Release in vitro of prothoracicotropic hormone from the brain of male Rhodnius prolixus during larval-adult development: Identification of novel and predicted release times

Abstract

This paper reports an in vitro study of the timing of release of prothoracicotropic hormone (PTTH) from isolated brain-retrocerebral complexes during larval-adult development in male Rhodnius. Brain complexes were removed on sequential days of development and tested for PTTH release by their ability to stimulate ecdysteroid synthesis in co-incubated prothoracic glands. This is the first systematic study to utilize ecdysteroid synthesis in vitro to detect PTTH release occurring naturally from intact brain complexes during development. It provides the most direct evidence to date of the times during development when PTTH is released. Brain complexes were found to release PTTH during three distinct periods of development. Brain complexes at other times and other ganglia released no significant activity. The first release is brief and occurs rapidly after a blood meal. The second release occurs at the head critical period for moulting on day 6. The existence of these two previously suspected release times is therefore confirmed. Both are associated in vivo with increases in ecdysteroid synthesis by prothoracic glands and in the haemolymph ecdysteroid titre. The third release time is novel and consisted of sporadic PTTH release spanning days 6–12. This release is not qualitatively necessary for the continuation of development. It is suggested that PTTH release may be episodic during this period and play a role in the previously reported circadian regulation of ecdysteroid synthesis in prothoracic glands. PTTH release during this period of development has not been reported previously in any insect.