We determined the mechanisms underlying caste differences in the brain levels of dopamine-related substances in adult honeybees (Apis mellifera L.). Brain levels of dopamine, DOPA (a dopamine precursor), and N-acetyldopamine (a dopamine metabolite, NADA) were significantly higher in three-day-old virgin queens than same-aged workers. Caste differences in dopamine and NADA levels were also found in the hemolymph. The in vitro enzymatic activities of DOPA decarboxylase (DDC) during dopamine synthesis in brains were not significantly different between castes. The DDC activity in adult queens was mainly found in the brain, but with lower levels of activity detected in the mandibular glands, salivary glands and ovaries. Oral application of DOPA to workers led to DOPA uptake in the brain and significantly higher dopamine and NADA levels in the brain, suggesting that dopamine synthesis could be controlled by the amount of DDC substrate. Royal jelly samples taken from queen cells had a >25-fold higher concentration of dopamine compared with honey samples collected from honey cells. However, oral application of the same concentration of dopamine did not significantly enhance the brain levels of dopamine and NADA. These results suggest that the higher levels of brain dopamine in queens compared with workers can be explained by the higher level of DDC substrate, rather than DDC activity in the brain and other tissues of queens as well as exogenous dopamine in the royal jelly.
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