Four Individually Identified Paired Dopamine Neurons Signal Reward in Larval Drosophila

Abstract

Dopaminergic neurons serve multiple functions, including reinforcement processing during associative learning [1-12]. It is thus warranted to understand which dopaminergic neurons mediate which function. We study larval Drosophila, in which only approximately 120 of a total of 10,000 neurons are dopaminergic, as judged by the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine biosynthesis [5, 13]. Dopaminergic neurons mediating reinforcement in insect olfactory learning target the mushroom bodies, a higher-order "cortical" brain region [1-5, 11, 12, 14, 15]. We discover four previously undescribed paired neurons, the primary protocerebral anterior medial (pPAM) neurons. These neurons are TH positive and subdivide the medial lobe of the mushroom body into four distinct subunits. These pPAM neurons are acutely necessary for odor-sugar reward learning and require intact TH function in this process. However, they are dispensable for aversive learning and innate behavior toward the odors and sugars employed. Optogenetical activation of pPAM neurons is sufficient as a reward. Thus, the pPAM neurons convey a likely dopaminergic reward signal. In contrast, DL1 cluster neurons convey a corresponding punishment signal [5], suggesting a cellular division of labor to convey dopaminergic reward and punishment signals. On the level of individually identified neurons, this uncovers an organizational principle shared with adult Drosophila and mammals [1-4, 7, 9, 10] (but see [6]). The numerical simplicity and connectomic tractability of the larval nervous system [16-19] now offers a prospect for studying circuit principles of dopamine function at unprecedented resolution.

Common microbehavioral “footprint” of two distinct classes of conditioned aversion

“…Thus, to this extent aversive and appetitive memories in Drosophila share the same circuit. Rewards (appetitive unconditioned stimuli in Pavlovian terminology) are signaled by a subset of dopaminergic neurons (Rohwedder et al 2016), similar to the case in vertebrates (Schultz 2015). Whether dopaminergic neurons in Drosophila convey a prediction error signal rather than the reward signal per se as implicated by learning theory (Rescorla and Wagner 1972) and demonstrated in vertebrates (Schultz 2015) remains to be shown in Drosophila.…”

“…Punishments (aversive unconditioned stimuli in Pavlovian terminology) are signaled by another subset of dopaminergic neurons in the fruit fly (Schroll et al 2006;Liu et al 2012;Aso et al 2014a,b;Rohwedder et al 2016), which is controversial in vertebrates (Lammel et al 2012;Schultz 2013). Thus, different sets of dopaminergic neurons were found to be necessary in Drosophila appetitive or aversive learning, respectively (e.g., Schroll et al 2006;Liu et al 2012;Aso et al 2014a,b;Rohwedder et al 2016).…”

“…Thus, different sets of dopaminergic neurons were found to be necessary in Drosophila appetitive or aversive learning, respectively (e.g., Schroll et al 2006;Liu et al 2012;Aso et al 2014a,b;Rohwedder et al 2016). These different sets of dopaminergic neurons innervate clearly separated compartments of the mushroom body (Pauls et al 2010;Aso et al 2014a).…”

“…The results can add to the conceptually interesting comparison of learning about rewards/punishments versus the lack of rewards/punishments. Furthermore, this study can serve as a behavioral framework for the on-going analyses of the underlying circuits (e.g., Berck et al 2016;Fushiki et al 2016;Rohwedder et al 2016). Together, these efforts can provide a detailed picture of how associative memory helps to organize conditioned approach and aversion in a simple nervous system, and how both are implemented in specific motor actions.…”

Big Lessons from Tiny Flies:<em> Drosophila Melanogaster </em>as a Model to Explore Dysfunction of Dopaminergic and Serotonergic Neurotransmitter Systems

“…In flies, these pathways have been dissected to yield a detailed understanding of input/output-relation, decision-making and memory formation [18,21,22,[69][70][71][72][73][74][75]: dopaminergic input impinges on the mushroom body output neurons: PPL1 dopaminergic neurons are involved in aversive olfactory reinforcement [76,77]. PAM dopaminergic neurons promote reward reinforcement [22,71,72,78]. Both, dopaminergic PAM and PPL1 neurons are subject to feedback control by mushroom body output neurons.…”