Neuroscience Letters 420 (2007) 268–272 Learned defense response to hypoxia in newborn mice Bieke Bollen a,b,1 , Myriam Bouslama a,c,1 , Boris Matrot a,c , Rudi D’Hooge b , Omer Van den Bergh d , Jorge Gallego a,c,∗ a INSERM, U676, Robert-Debr´ e Teaching Hospital, 75019 Paris, France b Laboratory of Biological Psychology, Department of Psychology, University of Leuven, B-3000 Leuven, Belgium c University Paris 7, Denis Diderot Medical School, IFR02, 10 av. de Verdun, 75010 Paris, France d Research Group for Stress, Health & Well-Being, Department of Psychology, University of Leuven, B-3000 Leuven, Belgium Received 8 February 2007; received in revised form 24 April 2007; accepted 6 May 2007 Abstract In newborns, hypoxia elicits defensive behaviors including awakening from sleep, body movements and crying. An inability to produce this defense response is a risk factor for sudden infant death syndrome and other respiratory control disorders. In this study, we examined the possibility that the defense response to hypoxia in newborns is partly determined by early exposure to hypoxia. We explored this possibility in 6-day-old mice, which resemble human preterm infants of approximately 25–30 weeks’ gestational age. Ultrasonic vocalizations (USVs) were recorded as a marker for the defense response to hypoxia. In a conditioning experiment, newborn mice were exposed to two artificial odors (conditioned stimuli, CS). For acquisition (two trials), pups were exposed to one odor (CS+) in a hypoxic gas mixture (10% O 2 , which was the unconditioned stimulus, US) and to another odor (CS-) in air. Then, the pups were exposed to each odor while breathing air. Newborn mice produced significantly more USVs when exposed to the odor previously paired with hypoxia than to the control odor. Thus, associative learning may shape the defense response to hypoxia in newborns. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Classical conditioning; Olfaction; Apnea In newborns, hypoxia elicits autonomic and behavioral responses that include increased ventilation, arousal from sleep, defensive movements, and alerting cries [18,32]. This sequence is triggered by chemoreceptors located in the carotid bodies. Chemoreceptors detect changes in arterial blood oxygen pres- sure (PaO 2 ) and send sensory information to the brainstem respiratory networks. Within seconds, after a PaO 2 decrease is sensed, ventilation increases. If hypoxia persists, venti- lation decreases while the metabolic rate falls [22]. In the event that PaO 2 is not rapidly restored by these autonomic adjustments, a defense response including arousal from sleep, movements, and crying is initiated [18]. Inability to produce this defense response to hypoxia is crucial in preterms, because respiratory control immaturity promotes the occurrence of hypoxic apneas [21]. This inability is associated with sudden ∗ Corresponding author at: INSERM U676, Hˆ opital Robert-Debr´ e, 48 Bd S´ erurier, 75019 Paris, France. Tel.: +33 1 40 03 47 81; fax: +33 1 40 03 47 70. E-mail address: gallego@rdebre.inserm.fr (J. Gallego). 1 These authors contributed equally to this study. infant death syndrome [18,29,33] and sleep apnea syndrome [17,32]. The aim of this study was to determine whether conditioning generated by previous exposure to hypoxia shapes the defense response to hypoxia. An intuitively appealing hypothesis is that classical conditioning, which is an important determinant of defensive behaviors [13], improves the ability of newborns to cope with hypoxia. This possibility is supported by the ability of newborns to learn a preference for a novel odor within the first few hours after birth [30]. Furthermore, infants repeatedly exposed to unpleasant medical interventions cry in anticipation of stimuli, indicating an ability to learn defense responses [31]. Based on these results, we reasoned that associative learning might be an important determinant of the defense response to hypoxia. We tested this hypothesis in 6-day-old mice (P6), which show similar brain development to preterms of 25–30 weeks’ gesta- tional age [20]. We analyzed the defense response to hypoxia by recording ultrasonic vocalizations (USVs) [8]. Acquisition consisted in exposing newborn mice to a novel odor (condi- tioned stimulus, CS+) while lowering the fraction of inspired O 2 0304-3940/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2007.05.012