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

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.