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PAIN MECHANISMS

Immobilizing Doses of Halothane, Isoflurane or Propofol, Do Not Preferentially Depress Noxious Heat-Evoked Responses of Rat Lumbar Dorsal Horn Neurons with Ascending Projections

Linda S. Barter, MVSc, PhD^*, Laurie O. Mark, BA, Steven L. Jinks, PhD^*, Earl E. Carstens, PhD, and Joseph F. Antognini, MD^*

From the *Department of Anesthesiology and Pain Medicine, University of California, Davis, California; School of Medicine, Indiana University, Indiana; and Section of Neurobiology, Physiology and Behavior, University of California, Davis, California.

Address correspondence to L. S. Barter, Department of Veterinary Surgical and Radiological Sciences, 2112 Tupper Hall, University of California, Davis, 95616. Address e-mail to lsbarter{at}ucdavis.edu.

Abstract

BACKGROUND: The spinal cord is an important site where volatile anesthetics decrease sensation and produce immobility. Beyond this knowledge, our understanding of a site of anesthetic action is limited. Previous evidence suggests that dorsal horn neurons with ascending projections may be more susceptible to depression by general anesthetics than local spinal interneurons. In this study we evaluated the effects of volatile and injectable general anesthetics on lumbar dorsal horn neurons with and without ascending projections.

METHODS: Thirty-seven adult male rats underwent laminectomies at C1, for placement of a stimulating electrode, and T13/L1, for extracellular recording from the spinal cord dorsal horn. Neuronal responses to heat were evaluated under two doses of halothane, isoflurane, or propofol anesthesia.

RESULTS: Under both halothane and isoflurane anesthesia, increasing the dose from 0.8 to 1.2 minimum alveolar concentration (MAC) had no significant effect on heat-evoked responses in neurons that had ascending projections identified via antidromic stimulation (AD) or those without ascending projections (nAD). Heat responses in AD neurons 1 min after IV administration of 3 and 5 mg/kg of propofol were reduced to 60% ± 18% (mean ± se) and 39% ± 14% of control respectively. Similarly, in nAD neurons responses were reduced to 56% ± 14% and 50% ± 10% of control by 3 and 5 mg/kg propofol respectively.

CONCLUSIONS: Our findings suggest, at peri-MAC concentrations, these general anesthetics do not preferentially depress lumbar dorsal horn neurons with ascending projections compared to those with no identifiable ascending projections.

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