Fluoxetine, but not paroxetine, alters the jaw-closing muscle activity during non-rapid eye movement sleep in mice.

Sleep bruxism is an involuntary, exaggerated jaw-closing activity during sleep. Selective serotonin reuptake inhibitor (SSRI) use is a risk factor for bruxism. However, the effect of various SSRIs on masseter (jaw-closing) muscle activity remains unclear. Here, we examined the effects of long-term administration of two SSRIs, fluoxetine (FLX) and paroxetine (PRX), for 14 days on masseter muscle activity during wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep for 24 h in mice. Vigilance states were scored based on electroencephalographic, electrooculography and neck electromyographic (EMG) activities. The EMG activity of the masseter muscle was quantified in 6 h periods. FLX and PRX did not affect the duration of the three vigilance states. Both drugs significantly prolonged the REM sleep episode duration while decreasing the number of episodes. FLX significantly increased REM sleep onset latency. Neither FLX nor PRX affected the mean masseter EMG activity during wakefulness. FLX significantly increased the relative time of masseter muscle activity in NREM sleep during 02:00-08:00 and 08:00-14:00, while PRX did not affect three vigilance states. Overall, FLX had a limited but significant effect on masseter muscle activity in NREM sleep during specific periods.

Optical imaging of neurons related to fictive swallowing using GCaMP6f in an arterially perfused rat preparation.

OBJECTIVE: It is difficult to comprehensively study the activity patterns and distribution of neurons in the brainstem that control the act of swallowing, as they are located deep in the brain. In this study, we aimed to evaluate the usefulness of calcium imaging using GCaMP6f in arterially perfused preparations to study the activity of swallowing-related neurons in the brainstem. METHODS: Arterially perfused rat preparations were prepared 3-4 weeks after the injection of a neuron-specific virus expressing GCaMP6f. Fictive swallowing was induced by repetitive electrical stimulation of the superior laryngeal nerve (SLN). Simultaneously, the activity of GCaMP6f-expressing neurons in the dorsal brainstem, between 0.1 and 4.8 mm rostral to the obex, was assessed by changes in the intracellular calcium concentration using confocal laser microscopy. RESULTS: Neurons responding to stimulation of the SLN included swallowing-related neurons (48%), which showed an increase in fluorescence intensity at the time of swallowing bursts in the cervical vagus nerve, and stimulation-related neurons (52%), which showed an increase in fluorescence intensity through stimulation, regardless of the swallowing bursts. Despite a broad search area, swallowing-related neurons were localized exclusively in and around the solitary nucleus. In contrast, most stimulation-related neurons were located in the brainstem reticular formation, which is more rostral than the solitary nucleus. CONCLUSIONS: Calcium imaging using GCaMP in arterially perfused rat preparations is useful for an efficient search of the activity pattern and distribution of neurons located in a wide area of the brainstem.

Involvement of ghrelin in the regulation of swallowing motor activity in an arterially perfused rat preparation.

Ghrelin, a peripheral peptide produced in the stomach, is involved in the neural networks that control food intake. Alterations in motor components, such as swallowing, are believed to be significant in the regulation food intake by orexigenic signals. However, there has been no detailed investigation of the relationship between ghrelin and swallowing activities induced in motor nerves innervating the pharyngeal and laryngeal muscles. In this study, we examined the effects of ghrelin administration on swallowing motor activity in arterially perfused rats. Injection of distilled water (0.5 ml) into the oral cavity or electrical stimulation of the superior laryngeal nerve evoked swallowing motor activity in the cervical vagus nerve. Administration of ghrelin (6 nM), but not des-acylated ghrelin (6 nM), into the perfusate increased the peak burst amplitude and burst duration, and shortened the first burst interval of water injection-induced swallowing. These ghrelin-induced changes in swallowing motor activity were blocked by the administration of JMV2959 (6 µM), a growth hormone secretagogue receptor antagonist. In preparations in which the hypothalamus was removed, ghrelin had no effect on swallowing motor activity. Furthermore, ghrelin-induced changes were counteracted by the administration of BIBO3304 (1 µM) or L-152,804 (1 µM), antagonists of neuropeptide Y Y1 and Y5 receptors, respectively, which are essential for ghrelin-induced enhancement of food intake. Ghrelin also increased the peak burst amplitude and burst duration of the swallowing motor activity evoked by electrical stimulation of the superior laryngeal nerve, although the effects of ghrelin on the number of swallowing bursts and burst intervals varied with stimulus intensity. These results suggest that ghrelin enhances the magnitude and frequency of bursts of swallowing motor activity by acting via the hypothalamic neural network, and that neuropeptide Y Y1 and Y5 receptors are involved in this enhancement.

Intrinsic properties and synaptic connectivity of Phox2b-expressing neurons in rat rostral parvocellular reticular formation.

The paired-like homeobox 2b gene (Phox2b) is critical for the development of the autonomic nervous system. We have previously demonstrated the distinct characteristics of Phox2b-expressing (Phox2b+) neurons in the reticular formation dorsal to the trigeminal motor nucleus (RdV), which are likely related to jaw movement regulation. In this study, we focused on Phox2b+ neurons in the rostral parvocellular reticular formation (rPCRt), a critical region for controlling orofacial functions, using 2-11-day-old Phox2b-EYFP rats. Most Phox2b+ rPCRt neurons were glutamatergic, but not GABAergic or glycinergic. Approximately 65 % of Phox2b+ rPCRt neurons fired at a low frequency, and approximately 24 % of Phox2b+ rPCRt neurons fired spontaneously, as opposed to Phox2b+ RdV neurons. Stimulation of the RdV evoked inward postsynaptic currents in more than 50 % of Phox2b+ rPCRt neurons, while only one Phox2b+ rPCRt neuron responded to stimulation of the nucleus of the solitary tract. Five of the 10 Phox2b+ neurons sent their axons that ramified within the trigeminal motor nucleus (MoV). Of these, the axons of the two neurons terminated within both the MoV and rPCRt. Our findings suggest that Phox2b+ rPCRt neurons have distinct electrophysiological and synaptic properties that may be involved in the motor control of feeding behavior.

Postnatal Maturation of Glutamatergic Inputs onto Rat Jaw-closing and Jaw-opening Motoneurons.

Motoneurons that innervate the jaw-closing and jaw-opening muscles play a critical role in oro-facial behaviors, including mastication, suckling, and swallowing. These motoneurons can alter their physiological properties through the postnatal period during which feeding behavior shifts from suckling to mastication; however, the functional synaptic properties of developmental changes in these neurons remain unknown. Thus, we explored the postnatal changes in glutamatergic synaptic transmission onto the motoneurons that innervate the jaw-closing and jaw-opening musculatures during early postnatal development in rats. We measured miniature excitatory postsynaptic currents (mEPSCs) mediated by non-NMDA receptors (non-NMDA mEPSCs) and NMDA receptors in the masseter and digastric motoneurons. The amplitude, frequency, and rise time of non-NMDA mEPSCs remained unchanged among postnatal day (P)2-5, P9-12, and P14-17 age groups in masseter motoneurons, whereas the decay time dramatically decreased with age. The properties of the NMDA mEPSCs were more predominant at P2-5 masseter motoneurons, followed by reduction as neurons matured. The decay time of NMDA mEPSCs of masseter motoneurons also shortened remarkably across development. Furthermore, the proportion of NMDA/non-NMDA EPSCs induced in response to the electrical stimulation of the supratrigeminal region was quite high in P2-5 masseter motoneurons, and then decreased toward P14-17. In contrast to masseter motoneurons, digastric motoneurons showed unchanged properties in non-NMDA and NMDA EPSCs throughout postnatal development. Our results suggest that the developmental patterns of non-NMDA and NMDA receptor-mediated inputs vary among jaw-closing and jaw-opening motoneurons, possibly related to distinct roles of respective motoneurons in postnatal development of feeding behavior.

Developmental changes in GABAergic and glycinergic synaptic transmission to rat motoneurons innervating jaw-closing and jaw-opening muscles.

Trigeminal motoneurons (MNs) innervating the jaw-closing and jaw-opening muscles receive numerous inhibitory synaptic inputs from GABAergic and glycinergic neurons, which are essential for oromotor functions, such as the orofacial reflex, suckling, and mastication. The properties of the GABAergic and glycinergic inputs of these MNs undergo developmental alterations during the period in which their feeding behavior proceeds from suckling to mastication; however, the detailed characteristics of the developmental patterns of GABAergic and glycinergic transmission in these neurons remain to be elucidated. This study was conducted to investigate developmental changes in miniature inhibitory postsynaptic currents (mIPSCs) in masseter (jaw-closing) and digastric (jaw-opening) MNs using brainstem slice preparations obtained from Wistar rats on postnatal day (P)2-5, P9-12, and P14-17. The frequency and amplitude of glycinergic mIPSCs substantially increased with age in both the masseter and digastric MNs. The rise time and decay time of glycinergic mIPSCs in both MNs decreased during development. In contrast, the frequency of GABAergic components in masseter MNs was higher at P2-5 than at P14-17, whereas that in the digastric MNs remained unchanged throughout the postnatal period. The proportion of currents mediated by GABA-glycine co-transmission was higher at P2-5, and then it decreased with age in both MNs. These results suggest that characteristics related to the development of inhibitory synaptic inputs differ between jaw-closing and jaw-opening MNs and between GABAergic and glycinergic currents. These distinct developmental characteristics may contribute to the development of feeding behaviors.

Responses evoked by electrical stimulation of the brainstem reticular formation in the jaw-opening and hypoglossal motor nerves of an arterially perfused rat preparation.

The interneuronal system in the brainstem reticular formation plays an important role in elaborate muscle coordination during various orofacial motor behaviors. In this study, we examined the distribution in the brainstem reticular formation of the sites that induce monosynaptic motor activity in the mylohyoid (jaw-opening) and hypoglossal nerves using an arterially perfused rat preparation. Electrical stimulation applied to 286 and 247 of the 309 sites in the brainstem evoked neural activity in the mylohyoid and hypoglossal nerves, respectively. The mean latency of the first component in the mylohyoid nerve response was significantly shorter than that in the hypoglossal nerve response. Moreover, the latency histogram of the first component in the hypoglossal nerve responses was bimodal, which was separated by 4.0 ms. The sites that induced short-latency (<4.0 ms) motor activity in the mylohyoid nerve and the hypoglossal nerve were frequently distributed in the rostral portion and the caudal portion of the brainstem reticular formation, respectively. Such difference in distributions of short-latency sites for mylohyoid and hypoglossal nerve responses likely corresponds to the distribution of excitatory premotor neurons targeting mylohyoid and hypoglossal motoneurons.

Enhancement of swallowing motor activity by the ACE inhibitor imidapril in an arterially perfused rat preparation.

Pharmacological agents that elevate dopamine and substance P concentrations have been suggested to prevent aspiration pneumonia and improve impaired swallowing processes. However, little is known about the effects of such agents on swallowing activities induced in motor nerves innervating the pharyngeal and laryngeal muscles. In this study, we examined the effects of imidapril, cilostazol, and amantadine, which are often prescribed for swallowing disorders, on swallowing motor activity. We recorded the efferent activities of the cervical vagal nerve, hypoglossal nerve, and phrenic nerve using arterially perfused rats aged between 21-35 postnatal days. The vagal nerve activity was used for evaluation of swallowing motor activity. When 1.25 ml of distilled water was injected into the oral cavity, or the superior laryngeal nerve was electrically stimulated, synchronized swallowing bursts were evoked in the vagal and hypoglossal nerves, while inspiratory discharges were inhibited in all the recorded nerves. Administration of imidapril (60 ng/ml) but not cilostazol (2.5 µg/ml) and amantadine (200 ng/ml) to the perfusate increased the mean peak amplitude of orally evoked swallowing bursts in the vagal nerve. Such increase in the peak amplitude by imidapril was antagonized by the administration of the NK1 receptor antagonist aprepitant (5 µg/ml) or the D1 receptor antagonist LE300 (2.5 µg/ml). In contrast, neither imidapril nor cilostazol caused a significant increase in swallowing bursts evoked by electrical stimulation of the superior laryngeal nerve. These results suggest that imidapril treatment may improve impaired swallowing by enhancing pharyngeal muscle activities via an increase in substance P and dopamine concentrations.

Serotonin1B receptor-mediated presynaptic inhibition of proprioceptive sensory inputs to jaw-closing motoneurons.

The proprioceptive sensory inputs from neurons in the mesencephalic trigeminal nucleus (MesV) to masseter motoneurons (MMNs) play an important role in regulating masseter muscle activity during mastication. Several histological studies have shown that serotonin (5-HT) fibers densely innervate both the MesV and the trigeminal motor nucleus. However, the functional roles of 5-HT in the regulation of the excitatory synaptic inputs from MesV afferents to MMNs remain to be clarified. Thus, using the whole-cell recording technique in brainstem slice preparations from juvenile Wistar rats aged between postnatal days 8 and 12, we examined the effects of 5-HT on the excitatory synaptic inputs from MesV afferents to MMNs. Bath application of 5-HT reduced the peak amplitude of excitatory postsynaptic potentials evoked in MMNs by electrical stimulation of the MesV afferents (eEPSPs), and this inhibitory effect of 5-HT on eEPSPs was replicated with the 5-HT1B receptor agonist CP-93129 but not by the 5-HT1A receptor agonist 8-OH-DPAT. Moreover, the 5-HT1B receptor antagonist SB-224289 but not the 5-HT1A receptor antagonist WAY-100635 antagonized the inhibitory effect of 5-HT on eEPSPs. CP-93129 increased the paired-pulse ratio and decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs), while it did not alter the mEPSC amplitude. These results suggest that presynaptic 5-HT1B receptors are involved in the inhibition of the excitatory synaptic inputs from MesV afferents to MMNs. Such inhibition may regulate MesV afferent activity during mastication.

5-HT2A receptor activation enhances NMDA receptor-mediated glutamate responses through Src kinase in the dendrites of rat jaw-closing motoneurons.

KEY POINTS: 5-HT increases the excitability of brainstem and spinal motoneurons, including the jaw-closing motoneurons, by depolarizing the membrane potential and decreasing the medium-duration afterhyperpolarization. In this study, we focused on how 5-HT enhances postsynaptic glutamatergic responses in the dendrites of the jaw-closing motoneurons. We demonstrate that 5-HT augments glutamatergic signalling by enhancing the function of the GluN2A-containing NMDA receptor (NMDAR) through the activation of 5-HT2A receptors (5-HT2A Rs) and Src kinase. To enhance glutamatergic responses, activation of the 5-HT2A Rs must occur within ∼60 µm of the location of the glutamate responses. 5-HT inputs to the jaw-closing motoneurons can significantly vary their input-output relationship, which may contribute to wide-range regulation of contractile forces of the jaw-closing muscles. ABSTRACT: Various motor behaviours are modulated by 5-HT. Although the masseter (jaw-closing) motoneurons receive both glutamatergic and serotonergic inputs, it remains unclear how 5-HT affects the glutamatergic inputs to the motoneuronal dendrites. We examined the effects of 5-HT on postsynaptic responses evoked by single- or two-photon uncaging of caged glutamate (glutamate responses) to the dendrites of masseter motoneurons in postnatal day 2-5 rats of either sex. Application of 5-HT induced membrane depolarization and enhanced the glutamate-response amplitude. This enhancement was mimicked by the 5-HT2A receptor (5-HT2A R) agonist and was blocked by the 5-HT2A/2C R antagonist. However, neither the 5-HT2B R nor the 5-HT2C R agonists altered glutamate responses. Blockade of the NMDA receptors (NMDARs), but not AMPA receptors, abolished the 5-HT-induced enhancement. Furthermore, the selective antagonist for the GluN2A subunit abolished the 5-HT-induced enhancement. 5-HT increased GluN2A phosphorylation, while the Src kinase inhibitor reduced the 5-HT-induced enhancement and GluN2A phosphorylation. When exposure to the 5-HT2A R agonist was targeted to the dendrites, the enhancement of glutamate responses was restricted to the loci of the dendrites near the puff loci. Electron microscopic immunohistochemistry revealed that both the NMDARs and the 5-HT2A Rs were close to each other in the same dendrite. These results suggest that activation of dendritic 5-HT2A Rs enhances the function of local GluN2A-containing NMDARs through Src kinase. Such enhancement of the glutamate responses by 5-HT may contribute to wide-range regulation of contractile forces of the jaw-closing muscles.

Electrophysiological properties of Ia excitation and recurrent inhibition in cat abdominal motoneurons.

Ia excitation and recurrent inhibition are basic neuronal circuits in motor control in hind limb. Renshaw cells receive synaptic inputs from axon collaterals of motoneurons and inhibit motoneurons and Ia inhibitory interneurons. It is important to know properties of Ia excitation and recurrent inhibition of trunk muscle such as abdominal muscles. The abdominal muscles have many roles and change those roles for different kind of functions. Intracellular recordings were obtained from the abdominal motoneurons of the upper lumbar segments in cats anesthetized. First, dorsal roots were left intact, and sensory and motor axons were electrically stimulated. Ia excitatory post-synaptic potentials were elicited in five of eight motoneurons at same segment stimulated. Second, dorsal roots were sectioned, and motor axons were electrically stimulated. Recurrent inhibitory post-synaptic potentials were elicited in one of 11 abdominal motoneurons. Renshaw cells extracellularly fired high-frequency bursts at short latency and at same segment stimulated.

Distinctive features of Phox2b-expressing neurons in the rat reticular formation dorsal to the trigeminal motor nucleus.

Phox2b encodes a paired-like homeodomain-containing transcription factor essential for development of the autonomic nervous system. Phox2b-expressing (Phox2b+) neurons are present in the reticular formation dorsal to the trigeminal motor nucleus (RdV) as well as the nucleus of the solitary tract and parafacial respiratory group. However, the nature of Phox2b+ RdV neurons is still unclear. We investigated the physiological and morphological properties of Phox2b+ RdV neurons using postnatal day 2-7 transgenic rats expressing yellow fluorescent protein under the control of Phox2b. Almost all of Phox2b+ RdV neurons were glutamatergic, whereas Phox2b-negative (Phox2b-) RdV neurons consisted of a few glutamatergic, many GABAergic, and many glycinergic neurons. The majority (48/56) of Phox2b+ neurons showed low-frequency firing (LF), while most of Phox2b- neurons (35/42) exhibited high-frequency firing (HF) in response to intracellularly injected currents. All, but one, Phox2b+ neurons (55/56) did not fire spontaneously, whereas three-fourths of the Phox2b- neurons (31/42) were spontaneously active. K+ channel and persistent Na+ current blockers affected the firing of LF and HF neurons. The majority of Phox2b+ (35/46) and half of the Phox2b- neurons (19/40) did not respond to stimulations of the mesencephalic trigeminal nucleus, the trigeminal tract, and the principal sensory trigeminal nucleus. Biocytin labeling revealed that about half of the Phox2b+ (5/12) and Phox2b- RdV neurons (5/10) send their axons to the trigeminal motor nucleus. These results suggest that Phox2b+ RdV neurons have distinct neurotransmitter phenotypes and firing properties from Phox2b- RdV neurons and might play important roles in feeding-related functions including suckling and possibly mastication.

Coordinated control of the tongue during suckling-like activity and respiration.

The tongue can move freely and is important in oral motor functions. Tongue movement must be coordinated with movement of the hyoid, mandible, and pharyngeal wall, to which it is attached. Our previous study using isolated brainstem-spinal cord preparations showed that application of N-methyl-D-aspartate induces rhythmic activity in the hypoglossal nerve that is coincident with rhythmic activity in the ipsilateral trigeminal motor nerve. Partial or complete midline transection of the preparation only abolishes activity in the trigeminal motor nerve; therefore, the neuronal network contributing to coordinated activity of the jaw/tongue muscles is located on both sides of the preparation and sends motor commands to contralateral trigeminal motoneurons. Arterially perfused decerebrate rat preparations exhibit stable inspiratory activity in the phrenic nerve, with efferent nerves innervating the upper airway muscles (the hypoglossal nerve, a branch of the cervical spinal nerve, the external branch of the superior laryngeal nerve, and the recurrent laryngeal nerve) under normocapnic conditions (5% CO2). During hypercapnia (8% CO2), pre-inspiratory discharges appear in all nerves innervating upper airway muscles. Such coordinated activity in the pre-inspiratory phase contributes to dilation of the upper airway and improves hypercapnia.

Coordinated Respiratory Motor Activity in Nerves Innervating the Upper Airway Muscles in Rats.

Maintaining the patency of the upper airway during breathing is of vital importance. The activity of various muscles is related to the patency of the upper airway. In the present study, we examined the respiratory motor activity in the efferent nerves innervating the upper airway muscles to determine the movements of the upper airway during respiration under normocapnic conditions (pH = 7.4) and in hypercapnic acidosis (pH = 7.2). Experiments were performed on arterially perfused decerebrate rats aged between postnatal days 21-35. We recorded the efferent nerve activity in a branch of the cervical spinal nerve innervating the infrahyoid muscles (CN), the hypoglossal nerve (HGN), the external branch of the superior laryngeal nerve (SLN), and the recurrent laryngeal nerve (RLN) with the phrenic nerve (PN). Inspiratory nerve discharges were observed in all these nerves under normocapnic conditions. The onset of inspiratory discharges in the CN and HGN was slightly prior to those in the SLN and RLN. When the CO2 concentration in the perfusate was increased from 5% to 8% to prepare for hypercapnic acidosis, the peak amplitudes of the inspiratory discharges in all the recorded nerves were increased. Moreover, hypercapnic acidosis induced pre-inspiratory discharges in the CN, HGN, SLN, and RLN. The onset of pre-inspiratory discharges in the CN, HGN, and SLN was prior to that of discharges in the RLN. These results suggest that the securing of the airway that occurs a certain time before dilation of the glottis may facilitate ventilation and improve hypercapnic acidosis.

Postnatal changes in glutamatergic inputs of jaw-closing motoneuron dendrites.

Dendrites of masseter (jaw-closing) motoneurons (MMNs) are well developed and ramify extensively throughout the trigeminal motor nucleus and often extend into the adjacent reticular formation. It is possible that the dendrites have active properties, which are altered with the development of the orofacial musculoskeletal system. Thus, we examined the changes in somatic voltage responses evoked by photostimulation of the MMN dendrites by laser photolysis of caged glutamate from postnatal day (P) 2-5 and 9-12 rats. We photostimulated 39 spots that were arranged around each recorded neuron in a concave shape and found that the dendritic stimulation induced somatic depolarization in the presence of tetrodotoxin in all MMNs. With increasing photostimulation intensity, the responses grew in amplitude up to a certain threshold, where a step-like increase in amplitude occurred. In 75% of P2-5 MMNs, the step-like increase in amplitude, which was blocked by 20µM D(-)-2-amino-5-phosphonovaleric acid application, corresponded to the NMDA spikes/plateau potentials. In contrast, at P9-12 the responses became significantly smaller in amplitude and shorter in duration and only one neuron out of 12 MMNs showed NMDA spikes/plateau potentials. These results suggest that the glutamatergic responses evoked by photostimulation of the MMN dendrites change during the first two postnatal weeks, and these changes may be involved in the transition from suckling to chewing during postnatal development.

Effects of citalopram on jaw-closing muscle activity during sleep and wakefulness in mice.

In this study, we investigated the effects of chronic administration of the selective serotonin reuptake inhibitor (SSRI) citalopram on sleep/wake cycles and masseter (jaw-closing) muscle electromyogram (EMG) activity over a 24-h period. From the dark to the light period, the times of wakefulness decreased, while those of non-rapid eye movement (NREM) and REM sleep increased. Citalopram did not induce major alterations in the temporal changes of sleep-wake distributions, except for leading to a decrease in the time of NREM sleep during the light period and an increase in the durations of REM sleep episodes. Moreover, citalopram did not modify mean masseter EMG activity during any of the vigilance states and did not affect the temporal changes related to the shifts between dark/light periods. However, citalopram increased the time engaged in masseter EMG activation during NREM sleep in the second and the first halves of the dark and light periods, respectively. These results suggest that chronic citalopram treatment does not affect the temporal changes of sleep-wake distributions, but has a limited facilitatory influence that fails to increase the number of epochs of high levels of masseter muscle activation.

Dark/light transition and vigilance states modulate jaw-closing muscle activity level in mice.

Bruxism is associated with an increase in the activity of the jaw-closing muscles during sleep and wakefulness. However, the changes in jaw-closing muscle activity across states of vigilance over a 24-h period are unclear. In this study, we investigated the effects of dark/light transition and sleep/wake state on EMG activity of the masseter (jaw-closing) muscle in comparison with the activity of the upper trapezius muscle (a neck muscle) over a 24-h period in mice. The activities of the masseter and neck muscles during wakefulness were much greater than during non-REM and REM sleep. In contrast, the activities of both muscles slightly, but significantly, decreased during the transition period from dark to light. Histograms of masseter activity during wakefulness and non-REM sleep showed bimodal distributions, whereas the neck muscle showed unimodal activation in all states. These results suggest that the activities of jaw-closing and neck muscles are modulated by both sleep/wake state and dark/light transition, with the latter being to a lesser degree. Furthermore, even during non-REM sleep, jaw-closing muscles display bimodal activation, which may contribute to the occurrence of exaggerated aberrant muscle activity, such as sleep bruxism.

Involvement of histaminergic inputs in the jaw-closing reflex arc.

Histamine receptors are densely expressed in the mesencephalic trigeminal nucleus (MesV) and trigeminal motor nucleus. However, little is known about the functional roles of neuronal histamine in controlling oral-motor activity. Thus, using the whole-cell recording technique in brainstem slice preparations from Wistar rats aged between postnatal days 7 and 13, we investigated the effects of histamine on the MesV neurons innervating the masseter muscle spindles and masseter motoneurons (MMNs) that form a reflex arc for the jaw-closing reflex. Bath application of histamine (100 µM) induced membrane depolarization in both MesV neurons and MMNs in the presence of tetrodotoxin, whereas histamine decreased and increased the input resistance in MesV neurons and MMNs, respectively. The effects of histamine on MesV neurons and MMNs were mimicked by an H1 receptor agonist, 2-pyridylethylamine (100 µM). The effects of an H2 receptor agonist, dimaprit (100 µM), on MesV neurons were inconsistent, whereas MMNs were depolarized without changes in the input resistance. An H3 receptor agonist, immethridine (100 µM), also depolarized both MesV neurons and MMNs without changing the input resistance. Histamine reduced the peak amplitude of postsynaptic currents (PSCs) in MMNs evoked by stimulation of the trigeminal motor nerve (5N), which was mimicked by 2-pyridylethylamine but not by dimaprit or immethridine. Moreover, 2-pyridylethylamine increased the failure rate of PSCs evoked by minimal stimulation and the paired-pulse ratio. These results suggest that histaminergic inputs to MesV neurons through H1 receptors are involved in the suppression of the jaw-closing reflex although histamine depolarizes MesV neurons and/or MMNs.

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus.

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

Coordination of NMDA-induced rhythmic activity in the trigeminal and hypoglossal nerves of neonatal mice in vitro.

Suckling is a rhythmic jaw movement that is symmetrical on the left and right side and is highly coordinated with tongue movement. Thus, we investigated the neuronal mechanisms of the left/right and jaw/tongue coordinations during N-methyl-d-aspartate (NMDA)-induced fictive suckling using isolated brainstem-spinal cord preparations obtained from neonatal mice. We observed synchronous low-frequency rhythmic activity in the left/right trigeminal motor nerves, which differed from respiration, and high-frequency rhythmic trigeminal activity, which was side-independent. The low-frequency rhythmic trigeminal activity was also synchronized with the hypoglossal nerve activity. After a complete midline separation of the preparation or a partial midline transection of the brainstem from the anterior inferior cerebellar artery to the junction of the vertebral artery, the low-frequency rhythmic trigeminal activity disappeared, whereas the high-frequency rhythmic trigeminal activity and low-frequency rhythmic hypoglossal activity still remained. These results suggest that the neuronal network that generates low-frequency rhythmic activity likely contributes to the synchronized activity of the left/right jaw muscles and to the jaw/tongue muscles, where it sends its command to the trigeminal motoneurons mainly via the commissural pathway that crosses the transected midline region. Such a neuronal network may underlie the coordinated movements of the jaw and tongue during suckling.