Biology

Mehrab Modi
CSHL / Janelia

Animals learn when stimuli of innate value, such as food or electric shocks, can be predicted by otherwise un-interesting stimuli like tones or odours. But the parameters of stimulus presentation most effective for conditioning are tuned differently from species to species, depending on the ecological niches they occupy. Drosophila melanogaster rapidly learns associations between neutral odours and reward or punishment, but this learning is best when the odour is presented for a full minute. I used two photon calcium-imaging to record the activity of the memory-forming neurons in the fly olfactory pathway – Kenyon cells (KCs) – while delivering one minute long odour stimuli. Some KC cell-bodies exhibit transient calcium responses at odour onset that fall back to baseline, early in the odour period. However, many KC cell-bodies also have plateau-shaped, sustained responses that last through the entire odour duration. In whole-cell electrophysiological recordings from KCs, I saw no spiking responses that remained sustained throughout a one-minute odour stimulation.  However subthreshold depolarizations were sustained, suggesting these may suffice to account for the sustained calcium-responses. We propose that the sustained calcium responses are part of a chemical signalling cascade that contributes to learning-related plasticity at the KC axon terminals. If this is true, it suggests that depolarizations that are below spike threshold can be above-threshold for chemical-signalling and can influence learning-related plasticity.