Brain and behavior: Searching for the biological basis of learning.

Abstract

The hippocampus is a brain structure known to be important for learning and memory, more specifically for the acquisition of spatial information. Hebb (1940) suggested that storage of information in the brain may involve modifications in the strength of synaptic connections. One example of an artificially-induced synaptic alteration that may share common mechanisms with memory formation is long-term synaptic enhancement (LTE). Recently, behaviorally-induced changes in hippocampal synapses have been discovered to occur in conjunction with exploratory behavior. This type of change has been called short-term exploratory modulation (STEM). It was proposed that STEM could share common mechanisms with artificially-induced LTE and memory formation in the hippocampus. The primary goals in this dissertation were to determine the relationship between STEM and LTE, to identify the mechanisms controlling these changes, and to determine whether STEM was a critical component of memory storage, a memory modulator, or an epiphenomenon. Synaptic changes in the hippocampus were measured by recording perforant-path evoked field potentials in the fascia dentata from awake behaving rats during rest and exploration or under sodium pentobarbital anesthesia. In the first experiment, a positive correlation was found between learning in the Morris swim task and STEM in young and aged rats. Comparisons of LTE and STEM indicated that STEM did not reflect the same type of synaptic change observed in LTE, such that the two phenomena did not interact with each other. Furthermore, the nature of the changes in the evoked potentials were observed to be different. Another feature that distinguishes STEM from LTE is that the induction of LTE is dependent on the NMDA receptor, whereas STEM is NMDA-receptor independent. When rats were anesthetized and their bodies warmed passively, they exhibited STEM-like changes which were highly correlated with body temperature. These temperature-induced changes in evoked potentials had little impact on the functional output of cells in the fascia dentata. It is therefore concluded that exploration-induced changes in the hippocampus are largely due to brain temperature changes and have minimal impact on the functioning of neurons as originally proposed.