What is it called when two hormones exert opposite effects?

1.14.1.2 Brief Introduction into Ovarian Hormones and the Hypothalamic–Pituitary–Gonadal Axis

The hypothalamic–pituitary–gonadal (HPG) axis is primarily responsible for regulating reproductive activity and the release of ovarian hormones in animals and humans (Couse et al., 2003; Meethal and Atwood, 2005). In this way, the HPG axis also plays a key role in promoting healthy brain function, as ovarian hormones exert neuroprotective effects and facilitate neurogenesis, neuronal differentiation and survival, and cognitive function (Blair et al., 2015).

The HPG axis is responsible for orchestrating the release of both centrally and peripherally produced ovarian hormones. Centrally produced regulatory hormones include gonadotropin-releasing hormone (GnRH) from the hypothalamus and gonadotropins from the pituitary, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH) (Meethal and Atwood, 2005). Recently, researchers identified a central regulator of GnRH; kisspeptin was identified as a neuromodulator that modulates GnRH release and subsequently, controls activity of the gonadotropins and the HPG axis (Skorupskaite et al., 2014). The other gonadotropins, LH and FSH, are instrumental in modulating behavioral and neuronal changes (Meethal and Atwood, 2005).

Receptors for estradiol, progesterone, and testosterone are widely distributed throughout the brain and are highly concentrated in regions outside of the hypothalamus and pituitary, such as the amygdala and cerebral cortex (Meethal and Atwood, 2005). Additionally, estradiol is locally generated within the hippocampus (Mukai et al., 2006), and levels of estradiol in this region can fluctuate based on circulating levels (Barker and Galea, 2009). Since estradiol is generated in the hippocampus, it not only has an effect on the negative-feedback loop regulating HPG axis activity, but estradiol, as well as progesterone and testosterone, can also have effects on different neuronal pathways (McEwen, 2002). In addition to ovarian hormones, there are peripherally produced protein complexes that are responsible for regulating the activity of the HPG axis. For example, activins and inhibins are dimeric paracrine protein complexes with opposing physiological effects on FSH secretion; activins are produced in all tissues, whereas inhibins are generated in the gonads (Meethal and Atwood, 2005). Activins are critical for stimulating FSH secretion, activating the HPG axis, and facilitating neuronal plasticity, whereas inhibins are responsible for inhibiting FSH secretion and inactivating and modulating the effects of activins. A third protein complex, follistatin, also works with inhibin to regulate the activin activity. Together, these protein complexes help regulate the onset and offset of HPG axis activity (Meethal and Atwood, 2005) in coordination with circulating levels of the ovarian hormones.

Neuroendocrine control

The hypothalamic–pituitary–gonadal (HPG) axis controls avian reproduction, similar to most other vertebrates. The initiating factor in this hormonal cascade are the GnRHs. These peptide hormones are transported to the anterior pituitary gland, which, in turn, stimulates the release of LH. The ability of GnRH to stimulate follicle-stimulating hormone (FSH) in avian species is unclear; therefore, some references use the term luteinizing hormone–releasing hormone (LHRH) rather than GnRH for birds.7,9 GnRH is released in a pulsatile fashion in response to environmental and tactile cues (i.e., photoperiod, food availability, rainfall, presence of a mate). LH stimulates the production of androgens, estrogens, and progestins; FSH appears to also be involved with steroidogenesis and ovarian folliculogenesis, but its role in avian species is still not completely clear.7

Most vertebrates, including birds, possess multiple forms of GnRH, which can be classified into three major forms—GnRH-I, GnRH-II, GnRH-III. GnRH-I is considered an “ancient” form that is conserved across species.9 In fact, avian GnRH-I differs from its mammalian counterpart by only one amino acid.9 Avian GnRH-II and GnRH-III have a greater number of genetic differences compared with the mammalian peptide hormones. Their ability to stimulate LH, and possibly FSH release from the anterior pituitary, is variable, depending on the species, sex, and reproductive status of the bird. In addition, multiple types of GnRH receptors have been identified and are subdivided into mammalian and nonmammalian receptors.10 The incongruities between avian and mammalian GnRH peptides and their receptors may explain the reduced efficacy of synthetic GnRH-agonists, including leuprolide acetate and deslorelin acetate in avian species.

Endogenously, avian reproduction is controlled by the HPG axis. However, there are numerous exogenous elements that have a considerable influence on reproduction. The exogenous and endogenous mechanisms of activation or suppression of the HPG axis and their roles in some common reproductive or reproductively associated problems are displayed in Figures 12-16 and 12-17. As such, any treatment that affects only the endogenous reproductive hormone cascade, including the use of GnRH agonists, will be rendered less effective if the environmental factors are not also addressed simultaneously. Therefore, GnRH agonist therapy should not be used as a sole therapy for suppression of reproductive activity in any avian species. Behavioral modification, for both the patient and the owner, is often one, if not the largest, component of most aspects of treatment and prevention. Figure 12-18 displays the level of influence on the HPG axis of antecedent arrangement and behavior-change strategies, GnRH agonist therapy, ovarian surgery, and oviductal surgery. Treatment options for common reproductive and reproductively associated problems are further compared with regard to therapy and prevention, in relation to outcome expectations, in Tables 12-2 and 12-3.

Hypothalamic-Pituitary-Gonadal Axis

The HPG axis is a major signaling pathway controlling gonadal sex change in fish. Unlike other vertebrates, gonadotropin releasing hormone (GnRH) neurons located in the preoptic area of the hypothalamus directly innervate the anterior pituitary in fish. In the anterior pituitary, GnRH is released and stimulates the synthesis and release of the gonadotropins luteinizing hormone and follicle stimulating hormone into circulation. Gonadotropins act through their receptors in the gonad, either on follicle cells in the ovary or leydig cells in the testis, to regulate the production of sex hormones (including estrogens and androgens) (Fig. 2).

In most teleost fish, the main estrogen and androgen are 17β-estradiol (E2) and 11-ketotestosterone (11-KT), respectively. The balance between these two sex steroids controls gonadal fate, with E2 promoting ovarian function, and 11-KT testicular function. Testosterone (T) acts as a prohormone in fish and is converted to E2 via the enzyme aromatase, and to 11-KT via the enzymes 11β-hydroxylase (11βH) and 11β-hydroxysteroid dehydrogenase 2 (11βHSD2) (Fig. 2). Therefore, relative functioning of these pathways in the gonad determines the balance of E2 and 11-KT, and subsequently gonadal fate.

Dramatic shifts in E2 and 11-KT blood serum levels occur across sex change in fish. In protogynous sex change, a rapid decrease in E2 is followed by a steady increase in 11-KT (Fig. 3). Treatment of adult females with either an aromatase inhibitor (AI) or 11-KT, can decrease E2 production and increase 11-KT plasma levels to induce complete sex change. It is thought that a decrease in E2 levels below a threshold will result in oocytes being unable to survive, while increasing 11-KT levels may accelerate ovarian atresia while also initiating proliferation of spermatogonia. The opposite pattern is seen in protandrous sex change: 11-KT levels drop before there is an increase in E2 production (Fig. 3). Similarly, AI administration to adult males can suppress natural sex change in protandrous species.

The importance of the HPG axis in controlling the E2–11-KT balance, and thus gonadal fate, has been demonstrated through several manipulative studies. For example, administration of GnRH and gonadotropins (or derivatives of these) in protogynous species results in a drop in E2 levels and an increase in 11-KT, inducing gonadal sex change. Thus, changes in gonadal sex steroid levels and subsequently gonadal sex change, can be controlled by the brain via the HPG axis.

3 Melatonin and Puberty

The hypothalamic-pituitary-gonadal axis is active in the human fetus but after the first year of life it becomes quiescent until approximately the age of 10 years, when an increase in the pulses of GnRH secretion lead to increased gonadotrophin levels (see Chapter 12). Melatonin levels peak during childhood and begin to fall around the end of the first decade, suggesting that the diminution of inhibition of the hypothalamic-pituitary axis by melatonin may be involved in the timing of the onset of puberty. In children with precocious puberty (prior to the age of 9–10) or delayed puberty (later than 13–14 years old) nocturnal levels of melatonin secretion are, relative to age-matched control children, low or high, respectively. These observations support an association between melatonin secretion and the timing of human puberty but do not provide evidence of a causal relationship between the two processes.

Hypothalamic Control: The Gonadotropin Releasing Hormone Pulse Generator

The hypothalamic-pituitary-gonadal (HPG) axis, which governs the reproductive system, is driven by the brain. Gonadotropin (FSH and LH) secretion by the anterior pituitary gland is controlled by the hypothalamus through the release of GnRH. GnRH, a 10-amino-acid peptide, is synthesized within several hypothalamic nuclei and released into the hypophysial portal circulation to stimulate the release of both gonadotropins. This direct vascular connection between the hypothalamus and the anterior pituitary allows for the rapid and undiluted transport of minute amounts of GnRH to its target, the pituitary gonadotroph. Gonadotropins, in turn, act on the gonads, the ovaries or testes, to induce specific morphological changes and to stimulate the release of gonadal steroids, such as testosterone, estradiol, and progesterone. Constant communication between the several levels of the HPG axis through the negative feedback loop is essential for normal function; estradiol or testosterone secreted by the ovaries or testes continuously feeds back to the hypothalamus and pituitary to adjust GnRH, LH, and FSH secretion. The interruption of this negative feedback loop, for instance, following castration, results in an increase of GnRH, FSH, and LH release.

Significantly, GnRH is released from the hypothalamus in a pulsatile fashion (the GnRH pulse generator), and thus tonic secretion of gonadotropins is pulsatile in nature. Each pulse of LH consists of an abrupt and brief release of the hormone followed by an exponential decrease according to its half-life. A pulsatile GnRH stimulus is required to increase the transcription of the gonadotropin subunit genes. Pathological events that interfere with the function of the GnRH pulse generator disrupt the pituitary-gonadal axis. For example, the complete absence of GnRH pulsatility results in a dormant reproductive axis, whereas lesser abnormalities, such as a decrease in GnRH pulse frequency, may interfere with proper reproductive function.

Hypothalamic–Pituitary–Gonadal Axis

The hypothalamic–pituitary–gonadal axis controls sexual development and reproduction. Gonadotrophs in the anterior pituitary secrete FSH and LH in response to GnRH. Due to inhibitory action of testosterone in male fetuses, circulating LH and FSH concentrations are generally low with the peak concentrations occur between 14 and 21 weeks of gestation. In female fetuses, LH and FSH concentrations are higher than males. The ratio of FSH/LH is also higher in females compared to males in utero. FSH and LH concentrations decline late in gestation due to the development of neuroendocrine responsiveness to negative gonadal feedback. Near term, pituitary hormones are needed for granulosa cell and follicular proliferation. The ovary also produces inhibin in utero though significantly less than males, which could also explain higher gonadotropin levels. Fetal sexual differentiation does not depend on gonadotropins from the pituitary but on the presence of the SRY (sex-determining region on the Y) gene and anti-Müllerian hormone. The hypothalamic–pituitary–gonadal axis continues to be fine-tuned during the neonatal period and throughout postnatal development, with most of the reproductive maturation occurring during puberty.

It is known that adverse in utero exposure to testosterone can produce reproductive (infertility) and metabolic deficiencies (obesity and insulin resistance) into adulthood. This has been reproduced in rats, nonhuman primates, and sheep. Exposure to testosterone between 30 and 60 days of gestation in fetal sheep resulted in growth restriction and consequent reductions in birth weight. Females also exhibited catch-up growth postnatally while males did not. Early pregnancy appears to be the critical window for fetal programming, which is concerning due to fetal exposure possibilities like continued contraceptive steroid use, unintentional environmental exposure, use of anabolic steroids, or from maternal diseases like polycystic ovarian syndrome, which increase circulating androgens (Manikkam et al., 2004). This knowledge allows growth restriction and catch-up growth to be a potential indicator for adult disease.

1.2 Effectors of Reproductive Functions

The HPG axis regulates reproductive functions in fishes (see Chapter 2, this volume) as in all vertebrates (see Volumes 2–5 in this series). At the brain and pituitary levels, multiple GnRH forms and GnRH receptors play central roles in coordinating reproductive endocrinology and behavior in teleosts (reviewed by Van Der Kraak, 2009). Gonadotropin-releasing hormone originating from the preoptic area (POA) induces GTH release from the pituitary, whereas the GnRH expressed outside the POA contributes to diverse neuromodulatory functions including the regulation of sexual behaviors (Soga, Ogawa, Millar, Sakuma, & Parhar, 2005; Kah et al., 2007). Gonadotropin-releasing hormone stimulates the secretion of the GTHs—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—from the pituitary gonadotropes (Dickey & Swanson, 2000; Vacher, Mananos, Breton, Marmignon, & Saligaut, 2000; Ando & Urano, 2005; Aizen, Kasuto, Golan, Zakay, & Levavi-Sivan, 2007). Whereas GnRH is probably the most important stimulator of GTH release, the control of GTH secretion is multifactorial and involves several additional stimulatory and inhibitory peptides and neurotransmitters (Trudeau et al., 2000; Chang et al., 2009; Van Der Kraak, 2009). Multiple peptides and neurotransmitters also are involved in regulating the production of GnRH in the POA. One key regulatory factor of the HPG axis in teleosts is dopamine (DA) (Dufour et al., 2005). Dopamine potently inhibits basal and GnRH-stimulated LH secretion in most teleost species. There is also evidence that DA can inhibit both the release of pituitary FSH and POA GnRH. While FSH promotes gametogenesis in females by stimulating the production of 17β-estradiol (E2) and the incorporation of Vtg into developing oocytes, in males FSH stimulates Sertoli cell proliferation and testosterone (T) production, and maintains spermatogenesis. LH promotes final sexual maturation by stimulating gonadal steroidogenesis in both sexes, oocyte maturation and ovulation in females, and spermiation in males (Zmora, Kazeto, Kumar, Schulz, & Trant, 2007; Van Der Kraak, 2009). Beyond the role of GTHs, ovarian and testicular functions in teleosts are regulated by several other secondary hormones and growth factors (Van Der Kraak, Chang, & Janz, 1998; Van Der Kraak, 2009; see also Chapters 3 and 4Chapter 3Chapter 4, this volume). While these secondary endocrine and paracrine signals also may be affected by stressors, such interactions are beyond the scope of this review. In general, sex steroids participate in spermatogonial and oogonial proliferation and E2 also plays a key role in the synthesis of the egg yolk precursor Vtg in the liver. Finally, E2 and T exert positive and negative effects on the HPG axis. Whereas the negative feedback effects of sex steroids on GTH release are mediated by indirect effects on GnRH release via dopaminergic fibers, the positive feedback effects can be exerted either directly at the pituitary level or indirectly by effects on GnRH in the POA (Yaron et al., 2003; Levavi-Sivan, Biran, & Fireman, 2006; Van Der Kraak, 2009).

5.15.2 Prepubertal Development

During the prenatal, infant, and prepubertal stages of life, the hypothalamic–pituitary–gonadal (HPG) axis undergoes several periods of changing activity. It is active in utero, has a peak of activity during infancy, and then enters a period of relative quiescence until activity resumes during puberty.

2.3 Hypothalamic–Pituitary–Gonadal (HPG) Axis

The hypothalamic–pituitary–gonadal (HPG) axis in reptiles is similar to that described in other vertebrates (Licht & Porter, 1987; Licht, 1995). It is important to note, however, that little is understood about the neural systems regulating the HPG axis in reptiles, and important clarifications with respect to neurohormones and gonadotropins (GTHs) are still to be made. As in mammals, NE appears to play an important role in ovarian function in female lizards (Jones, Desan, Lopez, & Austin, 1990). Noradrenergic activity in the contralateral hypothalamus predicts the pattern of ovarian growth and ovulation of A. carolinensis (Desan, Lopez, Austin, & Jones, 1992), which in female anoles entails ovulation of a single egg at a time and an alternating pattern in egg production between the left and right ovaries in successive ovulations (Smith, Sinelnik, Fawcett, & Jones, 1972). This hypothalamic regulatory control of ovarian function in A. carolinensis appears to be informed by afferent input from the ovaries to the brain (Jones et al., 1997). Ovariectomy influences not only noradrenergic activity but also brain hemisphere-specific serotonergic and dopaminergic activity.

Recent work in the leopard gecko, Eublepharis macularius, suggests that the most ubiquitous form of gonadotropin-releasing hormone (GnRH) in vertebrates, type II or chicken II (cGnRH-II or cGnRH2) shows gene, peptide, and receptor expression in reptiles (Ikemoto & Park, 2003; Ikemoto, Enomoto, & Park, 2004). However, like other vertebrate classes, there are multiple GnRH isoforms in reptiles. For example, Ikemoto and Park (2007) have identified two distinct forms of GnRH (cGnRH-I and cGNRH-II) and three forms of GnRH receptor in the leopard gecko, E. macularius. However, in A. carolinensis, there may be just one form, cGnRH II (Lescheid et al., 1997). According to Tsai and Licht (1993a), turtles such as T. scripta have at least two GnRH types (cGnRH-I and II), with cGnRH-I being the most abundant type in the median eminence. In addition to one of two GnRHs, another neurohormone, GTH-release inhibiting hormone (GnIH), appears to be present in reptiles as well as other vertebrates (Tsutui & Osugi, 2009; see also Volume 4, Chapter 2; Volume 5, Chapter 2).

A variety of GnRH subtypes are capable of stimulating luteinizing hormone (LH) release from pituitary gonadotropes in the turtle T. scripta (Tsai & Licht, 1993b). The pituitary gonadotropes of other reptiles, such as the North African spiny-tailed lizard, Uromastyx acanthinura, have been characterized as having only follicle-stimulating hormone (FSH)-like secretory capacity, at least as far as immunoreactivity to human LH and FSH antibodies may demonstrate (Hammouche, Gernigon, & Exbrayat, 2007). In the Keeled Indian Mabuya, M. carinata, the physiological effects of FSH in males, including the full complement of gonadotropic and steroidogenic effects, are subject to inhibition by stress and glucocorticoids (Yajurvedi & Nijagal, 2000; Yajurvedi & Menon, 2005). In female M. carinata, FSH also stimulates ovarian function (recrudescence) but is inhibited (as in mammals) by β-endorphin (Ganesh & Yajurvedi, 2003).

Although two GTHs, homologous to FSH and LH, are part of the generalized reptilian HPG axis, early studies had difficulty demonstrating separate reproductive functions or even that there were two GTH in some lizards (Licht et al., 1976; Licht, Wood, Owens, & Wood, 1979). In the green sea turtle, C. mydas, FSH and LH in females were demonstrated to be distinctly regulated and to have different functions in mating and oviposition (Licht et al., 1979). Similarly, in a more recent study in the lizard Calotes versicolor, a combined treatment of LH and FSH was more effective in stimulating testicular recrudescence than FSH alone, suggesting distinctive functions in males (Vijaykumar, Ramjaneyulu, Sharanabasappa, & Patil, 2002). In addition to the role of GTHs in stimulating gametogenesis and gonadal steroid hormone synthesis, there is some exciting new evidence from a study of the lizard Uta stansburiana that these pituitary hormones also may act directly to modulate a variety of physiological, morphological, and behavioral traits (Mills et al., 2008).

There is some evidence that the seasonality of reproductive function in reptiles is mediated by changes at the level of the hypothalamus (in terms of the areas that regulate GnRH release and thus pituitary GTH release) rather than at the level of the gonads (in terms of gonadal sensitivity to the actions of GTH). For example, FSH treatment of A. carolinensis was effective in stimulating testicular growth in males during the so-called refractory period, which occurs immediately after testicular regression in the late summer and fall (Summers, 1984a). In addition, evidence exists that opioid peptides may play a role in regulating seasonal reproductive activity in the lizard Podarcis sicula sicula as treatment of males of this species with the opioid antagonist naltrexone increased androgen levels in both plasma and testis (Ciarcia et al., 1994).

SCN Control of the Female Reproductive Cycle

The hypothalamic–pituitary–gonadal axis provides another example of the circadian control of endocrine physiology.131 In spontaneously ovulating animals, the ovulation-triggering luteinizing hormone (LH) surge occurs on the day of proestrus, typically just prior to the onset of activity. The LH surge proceeds only when a precisely timed signal from the SCN occurs in the presence of the high levels of estrogen produced as ovarian follicles approach maturity (reviewed in Ref. 131). Circadian regulation of the female reproductive cycle is mediated through SCN innervation of a complex hypothalamic network. GnRH neurons contain a molecular clock, the phase of which is altered by signals that reset the SCN circadian clock.132 The SCN projects directly onto a subset of GnRH neurons,133 as well as indirectly onto estrogen-responsive kisspeptin-containing neurons in the anteroventral periventricular nucleus (AVPV) and gonadotropin inhibitory hormone–containing neurons in the dorsomedial nucleus of the hypothamalus.131 VIPergic neurons arising in the core of the SCN synapse with VPAC2 receptor-containing GnRH neurons. The production of VIP by the SCN is important for triggering the activity of the GnRH neurons on the day of proestrus.131 Estrogen receptor α, the receptor subtype that mediates the positive feedback effect of estrogen in the hypothalamus, is notably absent in the SCN-receptive GnRH neuronal population.131,134 Thus, the integration of the circadian signal with the hormonal milieu must occur at an intermediate site. The AVPV is likely a key regulatory site (reviewed in Ref. 131). The AVPV receives afferent information from AVP-expressing neurons that arise in the SCN core. AVPV neurons express estrogen receptor α, project directly onto GnRH neurons, and are active at the time of the LH surge. Clock mutant mice display reduced levels of AVP in the SCN and do not produce an endogenous LH surge.135 AVP production by the SCN occurs consequent to GnRH secretion, and injection of AVP into the MPOA can produce an LH surge in SCN-lesioned animals. The AVPV neurons use the RFamide peptide kisspeptin to integrate the circadian and estrogen signals (reviewed in Ref. 136). Kisspeptin is upregulated by estrogen in the AVPV, and kisspeptin neurons are activated at the time of the LH surge. Finally, the SCN projects onto a population of neurons in the dorsomedial nucleus of the hypothalamus that express gonadotropin inhibitory hormone; production of this peptide suppresses the LH surge.137,138 The projections of the gonadotropin inhibitory hormone–containing neurons, in turn, lie in close apposition to those of GnRH neurons. It appears, then, that activation of the SCN in the proper hormonal environment stimulates GnRH neurons through a direct VIP-mediated signal and an indirect AVP-mediated signal with the kisspeptin-containing neurons of the AVPV and through inhibition of the gonadotropin inhibitory hormone–containing neurons of the DMH.