Modeling adolescent nicotine exposure: effects on cholinergic systems in rat brain regions

Smoking among teenagers is increasing and the initiation of tobacco use during adolescence is associated with subsequently higher cigarette consumption and lower rates of quitting. Few animal studies have addressed whether adolescent nicotine exposure exerts unique or lasting effects on brain structure or function. Initial investigations with a rat model of adolescent nicotine exposure have demonstrated that the vulnerable developmental period for nicotine-induced brain cell damage extends into adolescence. In the current study, we examined the effect of nicotine on cholinergic systems in male and female adolescent rats with an infusion paradigm designed to match the plasma levels found in human smokers or in users of the transdermal nicotine patch. Choline acetyltransferase activity (ChAT) and [3H]hemicholinium-3 binding (HC-3) were monitored; ChAT is a static marker that closely reflects the density of cholinergic innervation, whereas HC-3 binding, which labels the presynaptic high-affinity choline transporter, is responsive additionally to nerve impulse activity. Measurements were carried out in the midbrain, the region most closely involved in reward and addiction pathways, as well as in the cerebral cortex and hippocampus. During nicotine treatment and for 1 month after the termination of treatment, ChAT activity was reduced significantly in the midbrain but not in the other regions. HC-3 binding showed a substantial increase during the course of nicotine treatment and again, the effect was limited to the midbrain. Midbrain values returned to normal immediately after the cessation of nicotine exposure and then showed a subsequent, transient suppression of activity. Although the cerebral cortex showed little or no change in HC-3 binding during or after nicotine administration, activity was reduced persistently in the hippocampus. The regionally-selective effects of adolescent nicotine treatment on cholinergic systems support the concept that adolescence is a vulnerable developmental period for ultimate effects on behavior.