ADAPTIVE-CHANGES IN THE SOMATOTOPIC PROPERTIES OF INDIVIDUAL THALAMICNEURONS IMMEDIATELY FOLLOWING MICROLESIONS IN CONNECTED REGIONS OF THE NUCLEUS CUNEATUS

We examined the ability of thalamic neurons in the ventrobasal complex to show adaptive changes in receptive field properties following the loss of projections from the nucleus cuneatus. Thalamic responses to a ir jet stimulation were tested at multiple peripheral sites before and after making discrete microlesions in topographically-matched regions of the nucleus cuneatus. Prior to making a lesion, crosscorrelation a nalysis and orthodromic microstimulation were used to confirm the sour ce of cuneate neurons projecting to the thalamic recording site. A tot al of 69 thalamic neurons were recorded from 29 rats. Following placem ent of a microlesion (100-200 mu m diameter) in the nucleus cuneatus, 34 thalamic neurons did not show significant changes in stimulus-induc ed responses, possibly because the lesion was too small or because cri tical sites in the receptive field were not tested. The remaining 35 n eurons were affected by cuneate microlesions, but the change in respon siveness varied according to stimulation site. When the most responsiv e site in the receptive field was examined, 24 neurons exhibited signi ficant decreases and three neurons showed significant increases in res ponsiveness. Cuneate microlesions produced decreases at moderately res ponsive sites, but the reduction in response magnitude was smaller tha n at the most responsive site. When responses near the receptive field boundary were examined, 11 neurons displayed significant increases an d only four neurons showed significant decreases. For two neurons with out well-defined receptive field boundaries, cuneate microlesions caus ed new excitatory responses to emerge from sites that had formerly cau sed a slight inhibition of spontaneous activity. In all cases of incre ased responsiveness, the changes appeared on only one side of a neuron 's receptive field. This asymmetry may account for the fact that the p robability of detecting receptive field expansion increased from 27% ( six of 22 experiments) to 71% (five of seven experiments) when the num ber of stimulation sites located throughout the receptive field was in creased. These results indicate that the receptive field structure of individual neurons shows adaptive properties immediately after loss of the predominant ascending inputs. (C) 1996 Wiley-Liss, Inc.