Short-Term Adolescent Nicotine Exposure has Immediate and Persistent Effects on Cholinergic Systems: Critical Periods, Patterns of Exposure, Dose Thresholds

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Abstract
In adolescents, the symptoms of nicotine dependence can appear well before the onset of habitual smoking. We investigated short-term nicotine exposure in adolescent rats for corresponding cholinergic alterations. Beginning on postnatal day 30, rats were given a 1-week regimen of nicotine infusions or twice-daily injections, at doses (0.6, 2, and 6 mg/kg/day) set to achieve plasma levels found in occasional to regular smokers. In the cerebral cortex, midbrain, and hippocampus, we assessed nicotinic cholinergic receptor (nAChR) binding, choline acetyltransferase (ChAT) activity, a constitutive marker for cholinergic nerve terminals, and [3H]hemicholinium-3 (HC-3) binding to the high-affinity choline transporter, which responds to cholinergic synaptic stimulation. nAChR upregulation was observed with either administration route, even at the lowest dose; in the hippocampus, increases could be detected with as little as 2 days' treatment at 0.6 mg/kg/day. In the midbrain, upregulation was still significant even 1 month post-treatment. Adolescent nicotine treatment also produced lasting decrements in HC-3 binding that were separable from effects on ChAT, suggesting cholinergic synaptic impairment. Again, these effects were obtained at the lowest dose and remained significant 1 month post-treatment. Our results indicate that in adolescence, even a brief period of continuous or intermittent nicotine exposure, elicits lasting alterations in cholinergic systems in brain regions associated with nicotine dependence. As the effects are detected at exposures that produce plasma concentrations as little as one-tenth of those in regular smokers, the exquisite sensitivity of the adolescent brain to nicotine may contribute to the onset of nicotine dependence even in occasional smokers.