Acquisition and retention of habituation in the preweanling rat

The intensity of a fetal taste aversion is modulated by the anesthesia used during conditioning

Rat fetuses (E18) can learn a taste aversion in utero if experience with a sweet flavor (saccharin = Sac) is followed by a malaise-producing injection of lithium chloride (LiCl). Here we report that this phenomenon can be significantly modulated by the type of anesthesia administered to the pregnant dam before the conditioning procedure. Dams were anesthetized with one of the following drugs or drug combinations: (1) sodium pentobarbital; (2) ketamine hydrochloride and xylazine; or (3) sodium pentobarbital and ketamine hydrochloride. While under the influence of these anesthetics, rat fetuses received pairings of Sac + LiCl or one of the following sets of oral and systemic (i.p.) control injections: Sac + Saline, H2O + LiCl; H2O + Saline. At age 15 days neonatal rats were given a taste preference test by allowing them to select nipples painted with either saccharin or vehicle (H2O). After weaning, rats were given an additional taste preference test where they were allowed to drink from bottles filled with either 0.30% saccharin or water. Neonates that received Sac + LiCl injections avoided saccharin-painted nipples while neonates that received control injections in utero preferred saccharin-painted nipples. Rats that acquired the taste aversion under the influence of ketamine showed a significantly stronger conditioned taste aversion on the nipple preference test than did those from dams injected with sodium pentobarbital. The conditioned taste aversion was not detectable later during the bottle preference test. Since ketamine blocks N-methyl-D-aspartate (NMDA) glutamate receptors, and these receptors have been implicated in neural plasticity during development, our data suggest that NMDA antagonism can potentiate fetal learning. Ketamine has been used as an obstetrical and pediatric anesthetic.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the categorization of appetitive and aversive events during postnatal development of the rat

Environmental stimuli are not clearly categorized into appetitive and aversive classes during the first postnatal week. Three- and 6-day-old rats are highly activated by nominally appetitive (milk infusions) and aversive (foot shock) stimuli. Both evoked the same generalized behavioral responses (rolling, curling). By 12 days of age, these stimuli were responded to differently; mouthing and probing occurred in response to milk while flinching and escape responses were observed to shock. The affective properties of mild shock were found to be hedonically opposite for 6- and 12-day-old pups. Six-day-olds showed an acquired preference for an odor paired with shock which increased with intensity (.1-.5 mA) similar to that observed with milk infusions. An acquired odor aversion was displayed by 12 days old using these shock parameters. These results suggest that young rat pups may not differentiate many appetitive and aversive events behaviorally or affectively until the second postnatal week.

Ontogeny of maternal behavior in the laboratory rat: early origins in 18- to 27-day-old young

The responses of unweaned juveniles, 18 to 27 days of age, in brief tests with pups younger than 10 days were observed during initial exposure and at 4-6-day intervals. They showed no aversive responses and actively sought contact with the pups until 24 days when contact-seeking declined sharply. Young exposed to pups before 24 days continued to seek contact with them after 24 days showing an effect of the prior experience. Juveniles given a choice between young pups and a warm bowl chose the pups; given a choice between young pups and age mates, they chose the young pups until day 23, but chose the age mates after then. When housed continuously with young pups at 22 days of age, 5 of 9 retrieved and grouped pups with a latency of 1 day. Our results indicate that timidity or fear of novelty develops in rats in this situation about Day 24 and thereafter is a factor inhibiting the expression of positive social behavior toward pups.

Ontogenesis of spontaneous activity and habituation of activity in the rat pup

Individual components of activity and habituation of activity were determined throughout the 1st month of life in the rat pup. Total activity increased from 25% at 8 days of age to 68% at 22 days before declining to 49% at 26 days. Total slight activity (predominantly sniffing) increased to a maximum of 33% at 15 days whereas total very active behavior (predominantly ambulation) reached its maximum of 38% at 22 days. Habituation of activity expressed as the mean slope of decrement of activity over the 1st 30 min of the observation period was observed in rats as young as 8 days of age. By 12 days, habituation of total activity had increased significantly reflecting a 3-fold increase ihabituation of slight activity, an effect observed at 15 days as well. However, by 19 days the slope of activity decrement had declined to half of its 15-day value, indicating an impairment of habituation and reflecting the attenuation of very active behavior, predominantly ambulation. This decline in habituation continued through 22 days but by 26 days habituation of activity had increased again reaching a maximum for the 1st month of postnatal life. Our results suggest that the phenomenon of behavioral arousal observed in the developing rat pup 19 days of age reflects an inability of the organism to modulate his activity as effectively as the 15-day- or 26-day-old animal.

The ontogeny of exploration in the rat: habituation and effects of handling

Exploration, measured by head-dipping in a holeboard, was tested in male rats at 16, 21, 28, 56, and 84 days of age. Head-dipping increased with increasing age from Day 16 to Day 84, and so did the number of rears made. At all ages handled rats showed more head-dipping and made more rears than did their unhandled litter-mates. The age differences in exploration cannot be explained by different patterns of habituation: rats showed significant within-session habituation of head-dipping at all ages tested (Days 16, 21, and 28). Day 16 rats also showed significant between-session habituation.

Early isolation produces enduring hyperactivity in the rat, but no effect upon spontaneous alternation

Rearing rats in isolation between 23 and 45 days caused a lasting increase in photocell-cage activity, even if the animals were subsequently rehoused in social groups until 125 days of age. In contrast to its effects upon activity, isolation had no effect upon spontaneous alternation in a Y-maze. This dissociation argues for caution in relating hyperactivity and reduced alternation to a unitary deficit in an inhibitory system.