Termination of Nociceptive Bahaviour at the End of Phase 2 of Formalin Test is Attributable to Endogenous Inhibitory Mechanisms, but not by Opioid Receptors Activation.

Introduction: Formalin injection induces nociceptive bahaviour in phase I and II, with a quiescent phase between them. While active inhibitory mechanisms are proposed to be responsible for initiation of interphase, the exact mechanisms which lead to termination of nociceptive response in phase II are not clear yet. Phase II is a consequence of peripheral and central sensitization processes, which can lead to termination of the noxious stimuli responses; 45-60 minutes after formalin injection via possible recruitment of active inhibitory mechanisms which we have investigated in this study.