Differential effects of dopaminergic agents on locomotor sensitisation and on the reinstatement of cocaine-seeking and food-seeking behaviour

Effects of pramipexole on the acquisition of responding with opioid-conditioned reinforcement in the rat

RATIONALE

Dopamine D3 receptor-preferring ligands may be able to modify the conditioned reinforcing effects of drug-associated stimuli. In evaluating the effects of these compounds, it is important to clarify the extent to which responding depends on (1) conditioned reinforcement vs. other behavioral mechanisms and (2) dopamine D3 vs. D2 receptor activity.

OBJECTIVES

To use behaviorally stringent new-response acquisition procedures to characterize the effects of the D3-preferring agonist, pramipexole, on the conditioned reinforcing effects of a stimulus paired with the opioid agonist, remifentanil.

METHODS

First, in Pavlovian conditioning (PAV) sessions, rats received response-independent IV injections of remifentanil and presentations of a light-noise stimulus. In separate groups, injections and stimuli either always co-occurred ("paired PAV") or occurred with no consistent relationship ("random PAV" control). Next, in instrumental acquisition (ACQ) sessions, all animals could respond in two nose-poke manipulanda: an active nose-poke, which produced the stimulus alone, or an inactive nose-poke. Pramipexole was injected SC prior to ACQ sessions with or without pretreatments of the D3-preferring antagonist, SB-277011A, or the D2-preferring antagonist, L-741,626.

RESULTS

After paired PAV, but not random PAV, rats acquired nose-poke responding during ACQ (i.e., active > inactive). Pramipexole dose-dependently increased active responding without changing inactive responding. Pramipexole-induced increases in responding were blocked by pretreatment with L-741,626, but not SB-277011A.

CONCLUSIONS

Pramipexole specifically enhanced remifentanil-conditioned reinforcement: active responding was selectively increased only after the stimulus was paired with remifentanil. Although pramipexole is D3-preferring, the antagonist effects obtained presently suggest an important role for the D2 receptor in opioid-conditioned reinforcement.

Initial d2 dopamine receptor sensitivity predicts cocaine sensitivity and reward in rats

The activation of dopamine receptors within the mesolimbic dopamine system is known to be involved in the initiation and maintenance of cocaine use. Expression of the D₂ dopamine receptor subtype has been implicated as both a predisposing factor and consequence of chronic cocaine use. It is unclear whether there is a predictive relationship between D₂ dopamine receptor function and cocaine sensitivity that would enable cocaine abuse. Therefore, we exploited individual differences in behavioral responses to D₂ dopamine receptor stimulation to test its relationship with cocaine-mediated behaviors. Outbred, male Sprague-Dawley rats were initially characterized by their locomotor responsiveness to the D₂ dopamine receptor agonist, quinpirole, in a within-session ascending dose-response regimen (0, 0.1, 0.3 & 1.0 mg/kg, sc). Rats were classified as high or low quinpirole responders (HD₂ and LD₂, respectively) by a median split of their quinpirole-induced locomotor activity. Rats were subsequently tested for differences in the psychostimulant effects of cocaine by measuring changes in cocaine-induced locomotor activity (5 and 15 mg/kg, ip). Rats were also tested for differences in the development of conditioned place preference to a low dose of cocaine (7.5 mg/kg, ip) that does not reliably produce a cocaine conditioned place preference. Finally, rats were tested for acquisition of cocaine self-administration and maintenance responding on fixed ratio 1 and 5 schedules of reinforcement, respectively. Results demonstrate that HD₂ rats have enhanced sensitivity to the locomotor stimulating properties of cocaine, display greater cocaine conditioned place preference, and self-administer more cocaine compared to LD₂ animals. These findings suggest that individual differences in D₂ dopamine receptor sensitivity may be predictive of cocaine sensitivity and reward.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Cariprazine (RGH-188), a D₃-preferring dopamine D₃/D₂ receptor partial agonist antipsychotic candidate demonstrates anti-abuse potential in rats

RATIONALE

Cariprazine (RGH-188) is a D₃-preferring dopamine D₃/D₂ receptor partial agonist antipsychotic candidate for the treatment of schizophrenia and bipolar mania. Substance abuse is a frequent comorbidity of both disorders and is associated with serious health issues. Based on preclinical efficacy, dopamine D₂ and D₃ receptor partial agonists and antagonists are assumed to have relapse-preventing potential in human cocaine addiction.

OBJECTIVES

We investigated the anti-abuse potential of cariprazine in cocaine self-administration paradigms. Aripiprazole and bifeprunox were used as comparators because of their pharmacological similarity to cariprazine.

METHODS

The effects of compounds on cocaine's rewarding effect were investigated in a continuous self-administration regimen. The relapse-preventing potential of drugs was studied in rats with a history of cocaine self-administration after a period of complete abstinence in a relapse to cocaine-seeking paradigm.

RESULTS

Cariprazine, as well as aripiprazole and bifeprunox, were able to reduce the rewarding effect of cocaine (minimum effective doses were 0.17, 1, and 0.1 mg/kg, respectively) and attenuated relapse to cocaine seeking with half maximal effective dose [ED₅₀] values of 0.2, 4.2, and 0.17 mg/kg, respectively.

CONCLUSIONS

These results may predict a relapse-preventing action for cariprazine in humans in addition to its already established antipsychotic and antimanic efficacy.

Increased vulnerability to cocaine in mice lacking dopamine D3 receptors

Neuroimaging studies using positron emission tomography suggest that reduced dopamine D(2) receptor availability in the neostriatum is associated with increased vulnerability to drug addiction in humans and experimental animals. The role of D(3) receptors (D(3)Rs) in the neurobiology of addiction remains unclear, however. Here we report that D(3)R KO (D(3)(-/-)) mice display enhanced cocaine self-administration and enhanced motivation for cocaine-taking and cocaine-seeking behavior. This increased vulnerability to cocaine is accompanied by decreased dopamine response to cocaine secondary to increased basal levels of extracellular dopamine in the nucleus accumbens, suggesting a compensatory response to decreased cocaine reward in D(3)(-/-) mice. In addition, D(3)(-/-) mice also display up-regulation of dopamine transporters in the striatum, suggesting a neuroadaptative attempt to normalize elevated basal extracellular dopamine. These findings suggest that D(3)R deletion increases vulnerability to cocaine, and that reduced D(3)R availability in the brain may constitute a risk factor for the development of cocaine addiction.

Adenosine A2A receptors in the nucleus accumbens bi-directionally alter cocaine seeking in rats

Repeated cocaine administration enhances dopamine D(2) receptor sensitivity in the mesolimbic dopamine system, which contributes to drug relapse. Adenosine A(2A) receptors are colocalized with D(2) receptors on nucleus accumbens (NAc) medium spiny neurons where they antagonize D(2) receptor activity. Thus, A(2A) receptors represent a target for reducing enhanced D(2) receptor sensitivity that contributes to cocaine relapse. The aim of these studies were to determine the effects of adenosine A(2A) receptor modulation in the NAc on cocaine seeking in rats that were trained to lever press for cocaine. Following at least 15 daily self-administration sessions and 1 week of abstinence, lever pressing was extinguished in daily extinction sessions. We subsequently assessed the effects of intra-NAc core microinjections of the A(2A) receptor agonist, CGS 21680 (4-[2-[[6-amino-9-(N-ethyl-b-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride), and the A(2A) receptor antagonist, MSX-3 (3,7-dihydro-8-[(1E)-2-(3-methoxyphenyl)ethenyl]-7-methyl-3-[3-(phosphonooxy)propyl-1-(2-propynyl)-1H-purine-2,6-dione disodium salt hydrate), in modulating cocaine- and quinpirole-induced reinstatement to cocaine seeking. Intra-NAc pretreatment of CGS 21680 reduced both cocaine- and quinpirole-induced reinstatement. These effects were specific to cocaine reinstatement as intra-NAc CGS 21680 had no effect on sucrose seeking in rats trained to self-administer sucrose pellets. Intra-NAc treatment with MSX-3 modestly reinstated cocaine seeking when given alone, and exacerbated both cocaine- and quinpirole-induced reinstatement. Interestingly, the exacerbation of cocaine seeking produced by MSX-3 was only observed at sub-threshold doses of cocaine and quinpirole, suggesting that removing tonic A(2A) receptor activity enables behaviors mediated by dopamine receptors. Taken together, these findings suggest that A(2A) receptor stimulation reduces, while A(2A) blockade amplifies, D(2) receptor signaling in the NAc that mediates cocaine relapse.

Cav 1.3 L-type Ca ( 2+) channels mediate long-term adaptation in dopamine D2L-mediated GluA1 trafficking in the dorsal striatum following cocaine exposure

AMPA receptor (AMPAR) plasticity at glutamatergic synapses in the mesostriatal dopaminergic pathway has been implicated in persistent cocaine-induced behavioral responses; however, the precise mechanism underlying these changes remains unknown. Utilizing cocaine psychomotor sensitization in mice we find that repeated cocaine results in a basal reduction of Ser 845 GluA1 and cell surface GluA1 levels in the dorsal striatum (dStr) following a protracted withdrawal period, an adaptation that is dependent on Cav 1.3 channels but not those expressed in the VTA. We find that the basally-induced decrease in this phosphoprotein is the result of recruitment of the striatal dopamine D2 pathway, as evidenced by enhanced levels of D2 receptor (D2R) mRNA expression and D2R function as examined using the D2R antagonist, eticlopride, as well as alterations in the phosphorylation status of several downstream molecular targets of D2R's, including CREB, DARPP-32, Akt and GSK3β. Taken together with our recently published findings examining similar phenomena in the nucleus accumbens (NAc), these results underscore the utilization of divergent molecular mechanisms in the dStr, in mediating cocaine-induced persistent behavioral changes.

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

Behavioral sensitization to cocaine in rats: evidence for temporal differences in dopamine D3 and D2 receptor sensitivity

RATIONALE

Cocaine-induced changes in D(2) receptors have been implicated in the expression of sensitized behavioral responses and addiction-like behaviors; however, the influence of D(3) receptors is less clear.

OBJECTIVES

To characterize the effects of repeated cocaine administration on the sensitivity of rats to D(2)- and D(3)-mediated behaviors, as well as the binding properties of ventral striatal D(2)-like and D(3) receptors.

METHODS

Pramipexole was used to assess the sensitivity of rats to D(3)/D(2) agonist-induced yawning, hypothermia, and locomotor activity, 24 h, 72 h, 10, 21, and 42 days after repeated cocaine or saline administration. The locomotor effects of cocaine (42 day) and the binding properties of ventral striatal D(2)-like and D(3) receptors (24 h and 42 days) were also evaluated.

RESULTS

Cocaine-treated rats displayed an enhanced locomotor response to cocaine, as well as a progressive and persistent leftward/upward shift of the ascending limb (72 h-42 day) and leftward shift of the descending limb (42 days) of the pramipexole-induced yawning dose-response curve. Cocaine treatment also decreased B (max) and K (d) for D(2)-like receptors and increased D(3) receptor binding at 42 days. Cocaine treatment did not change pramipexole-induced hypothermia or locomotor activity or yawning induced by cholinergic or serotonergic agonists.

CONCLUSIONS

These studies suggest that temporal differences exist in the development of cocaine-induced sensitization of D(3) and D(2) receptors, with enhancements of D(3)-mediated behavioral effects observed within 72 h and enhancements of D(2)-mediated behavioral effects apparent 42 days after cocaine. These findings highlight the need to consider changes in D(3) receptor function when thinking about the behavioral plasticity that occurs during abstinence from cocaine use.

Effects of adenosine A2A receptor stimulation on cocaine-seeking behavior in rats

RATIONALE

Dopamine (DA) receptor stimulation in the nucleus accumbens (NAc) plays an important role in regulating cocaine-seeking behavior. Adenosine receptors antagonize the effects of DA receptor stimulation on intracellular signaling, neuronal output, and behavior.

OBJECTIVES

The goal of the present study is to determine the effects of adenosine A(2A) receptor stimulation on reinstatement of cocaine-seeking behavior in rats.

METHODS

Rats were trained to lever press for cocaine in daily self-administration sessions on a fixed-ratio 1 schedule for 3 weeks. After 1 week of abstinence, lever pressing was extinguished in six daily extinction sessions. We subsequently assessed the effects of the adenosine A(2A) receptor agonist, CGS 21680, on cocaine-, quinpirole (D(2) agonist)-, and cue-induced reinstatement to cocaine seeking. We also assessed the effects of CGS 21680 on sucrose seeking in rats extinguished from sucrose self-administration.

RESULTS

Pretreatment of CGS 21680 dose-dependently blunted cocaine-induced reinstatement (15 mg/kg, i.p.). Pretreatment with CGS 21680 (0.03 mg/kg, i.p.) also attenuated quinpirole- and cue-induced reinstatement. A minimally effective dose of CGS 21680 failed to alter cocaine-induced locomotor activity or sucrose seeking.

CONCLUSIONS

Stimulation of adenosine A(2A) receptors antagonizes reinstatement of cocaine seeking elicited by cocaine, DA D(2)-receptor stimulation, and cocaine-conditioned cues. These findings suggest that adenosine A(2A) receptor stimulation may oppose DA D(2) receptor signaling in the NAc that mediates cocaine relapse.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Dopamine D1 receptors in cocaine dependence measured with PET and the choice to self-administer cocaine

The goal of this study was to determine D(1) receptor availability in human cocaine-dependent (CD) subjects and matched healthy controls (HCs). In addition, the CD subjects performed cocaine self-administration sessions in order to explore the association between D(1) receptor availability and cocaine-seeking behavior. Twenty-five CD subjects (40+/-4 years, 19M/6 F) and 23 matched HCs (38+/-4 years, 19M/4F) were scanned with PET and the radiotracer [(11)C]NNC 112. During the cocaine self-administration sessions, CD volunteers were given the choice to self-administer cocaine (0, 6, and 12 mg) or to receive a monetary voucher worth $5. D(1) receptor availability was measured in the limbic, associative, and sensori-motor striatum in addition to cortical brain regions. No difference in D(1) receptor availability was seen between the two groups. A negative association was seen between D(1) receptor BP(ND) in the limbic striatum and the choice for the 6 mg dose of cocaine (r=-0.47, p=0.02, corrected for age). These results do not support the hypothesis that cocaine dependence is associated with a reduction in D(1) receptor availability in the striatum. However, within the CD subjects, low D(1) receptor availability in the ventral striatum was associated with the choice to self-administer cocaine, suggesting that low D(1) receptor availability may be associated with an increased risk of relapse in cocaine dependence.

Effects of intra-nucleus accumbens shell administration of dopamine agonists and antagonists on cocaine-taking and cocaine-seeking behaviors in the rat

RATIONALE

Dopamine signaling in the nucleus accumbens (NAc) plays an important role in regulating drug-taking and drug-seeking behaviors, but the role of D(1)- and D(2)-like receptors in this regulation remains unclear.

OBJECTIVES

Our objective was to study the role of NAc D(1)- and D(2)-like receptors in the reinstatement of cocaine-seeking behavior and the regulation of stabilized cocaine intake in rats.

METHODS

Using a within-session reinstatement procedure, whereby animals self-administer cocaine (90 min) and extinguish responding (150 min) in a single session, we assessed the ability of NAc microinfusions of the D(1) agonist SKF 81297 and the D(2) agonist 7-OH-DPAT to reinstate extinguished cocaine seeking. The effects of the D(1) antagonist SCH 23390 and the D(2) antagonist eticlopride pretreatment on agonist- and cocaine-primed reinstatement were also measured. Similar agonist and antagonist treatments were tested for their ability to modulate stabilized cocaine and sucrose self-administration.

RESULTS

Intra-NAc infusions of either SKF 81297 (0.3-3.0 microg) or 7-OH-DPAT (1.0-10.0 microg) dose-dependently reinstated cocaine seeking with greater efficacy in the medial core than in the shell subregion and at doses that also stimulated locomotor behavior. Intra-NAc shell infusions of SCH 23390 (1.0 microg) and eticlopride (3.0-10.0 microg) blocked cocaine-primed reinstatement (2.0 mg/kg, i.v.) and indiscriminately blocked reinstatement induced by either intra-NAc D(1) or D(2) agonists. Doses of agonists that triggered reinstatement failed to alter stabilized cocaine intake, whereas doses of antagonists that blocked reinstatement increased cocaine intake in the shell.

CONCLUSIONS

Both D(1) and D(2) receptors in the NAc play a prominent, and perhaps cooperative, role in regulating cocaine-taking and cocaine-seeking behaviors.