The effects of glutamatergic activation of the lateral supramammillary nucleus and the medial septum complex on perforant path-evoked potentials, and other physiological parameters, in the dentate gyrus of the rat

Carre, Geoffrey Philip (1993) The effects of glutamatergic activation of the lateral supramammillary nucleus and the medial septum complex on perforant path-evoked potentials, and other physiological parameters, in the dentate gyrus of the rat. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A large number of cells from the lateral aspect of the supramammillary nucleus (SUML) and the medial septum complex (MSC) innervate the dentate gyrus of the hippocampus. It has been demonstrated that electrical prestimulation of the SUML or the MSC enhances perforant path-dentate gyrus evoked field potentials. Considering the large number of fibres that pass through these regions, the effects glutamatergic stimulation of these regions had on dentate gyrus field potentials in urethane-anaesthetized rats was investigated. The perforant path was stimulated at a rate of 0.1 Hz, evoking an EPSP and a population spike in the dentate gyrus granule cell layer. L-glutamate was delivered by pressure injection (500 mM, 100-150 nl). In a second experiment, concomitant measures of hippocampal EEG, spontaneous unit activity, and the evoked potential recorded at the molecular layer were also taken. As well, the effects of glutamatergic activation of these areas on paired-pulse inhibition was investigated. -- Glutamate ejection to the SUML and MSC produced a significant enhancement of the population spike. The duration of enhancement ranged from 2 to 54 min (X = 18.4 min) and from 1 to 49 min (X = 10.5 min) after SUML and MSC activation respectively. A consistent, but relatively short increase in the EPSP slope was demonstrated after MSC activation but not after SUML activation. No consistent effects were found on the 2 latency measures. -- SUML and MSC activation induced theta in 4 of 7 (duration= 10-45 s) and 7 of 10 animals (duration= 20-112 s), respectively. Theta induction preceded spike enhancement and occurred for a shorter duration than the enhancement. -- After either SUML or MSC activation the effects on spontaneous unit activity were mixed. However, all changes in firing rate preceded spike enhancement, and their duration rarely coincided with the duration of the spike enhancement. -- Results of paired-pulses (lSI= 20-30 ms) given during SUML activation demonstrated evidence of reduced feed-back inhibition, despite an absence of the enhancement of the first spike of the pair. MSC activation produced a reduction of feed-back inhibition in one of three animals where the first spike of the pair was enhanced.

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