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 chronic electrolytic lesions of the nucleus reticularis thalami (nRT) in ketamine-anaesthetized rats after single electrical stimuli to the ventrobasal complex (VB). Extensive alterations of either the ongoing ("spontaneous") activity or the pattern of VB evoked responses were observed. Four major changes were observed in the activity of these intralaminar or pretectal neurones: 1) many neurones were silent, two times more frequently than in a parallel study with control intact rats; 2) the firing pattern of all the other neurones was in the form of tonic (stationary-like) discharge, without burst discharges as previously described in intact animals. They were ranked into classes according to their spontaneous discharge: class I, silent (no resting discharge) 12%, class II (1-15 Hz), 54 % and class III (> 16 Hz), 34%. Class III neurones were never found in intact rats; 3) electrical stimulation of the VB evoked a short latency orthodromic excitatory response in these neurones but this response was not followed by any slowing or depression of the spontaneous activity in more than 40% of recorded cells. When it occurred, this pause was shorter than that always observed in intact 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 ipsilateral nRT; 4) VB stimulation evoked prolonged excitatory responses lasting more than 150 ms in 13% of the responsive cells, and nRT stimulation led to a short latency response followed by a pause of activity. These findings suggest that the nRT is involved in sensory integration and modulation.