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Abstract
ObjectiveStudies on the influence of genetic factors on the ontogeny of cortisol circadian rhythm in infants are lacking. This study evaluated the influence of twinning and the heritability on the age of emergence of salivary cortisol rhythm.Design and subjectsA longitudinal study was performed using salivary samples obtained during morning and night, at 2, 4, 8, 12, 16, 20 and 24 weeks of postnatal life in 34 infants, 10 monozygotic (MZ) and 7 dizygotic (DZ) twin pairs. Salivary cortisol was determined by radioimmunoassay (RIA). Zigosity was verified by DNA analysis of at least 13 short tandem repeat polymorphisms. Difference of the emergence of cortisol circadian rhythm, within each twin pair, the intraclass correlation coefficient and the heritability index (h2) were calculated.ResultsThe mean (± SEM) age of emergence of salivary cortisol circadian rhythm was similar in MZ and DZ (7·8 ± 1·0 vs 7·4 ± 1·3 weeks). Seven pairs showed coincidence of the emergence of cortisol rhythm. Ten pairs were not coincident; among them the within-pair difference of emergence of salivary circadian rhythm was similar in both MZ and DZ groups. The intraclass correlation coefficients were rMZ = 0·60, P = 0·02; and rDZ = 0·65, P = 0·03, respectively. The heritability index (h2) was 0·21 (ns).ConclusionsSalivary circadian rhythm appeared at the same postnatal age in MZ and DZ twin infants. Although several physiological aspects might be involved, the heritability index, obtained in the present study, suggests less genetic than environmental impact on the age of the onset of the cortisol circadian rhythm. Our data also indicated that each twin-pair show synchrony because they probably shared prenatal and postnatal environmental synchronizers.
Highlights
The ACTH-corticosteroid rhythm is an endogenous rhythm regulated by the brain-adrenal neurohumoral circuit
There was no difference in clinical characteristics (Table 1) between MZ and DZ groups, except for a higher gestational age in MZ pairs (P = 0·01)
The circadian pattern emerged in MZ and DZ groups at
Summary
The ACTH-corticosteroid rhythm is an endogenous rhythm regulated by the brain-adrenal neurohumoral circuit. The major action of the environment is to synchronize the circadian rhythms by periodic factors, such as light–dark, rest–activity and sleep–wake cycles, food ingestion and social cues.[1,2,3] the effects of these synchronizers on circadian rhythm are superimposed on inherited characteristics of cortisol regulation. Salivary cortisol levels reflect free fraction of plasma cortisol concentrations. Saliva can be obtained at home, using a noninvasive and stress-free method. Salivary cortisol measurements represent a reliable tool to evaluate the hypothalamic-pituitary-adrenal (HPA) axis, including the cortisol circadian rhythm in infants and children.[4,5,6,7,8]
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