Response of first order vestibular neurons to lidocaine hydrochloride.

Changes in behavior and electrical activity of primary vestibular neurons were observed following injection of lidocaine hydrochloride into the middle ear of cats. After injection the cats exhibited head and ocular nystagmus, head and neck deviation and pupillary changes. Mean preinjection resting discharge rate for first order vestibular neurons was 46.0 spikes/sec. Two hours after lidocaine application the resting rate decreased to a mean of 22.2 spikes/sec and then recovered to 43.0 spikes/sec four hours, and 47.4 spikes/sec six hours after the experimental treatment. The increment sensitivity (increased rate of firing) of horizontal canal neurons to constant angular acceleration for the control period was 2.0 extra spikes/sec/deg/sec2; two hours after the application, 70% of the recorded neurons were unresponsive to angular acceleration or tilt. The sensitivity after four hours was 1.0 spikes/sec/deg/sec2 and 2.1 spikes/sec/deg/sec2 at six hours. The distinct depression of sensitivity by lidocaine at four hours compared to normal mean resting rate at this time suggests these functions may be governed by two modes of action in the receptor or first order afferents.