Poor inhibitory control predicts sex-specific vulnerability to nicotine rewarding properties in mice.

RATIONALE: The risk of becoming addicted to tobacco varies greatly from individual to individual, raising the possibility of behavioural biomarkers capable of predicting sensitivity to nicotine reward, a crucial step in the development of nicotine addiction. Amongst all of nicotine's pharmacological properties, one of central importance is the enhancement of cognitive performances, which depend on the balance between attentional processes and inhibitory control. However, whether the cognitive enhancement effects of nicotine are predictive of sensitivity to its rewarding properties is still unknown. OBJECTIVE: Using male and female mice, we investigated whether the effects of nicotine on cognitive performances are predictive of sensitivity to the rewarding properties of nicotine and, if so, whether this relationship is sex dependent. METHODS: Naïve male and female mice were first assessed for their performances in both baseline conditions and following nicotine injection (0.15 and 0.30 mg/kg) in a cued-Fixed Consecutive Number task (FCNcue) measuring both optimal (attention) and premature (inhibitory control) responding. Next, all mice underwent nicotine-induced conditioned place preference (CPP) in order to evaluate inter-individual differences in response to nicotine reward (0.30 mg/kg). RESULTS: Results showed that males and females benefited from the effect of nicotine as a cognitive enhancer in the FCNcue task. However, only those males displaying poor inhibitory control, namely high-impulsive animals, subsequently displayed sensitivity to nicotine reward. In females, sensitivity to nicotine reward was independent of FCNcue performances, in both basal and nicotine conditions. CONCLUSION: Thus, our study suggests that poor inhibitory control and its modulation by nicotine may be a behavioural biomarker for sensitivity to nicotine reward and consequent vulnerability to nicotine addiction in males but not females.

Detecting fine and elaborate movements with piezo sensors provides non-invasive access to overlooked behavioral components.

Behavioral phenotyping devices have been successfully used to build ethograms, but many aspects of behavior remain out of reach of available phenotyping systems. We now report on a novel device, which consists in an open-field platform resting on highly sensitive piezoelectric (electromechanical) pressure-sensors, with which we could detect the slightest movements (up to individual heart beats during rest) from freely moving rats and mice. The combination with video recordings and signal analysis based on time-frequency decomposition, clustering, and machine learning algorithms provided non-invasive access to previously overlooked behavioral components. The detection of shaking/shivering provided an original readout of fear, distinct from but complementary to behavioral freezing. Analyzing the dynamics of momentum in locomotion and grooming allowed to identify the signature of gait and neurodevelopmental pathological phenotypes. We believe that this device represents a significant progress and offers new opportunities for the awaited advance of behavioral phenotyping.

Impact of acute and chronic nicotine administration on midbrain dopaminergic neuron activity and related behaviours in TRPV1 knock-out juvenile mice.

The addictive properties of nicotine, the main alkaloid in tobacco and tobacco-derived products, largely depend on its action on the activity of midbrain dopamine (DA) neurons. The transient receptor potential vanilloid 1 (TRPV1) channel has also been examined as an emerging contributor to addiction-related symptoms due to its ability to modulate midbrain neurons. Thus, the objective of our study was to explore the role of TRPV1 receptors (TRPV1Rs) on nicotine-induced behaviours and associated response of DA neuron activity. Both wild type juvenile mice and juvenile mice with invalidation of the TRPV1R gene were exposed to acute or chronic nicotine 0.3 mg/kg administration. We analysed locomotor activity in response to the drug. In addition, we performed cell-attached and whole-cell recordings from ventral tegmental area (VTA) neurons after nicotine exposure. Our results showed that the genetic deletion of TRPV1Rs reduced nicotine-induced locomotor sensitization. In addition, it provided evidence in support of TRPV1Rs being regulators of inhibitory synaptic transmission in the VTA. However, TRPV1Rs did not seem to modulate either nicotine-induced conditioning place preference or nicotine-evoked electrical activity of DA neurons. In conclusion, TRPV1Rs modulate nicotine-induced psychomotor sensitization in mice independently of a control on VTA DA neuron activity. Thus, TRPV1R control may depend on another key player of the mesolimbic circuit.

Enhanced analgesic cholinergic tone in the spinal cord in a mouse model of neuropathic pain.

Endogenous acetylcholine (ACh) is an important modulator of nociceptive sensory processing in the spinal cord. An increased level of spinal ACh induces analgesia both in humans and rodents while interfering with cholinergic signaling is allodynic, demonstrating that a basal tone of spinal ACh modulates nociceptive responses in naïve animals. The plasticity undergone by this cholinergic system in chronic pain situation is unknown, and the mere presence of this tone in neuropathic animals is controversial. We have addressed these issues in mice through behavioral experiments, histology, electrophysiology and molecular biology, in the cuff model of peripheral neuropathy. Our behavior experiments demonstrate the persistence, and even increased impact of the analgesic cholinergic tone acting through nicotinic receptors in cuff animals. The neuropathy does not affect the number or membrane properties of dorsal horn cholinergic neurons, nor specifically the frequency of their synaptic inputs. The alterations thus appear to be in the neurons receiving the cholinergic signaling, which is confirmed by the fact that subthreshold doses of acetylcholinesterase (AChE) inhibitors in sham animals become anti-allodynic in cuff mice and by the altered expression of the ß2 nicotinic receptor subunit. Our results demonstrate that endogenous cholinergic signaling can be manipulated to relieve mechanical allodynia in animal models of peripheral neuropathy. Until now, AChE inhibitors have mainly been used in the clinics in situations of acute pain (parturition, post-operative). The fact that lower doses (thus with fewer side effects) could be efficient in chronic pain conditions opens new avenues for the treatment of neuropathic pain. SIGNIFICANCE STATEMENT: Chronic pain continues to be the most common cause of disability that impairs the quality of life, accruing enormous and escalating socio-economic costs. A better understanding of the plasticity of spinal neuronal networks, crucially involved in nociceptive processing, could help designing new therapeutic avenues. We here demonstrate that chronic pain modifies the spinal nociceptive network in such a way that it becomes more sensitive to cholinergic modulations. The spinal cholinergic system is responsible for an analgesic tone that can be exacerbated by acetylcholinesterase inhibitors, a property used in the clinic to relief acute pain (child birth, post-op). Our results suggest that lower doses of acetylcholinesterases, with even fewer side effects, could be efficient to relieve chronic pain.

Exercise craving potentiates excitatory inputs to ventral tegmental area dopaminergic neurons.

Physical exercise, which can be addictogenic on its own, is considered a therapeutic alternative for drug craving. Exercise might thus share with drugs the ability to strengthen excitatory synapses onto ventral tegmental area (VTA) dopaminergic neurones, as assessed by the ratio of AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) to NMDA receptor (NMDAR)-mediated EPSCs. As did acute cocaine, amphetamine, or Δ9 -tetrahydrocannabinol (THC) pretreatments, an acute 1-h wheel-running session increased the AMPAR/NMDAR ratio in VTA dopaminergic neurones. To dissect the respective influences of wheel-running seeking and performance, mice went through an operant protocol wherein wheel-running was conditioned by nose poking under fixed ratio schedules of reinforcement. Conditioned wheel-running increased the AMPAR/NMDAR ratio to a higher extent than free wheel-running, doing so although running performance was lower in the former paradigm than in the latter. Thus, the cue-reward association, rather than reward consumption, played a major role in this increase. The AMPAR/NMDAR ratio returned to baseline levels in mice that had extinguished the cued-running motivated task, but it increased after a cue-induced reinstatement session. The amplitude of this increase correlated with the intensity of exercise craving, as assessed by individual nose poke scores. Finally, cue-induced reinstatement of running seeking proved insensitive to acute cocaine or THC pretreatments. Our study reveals for the first time that the drive for exercise bears synaptic influences on VTA dopaminergic neurones which are reminiscent of drug actions. Whether these influences play a role in the therapeutic effects of exercise in human drug craving remains to be established.

Potential Involvement of Impaired BKCa Channel Function in Sensory Defensiveness and Some Behavioral Disturbances Induced by Unfamiliar Environment in a Mouse Model of Fragile X Syndrome.

In fragile X syndrome (FXS), sensory hypersensitivity and impaired habituation is thought to result in attention overload and various behavioral abnormalities in reaction to the excessive and remanent salience of environment features that would normally be ignored. This phenomenon, termed sensory defensiveness, has been proposed as the potential cause of hyperactivity, hyperarousal, and negative reactions to changes in routine that are often deleterious for FXS patients. However, the lack of tools for manipulating sensory hypersensitivity has not allowed the experimental testing required to evaluate the relevance of this hypothesis. Recent work has shown that BMS-204352, a BKCa channel agonist, was efficient to reverse cortical hyperexcitability and related sensory hypersensitivity in the Fmr1-KO mouse model of FXS. In the present study, we report that exposing Fmr1-KO mice to novel or unfamiliar environments resulted in multiple behavioral perturbations, such as hyperactivity, impaired nest building and excessive grooming of the back. Reversing sensory hypersensitivity with the BKCa channel agonist BMS-204352 prevented these behavioral abnormalities in Fmr1-KO mice. These results are in support of the sensory defensiveness hypothesis, and confirm BKCa as a potentially relevant molecular target for the development of drug medication against FXS/ASD.

Characterization of functional µ opioid receptor turnover in rat locus coeruleus: an electrophysiological and immunocytochemical study.

BACKGROUND AND PURPOSE: Regulation of µ receptor dynamics such as its trafficking is a possible mechanism underlying opioid tolerance that contributes to inefficient recycling of opioid responses. We aimed to characterize the functional turnover of µ receptors in the noradrenergic nucleus locus coeruleus (LC). EXPERIMENTAL APPROACH: We measured opioid effect by single-unit extracellular recordings of LC neurons from rat brain slices. Immunocytochemical techniques were used to evaluate µ receptor trafficking. KEY RESULTS: After near-complete, irreversible µ receptor inactivation with ß-funaltrexamine (ß-FNA), opioid effect spontaneously recovered in a rapid and efficacious manner. In contrast, α2 -adrenoceptor-mediated effect hardly recovered after receptor inactivation with the irreversible antagonist EEDQ. When the recovery of opioid effect was tested after various inactivating time schedules, we found that the longer the ß-FNA pre-exposure, the less efficient and slower the functional µ receptor turnover became. Interestingly, µ receptor turnover was slower when ß-FNA challenge was repeated in the same cell, indicating constitutive µ receptor recycling by trafficking from a depletable pool. Double immunocytochemistry confirmed the constitutive nature of µ receptor trafficking from a cytoplasmic compartment. The µ receptor turnover was slowed down when LC neuron calcium- or firing-dependent processes were prevented or vesicular protein trafficking was blocked by a low temperature or transport inhibitor. CONCLUSIONS AND IMPLICATIONS: Constitutive trafficking of µ receptors from a depletable intracellular pool (endosome) may account for its rapid and efficient functional turnover in the LC. A finely-tuned regulation of µ receptor trafficking and endosomes could explain neuroadaptive plasticity to opioids in the LC.

Loss of inhibitory tone on spinal cord dorsal horn spontaneously and nonspontaneously active neurons in a mouse model of neuropathic pain.

Plasticity of inhibitory transmission in the spinal dorsal horn (SDH) is believed to be a key mechanism responsible for pain hypersensitivity in neuropathic pain syndromes. We evaluated this plasticity by recording responses to mechanical stimuli in silent neurons (nonspontaneously active [NSA]) and neurons showing ongoing activity (spontaneously active [SA]) in the SDH of control and nerve-injured mice (cuff model). The SA and NSA neurons represented 59% and 41% of recorded neurons, respectively, and were predominantly wide dynamic range (WDR) in naive mice. Nerve-injured mice displayed a marked decrease in the mechanical threshold of the injured paw. After nerve injury, the proportion of SA neurons was increased to 78%, which suggests that some NSA neurons became SA. In addition, the response to touch (but not pinch) was dramatically increased in SA neurons, and high-threshold (nociceptive specific) neurons were no longer observed. Pharmacological blockade of spinal inhibition with a mixture of GABAA and glycine receptor antagonists significantly increased responses to innocuous mechanical stimuli in SA and NSA neurons from sham animals, but had no effect in sciatic nerve-injured animals, revealing a dramatic loss of spinal inhibitory tone in this situation. Moreover, in nerve-injured mice, local spinal administration of acetazolamide, a carbonic anhydrase inhibitor, restored responses to touch similar to those observed in naive or sham mice. These results suggest that a shift in the reversal potential for anions is an important component of the abnormal mechanical responses and of the loss of inhibitory tone recorded in a model of nerve injury-induced neuropathic pain.

Effect of ceftriaxone and topiramate treatments on naltrexone-precipitated morphine withdrawal and glutamate receptor desensitization in the rat locus coeruleus.

RATIONALE: Morphine withdrawal is associated with a hyperactivity of locus coeruleus (LC) neurons by an elevated glutamate neurotransmission in this nucleus. We postulate that reductions in the amount of glutamate in the LC by enhancing its reuptake or inhibiting its release could attenuate the behavioral and cellular consequences of morphine withdrawal. OBJECTIVES: We investigated the effect of chronic treatment with ceftriaxone (CFT), an excitatory amino acid transporter (EAAT2) enhancer, and acute administration of topiramate (TPM), a glutamate release inhibitor, on morphine withdrawal syndrome and withdrawal-induced glutamate receptor (GluR) desensitization in LC neurons from morphine-dependent rats. METHODS: Morphine withdrawal behavior was measured after naltrexone administration in rats implanted with a morphine (200 mg kg(-1)) emulsion for 3 days. GluR desensitization in the LC was assessed by performing concentration-effect curves for glutamate by extracellular electrophysiological recordings in vitro. RESULTS: Treatments with CFT or TPM reduced, in a dose-related manner, the total behavioral score of naltrexone-precipitated morphine withdrawal. CFT and TPM, at doses that were effective in behavioral tests, also reduced the induction of GluR desensitization normally occurring in LC neurons from morphine-dependent rats. Acute treatment with the specific EAAT2 inhibitor dihydrokainic acid (DHK) prevented the effect of CFT on withdrawal syndrome and GluR desensitization. Perfusion with TPM inhibited KCl-evoked but not glutamate-induced activation of LC neurons in vitro. CONCLUSIONS: Our results suggest that a reduction of synaptic concentrations of glutamate by enhancing EAAT2-mediated uptake or inhibiting glutamate release alleviates the behavioral response and the cellular changes in the LC during opiate withdrawal.