Altered somatosensory receptive fields in hamster colliculus after infraorbital nerve section and xylocaine injection

The effects of acute infraorbital (i.o.) nerve section upon the responses of somatosensory cells in the rostral part of the deep layers of the hamster's superior colliculus were studied using standard extracellular single-unit recording and receptive field mapping techniques. In nine animals a given cell's receptive field was determined both before and after i.o. nerve section and, in all cases, new areas of sensitivity were unmasked within 15 min after the nerve was cut. In a given electrode penetration where the i.o. nerve was sectioned (n = 13), somatosensory cells recorded after the nerve was cut, as the electrode was being withdrawn from the colliculus, exhibited receptive fields considerably different from those of somatosensory cells isolated during the descent of the recording electrode. Seventeen deep-layer somatosensory cells (in eight hamsters) were tested before and after subcutaneous injections of xylocaine into their receptive fields. This manipulation unmasked new areas of cutaneous sensitivity for sixteen units. Of these, the new receptive fields of nine cells disappeared as sensitivity in the original receptive field returned; five ultimately retained both the new and old receptive fields; in two instances, sensitivity in the original receptive field never returned over the 3 h of testing. Control experiments (n = 7) demonstrated that the changes observed did not result from spontaneous alterations in receptive field borders, changes induced by variations in the level of general anaesthesia, or non-specific trauma associated with the xylocaine injections or the surgery required to expose the i.o. nerve.