Elsevier

Neuroscience

Volume 80, Issue 3, 28 July 1997, Pages 939-949
Neuroscience

Formalin-induced nociceptive behavior and edema: involvement of multiple peripheral 5-hydroxytryptamine receptor subtypes

https://doi.org/10.1016/S0306-4522(97)00066-3Get rights and content

Abstract

The role of 5-hydroxytryptamine and its receptor subtypes in the development of acute inflammation was investigated using the rat paw formalin test as a model for pain (measured by flinching behavior) and edema formation (measured by plethysmometry). The role of endogenously released 5-hydroxytryptamine was assessed using 5-hydroxytryptamine receptor subtype-selective antagonists co-injected with 2.5% formalin, while the receptor subtypes involved in the inflammatory process were further defined by co-injection of 5-hydroxytryptamine or 5-hydroxytryptamine receptor subtype-selective agonists with 0.5% formalin in anticipation of an augmented response. When co-administered with 2.5% formalin, propranolol, tropisetron or GR113808A, but not ketanserin, effectively blocked nociceptive behavior. In the presence of 0.5% formalin, 5-carboxamidotryptamine, 1-(m-chlorophenyl) biguanide or 5-methoxytryptamine, but not (±)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane, augmented the flinching response. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in peripheral nociception. There may be some dissociation of nociception and edema formation, since no single 5-hydroxytryptamine receptor antagonist inhibited edema formation with 2.5% formalin; however, with 0.5% formalin, edema formation was enhanced by co-administration of 5-hydroxytryptamine, 5-carboxamidotryptamine, (±)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane or 5-methoxytryptamine, but not 1-(m-chlorophenyl) biguanide. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine2 and possibly 5-hydroxytryptamine4 receptors in edema formation.

These results confirm the involvement of 5-hydroxytryptamine1 and 5-hydroxytryptamine3 receptor subtypes in peripheral nociception associated with acute inflammation and further suggest an involvement of the more recently characterized 5-hydroxytryptamine4 receptor in this process. There appears to be a dissociation in 5-hydroxytryptamine receptors involved in peripheral nociception and edema formation.

Section snippets

Animals

All experiments were conducted on male Sprague–Dawley rats (Charles River, Quebec, Canada) weighing between 100 and 150 g. The animals were housed in groups of two to four in the animal care facility and maintained on a 12 h light/dark cycle with rat chow and water available ad libitum. A minimum 24 h acclimatization period was allowed after shipment to the facility. On the day of testing, rats were removed from the animal care facility to the testing area at least 1 h before testing. Each

Formalin dose–response relationship

Flinching behavior following formalin injection was determined for formalin concentrations of 0.5–5.0% and compared to saline. Only the higher concentrations of formalin (2.5% and 5.0%) produced flinching in phase 1 that was significantly greater (P<0.05) than a saline injection (Fig. 1A), whereas the flinching response to formalin was significantly greater than saline control in phase 2 (P<0.05) at all formalin concentrations tested (Fig. 1B). The lowest concentration of formalin tested

Discussion

This study, which used the low concentration formalin (0.5%) model to detect pronociceptive actions of inflammatory mediators, confirmed the involvement of 5-HT in the development of inflammation following injection of subcutaneous formalin in the rat hindpaw. As well as confirming the role of peripheral 5-HT1 and 5-HT3 receptors in nociception, this is the first study to suggest involvement of peripheral 5-HT4 receptors in augmentation of the pain signal. Activation of peripheral 5-HT2

Conclusions

This study shows 5-HT to be a key component of the inflammatory response to subcutaneous formalin injection in the rat. Multiple 5-HT receptor subtypes, including the 5-HT4 receptor, are involved. Peripherally active 5-HT4 receptor antagonists may represent a novel approach to analgesia in conditions of acute inflammation.

Acknowledgements

This work was supported by the Medical Research Council of Canada through a grant to J.S. and by the Canadian Anaesthetists' Society and Janssen Pharmaceutica through a fellowship to G.D.

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