Serotonin Receptor B May Lock the Gate of PTTH Release/Synthesis in the Chinese Silk Moth, Antheraea pernyi; A Diapause Initiation/Maintenance Mechanism?

The release of prothoracicotropic hormone, PTTH, or its blockade is the major endocrine switch regulating the developmental channel either to metamorphosis or to pupal diapause in the Chinese silk moth, Antheraea pernyi. We have cloned cDNAs encoding two types of serotonin receptors (5HTR(A and B)). 5HTR(A)-, and 5HTR(B)-like immunohistochemical reactivities (-ir) were colocalized with PTTH-ir in two pairs of neurosecretory cells at the dorsolateral region of the protocerebrum (DL). Therefore, the causal involvement of these receptors was suspected in PTTH release/synthesis. The level of mRNA(5HTRB) responded to 10 cycles of long-day activation, falling to 40% of the original level before activation, while that of 5HTR(A) was not affected by long-day activation. Under LD 16:8 and 12:12, the injection of dsRNA(5HTRB) resulted in early diapause termination, whereas that of dsRNA(5HTRA) did not affect the rate of diapause termination. The injection of dsRNA(5HTRB) induced PTTH accumulation, indicating that 5HTR(B) binding suppresses PTTH synthesis also. This conclusion was supported pharmacologically; the injection of luzindole, a melatonin receptor antagonist, plus 5th inhibited photoperiodic activation under LD 16:8, while that of 5,7-DHT, induced emergence in a dose dependent fashion under LD 12:12. The results suggest that 5HTR(B) may lock the PTTH release/synthesis, maintaining diapause. This could also work as diapause induction mechanism.