Pulsatile <scp>LH</scp> release in male and female Sprague Dawley and Wistar rats in different reproductive states

This study aims to provide a detailed characterization of luteinizing hormone (LH) pulsatility in male and female Sprague-Dawley and Wistar rats, utilizing a minimally invasive tail-tip blood sampling approach across various reproductive states. LH levels were measured in sequential blood samples collected from the tail tip using ultrasensitive ELISA, in both sexes and across different reproductive states. LH pulse frequency, amplitude, pattern and baseline levels were analyzed in intact and gonadectomized rats to assess strain differences and estrous cycle variation. Additionally, we evaluate LH hormonal changes in dams at day 7, 14, and 21 of gestation and lactation. LH pulse frequency and pattern were comparable between the strains in both sexes. In females, LH pulsatility varied across the estrous cycle, peaking during Metestrus and decreasing during Estrus. Gonadectomy elicited a marked increase in LH secretion in both sexes, reflected by higher amplitude and baseline levels at 6- and 10-min interval sampling, while the increase of pulse frequency was only observed at 6-min sampling. The LH surge during Proestrus exhibited substantial temporal variability, with most animals attaining peak levels between late afternoon and early night, aligning with previous observations. During gestation, LH secretion remained pulsatile, with stable frequency but increased amplitude. In lactating females, the restoration of baseline but not pulsatile LH secretion upon the removal of the sucking stimulus was unexpectedly rapid. Collectively, these results demonstrate conserved yet distinct patterns of LH secretion across sex, strain, and reproductive states, strengthening the idea that the rat is a good model for integrative neuroendocrine studies of the hypothalamic-pituitary-gonadal axis, and that the tail-tip sampling method is a reliable, humane, and minimally stressful approach for monitoring LH dynamics in rats.