Inhibition of hypothalamic and pituitary muscarinic receptor binding by progesterone

Allosteric Modulation of Muscarinic Receptors by Cholesterol, Neurosteroids and Neuroactive Steroids

Muscarinic acetylcholine receptors are membrane receptors involved in many physiological processes. Malfunction of muscarinic signaling is a cause of various internal diseases, as well as psychiatric and neurologic conditions. Cholesterol, neurosteroids, neuroactive steroids, and steroid hormones are molecules of steroid origin that, besides having well-known genomic effects, also modulate membrane proteins including muscarinic acetylcholine receptors. Here, we review current knowledge on the allosteric modulation of muscarinic receptors by these steroids. We give a perspective on the research on the non-genomic effects of steroidal compounds on muscarinic receptors and drug development, with an aim to ultimately exploit such knowledge.

Neurosteroids and steroid hormones are allosteric modulators of muscarinic receptors

The membrane cholesterol was found to bind and modulate the function of several G-protein coupled receptors including muscarinic acetylcholine receptors. We investigated the binding of 20 steroidal compounds including neurosteroids and steroid hormones to muscarinic receptors. Corticosterone, progesterone and some neurosteroids bound to muscarinic receptors with the affinity of 100 nM or greater. We established a structure-activity relationship for steroid-based allosteric modulators of muscarinic receptors. Further, we show that corticosterone and progesterone allosterically modulate the functional response of muscarinic receptors to acetylcholine at physiologically relevant concentrations. It can play a role in stress control or in pregnancy, conditions where levels of these hormones dramatically oscillate. Allosteric modulation of muscarinic receptors via the cholesterol-binding site represents a new pharmacological approach at diseases associated with altered cholinergic signalling.

Neuroactive steroids, WIN-compounds and cholesterol share a common binding site on muscarinic acetylcholine receptors

Endogenous neurosteroids and their synthetic analogues-neuroactive steroids-have been found to bind to muscarinic acetylcholine receptors and allosterically modulate acetylcholine binding and function. Using radioligand binding experiments we investigated their binding mode. We show that neuroactive steroids bind to two binding sites on muscarinic receptors. Their affinity for the high-affinity binding site is about 100 nM. Their affinity for the low-affinity binding site is about 10 µM. The high-affinity binding occurs at the same site as binding of steroid-based WIN-compounds that is different from the common allosteric binding site for alcuronium or gallamine that is located between the second and third extracellular loop of the receptor. This binding site is also different from the allosteric binding site for the structurally related aminosteroid-based myorelaxants pancuronium and rapacuronium. Membrane cholesterol competes with neurosteroids/neuroactive steroids binding to both high- and low-affinity binding site, indicating that both sites are oriented towards the cell membrane..

24 hour patterning in gene expression of pineal neurosteroid biosynthesis in young chickens (Gallus gallus domesticus L.)

The pineal gland, one of the three equivalent avian biological clock structures, is also the site of intensive neurosteroid synthesis (7α-hydroxypregnenolone and allopregnanolone). Pineal neurosteroid biosynthesis involves six enzymes: cytochrome P450 side-chain cleavage - Cyp11a1 encoded, cytochrome P450_7α - Cyp7b1, 3β-hydroxysteroid dehydrogenase - Hsd3b2, 5α-reductase - Srd5a1, 3α-hydroxysteroid dehydrogenase - Akr1d1, and 5β-reductase - Srd5a3. Regulation of neurosteroid biosynthesis is not fully understood; although it is known that the E4BP4 transcription factor induces activation of biosynthetic cholesterol genes, which are the targets for SREBP (element-binding protein transcription factor). SREBP principal activity in the pineal gland is suppression and inhibition of the Period2 canonical clock gene, suggesting our hypothesis that genes encoding enzymes involved in neurosteroidogenesis are under circadian clock control and are the Clock Control Genes (CCGs). Therefore, through investigation of daily changes in Cyp11a1, Cyp7b1, Hsd3b2, Akr1d1, Srd5a1, and Srd5a3, pineal genes were tested in vivo and in vitro, in cultured pinealocytes. Experiments were carried out on pineal glands taken from 16-day-old chickens in vivo or using in vitro cultures of pinealocytes collected from 16-day-old animals. Both the birds in the in vivo experiments and the pinealocytes were kept under controlled light conditions (LD 12:12) or in constant darkness (DD). Subsequently, materials were prepared for RT-qPCR analysis. Results revealed that three of the six tested genes: Cyp11a1, Cyp7b1, and Srd5a3 demonstrated significant 24-hour variation in in vivo and in vitro. Findings of this study confirm that these genes could be under clock control and satisfy many of the requirements to be identified as CCGs.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Non-genomic mechanisms of action of steroid hormones

Sex steroid hormones are known to act through intracellular receptors and their cognate hormone response elements, located in the promoters of hormone-regulated genes. However, this classical mechanism of action cannot account for a variety of rapid effects of steroids (within seconds or minutes). In this review, non-genomic modes of target cell responses to sex steroids are described. The prototypical example is the resumption of meiosis in amphibian oocytes, triggered by progesterone at the plasma membrane level. Membrane effects of progesterone may also account for sperm maturation. Other membrane-mediated effects of steroids are reviewed. Whether a steroid hormone might elicit responses from a single cell through both genomic and membrane mechanisms remains an open question.

Blockade of ionotropic receptor responses by progesterone in the ganglion cells of Aplysia

To compare nongenomic effects of progesterone on various receptor responses of neurons, Aplysia ganglion cells were pretreated with 30 microM progesterone for 5 min and various receptor responses were tested using a conventional voltage-clamp method. Progesterone reduced nicotinic receptor-activated Na(+)-currents, nicotinic receptor-activated Cl(-)-currents, gamma-aminobutyric acid receptor-activated Cl(-)-currents, and dopamine receptor-activated Na(+)-currents. These depressant effects are similar at two different agonist concentrations. On the other hand, progesterone affected neither muscarinic receptor-activated K(+)-currents nor dopamine receptor-activated K(+)-currents. The former four types of receptors are known to be ionotropic while the latter two types of receptors are known to be metabotropic. Therefore, progesterone selectively inhibited all the types of ionotropic receptor responses, presumably in a noncompetitive manner.

Rapid anxiolytic activity of progesterone and pregnanolone in male rats

The effect of different doses of progesterone (1.0, 3.0, 10.0, 30.0, and 100.0 mg/kg) and pregnanolone (1.0, 3.0, 10.0, and 30.0 mg/kg) upon burying defensive and elevated plus-maze (EPM) tests was investigated in adult male rats and compared with the effects of diazepam (0.25. 0.50, 1.0, and 2.0 mg/kg). All drugs were suspended in a 0.2% methylcellulose solution and administered intraperitoneally 30 min prior to testing. Progesterone and pregnanolone were found to produce anxiolytic-like effects similar to those of diazepam. Thus, at certain doses, both drugs significantly increased the latency for burying and decreased the cumulative burying behavior, without modifying the number of shocks, and increased the time spent in the open arms of the maze, without affecting the spontaneous locomotor activity. These data clearly demonstrate that the defensive burying paradigm is useful to detect the anxiolytic-like properties of pregnanolone. An important finding was that progesterone produces significant behavioral effects 30 min after its administration. This finding suggests a rapid bioconversion of progesterone to its active ring-A reduced metabolites; however, the possibility remains that rapid behavioral effects of progesterone are due to a direct interaction with specific steroid receptors located on the plasma membrane, independently from the gamma-aminobutyric acid(A) (GABA(A)) receptor complex modulation.

Muscarinic M1 and M3 receptors are present and increase intracellular calcium in adult rat anterior pituitary gland

Physiological and biochemical evidence indicates the existence of functional muscarinic cholinergic receptors in the anterior pituitary. The selectivity of these receptors has been characterised by studying the binding of [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]diphenyl-acetoxy-N-methyl-piperidine ([3H]4-DAMP) in membrane preparation of male rat anterior pituitary at 25 degrees C. Competition experiments with receptor selective muscarinic antagonists were used to characterise specific selective muscarinic receptor binding. Both [3H]QNB and [3H]4-DAMP bound to anterior pituitary membranes at low concentrations, binding was saturable and was potently displaced by 4-DAMP (M1, M3 subtypes selective antagonist) > atropine (general) > pirenzepine (M1). Methoctramine (M2) didn't antagonise the [3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca2+ level in 62% and 65% of cultured rat anterior pituitary cells in a dose-dependent manner, and this effect was prevented by pirenzepine. Based on these results we suggest that both M1 and M3 muscarinic receptors are present and active in the majority of cells in the rat anterior pituitary gland, but their physiological role in the adult rat remains to be examined.

Different types of steroids inhibit [3H]diprenorphine binding in mouse brain membranes

1. The binding of 60 drugs, mainly steroids, to opioid receptors was studied in crude membrane fractions from mouse brains. 2. Competition assays with the different drugs (5 x 10(-7)-10(-4)M) were performed by labeling opiate receptors with [3H]diprenorphine (0.3-0.4 nM). 3. Only 7 drugs (alpha,5beta-tetrahydrodeoxycorticosterone, megestrol acetate, mifepristone, 17alpha-ethynylestradiol, diethylstilbestrol, clomiphene citrate and tamoxifen citrate) inhibited [3H]diprenorphine binding more than 50% at the highest concentration assayed (10(-4) M). The IC50 values ranged between 6x10(-5) M. 4. Thus, the present results show that only a limited number of steroids, from diverse classes, bind to opiate receptors.

Antiprogestins inhibit the binding of opioids to mu-opioid receptors in nervous membrane preparations

The present study showed that the glucocorticoid/progesterone antagonists, 17 beta-hydroxy-1 1 beta-(4-dimethylamino-phenyl-1)-17-(prop-1-ynyl)estra-4,9-dien+ ++-3-one (RU486) and 17 beta-hydroxy-11 beta-(4-dimethylamino-phenyl-1)-17-(propan-3-ol)estra-4,9-dien-3-o ne (ZK 98299), inhibit the binding of labeled dihydromorphine to mu-opioid receptors present on membrane preparations derived from rat and mouse brain, as well as from human neuroblastoma cells. The inhibitory effect of RU486 was dose-dependent and linked to a decrease of the affinity of labeled dihydromorphine to the mu-opioid receptors. Kinetic experiments have shown that RU486 induces a decrease of the association rate constant (k + 1) of dihydromorphine. RU486 also proved able to dissociate the dihydromorphine-mu-opioid receptor complex, although at a rate slower than that exhibited by unlabeled dihydromorphine. Finally, the addition of NaCl (100 mM) to the incubation buffer induced a 50% decrease of the inhibitory effect of RU486. A 6-day treatment of neuroblastoma cells with RU486 eliminated the inhibitory effect morphine exerts on the intracellular accumulation of cyclic AMP induced by prostaglandin E1. These results indicate that RU-486 may interact with brain mu-opioid receptors in vitro, by decreasing the affinity of opioid ligands.

Glucocorticoid actions on synaptic plasma membranes: modulation of dihydropyridine-sensitive calcium channels

We have previously shown that glucocorticoids accelerate depolarization-induced 45Ca2+ influx in synaptosomes isolated from rat cerebral cortex, indicating that the steroids may modulate voltage-dependent Ca2+ channels. The present study was undertaken to characterize the biochemical action of glucocorticoids on dihydropyridine-sensitive voltage-dependent Ca2+ channels known to be present in brain synaptosomes. The [3H]dihydropyridine labeled site was used as a marker to determine the levels of functional Ca2+ channels. No effect on equilibrium binding of [3H]PN 200-110 was found when membranes from disrupted synaptosomes were incubated with corticosterone as high as 1 microM. However, when intact synaptosomes were first incubated with corticosterone at 37 degrees C and then disrupted, a significant increase in [3H]PN 200-110 binding was found. Steroid incubation of synaptosomes at 0 degree C was ineffective. It appears that metabolic processes requiring intracellular factors were involved in the steroid action. In examining this possibility, [3H]PN 200-110 binding was activated in disrupted membranes by MgATP and Ca(2+)-calmodulin, and corticosterone was found to enhance the activation in a concentration-dependent manner. [3H]PN 200-110 binding to membranes was also activated by incubation with MgATP and cAMP-dependent protein kinase, but this activation was not enhanced by the steroid. These findings are consistent with the interpretation that the steroid promotes Ca2+ channel activity by enhancing calmodulin-dependent activation of the channels. The action on voltage-dependent Ca2+ channels in synaptic terminals may well be a mechanism whereby glucocorticoids modulate neuronal activity.

Progesterone-facilitated sexual receptivity: a review of arguments supporting a nongenomic mechanism

Progesterone facilitates sexual receptivity in estradiol-primed female rats. While many experiments suggest a genomic mechanism for the behavioral action of progesterone, some results appear better explained by nongenomic mechanisms. Furthermore, the presence of membrane binding sites for progesterone and rapid modifications of neuronal excitability induced by this steroid suggest that a nongenomic action of progesterone could be membrane related. However, in spite of the discovery of such membrane-related actions of progesterone, their relation to progesterone-facilitated sexual behavior remains unclear.

Hypothalamic sites of progestin action on aggression and sexual behavior in female Syrian hamsters

The effects of intracranial implants of the synthetic progestin, promegestone (R5020), on aggression and sexual behavior were examined in female Syrian hamsters. Ovariectomized female hamsters showed high levels of aggression and no lordosis towards stimulus male and female hamsters, both prior to and after estradiol benzoate treatment. Forty-six hr after estradiol treatment (10 micrograms SC), 10% crystalline promegestone was applied bilaterally (27 ga cannulae) to the ventromedial hypothalamus. When tested 5-7 hr later, these animals had a significant reduction in the rate of attacks towards females, with no lordosis responding to a male. In contrast, promegestone in the caudal anterior hypothalamus activated low levels of lordosis, yet these females maintained high levels of aggression towards stimulus females. Females receiving promegestone in the rostral anterior hypothalamus, and control females receiving intracranial cholesterol, maintained high levels of aggression and no lordosis. Estradiol treatment alone enhanced pelvic movements in response to experimenter-applied perineal stimulation in a majority of females (18/22 compared with 4/22 prior to estradiol injection), with intracranial treatments having no further effect. These results indicate that progestins act on aggression and sexual behavior in female Syrian hamsters at different hypothalamic loci.

Binding of progesterone to nerve cell membranes of rat brain using progesterone conjugated to 125I-bovine serum albumin as a ligand

Radioiodinated bovine serum albumin conjugated to progesterone was used as a probe to examine binding parameters of steroids to membrane preparations from rat brain tissue. The binding of 11 alpha-hydroxyprogesterone-11-hemisuccinate-125I-bovine serum albumin conjugate reached saturation after 30 min of incubation at 5 degrees C. Several bovine serum albumin-conjugated steroids were then tested for competition displacement studies. Among these steroid conjugates, the bovine serum albumin conjugate at position 3 of progesterone had the highest affinity, with an estimated inhibition constant of 28.5 +/- 2.1 nM (n = 3), whereas bovine serum albumin itself and the 17 beta-estradiol 6-(O-carboxy-methyl)oxime-bovine serum albumin conjugate showed no specific displacement. In addition, the binding sites were localized in an axolemma-enriched fraction of rat brainstem. Specific binding was obtained in tissues from cerebral cortex, brainstem, cerebellum, corpus striatum, and hypothalamus, but little or no binding occurred in uterus, ovary, liver, and spleen. The present data indicate that progesterone-125I-bovine serum albumin conjugate can be used as a ligand to study progesterone-membrane receptor interactions.