PHOTOPERIODIC HISTORY AND HYPOTHALAMIC CONTROL OF PROLACTIN SECRETIONBEFORE BIRTH

We investigated whether the fetal lamb can construct a photoperiodic h istory in utero. We measured the fetal PRL response to a 12-h photoper iod in intact fetal sheep and in fetal sheep after hypothalamo-pituita ry 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 l ight (SL,n = 19) regimen from 57 days gestation until fetal HPD (pre-L L, 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 1 2-h photoperiod from surgery until 140 days gestation. In HPD fetal sh eep previously exposed to SL, fetal PRL concentrations were significan tly higher (P < 0.05) after 20 days in the 12-h L regimen than previou sly (0-5 days, 3.2 +/- 0.6 ng/ml; 21-25 days, 5.6 +/- 1.4 ng/ml). In t he HPD fetal sheep previously exposed to LL, however, fetal PRL concen trations significantly decreased (P < 0.06) after 5 days exposure to t he 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 < 0.001) in Fetal sheep previously exposed to either SL or LL. Fetal PRL concentrations were significantly highe r (P < 0.05) after 121 days gestation in the 12-h L regimen in all sha m fetal sheep (<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 mu g) was administered iv to all fetal sheep at 130-13 4 days gestation. There was a significant fetal PRL response to TRH in both the HPD (P = 20.9,P < 0.001) and sham if = 31.3, P < 0.001) grou ps. There was no difference, however, in the PRL response to TRH in fe tal sheep previously exposed to SL or LL in either the KPD or sham gro ups. The maximum percentage changes in PRL occurred at +10 min after T RH administration in the HPD (pre-SL, 421 +/- 75%; pre-LL, 555 +/- 76% ) and sham groups (pre-SL, 394 +/- 68% pre-LL, 369 +/- 59%). In summar y, therefore, we have demonstrated that there is an effect of photoper iodic history on the PRL response to an intermediate photoperiod in ut ero in HPD fetal sheep. It appears, however, that the effect of photop eriodic history on PRL secretion in intact fetal sheep is either maske d or suppressed by the stimulatory effect of factors associated with a n increase in gestational age acting at the fetal hypothalamus.