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We investigated whether the fetal lamb can construct a photoperiodic history in utero. We measured the fetal PRL response to a 12-h photoperiod in intact fetal sheep and in fetal sheep after hypothalamo-pituitary disconnection (HPD), following exposure of the ewe to either a long (16 h L) or short (8 h L) photoperiod for 50 days in early pregnancy. Ewes were maintained on either a long light (LL, n = 20) or a short light (SL, n = 19) regimen from 57 days gestation until fetal HPD (pre-LL, n = 7; pre-SL, n = 7) or sham surgery (pre-LL, n = 13; pre-SL, n = 12) was performed at 99-113 days gestation. All ewes were housed in a 12-h photoperiod from surgery until 140 days gestation. In HPD fetal sheep previously exposed to SL, fetal PRL concentrations were significantly higher (P &lt; 0.05) after 20 days in the 12-h L regimen than previously (0-5 days, 3.2 +/- 0.6 ng/ml; 21-25 days, 5.6 +/- 1.4 ng/ml). In the HPD fetal sheep previously exposed to LL, however, fetal PRL concentrations significantly decreased (P &lt; 0.05) after 5 days exposure to the 12-h L regimen (6.7 +/- 2.9 ng/ml) and remained low throughout the remaining study period (31-35 days, 1.7 +/- 0.5 ng/ml). In contrast, in the sham group there was no effect of photoperiodic history on the gestational age profile of fetal PRL, and PRL concentrations increased significantly (F = 22.4, P &lt; 0.001) in fetal sheep previously exposed to either SL or LL. Fetal PRL concentrations were significantly higher (P &lt; 0.05) after 121 days gestation in the 12-h L regimen in all sham fetal sheep (&lt;110 days, pre-SL 6.4 +/- 0.3 ng/ml, pre-LL 12.0 +/- 3.3 ng/ml; 121-125 days, pre-SL 20.0 +/- 3.9 ng/ml, pre-LL 25.9 +/- 4.4 ng/ml). TRH (50 microg) was administered i.v. to all fetal sheep at 130-134 days gestation. There was a significant fetal PRL response to TRH in both the HPD (F = 20.9, P &lt; 0.001) and sham (F = 31.3, P &lt; 0.001) groups. There was no difference, however, in the PRL response to TRH in fetal sheep previously exposed to SL or LL in either the HPD or sham groups. The maximum percentage changes in PRL occurred at +10 min after TRH administration in the HPD (pre-SL, 421 +/- 75%; pre-LL, 555 +/- 76%) and sham groups (pre-SL, 394 +/- 68%; pre-LL, 369 +/- 59%). In summary, therefore, we have demonstrated that there is an effect of photoperiodic history on the PRL response to an intermediate photoperiod in utero in HPD fetal sheep. It appears, however, that the effect of photoperiodic history on PRL secretion in intact fetal sheep is either masked or suppressed by the stimulatory effect of factors associated with an increase in gestational age acting at the fetal hypothalamus.

Photoperiodic History and Hypothalamic Control of Prolactin Secretion Before Birth*