β2- and β3-adrenergic receptors drive COMT-dependent pain by increasing production of nitric oxide and cytokines

Abstract Decreased activity of catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines, contributes to pain in humans and animals. Previously, we demonstrated that development of COMT-dependent pain is mediated by both β 2 - and β 3 -adrenergic receptors (β 2 ARs and β 3 ARs). Here we investigated molecules downstream of β 2 - and β 3 ARs driving pain in animals with decreased COMT activity. Based on evidence linking their role in pain and synthesis downstream of β 2 - and β 3 AR stimulation, we hypothesized that nitric oxide (NO) and proinflammatory cytokines drive COMT-dependent pain. To test this, we measured plasma NO derivatives and cytokines in rats receiving the COMT inhibitor OR486 in the presence or absence of the β 2 AR antagonist ICI118,551 + β 3 AR antagonist SR59320A. We also assessed whether the NO synthase inhibitor L-N G -nitroarginine methyl ester (L-NAME) and cytokine-neutralizing antibodies block the development of COMT-dependent pain. Results showed that animals receiving OR486 exhibited higher levels of NO derivatives, tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), interleukin-6 (IL-6), and chemokine (C-C motif) ligand 2 (CCL2) in a β 2 - and β 3 AR-dependent manner. Additionally, inhibition of NO synthases and neutralization of the innate immunity cytokines TNFα, IL-1β, and IL-6 blocked the development of COMT-dependent pain. Finally, we found that NO influences TNFα, IL-1β, IL-6, and CCL2 levels, whereas TNFα and IL-6 influence NO levels. Altogether, these results demonstrate that β 2 - and β 3 ARs contribute to COMT-dependent pain, at least partly, by increasing NO and cytokines. Furthermore, they identify β 2 - and β 3 ARs, NO, and proinflammatory cytokines as potential therapeutic targets for pain patients with abnormalities in COMT physiology.