Effects of metergoline and quipazine on locomotor activity of rats in novel and familiar environments.

Many studies have investigated the effects on locomotor activity of various manipulations of the brain's serotonin (5-hydroxytryptamine, 5-HT) systems but the results have not been consistent. However, besides employing different techniques for manipulating brain 5-HT, previous studies have differed in size of apparatus, amount of apparatus pre-exposure and length of session. To test the possibility that apparatus familiarity interacts with the effects of 5-HT manipulations on locomotor activity, this variable was manipulated in groups of rats treated with the 5-HT receptor blocker, metergoline or the agonist, quipazine. Within each drug treatment group, 18 rats had prior experience with the activity monitoring photocell chambers (pre-exposed condition) and 18 were not previously exposed (novel condition); each condition was further subdivided into 3 dose subgroups (n = 6). Testing consisted of 3 30-min sessions with subgroups receiving metergoline (0, 2.5, 5.0 mg/kg) or quipazine (0, 2.5, 5.0 mg/kg) 30 min before. Results with metergoline treatment revealed no significant drug effect in the pre-exposed groups but a decrease in activity in the novel condition. Quipazine, on the other hand, had no significant effect in the novel condition but produced a time-dependent effect on activity in the pre-exposed condition. These results suggest that the effects on locomotor activity of compounds affecting 5-HT neurotransmission may interact with the familiarity of the test apparatus and with the duration of testing. Interexperiment differences in these variables may account for some of the inconsistencies previously reported.