The nucleus accumbens is a critical locus
The nucleus accumbens is a critical locus for ER/mGluR signaling in the context of addiction
ER/mGluR signaling is regulated by caveolin proteins and palmitoylation Because of the vast implications of ER/mGluR association, it is important to understand what regulates their interaction. Such regulatory mechanisms are likely to be dynamic, allowing the coupling of these receptors in not only a sex-specific manner, but also a cell-specific manner. Cell culture experiments point to two regulatory mechanisms: interaction with caveolin proteins, and palmitoylation. First, caveolin proteins – structural membrane proteins – are required to traffic ERs to the plasma membrane where they can associate with mGluRs (Boulware et al., 2007; Razandi et al., 2002). The particular caveolin isoform (there are three) determines the character of the ER/mGluR pairing. That is, ERα with mGluR1a or mGluR5, or ERα or ERβ with mGluR2/3. In this way, caveolin creates functional microdomains within the membrane, clustering receptors with their effector proteins, and providing subcellular spatial tuning (see Fig. 1). In an attempt to better understand the involvement of caveolin in ER/mGluR-mediated enhancement of cocaine plasticity, and to bring our in vitro findings into an in vivo paradigm, we overexpressed Cav1 in neurons of the nucleus accumbens in ovariectomized rats (Fig. 2a) and measured differences in locomotor responses to cocaine (Fig. 2b). We used a dose of cocaine previously shown to not produce behavioral sensitization in ovariectomized rats without estradiol supplementation, and hypothesized that Cav1 overexpression would mimic the enhancement normally seen with estradiol. Indeed, Cav1 animals increased their locomotor responses from the first to last day of cocaine exposure, while control animals did not (Fig. 2b). This indicated that Cav1 overexpression facilitated cocaine-induced plasticity. A second source of regulation is palmitoylation – reversible post-translational lipidation. ERs must be palmitoylated in order to signal at the membrane. It is possible that neurons utilize a palmitoylation-depalmitoylation Macitentan pathway to divert greater or fewer ERs to the plasma membrane, or even to integrate membrane and neuronal estradiol signaling. Future studies of membrane-associated ER regulation and signaling will need to consider the recent advances in knowledge of depalmitoylation and local palmitoylation cycles (Fukata et al., 2013, Fukata et al., 2015, Fukata et al., 2016; Yokoi et al., 2016). Because palmitoylation is a dynamic process, it is possible that there could be an activity-dependent component of ER palmitoylation state (Tabatadze et al., 2013). We are only just beginning to understand the extent of palmitoylation influence on signaling mechanisms that rely on membrane-tethering of otherwise soluble proteins, including ER/mGluR activity. Finally, not only can ERs pair with different mGluRs in different brain regions, but it is becoming increasingly clear that the same mGluRs can pair with distinct downstream signaling partners to have differential effects both within and across brain regions (Gross et al., 2016; Mannaioni et al., 2001; Poisik et al., 2003; Valenti et al., 2002). In other cases, mGluR signaling may result in the same outcome, but through distinct pathways (Benquet et al., 2002; Thandi et al., 2002). These nuances in mGluR signaling are an important consideration, as they likely contribute to the varied effects of estradiol on structural plasticity. The flexibility and diversity of ER/mGluR signaling outcomes are thus conferred at multiple levels. Learning the precise mechanism that determines which mGluR an ER pairs with, and the nature of downstream effects of that mGluR, will clarify our understanding of how estradiol modulates neural systems in specific and complex ways.
Future directions and conclusions This review has focused on estrogen regulation of female motivational behavior, as estrogens have not been found to directly affect brain regions associated with the reward pathway in male mammals (Becker, 2016; Cummings et al., 2014). Nevertheless, estrogens are known to play essential roles in avian physiology and behavior (Cornil et al., 2012), including through ER/mGluR signaling mechanisms (Seredynski et al., 2015). Moreover, ER/mGluR signaling has recently been described in the cerebellum of male mice (Hedges et al., 2018), a brain region known to affect an ever increasing variety of cognitive functions (Strick et al., 2009; Galliano and De Zeeuw, 2014). Thus, it would be premature to exclude the possibility of estrogen affecting motivated behaviors in male mammals. That said, androgen receptors (AR) are also palmitoylated by the same DHHC enzymes as estrogen receptors (Pedram et al., 2012), and thus may be trafficked to the surface membrane as well. Hence, current work is examining potential AR/mGluR signaling and its impact upon the reward circuitry in the brains of male mammals, similar to the effects of estrogens in females.