Postnatal loss of brainstem serotonin neurones compromises the ability of neonatal rats to survive episodic severe hypoxia.

Non-Technical Summary Failure to withstand severe hypoxia (failed autoresuscitation) has been documented in sudden infant death syndrome (SIDS) cases, and SIDS is associated with a constellation of defects within the brainstem serotonin system, including reduced serotonin content. Neonatal mice with reduced numbers of serotonergic neurones beginning in utero also have major defects in autoresuscitation at the beginning of the second postnatal week. Here we injected a neurotoxin into the brainstems of 2- to 3-day-old rat pups, reducing serotonin content by ∼80%, and at P7–10 tested their ability to autoresuscitate when challenged with repeated episodes of environmental anoxia. Pups with reduced serotonin content have delayed gasping in response to each challenge, as well as a significantly higher mortality by the last episode. These new data imply that a postnatal loss of brainstem serotonin in infants could increase the risk of SIDS during severely hypoxic conditions. Abstract Pet-1 −/− mice with a prenatal, genetically induced loss of 5-hydroxytryptamine (5-HT, serotonin) neurones are compromised in their ability to withstand episodic environmental anoxia via autoresuscitation. Given the prenatal role of 5-HT neurones in the development of neural networks, here we ask if a postnatal loss of 5-HT neurones also compromises autoresuscitation. We treated neonatal rat pups at postnatal day (P)2–3 with an intra-cisternal injection of 5,7-dihydroxytryptamine (5,7-DHT; ∼40 μg; n= 8) to pharmacologically lesion the 5-HT system, or vehicle (control; n= 14). At P7–10 we exposed unanaesthetized treated and control pups to 15 episodes of environmental anoxia (97% N2, 3% CO2). Medullary 5-HT content was reduced 80% by 5,7-DHT treatment (P < 0.001). Baseline ventilation (), metabolic rate (), ventilatory equivalent (/), heart rate (HR), heart rate variability (HRV) and arterial haemoglobin saturation () were no different in 5-HT-deficient pups compared to controls. However, only 25% of 5-HT-deficient pups survived all 15 episodes of environmental anoxia, compared to 79% of control littermates (P= 0.007). High mortality of 5,7-DHT-treated pups was associated with delayed onset of gasping (P < 0.001), delayed recovery of HR from hypoxic-induced bradycardia (P < 0.001), and delayed recovery of eupnoea from hypoxic-induced apnoea (P < 0.001). Treatment with 5,7-DHT affected neither the gasping pattern once initiated, nor HR, / or during the intervening episodes of room air. A significant increase in HRV occurred in all animals with repeated exposure, and in 5-HT-deficient pups this increase occurred immediately prior to death. We conclude that a postnatal loss of brainstem 5-HT content compromises autoresuscitation in response to environmental anoxia. This report provides new evidence in rat pups that 5-HT neurones serve a physiological role in autoresuscitation. Our data may be relevant to understanding the aetiology of the sudden infant death syndrome (SIDS), in which there is medullary 5-HT deficiency and in some cases evidence of severe hypoxia and failed autoresuscitation.