Abstract
Projection-neurons (PNs) within the antennal lobe (AL) of the hawkmoth respond vigorously to odor stimulation, with each vigorous response followed by a period of suppression (dubbed the “afterhyperpolarization-phase,” or AHP-phase).
We conduct several physiological experiments to investigate the mechanisms underlying the AHP-phase, and find that pharmacological manipulation of the AL yields surprising results.
Specifically, (a) the application of picrotoxin (PTX) lengthens the AHP-phase and reduces PN activity, whereas (b) the application of Bicuculline-methiodide (BIC) reduces the AHP-phase and increases PN activity.
These results are curious, as both PTX and BIC are inhibitory-receptor antagonists.
To resolve this conundrum, we speculate that perhaps (a) PTX reduces PN activity through a disinhibitory circuit involving a heterogeneous population of local-neurons, and (b) BIC acts to hamper certain intrinsic currents within the PNs that contribute to the AHP-phase.
To support these hypotheses we build a computational model of the AL.
We find that, when benchmarked against our experimental observations, this model exhibits a particular kind of synchronous activity: namely, “multiple-firing-events” (MFEs).
These MFEs are causally-linked sequences of spikes which emerge stochastically, and turn out to have important dynamical consequences for all the experimentally observed phenomena we used as benchmarks.
Taking a step back, we extract a few predictions from our computational model pertaining to the real AL:
Some predictions deal with the MFEs we expect to see in the real AL, whereas other predictions involve the runaway synchronization that we expect when BIC-application hampers the AHP-phase.
By examining the literature we see support for the former, and we perform some additional experiments to confirm the latter.
The confirmation of these predictions validates, at least partially, our initial speculation above.
We conclude that the AL is poised in a state of high-gain; ready to respond vigorously to even faint stimuli.
Furthermore, we suspect that the AHP-phase might function to prevent runaway synchronization (after an initial odor response) and to “reset” the AL for another odor-specific response.