Ventilatory Responses In Rats Research Articles

Orexin facilitates the hypoxic ventilatory response in rats

Orexin is a hypothalamic neuropeptide involved in a variety of functions including arousal, body temperature regulation, and feeding. Orexin neurons are active in wakefulness (or the dark phase in rats) and mostly silent in sleep (light phase), implying the effects of orexin are greater in wakefulness and/or dark phase. Recent evidence implicates orexin in the control of cardiorespiratory function, including the respiratory response to CO2 and blood pressure regulation. Whether orexin influences normal breathing in a state-dependent manner is unresolved. In addition, although the role of orexin in the hypercapnic ventilatory response is well established, its effects on the acute hypoxic ventilatory response (HVR) has not been investigated. We hypothesized that orexin receptor blockade would lead to hypoventilation in wakefulness but not sleep and would reduce the HVR. To test these hypotheses, we conducted two separate experiments, both of which were performed in the light phase. First, we used whole-body plethysmography to measure the ventilation of adult rats in room air. Rats were implanted with EEG and EMG electrodes to determine vigilance state, which was confirmed by applying Fast Fourier Transform on the EEG signals. After a 1hr baseline period, we injected i.p. either suvorexant (an OxR1 and OxR2 antagonist, 20 mg/kg i.p.) or vehicle (100% DMSO, i.p.) and allowed the rats to freely behave for 4 hours in the chamber while breathing room air. Separate groups of rats, also instrumented with EEG and EMG electrodes, were exposed to graded hypoxia (15%, 13%, 11%, and 9% O2) following administration of either 20 mg/kg i.p. suvorexant or vehicle. We found that, irrespective of state of vigilance, OxR1 and OxR2 blockade had no effect on baseline breathing, but significantly reduced the HVR (treatment × O2 level: p=0.03). The effect on the HVR was especially evident at 13% and 11% O2 when the response was reduced by ~30%. We show that orexin facilitates the HVR in the light phase, even though the orexin neurons are silent. Whether orexin has a larger effect on the HVR during the dark phase, when orexin neurons are active, has yet to be determined.

Functional role of carbon dioxide on intermittent hypoxia induced respiratory response following mid-cervical contusion in the rat

Intermittent hypoxia induces respiratory neuroplasticity to enhance respiratory motor outputs and is a potential rehabilitative strategy to improve respiratory function following cervical spinal injury. The present study was designed to evaluate the functional role of intermittent and sustained carbon dioxide (CO2) on intermittent hypoxia-induced ventilatory responses in rats with mid-cervical spinal contusion. The breathing pattern of unanesthetized rats at the subchronic and chronic injured stages was measured in response to one of the following treatments: (1) Intermittent hypercapnic-hypoxia (10 × 5 min 10%O2 + 4%CO2 with 5 min normoxia interval); (2) Intermittent hypoxia with sustained hypercapnia (10 × 5 min 10%O2 + 4%CO2 with 5 min 21%O2 + 4%CO2 interval); (3) Intermittent hypoxia (10 × 5 min 10%O2 with 5 min normoxia interval); (4) Intermittent hypercapnia (10 × 5 min 21%O2 + 4%CO2 with 5 min normoxia interval); (5) Sustained hypercapnia (100 min, 21% O2 + 4% CO2); (6) Sustained normoxia (100 min, 21% O2). The results demonstrated that intermittent hypoxia associated with intermittent hypercapnia or sustained hypercapnia induced a greater ventilatory response than sustained hypercapnia during stimulus exposure. The tidal volume was significantly enhanced to a similar magnitude following intermittent hypercapnic-hypoxia, intermittent hypoxia with sustained hypercapnia, and intermittent hypoxia in subchronically injured animals; however, only intermittent hypercapnic-hypoxia and intermittent hypoxia were able to evoke long-term facilitation of the tidal volume at the chronic injured stage. These results suggest that mild intermittent hypercapnia did not further enhance the therapeutic effectiveness of intermittent hypoxia-induced respiratory recovery in mid-cervical contused animals. However, sustained hypercapnia associated with intermittent hypoxia may blunt ventilatory responses following intermittent hypoxia at the chronic injured stage.

Water deprivation enhances the hypercapnic ventilatory response in rats

Behavioral, neuroendocrine, autonomic and cardiovascular adjustments are triggered during water deprivation. However, the effects of water deprivation on ventilation remain to be determined. Herein, we evaluated the pulmonary ventilation after 48 h of water deprivation in male Holtzman rats (280–300 g) under unanesthetized conditions. Control rats (n=7) had continuous access to tap water and food, whereas water‐deprived (WD, n=6) rats had water, but not food removed for 48 h before the experiments. Rats were placed in whole‐body plethysmographic chamber (5 L) to measure the respiratory frequency (fR), tidal volume (VT) and minute ventilation (VE) under baseline condition (room air) and during hypercapnia (7% CO2 for 5 min). After the experiments, animals were decapitated for blood collection. WD rats had an increase in serum osmolality (307 ± 1 mOsmol/kg of water, vs. control: 297 ± 1 mOsmol/kg of water; p<0.05) and sodium concentration (148 ± 0.6 mmol/L, vs. control: 141 ± 0.5 mmol/L; p<0.05). WD rats exhibited increased VT (9.7 ± 0.5 mL.kg−1, vs. control: 7.7 ± 0.3 mL.kg−1; p<0.05) but reduced fR (76 ± 2 breaths.min−1, vs. control: 103 ± 7 breaths.min−1; p<0.05) compared to controls. As a result, baseline VE (746 ± 54 mL.kg−1.min−1, vs. control: 799 ± 64 mL.kg−1.min−1) were similar between groups. Under hypercapnia, the response of increases in VT were statistically similar between WD and control groups (ΔVT: 60 ± 16%, vs. control: 33 ± 7%). On the other hand, WD rats demonstrated amplified hypercapnia‐induced increases in fR (Δ: 47 ± 5%, vs. control: 19 ± 5%; p<0.05) and VE (Δ: 140 ± 35%, vs. control: 59 ± 9%; p<0.05). The results show that 48 h of water deprivation enhanced the ventilatory responses induced by hypercapnia in unanesthetized rats.Support or Funding InformationFAPESP; CNPqThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

ATP in the lateral hypothalamus/perifornical area enhances the CO2 chemoreflex control of breathing.

What is the central question of this study? ATP is known to modulate the chemosensitivity of some brain areas. However, whether the ATP contributes specifically to the mechanism of chemoreception in the lateral hypothalamus/perifornical area (LH/PFA) remains to be determined. What is the main finding and its importance? ATP, acting on the LH/PFA, enhances the hypercapnic ventilatory response in rats during wakefulness, in the dark period. Our results highlight the importance of ATP as a modulator of central chemoreception and provide new insight regarding the mechanisms involved in LH/PFA chemosensitivity and the sleep-wake differences in the CO2 /H+ -dependent drive to breathe. The lateral hypothalamus/perifornical area (LH/PFA) is a central chemoreceptor site, which acts in an arousal state-dependent manner. It has been shown that purinergic signalling through ATP influences the CO2 /H+ responsiveness of other chemosensitive regions, but it is unknown whether ATP is also involved in the mechanisms that underlie LH/PFA chemoreception. Here, we studied the effects of microdialysis of a P2X-receptor agonist [α,β-methylene ATP (α,β-meATP), 10mm] and a non-selective P2-receptor antagonist [pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 1mm] into the LH/PFA of conscious rats on ventilation in room air and in 7% CO2 . In the dark (active) phase, but not in the light, microdialysis of α,β-meATP caused an augmented hypercapnic ventilatory response during wakefulness, but not during non-REM sleep (P< 0.001). PPADS caused no change in CO2 ventilatory responses in either the dark period or the light period. Our data suggest that ATP in LH/PFA contributes to the hypercapnic ventilatory response in conscious rats during wakefulness in the dark phase of the diurnal cycle.

Toward a soft computing-based correlation between oxygen toxicity seizures and hyperoxic hyperpnea

Exposure to high levels of hyperbaric oxygen ( $$\hbox {HBO}_{2}$$ ) can induce central nervous system oxygen toxicity in humans and animals, a phenomenon characterized by repeated tonic–clonic seizures. The risk of developing this type of convulsions represents the limiting factor in using $$\hbox {HBO}_{2}$$ for a number of clinical and diving applications. Previously, using radio-telemetry in awake rats, a significant increase in the mean value of the ventilatory responses in rats to $$\hbox {HBO}_{2}$$ approximately 5–8 min before onset of seizures has been observed. This response has been termed “hyperoxic hyperpnea,” and it has been hypothesized it may serve as a predictor of an impending seizure while breathing $$\hbox {HBO}_{2}$$ . The purpose of the present study was to apply soft computing methods to determine whether there is a direct correlation between the onset of hyperoxic hyperpnea (i.e., the ventilatory response as defined by tidal volume and respiratory frequency) and the onset of seizure. In our experiments, we used Multilayer Perceptron, Naive Bayes, J48, and U-BRAIN aimed at evidencing the correlation between the respiratory feature vectors and the onset of an impending seizure in unanesthetized, freely behaving rats breathing 4, 5 or 6 atmospheres absolute (ATA) of oxygen. This strategy was also aimed to finding a set of rules relating physiological parameters and convulsive phenomena through inductive inference.

Effects of A2A and 5‐HT2A Antagonists on Hypoxic and Hypercapnic Ventilatory Response in Rats Exposed to Chronic Sustained Hypoxia

Different patterns and durations of hypoxic exposure cause different forms of plasticity in the neural control of ventilation (Pamenter &amp; Powell, F1000Prime Rep, 5: 23, 2013). For example, acute intermittent hypoxia (AIH) causes long term facilitation (LTF) of ventilation and phrenic nerve activity, while chronic sustained hypoxia (CSH) causes ventilatory acclimatization to hypoxia (VAH). In both LTF and VAH, ventilation in normoxia is increased after the hypoxic stimulus is removed. Previous work has shown that 5HT2a serotonin receptor activation (Gq pathway) is necessary for LTF but not VAH. However, recent work (Nichols et al., J Appl Physiol, 112: 1678–1688, 2012) shows A2A adenosine receptor activation (Gs pathway) is also involved in LTF with more severe AIH (PaO2 = 25–30 Torr vs. 35–45 Torr). We hypothesized that the Gs pathway also contributes to VAH and tested this in adult male rats by blocking the Gs and Gq pathways before and after 7 days of CSH (PO2 = 80 Torr) and measuring ventilation and the hypoxic ventilatory response (HVR) with barometric pressure plethysmography. Consistent with previous work, Gq blockade with a 5HT2a antagonist (Ketanserin, 1 mg/Kg i.p.) caused no significant changes before or after CSH. Gs blockade with an A2A agonist antagonist (MSX‐3, 1mg/Kg i.p.) had no effect before CSH but after CSH it significantly increased ventilation in acute normoxia (1777±352 vs. 2396±268 ml/min*kg) and hypoxia (2063±195 vs. 2403±450 ml/min*kg). This contrasts with the effect of Gs blockade during severe AIH to significantly decrease LTF and does not support our hypothesis. However, further experiments are necessary to determine the effects of blocking Gs and Gq pathways during CSH.Support or Funding InformationNIH RO1 HL‐081823

Hypoxic and hypercapnic ventilatory responses in rats with polycystic ovaries

In female rats, a single injection of estradiol valerate (EV) results in effects that are similar to those observed in women with polycystic ovary syndrome (PCOS). We hypothesized that EV-induced PCOS affects breathing control based on evidence showing an influence of sex hormones on ventilation. To test this hypothesis, we studied the effects of EV treatment on the ventilation of female rats in air, in 7% CO2 and in 7% O2, at 30, 45 and 60 days after EV injection. The group examined 30 days after EV treatment showed a 61% reduction in the hypercapnic ventilatory response compared to the control group. Basal ventilation, hypoxic ventilatory response, and body temperature were not affected. These results, suggest that the hormonal changes observed in PCOS may result in a temporary inhibition of the central chemoreflex but do not influence basal ventilation or the hypoxic peripheral chemoreflex.

Chronic Phenytoin Treatment Reduces Hypoxic Ventilatory Responses in Rats

Arterial afferents provide sensory information that participates in the peripheral control of ventilation. The carotid body chemosensory afferents increase their frequency of discharge in response to hypoxia, increasing ventilatory frequency and volume. Acute phenytoin application, a persistent Na+ current blocker, reduces the basal afferent activity and hypoxia‐induced ventilatory responses. Because phenytoin is used as an antiepileptic drug, it is of interest to investigate the long‐term effects of phenytoin treatment on ventilatory control. Male Sprague Dawley rats (180‐200 g) were anesthetized with isoflurane, an osmotic pump (Alzet 2ML4) placed subdermally, the incisions closed and the animals received antibiotic and antiinflammatory. The osmotic pump delivered a volume of 2.59 µL/hr, with a phenytoin dose of 10 mg/day. Between one and four weeks after implantation, animals were placed in a whole body plethysmograph (Buxco, USA) and the ventilatory volume and frequency was measured at different inspiratory oxygen fractions (FIO2: 100, 50, 21, 15, 10, 7). The animals were sacrificed after four weeks by anesthetic overdose (pentobarbitone, 120 mg/kg). Phenytoin treatment had no effect on basal ventilation, but produced a marked reduction in the hypoxia induced ventilatory responses both in frequency and volume. Thus, minute ventilation was reduced at all hypoxic levels, with approximately a 40% reduction at 7% FIO2. Our results indicate that chronic phenytoin treatment reduces reflex ventilatory responses to hypoxia. It is suggested that a detrimental reduction of ventilatory chemosensory peripheral drive could result from chronic phenytoin treatment in epileptic patients.

Effects of A2a Receptors on the Hypoxic Ventilatory Response in Rats Exposed to Chronic Sustained Hypoxia

Recent studies show that long term facilitation of phrenic nerve activity (LTF) with acute intermittent hypoxia (IH) involves two signaling pathways. In addition to the well‐known serotonergic (Gq) pathway, a Gs pathway can be activated by adenosine receptor 2 (A2a) agonists and cause LTF without IH (J Neurosci 28:2033‐42, 2008). Gs can inhibit the Gq pathway as A2a antagonists increase LTF from IH by disinhibiting Gq (J Physiol 588:255‐66, 2010). We tested the hypothesis that Gs also contributes to plasticity in ventilatory control during chronic sustained hypoxia (CH) by measuring the effects of A2a antagonists and agonists on ventilatory responses to hypoxia and hypercapnia in adult male rats before and after one week of CH (PO2 = 80 Torr in a hypobaric chamber). Ventilation measured in awake unrestrained rats with barometric pressure plethysmography increased after CH; acute ventilatory responses to hypoxia or hypercapnia increased also. Acute administration of A2a antagonist (MSX‐3, 1 mg/Kg i.p.) after CH did not change ventilation or acute ventilatory responses. A2a agonists (CGS 21680, 100 µg/Kg i.p.) before CH had no effect on ventilation or ventilatory responses, in contrast to stimulation reported for sleeping rats (J Neurosci 28:2033‐42, 2008). The results do not support a role for Gs in the increased ventilatory drive and hypoxic responses with CH, in contrast to their role in LTF in IH.Supported by NIH RO1 HL‐081823.

Systemic LPS attenuates the acute hypoxic ventilatory response: implications for a uniquely sensitive window of neonatal development (1177.4)

Sustained hypoxia exposure during a finite window of neonatal development (~10‐15 days of postnatal (P) age) attenuates the acute hypoxic (HVR) and hypercapnic (HCVR) ventilatory response in rats. In the current study, we investigated whether the HVR and HCVR are also modified by LPS exposure during a similar period of development. Rat pups received an intraperitoneal injection of either saline or 0.1mg/kg of LPS at either P5, P10, or P20; the acute HVR and HCVR were measured using whole‐body plethysmography ~2.5hrs later. LPS attenuated the HVR (but not the HCVR) in P10 rats (1.8 ml/g/min) compared to saline (2.4 ml/g/min) treated rats, whereas younger (P5) or older (P10) rats were unaffected. LPS was lethal in some rats at some time within 24 hours after LPS; P10 pups appeared particularly vulnerable with a higher rate of mortality (27% of pups) than P5 (9% of pups) or P20 (0% of pups) treated rats. These data reveal the presence of a developmental window (~P10) that may be at a heightened level of vulnerability to inflammatory challenges.

Hypoxic and hypercapnic ventilatory response in rats with estradiol valerate‐induced polycystic ovaries (LB773)

We evaluated pulmonary ventilation (VE) and body temperature (Tb) during hypercapnia (7%CO2) and hypoxia (7%O2) in a female rat model in which estradiol valerate (EV) (2mg/0.2mL/rat or oil (control) were administered intramuscularly in a single dose to induce Polycystic ovary syndrome (PCOS). The experiments were performed 30, 45 or 60 days after EV injection. The ovaries of 30, 45 and 60 days EV group displayed typical PCOS‐like changes. Plasma levels of estradiol (E2) and progesterone (P4) increased in 30 (E2: 74.5 ± 23.5 pg/mL; P4: 18.3 ±4.5 ng/mL), 45 (E2: 70.8 ± 17.5 pg/mL; P4: 21.3 ±7.6 ng/mL) and 60 (E2: 71.4 ± 4.7 pg/mL; P4: 23.7 ±4.5 ng/mL) days EV groups whereas testosterone (T) increased only in 45 (58.4±6.5 pg/mL) and 60 (79.5±18.3 pg/mL) days EV groups compared with control group (E2: 46.9±11.3 pg/mL; P4: 9.9±3.4 ng/mL; T: 22.6±1.9 pg/mL). 30, 45, 60 EV treatment did not affect resting ventilation and hypoxic ventilatory response. However, the increase in ventilation produced by hypercapnia was attenuated after 30 days EV group (1978.9 ± 85.7 mL.kg‐1.min‐1 in control group, 1651.2 ± 369.3 in 45 days group and 1617.4± 131.4 in 60 days group vs 771.0 ± 172.4 in 30 days group). Hypoxia induced Tb to reduce in all groups, a response that was amplified in 30 days EV group. Our results suggest that initial alterations promoted by EV treatment changes CO2‐drive to breathe and hypoxia‐induced drop in Tb, and these responses are compensated in 45 and 60 days EV females.

Differential effects of neonatal sustained hypoxia exposure on the acute hypoxic ventilatory response among different rat strains (1092.12)

Sustained hypoxia (SH) exposure during a vulnerable period of neonatal development (~11‐15 days of postnatal (P) age) attenuates the acute hypoxic (HVR) ventilatory response in rats. In the current study, we investigated whether the effects of SH exposure on the acute HVR varies among different rat strains. Sprague‐dawley (SD), Fisher (F), and Spontaneously Hypertensive (SHR) rats were exposed to SH (11% O2, 5 days) starting at P11. The acute HVR and Dejours test were assessed using whole‐body plethysmography the day after (P16) SH exposure ended. SH abolished the HVR in SHR, but not SD or F rats compared to normoxia raised animals. The transient ventilatory depression following brief (5 minutes) 100% O2 exposure (Dejours test) was also absent in SHR rats following SH treatment. These data indicate a strain difference in the vulnerability of the respiratory system to neonatal SH exposure; possible genetic determinants for this phenomenon requires further study.

Periaqueductal gray matter modulates the hypercapnic ventilatory response

The periaqueductal gray (PAG) is a midbrain structure directly involved in the modulation of defensive behaviors. It has direct projections to several central nuclei that are involved in cardiorespiratory control. Although PAG stimulation is known to elicit respiratory responses, the role of the PAG in the CO(2)-drive to breathe is still unknown. The present study assessed the effect of chemical lesion of the dorsolateral and dorsomedial and ventrolateral/lateral PAG (dlPAG, dmPAG, and vPAG, respectively) on cardiorespiratory and thermal responses to hypercapnia. Ibotenic acid (IBO) or vehicle (PBS, Sham group) was injected into the dlPAG, dmPAG, or vPAG of male Wistar rats. Rats with lesions outside the dlPAG, dmPAG, or vPAG were considered as negative controls (NC). Pulmonary ventilation (VE: ), mean arterial pressure (MAP), heart rate (HR), and body temperature (Tb) were measured in unanesthetized rats during normocapnia and hypercapnic exposure (5, 15, 30min, 7% CO(2)). IBO lesioning of the dlPAG/dmPAG caused 31% and 26.5% reductions of the respiratory response to CO(2) (1,094.3 ± 115mL/kg/min) compared with Sham (1,589.5 ± 88.1mL/kg/min) and NC groups (1,488.2 ± 47.7mL/kg/min), respectively. IBO lesioning of the vPAG caused 26.6% and 21% reductions of CO(2) hyperpnea (1,215.3 ± 108.6mL/kg/min) compared with Sham (1,657.3 ± 173.9mL/kg/min) and NC groups (1,537.6 ± 59.3). Basal VE: , MAP, HR, and Tb were not affected by dlPAG, dmPAG, or vPAG lesioning. The results suggest that dlPAG, dmPAG, and vPAG modulate hypercapnic ventilatory responses in rats but do not affect MAP, HR, or Tb regulation in resting conditions or during hypercapnia.

Selecting representative ages for developmental changes of respiratory irregularities and hypoxic ventilatory response in rats

Apnea frequency and the weak ventilatory response to hypoxia are a major clinical correlates of the immaturity of respiratory control system in preterm neonates. Rats are frequently used as model to study the respiratory control during development. However, little is known about the postnatal ages that best represent these respiratory irregularities and the hypoxic ventilatory response. Using plethysmography, we assessed baseline minute ventilation, ventilatory response to moderate hypoxia (FiO2 = 12%, 20 min) and apnea frequency in awake and non-anesthetized rats at the postnatal ages of 1, 4, 7, 12, 21 and 90 days old (P1, P4, P7, P12, P21, and P90, respectively). Baseline minute ventilation slightly increased in P4 (~25% vs P1) then gradually decreased with age (age effect: p < 0.05). The lowest level of ventilation was observed in P90 (p < 0.01 vs all ages). Minute ventilation (% from baseline) in response to hypoxia showed the well-known biphasic pattern in all rats at 12 days old or less. Minute ventilation at the initial phase of the hypoxic response was not significantly different between P1, P4, between P7, P12 and between P21, P90. The late phase of the hypoxic response was similar between P1, P4, and between P21, P90, but was significantly different between P7 and P12 (p < 0.05). Under baseline or hypoxic condition, the higher number of apnea frequency (spontaneous and post- sigh) was observed in P1, it then decreased progressively with age (age effect: p < 0.01 for baseline; p < 0.001 for hypoxia). These results suggest that when P4, P7 and P12 are selected to represent the age-dependent changes of the hypoxic ventilatory response in rats, the P1 rats should be included to better describe the age-dependence of apnea frequency.

Systemic inflammation suppresses hypoxic and hypercapnic ventilatory responses in rats.

Chemoreflex responses to hypoxia and hypercapnia are critical elements of ventilatory control. However, chemoreflexes may be undermined by factors frequently associated with lung disease, such as systemic inflammation. Here, we tested the hypothesis that systemic inflammation suppresses ventilatory responses to hypoxia and hypercapnia in rats. Ventilatory responses were assessed in unanesthetized adult rats via whole‐body plethysmography 1 day after lipopolysaccharide administration (LPS: 1 mg in saline, i.p.). In Lewis rats, LPS suppressed both hypoxic (10.5% O2; vehicle: 90±19%; LPS: 37±11%; p &lt; 0.05) and hypercapnic ventilatory responses (7% CO2; vehicle: 223±22%; LPS: 66±26%; p &lt; 0.05). Splenic iNOS mRNA increased 6.5‐fold (qPCR) in LPS‐treated Lewis rats, demonstrating systemic inflammation. In contrast, there was no evidence for suppressed hypoxic (vehicle: 127±14%; LPS: 92±21%) or hypercapnic ventilatory responses (vehicle: 216±19%; LPS: 239±27%) in LPS‐treated Sprague Dawley rats, nor were splenic iNOS mRNA levels increased. Thus, although LPS sensitivity varies among rat strains, systemic inflammation suppressed chemoreflex responses to hypoxia and hypercapnia. Suppressed chemoreflex responses may impair the ability to respond appropriately to ventilatory challenges in disease states associated with inflammation, including many lung diseases (NIH HL69064 and HL007654).Grant Funding Source: NIH HL69064 and HL007654

Modulatory Effect of Fentanyl on Bronchopulmonary C‐fiber (PCF)‐mediated Ventilatory (VE) Responses Is Age‐dependent in Anesthetized Rats

In adult rats, opioids administered into the pulmonary circulation produces an apnea and given systemically turns the phenylbiguanide (PBG)‐evoked rapid shallow breaths (RSBs) to an apnea by stimulating and facilitating PCFs, respectively. Fentanyl, an opioid μ‐receptor agonist, clinically has a lesser inhibitory effect on ventilation in children (&lt; 3 years old) than in adults (Barker, Pediatric Anesthesia 2007). Thus, we tested if the modulatory effect of opioids on the PCF‐mediated VE responses in rats were age‐dependent. VE responses to a right atrial bolus injection of PBG (before and after systemic administration of fentanyl) or fentanyl were recorded in anesthetized rat pups (3 week‐old) and adults. We found that: 1) PBG (5 μg/kg) evoked 5–10 RSBs in adults, but caused no change or 2–3 rapid breaths in pups; 2) systemic fentanyl (6 μg/kg, iv) reduced VE more greatly in adults (60%) than pups (27%, P &lt; 0.01) and switched the PBG‐induced RSB to an apnea in adults, but failed to alter the PBG effects on VE in pups; 3) right atrial injection of fentanyl (2 μg/kg) alone produced an apnea in adults but not in pups; and 4) the VE responses to right atrial injection of PBG or fentanyl were eliminated by bilateral vagotomy. Our data suggest that the PCF‐mediated RSBs and inspiratory inhibition in response to PBG and fentanyl are age‐dependent in rats, indicating a plasticity of PCFs as development. (ALA RT‐83131‐N).

The acute hypoxic ventilatory response under halothane, isoflurane, and sevoflurane anaesthesia in rats*

The relative order of potency of anaesthetic agents on the hypoxic ventilatory response has been tested in humans, but animal data are sparse. We examined the effects of 1.4, 1.6, 1.8, and 2.0 MAC halothane, isoflurane, and sevoflurane on phrenic nerve activity in euoxia (baseline) and during acute normocapnic hypoxia (inspired oxygen fraction 0.09) in adult male Sprague-Dawley rats. With halothane, all animals became apnoeic even in euoxia, and the hypoxic response was completely abolished at all anaesthetic levels. With isoflurane, 5 of 14 animals exhibited phrenic nerve activity in euoxia at 1.4 MAC and demonstrated a hypoxic response (302% of baseline activity), but all became apnoeic and lost the hypoxic response at higher doses. With sevoflurane, phrenic nerve activity and a hypoxic response was preserved in at least some animals at all doses (i.e. even the highest dose of 2.0 MAC). Similar to the rank order of potency previously observed in humans, the relative order of potency of depression of the hypoxic ventilatory response in rats was halothane (most depressive) > isoflurane > sevoflurane (p = 0.01 for differences between agents).

Neonatal intermittent hypoxia induces sex‐specific enhancement of hypoxic ventilatory response in rat pups

We tested the hypothesis that chronic intermittent hypoxia induces gender‐specific effects on hypoxic ventilatory response (HVR) in newborn rats. Rat pups were exposed with their dams to normal room air (controls) or to chronic intermittent hypoxia (IH: O2 levels cycled between 21% and 5% every 10 minutes for 1h followed by 1h under normoxia, 24h/day) during postnatal days 1–10. Minute ventilation (VE) was measured in 10‐day‐old male and female rat pups using whole‐body plethysmography. To assess HVR, VE was measured in normoxia and during acute hypoxia (5% achieved in 60 seconds), and expressed as % changes vs. baseline. IH induced a growth‐delay of similar magnitude in males and females. Under normoxia, respiratory frequency was slightly lower in both male and female IH rats vs. controls, but VE was similar. In males, HVR was higher in IH rats (+57±13%) vs. controls (+32±8%, p=0,001). In females, HVR was similar between neonatal IH (+35±13%) and control rats (+42±7%, p=ns). This study shows that neonatal IH increases HVR specifically in male rat pups, and suggests that apneas may have sex‐specific effects on respiratory control in neonates. (Founded by FRSQ, Sick kids foundation/CIHR).

Perinatal hyperoxia alters the expression of neurotrophic factors and their receptors in the rat carotid body

Perinatal hyperoxia attenuates the hypoxic ventilatory response in rats by altering development of the carotid body and its afferent neurons. We hypothesized that these changes in carotid body growth and innervation involve changes in the expression of neurotrophic factors or their receptors. Rats were born and raised in hyperoxia (60% O2) or normoxia (21% O2) until studied at 1–14 days of age (i.e., P1–P14). Carotid body volume was 30% smaller in hyperoxia‐reared rats than their normoxia‐reared counterparts by P4 (P=0.02) and showed little growth thereafter. In contrast, carotid bodies of normoxia‐reared rats more than doubled in size between P4 and P14. We measured mRNA expression for brain‐derived neurotrophic factor (BDNF) and its receptor tyrosine kinase B (Trk B), and glial cell line‐derived neurotrophic factor (GDNF) and its receptor Ret in carotid bodies at P3 via quantitative RT‐PCR. Ret expression was reduced ~30% in hyperoxia‐treated rats (P&lt;0.01), but no differences were detected for BDNF, TrkB, or GDNF. In addition, BDNF and GDNF protein expression levels were measured at P3 via ELISA. BDNF was reduced by ~70% in the carotid bodies of hyperoxia‐treated rats (P=0.02). GDNF appeared to decrease, depending on normalization methods. These data suggest a role for neurotrophic factors in hyperoxia‐induced respiratory plasticity. Supported by NIH grant P20 RR‐016463 (Maine INBRE).

Respiratory plasticity after perinatal hyperoxia is not prevented by antioxidant supplementation

Perinatal hyperoxia attenuates the hypoxic ventilatory response in rats by altering development of the carotid body and its chemoafferent neurons. In this study, we tested the hypothesis that hyperoxia elicits this plasticity through the increased production of reactive oxygen species (ROS). Rats were born and raised in 60% O(2) for the first two postnatal weeks while treated with one of two antioxidants: vitamin E (via milk from mothers whose diet was enriched with 1000 IU vitamin E kg(-1)) or a superoxide dismutase mimetic, manganese(III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP; via daily intraperitoneal injection of 5-10 mg kg(-1)); rats were subsequently raised in room air until studied as adults. Peripheral chemoreflexes, assessed by carotid sinus nerve responses to cyanide, asphyxia, anoxia and isocapnic hypoxia (vitamin E experiments) or by hypoxic ventilatory responses (MnTMPyP experiments), were reduced after perinatal hyperoxia compared to those of normoxia-reared controls (all P<0.01); antioxidant treatment had no effect on these responses. Similarly, the carotid bodies of hyperoxia-reared rats were only one-third the volume of carotid bodies from normoxia-reared controls (P <0.001), regardless of antioxidant treatment. Protein carbonyl concentrations in the blood plasma, measured as an indicator of oxidative stress, were not increased in neonatal rats (2 and 8 days of age) exposed to 60% O(2) from birth. Collectively, these data do not support the hypothesis that perinatal hyperoxia impairs peripheral chemoreceptor development through ROS-mediated oxygen toxicity.