Dopamine and the Temporal Dependence of Learning and Memory

Animal behavior is largely influenced by the seeking out of rewards and avoidance of punishments. Positive or negative reinforcements, like a food reward or painful shock, impart meaningful valence onto sensory cues in the animal’s environment. The ability of animals to form associations between a sensory cue and a rewarding or punishing reinforcement permits them to adapt their future behavior to maximize reward and minimize punishments. Animals rely on the timing of events to infer the causal relationships between cues and outcomes –– sensory cues that precede a painful shock in time become associated with its onset and are imparted with negative valence, whereas cues that follow the shock in time are instead associated with its cessation and imparted with positive valence. While the temporal requirements for associative learning have been well characterized at the behavioral level, the molecular and circuit mechanisms for this temporal sensitivity remain incompletely understood. Using the simple architecture of the mushroom body, an olfactory associative learning center in Drosophila, I examined how the relative timing of olfactory inputs and dopaminergic reinforcement signals is encoded at the molecular, synaptic, and circuit level to give rise to learned odor associations. I show that in Drosophila, opposing olfactory associations can be formed and updated on a trial-by-trial basis depending on the temporal relationship between an odor cue and dopaminergic reinforcement during conditioning. Additionally, both negative and positive reinforcements equivalently instruct appetitive and aversive olfactory associations –– odors preceding a negative reinforcement or following a rewarding reinforcement acquire an aversive valence, while odors instead following a negative reinforcement or preceding a rewarding reinforcement become attractive. Furthermore, functional imaging revealed that synapses within the mushroom body are bidirectionally modulated depending on the temporal ordering of odor and dopaminergic reinforcement, leading to synaptic depression when an odor precedes dopaminergic activity or synaptic facilitation when dopaminergic activity instead precedes an odor. Through the synchronous recording of neural activity and behavior, I found that the bidirectional regulation of synaptic transmission within the mushroom body directly correlates with the emergence of learned olfactory behaviors. This temporal sensitivity arises from two dopamine receptors, DopR1 and DopR2, that couple to distinct second-messengers and direct either synaptic depression or potentiation. Loss of either receptor renders the synapses of the mushroom body capable of only unidirectional plasticity and prevents the behavioral flexibility of writing opposing associations depending on the temporal structure of conditioning. Together, these results reveal how the distinct intracellular signaling pathways of two dopamine receptors can detect the order of events within an associative learning circuit to instruct opposing forms of synaptic and behavioral plasticity, providing a mechanism for animals to use both the onset and offset of a reinforcement signal to instruct distinct associations. Additionally, this bidirectional modulation allows animals to flexibly update olfactory associations on a trial-bytrial basis when temporal relationships are altered, permitting them to contend with a complex and changing sensory world