Spaced training forms complementary long-term memories of opposite valence inDrosophila

Forming long-term memory (LTM) in many cases requires repetitive experience spread over time. InDrosophila, aversive olfactory LTM is optimal following spaced training, multiple trials of differential odor conditioning with rest intervals. Studies often compare memory after spaced to that after massed training, same number of trials without interval. Here we show flies acquire additional information after spaced training, forming an aversive memory for the shock-paired odor and a ‘safety-memory’ for the explicitly unpaired odor. Safety-memory requires repetition, order and spacing of the training trials and relies on specific subsets of rewarding dopaminergic neurons. Co-existence of the aversive and safety memories can be measured as depression of odor-specific responses at different combinations of junctions in the mushroom body output network. Combining two particular outputs appears to signal relative safety. Learning a complementary safety memory thereby augments LTM performance after spaced training by making the odor preference more certain.

Ghrelin enhances food odor conditioning in healthy humans: an fMRI study

SummaryVulnerability to obesity includes eating in response to food cues, which acquire incentive value through conditioning. The conditioning process is largely subserved by dopamine, theorized to encode the discrepancy between expected and actual rewards, known as the reward prediction error (RPE). Ghrelin is a gut-derived homeostatic hormone that triggers hunger and eating. Despite extensive evidence that ghrelin stimulates dopamine, it remains unknown in humans if ghrelin modulates food cue learning. Here we show using functional magnetic resonance imaging that intravenously administered ghrelin increased RPE-related activity in dopamine-responsive areas during food odor conditioning in healthy volunteers. Participants responded faster to food odor-associated cues and perceived them to be more pleasant following ghrelin injection. Ghrelin also increased functional connectivity between hippocampus and ventral striatum. Our work demonstrates that ghrelin promotes the ability of cues to acquire incentive salience, and has implications for the development of vulnerability to obesity.

Oscillations synchronize amygdala-to-prefrontal primate circuits during aversive learning

SummaryThe contribution of oscillatory synchrony in the primate amygdala-prefrontal pathway to aversive learning remains unknown. We found increased power and phase synchrony in the theta range during aversive conditioning. The synchrony was linked to single-unit spiking and exhibited specific directionality between input and output measures in each region. Although it was correlated with the development of conditioned responses, it declined once the association stabilized. The results suggest that amygdala spikes aid to synchronize ACC activity and transfer error-signal information to support memory formation.HighlightsTone-odor conditioning induces theta phase-reset in primate amygdala and dACCA directional phase-locking develops between amygdala spikes and dACC ThetaInformation transfer from Amygdala to dACC decreases once memory stabilizes

Octopamine release impairs courtship conditioning in Drosophila melanogaster

Octopamine is known to have an appetitive role in odor conditioning paradigms in Drosophila melanogaster. We sought to test whether octopamine could also act as an appetitive stimulus in courtship conditioning, a paradigm in which training with an unreceptive female (such as a decapitated virgin) causes a subsequent decrease in courtship behavior in male Drosophila. To control octopamine release, we used the Tdc2-Gal4 and UAS-dTRPa1 genes in conjunction to depolarize octopaminergic neurons at 27 C in experimental flies. We hypothesized that inducing octopamine release during courtship training would decrease the aversive impact of training and cause less subsequent suppression of courtship behavior. Our findings confirmed this hypothesis: Tdc2-Gal4/UAS-dTRPa1 flies trained at 27 degrees showed significantly more courtship behavior than controls during testing, and in fact showed no significant effect of courtship training. This confirms that octopamine release counteracts the aversive stimulus of failure to copulate, indicating that octopamine may have an appetitive role in courtship.