The behavior and psychopharmacological sensitivity of periadolescent rats are examined in this review. Periadolescent rats are hyperactive and engage in more conspecific play behavior than younger or older rats. When compared with other-aged rats, periadolescents exhibit enhanced performance in simple active-avoidance learning tasks, but perform poorly in more complex appetitive and avoidance learning tasks in which increases in locomotor activity do not improve performance, perhaps as a result of age-specific alterations in selective attention or stimulus processing. Such behavioral "anomalies" of periadolescent animals observed in traditional laboratory situations may be in some way adaptive when considered in the context of the animals' natural habitat. In terms of psychopharmacological responsiveness, periadolescent rats, when compared with younger or older animals, are less sensitive to catecholaminergic agonists but are more responsive to the catecholaminergic antagonist haloperidol. This pattern of psychopharmacological sensitivity suggests that the catecholaminergic systems may be temporarily hyposensitive during the periadolescent period. Evidence is presented that a negative feedback system in the form of dopamine autoreceptors may become functionally mature in mesolimbic brain regions during the periadolescent period. The possibility is presented that maturation of these self-inhibitory autoreceptors might result in a temporary decrease in the efficacy of mesolimbic dopamine projections, perhaps contributing to the psychopharmacological and behavioral characteristics of periadolescent animals. In support of this suggestion, evidence is reviewed indicating that the behavior of adult animals with lesions of the ventral tegmental area, a region containing cell bodies from which these mesolimbic dopaminergic projections originate, resembles that of periadolescent rats.

When presented a novel olfactory stimulus while suckling a passive dam, 11- to 14-day-old rat pups acquire a conditioned preference for that stimulus. The magnitude of the conditioned preference is greater if the pups received milk while suckling than if they did not. The results indicate that infants are capable of learning while suckling and that milk delivery plays a role in this associative process.

In Experiment 1, 3-6-day-old rats were simultaneously exposed to an unfamiliar odor which they will normally avoid (orange extract) and the odor of maternal saliva, or to either orange alone or saliva alone. One hour later, in a two-odor choice test, those pups which were simultaneously exposed to orange and saliva exhibited an enhanced orientation to the orange odor. In Experiment 2, orange odor was presented while pups were exposed to the odor of saliva, or while they received one of two types of tactile stimulation (stroking or tailpinching). Pups in control groups were first presented with saliva or tactile stimulation and then the orange odor. Pups that had received any of the three simultaneous exposure treatments subsequently exhibited an enhanced orientation toward the orange odor, but not the pups in the control groups. The results suggest that the tendency of neonates to avoid a novel odor can be reversed by pairing that odor with events that elicit significant increases in behavioral activity. The possibility that this phenomenon reflects a classical conditioning process is discussed.

In Experiment I rats were trained for 2% days under partial (PRF) or continuous reinforcement (CRF) conditions starting at 18, 22,28, or 36 days of age and were then subjected to immediate extinction. At all ages there was a strong partial reinforcement extinction effect (PREE), and absolute size of PREE was greatest in the youngest rats. Rate of extinction increased as a function of age following both CRF and PRF. In Experiment II the youngest and oldest age groups of Experiment I were run under the two reward conditions of Experiment I and in a third condition, PRF with number of rewards rather than trials equated to CRF (PRF-R). The PRF-R and PRF groups were not different in extinction, and both were more persistent than CRF. The youngest rats were again more persistent than the oldest, particularly after PRF training. In Experiment III it was shown that the well-known paradoxical effect, greater reward in CRF acquisition leads to faster extinction, operates in our youngest and oldest animals, but is more pronounced in the oldest. The results are discussed in terms of whether they require different explanations than those often applied to extinction data from adult rats.We seem to know much more about the ontogeny of aversive (fear) conditioning in rats than about appetitive learning, perhaps because it seems easier to equate motivational factors at different ages in aversive than in appetitive learning (Campbell, 1967). However, we do have a little information on how level of maturity in rats affects learning in appetitive situations. Weanling rats are reported to be inferior to adults in learning a barpress response for food reward and in forming a lightdark discrimination (Campbell, Jaynes, & Misanin, 1968). Similarly, juvenile rats (less than 50 days of age) do less well on a spatial discrimination than adults (Bronstein & Spear, 1972). The younger rats in the studies of Campbell and his colleagues also show poorer retention of both a learned barpress and light-dark discrimination (Campbell et al., 1968), unless there are periodic reexposures of the rats to the experimental task over the retention interval (Campbell & Jaynes, 1969).While these experiments tell us something about the ontogeny of appetitive learning, they fall short of being decisive in several respects. First, the number of ages investigated is small over the range of ages investigated. This makes it difficult to determine the age at which an adult pattern first appears, and says little about the abruptness of the transition. A second related point is that, when traimng sessions are conducted over an extended period of time, it is difficult to separate changes in performance related to developmental factors from changes resulting from amount or duration of training. Finally, experimental treatment of the youngest subjects in these studies on appetitive learning usually begins in the fourth postnatal week or later,