| Abstract No.: | C-G3189 |
| Country: | Canada |
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| Title: | SPONTANEOUS AND SUSTAINED HIPPOCAMPAL THETA OSCILLATIONS IN VITRO: AN INVESTIGATION USING A COMPLETE SEPTO-HIPPOCAMPAL PREPARATION.
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| Authors/Affiliations: | 2 Romain Goutagny*; 1 Jesse Jackson; 1 Frederic Manseau; 1 Marc Danik; 2 Sylvain Williams;
1 Douglas Institute Research Center; 2 Douglas Institute Research Center-McGill University, Montreal, QC, Canada.
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| Content: | Objectives: During exploratory locomotion and rapid eye movement sleep in rodents, the hippocampus exhibits rhythmic oscillatory field potentials at frequencies ranging from 3 to 12 Hz (theta frequency). Theta oscillations are thought to represent the on-line state of the hippocampus, with the synchronization of hippocampal neurons during theta activity serving as a reference for information encoding by hippocampal place cells. Since one of the most important input to the hippocampus originates from neurons located within the medial septum and diagonal band complex (MS/DB), it has been an early postulate that the MS/DB represent the nodal point where hippocampal theta rhythm is generated. However, the exact mechanisms of hippocampal theta generation and pacing are yet to be determined.
Materials and Methods: To fill this gap, we used a new complete rat septo-hippocampal preparation in vitro that comprised the ipsilateral half septum and the adjoining hippocampus still attached together. For that purpose, young (P14-P22) Sprague-Dawley rats were deeply anaesthetised with isofluorane and subsequently decapitated and the brain was rapidly removed. After removal of the cerebellum and frontal part of the brain, a cut was made through the interhemispheric sulcus to separate the two hemispheres thereby exposing the hemisected septum. The half-septum and hippocampus were then carefully dissected out from each hemisphere by inserting a flat micro spatula in the lateral ventricle and sliding along the corpus callosum at both dorsal and ventral contours of hippocampus and septum. To evaluate the specific role of the septum in the generation of hippocampal theta oscillations, we used a custom made dual chamber bath. In these experiments, a Teflon wall was inserted between the septum and hippocampus at the level of the fornix/fimbria bundle so that the two structures could be perfused independently with aCSF containing different pharmacological agents.
Results: In this experimental paradigm, we were able to record spontaneous hippocampal oscillations in 24 septo-hippocampal preparations. Based on the main frequency of the oscillation in the CA1 area of the hippocampus, experiments were classified as delta (1-3 Hz, n = 11, mean frequency of 1.98±0.1 Hz) or theta (3-12 Hz, n =13, mean frequency of 5.2±0.4Hz) oscillations. We then studied the effect of the muscarinic receptor antagonist atropine (10µM) on these two types of oscillations. During delta oscillations, the specific application of atropine (n=3) only in the hippocampus produced no noticeable change in the frequency and power of the oscillation. On the other hand, the application of atropine to the hippocampus during theta oscillations (n=7) produced a decrease in the amplitude (- 32.9±8.6% decrease of the power at peak) without modification of the frequency of the theta oscillation.
Conclusion: Taken together, these results show for the first time that 1) it is possible to record spontaneous theta oscillations in-vitro without pharmacologically activating the hippocampus; and 2) these spontaneous theta oscillations are mediated by muscarinic and non-muscarinic receptors and have a pharmacological profile similar |
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