, 2005 and Milligan, 2009). The physiological relevance of heteromerization is clear for GPCRs that function only as “obligate dimers” (Jones et al., 1998 and Zhao et al., 2003). However, for the vast majority of GPCR combinations that have been studied, a physiological role for heteromers has C59 in vitro been difficult to establish, and an impediment to this characterization has been the in vivo experimental

challenge of differentiating receptor crosstalk from a direct protein-protein interaction. The handful of studies that indicate the formation of physiologically relevant heteromers have required numerous technical approaches to substantiate their conclusions and remain somewhat controversial (González-Maeso et al., 2008, Pei et al., 2010, Fribourg et al., 2011 and Liu et al., 2011). In this issue, Kern et al. (2012)

provide evidence supporting a physiologically relevant interaction between the dopamine D2 (D2R) and the ghrelin receptor (GHSR1a) that regulates feeding behavior in mice, and their observations are important in building a case for the in vivo relevance of GPCR heteromers. Additionally, their findings have important implications regarding the development not only of obesity-related therapies targeting ghrelin receptors but also of potential therapies targeting the dopamine-related reward systems underlying other neurological conditions. The motivation for HTS assay the present study stems from the authors’ previous intriguing observation that a subset of neurons in the hypothalamus coexpresses the D2R and GHSR1a, despite the virtual absence of ghrelin in the brain. Their hypothesis is that GHSR1a expression alone can modify D2R signaling, and they address this using several different approaches. In SH-SY5Y neuroblastoma that express

Gi/o-coupled D2R, coexpression of GHSR1a leads to an unexpected calcium mobilization response through D2R activation that ADP ribosylation factor is blocked by both D2R and GHSR1a antagonists. Importantly, they also detect an agonist-mediated D2R calcium signaling in primary cultures from hypothalamus, indicating that this form of signaling is also present in intact tissue. The authors further show that while GHSR1a mobilizes calcium through Gq/11, D2R-dependent Ca+2 signaling in the presence of GHSR1a occurs through a pertussis-toxin-sensitive Gβγ mechanism; another Gq/11-coupled GPCR cannot substitute for the GHSR1a; and D2R-dependent Ca+2 signaling is not dependent on GHSR1a constitutive activity. Thus, in the complex, the D2R remains coupled to and signals through Gi/o protein but the presence of the GHSR1a in the heteromer presumably recruits Gβγ subunits that have the ability to mobilize calcium from intracellular stores through the phospholipase C pathway. This observation is quite intriguing and could be a mechanistic basis for signaling diversity through heteromerization.