Physiology of the canine anoestrus and methods for manipulation of its length.
Progression from early to late anoestrus is characterized by the appearance of a larger number of gonadotrophin-releasing hormone (GnRH) pulses with a higher amplitude, an increase in the sensitivity of the pituitary to GnRH, an increase in ovarian responsiveness to gonadotrophins, and an increase in basal plasma follicle-stimulating hormone (FSH) concentration. A period of increased luteinizing hormone (LH) pulsatility has been observed shortly before the onset of pro-oestrus. Apart from these changes in the hypothalamus-pituitary-ovary axis, the initiation of a new follicular phase in the bitch is also stimulated by dopaminergic influences other than the accompanying plasma prolactin decrease. Metergoline, a drug which in a low dosage lowers the plasma prolactin concentration via a serotonin-antagonistic pathway, does not shorten the anoestrus; while bromocriptine, in a dosage insufficient to cause a decrease in the plasma prolactin concentration, does prematurely induce a follicular phase. These observations indicate that it is not the decrease in the plasma prolactin concentration, but another dopamine-agonistic influence that plays a crucial role in the transition to a new follicular phase. The dopamine-agonist induced oestrus is associated with a rapid rise in the basal plasma FSH concentration, similar to what is observed during the physiological late anoestrus. Administration of GnRH, eCG and oestrogens may also be used to induce oestrus but with variable results. Oestrus can be prevented surgically or medically, for which purpose progestagens are the most important drugs. The mechanism is still unclear, although it has been demonstrated that with continuing medroxyprogesterone acetate (MPA) treatment the FSH response to GnRH stimulation decreases and changes occur in the pulsatile release of the gonadotrophins. In general, LH pulses coincide with a FSH pulse, but during MPA treatment, LH pulses were observed while there was such a small increase in FSH that it was not recognized as significant FSH pulse.
de Gier
J
J
Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, University of Utrecht, Yalelaan, Utrecht, The Netherlands.
Beijerink
N J
NJ
Kooistra
H S
HS
Okkens
A C
AC
D000780:Anestrus / Q000097:blood / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001971:Bromocriptine; D004285:Dogs / Q000097:blood / Q000502:physiology*; D018491:Dopamine Agonists / Q000494:pharmacology*; D030762:Estrous Cycle / Q000097:blood / Q000187:drug effects* / Q000502:physiology; D004971:Estrus / Q000187:drug effects / Q000502:physiology; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood; D007987:Gonadotropin-Releasing Hormone / Q000097:blood; D007986:Luteinizing Hormone / Q000097:blood; D011388:Prolactin / Q000037:antagonists & inhibitors / Q000097:blood; D012702:Serotonin Antagonists / Q000494:pharmacology*; D013997:Time Factors
D018491:Dopamine Agonists; D012702:Serotonin Antagonists; D007987:Gonadotropin-Releasing Hormone; D001971:Bromocriptine; D011388:Prolactin; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone