Restoration of functionally impaired and degenerated sympathetic nerves in guinea-pig uterus following pregnancy

Abstract

The uterus is supplied with an extensive system of adrenergic nerves. The neurobiological properties of this innervation have been investigated using the guinea-pig as a model. The guinea-pig uterus is supplied from three different sources: from the paracervical plexus, containing short adrenergic neurons, from the inferior mesentric ganglion, and from a cranial source, probably the aortico-renal plexus, via nerves in the uterine suspensory ligaments. The nerve density is higher in the tubal end of the uterine horn and in the cervix than in the main part of the uterine horn. The levels of norepinephrine in the uterus are severely reduced in the course of pregnancy. The process of restoration of the uterine sympathetic transmitter following parturition has been elucidated by measurements of tissue norepinephrine in various uterine regions of guinea-pigs at four time intervals until six months post partum. The tubal end of the uterine horn showed no signs of altered sympathetic nerve function. In fact, this region of the uterine horn is the only part that appears unaffected by pregnancy. In the cervix the transmitter content was restored early, within one week post partum. Also in the sterile uterine horn (in case of previous unilateral pregnancy) the levels of the time-matched controls were reached, though this did not occur until about six months after delivery. In the previously fetus-containing (contralateral) uterine horn the restoration was very incomplete and there is reason to believe that this severe impairment of the adrenergic nerve plexus following a pregnancy is permanent. This would mean that the sympathetic innervation supplying the uterus before the first pregnancy does not represent a final stage of development, and that a “normal” degree of adrenergic innervation in the uterus is not established until after one pregnancy. The changes in the heart, which was used as control tissue, were restricted to those associated with the weight increase, reflecting the usual relationship between the sympathetic nerve plexus and volume of the target organ.