title: Differences in Motor Evoked Potentials Induced in Rats by Transcranial Magnetic Stimulation under Two Separate Anesthetics: Implications for Plasticity Studies.
abstract: Repetitive transcranial magnetic stimulation (rTMS) is primarily used in humans to change the state of corticospinal excitability. To assess the efficacy of different rTMS stimulation protocols, motor evoked potentials (MEPs) are used as a readout due to their non-invasive nature. Stimulation of the motor cortex produces a response in a targeted muscle, and the amplitude of this twitch provides an indirect measure of the current state of the cortex. When applied to the motor cortex, rTMS can alter MEP amplitude, however, results are variable between participants and across studies. In addition, the mechanisms underlying any change and its locus are poorly understood. In order to better understand these effects, MEPs have been investigatedin vivoin animal models, primarily in rats. One major difference in protocols between rats and humans is the use of general anesthesia in animal experiments. Anesthetics are known to affect plasticity-like mechanisms and so may contaminate the effects of an rTMS protocol. In the present study, we explored the effect of anesthetic on MEP amplitude, recorded before and after intermittent theta burst stimulation (iTBS), a patterned rTMS protocol with reported facilitatory effects. MEPs were assessed in the brachioradialis muscle of the upper forelimb under two anesthetics: a xylazine/zoletil combination and urethane. We found MEPs could be induced under both anesthetics, with no differences in the resting motor threshold or the average baseline amplitudes. However, MEPs were highly variable between animals under both anesthetics, with the xylazine/zoletil combination showing higher variability and most prominently a rise in amplitude across the baseline recording period. Interestingly, application of iTBS did not facilitate MEP amplitude under either anesthetic condition. Although it is important to underpin human application of TMS with mechanistic examination of effects in animals, caution must be taken when selecting an anesthetic and in interpreting results during prolonged TMS recording.
authors.affiliation: Brain Health Research Centre and Brain Research New Zealand Centre of Research ExcellenceDunedin, New Zealand; Department of Anatomy, University of OtagoDunedin, New Zealand; Experimental and Regenerative Neuroscience, School of Animal Biology, University of Western AustraliaPerth, WA, Australia.
authors.forename: Matthew
authors.initials: M
authors.lastname: Sykes
authors.affiliation: Brain Health Research Centre and Brain Research New Zealand Centre of Research ExcellenceDunedin, New Zealand; Department of Anatomy, University of OtagoDunedin, New Zealand.
authors.forename: Natalie A
authors.initials: NA
authors.lastname: Matheson
authors.affiliation: Brain Health Research Centre and Brain Research New Zealand Centre of Research ExcellenceDunedin, New Zealand; School of Physical Education, Sport and Exercise Sciences, University of OtagoDunedin, New Zealand.
authors.forename: Philip W
authors.initials: PW
authors.lastname: Brownjohn
authors.affiliation: Experimental and Regenerative Neuroscience, School of Animal Biology, University of Western Australia Perth, WA, Australia.
authors.forename: Alexander D
authors.initials: AD
authors.lastname: Tang
authors.affiliation: Experimental and Regenerative Neuroscience, School of Animal Biology, University of Western Australia Perth, WA, Australia.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Rodger
authors.affiliation: Brain Health Research Centre and Brain Research New Zealand Centre of Research ExcellenceDunedin, New Zealand; School of Physical Education, Sport and Exercise Sciences, University of OtagoDunedin, New Zealand.
authors.forename: Jonathan B H
authors.initials: JB
authors.lastname: Shemmell
authors.affiliation: Brain Health Research Centre and Brain Research New Zealand Centre of Research ExcellenceDunedin, New Zealand; Department of Anatomy, University of OtagoDunedin, New Zealand.
authors.forename: John N J
authors.initials: JN
authors.lastname: Reynolds
mesh_terms: D000777:Anesthetics / Q000494:pharmacology*; D000818:Animals; D004338:Drug Combinations; D019054:Evoked Potentials, Motor / Q000187:drug effects* / Q000502:physiology*; D008297:Male; D009044:Motor Cortex / Q000187:drug effects* / Q000502:physiology*; D009473:Neuronal Plasticity / Q000187:drug effects* / Q000502:physiology* / Q000528:radiation effects; D051381:Rats; D017208:Rats, Wistar; D013992:Tiletamine / Q000494:pharmacology; D050781:Transcranial Magnetic Stimulation / Q000187:drug effects / Q000379:methods*; D014520:Urethane / Q000494:pharmacology; D014991:Xylazine / Q000494:pharmacology; D015041:Zolazepam / Q000494:pharmacology
chemical_list: D000777:Anesthetics; D004338:Drug Combinations; C006131:tiletamine, zolazepam drug combination; D014991:Xylazine; D013992:Tiletamine; D014520:Urethane; D015041:Zolazepam
keywords: anesthesia; excitability; motor evoked potential; plasticity; rat; transcranial magnetic stimulation