Activity dependent plasticity in pathways between subcortical and cortical sites

Abstract

<p>Information storage between subcortical and cortical structures may include the occurrence of synaptic modifications such as those expressed following the induction of long-term potentiation, or LTP. It is well known that the hippocampus is important for memory, but certain adjacent and anatomically related cortical structures in the temporal lobe, especially entorhinal, perirhinal, and parahippocampal cortex also participate. It seems likely that the hippocampal system influences memory and consolidation primarily through an extensive set of reciprocal connections within the system and between this system and widespread areas of the neocortex. LTP has been extensively studied in the intrinsic connections of both the hippocampus and the neocortex. LTP in the pathways and structures thought to convey information between the hippocampus and the neocortex, however, have received little attention. It is unlikely that these structures simply relay information passively. Considerable additional processing must occur within them. Are they also involved in information storage? If so, and if LTP reflects a general synaptic encoding mechanism, then these systems should support LTP. Another focus of this thesis is the reciprocal thalamo-cortical system. Some of these pathways may also utilize LTP as a mechanism for information storage. Although much is known about the transmission of information from thalamus to cortex, little is known about the information transmitted along the reciprocal pathways back to the thalamus. In most cases these pathways are as large as, or larger than, those running from the thalamus to the cortex. The current studies were designed to investigate LTP in the reciprocal projections in both the hippocampal-neocortical and thalamo-neocortical systems. Whereas the latter is often viewed as a hardwired system, the former system is believed to remain plastic throughout the lifetime of all mammals. One of the goals of this thesis was to test this hypothesis, using the parameters required for induction of LTP in cortico-cortical pathways in the chronically implanted rat. LTP was found in all pathways in the hippocampal-neocortical system that were examined. There were, however, differences in induction and decay rates that are of particular interest. These results are discussed in relation to the presumed memory functions of the medial temporal lobe. None of the thalamocortical pathways sustained LTP with multiple stimulation sessions, but the corticothalamic pathway, showed a lasting potentiation. This finding is of great interest. These results are discussed in relation to a filtering mechanism and the response of thalamo-cortical systems to mismatch information.</p>