Muscarinic Regulation of Basal versus Thyrotropin-Releasing Hormone-Induced Prolactin Secretion in Rat AnteriorPituitary Cells

Abstract

Acetylcholine (ACh), synthesized in the pituitary, can act locally to modulate pituitary function. We used rat primary anterior pituitary (AP) cells to investigate how ACh affects pituitary prolactin (PRL) secretion in the presence or absence of known PRL regulators: thyrotropin-releasing hormone (TRH), 17β-estradiol (E₂) and triiodothyronine (T₃). Cultured AP cells were prepared from ovariectomized rats and pretreated with diluent, 0.6 nM E₂, 10 nM T₃, or E₂ plus T₃ for 5 days, then challenged with various doses of ACh or muscarinic receptor agonists (oxotremorine or carbachol) and TRH (100 nM) for 20 min. Significant ACh (10^–5M) suppression of both basal and TRH-induced PRL secretion was not evident in diluent-, E₂- or T₃-pretreated cells, but observed only in cells pretreated with both E₂ and T₃. Moreover, in E₂ plus T₃-pretreated cells, oxotremorine and carbachol, like ACh (10^–7–10^–5M), suppressed both responses in a dose- related manner. Pertussis toxin (PTX; 100 ng/ml) as well as atropine (a muscarinic receptor antagonist; 1 mM) blocked these effects of cholinomimetics. ACh also inhibited both PRL responses elicited by drugs elevating intracellular cAMP (10 µM forskolin) or Ca²⁺ (1 µM Bay K-8644) in a PTX-sensitive manner. ACh inhibition of basal PRL secretion was unaltered by intracellular Ca²⁺ mobilization blockers, TMB-8 (100 µM) and thapsigargin (1 µM), but abrogated by the nitric oxide synthase inhibitor (300 µM L-NAME). ACh inhibition of TRH-induced PRL secretion was accentuated by TMB-8 and alleviated by thapsigargin or L-NAME. In summary, muscarinic inhibition of either basal or TRH-induced PRL secretion was augmented by E₂ and T₃, and involved the PTX-sensitive cAMP/Ca²⁺ pathways. Furthermore, nitric oxide mediated the basal rather than TRH-induced PRL response to ACh, whereas the intracellular Ca²⁺ mobilization concerned the TRH-induced rather than the basal PRL response to ACh. Thus, ACh synthesized in the AP appears to inhibit basal vs. TRH-induced PRL secretion via different mechanisms.

Differential involvement of protein kinase C in basal versus acetylcholine‐regulated prolactin secretion in rat anterior pituitary cells during aging

“…4, left panel) or presence (Fig. 4, right panel) of TRH in E 2 þ T 3 pretreated AP cells [Pu et al, 1999]. ACh (10 À7 , 10 À5 , 10 À3 M) dose-dependently inhibited both basal (Fig.…”

“…It appears that in the old AP cells ACh suppresses TRH-induced PRL secretion mainly via the PKC (PMA-sensitive) pathway. Other intracellular signals such as the cAMP/Ca 2þ pathways, and the nitric oxide pathway essential for ACh-regulated PRL secretion in the young AP cells [Pu et al, 1999] may be downregulated in the aged AP cells. Consistent with this observation, reduction in pituitary phosphotidylinositol turnover and cAMP/cAMP binding protein has been found in aged as opposed to young rats [Arima, 1982;Tang and Tang, 1983;Bonetti et al, 1987].…”

“…Other inhibitory and stimulatory factors synthesized in the pituitary and hormones present in the circulation (i.e., estrogens and thyroid hormones) also affect PRL secretion [Maurer, 1982;Carmeliet et al, 1989;Dymshitz et al, 1992;Ray and Melmed, 1997;Bruhn et al, 1998;Cai et al, 1998;Kanyicska et al, 1998;Pu et al, 1999]. Among the autocrine/paracrine factors in the pituitary, angiotensin II (AII) and vasoactive intestinal peptide (VIP) have been shown to stimulate, and acetylcholine (ACh) to inhibit PRL secretion [Rudnick and Dannies, 1981;Carmeliet et al, 1989;Balsa et al, 1996;Díaz-Torga et al, 1998;Pu et al, 1999]. Recently, we have further demonstrated that the suppression by ACh of both basal and TRH-induced PRL secretion was enhanced in a 17b-estradiol (E 2 )-and triiodothyronine (T 3 )-dependent manner in AP cells prepared from ovariectomized young rats [Pu et al, 1999].…”

“…Among the autocrine/paracrine factors in the pituitary, angiotensin II (AII) and vasoactive intestinal peptide (VIP) have been shown to stimulate, and acetylcholine (ACh) to inhibit PRL secretion [Rudnick and Dannies, 1981;Carmeliet et al, 1989;Balsa et al, 1996;Díaz-Torga et al, 1998;Pu et al, 1999]. Recently, we have further demonstrated that the suppression by ACh of both basal and TRH-induced PRL secretion was enhanced in a 17b-estradiol (E 2 )-and triiodothyronine (T 3 )-dependent manner in AP cells prepared from ovariectomized young rats [Pu et al, 1999]. Furthermore, multiple intracellular pathways including nitric oxide are differentially involved in ACh actions on these two PRL responses in AP cells [Pu et al, 1999].…”

“…Recently, we have further demonstrated that the suppression by ACh of both basal and TRH-induced PRL secretion was enhanced in a 17b-estradiol (E 2 )-and triiodothyronine (T 3 )-dependent manner in AP cells prepared from ovariectomized young rats [Pu et al, 1999]. Furthermore, multiple intracellular pathways including nitric oxide are differentially involved in ACh actions on these two PRL responses in AP cells [Pu et al, 1999]. In this article, we investigated the mechanisms involved in aging-enhanced PRL secretion.…”