Individual return to Leydig cell function after GnRH-immunization of boars

Area of Science:

• Veterinary Immunology
• Reproductive Endocrinology
• Animal Science

Background:

• Immunization against gonadotropin-releasing hormone (GnRH) effectively suppresses testicular steroid synthesis in boars, reducing androstenone (boar taint).
• The duration of Leydig cell function suppression and the time required for its recovery following GnRH immunization are not well-established.

Purpose of the Study:

• To investigate the time course of Leydig cell function recovery after the second dose of GnRH vaccine (Improvac) in boars.
• To identify factors influencing the return of testicular steroid synthesis and explore potential reasons for delayed recovery.

Main Methods:

• Six boars received their second GnRH vaccine dose at 26 weeks of age.
• Blood samples were collected to monitor antibody titre, luteinizing hormone (LH), testosterone, and IGF-I levels.
• Leydig cell function was assessed by the return of testosterone levels exceeding 0.5 ng/mL.

Main Results:

• Antibody titres declined significantly by week 10 post-vaccination.
• Recovery of testicular function, indicated by rising testosterone, was preceded by increased pulsatile LH secretion.
• The onset of steroid synthesis was delayed and highly variable, occurring between 10 and 24 weeks, and was not correlated with initial antibody titres.
• A significant refractoriness to LH was observed, likely contributing to the delayed Leydig cell function recovery.

Conclusions:

• Recovery of Leydig cell function after GnRH immunization in boars is a prolonged and variable process.
• Variable refractoriness to LH appears to be a primary factor limiting the timely restoration of testicular steroidogenesis.
• These findings have implications for managing boar taint and reproductive status in immunized boars.