SummarySprague-Dawley rats were sacrificed by decapitation at 5,7, 12, 14,22, 26, 32, and 40 days of age. Adult animals (175 to 225 g) were also studied. Serum-free thyroxine (FT4) concentrations rose rapidly between 5 and 12 days to levels similar to adult concentrations, whereas the percentage of FT4 was relatively high between 5 and 12 days before declining to adult values by 14 days. Serumfree triiodothyronine (FT3) concentrations rose progressively to attain peak concentrations at 26 days and subsequently declined to adult levels by 40 days. The percentage FT3 rose in parallel with the FT3 concentrations to peak values at 22 to 26 days before declining to adult levels. FT4/thyroid-stimulating hormone (TSH) and FT3/TSH ratios increased progressively through 22 days of age in parallel with the FT3/FT4 ratio. These data indicate that free thyroid hormone concentrations follow essentially the same developmental profile as do total thyroid hormone concentrations. Progressive maturation of the negative feedback control mechanism for the pituitary-thyroid axis, as assessed by the FTUTSH and FT3/TSH ratios, occurs through 14 days. However, the continued rise in FT3 concentrations, FT3/TSH, and FT3/FT4 ratios through 26 days suggests a further resetting of the setpoint of the pituitary-thyroid axis possibly related to the stress of weaning. SpeculationMaturation of the pituitary-thyroid axis in the rat is principally a postnatal phenomenon. Careful study of the developmental profile of this axis under physiologic and pathologic states may afford considerable insight into the ontogenesis of the pituitarythyroid axis in the human fetus.The hypothalamo-pituitary-thyroid axis develops during the neonatal period in the rat (6). A recent report from this laboratory has documented the ontogenetic of thyroxine (T4) and triiodothvronine (T3) kinetics from birth to adulthood in this species (i). This stud; indicated that although T4 concentrations achieve adult levels by 12 days, there is a delay of approximately 12 days before peak circulating T3 concentrations are achieved. Although data are available for the ontogenesis of circulating free thryoid hormone concentrations in the human (7,10, 11,14) and ovine fetus (5, 9), little is known of the maturational processes by which negative feedback control of the pituitary-thyroid axis is controlled. Because free thyroid hormone concentrations in serum reflect more adequately the availability of active hormones at the tissue level, we undertook a study of the free concentrations of both T4 and T3 in the rat during postnatal development. MATERIALS AND METHODSVirgin Sprague-Dawley rats were mated at the supply house (Canadian Breeding Farms, St. Constant, Laprairie, Canada) and shipped 14 to 16 days later. Pregnant animals were housed in separate cages with free access to rat laboratory chow and water in a controlled (14 hr light) room. Humidity was not controlled.Animals were sacrificed by decapitation between 9:00 and 11:OO a.m. at 5, 7, 12, 14, 22, 26, 32, and 40 days of age. Ad...

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1977. The development of the hypothalamo-pituitary axis in the neonatal rat: sexual maturation in male and female rats as assessed by hypothalamic LHRH and pituitary and serum L H and FSH concentrations. Can. J. Physiol. Pharmaeol. 55, 109 1-1097.Using specific radioimmltnoassay techniques, we have measured hypothalamic LHRH concentration and pituitary and serum LH and FSH concentrations in neonatal rats from 0 to 60 days after birth. There were no sex differences demonstrable for hypothalamic EHRH concentration, which rose from minimal values at 2 days to peak concentrations at 22-28 days and declined significantly with the approach of puberty. Pituitary LH concentration in the two sexes rose to peak values at 19-25 days, with significantly higher values observed in females ( p < 0.01 ). Serum L H concentration was high in both sexes at birth, the values in females being significantly higher than those observed in males ( p < 0.01 ). While the serum E H concentration remained relatively stable throughout the study period in males, it declined rapidly to a nadir at 28 days in females. Pituitary FSW concentration was low at birth in both sexes. Females demonstrated a marked rise to peak concentrations at 16 days while males achieved significantly lower peak preplilberal values ( p < 0.01) at 28 days. Serum FSH concentration was significantly higher in females ( p < 0.01) at birth and rose further to peak values at 16 days. Males attained peak serum FSH concentrations at 35 days. These data demonstrate that the maturation of the hypothalan~o-pituitary-gonadal axis develops in the neonatal period in the rat and confirm the presence of significant differences in hypothalamic sexual differentiation and control. The possibility of this developmental process in the neonatal rat as a model for the study of the midgestational hypothalamic maturation of the human foetus is discussed. BUSSAUH.T, J. H.. WALKER, P., DUROIS, J. D. et LABRIE, F. 1977. The development of the hypothalamo-pituitary axis in the neonatal rat: sexual maturation in male and female rats as assessed by hypothalamic L H R H and pituitary and serum L H and FSH concentrations. Can. J. Physiol. Pharmacol. 55, 1091-1097.A l'aide de techniques sptcifiques de dosage radioimmunologique, nous avons mesurt la concentration hypothalamique de LHRH, ainsi que les concentrations pituitaires et striques de L H et de FSH chez des rats nouveaux-nCs de 0 B 60 jours aprks la naissance, On ne peut mettre en Cvidence de diffkrences significatives, selon le sexe. pour les concentrations hypothalamiq~ies de L H R H q i~i s'elevent de valeurs minimales i 2 jours jusqu'i des concentrations maximales B 22-28 jours et diminuent de fagon significative i I'approche de la puberth. La concentration de LH pituitaire chez les deux sexes s'eleve B des valeurs maximales 2 19-25 jours avec des valeurs significativement plus Clevkes chez les femelles (p < 0.81). Les concentrations driques de LH demeurent relativement stables chez les mfles au cours de la p6riode d'dtude, mais elles diminue...

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