Effects of short term forced oral breathing: physiological changes and structural adaptation of diaphragm and orofacial muscles in rats

Differential Recovery Patterns of the Maxilla and Mandible after Eliminating Nasal Obstruction in Growing Rats

Although nasal obstruction (NO) during growth causes maxillofacial growth suppression, it remains unclear whether eliminating the NO affects maxillary and mandibular growth differentially. We aimed to clarify whether eliminating NO can help regain normal maxillofacial growth and to determine the optimal intervention timing. Forty-two 4-week-old male Wistar rats were randomly divided into six groups. Their left nostril was sutured to simulate NO over different durations in the experimental groups; the sutures were later removed to resume nasal breathing. Maxillofacial morphology was assessed using microcomputed tomography. Immunohistochemical changes in hypoxia-inducible factor (HIF)-1α, osteoprotegerin (OPG), and receptor activator of nuclear factor kappa-B ligand (RANKL) of the condylar cartilage were evaluated to reveal the underlying mechanisms of these changes. Maxillary length was significantly lower in rats with NO for ≥5 weeks. In groups with NO for ≥7 weeks, the posterior mandibular length, ramus height, thickness of the hypertrophic cell layer in the condylar cartilage, HIF-1α levels, and RANKL levels were significantly lower and OPG levels and RANKL/OPG were significantly higher than those in the control group. Our findings suggest that eliminating NO is effective in regaining maxillofacial growth. Moreover, the optimal timing of intervention differed between the maxilla and mandible.

Nasal obstruction during the growth period modulates the Wnt/β-catenin pathway and brain-derived neurotrophic factor production in association with tyrosine kinase receptor B mRNA reduction in mouse hippocampus

There is accumulating evidence that nasal obstruction induces high‐level brain dysfunction, including memory and learning deficits. We previously demonstrated that unilateral nasal obstruction (UNO) during the growth period increases the expression of brain‐derived neurotrophic factor (BDNF). The expression of BDNF is regulated by the Wnt/β‐Catenin pathway, which is linked to neuronal differentiation, proliferation, and maintenance. However, little is known about whether Wnt3a protein expression could be an index for modulations analyses in the Wnt/β‐Catenin pathway caused by UNO during the growth period. This study aimed to investigate the effects of UNO during the growth period on the Wnt/β‐Catenin pathway in the hippocampus using combined behavioural, biochemical, and histological approaches. Male BALB/C mice were randomly divided into the control (CONT; n = 6) and experimental (UNO; n = 6) groups. Blood oxygen saturation (SpO₂) levels were measured, and a passive avoidance test was performed in mice aged 15 weeks. Brain tissues were subjected to immunohistochemistry, real‐time reverse transcription‐polymerase chain reaction, and western blot analysis. Compared with control mice, UNO mice had lower SpO₂ levels and exhibited memory/learning impairments during behavioural testing. Moreover, Wnt3a protein, BDNF mRNA, and tyrosine kinase receptor B (TrkB) mRNA expression levels were significantly lower in the hippocampus in the UNO group than in the CONT group. Our findings suggested that UNO during the growth period appeared to modulate the hippocampal Wnt/β‐catenin pathway and BDNF production in association with TrkB mRNA reduction, thereby resulting in memory and learning impairments.

Alveolar Bone Density Reduction in Rats Caused by Unilateral Nasal Obstruction

Background

Oral breathing can cause morphological changes in the oral and maxillofacial regions.

Aims

To investigate whether oral breathing affected structural changes in bone tissues.

Study Design

Animal experimentation.

Methods

A total of 48 8-day-old male Sprague−Dawley rats were divided into two groups: a breathing group and a sham (control) group. All Sprague−Dawley rats were killed at 7 weeks after unilateral nostril obstruction modeling. Then, structural changes in bone tissues were detected by micro-computed tomography, and the expression levels of receptor activator of nuclear factor-κB, osteoprotegerin, and receptor activator of nuclear factor-κB ligand in the signal pathway of bone metabolism within the local alveolar bone and serum of rats were detected by reverse transcription-quantitative polymerase chain reaction and Western blotting.

Results

The results showed that receptor activator of nuclear factor-κB ligand and receptor activator of nuclear factor-κB levels in bone tissues and serum in the oral breathing group were higher than those in the control group [Maxillary alveolar bone: receptor activator of nuclear factor-κB ligand (pRNA=0.009, pprotein=0.008), receptor activator of nuclear factor-κB (pRNA=0.008, pprotein=0.009); Mandibular alveolar bone: receptor activator of nuclear factor-κB ligand (pRNA=0.047, pprotein=0.042), receptor activator of nuclear factor-κB (pRNA=0.041, pprotein=0.007); Serum: receptor activator of nuclear factor-κB ligand (pRNA<0.001, pprotein<0.001), receptor activator of nuclear factor-κB (pRNA<0.001, pprotein<0.001)], along with decreased osteoprotegerin expression (Maxillary alveolar bone: pRNA=0.038, pprotein=0.048; Mandibular alveolar bone: pRNA<0.001, pprotein<0.001; Serum: pRNA=0.009, pprotein=0.006) and elevated receptor activator of nuclear factor-κB ligand/osteoprotegerin. Micro-computed tomography analysis indicated a significant difference in the level of bone volume fraction, as well as trabecular thickness in maxillary alveolar bone between the experimental and control groups (p=0.049, p=0.047). Meanwhile, trabecular thickness, and cortical thickness levels in mandibular alveolar bone also differed significantly between the experimental and control groups (p=0.043, p=0.024).

Conclusion

Structural changes of the respiratory system affect the alveolar bone structure and unilateral nasal obstruction may lead to a change in regional specific bone density.

Unilateral nasal obstruction alters sweet taste preference and sweet taste receptors in rat circumvallate papillae

Nasal obstruction causes mouth breathing, and affects the growth and development of craniofacial structures, muscle function in the stomatognathic system, and the taste perceptive system. However, the detailed mechanism underlying the effects of nasal obstruction on taste perception has not been fully elucidated. In this study, we investigated this mechanism using the two-bottle taste preference test, immunohistological analysis, and quantification of the mRNA expression of taste-related molecules in the circumvallate papillae. Neonatal male Wistar rats were divided randomly into control and experimental groups. Rats in the experimental group underwent unilateral nasal obstruction by cauterization of the external nostril at the age of 8 days. Arterial oxygen saturation (SpO₂) was recorded in awake rats using collar clip sensors. Taste preference for five basic taste solutions was evaluated. Immunohistochemical analysis and quantitative real-time polymerase chain reaction (RT-PCR) were conducted to evaluate the expressions of taste-related molecules in the taste cells of the circumvallate papillae. Body weights were similar between the two groups throughout the experimental period. The SpO₂ in the 7- to 12-week-old rats in the experimental group was significantly lower than that in the age-matched rats in the control group. In the two-bottle taste preference test, the sensitivities to sweet taste decreased in the experimental group. The mRNA expression of T1R2, T1R3, α-gustducin, and PLCβ2 was significantly lower in the experimental group than in the control group as determined by quantitative RT-PCR, and the immunohistochemical staining for α-gustducin and PLCβ2 was less prominent. These findings suggest that nasal obstruction may affect sweet taste perception via the reduced expression of taste-related molecules in the taste cells in rat circumvallate papillae.

Unilateral nasal obstruction affects motor representation development within the face primary motor cortex in growing rats

Postnatal growth is influenced by genetic and environmental factors. Nasal obstruction during growth alters the electromyographic activity of orofacial muscles. The facial primary motor area represents muscles of the tongue and jaw, which are essential in regulating orofacial motor functions, including chewing and jaw opening. This study aimed to evaluate the effect of chronic unilateral nasal obstruction during growth on the motor representations within the face primary motor cortex (M1). Seventy-two 6-day-old male Wistar rats were randomly divided into control (n = 36) and experimental (n = 36) groups. Rats in the experimental group underwent unilateral nasal obstruction after cauterization of the external nostril at 8 days of age. Intracortical microstimulation (ICMS) mapping was performed when the rats were 5, 7, 9, and 11 wk old in control and experimental groups (n = 9 per group per time point). Repeated-measures multivariate ANOVA was used for intergroup and intragroup statistical comparisons. In the control and experimental groups, the total number of positive ICMS sites for the genioglossus and anterior digastric muscles was significantly higher at 5, 7, and 9 wk, but there was no significant difference between 9 and 11 wk of age. Moreover, the total number of positive ICMS sites was significantly smaller in the experimental group than in the control at each age. It is possible that nasal obstruction induced the initial changes in orofacial motor behavior in response to the altered respiratory pattern, which eventually contributed to face-M1 neuroplasticity.NEW & NOTEWORTHY Unilateral nasal obstruction in rats during growth periods induced changes in arterial oxygen saturation (SpO₂) and altered development of the motor representation within the face primary cortex. Unilateral nasal obstruction occurring during growth periods may greatly affect not only respiratory function but also craniofacial function in rats. Nasal obstruction should be treated as soon as possible to avoid adverse effects on normal growth, development, and physiological functions.

Unilateral Nasal Obstruction during Later Growth Periods Affects Craniofacial Muscles in Rats

Nasal obstruction can occur at different life stages. In early stages of life the respiratory system is still under development, maturing during the growth period. Previous studies have shown that nasal obstruction in neonatal rats alters craniofacial function. However, little is known about the effects of nasal obstruction that develops during later growth periods. The aim of this study was to investigate the effects of nasal obstruction during later periods of growth on the functional characteristics of the jaw-opening reflex (JOR) and tongue-protruding muscles. In total, 102 6-day-old male Wistar rats were randomized into either a control or experimental group (both n = 51). In order to determine the appropriate timing of nasal obstruction, the saturation of arterial oxygen (SpO₂) was monitored at 8 days, and at 3, 5, 7, 9, and 11 weeks in the control group. Rats in the experimental group underwent unilateral nasal obstruction at the age of 5 weeks. The SpO₂ was monitored at 7, 9, and 11 weeks in the experimental group. The electromyographic responses of JOR and the contractile properties of the tongue-protruding muscles were recorded at 7, 9, and 11 weeks. In the control group, SpO₂ decreased until 5 weeks of age, and remained relatively stable until 11 weeks of age. The SpO₂ was significantly lower in the experimental group than in the control. In the experimental group, JOR changes included a longer latency and smaller peak-to-peak amplitude, while changes in the contractile properties of the tongue-protruding muscles included larger twitch and tetanic forces, and a longer half-decay time. These results suggest that nasal obstruction during later growth periods may affect craniofacial function.

Androgen modulation of Foxp1 and Foxp2 in the developing rat brain: impact on sex specific vocalization

Sex differences in vocal communication are prevalent in both the animals and humans. The mechanism(s) mediating gender differences in human language are unknown, although, sex hormones, principally androgens, play a central role in the development of vocalizations in a wide variety of animal species. The discovery of FOXP2 has added an additional avenue for exploring the origins of language and animal communication. The FOXP2 gene is a member of the forkhead box P (FOXP) family of transcription factors. Prior to the prenatal androgen surge in male fetuses, we observed no sex difference for Foxp2 protein levels in cultured cells. In contrast, 24 hours after the onset of the androgen surge, we found a sex difference for Foxp2 protein levels in cultured cortical cells with males having higher levels than females. Furthermore, we observed the potent nonaromatizable androgen dihydrotestosterone altered not only Foxp2 mRNA and protein levels but also Foxp1. Androgen effects on both Foxp2 and Foxp1 were found to occur in the striatum, cerebellar vermis, and cortex. Immunofluorescence microscopy and coimmunoprecipitation demonstrate Foxp2 and the androgen receptor protein interact. Databases for transcription factor binding sites predict a consensus binding motif for androgen receptor on the Foxp2 promoter regions. We also observed a sex difference in rat pup vocalization with males vocalizing more than females and treatment of females with dihydrotestosterone eliminated the sex difference. We propose that androgens might be an upstream regulator of both Foxp2 and Foxp1 expression and signaling. This has important implications for language and communication as well as neuropsychiatric developmental disorders involving impairments in communication.

It takes a mouth to eat and a nose to breathe: abnormal oral respiration affects neonates' oral competence and systemic adaptation

Mammalian, including human, neonates are considered to be obligate nose breathers. When constrained to breathe through their mouth in response to obstructed or closed nasal passages, the effects are pervasive and profound, and sometimes last into adulthood. The present paper briefly surveys neonates' and infants' responses to this atypical mobilisation of the mouth for breathing and focuses on comparisons between human newborns and infants and the neonatal rat model. We present the effects of forced oral breathing on neonatal rats induced by experimental nasal obstruction. We assessed the multilevel consequences on physiological, structural, and behavioural variables, both during and after the obstruction episode. The effects of the compensatory mobilisation of oral resources for breathing are discussed in the light of the adaptive development of oromotor functions.