Pregnancy-induced uterine neuronal degeneration in the rat

We studied the location, distribution, and density of uterine nerve bundles in virgin, full-term pregnant, and early postpartum female rats. In the virgin, a low-magnification analysis of uterine whole-mounts stained for acetylcholinesterase revealed fibers both in close association with blood vessels and coursing freely within muscle or connective tissue in the mesometrium and the uterine body. Higher magnification analysis of sections treated immunohistochemically with antibodies to the ubiquitous neuronal protein gene product 9.5, tyrosine hydroxylase, and calcitonin gene-related peptide indicated that the greatest density of nerve bundles was in the mesometrial triangle and intramyometrial region. Although the majority of bundles were associated with blood vessels, many coursed freely, particularly in the myometrial and endometrial layers. The density of innervation within the body of the uterus was heterogeneous. For instance, both innervated and noninnervated spiral arteries branched from the same innervated circumferential artery. In the term pregnant rat, we observed a profound denervation within all layers of the body of the uterus. The few remaining nerve bundles tended to be associated with blood vessels, and they were confined to the intramyometrial region. Within 48 h postpartum, both the number and density of nerve bundles increased. Long tortuous bundles, not seen in the full-term rat, were observed. The present study is the first to describe and illustrate the pattern of uterine innervation in virgin, full-term pregnant, and early postpartum rats over a wide range of magnifications.