Cigarette smoking is the leading preventable cause of death in the United States. Of those who smoke, 9 out of 10 report trying their first cigarette before the age of 18. Although most people who initiate tobacco use are teenagers, animal models for studying tobacco dependence have traditionally focused on how adult animals initiate, withdrawal from and relapse to cigarette smoking. Furthermore, cigarette smoke contains more than 7,000 constituents, including nicotine, yet pre-clinical research has focused on nicotine alone. Our lab began studying these constituents by creating cigarette smoke extract, CSE, a solution which contains the aqueous constituents present in cigarette smoke. Previous work from our lab found that CSE is more potent than nicotine alone and can enhance stress-induced reinstatement in adult male rats. In order to understand how the presence of tobacco smoke constituents may affect adolescents, I investigated the role of these constituents in models of smoking initiation and relapse. I found that the tobacco smoke constituents did not influence adolescent or adult acquisition of self-administration. Adolescents self-administered more low-dose nicotine than adults when their increased non-specific responding was corrected for. During reinstatement of drug seeking, I found that CSE enhanced stress- + cue-induced reinstatement of drug-seeking behavior with no effect of age. To investigate the role of tobacco smoke constituents on attenuation of adolescent self-administration, a novel smoking cessation pharmacotherapy, AT-1001, which is a selective α3β4 nAChR functional antagonist, was used. AT-1001 attenuated both CSE and nicotine self-administration in adolescent rats to a similar degree. My results suggest that both age and the presence of tobacco smoke constituents are important factors in establishing a proper model of tobacco dependence. Furthermore, my findings provide novel insights on adolescent initiation and relapse and offer an exciting potential for the development of a new tobacco dependence animal model that could help create innovative therapeutics to curb the addiction faced by many.

With the increasing use of e-cigarettes, teen nicotine exposure is becoming more widespread. This is a growing public health crisis, given findings from both clinical and preclinical studies that show adolescent brain to be uniquely sensitive to nicotine. Animal studies have shown that adolescent nicotine exposure results in increased sensitivity to cocaine and other drugs, although the mechanisms that underlie this are poorly understood. We now show microglia to be critical regulators of nicotine-induced increases in adolescent cocaine self-administration. Nicotine has age-dependent effects on microglial morphology and immune transcript profiles. Moreover, blocking D2 receptors eliminates both nicotine-induced increases in cocaine self-administration and microglial activation. Finally, we show nicotine causes microglial-dependent synaptic pruning and plasticity. Taken together, we demonstrate that adolescent microglia are uniquely sensitive to perturbations by drugs of abuse, crucial for nicotine-induced increases in cocaine-seeking, and D2 receptors play a mechanistic role in these phenomena.

Alcohol and nicotine, either from tobacco or increasingly from electronic cigarettes, are the two most commonly co-used drugs of abuse. Dual use has increased over time among teenagers and young adults, and represents a major public health concern among adolescents. During adolescence, the brain undergoes a profound reorganization in areas critical for executive function, reward processing, and motivated behavior that are necessary for adult autonomy, but do leave the adolescent brain vulnerable to the deleterious effects of drugs of abuse, including nicotine and alcohol. Neurobiological mechanisms mediating high rates of co-use are not well understood. However, as shown in this dissertation, developmental and sex-dependent changes in the dynorphin/kappa opioid receptor (KOR) system likely contribute.

In this dissertation, I emphasize the profound complexity of the KOR system in modulating acute drug- and mood-associated behaviors across development and between sexes. The present studies use behavioral pharmacological approaches to demonstrate that KOR activity differentially modulates the reinforcing properties of nicotine and/or alcohol. Anatomical studies suggest that distinct, drug-specific anatomical pathways containing KORs modulate the reinforcing properties of these drugs. I have also shown that drug-induced versus direct KOR activation differentially influences mood-associated behaviors. Concurrent nicotine and alcohol (Nic+EtOH) produced a KOR-dependent decrease in anxiety-like behavior in adult females that was paralleled by increased KOR function in the basolateral amygdala. In contrast, two selective KOR agonists exerted a robust anxiogenic effect in this group, that may suggest distinct circuitry mediating direct versus indirect KOR activation on anxiety. Both adolescent and adult males displayed increased anxiety following KOR agonist treatment, while only adolescent males were sensitive to the pro-depressant effects of direct KOR activation, and Nic+EtOH had a subtle anxiolytic effect in this group. I also demonstrate a unique and previously unknown immaturity of KORs in adolescent females, as they were largely impervious to the effects of KOR manipulation on drug- or mood-associated behaviors. As a whole, my work emphasizes the profound complexity of KOR signaling and highlights important maturational and sex-dependent changes in KOR function. These changes may act as a critical, but underrecognized, influence of nicotine and alcohol co-use among teenagers and women.

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Chronic pain represents a substantial burden to society and is difficult to manage for both physicians and patients due to lack of effective therapies. There are multiple dimensions of chronic pain, with symptoms such as anxiety, depression, fatigue, anhedonia, etc. Collectively known as negative affect, these symptoms exacerbate the pain experience and results in decreased efficacy of opioid analgesics. Endotoxins known to induce neuroinflammation produce anhedonia and depression in humans. Similarly, activation of kappa opioid receptors produces dysphoric and depressive symptoms in humans. This work investigates the contribution of neuroinflammation and kappa opioid receptors in multiple rodent models of chronic pain and the manifestation of negative affect in these models. We identified that kappa opioid receptors are upregulated within limbic brain regions of neuropathic pain animals and upregulation within ventral tegmental area dopamine neurons is responsible for the tonic-aversive component of pain. This effect was sex dependent where blocking or elimination of kappa opioid receptors in male but not female mice prevented pain-induced aversion.

Our lab previously identified that neuroinflammation within the ventral tegmental area in a model of neuropathic pain was responsible for altered reward signaling and dopamine release. We show that peripheral nerve injury model of neuropathic pain causes microglial activation in many brain structures but is not ubiquitous, and neuroinflammation is primarily restricted to limbic brain structures. Because neuroinflammation is a hallmark of traumatic brain injury, we asked whether neuroinflammation is also causative to chronic pain following head injury. Using a model of repeated mild traumatic brain injury (rmTBI), we show that blocking microglial activation one month after injury reverses cold hypersensitivity and affective-like behaviors but has no effect on rmTBI-induced hyper-locomotion or risk-taking behavior. Together, we provide insight to the complexity and importance of a global neuroinflammatory response and kappa opioid receptors to pain.