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                                         THE HYPOTHALAMUS


3.1   Morphology
3.2   Hypothalamic regulation of gonadotropic function
3.3   Gonadotropin-releasing factors
3.4   Effects of Mammalian Gonadotropin-releasing Hormone in Fishes


3.1 Morphology

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- -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
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incipient portal circulation in catfishes, Clarias batrachus and Heteropneustes fossilis, contrary to the
observations of Sundararaj and Viswanathan (1971) on the latter species.

3.2 Hypothalamic regulation of gonadotropic function

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 F2 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.

3.3 Gonadotropin-releasing factors

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
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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).

3.4 Effects of Mammalian Gonadotropin-releasing Hormone in Fishes

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.

				
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