Bupivacaine preferentially blocks ventral root axons in rats.
BACKGROUND
Clinically, bupivacaine can provide excellent sensory anesthesia with minimal impairment of motor function. However, the mechanisms by which local anesthetics produce differential sensory-motor nerve block is still unknown. The primary site of action for spinal and epidural anesthetics is thought to be the intradural segment of the spinal root. To determine the differential susceptibility of single motor and sensory nerve fibers to local anesthetic conduction block, bupivacaine effects on individual dorsal root (DR) and ventral root (VR) axons were studied.


METHODS
Lumbar DRs and VRs were excised from anesthetized adult male rats. Single-fiber dissection and recording techniques were used to isolate activity in individual axons. Supramaximal constant-voltage stimuli at 0.3 Hz were delivered to the root. During in vitro perfusion, each root was exposed to increasing concentrations of bupivacaine, and the minimum blocking concentration (C(m)) and the concentration that increased conduction latency by 50% (latency EC50) were measured.


RESULTS
Ventral root axons were significantly more sensitive to the steady-state conduction blocking effects of bupivacaine than were either myelinated or unmyelinated DR axons (DR-C(m), 32.4 microM; VR-C(m), 13.8 microM; P < 0.0001). In addition, VR axons were more susceptible to the latency-increasing effects of bupivacaine than were DR axons (DR-EC50 = 20.7 microM; VR-EC50 = 8.5 microM; P < 0.0001). Within axon groups, differential sensitivity as a function of conduction velocity (axon diameter), or length of nerve exposed to the anesthetic could not be demonstrated.


CONCLUSIONS
In contrast to clinical expectations, low concentrations of bupivacaine preferentially block motor (VR) axons in the rat.
Dietz
F B
FB
Department of Anesthesia, Stanford University School of Medicine, California 94305-5115, USA.
Jaffe
R A
RA
D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D001369:Axons / Q000187:drug effects; D002045:Bupivacaine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005727:Ganglia, Spinal / Q000187:drug effects*; D066298:In Vitro Techniques; D008297:Male; D009407:Nerve Block; D009431:Neural Conduction / Q000187:drug effects; D051381:Rats; D013126:Spinal Nerve Roots / Q000187:drug effects*
D000779:Anesthetics, Local; D002045:Bupivacaine