Breathing in fetal lambs: the effect of brain stem section.

Abstract

SUMMARY1. The effects of section of the brain stem caudally (through the upper pons or mid-collicular) or rostrally (through the caudal hypothalamus or anterior commissuresuprachiasmatic nucleus) were studied in fetal lambs from 118 days gestation, after recovery in utero.2. In lambs sectioned caudally, breathing movements and electrocortical activity were dissociated. After some days recovery breathing tended to become continuous, with an abnormal prolongation of inspiratory time. Isocapnic hypoxia caused an increase in the rate and amplitude of breathing. After carotid denervation hypoxia still caused an increase in the amplitude of breathing. 3. In lambs sectioned rostrally, there was also dissociation between breathing movements and electrocortical activity. Breathing remained episodic, with an incidence similar to that of intact fetal lambs. In two lambs after 10 days of recovery the breathing and electrocortical rhythms returned, from time to time, to their normal phasic relationship. Isocapnic hypoxia caused a diminution or arrest of breathing, as in intact lambs.4. The cardiovascular effects of transaction were examined. Baroreflex sensitivity was normal in those lambs sectioned caudally and enhanced in those sectioned rostrally.5. It is concluded first that as a result of rostral section, independent episodic rhythms of fetal breathing and electrocortical activity can be dissociated. Secondly, moderate isocapnic hypoxia causes arrest of fetal breathing indirectly, requiring the integrity of a suprapontine mechanism. And thirdly, after caudal section of the brain stem, hypoxia causes enhancement of fetal breathing efforts, independently of the carotid chemoreceptors. Possible mechanisms are discussed.

Developmental changes in central O2 chemoreflex in Rana catesbeiana: the role of noradrenergic modulation

“…Although the use of less severe hypoxia may be more physiologically relevant, we chose this level mainly to reproduce previous in vitro studies (Brockhaus et al, 1993;Winmill et al, 2005). Even so, results obtained in adult bullfrog brainstems were similar to the hypoxic responses reported in intact frogs (Rose and Drotman, 1967), newborn lambs (Dawes et al, 1983;Moore et al, 1996) and newborn rats in vitro (Brockhaus et al, 1993). This protocol allowed us to note that the fictive breathing response to central hypoxia is restricted to lung ventilation as fictive buccal movements were not affected by this stimulus.…”

“…Several factors can activate NA neurons to elicit neurotransmitter release and data show that during hypoxia, activation of pontine noradrenergic neurons such the locus coeruleus (A6) and the A5 group contributes to the changes in respiratory activity in response to this condition. For instance, activation of LC neurons is necessary for the ventilatory depression observed during hypoxia in newborn lamb as brainstem transections or focal cooling of LC neurons abolished this response (Dawes et al, 1983;Moore et al, 1996). However, the mechanisms linking noradrenergic modulation and hypoxic ventilatory depression in newborn animals are not well understood (Bissonnette, 2000).…”

The effect of cooling on breathing and shivering in unanaesthetized fetal lambs in utero.

“…Thermistors (sensitive to + 0-01 0() were placed in the fetal cranium to lie below the frontal lobes, amniotic sac and maternal inferior vena cava, and in some fetuses an additional thermistor was placed in either the rectum or stomach (Gunn & Gluckman, 1983). Pairs of stainless-steel wires were implanted onto the parietal dura, on the upper and lower eyelid, into the posterior nuchal muscles and into either a shoulder (latissimus dorsi) or limb muscle (biceps or triceps) (Dawes, Gardner, ,Johnston & Walker, 1983;Clewlow, Dawes, ,Johnston & Walker, 1983). In ten fetuses an 'external' cooling coil consisting of 5 m of McGaw V45 20 blood warming coil was placed around the fetal neck and thorax (Gunn & Gluckman, 1983).…”

The response to hypoxia of arterial chemoreceptors in fetal sheep and new‐born lambs.

“…This suggested that it is not functional immaturity of the chemoreceptors which prevents the normal adult response to hypoxia being seen in intact fetuses, but the intervention of some process which inhibits the reflex response. These observations (published in full by Dawes, Gardner, Johnston & Walker, 1983) made it necessary to re-examine the question of the activity and responses of the arterial chemoreceptors in fetal and new-born lambs.…”