The effect of water temperature on immobility in the forced swimming test in rats.

In the Porsolt swimming test intact rats acquire and retain the immobile response indistinguishably at 25 and 30 degrees C; at both temperatures retention is abolished by administration of the glucocorticoid antagonist RU38486 and the kappa-selective opiate receptor antagonist MR2266 when given together, but not when either is given alone. At 20 degrees C, however, the animals do not acquire the response, and have levels of retention significantly lower than at the higher temperature. This deficit is not restored by administration of glucocorticoids, kappa-opioid receptor-selective agonists, thyroid hormone or glucose.