Thiopental directly depresses lumbar dorsal horn neuronal responses to noxious mechanical stimulation in goats.

BACKGROUND

Thiopental has hypnotic actions in the brain, but it also depresses nociceptive transmission. In this study we examined whether thiopental had direct (spinal) and/or indirect (supraspinal) effects on the responses of single lumbar dorsal horn neurons to noxious mechanical stimulation, using a method to deliver thiopental differentially to either the torso or cranial circulation in goats.

METHODS

Goats (n=10) were anesthetized with isoflurane and neck dissections performed to permit cranial bypass. A lumbar laminectomy was made to permit single-unit recording of lumbar dorsal horn neuronal activity (1-2 neurons/animal). Isoflurane was maintained at 0.8+/-0.1% to both head and torso throughout the study. During cranial bypass, thiopental was separately administered to the torso (low dose, 1.5+/-0.5 mg/kg; high dose, 3.7+/-0.5 mg/kg) or cranial (low dose, 0.12+/-0.03 mg/kg; high dose, 0.2 mg/kg) circulation.

RESULTS

Thiopental administered to the torso significantly depressed dorsal horn neuronal responses to noxious stimulation at the high dose: 757+/-471 to 392+/-305 impulses/min at 1 min post-injection, P<0.006 (n=14); evoked responses recovered at 5 min post-injection. At the low dose, there was a similar numerical decrease, but this did not reach significance: 876+/-780 to 407+/-499 impulses/min at 1 min post-injection, P>0.05 (n=6). No significant change was observed when thiopental was administered to the cranial circulation: low dose, 1061+/-1167 to 965+/-874 impulses/min at 1 min post-injection, P>0.05 (n=10); high dose, 864+/-331 to 917+/-525 impulses/min at 1 min post-injection, P>0.05 (n=8).

CONCLUSION

Thiopental has a direct (spinal) depressant effect on dorsal neuronal responses to noxious stimulus, but no significant supraspinal effect.