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Response of prolactin to different photoperiods after surgical disconnection of the hypothalamus and pituitary in sheep fetuses

The aim of this study was to determine the impact of surgical disconnection of the fetal hypothalamus and pituitary on the fetal prolactin response to different photoperiods. Disconnection of the hypothalamus (HPD) or a sham operation was carried out at around day 110 of gestation (term = 145 ± 3 days). Before surgery, pregnant ewes were maintained under a photoperiod of 12 h light:12 h dark. After surgery, ewes carrying HPD fetuses (n = 10) or intact fetuses (n = 13) were exposed to either a long day (16 h light:8 h dark) or a short day (8 h light:16 h dark) regimen until day 143 of gestation. Thyrotrophin-releasing hormone (50 µg) was administered intrafetally at days 130–135 of gestation and chlorpromazine (a dopaminergic antagonist) was administered to all sheep fetuses at days 141–142 of gestation. Mean fetal prolactin concentrations were significantly higher in the long day group (HPD: 37.3 ± 11.3 ng ml^–1; intact: 71.0 ± 16.2 ng ml^–1) than in the short day group (HPD: 9.0 ± 4.8 ng ml^–1; intact: 34.2 ± 16.0 ng ml^–1). In the intact group, fetal prolactin concentrations increased significantly between day 6 and day 30 of exposure to either photoperiod. However, in the HPD group, fetal prolactin increased with increasing exposure to the long day photoperiod and decreased with increasing exposure to the short day photoperiod. The peak percentage increases in prolactin after administration of thyrotrophin-releasing hormone were significantly higher in the short day group (HPD: 696 ± 195%; intact: 521 ± 86%) than in the long day group (HPD: 211 ± 13%; intact: 224 ± 26%). There were no differences, however, between the intact and HPD groups in the percentage responses to thyrotrophin releasing hormone in either lighting regimen. In the intact group, the peak increase in prolactin occurred at 20 min after chlorpromazine in the short day group and at 40 min after chlorpromazine in the long day group. In the HPD group, there was no significant change in prolactin after chlorpromazine administration in either lighting regimen. In ewes carrying intact fetuses, maternal prolactin concentrations were significantly higher under the long day regimen than under the short day regimen. In the ewes carrying HPD fetuses, however, there was no consistent difference between maternal prolactin concentrations under either lighting regimen during the entire study period. In summary, we have demonstrated that there is a prolactin response to external photoperiod in sheep fetuses in which the hypothalamic–pituitary axis is either intact or surgically disconnected. The prolactin response is, therefore, unlikely to be entirely mediated by the inhibitory dopaminergic innervation of the pars distalis or median eminence.

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