Many animals show sophisticated navigational behavior based on their spatial memory. Path integration is one of the common strategies for navigation, in which the travel distance and direction are separately monitored and then the locational relationship between the current position and the goal was calculated. It has been well known that many insects can deduce their heading direction using a polarization pattern of the sky and use it for path integration. Although there are intensive studies focusing on sensory mechanisms underlying insect polarization vision, relatively few efforts has been taken for understanding central brain mechanisms. Here, I summarize the neural basis underlying polarization vision in insect, especially focusing on our recent progresses about possible brain mechanisms for encoding e-vector orientations.In the cricket optic lobe, information of the e-vector orientation is converged into the three types of polarization-sensitive neurons (POL1-neurons), each of which is tuned to the different e-vector orientation, suggesting that each e-vector orientation is encoded as a triplet signal of POL1-neurons. This idea is termed ‘instantaneous method’ because, using this method, the animal can deduce orientations ‘instantaneously’, i.e. without any rotation of the body. To confirm whether the insects can use this method, we constructed a neural network model that received inputs from POL1-neurons and that encoded any particular e-vector orientations unequivocally. In this model, each e-vector orientation was represented as a response pattern of ‘compass neurons’. For a reliable output of the model, a sufficient number of compass neuron types, each of which was narrowly tuned to a certain e-vector orientation, were required. Next, by intracellular recordings from the cricket brain, we found a group of central complex neurons exactly showing the response properties as we supposed for the compass neurons. We also examined the e-vector discriminative capability of honeybees by a classical conditioning paradigm using proboscis extension reflex and found that they could discriminate two 90° different e-vector orientations without any head movement. Taken these results together, we concluded that the insects could use ‘instantaneous method’ to detect e-vector orientation of the polarized light.

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Neurons in the silkmoth antennal lobe (AL) are well characterized in terms of their morphology and odor-evoked firing activity. However, their intrinsic electrical properties including voltage-gated ionic currents and synaptic connectivity remain unclear. To address this, whole-cell current- and voltage-clamp recordings were made from second-order projection neurons (PNs) and two morphological types of local interneurons (LNs) in the silkmoth AL. The two morphological types of LNs exhibited distinct physiological properties. One morphological type of LN showed a spiking response with a voltage-gated sodium channel gene expression, whereas the other type of LN was nonspiking without a voltage-gated sodium channel gene expression. Voltage-clamp experiments also revealed that both of two types of LNs as well as PNs possessed two types of voltage-gated potassium channels and calcium channels. In dual whole-cell recordings of spiking LNs and PNs, activation of the PN elicited depolarization responses in the paired spiking LN, whereas activation of the spiking LN induced no substantial responses in the paired PN. However, simultaneous recording of a nonspiking LN and a PN showed that activation of the nonspiking LN induced hyperpolarization responses in the PN. We also observed bidirectional synaptic transmission via both chemical and electrical coupling in the pairs of spiking LNs. Thus, our results indicated that there were two distinct types of LNs in the silkmoth AL, and their functional connectivity to PNs was substantially different. We propose distinct functional roles for these two different types of LNs in shaping odor-evoked firing activity in PNs.

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[in Japanese]

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