The effect of muscimol micro-injections into the fastigial nucleus on the optokinetic response and the vestibulo-ocular reflex in the alert monkey.

Eye movements of four macaque monkeys were investigated after unilateral micro-injections of the GABA agonist muscimol (1 microgram in 1 microliter NaCl) into the caudal fastigial nucleus, i.e. the fastigial oculomotor region. Spontaneous eye movements in the dark and in the light were tested, as well as those evoked by vestibular stimulation in the dark (sinusoidal: 0.1-0.2 Hz, +/- 40-100 deg/s, velocity trapezoid acceleration 40 deg/s2, constant velocity 120 deg/s), optokinetic stimulation (sinusoidal: 0.1-0.2 Hz, +/- 40-100 deg/s, constant velocity 60-100 deg/s), and visual-vestibular conflict stimulation. With these stimuli, smooth pursuit mechanisms (fast build-up of optokinetic slow phase velocity), the vestibulo-ocular reflex (VOR) and the velocity storage mechanism were investigated. Muscimol injections consistently led to specific eye movement changes which were maximal 30-60 min after the injection and lasted 4-6 h. The fast initial rise of OKN slow phase velocity to the contralateral side decreased by 45% (range 24%-82%) of its pre-injection value, while it was virtually unaltered on the ipsilateral side (average decrease of 1%, range from a decrease of 20% to an increase of 32%). For conflict ramp stimulation, the suppression of vestibular nystagmus was less (decrease of 50%, range 12-82%) towards the contralateral side while it remained unchanged on the ipsilateral side. The VOR in the dark and the velocity storage mechanism were not altered. For the latter, the slow build-up of optokinetic nystagmus velocity, the optokinetic afternystagmus (OKAN) and the time constant of decay for the vestibular nystagmus were evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)