Using of gonadotropins to fish induce breeding

june 12

1. Hypophysation
– Injection with crude pituitary extracts (PG)
– Replacement therapy: GtH from another fish take over when the breeding fish is not producing enough of it own
– Availability of pituitaries: powdered carp and salmon pituitaries or collected from mature fish, as fresh PG, and preserved in either alcohol or dried, after acetone extraction of fats
– A lot of disadvantages: hard to standardization, extracts are highly impure, contain accessory hormones and other components that may stimulate some fish but inhibit others and action is unpredictable

2. Mammalian GtH
– Human chorionic gonadotropin (HCG) is produced by the placenta and can be extract from urine of pregnant women
– Has the advantages of purity and long storage life
– However, its molecule is so unlike fish GtH, so high does is needed
– Work well in some species (most marine fish) but some fish not respond at all
– Usually use in combination with pituitary
– May have an immune reaction in fish repeatedly injected with HCG

3. GnRH analogues

– Mammalian GnRH was purified in 1970
– 10 amino acids joined together with an amine group (NH2) attached to position 10
– Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2
– Salmon GnRH only different from mGnRH in amino acids at position 7 and 8
– Because GnRH molecule are simple, they can be synthesizes and different amino acids can be inserted at any position to produce superactive analogues
– More than 2000 analogues have been synthesized
– In fish, the most potent analogues have 2 modifications: 1) Replace glycinamide (Gly-NH2) at position 10 with ethylamide (NH-CH2-CH3) which increases the binding affinity; 2) Replace Gly at position 6 with D-form (mirror image form of the naturally occurring L-form) of hydrophobic or aromatic which been found to be important in either increasing receptor binding affinity and/or an increasing hydrophobicity and providing greater resistance to enzymatic degradation
– 3 analogues useful in fish culture: 1) D-Ala6 mGnRHA (LHRHA); 2) D-Arg6 sGnRHA; 3) Buserelin
– GnRH acts early in the hormonal chain and cause fish to produce its own GtH
– GnRH is not highly species specific
– GnRH is simple and easy to manufacture

4. Combination of GnRHa with a dopamine antagonist
– Most cyprinids have a strong dopamine effect
– Dopamine effect also varies according to the ripeness of fish
– In carp, the combination is very effective
– Repeated use of DOM in combination with BUS has no negative effect on the induction of spawning or gonadal development in the Thai carp

recapped from N.Sukumasavin


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