Sex difference for parasympathetic vasodilatation of the rat masseter muscle

Objectives: Although masseter muscle disorders are well known to be more prevalent in females than males, the precise reason for the sex difference remains unknown. We have recently suggested that there are parasympathetic cholinergic and non-cholinergic vasodilator fibers in the rat masseter muscle and that these fibers would be involved in the hemodynamics of jaw muscles. Estrogen has been reported to influence cholinergic activity in the heart and brain. The present study was thus designed to examine whether there are sex difference for the effect of atropine on the parasympathetic vasodilatation (PSV) in the masseter muscle and on blood flow in common carotid artery (CCABF) derived from orofacial tissues in anesthetized male and female rats. Methods: PSV in the masseter muscle in the urethane-anesthetized, artificially ventilated rats, were monitored on the ipsilateral side using a laser-Doppler flowmeter. CCABF was recorded by an ultrasonic blood flowmeter. Results: Electrical stimulation of the central cut end of the lingual nerve elicited intensity- and frequency-dependent PSV and CCABF increase in both the male and female. The PSV was much more reduced by atropine in females (>90%) than males (<30%). Pretreatment with atropine significantly reduced the CCABF increases in females, but had no effect in males. Conclusion: These results indicate that the PSV in the masseter muscle seems to be largely due to cholinergic mechanism in female than in male, and the cholinergic PSV in female accounts for the CCABF increase, and suggest that there are abundant parasympathetic cholinergic vasodilator fibers, which would be regulated by estrogen, in the orofacial area in females.