Light-dark variation in response to chronic nicotine treatment and the density of hypothalamic α-bungarotoxin receptors

Chronic nicotine administration increased locomotor activity during the light, but not the dark, in rats maintained on a 12:12-hr light/dark cycle, but the period and peak of the circadian rhythm (CR) were not affected. In Experiment I, 24 male rats were implanted with battery-operated telemeters and locomotor activity was continuously measured for 10 days before and 10 days after the implantation of osmotic mini-pumps which delivered 0, 0.5, 3.0 or 10 mg/kg/day of (±)-nicotine tartrate. Nicotine increased locomotor activity during the light in a dose-dependent manner. Tolerance to the stimulant effects of nicotine during the light occurred in 5-6 days. To determine if the stimulant properties of nicotine were associated with light as opposed to disruption by the environmental stimuli normally present during the day in our animal facility, a second experiment was conducted in which rats were treated with saline or 10 mg/kg/day (±)-nicotine di(+)hydrate tartrate and maintained on a reversed light/dark cycle. Again nicotine increased activity during the light (21:00-09:00) but not dark (09:00-21:00). In a third experiment, the density of α-bungarotoxin binding sites was found to be significantly decreased when animals were sacrificed at 06:00 in comparison with animals sacrificed at 10:00 and 14:00.