Age-related changes in the ventilatory response to inspired CO2 in neonatal rats

Effect of intermittent hypercapnia on respiratory control in rat pups

Preterm infants are subject to fluctuations in blood gas status associated with immature respiratory control. Intermittent hypoxia during early postnatal life has been shown to increase chemoreceptor sensitivity and destabilize the breathing pattern; however, intermittent hypercapnia remains poorly studied. Therefore, to test the hypothesis that intermittent hypercapnia results in altered respiratory control, we examined the effects of daily exposure to intermittent hypercapnia on the ventilatory response to subsequent hypercapnic and hypoxic exposure in neonatal rat pups. Exposure cycles consisted of 5 min of intermittent hypercapnia (5% CO(2), 21% O(2), balance N(2)) followed by 10 min of normoxia. Rat pups were exposed to 18 exposure cycles each day for 1 week, from postnatal day 7 to 14. We analyzed diaphragm electromyograms (EMGs) from pups exposed to subsequent acute hypercapnic (5% CO(2)) and hypoxic (12% O(2)) challenges. In response to a subsequent hypercapnia challenge, there was no significant difference in the ventilatory response between control and intermittent hypercapnia-exposed groups. In contrast, intermittent hypercapnia-exposed rat pups showed an enhanced ventilatory response to hypoxic challenge with an increase in minute EMG to 118 +/- 14% of baseline versus 107 +/- 13% for control pups (p < 0.05). We speculate that prior hypercapnic exposure may increase peripheral chemoreceptor response to subsequent hypoxic exposures and result in perturbed neonatal respiratory control.

Breath-to-breath hypercapnic response in neonatal rats: temperature dependency of the chemoreflexes and potential implications for breathing stability

The breathing of newborns is destabilized by warm temperatures. We hypothesized that in unanesthetized, intact newborn rats, body temperature (T(B)) influences the peripheral chemoreflex response (PCR response) to hypercapnia. To test this, we delivered square-wave challenges of 8% CO(2) in air to postnatal day 4-5 (P4-P5) rats held at a T(B) of 30 degrees C (Cold group, n = 11), 33 degrees C (Cool group, n = 10), and 35 degrees C thermoneutral zone group [thermoneutral zone (TNZ) group, n = 11], while measuring ventilation (Ve) directly with a pneumotach and mask. Cool animals were challenged with 8% CO(2) balanced in either air or hyperoxia (n = 10) to identify the PCR response. Breath-to-breath analysis was performed on 30 room air breaths and every breath of the 1-min CO(2) challenge. As expected, warmer T(B) was associated with an unstable breathing pattern in room air: TNZ animals had a coefficient of variation in Ve (Ve CV%) that was double that of animals held at cooler T(B) (P < 0.001). Hyperoxia markedly suppressed the hypercapnic ventilatory response over the first 10 breaths (or approximately 4 s), suggesting that this domain is dominated by the PCR response. The PCR response (P = 0.03) and total response (P = 0.04) were significantly greater in TNZ animals compared with hypothermic animals. The total response had a significant, negative relationship with Vco(2) (R(2) = 0.53; P < 0.001). Breathing stability was positively related to the total response (R(2) = 0.36; P < 0.001) and to a lesser extent, the PCR response (R(2) = 0.19; P = 0.01) and was negatively related to Vco(2) (R(2) = 0.34; P < 0.001). ANCOVA confirmed a significant effect of T(B) alone on breathing stability (P < 0.01), with no independent effects of Vco(2) (P = 0.41), the PCR response (P = 0.82), or the total Ve response (P = 0.08). Our data suggest that in early postnatal life, the chemoreflex responses to CO(2) are highly influenced by T(B), and while related to breathing stability, are not predictors of stability after accounting for the independent effect of T(B).

Evolutionary trends in airway CO2/H+ chemoreception

In many species of air-breathing vertebrates CO2-sensitive airway receptors play an important role in ventilatory control. In ectotherms, olfactory receptors often inhibit breathing and prolong breath holding when environmental CO2 levels are high. CO2/H+ sensitive pulmonary receptors (intra pulmonary chemoreceptors (IPC) and pulmonary stretch receptors (PSR)) regulate breathing patterns in all vertebrates in a manner that reduces dead space ventilation and enhances the efficiency of CO2 excretion under conditions of environmental hypercarbia, and/or reduces CO2 loss from hyperventilation. The greater CO2 sensitivity of IPC may allow them to also serve as a venous CO2 receptor (at least transiently when levels of metabolically produced CO2 begin to rise), prevent alkalosis during hyperpnea/polypnea, and may have contributed to the evolution of the extremely thin air/blood barrier and increased diffusion capacity associated with the rigid avian lung. The presence of all three receptor groups with different degrees of CO2 sensitivity in most reptiles, however, gives rise to what appear to be anomalous responses to environmental CO2.

Lack of involvement of mu(1) opioid receptors in dermorphin-induced inhibition of hypoxic and hypercapnic ventilation in rat pups

The effects of dermorphin, a mu-selective opioid agonist, on respiratory responses to altered O(2) and CO(2) during postnatal development were investigated in conscious, unrestrained Wistar rats aged 2-21 days. Respiration was recorded by barometric plethysmography. Dermorphin (4 mg kg(-1)) was administered subcutaneously, and the ventilatory responses to hypoxia (11% O(2), 89% N(2)) in 2-21-day-old pups and hyperoxia (100% O(2)), and hypercapnia (8% CO(2), 92% O(2)) in 2-13-day-old pups were assessed in the presence and absence of the mu(1) receptor antagonist naloxonazine (10 mg kg(-1) s.c.) administered 1 day before testing. Six minutes of hypoxia increased ventilation in all age groups, largely via an increase in frequency. Dermorphin inhibited the ventilatory response to hypoxia, and this inhibition was insensitive to naloxonazine. After 5 min of hyperoxia, ventilation was the same as with air breathing except in the presence of dermorphin, when hyperoxic ventilation was depressed by a naloxonazine-insensitive decrease in frequency. Following this 5 min 100% O(2) exposure, pups were exposed to hypercapnia, and respiratory parameters were measured 5 min later. The ventilatory response to CO(2) was inhibited by dermorphin in a naloxonazine-insensitive manner. There was no evidence for endogenous mu(1) receptor modulation of the ventilatory responses to altered gases in rat pups of any age. Thus, mu opioid-induced inhibition of the hypoxic and hypercapnic responses in young rats does not occur via activation of mu(1) opioid receptors.

Olfactory CO(2) chemoreceptors

Amphibians and reptiles possess CO(2)-sensitive olfactory receptors that cause a dose-dependent decrease in breathing when stimulated by CO(2) concentrations ranging from 0.5 to 8%. In amphibians, it has been shown that inhibition of the enzyme, carbonic anhydrase (CA), attenuates the response of CO(2)-sensitive olfactory receptors to transient changes in nasal CO(2). Histology and electrophysiology studies in frogs show that identification of sites of CA activity can serve as markers for locations of CO(2) chemosensitivity in the olfactory epithelium. There is also growing evidence that CO(2) receptors may be present in the olfactory epithelium of mammals. The objectives of this review are to, (1) summarize the current state of knowledge of olfactory CO(2) receptors in amphibians, reptiles, and mammals; (2) present results from an experiment designed to determine the distribution and density of CA activity within the rat nasal cavity; (3) show results from an experiment that recorded the olfactory receptor response to CO(2) in areas of the rat nasal cavity exhibiting the highest densities of CA activity; and (4) discuss the presumed role of the olfactory CO(2) receptors in the control of breathing and in abnormalities of breathing, such as sudden infant death syndrome (SIDS).

Development of in vivo ventilatory and single chemosensitive neuron responses to hypercapnia in rats

We used pressure plethysmography to study breathing patterns of neonatal and adult rats acutely exposed to elevated levels of CO2. Ventilation (VE) increased progressively with increasing inspired CO2. The rise in VE was associated with an increase in tidal volume, but not respiratory rate. In all animals studied, the CO2 sensitivity (determined from the slope of the VE vs. inspired % CO2 curve) was variable on a day to day basis. Chemosensitivity was high in neonates 1 day after birth (P1) and fell throughout the first week to a minimum at about P8. Chemosensitivity rose again to somewhat higher values in P10 through adult rats. The developmental pattern of these in vivo ventilatory responses was different than individual locus coeruleus (LC) neuron responses to increased CO2. The membrane potential (V(m)) of LC neurons was measured using perforated patch (amphotericin B) techniques in brain slices. At all ages studied, LC neurons increased their firing rate by approximately 44% in response to hypercapnic acidosis (10% CO2, pH 7.0). Thus the in vivo ventilatory response to hypercapnia was not correlated with the V(m) response of individual LC neurons to hypercapnic acidosis in neonatal rats. These data suggest that CO2 sensitivity of ventilation in rats may exist in two forms, a high-sensitivity neonatal (or fetal) form and a lower-sensitivity adult form, with a critical window of very low sensitivity during the period of transition between the two (approximately P8).