Effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) after repeated administration on a conditioned...

8-OH-DPAT, a selective 5-HT1A agonist, has variously been found to impair, have no effect on or enhance the conditioned avoidance response (CAR). Procedural differences may account for the difference in results. In the first experiment in the present study rats were trained in the two-way active avoidance procedure to a criterion of 65% avoidance. Separate groups of rats were treated with 0.01, 0.1 or 1.0 mg/kg 8-OH-DPAT SC once per day for 14 days. The rats were tested in the CAR each day 5 min after treatment, using a 10 s light and tone conditioned stimulus and five 0.2 mA/0.5 s electric shocks. On the first day the doses of 0.1 and 1.0 mg/kg impaired avoidance, but by the end of training these two doses increased avoidance. This change in effect was accompanied by a 15-fold increase in the number of trials in which the subject crossed during a 10 s period of the ITI, which in turn led to a significant impairment in the discrimination ratio. The results of this experiment show that with repeated treatment 8-OH-DPAT changes from being antipsychotic like to being stimulant-like. The latter effect produces an improvement in avoidance, probably due to a non-specific increase in activity. In the second experiment, the rats were divided into groups based upon the undrugged performance. The avoidance-enhancing effect of 8-OH-DPAT was greater in magnitude in a group of poor performers, but was qualitatively similar in good performers. In the second stage of the experiment, gradual withdrawal from the drug was compared with sudden withdrawal. In the gradual withdrawal group, a reduction in the dose from 0.085 mg/kg to 0.01 mg/kg resulted in a gradual disappearance of the enhanced activity. There was an almost linear relationship between performance and the log dose of the drug, suggesting that the increase in activity seen after repeated administration of 8-OH-DPAT is directly related to the acute level of drug administered. This effect was evident in both good and poor performers. On the basis of these results it is suggested that many, but not all, antidepressant-like effects of 8-OH-DPAT may result from changes in activity.