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The effects of a sudden decrease or increase in daylength on prolactin secretion in the ram.

G A Lincoln, A S McNeilly, C L Cameron

    Journal of Reproduction and Fertility
    |March 1, 1978
    PubMed
    Summary
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    Artificial light cycles significantly impact prolactin levels in Soay rams. Changes in light duration rapidly alter prolactin secretion, with FSH levels inversely related to prolactin.

    Area of Science:

    • Reproductive Endocrinology
    • Animal Physiology
    • Chronobiology

    Background:

    • Seasonal variations in photoperiod influence reproductive cycles in many mammals.
    • Prolactin is a key hormone involved in reproduction and seasonal adaptation.
    • Soay rams exhibit seasonal breeding patterns influenced by light exposure.

    Purpose of the Study:

    • To investigate the effects of artificial photoperiod manipulation on prolactin secretion in adult Soay rams.
    • To determine the temporal dynamics of prolactin response to abrupt light changes.
    • To explore the relationship between prolactin and follicle-stimulating hormone (FSH) secretion under controlled light conditions.

    Main Methods:

    • Adult Soay rams were exposed to alternating long (16L:8D) and short (8L:16D) day photoperiods for over 10 months.

    Related Experiment Videos

  • Blood samples were collected hourly over 24-hour periods at various time points.
  • Plasma prolactin and FSH concentrations were measured using established immunoassays.

    • Main Results:

    • Abrupt shifts from long to short days caused a progressive decrease in plasma prolactin, while shifts to long days increased prolactin.
    • The prolactin response to light changes was rapid, initiating within 6 days.
    • A distinct 24-hour rhythm in prolactin was observed, with peaks during early light and dark phases, varying with long-term secretion levels.
    • Plasma FSH levels showed a strong inverse correlation with prolactin secretion.

    Conclusions:

    • Photoperiod is a critical regulator of prolactin secretion in Soay rams, with rapid adaptation to light changes.
    • The observed inverse relationship between prolactin and FSH suggests a role for prolactin in modulating gonadotropin release in this species.
    • These findings contribute to understanding the neuroendocrine mechanisms underlying seasonal reproduction in sheep.