Inhibitory effect of formalin administration on immobilization-induced epinephrine release

Injection of formalin is used as a classical painful stressor that produces a biphasic nociceptive response consisting of a 1- to 10-min early phase and a later phase 30 to 240 min after injection. The period between these two phases, called "interphase," is characterized by attenuated nociception. We evaluated the response of catecholamine plasma levels to formalin-induced pain stress with special attention to these three time periods. Subcutaneous injection of 4% formalin (0.2 mL/100 g bw) into the hind limb produced a slight reduction of plasma epinephrine levels in the first 15 min, which was followed by a significant increase that remained high up to 120 min after injection. Norepinephrine levels increased immediately after injections and remained high from 30 until 120 min. To test the effect of formalin injection in a stressful condition, we exposed animals to 2 h immobilization stress. In the first experiment, formalin was injected before the start of immobilization. A significant decrease of plasma epinephrine levels was measured up to 25 min post-injection, whereas plasma norepinephrine levels remained high. A second formalin injection during immobilization was as effective as the first one: It depleted plasma epinephrine levels from 5 to 15 min post-injection without significant changes in norepinephrine levels. In the second experiment, formalin given after the beginning of immobilization produced a significant decrease of epinephrine levels 15 min after the injection and produced a significant increase 60 min after injection. The plasma norepinephrine levels were significantly increased by 40 min post-injection. The data show that the inhibitory process during the interphase of formalin test is able to significantly decrease epinephrine release not only during basal conditions but also during exposure to a severe stressor, such as immobilization without suppression of plasma norepinephrine levels.