Learned immobility is also involved in the forced swimming test in mice

A modified version of the forced swimming test (FST) was utilised in order to test, for the first time in mice, the learned immobility hypothesis. Fro m this point of view, the subjects learn to be immobile in the first sessio n, being the second one a retention test. The development of habituation wa s observed by repeating the test. The forgetting was studied by allowing di fferent time intervals between the first and the second session. A decrease in the activity was observed with intervals of up to 18 days, but not with longer intervals of 21 or 24 days. Scopolamine (1 or 2 mg/kg), a cholinerg ic antagonist, did not modify the swimming activity in the second session. Physostigmine, a cholinergic agonist, at a dose of 0.05 mg/kg was ineffecti ve, and at a dose of 0.2 mg/kg decreased the swimming activity in the secon d session. These data extend to mice the findings previously obtained in ra ts, and lend additional support to the learned immobility hypothesis in the interpretation of the behaviour found in the FST.