Transformations of sensory input by the olfactory bulb (OB) are considered fundamental for downstream processing and memory. To understand the composition and wiring of the OB >400 neurons were reconstructed at ultra-structural resolution in an automatic segmentation of a SBEM volume (~18% of adult zebrafish OB volume).
Using multiple classification approaches we identified 10 distinct morphological classes of interneurons (INs) and three subclasses of mitral cells (MCs), the major type of projection neuron (PNs). The reconstruction of hundreds of PNs within the same brain allowed me to determine ruffed cells (RCs) to be an integral part of a zebrafish glomerulus and uncover anatomical evidence implicative of a paracrine interaction between MCs and RCs. Further, this investigation revealed that the majority of IN classes contradict the Law of Dynamic Polarization, i.e., axonless IN classes making reciprocal, dendro-dendritic synapses clearly dominate in zebrafish OB. In addition, I found the large and widespread anatomy of deep layer INs in agreement with a widely distributed interaction with MCs as it has been proposed for mammals.
A qualitative assessment of synaptic and non-synaptic physical interactions between INs and PNs revealed differences between superficial and deep layer IN classes. Reciprocal dendro-dendritic synapses were frequent and interactions suggestive of paracrine transmission were observed between specific neuron types. Furthermore, we discovered a striking preference in individual IN classes for specific interactions with one of the two mayor PN types.