Synapsin-like immunoreactivity is present in hair cells and efferent terminals of the toadfish crista ampullaris

The synapsins are presynaptic membrane-associated proteins involved in neurotransmitter release. They are differentially expressed in tissues and cells of the central and peripheral nervous system. In vestibular end organs of mammals, synapsin I-like immunoreactivity has been reported in efferent and afferent terminals and in afferent nerve calyces surrounding type I hair cells. In addition, synapsin I has recently been described in several non-neural cell lines. The present study was conducted to locate synapsin-like immunoreactivity in the neuronal and non-neuronal cells of the fish crista ampullaris, to examine the possibility that the non-neuronal sensory receptor cells express synapsins in vivo. Synapsin-like immunostaining was visualized by immunofluorescence detection in wholemounts of the toadfish crista ampullaris using multiphoton laser scanning microscopy and by electron microscopic visualization of post-embedding immunogold labeling. The results demonstrate that synapsin-like immunoreactivity is present in vestibular hair cells and efferent boutons of the toadfish crista ampullaris. Afferent endings are not labeled. Staining in hair cells is not associated with the synaptic ribbons, suggesting that there is an additional, non-synaptic role for the synapsins in some non-neuronal cells of vertebrates. Moreover, while the cristae of amniote and anamniote species share many functional attributes, differences in their synaptic vesicle-associated protein profiles appear to reflect their disparate hair cell populations.