Vestibular responses of flocculus and vestibular nuclei neurons in mice (B6CBA). Variation of stimulus amplitude and frequency.

Vestibular nuclei (Vn) neurons and floccular Purkinje (P) cells of unanesthetized paralyzed mice (B6CBA) responding to horizontal angular acceleration in the dark (type I and type II neurons) were studied by extracellular recordings with micropipettes while varying either the frequency (and velocity) or the amplitude (and velocity) of the sinusoidal rotation, keeping the respective third parameter constant. Phase and sensitivity were analyzed by a Fourier analysis and a "best sine fitting" program. Recording sites were localized by means of small iontophoretically applied horseradish peroxidase markings. The neuronal response amplitude at fundamental frequency (determined from peristimulus time histograms) increased with the frequency and amplitude of the sinusoidal rotation for both Vn and floccular neurons (0.05-0.5 Hz; +/- 60 degrees amplitude). Stimulus frequency/response amplitude and stimulus amplitude/response amplitude curves of floccular neurons were distinctly lower in magnitude than those of Vn neurons (P < 0.01) Accordingly, the sensitivity (re velocity) curves of Vn neurons and P cells differed in magnitude significantly (decreasing slightly with increasing stimulus frequency and amplitude in Vn neurons and more or less independent of stimulus parameters in floccular P cells). Response amplitudes of type I and type II neurons did not differ from each other. Phase advance relative to head angular velocity in the midfrequency range in Vn neurons was very small, indicating a head velocity signal carried by the Vn neurons. In floccular P cells phase advance was only small at 0.1 Hz (amplitude +/- 35 degrees), but increased with augmenting frequency to 140 degrees at 0.5 Hz. With a constant stimulus frequency (0.3 Hz) and varied stimulus amplitude, phase advance was 90 degrees at +/- 20 degrees amplitude and 60 degrees amplitude. Data are shown for the first time in which both the stimulus frequency and the stimulus amplitude have been varied in the same species and in the same neurons. The results demonstrate that the single data are in general well within the range of those found in other species, but they demonstrate further that phase behavior is dependent on the stimulus paradigm. The data provide the basis for comparative studies with mutant mice.