Journal of Neuroscience, Vol 15, 1660-1668, Copyright © 1995 by Society for Neuroscience
A sex difference in synaptic efficacy at the laryngeal neuromuscular junction of Xenopus laevis
ML Tobias, DB Kelley and M Ellisman
Department of Biological Sciences, Columbia University, New York, New York 10027.
Under physiological conditions, the response of Xenopus laevis laryngeal
muscle fibers to nerve stimulation is sexually differentiated; subthreshold
potentials are common in males and rare in females. This sex difference in
muscle fiber response is correlated with sex differences in vocal behavior.
Quantal analyses at male and female laryngeal synapses were performed to
determine if there is a sex difference in synaptic strength. Quantal
content at laryngeal synapses is significantly higher in females than in
males. Values for quantal content in males can be increased by raising
extracellular calcium concentration. There is no sex difference in
miniature endplate potential amplitude suggesting that ACh receptor number
or properties are not different in the sexes. Sex difference in synaptic
strength thus appear presynaptic in origin; transmitter release is less in
males. Ultrastructural analyses of the laryngeal motor terminal indicate
that there is no sex difference in the length of active zones or in the
number of channels per length of active zone. Thus, ultrastructural
characteristics of the laryngeal motor terminal do not account for the
pronounced sex difference in quantal content.