Adaptive changes in the somatotopic properties of individual thalamic neurons 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 air 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 analysis and orthodromic microstimulation were used to confirm the source of cuneate neurons projecting to the thalamic recording site. A total of 69 thalamic neurons were recorded from 29 rats. Following placement of a microlesion (100-200 microns diameter) in the nucleus cuneatus, 34 thalamic neurons did not show significant changes in stimulus-induced responses, possibly because the lesion was too small or because critical sites in the receptive field were not tested. The remaining 35 neurons were affected by cuneate microlesions, but the change in responsiveness varied according to stimulation site. When the most responsive site in the receptive field was examined, 24 neurons exhibited significant decreases and three neurons showed significant increases in responsiveness. Cuneate microlesions produced decreases at moderately responsive sites, but the reduction in response magnitude was smaller than at the most responsive site. When responses near the receptive field boundary were examined, 11 neurons displayed significant increases and only four neurons showed significant decreases. For two neurons without well-defined receptive field boundaries, cuneate microlesions caused new excitatory responses to emerge from sites that had formerly caused a slight inhibition of spontaneous activity. In all cases of increased responsiveness, the changes appeared on only one side of a neuron's receptive field. This asymmetry may account for the fact that the probability of detecting receptive field expansion increased from 27% (six of 22 experiments) to 71% (five of seven experiments) when the number of stimulation sites located throughout the receptive field was increased. These results indicate that the receptive field structure of individual neurons shows adaptive properties immediately after loss of the predominant ascending inputs.