Effects of Prolactin on the Luteinizing Hormone Response to Gonadotropin-Releasing Hormone in Primary Pituitary Cell Cultures During the Ovine Annual Reproductive Cycle

Abstract

In the sheep pituitary, the localization of prolactin (PRL) receptors in gonadotrophs and the existence of gonadotroph-lactotroph associations have provided morphological evidence for possible direct effects of PRL on gonadotropin secretion. Here, we investigated whether PRL can readily modify the LH response to GnRH throughout the ovine annual reproductive cycle. Cell populations were obtained from sheep pituitaries during the breeding season (BS) and the nonbreeding season (NBS), plated to monolayer cultures for 7 days, and assigned to receive one of the following treatments: 1) nil (control), 2) acute (90- min) bromocriptine (ABr), 3) chronic (7-day) bromocriptine (CBr), 4) ABr and PRL, 5) CBr and PRL, 6) PRL alone, or 7) thyrotropin-releasing hormone. Cells were treated as described above, with the aim of decreasing or increasing the concentrations of PRL in the culture, and simultaneously treated with GnRH for 90 min. The LH concentrations in the medium were then determined by RIA. GnRH stimulated LH in a dose-dependent manner during both stages of the annual reproductive cycle. During the NBS, single treatments did not significantly affect the LH response to GnRH. However, when PRL was combined with bromocriptine, either acutely or chronically, GnRH failed to stimulate LH release at all doses tested (P < 0.01). In contrast, during the BS, the LH response to GnRH was not affected by any of the experimental treatments. These results reveal no apparent effects of PRL alone, but an interaction between PRL and dopamine in the regulation of LH secretion within the pituitary gland, and a seasonal modulation of this mechanism.

Direct effects of prolactin and dopamine on the gonadotroph response to GnRH

“…Overall, the present findings are consistent with those of an early study in post-partum women where the LH response to exogenous GnRH was suppressed during the first month after parturition but enhanced above follicular phase levels in the second month (Keye & Jaffe 1976). In the case of dopamine, the blockade of the PRL-induced enhancement of the secretory response to GnRH by the co-application of bromocriptine is an important observation, which corroborates previous findings in primary cultures (Gregory et al 2004). If PRL is indeed inducing the observed enhancement through up-regulation of certain signalling components, dopamine is likely to be blocking these effects.…”

“…The optimal GnRH dose required to generate a maximal mRNA response (i.e. 10 K7 M) was previously determined by dose-response tests at 90 min and 24 h. The doses of Br (a specific dopamine-D 2 receptor agonist) and PRL employed in this study were selected on the basis of their previously observed effectiveness at inducing a biological response in preliminary studies, and in previous work in aT 3 -1 (Kanasaki et al 2002) and primary (Gregory et al 2004) cultures. Following incubation, the medium was aspirated off prior to cell lysis and RNA extraction.…”

“…An additional evidence for a direct pituitary action of PRL on the gonadotrophic axis was revealed by a study reporting the selective expression of PRL receptors in gonadotroph cells of the pars distalis and pars tuberalis of the ovine pituitary, a species that shows overt temporal changes in fertility throughout the year in response to photoperiod (Tortonese et al 1998). More recently, our laboratory demonstrated a seasonal modulation of the combined suppressive effects of PRL and dopamine on the gonadotrophin response to GnRH (Gregory et al 2004). …”

Seasonal effects of central leptin infusion and prolactin treatment on pituitary SOCS-3 gene expression in ewes

“…Indeed, the stimulating effect of PRL was evident during LDs, when the high concentration of PRL is observed in sheep plasma and during in vitro culture (March and May). One can suspect that, in vitro, the pituitary gland is isolated from dopaminergic inhibition and lactrotroph cells secrete a large amount of PRL into media; however, this was not observed in this study and in others (Gregory et al 2004). Concentration of PRL and other hormones secreted from pituitary cells into media is Figure 3 The mean expression of SOCS-3 mRNA (GS.E.M.)…”

Differential expression and seasonal modulation of VGF peptides in sheep pituitary

“…In sheep, lactotrope cells are increasingly stimulated from winter to summer, serum prolactin being higher during long days (from spring to summer) and lower during short days (from autumn to winter; Pelletier 1973, Ravault 1976). In the same species, the pituitary pars tuberalis is probably involved in stimulating prolactin secretion (Graham et al 2002), while prolactin may be involved in seasonal inhibition of gonadotropin release via a paracrine intrapituitary mechanism (Tortonese et al 1998), probably also involving dopamine (Gregory et al 2004). Conversely, thyrotropes can be stimulated in response to reduced ambient luminosity, in apparent connection with a higher need for heat production via increased thyroid hormone release during the winter months (Hassi et al 2001).…”