title: The emergence of the cortisol circadian rhythm in monozygotic and dizygotic twin infants: the twin-pair synchrony.
abstract: OBJECTIVE
  Studies 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.
  
  
  METHODS
  A 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 (h(2)) were calculated.
  
  
  RESULTS
  The 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 (h(2)) was 0.21 (ns).
  
  
  CONCLUSIONS
  Salivary 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.
authors.affiliation: Department of Pediatrics, School of Medicine of Ribeirao Preto, University of Sao Paulo, Ribeirao Preto, Brazil.
authors.forename: Rodrigo Jose
authors.initials: RJ
authors.lastname: Custodio
authors.forename: Carlos Eduardo Martinelli
authors.initials: CE
authors.lastname: Junior
authors.forename: Soraya Lopes Sader
authors.initials: SL
authors.lastname: Milani
authors.forename: Aguinaldo Luis
authors.initials: AL
authors.lastname: Simões
authors.forename: Margaret
authors.initials: M
authors.lastname: de Castro
authors.forename: Ayrton Custodio
authors.initials: AC
authors.lastname: Moreira
mesh_terms: D000704:Analysis of Variance; D002657:Child Development / Q000502:physiology*; D002940:Circadian Rhythm / Q000235:genetics*; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000032:analysis*; D007231:Infant, Newborn; D016014:Linear Models; D008137:Longitudinal Studies; D012463:Saliva / Q000737:chemistry*; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic
chemical_list: D006854:Hydrocortisone