Contribution of NMDA glutamate and nicotinic acetylcholine receptor mechanisms in the discrimination of ethanol-nicotine mixtures

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Role of mPFC and nucleus accumbens circuitry in modulation of a nicotine plus alcohol compound drug state

Combined use of nicotine and alcohol constitute a significant public health risk. An important aspect of drug use and dependence are the various cues, both external (contextual) and internal (interoceptive) that influence drug-seeking and drug-taking behavior. The present experiments employed the use of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and complementary Pavlovian drug discrimination procedures (feature-positive and feature-negative training conditions) in order to examine whether medial prefrontal cortex (prelimbic; mPFC-PL) projections to the nucleus accumbens core (AcbC) modulate sensitivity to a nicotine + alcohol (N + A) interoceptive cue. First, we show neuronal activation in mPFC-PL and AcbC following treatment with N + A. Next, we demonstrate that chemogenetic silencing of projections from mPFC-PL to nucleus accumbens core decrease sensitivity to the N + A interoceptive cue, while enhancing sensitivity to the individual components, suggesting an important role for this specific projection. Furthermore, we demonstrate that clozapine-N-oxide (CNO), the ligand used to activate the DREADDs, had no effect in parallel mCherry controls. These findings contribute important information regarding our understanding of the cortical-striatal circuitry that regulates sensitivity to the interoceptive effects of a compound N + A cue.

Ethanol→Nicotine & Nicotine→Ethanol drug-sequence discriminations: Conditional stimulus control with two interoceptive drug elements in rats

Self-administration of ethanol (E) and nicotine (N) occurs frequently in tandem orders (i.e., N→E vs. E→N) and thereby produces differing interoceptive profiles of subjective effects in humans. If the interoceptive stimulus characteristics of N→E differ from E→N, it is possible that such differences contribute to their co-dependence. The rationale for the present investigation was to determine whether ethanol, when preceded or followed by nicotine, produces different discriminative stimulus effects in rats. In two experiments, using a one-manipulandum operant drug discrimination procedure, rats were trained to discriminate temporal sequential administrations of ethanol (1.0 g/kg) that was followed or preceded by nicotine (0.3 mg/kg). Sessions alternated between food-reinforcement sessions on a variable interval 30-sec schedule (i.e., S^D) and non-reinforcement sessions (i.e., S^Δ). In Experiment 1, administrations of ethanol were followed or preceded by a 10-min interval of nicotine. Training sessions took place 10 min following the second drug injection. Four groups of rats were trained to discriminate only one sequence from sequential administrations of saline, and each drug sequence was counterbalanced across groups for their roles as S^D or S^Δ. There was robust stimulus control. N→E and E→N functioned equally well as S^D or S^Δ. Experiment 2 used two groups of rats. For one group, the E→N sequence functioned as the S^D and the N→E sequence functioned as the S^Δ. The drug sequences were counterbalanced for the other group. Brief non-reinforcement tests revealed significantly greater responding during the S^D sequence compared to the S^Δ sequence for both groups. These results suggest that different drug sequences of ethanol followed or preceded by nicotine established reliable discriminative stimulus control over operant responding, potentially because of characteristic differences in the overlapping pharmacokinetic profiles of the NE compound. The results are discussed in terms of: 1) conditional stimulus control among two interoceptive drug states; and 2) the clinical modulation of human alcohol consumption and tobacco smoking.

Cross-Species Translational Findings in the Discriminative Stimulus Effects of Ethanol

The progress on understanding the pharmacological basis of ethanol's discriminative stimulus effects has been substantial, but appears to have plateaued in the past decade. Further, the cross-species translational efforts are clear in laboratory animals, but have been minimal in human subject studies. Research findings clearly demonstrate that ethanol produces a compound stimulus with primary activity through GABA and glutamate receptor systems, particularly ionotropic receptors, with additional contribution from serotonergic mechanisms. Further progress should capitalize on chemogenetic and optogenetic techniques in laboratory animals to identify the neural circuitry involved in mediating the discriminative stimulus effects of ethanol. These infrahuman studies can be guided by in vivo imaging of human brain circuitry mediating ethanol's subjective effects. Ultimately, identifying receptors systems, as well as where they are located within brain circuitry, will transform the use of drug discrimination procedures to help identify possible treatment or prevention strategies for alcohol use disorder.

The nicotine + alcohol interoceptive drug state: contribution of the components and effects of varenicline in rats

RATIONALE

Nicotine and alcohol co-use is highly prevalent, and as such, individuals experience the interoceptive effects of both substances together. Therefore, examining sensitivity to a compound nicotine and alcohol (N + A) interoceptive cue is critical to broaden our understanding of mechanisms that may contribute to nicotine and alcohol co-use.

OBJECTIVES

This work assessed the ability of a N + A interoceptive cue to gain control over goal-tracking behavior and determined the effects of the α4β2 nicotinic partial agonist and smoking cessation compound varenicline on sensitivity to N + A.

METHODS

Two groups of male Long Evans rats were trained to discriminate N + A (0.4 mg/kg nicotine + 1 g/kg alcohol, intragastric gavage (IG)) from water under two different training conditions using a Pavlovian drug discrimination task. The effects of varenicline (0, 1, 3 mg/kg, intraperitoneally (IP)) administered alone and on sensitivity to N + A and the components were determined.

RESULTS

Under both training conditions, N + A rapidly gained control over behavior, with a greater contribution of nicotine to the N + A compound cue. Varenicline fully substituted for the N + A training dose, and varenicline (1 mg/kg) enhanced sensitivity to the lowest N + A dose (0.1 N + 0.1 A). Given the high selectivity of varenicline for the α4β2 receptor, this finding suggests a functional role for α4β2 nicotinic acetylcholine receptors (nAChRs) in modulating sensitivity to N + A.

CONCLUSIONS

The N + A compound cue is a unique cue that is modulated, in part, by activity at the α4β2 nAChR. These findings advance understanding of the interoceptive effects of nicotine and alcohol in combination and may have implications in relation to their co-use.