Decreased face primary motor cortex (face-M1) excitability induced by noxious stimulation of the rat molar tooth pulp is dependent on the functional integrity of medullary astrocytes.
We have recently shown that application of the small-fiber excitant and inflammatory irritant mustard oil (MO) to the rat molar tooth pulp can decrease face-M1 excitability, but increase the excitability of trigeminal medullary dorsal horn (MDH) nociceptive neurons and that application of the astrocytic inhibitor methionine sulfoximine (MSO) to the face-M1 or MDH can attenuate the MO-induced changes. The present study aimed to determine whether medullary MSO application could modulate the MO-induced decreased face-M1 excitability. Under ketamine general anesthesia, electromyographic (EMG) electrodes were implanted into the right anterior digastric (RAD, jaw-opening muscle) of adult male Sprague-Dawley rats. A microelectrode was positioned at a low-threshold (≤30 μA) site in the left face-M1. Then MO (n = 16) or control-solution (n = 16) was applied to the previously exposed molar tooth pulp, and intracortical microstimulation threshold intensities for evoking RAD EMG activities were monitored for 15 min. MSO (0.1 mM, n = 8) or phosphate-buffered saline (PBS, n = 8) was then applied to the MDH and RAD thresholds monitored every 15 min for 120 min. Statistics used ANOVA followed by post hoc Bonferroni as appropriate (p < 0.05). As compared to baseline, RAD thresholds significantly increased (i.e., decreased excitability) within 1 min (26.3 ± 7.9%, p = 0.007) and peaked at 15 min following pulpal MO application (49.9 ± 5.7%, p < 0.001) but not following control-solution. Following MSO (but not PBS) application to the medulla, RAD thresholds significantly decreased within 15 min (26.5 ± 3.0%, p = 0.05) and at 60 min approached 6.3 ± 2.4%, of baseline values (p = 0.1). These novel findings suggest that clinically related motor disturbances arising from dental pain may involve decreased face-M1 excitability that is modulated by medullary astrocytes.
Department of Oral Physiology, Faculty of Dentistry, University of Toronto, Toronto, ON, Canada.
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Pun
Department of Oral Physiology, Faculty of Dentistry, University of Toronto, Toronto, ON, Canada.
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Awamleh
Department of Oral Physiology, Faculty of Dentistry, University of Toronto, Toronto, ON, Canada.
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Lee
Department of Oral Physiology, Faculty of Dentistry, University of Toronto, Toronto, ON, Canada. limor.avivi.arber@utoronto.ca.|Department of Prosthodontics, Faculty of Dentistry, University of Toronto, 124 Edward St., Toronto, ON, M5G1G6, Canada. limor.avivi.arber@utoronto.ca.
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http://orcid.org/0000-0002-0567-6101
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Avivi-Arber
D000818:Animals; D001253:Astrocytes / Q000502:physiology*; D003782:Dental Pulp / Q000294:innervation / Q000502:physiology*; D004558:Electric Stimulation / Q000009:adverse effects; D004567:Electrodes, Implanted; D004576:Electromyography / Q000379:methods; D005145:Face / Q000294:innervation / Q000502:physiology; D008297:Male; D008526:Medulla Oblongata / Q000502:physiology*; D008963:Molar / Q000294:innervation / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
Brainstem; Dental; Glia; Intracortical microstimulation; Neuroplasticity; Pain