Plasma LH, FSH, testosterone, and age at puberty in ram lambs actively immunized against an inhibin α-subunit peptide

Active immunization against inhibin has been shown to advance puberty and increase ovulation rate in ewe lambs; but in ram lambs, effects on puberty and sperm production are equivocal. The objective of the present study was to determine whether active immunization against an inhibin asubunit peptide advances the onset of puberty in ram lambs. St. Croix hair sheep ram lambs were assigned to inhibin-immunized (n ¼ 7) and control (n ¼ 8) treatment groups. Lambs in the inhibinimmunized group were immunized against a synthetic peptide-carrier protein conjugate, a-(1–25)human a-globulin (ha-G), and control lambs were immunized against ha-G. Lambs were immunized at 3, 7, 13, 19, 25, 31, and 37 weeks of age. On the day of immunization a blood sample was collected and lambs were weighed. Another blood sample was collected 1 week following each immunization. At 20 weeks of age additional blood samples were collected at 20 min intervals for 8 h. Beginning at 20 weeks of age and at weekly intervals thereafter, scrotal circumference (SC) was measured and semen was collected using electroejaculation. A subsequent ejaculate was collected 1 week following onset of puberty, which was defined as the week of age when an ejaculate first contained � 50 � 10 6 sperm cells. In control lambs, plasma a-(1–25)-antibody (Ab) was nondetectable. In inhibinimmunized lambs, a-(1–25)-Ab titer increased from 7 to 25 weeks of age and then plateaued at a level that varied (P < 0:001) among animals. Body weight and SC of control and inhibinimmunized lambs were similar at the onset of puberty. At pubertal onset inhibin-immunized lambs were older than control lambs (31:9 � 0:5 vs. 29:5 � 0:7 weeks of age, P < 0:05). Plasma FSH concentrations were similar in control and inhibin-immunized lambs from 3 to 38 weeks of age. Plasma LH levels were lower (P < 0:01) in inhibin-immunized than control lambs. During the 8-h blood sampling period at 20 weeks of age, LH and testosterone concentrations were lower (P < 0:05) in inhibin-immunized than control ram lambs, and the LH pulse frequency was similar in the two groups of animals. The decreased LH secretion is consistent with the immunoneutralization of a