Introduction

Since the pioneering work of Houssay (1930, 1931) ovulation and spermiation has been induced in a wide range of teleosts. However, while it is evident that induced spawning techniques have been an essential component of the world-wide expansion of aquaculture, it is apparent that some fish are less responsive to induction procedures than others. Various studies have demonstrated that stress, prior to induced ovulation, can hinder successful spawning. Thus, Montalembert et al. (1978) reported that confinement reduced percent ovulation in Northern pike Esox lucius. Scott (1979) described atresia of vitellogenic oocytes in minnows Phoxinus phoxinus 48 h after their transfer to aquaria. Juario et al. (1984) demonstrated that captive milkfish Chanos chanos failed to respond to injection of maturational hormones. In this species, 80% of the females died following treatment, and, while males responded more favourably their sperm was of low quality. Furthermore, milt resorption occured 2-3 days post capture in some of the experimental animals. More recently, Rowland (1988) observed that handling of Murray cod Maccullochella peeli 14 days prior to the natural spawning period, resulted in the formation of atretic oocytes. The ovaries in this species were found to contain blood, mucus, oil and deformed translucent and partially resorbed oocytes in some of the handled animals.

From the above therefore, it is plain that to achieve successful induced ovulation in some species, alternative approaches to delivering maturational hormones must be developed. Clearly, the most desirable method of administering such effector molecules to fish is per os.

Induced Ovulation in Teleosts Following Oral Delivery of Maturational Hormones: An Overview

Various studies have demonstrated the passage of physiologically active peptides and proteins into the circulation of teleosts following oral administration (reviewed by McLean & Donaldson 1990). Early experiments (Tuchmann 1936; Regnier 1938) recorded that the feeding of guppies Girardinus guppi and swordtails Xiphophorous helleri with pituitary preparations influenced sexual development and growth performance. These reports provided the first evidence to suggest that quantitatively significant amounts of physiologically active, pituitary-derived hormones, were absorbed by the fish gut. More recent studies have sought to ratify these observations. Suzuki et al. (1988a,b) revealed that the oral administration of a pituitary extract derived from sexually mature chum salmon (Oncorhynchus keta) to goldfish Carassius auratus induced ovulation and spermiation in lysophosphatidylcholine, in order to elevate gastric pH and maximise gastrointestinal absorption respectively. Treated animals received a priming dose of 1 mg LHRHa/kg body weight, followed, 11 days later, by a second intubation containing 0.5 mg LHRHa/kg body weight. Control animals were intubated with the same solution minus the LHRHA.

At various time intervals after treatment, all animals were weighed to gain an indication of ovarian hydration. Following secondary LHRHA intubation, body mass of 3 of the treated fish was significantly greater (p < 0.01) than control animals. These animals subsequently ovulated, and were strip spawned within 7 days of secondary LHRHA delivery. All control animals, and one of the treated fish failed to ovulate. Harvested eggs were test fertilised using milt collected from spermiating males. Mean percent fertilization was 22.6+10.5%, while normal development of the embryos to the 4-cell stage was variable (7.0+4.8%).

Discussion

Present induced ovulation techniques demand that fish are captured, handled, anaesthetised and injected, sometimes upon more than one occasion. Such procedures are a major disadvantage and may be counter-productive for some species. An ability to deliver maturational hormones to fish using the oral route would avoid some of these problems. Unlike the reports of Suzuki et al. (1988b), in which oral delivery of salmon pituitary preparation failed to induce ovulation in gastric species of teleost, the present, and other studies (Thomas & Boyd 1989) demonstrate systemic bioavailability of LHRHA to both cold and warm-water species. Moreover, the present investigation provides primary evidence for successful oral priming of fish with LHRHa.

While it is difficult to draw conclusions upon the effectiveness of protecting the LHRHa from the gastric secretions, or enhancing its absorption utilising lysophosphatidylcholine, studies in mammals indicate that such methodologies enhance the uptake of peptides and proteins (Tagesson et al. 1985). The study of Solar et al. (1987), in which ovulation was induced in sablefish following injection of 0.2 mg LHRHa/kg body weight, when considered in context of oral LHRHa delivery, indicates that at maximum, only 7.5-fold the amount of LHRHa was required to induced spawning. Indeed, since the limited numbers of experimental animals precluded undertaking of a dose-response study for orally delivered LHRHa, it is likely that lower concentrations of LHRHa would have induced ovulation. These observations contrast to those of Thomas & Boyd (1989) who reported that for spotted sea trout, "at least 10 times more LHRHa is required to induce ovulation by oral administration than by intramuscular injection". From the present study therefore, it would appear that these initial estimations may be conservative particularly when protection-enhancement methodologies are employed for oral delivery.

The results of the present investigation thereby provide encouragement for the future application of oral delivery techniques for LHRHa and similar therapeutics (see McLean et al. 1990). Such systems of drug delivery may be equally applicable to other species of commercially important fish, and in particular to those which are either difficult to handle or easily stressed. While some teleosts demand dopamine antagonists be codelivered with LHRHa for induced ovulation (see Donaldson 1990), it is evident that the potent dopamine antagonist domperidone is also absorbed following oral delivery (Brogden et al. 1982). For such species, therefore, it might be possible to coadminister both LHRHa and domperidone in cocktail form. Further studies upon oral LHRHa delivery to sablefish, and other species, will take account of dose-response relationships.

References

1.  Brogden, R.N., Carmine, A.A., Heel, R.C., Speight, T.M. & Avery, G.S. (1982) Drugs 24: 360-400.

2.  Donaldson, E.M. (1990) In: McGraw-Hill Yearbook of Science & Technology, pp.15-18.

3.  McGraw-Hill Publ. Co., New York. Donaldson, E.M., McLean, E., Sherwood, N.M. & Warby, C.M. (1989) Aquatech '89, March 7-9, Vancouver, B.C., Abstract.

4.  Houssay, B.A. (1930) Rev. Soc. Argent. Biol. 6: 686-688.

5.  Houssay, B.A. (1931) C.R. Seances Soc. Biol. Ses Fil. 106: 3773 7 8.

6.  Juario, J.V., Duray, M.N., Duray, V.M., Nacario, J.F. & Almendras J.M.E. (1984) Aquaculture 36: 61-71.

7.  McLean, E. & Donaldson, E.M. (1990) J. Aqua. Anim. Health. in press.

8.  McLean, E., Dye, H.M. & Donaldson, E.M. (1990) This proceedings.

9.  Montalembert, G. de., Bry, C. & Billard, R. (1978) Am. Fish. Soc. Spec. Publ. 11: 217-225.

10. Nestor, J.J., Ho, T.L., Tahilramani, R., McRae, G.I. & Vickery, B.H. (1984) In: Labrie, F., Belanger, A. & Dupont, A. (eds.) LHRH & its Analogues: Basic & Clinical Aspects. pp. 24-35. Excerpta Medica, Amsterdam.

11. Regnier, M.T. (1938) Bull. Biol. 72: 385-395.

12. Rowland, S.J. (1988) Aquaculture 70: 371-389.

13. Scott, D.C.B. (1979) Symp. Zool. Soc. Lond. 44: 105-132.

14. Solar, I.I., Baker, I.J. & Donaldson, E.M. (1987) Aquaculture 62: 319-325.

15. Solar, I I., McLean, E., Baker, I.J., Sherwood, N.M. & Donaldson, E.M. Suzuki, Y., Kobayashi, M., Aida, K. & Hanyu, I. (1988a) J. Comp.Physiol. B, 157: 753-758.

16. Suzuki, Y., Kobayashi, M., Nakamura, O., Aida, K. & Hanyu, I. (1988b) Aquaculture 74: 379-384.

17. Taggesson, C., Franzen, L., Dahl, G. & Westrom, B. (1985) Lysophophatidylcholine increases rat ileal permeability to macromolecules. Gut 26: 369-377.

18. Thomas, P. & Boyd, N. (1989) Aquaculture 80: 363-370.

19. Tuchmann, H. (1936) Soc. Biol. Comptes Rendus 122: 162-165.