Page 485 - The Toxicology of Fishes

The Endocrine System 465

by androgens in teleosts, and gonadal concentrations of the receptor increase during the reproductive
season (Larsson et al., 2002; Sperry and Thomas, 1999a). Target tissue responsiveness in mammals and
probably in teleosts to estrogens and several other steroid hormones is also dependent on which nuclear
receptor subtypes are expressed. Two distinct nuclear ER subtypes (ERα and ERβ) are present in mammals
and three ERs (ERα, ERβa and ERβb) in advanced teleosts with distinct but overlapping tissue distribu-
tions and different steroid-binding affinities (Hawkins and Thomas, 2004; Hawkins et al., 2000; Kuiper
et al., 1997). Multiple ARs with different tissue distributions and ligand-binding properties have also been
identified in teleosts (Ikeuchi et al., 2001; Sperry and Thomas, 1999a,b, 2000). This multiplicity in nuclear
receptor subtypes modulates not only tissue responsiveness to natural steroid hormones but also interac-
tions with xenobiotic chemicals, a topic that is discussed later in this chapter.

Nongenomic Steroid Actions via Membrane Steroid Receptors

In addition to the classic mechanism of steroid action, extensive evidence now suggests that steroids also
exert rapid, nongenomic actions by binding to receptors on the surface of target cells and activating
signal transduction pathways, leading to a biological response (Figure 10.2) (Falkenstein et al., 2000;
Norman et al., 2004; Revelli et al., 1998; Watson and Gametchu, 1999). Progesterone treatment, for
example, causes a marked increase in intracellular calcium levels in mammalian sperm in less than a
minute, resulting in the acrosome reaction and hypermotility (Blackmore et al., 1991). Specific membrane
receptors and rapid nongenomic actions for estrogens, androgens, and progestins have recently been
identified throughout the reproductive system in vertebrates, including the hypothalamus, pituitary,
gonads, gametes, steroidogenic cells, and primary and secondary reproductive structures such as the
breast (Revelli et al., 1998). Nongenomic steroid actions have been shown to have important functions
in several reproductive processes such as the activation of sperm (Blackmore et al., 1991; Revelli et al.,
1998) in mammals, but their precise physiological roles in many other reproductive tissues remain unclear.

Membrane Steroid Receptors in Fish
Membrane receptors for all the sex steroids have been identified in teleosts (Thomas, 2000c, 2003). A
high-affinity membrane estrogen receptor (mER) has been characterized in Atlantic croaker testicular
membranes and is the likely intermediary in a rapid, cell-surface-initiated, nongenomic action of estro-
gens to downregulate testicular androgen production in this species (Loomis and Thomas, 2000). The
mER is also present in croaker ovaries and activates a stimulatory G-protein (G ), resulting in increased
s
cAMP production (Thomas et al., 2003). Androgens also can regulate gonadal steroidogenesis in Atlantic
croaker, causing downregulation of ovarian estrogen biosynthesis via a nongenomic mechanism (Braun
and Thomas, 2003). A membrane androgen receptor has been identified in croaker ovaries and fully
characterized in this species (Braun and Thomas, 2004). Probably the most intensively studied and best
characterized model of a cell-surface-initiated, nongenomic steroid action is the induction of oocyte
maturation (OM) in teleosts and amphibians by progestin MISs (Nagahama et al., 1994; Thomas, 1994;
Thomas et al., 2002). The mPRs on oocytes thought to mediate these actions of the fish MISs, 17,20β-P
and 20β-S, have been identified and fully characterized in several teleost species (Patiño and Thomas,
1990b; Thomas et al., 2002). MIS receptor concentrations are upregulated by gonadotropin during an
early stage of oocyte maturation, and this is associated with the oocytes becoming responsive to the
MISs and the completion of OM after hormone treatment (Thomas et al., 2001). In addition, an mPR
has been characterized on spotted seatrout sperm and is the likely intermediary in 20β-S stimulation of
sperm motility via increases in intracellular calcium and cAMP levels, resulting in increased fertilization
success in this species (Thomas, 2003; Thomas et al., 1998).
Recently, a novel gene was cloned and sequenced from a spotted seatrout cDNA ovarian library that
has all the characteristics of the mPR mediating MIS regulation of oocyte maturation in this species
(Thomas et al., 2002; Zhu et al., 2003a). Subsequently, 13 additional closely related cDNAs were
identified in other vertebrate species, including 3 in humans that represent 3 distinct clades and also
have characteristics of mPRs (Zhu et al., 2003b). These mPRs are not structurally related to nuclear
steroid receptors, but instead have 7 transmembrane domains, which is a characteristic of G-protein-
coupled receptors. Recent studies show the mPR in spotted seatrout is coupled to an inhibitory G-protein