Page 479 - The Toxicology of Fishes

The Endocrine System 459

Environmental stimuli
photoperiod, temperature stressors, pollutants
+ –

CNS
neurotransmitters
neuropeptides
+ or –
+ or –
HYPOTHALAMUS
GnRH neurotransmitters 5
(+) (+ or – )
female 2ndy sex + or – sex
PITUITARY 1 feedback steroids
characteristics and
FSH LH
(gametogenesis) (maturation) peptides
LIVER E OVARY TESTIS
ER E,T T, 11–KT + or –
vitellogenesis
vitellogenin, 2
zonaradiata gametogenesis
proteins 17,20β–P 3 17,20β–P
20β–S 20β–S + or –
male 2ndy sex
characteristics 6
pheromonal gamete maturation
regulation 4
FIGURE 10.1 Schematic representation of the hypothalamic–pituitary–gonadal axis controlling reproduction in fish.
2
1 Growth hormone, prolactin, and thyroid hormones also influence teleost reproduction. Only secondary (vitellogenic) oocyte
3
4
growth is under hormonal control. Either 17,20β-P or 20β-S is the primary progestin hormone in most species. Conjugated
metabolites of 17,20β-P and 20β-S in urine of periovulatory females influence reproductive behavior of conspecific males.
5 Feedback effects of steroids vary with the stage of the reproductive cycle; regulatory peptides include inhibin and activin.
6 Male secondary sex characteristics include male territorial behavior, gonopodia, breeding tubercles, and male coloration.
follicles and in Leydig cells in spermiating males (Miwa et al., 1994). The other receptor binds both
LH and FSH (GTH I) and is present in both thecal and granulosa cells during vitellogenesis but is only
present in the thecal cells in the preovulatory follicle. This receptor is present in the Sertoli cells in
males at all stages of spermatogenesis (Miwa et al., 1994). The specificities of the gonadotropin receptors
in other teleost species are currently unknown due to lack of purified homologous FSH. It was found
that mammalian LH and FSH can activate all the receptors identified in African catfish and channel
catfish to some extent (Kumar et al., 2001a,b; Vischer and Bogerd, 2003). Gonadotropin binding to the
receptors results in activation of G-proteins, adenylyl cyclase, and calcium-dependent second-messenger
signaling pathways (Kumar et al., 2001a,b; Miwa et al., 1994; Vischer and Bogerd, 2003). These
intracellular signals subsequently cause alterations in the production and secretion of steroid hormones,
growth factors, and regulatory peptides such as inhibin.
Steroid hormones are relatively stable structures, consisting of four fused hydrocarbon rings with
oxygen and carbon substitutions at different positions that are characteristic for each class of steroid
hormone. All steroid hormones are synthesized from a common precursor, cholesterol, via a series of
biosynthetic steps catalyzed by different steroidogenic enzymes. The protein that regulates the transfer
of cholesterol into the inner mitochondrial membrane, steroidogenic acute regulatory protein (StAR), is
a key regulatory (rate-limiting) step in steroid synthesis whose production is upregulated by gonadotro-
pins in the gonads and adrenocorticotropin in the adrenal cortex in mammals (Stocco and Clark, 1996)
and in fishes (Kasukabe et al., 2002). Cholesterol is converted to pregnenolone, a steroid with 21 carbon