Introduction Neuromedin U (NMU) is a neuropeptide enriched in the nucleus accumbens shell (NAcSh), a brain region associated with reward. While NMU and its receptor, NMU Receptor 2 (NMUR2), have been studied for its ability to regulate food reward, NMU has not been studied in the context of drugs of abuse (e.g. cocaine). Furthermore, the neuroanatomical pathways which express NMUR2 and its ultrastructural localization are unknown. Methods Immunohistochemistry was used to determine the synaptic localization of NMUR2 in the NAcSh and characterize which neurons express this receptor (n=17). The functional outcome of NMU on NMUR2 was examined using microdialysis (n=16). The behavioral effects of NMU microinjection directly to the NAcSh was investigated using cocaine-evoked locomotion (n=93). The specific effects of NMUR2 knockdown on cocaine-evoked locomotion was evaluated using viral-mediated RNAi (n=40). Results NMUR2 is localized to presynaptic GABAergic nerve terminals in the NAcSh originating from the dorsal raphe nucleus. Furthermore, NMU microinjection to the NAcSh decreased local GABA concentrations. Next, we evaluated the effects of NMU microinjection on behavioral sensitization to cocaine. When repeatedly administered throughout the sensitization regimen, NMU attenuated cocaine-evoked hyperactivity. Additionally, shRNA-mediated knockdown of presynaptic NMUR2 in the NAcSh using a retrograde viral vector potentiated cocaine sensitization. Conclusions Together, these data reveal that NMUR2 modulates a novel GABAergic pathway from the dorsal raphe nucleus to the NAcSh to influence behavioral responses to cocaine.

Highlights d Ensembles from both the OT and pPCX accurately encode conditioned odors d More OT neurons represent rewarded odors and do so in manners different from pPCX d OT D1 neurons flexibly represent rewarded odors during reversal learning d Activation of OT D1 neurons promotes engagement in a reinforcer-motivated task

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Animals choose between sensory stimuli, a highly complex behavior which includes detection, discrimination, preference, and memory processes. Rodents are reported to display robust preferences for some odors, for instance, in the context of choosing among possible mates or food items. In contrast to the apparent robustness of responses toward these and other "ethologically relevant" odors, little is known about the robustness of behaviors toward odors which have no overt role in the rodent ecological niche, so-called "nonethologically relevant" odors. We developed an apparatus for monitoring the nose-poking behavior of mice and used this apparatus to explore the prevalence and stability of choices among different odors both across mice, and within mice over successive days. Mice were tested with a panel of either ethologically relevant or nonethologically relevant odors in an olfactory multiple-choice test. Significant preferences to nonethologically relevant odors were observed across the population of mice, with longer investigation durations to some odors more than to others. However, we found substantial inter-mouse variability in these responses, and that responses to these odors even varied within mice across days of testing. Tests with ethologically relevant odors revealed that responses toward these odors were also variable across mice, but within individual mice, responses were somewhat stable. This work establishes an olfactory multiple-choice test for monitoring odor investigation, choice, and preference behaviors and the application of this apparatus to assess across- and within-mouse odor-preference choice stability. These results highlight that odor preferences, as assayed by measuring choice behaviors, are variable. (PsycINFO Database Record

SummaryLearning strengthens the strong emotional and behavioral responses odors are well known for eliciting. Presumably subserving this, several brain regions display experience-dependent plasticity during odor learning, yet the specific cellular systems involved and the actual influence of these systems on odor-directed behavior are less understood. Here we investigated the transformation of odor information throughout the association of odors with rewards and also sought to link those neural systems with displays of reinforcement-based task engagement. First, we investigated the representation of odor-reward associations within two areas recipient of dense olfactory bulb input, the posterior piriform cortex (pPCX) and olfactory tubercle (OT), using simultaneous multi-site electrophysiological recordings from mice engaged in a reward-based olfactory learning task. As expected, neurons in both regions represented conditioned odors and did so with similar information content, yet both the proportion of neurons recruited by conditioned rewarded odors and the magnitudes and durations of their responses were greater in the OT. Using fiber photometry, we found that OT D1-type dopamine receptor expressing neurons flexibly represent odors based upon reward associations. In both the recordings and imaging, statistically meaningful changes in activity occurred soon after odor onset. Finally, using optogenetics we show that OT D1-receptor expressing neurons strongly influence behavior to promote task engagement. Together our results contribute to a model whereby OT D1 neurons support odor-guided motivated behaviors.