Daily changes in plasma prolactin, corticosterone, and luteinizing hormone in the unrestrained, ovariectomized turkey...

Plasma concentrations of prolactin (PRL), corticosterone (B), and luteinizing hormone (LH) were determined in hourly samples collected over a 24-h period from unrestrained, long-term ovariectomized (OVX) turkey hens maintained in cages in an isolation room on a 14 h light: 10 h dark photoperiod. Mean plasma concentrations of all three hormones were found to vary significantly with the time of sampling. Mean plasma PRL levels reached a peak (6.50 +/- 1.70 ng/mL, means +/- SEM) during the latter half of the photophase, declined to a nadir (5.22 +/- 1.20 ng/mL) prior to the onset of darkness, and then rose gradually during the scotophase to reach another peak (8.96 +/- 1.80 ng/mL) 1 h prior to lights on. Mean plasma LH levels rose significantly (P less than .001) from a nadir of 4.11 + .29 ng/mL prior to lights out to a peak of 4.74 + .34 ng/mL during the dark period. No evidence of random fluctuations in LH levels was observed in any individual, suggesting that episodic LH release may not occur in OVX turkey hens. Individual patterns of B secretion varied substantially so that, although significant increases and decreases in mean plasma B levels were observed, no clearly defined peaks in mean plasma B levels were identified. Individual secretory profiles revealed that peaks in PRL secretion often coincided with peaks in B secretion, but B peaks were usually more numerous. It is concluded that PRL and LH are secreted in a daily rhythm in the OVX turkey hen, with the highest levels of both hormones occurring during the scotophase.