Neuroanatomical relations of prothoracicotropic hormone neurons with the circadian timekeeping system in the brain of larval and adult Rhodnius prolixus (Hemiptera)

May 31, 2007The Journal of comparative neurology

Brain connections between hormone-producing neurons and the internal clock in young and adult Rhodnius prolixus insects

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Abstract

The number of PTTH-producing neurons in Rhodnius prolixus increases from two in the last larval instar to five during larval-adult development.

PTTH neurons are localized in a single group in the lateral protocerebrum of the larvae.
A daily rhythm in the content of PTTH-positive material is observed in both larval and adult stages.
The rhythms of PTTH release correlate with evidence of circadian rhythms in brain samples.
Eight key clock neurons co-express pigment-dispersing factor and canonical clock proteins, influencing behavioral rhythms.
Intimate associations between PTTH neuron axons and clock neurons suggest a pathway for circadian control of hormone release.

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This paper reports the localization in the Rhodnius prolixus brain of neurons producing the key neuropeptide that regulates insect development, prothoracicotropic hormone (PTTH) and describes intimate associations of the PTTH neurons with the brain circadian timekeeping system. Immunohistochemistry and confocal laser scanning microscopy revealed that the PTTH-positive neurons in larvae are located in a single group in the lateral protocerebrum. Their number increases from two in the last larval instar to five during larval-adult development. In adults, there are two distinct groups of these neurons composed of two cells each. A daily rhythm in content of PTTH-positive material occurs in both the somata and the axons in both larval and adult stages. These rhythms correlate with previous evidence of a circadian rhythm of PTTH release from brains in vitro. The key circadian clock cells of Rhodnius are eight neurons, which co-express pigment-dispersing factor (PDF) and the canonical clock proteins PER and TIM; PDF fills the axons. Equivalent cells control behavioral rhythms in other insects. Double labeling revealed intimate associations between axons of larval PTTH neurons and clock neurons, indicating a neuronal pathway from the brain timekeeping system for circadian control of PTTH release. Additional PDF neurons appear in the adult, associated with the second group of PTTH neurons. These findings provide the first direct evidence that neurons of the insect brain timekeeping system control hormone rhythms. The range of functions regulated by this timekeeping system is quite similar to those of the vertebrate suprachiasmatic nucleus, for which the insect system is a valuable model.

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Neuroanatomical relations of prothoracicotropic hormone neurons with the circadian timekeeping system in the brain of larval and adult Rhodnius prolixus (Hemiptera)

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