Appetitive learning, patterned alternation, and extinction in 10-d-old rats with non-lactating suckling as reward

Abstract

HUGHLINGS-JACKSON¹ has expressed the view that, during ontogenesis, neural centres responsible for inhibition develop more slowly than centres responsible for excitation. Since then, the working hypothesis has been that the later-developing, phylogenetically ‘higher’ neural centres act to regulate (inhibit?) the excitation of the ‘lower’ centres. There has been some behavioural and physiological support for this position. In altricial mammalian species the hippocampus and neocortex become functionally mature relatively late in development², and the hippocampus, in particular, has been implicated in the expression of behavioural inhibition^3–5. Preweanling rats, during weeks 3 and 4 of life, seem to lack maturity for tasks requiring suppression of responding; for example, they are deficient in passive avoidance⁶ learning and in habituating to novel stimuli^7,8. Their performance in excitatory tasks, such as active avoidance⁹ and approach for food¹⁰, seems to be less deficient. These task-specific deficits resemble those shown in hippocampally lesioned adult animals¹¹. Campbell et al. have noted the correspondence between later-developing inhibitory centres in the rat and the decline in spontaneous activity occurring during ontogenesis¹², and have demonstrated that psychopharmacological agents¹³ and brain lesions¹⁴ specific to inhibitory systems are ineffective in increasing behavioural arousal early in development. In our laboratory, experiments on instrumental appetitive learning in preweanling rats have involved the use of dry food pellets as reinforcers in pups as young as 17 to 20 d of age. In animals as young as this, we have demonstrated acquisition, extinction, immediate persistence, and long term persistence using procedures that are similar to those for adult rats^15–17. We now report two experiments demonstrating behavioural plasticity in the 10-d-old rat, involving acquisition and subsequent extinction of an approach response where reward is the opportunity to suck briefly from an anaesthetised mother rat.