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Department of Basic Sciences and Neurosciences Research Center, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran

Introduction: There is some evidence demonstrating endogenous inhibitory pathways of pain involve in the inter-phase (phase between early and later phase) of the formalin test. We previously showed that swimming stress modulates the pain-related behaviors during the interphase of formalin test. In this study, we evaluated the role of endogenous opioid system in the modulating nociceptive responses of formalin test.
Material and methods: Swim stress was performed in different heights of water (5, 25, 50 cm) in a swimming tank. The mean nociceptive scores were measured during phase 1 (1-7 min.), interphase (8-14 min.) and phase 2 (15-90 min.) of the formalin test. Opioid receptor antagonist, naloxone (3 mg/kg; i.p.), was injected immediately before swim stress.
Results: Swim stress attenuated nociceptive behaviors in the first phase and increased duration of interphase in the formalin test, as compared to the control group, in a water height dependent manner. Naloxone significantly increased nociceptive behaviors at the first phase, interphase and phase 2 of formalin test, as compared to the control.
Conclusion: Stress could affect the nociceptive response. Swim stress in different heights of water could have different effects on the nociception in different phases of formalin test. In addition, involvement of endogenous opioid system is further demonstrated in the swim stress-induced modulation of pain behaviors in phase 1, phase 2 as well as interphase of formalin test in rats. 

Type of Study: Original | Subject: Behavioral Neuroscience
Received: 2016/05/17 | Accepted: 2018/03/3

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