Selective labeling of serotonin receptors by d-[3H]lysergic acid diethylamide in calf caudate.

Since it was known that d-lysergic acid diethylamide (LSD) affected catecholaminergic as well as serotoninergic neurons, the objective in this study was to enhance the selectivity of [3H]LSD binding to serotonin receptors in vitro by using crude homogenates of calf caudate. In the presence of a combination of 50 nM each of phentolamine (added to preclude the binding of [3H]LSD to alpha-adrenoceptors), apomorphine, and spiperone (added to preclude the binding of [3H]LSD to dopamine receptors), it was found by Scatchard analysis that the total number of [3H]LSD sites went down to 300 fmol/mg, compared to 1100 fmol/mg in the absence of the catecholamine-blocking drugs. The IC50 values (concentrations to inhibit binding by 50%) for various drugs were tested on the binding of [3H]LSD in the presence of 50 nM each of apomorphine (A), phentolamine (P) and spiperone (S). With this combination, the IC50 for serotonin was 35 nM (compared to 1000 nM without it), indicating that [3H]LSD had become considerably more selectively displaceable by serotonin under these conditions whereas the effects of norepinephrine and dopamine on [3H]LSD binding were eliminated. Various ergots had approximately equal IC50 values against [3H]serotonin and [3H]LSD but tryptamines were much more selective against [3H]serotonin; the data may indicate the existence of the two types of serotonin receptors.     

Thiazide diuretic drug receptors in rat kidney: identification with [3H]metolazone.

Thiazides and related diuretics inhibit NaCl reabsorption in the distal tubule through an unknown mechanism. We report here that [3H]metolazone, a diuretic with a thiazide-like mechanism of action, labels a site in rat kidney membranes that has characteristics of the thiazide-sensitive ion transporter. [3H]Metolazone bound with high affinity (Kd = 4.27 nM) to a site with a density of 0.717 pmol/mg of protein in kidney membranes. The binding site was localized to the renal cortex, with little or no binding in other kidney regions and 11 other tissues. The affinities of thiazide-type diuretics for this binding site were significantly correlated with their clinical potency. Halide anions (Cl-, Br-, and I-) specifically inhibited high-affinity binding of [3H]metolazone to this site. [3H]Metolazone also bound with lower affinity (Kd = 289 nM) to sites present in kidney as well as in liver, testis, lung, brain, heart, and other tissues. Calcium antagonists and certain smooth muscle relaxants had Ki values of 0.6-10 microM for these low-affinity sites, which were not inhibited by most of the thiazide diuretics tested. Properties of the high-affinity [3H]metolazone binding site are consistent with its identity as the receptor for thiazide-type diuretics.     

Dopamine D2 receptors in the cerebral cortex: distribution and pharmacological characterization with [3H]raclopride.

An apparent involvement of dopamine in the regulation of cognitive functions and the recognition of a widespread dopaminergic innervation of the cortex have focused attention on the identity of cortical dopamine receptors. However, only the presence and distribution of dopamine D1 receptors in the cortex have been well documented. Comparable information on cortical D2 sites is lacking. We report here the results of binding studies in the cortex and neostriatum of rat and monkey using the D2 selective antagonist [3H]raclopride. In both structures [3H]raclopride bound in a sodium-dependent and saturable manner to a single population of sites with pharmacological profiles of dopamine D2 receptors. D2 sites were present in all regions of the cortex, although their density was much lower than in the neostriatum. The density of these sites in both monkey and, to a lesser extent, rat cortex displayed a rostral-caudal gradient with highest concentrations in the prefrontal and lowest concentrations in the occipital cortex, corresponding to dopamine levels in these areas. Thus, the present study establishes the presence and widespread distribution of dopamine D2 receptors in the cortex.     

Sigma opiates and certain antipsychotic drugs mutually inhibit (+)-[3H] SKF 10,047 and [3H]haloperidol binding in guinea pig brain membranes.

The relationship between binding of antipsychotic drugs and sigma psychotomimetic opiates to binding sites for the sigma agonist (+)-[3H]SKF 10,047 (N-allylnormetazocine) and to dopamine D2 sites was investigated. In guinea pig brain membranes, (+)-[3H]SKF 10,047 bound to a single class of sites with a Kd of 4 X 10(-8) M and a Bmax of 333 fmol/mg of protein. This binding was different from mu, kappa, or delta opiate receptor binding. It was inhibited by opiates that produce psychotomimetic activities but not by opiates that lack such activities. Some antipsychotic drugs inhibited (+)-[3H]SKF 10,047 binding with high to moderate affinities in the following order of potency: haloperidol greater than perphenazine greater than fluphenazine greater than acetophenazine greater than trifluoperazine greater than molindone greater than or equal to pimozide greater than or equal to thioridazine greater than or equal to chlorpromazine greater than or equal to triflupromazine. However, there were other antipsychotic drugs such as spiperone and clozapine that showed low affinity for the (+)-[3H]SKF 10,047 binding sites. Affinities of antipsychotic drugs for (+)-[3H]SKF 10,047 binding sites did not correlate with those for [3H]spiperone (dopamine D2) sites. [3H]-Haloperidol binding in whole brain membranes was also inhibited by the sigma opiates pentazocine, cyclazocine, and (+)-SKF 10,047. In the striatum, about half of the saturable [3H]haloperidol binding was to [3H]spiperone (D2) sites and the other half was to sites similar to (+)-[3H]SKF 10,047 binding sites.     

Histamine H1 receptors identified in mammalian brain membranes with [3H]mepyramine.

The antihistamine [3H]mepyramine binds to H1 histamine receptors in mammalian brain membranes. Potencies of H1 antihistamines at the binding sites correlate with their pharmacological antihistamine effects in the guinea pig ileum. Specific [3H]mepyramine binding is saturable with a dissociation constant of about 4 nM in both equilibrium and kinetic experiments and a density of 10 pmol per gram of whole kinetic experiments and a density of 10 pmol per gram of whole brain. Some tricyclic antidepressants are potent inhibitors of specific [3H]mepyramine binding. Regional variations of [3H]mepyramine binding do not correlate with variations in endogeneous histamine and histidine decarboxylase activity.     

alpha-Adrenergic receptors in human adipocyte membranes: direct determination by [3H]yohimbine binding

The presence of alpha 2-adrenergic receptors in membranes derived from human sc adipose tissue was directly demonstrated with a new alpha 2-selective ligand, [3H]yohimbine. Binding of this radiolabeled antagonist to adipocyte membranes was of high affinity (Kd = 3.9 +/- 2.4 nM) and saturable. Computer modelling of [3H]yohimbine saturation curves demonstrated that it binds to a homogeneous class of sites with a density of 145.0 +/- 33.8 fmol/mg protein. Adrenergic agonists competed with [3H]yohimbine in the order expected of alpha-receptors, and their binding was strongly influenced by guanine nucleotides. Competition of alpha-antagonists with this radioligand demonstrated yohimbine to be more potent than prazosin, indicative of alpha 2-receptors. Antagonist binding was unaffected by guanine nucleotides. Paired saturation curves in these adipocyte membranes with the alpha 2-selective [3H]yohimbine and the nonsubtype selective alpha-antagonist [3H]dihydroergocryptine demonstrated similar receptor concentrations. [3H]Dihydroergocryptine has been previously shown to label both alpha 3- and alpha 2-receptors with equal affinity. Therefore, these data indicate that the vast majority of alpha-receptors in human sc fat are of the alpha 2-subtype. [3H]Yohimbine with its alpha 2-selectivity and high specific binding will provide an excellent tool for the clinical investigation of human adipocyte alpha-receptor mechanisms in both normal and pathological states.     

Adenosine receptors in brain membranes: binding of N6-cyclohexyl[3H]adenosine and 1,3-diethyl-8-[3H]phenylxanthine.

N6-Cyclohexyl[3H]adenosine ([3H]CHA) and 1,3-diethyl-8-[3H]phenylxanthine ([3H]DPX) to bind to adenosine receptors in brain membranes. The agonist [3H]CHA has high affinity in both bovine and guinea pig brain (Kd, 0.7 nM and 6 nM, respectively). [3H]CHA binding kinetics are slow (dissociation t1/2;60 min); binding is much higher at 25 degrees C than at 0 degrees C and is inhibited by guanine nucleotides. Potencies of nucleosides and xanthines in competing for [3H]CHA sites indicate that specific binding is entirely to A1 adenosine receptors. In bovine brain, the antagonist [3H]DPX exhibits high-affinity binding (Kd, 5 nM) to the same A1 receptors that bind [3H]CHA. Binding kinetics are rapid (dissociation t1/2, 1 min), and binding is moderately higher at 0 degrees C than at 25 degrees C. In guinea pig brain, [3H]DPX binding has only moderate affintiy (Kd 50 nM), and about 60% of specific binding is to sites that resemble A2 adenosine receptors.     

Alpha-adrenergic receptors in rat myocardium. Identification by binding of [3H]dihydroergocryptine.

[3H]Dihydroergocryptine ([3H]DHE) binds to sites in membranes derived from rat myocardium that have the characteristics expected of alpha-adrenergic receptors. The binding is saturable with 41 fmol [3H]DHE bound per mg of protein and of high affinity with KD = 2.9 nM. The binding is rapid and readily reversible. Adrenergic agonists compete with [3H]DHE for binding in the order: epinephrine greater than norepinephrine greater than isoproterenol; and adrenergic antagonists compete for binding in the order: phentolamine greater than propranolol. For comparison, (-)[3H]dihydroalprenolol [(-)[3h]dha] was used to bind to sites in the same membrane preparations having characteristics of beta-receptors. The number and affinity of beta-receptors were quite similar to those of the alpha-receptors with 46 fmol (-)[EH]DHA per mg protein bound at saturation and KD = 2.5 nM. These techniques allowed identification of both beta- and alpha-adrenergic receptors in membranes derived from isolated atria, right ventricular free walls, and left ventricles including interventricular septa. This is the first report documenting direct identification of myocardial alpha-receptors by radioligand-binding techniques and complements the literature previously reporting myocardial inotopic and electrophysiological responses to alpha-adrenergic stimulation.     

Multiple opiate receptors: [3H]ethylketocyclazocine receptor binding and ketocyclazocine analgesia.

The receptor binding of the kappa agonist [3H]ethylketocyclazocine to brain homogenates in vitro and ketocyclazocine (kappa) analgesia in vivo has been investigated and compared to morphine, a mu agonist. Saturation analysis of [3H]ethylketocyclazocine binding in both mice and rats yielded biphasic Scatchard plots similar to those of opiate mu agonists, antagonists, enkephalins, and endorphins. Treatment of brain membranes with monovalent and divalent cation, chelating agents, protein-modifying reagents, and enzymes affected [3H]ethylketocyclazocine binding in a manner similar to that of [3H]morphine. Naloxazone, a long-acting antagonist that selectively abolished high-affinity [3H-DAla2,Met5]enkephalinamide binding in vivo, also selectively blocked high-affinity [3H]ethylketocyclazocine binding. Evaluation of analgesia with writhing and tail-flick assays in animals whose high-affinity binding sites were blocked by naloxazone demonstrated a 6- to 7-fold increase in median effective dose (ED50) values of ketocyclazocine. This decrease in analgesic potency was comparable to morphine's decreased potency in similarly treated mice. These biochemical and pharmacological results suggest that the analgesic properties of both kappa and mu agonists may be mediated through the same subpopulation of receptors, the high-affinity binding sites.     

gamma-Aminobutyric acid receptors visualized in spinal cord cultures by [3H]muscimol autoradiography.

gamma-Aminobutyric acid (GABA) receptors were visualized in mouse spinal cord explant cultures by [3H]muscimol autoradiography over certain small neurons of the dorsal horn grey matter, in the interneuronal neuropil of dorsal and ventral horns, and (rarely) in small clusters on processes of anterior horn cells. Specificity was indicated in control experiments by inhibition of binding by [3H]muscimol after pretreatment with GABA or a GABA analogue, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP, isoxazole 16), or with receptor antagonists bicuculline and picrotoxin. Unlabeled muscimol exchanged effectively with [3H]muscimol and produced autoradiographic label in locations indistinguishable from those found in experiments with [3H]muscimol alone. Pretreatment with the GABA uptake and transport inhibitors (-)-nipecotic acid and guvacine did not affect binding with [3H]muscimol. These experiments indicate that explant culture systems can be used for demonstration of functional receptors.     

Comparative studies of the intestinal absorption of [3H]pteroylmonoglutamate and [3H]pteroylheptaglutamate in man.

Comparative efficiencies of absorption of crystalline folic acid polyglutamate and monoglutamyl folic acid were determined in 11 normal subjects by measurement of the excretion of radioisotope in the urine after oral administration of [3H]pteroylheptaglutamic acid ([3H]PteGlu7) synthesized in our laboratory and of [3H]pteroylglutamic acid ([3H]PGA). Following ingestion of 0.6 mumole of [3H]PteGlu7, urinary excretion of radioactivity over 48 hr averaged 56.1 +/- 11.2% of the total dose. By comparison the ingestion of 0.6 mumole of [3H]PGA resulted in an average urinary excretion of 70.8 +/- 13.0% for the same time period. Approximately 90% of the urinary radioactivity was excreted during the initial 24-hr collection period. The mean recovery of radioactivity in urine and stool was 94% and recovery exceeded 84% in all subjects. The principal radioactive compound in the urine chromatographed with standard pteroylmonoglutamates. By chromatography, urinary folates were monoglutamates whether [3H]PGA or [3H]PteGlu7 was administered. The time course of folate absorption for the study compounds was compared by measuring the rise in serum radioactivity after the oral folate dose. Peak values in serum folate radioactivity following [3H]PGA occurred at 1 hr, whereas the peak values after [3H]PteGlu7 more often occurred at 2 hr. Only monoglutamyl folate was detected in the serum. These studies demonstrate that in normal subjects physiological doses of crystalline monoglutamyl and crystalline heptaglutamyl folates are both absorbed with high degrees of efficiency.     

Biochemical characterization of putative central purinergic receptors by using 2-chloro[3H]adenosine, a stable analog of adenosine.

After pretreatment of rat brain synaptic membranes with adenosine deaminase to remove endogenous adenosine, 2-chloro[3H]adenosine, a stable analog of adenosine, binds to two sites with Kd values of 1.3 and 16 nM and corresponding Bmax values of 207 and 380 fmol/mg of protein. Binding is reversible, and the highest density of sites occurs in enriched synaptosomal fractions. In peripheral tissue, negligible binding is observed in heart, kidney, and liver, while testicle has 11 fmol of binding sites/mg of protein. In brain, caudate and hippocampus have the highest density of sites, and spinal cord and hypothalamus have the lowest. This high-affinity binding is stereospecific; the L diasteromer of N6-phenylisopropyladenosine is approximately 30-times more potent as a displacer of 2-chloro[3H]adenosine than the D isomer and is also sensitive to theophylline (IC50 = 8.8 microM) and other purine-related compounds. Several putative neurotransmitters, neurotransmitter antagonists, and other centrally active compounds have no effect on binding. The data are consistent with the hypothesis that 2-chloro[3H]adenosine is binding to central purinergic receptors.     

Neuroanatomical localization of sex steroid-concentrating cells in the Japanese quail (Coturnix japonica): autoradiography with [3H]-testosterone, [3H]-estradiol, and [3H]-dihydrotestosterone

Steroid autoradiography was undertaken to determine the neuroanatomical loci which might be involved in the activation of steroid-sensitive behaviors in the Japanese quail (Coturnix japonica). Male and female quail were either surgically gonadectomized or photically regressed and implanted with androgen or estrogen to restore normal sexual and courtship behavior. After gonadectomy or implant removal, each quail was injected with 250 microCi of [3H]-testosterone (3H-T), [3H]-estradiol (3H-E2), or [3H]-dihydrotestosterone (3H-DHT), sacrificed, processed for autoradiography, and the telencephalon, diencephalon, mesencephalon, and rhombencephalon were examined for labelled cells. Following 3H-T or 3H-E2 injection and autoradiography, labelled cells were found in nucleus septalis lateralis (SL), nucleus preopticus medialis (POM), nucleus paraventricularis (PVN), regio lateralis hypothalami (LHy), nucleus inferior hypothalami (IH), nucleus infundibuli (IN), nucleus intercollicularis (ICo), substantia grisea centralis (GCt), nucleus taeniae (Tn), and in the reticular formation near nucleus motorius nervi trigemini (MV). In addition, following 3H-E2 autoradiography, labelled cells were found around nucleus accumbens (Ac). Following 3H-DHT autoradiography, labelled cells were found only in SL, PVN, Tn, LHy, ICo, and CGt. No labelled cells were found in Ac, POM, IH, IN, or MV even after long exposure times. These results suggest that the nuclei labelled following 3H-E2 but not 3H-DHT administration bind exclusively the aromatized metabolites of T. Since quail show a sex difference in male-typical copulatory behavior in response to E2, labelled cells were counted in POM, LHy, IH, and Tn of male and female quail following 3H-E2 injection and autoradiography. No sex differences in the number of labelled cells were found in POM, LHy, or IH. Males were found to have more labelled cells than females in Tn. These results show that sex differences in male-typical copulatory behavior are not due to sex differences in the number of cells binding estrogens in POM. The results reported here constitute the most neuroanatomically extensive report of steroid binding cells to date for a galliform brain, the first comparison in a galliform bird of the distributions of cells labelled following injection of 3H-T, 3H-E2, and 3H-DHT and the first analysis of sex differences in numbers of estrogen-binding cells in four nuclei in the avian brain.     

[3H]xanthine amine congener of 1,3-dipropyl-8-phenylxanthine: an antagonist radioligand for adenosine receptors.

An amine-functionalized derivative of 1,3-dipropyl-8-phenylxanthine has been prepared in tritiated form as a xanthine amine congener ([3H]XAC) for use as an antagonist radioligand for adenosine receptors. [3H]XAC has higher receptor affinity, higher specific activity, lower nonspecific membrane binding, and more favorable hydrophilicity than 1,3-diethyl-8-[3H]phenylxanthine, the xanthine commonly used for adenosine receptor binding. In rat cerebral cortical membranes, [3H]XAC exhibits saturable, specific binding with a Kd of 1.23 nM and a Bmax of 580 fmol/mg of protein at 37 degrees C. N6-(R-Phenylisopropyl)adenosine is a more potent inhibitor of [3H]XAC binding than is 5'-N-ethylcarboxamidoadenosine, indicating that binding is to an A1-adenosine receptor. In the absence of GTP, the inhibition curves for adenosine agonists versus [3H]XAC binding are biphasic, indicating that [3H]XAC is binding to low- and high-affinity agonist states of the A1 receptor. In the presence of GTP, adenosine analogs exhibit monophasic, low-affinity inhibition of binding of [3H]XAC. Inhibition of [3H]XAC binding by theophylline or by various 8-phenylxanthines is monophasic, and the potencies are commensurate with the potencies of these xanthines as adenosine receptor antagonists. The receptor sites in calf brain membranes exhibit a higher affinity (Kd = 0.17 nM) for [3H]XAC, whereas sites in guinea pig exhibit a slightly lower affinity (Kd = 3.0 nM). Densities of [3H]XAC binding sites are similar in brain membranes from all species.     

Extensive in situ activation of nuclear estrogen receptors after exposure of murine uteri to [3H]estradiol or [3H]4-hydroxytamoxifen

Estrogen receptor activation has been examined in murine uteri by characterizing binding to ATP-Sepharose. Determinations were performed under conditions in which specific binding to estrogen receptors was demonstrated by both agonist and antagonist without participation by nonreceptor antiestrogen-binding components. Cell-free activation of estrogen receptors in cytosol was more effectively promoted by [3H]estradiol estradiol than by [3H]4-hydroxytamoxifen or [3H]tamoxifen aziridine. However, when in situ activation was examined after intact uteri were exposed to [3H]estradiol or [3H]4-hydroxytamoxifen, virtually all extracted nuclear receptors demonstrated activated binding to ATP-Sepharose within 20 min of hormone exposure. Profiles of nuclear receptor activation were remarkably similar after exposure to either agonist or antagonist. Estrogen receptors in cytosol prepared after exposing intact uteri to 3H-labeled ligands were characterized by much less ability to bind to ATP-Sepharose than nuclear receptors. After uteri were exposed to [3H]estradiol, the activated receptor fraction in the cytosol progressively increased in contrast to preparations obtained after uteri were exposed to [3H]4-hydroxytamoxifen, which demonstrated a constant level of activation. Thus, even when activation has occurred within the intact uterus, agonists and antagonists may be characterized by different apparent levels of receptor activation in the cytosol fraction. These differences in the cytosol, however, are considerably overshadowed by the extensive activation occurring within the nuclear fraction, which we have observed to be similar with agonist and antagonist. Since estrogen receptors appear to act within chromatin, and cytosol receptors may be produced by receptor redistribution during preparation, we interpret these observations to indicate that in situ receptor activation is very similar after exposure to either agonist or antagonist. Consequently, antagonism does not appear to be associated with antiestrogens that impede receptor activation within intact murine uteri.     

Histamine H3 receptors: an overview.

Histamine H3 receptors represent a new class of prejunctional receptors, which, first described as modulators of histamine synthesis and release in the central nervous system, were subsequently found to occur also in peripheral tissues. They may behave both as autoreceptors located in histaminergic neurons and as heteroreceptors localized in adrenergic, cholinergic, serotoninergic and NANC nerve endings. As far as the gastrointestinal tract is concerned, H3 receptors seem to negatively control gastric acid secretion induced by endogenous cholinergic stimuli as well as gastrointestinal motility, as observed in different isolated preparations electrically stimulated. So far experimental evidence suggests that these receptors are absent in the effector organs. The most selective agonist and antagonist of H3 receptors are (R) alpha-methylhistamine and thioperamide, respectively; however, a lot of similar compounds are being synthesized in a few laboratories. It is easy to foresee that in the near future the availability of such substances will help the investigators in enlarging our knowledge in this interesting field.     

Identification of cardiac beta-adrenergic receptors by (minus) [3H]alprenolol binding.

(Minus) [3-H] alprenolol, a potent beta-adrenergic antagonist, was used to identify binding sites in a fraction of canine cyocardium. Beta adrenergic agonists and antagonists compete for these binding sites in a manner which directly parallels their known affinity for the cardiac beta-adrenergic receptor. Thus, binding was highly stereo-specific, with the (minus) isomers of beta-adrenergic agonists or antagonists being at least two orders of magnitude more potent than were the (plus) isomers in competing for these sites. The order of potency for inhibition of binding by beta-adrenergic agonists was (minus) isoproterenol greater than (minus) epinephrine greater than (minus) norepinephrine. The dissociation constant (KD) of (minus) alprenolol for the beta-adrenergic receptors was 7-11 nM as determined independently by direct binding studies or by inhibition of isoproterenol-stimulated adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. The beta-adrenergic antagonist (minus) propranolol also had high affinity for the binding sites (KD equals 12 nM). The physiologically inactive catechol-containing compounds pyrocatechol and (plus or minus) dihydroxymandelic acid, as well as the metabolite (plus or minus) normetanephrine, and the alpha-adrenergic antagonist phentolamine did not compete for the binding sites at a concentration of 160 muM. Binding was rapid (t1/2 less than 30 sec) and was rapidly reversible (t1/2 less than 15 sec). The binding sites were saturable and bound 0.35 pmol of (minus) [3-H] alprenolol per mg of membrane protein. These characteristics suggest that these binding sites represent the cardiac beta-adrenergic receptors.     

Neurosteroids, via sigma receptors, modulate the [3H]norepinephrine release evoked by N-methyl-D-aspartate in the rat hippocampus.

N-Methyl-D-aspartate (NMDA, 200 microM) evokes the release of [3H]norepinephrine ([3H]NE) from preloaded hippocampal slices. This effect is potentiated by dehydroepiandrosterone sulfate (DHEA S), whereas it is inhibited by pregnenolone sulfate (PREG S) and the high-affinity sigma inverse agonist 1,3-di(2-tolyl)guanidine, at concentrations of > or = 100 nM. Neither 3 alpha-hydroxy-5 alpha-pregnan-20-one nor its sulfate ester modified NMDA-evoked [3H]NE overflow. The sigma antagonists haloperidol and 1-[2-(3,4-dichlorophenyl)-ethyl]-4-methylpiperazine, although inactive by themselves, completely prevented the effects of DHEA S, PREG S, and 1,3-di(2-tolyl)guanidine on NMDA-evoked [3H]NE release. Progesterone (100 nM) mimicked the antagonistic effect of haloperidol and 1-[2-(3,4-dichlorophenyl)ethyl]-4-methyl-piperazine. These results indicate that the tested steroid sulfate esters differentially affected the NMDA response in vitro and suggest that DHEA S acts as a sigma agonist, that PREG S acts as a sigma inverse agonist, and that progesterone may act as a sigma antagonist. Pertussis toxin, which inactivates the Gi/o types of guanine nucleotide-binding protein (Gi/o protein) function, suppresses both effects of DHEA S and PREG S. Since sigma 1 but not sigma 2 receptors are coupled to Gi/o proteins, the present results suggest that DHEA S and PREG S control the NMDA response via sigma 1 receptors.