Vagal modulation of pre-inspiratory activity in hypoglossal discharge in the decerebrate rat.

Respiration consists of three phases--inspiration (I), post-inspiration (post-I), and late expiration (E2). Pre-I is a subphase occurring at the end of E2. Hypoglossal (XII) discharge contains I and occasionally pre-I activity. Functionally, XII pre-I underlies tongue muscle contraction and expansion of the upper airway, causing a decrease in airway resistance in anticipation of the succeeding inspiratory effort. It has been shown that vagotomy causes an increase in pre-I activity in XII in anesthetized animals. Also, in anesthetized artificially-ventilated animals, XII onset is synchronized with that of inspiratory phrenic nerve (PhN) activity. Therefore, we sought to systematically test the hypothesis that XII pre-I is present in vagus-intact unanesthetized decerebrate animals and vagal afferents negatively modulate XII pre-I discharge in decerebrate rats, in the absence of confounding anesthesia. Experiments were performed on seven Sprague-Dawley unanesthetized decerebrate adult male rats and bilateral PhN and XII recordings performed. In three animals, vagotomy was performed during PhN recordings and one animal was vagotomized during initial surgical preparation prior to recordings. In vagus-intact animals, XII pre-I duration averaged 12.4 ms. Vagotomy was associated with greater XII pre-I duration, expressed in absolute time (89.5 vs. 12.4 ms; p<0.01) as well as relative to the XII bursting period (18.9 vs. 3.4%; p<0.01). Vagal deafferentation was also associated with a larger relative amplitude of the pre-I XII activity relative to total XII discharge (12.4 vs. 2.1%; p<0.01). We conclude that pre-I discharge is present in vagus-intact artificially-ventilated unanesthetized decerebrate animals and is negatively modulated by vagal afferents.