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.
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.
DISCUSSION: 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.