Molecular properties of biogenic amine receptors cloned from honeybee (Apis mellifera) brain

W. Blenau 1 & A. Baumann 2

1 University of Otago, Department of Zoology, P.O. Box 56, Dunedin, New Zealand; 2 Forschungszentrum Juelich, IBI-1, Postfach 1913, 52425 Juelich, Germany

Biogenic amines and their receptors are important regulators of physiological and behavioural functions in both vertebrates and invertebrates. We have cloned the cDNAs encoding a dopamine D1 receptor (AmDOP1) and a tyramine receptor (AmTYR1) from the honeybee brain. In the transmembrane regions, AmDOP1 shares ~75% similarity with vertebrate D1/D5-receptors. The pharmacological profile of the heterologously expressed AmDOP1 is very similar to that of native receptors, which were previously analysed in membrane preparations of honeybee brains. [ 3 H]LSD specifically binds to the heterologously expressed receptor with KD ~5 nM. Dopaminergic agonists and antagonists are potent displacers of [ 3 H]LSD binding. Of all biogenic amines tested, dopamine was the most potent competitor (Ki = 56 nM). Similar to dopamine D1-receptors from vertebrates and Drosophila, activation of AmDOP1 leads to cyclic AMP production. In situ hybridization revealed that the mRNA is expressed in perikarya of different brain neuropils, including those of intrinsic mushroom body neurons. From the cDNA library we isolated another clone which displayed the highest sequence similarity to tyramine receptors cloned from Locusta and Drosophila as well as to an octopamine receptor from Heliothis. Functional properties of the AmTYR1 receptor were studied in stably transformed HEK 293 cells. Tyramine reduces forskolin-induced cyclic AMP production in a dose-dependent manner with an EC50 ~130 nM. A similar effect of tyramine was observed in membrane preparations of honeybee brains. Octopamine also reduces cyclic AMP production in the transformed cell line but is both less potent (EC50 ~3 µM) and less efficacious than tyramine. Receptor-encoding mRNA has a widespread distribution in the brain and suboesophageal ganglion of the honeybee, suggesting that the tyramine receptor is involved in sensory signal processing as well as higher order brain functions. This work was supported by grants from the Deutsche Forschungsgemeinschaft (Ba 1541/2-2; Bl 469/1-2) and a Feodor Lynen Fellowship of the Humboldt Foundation to W. Blenau.

Index terms: biogenic amine receptors, dopamine, cyclic AMP, tyramine.

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