Prothoracicotropic hormone (PTTH)

Definition

A neuropeptide hormone produced by neurosecretory cells in the insect brain (typically the lateral neurosecretory cells of the protocerebrum) that stimulates the prothoracic glands to synthesize and release ecdysteroids, thereby initiating the molting cascade. PTTH is the primary upstream signal that triggers ecdysis in insects and some other arthropods, integrating environmental and internal cues (photoperiod, body size, nutritional status) to regulate developmental timing. In lepidopterans, the hormone is a homodimeric glycoprotein of approximately 30 kDa; its structure and signaling pathway via the receptor tyrosine kinase Torso are well characterized in model species such as *Bombyx mori* and *Manduca sexta*.

Etymology

From Greek *pro-* (before, in front of) + *thorax* (chest, referring to the prothoracic glands) + *-tropic* (turning toward, influencing), indicating the hormone's target tissue; initialism PTTH standard in insect endocrinology literature.

Example

In the tobacco hornworm *Manduca sexta*, a surge of PTTH released from the corpus cardiacum into the hemolymph on the final day of the fifth larval instar triggers a peak in ecdysteroid production, committing the larva to metamorphosis and the formation of the pupal stage.

Synonyms

• PTTH
• ecdysiotropin
• molting neurohormone

Related Terms

• ecdysteroid
• prothoracic gland
• corpus cardiacum
• Ecdysis
• Juvenile hormone
• neurosecretory cell
• Torso receptor
• Ecdysone
• molting cascade
• endocrine regulation

Usage Notes

PTTH is often distinguished from direct ecdysteroid administration; the term specifies the brain-derived tropic signal rather than the glandular product. In some literature, 'ecdysiotropin' appears as a functional synonym, though PTTH predominates in modern usage. The hormone is absent or non-homologous in crustaceans, where the Y-organ is regulated by molt-inhibiting hormone rather than PTTH, making PTTH a definitive marker of insect (and some chelicerate) endocrine architecture. Quantitative assays for PTTH (e.g., the *in vitro* prothoracic gland activation bioassay) remain standard for studying developmental timing in physiological ecology.