Effects of spinally administered P2X receptor agonists and antagonists on the responses of dorsal horn neurones recorded in normal, carrageenan‐inflamed and neuropathic rats

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Abstract
The function and role of P2X receptors in the spinal transmission of nociception was investigated using the selective P2X receptor agonists, α,β‐methylene ATP (α,β‐me ATP) and β,γ‐methylene‐L‐ATP (β,γ‐me‐L‐ATP) and the P2X receptor antagonists pyridoxal‐phosphate‐6‐azophenyl‐2′,4′‐disulphonate (PPADS) and suramin. Intrathecal administration of 5 and 50 μg of β,γ‐me‐L‐ATP produced a significant facilitation of the C‐fibre evoked response and a tendency towards increased excitability of the post‐discharge, but not Aβ‐fibre evoked response of dorsal horn neurones recorded in normal animals. Administration of similar doses of α,β‐me ATP did not produce an overall change in the response of the neuronal population. Peripheral administration of 20 μg of these agonists into the paw of the rat evoked firing in the dorsal horn neurones. Intrathecal administration of the antagonists, suramin (50 and 500 μg) and PPADS (5, 50 and 500 μg), to normal animals and to animals with a model of neuropathy induced by spinal nerve ligation did not alter the evoked neuronal responses. In contrast, intrathecal administration of 500 μg of suramin to animals 3 h after the induction of carrageenan inflammation produced a significant inhibition of the C‐fibre evoked response of the neurones. Similar inhibitions were also seen following high doses of intrathecal PPADS, although this did not reach significance. These results suggest that spinal P2X receptors may play a role in the modulation of spinal nociceptive transmission following the development of inflammation, but that these receptors play at most a minor role in spinal nociceptive processing in normal and neuropathic animals. British Journal of Pharmacology (2000) 129, 351–359; doi:10.1038/sj.bjp.0703047