ABNORMAL NEURONEAL ACTIVITIES IN INTRALAMINAR THALAMIC NUCLEI FOLLOWING CHRONIC LESIONS OF NUCLEUS-RETICULARIS THALAMI IN RATS

Extracellular single unit activity in the intralaminar thalamic nuclei (ncl. centralis lateralis, CL, n=77 and ncl. parafascicularis, Pf, n= 163) and in the pretectal area (Pt, n=75) was examined following chron ic electrolytic lesions of the nucleus reticularis thalami (nRT) in ke tamine-anaesthetized rats after single electrical stimuli to the ventr obasal complex (VB). Extensive alterations of either the ongoing (''sp ontaneous'') activity or the pattern of VB evoked responses were obser ved. Four major changes were observed in the activity of these intrala minar or pretectal neurones: 1) many neurones were silent, two times m ore frequently than in a parallel study with control intact rats; 2) t he firing pattern of all the other neurones was in the form of tonic ( stationary-like) discharge, without burst discharges as previously des cribed in intact animals. They were ranked into classes according to t heir spontaneous discharge: class I, silent (no resting discharge) 12 %, class II (1-15 Hz), 54 % and class III (>16 Hz), 34 %. Class III ne urones were never found in intact rats; 3) electrical stimulation of t he VB evoked a short latency orthodromic excitatory response in these neurones but this response was not followed by any slowing or depressi on of the spontaneous activity in more than 40 % of recorded cells. Wh en it occurred, this pause was shorter than that always observed in in tact rats by more than 35 % and longer in 7 % of the responsive cells. All these changes were correlated with the extent of damage to the ip silateral nRT; 4) VB stimulation evoked prolonged excitatory responses lasting more than 150 ms in 13 % of the responsive cells, and nRT sti mulation led to a short latency response followed by a pause of activi ty. These findings suggest that the nRT is involved in sensory integra tion and modulation.