title: 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.
abstract: BACKGROUND
  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.
  
  
  METHODS
  To test our hypothesis, in the first set of experiments, we evaluated nociceptive response after two consecutive injection of formalin (50µL, 2%), with intervals of 5 or 60 minutes. In the next set, formalin tests were carried out in companion with injection of Naloxone Hydrochloride, a non-selective antagonist of opioid receptors, pre-formalin injection and 30 and 45 minutes post formalin injection.
  
  
  RESULTS
  While normal nociceptive behaviour was observed in the group receiving one injection of formalin, a diminished response was observed in phases I and II of those receiving consequent injection of formalin, 60 minute after first injection. While second injection of formalin, 5 minute after first injection, had no effect. Administration of naloxone (1mg/kg) decreased nociception in phase 2A; but had no effect on delayed termination of formalin test.
  
  
  CONCLUSIONS
  The results of this study suggest the existence of an active inhibitory mechanism, other than the endogenous opioids, that is responsible for termination of nociceptive behaviour at the end of formalin test.
authors.affiliation: Department of Basic Sciences, Torbat heydariyeh University of Medical Sciences, Torbat heydariyeh, Iran.
authors.forename: Hassan
authors.initials: H
authors.lastname: Azhdari-Zarmehri
authors.affiliation: Department of Physiology & Cellular and Molecular Research Center, Sabzevar University Of medical Sciences, Sabzevar, Iran.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Mohammad-Zadeh
authors.affiliation: Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors.forename: Masoud
authors.initials: M
authors.lastname: Feridoni
authors.affiliation: Medical Students Research Committee, Kerman Medical University, Kerman, Iran.
authors.forename: Masoud
authors.initials: M
authors.lastname: Nazeri
keywords: Formalin Test; Naloxone; Nociception