Tooth pulp- and facial hair mechanoreceptor-evoked responses of trigeminalsensory neurons are attenuated during ketamine anesthesia

Background: Evidence exists that ketamine, administered systemically using a dose required for inducing a state of anesthesia, may antagonize nocicept ive but not innocuous input to lumbar dorsal horn neurons. However, it is u nclear whether ketamine exerts this selective action on sensory inputs to t rigeminal sensory neurons. The current study was undertaken to compare the responses evoked in trigeminal sensory neurons by electrical stimuli applie d to the tooth pulp versus air-puff stimuli applied to facial hair mechanor eceptors (FHMs) during quiet wakefulness versus ketamine anesthesia. Methods: Accordingly, responses of rostral trigeminal sensory nuclear compl ex (TSNC) and trigeminothalamic tract neurons evoked by tooth pulp (a sourc e of small-diameter fiber input) and FHMs (a source of larger-diameter fibe r input) were recorded extracellularly from chronically instrumented cats b efore, during, and after recovery from the anesthetic state induced by a si ngle (2.2 mg/kg) intravenous injection of ketamine. Results: Overall, tooth pulp-evoked responses of TSNC neurons were maximall y suppressed by 50% within 5 min after the intravenous administration of ke tamine. Ketamine also suppressed the FHM-evoked responses of TSNC and trige minothalamic neurons by 45%. The time course of ketamine's suppressive acti on was equivalent for tooth pulp and FHM-evoked responses. However, the rec overy of tooth pulp-evoked TSNC neuronal responses at suprathreshold intens ities was markedly prolonged compared with neuronal responses driven by thr eshold stimuli or FHM. Conclusions: These elecrrophysiologic results in the chronically instrument ed cat preparation indicate that a nonselective suppression of orofacial so matosensory information occurs during ketamine anesthesia. The prolonged re covery of su prathreshold responses of TSNC neurons mediated by small-diame ter afferent fiber input may partly underlie the analgesic action of ketami ne that is clinically relevant at subanesthetic doses.