Stéphanie Caillé

Specific Alterations of Extracellular Endocannabinoid Levels in the Nucleus Accumbens by Ethanol, Heroin, and Cocaine Self-Administration

Ethanol and opiate self-administration are sensitive to manipulations of cannabinoid CB1 receptor function and, from this, a role for the endogenous cannabinoid system in the modulation of drug reward has been hypothesized. However, direct in vivo evidence of drug-induced alterations in brain endocannabinoid (eCB) formation has been lacking. To address this issue, we explored the effect of drug self-administration on interstitial eCB levels in the nucleus accumbens (NAc) shell using in vivo microdialysis. Ethanol, heroin, and cocaine were compared because the rewarding properties of ethanol and heroin are reduced by CB1 receptor inactivation, whereas cocaine reward is less sensitive to these manipulations. Ethanol self-administration significantly increased dialysate 2-arachidonoylglycerol (2-AG) levels with no concomitant change in dialysate anandamide (AEA) concentrations. Conversely, heroin self-administration significantly increased dialysate AEA levels, and induced a subtle but significant decrease in dialysate 2-AG levels. In each case, the relative change in dialysate eCB content was significantly correlated with the amount of drug consumed. In contrast, cocaine self-administration did not alter dialysate levels of either AEA or 2-AG. Local infusion of the CB1 antagonist SR 141716A into the NAc significantly reduced ethanol, but not cocaine, self-administration. Together with our previous observation that intra-NAc SR 141716A reduces heroin self-administration, these data provide novel in vivo support for an eCB involvement in the motivational properties of ethanol and heroin but not cocaine. Furthermore, the selective effects of ethanol and heroin on interstitial 2-AG and AEA provide new insight into the distinct neurochemical profiles produced by these two abused substances.

Neural correlates of the motivational and somatic components of naloxone-precipitated morphine withdrawal.

In morphine‐dependent rats, low naloxone doses have been shown to induce conditioned place aversion, which reflects the negative motivational component of opiate withdrawal. In contrast, higher naloxone doses are able to induce a ‘full’ withdrawal syndrome, including overt somatic signs. The c‐fos gene is commonly used as a marker of neuronal reactivity to map the neural substrates that are recruited by various stimuli. Using in situ hybridization, we have analysed in the brain of morphine‐dependent rats the effects of acute withdrawal syndrome precipitated by increasing naloxone doses on c‐fos mRNA expression. Morphine dependence was induced by subcutaneous implantation of slow‐release morphine pellets for 6 days and withdrawal was precipitated by increasing naloxone doses inducing the motivational (7.5 and 15 µg/kg) and somatic (30 and 120 µg/kg) components of withdrawal. Our mapping study revealed a dissociation between a set of brain structures (extended amygdala, lateral septal nucleus, basolateral amygdala and field CA1 of the hippocampus) which exhibited c‐fos mRNA dose‐dependent variations from the lowest naloxone doses, and many other structures (dopaminergic and noradrenergic nuclei, motor striatal areas, hypothalamic nuclei and periaqueductal grey) which were less sensitive and recruited only by the higher doses. In addition, we found opposite dose‐dependent variations of c‐fos gene expression within the central (increase) and the basolateral (decrease) amygdala after acute morphine withdrawal. Altogether, these results emphasize that limbic structures of the extended amygdala along with the lateral septal nucleus, the basolateral amygdala and CA1 could specifically mediate the negative motivational component of opiate withdrawal.

Cannabinoid Modulation of Opiate Reinforcement through the Ventral Striatopallidal Pathway

Recent evidence indicates that cannabinoid-1 (CB1) receptors play a role in the mediation of opiate reward, though the neural mechanisms for this process have not been characterized. The present experiments investigated the influence of CB1 receptors in the ventral striatopallidal system on opiate-induced neurochemical events and opiate self-administration behavior in rats. Acute morphine administration (3 mg/kg) significantly reduced ventral pallidal GABA efflux in a manner similar to that produced by heroin self-administration. This neurochemical effect was reversed by doses of the selective CB1 antagonist SR 141716A (Rimonabant; 1 and 3 mg/kg) that also significantly reduce opiate reward. Morphine-induced increases in nucleus accumbens dopamine levels were unaltered by SR 141716A. Intravenous heroin self-administration (0.02 mg/infusion) was significantly reduced by intra-accumbens, but not intraventral pallidal SR 141716A infusions (1 and 3 μg/side), implicating nucleus accumbens CB1 receptors in the modulation of opiate reinforcement. In contrast, SR14716A did not alter cocaine self-administration (0.125 mg/inf), cocaine-induced (10 mg/kg) decrements in ventral pallidal GABA efflux or cocaine-induced increases in accumbens dopamine. This is consistent with evidence that selective inactivation of CB1 receptors reduces opiate-, but not psychostimulant-maintained self-administration. The CB1 receptor agonist WIN 55,212-2 (5 mg/kg) reduced pallidal GABA efflux in a manner similar to morphine, and this effect was reversed by the opiate receptor antagonist naloxone. Collectively these findings suggest that CB1 receptors modulate opiate reward through the ventral striatopallidal projection and that the modulation of this projection system may be involved in the reciprocal behavioral effects between cannabinoids, and opioids.

The addition of five minor tobacco alkaloids increases nicotine-induced hyperactivity, sensitization and intravenous self-administration in rats

Several minor tobacco alkaloids have been found to exhibit properties pharmacologically relevant to the addictive profile of tobacco; however, little is known of their effects on a behavioural model of drug addiction. In this study we compared the locomotor and reinforcing effects of intravenous nicotine (30 microg/kg per infusion) vs. a cocktail of nicotine plus five minor alkaloids found in tobacco smoke (anabasine, nornicotine, anatabine, cotinine and myosmine). Rats were initially tested for their locomotor response to nicotine or nicotine plus the minor alkaloids with six intravenous injections over 1 h. We then assessed the spontaneous acquisition of intravenous self-administration with nicotine or nicotine plus the minor alkaloids, under a fixed-ratio 1 schedule followed by responding on a fixed-ratio 5 schedule, progressive-ratio schedule and a single within-session ascending dose-response test. The activity test was repeated following the progressive-ratio phase to assess locomotor sensitization. A second group of rats were then tested on the locomotor procedure to better clarify the role of each individual minor alkaloid in nicotine-induced locomotor activity. Compared to nicotine alone, addition of the minor tobacco alkaloids increased locomotor activity and increased locomotor sensitization following self-administration. During fixed-ratio 5, progressive ratio and the dose-response test, rats receiving nicotine plus the minor alkaloids responded significantly more than those receiving nicotine alone. Testing of each minor alkaloid in the second experiment indicated that anatabine, cotinine and myosmine individually increased nicotine-induced locomotor activity. These results suggest that the minor tobacco alkaloids, particularly anatabine, cotinine and myosmine, may increase the motivation for nicotine and thus facilitate smoking behaviour.

SR141716A reduces the reinforcing properties of heroin but not heroin-induced increases in nucleus accumbens dopamine in rats.

The present experiments tested the hypothesis that the selective CB1 receptor antagonist SR141716A alters heroin self‐administration by attenuating heroin‐induced increases in nucleus accumbens dopamine levels. SR141716A pretreatment dose‐dependently (0.3–3 mg/kg, i.p.) reduced operant heroin self‐administration by male Wistar rats under a fixed ratio schedule of reinforcement, and significantly lowered the breaking point of responding for heroin under a progressive ratio schedule of reinforcement. These observations are consistent with recent reports that CB1 receptor inactivation reduces the rewarding properties of opiates. Operant responding for water reinforcement by water‐restricted rats was unaltered by these SR141716A doses. Microdialysis tests revealed that heroin self‐administration significantly increases interstitial dopamine levels in the nucleus accumbens shell of vehicle‐pretreated control rats. However, whereas SR141716A pretreatment dose‐dependently reduced heroin self‐administration, it did not alter the heroin‐associated increase in nucleus accumbens dopamine. These findings suggest that the CB1 antagonist‐induced attenuation of heroin reward does not involve dopaminergic mechanisms in the nucleus accumbens shell.

Opiate withdrawal-induced place aversion lasts for up to 16 weeks

Abstract Rationale: Administration of low doses of opiate antagonists to morphine-dependent rats produces an aversive response as measured by a conditioned place aversion, but the time course of such a learned aversion is largely unknown. Objectives: The purpose of this experiment was to examine the time course for the expression of a place aversion to opiate withdrawal. Methods: Morphine-dependent rats were tested in a three-chamber place- aversion apparatus. The conditioning phase consisted of three pairings of either naloxone (15 µg/kg s.c.) or vehicle with two compartments, with the most similar time allotments during the preconditioning test. During the testing phase, rats were again allowed to explore the entire apparatus. Different groups were tested at 24 h, 1 week, 2 weeks, 4 weeks, 8 weeks, and 16 weeks post-conditioning (morphine-free tests). Results: A robust place aversion was recorded at every time point tested, including at 16 weeks. In previously published work, placebo-pelleted rats tested with naloxone at the same dose failed to show a place aversion and nondependent rats showed a stable lack of aversion at tests up to 56 days. Dependent animals without naloxone also failed to show a place aversion at any of those time points. Conclusions: In the absence of any active intervention, the place aversion produced by opiate withdrawal is very long lasting and provides a model for protracted abstinence that may be useful for delineating the neurobiological substrate for vulnerability to relapse.

Intravenous heroin self‐administration decreases GABA efflux in the ventral pallidum: an in vivo microdialysis study in rats

Several lines of evidence suggest that opiate‐induced disinhibition of the ventral pallidum participates in the mediation of opiate reward, though direct in vivo evidence to support this hypothesis has been lacking. The present experiment tested this hypothesis by investigating alterations in ventral pallidal amino acid efflux using in vivo microdialysis during ongoing intravenous heroin self‐administration in rats. Concentrations of the inhibitory amino acid GABA in ventral pallidal dialysates were significantly reduced within the first 10 min of heroin self‐administration (0.02 mg per infusion; FR‐1), and remained ≈ 65% of presession baseline levels for the remainder of the 3‐h self‐administration session. Dialysate glutamate levels were unaltered during the first hour of heroin intake but significantly increased to a stable level of ≈ 120% presession values during the subsequent 2 h of self‐administration. Thus, heroin self‐administration is associated with both decreased GABA efflux and a late phase increase in glutamate efflux in the ventral pallidum. These observations are consistent with the hypothesis that heroin self‐administration results in a disinhibition and/or excitation of the ventral pallidum.

The effects of response operandum and prior food training on intravenous nicotine self-administration in rats

RationaleNicotine intravenous self-administration (IVSA) in rats has been conducted using a variety of methodological procedures with equally variable results.ObjectivesHere, we addressed the importance of the type of response operandum and prior instrumental training with a natural reinforcer on nicotine IVSA and reinstatement.MethodsRats were tested for spontaneous acquisition of IVSA using either nose poke (NP) or lever press (LVR) operandum. A dose-response test was then conducted, followed by extinction and cue- and nicotine-induced reinstatement.ResultsThe use of the NP operandum resulted in markedly higher levels of IVSA across acquisition and across dose-response testing compared with the LVR group. Whereas both groups reinstated following a nicotine prime, only the LVR group demonstrated cue-induced reinstatement. As a positive control, the experiment was repeated with cocaine as the reinforcer: equivalent levels of IVSA were observed across all tests, irrespective of operandum. When rats self-administering nicotine received instrumental training with a sucrose reinforcer prior to IVSA, a facilitated acquisition of IVSA was observed in both LVR and NP groups to a similar extent (the effect of operandum remained), but had little effect on responding thereafter. During reinstatement testing, both groups now displayed cue- and nicotine-induced reinstatement, but this was also evident in saline control animals that had never received nicotine.ConclusionsThese results suggest that, unlike cocaine, an increased physical response requirement can decrease nicotine intake. It also indicates that operandum and prior sucrose training may influence the role that visual cues play in nicotine dependence.

Voluntary Nicotine Consumption Triggers In Vivo Potentiation of Cortical Excitatory Drives to Midbrain Dopaminergic Neurons

Active response to either natural or pharmacological reward causes synaptic modifications to excitatory synapses on dopamine (DA) neurons of the ventral tegmental area (VTA). Here, we examine these modifications using nicotine, the main addictive component of tobacco, which is a potent regulator of VTA DA neurons. Using an in vivo electrophysiological technique, we investigated the role of key components of the limbic circuit, the infralimbic cortex (ILCx) and the bed nucleus of the stria terminalis (BNST), in operant behaviors related to nicotine reward. Our results indicated that nicotine self-administration in rats, but not passive delivery, triggers hyperactivity of VTA DA neurons. The data suggest that potentiation of the ILCx-BNST excitatory pathway is involved in these modifications in VTA DA neurons. Thus, recruitment of these specific excitatory inputs to VTA DA neurons may be a neural correlate for the learned association between active responding and the reward experience.

D1 dopamine receptor-mediated LTP at GABA synapses encodes motivation to self-administer cocaine in rats.

Enhanced motivation to take drugs is a central characteristic of addiction, yet the neural underpinning of this maladaptive behavior is still largely unknown. Here, we report a D1-like dopamine receptor (DRD1)-mediated long-term potentiation of GABAA-IPSCs (D1-LTPGABA) in the oval bed nucleus of the stria terminalis that was positively correlated with motivation to self-administer cocaine in rats. Likewise, in vivo intra-oval bed nucleus of the stria terminalis DRD1 pharmacological blockade reduced lever pressing for cocaine more effectively in rats showing enhanced motivation toward cocaine. D1-LTPGABA resulted from enhanced function and expression of G-protein-independent DRD1 coupled to c-Src tyrosine kinases and required local release of neurotensin. There was no D1-LTPGABA in rats that self-administered sucrose, in those with limited cocaine self-administration experience, or in those that received cocaine passively (yoked). Therefore, our study reveals a novel neurophysiological mechanism contributing to individual motivation to self-administer cocaine, a critical psychobiological element of compulsive drug use and addiction.