Effect of immunization against melatonin on prolactin concentrations and the timing of reproductive transitions in ewes

The objective of this experiment was to develop a procedure for immunizing ewes against melatonin that would alter the effects of changing photoperiod on seasonal reproduction and prolactin secretion. Ewes were immunized agai nst human serum albumin (HSA) as controls (n = 9) or a melatonin-human seru m albumin conjugate (0.25 mg: n = 10) on December 14th (Day 0) and boosted 9 times. They were maintained on natural photoperiod and then transferred i ndoors and exposed to long days for 35 d, followed by short days for 146 d, long days for 93 d, and short days for a further 123 d. Antibody titers to melatonin (at a serum dilution of 1:1,250) were significantly higher in im munized ewes (27.3 +/- 6.6%) than controls (0.7 +/- 0.1%; P < 0.001). At th e end of the experiment, antibody titers in immunized ewes (at dilution of 1:50) were higher in blood (43.7 +/- 8.2%) than in cerebrospinal fluid (10. 8 +/- 3.9%; P < 0.05), and highly correlated (r(2) = 0.746). Onset of the b reeding season was advanced slightly after the second transfer from long to short days in immunized ewes (April 12 +/- 3 d) compared with controls (Ap ril 25 +/- 3 d; P < 0.05). Mean serum prolactin concentrations were lower ( P < 0.05) in melatonin-immunized ewes compared with controls on natural pho toperiod, after transfer from long to short days, during long days, and aft er the second transfer from long to short days. In conclusion, despite mela tonin-immunization increasing antibody titers in blood and cerebrospinal fl uid, and decreasing prolactin concentrations over much of the experiment, m inimal effects on the timing of reproductive transitions in the ewes were e vident. This discrepancy between the response of the prolactin and reproduc tive axes to melatonin immunization supports the hypothesis of a dual site of action of melatonin, with melatonin acting in the pituitary gland to med iate the effects of photoperiod on prolactin secretion and in the mediobasa l hypothalamus to affect reproductive responses. (C) Elsevier Science Inc. 1999.