Smoking remains the primary cause of preventable death in the United States and smoking related illness costs more than $300 billion annually. Nicotine (the primary reinforcer in cigarettes) causes changes in behavior and neurochemistry that lead to increased probability of relapse. Given the role of mesolimbic dopamine projections in motivation, substance use disorder, and drug relapse, we examined the effect of repeated nicotine on rapid dopamine signals in the nucleus accumbens (NAc) of rats. Adult, male Sprague-Dawley rats were exposed to nicotine (0.2 or 0.4 mg/kg, subcutaneous) once daily for 7 days. On day 8, dopamine release and uptake dynamics, and their modulation by nicotinic receptor agonists and antagonists, were assessed using fast scan cyclic voltammetry in the NAc core. Nicotine exposure decreased electrically-stimulated dopamine release across a range of stimulation frequencies and decreased α6β2-containing nicotinic receptor control over dopamine release. Additionally, nicotine locomotor sensitization correlated with accumbal dopamine modulation by nicotine and mecamylamine. Taken together, our study suggests that repeated exposure to nicotine blunts dopamine release in the NAc core through changes in α6β2 modulation of dopamine release and individual differences in the sensitivity to this outcome may predict variation in behavioral models of vulnerability to substance use disorder.Smoking tobacco is the number one cause of preventable death in the United States, with 480,000 individuals dying each year from cigarette use and second-hand smoke exposure 1 . Nicotine, the main reinforcer in tobacco, is a primary reinforcer that has been shown to support self-administration, increase and sensitize locomotor activity, and drive drug-seeking behavior 2,3 . Additionally, nicotine enhances the reinforcing effects and incentive motivation of stimuli that accompany tobacco use 4 .Nicotinic acetylcholine receptors (nAChR) are necessary for both the primary reinforcing and reinforcement enhancing effects of nicotine. Activation of nAChRs in the nucleus accumbens or in the VTA can directly increase dopamine release in the striatum 5,6 and systemic nAChR antagonism decreases nicotine self-administration 7-9 . In addition, NAc nAChRs modulate dopamine release in a frequency dependent manner 10 . Dopamine neurons switch between tonic (~4-5 Hz) and phasic (2-5 spikes at 20-100 Hz) patterns of firing during the presentation of reinforcers or reward-related cues [11][12][13] . Nicotine is thought to enhance the contrast between baseline firing and reward-related firing by decreasing dopamine release to tonic firing rates while increasing dopamine release to phasic firing patterns in the NAc 14 . This is hypothesized to enhance the salience of reward-related cues and play a role in the reinforcement enhancing effects of nicotine. Further supporting this hypothesis, systemic antagonism of nAChRs decreases nicotine-induced enhancement of reinforcers, although the brain regions necessary for this effect have not yet ...

Metabotropic glutamate (mGlu) receptors are regulators of glutamate release and targets for development of therapies for hyperactive glutamatergic signaling. However, the effects of long‐term stimulation of mGlu receptors on cellular signaling in the brain have not been described. This study investigated the effects of 2‐day and 14‐day osmotic mini‐pump administration of the mGlu2,3 agonist LY379268 (3.0 mg kg−1 day−1) to rats on receptor‐mediated G‐protein activation and signaling in mesocorticolimbic regions in rat brain sections. A significant reduction in LY379268‐stimulated [35S]GTPγS binding was observed in the 14‐day group in some cortical regions, prefrontal cortex, nucleus accumbens, and ventral pallidum. The 14‐day LY379268 treatment group exhibited mGlu2 mRNA levels significantly lower in hippocampus, nucleus accumbens, caudate, and ventral pallidum. In both 2‐day and 14‐day treatment groups immunodetectable phosphorylated cAMP Response Element‐Binding protein (CREB) was significantly reduced across all brain regions. In the 2‐day group, we observed significantly lower immunodetectable CREB protein across all brain regions, which was subsequently increased in the 14‐day group but failed to achieve control values. Neither immunodetectable extracellular signal‐regulated kinase (ERK) protein nor phosphorylated ERK from 2‐day or 14‐day treatment groups differed significantly from control across all brain regions. However, the ratio of phosphorylated ERK to total ERK protein was significantly greater in the 14‐day treatment group compared with the control. These results identify compensatory changes to mGlu2,3 signal transduction in rat brains after chronic systemic administration of agonist, which could be predictive of the mechanism of action in human pharmacotherapies.

Smoking remains the primary cause of preventable death in the United States and smoking related illness costs more than $300 billion annually. Nicotine (the primary reinforcer in cigarettes) causes changes in behavior and neurochemistry that lead to increased probability of relapse. Given the role of mesolimbic dopamine projections in motivation, substance use disorder, and drug relapse, we examined the effect of repeated nicotine on rapid dopamine signals in the nucleus accumbens (NAc) of rats. Adult, male Sprague-Dawley rats were exposed to nicotine (0.2 or 0.4 mg/kg, subcutaneous) once daily for 7 days. On day 8, dopamine release and uptake dynamics, and their modulation by nicotinic receptor agonists and antagonists, were assessed using fast scan cyclic voltammetry in the NAc core. Nicotine exposure decreased electricallystimulated dopamine release across a range of stimulation frequencies and decreased 62containing nicotinic receptor control over dopamine release. Additionally, nicotine locomotor sensitization correlated with accumbal dopamine modulation by nicotine and mecamylamine.Taken together, our study suggests that repeated exposure to nicotine blunts dopamine release in the NAc core through changes in 62 modulation of dopamine release and individual differences in the sensitivity to this outcome may predict variation in behavioral models of vulnerability to substance use disorder.To examine the effects of chronic nicotine on nAChR modulation of dopamine release in the NAc core, we used ex vivo fast-scan cyclic voltammetry (FSCV) to measure dopamine release in rats following seven days of nicotine exposure. Various stimulation parameters were used to model a range of dopamine neuron firing patterns. Then, non-selective and selective nAChR antagonists were used to examine whether repeated nicotine altered nAChR modulation of NAc dopamine release. We then assessed whether magnitude of locomotor sensitization following repeated nicotine correlated with nicotine-induced modulation of dopamine release across tonic and phasic stimulations. Materials and Methods AnimalsAdult male Sprague-Dawley rats (325-350 grams, Harlan Sprague Dawley, Inc., Madison, WI) were maintained on a 12:12 h reverse light/dark cycle (4:00 a.m. lights off; 4:00 p.m. lights on) with food and water available ad libitum. All animals were maintained according to the National Institutes of Health guidelines in Association for Assessment and Accreditation of Laboratory Animal Care accredited facilities. The experimental protocol was approved by the Institutional Animal Care and Use Committee at Wake Forest School of Medicine. Locomotor assessment and nicotine exposureRats were given at least a week to acclimate to the housing environment and light cycle prior to the start of experiments. All locomotor testing occurred during the dark/active cycle (9:00AM) to prevent sleep from contributing to variability in locomotor activity. Rats were first transferred to the dark locomotor testing room for one hour to habituate in their home cages....

Drug addiction is a growing issue that currently has no FDA‐approved pharmacotherapy; therefore, the concern on how to treat this rapidly increasing problem. The neuroadaptation of neurotransmitter release and signal transduction results in changes in cognitive processes that make it harder for users to stop using the addictive drug. It is believed that restoration of cognition dysfunction may be an effective treatment for addiction. Recent studies in our NIDA‐supported “Center for the Neurobiology of Addiction Treatment” indicated that the cognitive remediator LY379268, an mGluR2/3 agonist, is able to reverse self‐administration of cocaine in rats (Karkhanis, Beveridge, Blough, Jones, & Ferris, 2016, Drug and Alcohol Dependence, 166, 51–60). We hypothesized that the mechanism of LY379268 is to alter cellular signaling in parts of the brain critical for cognition. To test this hypothesis, we examined changes in cellular signal transduction, determined as mGluR2/3‐mediated Gi/o protein activation and the phosphorylation of extracellular‐regulated kinase 1 and 2 (ERK1/2) and cyclic AMP response element binding protein (CREB) in specific regions of the brain related to drug addiction and cognitive processes. Groups of Sprague‐Dawley rats were treated with acute or chronic dosing of LY379268 via subcutaneous Alzet minipump that released 1mg/kg of LY379268 for 36 hours or 14 days, respectively. The control group was subjected to the surgical procedure but did not have a pump inserted. After sacrifice, frozen brain sections were examined for LY379268‐stimulated [35S]GTPgammaS binding, and densities were quantitated in identified brain regions. Immunohistochemistry was performed in intervening sections to quantitate changes in phosphorylation levels using primary antibodies to phospho‐ or total CREB or ERK1/2, and secondary antibodies coupled to infrared dyes. The stained brain sections were imaged and densities were analyzed for those same brain regions that responded to LY379268 in G protein activation studies using LiCor Odyssey software. We observed that desensitization of the G protein‐coupled receptor resulted after the chronic treatment with LY379268. We also observed an increase in CREB phosphorylation in chronic compared with acute treatment but there were no differences between groups in ERK phosphorylation. These studies suggest that chronic treatment with an mGluR2/3 agonist results in sustained neurochemical changes that might be responsible for behavioral modifications seen in chronically‐treated animals.Support or Funding InformationNIDA grant P50‐DA006634This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Adolescence is a developmental period characterized by increased vulnerability to substance use and dependence. However, mechanisms mediating this vulnerability are not well understood. Previous work has shown that locomotor responses in a novel environment predict acquisition of nicotine and cocaine self‐administration in adult rats, with high‐responders (HR) acquiring faster and at lower doses than low‐responders (LR). Additionally, adult rats with higher vulnerability have more sensitive nicotine acetylcholine receptor (nAChR) modulation of striatal dopamine release to phasic firing‐like stimulation, which is known to be important in acquisition of self‐administration. Here we examined how antagonism of nAChRs altered dopamine release in the nucleus accumbens core in adolescent and adult rats using fast scan cyclic voltammetry. We found that non‐selective antagonism of nAChRs, by mecamylamine (2 uM) or a desensitizing dose of nicotine (500 nM), increased dopamine release in HRs to stimulation that models phasic dopamine release, but suppressed dopamine release in LR rats. Further, we report a‐conotoxin PIA (100 nM), an a6‐selective antagonist, produced similar effects as the non‐selective antagonists, indicating that facilitation of dopamine in the HR rats is mediated by a6‐containing nAChR. When dopamine release in adolescent rats was tested under the same parameters, nAChR modulation of dopamine release was similar to HR adult rats, with non‐selective and a6‐selective antagonists facilitating dopamine release following phasic firing‐like stimulation. This suggests a role for a6‐containing nAChR modulation of striatal dopamine release in mediating vulnerability to drug abuse.Support or Funding InformationThis work is supported by NIDA grant R00 DA031791 awarded to MJFThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.