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Monday, July 20 • 9:00pm - 10:00pm
P145: Possible roles of non-synaptic interactions between olfactory receptor neurons in insects

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Mario Pannunzi
, Thomas Nowotny

In insects, olfactory receptor neurons (ORNs) are grouped in hairs (sensilla) in a stereotypical way. For example, in _Drosophila_ each sensillum houses 2 or 4 ORNs. ORNs that are co-housed in the same sensillum interact with each other via a non-synaptic mechanism (NSI, see panel a), a mechanism which is still not fully understood. The mechanism could simply be a spandrel or instead improve the function of the insect olfactory system. A number of hypotheses have been suggested [1] trying to explain the potential role of NSIs.

We analyzed two hypotheses that suggest that NSIs play a role in odor sensing in mixtures with a computational model of the first two layers of the _Drosophila_ olfactory system - the ORNs on the antennae and the glomeruli, with projection neurons (PNs) and local neurons (LNs), in the antennal lobe (AL, see panel b). The model is the first to consider NSIs between ORNs in the context of the circuits of the first and second layer of processing in the insect olfactory pathway. We constrained the model by reproducing the responses to a set of typical odor stimuli reported in the literature. Then, we tested the feasibility of the hypotheses and compared the advantages of having NSIs against a control network which lacked any interaction between ORNs or PNs, and with a network without NSIs but strong lateral inhibition in the AL, a mechanism proposed to be a valid alternative to NSIs.

The two tested hypotheses were: 1. NSIs could improve the concentration ratio identification of a mixture of odorants by increasing the dynamic range over which it can be perceived without distortion. 2. NSIs could help insects to distinguish mixtures of odorants emanating from a single source against those emanating from two separate sources, by improving the capacity to encode the correlation between olfactory stimuli.

For the first hypothesis, we observed that: 1. When comparing the capacity to encode the ratio of the concentration of short synchronous whiffs via PN responses, both networks, the one with NSIs and the one with AL inhibition, outperform the ‘control network’. Moreover, the NSIs help more than the LN inhibition for this task. This effect is stronger for very short stimuli (<100ms) than for longer stimuli. 2. When a network with LN inhibition (with NSIs) is stimulated with asynchronous whiffs of two odorants, its PN outputs in response to the second whiffs are strongly (mildly) altered by the response to the first whiff.  More complex interpretations are needed when assessing the capacity to encode the correlation between two odorants. We noted that: 1. In terms of average PN activity, the network with LN inhibition is able to encode stimulus correlation but the network with NSI mechanism is not, but  2. In terms of peak PN activity (response higher than a given threshold), the network with NSI mechanism encodes correlations better than the one with LN inhibition. This improvement is bigger for shorter whiff durations (<100 ms). 

Acknowledgments
This research was funded by the Human Frontiers Science Program, grant RGP0053/2015 (Odor Objects), the European Union under grant agreement 785907 (HBP SGA2) and a Leverhulme Trust Research Project Grant.
References
  1. Todd JL, Baker TC. Function of peripheral olfactory organs. Insect olfaction, 1999: p. 67-96; Su CY et al. Non-synaptic inhibition between grouped neurons in an olfactory circuit. Nature 2012: 492(7427) p. 66.

Speakers
avatar for Mario Pannunzi

Mario Pannunzi

Post doc, Department of Informatics, University of Sussex



Monday July 20, 2020 9:00pm - 10:00pm CEST
Slot 14