Tyrosine hydroxylase and dopa decarboxylase activities in the guinea-pig uterus: Further evidence for functional adrenergic denervation in association with pregnancy

Abstract

The capacity for the synthesis of neuronal noradrenaline was studied during pregnancy and post partum in the guinea-pig uterus, using animals with bilateral or unilateral pregnancies. The activities of the noradrenaline-synthesizing enzymes, tyrosine hydroxylase and in some experiments also l-dopa decarboxylase were measured in various parts of the uterus and the adjacent paracervical region, which contains adrenergic ganglion cells contributing to the uterine adrenergic innervation. The submandibular gland was used for comparison. During advancing pregnancy the tyrosine hydroxylase activity in the fetus-containing uterine horn was progressively reduced, to reach undetectable levels at term. This change was not seen in the cervix and the submandibular gland. By analogy with previous fluorescence microscopic observations on the uterine adrenergic innervation, it is suggested that the very pronounced reduction in tyrosine hydroxylase activity in the fetus-containing uterine horn during advancing pregnancy is due to degeneration of the adrenergic terminal network. In the contralateral empty uterine horn of a unilateral pregnancy the tyrosine hydroxylase activity was decreased by about 90% at term despite evidence for a relatively intact adrenergic nerve plexus. In the post partum period the tyrosine hydroxylase activity increased very slowly in both the previously empty and fetus-containing horns. In the latter tissue, even 6 months after delivery, the activity was only 14% of that in uterine horns of virgin animals. In view of the marked regional heterogeneity in the changes, also related to the position and size of the onceptus, it is assumed that they are caused mainly by local humoral and probably also mechanical factors within the uterus.