Electrophysiological effects of temporary deafferentation on two characterized cell types in the nucleus gracilis of the rat.

Single- and multiunit recordings were made in the nucleus gracilis of anaesthetized rats in order to study the characteristics of the responses to natural cutaneous stimulation before and during local anaesthetic-induced deafferentation. Two types of cells were found which exhibited different electrophysiological features at rest and in response to stimulation of their receptive fields (RFs). Low-frequency (LF) neurons (77%) had very low spontaneous activity, and most could be antidromically activated from the medial lemniscus. High-frequency (HF) cells (23%) had a much higher spontaneous discharge rate, with shorter spike duration, and did not project through the lemniscus. Both cell types generated phasic responses with similar latencies following cutaneous stimulation. Recordings of lemniscal axons had similar characteristics to those of LF neurons. Within minutes after anaesthetizing the functional centre of the RF, the LF and HF cells displayed new RFs, and enhanced responses to stimuli delivered at the periphery of the original fields. Firing rates increased during anaesthesia, but only in LF cells. Using a paired-stimulation paradigm, many LF neurons displayed during anaesthesia a decrease of the normal inhibition that the conditioning stimulus evoked on the responses to the test stimulus; the opposite effect was observed in all HF cells. These results suggest that (i) LF neurons correspond to thalamic projection cells, and HF neurons may be inhibitory interneurons; (ii) by disinhibiting LF (but not HF) cells, temporary deafferentation may increase neuronal responsiveness to peripheral stimulation, and thus contribute to reveal new RFs, and (iii) these changes in the nucleus gracilis may partly account for the reorganization of representational maps at higher levels of the somatosensory pathways.