Effect of artificial rearing on the contractile properties and myosin heavy chain isoforms of developing rat tongue musculature

Histological and contractile changes in the genioglossus muscle after nasal obstruction in growing rats

The aim of the study was to address the genioglossus muscle physiological and histological changes after unilateral nasal obstruction in growing rats. Fifty-four 6-day-old male Wistar albino rats were randomly divided into control (n = 27) and experimental (n = 27) groups. Unilateral nasal obstruction was performed at 8 days old. Contractile properties of the genioglossus whole muscle were measured at 5-, 7- and 9-week-old, including the twitch and tetanic forces, contraction time, half-decay time, and fatigue index. The histological characteristics of the genioglossus were also evaluated at 5-, 7- and 9-week-old, analyzing the myosin heavy chain composition of the slow, fast, IIa and IIb muscle fiber type, by measuring the number, rate, diameter and cross-sectional area. The maximal twitch force, and tetanic force at 60 Hz and 80 Hz force was significantly increased at all ages after nasal obstruction. The fatigue index was decreased at 5 weeks-old after nasal obstruction. The diameter and cross-sectional area of the fast, IIa and IIb muscle fiber types were increased at 7 and 9 weeks after nasal obstruction, while only the diameter of IIa type and cross-sectional area of IIb type were increased at 5 weeks-old after nasal obstruction. Nasal obstruction during growth affects the whole genioglossus muscle contractile properties and histological characteristics, increasing its force, the diameter and area of its muscle fibers. These changes in the genioglossus muscle may affect the normal growth, development and function of the craniofacial complex.

Tongue muscle contractile, fatigue, and fiber type properties in rats

The intrinsic and extrinsic tongue muscles manipulate the position and shape of the tongue and are activated during many oral and respiratory behaviors. In the present study, in 6-mo-old Fischer 344 rats, we examined mechanical and fatigue properties of tongue muscles in relation to their fiber type composition. In an ex vivo preparation, isometric force and fatigue was assessed by direct muscle stimulation. Tongue muscles were frozen in melting isopentane and transverse sections cut at 10 µm. In hematoxylin-eosin (H&E)-stained muscle sections, the relative fractions of muscle versus extracellular matrix were determined. Muscle fibers were classified as type I, IIa and IIx, and/or IIb based on immunoreactivity to specific myosin heavy chain isoform antibodies. Cross-sectional areas (CSAs) and proportions of different fiber types were used to calculate their relative contribution to total muscle CSAs. We found that the superior and inferior longitudinal intrinsic muscles (4.4 N/cm²) and genioglossus muscle (3.0 N/cm²) generated the greatest maximum isometric force compared with the transversalis muscle (0.9 N/cm²). The longitudinal muscles and the transversalis muscle displayed greater fatigue during repetitive stimulation consistent with the greater relative contribution of type IIx and/or IIb fibers. By contrast, the genioglossus, comprising a higher proportion of type I and IIa fibers, was more fatigue resistant. This study advances our understanding of the force, fatigue, and fiber type-specific properties of individual tongue musculature. The assessments and approach provide a readily accessible muscular readout for scenarios where motor control dysfunction or tongue weakness is evident.NEW & NOTEWORTHY For the individual tongue muscles, relatively little quantification of uniaxial force, fatigue, and fiber type-specific properties has been documented. Here, we assessed uniaxial-specific force generation, fatigability, and muscle fiber type-specific properties in the superior and inferior longitudinal muscles, the transversalis, and the genioglossus in Fischer 344 rats. The longitudinal muscles produced the greatest isometric tetanic-specific forces. The genioglossus was more fatigue resistant and comprised higher proportions of I and IIa fibers.

Abnormal Nutritive Sucking as an Indicator of Neonatal Brain Injury

A term neonate is born with the ability to suck; this neuronal network is already formed and functional by 28 weeks gestational age and continues to evolve into adulthood. Because of the necessity of acquiring nutrition, the complexity of the neuronal network needed to suck, and neuroplasticity in infancy, the skill of sucking has the unique ability to give insight into areas of the brain that may be damaged either during or before birth. Interpretation of the behaviors during sucking shows promise in guiding therapies and how to potentially repair the damage early in life, when neuroplasticity is high. Sucking requires coordinated suck-swallow-breathe actions and is classified into two basic types, nutritive and non-nutritive. Each type of suck has particular characteristics that can be measured and used to learn about the infant's neuronal circuitry. Basic sucking and swallowing are present in embryos and further develop to incorporate breathing ex utero. Due to the rhythmic nature of the suck-swallow-breathe process, these motor functions are controlled by central pattern generators. The coordination of swallowing, breathing, and sucking is an enormously complex sensorimotor process. Because of this complexity, brain injury before birth can have an effect on these sucking patterns. Clinical assessments allow evaluators to score the oral-motor pattern, however, they remain ultimately subjective. Thus, clinicians are in need of objective measures to identify the specific area of deficit in the sucking pattern of each infant to tailor therapies to their specific needs. Therapeutic approaches involve pacifiers, cheek/chin support, tactile, oral kinesthetic, auditory, vestibular, and/or visual sensorimotor inputs. These therapies are performed to train the infant to suck appropriately using these subjective assessments along with the experience of the therapist (usually a speech therapist), but newer, more objective measures are coming along. Recent studies have correlated pathological sucking patterns with neuroimaging data to get a map of the affected brain regions to better inform therapies. The purpose of this review is to provide a broad scope synopsis of the research field of infant nutritive and non-nutritive feeding, their underlying neurophysiology, and relationship of abnormal activity with brain injury in preterm and term infants.

Comparison of tongue muscle characteristics of preterm and full term infants during nutritive and nonnutritive sucking

BACKGROUND

Independent oral feeding requires coordination of suck, swallow and breathe and the lingual musculature plays a significant role in this coordinative action. However, clinical benchmarks of lingual function fundamental to successful feeding have not been explored.

AIMS

The present study tests our model for quantifying infant lingual force and size and compares the muscle measures of interest in two cohorts: healthy full-term infants (FT) (N=5) and healthy preterm infants (PT) (N=6).

METHOD

Using an instrumented pacifier and bottle nipple, we determined the resultant compressive forces applied to the nipple by the tongue during nutritive (NS) and nonnutritive sucking (NNS). Muscle size was estimated from measures of posterior tongue thickness using ultrasonography.

RESULTS

After controlling for weight and post menstrual age, statistically significant differences were found between FT and PT infants beginning to feed for NNS frequency and NS tongue force. Clinically significant differences were detected for NNS tongue force and posterior tongue thickness. Additionally, PT infants demonstrated a significant difference in mean tongue force between NS and NNS and FT infants did not. FT infants demonstrated a significant difference in mean frequency between NS and NNS and PT infants did not. Linear regression indicated that mean posterior tongue thickness alone predicted 55% of the variance in NS force.

CONCLUSIONS

Results demonstrate the feasibility of our approach and suggest that infant tongue muscle characteristics necessary for successful feeding differ between healthy full term infants and preterm infants who are beginning oral feeding.

Tongue anatomy and physiology, the scientific basis for a novel targeted neurostimulation system designed for the treatment of obstructive sleep apnea

INTRODUCTION

 Obstructive sleep apnea (OSA) is a chronic condition that affects millions adults. The effective standard treatment is positive airway pressure (PAP). However, approximately half of the patients that are prescribed PAP are unable or unwilling to comply with this therapy. Untreated OSA ultimately leads to very serious comorbidities. An alternative therapy for this patient population, therefore, is desirable. Hypoglossal nerve (HGN) stimulation is under investigation by multiple groups as a possible alternative therapy for OSA.

OBJECTIVE

To understand the underlying mechanisms of actions related to HGN stimulation, and the implication of this knowledge for specifying and designing a neurostimulation system for the treatment of OSA.

RESULTS

 Loss of lingual and pharyngeal tone within a narrow airway is the primary mechanism for OSA. Posterior and anterior tongues are different in their anatomy and physiology. Muscle fibers in the posterior tongue are predominantly fatigue resistant that are responsible for the long sustained tonic activities required for maintaining the tongue's position and preventing its mass from falling into the retroglossal airway. The human tongue is a muscular hydrostat and hence would benefit from a sophisticated HGN stimulation system that is capable of achieving a concerted spatio-temporal interplay of multiple lingual muscles, including retrusors.

CONCLUSION

 Targeted neurostimulation of the proximal HGN presents as a viable system approach that is far more versatile and physiologic and quite different than prior systems.

Orosensory deprivation alters taste-elicited c-Fos expression in the parabrachial nucleus of neonatal rats

In the present study we examined the effects of neonatal orosensory deprivation on taste-elicited gustatory activity in the rat parabrachial nucleus (PBN) using the functional anatomical marker c-Fos. Animals in three groups (GG, GO and GM) received gastric cannula implantation surgery on postnatal day 9 (P9). Animals in the fourth group (MR) did not receive any surgery. GG rats were fed by infusion of artificial milk directly into the stomach. GO rats were fed by intraoral infusion of artificial milk. GM and MR rats were reared by their mother with free access to mother's milk, water and rat chow. Rats from all groups were similar in body weight and length by P21. On P21 rats in all groups were intraorally presented with 0.5M sucrose solution and the brains were extracted and processed for c-Fos immunohistochemistry. Taste-elicited c-Fos expression in both the gustatory waist area, and the external lateral subnucleus of the PBN in rats in the GG group was significantly more robust than in the other three groups. These findings suggest a substantial alteration in orosensory-evoked neuronal response in this nucleus, due to sensory or motor deprivation during a critical developmental stage.

Effects of artificial rearing on contractile properties of genioglossus muscle in Sprague–Dawley rat

Mammals suckle from a nipple during the early neonatal period to obtain nourishment. The genioglossus muscle helps position and move the tongue for efficient suckling. The purpose of this study was to examine the contractile properties and myosin heavy chain (MHC) phenotype of the genioglossus following an early period of artificial rearing, which reduced nutritive suckling. Beginning at 3 days of age, rats were fed via gastric cannula until postnatal day 14 (P14). At P14, artificially reared rat pups were either allowed to grow to postnatal day 42 (P42) or anaesthetised and prepared for experimentation. Comparisons were made between artificially reared and dam reared groups at P14 and P42. At P14 maximum tetanic tension and fatigue index were lower in the artificially reared group than the dam reared group. By P42, artificially reared rats had a higher fatigue index and lower percentage of MHCIIa than dam reared rats. The artificial rearing technique employed in this study was adequate to produce chronic changes in fatigue resistance and MHC distribution in genioglossus muscle of rat; the changes observed here may be similar to changes that occur in premature human infants requiring early artificial feedings.

Contractile properties and myosin heavy chain composition of rat tongue retrusor musculature show changes in early adulthood after 19 days of artificial rearing

Previously, we showed that artificial rearing using the “pup in a cup” model results in decreased tongue activity and caused some minor alterations in the tongue retrusor musculature. However, the artificial rearing time frame previously chosen was brief (11 days). The purpose of the present investigation was to extend the artificial rearing period from postnatal days 3 to 21 (P21) to determine whether significant alterations occur as a result of this reduced tongue use. Several changes in contractile properties due to the artificial rearing process were observed, which fully recovered by postnatal days 41 to 42 (P41–2). These changes included a shorter twitch contraction time, shorter twitch half-relaxation time, and decreased fatigue resistance. Styloglossus muscle exhibited more neonatal myosin heavy chain (MHC) isoform at P21 for the artificially reared (AR) group. Changes that were persistent at P41–2 were also observed. Maximum tetanic tension was lower for the AR group at P21 and P41–2 compared with their dam-reared counterparts. Twitch tension was also lower by P41–2 in the AR group. At P41–2, the AR group exhibited an increase in MHC IIa and a decrease in MHC IIb for the styloglossus muscle. In addition, the AR group exhibited a decreased MHC IIb for the long head of the biceps brachii at P41–2. Our results are similar to other models of hindlimb immobilization and suspension. By extending our artificial rearing period, this reduced tongue activity induced acute changes and alterations in the tongue retrusor musculature that persisted into early adulthood.

Effects of 12 days of artificial rearing on morphology of hypoglossal motoneurons innervating tongue retrusors in rat

The purpose of this study was to examine the influence of reduced tongue activity by artificial rearing on the morphology of motoneurons innervating the extrinsic tongue retrusors. Artificially reared rat pups were fed via gastric cannula from postnatal day 3 to postnatal day 14. Artificially reared animals and dam-reared controls had cholera toxin (subunit B) conjugate of horseradish peroxidase injected into the styloglossus to label motoneurons innervating hyoglossus and styloglossus on postnatal day 13 and postnatal day 59. Following perfusion on postnatal days 14 and 60, serial transverse sections treated with tetramethyl benzidine and counterstained neutral red were used to analyze motoneuron morphology. The shorter diameter of hyoglossus motoneurons increased with age for the dam-reared but not the artificially reared group. There was a tendency for a similar pattern for styloglossus motoneurons across the two rearing groups. The changes in form factor reflected the changes in shorter diameter for both motoneuron pools. Therefore, reducing suckling activity during normal postnatal development leads to diminished motoneuron somal growth in rats. This may also be the case in premature infants necessarily fed artificially.

Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of -61.8 mV. The input resistance and time constant diminished from 82.9-53.1 M omega and from 9.4-4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1-5 days postnatally). The motoneurons' rheobase gradually decayed from 0.29-0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3-1.2 ms, and the medium afterhyperpolarization from 184.8-94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.

Inhibitory synaptic transmission governs inspiratory motoneuron synchronization

Neurons within the intact respiratory network produce bursts of action potentials that cause inspiration or expiration. Within inspiratory bursts, activity is synchronized on a shorter timescale to generate clusters of action potentials that occur in a set frequency range and are called synchronous oscillations. We investigated how GABA and glycine modulate synchronous oscillations and respiratory rhythm during postnatal development. We recorded inspiratory activity from hypoglossal nerves using the in vitro rhythmically active mouse medullary slice preparation from P0-P11 mice. Average oscillation frequency increased with postnatal development, from 17 +/- 12 Hz in P0-P6 mice (n = 15) to 38 +/- 7 Hz in P7-P11 mice (n = 37) (P < 0.0001). Bath application of GABAA and GlyR antagonists significantly reduced oscillation power in neonates (P0-P6) and juveniles (P7-P10) and increased peak integrated activity in both age groups. To test whether elevating slice excitability is sufficient to reduce oscillation power, Substance P was bath applied alone. Substance P, although increasing peak integrated activity, had no significant effect on oscillation power. Prolonging the time course of GABAergic synaptic currents with zolpidem decreased the median oscillation frequency in P9-P10 mouse slices. These data demonstrate that oscillation frequency increases with postnatal development and that both GABAergic and glycinergic transmission contribute to synchronization of activity. Further, the time course of synaptic GABAergic currents is a determinant of oscillation frequency.

Phenotype and contractile properties of mammalian tongue muscles innervated by the hypoglossal nerve

The XIIth cranial nerve plays a role in chewing, respiration, suckling, swallowing, and speech [Lowe, A.A., 1981. The neural regulation of tongue movements. Prog. Neurobiol. 15, 295-344.]. The muscles innervated by this nerve are functionally subdivided into three categories: those causing protrusion, retrusion, and changing the shape of the tongue. Myosin heavy chain (MHC) II isoform makes up the majority of the MHC phenotype with some variability among mammalian species and some evidence suggests between genders. In addition, there are regional differences in fiber type within some of these muscles that suggest functional compartmentalization. The transition from developmental MHC isoforms to their adult phenotype appears to vary not only from muscle to muscle but also from species to species. Motor units within this hypoglossal motor system can be categorized as predominantly fast fatigue resistant. Based on twitch contraction time and fatigue index, it appears that hypoglossal innervated muscles are more similar to fast-twitch muscles innervated by spinal nerves than, for example, extraocular muscles.

Postnatal development of hypoglossal motoneurons that innervate the hyoglossus and styloglossus muscles in rat

Postnatal development of hyoglossus and styloglossus motoneurons was studied in this investigation of the hypoglossal nucleus. Our findings show separate and distinct locations for hyoglossus and styloglossus motoneurons within the retrusor (dorsal) subdivision of the hypoglossal nucleus for all age groups. Hyoglossus and styloglossus motoneuron cross-sectional area reached their adult size at different times (by weeks 2 and 3, respectively). Cell roundness, as measured by form factor (measure of cell perimeter relative to its area), decreased with advancing postnatal age for both populations of motoneurons. Differences in the direction of the dendritic projection between hyoglossus and styloglossus motoneurons were found. Hyoglossus and styloglossus motoneuron development was compared to genioglossus motoneuron postnatal development.