ALTERED RECEPTIVE-FIELDS AND SENSORY MODALITIES OF RAT VPL THALAMIC NEURONS DURING SPINAL STRYCHNINE-INDUCED ALLODYNIA

Allodynia is an unpleasant sequela of neural injury or neuropathy that is characterized by the inappropriate perception of light tactile sti muli as pain. This condition may be modeled experimentally in animals by the intrathecal (i.t.) administration of strychnine, a glycine rece ptor antagonist. Thus after i.t. strychnine, otherwise innocuous tacti le stimuli evoke behavioral and autonomic responses that normally are elicited only by noxious stimuli. The current study was undertaken to determine how i.t. strychnine alters the spinal processing of somatose nsory input by examining the responses of neurons in the ventropostero lateral thalamic nucleus. Extracellular, single-unit recordings were c onducted in the lateral thalamus of 19 urethan-anaesthetized, male, Wi star rats (342 +/- 44 g; mean +/- SD). Receptive fields and responses to noxious and innocuous cutaneous stimuli were determined for 19 unit s (1 per animal) before and immediately after i.t. strychnine (40 mu g ). Eighteen of the animals developed allodynia as evidenced by the abi lity of otherwise innocuous brush or air jet stimuli to evoke cardiova scular and/or motor reflexes. All (3) of the nociceptive-specific unit s became responsive to brush stimulation after i.t. strychnine, and on e became sensitive to brushing over an expanded receptive field. Expan sion of the receptive field, as determined by brush stimulation, also was exhibited by all of the low-threshold mechanoreceptive units (14) and wide dynamic range units (2) after i.t. strychnine. The use of air jet stimuli at fixed cutaneous sites also provided evidence of recept ive field expansion, because significant unit responses to air jet dev eloped at 13 cutaneous sites (on 7 animals) where an identical stimulu s was ineffective in evoking a unit response before i.t. strychnine. H owever, the magnitude of the unit response to cutaneous air jet stimul ation was not changed at sites that already had been sensitive to this stimulus before i.t. strychnine. The onset of allodynia corresponded with the onset of the altered unit responses (i.e., lowered threshold/ receptive field expansion) for the majority of animals (9), but the al tered unit response either terminated concurrently with symptoms of al lodynia (6) or, more frequently, outlasted the symptoms of allodynia ( 10) as the effects of strychnine declined. The present results demonst rate that the direct, receptor-mediated actions of strychnine on the s pinal processing of sensory information are reflected by changes in th e receptive fields and response properties of nociceptive and nonnocic eptive thalamic neurons. These changes are consistent with the involve ment of thalamocortical mechanisms in the expression of strychnine ind uced allodynia and, moreover, suggest that i.t. strychnine also produc es changes in innocuous tactile sensation.