Estrogen and laryngeal synaptic strength in Xenopus laevis: Opposite effects of acute and chronic exposure

Synaptic transmission at the vocal synapse, the laryngeal neuromuscular jun ction, of Xenopus laevis has been shown to be regulated by long-term change s in circulating estrogen. In females, high levels of circulating estrogen also accompany gonadotropin-induced ovulation and oviposition and the switc h from sexually unreceptive to receptive states, including changes in vocal behaviors (ticking to rapping). Here we examine the effects of gonadotropi n injection on laryngeal synaptic strength and call type. Gonadotropin acut ely reduced quantal content values of laryngeal synapses in intact, adult f emales; the lowest values were attained by 12 h post-injection. Estrogen an d progesterone levels increased following human chorionic gonadotropin (hCG ) injection; the time course was similar to, but negatively correlated with , changes in synaptic strength. In ovariectomized frogs, exogenous estrogen , but not progesterone or hCG, mimicked the acute effects of hCG in weakeni ng laryngeal synapses of intact frogs. hCG injection suppressed ticking and sometimes induced rapping. Females could tick with either strong or weaken ed laryngeal synapses while rapping was only produced during the weakening action of hCG. The normally strong synapses of females may enable vocal pro duction even when laryngeal synapses are weakened by hormones that induce o vulation. In contrast to the acute effect of estrogen on weakening laryngea l synapses, juveniles required more than 2 weeks of estrogen treatment to s trengthen laryngeal synapses while at least 4 weeks postovariectomy were re quired to weaken synapses in adult females. We conclude that acute (hours) increases in circulating levels of estrogen weaken synapses while chronic ( weeks) increases strengthen laryngeal synapses. Copyright (C) 2001 S. Karge r AG. Basel.