Contextual renewal of nicotine seeking in rats and its suppression by the cannabinoid-1 receptor antagonist Rimonabant (SR141716A).

Nicotine-associated paraphernalia such as cigarettes and ashtrays are potent smoking relapse triggers. In addition to these discrete cues, environmental contexts previously associated with smoking elicit strong cigarette craving, indicating that contextual stimuli also contribute to high smoking relapse rates. Nonetheless, little is known about the precise role of these stimuli in smoking relapse and the neuropharmacological mechanisms implicated herein. To address this issue, we determined whether re-exposure to the nicotine self-administration context after long-term extinction reinstates nicotine seeking behavior in rats. Further, we examined the effects of SR141716A (Rimonabant), a selective cannabinoid CB1 receptor antagonist which has been shown to attenuate cue-induced relapse to nicotine seeking, on context-induced reinstatement of nicotine seeking. Rats were trained to self-administer nicotine intravenously (30microg/kg/infusion). Nicotine infusions were paired with an audiovisual compound stimulus. Subsequently, nose poking behavior was extinguished in the presence of this discrete cue in a context different from the self-administration context. Hereafter, rats were injected with 0, 1, or 3mg/kg Rimonabant (i.p.) prior to re-exposure to either the self-administration or the extinction context. Re-exposure to the self-administration context, but not to the extinction context robustly reinstated responding for the discrete nicotine cues, an effect that was dose-dependently attenuated by Rimonabant. This is the first demonstration of contextual renewal of nicotine seeking in rodents after prolonged withdrawal. Further, our results indicate that the endocannabinoid system is involved in context-induced relapse to nicotine seeking, and as such these data provide further evidence for the use of CB1 antagonists in smoking cessation.

Strain specificity and cholinergic modulation of visuospatial attention in three inbred mouse strains.

The tremendous increase in the use of mouse inbred strains and mutant mice to study the molecular basis of psychiatric disorders urges for a better understanding of attentional performance in mice. To this aim, we investigated possible strain differences in performance and cholinergic modulation of visuospatial attention in three widely used mouse inbred strains (129S2/SvHsd, C57BL/6JOlaHsd and DBA/2OlaHsd) in the five-choice serial reaction time task. Results indicated that after extended training, performance of 129S2/SvHsd mice was superior to that of C57BL/6JOlaHsd and DBA/2OlaHsd mice in terms of attention, omission errors, inhibitory control and latencies to correct choice. Increasing the attentional load resulted in comparable decrements in attention in all strains and inhibitory control impairments that were most pronounced in DBA/2OlaHsd mice. Further pharmacological evaluation indicated that all strains showed attentional impairments after treatment with the muscarinic and nicotinic antagonists scopolamine and mecamylamine, respectively. 129S2/SvHsd mice were less sensitive, whereas DBA/2OlaHsd mice appeared more sensitive to the detrimental effects of mecamylamine. In addition, subchronic, but not acute, nicotine treatment slightly improved attentional performance in all strains to the same extent. In conclusion, our data indicate strain specificity with particularly good performance of 129S2/SvHsd mice and strong cholinergic involvement in visuospatial attention in mice.

Characterization of the serotonin transporter knockout rat: a selective change in the functioning of the serotonergic system.

Serotonergic signaling is involved in many neurobiological processes and disturbed 5-HT homeostasis is implicated in a variety of psychiatric and addictive disorders. Here, we describe the functional characterization of the serotonin transporter (SERT) knockout rat model, that is generated by N-ethyl-N-nitrosurea (ENU)-driven target-selected mutagenesis. Biochemical characterization revealed that SERT mRNA and functional protein are completely absent in homozygous knockout (SERT-/-) rats, and that there is a gene dose-dependent reduction in the expression and function of the SERT in heterozygous knockout rats. As a result, 5-HT homeostasis was found to be severely affected in SERT-/- rats: 5-HT tissue levels and depolarization-induced 5-HT release were significantly reduced, and basal extracellular 5-HT levels in the hippocampus were ninefold increased. Interestingly, we found no compensatory changes in in vitro activity of tryptophan hydroxylase and monoamine oxidase, the primary enzymes involved in 5-HT synthesis and degradation, respectively. Similarly, no major adaptations in non-serotonergic systems were found, as determined by dopamine and noradrenaline transporter binding, monoamine tissue levels, and depolarization-induced release of dopamine, noradrenaline, glutamate and GABA. In conclusion, neurochemical changes in the SERT knockout rat are primarily limited to the serotonergic system, making this novel rat model potentially very useful for studying the behavioral and neurobiological consequences of disturbed 5-HT homeostasis.

Interactions between CB1 cannabinoid and mu opioid receptors mediating inhibition of neurotransmitter release in rat nucleus accumbens core.

We examined the occurrence of functional interactions between CB1 cannabinoid and mu opioid receptors in the core of rat nucleus accumbens (NAc core). To that end, receptor-mediated inhibition of depolarization (4-aminopyridine)-induced [3H]glutamate release and glutamate (NMDA) receptor-stimulated [14C]acetylcholine (ACh) and [3H]GABA release was studied in superfused NAc core slices. The inhibitory effects of the mu receptor agonist morphine and the CB1 receptor agonist HU210 on the release of these neurotransmitters were selectively antagonized by the mu receptor antagonist naloxone and the CB1 receptor antagonist SR141716A, respectively. Surprisingly, naloxone prevented the antagonistic action of SR141716A at CB1 receptors and SR141716A abolished that of naloxone at mu receptors mediating inhibition of [3H]glutamate and [3H]GABA release. Therefore, these antagonists seem to allosterically interact, indicating the involvement of physically associated mu opioid and CB1 cannabinoid receptors. Such an interaction between antagonists was not observed at the receptors mediating inhibition of [14C]ACh release. Moreover, dose-response curves of the agonists showed that mu and CB1 receptors mediating inhibition of [3H]glutamate release display a non-additive interaction, whereas these receptors synergistically interact regarding their inhibitory control of [3H]GABA release. Finally, the apparent allosteric interaction between antagonists was also observed regarding the effects of other receptor-selective agonists and antagonists at mu opioid and CB1 cannabinoid receptors (mediating inhibition of NMDA-induced [3H]GABA release) and must therefore be a unique property of the receptors involved. These data suggest the existence of physically associated mu opioid and CB1 cannabinoid receptors, whereby activation of these receptors results in either a non-additive (glutamate release) or a synergistic (GABA release) effect. It is proposed that these allosterically interacting mu and CB1 receptors in the NAc core may represent G-protein coupled heterodimeric receptor complexes.

Neuroadaptive changes in mesocorticolimbic dopamine and acetylcholine neurons following cocaine or saline self-administration are dependent on pre-existing individual differences.

Previously, we demonstrated that stress-induced self-grooming behaviour in rats predicted an enhanced motivation to self-administer cocaine as determined under a progressive ratio schedule of reinforcement. The enhanced motivation of high grooming (HG) rats was associated with a reduced reactivity of dopaminergic neurons in the medial prefrontal cortex and amygdala, but not nucleus accumbens. In the present study, we studied the effect of cocaine and saline self-administration on these pre-existing differences in neurochemical profile by determining the electrically evoked release of [3H]dopamine and [14C]acetylcholine from superfused slices of the nucleus accumbens shell and core, medial prefrontal cortex and amygdala of HG and low grooming (LG) rats. Although HG and LG rats did not differ in acquisition of cocaine and saline self-administration, both conditions induced substantially different neuroadaptations in these rats. Differences in depolarisation-induced dopamine and acetylcholine release were maintained in the medial prefrontal cortex, emerged in the nucleus accumbens and dissipated in the amygdala. These results indicate that altered reactivity of mesocorticolimbic dopaminergic and cholinergic neurons due to exposure to cocaine and environmental stimuli (saline) is dependent on pre-existing neurochemical differences and displays region-specificity. These pre-existing differences and the cocaine- and environmental-induced neuroadaptations seem to act in concert to produce an enhanced motivational state to self-administer cocaine.

Individual differences in the effects of serotonergic anxiolytic drugs on the motivation to self-administer cocaine.

Numerous clinical studies have indicated that lifetime anxiety is highly prevalent in drug addicts. In the treatment of drug abuse, dually diagnosed drug addicts may benefit from pharmacological intervention strategies that alleviate the psychiatric symptomatology. We have previously shown that rats with different coping strategies in a stressful environment show strong differences in the motivation to self-administer cocaine. That is, cocaine self-administration under a progressive ratio (PR) schedule of reinforcement was enhanced in high grooming (HG) rats as compared with low grooming (LG) rats. To identify the pharmacological basis of these differences, we tested the acute effects of several anxiolytic drugs on cocaine self-administration in HG and LG rats under a PR schedule of reinforcement. Chlordiazepoxide increased PR responding in both the HG and LG rats, while the selective corticotrophin releasing hormone 1 receptor antagonist R121919 had no effect on cocaine self-administration under the PR schedule. Interestingly, buspirone and fluoxetine decreased PR responding in HG rats only and thereby abolished the individual differences in PR responding between HG and LG rats. In support of the differential effects of the serotonergic drugs on PR responding in HG and LG rats, we found that the in vitro electrically evoked release of [3H]serotonin from mesocorticolimbic brain slices was reduced in the medial prefrontal cortex, substantia nigra and nucleus accumbens core, and increased in the nucleus accumbens shell of HG rats relative to LG rats. These findings show that serotonergic anxiolytics abolish the pre-existing individual differences in cocaine self-administration between HG and LG rats, which show differences in the reactivity of serotonergic neurons. This suggests that the effectiveness of pharmacological interference may depend on the neurochemical and motivational state of the individual.

Effects of amphetamine on dopamine release in the rat nucleus accumbens shell region depend on cannabinoid CB1 receptor activation.

The psychostimulant drug amphetamine is often prescribed to treat Attention-Deficit/Hyperactivity Disorder. The behavioral effects of the psychostimulant drug amphetamine depend on its ability to increase monoamine neurotransmission in brain regions such as the nucleus accumbens (NAC) and medial prefrontal cortex (mPFC). Recent behavioral data suggest that the endocannabinoid system also plays a role in this respect. Here we investigated the role of cannabinoid CB1 receptor activity in amphetamine-induced monoamine release in the NAC and/or mPFC of rats using in vivo microdialysis. Results show that systemic administration of a low, clinically relevant dose of amphetamine (0.5mg/kg) robustly increased dopamine and norepinephrine release (to ∼175-350% of baseline values) in the NAC shell and core subregions as well as the ventral and dorsal parts of the mPFC, while moderately enhancing extracellular serotonin levels (to ∼135% of baseline value) in the NAC core only. Although systemic administration of the CB1 receptor antagonist SR141716A (0-3mg/kg) alone did not affect monoamine release, it dose-dependently abolished amphetamine-induced dopamine release specifically in the NAC shell. SR141716A did not affect amphetamine-induced norepinephrine or serotonin release in any of the brain regions investigated. Thus, the effects of acute CB1 receptor blockade on amphetamine-induced monoamine transmission were restricted to dopamine, and more specifically to mesolimbic dopamine projections into the NAC shell. This brain region- and monoamine-selective role of CB1 receptors is suggested to subserve the behavioral effects of amphetamine.

SLV330, a cannabinoid CB(1) receptor antagonist, attenuates ethanol and nicotine seeking and improves inhibitory response control in rats.

Cannabinoid CB(1) receptor (CB(1)R) signaling has been shown to play a role in the regulation of addictive behavior. In the present study, our aim was to investigate whether the CB(1)R antagonist SLV330 could reduce ethanol and nicotine self-administration and cue-induced reinstatement of ethanol and nicotine seeking behavior in Wistar rats. In operant chambers, rats were learned to emit a specific response (nose poke) in order to receive an ethanol solution or intravenous injections of nicotine. Discrete light and tone cues were presented during ethanol and nicotine delivery. These cues are particularly important for drug self-administration behavior and, through Pavlovian conditioning, acquire conditioned reinforcing and motivational properties and are therefore able to generate and maintain drug-seeking behavior. Subsequently, the CB(1)R antagonist SLV330 (doses ranging from 1 to 10mg/kg, given orally, p.o.) was administered to investigate the effects on drug self-administration. In addition, responding for ethanol and nicotine was extinguished. Then, the animals were tested for cue-induced reinstatement of ethanol and nicotine seeking and treated with vehicle or SLV330. Finally, the effects of SLV330 were studied on the number of anticipatory responses in the 5-choice serial reaction time task (5-CSRTT) in order to determine whether this compound could also increase impulse control in Wistar rats. The CB(1) antagonist SLV330 was effective in reducing ethanol self-administration at a lowest effective dose (LED) of 10mg/kg (p.o.) and reinstatement of ethanol seeking at a LED of 3mg/kg (p.o.). SLV330 was also effective in reducing nicotine self-administration and reinstatement of nicotine seeking, although at a LED of 10mg/kg (p.o.). Finally, SLV330 decreased time delay-dependent anticipatory responding (LED of 3.0mg/kg, p.o.), indicating an increased inhibitory control. These findings are in agreement with results reported with other CB(1) antagonists. The combined action of reducing the reinforcing and motivational properties of nicotine and alcohol and the improvement of impulse control supports the idea that the cannabinoid system is a promising target for anti-relapse medication.

The antioxidant anethole dithiolethione inhibits monoamine oxidase-B but not monoamine oxidase A activity in extracts of cultured astrocytes.

Anethole dithiolethione (ADT) is a clinically available, pluripotent antioxidant proposed as a neuroprotectant for Parkinson's disease (PD). Here, using extracts from cultured astrocytes, containing both monoamine oxidase (MAO) A and B activity, we demonstrate that ADT concentration-dependently inhibits MAO-B activity in a clinically relevant concentration range (0.03-30 microM, IC-50 = 0.5 microM) without affecting MAO A activity. Considering the alleged contribution of MAO activity in general, and MAO-B in particular, to oxidative stress and neurodegeneration in PD, our data further support the neuroprotective potential of ADT.