Involvement of two different mechanisms in trigeminal ganglion-evoked vasodilatation in the cat lower lip: role of experimental conditions

The present study was designed to examine the vasodilator mechanisms elicit ed by electrical stimulation of trigeminal ganglion (TG) in cat lower lip o f the cats. When vago-sympathectomized cats were fixed into a stereotaxic f rame by means of ear-bars, etc., the Lip blood flow (LBF) increase evoked b y lingual nerve (LN) stimulation (parasympathetic reflex response) was almo st abolished in 15 out of 34 animals, but unaffected in the other 19. With the animal in the stereotaxic frame, electrical stimulation at sites within the TG evoked an LBF increase whether or not the LN stimulation-induced re flex response was intact. However, hexamethonium abolished the TG stimulati on-induced LBF increase in animals whose brainstem parasympathetic reflex w as intact, but reduced it by only 50% in animals whose reflex was impaired. This difference was seen in all experiments in which the electrode site wa s within the TG proper, regardless of its exact position. Although the unde rlying mechanism is unclear, these data suggest that when the TG is stimula ted the LBF increase is entirely mediated via the parasympathetic reflex me chanism in animals whose brainstem reflex is intact, and that an antidromic vasodilatation occurs only in animals whose brainstem parasympathetic refl ex is impaired. (C) 2000 Elsevier Science B.V. All rights reserved.