Aminergic control and modulation of honeybee behaviour

Abstract

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, IP(3) and/or Ca(2+). 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.

Keywords: Serotonin; amine receptors; behaviour; division of labour; dopamine; honeybee; octopamine; tyramine.

Fig. (1)

A. Schematic drawing of major compartments of the honeybee brain. The protocerebrum contains the optic lobes, the lamina (la), medulla (me) and lobula (lo), a pair of mushroom bodies (mb) and the central complex (cc). Each mushroom body consists of a lateral calyx (lc) and a median calyx (mc), which are connected via the pedunculus (ped) to the α-lobe (α) and the β-lobe (β). The antennal lobe (al) is part of the deutocerebrum, which also contains the dorsal lobe (not shown). The tritocerebrum (not shown) is composed of two small bilateral lobes at the base of the brain. B. Schematic drawing of a mushroom body with the three subpopulations of Kenyon cells. The outer compact cells (occ) are born first and later pushed outward by the non-compact cells (ncc). These cells are later pushed outward by the inner compact cells (icc), which reside at the centre of each calycal cup (after [47]).

Fig. (2)

Signalling cascades following activation of a G-protein coupled receptor (GPCR). Biogenic amine receptors are activated after binding of the respective ligand. The activated GPCR in turn can activate a stimulatory G-protein (G_s) and thus initiate a cAMP signalling pathway (A) or it can activate a G-protein of the G_q/o family (G_q/o), which couples the amine receptor to IP₃/DAG signalling pathways (B). A. The activated G-protein stimulates an adenylyl cyclase (AC), which catalyses the production of cAMP from ATP. The increasing intracellular concentration of cAMP activates cAMP-dependent protein kinase (PKA). This kinase can phosphorylate a number of target proteins on serine and threonine residues. B. Members of the G_q/o-protein-family activate phospholipase C (PLC). This enzyme hydrolyses phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds to specific receptors (IP₃-R), which form ligand-gated ion channels in the membrane of the endoplasmatic reticulum. Opening of IP₃-Rs causes an efflux of Ca²⁺ from the endoplasmic reticulum into the cytoplasm. DAG and the increased intracellular Ca²⁺ levels lead to activation of protein kinase C (PKC) which, like PKA, can phosphorylate different target proteins.

Fig. (3)

Phylogenetic relationship between biogenic amine receptors of the honeybee and human aminergic receptors. Alignments were performed with the complete amino-acid sequence of each receptor. The receptor sequences, followed by their GenBank accession numbers (#), are listed below in the order illustrated: human 5-HT_1B (#NP_000854), human 5-HT_1D (#NP_000855), human 5-HT_1A (#NP_000515), human 5-HT₄ isoform b (#NP_000861), human 5-HT₇ isoform a(#NP_000863),human α_1A-adrenergic isoform 1 (#NP_000671), human α_1B-adrenergic (#NP_000670), human α_1D-adrenergic (#NP_000669), Apis mellifera dopamine 2 (AmDOP2; #NP_001011567), A. mellifera octopamine 1 (AmOA1; #NP_001011565), human β₁-adrenergic (#NP_000675), human β₂-adrenergic (#NP_000015), human β₃-adrenergic (#NP_000016), human β_2A-adrenergic (#NP_ 000672), human β_2C-adrenergic (#NP_000674), human β_2B-adrenergic (#NP_000673), A. mellifera tyramine 1 (AmTYR1; #NP_001011594), human D₁ (#NP_000785), human D₅ (#NP_000789), A. mellifera dopamine 1 (AmDOP1; #NP_001011595), human D₂ isoform short (#NP_057658), human D₃ isoform a (#NP_000787), A. mellifera D2likedopamine (AmDOP3;#NP_001014983),human D₄ (#NP_000788), human 5-HT_2A (#NP_000612), human 5-HT_2C (#NP_000859), human 5-HT_2B (#NP_000858), and Drosophila melanogaster FMRFamide (DmFR; CG2114-PA; #AAF47700). The genetic distance between sequences was calculated with ClustalX (version 1.81). A neighbor-joining tree was constructed with ClustalX by using 1000-fold boot-strap re-sampling. The resulting tree was displayed graphically by TreeView using the divergent D. melanogaster FMRFamide receptor as an outgroup. The numbers at the nodes of the branches represent the percentage bootstrap support for each branch. The scale bar allows conversion of branch lengths in the dendrogram to genetic distance between clades (0.1 = 10% genetic distance).

Fig. (4)

Schematic drawing of [³H]OA binding-site distribution in the main neuropils of the bee brain. The brains were incubated with 9 nM [³H]OA and the densities of radioactive labelling are indicated by the degree of shading. The left part of the figure shows anterior parts of the brain whereas the right part shows the posterior parts of the bee brain. Abbreviations: α alpha lobe, β beta lobe of the mushroom bodies. a anterior axis, al antennal lobe, an antennal nerve, br basal ring of the calyx, cb central body, co collar of the calyx, d dorsal axis, dl dorsal lobe, la lamina, lca lateral calyx, lip lip of the calyx, lo lobula, mb, mushroom body, mca median calyx, me medulla, of oesophageal foramen, p posterior axis, ped pedunculus of the mushroom body, sog subesophageal ganglion, som layer of somata, v ventral axis. The figure was taken from [122] with friendly permission of the publisher, © Springer-Verlag 2002.

Fig. (5)

Schematic drawing of [³H]5-HT binding-site distribution in the bee brain. The brains were incubated with 10 nM [³H]5-HT and the densities of radioactive labelling are indicated by the degree of shading. The left part of the figure shows anterior parts whereas the right part shows the posterior parts of the bee brain. Abbreviations as in Fig. (4). The figure was taken from [122] with friendly permission of the publisher, © Springer-Verlag 2002.

Fig. (6)

Proboscis extension response (PER) in the honeybee. When the antennae of a fixed bee are stimulated with a droplet of sucrose solution above the individual response threshold, the bee reflexively extends its proboscis in expectation of food. This behaviour is employed for different non-associative and associative learning paradigms.

Fig. (7)

Acquisition curve in tactile antennal learning. The x-axis shows the number of acquisition trials. The y-axis displays the percentage of bees showing the conditioned proboscis extension response (PER). After only two conditioning trials, the bees reach a stable plateau in their learning performance. The inlet displays a bee showing conditioned proboscis extension while it scans the conditioned stimulus, a small metal plate.