title: Nerve growth factor alters the sensitivity of rat masseter muscle mechanoreceptors to NMDA receptor activation.
abstract: Intramuscular injection of nerve growth factor (NGF) into rat masseter muscle induces a local mechanical sensitization that is greater in female than in male rats. The duration of NGF-induced sensitization in male and female rats was associated with an increase in peripheral N-methyl-d-aspartate (NMDA) receptor expression by masseter muscle afferent fibers that began 3 days postinjection. Here, we investigated the functional consequences of increased NMDA expression on the response properties of masseter muscle mechanoreceptors. In vivo extracellular single-unit electrophysiological recordings of trigeminal ganglion neurons innervating the masseter muscle were performed in anesthetized rats 3 days after NGF injection (25 μg/ml, 10 μl) into the masseter muscle. Mechanical activation threshold was assessed before and after intramuscular injection of NMDA. NMDA injection induced mechanical sensitization in both sexes that was increased significantly following NGF injection in the male rats but not in the female rats. However, in female but not male rats, further examination found that preadministration of NGF induced a greater sensitization in slow Aδ-fibers (2-7 m/s) than fast Aδ-fibers (7-12 m/s). This suggests that preadministration of NGF had a different effect on slowly conducting mechanoreceptors in the female rats compared with the male rats. Although previous studies have found an association between estrogenic tone and NMDA activity, no correlation was observed between NMDA-evoked mechanical sensitization and plasma estrogen level. This study suggests NGF alters NMDA-induced mechanical sensitization in the peripheral endings of masseter mechanoreceptors in a sexually dimorphic manner.
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, British Columbia, Canada;
authors.forename: Hayes
authors.initials: H
authors.lastname: Wong
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, British Columbia, Canada; College of Stomatology, Tianjin Medical University, Tianjin, China; and.
authors.forename: Xu-Dong
authors.initials: XD
authors.lastname: Dong
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, British Columbia, Canada; Center for Sensory Motor Interaction, Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Aalborg East, Denmark brcairns@mail.ubc.ca.
authors.forename: Brian E
authors.initials: BE
authors.lastname: Cairns
mesh_terms: D000818:Animals; D004967:Estrogens / Q000097:blood; D005260:Female; D008297:Male; D008406:Masseter Muscle / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D008465:Mechanoreceptors / Q000187:drug effects / Q000378:metabolism*; D009413:Nerve Fibers, Myelinated / Q000502:physiology; D020932:Nerve Growth Factor / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000235:genetics / Q000378:metabolism*; D012737:Sex Factors; D014276:Trigeminal Nerve / Q000502:physiology
chemical_list: D004967:Estrogens; D016194:Receptors, N-Methyl-D-Aspartate; D020932:Nerve Growth Factor
keywords: NMDA receptors; masseter muscle; mechanoreceptors; nerve growth factor; sensitization