Putative axonal input and output pathways of the circadian clock system in the brain of the insect Rhodnius prolixus (Hemiptera)

The lateral clock neurons (LN s) arborize in the accessory medulla ( aMe) and the dorsal midbrain. In hemimetabolous insects, this latter region appears highly developed and has been extensively studied in the insect Rhodnius prolixus within which it is called the principle protocerebral arborization area (PPA). This study investigated the importance of the PPA in the Rhodnius brain clock system. Double label immunohistochemistry and laser scanning confocal microscopy allowed observation of close associations between neurons stained with antibodies against four neurochemicals and the LNs. Neuronal projections more densely innervated the PPA than the aMe. All neurochemicals were produced and released with a daily rhythm, implying clock control, likely via PPA axons. Taken together, these results suggest that the PPA is important in the integration of timing information in the Rhodnius brain clock system, perhaps as important as the aMe, containing many potential pathways for input to and output from the system.