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REGIONAL ANESTHESIA AND PAIN MANAGEMENT

Activation of the Rostral Ventrolateral Medulla in an Acute Anesthetized Rodent Strychnine Model of Allodynia

Sean R. Hall, MSc^*, Louie Wang, MD^*, Brian Milne, MD^*, and Christopher Loomis, PhD

Departments of *Anaesthesia and Pharmacology & Toxicology, Queens University, Kingston, Ontario; and School of Pharmacy, Memorial University, St. John's, Newfoundland, Canada

Address correspondence and reprint requests to Dr. Louie Wang, Department of Anaesthesia, Kingston General Hospital, 76 Stuart Street, Kingston, Ontario, Canada K7L 2V7. Address e-mail to wangl{at}KGH.KARI.NET

After the administration of intrathecal strychnine, allodynia is manifested as activation of supraspinal sites involved in pain processing and enhancement of cardiovascular responses evoked by normally innocuous stimuli. The objective of this study was to investigate the effect of strychnine-induced allodynia on adrenergic neuronal activity in the C1 area of the rostral ventrolateral medulla (RVLM), a major site involved in cardiovascular regulation. The effect of intrathecal strychnine (40 µg) or saline followed by repeated hair deflection to caudal lumbar dermatomes in the urethane-anesthetized rat was assessed by measuring voltammetric changes in the RVLM catechol oxidation current (CA · OC), mean arterial pressure (MAP), and heart rate (HR). After the administration of intrathecal strychnine, hair deflection evoked a significant and sustained increase in the RVLM CA · OC and MAP (peak 146.4% ± 5.6% and 159% ± 18.4% of baseline, respectively; P < 0.05). There was a nonsignificant increase in HR (peak 128% ± 8.2%). In the absence of hair deflection, there was no demonstrable change. Intrathecal saline-treated rats failed to demonstrate changes in RVLM CA · OC, MAP, or HR. In the present study, we demonstrated that, after the administration of intrathecal strychnine, innocuous hair deflection evokes temporally related neuronal activation in the rat RVLM and an increase in MAP. This suggests that the RVLM mediates, at least in part, the cardiovascular responses during strychnine allodynia.

Implications: Neural injury-associated pain, as manifested by allodynia, is resistant to conventional treatment. In a rat model of allodynia, we demonstrated activation of the brain region involved in sympathetic control. Innovative therapies that target this region may be successful in managing this debilitating condition.