SPINAL STRYCHNINE ALTERS RESPONSE PROPERTIES OF NOCICEPTIVE-SPECIFIC NEURONS IN RAT MEDIAL THALAMUS

Experiments in both conscious and anesthetized animals indicate that i ntrathecal (i.t.) strychnine (STR; glycine receptor antagonist) produc es acute, reversible allodynia, as evidenced by inappropriate behavior al and autonomic responses to cutaneous tactile stimuli. Although STR is known to produce disinhibition of afferent input to the spinal cord , changes in spinal reflexes cannot fully explain the complex behavior s observed following i.t. STR. Which supraspinal sites are involved in STR-dependent allodynia and how this abnormal somatosensory message i s relayed to these sites remain to be determined. The medial thalamus contains many nociceptive-specific (NS) neurons and is believed to be involved in mediating the affective-motivational aspects of pain. It i s thus important to determine whether spinally administered STR elicit s changes in the responses of medial thalamic NS neurons. Extracellula r single-unit recordings were conducted in urethan-anesthetized rats ( 290-490 g). A detailed characterization of 20 thalamic NS units (1 per rat; 2 in 1 case) was conducted before and immediately after i.t. STR (40 mu g). Initially, all of the units in this study were classified as NS, because they were excited by noxious pinch but not by innocuous tactile stimuli. After i.t. STR, all (formerly NS) units exhibited si gnificant responses to innocuous tactile stimuli (brush and/or air jet ) applied to lumbar or sacral dermatomes. This effect of STR on thalam ic NS neurons was acute and reversible. The majority of units (11 of 2 0) also exhibited an increase in spontaneous firing rate. Although the complete pinch receptive field (RF) could not be determined for all u nits, the available data indicate that the RFs for brush stimulation a fter i.t. STR were substantially different from the pre-STR pinch RFs for all but three units. The same i.t. STR injection that caused the o bserved changes in medial thalamus also produced allodynia, in the for m of brush-evoked cardiovascular or motor responses, in 18 of the 19 r ats. The ability of NS cells in medial thalamus to respond to tactile input after i.t. STR suggests that the STR lowers the threshold of noc iceptive neurons that project directly and/or indirectly to medial tha lamus. These observations suggest that ascending nociceptive pathways and medial thalamic structures contribute to the expression of STR-dep endent allodynia.