Biology of gonadotrophin secretion in adult and prepubertal female dogs.

Studies in the female domestic dog demonstrate that luteinizing hormone (LH) and follicle-stimulating hormone (FSH) have secretion patterns that are pulsatile, are inhibited by oestradiol during pro-oestrus and surge to maximal values before ovulation. Studies in ovariectomized bitches suggest that the periovulatory surge is triggered by a preovulatory decline at late pro-oestrus in the oestrogen:progesterone ratio. During the 3-8-month non-seasonal anoestrus, FSH concentrations usually are 50-100% of those of the periovulatory peak, whereas LH concentrations are only 10-20% of peak values. In ovariectomized bitches FSH concentrations are often 5-10 times preovulatory peak values, whereas LH concentrations are only the same as, or double, peak values. Increased LH concentration and pulse frequency are associated with the termination of anoestrus. Treatment with gonadotrophin-releasing hormone (GnRH) pulses or infusions of GnRH agonists can induce fertile oestrus in early anoestrous bitches, as can treatment with a dopamine agonist, presumably by suppression of prolactin secretion. Between 4 months of age and pubertal pro-oestrus at 8-12 months of age, serum concentrations of FSH and LH are similar to those in adult anoestrus, and are suppressed during chronic infusion of GnRH agonist. The latter resulted in a reversible inhibition of puberty during 1 year of treatment. Studies in vivo have shown that LH and prolactin are luteotrophic throughout most of the luteal phase. LH stimulated progesterone synthesis by bitch luteal cells in vitro in the presence or absence of stimulant factors or steroid precursors present in serum.