Parasympathetic vasodilation in masseter muscle evoked by vagal afferent stimulation

Objectives: Vagal afferent (VA) input have been considered to play a role in the control of blood flow to the craniofacial tissues because activation of the VAs is suggested to induce blood flow increases of these tissues mediated by parasympathetic reflex mechanism. We have recently reported that there are parasympathetic vasodilator fibers in rat masseter muscle, and that activation of these fibers mediated by stimulation of trigeminal afferents would be involved in the regulation of the hemodynamics of jaw muscles. Although the cervical vagal nerves as well as trigeminal nerves contain a large number of afferent fibers originated from cardiovascular, respiratory and gastrointestinal systems, it is still questionable whether activation of these afferents contributes to the parasympathetic reflex vasodilation of the jaw muscles. We measured the changes of blood flow in the masseter muscle (MBF) evoked by electrical stimulation of the central stump of cervical VAs in order to examine the possible role of general VA input in the regulation of the hemodynamics of jaw muscles in urethane-anesthetized, sympathectomized rats. Methods: Changes in blood flow in the masseter muscle (MBF) were monitored on both sides using a laser-Doppler flowmeter. The systemic arterial blood pressure (SABP) was recorded from the femoral catheter via a Statham pressure transducer. Results: Electrical stimulation of the cervical VAs elicited intensity- and frequency-dependent increases of the MBF in the bilateral sides. The magnitude of the MBF increase evoked by VA stimulation was independent of changes in the SABP. The VA stimulation-induced MBF increases were profoundly reduced by intravenous administration of the autonomic cholinergic ganglion blocker hexamethonium (10 mg/kg). Conclusion: Our results indicate that the vasodilator response occurs by VA stimulation in the rat masseter muscle via an activation of parasympathetic reflex mechanism, and suggest that vagal afferent input may influence the hemodynamics of the jaw muscles.