Classification of cat ventrolateral spinal axons and their response to an itch-producing stimulus (cowhage).

Unit activity was recorded from 92 ventrolateral spinal axons in seven anemically decerebrated spinal cat preparations, using a microdissection technique. Axons were classified into four categories according to their responsiveness to mechanical stimuli applied to skin and/or other tissues: (1) low threshold spinal axons (5%), (2) wide dynamic range spinal axons (57%), (3) high threshold spinal axons (23%), and (4) an "other" category which included axons responding to mechanical stimulation applied to visceral or deep tissue and axons responding to changes in limb or tail position. Sixty-one of the 92 spinal axons had an ongoing resting discharge. Hence, they were divided into three types on the basis of their resting discharge pattern: (1) units with no resting activity (34%), (2) units with intermittent resting discharge (23%), and (3) units having continuous resting discharge (43%). Consequently, each category of mechanically sensitive ventrolateral spinal axons was further subgrouped on the basis of resting discharge. In general, the fields of low threshold spinal axons were large and bilateral; the fields of wide dynamic range spinal axons were intermediate in size and ipsilateral; and the fields of high threshold spinal axons were small and contralateral. The approximate location in the ventrolateral spinal white matter of different categories of spinal axons was mapped but no significant segregation was found. A comparison was made between different categories of mechanically sensitive ventrolateral spinal axons to determine their sensitivity to the itch-producing stimulus, cowhage. The wide dynamic range spinal axons were significantly affected. Of 34 wide dynamic range units which were tested, 23% demonstrated a relatively high sensitivity to cowhage and hence might be regarded as pruritogen-responsive spinal axons. Sixty percent of the pruritogen-responsive units had an intermittent resting discharge pattern. Cowhage was applied on 14 high threshold and three low threshold ventrolateral spinal axons but no significant effect was demonstrated.