Ultrastructure of cisternal synapses on outer hair cells of the mouse cochlea

• Published in: Journal of comparative neurology (1911) 2014-02, Vol.522 (3), p.717-729
• Main Authors: Fuchs, Paul Albert, Lehar, Mohamed, Hiel, Hakim
• Format: Article
• Language: English
• Subjects: Acetylcholine receptors; Animals; calcium store; cholinergic inhibition; cochlea; Cochlea - cytology; hair cell; Hair Cells, Auditory, Outer - cytology; Imaging, Three-Dimensional; Mice; Mice, Inbred C57BL; Mice, Transgenic; Microscopy, Electron, Transmission; Mutation - genetics; Receptors, Nicotinic - genetics; Small-Conductance Calcium-Activated Potassium Channels - genetics; Synapses - genetics; Synapses - ultrastructure; synaptic cistern
• ISSN: 0021-9967; 1096-9861
• DOI: 10.1002/cne.23478

ABSTRACT C (cisternal) synapses with a near membrane postsynaptic cistern are found on motor neurons and other central neurons, where their functional role is unknown. Similarly structured cisternal synapses mediate cholinergic inhibition of cochlear hair cells via α9α10‐containing ionotropic receptors and associated calcium‐activated (SK2) potassium channels, providing the opportunity to examine the ultrastructure of genetically altered cisternal synapses. Serial section electron microscopy was used to examine efferent synapses of outer hair cells (OHCs) in mice with diminished or enhanced cholinergic inhibition. The contact area of efferent terminals, the appositional area of the postsynaptic cistern, the distance of the cistern from the plasma membrane, and the average width of the cisternal lumen were recorded. The synaptic cisterns of wild‐type OHCs were closely aligned (14‐nm separation) with the hair cell membrane and coextensive with the micrometers‐long synaptic terminals. The cisternal lumen averaged 18 nm so that the cisternal volume was approximately 30% larger than that of the cytoplasmic space between the cistern and the plasma membrane. Synaptic ultrastructure of α9L9′T knockin OHCs (acetylcholine receptor gain of function) were like those of wild‐type littermates except that cisternal volumes were significantly larger. OHCs of SK2 knockout mice had few small efferent terminals. Synaptic cisterns were present, but smaller than those of wild‐type littermates. Taken together, these data suggest that the cistern serves as a sink or buffer to isolate synaptic calcium signals. J. Comp. Neurol. 522:717–729, 2014. © 2013 Wiley Periodicals, Inc. A near‐membrane (14‐nm) postsynaptic cistern is coextensive with cholinergic efferent synapses on cochlear hair cells. Serial section electron microscopy of synapses on hair cells of genetically altered mice suggests that the cistern may serve to segregate efferent and afferent synaptic calcium signals. Scale bar = 1 μm.