Dissociation of patterned alternation learning and the partial reinforcement extinction effect in preweanling rats

Common processes may contribute to extinction and habituation

Psychologists routinely attribute the characteristics of conditioned behavior to complicated cognitive processes. For example, many of the characteristics of behavior undergoing extinction have been attributed to retrieval from memory. The authors argue that these characteristics may result from the simpler process of habituation. In particular, conditioned responding may decrease during extinction partially because habituation occurs to the stimuli that control responding when those stimuli are presented repeatedly or for a prolonged time (e.g., the experimental context, the conditioned stimulus in classical conditioning). This idea is parsimonious, has face validity, and evokes only processes that are well established by other evidence. In addition, behavior undergoing extinction shows 12 of the fundamental properties of behavior undergoing habituation. However, this model probably cannot provide a complete theory of extinction. It provides no obvious explanation for some of the other characteristics of extinguished behavior.

Alleviation of x-irradiation-based deficit in memory-based learning by D-amphetamine: suggestions for attention deficit-hyperactivity disorder

Selective exposure to x-irradiation during infancy, from postnatal days (PND) 2-11 in the rat, results in severe hippocampal granule cell hypoplasia. Preweanling (PND 17-18) rats, which suffer such hippocampal granule-cell agenesis, show deficits in patterned single alternation (PSA), a form of memory-based learning. Deficits in short-term memory along with increased arousal have been suggested as characteristic of children diagnosed with attention deficit-hyperactivity disorder (ADHD). We report here on the ameliorating effects of D-amphetamine, a drug commonly used in the treatment of ADHD, before Ritalin, on PSA, after infantile (PND 2-15) exposure to x-irradiation. After i.p. injections of 0.3 mg/kg D-amphetamine, the onset and magnitude of the PSA memory-based discrimination in the x-irradiated preweanling rats was restored to about the level of controls. These results, showing alleviation of x-irradiation-related deficits in short-term memory by D-amphetamine injections, along with our earlier and present results, showing substantial deficits after x-irradiation alone, encourage the hypothesis that hippocampal granule-cell hypoplasia, which would occur in humans prenatally and is Altman's model of "minimal brain dysfunction" [Altman, J. (1986) in Learning Disabilities and Prenatal Risk, ed. Lewis, M. (Univ. of Illinois Press, Urbana), pp. 241-304], may be a factor in at least some forms of ADHD and may provide a basis for an animal model of the disease.

Neurotoxic effects of neonatal triethyltin (TET) exposure are exacerbated with aging

Neonatal Long-Evans rats dosed with TET (5 mg/kg; IP) or saline on postnatal day (PND) 10 were examined across the life span for neural damage and performance on spatial learning tasks. A subset of rats were sacrificed to assess early damage with Nissl-staining, Timm's histochemistry, and glial fibrillary acidic protein (GFAP) immunohistochemistry 2, 7, or 14 days after dosing. Littermates were tested behaviorally in a T-maze spatial delayed alternation task on PND 23 or PND 90, and in a Morris water maze place learning task at 3, 12, or 24 months postdosing and then sacrificed for histological analysis. In neonatal rats, histological analysis indicated gliosis in discrete cortical regions, loss of Nissl-stained neurons in the hippocampal formation, entorhinal cortex and piriform cortex, and loss of Timm's staining in the entorhinal cortex. The behavioral assessment at PND 23 indicated a significant impairment in the T-maze. However, no significant impairments were observed in the T-maze at 3 months or the water maze at 3 or 12 months postdosing. At 24 months, TET-treated rats showed significant deficits in acquisition and retention of the water maze task compared with age-matched controls. Both groups of 24 months old rats were significantly impaired compared with young controls. At 24 months, there was a general age-related decrease in the optical density of Timm's staining in cortical regions (9%), compounded by a further decrease in the entorhinal cortex and outer molecular layer of the dentate gyrus of the hippocampus in TET treated rats (30%). These data indicate that early developmental exposure to an organometal resulted in morphological damage that was apparent behaviorally only during early postnatal development and with advanced aging.