Unique synaptic topography of crest-type synapses in the interpeduncular nucleus.

Neurons in the central nervous system display a great diversity of synaptic architecture. While much of our knowledge on the excitatory synapse morphology derives from the prototypical asymmetric synapses, little has been studied about the atypical crest-type synapse that exists in the restricted brain regions. Here, we used focused ion beam scanning electron microscopy (FIB/SEM) to image a neuropil volume of interpeduncular nucleus (IPN) and manually reconstructed several dendrites to obtain an insight about the topography and quantitative features of crest synapses. Three-dimensional reconstruction showed numerous U-shaped structures protruding from the IPN dendrites. On either faces of the U-shaped structure, a pair of crest synapses are aligned in parallel such that there exists a positive correlation between the postsynaptic density (PSD) area of synapses that participate in pair formation. Interestingly, mitochondria are excluded from the site of crest synapses. Several presynaptic axons run through the hollow, cylindrical space of the U-shape grooves such that the plasma membrane of the axon and the dendrite are organized in a tight opposition without any intervening glial membrane. Unlike the peculiar dendritic morphology, IPN neurons possess typical somatic morphology with an oval, centrally located nucleus. In conclusion, our data reveals a hitherto unknown unique topographical feature of crest synapses in the IPN.