The ontogeny of spatial memory in rats

Abstract

Been able to navigate through the environment is essential for every mobile species. Up to date, it is quite understood how spatial representations are formed, how these representations serve our everyday memories and what are the brain mechanisms that underlie these processes. Most of this knowledge has been acquired upon studies in adults while the development of the spatial memory capability across ontogeny remains less understood. Recent findings have shown that the brain structures that support spatial cognition, such as the hippocampus, have a protracted development including a critical period. Yet, the studies conducted to investigate the developmental trajectory of hippocampal-dependent spatial capabilities are scarce. The present thesis addresses three open questions regarding the ontogeny of the spatial memory capability in rats. Study 1 aimed at evaluating the developmental trajectory of spatial memory by using the object-place recognition task. Study 2 was designed to further understand how early infant rats use spatial information to drive spatial behavior. Study 3 was designed to unveil the effect of early spatial experiences in the adult spatial memory capability. Results of Study 1 showed that spatial memory was first expressed during the infantile period as familiarity preference, while the adult-like expression of the memory (i.e., novelty preference) was exhibited during the adolescence period without further changes. The object-place recognition task was unable to validly assess spatial memory capabilities in early infant pups. By using a simple spatial habituation task, study 2 showed that rats were able to form persistent spatial representations already during early infancy. Lastly, the results of Study 3 showed that rats subjected to spatial experiences during infancy had an enhanced spatial memory performance in adulthood. This enhanced effect was experience-specific, context dependent, restricted to the infantile period, and it required the occurrence of sleep after the infantile experiences. Furthermore, infantile spatial experiences increased neuronal activity in the prelimbic (PL) region of the medial prefrontal cortex, a key area involved in the processing of schema information. Three major conclusions can be drawn from the present thesis. First, rats are able to form spatial representations and use it to drive behavior from early infancy. Second, the expression of spatial memory is dynamic and change across ontogeny and third, the adult spatial capability is built upon early spatial experiences.