Cellular Changes in the Prothoracic Glands of Diapausing Pupae of Manduca Sexta

Prothoracic glands from diapausing pupae of the tobacco hornworm, Manduca sexta, synthesize markedly less ecdysone in vitro in response to prothoracicotropic hormone (PTTH) than do glands from non-diapausing pupae. Impaired steroidogenesis is also observed in glands from diapausing animals exposed to agents that enhance ecdysone synthesis in non-diapausing pupal glands by increasing intracellular levels of cAMP (1-methyl-3-isobutylxanthine, dibutyryl cAMP, and the calcium ionophore A23187). In contrast, prothoracic glands from diapausing pupae synthesize significantly more cAMP in response to PTTH and A23187 than do those from non-diapausing pupae. These observations indicate that the PTTH-refractoriness characteristic of prothoracic glands during diapause results from a lesion in the steroidogenic pathway occurring beyond the level of the PTTH receptor-adenylate cyclase system. The diapause condition of the prothoracic glands (reduced ecdysone synthesis accompanied by enhanced cAMP formation) can be mimicked by extirpation of the brain of a non-diapausing pupa. Thus, cellular changes in the prothoracic glands associated with diapause may arise as a result of the absence of some factor produced by the pupal brain (e.g. PTTH).