System-Like Consolidation of Olfactory Memories in Drosophila

Time requirements for synaptic output from MBN subsets for the retrieval of aversive olfactory memory. A, All flies were trained using an aversive unconditioned stimulus at 24°C and shifted to 32°C 10 min before a retrieval test. A synaptic blockade imposed by the expression and activation of shi^ts in the α′/β′ MBNs significantly reduced memory expression from 15 min to 1.5 h after training (Mann–Whitney pairwise comparisons, p ≤ 0.0043). No significant difference was observed during retrieval of memory at 3 or 24 h after training (Mann–Whitney pairwise comparisons, p ≥ 0.4452). A small decrement in expression of aversive memory was observed during retrieval at early time points at restrictive temperature in control flies carrying only the c305a-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.0022) due to heat stress. This decrement was not observed at subsequent time points (Mann–Whitney pairwise comparisons, p ≥ 0.0931) and is discussed further in the text. B, Synaptic blockade of γ MBNs significantly reduced aversive olfactory memory expression from 15 min to 3 h after training (Mann–Whitney pairwise comparisons, p ≤ 0.0260). No significant difference was observed during retrieval of memory at 24 h after training (Mann–Whitney pairwise comparison, p = 0.4286). A small decrement in performance of aversive memory was observed during retrieval of 15 min to 1.5 h memory at restrictive temperature in control flies carrying only the 1471-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.0152). This impairment was not observed at subsequent time points (Mann–Whitney pairwise comparisons, p ≥ 0.6623). C, Synaptic blockade of α/β MBNs significantly reduced aversive olfactory memory expression from 15 min to 24 h after training (Mann–Whitney pairwise comparisons, p ≤ 0.0095). A small decrement in performance of aversive memory was observed during retrieval at 15 min and 1 h at restrictive temperature in control flies carrying only the c739-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.0411). This impairment was not observed at subsequent time points (Mann–Whitney pairwise comparison, p ≥ 0.1320). D, A slight decrement in performance of aversive memory expression was observed during retrieval from 15 min to 1.5 h at restrictive temperature in control flies carrying only the uas-shi^ts genetic element (Mann–Whitney pairwise comparison, p ≥ 0.0303). This impairment was not observed at subsequent time points (Mann–Whitney pairwise comparison, p ≥ 0.3095).

Time requirements for synaptic output from MBN subsets for the retrieval of appetitive olfactory memory is odor independent. A, All flies were conditioned using an appetitive unconditioned stimulus at 24°C and shifted to 32°C 10 min before a retrieval test. Conditioned odors used were Ben and Mes. A synaptic blockade imposed by the expression and activation of shi^ts in the α′/β′ MBNs significantly reduced memory expression at 1 h but not 24 h after training (Mann–Whitney pairwise comparisons, p = 0.0022 and p = 0.3776, respectively). No significant difference was observed between permissive and restrictive temperatures for control flies carrying only the c305a-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.0649). B, Synaptic blockade of γ MBNs significantly reduced 1 h but not 24 h appetitive olfactory memory expression (Mann–Whitney pairwise comparison, p = 0.0135 and p = 0.6304, respectively). No significant difference was observed between permissive and restrictive temperatures for control flies carrying only the 1471-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.7483). C, Synaptic blockade of α/β MBNs significantly reduced appetitive olfactory memory expression during retrieval at 1 and 24 h after training (Mann–Whitney pairwise comparisons, p ≤ 0.0081). No significant difference was observed between permissive and restrictive temperatures for control flies carrying only the c739-gal4 element (Mann–Whitney pairwise comparisons, p ≥ 0.4848). D, No significant difference was observed between permissive and restrictive temperatures for control flies carrying only the uas-shi^ts element (Mann–Whitney pairwise comparisons, p ≥ 0.8182). Asterisks indicate a statistically significant difference assessed by Kruskal-Wallis analysis followed by Mann–Whitney pairwise comparison (n = 6 for each group).

Retrieval of short- and long-term appetitive and aversive memories induces an increased retrieval signal to the learned odor in different MBN subsets. A, Response ratio of the CS⁺/CS⁻ as a function of time measured in the vertical branch of the α′/β′ MBNs after appetitive olfactory conditioning and in naive flies. A robust increase in calcium influx was detected in the vertical branch during the retrieval of 1 h appetitive memory (Mann–Whitney pairwise comparison, p = 0.0041). This increase was not detected during the retrieval of the appetitive memory 24 h after training (Mann–Whitney pairwise comparison, p = 0.8361). No change was observed at either time point in naive animals. Asterisks indicate a statistically significant difference compared with the naive group. n = 10–16 for each group. B, Response ratio of the CS⁺/CS⁻ as a function of time measured in the vertical branch of the α/β MBNs after appetitive olfactory conditioning and in naive flies. A robust increase in calcium influx was detected in the vertical branch during the retrieval of 24 h appetitive memory (Mann–Whitney pairwise comparison, p = 0.0183). This increase was not detected during the retrieval of the appetitive memory 1 h after training (Mann–Whitney pairwise comparison, p = 1). No change was observed at either time point in naive animals. Asterisks indicate a statistically significant difference compared with the naive group (n = 10–16 for each group). C, Response ratio of the CS⁺/CS⁻ as a function of time measured in the vertical branch of the α′/β′ MBNs after aversive olfactory conditioning and in naive flies. A robust increase in calcium influx was detected in the vertical branch during the retrieval of 1 h aversive memory (Mann–Whitney pairwise comparison, p = 0.0011). This increase was not detected during the retrieval of the aversive memory 24 h after training (Mann–Whitney pairwise comparison, p = 0.2618). No change was observed at either time point in naive animals. Asterisks indicate a statistically significant difference compared with the naive group (n = 10–16 for each group). D, Response ratio of the CS⁺/CS⁻ as a function of time measured in the vertical branch of the α/β MBNs after aversive olfactory conditioning and in naive flies. A robust increase in calcium influx was detected in the vertical branch during the retrieval of 24 h aversive memory (Mann–Whitney pairwise comparison, p = 0.0027). This increase was not detected during the retrieval of the aversive memory 1 h after training (Mann–Whitney pairwise comparison, p = 0.2225). No change was observed at either time point in naive animals. Asterisks indicate a statistically significant difference compared with the naive group (n = 10–16 for each group). E, Expression of the activity-dependent gene c-fos in the hippocampus CA1 region after recent and remote memory tests for contextual fear conditioning in mice. c-fos expression is shown as a percentage relative to controls that were not given foot shock. Figure is adapted with permission from Frankland et al. (2004). F, Expression of the activity-dependent gene c-fos in the anterior cingulate cortex after recent and remote memory tests for contextual fear conditioning in mice. c-fos expression is shown as a percentage relative to controls that were not given footshock. Figure is adapted with permission from Frankland et al. (2004).