A hierarchy of neural control of mastication in the rat

High fat diet disrupts endoplasmic reticulum calcium homeostasis in the rat liver

BACKGROUND & AIMS

Disruption to endoplasmic reticulum (ER) calcium homeostasis has been implicated in obesity, however, the ability to longitudinally monitor ER calcium fluctuations has been challenging with prior methodologies. We recently described the development of a Gaussia luciferase (GLuc)-based reporter protein responsive to ER calcium depletion (GLuc-SERCaMP) and investigated the effect of a high fat diet on ER calcium homeostasis.

METHODS

A GLuc-based reporter cell line was treated with palmitate, a free fatty acid. Rats intrahepatically injected with GLuc-SERCaMP reporter were fed a cafeteria diet or high fat diet. The liver and plasma were examined for established markers of steatosis and compared to plasma levels of SERCaMP activity.

RESULTS

Palmitate induced GLuc-SERCaMP release in vitro, indicating ER calcium depletion. Consumption of a cafeteria diet or high fat pellets correlated with alterations to hepatic ER calcium homeostasis in rats, shown by increased GLuc-SERCaMP release. Access to ad lib high fat pellets also led to a corresponding decrease in microsomal calcium ATPase activity and an increase in markers of hepatic steatosis. In addition to GLuc-SERCaMP, we have also identified endogenous proteins (endogenous SERCaMPs) with a similar response to ER calcium depletion. We demonstrated the release of an endogenous SERCaMP, thought to be a liver esterase, during access to a high fat diet. Attenuation of both GLuc-SERCaMP and endogenous SERCaMP was observed during dantrolene administration.

CONCLUSIONS

Here we describe the use of a reporter for in vitro and in vivo models of high fat diet. Our results support the theory that dietary fat intake correlates with a decrease in ER calcium levels in the liver and suggest a high fat diet alters the ER proteome. Lay summary: ER calcium dysregulation was observed in rats fed a cafeteria diet or high fat pellets, with fluctuations in sensor release correlating with fat intake. Attenuation of sensor release, as well as food intake was observed during administration of dantrolene, a drug that stabilizes ER calcium. The study describes a novel technique for liver research and provides insight into cellular processes that may contribute to the pathogenesis of obesity and fatty liver disease.

Descriptive and functional neuroanatomy of locus coeruleus-noradrenaline-containing neurons involvement in bradykinin-induced antinociception on principal sensory trigeminal nucleus

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.

Jaw movement responses to electrical stimulation of different parts of the human temporalis muscle

Previous EMG studies have provided indications for the differential activation of the human temporalis muscle. However, in these studies the contribution of different parts of the temporalis muscle could not be separated from the contributions of other muscles, since contraction was performed by voluntary activation. Therefore, the aim of this study was to examine functional differences among various parts of the human temporalis muscle by means of recording the incisal point movement response to electrical stimulation of parts of the muscle. Five healthy male subjects participated in this study. Three locations (anterior, middle, and posterior temporalis muscle) were stimulated, by means of monopolar wire electrodes and rectangular pulses. The insertion depths of the stimulation electrodes were determined by means of magnetic resonance images. Stimulation was performed in four jaw positions (resting position, 50% maximum mouth opening, 1 cm to the left, and 1 cm to the right). Movement responses to stimulation of the different muscle parts were recorded with the OKAS-3D jaw movement analysis system. The movement responses were expressed in polar coordinates. The variation in the direction of the jaw movement response was partly explained by the factors 'stimulation location' and 'jaw position' (ANOVA, p < 0.001). When the stimulation location shifted in an antero-posterior direction, the response changed from a vertical-lateral incisal point movement to a lateral-posterior movement with a smaller vertical component. The jaw position during stimulation also influenced the movement response. A functional subdivision of the temporalis muscle into at least three parts is favored.

The control of multi-muscle systems: human jaw and hyoid movements

A model is presented of sagittal plane jaw and hyoid motion based on the lambda model of motor control. The model, which is implemented as a computer simulation, includes central neural control signals, position- and velocity-dependent reflexes, reflex delays, and muscle properties such as the dependence of force on muscle length and velocity. The model has seven muscles (or muscle groups) attached to the jaw and hyoid as well as separate jaw and hyoid bone dynamics. According to the model, movements result from changes in neurophysiological control variables which shift the equilibrium state of the motor system. One such control variable is an independent change in the membrane potential of alpha-motoneurons (MNs); this variable establishes a threshold muscle length (lambda) at which MN recruitment begins. Motor functions may be specified by various combinations of lambda s. One combination of lambda s is associated with the level of coactivation of muscles. Others are associated with motions in specific degrees of freedom. Using the model, we study the mapping between control variables specified at the level of degrees of freedom and control variables corresponding to individual muscles. We demonstrate that commands can be defined involving linear combinations of lambda change which produce essentially independent movements in each of the four kinematic degrees of freedom represented in the model (jaw orientation, jaw position, vertical and horizontal hyoid position). These linear combinations are represented by vectors in lambda space which may be scaled in magnitude. The vector directions are constant over the jaw/hyoid workspace and result in essentially the same motion from any workspace position. The demonstration that it is not necessary to adjust control signals to produce the same movements in different parts of the workspace supports the idea that the nervous system need not take explicit account of musculo-skeletal geometry in planning movements.

The equilibrium point hypothesis and its application to speech motor control

In this paper, we address a number of issues in speech research in the context of the equilibrium point hypothesis of motor control. The hypothesis suggests that movements arise from shifts in the equilibrium position of the limb or the speech articulator. The equilibrium is a consequence of the interaction of central neural commands, reflex mechanisms, muscle properties, and external loads, but it is under the control of central neural commands. These commands act to shift the equilibrium via centrally specified signals acting at the level of the motoneurone (MN) pool. In the context of a model of sagittal plane jaw and hyoid motion based on the lambda version of the equilibrium point hypothesis, we consider the implications of this hypothesis for the notion of articulatory targets. We suggest that simple linear control signals may underlie smooth articulatory trajectories. We explore as well the phenomenon of intraarticulator coarticulation in jaw movement. We suggest that even when no account is taken of upcoming context, that apparent anticipatory changes in movement amplitude and duration may arise due to dynamics. We also present a number of simulations that show in different ways how variability in measured kinematics can arise in spite of constant magnitude speech control signals.

Influence of stress and occlusal interference on the EMG activity of some masticatory muscles during a single mastication cycle

The aim of the study was to investigate the influence of stress on the open-close activity in mastication, using an animal model. For this, we have compared the single-motor activities of three muscles: anterior digastric (AD), anterior temporalis (AT) and the superficial masseter (MS) in different groups of rats classed according to emotional state and dental occlusion. The EMG graphs have been analysed by the method proposed by the Weijs & Dantuma (1975). The results show that a dominant action of the stress on the AD and a combined action of the stress and the occlusal state on AT and MS occur during a single mastication cycle. A suppression of the reflex myotatic inhibition for AT and MS occurred in the presence of an occlusal interference.

An electromyographic study of whether the digastric muscles are controlled by jaw-closing proprioceptors in man

Whether in the oral system the digastric muscles (which lack muscle spindles) are under the control of proprioceptive information from the masseter muscles (which contain muscle spindles) was investigated by analysing whether and how the masseters and digastrics showed coordinated behaviour during a static, forceful bite. Subjects were asked to maintain a 100-N force for 15 s with and without visual guidance; bite force exerted, and masseter and digastric electromyograms (EMGs) were recorded. Under visual guidance all subjects co-contracted their digastric muscles during the isometric bite. They held the force for a short time, followed by periods with fluctuations (peak-to-peak force amplitude about 15-20 N). Fluctuations in bite force correlated with the masseter EMGs, the maximum in the correlogram occurring at about -50 ms with the force lagging the masseter. In 75% of the subjects a significant periodic component in the masseter and in the force spectra was found at about 4 Hz. This was also seen in the amplitude spectra of the forces, which showed in 80% of the subjects a significant elevation between 7-10 Hz as well. No correlation between the digastric EMGs and the bite forces, and between the EMGs of masseter and digastric could be detected. Spectra of digastric EMGs showed no prominent maxima. When subjects were deprived of visual feedback, maxima at -50 ms in the cross-correlation functions of the masseters and the forces were reduced considerably; periodicities of +/- 250 ms disappeared.(ABSTRACT TRUNCATED AT 250 WORDS)

Timing in the movement of jaws, tongue, and hyoid during feeding in the hyrax, Procavia syriacus

The anatomical structures used during mammalian feeding are morphologically linked by their connecting musculature, suggesting a predictable timing relationship among their movements. Cine-x-ray (100 frames per second) was used to record feeding behavior in four adult hyraxes (Procavia syriacus), herbivores. Movement of jaws, tongue, and hyoid bone was cyclic, and prior to the first swallow cycle, cycle duration was constant through time for all structures. Minimum gape, beginning of forward movement of the tongue and hyoid, and beginning of intrinsic tongue expansion occurred simultaneously over a large number of cycles. However, maximum gape, maximum protrusion of hyoid, and maximum forward position of the tongue happened at statistically different points in time. After the first swallow, cycle duration increased. Most of the variation in cycle duration can be explained by variation in the opening or forward phase of movement; the closing and return phases are constant in duration. These results are a quantitative description of the coordination that exists during different feeding behaviors (ingestion, intraoral transport, mastication, and swallowing) in normal, freely functioning hyraxes. The patterns of phase duration differ from some results obtained by using anesthetized animals.

Effects of anaesthesia of human oral structures on masticatory performance and food particle size distribution

Ten persons with intact dentitions performed a series of 6 masticatory tests, employing 5, 10, 20, 40, 80 and 160 strokes to chew standard quantities of peanuts on their preferred chewing side for each of three test conditions: (1) before anaesthesia: (2) after maxillary anaesthesia; and, (3) after maxillary and mandibular anaesthesia (unilateral). A seventh test employing 20 strokes was also repeated on the non-anaesthetized contralateral side. The chewed food was sieved through 5, 10, 20, 40, 80 and 100 mesh screens. The percentage of the ratio of the volume of peanuts that passed through the sieve and the total volume of recovered food provided the performance score for the given sieve. The performance scores increased significantly with the number of stokes and dropped markedly after anaesthesia. The maximum reduction of 19.6% in the mean masticatory performance and 46% in the mean masticatory efficiency occurred after unilateral anaesthesia at 10 mesh particle size in the 20-stroke test. An average of 40 strokes was required after unilateral anaesthesia to achieve almost the same performance achieved with 20 strokes before anaesthesia. The regression slopes, derived from the 5, 10 and 20 mesh particle distributions showed that coarse particles were ground more rapidly than fine particles before anaesthesia. This preferential comminution became less evident after maxillary anaesthesia and was least evident after unilateral anaesthesia. The regression slope for the control peaked at 10 strokes as compared to 20 strokes after unilateral anaesthesia. Thus peripheral sensory impairment affects masticatory efficiency in dentate persons.

Treatment of muscle spasms with oral dantrolene sodium

A case of severe muscle spasms relieved by oral administration of dantrolene sodium is presented. The patient had undergone full-mouth extraction 2 days prior to onset of her spasms. The treatment consisted of intravenous diazepam followed by oral dantrolene sodium. The spasms lasted for approximately 10 days.

Comparison of automatic and voluntary chewing patterns and performance

Chewing, like respiration, is ordinarily performed as an automatic motor act, yet both can be voluntarily controlled. No in-depth analyses of voluntary chewing exist. Therefore, we have analyzed on a cycle-by-cycle basis voluntarily controlled chewing, and compared it with automatic chewing. We assessed the performance during voluntarily controlled chewing by obtaining constant error and variable error scores. Nine healthy adults with full dentition were subjects (Ss). Their three-dimensional jaw movements and movement times were derived from Kinesiograph recordings obtained while chewing a standard piece of gum on the right side of the mouth. Burst durations and onset latencies of masseter activity were obtained from surface-recorded EMGs. Frequency during automatic chewing was obtained from data recorded while the subject viewed a film. Next, the subject chewed in time with a metronome set at this "automatic" rate. Intrasubject variability among 30 consecutive chewing cycles during voluntary was less than during automatic chewing. In every S gape and ipsilateral jaw excursions and the variability of burst durations of masseter activity were less during voluntary than during automatic chewing, showing that both the spatial and temporal aspects of the two types of chewing differ significantly. Ss varied in ability to follow the metronome. A S's constant error might be small, yet his variable error might be large, as if feed-back-based corrections influenced cycle-to-cycle variability. Fast chewers had smaller constant and variable error scores than did slow chewers, suggesting a speed-accuracy relationship. In summary, both temporal and spatial aspects of voluntary chewing were modified compared with those of automatic chewing. During voluntarily controlled chewing, cycle-to-cycle variability was less compared with automatic chewing due to reductions in variability of occlusal phase and the masseter's burst durations, and total jaw excursions were less because gapes and ipsilateral deviations during closing were reduced.

The effect of infrahyoid-muscle tenotomy on orofacial motor activity in the rat

The effects of disturbed proprioceptive feedback from the infrahyoid muscles on the control of chewing and drinking were studied in the unrestrained rat. Rats were trained to lick and to eat in a fixed body position in front of a video camera. After training, the left and right sternohyoid, omohyoid and sternothyroid muscles were cut close to the hyoid bone. EMG-wire electrodes were inserted into the superficial masseter, the anterior and posterior digastic and sternhyoid muscles. EMG-recordings were made at the third, the tenth and the eighteenth day after tenotomy. After about a week the cut ends of the muscles became encapsulated in connective tissue and began to reattach to the hyoid bone; in some animals the sternohyoid muscle reattached to the trachea. In the period before reattachment the mean cycle duration for chewing (but not for drinking) increased. After reattachment the mean cycle duration returned to control values. The contraction patterns after tenotomy were similar to those of the controls during chewing and drinking. The mean burst duration of the sternohyoid increased after tenotomy during chewing but not drinking; it decreased in the digastric muscles and did not change in the masseter.(ABSTRACT TRUNCATED AT 250 WORDS)