Introduction

The culture of finfish is rapidly increasing in significance worldwide. The cultured fish species are utilized as inexpensive protein sources, luxury foods, wild stock enhancement or for aquarium pets.

The huge demand for the 1000 or more species desired for aquaria or food fish production has been met to a large degree by harvest of juveniles or adults from wild stocks. Often this capture has involved the use of chemicals which are damaging to the fish and to the rest of the ecosystem. Many of the species are supplied from Asia where the intensive harvest and loss of the environment to urbanization or pollution have seriously depleted some species to the point of endangerment. International trade in some species has had to be prohibited by C.I.T.E.S., for example a rowana species from Malaysia.

The solution to maintenance of a supply of fish for aquaria and food production is the development of technology to optimize the reproduction of the species. Not only will induced spawning of fish in captivity ensure the supply, but will also greatly enhance conservation of species by reducing wild harvest pressures, and may contribute significantly to restoration of some endangered species.

A major factor limiting the reproductive, and therefore culture, potential of many species in captivity and in areas remote from their native distribution is the lack of natural spawning stimuli. Both gonadal maturation and spawning behaviour are known to occur in response to environmental stimuli: temperature, hours of daylight, rainfall, etc. (Figure 1) The use of induced spawning hormones will allow the spawning of most species in the absence of some or all of the natural stimuli, will overcome ovarian follicular atresia found in some species stressed by holding in captivity, and allow fish culturists to become less dependent on capture and importations.

Induced Spawning

The widespread use of synthetic hormones to induce spawning in fish represents the application of leading edge technology in fish reproduction and endocrinology. The technology has developed over the years starting with the use of crude pituitary gland extracts introduced in the 1930's and continuing with the use of human chorionic gonadotropin (HCG), partially purified teleost gonadotropins, luteinizing hormone-releasing hormone (LHRH) and its analogues and finally the superactive teleost gonadotropin releasing hormone (GnRH) analogues, including Syndel's patented salmon GnRH analogue (sGnRHa).

Although still used, the pituitary gland extracts suffer from variability in potency, the presence of other pituitary hormones which interfere with fish physiology, the difficulty of supply of sexually mature donors and the high cost of the glands. (Lam, 1982, Fontaine, 1976) Increasing knowledge of fish reproductive physiology including the role of gonadotropins led to the use of HCG and partially purified pituitary extracts, but problems with consistency of effects within and among species and with cost of treatment remained. (Lam, 1982, Jalabert et al., 1978, Hunter et al., 1981, Idler, 1982)

The recent advancements in knowledge of the hormonal factors controlling gonadotropin release have further refined the induced spawning techniques. (Peter and Crim, 1979, Peter, 1982) These hormonal factors consist largely of GnRH stimulation and dopamine inhibition. (see Figure 2) The use of LHRH and its analogues (LHRHa) to mimic GnRH stimulation made induction of spawning more consistent. (Crim et al., 1987) However, the isolation of a teleost GnRH from salmon has led to identification of homologues in studied species and the synthesis of an analog, sGnRHa, which is superactive. (Sherwood, 1983, Van Der Kraak et al., 1987, Habibi et al., 1987, 1989, Peter et al., 1985, 1987) This superactivity is accounted for by many factors including increased receptor affinity, resistance to metabolic degradation and increased hydrophobicity. (DeLeeuw et al., 1988, Habibi et al., 1989, Zohar et al., 1989).

The use of a dopamine antagonist overcomes the inhibitory effect on gonadotropin release allowing for spawning and in many cases a reduction in the required dose of sGnRHa. For many species, including the goldfish, the use of a dopamine antagonist such as domperidone is essential to produce a rapid ovulatory surge of gonadotropins to induce spawning. (Peter et al., 1988, Sokolowska et al., 1984) The inhibitory role of dopamine in goldfish and other species has been studied and reported. (Chang and Peter 1983, Chang et al., 1984, Deleeuw et al., 1988, Omeljanuik et al 1989) The review of available antagonistic compounds ha s led the selection of domperidone for its safety and efficacy. Domperidone is a highly specific and potent dopamine receptor antagonist which does not cross the blood-brain barrier in the goldfish, thereby reducing the potential for undesirable sideeffects due to action on the central catecholaminergic system. (Omeljaniuk et al., 1987)The combination of the patented analogue sGnRha and domperidone in a single injectable solution is produced and supplied by Syndel Laboratories as "Ovaprim". In some circumstances oral administration of hormones to induce spawning may be more desirable than the proven method of injection. It is now well known that the fish gut is capable of absorption of intact peptides. The mechanism of this process has been reviewed. (McLean and Donaldson, 1990) The successful release of gonadotropin or successful induction of spawning has followed the oral administration of LHRH and LHRHa in coho salmon, (McLean et al, submitted), LHRHa in spotted seatrout (Thomas and Boyd, 1989) and LHRHa in sablefish (Solar et al., submitted) and goldfish have been induced to spawn using oral salmon pituitary extract. (Suzuki et al., 1988) Further research investigating the oral administration of Ovaprim is now underway.

Field Trials

A group of fixed criteria have been established to judge the effectiveness of induced ovulation techniques. (Peter, 1988) These criteria are:

1.  high ovulation rate

2.  complete ovulation i.e. no retention of eggs

3.  high rate of ovulation repeatable from one group to another within a common species

4.  time to ovulation after injection short and predictable

5.  fertile and viable ovulated eggs

6.  no adverse effects in subsequent reproductive cycles by the same broodfish.

If the results of a field trial meet these criteria, then the trial is considered successful. Table I shows the results of several trials using domperidone and either sGnRH-A or LHRH-A. Table 2 shows results of trials using Ovaprim.

Field trials are now actively being sought in all species of interest to fish culturists. These trials are performed under controlled protocols in order to assess the potential benefits of the use of Ovaprim in improving the biological and economic success of culture of the species.

Figure 1. Physiological pathway resulting from the reception of environmental stimuli to the maturation of gametes.

Figure 2. Gonadotropin releasing hormone (GnRH) has a positive influence on the pituitary receptor sit on the release of gonadotropin while the dopaminergic system has a negative influence. The application of GnRH against peptides has a positive influence on the pituitary and the release of gonadotropin. The application of dopaminergic inhibitors such as a domperidone or pimoxide potentiates an increased release of gonadotropin.

Table 1: Field Trial doses used to induce ovulation and spawning in several teleost species: using simutaneous injections of domperidone (dam) and sGnRH-A or LHRH-A

Table 2. Successful OVAPRIM doses used to induce spawning in several species

References

1.  Brogden, R.N., A.A. Carmine, R.C. Heel, T.M. Speight and G.S. Avery, 1982. Domperidone: a review of its pharmacological activity, pharmacokinetics and therapeutic efficacy in the symptomatic treatment of chronic dyspepsia and as an antiemetic. Drugs 24; 360-400.

2.  Chang, J.P. and R.E. Peter, 1983. Effects of dopamine on gonadotropin release in female goldfish, Carassius auratus. Neuroendocrinology 36:351-357.

3.  Chang, J.P., R.E. Peter and L.W. Crim, 1984. Effects of dopamine and apomorphine on gonadotropin release from the transplanted pars distalis in goldfish. Gen. Comp. Endocrinol. 55: 3473 5 0.

4.  Crim, L.W., R.E. Peter and G. Van Der Kraak, 1987. The use of LHRH analogs in aquaculture. In: B.H. Vickery and J.J. Nestor, Jr. (Editors), LHRH and Its Analogs, Part 2. MTP Press, Lancaster, England, pp. 489-498.

5.  DeLeeuw, R., C. Van't Veer, W. Smit-Van Dijk, H.J.Th. Goos and P.G.W.J. Van Oordt, 1988. Binding affinity and biological activity of gonadotropin-releasing hormone analogs in the African catfish, Clarlas gariepinus. Aquaculture, 71:119-131.

6.  Fontaine, M., 1976. Hormones and the control of reproduction in aquaculture. J. Fish. Res. Board Can. 33:922-939.

7.  Habibi, H.R., R.E. Peter, M. Sokolowska, J.E. Rifier and W.W. Vale, 1-987. Characterization of gonadotropin-releasing hormone (GnRH) binding to pituitary receptors in goldfish (Carassius auratus). Biol. Reprod. 36:844-853.

8.  Habibi, H. R., T.A. Marchant, C.S. Nahorniak, H. Van Der Loo, R.E. Peter, J.E. Rivier and W.W. Vale, 1989.

9.  Functional relationship between receptor binding and biological activity for analog . of mammalian and salmon gonadotropin-releasing hormones in the pituitary of the goldfish (Carassius auratus). Biol. Reprod. 40:1152-1161.

10. Hunter, C,.A., E.M. Donaldson and H. M. Dye. 1981. Induced ovulation in coho salmon (Oncorhynchus kisutch). 1. Further studies on the use of salmon pituitary preparations. Aquaculture. 26:117-127.

11. Idler, D.R., 1982. Some perspectives on fish gonadotropins. In: C.J.J. Richter and H. J. Th. Goos (comp), Proceedings of the International Symposium on Reproductive Physiology of Fish, Wageningen, the Netherlands: Pudoc. -Ill, 4-13.

12. Jalabert, B., F.W. Goetz, B. Breton, A. Fostier and E.M. Donaldson. 1978. Precocious induction of oocyte maturation and ovulation in coho salmon, oncorhyncus-kisutch. J. Fish. Res. Board Can. 35:1423-1429.

13. Lam, T.J., 1982. Applications of endocrinology to fish culture. Can. J. Fish. Aquat. Sci. 39:1.11-137.

14. McLean, E. and E.M. Donaldson, 1990. Absorption of bioactive proteins by the gastrointestinal tract of fish: a review. Aquatic Animal Health 2:(in press).

15. McLean, E., D. Parker, C.M. Warby, N.M. Sherwood and E.M. Donaldson. Gonadotropin release following oral delivery of luteinizing hormone-releasing hormone & its superactive analogue (des-Gly10 [D-Ala6l LHRH ethlamide) to 17Bestradiol­primed coho salmon. (submitted).

16. Omeljaniuk, R.J., S.H.Shih and R.E. Peter, 1987. In vivo evaluation of dopamine receptor-mediated inhibition of gonadotropin secretion from the pituitary of the goldfish, Carassius auratus. J. Endocrinol., 114:449-458.

17. Omeljaniuk, R.J., M.C. Tonon and R. E. Peter, 1989. Dopamine inhibition of gonadotropin (GtH) and a-melanocyte stimulating hormone (a-MSH) release in-vitro from the pituitary of the goldfish (Carassius auratus). Gen. Comp. Endocrinol., (in press).

18. Peter, R.E. and L. W. Crim, 1979. Reproductive endocrinology of fishes: gonadal cycles and gonadotropin in teleosts. Ann. Rev. Physiol. 41:323-335.

19. Peter, R.E., 1982. Nature, localization and actions of neurohormones regulating gonadotropin secretion in teleosts. In: C.J.J. Richter and H. J. Th. Goos (comp), Proceedings of the International Symposium on Reproductive Physiology of Fish, Wageningen, the Netherlands: Pudoc. -Ill, 30-39.

20. Peter, R.E., C.S. Nahorniak, M. Sokolowska, J.P. Chand, J.E. Rivier, W.W. Vale, J.A. King and R.P. Millar, 1985. Structure-activity relationships of mammalian, chicken, and salmon gonadotropin releasing hormones in vivo in goldfish. Gen Comp. Endocrinol. 58:231-242.

21. Peter, R.E., H.R. Habibi, T.A. Marchant and C.S. Nahorniak, 1987. Vertebrate gonadotropin-releasing hormones: phylogeny and structure-function relationships. In: The Terminal Nerve (Nervus Terminalis): Structure, Function, and Evolution. L.S. Demski and M. Schwanzel-Fukuda, Eds. Annal New York Acad. Sci. 519:299-309.

22. Peter, R.E., H.R. Lin and G. Van Der Kraak, 1988. Induced ovulation and spawning of cultured freshwater fish in China: Advances in application of GnRH analogues and dopamine antagonists. Aquaculture, 74:1-10.

23. Sherwood, N., L. Eiden, M. Brownstein, J. Spiess, J. Rivier and W.Vale, 1983. Characterization of a teleost gonadotropinreleasing hormone. Proc. Natl. Acad. Sci. 80:2794-2798.

24. Sokolowska, M., R.E. Peter, C.S. Nahorniak, C.H. Pan, J. P. Chang L. W. Crim and C. Weil, 1984. Induction of ovulation in goldfish, Carassuis auratus, by pimozide and analogues of LHRH. Aquaculture, 36:71-83.

25. Solar, I.I., E. McLean, I.J. Baker, N. M. Sherwood and E.M. Donaldson. Induced ovulation of sablefish (Anopoploma fimbria) following oral administration of des Gly10-(DAla6)LH-RH ethylamide. (submitted).

26. Suzuki, Y., M. Kobayashi, 0. Nakamura, K. Aida and I. Hanyu, 1988. Induced ovulation of the goldfish by oral administration of salmon pituitary extract. Aquaculture, 74:379-384.

27. Thomas, P. and N.W. Boyd, 1989. Dietary administration of an LHRH analogue induces spawning of spotted seatrout (Cynoscion nebulosus). Aquaculture, 80:363-370.

28. Van Der Kraak, G., E.M. Donaldson, H.M. Dye, G.A. Hunter, J.E. Rivier and W.W. Vale, 1987. Effects of mammalian and salmon gonadotropin-releasing hormones and analogues on plasma gonadotropin levels and ovulation in coho salmon (Oncorhynchus kisutch). Can. J. Fish. Aquat. Sci. 44:19301935.

29. Zohar, Y., A. Goren, M. Tosky, G. Pagelson, D. Lelbovitz and Y. Kock, 1989. Physiological regulation of GnRH activity in the gilthead seabream, Sparus aurata. Fish Physiol. Biochem., (in press).