Mechanism of antinociceptive action of clonidine in nonmyelinated nerve fibres.
Despite a large body of clinical evidence in favour of a local anesthetic effect of clonidine, the underlying mechanism has not yet been elucidated. In this study we have used the sucrose-gap method to measure the effects of clonidine on the electrophysiological properties of nonmyelinated nerve fibers in the rabbit vagus nerve. The results showed that clonidine enhanced the hyperpolarizing and reduced the depolarizing afterpotential that follow compound action potentials during electrical activity. We showed that summation of these afterpotentials shifts the membrane potential toward more negative values, thus creating a region of low safety conduction, where the local circuit currents might fail to depolarize the axonal membrane to the threshold value needed to open voltage-dependent Na(+) channels. Yohimbine did not reverse the inhibitory effects of clonidine on impulse propagation, indicating that the observed effects of clonidine relies on mechanisms not mediated by alpha(2)-adrenoceptors.
Erne-Brand
F
F
Department of Anesthesia and Research, University of Basel/Kantonsspital, Hebelstr. 20, CH 4031, Basel, Switzerland.
Jirounek
P
P
Drewe
J
J
Hampl
K
K
Schneider
M C
MC
D000200:Action Potentials / Q000187:drug effects; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000700:Analgesics / Q000494:pharmacology*; D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D004558:Electric Stimulation; D004594:Electrophysiology; D066298:In Vitro Techniques; D009186:Myelin Sheath / Q000502:physiology; D009412:Nerve Fibers / Q000187:drug effects* / Q000502:physiology; D011817:Rabbits; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000502:physiology; D015222:Sodium Channels / Q000187:drug effects*; D013997:Time Factors; D014630:Vagus Nerve / Q000187:drug effects / Q000648:ultrastructure; D015016:Yohimbine / Q000494:pharmacology
D000317:Adrenergic alpha-Antagonists; D000700:Analgesics; D000779:Anesthetics, Local; D018341:Receptors, Adrenergic, alpha-2; D015222:Sodium Channels; D015016:Yohimbine; D003000:Clonidine