Social isolation modifies nicotine's effects in animal tests of anxiety

Abstract

1. These experiments determined whether the housing conditions of rats influenced the effects of nicotine in two animal tests of anxiety, social interaction and elevated plus-maze tests. 2. In animals housed singly for 7 days, (-)nicotine (0.025 mg kg(-1) s.c.) was ineffective, but 0.05, 0.1 and 0.25 mg kg(-1) (s.c.) significantly increased the time spent in social interaction, without changing locomotor activity, thus indicating anxiolytic actions. (-)Nicotine (0.45 mg kg(-1) s.c.) significantly reduced social interaction, indicating an anxiogenic effect. 3. However, in group-housed animals, (-)nicotine (0.025 mg kg(-1) s.c.) had a significant anxiolytic effect in the social interaction test, but 0.01, 0.05, 0.1, 0.25 and 0.45 mg kg(-1) were ineffective. (-)Nicotine (1 mg kg(-1)) reduced motor activity and social interaction in the group-housed animals. 4. In the elevated plus-maze, the time-course and the dose-response curve to nicotine were investigated. In both singly- and group-housed rats, (-) nicotine (0.1 - 0.45 mg kg(-1) s.c.) decreased the per cent entries into, and per cent time spent on, the open arms, indicating anxiogenic effects. 5. The housing condition influenced the time course, with significant effects at 5 and 30 min after injection in group-housed rats, and significant effects at 30 and 60 min in singly-housed rats. 6. In the social interaction test there was no difference in the scores of the first and last rats removed from group cages, whereas the order of removal from the cages did affect the scores in the elevated plus-maze. 7. These results provide further evidence that the two animal tests model distinct states of anxiety, and show how social isolation powerfully modifies both anxiolytic and anxiogenic effects of nicotine.

Figure 1

Mean (±s.e.mean) time(s) spent in social interaction by rats housed singly (hatched bars) or in social groups of five (black bars) for 7 days and then tested 30 min after injection with vehicle or (−) nicotine (0.025 – 0.45 mg kg⁻¹ s.c.). ^**P<0.01 compared with appropriate vehicle control group. ++P<0.01 comparing vehicle-injected singly and group housed animals.

Figure 2

Mean (±s.e.m.) per cent time spent on the open arms of the elevated plus maze by rats housed singly or in social groups of five for 7 days and then tested 5, 30 or 60 min after injection with vehicle or (−) nicotine (0.05 – 0.45 mg kg⁻¹). ^*P<0.05, ^**P<0.001 compared with appropriate vehicle control group.