Postembryonic Development Of The Brain Structure And5-HT/TPH-IR Neurons In Ant Camponotus Japonicus

Ant is holometabolous social insect, which is a classic model for studying the changes ofbrain system and neurotransmitters during individual development. Using BrdU incorporation,CP embedding method, and immuno-colocalization of5-HT/TPH on adjacent sections, weinvestigated the postembryonic development of the brain structure and serotonergic neuronsin ant Camponotus japonicus. The results showed that the brain system developed rapidly atthe larval and prepupal stages, but the BrdU-labeled cells decreased their proliferativity afterentering the early pupal stages. During postembryonic development, the optic lobe and centralbody appeared at the larval stages, and the BrdU-labeled cells in the optic lobe increased tothe peak at the prepupal stages, over1300; the mushroom body and antennal lobe were visibleuntil the prepupal stages; and the proliferativity of the mushroom body remained to themid-pupal stage; and the volumes of most neuropils increased form larvae to forager ant,which suggested that the aging and task division also influenced the brain maturation. Inaddition, this study has also compared the similarities and differences of the braindevelopment between the holometabolous and hemimetabolous insects.Using immuno-colocalization of5-HT and TPH, we found that two types of5-HT-IRneurons were identified, including strongly-stained immunoreactive neurons (SINs) andweakly-stained immunoreactive cells (WINs). The SINs showed both dense5-HT-andTPH-immunoreactivity in the corresponding somata and axons, which were mainlydistributed at the posterior medial protocerebrum, optic lobe, deutocerebrum andsuboesophageal ganglion, and the number and the size of these SINs somata graduallyincreased during ant development. As for the WINs, the somata showed weak5-HT-IRY butstrong TPH-IRY, which were first discovered in ant brain. These somata emerged from theearly pupal stage and formed two large groups: the ALCa group at the anterior lateral calyx ofthe mushroom body, and the AMP group at the anterior medial protocerebrum. Most WINs’somata were small, round, without neurites and close to the perineurium, with the exceptionof a few large ones with long axons in the ALCa group.In the nervous systems of vertebrates and invertebrates,5-HT acts as a neurotransmitter,neuromodulator or neurohormone affecting neuronal activity and behavior. According to thecell morphologies, the5-HT levels in the corresponding synthesis sites, and the patterns of5-HT-mediated transmission, we speculated that5-HT might plays different physiologicalroles in5-HT-IR cells with different morphologies. First,5-HT in the SINs (large somata withlong axons) might act as neurotransmitter. Second, in the small WINs cells without neurites,and close to the perineurium, so5-HT probable be released into hemolymph to act as neurohormone. While, in the large WINs cells with long axons,5-HT may be released into theprojections terminals affecting target cells or organs, acting as neuromodulator.