VPT 311: General and Systemic Veterinary Pharmacology (2+1)

HORMONES INFLUENCING REPRODUCTIVE FUNCTIONS

Gonadotrophin releasing hormone (GnRH)

Effects

• Administration of GnRH releases LH and FSH from the anterior pituitary gland. The effects of GnRH on LH/FSH release are dependent on
  • the dose and route of administration
  • the endocrine status of the animal at the time of treatment and
  • the frequency of injection
• Generally the release of LH and FSH are higher following GnRH injection when the animal is under the influence of estrogen and lower in progesterone dominated animals.

Uses

• Injection of a single large quantity of GnRH during the follicular phase of the estrous cycle stimulates ovulation 24-48 hours later.
  • Injection at the end of a synchronization schedule in order to control the time of ovulation for scheduling times of insemination
  • To overcome delayed ovulation in dairy cows, particularly in high yielding post partum cows bred within 60 days of parturition
  • Injection of mares during the estrous to time the ovulation and covering in order to maximize the chances of conception
  • Injection of cows at the time of insemination to increase conception rate
• GnRH stimulates LH and FSH in the male and may have a use in animals with poor semen producing ability.
• Synthetic GnRH like gonadorelin, buserelin and fertirelin have been produced which stimulate release of both FSH and LH.
• Buserelin was developed by substituting two aminoacids in natural GnRH.
• This modification helped in increasing resistance to enzymatic degradation and an increased plasma concentration of FSH and LH which persisted for several hours was noticed in mares and cows at therapeutic concentrations.