Short communicationInvolvement of mast cells, sensory afferents and sympathetic mechanisms in paw oedema induced by adenosine A1 and A2B/3 receptor agonists
Introduction
The adenosine A1 receptor agonist N6-cyclopentyladenosine and the adenosine A2B/3 receptor agonist N6-benzyl-5′-N-ethylcarboxamido adenosine (N6-B-NECA) produce opposing effects on nociceptive signalling when administered locally into the hindpaw of the rat, producing suppression Aley et al., 1995, Liu and Sawynok, 1998 and facilitation (Sawynok et al., 1997) of pain behaviours, respectively. Despite these opposing behavioural effects, they both produce an intrinsic paw oedema following local injection (Sawynok et al., 1999). Paw oedema by both agents is blocked by mepyramine and ketanserin, antagonists for histamine H1 and 5-HT2 receptors, respectively, indicating an involvement of mast cells, as well as by phentolamine, an α-adrenoceptor antagonist, indicating an involvement of the sympathetic nervous system in these responses (Sawynok et al., 1999). Rat mast cells express adenosine A2B and A3 receptors, with activation resulting in release of mediators Linden, 1994, Feoktistov and Biaggioni, 1997, but do not express adenosine A1 receptors (Ramkumar et al., 1993). Mast cells, however, also can contribute to oedema by a neurogenic mechanism. In this case, the mast cell is activated by the sensory afferent nerve as part of a local axon reflex that results in peripheral release of substance P and calcitonin gene-related peptide from the nerve terminal with a subsequent activation and degranulation of mast cells (reviewed by Holzer, 1988). Accordingly, plasma extravasation induced by capsaicin, which activates sensory nerves directly, is inhibited by pretreatment with both capsaicin acting as a sensory neurotoxin, as well as compound 48/80, which depletes mast cells Arvier et al., 1977, Coderre et al., 1989. The nature of the involvement of the sympathetic nervous system in these paw oedema responses is not clear. Sympathetic mechanisms have been implicated in peripheral inflammatory responses in some (Nakamura and Ferreira, 1987), but not all Donnerer et al., 1991, Perrot et al., 1994, studies with differing outcomes perhaps reflecting different phases or aspects of the inflammatory response (Woolf et al., 1996).
In the present study, we evaluated the involvement of mast cells (by using compound 48/80), sensory afferent nerve terminals (by using capsaicin) and sympathetic nerves (by using 6-hydroxydopamine) on paw oedema induced by N6-cyclopentyladenosine and N6-B-NECA in order to gain a clearer understanding of the mechanisms by which these two agents produce such actions.
Section snippets
Animals
Experiments were performed on male Sprague–Dawley rats 120–160 g. Rats were housed in pairs and given ad libitum access to food and water. All procedures were approved by the University Committee on Laboratory Animals.
Paw volume determinations
Paw volumes were determined by volume displacement using a commercially available plethysmometer (Ugo Basile, Italy) as previously described (Sawynok et al., 1999). The hindpaw was immersed to the junction of the hairy skin, and volumes determined in triplicate prior to and at 30,
Results
Both N6-cyclopentyladenosine and N6-B-NECA produce a dose-related increase in paw volume when tested over a 3-h interval following s.c. injection into the rat hindpaw (Sawynok et al., 1999). Doses of N6-cyclopentyladenosine and N6-B-NECA producing equivalent increases in paw volume (5 and 0.5 nmol, respectively) were selected to determine the effects of the various pretreatment regimens. Pretreatment with compound 48/80 caused a marked reduction in the paw oedema produced by both N6
Discussion
The present study demonstrates that paw oedema produced by the adenosine A1 receptor agonist N6-cyclopentyladenosine is blocked by compound 48/80 and capsaicin but not 6-hydroxydopamine, indicating an involvement of mast cells and sensory afferents but not sympathetic nerve terminals in this action. It is proposed that N6-cyclopentyladenosine activates sensory afferent nerve terminals, and this leads to a neurogenic involvement of mast cells to release histamine and 5-HT, which then produce paw
Acknowledgements
This work was supported by the Medical Research Council of Canada.
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