April E. Ronca

Heart rate response of the rat fetus and neonate to a chemosensory stimulus

Resting heart rate (HR) and HR responses of fetal and neonatal rats are described before and after intraoral infusion of isotonic saline or lemon solution. Stable measurements of resting HR were obtained for fetuses over the last three days of gestation (E19, E20, E21) and pups on the day of birth (P0) and four subsequent postnatal ages (P1, P3, P5, P7). Resting HR decreased significantly on P0 relative to the three prenatal ages and exhibited a linear increase thereafter. Variability in resting HR was pronounced on E21, decreased sharply after birth, and gradually increased through P7. Developmental changes in the HR response of fetuses and pups were evident following infusion of lemon. Fetal HR responses to lemon were characterized by bradycardia, which increased in magnitude through P1, diminished after P1, and eventually changed to tachycardia by P7. Both resting HR and HR responses to chemosensory stimulation point to the immediate perinatal period as a time of quantitative and qualitative change during sensory development.

Simulated uterine contractions facilitate fetal and newborn respiratory behavior in rats

We tested the hypothesis that sensory and nonsensory factors associated with birth stimulate respiratory behavior in the fetal and newborn rat. Late gestation (Day 21) rat fetuses were externalized from the uterus with intact umbilical connections to the dam and exposed to stimuli normally associated with labor and delivery. Onset and maintenance of respiratory movements were monitored. In the first experiment, rat fetuses were exposed to either: (i) simulated uterine contractions; (ii) cooling (26 degrees C); (iii) umbilical cord occlusion; or (iv) air heated to intrauterine temperature (37.5 degrees C). Subjects were videotaped for 1 h, and respiratory movements counted during tape review. Fetuses in each group showed some respiratory behavior although compression significantly elevated respiratory rate compared to other experimental conditions. All subjects in each group were respiring after 1 h, except for pups that received umbilical cord occlusion. The 100% attrition rate of the cord occlusion-alone group was reversed by combining cord occlusion with compression, or with compression and cooling, but not by combining cord occlusion with cooling. Simulated birth pups (i.e., those exposed to compression, cooling and umbilical cord occlusion) and normal, vaginally delivered pups breathed at identical rates and showed a similar pattern of postpartum breathing. These results suggest that mechanical stimulation of the fetus associated with uterine contractions plays a critical role in the maintenance of continuous respiration at birth. Possible mechanisms for the facilitative effects of compression on perinatal breathing are discussed.

Heart rate development and sensory-evoked cardiac responses in perinatal rats

Sensory-evoked changes in heart rate (HR) are often used to assess sensory function in infants, but HR measures have not been applied in studies of perinatal sensory function. Normative HR data are needed to judge whether baseline characteristics and response properties of the late fetal and newborn heart are appropriate as a dependent measure in sensory studies. We compared parameters of HR development and phasic cardiac responses to stimulation across the perinatal period in Norway rats. EKG was measured in cesarean-delivered prenatal rats (embryonic day 20-day of birth; conception = E0) and compared to both cesarean and vaginally delivered neonates (day of birth-postnatal day 2; birth = P0). Resting heart rate increased monotonically across the perinatal period, except between E21 and DOB (Experiment 1). During an hour-long test, HR was strong and stable. Variations in HR across successive measurement intervals (10 sec) were uniformly low (less than 2%) in all age groups. In Experiment 2, fetuses and neonates responded with dramatic HR decelerations (20-80 bpm) to an intraoral infusion of lemon extract. HR responses did not appear to be reflexively mediated by changes in respiration. These findings suggest that parameters of resting HR and cardiac responses to stimulation are well suited to studies of perinatal sensory function.

The experience of being born: a natural context for learning to suckle.

Understanding the developmental origins of congenital capabilities such as sucking is fundamental knowledge that can contribute to improving the clinical management of early feeding and facilitate the onset of oral ingestion. We describe analyses in rats showing that sensory stimulation in utero and during birth establishes the newborns sucking responses to maternal cues. We mimicked elements of labor and delivery (viz., compressions simulating labor contractions, stroking simulating postnatal maternal licking of the newborn, and postnatal thermal flux), and used them to induce postnatal respiration and nipple attachment in caesarian-delivered pups. We report herein new data showing that, by simulating a fetal rats experience of being born, specific components of vaginal birth provide sufficient conditions for the odor learning that guides newborns sucking responses. In contrast, the absence of in utero compressions was associated with poor sucking onset. Knowing how birth stimuli contribute to the first nipple attachment and constitute a context for learning to suckle is an important step toward better management of some early feeding problems. It can serve also as a foundation for understanding the challenges of facilitating sucking by babies born prematurely so that they do not experience the typical contingencies mediating onset of oral ingestion.