Epinephrine Produces a β-Adrenergic Receptor-Mediated Mechanical Hyperalgesia and In Vitro Sensitization of Rat Nociceptors

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Abstract
Epinephrine produces a β-adrenergic receptor-mediated mechanical hyperalgesia and in vitro sensitization of nociceptor-like neurons in the rat. Hyperalgesic and nociceptor sensitizing effects mediated by the β-adrenergic receptor were evaluated in the rat. Intradermal injection of epinephrine, the major endogenous ligand for the β-adrenergic receptor, into the dorsum of the hindpaw of the rat produced a dose-dependent mechanical hyperalgesia, quantified by the Randall-Selitto paw-withdrawal test. Epinephrine-induced hyperalgesia was attenuated significantly by intradermal pretreatment with propranolol, a β-adrenergic receptor antagonist, but not by phentolamine, an α-adrenergic receptor antagonist. Epinephrine-induced hyperalgesia developed rapidly; it was statistically significant by 2 min after injection, reached a maximum effect within 5 min, and lasted 2 h. Injection of a more β-adrenergic receptor-selective agonist, isoproterenol, also produced dose-dependent hyperalgesia, which was attenuated by propranolol but not phentolamine. Epinephrine-induced hyperalgesia was not affected by indomethacin, an inhibitor of cyclo-oxygenase, or by surgical sympathectomy. It was attenuated significantly by inhibitors of the adenosine 3′,5′-cyclic monophosphate signaling pathway (the adenylyl cyclase inhibitor, SQ 22536, and the protein kinase A inhibitors, Rp-adenosine 3′,5′-cyclic monophosphate and WIPTIDE), inhibitors of the protein kinase C signaling pathway (chelerythrine and bisindolylmaleimide) and a μ-opioid receptor agonist DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin). Consistent with the hypothesis that epinephrine produces hyperalgesia by a direct action on primary afferent nociceptors, it was found to sensitize small-diameter dorsal root ganglion neurons in culture, i.e., to produce an increase in number of spikes and a decrease in latency to firing during a ramped depolarizing stimulus. These effects were blocked by propranolol. Furthermore epinephrine, like several other direct-acting hyperalgesic agents, caused a potentiation of tetrodotoxin-resistant sodium current, an effect that was abolished by Rp-adenosine 3′,5′-cyclic monophosphate and significantly attenuated by bisindolylmaleimide. Isoproterenol also potentiated tetrodotoxin-resistant sodium current. In conclusion, epinephrine produces cutaneous mechanical hyperalgesia and sensitizes cultured dorsal root ganglion neurons in the absence of nerve injury via an action at a β-adrenergic receptor. These effects of epinephrine are mediated by both the protein kinase A and protein kinase C second-messenger pathways.