Development of fear-related heart rate responses in neonatal rabbits.
Classical simple conditioning of heart rate (HR) was studied in rabbits between the 1st and 18th neonatal day. An auditory stimulus (1000 Hz, 5 s) served as the conditioned stimulus (CS), and a train of electric impulses (100 Hz, 500 ms, 1-1.5 mA) was used as the unconditioned stimulus (US). HR responses developed during orientation session (CS-alone) as well during acquisition (CS-US paired) were analyzed and compared to those developed by young adult rabbits (3-month-old). In all neonatal animals tested, baseline HR measured during an adaptation session preceding conditioning, was similar though significantly higher than that measured in adult rabbits (Newman-Keuls P < 0.05). Before the 10th neonatal day, the animals did not show either somatomotor or HR orienting responses to the CS-alone presentations. Consequently, since orienting reactions play a necessary role in the formation and manifestation of conditioned reflexes, 1 to 10-day-old infant rabbits were not submitted to the acquisition session. All the other neonatal groups, while showing orienting behaviours similar to those exhibited by adults (head and pinna movement), presented different patterns of HR orienting responses (no response, bradycardia, tachycardia, bradycardia/tachycardia etc.). As for the acquisition session, the first bradycardic response, similar to that developed by adult rabbits, was found in 18-day-old rabbits. However, also in this neonatal group the amplitude of the conditioned response was significantly smaller when compared to that exhibited by young adults (Newman-Keuls P < 0.01). In addition, in some of the 10-day-old neonates, HR appeared very unstable and dropped to very low values (as low as 146 beats/min) early during conditioning, apparently as a consequence of CS-US association. As for the unconditioned response, no differences were found between adult rabbits and the neonatal animals older than 12 days. In contrast, most of the 10-day-old rabbits showed either bradycardia or no response to the unconditioned stimulus. Considering the ability of mammalian infants to learn somatomotor conditioned responses at early stages of maturation, conditioning of HR responses occurs late during ontogeny. Since this incapacity to show HR conditioned responses before the 18th postnatal day cannot be ascribed to their inability to show phasic HR changes nor to a failure in detecting the auditory stimulus, these results suggest the possibility that HR conditioned responses may be mediated by neural structures developing later during maturation.