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Contrary Effects of Octopamine Receptor Ligands on Behavioral and Neuronal Changes in Locomotion of Lymnaea

¹ Biology Club, Hokkaido Sapporo Okadama High School, 2-chome, Kitaokadama 1-jo, Higashi-ku, Sapporo 007-0881, Japan
² Hokkaido Science Education Center, The Hokkaido Education Research Institute, 42 Bunkyodaihigashi-machi, Ebetsu 069-0834, Japan
³ Department of System Neuroscience, School of Medicine, Sapporo Medical University, Minami 1-jo, Nishi 17-chome, Chuo-ku, Sapporo 060-8556, Japan
⁴ Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita 13-jo, Nishi 8-chome, Kita-ku, Sapporo 060-8628, Japan
⁵ Laboratory of Functional Biology, Kagawa School of Pharmaceutical Sciences, Tokushima Bunri University, 1314-1 Shido, Sanuki 769-2193, Japan
⁶ Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Dr. NW, Calgary AB, Canada T2N 4N1

^* To whom correspondence should be addressed. E-mail: eito{at}kph.bunri-u.ac.jp

The pond snail Lymnaea stagnalis moves along the sides and bottom of an aquarium, but it can also glide upside down on its back below the water's surface. We have termed these two forms of locomotion "standard locomotion" and "upside-down gliding," respectively. Previous studies showed that standard locomotion is produced by both cilia activity on the foot and peristaltic contraction of the foot muscles, whereas upside-down gliding is mainly caused by cilia activity. The pedal A neurons are thought to receive excitatory octopaminergic input, which ultimately results in increased cilia beating. However, the relationship between locomotory speed and the responses of these neurons to octopamine is not known. We thus examined the effects of both an agonist and an antagonist of octopamine receptors on locomotory speed and the firing rate of the pedal A neurons. We also examined, at the electron and light-microscopic levels, whether structural changes occur in cilia following the application of either an agonist or an antagonist of octopamine receptors to the central nervous system (CNS). We found that the application of an octopamine antagonist to the CNS increased the speed of both forms of locomotion, whereas application of octopamine increased only the firing rate of the pedal A neurons. Microscopic examination of the cilia proved that there were no changes in their morphology after application of octopamine ligands. These data suggest that there is an unidentified octopaminergic neuronal network in the CNS whose activation reduces cilia movement and thus locomotory speed.

Abbreviations: ANOVA, analysis of variance • CNS, central nervous system • LPe, left pedal • PeA, pedal A • PNS, peripheral nervous system • RPe, right pedal • SEM, standard error of the mean • SSW, standard snail water

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