Temporal expression patterns and functions of serotonin receptor subtypes in the honey bee, Apis mellifera
Final Report Abstract
Serotonin plays a pivotal role in regulating and modulating physiological and behavioral processes in both vertebrates and invertebrates. In the honeybee (Apis mellifera), serotonin has been implicated in division of labor, visual processing, and learning processes. We have investigated the honeybee 5-HT2 receptor class which consists of two subtypes, Am5-HT2α and Am5-HT2β. Each receptor gene also gives rise to alternatively spliced mRNAs that possibly code for truncated receptors. Only activation of the full-length receptors with serotonin caused an increase in the intracellular Ca2+ concentration. The effect was mimicked by the agonists 5-methoxytryptamine and 8-OH-DPAT at low micromolar concentrations. Receptor activities were blocked by established 5-HT receptor antagonists such as clozapine, methiothepin, or mianserin. High transcript numbers were detected in exocrine glands suggesting that 5-HT2 receptors participate in secretory processes in the honeybee. In cooperation with the group of G. Wright (Newcastle University, UK), we tested the role of serotonin as a neurotransmitter in feeding and crop contractions and its role as a systemic hormone that affected feeding in adult foraging honeybees. We found 5-HT-immunoreactive processes throughout the gut, including on the surface of the oesophagus, crop, proventriculus, and the midgut, as well as in the ventral nerve cord. Transcripts for all four honeybee 5-HT receptors (Am5-ht1A,2α,2β,7) were expressed in the crop and the midgut suggesting a functional role for 5-HT in these locations. Application of a cocktail of antagonists with activity against these known receptors to the entire gut in vivo reduced the rate of spontaneous contraction in the crop and proventriculus. Although feeding with sucrose caused a small elevation of endogenous 5-HT levels in the haemolymph, injection of exogenous 5-HT directly into the abdomen of the bee to elevate 5-HT in the haemolymph did not alter food intake. However, when 5-HT was injected into directly into the brain there was a reduction in intake of carbohydrate, amino acid, or toxin-laced food solutions. These data demonstrate that 5-HT inhibits feeding in the brain and excites muscle contractions in the gut, but general elevation of 5-HT in the bee‟s haemolymph does not affect food intake. The results of this research project should facilitate further attempts to unravel central and peripheral effects of serotonin mediated by specific receptors subtypes in the honeybee and other insects.
Publications
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(2011) Distribution of serotonin (5-HT) and its receptors in the insect brain with focus on the mushroom bodies. Lessons from Drosophila melanogaster and Apis mellifera. Arthropod Struct. Dev. 40, 381-394
Blenau, W. and Thamm, M.
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(2012) Molecular and pharmacological characterization of serotonin 5-HT2α and 5-HT7 receptors in the salivary glands of the blowfly Calliphora vicina. PLoS One 7, e49459
Röser, C., Jordan, N., Balfanz, S., Baumann, A., Walz, B., Baumann, O. and Blenau, W.
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(2012) Plant essential oils and formamidines as insecticides/acaricides. What are the molecular targets? Apidologie 43, 334-347
Blenau, W., Rademacher, E. and Baumann, A.
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(2013) Function and distribution of 5-HT2 receptors in the honeybee (Apis mellifera). PLoS One 8, e82407
Thamm, M., Rolke, D., Jordan, N., Schiffer, C., Baumann, A. and Blenau, W.
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(2013) Serotonin in insects: distribution, biosynthesis, uptake, inactivation, receptors, functions, and implications for human health. In: F. S. Hall (Ed.) Serotonin: Biosynthesis, Regulation and Health Implications. Nova Publishers, New York, 1-26
Blenau, W., Thamm, M. and Baumann, A.
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(2014) Characterization of an Invertebrate-Type Dopamine Receptor of the American Cockroach, Periplaneta Americana. Int. J. Mol. Sci. 15, 629-653
Troppmann, B., Balfanz, S., Krach, C., Baumann, A. and Blenau, W.
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(2015) Cockroach Gabab Receptor Subtypes: Molecular Characterization, Pharmacological Properties and Tissue Distribution. Neuropharmacology 88, 134-144
Blankenburg, S., Balfanz, S., Hayashi, Y., Shigenobu, S., Miura, T., Baumann, O., Baumann, A. and Blenau, W.
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(2015) Serotonin Receptor Technologies, Neuromethods, Vol. 95, Humana Press
Blenau, W. and Baumann, A. (Eds.)
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(2016) Octopaminergic and tyraminergic signaling in honeybee (Apis mellifera) brain: behavioral, pharmacological and molecular aspects. In: T. Farooqui and A. Farooqui (Eds.) Trace Amines and Neurological Disorders: Potential Mechanisms and Risk Factors. Elsevier, 203-219
Blenau, W. and Baumann, A.
