Effect of Reproductive Condition on Luteinizing Hormone and Prolactin Release Induced by Electrical Stimulation of the Turkey Hypothalamus

In the turkey, the onset or incubation behavior is associated with decreased luteinizing hormone (LH) and increased prolactin (PRL). This study was designed to clarify the contribution of the hypothalamus and the anterior pituitary to the changes in plasma LH during the reproductive cycle of the turkey. Plasma LH and PRL were measured in anesthetized turkey before, during, and after electrical stimulation in the median eminence. In one experiment, luteinizing hormone releasing hormone (LHRH; 4 μg/kg) was injected intramuscularly 30 min after termination of electrical stimulation, and blood samples were obtained 5, 10, 20, and 30 min after injection. Electrical stimulation in the median eminence significantly increased (P < 0.05) plasma LH of laying (LAY), nest-deprived, previously incubating (NEST DEP), and photorefractory (REFRAC) hens, but not of photosensitive short-day (SHORT DAY) birds (P > 0.05). Plasma LH of LAY hens peaked at 4.06 ± 0.78 ng/ml from a prestimulation baseline or 2.30 ± 0.21 ng/ml and that in NEST DEP birds increased from 1.08 ± 0.18 ng/ml to 2.57 ± 0.53 ng/ml. Administration of LHRH increased plasma LH levels in SHORT DAY, LAY, and NEST DEP hens with the increase being 2.0-. 2.5-, and 6.1-fold, respectively. Electrical stimulation in the median eminence increased plasma PRL (P < 0.05) in all the reproductive groups tested, with peak response being greatest for NEST DEP birds (661 ± 126 ng/ml followed by LAY (317 ± 26 ng/ml). REFRAC (50 ± 7 ng/ml. and SHORT DAY (39 ± 12 ng/ml) hens. These results are interpreted to indicate that the reduced plasma LH levels in NEST DEP hens appear to be due to functional limitations of central neural mechanisms afferent to the median eminence and not to the inability of either median eminence or anterior pituitary to respond.