In a variety of species, the LH secretory response to GnRH is completely suppressed by the combined actions of prolactin (PRL) and dopamine (DA). In sheep, this effect is only observed under long days (non-breeding season -NBS). To investigate the level at which these mechanisms operate, we assessed the effects of PRL and bromocriptine (Br), a DA-agonist, on the gonadotropin secretory and mRNA responses to GnRH in pituitary cell cultures throughout the ovine annual reproductive cycle. As expected, the LH secretory response to GnRH was only abolished during the NBS following combined PRL and Br application. Conversely, the LHB-subunit response to GnRH was reduced during both the BS and NBS by the combined treatment and Br alone. Similar results were obtained in pars distalis-only cultures, indicating that the effects are pars tuberalis (PT)-independent. Further signaling studies revealed that PRL and Br alter the LH response to GnRH via convergence at the level of PLC and PKC. Results for FSH generally reflected those for LH, except during the BS where removal of the PT allowed PRL and Br to suppress the FSH secretory response to GnRH. These data show that suppression of the LH secretory response to GnRH by PRL and DA is accompanied by changes in mRNA synthesis and that the photoperiodic modulation of this inhibition operates primarily at the level of LH release through alterations in PKC and PLC. Furthermore, the suppressive effects of PRL and DA on the secretion of FSH are photoperiodically regulated in a PT-dependent manner.
|Article number||DOI 10.1095|
|Number of pages||9|
|Journal||Biology of Reproduction|
|Early online date||1 Feb 2012|
|Publication status||Published - 2012|
Bibliographical noteSenior and corresponding author. Planned, directed and supervised the research. Helped to analyse the data and write the paper. The first author (Hodson) and the second author (Henderson) were my PhD students.
Rating 3*. Best journal in the field: Impact Factor 4.0. Pioneering studies in this topic, which unravelled the specific modulatory effects of season on the ability of prolactin and dopamine to suppress the LH and FSH responses to GnRH and their differential actions at mRNA and secretory levels.
- prolactin, dopamine, LH, GnRH, pituitary, sheep