Effects of the NMDA receptor antagonist ketamine in electrically induced A delta-fiber pain.
OBJECTIVE
Both ketamine and pethidine have previously been found to have analgesic effects in experimentally induced C-fiber pain. The aim of the present study was to examine the effects of ketamine and pethidine in A delta-fiber-mediated pain induced by electrical tooth stimulation.


METHODS
In this double-blind crossover study, an upper right central incisor was stimulated by a Bofors electrical stimulator before and after the intravenous administration of drugs. According to a randomized protocol and with a one-week wash-out period, 9 healthy female volunteers were given either racemic ketamine 0.3 mg/kg or pethidine 0.7 mg/kg, and 11 participants were given (R)-ketamine 0.5 mg/kg or (S)-ketamine 0.15 mg/kg. Pain thresholds were registered by an amperemeter on the stimulator. Drug-induced side effects were registered by use of a standardized questionnaire and visual analog scales.


RESULTS
Both ketamine and pethidine gave significant but short-lasting increases in pain thresholds compared to no medication. The participants reported more pronounced mental side effects after ketamine injection. A comparison of (R)- and (S)-ketamine revealed no statistical difference in the effect on pain thresholds of the two enantiomers at the present doses, but 7 out of 11 participants reported to have fewer and less pronounced side effects from (R)-ketamine than from (S)-ketamine.


CONCLUSIONS
Ketamine and pethidine have only a marginal effect on pain thresholds in electrically induced A delta-fiber-mediated tooth pulp pain, in contrast to the analgesic effects previously found in experimental and postoperative C-fiber-mediated pain. The ketamine enantiomers at the doses used in this study showed equal analgesic properties but different side effect profiles.
Rabben
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Section of Dental Pharmacology and Pharmacotherapy, Oslo University School of Dentistry, Norway. rabben@odont.uio.no
D000328:Adult; D000700:Analgesics / Q000494:pharmacology*; D018592:Cross-Over Studies; D004311:Double-Blind Method; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D007649:Ketamine / Q000494:pharmacology*; D008614:Meperidine / Q000494:pharmacology; D009413:Nerve Fibers, Myelinated / Q000187:drug effects* / Q000502:physiology; D017288:Pain Threshold / Q000187:drug effects*; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*; D013237:Stereoisomerism; D014070:Tooth / Q000294:innervation
D000700:Analgesics; D016194:Receptors, N-Methyl-D-Aspartate; D007649:Ketamine; D008614:Meperidine