Behavioral differences between late preweanling and adult female Sprague–Dawley rat exploration of animate and inanimate stimuli and food

Abstract

The late preweanling rat has potential as a preclinical model for disorders initially manifested in early childhood that are characterized by dysfunctional interactions with specific stimuli (e.g., obsessive-compulsive disorder and autism). No reports, however, of specific-stimulus exploration in the late preweanling rat are found in the literature. We examined the behavioral responses of normal late preweanling and adult rats when presented with exemplars of categorically-varied stimuli, including inanimate objects systematically varied in size and interactive properties, biological stimuli, and food. Preweanlings were faster to initiate specific stimulus exploration and were more interactive with most specific stimuli than adults; the magnitude of these preweanling-adult quantitative differences ranged from fairly small to very large depending upon the stimulus. In contrast, preweanlings were adult-like in their interaction with food and prey. Preweanling response to some stimuli, for example to live pups, was qualitatively different from that of adults; the preweanling behavioral repertoire was characterized by pup-seeking while the adult response was characterized by pup-avoidance. The specific stimulus interactions of preweanlings were less impacted than those of adults by the time of day of testing and placement of a stimulus in an anxiety-provoking location. The impact of novelty was stimulus dependent. The differences in interactions of preweanlings versus adults with specific stimuli suggests that CNS systems underlying these behavior patterns are at different stages of immaturity at PND 18 such that there may be an array of developmental trajectories for various categories of specific stimuli. These data provide a basis for the use of the preweanling as a preclinical model for understanding and medicating human disorders during development that are characterized by dysfunctional interactions with specific stimuli.

Behavior in the elevated plus maze is differentially affected by testing conditions in rats under and over three weeks of age

“…In fact, the observed age differences in open arm time were carried entirely by the time in the center square. These findings align with conclusions made by Smith and Morrell ( 2011 ) that late preweanling rats display differential sensitivities to environmental stimuli than adults, while at the same time expressing reduced fear of open spaces, even within the home cage (Smith and Morrell, 2007 ). In general, the findings support the idea that behavior of juvenile rats on the EPM results from overlapping developmental trajectories for multiple neural systems involved in sensorimotor ability, anxiety, and risk-assessment.…”

“…While rhythmic activity-rest cycles across the day are present as early as P10 (Reppert et al, 1984 ), it is not until after 3 weeks of age that circadian activity rhythms are entrained to the light cycle (Honma and Honma, 1986 ) so that by 2 months of age, peak activity occurs during the dark phase (Andrade et al, 2003 ). In developing rats, before the circadian shift occurs, locomotor activity is highest during the first quartile of the day, compared to heightened locomotion in the third quartile in older animals (Smith and Morrell, 2011 ). Our findings agree in that young animals explore the EPM most during the AM, and only P22–24 rats exhibit diurnal differences in stress reactivity.…”