Imaging olfactory network activity in a semi-intact nose-brain preparation: On the origin of spontaneous theta oscillations in the olfactory bulb
Sensory and Behavioural Biology
Molecular Biology and Physiology of Neurons and Glial Cells
Final Report Abstract
Neural network oscillations are very prominent in the olfactory system. In the bulbus olfactorius, the first processing station for odor information, slow oscillations occur that are linked to the respiratory rhythm. These so-called theta oscillations play an important role in the encoding of olfactory information by providing an odorant-specific clocking of the activity of the input channels. The starting point of the project was the observation that in a semi-intact nose-brain preparation we had previously developed, oscillations of the local field potential occur at a frequency comparable to that of respiration, despite the absence of respiration from the preparation. The working hypothesis derived from this finding was that these slow oscillations are an intrinsic resonance of the neuronal network of the bulb. Regarding the mechanism, it was hypothesized that the external tufted cells in the glomerular layer of the bulb, which spontaneously fire in "bursts," act as pacemaker cells and thus generate these oscillations. Furthermore, these oscillations should be entrained by respiration and thus underlie the theta oscillations mentioned above. It was planned to first further characterize the phenomenon of oscillations electrophysiologically and pharmacologically and then to observe the presumed pacemaker cells in action by means of 2-photon Ca2+ imaging. However, the results of the pharmacological manipulations were contradictory, and also certain properties of the oscillations made the working hypothesis of a neuronal network oscillation seem increasingly questionable. On the other hand, there were the observations that the oscillations were of neurogenic origin and that the spontaneous firing of mitral cells, the principal neurons of the bulb, was partially synchronized to the oscillations - which could not be explained by an artifact (electrical or/and mechanical) alone. After two years of experimental work, it was finally found that perfusion pump-evoked pulsations of perfusion pressure provide an adequate stimulus for excitatory mechanoreceptors in mitral cells which then mediate the oscillatory potential. This finding led to the prediction that heartbeat-induced arterial pulsations in vivo - which are comparable in amplitude and time course to pump-induced pressure pulsations - may also influence mitral cell firing. The proof of this hypothesis was achieved in collaboration with scientists from Hamburg. Thus, certain neurons of the brain can directly detect the heartbeat, a previously unknown pathway of interoception. The project exemplifies how novel mechanisms are discovered through initially inexplicable findings.
Publications
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Blood pressure pulsations modulate olfactory bulb neuronal activity via mechanosensitive ion channels. Cold Spring Harbor Laboratory.
Salameh, Luna Jammal; Bitzenhofer, Sebastian H.; Hanganu-Opatz, Ileana L.; Dutschmann, Mathias & Egger, Veronica
