The response of plasma catecholamines in rats simultaneously exposed to immobilization and painful stimuli.

Immobilization represents a strong stressor inducing a profound increase in plasma epinephrine and norepinephrine levels. We have previously demonstrated that a subcutaneous injection of formalin (0.2 mL of 4% solution/100 g bw) attenuated the immobilization-induced elevation of plasma epinephrine levels in rats. In the present study, we investigated whether other painful and stressful stimuli, such as capsaicin, hydrochloric acid, mechanical pressure, heat, and cold, might also attenuate the increase of plasma epinephrine in rats exposed to acute immobilization stress. With the exception of formalin, all of the painful stimuli applied failed to attenuate the increase of plasma epinephrine levels in immobilized animals. Our data suggest that the attenuation of an immobilization-induced increase in plasma epinephrine levels is specific for subcutaneous formalin administration.