EFFECTS OF RED NUCLEUS INACTIVATION ON BURST DISCHARGE IN TURTLE CEREBELLUM
IN VITRO: EVIDENCE FOR POSITIVE FEEDBACK.
Department of Anatomy & Structural Biology, University of South Dakota
School of Medicine, Vermillion, SD 57069.
APStracts 3:0130N, 1996.
SUMMARY AND CONCLUSIONS
1. In behaving animals the red nucleus produces sustained action potential
discharge during movements of the limbs. These bursts are thought to encode
parameters of movement and thereby represent motor commands. Similar bursts
can be recorded in the in vitro brainstem-cerebellum from the turtle. In this
preparation, sustained discharge of red nucleus neurons was postulated to be
generated by NMDA-mediated cellular mechanisms acting in combination with
positive feedback in a recurrent cerebellorubral network. The present study
was designed to test this positive feedback hypothesis. While recording
sustained discharge in the deep cerebellar nuclei and cortex, the red nucleus
was reversibly inactivated by microinjection. The positive feedback hypothesis
would be supported if activity in the cerebellum was attenuated by
inactivation of the red nucleus. A non-recurrent source of excitation would
have to be postulated if cerebellar activity was unaffected. 2. Extracellular
single-unit recordings were made from neurons in the deep cerebellar nuclei,
cerebellar cortex and vestibular nuclei. Burst discharges were evoked by brief
electrical stimuli applied to the spinal cord that activated sensory
structures. During inactivation of the red nucleus, sensory projections to the
cerebellum that may evoke burst discharge were unaffected. Pressure
microinjections of cobalt, lidocaine, GABA or CNQX were used to reversibly
inactivate the red nucleus. Saline injections were also tested. 3. Sustained
discharge of all neurons recorded in the lateral cerebellar nucleus was
greatly attenuated or blocked completely by injection of the pharmacological
agents into the red nucleus. These effects were reversible. Of the recordings
in the cerebellar cortex, 63% of these were blocked. All four compounds tested
were effective blockers of the bursts, although the effects of GABA were less
potent than the others. Saline injections into the red nucleus showed no
effect. Burst discharges of single-units recorded in either the medial
cerebellar nucleus or the vestibular complex, which do not receive input from
the red nucleus, showed no effect of red nucleus inactivation. 4. The results
showed that sustained discharge in the cerebellum was significantly attenuated
by inactivation of the red nucleus even though sensory input that may trigger
the bursts was intact. These data support the hypothesis that sustained
discharge in the cerebellorubral circuit is generated by a distributed
neuronal network that uses positive feedback. The results have implications
for mechanisms underlying normal brain function and some motor disorders.
Received 28 March 1996; accepted in final form 6 June 1996.
APS Manuscript Number J260-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 28 June 96