α1-Adrenergic Receptors Augment P2X3 Receptor–Mediated Nociceptive Responses in the Uninjured State

In the present study, the adrenergic receptor (AR) subtype mediating adrenergic augmentation of P2X3 receptor–mediated nociceptive responses on sensory nerve endings was examined by using selective AR receptor agonists and antagonists in Sprague Dawley rats in the uninjured state. Local administration of αβ-methyleneATP (ligand for P2X3/P2X2/3 receptors) into the plantar hind paw produced few pain behaviors when given alone in this strain of rats; combination with adrenaline (α1- and α2-AR agonist) and phenylephrine (α1-AR agonist) but not clonidine or UK 14,304 (α2-AR agonists) increased flinching behaviors. Flinching produced by noradrenaline (NA)/αβ-methyleneATP was suppressed by low doses of prazosin (α1-AR antagonist), and this reduction was selective compared with yohimbine (α2-AR antagonist). Prazosin also reduced flinching produced by phenylephrine/αβ-methyleneATP. Using thermal threshold determinations, adrenaline and phenylephrine but not clonidine or UK 14,304, mimicked the action of NA in augmenting reductions in thermal thresholds produced by αβ-methyleneATP. Terazosin (another α1-AR antagonist) inhibited hyperalgesia produced by NA/αβ-methyleneATP. These results provide evidence for α1-AR involvement in adrenergic augmentation of P2X3/P2X2/3 receptor–mediated responses on sensory nerve endings in the uninjured state in Sprague Dawley rats.PerspectiveThis study indicates the α1-adrenergic receptor subtype mediates adrenergic augmentation of the activation of sensory nerves by purinergic P2X3 receptors (respond to ATP) in the periphery. Observations are potentially relevant to chronic pain conditions in which sympathetic nerves influence sensory nerves.