Identification of Dopaminergic Neurons That Can Both Establish Associative Memory and Acutely Terminate Its Behavioral Expression

Abstract

An adaptive transition from exploring the environment in search of vital resources to exploiting these resources once the search was successful is important to all animals. Here we study the neuronal circuitry that allows larval Drosophila melanogaster of either sex to negotiate this exploration-exploitation transition. We do so by combining Pavlovian conditioning with high-resolution behavioral tracking, optogenetic manipulation of individually identified neurons, and EM data-based analyses of synaptic organization. We find that optogenetic activation of the dopaminergic neuron DAN-i1 can both establish memory during training and acutely terminate learned search behavior in a subsequent recall test. Its activation leaves innate behavior unaffected, however. Specifically, DAN-i1 activation can establish associative memories of opposite valence after paired and unpaired training with odor, and its activation during the recall test can terminate the search behavior resulting from either of these memories. Our results further suggest that in its behavioral significance DAN-i1 activation resembles, but does not equal, sugar reward. Dendrogram analyses of all the synaptic connections between DAN-i1 and its two main targets, the Kenyon cells and the mushroom body output neuron MBON-i1, further suggest that the DAN-i1 signals during training and during the recall test could be delivered to the Kenyon cells and to MBON-i1, respectively, within previously unrecognized, locally confined branching structures. This would provide an elegant circuit motif to terminate search on its successful completion.

Switch-like and persistent memory formation in individualDrosophilalarvae

“…In other associative conditioning experiments using DAN-i1 activation as a reward, decreased attraction to the odor was observed in the reverse-paired groups [9,43,44]. In our experiments, we did not see any evidence of increased aversion in the reverse-paired groups.…”

“…Here our results differed from those previously reported. In other as-says, presenting the reward (including via activation of DAN-i1) prior to presenting the conditioned odor results in decreased attraction/increased avoidance [9,44] of that odor. We found that such "reversepairings" neither increased nor decreased a larva's avoidance of CO 2 .…”

The connectome of the adult Drosophila mushroom body provides insights into function

“…DANs also make direct connections with MBONs in each MB compartment, and optogenetic activation of PAM11 DANs can directly excite MBON07 with slow dynamics ( Takemura et al, 2017 ). This might provide a mechanism to temporarily suppress memory expression without impairing the underlying memory, which is stored as depressed KC-to-MBON synapses ( Krashes et al, 2009 ; Schleyer et al, 2020 ; Senapati et al, 2019 ; Takemura et al, 2017 ). For example, DANs are known to control hunger and thirst-dependent memory expression ( Krashes et al, 2009 ; Senapati et al, 2019 ), and their excitation of MBONs could provide a possible mechanism.…”

Aversive teaching signals from individual dopamine neurons in larval Drosophila show qualitative differences in their temporal “fingerprint”

“…We note that transferring the brains directly to pure Bouin’s solution would lead to a collapse of the tissue. The fixation time was 7 min, at room temperature on a shaker; initial experiments with fixation in 4% paraformaldehyde (PFA) had failed for the primary mouse anti‐ChR2 antibody mentioned below (data not shown, see also Weiglein, Gerstner, Mancini, Schleyer, & Gerber, 2019, Schleyer et al, 2020). Then, the larval brains were washed three times consecutively in fresh washing solutions for 10 min each time, using 0.2% PBT (Triton‐X‐100, CAS: 9036‐19‐5, Roth, Karlsruhe, Germany; in 1x PBS).…”