Central neural respiratory response to carotid sinus nerve stimulation in newborns

Mechanisms regulating hypoxic respiratory depression during fetal and postnatal life

Selected topics in the respiratory response to acute hypoxia in the fetus and newborn are reviewed. Peripheral chemoreceptors acting through ionotrophic glutamate receptors play an important role in affecting the initial augmentation phase. Whether fall off in peripheral chemoreceptor activity contributes to the secondary depressive phase remains controversial. A number of approaches including permanent electrolytic and reversible cooling lesions, Fos protein activation, and double-labeling immunohistochemistry has converged to show that an area in and around the locus ceruleus in the rostral pons affects the central depression. There is evidence that this is mediated by catecholamines acting at alpha(2)-adrenergic receptors. Tonic activity in early expiratory (postinspiratory) neurons may contribute to hypoxia-induced apneic episodes in the fetus and newborn. Desensitization of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors has been demonstrated in respiratory-related neurons both in vivo and in vitro. The role that this process might play in the depressive phase of the hypoxic ventilatory response has not been established. In vitro experiments with isolated brain stem-spinal cord preparations or transverse brain stem slices usually involve anoxia, whereas whole animal experiments use 8-15% O(2). Therefore, caution must be exercised in attempting to construct a unifying framework from these two approaches.

Carotid bodies and ventilatory response to hypoxia in aminophylline-treated piglets

Peripheral chemoreceptors may be immature in neonatal animals, exhibiting maturational changes in the perinatal period. Even though methylxanthines are respiratory stimulants, many premature neonates do not respond to them. Thus, we hypothesized that carotid body activity is necessary for aminophylline to reverse hypoxia-induced respiratory depression. We exposed 16 anesthetized newborn piglets (age 2-7 days) to hypoxia (inhalation of 12% oxygen) for 5 min. Aminophylline (15 mg/kg iv) was administered either prior to (11 piglets) or following (5 piglets) carotid body denervation (CBD). Before CBD, hypoxia elicited transient initial increases in tidal volume (from 79 +/- 4 to 99 +/- 1% of maximum, mean +/- SE), minute ventilation (from 64 +/- 5 to 93 +/- 4%), and peak phrenic electroneurogram (from 63 +/- 8 to 91 +/- 6%, all P < 0.05). This was followed by a decrease in tidal volume, minute ventilation and phrenic electroneurogram (all P < 0.05). Prior to CBD, aminophylline pretreatment prevented the decrease in all the measures of respiratory output during late hypoxia. After CBD, hypoxia induced an initial and sustained depression of ventilation (tidal volume from 100 to 33 +/- 14%; frequency from 94 +/- 4 to 42 +/- 17%; minute ventilation from 100 to 32 +/- 14%, all P < 0.05) and phrenic electroneurogram (peak phrenic from 100 to 47 +/- 18%; minute phrenic from 85 +/- 6 to 55 +/- 21%, both P < 0.05). Administration of aminophylline after CBD did not prevent the profound respiratory depression elicited by hypoxia in the chemodenervated piglets.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of hypoxia on the ventilatory response to sudden changes in CO2 in newborn piglets

1. The ventilatory response to square-wave challenges in end-tidal partial pressure of CO2 (PCO2) was investigated at three levels of arterial PO2 (Pa,O2) in nineteen anaesthetized 2- to 11-day-old piglets. 2. The ventilatory responses, measured on a breath-to-breath basis, were separated into a peripheral and a central component using a two-compartment model. Both components were described by a CO2 sensitivity, a time constant, a time delay and a single offset. 3. Fifty-six responses were analysed against a background of normoxaemia (Pa,O2 = 12.70 +/- 0.72 kPa, mean +/- S.D.), fifty-three against a background of moderate hypoxaemia (Pa,O2 = 8.63 +/- 0.34 kPa) and fifty-one against a background of severe hypoxaemia (Pa,O2 = 4.98 +/- 0.30 kPa). 4. The sensitivity of the peripheral chemoreceptors in mediating the response to CO2 increased from 38.3 +/- 17.0 ml min-1 kPa-1 kg-1 during normoxaemia to 48.8 +/- 15.3 ml min-1 kPa-1 kg-1 during moderate hypoxaemia and to 72.9 +/- 24.0 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 5. As compared with the central CO2 sensitivity during moderate hypoxaemia and normoxaemia (104.0 +/- 39.0 and 100.8 +/- 41.6 ml min-1 kPa-1 kg-1, respectively) it decreased to 85.9 +/- 54.1 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 6. We conclude that in newborn piglets there is a positive interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors while severe hypoxaemia reduced the CO2 sensitivity centrally.

Maturation of carotid chemoreceptor sensitivity to hypoxia: in vitro studies in the newborn rat

1. A preparation was developed to record single-fibre chemoreceptor afferent activity from carotid bodies of newborn and adult rats in vitro. The response to severe hypoxia was studied as a function of developmental age in four age groups: 1-2, 4-7, 10-15 days and adult (25-30 days). 2. During superfusion with HEPES-saline at room PO2 and at 26 or 35 degrees C, afferent chemoreceptor activity could be recorded in all age groups. No significant difference was found among groups in baseline discharge frequency at 26 or 35 degrees C. 3. All chemoreceptors responded to anoxia (PO2 congruent to 0 Torr) with a rapid increase in discharge frequency. At 35 degrees C, peak discharge frequency of single chemoreceptor afferents was significantly greater in the adult (15.7 +/- 1.6 Hz, mean +/- S.E.M., n = 18) and 10-15 days (11.2 +/- 4.2, n = 8) compared to rats of 1-2 days (4.3 +/- 0.7, n = 14) and 4-7 days of age (3.9 +/- 1.0, n = 7). 4. At 2 min into the anoxia period, all chemoreceptor activities were reduced from their peak discharge levels. At 35 degrees C, this decrease was significantly greater in the adult compared to the newborn. 5. During the period of decreased activity during anoxia, the chemoreceptor discharge could not be increased by inter-stream injection of acetylcholine (100 micrograms) or dopamine (100 micrograms), although these drugs were effective in increasing discharge rate prior to hypoxia. 6. We conclude that: (1) postnatal maturation of chemoreceptor sensitivity to hypoxia is present in vitro, (2) maturation occurs between the first and second week after birth in the rat, and (3) the decrease in activity during prolonged anoxia is not greater in the newborn compared to the adult. Thus, maturational changes occur in the sensitivity of the glomus cell-nerve ending complex to hypoxia.

Efferent phrenic nerve and respiratory neuron activities in the developing kitten: spontaneous discharges and hypoxic responses

Efferent phrenic nerve and medullary respiratory neuron discharges were examined for age-dependent changes of activities during normocapnic hyperoxia and hypoxia in anesthetized and decerebrate kittens (22-150 days old). In animals less than 39 days of age, phrenic power spectra during hyperoxia were dominated by components in the medium-frequency band (20-50 Hz), whereas spectra of animals of at least 39 days of age were dominated by components in the high-frequency band (50-100 Hz). Such high-frequency oscillations were also observed in the power spectra of some inspiratory neurons in animals of at least 43 days old. In hypoxia, the amplitude of phrenic discharge exhibited an initial facilitation followed by a diminution (i.e. biphasic response) in animals 39 days old or younger. In animals older than 39 days, however, hypoxia elicited a sustained facilitation of phrenic discharge amplitude. In contrast, no such age-dependent change in response pattern to hypoxia was observed for neuronal discharges; rather, responses of most neurons consisted of either decreases of discharge frequency, or complete abolishment of discharges.

Discharge properties of dorsal medullary inspiratory neurons in newborn pigs

The discharges of medullary inspiratory neurons were recorded in newborn pigs. They were classified by discharge pattern; response to lung inflation; synaptic relation to phrenic motoneurons. Our results showed: these neurons have similar discharge patterns and responses to lung inflation as adult cats; most neurons do not project to phrenic motoneurons. It is suggested that our sampled population of neurons is involved in integrating pulmonary afferent inputs.