Putative Neural Network Within an Olfactory Sensory Unit for Nestmate and Non-nestmate Discrimination in the Japanese Carpenter Ant: The Ultra-structures and Mathematical Simulation

Ants are known to use a colony-specific blend of cuticular hydrocarbons (CHCs) as a pheromone to discriminate between nestmates and non-nestmates and the CHCs were sensed in the basiconic type of antennal sensilla (S. basiconica). To investigate the functional design of this type of antennal sensilla, we observed the ultra-structures at 2D and 3D in the Japanese carpenter ant, Camponotus japonicus, using a serial block-face scanning electron microscope (SBF-SEM), and conventional and high-voltage transmission electron microscopes. Based on the serial images of 352 cross sections of SBF-SEM, we reconstructed a 3D model of the sensillum revealing that each S. basiconica houses > 100 unbranched dendritic processes, which extend from the same number of olfactory receptor neurons (ORNs). The dendritic processes had characteristic beaded-structures and formed a twisted bundle within the sensillum. At the “beads,” the cell membranes of the processes were closely adjacent in the interdigitated profiles, suggesting functional interactions via gap junctions (GJs). Immunohistochemistry with anti-innexin (invertebrate GJ protein) antisera revealed positive labeling in the antennae of C. japonicus. Innexin 3, one of the five antennal innexin subtypes, was detected as a dotted signal within the S. basiconica as a sensory organ for nestmate recognition. These morphological results suggest that ORNs form an electrical network via GJs between dendritic processes. We were unable to functionally certify the electric connections in an olfactory sensory unit comprising such multiple ORNs; however, with the aid of simulation of a mathematical model, we examined the putative function of this novel chemosensory information network, which possibly contributes to the distinct discrimination of colony-specific blends of CHCs or other odor detection

Visual and olfactory input segregation in the mushroom body calyces in a basal neopteran, the American cockroach

The cockroach Periplaneta americana is an evolutionary basal neopteran insect, equipped with one of the largest and most elaborate mushroom bodies among insects. Using intracellular recording and staining in the protocerebrum, we discovered two new types of neurons that receive direct input from the optic lobe in addition to the neuron previously reported. These neurons have dendritic processes in the optic lobe, projection sites in the optic tracts, and send axonal terminals almost exclusively to the innermost layer of the MB calyces (input site of MB). Their responses were excitatory to visual but inhibitory to olfactory stimuli, and weak excitation occurred in response to mechanosensory stimuli to cerci. In contrast, interneurons with dendrites mainly in the antennal lobe projection sites send axon terminals to the middle to outer layers of the calyces. These were excited by various olfactory stimuli and mechanosensory stimuli to the antenna. These results suggest that there is general modality specific terminal segregation in the MB calyces and that this is an early event in insect evolution. Possible postsynaptic and presynaptic elements of these neurons are discussed