Correlates of motivation and reward

National Institute On Drug Abuse

Investigators

Linked publications, trials & patents

Abstract

I discuss here two projects that have provided notable progress for the present fiscal year. Project 1 concerns paradoxical responses of medial septal GAD2 GABAergic neurons to rewarding and aversive stimuli. It is fundamental for humans and animals to seek rewards and safety from threats. To begin understanding neural mechanisms for such processes, we employed intracranial self-stimulation (ICSS), a model procedure for reward-seeking behavior, using optogenetic manipulations. In particular, we examined the roles of medial septal GABAergic (MS GABA) neurons using vGAT-Cre and GAD2-Cre mice. Mice quickly learned to stimulate MS GAD2 neurons (n=6), but not vGAT neurons (n=6). Next, we examined how these neuronal populations respond during appetitive and aversive contexts. AAV9-syn-FLEX-jGCaMP7f-WPRE was injected into the MS, and a probe was implanted in vGAT- and GAD2-Cre mice for fiber photometry. We then performed Pavlovian conditioning procedures with three different tones paired with a 100%, 50%, or 0% chance of a water reward or foot shock. MS vGAT neurons displayed heterogenous responses between mice (n= 6): vGAT neurons did not consistently respond to water rewards or cues predicting water rewards. By contrast, both certain and uncertain cues and water rewards decreased the activity of MS GAD2 neurons (n=4). During the shock procedure, MS vGAT neurons increased activity in response to foot shock, but not shock-paired tones, while MS GAD2 neurons displayed ramping activity during shock-paired tones and increased activity in response to shock. In sum, the results suggest that while vGAT is experessed in functionally heterogenous populations of MS GABA neurons, GAD2 is expressed in relatively homogenous MS GABA neurons. Concering MS GAD2 neurons, we obtained paradoxical results: First, we found that MS GAD2 neurons are involved in reward-seeking behavior as indicated by ICSS. Second, MS GAD2 neurons display decreased activity to rewards and cues predicting rewards. Third, MS GAD2 neurons increased activity in response to punishment and cues predicting punishment. We are currently examining our hypothesis that MS GAD2 neurons play a role in seeking for safety from threats, but not seeking for classical rewards. NIDA-IRP and the Center on Compulsive Behaviors supported this work. Project 2 concerns that supramammillary neurons projecting to the lateral preoptic area modulate reward-seeking behavior. Midbrain dopamine neurons are known to be critical in reward-seeking behavior. However, it is not well understood how other neural systems interact with dopamine neurons in reward-seeking behavior. Previous studies suggest that the supramammillary nucleus (SuM), particularly SuM glutamatergic neurons (GluN) projecting to the medial septum (MS), are important in reward-seeking behavior. In addition, SuM neurons appear to regulate reward-seeking behavior by coordinating the activities of multiple brain regions. To further understand such coordination role of the SuM and its mechanisms, first, we examined the hypothesis that SuM-MS neurons send collateral projections to multiple brain regions. We confirmed that about 95% of SuM-MS neurons were GluN using retrograde tracing and mRNA in situ hybridization procedures. Then, we examined collateral projections of SuM-MS GluN by injecting a retrograde AAV-Cre into the MS and an AAV-FLEX-mGFP-2A-SYP-mRuby. We found that SuM-MS neurons densely project to the dorsal tenia tecta, vertical and horizontal limb of the diagonal band, MS, substantia innominata, lateral preoptic area (LPO), lateral hypothalamus, and hippocampal CA1/CA2. Because the LPO is known to modulate reward-seeking behavior, we focused on the SuM-LPO pathway. Using the anterograde transsynaptic property of AAV serotype 1, we injected an AAV1-Cre into the SuM and found LPO neurons expressed Cre, confirming that SuM neurons have synaptic contacts with LPO neurons. We then examined whether optogenetic stimulation of SuM-LPO GluN instigates motivated behavior using an intracranial optogenetic self-stimulation test. We expressed channelrhodopsin-2 in SuM GluN-LPO of Vglut2-Cre mice. Mice quickly learned to press the lever that activated SuM GluN-LPO, suggesting that the stimulation of SuM-LPO GluN instigates and reinforces reward-seeking behavior. Finally, we tested SuM-LPO neuronal activity during reward-seeking behavior using a water-seeking test with GCaMP7s recording. We found that when mice nose-poked into water reward-port to drink water, SuM-LPO neuronal activity decreased, suggesting that SuM-LPO neurons are active during the reward-seeking phase, but not consummatory phase. In summary, our research suggests that SuM-MS GluN have collateral projections to multiple regions, including the LPO, and that SuM-LPO GluN instigate and reinforce reward-seeking behavior. Future studies will examine how SuM-LPO GluN interact with midbrain dopamine neurons in reward-seeking behavior.