SYNAPTIC PLASTICITY OF THE INTERPOSITO-RUBRAL PATHWAY THAT IS FUNCTIONALLY RELATED TO FORELIMB FLEXION MOVEMENTS. PANANCEAU, MARC, LUCIE RISPAL-PADEL AND EL MEHDI MEFTAH. Centre de Recherche "Cerveau et Cognition", UMR 9940, Centre National de la Recherche Scientifique, Facult[acute]e de M[acute]edecine de Toulouse Rangueil, 31062 Toulouse, France.
APStracts 3:0014N, 1996.
1- Some connections from the afferents to the magnocellular red nucleus (RNm), like the cortico-rubral synapses, have plastic properties which are thought to contribute to long-term changes such as functional re-adaptation, motor learning and the establishment of conditioned responses. Since previous studies had focused on cortico-rubral synaptic reorganization after these events, we attempted to investigate cerebello-rubral connections in intact adult cats during associative conditioning by pairing electrical stimulation of interpositus nucleus (the CS) with electrical simulation of the forelimb (the UCS). A large increase in the amplitude of the forelimb flexion (CR) to the CS was observed after several days of paired CS-UCS presentations. 2- For this purpose, both behavioral and electrophysiological methods were used to correlate synaptic plasticity with changes in the motor responses. The somatotopically organized sensorimotor network functionally related to the control of the elbow joint movements was studied in awake adult cats. This circuit was defined on the basis of sites at which elbow flexions could be evoked both as a conditioned (CS) and unconditioned (UCS) stimulus. The CS was applied in the cerebellar interpositus nucleus site (IN) and the UCS was given to the skin on the dorsum of the distal part of the forepaw. Daily classical conditioning consisted of repetitive pairings of CS and UCS with an interstimulus interval (ISI) of 100ms. 3- The transmission efficacy resulting from the conditioning was tested in various targets of the cerebellar efferent pathway, including the RNm. Electrophysiological responses evoked in these relay structures by the CS and the forelimb angular deviations were simultaneously recorded throughout each daily conditioning session. The surface areas of the rubral responses to CS and the percentage response rate, the angular deviation (amplitude) and latency of the motor responses were systematically measured throughout the conditioning procedure. Test sessions were also performed before and after each period of conditioning. Quantification and statistical analysis were carried out to determine whether changes observed in interposito-rubral synaptic transmission and in the motor responses evoked by the CS were correlated. 4- Daily repetition of paired CS and UCS according to a predefined and fixed temporal schedule led to an increase in the response rate and amplitude of the forelimb flexions. A procedure with repeated presentation of CS preceded by UCS was used to produce extinction of the enhanced motor responses. The associative nature of these changes was confirmed by the fact that the CS given alone for eleven days in a control condition failed to produce any modification of the motor response. 5- The changes in the flexion movements were accompanied by a nearly parallel increase of the amplitude of the "post-synaptic field potentials" evoked in the RNm by the CS (interpositus nucleus stimulation). Changes in the transmission efficacy of the interposito-rubral synapses stayed stable even after several days of interruption and remained constant up the extinction period. Changes affecting both the motor and the central responses were significantly correlated suggesting that modifications in the interposito- rubral transmission efficacy might be one of the plastic correlates of forelimb flexion conditioning. 6- Examination of the neuronal excitability within either the interpositus nucleus or the RNm or in the spinal cord failed to show any evidence of facilitation suggesting that the increases in the post-synaptic rubral field potential were attributable to a plasticity of the interposito-rubral connections. The long lasting duration of the increase of cerebello-rubral synaptic transmission suggests that structural changes were induced by conditioning in the intact animal. These changes could either take part in the refinement of the pathways involved in the newly learned movements or mediate the adaptation of the motor command in response to the external constraints. Thus, the "activity-dependent" synaptic plasticity could be one of the neuronal correlates of behavioral adaptation processes but it could also indicate the RNm participation in some aspects of motor learning.

Received 7 July 1995; accepted in final form 28 December 1995.
APS Manuscript Number J431-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 January 96