A method for equating thermal stress parameters for neurobehavioral toxicity studies

Rats were immersed in 20 cm of cold (15 or 20 degrees C) water for 1, 2.5 or 5 min and were tested 15 min later on fixed ratio 15 (FR15) operant behavior for 30 min. Linear regression analyses predicted that 4 min exposure to 20 degrees C would suppress FR15 behavior 50%. Subsequent exposure to 20 degrees C water for this time period resulted in 53% suppression of FR15 responding. Rats were then immersed in heated water for 4 min and tested 15 min later, exactly as in the cold stress experiment. Exposure to 44, 45 or 46 degrees C water suppressed FR15 responding 21, 46 and 94%, respectively. Significant alterations in colonic temperature indicated that the manipulations disrupted thermoregulatory mechanisms and were probably stressful. The procedure is an alternative to other methods used to index the intensity of stress. By manipulating selected variables (e.g., water temperature or exposure duration), qualitatively different stressors can be matched behaviorally prior to studying interactions between stress and various other manipulations such as drugs or toxic compounds.