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Biogenic amines are important messenger substances in the central nervous system and in peripheral organs of vertebrates and of invertebrates. The honeybee, Apis mellifera, is excellently suited to uncover the functions of biogenic amines in behaviour, because it has an extensive behavioural repertoire, with a number of biogenic amine receptors characterised in this insect. In the honeybee, the biogenic amines dopamine, octopamine, serotonin and tyramine modulate neuronal functions in various ways. Dopamine and serotonin are present in high concentrations in the bee brain, whereas octopamine and tyramine are less abundant. Octopamine is a key molecule for the control of honeybee behaviour. It generally has an arousing effect and leads to higher sensitivity for sensory inputs, better learning performance and increased foraging behaviour. Tyramine has been suggested to act antagonistically to octopamine, but only few experimental data are available for this amine. Dopamine and serotonin often have antagonistic or inhibitory effects as compared to octopamine. Biogenic amines bind to membrane receptors that primarily belong to the large gene-family of GTP-binding (G) protein coupled receptors. Receptor activation leads to transient changes in concentrations of intracellular second messengers such as cAMP, IP3 and/or Ca2+. Although several biogenic amine receptors from the honeybee have been cloned and characterised more recently, many genes still remain to be identified. The availability of the completely sequenced genome of Apis mellifera will contribute substantially to closing this gap. In this review, we will discuss the present knowledge on how biogenic amines and their receptor-mediated cellular responses modulate different behaviours of honeybees including learning processes and division of labour.
The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) plays a key role in regulating and modulating various physiological and behavioral processes in both protostomes and deuterostomes. The specific functions of serotonin are mediated by its binding to and subsequent activation of membrane receptors. The vast majority of these receptors belong to the superfamily of G-protein-coupled receptors. We report here the in vivo expression pattern of a recently characterized 5-HT(1) receptor of the honeybee Apis mellifera (Am5-HT(1A)) in the mushroom bodies. In addition, we summarize current knowledge on the distribution of serotonin and serotonin receptor subtypes in the brain and specifically in the mushroom bodies of the fruit fly Drosophila melanogaster and the honeybee. Functional studies in these two species have shown that serotonergic signaling participates in various behaviors including aggression, sleep, circadian rhythms, responses to visual stimuli, and associative learning. The molecular, pharmacological, and functional properties of identified 5-HT receptor subtypes from A. mellifera and D. melanogaster will also be summarized in this review.