Role of the Cerebellar Vermis in the Control of Eye Position. II. Functional Role of Cerebello-Prepositus Circuit in the Cat

In the previous study, we reported the properties of eye movements evoked by the stimulation of the vermis, lobules V-VII in the cat, and suggested that the vermis played an important role in the control of eye position. In the present study, the neuronal connection between the cerebellum and the prepositus hypoglossi nucleus (PHN) which has recently been regarded as a premotor center encoding mainly horizontal eye position was investigated in encephale isole cats. 1) The single unit activities of about 100 neurons in PHN as well as the neighboring dorsomedial medullary reticular formation were recorded during spontaneous eye movements or vestibular nystagmus. The activities of 49 neurons were obviously related to eye movements. The discharge rate in 36 (73%) of the neurons was closely proportional to the eye position. Anti-and monosynaptic orthodromic spikes evoked by the stimulation of PHN or fastigial nucleus (FN) were recorded in the contralateral FN or PHN, respectively. PHN neurons were found to be mutually connected with the contralateral FN neurons by excitatory monosynaptic pathways. 2) Repetitive stimulation (200Hz, 10 μA) of FN increased the tonic discharge of the contralateral PHN neurons, and this activation continued for 3-5 seconds after the cessation of the stimulation. Repetitive stimulation (200 Hz, 20 μA) of the vermis, lobule VI reduced the tonic discharge of the contralateral PHN neurons which encoded the ipsilateral horizontal eye position, and increased the tonic discharge of the ipsilateral PHN neurons. The eye deviated to the ipsilateral horizontal direction. The stimulation of lobule V or VII had no effect on the tonic discharge of the contralateral PHN neurons which encoded the ipsilateral horizontal eye position. 3) Microinjection of lidocaine hydrochloride into FN reduced the excitability of the contralateral PHN neurons, and the cats were unable to maintain an eccentric eye position. The eye returned to the neutral position by post-saccadic drift. The post-saccadic drift was exponential in shape and had a time constant of 1.4 sec. Microinjection of bicuculline (GABA antagonist) into FN increased the tonic discharge of the contralateral PHN neurons, and the eye deviated to the contralateral horizontal direction. These findings strengthen the view that there exists a mutual excitatory connection (reverberating circuit) between PHN and FN which generate background facilitation in PHN neurons and the excitability of this circuit is controlled by the inhibitory output of the vermis, lobule VI