Activation of progestin receptors in female reproductive behavior: Interactions with neurotransmitters

Abstract

The steroid hormone, progesterone (P), modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and reproductive behavior in female mammals. A wide body of evidence indicates that these neural effects of P are predominantly mediated via their intracellular progestin receptors (PRs) functioning as "ligand-dependent" transcription factors in the steroid-sensitive neurons regulating genes and genomic networks. In addition to P, intracellular PRs can be activated by neurotransmitters, growth factors and cyclic nucleotides in a ligand-independent manner via crosstalk and convergence of pathways. Furthermore, recent studies indicate that rapid signaling events associated with membrane PRs and/or extra-nuclear, cytoplasmic PRs converge with classical PR activated pathways in neuroendocrine regulation of female reproductive behavior. The molecular mechanisms, by which multiple signaling pathways converge on PRs to modulate PR-dependent female reproductive behavior, are discussed in this review.

Fig. 1

Schematic representation of structure and functional organization of progestin receptor (A) and isoforms (B). (A) Progestin receptors (PRs) have a highly conserved DNA binding domain (DBD) and a ligand-binding domain (LBD) connected by a variable Hinge region (H). The N-terminal region contains a transactivation function -1 (AF1). The LBD region contains the AF2 domain. The third activation function domain, AF3 (BUS), is present in the N-terminal segment, and is unique to PR-B. (B) Schematic representation of PR isoforms and splice variants. Nuclear PR gene is composed of 8 exons with 3100-bp coding region and 5′- and 3′-untranslated regions. PR-B and PR-A isoforms are transcribed from two alternate transcription initiation sites. PR-C isoform results from an in-frame initiation of translation and lacks exon 1.

Fig. 2

Mechanisms of PR activation. Unliganded PR is present as an inactive complex associated with heat shock proteins (HSP) and chaperone proteins (p29, p56) in the cytoplasm. In the classical ligand dependent pathway of activation (LDA), progesterone and other progestins bind to the PR to induce conformational change, dissociation of HSPs and chaperone proteins. PRs undergo dimerization and bind to the hormone response element (HRE) in the target DNA. Ligand-induced conformational change facilitates the recruitment of cofactors and other general transcription factors (GTFs) to the promoter, producing a transcriptionally active complex that can direct gene transcription. Compounds such as cyclic nucleotides, neurotransmitters (NTs), growth factors (GFs) and neurosteroids can activate second messengers and protein kinase pathways to activate PR and/or coactivators in a ligand-independent manner.

Fig. 3

Ligand-dependent and –independent activation of nuclear PRs in female reproductive behavior in mice. (a) Ovariectomized, wild type (+/+) and homozygous (−/−) PR mutant mice were primed with estradiol benzoate (EB), followed by intracerebroventricular administration of progesterone (P) or D₁ agonist, SKF38393 (SKF) 48 h later. Female receptive behavior in the presence of a male mouse was quantitated and represented as lordosis quotient (LQ). Statistically significant differences were seen in P- and SKF-facilitated lordosis responses of the −/− compared to their +/+ littermates (*P < 0.001). Adapted from Mani et al [165]. (b) Ligand dependent- and -independent induction of sexual receptivity in PR isoform-specific null mutant mice. Ovariectomized PRAKO^−/−, PRBKO^−/− and PR^+/+ mice were primed with EB for 48 h, followed by icv administration of progesterone (P), dopamine D₁ agonist SKF 81297 (SKF) or 8-Bromo-cAMP (8-Br-cAMP). P-facilitation of lordosis response was significantly lower in PRAKO^−/− null mutants compared to the wild type animals (**P < 0.05). Statistically significant differences (#P < 0.01) were observed in SKF- and 8-Br-cAMP-treated animals compared with EB-treated controls. Adapted from Mani et al [168].

Fig. 4

Crosstalk between intracellular signaling pathways in female reproductive behavior. The schematic representation of potential signaling pathways operating in the hypothalamus and preoptic areas. (1) Classical genomic mechanism of action by progesterone- and ring-A class of progestins, mediated by nuclear PRs, plays a predominant role. The ligands allosterically bind to their cognate nuclear receptors and activate PRs to promote interactions with coactivator proteins (2) Progesterone effects mediated by second messengers (cAMP, cGMP) and extra-nuclear signaling kinases (PKA, PKC, CaMKII), activates MAPK signal transduction cascade, leading to plausible phosphorylation of nuclear TFs, PRs/PR coactivators, CREB, and/or its associated protein CBP. (3) Progesterone and progestins, act via the Src kinase, to interact with extranuclear PRs and activate MAPK cascade. (4) Progesterone acting via the extra-nuclear PKA/MAPK/DARPP-32 pathway can cause a decrease in phosphatase activity and an increase in phosphorylation of PR and/or its coactivators. (5) Mating stimuli (VCS) and dopamine D₁ agonist can stimulate PKA activation. D₁ agonist-stimulated PKA-mediated pathway phosphorylates DARPP-32, which inhibits PP1, leading to the activation of CREB/PR/coactivators. VCS-stimulated PKA activation can also interact with MAPK cascade. (6) Neuropeptides, nucleotides, GnRH and PGE2 can act through various receptor- and/or second messengers (cAMP, cGMP, NO) and transmit signals to the nuclear PRs or other TFs. Interactions between the signal transduction pathways may serve as an amplification mechanism to converge on nuclear TFs and/or coactivators to regulate gene transcription and translation to facilitate female reproductive behavior.

Fig. 5

Mechanism of dopamine action in the hypothalamus. Dopamine acting via D₁ receptor subtype stimulates an increase in cAMP levels and PKA activity, leading to enhanced phosphorylation of the neuronal phosphoprotein DARPP-32 on Thr³⁴. Phosphorylation of DARPP-32 converts the phosphoprotein to a potent inhibitor of protein phosphatase 1 (PP-1). Phosphatase inhibition leads to increased kinase activity and increased phosphorylation of target proteins including PR and/or coactivators.