Gonadotrophin-releasing hormones

By | 2011-08-18

Endogenous gonadotrophin-releasing hormone (GnRH) is a decapeptide secreted by the hypothalamus. Gonadotrophin releasing-hormone causes release of both LH and FSH from the anterior pituitary gland. Fertirelin and gonadorelin are synthetic forms of GnRH. Buserelin and lecirelin are synthetic analogues of gonadotrophin-releasing hormone in which specific amino acid substitutions have been made in their molecular structure, resulting in reduced susceptibility to proteolytic enzymes and greater affinity for binding to GnRH receptors; a tenfold increase in potency is claimed. Deslorelin is a synthetic analogue with more than 100 times greater potency than naturally occurring gonadotrophin-releasing hormone.

The increase in LH concentration, that follows treatment with gonadotrophin-releasing hormone, can be used to induce ovulation in horses, cattle, and rabbits. In mares, ovulation is induced in animals with prolonged oestrus during the transitional phase from winter anoestrus to the onset of normal cyclical ovarian activity, and also after mating. These drugs are used for treatment in cattle with follicular cysts. Administration at the time of service or insemination may improve conception rates in mares and cows. In cows, administration 11 or 12 days post service may increase pregnancy rates. GnRHs are also used in conjunction with progestogens and prostaglandin F2alpha in oestrus synchronisation treatment regimens to control follicular growth in cows, thereby improving pregnancy rates. Buserelin is also used in the fish farming industry.

BUSERELIN

Indications. See under Dose

Warnings. Avoid contamination of product with traces of disinfectant or alcohol

Dose:

Horses: by subcutaneous, intramuscular (preferred), or intravenous injection.

Induction of ovulation (see notes above), 40 micrograms 6 hours before insemination, repeat after 1 day if required

Cattle: by subcutaneous, intramuscular (preferred), or intravenous injection.

Anoestrus, 20 micrograms, repeat after 8-22 days if required

Delayed ovulation, 10 micrograms 6-8 hours before or at time of insemination

Improvement in pregnancy rate, 10 micrograms 6-8 hours before or at time of insemination or 11-12 days after insemination

Follicular cysts, 20 micrograms, repeat after 10-14 days if required

Synchronisation of oestrus

(‘Bovsynch’), 10 micrograms buserelin, followed by pros-taglandin F2aplha after 8 days, then 10 micrograms buserelin after 2 days. Insemination 20-24 hours later Use in conjunction with intravaginal progesterone (Eazi-Breed CIDR), 10 micrograms buserelin at the time of insertion. Prostaglandin F2aplha on day 7, 8, or 9 followed by withdrawal of intravaginal device 24 hours later Rabbits: by subcutaneous injection. Induction of ovulation post partum, 800 nanograms (0.2 mL) 24 hours after parturition, and followed by insemination

Improvement of conception rate, 800 nanograms at time of insemination

Rainbow trout: by intramuscular injection. To facilitate stripping and to reduce mortality due to egg binding, 3-4 micrograms/kg. Stripping performed 2-3 days after treatment

Prescription-only medicine: Receptal (Intervet) UK

Injection, buserelin 4 micrograms/mL, for horses, cattle, rabbits, rainbow trout

Withdrawal Periods. Horses, cattle, rabbits: slaughter withdrawal period nil, milk withdrawal period nil. Should not be used in fish intended for human consumption

GONADORELIN

Indications. See under Dose

Contra-indications. Hypersensitivity to the drug; pregnant animals

Warnings. Avoid contamination of product with traces of disinfectant or alcohol

Dose:

Cattle: by intramuscular injection. Improvement of conception rate, 250 micrograms on day of insemination

Follicular cysts, 500 micrograms, repeat if required Anoestrus post partum, 250 micrograms less than 40 days post partum or 500 micrograms after 40 days post partum, repeat after 1-3 weeks Synchronisation of oestrus 250 micrograms gonadorelin, followed by prostaglandin F2aplha after 8 days, then 250 micrograms gonadorelin 54-56 hours after prostaglandin. Insemination 16-18 hours later Use in conjunction with intravaginal progesterone (Eazi-Breed CIDR), 250 micrograms gonadorelin at the time of insertion

Prescription-only medicine: Fertagyl (Janssen) UK

Injection, gonadorelin 100 micrograms/mL, for cattle

Withdrawal Periods.

Cattle: slaughter withdrawal period nil, milk withdrawal period nil