Neuroscience Letters 535 (2013) 73–77 Contents lists available at SciVerse ScienceDirect Neuroscience Letters jou rn al h om epage: www.elsevier.com/locate/neulet Effects of chewing and swallowing behavior on jaw opening reflex responses in freely feeding rabbits Aki Yamada a , Yuka Kajii a , Shogo Sakai a , Takanori Tsujimura a , Yuki Nakamura a , Sajjiv Ariyasinghe b , Jin Magara a , Makoto Inoue a,∗ a Division of Dysphagia Rehabilitation, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan b Division of Physiology, Department of Basic Sciences, Faculty of Dental Sciences, University of Peradeniya, Peradeniya, Sri Lanka h i g h l i g h t s ◮ The inferior alveolar nerve was stimulated to evoke jaw opening reflex in rabbits. ◮ Jaw opening reflex responses were inhibited during chewing and swallowing. ◮ Inferior alveolar nerve stimulation itself did not change feeding behaviors. ◮ Undesirable jaw movements are inhibited during chewing and swallowing. a r t i c l e i n f o Article history: Received 14 October 2012 Received in revised form 26 December 2012 Accepted 27 December 2012 Keywords: Swallowing Chewing Jaw opening reflex Rabbit Inferior alveolar nerve stimulation a b s t r a c t It has been reported that the jaw opening reflex (JOR) evoked by intra-oral innocuous stimulation was suppressed during a reflex swallow in anesthetized animals only. However, the mechanism of JOR inhi- bition during swallowing has not yet been elucidated. The aim of the present study was to investigate the effects of peripheral nerve stimulation on masticatory behaviors, as well as the modulation of low threshold afferent evoked JOR responses during chewing and swallowing in freely feeding animals. The JOR in the digastric muscle was evoked by low threshold electrical stimulation of the inferior alveolar nerve (IAN). Changes in the peak-to-peak amplitude of digastric electromyographic responses were com- pared among the phases of chewing and swallowing. IAN stimulation did not produce any differences in cycle duration, gape of the jaw in one cycle, or swallowing interval, suggesting a minimal effect on feeding behaviors. The JOR amplitude during the fast-closing, slow-closing, and slow-opening phases of chewing was significantly smaller than that of the control (recorded when the animal was at rest) and fast-opening phase. During swallowing, the JOR amplitude was significantly less than the control. Inhi- bition of the JOR during swallowing is assumed to prevent unnecessary opposing jaw opening motion. © 2013 Elsevier Ireland Ltd. All rights reserved. Introduction It has been reported that the amplitude of the jaw opening reflex (JOR) evoked by innocuous stimulation of the trigeminal nerve is subjected not only to tonic suppression, but also to phase- dependent modulation during mastication or fictive mastication, as demonstrated in conscious [10] and anesthetized [11] animals. This inhibition has been attributed to presynaptic inhibition of synaptic Abbreviations: Dig, digastric; IAN, inferior alveolar nerve; JOR, jaw opening reflex; MH, mylohyoid; SLN, superior laryngeal nerve. ∗ Corresponding author at: Division of Dysphagia Rehabilitation, Niigata Uni- versity Graduate School of Medical and Dental Sciences, 2-5274 Gakkocho-dori, Chuo-ku, Niigata 951-8514, Japan. Tel.: +81 25 227 0733; fax: +81 25 227 0733. E-mail address: inoue@dent.niigata-u.ac.jp (M. Inoue). transmission of peripheral inputs from the trigeminal afferents in the reflex pathway [9], and to inhibitory modulation of trigemi- nal interneuron excitability [17,18]. These studies propose that the central network that generates chewing movements, the mastica- tory central pattern generator (CPG), is involved in the modulation of JOR responses. It has also been demonstrated that the JOR is suppressed during reflex swallowing [2,19]. However, modulation of JOR responses during swallowing has been reported in anesthetized animals only, and the neuronal mechanisms of JOR modulation have not yet been elucidated. Additionally, in these studies, swallowing was evoked by electrical stimulation of the superior laryngeal nerve (SLN). The SLN includes not only low- and high-threshold sensory afferents from the pharynx and larynx, but also potentially autonomic nerves [16]; therefore, SLN stimulation may have a range of effects worth considering. 0304-3940/$ – see front matter © 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2012.12.047