Table 2.

Receptor Characteristics (Developmental Baseline and TCDD-Induced mRNA Expression, Endogenous Ligands and Roles, and Binding Partners) of AHR2, AHR1a, and AHR1b (See Text for Citations)

Characteristic	AHR2	AHR1a	AHR1b
Earliest detected expression	5 hpf   (Andreasen et al., 2002b; Tanguay et al., 1999)	24 hpf   (Andreasen et al., 2002a; Karchner et al., 2005)	24 hpf   (Karchner et al., 2005)
mRNA localization during development	Several regions including the head and trunk   (Andreasen et al., 2002b; Sugden et al., 2017)	Liver at 52 hpf (Sugden et al., 2017), regenerating fin (Sugden et al., 2017)	Developing eye   (Karchner et al., 2017; Sugden et al., 2017)
mRNA localization in adults	Brain, heart, muscle, swim bladder, liver, gill, skin, eye, kidney, fin   (Andreasen et al., 2002a)	Brain (Webb et al., 2009), liver, heart, swim bladder, and kidney (Andreasen et al., 2002a)	Unknown
Effect of TCDD exposure on mRNA expression	Increase in expression   (Andreasen et al., 2002b; Garcia et al., 2018a; Karchner et al., 2005; Tanguay et al., 1999)	Increase in expression   (Andreasen et al., 2002a; Karchner et al., 2005)	No change in expression   (Karchner et al., 2005; Ulin et al., 2019)
Endogenous roles	Several at both embryonic/larval and adult life stages   (see Endogenous AHR Roles in Zebrafish)	Possible roles in hypocretin/orexin signaling   (Seifinejad et al., 2019)	Crosstalk detected with NRF signaling   (Ulin et al., 2019)
Known endogenous ligands	FICZ (Jonsson et al., 2009; Wincent et al., 2016), 3α,5α-tetrahydrocorticosterone and 3α,5β-tetrahydrocorticosterone (5α- and 5β-THB) (Wu et al., 2019)	None identified	FICZ   (Jonsson et al., 2009)
Endogenous Cyp1a expression regulation	Expression in the developing zebrafish eye but not in the trunk or brain   (Sugden et al., 2017)	None	None
In vitro binding with ARNTs	ARNT1b, 1c, 2 b, 2c   (Prasch et al., 2006; Tanguay et al., 2000)	ARNT2b   (Andreasen et al., 2002a)	ARNT2b   (Karchner et al., 2005)
In vitro binding with TCDD	Yes   (Prasch et al., 2006; Tanguay et al., 2000)	No   (Andreasen et al., 2002a; Karchner et al., 2005)	Yes   (Karchner et al., 2005)
In vitro transactivation activity with TCDD	Yes   (Prasch et al., 2006; Tanguay et al., 2000)	Not applicable	Yes but less sensitive than AHR2   (Karchner et al., 2005)
ARNT required for in vivo activation	ARNT1   (Antkiewicz et al., 2006; Prasch et al., 2004, 2006)	Not applicable	Unknown