SYNAPTIC PLASTICITY IN THE THALAMOCORTICAL PATHWAY AS ONE OF THE NEUROBIOLOGICAL CORRELATES OF FORELIMB FLEXION CONDITIONING - ELECTROPHYSIOLOGICAL INVESTIGATION IN THE CAT

1. In a previous study, using a chronic cat preparation subjected to an associative conditioning procedure, we described the plasticity of the thalamo-cortical pathway by qualitatively and quantitatively analyzing the motor responses induced by stimulating each of the relays on the cerebello-thalamo-cortical pathway. In the present study, it was proposed to analyze the effects on the synapses located between thalamic endings and cortical neurones, using a twofold behavioral and electrophysiological approach, with a view to correlating the patterns of synaptic plasticity with the changes in the motor responses recorded. 2. For this purpose, a reduced, functionally organized sensorimotor circuit, which can be taken to be a neuronal analog of associative conditioning, was studied in an awake chronic animal preparation. This circuit was defined on the basis of the sites at which conditioned (CS) and unconditioned stimuli (UCS) were applied: the CS was applied at a site on the cerebellar interpositus nucleus which activated the forepaw musculature so as to induce flexion movements and the UCS was applied to the skin of the distal part of that paw so as to induce reflex flexion movements. By repetitively activating the central nervous pathways by the associated CS and UCS according to a predefined temporal pattern, the efficiency of the thalamo-cortical pathway's contribution to the movement production was enhanced, and its capacity to convey the cerebellar inputs to neurons in the motor cortex increased. 3. The associative nature of the conditioning was tested using previously established criteria. The setting up of motor and central changes in response to the repetitive presentation of paired CS and UCS, the fact that these changes were reversible because they could be abolished by applying extinction procedures, and the consistency of their occurrence whenever the CS was applied repeatedly alone for several days to naive animals, all showed that the stimuli of both kinds (CS and UCS) had to be applied together for the plasticity of the thalamo-cortical pathway to be expressed. 4. By determining whether the waves constituting the cerebello-cortical responses were excitatory or inhibitory, the nature of the changes in the transmission of the cerebellar impulses to neurons in the motor cortex was established. A persistent increase in the amplitude and duration of the excitatory responses arising in the elbow motor representation area, which is closely linked to the conditioned sensorimotor circuit, and an enhancement of the amplitude of the inhibitory responses arising in the shoulder representation area of the cortex were in fact recorded in parallel with the increase in both the rates of occurrence and the amplitudes of the forelimb movements triggered by applying the CS. Upon quantifying these central and motor changes and comparing them using statistical correlation tests, close relationships were found to exist between these motor and central events. 5. Judging from the analysis carried out on the pattern of transmission occurring at each of the relays in the cerebello-thalamocortical circuit, the cerebello-cortical and motor changes were taken to be attributable to a lasting change in the efficiency of the synaptic transmission both at the thalamo-cortical level and in the intracortical interneuronal networks. 6. In conclusion, our results show that the thalamo-cortical synapses are morphological sites showing considerable plasticity, which may be involved in the initial stage of motor learning. This plasticity, however, seems to subserve mainly the setting up of the circuits conveying cerebellar inputs to the musculature. The mechanisms possibly underlying this plasticity are discussed in the light of the data available in the literature on the plasticity of the motor cortex. The functional implications of this plasticity also are discussed in terms of the role that the motor cortex is known to play in determining the kinematic and dynamic parameters of voluntary movements.