Octopamine receptors in the honey bee and locust nervous system: pharmacological similarities between homologous receptors of distantly related species

Abstract

Honey bees are perhaps the most versatile models to study the cellular and pharmacological basis underlying behaviours ranging from learning and memory to sociobiology. For both aspects octopamine (OA) is known to play a vital role. The neuronal octopamine receptor of the honey bee shares pharmacological similarities with the neuronal octopamine receptor of the locust. Both, agonists and antagonists known to have high affinities for the locust neuronal octopamine receptor have also high affinities for the bee neuronal octopamine receptor. The distribution of receptors is more or less congruent between locusts and bees. Optic lobes and especially the mushroom bodies are areas of greatest octopamine receptor expression in both species, which mirrors the physiological significance of octopamine in the insect nervous system. The neuronal octopamine receptor of insects served as a model to study the pharmacological similarity of homologous receptors from distantly related species, because bees and locusts are separated by at least 330 million years of evolution.

Figure 1

Saturation analysis of [³H]-NC-5Z binding to honey bee nervous tissue membranes. Three different parts of the honey bee brain, the mushroom bodies, the optic lobes and the remainder of the brain (central brain without mushroom bodies and antennal lobes) were prepared. [³H]-NC-5Z concentrations ranging from 0.1–2 nmol were used. Each concentration was tested at least three to four times in triplicate. s.d. is given as vertical bars (top). A Scatchard-plot of the saturation data is shown in the lower part of the figure.

Figure 2

Affinity of selected high affinity agonists for the honey bee and locust neuronal octopamine receptor. Increasing concentrations of six different high affinity agonists were used to displace specific [³H]-NC-5Z binding in the bee and locust nervous system. Each concentration is tested at least three times in triplicate.

Figure 3

High affinity antagonists of the honey bee and locust octopamine receptor. The effect of increasing concentrations of six different antagonists on the displacement of specific [³H]-NC-5Z binding is plotted for the honey bee and locust neuronal octopamine receptor. Details see Figure 2.

Figure 4

Comparison of the pharmacological profile of the bees neuronal octopamine receptor with those of all four octopamine receptors of the locust. The affinities (pK_i-values) of the substances tested on the honey bee neuronal OAR were compared with the corresponding values for the four different octopamine receptors of the locust. Each point represents a specific substance as indicated in the plot. Regression analysis of these points revealed different slopes (s), correlation coefficients (r²), and probabilities (P). CDM=chlordimeform, DCDM=demethylchlordimeform, epi.=epinastine, mian.=mianserin, octop.=octopamine.

Figure 5

Concentration of octopamine receptors in different parts of the nervous systems of the honey bee and the locust. The octopamine receptor concentration in the retinae (ret), the optic lobes (ol), the mushroom bodies (mb), the antennal lobes (al), the remainder of the brain (rb), the suboesophageal ganglion (sog), and the thoracic ganglia (tg) of the honey bee (white) and the locust (black) was evaluated and plotted as per cent of maximal binding.