Protocols adhered to APS/NIH guidelines for humane use of animals for research and were prospectively approved by the IACUC at the University of Utah Health Sciences Center. We used the brain of preterm lambs of both sexes for which pulmonary outcomes are reported5759 to secondarily analyze the hippocampus.

Preterm lambs (~130 days gestation; term ~150 days) were delivered via cesarean section and divided into two groups: IMV or NRS.5759 Matched subgroups were managed for 3 days (n = 4/group) or 21 days (n = 4/group; Table 1; Supplementary Material Fig. 1). Block randomization was used for assignment before delivery.

Demographic characteristics of lambs stratified by group.

IMV invasive mechanical ventilation, NRS noninvasive respiratory support.

No statistical differences, except as indicated: *p < 0.05 compared to the preterm lamb groups and unventilated control fetal group by ANOVA and Fisher’s PLSD post hoc test.

Fetal lambs were exposed to antenatal steroid, perinatal surfactant, and postnatal caffeine citrate (details are provided in the Supplementary Material). Lambs were endotracheally intubated for resuscitation and continuing IMV (Bird VIP ventilator; model 15215, Palm Springs, CA). Initial ventilator settings were respiratory rate of 60 breaths/min, inspiratory time of 0.32 s, peak inspiratory pressure (PIP) of 21 cmH2O, and positive end-expiratory pressure of 8 cmH2O. Fractional inspired oxygen started at 0.50 and was adjusted to attain target oxygen saturation 88–92% by pulse oximetry (model SurgiVet V9200IBP/Temp, Smith Medical ASD Inc., St. Paul, MN). PIP was adjusted to attain target ventilation (PaCO2) between 45 and 60 mmHg, resulting in pH between 7.25 and 7.35. Target expiratory tidal volume, measured by the ventilator, was 5–7 mL/kg. Details of monitoring, feeding, fluid homeostasis, and treatment for infection are provided in the Supplementary Material.

Management of anesthesia was different between the preterm groups. The IMV group was kept anesthetized (3–5 mg/kg pentobarbital sodium, intravenously (i.v.)) and given buprenorphine hydrochloride (5 μg/kg, i.v., every 6 h; Reckitt and Colman Pharmaceuticals, Richmond, VA). Pentobarbital sodium was given as needed to prevent breathing over the ventilator, swallowing, or chewing. The NRS group was extubated ~3 h after preterm birth. At transition, an uncuffed 3.0 Murphy tube was placed in one nostril.5759 Preterm lambs managed by NRS were given buprenorphine hydrochloride only to allow effective spontaneous breathing.

At the end of the 3-day study or the 21-day study, preterm lambs were given heparin (1000 U, i.v.). The NRS group was reintubated and briefly managed with the same ventilator settings during NRS. All lambs were euthanized, using Beuthanasia solution (65 mg/kg; Intervet Inc., Madison, NJ). The brain was removed and chilled during dissection to remove the hippocampus, which was snap frozen in liquid nitrogen and stored at −80 °C.

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