Presence of Parasympathetic Vasodilator Fibers in Lower Lip of Guinea-pig

Objectives: When we evoked changes in lower lip blood flow (LBF) and systemic arterial blood pressure (SABP) by electrically stimulating the central cut ends of the lingual nerve (LN) in animals such as cats, rats, monkeys, dogs, rabbits, and guinea-pigs, the ipsilateral LBF increases occurred consistently in all of the animals examined except in guinea-pigs, although the SABP changes were species-dependent(J. Comp. Physiol., 173 629-636, 2003). These led us to speculate that inability of LBF increase in response to the LN stimulation in guinea-pigs would be due to either i) an absence of the parasympathetic vasodilator fibers, or ii) a profound SABP decrease which would mask the parasympathetic-mediated LBF increases, even though the presence of these fibers. To address this question, we compared the effects of the LN stimulation on the LBF in guinea-pigs pretreated with or without guanethidine (postganglionic sympathetic nerve blocker). Methods: Experiments were performed on male guinea-pigs. Animals were divided into two groups. One is control group and other is guanethidine pretreated group. Guanethidine was injected 24h prior to experiments. Changes in LBF were monitored using a laser-Doppler flowmeter as described previously (J. Physiol. (Lond.), 450, 191-202, 1992; ibid, 510, 915-921, 1998; ibid, 569, 617-629, 2005). Result: The ipsilatelal LBF increase was observed in the guanethidine group, but not in the control group. Marked SABP decreases were not observed in the guanethidine group. The LBF increase was reduced by hexamethonium in time/dose-dependent manners. We are now under investigation for immunohistochemical analysis of parasympathetic vasodilator fibers in the orofacial regions of guinea-pigs. Conclusion: Present results suggest that i) the parasympathetic vasodilator fibers exist in the guinea-pig lower lip, and that ii) the parasympathetic mediated vasodilatory response would be masked by a passive fall due to the profound SABP decrease elicited by LN stimulation.