No abstract

We have studied the effects of independently altering the time of darkness and the phase of the daily melatonin rhythm during a 12-h photoperiod on the diurnal rhythms of fetal breathing movements (FBMs), low voltage electrocortical activity, and maternal and fetal plasma concentrations of PRL. Thirteen pregnant ewes were pinealectomized at 99-100 days gestation and held under a normal lighting regime (12-h photoperiod, lights off at 1900 h) until 135 days gestation. All ewes were then exposed to an altered lighting regime (12-h photoperiod, lights off at 1100 h) until 145 days gestation. Six of these ewes were infused with melatonin (12-15 micrograms/h iv) from 1100 h for 12 h each day between 125 and 135 days gestation while in the normal lighting regime, and from 1900 h while in the altered lighting regime. The remaining seven ewes were infused with saline at corresponding times. Infusion of melatonin produced a significant daily rhythm in maternal and fetal melatonin concentrations; concentrations were highest throughout the 12-h infusion period in each lighting regime. In the normal lighting regime (i.e. lights off at 1900 h) the peak incidence of FBM occurred at between 1000 and 1300 h in the ewes infused with saline or melatonin. Under the altered lighting conditions there was no 24-h variation in the incidence of FBMs in the saline-infused group, but a significant daily variation was present in the melatonin-infused group with peak incidence occurring at 1900-2200 h. The daily variation in maternal PRL concentrations was not affected by changes in the time of onset of melatonin infusion but was different in each of the two lighting regimes. In the normal lighting regime, PRL concentrations were highest at 2100 h (56.2 +/- 21.6 ng/ml) whereas in the altered lighting conditions the peak in maternal PRL occurred at 1700 h (90.4 +/- 25.1 ng/ml). The daily variation in fetal PRL concentrations was also not affected by changes in the time of onset of melatonin infusion but was different in the two lighting regimes; in the normal lighting regime, fetal PRL reached a peak (13.0 +/- 3.7 ng/ml) at 0100-0300 h whereas in the altered lighting regime the peak (29.2 +/- 9.8 ng/ml) occurred at 1700-2100 h. We have presented evidence therefore that the daily melatonin rhythm may entrain the daily rhythm in fetal breathing activity.(ABSTRACT TRUNCATED AT 400 WORDS)

We have investigated the effect of surgical disconnection of the fetal hypothalamus and pituitary (HPD) on generation of the daily rhythm in fetal plasma melatonin and PRL concentrations under long and short photoperiods. Fetal HPD or a sham operation was carried out at around 110 days gestation. Ewes carrying either HPD fetal sheep (n = 10) or intact fetal sheep (n = 12) were exposed to a long light (LL; 16 h of light and 8 h of darkness) or a short light (SL; 8 h of light and 16 h of darkness) regimen for the remainder of gestation. All ewes were subjected to a 24-h blood-sampling experiment (13 samples collected between 0900-0900 h the following day) between 135-140 days gestation, and fetal and maternal plasma melatonin and PRL concentrations were measured using specific RIAs. The hormonal data were analyzed using multifactorial analysis of variance and cosinor analysis. There was an increase in maternal melatonin concentrations during the dark phase in each lighting regimen in ewes carrying HPD or intact fetal sheep. In the SL regimen, there was also a significant increase in fetal melatonin concentrations during the dark phase in the HPD and intact groups. Under LL conditions, however, fetal melatonin concentrations were only consistently increased during the dark phase in the intact, not the HPD, group. The 24-h mean fetal plasma concentrations of PRL were significantly higher (P < 0.001) in both intact and HPD fetuses in the LL (intact, 111.0 +/- 22.0 pg/ml; HPD, 37.6 +/- 7.3 pg/ml) than in the SL regimen (intact, 37.8 +/- 18.4 pg/ml; HPD, 6.7 +/- 4.3 pg/ml). There was also a significant interaction (P < 0.001) between the effects of fetal surgical treatment and time of day on fetal PRL concentrations. In the intact group, fetal PRL concentrations were significantly higher (P < 0.05) at 1300 and 1700 h than between 0300-0700 h in both lighting conditions. Cosinor analysis also identified a significant rhythm in 8 of the 12 fetal PRL profiles in the intact group. In contrast, in the HPD group, there was no significant effect of time of day on fetal PRL in either the LL or SL regimen, and cosinor analysis only identified a significant rhythm in 2 of the 10 fetal PRL profiles in this group. We have, therefore, demonstrated that in the fetal sheep, HPD resulted in abolition of the diurnal melatonin rhythm under LL conditions and in the loss of the diurnal PRL rhythm under LL and SL conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

To evaluate the hypothalamic contribution to the development of anterior pituitary (AP) cells we surgically disconnected the hypothalamus from the pituitary (hypothalamo-pituitary disconnection, HPD) in fetal sheep and collected pituitaries 31 days later. Pituitaries (n = 6 per group) were obtained from fetal sheep (term = 147 ± 3 days) at 110 days (unoperated group) of gestation and at 141 days from animals that had undergone HPD or sham surgery at 110 days. Cells were identified by labelling pituitary sections with antisera against the six AP hormones. Additionally, we investigated the colocalization of glycoprotein hormones. The proportions of somatotrophs and corticotrophs were unchanged by age or HPD. Lactotrophs increased 80% over time, but the proportion was unaffected by HPD. Thyrotrophs, which were unaffected by age, increased 70% following HPD. Gonadotrophs increased with gestational age (LH+ cells 55%; FSH+ cells 19-fold), but this was severely attenuated by HPD. We investigated the possible existence of a reciprocal effect of HPD on multipotential glycoprotein-expressing cells. Co-expression of LH and TSH was extremely rare (< 1%) and unchanged over the last month of gestation or HPD. The increase of gonadotrophs expressing FSH only or LH and FSH was attenuated by HPD. Therefore, the proportions of somatotrophs, lactotrophs and corticotrophs are regulated independently of hypothalamic input in the late gestation fetal pituitary. In marked contrast, the determination of the thyrotroph and gonadotroph lineages over the same time period is subject to complex mechanisms involving hypothalamic factors, which inhibit differentiation and/or proliferation of thyrotrophs, but stimulate gonadotrophs down the FSH lineage. Development of a distinct population of gonadotrophs, expressing only LH, appears to be subject to alternative mechanisms. The anterior pituitary is a highly heterogeneous organ composed of a variety of cells located throughout the gland. The process by which the individual secretory cells differentiate and proliferate during development is largely unknown. Considerable progress has been made in understanding the role played by transcription factors, but we still know relatively little about the input of the fetal hypothalamus in pituitary cell development. After birth, hypothalamic factors are critical in regulating anterior pituitary (AP) function, but the role played before birth remains largely unknown.Recent studies suggest that the timely expression of various transcription factors is wholly responsible for the composition of the AP, independent of the hypothalamus (Dasen & Rosenfeld, 1999Melmed, 2003). A number of experiments in genetically engineered mice are consistent with this view. For example, corticotrophin-releasing hormone (CRH) receptor knockout mice (Timpl et al. 1998) and thyrotrophin-releasing hormone (TRH) knockout mice have a normal complement of all pituitary cells (Shibusawa et al. 2000), clearly suggesting that these two hypothalamic factors are not required for normal...

We have investigated the effect of surgical disconnection of the fetal hypothalamus and pituitary (HPD) on generation of the daily rhythm in fetal plasma melatonin and PRL concentrations under long and short photoperiods. Fetal HPD or a sham operation was carried out at around 110 days gestation. Ewes carrying either HPD fetal sheep (n = 10) or intact fetal sheep (n = 12) were exposed to a long light (LL; 16 h of light and 8 h of darkness) or a short light (SL; 8 h of light and 16 h of darkness) regimen for the remainder of gestation. All ewes were subjected to a 24-h blood-sampling experiment (13 samples collected between 0900-0900 h the following day) between 135-140 days gestation, and fetal and maternal plasma melatonin and PRL concentrations were measured using specific RIAs. The hormonal data were analyzed using multifactorial analysis of variance and cosinor analysis. There was an increase in maternal melatonin concentrations during the dark phase in each lighting regimen in ewes carrying HPD or intact fetal sheep. In the SL regimen, there was also a significant increase in fetal melatonin concentrations during the dark phase in the HPD and intact groups. Under LL conditions, however, fetal melatonin concentrations were only consistently increased during the dark phase in the intact, not the HPD, group. The 24-h mean fetal plasma concentrations of PRL were significantly higher (P < 0.001) in both intact and HPD fetuses in the LL (intact, 111.0 +/- 22.0 pg/ml; HPD, 37.6 +/- 7.3 pg/ml) than in the SL regimen (intact, 37.8 +/- 18.4 pg/ml; HPD, 6.7 +/- 4.3 pg/ml). There was also a significant interaction (P < 0.001) between the effects of fetal surgical treatment and time of day on fetal PRL concentrations. In the intact group, fetal PRL concentrations were significantly higher (P < 0.05) at 1300 and 1700 h than between 0300-0700 h in both lighting conditions. Cosinor analysis also identified a significant rhythm in 8 of the 12 fetal PRL profiles in the intact group. In contrast, in the HPD group, there was no significant effect of time of day on fetal PRL in either the LL or SL regimen, and cosinor analysis only identified a significant rhythm in 2 of the 10 fetal PRL profiles in this group. We have, therefore, demonstrated that in the fetal sheep, HPD resulted in abolition of the diurnal melatonin rhythm under LL conditions and in the loss of the diurnal PRL rhythm under LL and SL conditions.(ABSTRACT TRUNCATED AT 400 WORDS)