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

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% N(2), 3% CO(2)). Medullary 5-HT content was reduced 80% by 5,7-DHT treatment (P < 0.001). Baseline ventilation (V(E)), metabolic rate (V(O(2))), ventilatory equivalent (V(E)/V(O(2))), heart rate (HR), heart rate variability (HRV) and arterial haemoglobin saturation (S(aO(2))) 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, V(E)/V(O(2)) or S(aO(2)) 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.

Failed heart rate recovery at a critical age in 5-HT-deficient mice exposed to episodic anoxia: implications for SIDS.

Mice deficient in the transcription factor Pet-1⁻/⁻ have a ∼70% deficiency of brainstem serotonin [5-hydroxytryptamine (5-HT)] neurons and exhibit spontaneous bradycardias in room air at postnatal day (P)5 and P12 and delayed gasping in response to a single episode of anoxia at P4.5 and P9.5 (Cummings KJ, Li A, Deneris ES, Nattie EE. Am J Physiol Regul Integr Comp Physiol 298: R1333-R1342, 2010; and Erickson JT, Sposato BC. J Appl Physiol 106: 1785-1792, 2009). We hypothesized that at a critical age Pet-1⁻/⁻ mice will fail to autoresuscitate during episodic anoxia, ultimately dying from a failure of gasping to restore heart rate (HR). We exposed P5, P8, and P12 Pet-1⁻/⁻ mice and wild-type littermates (WT) to four 30-s episodes of anoxia (97% N2-3% CO2), separated by 5 min of room air. We observed excess mortality in Pet-1⁻/⁻ only at P8: 43% of Pet-1⁻/⁻ animals survived past the third episode of anoxia while ∼95% of WT survived all four episodes (P = 0.004). No deaths occurred at P5 and at P12, and one of six Pet-1⁻/⁻ mice died after the fourth episode, while all WT animals survived. At P8, dying Pet-1⁻/⁻ animals had delayed gasping, recovery of HR, and eupnea after the first two episodes of anoxia (P < 0.001 for each); death ultimately occurred when gasping failed to restore HR. Both high- and low-frequency components of HR variability were abnormally elevated in dying Pet-1⁻/⁻ animals following the first episode of anoxia. Dying P8 Pet-1⁻/⁻ animals had significantly fewer 5-HT neurons in the raphe magnus than surviving animals (P < 0.001). Our data indicate a critical developmental window at which a brainstem 5-HT deficiency increases the risk of death during episodes of anoxia. They may apply to the sudden infant death syndrome, which occurs at a critical age and is associated with 5-HT deficiency.