Spike frequency adaptation and coding of transient stimuli in an insect CO2 sensory system

A BEYERLEIN; P G GUERENSTEIN; J G HILDEBRAND

Chicago, EEUU

Congreso; Annual Meeting of the Society for Neuroscience; 2009

Society for Neuroscience

Due to their volatile nature, odorants released from point sources are necessarily encountered by insects as intermittent stimuli. CO2, an odorant known to influence insect behavior is an exception to this rule. Although natural CO2 levels exhibit variability at multiple temporal scales, CO2 is never absent from ambient air. For the hawkmoth Manduca sexta, transient upward fluctuations in CO2 levels are sufficient to elicit upwind flight and location of the odor source. Furthermore, floral nectar sources Manduca is known to visit in the field are sources of locally elevated levels of CO2 whose detection appear to aid in foraging efficiency.   We are investigating neural encoding of behaviorally relevant transient changes from constant background CO2 levels by the first two levels of the Manduca olfactory pathway. Previously our group showed that Manduca detects CO2 exclusively through receptor cells (CO2 RCs) housed within the labial palp organ (LPO). The axons of CO2 RCs terminate in a single glomerulus (the LPOG) within the antennal lobe (AL). In common with many typical olfactory receptor cells (ORCs), CO2 RCs are excited by increases in stimulus concentration. Unlike typical ORCs, the firing rate of CO2 RCs adapts during prolonged stimulation to a constant firing frequency that encodes the unchanging background concentration. Here, we test the effects of sensory adaptation on the neural representation of CO2 stimuli at two stages in the olfactory network. We recorded from both CO2 receptor neurons and LPOG PNs to analyze the effect of spike-frequency adaptation caused by a constant, natural background CO2 level upon the encoding of superimposed transient CO2 changes. In doing so, we took advantage of a previously developed model of spike-frequency adaptation (Benda & Herz, Neural Computation, 2003), and applied it to the olfactory system in order to quantify the influence of adaptation on the processing of transient CO2 stimuli. We describe a transfer function that links adaptation in CO2 RCs and the output of the LPOG.