Strawberry Additive Increases Nicotine Vapor Sampling and Systemic Exposure But Does Not Enhance Pavlovian-Based Nicotine Reward in Mice.

Nicotine is an addictive drug whose popularity has recently increased, particularly among adolescents, because of the availability of electronic nicotine devices (i.e., "vaping") and nicotine e-liquids containing additives with rich chemosensory properties. Some efforts to understand the role of these additives in nicotine reward suggest that they increase nicotine reward and reinforcement, but the sensory contributions of additives, especially in their vapor forms, are largely untested. Here, to better understand how a fruit-flavored (i.e., strawberry) additive influences nicotine reward and aversion, we used a conditioned place preference (CPP) procedure in which nicotine and a strawberry additive were delivered as a vapor to male and female adolescent mice. We found that nicotine vapor alone can lead to a dose-dependent CPP when using a biased design. The strawberry additive did not produce CPP on its own, and we did not observe an effect of the strawberry additive on nicotine vapor-induced reward. Nevertheless, mice exposed to nicotine plus strawberry additive vapor had higher plasma cotinine concentrations, which did not appear to reflect altered nicotine metabolism. Instead, by directly measuring vapor sampling through respiration monitoring, we uncovered an increase in the amount of sniffing toward strawberry-containing nicotine vapor compared with nicotine vapor alone. Together these data indicate that chemosensory-rich e-liquid additives may enhance the perceived sensory profile of nicotine vapors rather than the reward value per se, which leads to overall increased nicotine exposure.

CHRNA5 gene variation affects the response of VTA dopaminergic neurons during chronic nicotine exposure and withdrawal.

Nicotine is the principal psychoactive component in tobacco that drives addiction through its action on neuronal nicotinic acetylcholine receptors (nAChR). The nicotinic receptor gene CHRNA5, which encodes the α5 subunit, is associated with nicotine use and dependence. In humans, the CHRNA5 missense variant rs16969968 (G > A) is associated with increased risk for nicotine dependence and other smoking-related phenotypes. In rodents, α5-containing nAChRs in dopamine (DA) neurons within the ventral tegmental area (VTA) powerfully modulate nicotine reward and reinforcement. Although the neuroadaptations caused by long-term nicotine exposure are being actively delineated at both the synaptic and behavioral levels, the contribution of α5-containing nAChRs to the cellular adaptations associated with long-term nicotine exposure remain largely unknown. To gain insight into the mechanisms behind the influence of α5-containing nAChRs and the rs16969968 polymorphism on nicotine use and dependence, we used electrophysiological approaches to examine changes in nAChR function arising in VTA neurons during chronic nicotine exposure and multiple stages of nicotine withdrawal. Our results demonstrate that CHRNA5 mutation leads to profound changes in VTA nAChR function at baseline, during chronic nicotine exposure, and during short-term and prolonged withdrawal. Whereas nAChR function was suppressed in DA neurons from WT mice undergoing withdrawal relative to drug-naïve or nicotine-drinking mice, α5-null mice exhibited an increase in nAChR function during nicotine exposure that persisted throughout 5-10 weeks of withdrawal. Re-expressing the hypofunctional rs16969968 CHRNA5 variant in α5-null VTA DA neurons did not rescue the phenotype, with α5-SNP neurons displaying a similar increased response to ACh during nicotine exposure and early stages of withdrawal. These results demonstrate the importance of VTA α5-nAChRs in the response to nicotine and implicate them in the time course of withdrawal.

Individual differences in dopamine uptake in the dorsomedial striatum prior to cocaine exposure predict motivation for cocaine in male rats.

A major theme of addiction research has focused on the neural substrates of individual differences in the risk for addiction; however, little is known about how vulnerable populations differ from those that are relatively protected. Here, we prospectively measured dopamine (DA) neurotransmission prior to cocaine exposure to predict the onset and course of cocaine use. Using in vivo voltammetry, we first generated baseline profiles of DA release and uptake in the dorsomedial striatum (DMS) and nucleus accumbens of drug-naïve male rats prior to exposing them to cocaine using conditioned place preference (CPP) or operant self-administration. We found that the innate rate of DA uptake in the DMS strongly predicted motivation for cocaine and drug-primed reinstatement, but not CPP, responding when "price" was low, or extinction. We then assessed the impact of baseline variations in DA uptake on cocaine potency in the DMS using ex vivo voltammetry in naïve rats and in rats with DA transporter (DAT) knockdown. DA uptake in the DMS of naïve rats predicted the neurochemical response to cocaine, such that rats with innately faster rates of DA uptake demonstrated higher cocaine potency at the DAT and rats with DAT knockdown displayed reduced potency compared to controls. Together, these data demonstrate that inherent variability in DA uptake in the DMS predicts the behavioral response to cocaine, potentially by altering the apparent potency of cocaine.

Suvorexant, an orexin/hypocretin receptor antagonist, attenuates motivational and hedonic properties of cocaine.

Orexins ('hypocretins') are peptides produced by neurons of the hypothalamus that project to structures implicated in reward and emotion processing. Converging evidence demonstrates functional roles of orexin signaling in arousal, sleep/wakefulness and motivated behaviors for natural and drug rewards. Suvorexant, a dual orexin receptor antagonist, recently received approval from the US Food and Drug Administration to treat insomnia. In Experiment 1, rats self-administered cocaine under a progressive-ratio schedule of reinforcement and the effects of suvorexant on motivation to self-administer cocaine were measured. In Experiment 2, the effects of suvorexant on cocaine reward were assessed by using a place conditioning paradigm, and 50-kHz ultrasonic vocalizations were also recorded to track changes in hedonic reactivity to cocaine. To rule out potentially confounding effects of suvorexant-induced somnolence, locomotor activity was also measured. In Experiment 3, the effects of suvorexant on cocaine-evoked elevations in ventral striatal dopamine were examined. Data reveal that suvorexant (i) reduced the number of cocaine infusions earned during progressive-ratio self-administration; (ii) attenuated initial positive hedonic reactivity to cocaine and prevented cocaine place preference; (iii) did not affect cocaine-induced hyperlocomotion and (iv) reduced cocaine-induced elevations in extracellular ventral striatal dopamine. The present study examined the therapeutic potential of suvorexant in rodent models of cocaine use disorder. These results contribute toward a growing literature supporting therapeutic roles of orexin receptor antagonists in treating substance use disorders.

Hypocretin/orexin knock-out mice display disrupted behavioral and dopamine responses to cocaine.

The hypocretin/orexin (HCRT) system is implicated in reward and reinforcement processes through actions on the mesolimbic dopamine (DA) system. Here we provide evidence for the relationship between HCRT and DA in vivo in anesthetized and freely moving mice. The ability of cocaine to elicit reward-related behaviors in mice lacking the HCRT prepro-peptide (HCRT knock-out; KO) and wild-type controls was determined using conditioned place preference. Using a combination of microdialysis and in vivo fast scan cyclic voltammetry in anesthetized and freely moving mice, we investigated the underlying role of HCRT in the regulation of DA release and uptake. We show that, unlike wild-type mice, HCRT KO mice fail to develop characteristic conditioned place preference for cocaine. These mice also demonstrated reduced DA release and uptake under baseline conditions in both anesthetized and freely moving experiments. Further, diminished DA signaling in HCRT KO mice persists following administration of cocaine. These findings indicate that HCRT is essential for the expression of behaviors associated with the rewarding effects of cocaine, and suggest that HCRT regulation of reward and reinforcement may be related to disruptions to DA neurotransmission.

Clozapine N-Oxide Administration Produces Behavioral Effects in Long-Evans Rats: Implications for Designing DREADD Experiments.

Clozapine N-oxide (CNO) is a ligand for a powerful chemogenetic system that can selectively inhibit or activate neurons; the so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADD) system. This system consists of synthetic G-protein-coupled receptors, which are not believed to be activated by any endogenous ligand, but are activated by the otherwise inert CNO. However, it has previously been shown that the administration of CNO in humans and rats leads to detectable levels of the bioactive compounds clozapine and N-desmethylclozapine (N-Des). As a follow-up, experiments were conducted to investigate the effects of CNO in male Long-Evans rats. It was found that 1 mg/kg CNO reduced the acoustic startle reflex but had no effect on prepulse inhibition (PPI; a measure of sensorimotor gating). CNO (2 and 5 mg/kg) had no effect on the disruption to PPI induced by the NMDA antagonist phencyclidine or the muscarinic antagonist scopolamine. In locomotor studies, CNO alone (at 1, 2, and 5 mg/kg) had no effect on spontaneous locomotion, but 5 mg/kg CNO pretreatment significantly attenuated d-amphetamine-induced hyperlocomotion. In line with the behavioral results, fast-scan cyclic voltammetry found that 5 mg/kg CNO significantly attenuated the d-amphetamine-induced increase in evoked dopamine. However, the effects seen after CNO administration cannot be definitively ascribed to CNO because biologically relevant levels of clozapine and N-Des were found in plasma after CNO injection. Our results show that CNO has multiple dose-dependent effects in vivo and is converted to clozapine and N-Des emphasizing the need for a CNO-only DREADD-free control group when designing DREADD-based experiments.