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The mechanisms whereby leptin regulates testosterone secretion are complex and are likely to involve actions at different levels of the hypothalamus-pituitary-gonadal axis. In the present study, the effect of leptin on testicular steroidogenesis at different developmental stages in mice and sheep was investigated. Testosterone data from testicular slice and Leydig cells of immature and adult mice testes demonstrated that the action of leptin in the regulation of steroidogenesis appears to be dependent on the developmental stage of the testis. Leptin biphasically modulates basal testosterone production in immature testicular slice cultures: at relatively low concentrations (6.25-12.5 ng mL⁻¹) leptin exerts a significant inhibitory effect, but has less of an effect at very low (1.25 ng mL⁻¹) or high concentrations (25 ng mL⁻¹). However, leptin failed to modulate basal testosterone levels in Leydig cell preparations. In contrast with immature testes, leptin was unable to regulate either basal or human chorionic gonadotrophin (10 IU mL⁻¹)-stimulated testosterone production in adult testicular slices or Leydig cell cultures. The age- and concentration-dependent regulation pattern was confirmed using sheep testicular slice culture. Leptin (1.56-25 ng mL⁻¹) significantly inhibited basal testosterone production in the testis from birth to Day 21, but had no effect on Day 27 or older testes. However, the plasma and testicular concentrations of leptin and testosterone data in the ram indicate that such a regulatory effect of leptin on testis steroidogenesis 'in vitro' is unable to efficiently influence testosterone concentrations 'in vivo'. This does not exclude the possibility of a non-competitive mechanism of interaction between leptin and luteinising hormone to regulate testosterone production. Thus, we hypothesise that leptin is not an important independent regulator of testosterone concentration in the normal physiological state. The physiological significance and mechanism of leptin regulation of basal testosterone production are not known; further studies are required to elucidate these important issues. |
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