Cellular and molecular mechanisms underlying plasticity in uterine sympathetic nerves

Dynamic responses of uterine sympathetic nerves to changes in the circulating levels of sex hormones represent one of the most remarkable examples of physiological plasticity in the adult autonomic nervous system. The density of uterine sympathetic nerves is markedly and irreversibly reduced following puberty, and shows phases of degeneration and regeneration during the natural oestrous cycle. Even more remarkable, uterine sympathetic nerves degenerate during normal pregnancy and regenerate following delivery. Plasticity in uterine sympathetic nerves was initially interpreted as a selective effect of sex hormones on the system of paracervical short adrenergic neurons supplying the uterus. In the last decade, the alternative explanation that sex hormones might alter the ability of the uterine tissue to support its innervation began to be explored and current evidence indicates that oestrogen and pregnancy elicit changes in the neuritogenic properties of the target uterine tissue. In addition, there are indications that sex hormones may also affect the receptivity of uterine-related sympathetic neurons to target-derived signals. Although the nature of these signals is still fragmentary, there is evidence for the contribution of a range of molecules, including neurotrophins, pro-neurotrophins and chemorepulsive signals of the semaphorin family. This review summarizes some general features of plasticity in uterine sympathetic nerves and highlights recent investigations of the cellular and molecular mechanisms underlying this dramatic model of natural plasticity.