3.2 Hypothalamic regulation of gonadotropic function
3.3 Gonadotropin-releasing factors
3.4 Effects of Mammalian Gonadotropin-releasing Hormone in Fishes
The regulatory influence of the hypothalamus on the reproductive functions of the pituitary has been established by demonstrating that transplantation of the pituitary from its normal location to other areas or placing electrolytic lesions in certain regions of the hypothalamus results in regression of gonadotropic cells in the pituitary and atrophy of the gonads (Ball and Baker, 1969; Dodd, Follett and Sharp, 1971; Ball et al., 1972; Peter, 1973; Peter and Crim, 1978, 1979; Peter and Fryer, 1979). In a few species, investigations have been conducted to determine the location of the hypophysiotropic centres in the hypothalamus and their functional relationship with the pituitary, as well as the chemical nature of the neurosecretory materials (see Goos, 1978; Crim, Dickhoff and Gorbman, 1978).
The hypothalamus of teleost fishes comprises the Gomori-positive nucleus preopticus (NPO) and the Gomori-negative nucleus lateralis tuberis (NLT) (Follénius, 1965; Ball and Baker, 1969; Perks, 1969; Sage and Bern, 1971; Peter, 1973; Holmes and Ball, 1974), The NPO, which is composed of the pars magnocellularis and the pars parvocellularis, is situated on both sides of the preoptic recess. The axons (Type A peptidergic fibres containing 100-200 nm granules) originating from the cells of the NPO form the preopticohypophysial tract which penetrates the pituitary, ramifies and terminates in the neurointermediate lobe (Bargmann, 1966; Knowles and Vollrath, 1966c, d). The NLT is located in the more caudal part of the hypothalamus and is subdivided into the pars rostralis, pars medialis, pars ventrolateralis and pars lateralis; the presence or absence of these NLT subdivisions varies in different teleost species (see Terlou and Ekengren, 1979). The pars medialis and pars lateralis have been described in the hypothalamus of Mugil cephalus and Mugil capito (Stahl, 1957; Blanc-Livni and Abraham, 1970), whereas in the rainbow trout all the four parts of the NLT have been described (Follénius, 1963; Terlou and Ekengren, 1979). Follénius and Dubois (1977) using anti-a -endorphin have demonstrated immunocytochemically a positive reaction in a number of pars lateralis cells of NLT in Cyprinus carpio. The axons (Type B aminergic fibres containing 90-100 nm granules) from the NLT nuclei terminate at the adenoneurohypophysial interface (Falck et al., 1962). Ultrastructural studies of the NLT cells have shown similarities between the size of dense core vesicles in them and nerve terminals in the pituitary, thereby supporting the concept of NLT-pituitary relationship (Zambrano, 1972). This concept has been supported by Bern, Nishioka and Nagahama (1974), who have traced the NLT axons by cobalt chloride iontophoresis back to the sectioned pituitary stalk in Sarotherodon (Tilapia) mossambicus. The NLT may be regulated by other brain centres. Ekengren and Terlou ("1978) and Terlou, de Jong and van Oordt (1978) have shown that NLT cells in the rainbow trout are innervated by monoaminergic fibres.
All parts of the neurohypophysis have rich innervation from the NPO and possibly aminergic fibres coming from the paraventricular organ (PVO). The NPO receives aminergic fibres from the PVO, and the NLT has a high input of NPO and aminergic fibres from PVO. These contacts among the three hypothalamic nuclei such as the NPO, the NLT and the PVO as well as those with the pituitary suggest interesting interactions (Ekengren and Terlou, 1978). A stereotaxic atlas of hypothalamic nuclei is available for the goldfish (Peter and Gill, 1975) and the killifish, Fundulus heteroclitus (Peter, Macey and Gill, 1975).
In teleost fishes there is no real homologue of the tetrapod hypothalamo-hypophysial portal system (Peter, 1973; Dodd, 1975). But Sathyanesan (1970, 1971, 1972) has reported the presence of an incipient portal circulation in catfishes, Clarias batrachus and Heteropneustes fossilis, contrary to the observations of Sundararaj and Viswanathan (1971) on the latter species.
The hypothalamic regulation of the activity of the gonadotropic cells in the pituitary has been studied in a number of fishes. The NLT fibres invade the rostral and proximal pars distalis and either innervate the gonadotropic cells or discharge the secretions into peri-vascular spaces surrounding these cells (Knowles and Vollrath, 1966a, b; Vollrath, 1967; Leatherland, 1970a, b; Zambrano, 1970a, b; van Oordt and Ekengren, 1978; Peter and Fryer, 1979). Haider and Sathyanesan (1972a, b) have suggested that in the catfish, Heteropneustes fossilis, the NLT secretory products may also be released into blood vessels passing the perikarya. In many teleost fishes such as the gobiid fish, Gillichthys mirabilis (Zambrano, 1970a, b), the goldfish, Carassius auratus (Kaul and Vollrath, 1974), the black molly, Poecilia latipinna (Peute et al., 1976) the gonadotrophs are directly innervated by fibres similar to those of the NLT, whereas in primitive teleost fishes such as the salmonids, the gonadotropic cells are not directly innervated (Friedberg and Ekengren, 1977). The NLT fibres may exert a stimulatory influence on gonadotropic cells (Zambrano, 1970a, b; 1971; Knowles and Vollrath, 1966c, d; Peter and Crim, 1979).
Secretory activity in the NLT has been correlated with reproduction in a number of teleost fishes (see de Vlaming, 1974; Viswanathan and Sundararaj, 1974a). However, Peter (1970, 1973) provided a direct evidence for the involvement of the NLT in gonadal activity by demonstrating that lesions in certain parts of the NLT result in loss of gonadal activity.
In a number of species including goldfish and black molly the gonadotrophs are also directly innervated by fibres originating in the NPO. This implicates both the NPO and the NLT in the regulation of gonadotropic functions of the pituitary (Peter and Fryer, 1979). Terlou, de Jong and van Oordt (1978) using scanning cytophotometry, have correlated the activity of the NPO with the annual gonadal cycle in the rainbow trout; the NPO is active during the vitellogenic and the spawning periods (June to January) and inactive in the sexually quiescent period (February to June). Similarly, seasonal fluctuations in the quantity of neurosecretory material in the NPO have been correlated with the gonadal activity in the catfish, Heteropneustes fossilis (Viswanathan and Sundararaj, 1974a).
The NPO shows changes after gonadectomy or after administration of steroids in Clarias batrachus (Dixit, 1970; Rao and Betole, 1973) and in Heteropneustes fossilis (Viswanathan and Sundararaj, 1974a, b). Rao, Subhedar and Ganesh (1979) have recently reported seven different subdivisions of the NPO in the catfish, Clarias batrachus. Of these only the medial, lateral and posterolateral subdivisions of NPO paraventricularis show stimulatory changes associated with hypertrophy of gonadotropic cells in the pituitary following ovariectomy and these changes are reversed by estradiol treatment.
Recent work conducted on the goldfish indicates that in the sexually mature gravid female there is a tonic inhibition of gonadotropin (GtH) secretion which must be abolished to allow the ovulatory surge of GtH (see Peter et al., 1978; Stacey, Cook and Peter, 1979). Since lesions in the NLT block gonadal recrudescence and inasmuch as similar lesions cause ovulatory surge in the mature goldfish, the NLT may be the source of gonadotropin-releasing as well as gonadotropin-inhibiting factors (Peter and Crim, 1979). Prostaglandins F2a and F2 have been reported to be a part of the inhibitory mechanism (Peter and Billard, 1976). Further work would be necessary to clarify the role of the NPO vis-à-vis that of the NLT in regulation of gonadotropic functions in teleost fishes.
Attempts have been made to demonstrate the presence of gonadotropin-releasing factor (GRF) in the brain of teleost fishes by immunofluorescent techniques (see Goos, 1978; Jackson, 1978; Crim, Dickhoff and Gorbman, 1978). Goos and Murathanoglu (1977) have localized GRF in the area dorsalis pars medialis of the telencephalon of rainbow trout, Crim and Evans (1980) have reported that GRF may also be found in an extrahypothalamic site in the winter flounder. Dubois, Billard and Breton (1.978) have recently reported that the mammalian luteinizing hormone-releasing hormone. (LH-RH) immunoreactive fibres terminate only in the meso-adenohypophysis of the rainbow trout, while Goos and van Oordt (1978) have shown numerous immunoreactive LH-RH positive fibres in the lateral walls of the diencephalon that end in the pituitary stalk.
GRF activity has been demonstrated in crude hypothalamic extracts from the common carp (Breton et al., 1971a, 1972a; Breton and Well, 1973; Well, Breton and Reinaud, 1975), the golden shiner (de Vlaming and Vodicnik, 1975), the goldfish (Crim, Peter and Billard, 1976), as well as the plaice, the winter flounder, the Atlantic parr salmon and the rainbow trout (Crim and Evans, 1980) by the ability of these extracts to promote release of gonadotropins when injected into intact recipients. However, the. time of autopsy and condition of the donor and recipient fishes are not always clearly indicated and evaluation is made difficult by the lack of control fish treated with the brain extract. Crim and Evans (1980) have recently developed an in vitro assay for detection of teleost GRF. Breton, Jalabert and Well (1975) have partially characterized the GRF from hypothalamic extracts of the common carp which has a molecular weight of less than 5 000. It is not identical with the mammalian LH-RH and does not cross react with antimammalian-LH-RH (Breton and Well, 1973; Deery, 1974).
The mammalian LH-RH is biologically active in several teleost fishes (Crim, Dickhoff and Gorbman, 1978). Mammalian synthetic LH-RH or its analogues in large doses bring about release of gonadotropin in the common carp, Cyprinus carpio (Breton, Well and Jalabert, 1972; Breton and Well, 1973; Fish Reproductive Physiology Research Group and Peptide Hormone Group, 1.978), the brown trout, Salmo trutta (Breton and Well, 1973; Crim and Cluet, 1974), and the goldfish, Carassius auratus (Crim, Peter and Billard, 1976). Peter (1980) has reported that the superactive analogue of LH-RH brings about a longer duration of gonadotropin release response in the goldfish than LH-RH itself. Multiple injections of LH-RH over several, days induce ovulation in the ayu, Plecoglossus altivelis (Hirose and Ishida, 1974), the goldfish (Lam et al., 1978), the common carp (Sokolowska, Popek and Bieniarz, 1978), and plaice and goby (Aida et al., 1978). This effect of LH-RH on the pituitary-gonadal system is of great potential importance in aquaculture for spawning cultivated fishes. Chinese carps, Aristichthys nobilis, Ctenopharyngodon idella, Megalobrama amblyocephala and Hypophthalmichthys molitrix, have been induced to spawn by injecting synthetic LH-RH or its nonapeptide analogue (Arimura et al., 1974) D-Ala6, des-Gly10-LH-RH ethylamide (Cooperative Team for Hormonal Application in Pisciculture, 1977; Fukien-Kiangsu-Chekiang-Shanghai Cooperative Group for Artificial Reproduction of Fresh-water Economic Fishes, 1977). However, the LH-RH and its analogue have been administered along with pituitary extracts and this makes evaluation of the contribution of LH-RH or its analogue very difficult. However, an unexpected benefit was an overall drop in mortality in breeders. Donaldson, Hunter and Dye (1978) have found the nonapeptide to be more potent than LH-RH in inducing final oocyte maturation and ovulation in the coho salmon, Oncorhynchus gorbuscha. Another superactive analogue of LH-RH [D-Ser(But)6] LH-RH ethylamide (Hoechst, 766) was even more potent in inducing ovulation in the coho salmon when preceded by a priming dose of partially-purified salmon gonadotropin (Donaldson, Hunter and Dye, 1979).
Pituitary response to mammalian LH-RH has been studied in male and female rainbow trout at different stages of gametogenesis. The male is barely sensitive at the beginning of spermatogenesis, but a response occurs at the spermatid stage and continues during the rest of spermatogenesis and spermiation. The response to LH-RH is low in immature females and in those in early stages of maturation and becomes stronger at vitelline maturation (Well et al., 1978).
The above discussion shows conclusively that the hypothalamus regulates the reproductive functions of the pituitary gland, the structure and physiology of which is discussed in the next section.