Conditional Knockout of NMDA Receptors in Dopamine Neurons Prevents Nicotine-Conditioned Place Preference

Nicotine from smoking tobacco produces one of the most common forms of addictive behavior and has major societal and health consequences. It is known that nicotine triggers tobacco addiction by activating nicotine acetylcholine receptors (nAChRs) in the midbrain dopaminergic reward system, primarily via the ventral tegmental area. Heterogeneity of cell populations in the region has made it difficult for pharmacology-based analyses to precisely assess the functional significance of glutamatergic inputs to dopamine neurons in nicotine addiction. By generating dopamine neuron-specific NR1 knockout mice using cre/loxP-mediated method, we demonstrate that genetic inactivation of the NMDA receptors in ventral tegmental area dopamine neurons selectively prevents nicotine-conditioned place preference. Interestingly, the mutant mice exhibit normal performances in the conditioned place aversion induced by aversive air puffs. Therefore, this selective effect on addictive drug-induced reinforcement behavior suggests that NMDA receptors in the dopamine neurons are critical for the development of nicotine addiction

Naltrexone modulates dopamine release following chronic, but not acute amphetamine administration : a translational study

The opioid antagonist naltrexone has been shown to attenuate the subjective effects of amphetamine. However, the mechanisms behind this modulatory effect are currently unknown. We hypothesized that naltrexone would diminish the striatal dopamine release induced by amphetamine, which is considered an important mechanism behind many of its stimulant properties. We used positron emission tomography and the dopamine D2-receptor radioligand [C-11]raclopride in healthy subjects to study the dopaminergic effects of an amphetamine injection after pretreatment with naltrexone or placebo. In a rat model, we used microdialysis to study the modulatory effects of naltrexone on dopamine levels after acute and chronic amphetamine exposure. In healthy humans, naltrexone attenuated the subjective effects of amphetamine, confirming our previous results. Amphetamine produced a significant reduction in striatal radioligand binding, indicating increased levels of endogenous dopamine. However, there was no statistically significant effect of naltrexone on dopamine release. The same pattern was observed in rats, where an acute injection of amphetamine caused a significant rise in striatal dopamine levels, with no effect of naltrexone pretreatment. However, in a chronic model, naltrexone significantly attenuated the dopamine release caused by reinstatement of amphetamine. Collectively, these data suggest that the opioid system becomes engaged during the more chronic phase of drug use, evidenced by the modulatory effect of naltrexone on dopamine release following chronic amphetamine administration. The importance of opioid-dopamine interactions in the reinforcing and addictive effects of amphetamine is highlighted by the present findings and may help to facilitate medication development in the field of stimulant dependence

PPARα regulates cholinergic-driven activity of midbrain dopamine neurons via a novel mechanism involving α7 nicotinic acetylcholine receptors.

Ventral tegmental area dopamine neurons control reward-driven learning, and their dysregulation can lead to psychiatric disorders. Tonic and phasic activity of these dopaminergic neurons depends on cholinergic tone and activation of nicotinic acetylcholine receptors (nAChRs), particularly those containing the beta 2 subunit (beta 2(star)-nAChRs). Nuclear peroxisome proliferator-activated receptors type-alpha (PPAR alpha) tonically regulate beta 2(star)-nAChRs and thereby control dopamine neuron firing activity. However, it is unknown how and when PPAR alpha endogenous ligands are synthesized by dopamine cells. Using ex vivo and in vivo electrophysiological techniques combined with biochemical and behavioral analysis, we show that activation of alpha 7-nAChRs increases in the rat VTA both the tyrosine phosphorylation of the beta 2 subunit of nAChRs and the levels of two PPAR alpha endogenous ligands in a Ca2+-dependent manner. Accordingly, in vivo production of endogenous PPAR alpha ligands, triggered by alpha 7-nAChR activation, blocks in rats nicotine-induced increased firing activity of dopamine neurons and displays antidepressant-like properties. These data demonstrate that endogenous PPAR alpha ligands are effectors of alpha 7-nAChRs and that their neuromodulatory properties depend on phosphorylation of beta 2(star)-nAChRs on VTA dopamine cells. This reveals an autoinhibitory mechanism aimed at reducing dopamine cell overexcitation engaged during hypercholinergic drive. Our results unveil important physiological functions of nAChR/PPAR alpha signaling in dopamine neurons and how behavioral output can change after modifications of this signaling pathway. Overall, the present study suggests PPAR alpha as new therapeutic targets for disorders associated with unbalanced dopamine-acetylcholine systems

PPAR Regulates Cholinergic-Driven Activity of Midbrain Dopamine Neurons via a Novel Mechanism Involving 7 Nicotinic Acetylcholine Receptors

Ventral tegmental area dopamine neurons control reward-driven learning, and their dysregulation can lead to psychiatric disorders. Tonic and phasic activity of these dopaminergic neurons depends on cholinergic tone and activation of nicotinic acetylcholine receptors (nAChRs), particularly those containing the 2 subunit (2*-nAChRs). Nuclear peroxisome proliferator-activated receptors type- (PPAR) tonically regulate 2*-nAChRs and thereby control dopamine neuron firing activity. However, it is unknown how and when PPAR endogenous ligands are synthesized by dopamine cells. Using ex vivo and in vivo electrophysiological techniques combined with biochemical and behavioral analysis, we show that activation of7-nAChRs increases in the rat VTA both the tyrosine phosphorylation of the 2 subunit of nAChRs and the levels of two PPAR endogenous ligands in a Ca2-dependent manner. Accordingly, in vivo production of endogenous PPAR ligands, triggered by7-nAChR activation, blocks in rats nicotine-induced increased firing activity of dopamine neurons and displays antidepressant-like properties. These data demonstrate that endogenous PPAR ligands are effectors of 7-nAChRs and that their neuromodulatory properties depend on phosphorylation of2*-nAChRs on VTA dopamine cells. This reveals an autoinhibitory mechanism aimed at reducing dopamine cell overexcitation engaged during hypercholinergic drive. Our results unveil important physiological functions of nAChR/PPAR signaling in dopamine neurons and how behavioral output can change after modifications of this signaling pathway. Overall, the present study suggests PPAR as new therapeutic targets for disorders associated with unbalanced dopamine–acetylcholine systems

D-Amino Acid Aberrations in Cerebrospinal Fluid and Plasma of Smokers

<p>The glutamatergic neurotransmission system and the N-methyl-D-aspartate receptor (NMDAR) have been implicated in smoking and alcohol consumption behavior. Preclinical studies have demonstrated that nicotine and ethanol influence NMDAR functionality, which may have a role in tendencies to consume these substances. Nonetheless, little is known about concentrations of NMDAR coagonists in the cerebrospinal fluid (CSF) and plasma of individuals who smoke or consume alcohol. Glycine and L- and D-stereoisomers of alanine, serine, and proline were therefore measured using ultra-high-performance liquid chromatography-tandem mass spectrometry in 403 healthy subjects. Nicotine and alcohol consumption were quantified using questionnaires. Possible differences in NMDAR coagonist concentrations in plasma and CSF were investigated using ANCOVA with age, body mass index, and storage duration as covariates. The significance threshold was Bonferroni corrected (alpha = 0.00625). Compared with non-smokers, smokers displayed lower levels of D-proline in plasma (p = 0.0027, Cohen's d = -0.41) and D-proline in CSF (p = 0.0026, Cohen's d = -0.43). D-Serine in CSF was higher in smokers than in non-smokers (p = 0.0052, Cohen's d = 0.41). After subdividing participants based on smoking quantity, dose-dependent decreases were demonstrated in smokers for D-proline in plasma (F = 5.65, p = 0.0039) and D-proline in CSF (F = 5.20, p = 0.0060). No differences in NMDAR coagonist levels between alcohol consumption groups were detected. To our knowledge, this is the first report to implicate D-amino acids in smoking behavior of humans. Whether such concentration differences lie at the root of or result from smoking habits may be addressed in prospective studies.</p>

A role for glutamate in subjective response to smoking and its action on inhibitory control

Rationale Our previous study using memantine in smokers suggests that there may be a differential role for N-methyl-d-aspartate (NMDA) receptors in the subjective and cognitive effects of smoking. Objectives This study was designed to investigate if d-cycloserine (DCS) would modulate the subjective and cognitive effects of limited smoking. Methods Forty-eight habitual smokers abstinent for a minimum of 2 h were randomly allocated to receive either placebo or 50 mg DCS (double-blind) and were subsequently required either to smoke half of one cigarette or to remain abstinent. Subjective and physiological effects of DCS were measured at baseline, 90 min postcapsule, and again after the partial-smoking manipulation, while the effects on sustained attention (rapid visual information processing test—RVIP) and cognitive flexibility (intra–extra dimensional set-shift test—IED) were evaluated only after the partial-smoking manipulation. Results DCS alone did not produce significant subjective effects other than an increase in ratings of “Stimulated”. In combination with partial smoking, however, DCS blocked the smoking-induced increase in “Stimulated” and the decrease in “Relaxed” ratings. Furthermore, in combination with smoking, DCS reduced the number of false alarms during the RVIP test (an index of inhibitory control) and produced a small increase in diastolic blood pressure. DCS failed to modulate IED performance. Conclusions These findings provide further evidence of a role for glutamate release in the subjective effects of smoking but not the effects on attention and cognitive flexibility. Furthermore, our results indicate that glutamate release may also be involved in the effect of smoking on inhibitory control