Prothoracicotropic Hormone Release Research Articles

Temperature-sensitive mechanism regulating diapause in Heliothis zea

Pupal diapause in Heliothis zea is regulated by a temperature-sensitive mechanism which prevents ecdysone production despite the release of prothoracicotropic hormone. To determine how this mechanism functioned, donor prothoracic glands were implanted into prothoracic gland-ablated hosts to test their ability to produce ecdysone in a diapause-sustaining temperature of 19°C. Results of these experiments ruled out the possibility that ecdysis production was regulated by the nervous system or by a mechanism intrinsic to the prothoracic glands, and suggested that a humoral factor was required for diapause termination. Haemolymph injection experiments supported this humoral factor hypothesis, i.e. haemolymph from non-diapausing donor pupae terminated diapause in hosts maintained at 19°C, whereas haemolymph from diapausing donor pupae had no such effect. These findings indicate that the temperature-sensitive mechanism regulating H. zea diapause functions by controlling the availability of a humoral factor necessary for ecdysone production by the prothoracic glands.

Cytophysiological correlations between prothoracic gland activity and hemolymph ecdysteroid concentrations in Rhodnius prolixus during the fifth larval instar: Further studies in normal and decapitated larvae

Hemolymph ecdysteroid titers in fifth instar larvae of Rhodnius prolixus were determined by radioimmunoassay, and their prothoracic glands were excised and examined by electron microscopy. During the last larval instar, the titer of ecdysteroid increased between the head-critical period until Day 13, at which time the peak titer was 3100 pg 20-hydroxyecdysone equivalents/μl. The activation of secretory cells at the time of the second period of prothoracicotropic hormone release was correlated with the development of major cellular organelles. The smooth endoplasmic reticulum first appeared at the head-critical period and the proliferated in close relation to the increase in ecdysteroid titer until Day 13, after which time it disappeared. Mitochondria expand and develop tubular cristae. They are closely associated with the smooth endoplasmic reticulum. When insects were decapitated, hemolymph ecdysteroid titer remained below 10 pg/μl and the prothoracic gland cells failed to develop smooth endoplasmic reticulum. We conclude that in the prothoracic gland cells as well as other steroidogenic tissues the smooth endoplasmic reticulum in association with mitochondria is involved in ecdysone biosynthesis.

Temporal organization of endocrine events underlying larval-larval ecdysis in the silkworm, Bombyx mori

The timing of ecdysis in the penultimate instar of Bombyx mori was demonstrated to be under the control of a circadian clock. The temporal organization of secretion of prothoracicotropic hormone (PTTH), ecdysone and juvenile hormone was studied with particular attention to the circadian control of the timing of hormone release. PTTH release occurs, at least, in the second and third night. The latter is responsible for evoking the larval ecdysis. Prothoracic gland initiates ecdysone secretion abruptly with a very short span after the second PTTH release and secrete enough amount of ecdysone for larval moulting, which takes place 11 h later. Juvenile hormone titer is relatively high before the second PTTH release and corpus allatum becomes dispensable for ensuring the larval moulting in 1.5 h. Based on these findings, interpretations for the endocrine system underlying precocious pupation and formation of intermediates, which are produced by neck ligation, are presented.

Prothoracic glands of the saturniid moth Samia cynthia ricini possess a circadian clock controlling gut purge timing

In the moth Samia cynthia ricini, timing of the release of prothoracicotropic hormone is controlled by a circadian clock in the cephalic organ (brain); while this hormone release is necessary for gut purge, the final timing of this event is controlled by a circadian clock in the prothoracic glands that gates release of ecdysone. The photoreceptor of the prothoracic gland clock(s) is extraocular and evidently in the glands themselves. Demonstration that the clock and its photoreceptor are in the prothoracic glands is based on a mixture of localized illuminations and the transplantation of glands from one larva to another.

Temporal organization of endocrine events in relation to the circadian clock during larval-pupal development in Samia cynthia ricini

The temporal organization of secretion of the prothoracicotropic hormone (PTTH) and ecdysone during larval-pupal development of Samia cynthia ricini was studied by ligations, with particular attention to the circadian control of the timing of hormone release. PTTH and ecdysone are required first for the induction of prodromes of pupation and again later for pupal-cuticle formation. PTTH release in the first step occurs during the second or third photophase after the last-larval ecdysis under a photoperiod of 12 hr light and 12 hr darkness and is thought to be under the control of a circadian clock. Ecdysone release follows 1.5 days later, i.e. during the scotophase that precedes the gut purge. In the second secretory step, PTTH is released 2 days after purging the gut, and ecdysone release follows 6 hr later. The PTTH release at this time occurs at a fixed time after the gut purge irrespective of light conditions, accounting for light insensitivity of the timing for pupal ecdysis. Possible mechanisms relating to the inconsistent association of a circadian clock with PTTH release, and those underlying the determination of timing of the gut purge are discussed.

Haemolymph ecdysteroid titres during larval-adult development in Rhodnius prolixus: Correlations with moulting hormone action and brain neurosecretory cell activity

A haemolymph ecdysteroid titre of the fifth (last)-larval instar of the hemipteran, Rhodnius prolixus has been determined by radioimmunoassay. During the last-larval stadium the ecdysteroid titre increases from a negligible level in the unfed insect to a detectable level within minutes following a blood meal. The titre reaches a plateau of ∼50–70 ng/ml at 3–4 hr and this level is maintained until day 5–6, the time of the head-critical period in Rhodnius. At the head-critical period the titre begins to increase again, this time dramatically, reaching a peak of ∼ 3500 ng/ml at day 13. From day 14 to ecdysis (day 21) the titre declines to a low level, ∼ 30 ng/ml. Basal levels of ecdysteroids, ∼ 15 ng/ml, were detectable in young adult males and females. A survey of haemolymph volumes during the last-larval instar indicates that the changes in the ecdysteroid titre reflect changes in the rates of ecdysteroid synthesis, and not changes in haemolymph volume. Excretion of ecdysteroids varies systematically during the instar, suggesting that control of ecdysteroid excretion may be important in regulation of the haemolymph titre. Qualitative analysis of the haemolymph ecdysteroid RIA activity revealed the presence of only ecdysone and 20-hydroxy-ecdysone. For the large peak preceding larval-adult ecdysis, 20-hydroxy-ecdysone was the predominant hormone. These results indicate that there may be two periods of release of prothoracicotropic hormone (PTTH) from the brain in Rhodnius, one immediately following the blood meal and the second on day 5 or 6. The significance of these times of PTTH release is discussed in relation to classical evidence of the timing of moulting hormone action, the response of target tissues, and with more recent findings on the timing of release of neurosecretory material from the brain of Rhodnius during moulting.

Spontaneous and evoked release of prothoracicotropin from multiple neurohemal organs of the tobacco hornworm

Release of neurohormone from putative cephalic neurohemal organs was directly demonstrated in an insect. The prothoracicotropic hormone (PTTH) of the tobacco hornworm, Manduca sexta, was measured indirectly by its ability to stimulate the secretion of alpha-ecdysone by inactive prothoracic glands; the ecdysone was measured by radioimmunoassay. The PTTH released spontaneously from intact brain-retrocerebral complexes was localized to the retrocerebral complex by placing a waxy barrier across the nerves connecting the corpora cardiaca to the brain. Isolated corpora allata spontaneously released much more PTTH than did either isolated corpora cardiaca or isolated brains. Media containing 100 mM potassium stimulated PTTH release from both isolated corpora allata and isolated corpora cardiaca. In calcium-free media, spontaneous PTTH release was diminished and release could not be stimulated by high potassium. These results indicate that depolarization of the neurosecretory cells is correlated with calcium-dependent neurohormone release and that there are multiple neurohemal organs for PTTH. The biological activities of stored and circulating PTTH are compared.

Growth parameters associated with endocrine events in larval Trichoplusia ni (Hübner) and timing of these events with developmental markers

Numerous behavioural and morphological markers were found during the last instar of Trichoplusia ni which permit selection of highly synchronous groups of larvae for physiological or biochemical experiments. Growth parameters were also examined and it was found that the occurrence of a 6th instar was associated with a head-capsule width below the critical threshold of 1.66 mm. Starvation experiments indicated that the critical body weight triggering the first release of prothoracicotropic hormone (PTTH) was not the same for all larvae and was associated closely with a critical ratio of body size: head-capsule size and/or body size: initial body size at the beginning of the instar. The time of prothoracicotropic hormone-ecdysone release in preultimate instars was also associated with a similarly calculated ratio. The ratio was very similar from instar to instar. Neck or thoracic-abdominal ligation of larvae attaining various markers provided indications of times of release of critical amounts of prothoracicotropic hormone, ecdysone and juvenile hormone. The time of peak juvenile-hormone-esterase (JHE) activity in the haemolymph during the prepupal stage was determined with these markers.

Critical Periods in the Regulation of the Pupal Molt of the Tobacco Budworm, Heliothis Virescens12

Behavioral and morphological changes which occur during the last larval stage of Heliothis Virescens (Fabr.) have been used to establish five discrete phases within the instar. Using these criteria, critical periods for prothoracicotropic hormone (PTTH) and ecdysone release in the last instar were determined by percent molting after neck and thoracic ligations, respectively. We observed one critical period for PTTH release in animals reared at a nondiapause-inducing regimen (30°C, long day photoperiod) and four critical periods for PTTH release in animals reared at a diapause-inducing regimen (20°C, short day photoperiod). Only one critical period was observed for ecdysone release under both rearing conditions. In larvae with the thorax ligated, where the prothoracic glands were isolated from the posterior portion, 20-hydroxyecdysone injected into abdomens prior to the ecdysone critical period did not readily induce pupation, even at high doses.

Prothoracicotropic hormone in Manduca sexta: localization by a larval assay.

1. A new method for the assay of insect prothoracicotropic hormone (PTTH) is described, using fourth instar larvae of Manduca sexta. Larvae neck-ligated at a critical time to prevent release of PTTH from the head fail to undergo the next larval moult. Such ligated larvae moult to fifth instar larvae or larval-pupal intermediates after injection of brain homogenates from Manduca larvae, pupae or pharate adults. The degree of response is proportional to the concentration of brain homogenate injected. 2. The source of PTTH in the pupal brain is the dorsal region of the protocerebrum containing the lateral neurosecretory cells. Microhomogennates of single pieces of brain showed activity with this method. 3. PTTH activity in partially purified extracts is water soluable, stable to boiling for 10 min, and is destroyed by Pronase or trypsin.

Release of prothoracicotropic hormone and potentiation of developmental ability during diapause in the bollworm, Heliothis zea

In Heliothis zea, pupal diapause is not due to a deficiency of the prothoracicotropic hormone (PTTH), as it is in many other insects. However, PTTH is essential for diapause termination and adult development. Removal of the pupal brain 4 hr after larval-pupal ecdysis blocks the insect's ability to initiate adult development. Transplantation of brain neurosecretory cells restores this ability, whereas other tissues such as corpora allata have no effect. In the diapausing pupa, PTTH is released from the brain within 24 hr after larval-pupal ecdysis. Subsequent removal of the brain fails to block the ability for diapause termination, because PTTH potentiates the ability for adult development. Since diapause termination is suppressed in a temperature of 21°C, the bollworm retains the ability to initiate development in 27°C whereas it remains in diapause in 21°C. Diapause continues even though pupae are supplied with additional PTTH via neurosecretory cell transplantation. Ecdysone injection and prothoracic gland-ablation experiments indicate that the prothoracic glands are the source of the prohormone α-ecdysone, and that diapause is maintained by an α-ecdysone deficiency. This evidence, in conjunction with the above results, suggests that PTTH release potentiates prothoracic gland function in the diapausing pupa which is then regulated by a temperature dependent process.

Ecdysone as a feedback regulator for the neurosecretory brain cells in Mamestra brassicae

The effect of β-ecdysone injection or prothoracic gland removal on the neurosecretory brain cells in the cabbage armyworm, Mamestra brassicae was investigated. The prothoracic gland removal of the 6-day-old last instar larvae induced the release of stainable neurosecretory material from type II cells in the pars intercerebralis 24 hr after treatment. The injection of β-ecdysone into the prothoracic gland-extirpared larva stimulated both the synthesis of stainable material in the type II cells and transportation of stainable material from neurosecretory cells to axon tracts. These results suggest that ecdysone may have a rôle as a positive feedback regulator for the synthesis and release of prothoracicotropic hormone in M. brassicae brain.

Control of moulting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): growth of the last-instar larva and the decision to pupate.

ABSTRACT The dynamics of growth and the timing of release of the brain ‘s pro-thoracicotropic hormone (PTTH) in final instar larvae of Manduca sexta are consistent with the following hypothesis. When a 5th-stage larva reaches a critical weight of about 5 g an unidentified process is initiated which requires 24 h to be completed. At the completion of this process the brain is rendered competent to release PTTH. The actual release of PTTH is gated by the photoperiod and occurs when the gate opens during the very next photophase.

Control of moulting and metamorphosis in the tobacco hornworm, Manduca sexta (L.): cessation of juvenile hormone secretion as a trigger for pupation.

ABSTRACT During the final larval instar of the tobacco hornworm the presence of juvenile hormone (JH) inhibits the secretion of the brain ‘s prothoracicotropic hormone (PTTH). The corpora allata cease to secrete JH when the larvae attain a weight of approximately 5 g. The JH is cleared from the haemolymph in about 24 h. This process in itself renders the brain competent to release PTTH. The actual release of PTTH occurs at the very first photoperiodic gate after the JH has disappeared from the haemolymph. A functional failure of this normal mechanism is apparently responsible for the developmental standstill of Lepidoptera which diapause as mature larvae.