Estrogen and Laryngeal Synaptic Strength in Xenopus laevis: Opposite Effects of Acute and Chronic Exposure

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Abstract
Synaptic transmission at the vocal synapse, the laryngeal neuromuscular junction, of Xenopus laevis has been shown to be regulated by long-term changes in circulating estrogen. In females, high levels of circulating estrogen also accompany gonadotropin-induced ovulation and oviposition and the switch from sexually unreceptive to receptive states, including changes in vocal behaviors (ticking to rapping). Here we examine the effects of gonadotropin injection on laryngeal synaptic strength and call type. Gonadotropin acutely reduced quantal content values of laryngeal synapses in intact, adult females; the lowest values were attained by 12 h post-injection. Estrogen and 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 weakening laryngeal synapses of intact frogs. hCG injection suppressed ticking and sometimes induced rapping. Females could tick with either strong or weakened laryngeal synapses while rapping was only produced during the weakening action of hCG. The normally strong synapses of females may enable vocal production even when laryngeal synapses are weakened by hormones that induce ovulation. In contrast to the acute effect of estrogen on weakening laryngeal synapses, juveniles required more than 2 weeks of estrogen treatment to strengthen laryngeal synapses while at least 4 weeks postovariectomy were required 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.