In vitro photosensitivity of ecdysteroid synthesis by prothoracic glands of Rhodnius prolixus (hemiptera)

The effects of light:dark transitions on in vitro ecdysteroid synthesis by prothoracic glands (PGs) of male fifth instar Rhodnius have been studied. PGs were rendered arrhythmic by prolonged maintenance of the animals in continuous light (LL). PGs were then explanted and ecdysteroid synthesis was measured hourly by radioimmunoassay. PGs were transferred from LL to darkness at various times after explanation and hourly synthesis was measured for several more hours. Transfer of PGs to darkness in vitro invariably elicited a prompt and substantial (up to fivefold) increase in ecdysteroid synthesis. This response was unaffected by the presence of tetrodotoxin in the medium at a concentration that blocks peripheral nervous activity. It is concluded that the PGs are themselves directly photosensitive. In contrast, PGs from animals raised in continuous darkness and incubated in vitro in darkness showed no response when exposed to light. It is inferred that PGs do not respond to maintained states of either light or dark, or to "lights on." Rather, it is argued that transfer from LL to darkness in vitro constitutes a "lights-off" Zeitgeber which is detected by a photosensitive circadian clock in the PGs. This is the first report of photosensitivity in a steroidogenic tissue and provides primary evidence of a photosensitive pacemaker in a nonneural endocrine tissue.