Koutaroh Okada

Visualization of modulatory effects of serotonin in the silkmoth antennal lobe

SUMMARY A unique serotonin-immunoreactive neuron innervates every glomerulus of the contralateral antennal lobe (AL), the primary olfactory center, of the male silkmoth Bombyx mori. In order to examine the possible modulatory effects of serotonin in the AL, we utilized high-speed optical imaging with a voltage-sensitive dye combined with bath application of serotonin. We found that serotonin at 10-4moll-1 caused significant and reversible increases in the optical responses in both the macroglomerular complex (MGC) and the ordinary glomeruli (Gs) evoked by electrical stimulation of the antennal nerve. Optical responses in both the MGC and Gs were also significantly longer lasting following serotonin application. Serotonin exerted a significantly greater enhancing effect in the toroid glomerulus of the MGC than in the cumulus, and the effects of serotonin were also non-homogenously distributed in the Gs. Our results are evidence that serotonin acts in both the MGC and Gs to modulate the responses of neuronal populations.

High-speed voltage-sensitive dye imaging of an in vivo insect brain

We have developed an improved optical recording technique with high spatio-temporal resolution to investigate neural activity from an in vivo insect brain. Optical images were acquired from hawk moth antennal lobe (AL) neurons. When activity was elicited by electrical stimulation of the antennal nerve using a stereoscopic microscope, depolarization followed by hyperpolarization was consistently recorded in the AL. When gamma-aminobutyric acid (GABA)-mediated synaptic transmission was blocked by the picrotoxin, the typical hyperpolarization was greatly reduced while no changes were observed in the depolarization. This suggests that the hyperpolarization results from inhibitory postsynaptic potentials (IPSPs) originating from GABA-sensitive chloride channels on the AL neurons.

Localization of odor-induced oscillations in the bumblebee antennal lobe.

Odor-induced oscillatory signals have been reported in odor discrimination in various insects. To understand dynamic olfactory coding in the first order olfactory center, the antennal lobe, localization of oscillatory signals was investigated using an optical recording technique with a voltage-sensitive dye. We present here the first report of visualization of the spatial distribution pattern of odor-induced oscillations in the bumblebee antennal lobe. Analysis of the odor-induced optical responses by a maximum entropy method allowed visualization of oscillatory regions in the antennal lobe. The oscillatory signals were usually localized to regions corresponding to a single or several glomerular structures. Our results suggest that glomerular structures may be functional units of odor processing from the viewpoint of odor-induced population responses, the oscillations.

Spatio-temporal activities in the antennal lobe analyzed by an optical recording method in the male silkworm moth Bombyx mori

Optical recordings with a voltage-sensitive dye showed that the spatio-temporal pattern of depolarizing responses evoked by electrical stimulation of antennal nerve (AN) was non-homologously distributed in the antennal lobe (AL) of the male silkworm moth, Bombyx mori. Time courses of postsynaptic activities and GABAergic inhibitory potentials of AL neurons were individually demonstrated by pharmacological experiments, i.e. Ca2+ free and bicuculline conditions. GABAergic inhibitory potentials began with a ca. 3 ms delay from the beginning of the postsynaptic activities. Intensity of the postsynaptic activities and GABAergic inhibitory potentials were non-homologously distributed in the AL. Relatively strong postsynaptic activities and GABAergic inhibitory potentials were consistently observed in some parts of the macroglomerular complex (MGC) and/or in some ordinary glomeruli (Gs) in the medial and ventral part of the AL.