Hypothalamic Neuropeptide Y and Its Gene Expression: Relation to Light/Dark Cycle and Circulating Corticosterone

Jun 1, 1994Molecular and cellular neurosciences

Brain hunger signal and its gene activity linked to daily light cycle and stress hormone levels

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Abstract

NPY mRNA levels in the mediobasal hypothalamus vary significantly in relation to the light/dark cycle, demonstrating a sharp rise 4-6 hours before dark onset.

NPY mRNA in the mediobasal hypothalamus peaks 4-6 hours before dark onset, followed by high levels for 3-4 hours and a sharp decline 1 hour before.
At dark onset, NPY peptide levels in the mediodorsal hypothalamus increase as levels in the mediobasal hypothalamus decrease.
Circulating corticosterone levels rise from baseline (<0.3 microgram/dl) to a peak of 26 micrograms/dl around dark onset.
Corticosterone levels show a positive relationship with NPY mRNA and peptide levels in the mediobasal hypothalamus before dark onset.
In the mediodorsal hypothalamus, a positive relationship between corticosterone and NPY peptide levels occurs at dark onset.

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The hypothalamic neuropeptide Y (NPY) system, along with levels of circulating corticosterone (CORT), were examined in rats at different times across the light/dark cycle. Tissue samples were taken from the mediobasal hypothalamus (MBH), which contains the primary hypothalamic NPY cell group of the arcuate nucleus (ARC), and the mediodorsal (MDH) hypothalamus, which contains the paraventricular and dorsomedial nuclei that receive a dense NPY innervation from the ARC. In these dissections, measurements of NPY mRNA and peptide levels were taken using a solution hybridization/nuclease protection assay procedure and radioimmunoassay. The results demonstrate that (i) NPY mRNA levels in the MBH, but not MDH, vary significantly in relation to the light/dark cycle, showing a sharp rise 4-6 h before dark onset, sustained high levels over the next 3-4 h and then, a sharp decline 1 h before dark onset; (ii) this rise in NPY mRNA in the MBH before dark onset, while associated with stable levels of MBH NPY during this time, is followed 2-4 h later, around dark onset, by a rise in NPY peptide levels of the MDH simultaneous to a decrease in NPY levels of the MBH; (iii) levels of circulating CORT shift dramatically across the light-dark cycle, exhibiting an increase from basal levels (< 0.3 microgram/dl) to 5 micrograms/dl approximately 4 h before dark onset, a further rise that peaks at 26 micrograms/dl around dark onset, and then a significant decline to 16 micrograms/dl at 2 h after dark onset; and (iv) there exists a positive relationship between CORT and NPY mRNA or peptide levels in the MBH during the 4-6 h before dark onset, while in the MDH, a positive relationship between this steroid and NPY peptide levels is obtained at dark onset. It is proposed that these rhythms, involving a predark rise in CORT and NPY gene expression leading to a peak in CORT and peptide levels at dark onset, are active in stimulating feeding behavior, particularly carbohydrate ingestion, which predominates at that time.

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Hypothalamic Neuropeptide Y and Its Gene Expression: Relation to Light/Dark Cycle and Circulating Corticosterone

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