Effects of Developmental Nicotine Exposure on the Brain and Behavior

• Other Authors: Martin, Melissa M. (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Neurosciences; Biology; Medical sciences
• Online Access: http://purl.flvc.org/fsu/fd/2019_Fall_Martin_fsu_0071E_15481

Cigarette smoking during pregnancy is a major public health concern, resulting in detrimental health effects in the mother and her offspring. The adverse behavioral consequences for children with developmental nicotine exposure include increased risk for attention deficit hyperactivity disorder, working memory deficits, epilepsy, novelty-seeking, and risk-taking behaviors. Interestingly, these behavioral conditions are consistent with altered inhibitory (GABA) neurotransmitter signaling. Therefore, the goal of my dissertation research was to test the hypothesis that early exposure to nicotine alters the development of the GABA system, which can produce functional changes in the brain. In order to test the hypothesis that developmental nicotine exposure produces long-lasting effects in the adult mouse brain, specifically the GABA system, I used a GAD67-GFP knock-in mouse in which GFP is intrinsically expressed under the GAD67 promoter. I first examined GABA neuron numerical densities, non-GABA neuron numerical densities, as well as the GABA-to-non-GABA neuron ratio for male and female offspring that received plain drinking water (WATER) or drinking water containing either 100 µg/ml (Nic100) or 200 µg/ml (Nic200) nicotine during development. I performed these analyses in the prefrontal and medial prefrontal cortices, two brain regions which are important for executive function and regulate behaviors known to be altered in developmental disorders such as attention deficit/hyperactivity disorder. In addition, these frontal cortical brain regions were subdivided into cortical layers (II-III, V, and VI) since each cortical layer is defined based on their cortical connections and hence have different functions. Overall, I found that developmental nicotine exposure does not alter the numerical density of GABA or non-GABA cells in the frontal cortex, however, I saw a significant reduction in the GABA-to-non-GABA ratio in the Nic200 treatment group compared to the WATER and Nic100 groups across both brain regions (prefrontal and medial prefrontal cortices) and across all cortical layers. Next, in order to test the hypothesis that developmental nicotine exposure produces long-lasting effects on behavioral phenotypes, I performed a battery of tests that examined locomotor activity, approach-avoidance behavior, exploratory behavior, spatial working memory, and object-based attention beginning at 90 days of age. Since I found a significant reduction in the GABA-to-non-GABA ratio for the Nic200 treatment group and not for the Nic100 group, I performed behavioral analyses in the WATER and Nic200 treatment groups. For these analyses, I used both male and female offspring as well as GAD67-GFP and wild-type offspring. I found that developmental nicotine exposure increases novelty-induced locomotor activity, increases exploratory behavior, and alters approach-avoidance behavior in favor of the approach behavior. During embryonic development, the majority of the GABA neurons originate in the medial ganglionic eminence of the basal forebrain and migrate tangentially to regions of the dorsal forebrain. At the same time, neurogenesis in the ventricular and subventricular zones is occurring in the dorsal forebrain. Therefore, I wanted to examine the immediate effects of developmental nicotine exposure on GABA neuron migration and neurogenesis in the embryonic brain. To do this, I collected the brains from embryonic day 13 and 15 embryos; two time points during embryonic development in which GABA neurons are migrating through regions of the dorsal forebrain and neurogenesis is occurring in the dorsal forebrain. Using GAD67-GFP embryos, I found that nicotine exposure significantly increases the number of GABA neurons in the prefrontal and medial prefrontal cortices. Similarly, using an in-vitro culture assay, nicotine exposure increased the number of GABA neurons migrating out from a medial ganglionic eminence explant. Taken together these results suggest that nicotine exposure during embryonic development alters GABA neuron migration. In addition, using wild-type embryos, I found that nicotine exposure significantly reduces the cell output in the future prefrontal and medial prefrontal cortices. Overall, developmental nicotine exposure produced dose-dependent decreases in GABA-to-non-GABA neuron ratios in the prefrontal and medial prefrontal cortices without perturbing the intrinsic differences in cortical thickness and laminar distribution of GABA or non-GABA neurons between these regions. A significant increase in exploratory behavior and a shift toward “approach” in the approach-avoidance paradigm were also observed. Thus, developmental nicotine exposure shifts the cortical excitation-inhibition balance toward excitation and produces behavioral changes consistent with novelty-seeking behavior. Lastly, I found that nicotine exposure during embryonic development produces a significant increase in GABA neuron migration and reduces the cell output in the dorsal forebrain. Collectively, these results suggest that developmental nicotine exposure alters embryonic brain development and that continued exposure during the embryonic and early postnatal period produces long-lasting structural and functional changes in the brain. === A Dissertation submitted to the Department of Biomedical Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === 2019 === September 11, 2019. === Acetylcholine, Approach-avoidance behavior, Brain development, Exploratory behavior, GABA, Prenatal nicotine exposure === Includes bibliographical references. === Pradeep G. Bhide, Professor Directing Dissertation; Jonathan H. Dennis, University Representative; Richard S. Nowakowski, Committee Member; Gregg D. Stanwood, Committee Member; Christine Metin, Committee Member.