Delayed uterine clearance is the most common cause of subfertility in the mare.
Previous studies have shown that mares that accumulate intrauterine fluid have
impaired myometrial activity. Uterine contractility is a complex mechanism
controlled by coordination of myogenic, neurogenic and hormonal factors. Despite
the importance of the nervous system in controlling uterine contractility, there have
been no studies on uterine innervation in the mare. Furthermore, there have been no
detailed information on oxytocin release in mares during oestrus, the time when the
uterus is challenged by breeding. The aims of this study were to investigate the
factors which are associated with uterine contractility in mares. This study was
designed to describe uterine innervation, to identify the presence and location of
oxytocin in the equine endometrium, and to measure circulating concentrations of
oxytocin around oestrus and after ovulation. The uterus of the mare was well
supplied by a variety of nerve fibres. Using general neuronal marker, PAN-N and
PGP 9.5, it showed a general view of equine uterine innervation. Within the regions
of the uterus, nerve density score for both PGP-immunoreactive (IR) and PAN-N-IR
was greatest in the myometrium. There were no marked differences between the
supply to uterine horn or body, but nerve density was significantly greater (P<0.05)
in the cervix. The nerve supply was predominantly adrenergic and was distributed
throughout all regions of the uterus. With adrenergic nerves, the density score was
the greatest in the myometrium within the structure and in the cervix within the
regions. Peptidergic nerves were also seen and were slightly denser in the cervix than
in the uterine horn or uterine body. Among the peptidergic nerves, neuropeptide Y was the most abundant, whereas vasoactive intestinal polypeptide and calcitonin
gene-related peptide were less frequent. Substance P was rarely observed. The
presence of oxytocin and neurophysin in the uterus was demonstrated using
immunohistochemistry. Ultrastructural studies showed that these hormones were
stored in the secretory vesicles of the luminal secretory cells and the secretory cells
in the superficial endometrial glands. Ciliated cells in the luminal epithelium and
endometrial glands did not show any positive staining for either oxytocin or
neurophysin. Mean plasma oxytocin concentrations in genitally-normal mares (n=5)
were significantly higher (P<0.02) in oestrus (day -5 to day -2) than the day of
ovulation (day 0). On the day 1 post ovulation, mean plasma oxytocin concentrations
were the lowest level. Plasma oxytocin concentrations in day 2 oestrus were
significantly higher (P<0.01) in genitally-normal mares (n=5) than in mares with
delayed uterine clearance (n=5). This study has provided fundamental information on
factors associated with uterine contractile function. This information will be used in
further studies to investigate mares with dysfunction in uterine contractility. Uterine
innervation is a main key function to regulate myometrial contractility and is
coordinated by hormones such as sex steroid hormones, oxytocin and PGF₂α- Studies
are now needed to investigate further differences in uterine innervation, the
distribution of α and ß adrenergic receptors, uterine oxytocin and oxytocin receptors
in mares between genitally-normal mares and mares with delayed uterine clearance
