Infantile handling and successive spatial reversal learning in rats

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Early life manipulations alter learning and memory in rats

Much research shows that early life manipulations have enduring behavioral, neural, and hormonal effects. However, findings of learning and memory performance vary widely across studies. We reviewed studies in which pre-weaning rat pups were exposed to stressors and tested on learning and memory tasks in adulthood. Tasks were classified as aversive conditioning, inhibitory learning, or spatial/relational memory. Variables of duration, type, and timing of neonatal manipulation and sex and strain of animals were examined to determine if any predict enhanced or impaired performance. Brief separations enhanced and prolonged separations impaired performance on spatial/relational tasks. Performance was impaired in aversive conditioning and enhanced in inhibitory learning tasks regardless of manipulation duration. Opposing effects on performance for spatial/relational memory also depended upon timing of manipulation. Enhanced performance was likely if the manipulation occurred during postnatal week 3 but performance was impaired if it was confined to the first two postnatal weeks. Thus, the relationship between early life experiences and adulthood learning and memory performance is multifaceted and decidedly task-dependent.

Cognitive performance in rhesus monkeys varies by sex and prenatal androgen exposure

Men and women differ on performance and strategy on several spatial tasks. Rodents display similar sex differences, and manipulations of early hormone exposure alter the direction of these differences. However, most cognitive testing of nonhuman primates has utilized sample sizes too small to investigate sexually differentiated behaviors. This study presents an investigation of sex differences and the effects of prenatal androgen on spatial memory and strategy use in rhesus monkeys. Monkeys prenatally exposed to vehicle, testosterone, or the androgen receptor blocker flutamide performed a search task in which 5 of 12 goal boxes contained food rewards. Spatial consistency and the presence of local landmarks were varied. Performance when both spatial and marker cues were available did not differ by sex or prenatal treatment. Contrary to predictions, females easily solved the task when local markers were removed, and their performance outscored males. Although eliminating spatial consistency and requiring subjects to use local markers impaired performance by all monkeys, females continued to locate correct goal boxes at higher than chance levels and scored better than males. Blocking prenatal androgen exposure in males improved use of local markers. These findings suggest that the tendency to attend to landmarks and to use them in solving spatial problems is typical of females across many species, including rodents, humans, and rhesus monkeys. In rhesus monkeys and rodents, developmental androgen eliminates this specialization. However, these results are the only known example of better performance of females than males when salient markers are removed.

The effects of number of trials per day, retention interval, gender and time of day on acquisition of a two-choice, win-stay water-escape working memory task in the rat

A 2 x 2 x 2 factorial study was conducted to examine the effects of gender, trials/day (TPD), and retention interval (RI) on acquisition of a two-choice win-stay water-escape task in 4-month-old Sprague-Dawley rats. Variations of this task have previously been shown to be sensitive to changes in working memory that occur with age (Means and Kennard, 1991) or to follow treatment with an anxiolytic (Bass et al., 1989) or with cognitive enhancers (Comer and Moore, in press) in males or to estrogen in ovariectomized females (O'Neal et al., 1990). Both training with 1 TPD and with a 5-min RI, time between an information "run" and a test "run", reduced errors during acquisition as compared to giving 3 TPD and a 1-min RI (P less than 0.05 in all cases). Males made fewer errors than did females (P less than 0.05). The three effects were additive: males trained with 1 TPD and a 5-min RI performed best and females trained with 3 TPD and a 1-min RI performed worst. Also, rats given 1 TPD made fewer acquisition errors when trained 2 h before the lights were turned off than when trained during the middle of the light portion of the light/dark cycle (P less than 0.05). Finally, when given 3 TPD, the rats, especially the females, performed significantly worse on their third trial (P less than 0.05). It was concluded that two-choice win-stay water-escape is a very sensitive assay of working memory with much potential for neurobehavioral research.

Effects of early postnatal gonadal steroids on acquisition and extinction of a single alternation response in discrete-trials lever pressing

The present report studies the effect of sex on the acquisition and extinction of a single alternation schedule of reinforced (R) and nonreinforced (N) trials in a Skinner box. In addition to the investigation of the basic sex differences in Experiment 1, the effects of early gonadal steroids treatment and postpubertal gonadectomy of male and female rats were examined in Experiments 2 and 3, respectively. Both acquisition and extinction performances were evaluated by means of latency. In Experiment 1, males showed higher latencies on N trials and extinguished faster than female rats. In Experiment 2, female androgenization and male orchidectomy on day one after birth reversed the direction of the sex differences found in the previous experiment. However, postpubertal gonadectomy (Experiment 3) does not affect the performance of male and female rats.

Effects of early postnatal gonadal steroids on the successive conditional discrimination reversal learning in the rat

In the present study the existence of sex differences in the acquisition, retraining and reversal of a successive conditional discrimination learning (Experiment 1) and the role of the early postnatal gonadal steroids on these discrimination tasks (Experiment 2) were investigated. In Experiment 1 two groups of experimentally naive rats (males and females) were exposed to a black-white successive conditional discrimination task in a T-maze. No sex differences were found in the acquisition or retraining. However, in the reversal phase females made fewer errors and reached the discrimination criterion (90% of correct choices) sooner than males. In Experiment 2, the absence of sex differences in the acquisition or retraining phases and the existence of sexual dimorphism in the reversal period were confirmed. In addition, female androgenization and male orchidectomy, on day one after birth, reversed the direction of the sex differences found in the successive conditional discrimination reversal learning.

The effects of early handling on latent inhibition in male and female rats

Latent inhibition (LI) is a behavioral paradigm in which repeated exposure to stimuli not followed by meaningful consequences renders these stimuli ineffective for subsequent learning. The development of LI is considered to reflect learning not to attend to, ignore, or tune out irrelevant stimuli. The present study investigated the differences in the development of LI between handled and nonhandled males and females. Infantile handled (Days 1-22) and nonhandled, male and female Wistar rats were tested in maturity in the LI paradigm. The LI procedure consisted of two stages: pre-exposure, where animals received 60 presentations of the to-be-conditioned stimulus (tone) and test, where the animals acquired a two-way active avoidance response with the tone serving as the warning signal. Handled animals reached higher percentage of avoidance responses as compared with nonhandled animals. Latent inhibition was obtained in both the handled and the nonhandled females, but only the handled males showed the LI effect. Nonhandled males failed to develop LI. The results indicate that the effects of handling are evident in learning tasks that do not involve motivational-emotional variables, i.e., learning to ignore irrelevant stimuli; handling differentially affects males and females, with a much greater impact on males and the nonhandling procedure has significant deleterious consequences on adult behavior.

Effects of early visual and complex stimulation on learning, brain biochemistry, and electrophysiology

A complex stimulation regimen (visual, auditory, and somesthetic-kinesthetic with forced movements, 30 times for 30 min each within 14 days) increased significantly the amplitudes of visual cortical evoked potentials (EPs) in adult rats if applied during the second postnatal fortnight. The EP increase after stimulation during the first 14 days after birth was not significant. Visual stimulation alone was compared with complex stimulation (visual plus forced movements) during the 2nd postnatal fortnight. More specific local changes in the visual cortex were revealed in brain biochemistry (lower DNA concentration, more RNA and protein per cell) and cortical electrogenesis (enhanced visual EPs) after visual stimulation alone, whereas complex stimulation induced more diffuse changes and rather profoundly influenced higher nervous functions (viz., memory retrieval - improved 24-h). Involvement of both specific and nonspecific mechanisms in the aftereffects of early stimulation is indicated.