Elsevier

Theriogenology

Volume 60, Issue 5, 15 September 2003, Pages 933-941
Theriogenology

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

https://doi.org/10.1016/S0093-691X(03)00104-3Get rights and content

Abstract

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 α-subunit peptide advances the onset of puberty in ram lambs. St. Croix hair sheep ram lambs were assigned to inhibin-immunized (n=7) and control (n=8) treatment groups. Lambs in the inhibin-immunized group were immunized against a synthetic peptide-carrier protein conjugate, α-(1–25)-human α-globulin (hα-G), and control lambs were immunized against hα-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 ≥50×106 sperm cells. In control lambs, plasma α-(1–25)-antibody (Ab) was nondetectable. In inhibin-immunized lambs, α-(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 inhibin-immunized 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 putative inhibin α-subunit-related peptide that stimulates LH secretion in ram lambs. Present findings show that active immunization against an inhibin α-peptide delays rather than advances puberty in ram lambs.

Introduction

Development of a method to advance puberty in ram lambs may prove worthwhile by enabling younger males to be used in the breeding flock. Use of younger males may reduce production costs, accelerate benefits of genetic selection, and allow earlier progeny and libido testing. Benefits of advanced puberty in ram lambs may be accentuated in year-round breeding programs that can take advantage of development of early fertility. Active immunization against inhibin as a method to induce early and enhanced gametogenesis has been shown to advance puberty and to increase ovulation rate in ewe lambs [1], [2]. In ram lambs, however, Al-Obaidi et al. [1] reported that active immunization against inhibin did not advance puberty. The authors noted that observations may have been started too late to reveal differences in the first appearance of spermatozoa in ejaculates.

Different types of inhibin immunogens have been employed [3]. These include partially purified (PP) inhibin preparations derived from follicular fluid, recombinant DNA-produced inhibin α-subunit, and synthetic α-subunit peptides. Immunogens comprised of PP inhibin or a synthetic α-subunit peptide have been used successfully to advance puberty in ewe lambs [1], [2]. Synthetic α-subunit peptides afford purity, chemical consistency, and availability compared to the other types of immunogens.

The objective of the present study was to determine whether active immunization against an inhibin α-subunit peptide advances the onset of puberty in St. Croix hair sheep ram lambs in St. Croix, US Virgin Islands. At this location, most St. Croix ewes express estrus during each month of the year [4].

Section snippets

Materials and methods

St. Croix ram lambs born from November 12 to 19 were assigned alternately to control (n=8) and inhibin-immunized (n=7) treatment groups. Lambs assigned to the latter group were immunized against a synthetic peptide, α-(1–25), that matched in sequence the N-terminal 25 amino acids of the αC segment [5] of the ovine inhibin α-subunit (residues 233–257 as reported by Forage et al. [6]). The α-(1–25) peptide was coupled through a glycine–tyrosine C-terminal extension with human α-globulin (hα-G).

Results

Plasma samples collected from all ram lambs at 3 and 4 weeks of age and from control lambs throughout the study contained little if any α-(1–25)-Ab as binding of 125I-α-(1–25) was nondetectable with few exceptions. In inhibin-immunized lambs, α-(1–25)-Ab titer was detected consistently in the plasma sample collected 1 week following the second immunization, which was administered at 7 weeks of age. Thereafter, α-(1–25)-Ab titer increased to 25 weeks and then plateaued (Fig. 1). Each boost was

Discussion

Active immunization against an inhibin α-subunit peptide conjugate did not promote early or enhanced sperm output in St. Croix ram lambs. Ram lambs were immunized against α-(1–25)-hα-G beginning at 3 weeks of age. Plasma α-(1–25)-Ab titer was detectable within 1 week following the first booster immunization administered at 7 weeks of age. Thereafter, α-(1–25)-Ab titer increased and subsequently plateaued at a level that varied among rams. Characteristics of the immune response were similar to

Acknowledgements

We thank the National Hormone and Pituitary Program, the National Institute of Diabetes and Digestive and Kidney Diseases, and Dr. A.F. Parlow for purified hormones and antisera. Appreciation is expressed to Carla Dean, Bryan Pannagl, and William Gonzales for technical assistance. The study was supported in part by the Minnesota Agricultural Experiment Station and the North Central Regional Research Project NC-111.

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