Abstract
Several experiments indicate that there exists substantial synaptic-depression at the synapses between olfactory receptor neurons (ORNs) and neurons within the drosophila antenna lobe (AL). This synaptic-depression may be partly caused by vesicle-depletion, and partly caused by presynaptic-inhibition due to the activity of inhibitory local neurons within the AL. While it has been proposed that this synaptic-depression contributes to the nonlinear relationship between ORN and projection neuron (PN) firing-rates, the precise functional role of synaptic-depression at the ORN synapses is not yet fully understood. In this paper we propose two hypotheses linking the information-coding properties of the fly AL with the network mechanisms responsible for ORNAL synaptic-depression. Our first hypothesis is related to variance coding of ORN firing-rate information — once stimulation to the ORNs is sufficiently high to saturate glomerular responses, further stimulation of the ORNs increases the regularity of PN spiking activity while maintaining PN firing-rates. The second hypothesis proposes a tradeoff between spike-time reliability and coding-capacity governed by the relative contribution of vesicle-depletion and presynaptic-inhibition to ORNAL synaptic-depression. Synaptic-depression caused primarily by vesicle-depletion will give rise to a very reliable system, whereas an equivalent amount of synaptic-depression caused primarily by presynaptic-inhibition will give rise to a less reliable system that is more sensitive to small shifts in odor stimulation. These two hypotheses are substantiated by several small analyzable toy models of the fly AL, as well as a more physiologically realistic large-scale computational model of the fly AL involving glomerular channels.
Highlights
The early stages of the drosophila olfactory system include a primary sensory structure called the antenna lobe (AL)
In order to investigate the possible function associated with these network mechanisms, we have designed and built a scaled down computational network model of the fly AL
Using our large-scale model for the fly antennal lobe, we have been able to put forth two hypotheses which concern the functional role played by synaptic-depression at the olfactory receptor neurons (ORNs) synapses within the fly AL
Summary
The early stages of the drosophila olfactory system include a primary sensory structure called the antenna lobe (AL). Each glomerulus within the AL contains dendrites of local neurons (LNs) whose projections are limited to the AL, as well as projection neurons (PNs) whose axons extend beyond the AL deeper into the fly brain [6]. Various experiments indicate that there exists substantial synaptic-depression at the synapses between olfactory receptor neurons (ORNs) and neurons within the drosophila antenna lobe (AL); by ‘synaptic-depression’, we refer to any mechanism which gives rise to short-term depression of the ORN-induced EPSCs within the AL following an increase in ORN activity. To investigate the relationship between synaptic-depression and the coding properties of the fly AL, we created and analyzed the dynamics of several models of the fly AL. We have been able to distill two hypotheses linking the information-coding properties of the fly AL with the network mechanisms responsible for ORN?AL synaptic-depression
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