Progesterone reduces immobility in rats forced to swim

Background: Among its behavioral actions, progesterone reduces anxiety in s everal species including humans; however, any antidepressant-like propertie s remain to be explored. Methods. In the present study, Wistar rats received injections (i.p.) of pr ogesterone (0.20-3.0 mg/kg) 24 and 2 h before being submitted to the forced swim, a test in which antidepressants regularly reduce immobility and exer t few or no actions on locomotor activity. In order to discard the cumulati ve effects of progesterone and a possible effect from the repetition of the swimming test, all animals received one of several progesterone doses in a different sequence following a completely randomized experimental intrasub ject design. Results. Ovariectomy did not modify immobility in the forced swim test as c ompared to control tests practiced before surgery. A dose of 0.80 mg/kg str ongly reduced the total time of immobility in forced swim test (p<0.001), b ut did not modify locomotor activity. In a drug-free control test applied I week after the last injection of progesterone, immobility returned to the higher values observed in the control tests. Conclusions. From these results, it is concluded that progesterone may poss ess some antidepressant-like activity. (C) 1999 IMSS. Published by Elsevier Science Inc.