Beta 1 Adrenergic Receptor Activation and Inhibition of Inflammatory Mediators

Objectives: Our previous studies characterized the expression of the beta adrenergic receptors (βAR) in the trigeminal ganglia and demonstrated colocalization of the β1AR with two markers of nocicpetive neurons, Transient Receptor Potential Vanilloid 1 (TRPV1) and the bradykinin 2 receptor. In the present study, we evaluated whether the β1AR exerts a functional effect on nociceptors by measuring in vitro intracellular calcium accumulation ([Ca]i) and in vivo thermal withdrawal thresholds.

Methods: Rat trigeminal ganglia were harvested, cultured for 5 days and then labeled with fura2 to measure real time [Ca]i levels after exposure to vehicle, test drugs (βAR agonists/antagonists) or BK/prostaglandin E2 (PGE2). The plantar test was used to evaluate the effects of β1AR agonists on bradykinin (BK)-evoked thermal hyperalgesia using blinded observers. Data were analyzed by ANOVA.

Results: The in vitro studies demonstrated that application of denopamine, a β1AR agonist, had no effect on the accumulation of [Ca]i induced by BK/PGE2 under basal conditions. However, a brief pretreatment with BK rapidly induced competency of the β1AR such that subsequent application of denopamine produced a significant inhibition of BK/PGE2 evoked increase in [Ca]i. Furthermore, the denopamine effect was blocked by pretreatment with a β1 antagonist, atenolol, but not with a β2 antagonist, ICI 118,551. The in vivo studies demonstrated that denopamine inhibited BK evoked thermal hyperalgesia, but again only under conditions of a BK priming pretreatment.

Conclusion: These studies demonstrate that activation of β1AR produces a significant inhibition of both cellular and behavioral measures of nociceptor function, but that priming with BK is necessary for development of competency of the β1AR. These studies provide a model system to evaluate mechanisms mediating ligand-dependent signaling of GPCRs in nociceptive neurons.

Supported in part by F30 DE017307.