Role of Neuromedin S-expressing Ventral Tegmental Area Neurons in Morphine Behavior

Abstract

<b>Abstract ID 13625</b> <b>Poster Board 269</b> <b>Objective:</b> Opioid dependence and addiction constitute a major health and economic burden, but our limited understanding of the underlying neurobiology limits better interventions. Alteration in the activity and output of dopamine (DA) neurons in the ventral tegmental area (VTA) is known to contribute to drug effects, but the mechanisms underlying these changes remain relatively unexplored. We used TRAP to identify gene expression changes in VTA DA neurons following chronic morphine and found that Neuromedin S (NMS) is enriched in VTA DA neurons, and its expression is robustly increased by morphine. However, whether all VTA DA neurons express NMS, and their potential functional impact has yet to be determined. We hypothesize that NMS neurons represent a novel subset of VTA neurons that contribute to morphine-elicited behavior. Specifically in these studies, we hypothesize that activating and inhibiting VTA-NMS neurons will promote and inhibit morphine behaviors, respectively. <b>Methods:</b> To test this, adult male and female NMS-Cre mice and wild-type littermates were used. Cre-dependent viral vectors were stereotaxically injected into the VTA to allow for DREADD-mediated activation (Gq) or inhibition (Gi) of VTA-NMS neurons. Behavioral analyses were completed two weeks after surgery. Locomotor activity was assessed following saline (d1), saline + Clozapine-N-oxide (CNO, 0.3mg/kg, ip, d2-d3), and morphine (15mg/kg) + CNO (d4-d8). A morphine + CNO challenge was done 1 week following d8. Conditioned place preference (CPP) was also performed. Mice underwent a 20 min. pre-test, followed by conditioning sessions where they received vehicle + saline in the morning and CNO + morphine (0.3mg/kg and 15mg/kg, respectively) in the afternoon for 4 days. Morphine preference was assessed during a 20 min. post-test. Significant differences (p&lt;0.05) were determined using a repeated-measures two-way ANOVA for locomotor behavior and paired t-tests for CPP (pre-test vs. post-test). <b>Results:</b> We find that both male and female NMS-Gq mice exhibit increased morphine-induced locomotor activity compared to controls. Additionally, locomotor response to a challenge morphine + CNO injection was significantly increased in NMS-Gq mice compared to controls. NMS-Gi mice show a trend for the opposite effect, with a decreased locomotor response to a challenge morphine + CNO injection. In CPP assays, NMS-Gq mice displayed a similar morphine-CNO CPP to controls. However, NMS-Gi mice exhibited decreased morphine-CNO CPP compared to controls. <b>Conclusions:</b> Thus, manipulation of VTA-NMS neuronal activity alters morphine-elicited behaviors including locomotion, sensitization, and CPP. Future studies will determine whether VTA-NMS neuronal activity modulates other morphine behaviors. Our current data suggest that VTA-NMS neurons represent a subset of neurons that may be functionally relevant for morphine responses. Financial support provided by NIDA (R01 DA039895, MMR), the McManus Foundation, NSF (DGE-1848739, CMRQ) and the Howard Hughes Medical Institute.