Prothoracicotropic hormone in Rhodnius prolixus: in vitro analysis and changes in amounts in the brain and retrocerebral complex during larval—adult development

Abstract

Prothoracicotropic hormone (PTTH) was extracted from the brain-retrocerebral complex of fifth instar larvae of Rhodnius prolixus and was assayed in vitro by its ability to stimulate a statistically significant increase in ecdysteroid synthesis by prothoracic glands (PGs). PTTH stimulated synthesis in a dose-dependent manner with linear increase between 0.25–1.0 brain complex equivalent. Significant stimulation was observed with 0.4 eqt and maximum stimulation with 1.0 eqt, which stimulated ecdysteroid synthesis 3.5-fold. The dose-response curves for PTTH extracted from unfed and day 4 fed animals were closely similar. PTTH activity was found in both brain and retrocerebral complex but not in other ganglia. The activity was destroyed by pronase. PGs were maximally activated by exposure to PTTH for 20 min. Longer exposure did not increase the response. PGs of young fifth instar larvae are highly responsive to stimulation by PTTH, whereas PGs from animals later in the moult cycle were refractory to further stimulation. The content of PTTH in the brain and in the retrocerebral complex both exhibit large, systematic changes during larval-adult development. Complete loss of PTTH activity in both structures occurred at previously documented times of PTTH release. Large increases in PTTH content occurred at previously described times of active synthesis and transport in the medial neurosecretory cells of the brain. PTTH lost by release was replenished in the latter part of development such that the adult insect after ecdysis contained an amount of PTTH comparable to that of the unfed larva.