mGluR5 activation in the nucleus accumbens is not essential for sexual behavior or cross-sensitization of amphetamine responses by sexual experience

Highlights

• Nucleus accumbens mGluR5s do not regulate sexual behavior.

• Nucleus accumbens mGluR5 antagonism during mating does not prevent sex-induced sensitized responses to amphetamine.

• Sexual experience followed by a prolonged abstinence period decreases nucleus accumbens mGluR5.

• Antagonism of nucleus accumbens mGluR5 causes sensitization of amphetamine-induced locomotor activity.

• Antagonism of nucleus accumbens mGluR5 causes enhanced amphetamine conditioned reward.

Abstract

Natural rewards and psychostimulants cause similar neural plasticity in the nucleus accumbens (NAc). In addition, sexual experience in male rats causes increased locomotor activity and conditioned place preference (CPP) induced by d-Amphetamine (amph). The latter is dependent on a period of abstinence from sexual reward. In this study, the role of mGluR5 activation in the NAc for expression of mating and the cross-sensitizing effects of sexual experience was tested. First, intra-NAc infusions of mGluR5 antagonists MPEP (1 or 10 μg/μL) or MTEP (1 μg/μL) 15 min prior to mating during 4 daily sessions had no effect on male rat sexual behavior. Subsequently, these sexually experienced males were tested for amph-induced locomotor activity and CPP after one week of abstinence from sexual reward. In addition, sexually naïve males that received MPEP, MTEP or vehicle infusions prior to 4 daily handling sessions were included. Cross-sensitization of locomotion or CPP was not prevented by NAc mGluR5 antagonism during acquisition of sexual experience. Instead, sexually naive animals that received NAc mGluR5 antagonists without mating demonstrated sensitized amph-induced locomotor responses and enhanced CPP on par with sexually experienced males. Finally, we showed that sexual experience caused prolonged down-regulation of mGluR5 protein in the NAc, dependent on abstinence from sexual behavior. Together, these findings suggest that mGluR5 activation in the NAc is not essential for the expression of mating, but that experience-induced reduction in mGluR5 protein may contribute to the cross-sensitization of amph responses by sexual experience and abstinence.

Introduction

Natural reward behavior can influence drug responses and drug-associated reward (Balfour et al., 2004, Bradley and Meisel, 2001, Frohmader et al., 2010a, Gipson et al., 2011, Olsen, 2011, Pitchers et al., 2010a, Pitchers et al., 2014, Pitchers et al., 2013, Stairs et al., 2011). We have previously demonstrated that sexual experience in male rats causes long-lasting sensitization of behavioral responses to amphetamine (amph), including amph-induced locomotor activity (cross-sensitization) and conditioned place preference (CPP) (Frohmader et al., 2010a, Frohmader et al., 2011, Pitchers et al., 2010a, Pitchers et al., 2014, Pitchers et al., 2010b). The latter is dependent on a period of abstinence from sexual reward (Pitchers et al., 2010a, Pitchers et al., 2012, Pitchers et al., 2013). Male rat sexual behavior is rewarding and reinforcing: sexually experienced male rats form a CPP for copulation (Agmo and Berenfeld, 1990, Agmo and Gomez, 1993, Tenk et al., 2009), perform operant tasks to gain access to sexually receptive females (Everitt et al., 1987, Everitt and Stacey, 1987), and develop faster running speeds in straight-arm runway (Lopez and Ettenberg, 2002), T-maze (Kagan, 1955) and hurdle climbing tests (Sheffield et al., 1951). In addition, sexual experience in male rats causes facilitation of subsequent sexual behavior, indicated by increased sexual motivation and performance (Balfour et al., 2004, Beloate et al., 2016, Pitchers et al., 2010a, Pitchers et al., 2010b, Pitchers et al., 2012, Pitchers et al., 2013, Pitchers et al., 2014).

Studies investigating the mediators of changes in drug responses caused by sexual experience and abstinence have primarily focused on the mesolimbic system, specifically the nucleus accumbens (NAc) and ventral tegmental area. Dopaminergic neurons in the ventral tegmental area undergo morphological changes following sexual experience, an effect mediated by endogenous opioids (Pitchers et al., 2014). In the NAc, deltaFosB and dendritic spines are increased, which are D1 dopamine receptor dependent processes (Pitchers et al., 2013). Together, these changes mediate the long-term outcomes of sexual experience on sensitized amph-CPP (Beloate et al., 2016, Pitchers et al., 2014, Pitchers et al., 2010b, Pitchers et al., 2013). Neuroplasticity in glutamate signalling in the NAc also seems to play a role in mediating the effects of sexual experience (Pitchers et al., 2012, Wolf, 2010) since it is associated with altered ionotropic receptor trafficking and function in the NAc (Pitchers et al., 2012). In addition to ionotropic glutamate receptors, there is evidence that type 5 metabotropic glutamate receptors (mGluR5) are important mediators of addictive drugs (Carroll, 2008, Kalivas, 2009, Kim et al., 2015, Lou et al., 2014). Yet, mGluR5 remains understudied regarding its role in mediating natural reward behaviors, or the interactions between natural and drug behaviors.

There are 8 subtypes of metabotropic glutamate receptors (mGluR1-8) organized into 3 groups based on sequence homology, pharmacological profile and cell signalling mechanisms (Conn and Pin, 1997). mGluRs act via G-proteins to regulate intracellular processes (G-protein coupled receptors; GPCRs). Of particular interest in the current study, mGluR5 is a group I metabotropic receptor that is coupled to Gα_(q/11) protein and activates phospholipase C and increases intracellular calcium. There is a high density of mGluR5 in the NAc (Conn and Pin, 1997, Romano et al., 1995, Testa et al., 1994, Testa et al., 1995), located mainly outside the synaptic cleft on the post-synaptic membrane (Mitrano and Smith, 2007). The antagonism of mGluR5 attenuates drug self-administration (Kenny et al., 2005, Platt et al., 2008), psychostimulant-induced locomotor activity (Herzig and Schmidt, 2004, Martinez et al., 2014), alcohol consumption (Cozzoli et al., 2012) and reinstatement of cocaine, methamphetamine, heroin, nicotine and alcohol (Backstrom et al., 2004, Backstrom and Hyytia, 2006, Besheer et al., 2008, Chesworth et al., 2013, Gass et al., 2009, Herrold et al., 2013, Kenny et al., 2005, Kim et al., 2015, Kumaresan et al., 2009, Lee et al., 2005, Lou et al., 2014, Martin-Fardon et al., 2009, McGeehan and Olive, 2003, Paterson and Markou, 2005, Platt et al., 2008). Much less work has been conducted to study the antagonism of mGluR5 on non-drug reward behavior, especially in a brain site-specific manner. To this point, systemic administration of mGluR5 antagonists has been show to inhibit sex-seeking and sexual behavior (Li et al., 2013), but have no effect on food self-administration (Tessari et al., 2004), food-CPP (Herzig et al., 2005), or sweetened or condensed milk self-administration (Martin-Fardon et al., 2009). Therefore, the goal of the current study was to determine whether mGluR5 activation in the NAc during sexual behavior affects the expression of this behavior and its facilitation with experience. The second objective was to test the hypothesis that NAc mGluR5 activation mediates the long-term consequences of sexual experience and abstinence on sensitized amph responses, including enhanced amph-induced locomotor activity and CPP.