Effects of Conditioning upon Stress Responses in the Rat. ∗

In a study (1) of the role of histamine-mediation of pituitary-adrenal activation by a variety of drugs and other stressful stimuli such as cold exposure in the rat, the problem of obtaining stable control values for plasma corticosterone levels was evident. The widespread use of stress as a tool for studying induction of enzyme synthesis and conclusions regarding enzyme regulation drawn therefrom, suggested the desirability of a controlled study to determine optimal methods of animal handling during such studies. Adaptation to handling and intravenous injection of saline was assessed by measuring plasma corticosterone elevation caused by subsequently administered histamine or saline. The combined effect of housing and conditioning to injection was also evaluated in view of the report(2) that resting levels of plasma corticosterone in rats are influenced by the number of animals maintained per cage. Variation in resting levels of plasma corticosterone and adrenal ascorbic acid was measured over a 12-month period since recent reports(3) show that susceptibility of the rat to various stressors appears to be seasonally-dependent.

Methods and materials. Male rats of the Holtzman strain weighing between 180 and 280 g were used throughout this study and were maintained on commercial laboratory chow and water. Animals were held for 7 days after receipt from the supplier before use in stress studies and were housed at 76°F with alternating 12-hour periods of light and darkness. Drugs were administered and the animals sacrificed in the animal quarters between 8:30 A.M. and 11:30 A.M. Animals were sacrificed by decapitation within 30 seconds of removal from the cage. Free-flowing blood was collected in tubes containing a few mg of potassium oxalate, centrifuged to remove red cells, and the plasma frozen and stored until assayed for plasma corticosterone.