Modulation of BzATP and formalin induced nociception: attenuation by the P2X receptor antagonist, TNP‐ATP and enhancement by the P2X3 allosteric modulator, cibacron blue

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Abstract
1. Exogenous ATP produces acute and localized pain in humans, and P2X receptor agonists elicit acute nociceptive behaviours in rodents following intradermal administration to the hindpaw. The predominant localization of P2X(3) mRNA in sensory neurones has led to the hypothesis that activation of P2X(3) and/or P2X(2/3) receptors contributes to nociception. 2. The local administration of the P2X receptor agonist, BzATP (100--1000 nmol paw(-1), s.c.) into the rat hindpaw produced an acute (<15 min) paw flinching response that was similar to that observed in the acute phase of the formalin (5%) test. 3. The co-administration of the potent P2X receptor antagonist, TNP-ATP (30--300 nmol paw(-1)), but not an inactive analogue, TNP-AMP, with BzATP into the rat hindpaw attenuated BzATP-induced nociception. Similarly, co-administration of TNP-ATP, but not TNP-AMP, with 5% formalin reduced both acute and persistent nociception in this test. 4. Co-administration of cibacron blue (30 and 100 nmol paw(-1)), a selective allosteric enhancer of P2X(3) and P2X(2/3) receptor activation, with BzATP (30 and 100 nmol paw(-1)) into the rat hindpaw produced significantly greater nociception as compared to the algogenic effects of BzATP alone. Intradermal co-administration of cibacron blue (30 and 100 nmol paw(-1)) with formalin (1 and 2.5%) into the rat hindpaw also produced significantly greater nociceptive behaviour as compared to formalin alone. 5. The ability of TNP-ATP and cibacron blue to respectively attenuate and enhance nociceptive responses elicited by exogenous BzATP and formalin provide further support for the hypothesis that activation of peripheral P2X(3) containing channels contributes specifically to both acute and persistent nociception in the rat.