Progesterone modulates a neuronal nicotinic acetylcholine receptor.

Abstract

The major brain nicotinic acetylcholine receptor is assembled from two subunits termed a4 and nal. When expressed in Xenopus oocytes, these subunits reconstitute a functional acetylcholine receptor that is inhibited by progesterone levels similar to those found in serum. In this report, we show that the steroid interacts with a site located on the extracellular part of the protein, thus confirming that inhibition by progesterone is not due to a nonspecific perturbation of the membrane bilayer or to the activation of second messengers. Because inhibition by progesterone does not require the presence of agonist, is voltage-independent, and does not alter receptor desensitization, we conclude that the steroidis not an open channel blocker. In addition, we show that progesterone is not a competitive inhibitor but may interact with the acetylcholine binding site and that its effect is independent of the ionic permeability of the receptor.Steroid hormones are synthesized from cholesterol in the adrenal gland (the gluco-and mineralocorticoids) and in the gonads and placenta (androgens and estrogens). Their lipophilicity explains their passage of the blood-brain barrier. Steroids have also been demonstrated to reduce brain activity and alphaxalone is used clinically. The potency of anesthetics has been correlated with their liposolubility (2, 3), yet the molecular basis of these effects remains obscure.Some steroids, including 5a-pregnan-3a-hydroxy-20-one, enhance chloride fluxes at the y-aminobutyric acid (GABA) synapses of the type A GABA (GABAA) receptors (4, 5) and in transfected cells expressing GABAA receptors (6). In addition, progesterone attenuates cation fluxes evoked by excitatory amino acids in cerebellar Purkinje cells (7) and current induced by acetylcholine (AcCho) in chromaffin cells (8) and in reconstituted brain nicotinic AcCho receptor (nAcChoR) (9). In contrast, pregnenolone inhibits GABAA receptors in rat cortex neurons (10) and a progesteroneinduced reduction of glycine-evoked current has been reported for spinal cord neurons (11).The aim of this work is to investigate the mode of action of progesterone on the major brain neuronal nAcChoR a4/nal reconstituted in Xenopus oocytes. MATERIALS AND METHODSOocyte preparation and recording procedures were as described (12). Data acquisition, storage, and analysis were done on an IBM-PC/AT using the software DATAC (13).Inhibition curves were fitted to the empirical Hill equation. Inhibition by progesterone as a function of the AcCho concentration was fitted by using a Michaelis equation in the form:where y is the normalized current, a is a constant, x is the AcCho concentration, and IC50 is apparent inhibition constant. All steroids were purchased from Sigma. Stock solutions of steroids and progesterone-3-(O-carboxymethyl)oxime (P-3; Sigma P-3277) were at 0.01 M, in ethanol. Stock solutions were kept at -20TC and diluted in OR-2 (12) just before use. P-3-conjugated bovine serum albumin (P-3-BSA; Sigma P-4778, progesterone/BSA ratio = 38:1) and Hahyd...

Quercetin Inhibits α3β4 Nicotinic Acetylcholine Receptor-Mediated Ion Currents Expressed inXenopusOocytes

“…On the other hand, the previous reports have shown that various agents such as serotonin, strychnine, Ca 2＋ channel blockers, polyamines, steroids such as progesterone and hydrocortisone, ethanol, and metal ion like Zn 2＋ , regulate muscle or neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes [28][29][30][31][32][33][34][35]. Interestingly, the mechanism (i.e., voltage dependence or competition with acetylcholine for binding site) by which these substances regulate nicotinic acetylcholine receptors depend on the receptor subunit composition.…”

Evidence for a Role of Progesterone in Menstrual Cycle-Related Variability in Prepulse Inhibition in Healthy Young Women

“…This may be related to the involvement of progesterone in multiple receptor systems. Progesterone is known to modulate the release of dopamine (Dluzen and Ramirez, 1990;Ramirez and Zheng, 1996), to act as a functional antagonist at 5-HT3 receptors (Wetzel et al, 1998), as well as to play a role in the control of nicotinic cholinergic receptors (Valera et al, 1992). Given the known sensitivity of PPI to dopaminergic, serotonergic, glutamatergic, and cholinergic systems (Swerdlow and Geyer, 1998;Geyer et al, 2001), the data available so far do not allow us to speculate, which of these systems might be most pertinent to the effects of progesterone in PPI of healthy young women.…”

Unconventional ligands and modulators of nicotinic receptors

“…However, evidence is accumulating that neurosteroids can interact with and regulate the activity of neuronal nAChRs. For instance, progesterone and testosterone inhibit noncompetitively the activity of rat ␣4* receptors ectopically expressed in Xenopus oocytes (Bertrand et al, 1991;Valera et al, 1992;Paradiso et al, 2000). Likewise, progesterone, estradiol, and corticosterone act as noncompetitive antagonists at human ␣3␤4 nAChRs expressed by SH-SY5Y cells (Ke and Lukas, 1996).…”

“…Likewise, progesterone, estradiol, and corticosterone act as noncompetitive antagonists at human ␣3␤4 nAChRs expressed by SH-SY5Y cells (Ke and Lukas, 1996). Inhibition of the activation of rat ␣4* nAChRs and human ␣3␤4 nAChRs can be observed within a few seconds of exposure of the receptors to permeant or nonpermeant forms of steroids (Valera et al, 1992;Ke and Lukas, 1996), and is, therefore, the result of the interaction of these compounds with a site(s) located in the extracellular region of the receptors.…”