Vertical Vestibular and Saccade Neurons in Alert Cats. Fukushima, K., T. Ohashi, J. Fukushima, and C. R. S. Kaneko. Department of Physiology, Hokkaido University School of Medicine, Sapporo 060, Japan; and Department of Physiology and Biophysics and Regional Primate Research Center, University of Washington, Seattle, Washington.
APStracts 2:0012N, 1995.
SUMMARY AND CONCLUSIONS
1. The interstitial nucleus of Cajal (INC) is involved in generation of vertical eye movements. To further explore the neural substrates of INC involvement in eye movements, we examined the discharge characteristics of neurons in the rostral midbrain that discharge for vestibular stimulation and ccades in alert cats. We analyzed 75 vestibular and saccade neurons (VSNs) that showed characteristic discharge during pitch rotation. Of these, 50 exhibited gradually increasing activity during upward slow phases and bursts of action potentials during downward fast phases induced by downward pitch. The remaining 25 cells showed the opposite response pattern: gradually increasing activity during downward slow phases and a burst during upward fast phases induced by upward pitch. We classified these cells as downward VSNs and upward VSNs, respectively. The effects of electrical stimulation of the contralateral vestibular nerve were tested for 13 downward VSNs and 2 upward VSNs; all of them were activated at short latencies. 2. All vertical VSNs also showed burst activity associated with spontaneous saccades. The preferred direction for the burst was always the same for saccades and vestibular fast phases. The onset of burst activity preceded the onset of fast eye movement by an average of 35 +/- 14 ms (mean +/- SD). During fast eye movement in other directions, many of these cells also showed an inconsistent burst and the burst often began after the onset of saccades. Half of the upward VSNs examined showed a pause in activity during downward fast eye movement. 3. Burst parameters in the preferred directions were correlated with saccade parameters in half of the vertical VSNs examined, although the correlation coefficients were typically 0.5 - 0.6. 4. All vertical VSNs had irregular resting activity. Half of the downward VSNs examined showed eye-position sensitivity toward the direction opposite to the preferred direction for the burst activity but only if the period of analysis was restricted to discharge shortly before and after saccades. 5. All downward VSNs examined during sinusoidal rotation in several vertical planes received strong excitatory input from the contralateral anterior canal. Three of four upward VSNs examined received contralateral posterior canal excitation; the exception received ipsilateral posterior canal excitation. 6. Downward VSNs were found within and near the INC, whereas upward VSNs were found in the reticular formation lateral and caudal to the INC. 7. Bilateral muscimol infusion into the downward VSN areas produced loss of downward fast eye movement. Upward saccades were followed by a failure to maintain upward eccentric eye position and were characterized by a postsaccadic drift with a mean time constant of 0.39 +/- 0.15 s. Bilateral muscimol infusion into the upward VSN areas significantly decreased the generation and number of upward saccades. Effects on ability to hold eye position after downward saccades were variable. Small saccades were followed by postsaccadic drifts with a mean time constant of 0.60 +/- 0.30 s. Larger saccades were less affected and some saccades showed no effect at all. 8. These results indicate that the midbrain reticular formation within and near the INC contains a class of cells whose activity is similar to previously described pontine neurons that may be involved in generation of vertical fast eye movements in alert cats. The discharge characteristics of VSNs suggests that they also may contribute to the process of neural integration. 

Received 2 August 1994; accepted in final form 7 February 1995.
APS Manuscript Number J482-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.