Radioligand binding assays of mu opioid receptors, for example, detect little downregulation in most brain regions even after prolonged administration of agonist drugs (Sim-Selley et al., 2000; Yoburn et al., 1993). Instead, it is thought that the major trafficking itinerary of receptors after ligand-induced endocytosis is nondestructive recycling to the plasma membrane, which can occur

repeatedly and efficiently under conditions of prolonged agonist exposure (Tanowitz and von Zastrow, 2003) (Figure 1B). 7TMR recycling has long been recognized to be one means for supporting the ability of cells to sustain cellular responsiveness to a neuromodulator or for achieving efficient recovery of responsiveness after a period of functional desensitization (Gainetdinov et al., 2004). LGK-974 manufacturer An important caveat is that most studies investigating the functional consequences of 7TMR recycling are limited to cultured cell systems. However, the rapid recycling pathway traversed by adrenergic catecholamine receptors is essential for maintaining physiological catecholamine responsiveness of the heart (Odley et al., 2004). Conversely,

disrupting the ability of mu opioid receptors to recycle efficiently signaling pathway in vivo produces enhanced physiological tolerance to the antinociceptive effects of opioids (Enquist et al., 2011). Differences in the endocytic trafficking fate of otherwise similar 7TMRs can confer essentially opposite functional effects on longer-term cellular signaling responsiveness (Cao et al., 1999; Tanowitz and von Zastrow, 2003). In principle, discrete endocytic fates could be mediated by altogether different endocytic mechanisms or by molecular sorting of receptors after endocytosis. The former possibility has not been fully ruled out and may apply to the oxyclozanide regulation of some 7TMRs. However, there is compelling

evidence that opioid and catecholamine receptors are subject to exquisitely selective molecular sorting after endocytosis by a shared, CCP-dependent early endocytic pathway (Tsao and von Zastrow, 2000; Puthenveedu et al., 2010). The following discussion will focus on such “postendocytic” sorting of 7TMRs and the recycling-versus-degradation decision as a relatively extensively studied example. Many signaling receptors require ubiquitylation for endocytic delivery to lysosomes and recycle efficiently to the plasma membrane when their ubiquitylation is prevented (Raiborg and Stenmark, 2009; Eden et al., 2012). Ubiquitin-directed sorting is mediated by a complex endosome-associated machinery, extensively conserved from yeast to humans, which is collectively called the endosomal sorting complex required for transport (ESCRT, Figure 2A). A great deal is presently known about ESCRT structure and function, as discussed in excellent recent reviews (Hurley and Hanson, 2010; Henne et al., 2011; Raiborg and Stenmark, 2009).