Table 1.

Summarization of in vivo trafficking of endogenous opioid receptors

Receptor	Method	Animal	Region	Stimulation	Findings	Reference
MOPR	immunohistochemistry + confocal microscopy	guinea pig	myenteric plexus	acute etorphine or morphine	internalized by etorphine, but not by morphine	(Sternini et al. 1996)
		rat	dorsal horn of spinal cord	DAMGO, remifentanil, endomorphin-1 or morphine; noxious stimuli	internalized by DAMGO, remifentanil, endomorphin-1, but not by morphine or noxious stimuli	(Trafton et al. 2000)
		morphine tolerant rat	dorsal horn of spinal cord	DAMGO	DAMGO caused similar magnitudes of internalization in control and tolerant rats	(Trafton and Basbaum 2004)
		rat	dorsal horn of spinal cord	DAMGO (low dose that did not promote internalization) + Morphine	Promoted internalization	(He et al. 2002)
		rat	Lateral septum	acute morphine	MOPR1C internalized	(Abbadie And Pasternak 2001)
		OVX rat	medial preoptic nucleus; posterodorsal medial amygdale	estrogen	internalization	(Eckersell et al. 1998)
	Immunogold-labeling + electronmicroscopy	rat	locus coeruleus	acute etorphine or morphine	internalized by etorphine, but not by morphine	(Van Bockstaele And Commons 2001)
			nucleus accumbens	acute morphine	internalized in dendrites, but not in cell bodies	(Haberstock-Debic et al. 2003)
			rostral ventrolateral medulla	acute morphine	internalized only in the dendrites with diameter <1.4 µm	(Drake et al. 2005)
DOPR	Confocal microscopy	knock-in mouse	striatum	SNC80	internalization correlated with tolerance	(Scherrer et al. 2006)
	immunogold-labeling + electron microscopy	rat	dorsal horn of spinal cord	Chronic inflammatory pain	up-regulation and translocation to surface	(Cahill et al. 2003)
				Chronic morphine treatment	translocation to surface	(Cahill et al.2001b)
KOPR	immunogold-labeling + electron microscopy	rat	dorsal horn of spinal cord	U50,488H or dynorphin A	internalized by dynorphin A	(Wang et al. submitted)
			hypothalamus	salt loading	inserted into cell surface	(Shuster et al. 1999)