Hypergravity and aging in Drosophila melanogaster. 9. Conditioned suppression and habituation of the proboscis extension response.

In a first experiment, the conditioned suppression of the proboscis extension response (PER) to sucrose was measured in young, middle-aged and old male Drosophila melanogaster flies living at either 1, 3 or 5 g. Flies were starved and then subjected to a learning task involving a sucrose stimulus, followed by an aversive one applied to their forelegs. In this learning task, flies learn to not extend their proboscis when walking on sucrose. Flies which have lived in hypergravity (HG) had a lower number of PER suppressions than 1 g ones, and this finding was mainly due to young and middle-aged flies. In a second experiment, the habituation of the PER was studied using as stimulation sucrose solutions 2-fold (first experiment), 4-fold (second one) or 8-fold (third one) higher than the individual sucrose threshold. Middle-aged and old flies habituated more slowly than young flies in the second and third experiments. In the third experiment, a decreasing speed of habituation was observed when gravity increased; this result was mainly due to young flies, and no gravity effect was observed in the other two age groups. This whole set of results suggests that HG-kept flies do not age faster than 1 g ones, as far as these learning and habituation tasks are concerned. It seems possible that HG acts like a mild stress to which flies adapt; if applied for a long time, HG could induce a premature aging, as observed in the previous papers of this series.