Factors And Mechanisms That Influence The Intrinsic Optical Signals In The Olfactory Bulb

Olfactory is a crucial chemical sensory for mammal’s survival and daily life. The olfactory system responds to the same odor stimulus with high fidelity. Meanwhile, the olfactory system itself is constantly modulated by both external and internal factors. The olfactory bulb (OB) is the first relay station in the olfactory system and is essential for coding, processing and transmitting odor information. Spatial and temporal coding of the odorants in the OB of rodent have been studied extensively using various techniques which utilized sources of contrast of electrophysiological, radioisotopic, magnetic and optical origins. Among theses techniques, intrinsic optical signal (IOS) imaging was the first imaging technique to show odor map at a fine spatial scale. However, the effects of internal factors on the IOS and its biophysical origins are still not fully understood.According to the previously studies, the odor concentration and structures (e.g., the functional group and the length of the carbon chain) could affect the IOS in the OB. In the present study, we focused on the effects of odor duration on the odor-evoked IOS. Surprisingly, it was found that IOS response induced by odor stimuli with prolonged duration (3min) was consistent during the stimulation. Unlike the local field potential signals at β band, there was no observable adaptation shown in the time course of IOS.Differ to other sensory centers, the OB has heavy intermodal neural projections to other brain regions related to emotion and memory (e.g., hippocampus and amygdala), which are sensitive to the internal factors such as depth of anesthesia, metabolic conditions and alerting. Hence, the IOS responses to the same odorant under two different anesthesia levels (assessed by the EEG signal of the animal) were compared in the present study. It was found that the base line and the response intensity of the IOS elicited in the dorsal OB by a given odorant were significantly affected by the anesthesia level, but the topography was highly similar across different anesthesia level.At last, to assess the contribution of the postsynaptic activities to the IOS, we employed the optogenetic techniques. By activating the channelrhodopins-2-expressing mitral cells directly through a DMD-based light stimulation system, we found that the activation of postsynaptic neuron was sufficient for the production of IOS. Moreover, pharmacological experiments demonstrated that this process was dependent of astrocyte-mediated glutamate uptake.