Capsaicin induces a punctate pattern of plasma extravasation on the tongue

Objectives: Ongoing studies indicate that intraoral capsaicin suppresses gustatory responses of neurons in the nucleus solitarius of the rat. Capsaicin is known to induce plasma extravasation in the peripheral vasculature via the release of neuropeptides (substance-P, CGRP) from C-fibers. We presently tested if oral capsaicin induces plasma extravasation in the tongue, with attention to taste papillae that are innervated by trigeminal and chorda tympani afferent nerve fibers. Methods: Sprague-Dawley rats anesthetized with sodium pentobarbital were injected with Evans Blue dye (50 mg/kg). Capsaicin (100 ppm in 5% ethanol) was flowed over the tongue surface for 7 min (n=6). Controls received saline (n=6) or 5% ethanol (n=6). Rats were perfused with saline. Tongues were removed and photographed, macerated and incubated in formamide at 50°C for 48 hr. Tissue dye concentrations were analyzed by spectrophotometry at 620 nm. Results: Spectrophotometric analysis revealed no significant between-group differences. However, capsaicin application resulted in spot-like regions of blue dye in a pattern matching the topographic distribution of fungiform papillae. The mean number of blue spots per tongue (110.6 ± 7.6 SD) was significantly greater (p<0.001) compared to saline or ethanol controls (or 2 animals receiving 0.6 M nicotine) which exhibited no spots. Conclusions: The punctate distribution of dye labeling suggests that capsaicin induced a limited amount of plasma extravasation, but not enough to significantly increase dye concentration in the whole tongue. We speculate that the blue spot-like regions represent fungiform papillae which accumulated dye by plasma extravasation via release of substance P from nearby C-fibers. Furthermore, the vascular actions of capsaicin may have induced edema of the lingual epithelium, closing taste pores and thus trapping dye within the lumen of the taste papilla. This mechanism could prevent tastants from accessing taste receptors, resulting in capsaicin suppression of taste.