Pattern of prolactin secretion and pituitary responsiveness to LRH in pseudopregnant rats maintained in constant light

Secretion of LH in response to an LRH infusion (104 ng/h during 21 h), secretion of progesterone (P) and 20 α-dihydroprogesterone (DHP), as well as the 24 h rhythm of prolactin (Prl) secretion were investigated in rats rendered persistently oestrous by exposure to constant light and in which subsequently pseudopregnancy (PSP) had been induced by cervical stimulation after induction of ovulation by hCG. Similar persistently oestrous rats, but otherwise untreated, served as controls. LRH was infused through an intra-jugular cannula and blood samples for assay of LH were taken via an intra-carotid cannula. LH responses were judged on the basis of the mean maximal height (MH; ng LH ml plasma) and the rate of decrease or half life (t½ of the plasma LH concentrations after 2 h of LRH infusion. For assay of P and DHP blood samples were taken from the orbitalplexus. In another series of animals blood, for assay of Prl, was sampled every 2 h for 24 h from a cannula with the tip in the right atrium. It was observed that in persistently oestrous rats LH-responses were high (MH = 3326 ± 305) and steep (t½ = 65 ± 10 min). On the day after the night of ovulation (day 0 of PSP) and on days 1, 5 and 9 of PSP the responses were lower (MH between 1315 and 1041) and more blunted (t½ varied between 236 and 142 min), whilst on day 12 of PSP they were again higher (MH = 4416 ± 575) and steeper (t½ = 101 ± 12 min). P and DHP concentrations were low in persistently oestrous rats and high during days 5 and 8 of PSP. On day 12 of PSP P concentrations were low again, while the DHP concentrations had increased further. In persistently oestrous rats the plasma Prl levels were low with incidental elevations (up to 141 ng Prl/ml aAntoni van Leeuwenhoekhuis The Netherlands Cancer Institute, Amsterdam, The Netherlands plasma). During days 8 – 9 of PSP Prl was secreted at irregular intervals (1 to 3 peaks/24 h) with peak levels of up to 400 ng Prl/ml plasma. On days 11 – 12 of PSP, Prl peaks were lacking almost completely: only a few minor elevations (up to 41 ng Prl/ml plasma) were measured. It is concluded that during days 0 to 10 of pseudopregnancy under constant light the endocrine state of the animals (in terms of LRH-responsiveness and P and DHP secretion) differs significantly form that during the previous and the following days. As similar changes are observed in PSP rats maintained under standard lighting conditions, it is apparent that the lighting conditions do not influence these changes. The pattern of Prl secretion during PSP, on the other hand is affected by the lighting conditions.