The circadian timing system in the brain of the fifth larval instar of Rhodnius prolixus (hemiptera).

The brain of larval Rhodnius prolixus releases neurohormones with a circadian rhythm, indicating that a clock system exists in the larval brain. Larvae also possess a circadian locomotor rhythm. The present paper is a detailed analysis of the distribution and axonal projections of circadian clock cells in the brain of the fifth larval instar. Clock cells are identified as neurons that exhibit circadian cycling of both PER and TIM proteins. A group of eight lateral clock neurons (LNs) in the proximal optic lobe also contain pigment-dispersing factor (PDF) throughout their axons, enabling their detailed projections to be traced. LNs project to the accessory medulla and thence laterally toward the compound eye and medially into a massive area of arborizations in the anterior protocerebrum. Fine branches radiate from this area to most of the protocerebrum. A second group of clock cells (dorsal neurons [DNs]), situated in the posterior dorsal protocerebrum, are devoid of PDF. The DNs receive two fine axons from the LNs, indicating that clock cells throughout the brain are integrated into a timing network. Two axons of the LNs cross the midline, presumably coordinating the clock networks of left and right sides. The neuroarchitecture of this timing system is much more elaborate than any previously described for a larval insect and is very similar to those described in adult insects. This is the first report that an insect timing system regulates rhythmicity in both the endocrine system and behavior, implying extensive functional parallels with the mammalian suprachiasmatic nucleus.