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 +/- 303 impulses/min at I 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 I 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 postinjection, 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.