Pharmacotherapeutic group: Gonadotropin-releasing hormones. ATCvet code: QH01CA01
Gonadorelin (as diacetate) is a synthetic hormone physiologically and chemically identical to the Gonadotropin Releasing Hormone (GnRH) synthesized in mammalian species.
Gonadorelin stimulates the synthesis and release of the pituitary gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). Its action is mediated by a specific plasma membrane receptor. Only 20% GnRH receptor occupancy is required to induce 80% of the maximum biological response. The binding of GnRH to its receptor activates protein kinase C (PKC) and also mitogen-activated protein kinase (MAPK) cascades which provide an important link for the transmission of signals from the cell surface to the nucleus allowing synthesis of the gonadotropin hormones.
In repeat breeding animals, one of the most prominent findings is the delayed and smaller preovulatory LH surge leading to delayed ovulation. Injection of GnRH during oestrus increases the spontaneous LH peak and prevents delay in ovulation in repeat breeding animals.
After intramuscular administration of 100 µg of gonadorelin (as diacetate) to the animal, absorption of GnRH is rapid. The maximum concentration (Cmax) of 120.0 ± 34.2 ng / litre is obtained after 15 min (Tmax). Concentrations of GnRH decreased rapidly in plasma.
The absolute bioavailability of gonadorelin (IM versus IV) was estimated to be around 89%.
24 hours after intramuscular administration of 100µg of radiolabelled gonadorelin (as diacetate), the greatest amounts of radioactivity in tissues were measured in the main organs of excretion: liver, kidney and lungs.
8 or 24 hours after the administration, gonadorelin shows an extensive plasma protein binding of 73%.
Gonadorelin is a naturally occurring peptide which is rapidly broken down into inactive metabolites.
After intramuscular administration of gonadorelin to the dairy cow, the principal excretion route is milk followed by urine and faeces. A high percentage of the administered dose is excreted as carbon dioxide in expired air.