Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Physiology and Pharmacology

Supervisor

Dr. Stan Leung

Abstract

The hippocampal theta rhythm facilitates memory formation. This study investigated the temporal relation of long-term potentiation (LTP) with the hippocampal theta rhythm. Theta rhythm consists of a wave of somatodendritic depolarization, but the depolarization of apical and basal dendrites of hippocampal CA1 pyramidal cells peak at a similar theta phase. Thus, we hypothesize that the population spike excitability evoked by excitation of the apical and basal dendrites peak at a similar phase of the theta rhythm. We also expect that LTP at the basal and apical dendritic synapses to be maximal at a similar theta phase.

Rats (~300 g) were anesthetized by urethane. Stimulating electrodes were placed in stratum radiatum and stratum oriens, to excite apical and basal dendrites of CA1 pyramidal cells, respectively. A 16-channel electrode array (50 µm between electrodes) recorded field potentials in CA1. Burst stimulation (5 pulses at 200 Hz, 3 x response threshold) was used to induce LTP, and a single pulse (~1.5x population spike threshold) was used to evoke a population spike. Apical (or basal) LTP was measured by the rising slope of the excitatory current sink at the apical (or basal) dendritic layer. Theta phase was derived from the theta field potentials recorded at stratum lacunosum-moleculare (90° = positive peak).

Maximal population spikes evoked by single-pulse stimulation of the apical and basal dendrites occurred at the falling phase of theta (120°-160°). LTP was maximal when the burst stimulation occurred during the rising phase of theta, which was 306°-30° (basal LTP) and 333°-24° (apical LTP). The magnitude of LTP decreased gradually when tetanized at a phase different from the optimal phase.

We conclude that theta rhythm sets up a temporal (or phase) gradient of synaptic plasticity at the basal and apical dendrites of hippocampal pyramidal cells. Both basal and apical LTP were optimal during the rising phase of the distal apical dendritic theta rhythm, distinctly different from the phase of maximal spike excitability that occurs in the falling theta phase. Possible mechanisms will be discussed.

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