Oculomotor Effects Of Reversible Inactivation Of The Primate Mesencephalic Reticular Formation (MRF). II. Hypometric Vertical Saccades. David M. Waitzman, Valentine L. Silakov, Stacy DePalma-Bowles, and Amanda S. Ayers. Department of Neurology, University of Connecticut Health Center, Farmington, CT 06030.
APStracts 6:0512N, 1999.
Electrical microstimulation and single unit recording has suggested that a group of long-lead burst neurons (LLBNs) in the Mesencephalic Reticular Formation (MRF) just lateral to the interstitial nucleus of Cajal (INC) (the peri-INC MRF, piMRF) may play a role in the generation of vertical rapid eye movements. Inactivation of this region with muscimol (a GABAA agonist) rapidly produced vertical saccade hypometria (6 injections). In 3 out of 6 injections there was a marked reduction in the velocity of vertical saccades out of proportion to saccade amplitude (i.e., saccades fell below the main sequence). This was associated with a moderate increase in saccade duration. Inadvertent inactivation of the INC could not account for these observations since vertical, post-saccadic drift was not observed. Similarly, pure downward saccade hypometria, the hallmark of rostral interstitial MLF (riMLF) inactivation, was always preceded by loss of upward saccades in our experiments. We also found a downward and ipsiversive displacement of initial eye position, and evidence of a contraversive head tilt following peri-INC MRF injections. Saccade latency was shorter following 2 of 6 injections. Simulation of a local feedback model provided 3 possible explanations for vertical saccade hypometria: 1) a shift in the input to the model to request smaller saccades; 2) a reduction of long lead burst neuron input to the vertical saccade medium lead burst neurons (MLBNs); or 3) an increase in the gain of the feedback pathway. However, when the second hypothesis was coupled to a shortened duration of the saccade trigger (i.e. the discharge of the omnipause neurons), the physiologic observations of piMRF inactivation could be replicated. This suggested that muscimol had targeted structures that provided both long-lead burst activity to the MLBNs in the riMLF and were critical for reactivation of the omnipause neurons. Evidence of markedly reduced vertical saccade amplitude, curved saccade trajectories, increased saccade duration, and saccades which fall below the amplitude/velocity main sequence in these monkeys closely parallels the oculomotor findings of patients with progressive supranuclear palsy (PSP).
Received 9 August 1999; accepted in final form 8 October 1999.
APS Manuscript Number J675-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999