American-Eurasian Journal of Scientific Research 2 (1): 29-32, 2007 ISSN 1818-6785 © IDOSI Publications, 2007 Corresponding Author: Mr. A.S. Oyadeyi, Department of Physiology, Ladoke Akintola University of Technology, P.M.B. 4000, Ogbomoso, Oyo State, Nigeria 29 Reduced Formalin Nociceptive Responses in a Rat Model of Post - Surgical Pain A.S. Oyadeyi, A.O. Afolabi, F.O. Ajao and G.F. Ibironke 1 1 1 2 Department of Physiology, Ladoke Akintola University of Technology, 1 P.M.B. 4000, Ogbomoso, Oyo State, Nigeria Department of Physiology, College of Medicine, University of Ibadan, Ibadan, Nigeria 2 Abstract: The responses to inflammatory pain were assessed in a rat model of post surgical pain using the formalin test. There were significantly reduced nociceptive responses in the second phase of the formalin test in both ipsilateral and contralateral limbs. The results either reflect reduced activity in mechanically insensitive afferents, A and C fibers that mediate the second phase response, or a diffuse noxious inhibitory controls mechanism (DNIC). Key words: Pain surgery incision formalin DNIC INTRODUCTION possibility also exists post surgically. One common One out of every two patients suffers intense or responses is the formalin test [10]. The test involves a very intense pain during the first few days post subcutaneous injection of dilute formaldehyde into surgery [1], clinical and preclinical models of post-surgical the plantar surface of the hind paw that causes a pain are therefore becoming increasingly important for characteristic pattern of behavioural responses such investigating the patho physiological mechanisms of as elevation and licking of the injected paw. There is an this condition [2-5]. Recently, a new animal model of initial period of responding during the first 5-10 min of postoperative pain has been introduced [6], where the the test that is followed by a 5-10 min period of quantifiable mechanical allodynia and non-evoked pain diminished responses that gradually return and behaviour produced parallel the postoperative course of continue for an additional 20-40 min. The second patients. period of responding is thought to reflect an It is now known that there are two types of inflammatory process [11], signaled by peripheral postoperative pain. Pain during rest (Rest pain) and pain mechanically insensitive afferents (MIAs) and C - fibres during function (Incident pain) [7]. Four recent studies [12]. have investigated the action of cyclooxygenase (COX) Combining the methods of paw incision to model inhibitors on post surgical pain, three of these [3, 8] post surgical pain and the formalin test to model demonstrated that post-surgical intrathecal administration inflammatory pain, the purpose of the present experiment of indomethacin or a selective COX- 2 inhibitor (JTE 522) was to determine if an increase in inflammatory responses 5 minutes following plantar incision can reduce tactile developed following the onset of a post surgical pain allodynia. condition. From the fore-going, it appears there is considerable information about the physiological and pharmacological MATERIALS AND METHODS processes that underlie post surgical pain while there is a relative paucity of experiments that have examined if Animals: Adult (150-200 g) male Sprague - Dawley rats enhanced responses during chemogenic or inflammatory were used. They were housed in the faculty animal house conditions can also develop post surgically. Indeed, with free access to food and water. LaBuda and co [9] had earlier hypothesized the possibility of inflammatory conditions been a significant clinical Surgery: Surgery was performed as previously described problem in conditions of nerve damage and given the [6], with minor modifications. Briefly, rats were similarity between nerve damage and surgery, this anaesthetized with sodium pentobarbital (40 mg kg i.p experimental condition to explore inflammatory pain 1