Distinctive Neuroanatomic Regions Involved in Cocaine-Induced Behavioral Sensitization in Mice.

The present study aimed to characterize the phenomenon of behavioral sensitization to cocaine and to identify neuroanatomical structures involved in the induction and expression phases of this phenomenon. For this, in experiment 1 (induction phase), mice were treated with saline or cocaine every second day for 15 days (conditioning period), in the open-field or in their home-cages. In experiment 2 (expression phase), the same protocol was followed, except that after the conditioning period the animals were not manipulated for 10 days, and after this interval, animals were challenged with cocaine. Neuroanatomical structures involved in the induction and expression phases were identified by stereological quantification of c-Fos staining in the dorsomedial prefrontal cortex (dmPFC), nucleus accumbens core (NAc core and shell (NAc shell), basolateral amygdala (BLA), and ventral tegmental area (VTA). Neuroanatomical analysis indicated that in the induction phase, cocaine-conditioned animals had higher expression of c-Fos in the dmPFC, NAc core, BLA, and VTA, whereas in the expression phase, almost all areas had higher expression except for the VTA. Therefore, environmental context plays a major role in the induction and expression of behavioral sensitization, although not all structures that compose the mesolimbic system contribute to this phenomenon.

Context-dependent effects of the CB1 receptor antagonist rimonabant on morphine-induced behavioral sensitization in female mice.

Introduction: The endocannabinoid system has been implicated in the neurobiology of opioid use disorder. While the CB1 receptor antagonist rimonabant has been shown to block some of the behavioral effects of opioids, studies suggest that the treatment environment (i.e., receiving treatment in the drug-associated environment, and/or novelty) can influence its effects. In the present study, we investigated the role of the treatment environment in the effects of rimonabant on the expression of morphine-induced behavioral sensitization. Methods: Adult female Swiss mice were submitted to a behavioral sensitization protocol, during which they received morphine (20 mg/kg, i.p.) in the open-field apparatus, and were subsequently treated with vehicle or rimonabant (1 or 10 mg/kg, i.p.) either in the open-field, in the home-cage or in an activity box (novel environment). The expression of conditioned locomotion (increased locomotor activity in the open-field apparatus in the absence of morphine) and of morphine-induced behavioral sensitization (increased locomotor activity in animals sensitized to morphine) was evaluated during asubsequent saline and morphine challenge, respectively. Results: Animals treated with morphine expressed behavioral sensitization, showing a significant increase in locomotor activity over time. Animals sensitized to morphine and treated with vehicle in the home-cage expressed conditioned locomotion, an effect that was blocked by home-cage treatment with rimonabant. During a saline challenge, only animals sensitized to morphine and treated with saline in the home-cage expressed morphine-induced conditioned locomotion. All morphine-treated animals that received saline during the treatment phase (control groups) expressed behavioral sensitization during the morphine challenge. Treatment with rimonabant in the open-field and in the activity box, but not in the home-cage, blocked the expression of morphine-induced behavioral sensitization. Discussion: Our findings suggest that CB1 receptor antagonism can modulate conditioned responses to morphine even when administered in the home-cage. However, exposure to the drug-associated environment or to a novel environment is necessary for the expression of rimonabant's effects on morphine-induced behavioral sensitization during a morphine challenge.

Ibogaine Blocks Cue- and Drug-Induced Reinstatement of Conditioned Place Preference to Ethanol in Male Mice.

Ibogaine is a psychedelic extracted from the plant Tabernanthe iboga Baill. (Apocynaceae), natural from Africa, and has been proposed as a potential treatment for substance use disorders. In animal models, ibogaine reduces ethanol self-administration. However, no study to date has investigated the effects of ibogaine on ethanol-induced conditioned place preference (CPP). The present study aimed to investigate the effects of repeated treatment with ibogaine on the reinstatement of CPP to ethanol in male mice. The rewarding effects of ethanol (1.8 g/kg, i. p.) or ibogaine (10 or 30 mg/kg, p. o.) were investigated using the CPP model. Furthermore, we evaluated the effects of repeated treatment with ibogaine (10 or 30 mg/kg, p. o.) on the reinstatement of ethanol-induced CPP. Reinstatement was evaluated under two conditions: 1) during a priming injection re-exposure test in which animals received a priming injection of ethanol and had free access to the CPP apparatus; 2) during a drug-free test conducted 24 h after a context-paired re-exposure, in which subjects received an injection of ethanol and were confined to the compartment previously conditioned to ethanol. Our results show that ethanol, but not ibogaine, induced CPP in mice. Treatment with ibogaine after conditioning with ethanol blocked the reinstatement of ethanol-induced CPP, both during a drug priming reinstatement test and during a drug-free test conducted after re-exposure to ethanol in the ethanol-paired compartment. Our findings add to the literature suggesting that psychedelics, in particular ibogaine, may have therapeutic properties for the treatment of alcohol use disorder at doses that do not have rewarding effects per se.

Ethanol-induced locomotor sensitization: Neuronal activation in the nucleus accumbens and medial prefrontal cortex.

Ethanol consumption may promote neuroplasticity and alterations in synapses, resulting in modifications in neuronal activity. Here, we treated male Swiss mice with ethanol (2.2 g/kg) or saline once per day for 21 consecutive days. Nine days after the last ethanol administration, they received a challenge injection of ethanol or saline, and we assessed locomotor activity. After the behavioral analysis, we evaluated neuronal activation in the medial Prefrontal Cortex (Cingulate, Prelimbic, and Infralimbic) and the Nucleus Accumbens (Shell and Core) using Fos/DAB immunohistochemistry. In another group of animals, we performed the quantitative analysis of the ARC and PSD-95 protein levels by Western blotting in the medial prefrontal cortex and nucleus accumbens brain areas. Repeated ethanol administration produced locomotor sensitization, accompanied by an increase in the nucleus accumbens shell's activation but not core. Furthermore, the ethanol pretreatment reduced ARC expression in the nucleus accumbens and medial prefrontal cortex. Our results suggest the participation of the nucleus accumbens shell in ethanol behavioral sensitization and add new pieces of evidence that neuroplasticity in synapses may contribute to the mechanism underlying this behavior.

Ayahuasca blocks the reinstatement of methylphenidate-induced conditioned place preference in mice: behavioral and brain Fos expression evaluations.

RATIONALE: Accumulating evidence suggests that ayahuasca, a hallucinogenic beverage used in traditional Amazonian communities for ritualistic and curative purposes, has been associated with reduced rates of substance use disorders. However, the brain mechanisms underlying the therapeutic effects of ayahuasca have not yet been fully elucidated. OBJECTIVES: The aim of the present study was to investigate the effects of treatment with ayahuasca on the rewarding properties of the psychostimulant methylphenidate. METHODS: The rewarding properties of ayahuasca (100 mg/kg, orally) and methylphenidate (10 mg/kg, i.p.) were investigated using the conditioned place preference (CPP) model. Furthermore, we evaluated the effects of repeated treatment with ayahuasca on the reinstatement of methylphenidate-induced CPP. Fos expression was evaluated in different limbic structures (cingulate cortex-area 1, prelimbic cortex, infralimbic cortex, orbitofrontal cortex-lateral orbital area, nucleus accumbens core and shell, ventral tegmental area, dorsal striatum, and basolateral amygdala) upon each experimental phase. RESULTS: Both ayahuasca and methylphenidate induced CPP in mice. However, ayahuasca had limited effects on Fos expression, while methylphenidate altered Fos expression in several brain regions associated with the behavioral effects of drugs of abuse. Treatment with ayahuasca after conditioning with methylphenidate blocked the reinstatement of methylphenidate-induced CPP. Those behavioral effects were accompanied by changes in Fos expression patterns, with ayahuasca generally blocking the changes in Fos expression induced by conditioning with methylphenidate and/or reexposure to methylphenidate. CONCLUSIONS: Our findings suggest that ayahuasca restored normal brain function in areas associated with the long-term expression of drug wanting/seeking in animals conditioned to methylphenidate.

Participation of Dopamine D1 and D2 Receptors in the Rapid-Onset Behavioral Sensitization to Modafinil.

Studies on the abuse potential of modafinil, a psychostimulant-like drug used to treat narcolepsy, are still controversial. While some studies claim no potential for abuse, increasing evidence suggests that modafinil induces abuse-related effects, including rapid-onset behavioral sensitization (i.e., a type of sensitization that develops within hours from the drug priming administration). The rapid-onset sensitization paradigm is a valuable tool to study the neuroplastic changes that occur quickly after drug administration, and shares neuroadaptations with drug abuse in humans. However, the mechanisms involved in the rapid-onset behavioral sensitization induced by modafinil are uncertain. Our aim was to investigate the possible involvement of dopamine D1 and D2 receptors on acute modafinil-induced hyperlocomotion and on the induction and expression of rapid-onset behavioral sensitization induced by modafinil in male Swiss mice. Treatment with the D1 receptor antagonist SCH 23390 or the D2 receptor antagonist sulpiride attenuated the acute modafinil-induced hyperlocomotion in a dose-dependent manner. Pretreatment with either antagonist before the priming injection of modafinil prevented the development of sensitization in response to a modafinil challenge 4 h later. However, only SCH 23390 decreased the expression of modafinil-induced rapid-onset behavioral sensitization. Taken together, the present findings provide evidence of the participation of D1 and D2 receptors on the development of rapid-onset behavioral sensitization to modafinil, and point to a prominent role of D1 receptors on the expression of this phenomenon.

Post-sensitization treatment with rimonabant blocks the expression of cocaine-induced behavioral sensitization and c-Fos protein in mice.

CB1 receptor antagonists have been shown to prevent acute and long-term behavioral effects of cocaine. Here we evaluate the effectiveness of the CB1 receptor antagonist rimonabant to modify sensitized responses to cocaine. Mice were treated with saline or cocaine injections in a 15-day intermittent sensitization treatment and subsequently treated with either vehicle, 1 or 10mg/kg rimonabant in the drug-associated environment for 8 consecutive days. Animals were then challenged with saline and cocaine in the open-field apparatus on subsequent days to evaluate the expression of conditioned and sensitized effects to cocaine. c-Fos protein expression was evaluated in the nucleus accumbens (NAcc), ventral tegmental area (VTA), basolateral amygdala (BLA), medial prefrontal cortex (mPFC) and caudate-putamen (CPu) after the last (cocaine) challenge. Previous treatment with 10mg/kg rimonabant blocked the expression of conditioned hyperlocomotion and behavioral sensitization to cocaine, but not acute cocaine-induced hyperlocomotion. These behavioral effects were accompanied by significant changes in c-Fos expression in the brain reward system. Chronic cocaine sensitization blunted a subsequent acute cocaine-induced increase in c-Fos protein in the NAcc, effect that was reversed by previous treatment with rimonabant. Treatment with 10mg/kg rimonabant also attenuated the significant increase in c-Fos expression in the CPu, mPFC and BLA induced by previous chronic sensitization with cocaine. Our findings add to the evidence that drugs targeting CB1 receptors are good candidates for the treatment of cocaine abuse and provide further insights into the mechanisms underlying endocannabinoid signaling within the brain reward system in the context of cocaine abuse.

Modafinil Induces Rapid-Onset Behavioral Sensitization and Cross-Sensitization with Cocaine in Mice: Implications for the Addictive Potential of Modafinil.

There is substantial controversy about the addictive potential of modafinil, a wake-promoting drug used to treat narcolepsy, proposed as pharmacotherapy for cocaine abuse, and used indiscriminately by healthy individuals due to its positive effects on arousal and cognition. The rapid-onset type of behavioral sensitization (i.e., a type of sensitization that develops within a few hours from the drug priming administration) has been emerged as a valuable tool to study binge-like patterns of drug abuse and the neuroplastic changes that occur quickly after drug administration that ultimately lead to drug abuse. Our aim was to investigate the possible development of rapid-onset behavioral sensitization to modafinil and bidirectional rapid-onset cross-sensitization with cocaine in male Swiss mice. A priming injection of a high dose of modafinil (64 mg/kg) induced rapid-onset behavioral sensitization to challenge injections of modafinil at the doses of 16, 32, and 64 mg/kg, administered 4 h later. Furthermore, rapid-onset cross-sensitization was developed between modafinil and cocaine (64 mg/kg modafinil and 20 mg/kg cocaine), in a bidirectional way. These results were not due to residual levels of modafinil as the behavioral effects of the priming injection of modafinil were no longer present and modafinil plasma concentration was reduced at 4 h post-administration. Taken together, the present findings provide preclinical evidence that modafinil can be reinforcing per se and can enhance the reinforcing effects of stimulants like cocaine within hours after administration.

Effects of rimonabant on the development of single dose-induced behavioral sensitization to ethanol, morphine and cocaine in mice.

RATIONALE: The endocannabinoid system has been implicated in the neurobiological mechanism underlying drug addiction, especially the primary rewarding dopamine-dependent processes. Therefore, endocannabinoid receptor antagonists, such as the CB1 cannabinoid antagonist rimonabant, have been proposed as candidates for preventive addiction therapies. OBJECTIVES: Investigate the possible involvement of CB1 receptors in the development of behavioral sensitization to ethanol, morphine and cocaine in mice. METHODS: We compared the effects of different doses of rimonabant (0.3, 1, 3 and 10mg/kg) on spontaneous locomotor activity in the open-field, hyperlocomotion induced by acute administration of ethanol (1.8g/kg), morphine (20mg/kg) or cocaine (10mg/kg) and on subsequent drug-induced locomotor sensitization using a two-injection protocol in mice. We also investigated a possible depressive-like effect of an acute rimonabant challenge at the highest dose and its potential anxiogenic property. RESULTS: At the highest dose, rimonabant abolished ethanol- and cocaine-induced hyperlocomotion and behavioral sensitization without modifying spontaneous and central locomotor activity or inducing depressive-like behavior on the forced swim test in mice. The other doses of rimonabant also selectively blocked acute ethanol-induced central hyperlocomotion. Although rimonabant at 0.3 and 1mg/kg potentiated the central hyperlocomotion induced by acute morphine injection, it was effective in attenuating morphine-induced behavioral sensitization at all doses. CONCLUSIONS: Because the neural basis of behavioral sensitization has been proposed to correspond to some components of addiction, our findings indicate that the endocannabinoid system might be involved in ethanol, cocaine and morphine abuse.