Mu-Opioid Receptor - pAKT signaling in the Ventral Tegmental Area is Critical for the Behavioral and Cellular Consequences of Social Stress

Social defeat stress is naturalistic model that induces social avoidance and long-lasting cross-sensitization to drugs of abuse, as well as a corresponding upregulation of ventral tegmental area (VTA) mu-opioid receptor (MOR) mRNA. In the VTA, MOR activation is generally thought to inhibit a subpopulation GABA neurons thus disinhibiting VTA dopamine neurons and providing a role for VTA MORs in the regulation of psychostimulant sensitization. The present study determined the effect of lentivirus-mediated MOR knockdown in the VTA on the consequences of intermittent social defeat stress, a salient and profound stressor in humans and rodents. Social stress exposure induced social avoidance and attenuated weight gain in animals with non-manipulated VTA MORs, but both these effects were prevented by VTA MOR knockdown. Rats with non-manipulated VTA MOR expression exhibited cross-sensitization to amphetamine challenge (1.0 mg/kg, i.p.), evidenced by a significant augmentation of locomotion. By contrast, knockdown of VTA MORs prevented stress-induced cross-sensitization without blunting the locomotor-activating effects of amphetamine. Elucidating VTA MOR regulation of stress effects on the mesolimbic system may provide new therapeutic targets for treating stressinduced vulnerability to substance abuse. INTRODUCTION In humans, stress is one variable that influences the transition from recreational drug use to abuse, and it has been correlated with increased risk of substance abuse and relapse (Sinha, 2001, 2008, Razzoli et al., 2009, Sinha, 2011). Rodent studies have shown that repeated social 1 All of the data in this chapter have been published, however this chapter does not include the immunohistochemical data included in Johnston et al. (2015) – to see this publication in its entirety, please refer to Appendix A. 45 defeat stress exposure consistently produces social avoidance (Berton et al., 2006, Krishnan et al., 2007, Fanous et al., 2011, Komatsu et al., 2011) and augments the effect of psychomotor stimulants, a phenomena known as ‘cross-sensitization’ (Covington and Miczek, 2001, Nikulina et al., 2004, Miczek et al., 2011a, Nikulina et al., 2012). Genetic mu-opioid receptor (MOR) knockout mice do not exhibit social avoidance following continuous social defeat (Komatsu et al., 2011), suggesting that MORs play a critical role in stress-induced changes in long-term neuroplasticity. In fact, even acute social defeat stress has been shown to rapidly upregulate MOR mRNA expression in the ventral tegmental area (VTA; Nikulina et al., 1999), while repeated social stress exposure increases VTA MOR mRNA expression for up to 14 days after the last episode (Nikulina et al., 2008). In the VTA MORs are expressed by gamma-aminobutyric acid (GABA) neurons (Sesack and Pickel, 1995, Garzon and Pickel, 2002), which are hyperpolarized in response to MOR stimulation, thus disinhibiting local dopamine (DA) transmission and facilitating response to drugs of abuse (Johnson and North, 1992, Bergevin et al., 2002, Vargas-Perez et al., 2009b, Dacher and Nugent, 2011). Rats exposed to repeated social defeat stress, then challenged with an intraVTA infusion of a MOR-specific agonist exhibited sensitized locomotor activity (Nikulina et al., 2005, Nikulina et al., 2008). This VTA opiate-induced sensitized locomotor activity was present at the same time point that social stress-induced cross-sensitization to psychomotor stimulants was observed (Covington and Miczek, 2001, Nikulina et al., 2004, Nikulina et al., 2012). Taken together, these findings indicate that increased VTA MOR expression might play a role in social stress-induced psychostimulant sensitization. Consistent with this view, MOR knockout mice exhibit reduced cocaine self-administration and increased VTA GABA transmission (Mathon et al., 2005). Furthermore, the expression of amphetamine sensitization is associated with persistent VTA MOR upregulation, and can be blocked by a treatment with MOR antagonist (Magendzo and Bustos, 2003, Trigo et al., 2010). Although research has implicated VTA MORs in drug sensitization and social behaviors (Van Ree et al., 2000, Miczek et al., 2011l, Nikulina et al., 2014, Pitchers et al., 2014), it is unknown whether upregulation of VTA MORs causes the behavioral and biological effects of 46 social defeat stress exposure. To address this question, the present study used lentivirusmediated gene transfer and RNA interference to knockdown MORs in the VTA, and then assessed social stress-induced cross-sensitization to amphetamine. Given that social avoidance is altered in MOR knockout mice after continuous social stress (Komatsu et al., 2011), the effect of VTA MOR knockdown on social stress-induced social avoidance was also examined. Finally, the effect of VTA MOR knockdown on stress-induced deficits of weight gain was investigated.