Neural mechanisms underlying mechanosensation through rodent olfactory system

Abstract

The sense organs are dedicated to collect information from the external environment and convert the incoming physical/chemical energy into neural representations. While most of the sensory systems encode various features of a single sensory stimulus through multiplexing, the rodent olfactory sensory neurons (OSNs) can process completely distinct cues/stimuli – the mechanical stimulation associated with the airflow that carries odor molecules and the chemical sensation of odors themselves. Although the neural mechanisms of the latter have been investigated in detail, pathways processing the mechanical information remain largely elusive. Therefore, probing the multimodal aspects of olfactory information processing is fundamentally important. We focused on studying the neural mechanisms of airflow information processing using the mouse model system. We investigated the role of rodent olfactory system in detecting and discriminating airflow rates in presence and absence of odorant molecules. Our results showed that mice can learn to discriminate a range of airflow rates even in the absence of whiskers, which are known to be specialized sense organs that processes airflow-related information. Further, the discrimination abilities of the animals were lost when OSNs were ablated either by intranasal application of zinc sulphate or intra-peritoneal administration of olfacto-toxic drug methimazole. This confirmed their role in mechano-sensation. These results led us to further probe the role of olfactory bulb (OB) in processing the mechanical information. Our experimental results on airflow discriminations before and after olfactory bulbectomy proved the role of OB in processing airflow information. On investigating the neuronal activation patterns using c-Fos, an activity marker, we found the involvement of OB inhibitory circuits in discriminating airflow rates. This was further confirmed by recording calcium dynamics from a population of GABAergic (GAD65 expressing) interneurons using in vivo opto-physiology. These interneurons showed activations in response to the airflows in a stimulus strength dependent manner under anesthetized state. Further, the learning-dependent refinement of the interneuron activity was also observed during airflow discrimination training. This led us to causally investigate their role in refining mechanical stimuli information in the context of olfactory perception. We modified the function of this inhibitory circuit in vivo in a bidirectional way using optogenetics. While the enhancement of inhibition resulted in a poor discrimination of airflow rates, decrease of inhibition exhibited a contrasting behavioral phenotype. However, the same optogenetic modulations during odor discriminations ensued opposite behavioral phenotypes compared to airflow discriminations, indicating that optimal synaptic inhibition needed for refining olfactory and mechanosensory information varies. By altering both airflow and odorant components i.e. by applying combined stimuli of Flow1+odorant1 vs. Flow2+odorant2, the opposing effect of optogenetic modifications were nullified, thus confirming the association between mechanical and odorant information in olfactory perception (Mahajan et al., under preparation)1. Further, these results point towards the essentiality of dissecting the molecular and synaptic mechanisms underlying mechano-sensation through rodent olfactory circuits. Previous studies from our lab proved learning-dependent refinement of sampling (sniffing) behavior during odor discriminations in mice. We further wanted to explore the breathing patterns during olfaction-based cognitive tasks in healthy human subjects. To accomplish this, we coupled a breathing sensor with a custom-built olfactory-action meter that has already been established to examine olfactory abilities of human subjects (Bhowmik, Pardasani & Mahajan et al., 2022)2. In presence of odor stimuli, we observed longer inhalations of higher amplitude and shorter exhalations, resulting in a concomitant increase of breathing frequency during decision-making. This novel experimental strategy provided us with a robust method to quantify odor sampling behaviors shown by patients with olfactory deficits. When the breathing patterns of COVID-19 recovered subjects were examined, differences in the amplitudes of inhalation and exhalation were found (Mahajan et al., under preparation)3. Further, it prompts us to ask whether mechanosensation-driven modulation of olfactory perception prevails in humans. References: 1. Mahajan S., Tamboli S., Bhattacharjee A. S., Srikanth P., Pardasani M., Abraham, N. M. Mouse olfactory system acts as anemo-detector and -discriminator. (Manuscript under preparation). 2. Bhowmik R.*, Pardasani M.*, Mahajan S.*, Bhattacharjee A. S., Konakamchi S., Phadnis S., Musthafa T., McGowan E., Srikanth P., Marathe S. D., Abraham N. M. Uncertainty revealed by delayed responses during olfactory matching (*Equal contribution). Under Review, bioRxiv: https://doi.org/10.1101/2022.09.11.507462 (2022).