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

Abstract

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, P0.05 (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.