Sexual differentiation and hormonal regulation of the laryngeal synapse in Xenopus laevis

Abstract

In Xenopus laevis frogs, sex differences in adult laryngeal synapses contribute to sex differences in vocal behavior. This study explores the development of sex differences in types of neuromuscular synapses and the development and hormone regulation of sex differences in transmitter release. Synapses in the juvenile larynx have characteristics not found in adults: juvenile muscle fibers can produce subthreshold or suprathreshold potentials in response to the same strength of nerve stimulation and can also produce multiple spikes to a single nerve stimulus. Juvenile laryngeal muscle also contains the same synapse types (I, II, and III) as are found in adult laryngeal muscle. The distribution of laryngeal synapse types in juveniles is less sexually dimorphic than the distribution in adults. Analysis of quantal content indicates that laryngeal synapses characteristically release low amounts of transmitter prior to sexual differentiation. Quantal content values from male and female juveniles are similar to values for adult males and are lower than values for adult females. When juveniles are gonadectomized and treated with exogenous estrogen, quantal content values increase significantly, suggesting that this hormone may increase transmitter release at laryngeal synapses during development. Gonadectomy alone does not affect quantal content of laryngeal synapses in either sex. Androgen treatment decreases quantal content in juvenile females but not males; the effect is opposite to and smaller than that of estrogen. Thus, muscle fiber responses to nerve stimulation and transmitter release are not sexually dimorphic in juvenile larynges. Transmitter release is strengthened, or feminized, by the administration of estradiol, an ovarian steroid hormone.

Androgens Modulate NMDA Receptor–Mediated EPSCs in the Zebra Finch Song System

“…In electric fish, androgens and estrogens reciprocally modulate voltage-dependent sodium channels that underlie sexually dimorphic electric organ discharges (Dunlap et al 1997;Ferrari et al 1995), and in Xenopus, estrogens increase quantal content at laryngeal neuromuscular junctions (Tobias and Kelley 1995). In the mammalian CNS, estrogens can modulate synaptic transmission through a variety of mechanisms (Joels 1997), including the augmentation of NMDA receptor currents in the CA1 region of the rat hippocampus during estrus (Woolley et al 1997).…”

Estrogen and Laryngeal Synaptic Strength in <i>Xenopus laevis:</i> Opposite Effects of Acute and Chronic Exposure

“…However, rising levels of endogenous gonadal steroids during development, as well as exogenous estrogen treatment, induce strong laryngeal synapses [5, 6]. We thus examined steroid levels in hCG-injected females and compared them to synaptic strength.…”

“…As in previous studies in larynx [4, 5], quantal contents were determined using the failure analysis described by Del Castillo and Katz [9]. We did not compare spontaneous (miniature) to evoked end-plate potential amplitudes because mepp frequency (∼2/min) is so low; stable recordings for more than 1 h would be required for measuring quantal content using amplitude.…”

“…The estradiol dosage was either 100–150 or 30–70 µg/g body wt (in two different sets of experiments). The first dose is based on that used by Tobias and Kelley [5]while the second yields circulating concentrations in the physiological range. For gonadectomized adult females, estradiol-containing Silastic blocks were implanted under local anesthesia (10% Lidocaine, Sigma, in absolute ethanol was applied to the skin 1 min before incision) into the dorsal lymph sac 12 h before larynx removal.…”

“…The laryngeal synapse is estrogen sensitive; females have stronger synapses than males and synaptic strength relies on estrogen exposure during development [5, 6]. Thus, juveniles of both sexes have similarly weak synapses; males maintain weak synapses throughout development while synaptic strength in females increases in response to rising titers of estrogen.…”