Ingrid B. Johanson

Prenatal cocaine exposure: Effects on play behavior in the juvenile rat

Prenatal exposure to cocaine has become a growing problem in the United States. This study examined the effects of such exposure on social behavior in young rats. The subjects were offspring of Sprague-Dawley dams exposed to 40 mg/kg cocaine HCl (SC) daily on gestational days 8-20, pair-fed dams injected daily with 0.9% saline, and saline-injected controls. The pups were tested at 28, 30, 32, 34, and 36 days of age for instances of pinning, a reliable measure of play behavior. Although showing no differences on such developmental indices as body weight or age of eye opening, offspring prenatally exposed to cocaine showed significantly less pinning than saline controls and were more likely to be pinned. Although males pinned more than females across the treatment groups, cocaine-exposed rats of both sexes showed play deficits. Gestational cocaine exposure thus appears to result in suppressed levels of play in juvenile rats.

Unilateral olfactory conditioning in 6-day-old rat pups.

The potential that early olfactory learning might be laterally organized in the brain was investigated in 6-day-old rats. This hypothesis is based on the finding that the commissural systems that subserve bilateral olfactory communication do not mature until the second week of postnatal life. Pups were trained with pairings of cedar odor and intraoral infusions of milk while one nostril was occluded. Animals expressed a conditioned orientation towards cedar if tested with the trained nostril open. No such conditioning was observed if the untrained nostril was open during testing. Further, when individual pups received cedar odor/milk pairings with one nostril open and orange odor/milk pairings with the other open, they expressed a conditioned preference for orange when tested with the orange-trained nostril open, and a preference for cedar when tested with the cedar-trained nostril open. Classically conditioned oral responses (mouthing) also appeared to be lateralized. However, no such unilateral conditioning occurred with respect to behavioral activation, which is also conditioned in this paradigm. Increases in activity to the odor CS were observed regardless of whether the trained or untrained nostril was open during testing. These results suggest that in developing rodents, olfactory memories may be partly represented in structures that can be unilaterally accessed during training and testing. They provide a starting point for isolation of neural substrates of the olfactory conditioning process.

Learning in Infancy

Several decades ago, the prevailing view of how learning and memory mechanisms might develop was based on the idea that altricial neonates are simply incompletely formed adults. Infant learning tasks were designed with the adult in mind, and performance differences between infants and adults were considered proof of the immature learning and performance capacities of infants. These immature capacities were attributed, in turn, to an immature nervous system (e.g., Cornwell and Fuller, 1961; Fuller, Easler, and Banks, 1950).

The development of aversive responses to quinine in hyperthyroid rats

Normal and hyperthyroid rat pups received intraoral infusions of 0.25% quinine hydrochloride at 8, 10, 12, 14, or 16 days of age. Chin scraping and paw treading, aversive behaviors that adult rats show to quinine, emerged at 12 to 14 days in normal animals and were accelerated in hyperthyroid animals. Once these behaviors emerged, the hyperthyroid pups showed levels of both paw treading and chin scraping that were typical of controls several days older. Hyperthyroid and control pups did not differ in their intake of quinine at ages at which there were marked differences in chin scraping and paw treading. Other types of responses that pups show to aversive stimuli, such as paw treading. Other types of responses were not affected by the thyroid treatment. These findings suggest that the appearance of chin scraping and paw treading at 12-14 days of age in normal animals results from maturation of motor systems underlying these behaviors, and that this maturation can be accelerated by treatment with thyroid hormones.