Inhibition of swallowing reflex following phosphorylation of extracellular signal-regulated kinase in nucleus tractus solitarii neurons in rats with masseter muscle nociception

Inhibition of Water-Evoked Swallowing During Noxious Mechanical Stimulation of Tongue in Anesthetized Rats

Dysphagia is sometimes accompanied by pain. Because orofacial structures subserve mastication and swallowing, orofacial pain might impair both functions. Tongue biting can occur not only accidentally while eating but also in some pathological conditions. However, it remains unclear whether noxious mechanical stimulation of the tongue affects swallowing. To explore this question, we evaluated the effects of lingual pinch stimulation on the initiation of swallowing evoked by distilled water (DW) infusion with a flow rate of 5.0 µL/s for 20 s into the pharyngolaryngeal region in anesthetized rats. The swallowing reflex was identified by electromyographic (EMG) bursts in the suprahyoid muscles which include the anterior belly of the digastric muscle, mylohyoid and geniohyoid muscles, and laryngeal elevation by visual inspection. The number of DW-evoked swallows during pinch stimulation was significantly smaller than that in a control condition or during pressure stimulation. The onset latency of the first swallow during pinch stimulation was significantly longer than that in the control condition. DW-evoked swallowing was almost abolished following bilateral transection of the superior laryngeal nerve (SLN) compared with the control condition, suggesting that the SLN plays a crucial role in the initiation of DW-evoked swallowing. Finally, electrophysiological data indicated that some SLN-responsive neurons in the nucleus tractus solitarii (nTS) exhibited delayed latency from a single SLN stimulation during lingual pinch stimulation. These results suggest that noxious mechanical stimulation of the tongue inhibits the initiation of swallowing and modulates neuronal activity in the nTS.

Swallowing dysfunction following radiation to the rat mylohyoid muscle is associated with sensory neuron injury

Radiation-based treatments for oropharyngeal and hypopharyngeal cancers result in impairments in swallowing mobility, but the mechanisms behind the dysfunction are not clear. The purpose of this study was to determine if we could establish an animal model of radiation-induced dysphagia in which mechanisms could be examined. We hypothesized that 1) radiation focused at the depth of the mylohyoid muscle would alter normal bolus transport and bolus size and 2) radiation to the mylohyoid muscle will induce an injury/stress-like response in trigeminal sensory neurons whose input might modulate swallow. Rats were exposed to 48 or 64 Gy of radiation to the mylohyoid given 8 Gy in 6 or 8 fractions. Swallowing function was evaluated by videofluoroscopy 2 and 4 wk following treatment. Neuronal injury/stress was analyzed in trigeminal ganglion by assessing activating transcription factor (ATF)3 and GAP-43 mRNAs at 2, 4, and 8 wk post treatment. Irradiated rats exhibited decreases in bolus movement through the pharynx and alterations in bolus clearance. In addition, ATF3 and GAP-43 mRNAs were upregulated in trigeminal ganglion in irradiated rats, suggesting that radiation to mylohyoid muscle induced an injury/stress response in neurons with cell bodies that are remote from the irradiated tissue. These results suggest that radiation-induced dysphagia can be assessed in the rat and radiation induces injury/stress-like responses in sensory neurons.NEW & NOTEWORTHY Radiation-based treatments for head and neck cancer can cause significant impairments in swallowing mobility. This study provides new evidence supporting the possibility of a neural contribution to the mechanisms of swallowing dysfunction in postradiation dysphagia. Our data demonstrated that radiation to the mylohyoid muscle, which induces functional deficits in swallowing, also provokes an injury/stress-like response in the ganglion, innervating the irradiated muscle.

Adaptations to Oral and Pharyngeal Swallowing Function Induced by Injury to the Mylohyoid Muscle

Muscle injury is a frequent side effect of radiation treatment for head and neck cancer. To understand the pathophysiology of injury-related dysfunction, we investigated the effects of a single muscle injury to the mylohyoid on oropharyngeal swallowing function in the rat. The mylohyoid protects the airway from food/liquid via hyolaryngeal elevation and plays an active role during both oral and pharyngeal swallowing. We hypothesized (1) that fibrosis to the mylohyoid alters swallowing bolus flow and licking patterns and (2) that injury to the mylohyoid changes normal activity of submental, laryngeal, and pharyngeal muscles during swallowing. A chilled cryoprobe was applied to the rat mylohyoid muscle to create a localized injury. One and two weeks after injury, swallowing bolus transit was assessed via videofluoroscopy and licking behavior via an electrical lick sensor. The motor activity of five swallow-related muscles was analyzed immediately after injury using electromyography (EMG). Comparisons were made pre- and post-injury. Fibrosis was confirmed in the mylohyoid at 2 weeks after injury by measuring collagen content. One week after injury, bolus size decreased, swallowing rate reduced, and licking patterns were altered. Immediately post-injury, there was a significant depression in mylohyoid and thyropharyngeus EMG amplitudes during swallowing. Our results demonstrated that injury to the mylohyoid is sufficient to cause changes in deglutition. These disruptions in oral and pharyngeal swallowing were detected prior to long-term fibrotic changes, including delays in tongue movement, alterations in bolus flow, and changes in sensorimotor function. Therefore, injuring a single important swallowing muscle can have dramatic clinical effects.

Immediate effect of laryngeal surface electrical stimulation on swallowing performance

Surface electrical stimulation of the laryngeal region is used to improve swallowing in dysphagic patients. However, little is known about how electrical stimulation affects tongue movements and related functions. We investigated the effect of electrical stimulation on tongue pressure and hyoid movement, as well as suprahyoid and infrahyoid muscle activity, in 18 healthy young participants. Electrical stimulation (0.2-ms duration, 80 Hz, 80% of each participant's maximal tolerance) of the laryngeal region was applied. Each subject swallowed 5 ml of barium sulfate liquid 36 times at 10-s intervals. During the middle 2 min, electrical stimulation was delivered. Tongue pressure, electromyographic activity of the suprahyoid and infrahyoid muscles, and videofluorographic images were simultaneously recorded. Tongue pressure during stimulation was significantly lower than before or after stimulation and was significantly greater after stimulation than at baseline. Suprahyoid activity after stimulation was larger than at baseline, while infrahyoid muscle activity did not change. During stimulation, the position of the hyoid at rest was descended, the highest hyoid position was significantly inferior, and the vertical movement was greater than before or after stimulation. After stimulation, the positions of the hyoid at rest and at the maximum elevation were more superior than before stimulation. The deviation of the highest positions of the hyoid before and after stimulation corresponded to the differences in tongue pressures at those times. These results suggest that surface electrical stimulation applied to the laryngeal region during swallowing may facilitate subsequent hyoid movement and tongue pressure generation after stimulation. NEW & NOTEWORTHY Surface electrical stimulation applied to the laryngeal region during swallowing may facilitate subsequent hyoid movement and tongue pressure generation after stimulation. Tongue muscles may contribute to overshot recovery more than hyoid muscles.

Effects of chewing and swallowing behavior on jaw opening reflex responses in freely feeding rabbits

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 inhibition 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 compared 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. Inhibition of the JOR during swallowing is assumed to prevent unnecessary opposing jaw opening motion.

Changes of timing variables in swallowing of boluses with different viscosities in patients with dysphagia

OBJECTIVES

To evaluate the timing of the swallowing process and the effect of bolus viscosity on swallowing.

DESIGN

Prospective observational study.

SETTING

General teaching hospital, rehabilitation unit.

PARTICIPANTS

We enrolled patients with dysphagia (n=82) in a videofluoroscopic swallowing study (VFSS) from January 13, 2009, to October 22, 2009. Based on VFSS results, we classified patients as "thin-fluid aspirators" (n=40) or as "nonaspirators" (n=42).

INTERVENTIONS

Swallowing of a 5-mL thick bolus and a 5-mL thin bolus in all patients.

MAIN OUTCOME MEASURES

Kinematic analysis of various variables during the swallowing process (pharyngeal phase), including epiglottis contact with the bolus, laryngeal elevation, pharyngeal constriction, and upper esophageal sphincter opening.

RESULTS

In both groups, the thin bolus arrived at the vallecular pouch earlier than the thick bolus. During swallowing of the thick bolus, the thin-fluid aspirators had a delayed latency of upper esophageal sphincter opening, delayed laryngeal elevation to peak level, and significantly longer rise time of laryngeal elevation.

CONCLUSIONS

Our results indicate clear differences in the degree of adaptation to bolus viscosity between patients classified as thin-fluid aspirators and as nonaspirators. These differences were mainly in activities of laryngeal elevators rather than pharyngeal constrictors.

GABAB receptors in the NTS mediate the inhibitory effect of trigeminal nociceptive inputs on parasympathetic reflex vasodilation in the rat masseter muscle

The present study was designed to examine whether trigeminal nociceptive inputs are involved in the modulation of parasympathetic reflex vasodilation in the jaw muscles. This was accomplished by investigating the effects of noxious stimulation to the orofacial area with capsaicin, and by microinjecting GABAA and GABAB receptor agonists or antagonists into the nucleus of the solitary tract (NTS), on masseter hemodynamics in urethane-anesthetized rats. Electrical stimulation of the central cut end of the cervical vagus nerve (cVN) in sympathectomized animals bilaterally increased blood flow in the masseter muscle (MBF). Increases in MBF evoked by cVN stimulation were markedly reduced following injection of capsaicin into the anterior tongue in the distribution of the lingual nerve or lower lip, but not when injected into the skin of the dorsum of the foot. Intravenous administration of either phentolamine or propranolol had no effect on the inhibitory effects of capsaicin injection on the increases of MBF evoked by cVN stimulation, which were largely abolished by microinjecting the GABAB receptor agonist baclofen into the NTS. Microinjection of the GABAB receptor antagonist CGP-35348 into the NTS markedly attenuated the capsaicin-induced inhibition of MBF increase evoked by cVN stimulation, while microinjection of the GABAA receptor antagonist bicuculline did not. Our results indicate that trigeminal nociceptive inputs inhibit vagal-parasympathetic reflex vasodilation in the masseter muscle and suggest that the activation of GABAB rather than GABAA receptors underlies the observed inhibition in the NTS.

Organization of pERK-immunoreactive cells in trigeminal spinal nucleus caudalis, upper cervical cord, NTS and Pa5 following capsaicin injection into masticatory and swallowing-related muscles in rats

Many phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) cells are expressed in the trigeminal spinal subnucleus caudalis (Vc), upper cervical spinal cord (C1-C2), nucleus tractus solitarii (NTS) and paratrigeminal nucleus (Pa5) after capsaicin injection into the whisker pad (WP), masseter muscle (MM), digastric muscle (DM) or sternohyoideus muscle (SM). The pERK-IR cells also showed NeuN immunoreactivity, indicating that ERK phosphorylation occurs in neurons. The pERK-IR cells were significantly reduced after intrathecal injection of MEK 1/2 inhibitor PD98059. The pERK-IR cells expressed bilaterally in the Vc and C1-C2 after capsaicin injection into the unilateral DM or SM, whereas unilaterally in the Vc and C1-C2 after unilateral WP or MM injection. After capsaicin injection into the WP or MM, the pERK-IR cell expression in the Vc was restricted rostrocaudally within a narrow area. However, the distribution of pERK-IR cells was more wide spread without a clear peak in the Vc and C1-C2 after capsaicin injection into the DM or SM. In the NTS, the unimodal pERK-IR cell expression peaked at 0-720μm rostral from the obex following capsaicin injection into WP, MM, DM or SM. In the ipsilateral Pa5, many pERK-IR cells were observed following capsaicin injection into the SM. The number of swallows elicited by distilled water administration was significantly smaller after capsaicin injection into the WP, MM or DM but not SM compared to that of vehicle-injected rats. Various noxious inputs due to the masticatory or swallowing-related muscle inflammation may be differentially involved in muscle pain and swallowing reflex activity.

Neural mechanisms of swallowing and effects of taste and other stimuli on swallow initiation

Swallowing involves several motor processes such as bolus formation and intraoral transport of a food bolus (oral stage) and a series of visceral events that occur in a relatively fixed timed sequence but are to some degree modifiable (pharyngeal stage or swallow reflex). Reflecting the progressive aging of society, patients with swallowing disorders (i.e., dysphagia) are increasing. Therefore, there is expanding social demand for the development of better rehabilitation treatment of dysphagic patients. To date, many dysphagia diets have been developed and are available commercially to help bring back the pleasure of mealtimes to dysphagia patients. Texture modification of food to make the food bolus easier to swallow with less risk of aspiration is one of the important elements in dysphagia diets from the viewpoint of safety assurance. However, for the further development of dysphagia diets, new attempts based on new concepts are needed. One of the possible approaches is to develop dysphagia diets that facilitate swallow initiation. For this approach, an understanding of the mechanisms of swallow initiation and identification of factors that facilitate or suppress swallow initiation are important. In this review, we first summarize the neural mechanisms of swallowing and effects of taste and other inputs on swallow initiation based on data mainly obtained from experimental animals. Then we introduce a recently established technique for eliciting swallowing using electrical stimulation in humans and our ongoing studies using this technique.

Purinergic receptors are involved in tooth-pulp evoked nocifensive behavior and brainstem neuronal activity

Background

To evaluate whether P2X receptors are involved in responses to noxious pulp stimulation, the P2X₃ and P2X_2/3 receptor agonist α,β-methyleneATP (α,β-meATP) was applied to the molar tooth pulp and nocifensive behavior and extracellular-signal regulated kinase (ERK) phosphorylation in trigeminal spinal subnucleus caudalis (Vc), trigeminal spinal subnucleus interpolaris (Vi), upper cervical spinal cord (C1/C2) and paratrigeminal nucleus (Pa5) neurons were analyzed in rats.

Results

Genioglossus (GG) muscle activity was evoked by pulpal application of 100 mM α,β-meATP and was significantly larger than GG activity following vehicle (phosphate-buffered saline PBS) application (p < 0.01). The enhanced GG muscle activity following 100 mM α,β-meATP was significantly reduced (p < 0.05) by co-application of 1 mM TNP-ATP (P2X₁, P2X₃ and, P2X_2/3 antagonist). A large number of pERK-LI cells were expressed in the Vc, Vi/Vc, C1/C2 and Pa5 at 5 min following pulpal application of 100 mM α,β-meATP compared to PBS application to the pulp (p < 0.05). The pERK-LI cell expression and GG muscle activity induced by 100 mM α,β-meATP pulpal application were significantly reduced after intrathecal injection of the MAPK/ERK kinase (MEK) inhibitor PD 98059 and by pulpal co-application of 1 mM TNP-ATP (p < 0.05).

Conclusions

The present findings suggest that activation of P2X₃ and P2X_2/3 receptors in the tooth pulp is sufficient to elicit nociceptive behavioral responses and trigeminal brainstem neuronal activity.