Role of Vasoactive Intestinal Peptide and 5-HT₂ Receptor Subtype in Serotonin Stimulation of Basal and Thyrotropin-Releasing-Hormone- Induced Prolactin Release in vitro from Rat Pituitary Cells

Abstract

We have previously demonstrated that 5-HT stimulates not only basal but also thyrotropin-releasing-hormone (TRH)-induced prolactin (PRL) release by acting directly at the pituitary gland level. In the present report, the participation of an autoparacrine action of VIP in the stimulatory effects of 5-HT and the involvement of the 5-HT₂ receptor type in mediating serotonin-induced PRL release have been examined. Cultured anterior pituitary cells from ovariectomized adult rats were incubated for 1 h in 1 ml of T₃-supplemented medium with or without the test substances. The results obtained in the presence of T₃ confirm our previous observations, since treatment of the cells with 5-HT caused dose-dependent increases in basal PRL release, with an approximate EC₅₀ of 3.68 × 10^–8 M, and led to a significant potentiation (1.3-fold) of the TRH-induced PRL release. In order to evaluate the possible participation of vasoactive intestinal peptide (VIP) as mediator of the effects of 5-HT on PRL release, cells were incubated in the presence of 5-HT alone (3–1,000 nM) or 100 nM 5-HT plus 30 nM TRH, with or without 200 nM VIP antagonist (VIP-At): [D,4-Cl-Ph⁶,Leu¹⁷]VIP. VIP-At partially inhibited the release of PRL induced by 5-HT, both basal and TRHstimulated release. The stimulatory effect of 5-HT, however, was not eliminated by VIP-At, since the PRL released in response to 5-HT was still over the respective control ones. These results further support the findings suggesting that 5-HT acts directly at pituitary level by stimulating PRL release. Addition of the 5-HT₂ receptor antagonist, ketanserin (1 µM) into the incubation medium resulted in the loss of cellular responsiveness to 5-HT, preventing not only the stimulatory effect of 5-HT on the basal but also on the TRH-induced PRL release. In conclusion, the results further strengthen the possibility that 5-HT increases the basal PRL release and potentiates the stimulatory effect of TRH by acting directly at the level of the lactotropes. These effects are not simply a consequence of autoparacrine action of VIP. In addition, it was shown that ketanserin completely antagonizes PRL response to 5-HT, indicating the involvement of the 5-HT₂ receptor type in mediating PRL release.

Pulsatile Gonadotropin-Releasing Hormone (GnRH) Secretion Is an Inherent Function of GnRH Neurons, as Revealed by the Culture of Medial Olfactory Placode Obtained from Embryonic Rats

“…Those GnRH neurons in cultures released GnRH into the medium in a pulsatile fashion at a mean interpulse interval of 47 min, which was very similar to that observed in ovariectomized rhesus monkeys in vivo [27, 28, 29]. Furthermore, the immortalized mouse GnRH neurons, the GT1-1 and GT1-7 neurons, were found to release GnRH in a pulsatile fashion with a mean interpulse interval of 36 min [9]or about 25 min [8, 30], similar to the interval reported for LH in castrated male mice in vivo, 28 min [31]. Altogether, these characteristics of cultured GnRH neurons seem to indicate that the GnRH pulse generator may reside within the GnRH neuron itself or the network of GnRH neurons and that inputs from other neurons are not needed to maintain the intermittent releasing activity.…”

Rapid prolactin induction in adult male rats after treatment with diethylstilbestrol

“…Physiological control of PRL secretion induced by 5‐HT is mediated, at least in part, by hypothalamic VIP release into the hypophysial portal blood in the rat . VIP antagonist ([D,4‐Cl‐Ph6,Leu17]VIP) partially inhibits the release of PRL induced by 5‐HT, comprising the basal and thyrotrophin‐releasing‐hormone‐stimulated releases . VIP increased 24 hours after DES treatment only in male rats (Table ), which suggests that the increase in PRL after 48 hours might be dependent on the increase in VIP.…”

Identification of 5-hydroxytryptamine receptor gene polymorphisms modulating hyperprolactinaemia in antipsychotic drug-treated patients with schizophrenia

“…At least three physiologically active substances play a role in promoting PRL release from pituitary cells: TRH, vasoactive intestinal polypeptide (VIP) and serotonin (also known as 5-HT) (Apfelbaum 1998). However, 5-HT stimulates PRL secretion through a complex, multi-level action on both the hypothalamus and the pituitary gland (Jørgensen 2007;Fitzgerald and Dinan 2008).…”

“…However, later studies showed that stimulation of HTR2 affects PRL release by acting directly at the pituitary gland level (Meltzer et al 1983;Apfelbaum 1987;Balsa et al 1998). Stimulation of HTR2 not only promoted PRL release via the local autoparacrine action of VIP, but also directly by activating HTR2s on lactotrophs (Apfelbaum 1998 Souza (1986) described the presence of 5-HT2 binding sites in all three lobes of the rat pituitary gland. However, the density in the anterior lobe was much lower than the density in the intermediate and posterior lobes.…”