The University of Osaka elucidates the function of the neuropeptide "corazonin"

Many insects use day length (photoperiod) as a cue to predict seasonal changes and survive harsh seasons like winter by entering diapause, a physiological state in which reproduction is suppressed. Although molecules and neurons that promote oviposition in the bean bug have been identified, the molecules that induce diapause, which is important for seasonal adaptation, and the details of their neural circuits had not been clarified.

A research group led by Specially Appointed Assistant Professor Jili Xi at the International College, the University of Osaka (also affiliated with the Graduate School of Science), as well as Lecturer Yoshitaka Hamanaka and Professor Sakiko Shiga at the Graduate School of Science at the University of Osaka, clarified the function of the neuropeptide "corazonin." This neuropeptide is an important molecule that regulates reproductive activity according to season in the bean bug (Riptortus pedestris), a pest of leguminous plants. The research was published in the Journal of Experimental Biology.

The research group identified the corazonin molecule that suppresses reproduction in the bean bug and identified corazonin neurosecretory cells in the pars lateralis (important for diapause induction) by combining analytical methods such as neuroanatomy, RNA interference, and PCR. Anatomically, they further revealed neural connections with pigment-dispersing factor (PDF) neurons located near the circadian clock.

Insect corazonin is known to be evolutionarily related to gonadotropin-releasing hormone (GnRH) in vertebrates and involved in regulating metabolism and stress responses. The research group focused on this neuropeptide and confirmed that suppression of gene expression by RNA interference inhibited diapause in short-day females, causing ovarian development and oviposition. In contrast, long-day females with suppressed corazonin gene expression oviposited normally, with no clear effect on reproduction observed. These results indicate that corazonin is involved in short-day-dependent reproductive suppression.

Furthermore, brain immunohistochemistry revealed that corazonin is expressed in three pairs of neurons in the protocerebrum. One of these pairs consisted of pars lateralis neurons involved in diapause induction. These pars lateralis corazonin cells were found to have neural connections with PDF neurons that have cell bodies in the anterior region of the base of the medulla in the optic lobe. This suggests a neural pathway where photoperiodic information received by the compound eyes is transmitted through the optic lobe and relayed to corazonin-producing neurosecretory cells via PDF neurons.

Additionally, axons of corazonin-producing neurons were suggested to project to the corpus cardiacum-corpus allatum complex located posterior to the brain and secrete corazonin into the hemolymph. Since corazonin receptors are not present in the corpus cardiacum-corpus allatum complex but are expressed in the fat body and ovary, it is thought that corazonin regulates reproductive activity by acting directly on peripheral tissues without going through classical endocrine organs.

These findings provide important clues for understanding how photoperiodic information is processed and transmitted in the insect brain to control seasonal reproduction and are expected to contribute to the future development of insect seasonal adaptation mechanisms and pest management technologies.

Xi commented: "In this research, we employed a multifaceted approach integrating neural staining, gene expression suppression and confirmation, and physiological observations and analyses to clarify the distribution, brain expression, and physiological functions of the neuropeptide 'corazonin.' In particular, to achieve both observation of minute neurons and axons and evaluation of reproductive phenotypes, numerous individuals, long-term healthy rearing management, and careful experimental techniques were essential. Through these accumulated efforts, we were able to obtain highly reproducible results, which gave us a great sense of accomplishment."

Journal Information
Publication: Journal of Experimental Biology
Title: Corazonin mediates photoperiodically induced diapause in the bean bug Riptortus pedestris
DOI: 10.1242/jeb.250528