Hibernating mammals exhibit remarkable reductions in metabolism, body temperature, and physical activity when faced with limited resources. Harnessing this hibernation phenotype has long been pursued for potential medical applications, spanning from obesity treatment to emergency critical care on Earth and even extended space travel. Yet, the precise biological mechanisms that enable natural hibernators to enter torpor remain incompletely understood. Recent progress in synthetic torpor techniques has provided valuable insights into how torpor may be induced, though many approaches still fall short of replicating the profound metabolic suppression characteristic of true hibernation. Multiple studies have identified a link between CO 2 dynamics and active metabolic suppression during torpor, showing that abrupt increases in CO 2 retention coincide with rapid, temperature-independent reductions in metabolism at the onset of torpor. Our recent work demonstrated that regulated changes in ventilation enable animals to deliberately retain or expel excess CO 2 at the onset of torpor entry and during arousal. We propose that the regulation of CO 2 within blood and tissues plays a pivotal role in driving the active metabolic suppression characteristic of hibernating species. In the present study, we subjected thirteen-lined ground squirrels transitioning into or out of torpor to elevated environmental CO 2 concentrations (3%, 5%, and 7%) to assess whether hypercapnia could enhance metabolic suppression during entrance, and impede the subsequent rise in metabolism during arousal from torpor. Hypercapnia was applied both at the onset of torpor entry and arousal, as well as two hours prior to these state transitions. Metabolic and temperature responses were monitored throughout to evaluate whether CO 2 flux plays a critical role in regulating the torpor–arousal cycle. Our findings revealed significant effects of hypercapnia on metabolic suppression and torpor dynamics. When high CO 2 (5% and 7%) was introduced at the onset of torpor entry, metabolism declined more rapidly, shortening overall entrance by approximately 40 minutes. During arousal, hypercapnic exposure reduced the peak metabolic rate by about 20%, but did not otherwise alter the progression of recovery. Pre-exposure to hypercapnia prior to torpor entry produced no additional changes in the likelihood of entering torpor or in the degree of metabolic suppression beyond those observed when exposure occurred at torpor onset. In contrast, pre-exposure to hypercapnia before arousal completely inhibited recovery, with animals remaining in torpor until CO 2 levels were normalized. These results underscore a critical role of CO 2 regulation in the torpor–arousal cycle, although the precise mechanisms by which CO 2 modulates metabolism remain unresolved. Funded by NIH R01HL142752 (JJW and TLB) and NSERC 22R87150 (WKM). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Substance P (SubP) and endomorphin-2 (Endo2) are co-localized presynaptically in vesicles of neurons adjacent to inspiratory rhythm-generating pre-Botzinger Complex (preBotC) neurons but the effects of co-released and Endo2 on respiratory motor control are not known. To address this question, SubP alone or a combination SubP and Endo2 (SubP/Endo2) were bath-applied in a sustained (15-min) or intermittent (5-min application, min washout, x3) pattern at 10-100 nM to neonatal rat brainstem-spinal cord preparations. During neuropeptide application, SubP/Endo2 co-applications generally attenuated SubP-induced increases in burst frequency decreases in burst amplitude. With respect to frequency plasticity (long-lasting increase in burst frequency 60 min post-neuropeptide application), SubP-induced frequency plasticity was increased with sustained Endo2 co-applications at 20 and 100 nM. Intermittent SubP/Endo2 co-applications tended to decrease the of frequency plasticity induced by intermittent SubP alone applications. SubP/Endo2 co-applications revealed potentially new functions for neurokinin-1 (NK1R) and mu-opioid (MOR) receptors on respiratory rhythm generating medullary neurons.

Premature and newborn infants often have prolonged apneas and are susceptible to bacterial infections that further disrupt breathing. Phoshodiesterase-4 (PDE4) inhibitor drugs increase inspiratory motor activity and appear to induce a long-lasting increase in inspiratory frequency ("frequency plasticity"). To test whether a PDE4 inhibitor drug induces frequency plasticity, neonatal rat brainstem-spinal cords were isolated and exposed to bath-applied roflumilast (10 min, 0.02-1.0 µM). Roflumilast acutely increased burst frequency and induced frequency plasticity in a concentration-dependent manner. Blockade of protein kinase A (PKA) or exchange protein activated by cAMP (EPAC) signaling pathways abolished the induction, but not the maintenance, of roflumilast-induced frequency plasticity. Brainstem-spinal cords isolated from neonatal rats injected with lipopolysaccharide (LPS, 0.1 mg/kg, 3 h prior) expressed frequency plasticity following bath-applied roflumilast at 0.05-0.5 µM, but not at lower concentrations. This shows that roflumilast-induced frequency plasticity is largely resistant to LPS-induced inflammation. Thus, roflumilast increases inspiratory burst frequency acutely and induces frequency plasticity even during ongoing inflammation, which could have important clinical implications.

Reduced respiratory neural activity elicits a rebound increase in phrenic amplitude once respiratory neural activity is restored, a form of spinal plasticity termed inactivity‐induced phrenic motor facilitation (iPMF). We tested the hypothesis that the zeta isoform of atypical PKC (PKCζ) is necessary within phrenic motor neurons (PMNs) for iPMF. Rats received bilateral intrapleural injections of siRNA (Sieck et al., 2012; EB Abstract #1147.2) targeting PKCζ (30ul of 100pmol siRNA/side) once daily for 3 days to knock down PKCζ expression specifically within phrenic motor neurons. Control rats received a non‐targeting siRNA. The fourth day, rats were anesthetized and mechanically ventilated; the phrenic nerve was prepared for electrophysiological recordings. After baseline phrenic amplitude was established, respiratory neural activity was reversibly suppressed for 30min by hyperventilating until phrenic activity ceased. 60min following restoration of baseline conditions, rats treated with control injections expressed significant iPMF (82±19 %baseline, p<0.05), whereas iPMF was significantly attenuated (13±10 %baseline, p>;0.05) in siPKCζ treated rats. PKCζ knockdown was confirmed within ventral cervical spinal segments containing PMNs with western blot analysis (−31±3 %control; p<0.05). These data indicate that expression of PKCζ specifically within PMNs is necessary for iPMF. NIH105511

Obstructive sleep apnea (OSA), a respiratory sleep disorder associated with cardiovascular diseases, is more prevalent in men. However, OSA occurrence in pregnant women rises to a level comparable to men during late gestation, creating persistent effects on both maternal and offspring health. The exact mechanisms behind OSA-induced cardiovascular diseases remain unclear, but inflammation and oxidative stress play a key role. Animal models using intermittent hypoxia (IH), a hallmark of OSA, reveal several pro-inflammatory signaling pathways at play in males, such as TLR4/MyD88/NF-κB/MAPK, miRNA/NLRP3, and COX signaling, along with shifts in immune cell populations and function. Limited evidence suggests similarities in pregnancies and offspring. In addition, suppressing these inflammatory molecules ameliorates IH-induced inflammation and tissue injury, providing new potential targets to treat OSA-associated cardiovascular diseases. This review will focus on the inflammatory mechanisms linking IH to cardiovascular dysfunction in males, pregnancies, and their offspring. The goal is to inspire further investigations into the understudied populations of pregnant females and their offspring, which ultimately uncover underlying mechanisms and therapeutic interventions for OSA-associated diseases.

Background: Locoregional anesthesia is an essential component of multimodal analgesic and anesthetic techniques. However, few are described in rabbits, especially those aimed at the cranial abdominal wall. The aim of this study was to describe the external oblique intercostal (EOI) block technique in rabbits and compare the spread of two injectate volumes across the paracostal region and cranial abdominal wall. Methods: Eight rabbit cadavers (16 hemithoraces) were randomized to receive ultrasound-guided injections between the external abdominal oblique and external intercostal muscles. Each rabbit received 0.25 mL/kg (LV) and 0.5 mL/kg (HV) of 1% new methylene blue with immediate tissue dissection. Site of needle insertion, spread of injectate, and number of stained intercostal spaces and nerves were assessed by a blinded investigator. Results: Injection points ranged from intercostal spaces 5-12. There were no differences between HV and LV groups in the number of covered intercostal spaces or dorsal, caudal, and total (cranial + caudal) spread. There was significantly more ventral and cranial spread in the HV group as well as staining of intercostal nerves. Conclusions and Clinical Relevance: Injection within the EOI fascial plane successfully stained intercostal nerves associated with the thoracic wall and cranial abdomen; higher volumes provided significantly more spread in the cranial and ventral directions and stained more intercostal nerves. While the EOI block using a volume of 0.5 mL/kg could be a technique used to provide regional anesthesia of the rabbit paracostal region and cranial abdomen, future studies would be needed to determine clinical safety and efficacy.

OBJECTIVE:To investigate thermoregulation, thermal antinociception, food/kaolin intake, fecal output, and behavior following long-acting buprenorphine preparations in rats. ANIMALS:8 adult male rats (Rattus norvegicus) were administered long-acting SC buprenorphine (SB; 0.65 mg/kg), transdermal buprenorphine (TB; 10 mg/kg), and controls in a randomized, cross-over design. METHODS:Body temperature, self-injury, sedation, food/kaolin intake, fecal output, and thermal withdrawal latencies were measured 1, 4, 8, 12, 24, 48, and 72 hours posttreatment. Data analysis was performed with mixed linear models. RESULTS:Self-injury was present between 1 and 12 hours and 4 and 12 hours following TB and SB, respectively; sedation was associated with TB at 12 to 24 hours. Withdrawal latencies were longer in both TB and SB groups than in the control group. Food intake decreased with time in all groups but was significantly lower 24 to 48 hours after TB and 24 to 72 hours after SB versus controls. Kaolin intake decreased from baseline 48 to 72 hours in the control group. Fecal output decreased from baseline 24 to 72 hours in all groups but was significantly lower than controls 24 hours following TB and 24 to 48 hours in SB. Body temperature increased from baseline at 1 hour, 1 to 12 hours, and 1 to 24 hours in the control, TB, and SB groups, respectively, and was significantly higher than the control group 1 to 72 hours following TB and 4 to 24 hours after SB. Transdermal buprenorphine and SB in normal rats produced antinociception, self-injurious behavior, hyperthermia, and decreased food/fecal output. CLINICAL RELEVANCE:Although these buprenorphine preparations may produce antinociception, untoward effects such as hyperthermia, self-injurious behavior, and reduced food intake/fecal output may be seen.

Low level activation of mu-opioid receptors (MORs) in neonatal rat brainstem-spinal cord preparations increases inspiratory burst amplitude recorded on cervical spinal roots. We tested whether: (1) MOR activation with an endogenous ligand, such as endomorphin-2, increases inspiratory burst amplitude, (2) disinhibition of GABAergic or glycinergic inhibitory synaptic transmission is involved, and (3) inflammation alters endomorphin2 effects. Using neonatal rat (P0-P3) brainstem-spinal cord preparations, bath-applied endomorphin-2 (10-200 nM) increased inspiratory burst amplitude and decreased burst frequency. Blockade of GABAA receptors (picrotoxin), glycine receptors (strychnine), or both (picrotoxin and strychnine) did not abolish endomorphin-2induced effects. In preparations isolated from neonatal rats injected 3 h previously with lipopolysaccharide (LPS, 0.1 mg/kg), endomorphin-2 continued to decrease burst frequency but abolished the burst amplitude increase. Collectively, these data indicate that disinhibition of inhibitory synaptic transmission is unlikely to play a role in endomorphin-2-induced changes in inspiratory motor output, and that different mechanisms underlie the endomorphin-2-induced increases in inspiratory burst amplitude and decreases in burst frequency.

An adverse perinatal environment can increase long-term cancer risk, although the precise nature of associated perinatal triggers remain unknown. Sleep apnea is a common condition during pregnancy, characterized by recurrent cessations in breathing during sleep, and the potential consequences of sleep apnea during pregnancy as it relates to breast cancer risk in offspring have not been explored. To model sleep apnea, Sprague-Dawley dams were exposed during gestation to nightly intermittent hypoxia (GIH) or normoxia (GNx), and the mammary glands of female offspring were examined. GIH offspring demonstrated increased epithelial stem and progenitor cell populations, which are associated with diminished transforming growth factor beta (TGFβ) activity. Elevations in adipose tissue stem cells in the mammary gland were also identified in GIH offspring. In aging females, mammary tumors formed in GIH offspring. These tumors displayed a dramatic increase in stroma compared to tumors from GNx offspring, as well as distinct patterns of expression of stem cell-related pathways. Together, these results suggest that exposure to sleep apnea during pregnancy leads to lasting changes in the mammary glands of female offspring. Increased stem and progenitor cell populations as a result of GIH exposure could enhance long-term breast cancer risk, as well as alter the clinical behavior of resulting breast tumors.

We report an important role for microglia in regulating neuroplasticity within phrenic motor neurons. Brief episodes of low oxygen (acute intermittent hypoxia; AIH) elicit a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF) that is regulated by the balance of competing serotonin vs adenosine-initiated cellular mechanisms. Serotonin arises from brainstem raphe neurons, but the source of adenosine is unknown. We tested if hypoxic episodes initiate phrenic motor neuron to microglia fractalkine signaling that evokes extracellular adenosine formation using a well-defined neurophysiology preparation in male rats. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF whereas severe AIH induces pLTF by the adenosine-dependent mechanism. Consequently, phrenic motor neuron fractalkine knockdown, microglial fractalkine receptor inhibition, and microglial ablation enhance moderate AIH, but suppress severe AIH-induced pLTF. We conclude, microglia play important roles in healthy spinal cords, regulating plasticity in motor neurons responsible for breathing. The role of microglia in regulating spinal neuroplasticity is unknown. Here, the authors show that spinal microglia regulate plasticity in motor neurons that drive breathing through reciprocal fractalkine and adenosine signaling.

Microglia are innate CNS immune cells that play key roles in supporting key CNS functions including brain plasticity. We now report a previously unknown role for microglia in regulating neuroplasticity within spinal phrenic motor neurons, the neurons driving diaphragm contractions and breathing. We demonstrate that microglia regulate phrenic long-term facilitation (pLTF), a form of respiratory memory lasting hours after repetitive exposures to brief periods of low oxygen (acute intermittent hypoxia; AIH) via neuronal/microglial fractalkine signaling. AIH-induced pLTF is regulated by the balance between competing intracellular signaling cascades initiated by serotonin vs adenosine, respectively. Although brainstem raphe neurons release the relevant serotonin, the cellular source of adenosine is unknown. We tested a model in which hypoxia initiates fractalkine signaling between phrenic motor neurons and nearby microglia that triggers extracellular adenosine accumulation. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF; in contrast, severe AIH drives pLTF by a unique, adenosine-dominant mechanism. Phrenic motor neuron fractalkine knockdown, cervical spinal fractalkine receptor inhibition on nearby microglia, and microglial depletion enhance serotonin-dominant pLTF with moderate AIH but suppress adenosine-dominant pLTF with severe AIH. Thus, microglia play novel functions in the healthy spinal cord, regulating hypoxia-induced neuroplasticity within the motor neurons responsible for breathing.

Sleep‐disordered breathing is a respiratory disorder commonly experienced by pregnant women. The recurrent hypoxaemic events associated with sleep‐disordered breathing have deleterious consequences for the mother and fetus. Adult male (but not female) rats born to dams subjected to gestational intermittent hypoxia (GIH) have a higher resting blood pressure than control animals and show behavioural/neurodevelopmental disorders. The origin of this persistent, sex‐specific effect of GIH in offspring is unknown, but disruption of the neuroendocrine stress pathways is a key mechanism by which gestational stress increases disease risk in progeny. Using FosB immunolabelling as a chronic marker of neuronal activation, we determined whether GIH augments basal expression of FosB in the perikaryas of cells in the paraventricular nucleus of the hypothalamus (PVN), a key structure in the regulation of the stress response and blood pressure. From gestational day 10, female rats were subjected to GIH for 8 h/day (light phase) until the day before delivery (gestational day 21); GIH consisted of 2 min hypoxic bouts (10.5% O 2 ) alternating with normoxia. Control rats were exposed to intermittent normoxia over the same period (GNX). At adulthood (10–15 weeks), the brains of male and female rats were harvested for FosB immunohistochemistry. In males, GIH augmented PVN FosB labelling density by 30%. Conversely, PVN FosB density in GIH females was 28% lower than that of GNX females. We conclude that GIH has persistent and sex‐specific impacts on the development of stress pathways, thereby offering a plausible mechanism by which GIH can disturb neural development and blood pressure homeostasis in adulthood.

Alzheimer’s disease (AD) is a neurodegenerative disease commonly associated with aging. Along with a decline in cognition and memory, individuals with AD often have respiratory disturbances such as sleep apnea, insufficient ventilation during sleep, and shortness of breath. Mechanisms underlying respiratory dysfunction in AD are unknown but may involve chemosensory deficits since at least one chemosensitive region, the locus coeruleus, undergoes significant neurodegeneration during AD progression in humans. To test the hypothesis that chemoreflexes are impaired in a mouse model of AD, we measured the hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses using whole body plethysmography in 5XFAD mice as amyloid pathology progressed. We evaluated 2 timepoints of amyloid beta (Aß) pathology: early (4 months of age) and late (8 months of age). Compared to WT mice, 8-month-old male and female 5XFAD mice had a significantly reduced capacity to increase ventilation following challenge with 5% inspired CO 2 (males: 5XFAD 99 ± 11% vs WT 168 ± 18%; females: 5XFAD 126 ± 12% vs. WT 212 ± 12%) or 5% inspired O 2 (males: 5XFAD 141 ± 20% vs. WT 201 ± 20%; females: 5XFAD 75 ± 7.6% vs. WT 134 ± 12%). In addition, both male and female 5XFAD mice experienced significantly more apneic events (~120 apneas per/h) than their WT counterparts (~50 apneas/h) during presumptive sleep. Consistent with impaired ventilation, Aß plaques were abundant in medullary regions of the brainstem where both central chemosensors and rhythm generating neurons reside. Preliminary data suggest that chemoreflex dysfunction may emerge early in disease progression since a trend toward a reduced HCVR (5XFAD 133 ± 23% vs WT 206 ± 39%, combined sexes) was apparent in 4-month-old 5XFAD mice. Surprisingly, and contrary to findings at 8 months, preliminary data in 4-month-old 5XFAD mice indicate that the HVR may be slightly higher indicating that 5XFAD mice may be hypersensitive to hypoxia early in disease progression. Together, these data suggest that responses to ventilatory challenges become significantly disrupted by Aß pathology late in disease progression, deficits which may manifest early in disease and could promote disease progression. Results from our studies will identify the contributions of Aß deposition to respiratory dysfunction in 5XFAD mice, and may have implications for therapeutic interventions in individuals with AD. NIH R01 HL142752, HL142752S1, and T32 AG000213 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Sleep disordered breathing (SDB) during pregnancy is a growing concern because it causes adverse outcomes for infant offspring. Recently, our lab developed a rat model of SDB during pregnancy in which pregnant dams were exposed to gestational intermittent hypoxia (GIH; 15 episodes/hr of 10.5% oxygen for 8h daily from days 10-20 of pregnancy). Control dams were exposed in parallel to gestational intermittent normoxia (GNX). We found that male (but not female) GIH offspring had increased apneas in adulthood compared to their GNX control counterparts. Thus, in this study, we tested the hypothesis that GIH would increase respiratory instability (i.e. increase the number of apneas) in neonatal male, not female, offspring. Plethysmography was performed on GIH and GNX male and female pups at postnatal (P) days 0, 10, and 15, and the number of apneas was quantified. An apnea was defined as two missed breaths (2x the average period between breaths). Preliminary data suggest that apnea frequency was sexually dimorphic and changed both with development and treatment. At P10 and P15 male GIH offspring had more apneas compared to GNX offspring (p<0.05). However, at P0, male GIH offspring (16 ± 2.4 apneas/10m) had surprisingly fewer apneas compared to GNX offspring (28 ± 1.9 apneas/10m). In female offspring in contrast, there was no difference in apnea frequency between GIH and GNX treatment at P0, but at P10 and P15 respectively, female GIH offspring (17 ± 2.8 and 9 ± 2.3 apneas/10m) had more apneas compared to their GNX counterparts (8 ± 1.2 and 2.9 ± 0.57 apneas/10m). Apnea frequency decreased with increasing age in both sexes, regardless of GIH and GNX treatment. Together, as expected, these results suggest that the increased apnea phenotype observed in the adult GIH male emerges as early as postnatal day 10. Unexpected however is the observation that female neonatal GIH offspring also experience respiratory instability in during the postnatal period despite its absence in adult female GIH offspring. Studies are currently underway to better understand the mechanisms underlying these sex differences in respiratory neural control, and the protective responses utilized by adult female GIH offspring to mitigate these respiratory deficits induced by intermittent hypoxia exposure in utero. R01 HL142752 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Alzheimer’s disease (AD) is a neurodegenerative disease commonly associated with aging. Along with a decline in cognition and memory, individuals with AD often have respiratory disturbances such as sleep apnea, insufficient ventilation during sleep, and shortness of breath. Mechanisms underlying respiratory dysfunction in AD are unknown but may involve chemosensory deficits since at least one chemosensitive region, the locus coeruleus, undergoes significant neurodegeneration during AD progression in humans. To test the hypothesis that chemoreflexes are impaired in a mouse model of AD, we measured the hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses using whole body plethysmography in 5XFAD mice as amyloid pathology progressed. We evaluated 2 timepoints of amyloid beta (Aß) pathology: early (4 months of age) and late (8 months of age). Compared to WT mice, 8-month-old male and female 5XFAD mice had a significantly reduced capacity to increase ventilation following challenge with 5% inspired CO2 (males: 5XFAD 99 ± 11% vs WT 168 ± 18%; females: 5XFAD 126 ± 12% vs. WT 212 ± 12%) or 5% inspired O2 (males: 5XFAD 141 ± 20% vs. WT 201 ± 20%; females: 5XFAD 75 ± 7.6% vs. WT 134 ± 12%). In addition, both male and female 5XFAD mice experienced significantly more apneic events (~120 apneas per/h) than their WT counterparts (~50 apneas/h) during presumptive sleep. Consistent with impaired ventilation, Aß plaques were abundant in medullary regions of the brainstem where both central chemosensors and rhythm generating neurons reside. Preliminary data suggest that chemoreflex dysfunction may emerge early in disease progression since a trend toward a reduced HCVR (5XFAD 133 ± 23% vs WT 206 ± 39%, combined sexes) was apparent in 4-month-old 5XFAD mice. Surprisingly, and contrary to findings at 8 months, preliminary data in 4-month-old 5XFAD mice indicate that the HVR may be slightly higher indicating that 5XFAD mice may be hypersensitive to hypoxia early in disease progression. Together, these data suggest that responses to ventilatory challenges become significantly disrupted by Aß pathology late in disease progression, deficits which may manifest early in disease and could promote disease progression. Results from our studies will identify the contributions of Aß deposition to respiratory dysfunction in 5XFAD mice, and may have implications for therapeutic interventions in individuals with AD. NIH R01 HL142752, HL142752S1, and T32 AG000213 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Alzheimer’s disease (AD) is a neurodegenerative disease commonly associated with aging. Along with regression in cognition and memory, AD patients often have respiratory disturbances such as sleep apnea, insufficient ventilation during sleep, and shortness of breath. Further, at least one chemosensitive region located in the pons, the locus coeruleus, undergoes dramatic degeneration during AD progression. These observations suggest chemoreception may be altered during the progression of AD. To begin testing this hypothesis, we examined the hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses on 5XFAD mice (>6months) using whole body plethysmography. Preliminary data showed a reduced capacity to lower metabolic rate and body temperature during hypoxia (5.5% inspired O2) in 5XFAD mice, such that average oxygen consumption was reduced by 34.8 ± 2.8% (S.E.M) and body temperature fell to 33.5 ± 0.4°C. In wild type (WT) mice oxygen consumption was reduced by 53% and body temperature fell to 32.8°C. Further, 5XFAD mice sustained a slight ventilatory increase (1.3 ± 3.6% increase from baseline) during hypoxia while WT mice reduced ventilation by 5%. 5XFAD mice also displayed a reduced hypercapnic ventilatory response, showing little increase in tidal volume in response to hypercapnia; but they appeared to augment this with a greater increase in breathing frequency compared to WT mice. The net result, however, was an increase in ventilation by 122 ± 22% from baseline in response to 5% CO2 while WT mice had a 181 ± 26% increase from baseline to the same challenge. Our data suggest that ventilatory chemoresponses are altered by disease progression in the 5XFAD mouse model. These alterations may contribute to the respiratory disturbances observed in AD patients.

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

Mammalian hibernation initiates dramatic changes in physiology that include large reductions in ventilation (V̇E), heart rate, oxygen consumption rate (V̇o2), and body temperature (Tb) upon entrance into torpor, effects that reverse upon arousal. Several physiologic processes must maintain function throughout a wide range of temperatures. Recent evidence indicates that neuronal synapses loosen and retract in regions of the brain that are silenced during a torpor bout, although little is understood yet about how these connections are regulated during hibernation. Microglia, brain resident immune cells, are potential facilitators of these essential neural homeostatic activities due to their critical roles in neuronal support and synaptic strengthening and pruning. Thus, we hypothesized that microglia are required for maintaining normal ventilatory neural control during hibernation. To test this, we treated 13‐lined ground squirrels with either vehicle or colony‐stimulating factor receptor‐1 (CSF1R) antagonist, PLX3397 (80mg/kg, po), a receptor whose activation is necessary for microglial survival in the adult nervous system, and measured V̇E, V̇o2, and Tb throughout a torpor bout. During entrance, PLX‐treated squirrels took ~1.5 hours longer to reach their minimum torpor temperature (Tb; 12°C) compared to control animals (Tb; 9.3°C) (ambient temperature; 5‐6°C). Similarly, arousal time in microglia‐depleted animals was as much as 1 hour longer relative to controls, despite starting at a higher minimum Tb. In both treatment groups, V̇o2 during entrance fell and spiked periodically. However, in PLX‐treated animals, these spikes were much greater and more frequent. Minimum V̇o2 in torpor was ~3 times greater in PLX‐treated animals compared to control animals. Upon arousal from torpor, V̇o2 remained elevated, longer, in PLX‐treated squirrels. Total V̇E tended to be elevated in PLX‐treated animals through entrance, steady state torpor, and arousal, but it was most striking during steady state torpor, where V̇E in PLX‐treated animals was 15‐fold higher than control animals. The net result in torpor was an elevated air convection requirement, thus microglia‐depleted animals hyperventilated throughout the torpor bout. Together, these results suggest that microglia play an important regulatory role in normal ventilatory and temperature control throughout torpor and influence the rate at which animals enter into and arouse from torpor. Mechanisms whereby microglia contribute to hibernation neurophysiology are currently under investigation.

Mu-opioid receptors (mORs) located on brainstem respiratory neurons are part of an intricate peptidergic neuromodulatory system that regulates breathing. The endogenous role of mORs in respiratory motor control, however, is poorly understood. To explore this question, very low (nanomolar) concentrations of mOR agonist drugs were bath-applied to neonatal rat (P1-P3) brainstem-spinal cord preparations while measuring respiratory-related motor output from spinal C4-C5 rootlets containing phrenic motor neuron axons. When DAMGO (synthetic mOR agonist) or endomorphin-2 (endogenous mOR ligand) were bath-applied for 15 min at 20-100 nM, respiratory motor burst amplitude increased dramatically with a modest decrease in burst frequency. Burst amplitude facilitated by 45 ± 38% and 32 ± 24% (mean ± stdev) above baseline for 50 nM DAMGO (n=8) and 100 nM endomorphin-2 (n=10), respectively (p<0.001). To test whether respiratory motor output on thoracic ventral roots was altered by mOR activation, brainstem-spinal cord preparations were isolated to include spinal segments C1-T8. Bath-applied DAMGO (50 nM) induced burst amplitude facilitation on thoracic roots T4-T6 and cervical roots C4-C5 by 62 ± 19% and 38 ± 36%, respectively (p<0.012). To test whether mOR-induced burst facilitation is due to brainstem or spinal mechanisms, a split-bath chamber with a barrier at C1-C2 was used to separately bathe the brainstem and spinal cord compartments. DAMGO (50 nM) application to the spinal cord (C2-C8) had little effect on respiratory motor output measured from C4-C5 rootlets, whereas DAMGO application to the brainstem increased C4-C5 motor output by 34 ± 19% (p<0.004). These data suggest that mOR activation in the brainstem can transform the respiratory network to produce a motor pattern with decreased frequency balanced by increased amplitude, with relatively little change in overall neural output. Given that mOR-induced facilitation was observed in both cervical and thoracic motor neuron pools, we hypothesize that burst amplitude facilitation is due to mOR activation on rhythm-generating neurons or premotor neurons in the brainstem. This study reveals that endogenous mOR activation modulates the neural control of breathing in a manner that is potentially different from respiratory depression, which is more commonly associated with opioids.