Differential expression of nicotine withdrawal as a function of developmental age in the rat

Cigarette smoking and resultant nicotine dependence remain major public health problems. Most smokers begin before the age of 18, yet preclinical models have insufficiently characterized the development of nicotine dependence in adolescence. To categorize the short-term effects of chronic nicotine administration throughout adolescence and adulthood, we exposed male Sprague Dawley rats to 14 days of continuously delivered nicotine (0, 1.2 or 4.8 mg/kg/d) using a subcutaneous osmotic minipump, starting between postnatal day 33 (p33) and p96. Next, to explore the effects of extended exposure to chronic nicotine, we exposed male Sprague Dawley rats to 42 days of continuous nicotine starting in adolescence (p33) or early adulthood (p68). Somatic and affective signs of precipitated withdrawal (PW) were observed after a mecamylamine (1.5 mg/kg, i.p.) challenge as compared to a saline injection. Short term nicotine exposure starting at p96, well within the adult period, elicited a significant increase in somatic PW as measured by a composite behavioral score. In contrast, adolescent exposure to nicotine elicited a unique behavioral profile, dependent on the starting age of exposure. Late adolescence exposure was characterized by scratching while adult exposure was characterized by facial tremors and yawns. Extended exposure to nicotine resulted in age specific characteristic nicotine withdrawal behaviors, including scratches, ptosis and locomotion, distinct from the short-term exposure. Thus, nicotine dependence severity, based on the expression of total somatic PW behaviors, is not observed until the adult period, and differences between adolescents and adults are observed using a more nuanced behavioral scoring approach. We conclude that age of nicotine initiation affects somatic withdrawal signs and their magnitude. These data serve as a foundation for understanding the underlying brain mechanisms of nicotine dependence and their development over adolescence and early adulthood.