Dissociations between the locomotor stimulant and depressant effects of nicotinic agonists in rats.

The effects of nicotine and related compounds on locomotor activity were compared in experimentally naive rats and in animals chronically exposed to nicotine and the photocell test chambers. In experimentally naive rats, all nicotinic compounds decreased locomotion in a dose-related manner and the rank order of potency was (-)-nicotine > (+)-nornicotine > (+)-nicotine > cytisine > lobeline > anabasine. Mecamylamine attenuated the locomotor depressant effects of most of the agonists, except lobeline. In rats previously exposed to nicotine and the test apparatus for several weeks, (-)-nicotine increased locomotor activity in a dose-related manner, with a maximal increase to 400% of baseline at a dose of 0.4 mg/kg. One or more doses of (+)-nicotine, (+)-nornicotine and anabasine also increased locomotor activity in these animals, although the maximal effects seen were in all cases less than the maximal effect of (-)-nicotine. Cytisine and lobeline failed to increase locomotor activity at any dose tested. These conclusions were not altered by consideration of the time-courses for the effects of the different drugs. Thus, the results confirm that the locomotor stimulant and depressant effects of nicotine can be dissociated from each other, a finding that may be explained by differences in their actions at nicotinic receptors.