Characteristics of Hippocampal Theta, Gamma, and Theta-Gamma Coupling:Effects of Aging, Environmental Change, and Cholinergic Activation

Date of Completion

January 2011

Keywords

Biology, Neurobiology|Psychology, Physiological

Degree

Ph.D.

Abstract

Hippocampal rhythmic oscillations in the theta and gamma range have been purported to underlie information processing and are prominent during exploration. Theta and gamma oscillations may serve to synchronize activity within the hippocampus and across distributed regions of the brain during learning. CA1 receives both intrahippocampal and extrahippocampal projections, and has been postulated to serve as a comparator, allowing for orthogonalization of incoming sensory signals and stored representations, such that the hippocampus rapidly alternates between an encoding and retrieval state. We further examined possible functional aspects of these oscillations and the relationship between different bands by comparing adult and aged rats. In both humans and rodents aging is linked with impairments in hippocampal processing of new spatial information. Therefore we recorded local field potential as animals traversed both a highly familiar and novel runway configuration. The cholinergic system is degraded during aging, and cholinomimetics can reverse some age-related memory impairments, as well as modulate oscillatory activity in the hippocampus. Therefore the effects of the cholinomimetic physostigmine (an actylcholinesterase inhibitor) on theta and gamma oscillations was determined. ^ Exploration of a novel environment increased theta, but not gamma activity in both age groups. Physostigmine selectively increased theta power in the lower range only in old rats, and shifted power to the lower range in young adults. There were marked differences in measures of theta-gamma coupling in adult and old rats, though there was little effect of either the novel situation or treatment with physostigmine. ^

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