Peripheral adenosine 5′-triphosphate enhances nociception in the formalin test via activation of a purinergic p2X receptor
Introduction
There is an increasing interest in the role of locally released adenosine 5′-triphosphate (ATP) with a subsequent activation of cell surface P2 receptors in the regulation of the inflammatory response (Dubyak and El-Moatassim, 1993) and in pain initiation at sensory nerve terminals (Burnstock and Wood, 1996). ATP released under inflammatory conditions could originate from a cytosolic source in a number of cells following lysis or hypoxic-stress, or from more discrete stores in cells such as platelets, mast cells, or sensory or sympathetic nerves. ATP has long been known to stimulate sensory nerve endings, producing an algogenic or pain initiating response (Keele and Armstrong, 1964; Bleehen and Keele, 1977). ATP depolarizes sensory neuron cell bodies by activation of cation channels (Jahr and Jessell, 1983; Krishtal et al., 1988; Bean, 1990); a similar action is presumed to occur at the peripheral sensory nerve terminal accounting for the effect on sensory transmission. Recently, P2X purinoceptor subtypes have been cloned, directly implicated in the fast depolarization of sensory neurons by ATP and proposed to play a selective role in nociceptive activation (Chen et al., 1995; Lewis et al., 1995; Burnstock and Wood, 1996).
While a direct activation of sensory neurons may mediate some aspects of pronociceptive effects of ATP, additional indirect mechanisms via interactions with inflammatory mediators or inflammatory cells may occur (Green et al., 1991; Dubyak and El-Moatassim, 1993). The formalin test is a pain model with two distinct components, an initial phase which reflects a direct sensory nerve activation and a later phase which may reflect an inflammatory component (reviewed in Tjølsen et al., 1992). A variation of this test which uses lower concentrations of formalin (0.5–1.0%) has recently been used to evaluate pronociceptive effects of adenosine (Karlsten et al., 1992; Doak and Sawynok, 1995). In the present study, we have determined whether the low concentration formalin model can reveal pronociceptive effects of ATP and whether it can be used to ascertain the role of particular P2 receptor subtypes in pronociception by evaluating the effect of selective agonists (α,β-methylene-ATP, 2-methylthio-ATP) and antagonists (suramin; pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid or PPADS) on the response to formalin (see Section 4for a consideration of receptor selectivity). Such a model could be useful for examining potential interactions of ATP with other inflammatory mediators in modulating the sensory afferent pain signal.
Section snippets
Animals
Male Sprague Dawley rats weighing 100–200 g (Charles River, Montreal, Canada) were used in all experiments. Rats were housed in groups of 2–3 at a temperature of 22±1°C on a 12/12 h light/dark cycle, with food and water freely available. Rats which received 0.5% formalin were used twice, with an interval of 5–7 days between trials and the second injection into the contralateral hindpaw. Each experiment was completed and controlled for within a given trial and this accounts for the variability
Effects of coadministration of ATP and related agonists with formalin on flinching behaviours
The s.c. injection of formalin 0.5% produced a modest but significant phase 2 behavioural response when compared to saline, but there was no significant phase 1 response (Fig. 1A inset). Coadministration of ATP 5–500 nmol with formalin 0.5% produced no significant change in the phase 1 response, but a dose-related increase in phase 2 responses (Fig. 1A and 2). A similar increase in phase 2 responses was seen with α,β-methylene-ATP 0.5–50 nmol but not with 2-methylthio-ATP 50–500 nmol (Fig. 1B,
Discussion
The present study demonstrates a delayed pain facilitatory (pronociceptive) effect of ATP and α,β-methylene-ATP in a behavioural paradigm in which flinching behaviours are assessed following coadministration with a low concentration of formalin. These agents are inactive in producing this response when administered in the absence of formalin. The enhancement of formalin responses by ATP is blocked both by suramin and PPADS. Both the agonist and antagonist profiles (Fredholm et al., 1994) are
Acknowledgements
This work was supported by the Medical Research Council of Canada.
References (39)
- et al.
The formalin test: Scoring properties of the first and second phases of the pain response in rats
Pain
(1995) - et al.
Observations on the algogenic actions of adenosine compounds on the human blister base preparation
Pain
(1977) - et al.
Purinergic receptors: Their role in nociception and primary afferent neurotransmission
Curr. Opin. Neurobiol.
(1996) - et al.
The formalin test: A validation of the weighted-scores method of behavioural pain rating
Pain
(1993) - et al.
Complex role of peripheral adenosine in the genesis of the response to subcutaneous formalin in the rat
Eur. J. Pharmacol.
(1995) - et al.
Spinal antinociceptive effects of adenosine compounds in mice
Eur. J. Pharmacol.
(1987) - et al.
Antinociceptive effect of intrathecally administered P2-purinoceptor antagonists in rats
Brain Res.
(1994) - et al.
Behavioural effects of intraplantar injection of inflammatory mediators in the rat
Neurosci.
(1994) - et al.
How should P2X purinoceptors be classified pharmacologically?
Trends Pharmacol. Sci.
(1995) - et al.
Cationic channels activated by extracellular ATP in rat sensory neurons
Neurosci.
(1988)