ACTIVE IMMUNIZATION AGAINST VASOACTIVE-INTESTINAL-PEPTIDE PREVENTS PHOTOINDUCED PROLACTIN SECRETION IN TURKEYS

Photostimulation initiates and maintains the rise in circulating prola ctin (PRL) observed during the reproductive cycle of the female turkey . Vasoactive intestinal peptide (VIP) is the principal PRL-releasing f atter. This study tested the hypothesis that gonadal stimulatory photo periods stimulate PRL secretion by releasing hypothalamic VIP. Therefo re, an experiment was designed to determine if VIP immunoneutralizatio n altered photoinduced PRL secretion, Reproductively quiescent female turkeys were divided into two groups comprising turkeys actively immun ized with synthetic VIP conjugated to keyhole limpet hemocyanin (VIP-K LH; immunized; n = 48) or KLH alone (control; n = 48). The first immun ization was administered 6 weeks before photostimulation. Blood sample s were collected at frequent intervals prior to and following photosti mulation, and plasma PRL concentrations were determined, Vasoactive in testinal peptide antibody titer was estimated from the percentage of I -125-labeled VIP bound to plasma diluted 1:1000. At the onset of photo stimulation (Day 0), plasma PRL levels were similar for immunized and control turkeys (9.1 +/- 0.3 versus 8.9 +/- 0.3 ng/ml, respectively). Plasma PRL of control birds increased (P < 0.05) by Day 16 of photosti mulation, reaching a peak value of 724.9 +/- 90.1 ng/ml on Day 84. In contrast, plasma PRL remained essentially unchanged in immunized birds , Titer of anti-VIP antibodies expressed as I-125-VIP bound by plasma in immunized birds was 10.9 +/- 1.5% on the day of photostimulation, I ncubation behavior was blocked in immunized birds, whereas 75% of cont rols exhibited incubation behavior. The control group laid 1.83 eggs/w eek/hen compared to 3.40 egg/week/hen in immunized hens. These finding s suggest that photoperiodic modulation of PRL secretion in the turkey is influenced by hypothalamic VIP neuronal system. (C) 1996 Academic Press, Inc.