Abstract
Value coding of external stimuli in general, and odor valence in particular, is crucial for survival. In flies, odor valence is thought to be coded by two types of neurons: mushroom body output neurons (MBONs) and lateral horn (LH) neurons. MBONs are classified as neurons that promote either attraction or aversion, but not both, and they are dynamically activated by upstream neurons. This dynamic activation updates the valence values. In contrast, LH neurons receive scaled, but non-dynamic, input from their upstream neurons. It remains unclear how such a non-dynamic system generates differential valence values. Recently, PD2a1/b1 LH neurons were demonstrated to promote approach behavior at low odor concentration in starved flies. Here, we demonstrate that at high odor concentrations, these same neurons contribute to avoidance in satiated flies. The contribution of PD2a1/b1 LH neurons to aversion is context dependent. It is diminished in starved flies, although PD2a1/b1 neural activity remains unchanged, and at lower odor concentration. In addition, PD2a1/b1 aversive effect develops over time. Thus, our results indicate that, even though PD2a1/b1 LH neurons transmit hard-wired output, their effect on valence can change. Taken together, we suggest that the valence model described for MBONs does not hold for LH neurons.
Highlights
In order to survive, animals must attach value to external stimuli, be it due to innate or learned behavior
PD2a1/b1 neurons belong to the lateral horn output neurons (LHON)[15]
Reciprocal LHON input accounts for about 20%, and a varying amount of ipsi- and contralateral axoaxonic input in the superior intermediate protocerebrum (SIP) comes from the mushroom body output neuron (MBON)-α2sc[15]
Summary
Animals must attach value to external stimuli, be it due to innate or learned behavior. PD2a1/b1 neurons were shown to be required for aversive conditioning and it was suggested that reduced activation of PD2a1/b1 neurons following aversive conditioning was responsible for the reduced odor approach[15] These observations are in agreement with current knowledge about learning and memory processes occurring at the MB and MBONs. MBONs are divided into neurons that drive either attraction or aversion, and plasticity between MB and MBONs shifts the balance between attraction and aversion for each odor[22,23,24,25,26,27,28,29,30]. Starvation abolished the contribution of PD2a1/b1 neurons to an aversive response to odors even though there was no effect on the PD2a1/b1 neuron odor responses
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