Acute effects of nicotine amplify accumbal neural responses during nicotine-taking behavior and nicotine-paired environmental cues

Nucleus accumbens shell neurons' early sensitivity to cocaine is associated with future increases in drug intake

The striatum, both dorsal and ventral, is strongly implicated in substance use disorder. Chronic consumption of abused substances, such as cocaine, can cause an oversaturation of mesostriatal dopamine, which results in alterations in the firing of striatal neurons. While most preclinical studies of drug self-administration (S-A) are focused on these alterations, individual differences in a subject's early responses to drugs can also account for substantial differences in addiction susceptibility. In this study, we modeled longitudinal pharmacokinetics using data from a previous longitudinal study (Coffey et al., 2015) and aimed to determine if firing in specific dorsal and ventral striatal subregions was subject to changes across chronic cocaine S-A, and if individual animal differences in striatal firing in response to early drug exposure correlated with increases in drug intake. We observed that the firing patterns of nucleus accumbens (NAc) core and shell neurons exhibited increasing sensitivity to cocaine over the first 6 S-A sessions and maintained a strong negative correlation between drug intake and neuronal firing rates across chronic S-A. Moreover, we observed that the early sensitivity of NAc shell neurons to cocaine correlated with future increases in drug intake. Specifically, rats whose NAc shell neurons were most inhibited by increasing levels of cocaine upon first exposure exhibited the strongest increases in cocaine intake over time. If this difference can be linked to a genetic difference, or druggable targets, it may be possible to screen for similar addiction susceptibility in humans or develop novel preemptive pharmacotherapies.

Altered neuronal activity in the ventromedial prefrontal cortex drives nicotine intake escalation

Nicotine addiction develops after prolonged drug use and escalation of drug intake. However, because of difficulties in demonstrating escalation of nicotine use in rats, its underlying neuroadaptations still remain poorly understood. Here we report that access to unusually high doses of nicotine (i.e., from 30 µg to 240 µg/kg/injection) for self-administration precipitated a rapid and robust escalation of nicotine intake and increased the motivation for the drug in rats. This nicotine intake escalation also induced long-lasting changes in vmPFC neuronal activity both before and during nicotine self-administration. Specifically, after escalation of nicotine intake, basal vmPFC neuronal activity increased above pre-escalation and control activity levels, while ongoing nicotine self-administration restored these neuronal changes. Finally, simulation of the restoring effects of nicotine with in vivo optogenetic inhibition of vmPFC neurons caused a selective de-escalation of nicotine self-administration.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Nicotine and ethanol co-use in Long-Evans rats: Stimulatory effects of perinatal exposure to a fat-rich diet

Clinical studies demonstrate frequent co-existence of nicotine and alcohol abuse and suggest that this may result, in part, from the ready access to and intake of fat-rich diets. Whereas animal studies show that high-fat diet intake in adults can enhance the consumption of either nicotine or ethanol and that maternal consumption of a fat-rich diet during pregnancy increases operant responding for nicotine in offspring, little is known about the impact of dietary fat on the co-abuse of these two drugs. The goal of this study was to test in Long-Evans rats the effects of perinatal exposure to fat on the co-use of nicotine and ethanol, using a novel paradigm that involves simultaneous intravenous (IV) self-administration of these two drugs. Fat- vs. chow-exposed offspring were characterized and compared, first in terms of their nicotine self-administration behavior, then in terms of their nicotine/ethanol self-administration behavior, and lastly in terms of their self-administration of ethanol in the absence of nicotine. The results demonstrate that maternal consumption of fat compared to low-fat chow during gestation and lactation significantly stimulates nicotine self-administration during fixed-ratio testing. It also increases nicotine/ethanol self-administration during fixed-ratio and dose-response testing, with BEC elevated to 120 mg/dL, and causes an increase in breakpoint during progressive ratio testing. Of particular note is the finding that rats perinatally exposed to fat self-administer significantly more of the nicotine/ethanol mixture as compared to nicotine alone, an effect not evident in the chow-control rats. After removal of nicotine from the nicotine/ethanol mixture, this difference between the fat- and chow-exposed rats was lost, with both groups failing to acquire the self-administration of ethanol alone. Together, these findings suggest that perinatal exposure to a fat-rich diet, in addition to stimulating self-administration of nicotine, causes an even greater vulnerability to the excessive co-use of nicotine and ethanol.

Contribution of nucleus accumbens core (AcbC) to behavior control during a learned resting period: introduction of a novel task and lesion experiments

In recent years, the study of resting state neural activity has received much attention. To better understand the roles of different brain regions in the regulation of behavioral activity in an arousing or a resting period, we developed a novel behavioral paradigm (8-arm food-foraging task; 8-arm FFT) using the radial 8-arm maze and examined how AcbC lesions affect behavioral execution and learning. Repetitive training on the 8-arm FFT facilitated motivation of normal rats to run quickly to the arm tips and to the center platform before the last-reward collection. Importantly, just after this point and before confirmation of no reward at the next arm traverse, locomotor activity decreased. This indicates that well-trained rats can predict the absence of the reward at the end of food seeking and then start another behavior, namely planned resting. Lesions of the AcbC after training selectively impaired this reduction of locomotor activity after the last-reward collection without changing activity levels before the last-reward collection. Analysis of arm-selection patterns in the lesioned animals suggests little influence of the lesion in the ability to predict the reward absence. AcbC lesions did not change exploratory locomotor activity in an open-field test in which there were no rewards. This suggests that the AcbC controls the activity level of planned resting behavior shaped by the 8-arm FFT. Rats receiving training after AcbC lesioning showed a reduction in motivation for reward seeking. Thus, the AcbC also plays important roles not only in controlling the activity level after the last-reward collection but also in motivational learning for setting the activity level of reward-seeking behavior.

AMPAR-independent effect of striatal αCaMKII promotes the sensitization of cocaine reward

Changes in CaMKII-regulated synaptic excitability are a means through which experience may modify neuronal function and shape behavior. While behavior in rodent addiction models is linked with CaMKII activity in the nucleus accumbens (NAc) shell, the key cellular adaptations that forge this link are unclear. Using a mouse strain with striatal-specific expression of autonomously active CaMKII (T286D), we demonstrate that while persistent CaMKII activity induces behaviors comparable to those in mice repeatedly exposed to psychostimulants, it is insufficient to increase AMPAR-mediated synaptic strength in NAc shell. However, autonomous CaMKII upregulates A-type K(+) current (IA) and decreases firing in shell neurons. Importantly, inactivating the transgene with doxycycline eliminates both the IA-mediated firing decrease and the elevated behavioral response to cocaine. This study identifies CaMKII regulation of IA in NAc shell neurons as a novel cellular contributor to the sensitization of cocaine reward.