Neurochemistry of the sympathetic innervation to the uterus

1. The uterus is supplied by numerous noradrenergic sympathetic nerve fibres, which supply the intramural vasculature and also, in some species at least, the myometrium. 2. Pregnancy is associated with progressive loss of catecholamine from these nerves. Although some direct mechanical damage due to stretching of the uterine wall contributes to this effect, it is primarily due to hormonal influences. 3. In animal experiments, the changes in uterine catecholamine occurring during pregnancy are mimicked by intra-uterine administration of progesterone. 4. The action of progesterone appears not to be selective for pelvic sympathetic neurons, or those in which transmitter turnover rate is particularly slow. The mechanism of depletion is probably through inhibition of tyrosine hydroxylase activity. 5. The functional role of this phenomenon during pregnancy, if one exists, seems likely to be related to protection of the fetoplacental blood supply against sympathetically mediated vasospasm. It is also possible that circulating progesterone may have a general inhibitory action on the female sympathetic nervous system.