Methods: Rat trigeminal ganglia were harvested and prepared for combined in situ hybridization, and immunohistochemistry of the B-AR1. Functional assays used primary rat trigeminal cultures and evaluated B-AR modulation of iCGRP release (measured by RIA) and changes in intracellular calcium ([Ca]i; assessed by changes in fura-2 fluorescence). Data were analyzed by ANOVA.
Results: The anatomical expression studies indicate that the B-AR1 is present in 39.7% of trigeminal neurons, and importantly, exhibits a65.2% co-expression with TRPV1, a marker for a major subset of trigeminal nociceptors. Pretreatment with denopamine, a B-AR1 agonist, had no effect on bradykinin-evoked release of iCGRP or [Ca]i levels under basal conditions. However, previous studies have demonstrated that BK primes GPCR signaling in trigeminal neurons (Patwardhan et al., J Neuroscience 2005). Therefore, we pre-treated cultures with BK (10 uM) and observed the development of a rapid (<10min), significant (p<0.01) and concentration-dependent denopamine inhibition of both BK-evoked CGRP release and changes in [Ca]i.
Conclusions: Collectively, these preliminary studies are consistent with the hypothesis that activation of the beta adrenergic receptor inhibits trigeminal nociceptors under prior conditions that involve release of the inflammatory mediator bradykinin. This work further suggests the prospect of utilizing the beta adrenergics in the treatment of pain. Supported in part by F30 DE017307.