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.