A genetic variant of fatty acid amide hydrolase (FAAH) exacerbates hormone-mediated orexigenic feeding in mice.

Fatty acid amide hydrolase (FAAH) degrades the endocannabinoid anandamide. A polymorphism in FAAH (FAAH C385A) reduces FAAH expression, increases anandamide levels, and increases the risk of obesity. Nevertheless, some studies have found no association between FAAH C385A and obesity. We investigated whether the environmental context governs the impact of FAAH C385A on metabolic outcomes. Using a C385A knock-in mouse model, we found that FAAH A/A mice are more susceptible to glucocorticoid-induced hyperphagia, weight gain, and activation of hypothalamic AMP-activated protein kinase (AMPK). AMPK inhibition occluded the amplified hyperphagic response to glucocorticoids in FAAH A/A mice. FAAH knockdown exclusively in agouti-related protein (AgRP) neurons mimicked the exaggerated feeding response of FAAH A/A mice to glucocorticoids. FAAH A/A mice likewise presented exaggerated orexigenic responses to ghrelin, while FAAH knockdown in AgRP neurons blunted leptin anorectic responses. Together, the FAAH A/A genotype amplifies orexigenic responses and decreases anorexigenic responses, providing a putative mechanism explaining the diverging human findings.

Endocannabinoid regulation of homeostatic feeding and stress-induced alterations in food intake in male rats.

BACKGROUND AND PURPOSE: Stress is known to reduce food intake. Many aspects of the stress response and feeding are regulated by the endocannabinoid system, but the roles of anandamide (AEA) and 2-arachidonoyl glycerol (2-AG) in stress-induced anorexia are unclear. EXPERIMENTAL APPROACH: Effects of acute restraint stress on endocannabinoids were investigated in male Sprague-Dawley rats. Systemic and central pharmacological inhibition of fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL) was used to assess the effects of elevated AEA and 2-AG on homeostatic feeding and on food consumption after stress. Animals were pretreated with the FAAH inhibitor, PF-04457845, or the MAGL inhibitor, MJN110, before 2 h acute restraint stress or 2 h homecage period without food. KEY RESULTS: Restraint stress decreased hypothalamic and circulating AEA, with no effect in the gastrointestinal tract, while 2-AG content in the jejunum (but not duodenum) was reduced. PF-04457845 (30 µg), given i.c.v., attenuated stress-induced anorexia via CB1 receptors, but reduced homeostatic feeding in unstressed animals through an unknown mechanism. On the other hand, systemic administration of MJN110 (10 mg·kg-1 ) reduced feeding, regardless of stress or feeding status and inhibited basal intestinal transit in unstressed rats. The ability of MAGL inhibition to reduce feeding in combination with stress was independent of CB1 receptor signalling in the gut as the peripherally restricted CB1 receptor antagonist, AM6545 did not block this effect. CONCLUSIONS AND IMPLICATIONS: Our data reveal diverse roles for 2-AG and AEA in homeostatic feeding and changes in energy intake following stress. LINKED ARTICLES: This article is part of a themed section on 8th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Role for fatty acid amide hydrolase (FAAH) in the leptin-mediated effects on feeding and energy balance.

Endocannabinoid signaling regulates feeding and metabolic processes and has been linked to obesity development. Several hormonal signals, such as glucocorticoids and ghrelin, regulate feeding and metabolism by engaging the endocannabinoid system. Similarly, studies have suggested that leptin interacts with the endocannabinoid system, yet the mechanism and functional relevance of this interaction remain elusive. Therefore, we explored the interaction between leptin and endocannabinoid signaling with a focus on fatty acid amide hydrolase (FAAH), the primary degradative enzyme for the endocannabinoid N-arachidonoylethanolamine (anandamide; AEA). Mice deficient in leptin exhibited elevated hypothalamic AEA levels and reductions in FAAH activity while leptin administration to WT mice reduced AEA content and increased FAAH activity. Following high fat diet exposure, mice developed resistance to the effects of leptin administration on hypothalamic AEA content and FAAH activity. At a functional level, pharmacological inhibition of FAAH was sufficient to prevent leptin-mediated effects on body weight and food intake. Using a novel knock-in mouse model recapitulating a common human polymorphism (FAAH C385A; rs324420), which reduces FAAH activity, we investigated whether human genetic variance in FAAH affects leptin sensitivity. While WT (CC) mice were sensitive to leptin-induced reductions in food intake and body weight gain, low-expressing FAAH (AA) mice were unresponsive. These data demonstrate that FAAH activity is required for leptin's hypophagic effects and, at a translational level, suggest that a genetic variant in the FAAH gene contributes to differences in leptin sensitivity in human populations.

The Lateral Habenula Directs Coping Styles Under Conditions of Stress via Recruitment of the Endocannabinoid System.

BACKGROUND: The ability to effectively cope with stress is a critical determinant of disease susceptibility. The lateral habenula (LHb) and the endocannabinoid (ECB) system have independently been shown to be involved in the selection of stress coping strategies, yet the role of ECB signaling in the LHb remains unknown. METHODS: Using a battery of complementary techniques in rats, we examined the localization of type-1 cannabinoid receptors (CB1Rs) and assessed the behavioral and neuroendocrine effects of intra-LHb CB1R manipulations. We further tested the extent to which the ECB system in the LHb is impacted following chronic unpredictable stress or social defeat stress, and whether manipulation of LHb CB1Rs can bias coping strategies in rats with a history of chronic stress. RESULTS: Electron microscopy studies revealed CB1R expression on presynaptic axon terminals, postsynaptic membranes, mitochondria, and glial processes in the rat LHb. In vivo microdialysis experiments indicated that acute stress increased the amount of 2-arachidonoylglycerol in the LHb, while intra-LHb CB1R blockade increased basal corticosterone, augmented proactive coping strategies, and reduced anxiety-like behavior. Basal LHb 2-arachidonoylglycerol content was similarly elevated in rats that were subjected to chronic unpredictable stress or social defeat stress and positively correlated with adrenal weight. Finally, intra-LHb CB1R blockade increased proactive behaviors in response to a novel conspecific, increasing approach behaviors irrespective of stress history and decreasing the latency to be attacked during an agonistic encounter. CONCLUSIONS: Alterations in LHb ECB signaling may be relevant for development of stress-related pathologies in which LHb dysfunction and stress-coping impairments are hallmark symptoms.

Sex- and hormone-dependent alterations in alcohol withdrawal-induced anxiety and corticolimbic endocannabinoid signaling.

Alcohol dependence is associated with anxiety during withdrawal. The endocannabinoid (ECB) system participates in the neuroendocrine and behavioral response to stress and changes in corticolimbic ECB signaling may contribute to alcohol withdrawal-induced anxiety. Moreover, symptoms of alcohol withdrawal differ between sexes and sexual dimorphism in withdrawal-induced ECB recruitment may be a contributing factor. Herein, we exposed intact male and female rats and ovariectomized (OVX) female rats with or without estradiol (E2) replacement to 6 weeks of chronic intermittent alcohol vapor and measured anxiety-like behavior, ECB content, and ECB-related mRNA in the basolateral amygdala (BLA) and ventromedial prefrontal cortex (vmPFC). Acute alcohol withdrawal increased anxiety-like behavior, produced widespread disturbances in ECB-related mRNA, and reduced anandamide (AEA) content in the BLA and 2-arachidonoylglycerol (2-AG) content in the vmPFC of male, but not female rats. Similar to males, alcohol-exposed OVX females showed reductions in Napepld mRNA in the BLA, decreased AEA content in the BLA and vmPFC, and reductions in all ECB-related genes measured in the vmPFC. Importantly, E2 replacement prevented withdrawal-induced alterations in ECB content (but not mRNA) in OVX females, and although alcohol-exposed OVX females failed to exhibit more anxiety compared to their respective control, chronic alcohol exposure abolished the anxiolytic properties of E2 in OVX rats. These data indicate that ovarian sex hormones (but not E2 alone) protect against withdrawal-induced alterations in corticolimbic ECB signaling but do not impart resilience to withdrawal-induced anxiety. Thus, the mechanisms implicated in the manifestation of alcohol withdrawal-induced anxiety are most likely sex-specific. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

A comparison of novel, selective fatty acid amide hydrolase (FAAH), monoacyglycerol lipase (MAGL) or dual FAAH/MAGL inhibitors to suppress acute and anticipatory nausea in rat models.

RATIONALE: Drugs that block fatty acid amide hydrolase (FAAH, which elevates anandamide [AEA]) and drugs which block monoacylglycerol (MAGL, which elevates 2-arachidonyl glycerol [2-AG]) have promise in treating both acute and anticipatory nausea in human patients. OBJECTIVE: This study aims to evaluate the relative effectiveness of dual MAGL/FAAH inhibition with either alone to reduce acute and anticipatory nausea in rat models. MATERIALS AND METHODS: AM4302, a new dual MAGL/FAAH inhibitor, was compared with a new selective MAGL inhibitor, AM4301, and new selective FAAH inhibitor, AM4303, for their potential to reduce acute nausea (gaping in taste reactivity) and anticipatory nausea (contextually elicited conditioned gaping) in two rat models. RESULTS: Our in vitro studies indicate that AM4302 blocks human and rat FAAH: IC50 60 and 31 nM, respectively, with comparable potencies against human MAGL (IC50 41 nM) and rat MAGL (IC50 200 nM). AM4301 selectively blocks human and rat MAGL (IC50 8.9 and 36 nM, respectively), while AM4303 selectively inhibits human and rat FAAH (IC50 2 and 1.9 nM), respectively. Our in vivo studies show that the MAGL inhibitor, AM4301, suppressed acute nausea in a CB1-mediated manner, when delivered systemically or into the interoceptive insular cortex. Although the dual FAAH/MAGL inhibitor, AM4302, was equally effective as the FAAH inhibitor or MAGL inhibitor in reducing acute nausea, it was more effective than both in suppressing anticipatory nausea. CONCLUSIONS: Dual FAAH and MAGL inhibition with AM4302 may be an especially effective treatment for the very difficult to treat symptom of anticipatory nausea.

Endocannabinoid regulation of nausea is mediated by 2-arachidonoylglycerol (2-AG) in the rat visceral insular cortex.

Cannabinoid (CB) agonists suppress nausea in humans and animal models; yet, their underlying neural substrates remain largely unknown. Evidence suggests that the visceral insular cortex (VIC) plays a critical role in nausea. Given the expression of CB1 receptors and the presence of endocannabinoids in this brain region, we hypothesized that the VIC endocannabinoid system regulates nausea. In the present study, we assessed whether inhibiting the primary endocannabinoid hydrolytic enzymes in the VIC reduces acute lithium chloride (LiCl)-induced conditioned gaping, a rat model of nausea. We also quantified endocannabinoid levels during an episode of nausea, and assessed VIC neuronal activation using the marker, c-Fos. Local inhibition of monoacylglycerol lipase (MAGL), the main hydrolytic enzyme of 2-arachidonylglycerol (2-AG), reduced acute nausea through a CB1 receptor mechanism, whereas inhibition of fatty acid amide hydrolase (FAAH), the primary catabolic enzyme of anandamide (AEA), was without effect. Levels of 2-AG were also selectively elevated in the VIC during an episode of nausea. Inhibition of MAGL robustly increased 2-AG in the VIC, while FAAH inhibition had no effect on AEA. Finally, we demonstrated that inhibition of MAGL reduced VIC Fos immunoreactivity in response to LiCl treatment. Taken together, these findings provide compelling evidence that acute nausea selectively increases 2-AG in the VIC, and suggests that 2-AG signaling within the VIC regulates nausea by reducing neuronal activity in this forebrain region.

Cannabinoid Regulation of Acute and Anticipatory Nausea.

Chemotherapy-induced nausea is one of the most distressing symptoms reported by patients undergoing treatment, and even with the introduction of newer antiemetics such as ondansetron and aprepitant, nausea remains problematic in the clinic. Indeed, when acute nausea is not properly managed, the cues of the clinic can become associated with this distressing symptom resulting in anticipatory nausea for which no effective treatments are available. Clinical trials exploring the potential of exogenous or endogenous cannabinoids to reduce chemotherapy-induced nausea are sparse; therefore, we must rely on the data from pre-clinical rat models of nausea. In this review, we explore the human and pre-clinical animal literature examining the potential for exogenous and endogenous cannabinoid treatments to regulate chemotherapy-induced nausea. The pre-clinical evidence points to a compelling need to evaluate the antinausea potential of cannabidiol, cannabidiolic acid, and treatments that boost the functioning of the endocannabinoid system in human clinical trials.

Endocannabinoid Mechanisms Influencing Nausea.

One of the first recognized medical uses of Δ(9)-tetrahydrocannabinol was treatment of chemotherapy-induced nausea and vomiting. Although vomiting is well controlled with the currently available non-cannabinoid antiemetics, nausea continues to be a distressing side effect of chemotherapy and other disorders. Indeed, when nausea becomes conditionally elicited by the cues associated with chemotherapy treatment, known as anticipatory nausea (AN), currently available antiemetics are largely ineffective. Considerable evidence demonstrates that the endocannabinoid system regulates nausea in humans and other animals. In this review, we describe recent evidence suggesting that cannabinoids and manipulations that enhance the functioning of the natural endocannabinoid system are promising treatments for both acute nausea and AN.

Intraperirhinal cortex administration of the synthetic cannabinoid, HU210, disrupts object recognition memory in rats.

Cannabinoids disrupt learning and memory in human and nonhuman participants. Object recognition memory, which is particularly susceptible to the impairing effects of cannabinoids, relies critically on the perirhinal cortex (PRh); however, to date, the effects of cannabinoids within PRh have not been assessed. In the present study, we evaluated the effects of localized administration of the synthetic cannabinoid, HU210 (0.01, 1.0 µg/hemisphere), into PRh on spontaneous object recognition in Long-Evans rats. Animals received intra-PRh infusions of HU210 before the sample phase, and object recognition memory was assessed at various delays in a subsequent retention test. We found that presample intra-PRh HU210 dose dependently (1.0 µg but not 0.01 µg) interfered with spontaneous object recognition performance, exerting an apparently more pronounced effect when memory demands were increased. These novel findings show that cannabinoid agonists in PRh disrupt object recognition memory.

Effect of selective inhibition of monoacylglycerol lipase (MAGL) on acute nausea, anticipatory nausea, and vomiting in rats and Suncus murinus.

RATIONALE: To determine the role of the endocannabinoid, 2-arachodonyl glycerol (2-AG), in the regulation of nausea and vomiting. OBJECTIVE: We evaluated the effectiveness of the potent selective monoacylglycerol lipase (MAGL) inhibitor, MJN110, which selectively elevates the endocannabinoid 2-AG, to suppress acute nausea and vomiting, as well as anticipatory nausea in rat and shrew models. METHODS: The rat gaping models were used to evaluate the potential of MJN110 (5, 10, and 20 mg/kg, intraperitoneally [IP]) to suppress acute nausea produced by LiCl and of MJN110 (10 and 20 mg/kg, IP) to suppress anticipatory nausea elicited by a LiCl-paired context. The potential as well of MJN110 (10 and 20 mg/kg, IP) to suppress vomiting and contextually elicited gaping in the Suncus murinus was evaluated. RESULTS: MJN110 suppressed acute nausea in rats, LiCl-induced vomiting in shrews and contextually-elicited anticipatory nausea in both rats (accompanied by elevation of 2-AG in the visceral insular cortex) and shrews. These effects were reversed by the CB1 antagonist/inverse agonist, SR141716. The MAGL inhibitor did not modify locomotion at any dose. An activity-based protein profiling analysis of samples of tissue collected from the visceral insular cortex in rats and whole brain tissues in shrews revealed that MJN110 selectively inhibited MAGL and the alternative 2-AG hydrolase, ABHD6. CONCLUSIONS: MAGL inhibition by MJN110 which selectively elevates endogenous 2-AG has therapeutic potential in the treatment of acute nausea and vomiting as well as anticipatory nausea, a distressful symptom that is resistant to currently available treatments.

Second-order conditioning of LiCl-induced gaping with flavor and contextual cues.

Conditioned gaping occurs through a classically conditioned association between a flavor or a context (CS) and an unconditioned stimulus (US) that produces nausea, such as lithium chloride (LiCl; US). Rats display conditioned gaping to a flavor or context previously associated with nausea; thus, our aim was to investigate whether rats acquire second-order conditioning to a flavor experienced in a nausea-paired context. In Experiment 1, rats were assigned to one of three groups, based upon the contingency of the first order pairing (CS1 context and LiCl) and the contingency of the second-order pairing (CS2 saccharin CS1 context) including: Group Paired/Paired (P/P), Group Paired/Unpaired (P/U) and Group Unpaired/Paired (U/P). In the initial context conditioning, rats were injected with LiCl (Paired) or Saline (Unpaired) prior to a 30 min confinement in a distinctive context (CS1). Drug-free second-order conditioning training among Groups P/P and U/P then consisted of a 5 min intraoral infusion of 0.1 % saccharin (CS2) in the context (CS1), while Group P/U received saccharin in the home cage 24 hr prior to the CS1 exposure. Twenty four hr later, the rats were tested for second-order conditioning during a 2 min taste reactivity (TR) test. Saccharin (CS2) elicited gaping in Group P/P, but not Groups P/U or U/P. Experiment 2 revealed that second-order conditioning was produced in rats given 4 or 8 first-order conditioning trials, but not 2 trials. These results demonstrate that an excitatory contextual CS+ has the potential to confer second-order conditioning to a novel flavor in the absence of any direct pairing with LiCl.

CB1 receptor antagonism in the granular insular cortex or somatosensory area facilitates consolidation of object recognition memory.

Cannabinoid agonists typically impair memory, whereas CB1 receptor antagonists enhance memory performance under specific conditions. The insular cortex has been implicated in object memory consolidation. Here we show that infusions of the CB1 receptor antagonist SR141716 enhances long-term object recognition memory in rats in a dose-dependent manner (facilitation with 1.5, but not 0.75 or 3 µg/µL) when administered into the granular insular cortex; the SR141716 facilitation was seen with a memory delay of 72 h, but not when the delay was shorter (1 h), consistent with enhancement of memory consolidation. Moreover, a sub-group of rats with cannulas placed in the somatosensory area were also facilitated. These results highlight the robust potential of cannabinoid antagonists to facilitate object memory consolidation, as well as the capacity for insular and somatosensory cortices to contribute to object processing, perhaps through enhancement of tactile representation.

A comparison of cannabidiolic acid with other treatments for anticipatory nausea using a rat model of contextually elicited conditioned gaping.

RATIONALE: The effectiveness of cannabidiolic acid (CBDA) was compared with other potential treatments for anticipatory nausea (AN), using a rat model of contextually elicited conditioned gaping reactions. OBJECTIVE: The potential of ondansetron (OND), Δ(9)-tetrahydrocannabinol (THC), chlordiazepoxide (CDP), CBDA, and co-administration of CBDA and tetrahydrocannabinolic acid (THCA) to reduce AN and modify locomotor activity was evaluated. MATERIALS AND METHODS: Following four pairings of a novel context with lithium chloride (LiCl), the rats were given a test for AN. On the test trial, they received pretreatment injections of the following: vehicle, OND (0.1 or 1.0 mg/kg), THC (0.5 mg/kg), CBDA (0.0001, 0.001, 0.01, 0.1 mg/kg or 1.0 mg/kg), CDP (1, 5, or 10 mg/kg) or co-administration of subthreshold doses of CBDA (0.1 µg/kg), and THCA (5 µg/kg). Immediately following the AN test trial in all experiments, rats were given a 15 min locomotor activity test. Finally, the potential of CBDA (0.001, 0.01, 0.1, and 1 mg/kg) to attenuate conditioned freezing to a shock-paired tone was assessed. RESULTS: THC, CBDA, and CDP, but not OND, reduced contextually elicited gaping reactions. Co-administration of subthreshold doses of CBDA and THCA also suppressed AN, and this effect was blocked by pretreatment with either a cannabinoid receptor 1 (CB1) receptor antagonist or a 5-hydroxytryptamine 1A (5-HT1A) receptor antagonist. CDP (but not CBDA, THC or CBDA and THCA) also suppressed locomotor activity at effective doses. CBDA did not modify the expression of conditioned fear. CONCLUSIONS: CBDA has therapeutic potential as a highly potent and selective treatment for AN without psychoactive or locomotor effects.

Evaluation of the potential of the phytocannabinoids, cannabidivarin (CBDV) and Δ(9) -tetrahydrocannabivarin (THCV), to produce CB1 receptor inverse agonism symptoms of nausea in rats.

BACKGROUND AND PURPOSE: The cannabinoid 1 (CB1 ) receptor inverse agonists/antagonists, rimonabant (SR141716, SR) and AM251, produce nausea and potentiate toxin-induced nausea by inverse agonism (rather than antagonism) of the CB1 receptor. Here, we evaluated two phytocannabinoids, cannabidivarin (CBDV) and Δ(9) -tetrahydrocannabivarin (THCV), for their ability to produce these behavioural effect characteristics of CB1 receptor inverse agonism in rats. EXPERIMENTAL APPROACH: In experiment 1, we investigated the potential of THCV and CBDV to produce conditioned gaping (measure of nausea-induced behaviour) in the same manner as SR and AM251. In experiment 2, we investigated the potential of THCV and CBDV to enhance conditioned gaping produced by a toxin in the same manner as CB1 receptor inverse agonists. KEY RESULTS: SR (10 and 20 mg·kg(-1) ) and AM251 (10 mg·kg(-1) ) produced conditioned gaping; however, THCV (10 or 20 mg·kg(-1) ) and CBDV (10 or 200 mg·kg(-1) ) did not. At a subthreshold dose for producing nausea, SR (2.5 mg·kg(-1) ) enhanced lithium chloride (LiCl)-induced conditioned gaping, whereas Δ(9) -tetrahydrocannabinol (THC, 2.5 and 10 mg·kg(-1) ), THCV (2.5 or 10 mg·kg(-1) ) and CBDV (2.5 or 200 mg·kg(-1) ) did not; in fact, THC (2.5 and 10 mg·kg(-1) ), THCV (10 mg·kg(-1) ) and CBDV (200 mg·kg(-1) ) suppressed LiCl-induced conditioned gaping, suggesting anti-nausea potential. CONCLUSIONS AND IMPLICATIONS: The pattern of findings indicates that neither THCV nor CBDV produced a behavioural profile characteristic of CB1 receptor inverse agonists. As well, these compounds may have therapeutic potential in reducing nausea.

Inhibition of monoacylglycerol lipase attenuates vomiting in Suncus murinus and 2-arachidonoyl glycerol attenuates nausea in rats.

BACKGROUND AND PURPOSE: To evaluate the role of 2-arachidonoyl glycerol (2AG) in the regulation of nausea and vomiting using animal models of vomiting and of nausea-like behaviour (conditioned gaping). EXPERIMENTAL APPROACH: Vomiting was assessed in shrews (Suncus murinus), pretreated with JZL184, a selective monoacylglycerol lipase (MAGL) inhibitor which elevates endogenous 2AG levels, 1 h before administering the emetogenic compound, LiCl. Regulation of nausea-like behaviour in rats by exogenous 2AG or its metabolite arachidonic acid (AA) was assessed, using the conditioned gaping model. The role of cannabinoid CB(1) receptors, CB(2) receptors and cyclooxygenase (COX) inhibition in suppression of vomiting or nausea-like behaviour was assessed. KEY RESULTS: JZL184 dose-dependently suppressed vomiting in shrews, an effect prevented by pretreatment with the CB(1) receptor inverse agonist/antagonist, AM251. In shrew brain tissue, JZL184 inhibited MAGL activity in vivo. In rats, 2AG suppressed LiCl-induced conditioned gaping but this effect was not prevented by AM251 or the CB(2) receptor antagonist, AM630. Instead, the COX inhibitor, indomethacin, prevented suppression of conditioned gaping by 2AG or AA. However, when rats were pretreated with a high dose of JZL184 (40 mg·kg(-1) ), suppression of gaping by 2AG was partially reversed by AM251. Suppression of conditioned gaping was not due to interference with learning because the same dose of 2AG did not modify the strength of conditioned freezing to a shock-paired tone. CONCLUSIONS AND IMPLICATIONS: Our results suggest that manipulations that elevate 2AG may have anti-emetic or anti-nausea potential. LINKED ARTICLES: This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Reacquisition of heroin and cocaine place preference involves a memory consolidation process sensitive to systemic and intra-ventral tegmental area naloxone.

To investigate the effect of naloxone on a putative memory consolidation process underlying reacquisition of heroin and cocaine conditioned place preference, four studies were conducted in male Sprague-Dawley rats using a common procedure involving: place conditioning (0.3 or 1mg/kg heroin or 20mg/kg cocaine; x4 sessions), extinction (vehiclex4 sessions), and reconditioning (0 or 1mg/kg heroin or 20mg/kg cocaine; x1 session). Systemic naloxone injections (0, 1 and 3mg/kg) or bilateral intra-ventral tegmental area (VTA) naloxone methiodide infusions (2 nmol in 0.5 microl x side) were administered at different times following reconditioning. Post-reconditioning administration of naloxone dose-dependently blocked, attenuated and had no effect on reacquisition of heroin CPP when administered immediately, 1h and 6h after reconditioning, respectively. The highest dose of naloxone also blocked reacquisition of cocaine CPP, and did not produce a conditioned place aversion in heroin-naïve and heroin pre-treated animals. Post-reconditioning infusions in the VTA, but not in adjacent structures, blocked reacquisition of heroin CPP when administered immediately, but not 6h, after reconditioning. These data suggest that reacquisition of drug-cues associations involves a memory consolidation process sensitive to manipulations of the endogenous opioid system, and indicate that opioid receptors in the VTA may be critically involved in the re-emergence of drug seeking behavior.