Capsaicin-induced rapid receptive field reorganization in cuneate neurons.

Abstract

1. To determine whether activity in unmyelinated or thinly myelinated primary afferents affects the organization of cutaneous receptive fields (RFs) in the cuneate nucleus, subcutaneous injections of capsaicin were made into the RFs of cuneate neurons in anesthetized cats. The effects of capsaicin injection on RF size and position, response properties, and spontaneous firing rates were determined during single-unit recordings. 2. Subcutaneous injection of capsaicin (10% dissolved in 70% ethanol) into peripheral RFs produced rapid RF reorganization in all neurons tested (n = 20), even when the original RF remained responsive to tactile stimulation. RF reorganization was marked by the appearance of newly responsive RFs. These RFs appeared as either contiguous expansions of the original RF or as new, noncontiguous fields. Control injections of the vehicle alone never produced RF reorganization (n = 9). RF reorganization was not related to changes in spontaneous activity. 3. Following capsaicin-induced RF reorganization, subcutaneous injections of lidocaine were made into the original RFs (n = 8). These injections produced no additional RF reorganization in seven of the eight neurons, even though in each case the lidocaine injections produced at least partial blocks of responsiveness from within the original RF. 4. In approximately one-half of the neurons (9 of 20), the reorganized RFs had response properties that were different from those of the original RF. This result suggests that, for neurons in the dorsal column nuclei (DCN), physiological mechanisms that normally mask inputs and restrict RF size can give rise to response specificity. 5. The results of these experiments demonstrate that capsaicin-induced RF reorganization, which has been reported previously in ventrobasal thalamus and primary somatosensory cortex, can arise within the dorsal column-medial lemniscal system, at the level of the DCN. The present results also provide evidence that rapid RF reorganization in cuneate neurons can be produced by blockade of a subset of peripheral afferents that are sensitive to capsaicin.