow-onset, long-lasting increase (300-400%) in extracellular nucleus accumbens dopamine levels, as measured by in vivo brain microdialysis in awake laboratory rats. Slow-onset, long-lasting decreases (40-80%) in the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid, and the serotonin metabolite 5-hydroxyindoleacetic acid were also seen. Compound 30,640 (3 or 5 mg/kg i.p.) also produced a significant (approximately 30%) slow-onset, long-lasting enhancement of electrical brain-stimulation reward, which was additive with that of cocaine (5 mg/kg i.p.). When given to cocaine-administering rats, 30,640 (2.5, 3, 5, or 10 mg/kg i.p.) significantly inhibited (30-60%) intravenous cocaine self-administration, with a pronounced long-lasting profile. In sum, 30,640 showed cocaine-like effects, but with a marked slow-onset, long-lasting profile. We conclude that the prodrug strategy employed in the design of 30,640 achieved its goal. We suggest that such compounds may be useful as maintenance pharmacotherapies for psychostimulant addiction.

Critical assessment of how to study addiction and its treatment: human and non-human animal models

Laboratory models, both animal and human, have made enormous contributions to our understanding of addiction. For addictive disorders, animal models have the great advantage of possessing both face validity and a significant degree of predictive validity, already demonstrated. Another important advantage to this field is the ability of reciprocal interplay between preclinical and clinical experiments. These models have made important contributions to the development of medications to treat addictive disorders and will likely result in even more advances in the future. Human laboratory models have gone beyond data obtained from patient histories and enabled investigators to make direct observations of human drug self-administration and test the effects of putative medications on this behavior. This review examines in detail some animal and human models that have led not only to important theories of addiction mechanisms but also to medications shown to be effective in the clinic.

Sensitized Fos expression in subterritories of the rat medial prefrontal cortex and nucleus accumbens following amphetamine sensitization as revealed by stereology

Behavioral sensitization to the locomotor activating effects of amphetamine refers to the progressive, long lasting increase in locomotor activity that occurs with repeated injections. This phenomenon is thought to result from neuroadaptations occurring in the projection fields of mesocorticolimbic dopaminergic neurons. In the present study, we investigated the effects of amphetamine sensitization on Fos immunoreactivity (Fos-IR) in subterritories of the nucleus accumbens (core and shell) and medial prefrontal cortex (mPFC; dorsal and ventral) using stereology. Rats received five daily injections of amphetamine (1.5 mg/kg, i.p.) or saline. Behavioral sensitization was measured 48 h following the last injection, in response to a challenge injection of 1.5 mg/kg amphetamine. Sensitized rats showed a greater enhancement of locomotor activity upon drug challenge compared with their saline counterparts. Densities of Fos-positive nuclei were enhanced more in the dorsal than the ventral mPFC subterritory, whereas in the nucleus accumbens, densities of Fos-positive nuclei were increased more in the core than the shell of amphetamine-sensitized rats compared to controls. These results represent, to our knowledge, the first published report using stereological methods to quantify Fos-IR in the brain and suggest functional specialization of cortical and limbic regions in the expression of behavioral sensitization to amphetamine.

Cocaine intake by rats correlates with cocaine-induced dopamine changes in the nucleus accumbens shell

Extracellular dopamine levels were determined by microdialysis in the core and shell of the nucleus accumbens and the frontal cortex of rats before and after an injection of cocaine (20 mg/kg, IP). After removal of the probes, these same animals were then tested for their voluntary intake of cocaine using the two-bottle, free-choice paradigm. Baseline dopamine levels and their responses to an injection of cocaine differed among the three brain areas. No significant correlations were found between baseline dopamine levels in any of the three brain regions and the voluntary cocaine consumption. A significant negative correlation was found between cocaine-induced increases in extracellular dopamine in the shell of the nucleus accumbens and the voluntary intake of cocaine (r = -0.73, p < 0.01). No such correlations were observed in the accumbens core region or the frontal cortex. These results provide further evidence of the role of the accumbal shell region in cocaine preference, and indicate that cocaine-induced increases in dopamine levels play a role in oral cocaine self-administration or preference. In addition, this relatively novel approach in using the same animals for both cocaine induced neurotransmitter responses and cocaine preference studies can also be applied for the study of other neurotransmitters and drugs of abuse.

Simple microbore high-performance liquid chromatographic method for the determination of dopamine and cocaine from a single in vivo brain microdialysis sample

A microbore chromatographic method for the analysis of both dopamine and cocaine from in vivo brain microdialysis samples is described. To eliminate the need for separate chromatographic systems for each analyte, post-column electrochemical and ultraviolet detection systems were arranged in series. The limit of quantitation for dopamine (5 fmol) was well within range for detecting dialysate concentrations of this neurotransmitter in rats which were in a baseline, drug-free state. The limit of quantitation for cocaine (0.5 pmol) was sufficient to detect brain cocaine levels following the peripheral administration of a low dose of this psychostimulant (5 mg/kg, i.p.). Estimates of dialysate dopamine and cocaine concentrations after 5, 10 and 20 mg/kg cocaine (i.p.) were in agreement with reports which utilized separate HPLC analyses for each analyte.

Differential effects of SCH 23390 on the apomorphine subsensitivity in the substantia nigra and ventral tegmental area 1 day following withdrawal from continuous or intermittent cocaine pretreatment

Using extracellular single-unit recordings in rats, the effects of chronic intermittent injections and continuous infusion of cocaine on single dopamine neurons were directly compared in the substantia nigra and ventral tegmental area. After 1-day withdrawal we determined: (1) the neuronal sensitivity to the mixed D1/D2 agonist apomorphine and (2) its modulation by the D1 antagonist SCH 23390. The nigral dopamine neurons exhibited subsensitivity to the impulse-inhibiting effects of apomorphine following both intermittent and continuous regimens. SCH 23390 selectively reversed the apomorphine subsensitivity in the intermittent group, while having minimal effects in the other group. Dopamine neurons in the ventral tegmental area, on the other hand, were sub- and normosensitive to apomorphine following intermittent and continuous dosing regimens, respectively. In contrast to the substantia nigra, SCH 23390 failed to alter the apomorphine sensitivity in either of the pretreatment groups. Possible mechanisms underlying these distinctive changes in the substantia nigra and ventral tegmental area following intermittent and continuous cocaine pretreatment regimens are discussed.