Distribution Of Serotonin Immunoreactive System In The Brain Of The Wasp Vespula Rufa Rufa And Vespula Germanica

Applying Colophony-Paraffin (CP) tissue embedding technique combined with immunocytochemical Streptavidin-Peroxidase (SP) method, we had investigated the distribution of serotonin (5-HT) immunoreactive neuron system in the protocerebrum of Vespula rufa rufa and Vespula germanica. Results showed that the brain of wasps is developed, and the protocerebrum is the largest in the brain,almost occupying three fourths of the whole brain volume. The brain structures are similar in the two wasps, and the protocerebrum is composed of several obvious structured neuropils including the mushroom body (MB), central complex (CC), optic lobes (OL), lateral accessory lobe (LAL), and anterior optic tubercle (AOTu). Mushroom body is paired and symmetrical distributed. Optic lobe is the vision center of the insect, which consists of three concentric zones, the lamina, medulla, and lobula. Serotonin is widely distributed throughout all areas of the brain. The number of serotonin-like immunoreactive somata in the brain is a bit small, and the positive neurons are mainly symmetrically localized dorsal or posterior to the brain. In addition, about 74 and 76 serotonin-positive somata, clustering into 8 groups, are respectively observed in the protocerebrum of Vespula rufa rufa and Vespula germanica. In both the two wasps, the serotonin immunostaining in the anterior optic tubercle is weaker than that in the surrounding neuropils. However, it is much stronger in Vespula rufa rufa than that in Vespula germanica. Intense serotonin immunoreactivity can be found in the peduncles of mushroom bodies, the lateral accessory lobes, the lateral neuropils of precerebral bridge, and the large region of the central body and its surrounding neuropils. Moreover, the serotonin-immunoreactive varicose fibres form thick neural network in the central body surrounding neuropils and the suboesophageal ganglion (SOG). There are notable serotonin-positive fibre projections in the calyces, optic lobes, and antennal lobes, but the Kenyon cells and the postcerebral bridge are negatively labelled. The serotonin immunoreactivity in the dorsal lobes, tritocerebrum, and suboesophageal ganglion is relatively strong. The serotonin-positive neurons in the suboesophageal ganglion send most processes into the neuropil, where they form a few thick fibre tracts. On the whole, no obvious differences of serotonin distribution pattern are detected in the neuropils between the two insect species, while the location of serotonin-immunoreactive neurons varied from one wasp to the other. It is suggested that 5-HT mainly modulates distant neural interactions.