Phencyclidine interactions with the ionic channel of the acetylcholine receptor and electrogenic membrane

Interactions of Nereistoxin and Its Analogs with Vertebrate Nicotinic Acetylcholine Receptors and Molluscan ACh Binding Proteins

Nereistoxin (NTX) is a marine toxin isolated from an annelid worm that lives along the coasts of Japan. Its insecticidal properties were discovered decades ago and this stimulated the development of a variety of insecticides such as Cartap that are readily transformed into NTX. One unusual feature of NTX is that it is a small cyclic molecule that contains a disulfide bond. In spite of its size, it acts as an antagonist at insect and mammalian nicotinic acetylcholine receptors (nAChRs). The functional importance of the disulfide bond was assessed by determining the effects of inserting a methylene group between the two sulfur atoms, creating dimethylaminodithiane (DMA-DT). We also assessed the effect of methylating the NTX and DMA-DT dimethylamino groups on binding to three vertebrate nAChRs. Radioligand receptor binding experiments were carried out using washed membranes from rat brain and fish (Torpedo) electric organ; [³H]-cytisine displacement was used to assess binding to the predominantly high affinity alpha4beta2 nAChRs and [¹²⁵I]-alpha-bungarotoxin displacement was used to measure binding of NTX and analogs to the alpha7 and skeletal muscle type nAChRs. While the two quaternary nitrogen analogs, relative to their respective tertiary amines, displayed lower α4β2 nAChR binding affinities, both displayed much higher affinities for the Torpedo muscle nAChR and rat alpha7 brain receptors than their respective tertiary amine forms. The binding affinities of DMA-DT for the three nAChRs were lower than those of NTX and MeNTX. An AChBP mutant lacking the C loop disulfide bond that would potentially react with the NTX disulfide bond displayed an NTX affinity very similar to the parent AChBP. Inhibition of [³H]-epibatidine binding to the AChBPs was not affected by exposure to NTX or MeNTX for up to 24 hr prior to addition of the radioligand. Thus, the disulfide bond of NTX is not required to react with the vicinal disulfide in the AChBP C loop for inhibition of [³H]-epibatidine binding. However, a reversible disulfide interchange reaction of NTX with nAChRs might still occur, especially under reducing conditions. Labeled MeNTX, because it can be readily prepared with high specific radioactivity and possesses relatively high affinity for the nAChR-rich Torpedo nAChR, would be a useful probe to detect and identify any nereistoxin adducts.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

M2 mutations of the nicotinic acetylcholine receptor increase the potency of the non-competitive inhibitor phencyclidine

Phencyclidine (PCP) is a non-competitive inhibitor of the nicotinic acetylcholine receptor (nAChR) with biphasic characteristics. At low and high micromolar concentrations, PCP inhibits nAChR from fetal mouse muscle, whereas at intermediate concentrations PCP does not inhibit the receptor. The present study was performed to determine whether the high and low concentration effects of PCP on mouse nAChR were due to interactions of this blocker with channel lining amino acids. In order to test this hypothesis, we examined the ability of PCP to inhibit acetylcholine-induced currents from wild-type nAChR and nAChR in which amino acid substitutions were made in the 6', 8' and 10' positions of the M2 transmembrane segments of the receptor. Fetal mouse nAChR from BC(3)H-1 cells were expressed in Xenopus laevis oocytes and studied using the two-electrode voltage clamp technique. The results of this study reveal that in native fetal muscle receptor, PCP potency is not affected by membrane potential between -80 mV and -30 mV. The potency of PCP is increased by mutations in M2 6', 8', and 10' positions. This increase in potency cannot be explained merely by either changes in hydrophobicity/hydrophilicity of amino acids at these positions or by side-chain size. A model proposing extra-luminal inhibitory and regulatory sites for PCP explains the lack of voltage-dependency, the biphasic effect of PCP, and the fact that all M2 mutations increased PCP potency (by disrupting the link with the regulatory sites).

Determinants of phencyclidine potency on the nicotinic acetylcholine receptors from muscle and electric organ

1. Phencyclidine (PCP) is an inhibitor of the nicotinic acetylcholine receptor (AChR) with characteristics of an open-channel blocker. The location of PCP binding site on the AChR molecule is unknown. 2. PCP inhibits the AChR from electric organ with a higher potency than muscle AChR. To find the molecular basis of this difference, we expressed the two native and six hybrid receptors, and two receptors containing mutated mouse gamma subunits in Xenopus laevis oocytes. The inhibition of ACh-induced current in these receptors by PCP was studied using whole-cell voltage-clamp. All hybrid receptors generated robust ACh-induced currents, while incomplete receptors (gamma-less or delta-less) did not. 3. PCP potency was higher on hybrids containing Torpedo beta and gamma subunits regardless of the alpha and delta subunit origin. A mouse gamma subunit containing the asparagine 6' to the serine mutation in the M2 segment conferred a high sensitivity to PCP. 4. These results support the conclusion that the amino acid residues at the position 6' of the M2 segments contribute to the PCP potency difference between Torpedo and mouse receptors. 5. Another noncompetitive inhibitor of the AChR, the cembranoid eupalmerin acetate (EUAC), also inhibited the electric organ receptor with a somewhat higher potency than muscle AChR. However, the IC50 values for EUAC inhibition of hybrid receptors did not follow the pattern observed for PCP. Therefore, these two inhibitors interact differently with the AChR molecule.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Agonist self-inhibitory binding site of the nicotinic acetylcholine receptor

A major focus of current research on the nicotinic acetylcholine receptor (AChR) has been to understand the molecular mechanism of ion channel inhibition. In particular, we put special emphasis on the description of the localization of the agonist self-inhibitory binding site. Binding of agonist in the millimolar concentration range to this particular site produces inhibition of the ion flux activity previously elicited by the same agonist at micromolar concentrations. Due to the similitude in the pharmacological and electrophysiological behavior in inhibiting the ion channel of both high agonist concentrations and noncompetitive antagonists, we first describe the localization of noncompetitive inhibitor binding sites on the AChR. There is a great body of experimental evidence for the existence and location of luminal high-affinity noncompetitive inhibitor binding sites. In this regard, the most simple mechanism to describe the action of noncompetitive inhibitors which bind to luminal sites and, by its semblance, the agonist self-inhibition itself, is based on the assumption that these compounds enter the open channel, bind to different rings within the M2 transmembrane domain of the receptor, and block cation flux by occluding the receptor pore. However, the existence of high-affinity nonluminal noncompetitive inhibitor binding sites is not consistent with the open-channel-blocking mechanism. Instead, the presence of the quinacrine locus at the lipid-protein (alpha M1) interface approximately 7 A from the lipid-water interface and the ethidium domain located approximately 46 A from the membrane surface in the wall of the vestibule open the possibility for the regulation of cation permeation by an allosteric process. Additionally, the observed (at least partially) overlapping between the quinacrine and the agonist self-inhibitory binding site also suggests an allosteric process for agonist self-inhibition. For this alternative mechanism, cholinergic agonist molecules first need to be partitioned into (or to be adsorbed onto) the lipid membrane to further interact with its binding site located at the lipid-protein interface.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

The histrionicotoxin-sensitive ethidium binding site is located outside of the transmembrane domain of the nicotinic acetylcholine receptor: a fluorescence study

A novel, relatively photostable, long-wavelength fluorescent membrane probe, N-(Texas Red sulfonyl)-5(and 6)-dodecanoylamine (C12-Texas Red), was synthesized and used as an electronic energy acceptor for Förster fluorescence resonance energy transfer (FRET) between ethidium bound to a histrionicotoxin-sensitive binding site on the Torpedo nicotinic acetylcholine receptor (AChR) and the lipid membrane surface. FRET from membrane-partitioned 5-(N-dodecanoylamino)fluorescein (C12-fluorescein) to the membrane-partitioned C12-Texas Red was also determined with a parallel set of cuvettes to (1) compare FRET results with a donor in a known position in the membrane and (2) assess the surface density of the membrane-partitioned C12-Texas Red. Stern-Volmer analysis of the FRET results showed that C12-Texas Red quenched membrane-partitioned C12-fluorescein fluorescence 2.9 times more effectively than it quenched the receptor-bound ethidium fluorescence even though the Förster critical distances for the two donor-acceptor pairs were very similar (49.9 and 54.3 A, respectively). Analysis of the ethidium to C12-Texas Red FRET as a function of acceptor surface density with the assumptions that the donor is attached along the major axis of symmetry of a cylindrical protein embedded perpendicularly into the membrane (On-Axis FRET model) suggested that the distance of closest approach between the receptor-bound ethidium and the membrane surface was approximately 52 A. Because the minimum distance between the surface of the lipid-membrane domain and the major symmetry axis of the AChR is approximately 28 A, the FRET results strongly suggest that the ethidium binding site is not located near the entrance of the luminal transmembrane domain is generally assumed.(ABSTRACT TRUNCATED AT 250 WORDS)

An electrophilic affinity ligand based on (+)-MK801 distinguishes PCP site 1 from PCP site 2

The electrophilic affinity ligand, (+)-3-isothiocyanato-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycl ohepten-5,10 - imine hydrochloride [(+)-MK801-NCS] was characterized for its ability to acrylate phencyclidine (PCP) and sigma binding sites in vivo. Initial studies, conducted with mouse brain membranes, characterized the binding sites labeled by [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP). The Kd values of [3H]TCP for PCP site 1 (MK801-sensitive) and PCP site 2 (MK801-insensitive) were 12 nM and 68 nM, with Bmax values of 1442 and 734 fmol/mg protein, respectively. Mice were sacrificed 18-24 hours following intracerebroventricular administration of the acylator. The administration of (+)-MK801-NCS increased [3H]TCP binding to site 2, but not to site 1. Although (+)-MK801-NCS decreased [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d; ccyclohepten-5,10-imine maleate ([3H](+)-MK801) binding to site 1, it had no effect on [3H]TCP binding to site 1. Viewed collectively with other published data, these data support the hypothesis that PCP sites 1 and 2 are distinct binding sites, and that [3H]TCP and [3H](+)-MK801 label different domains of the PCP binding site associated with the NMDA receptor.

Isoflurane inhibits nicotinic acetylcholine receptor-mediated 22Na+ influx and muscarinic receptor-evoked cyclic GMP production in cultured bovine adrenal medullary cells

The effects of isoflurane on 22Na+ influx, 45Ca2+ influx, catecholamine secretion and cyclic GMP production induced by three kinds of secretagogue (nicotinic agonists, veratridine and a high concentration of K+) have been investigated using cultured bovine adrenal medullary cells. (1) Isoflurane (1-6%) inhibited catecholamine secretion stimulated by carbachol, nicotine and dimethyl-4-phenylpiperazinium in a concentration-dependent manner. Isoflurane suppressed carbachol-evoked 22Na+ influx and 45Ca2+ influx at concentrations similar to those which suppressed catecholamine secretion. The inhibition of catecholamine secretion by isoflurane was not overcome by increasing the concentration of carbachol. (2) The inhibitory effects of isoflurane on veratridine-induced 22Na+ influx, 45Ca2+ influx and catecholamine secretion became evident when the concentration of isoflurane was raised to 4-6%, i.e. 2-3 fold higher than the concentrations (1-2%) employed clinically. (3) High K(+)-evoked 45Ca2+ influx and catecholamine secretion were not affected by isoflurane (1-6%). (4) Isoflurane (1-6%) attenuated the production of cyclic GMP caused by muscarine, but not that caused by atrial natriuretic peptide or by sodium nitroprusside. These results suggest that isoflurane, at clinical anesthetic concentrations, inhibits nicotinic acetylcholine receptor-mediated cell responses as well as muscarinic receptor-mediated cyclic GMP production in adrenal medullary cells.

The ion channel of muscle and electric organ acetylcholine receptors: differing affinities for noncompetitive inhibitors

1. Muscle and electric organ acetylcholine receptors (AChR's) were expressed in Xenopus laevis oocytes and differential effects of noncompetitive blockers on each type of receptor were analyzed using a two-electrode voltage clamp. 2. The positively charged channel blockers, phencyclidine (PCP) and tetracaine, displayed a much lower potency on muscle receptor than on the electric organ receptor. The IC50 for both blockers at the electrocyte receptor was close to 1 microM at -60 mV and even lower at more hyperpolarized voltages. In contrast, with muscle receptor IC50's were 20 to 40 microM at -60 or -80 mV. 3. Eupalmerin acetate, an uncharged noncompetitive inhibitor that displaces [3H]PCP from its high-affinity binding site, inhibited both receptors with a similar potency: IC50 of 4.9 and 6.4 microM for electrocyte and muscle receptors, respectively. However, eupalmerin acetate affected the desensitization process in each receptor type differently and triggered an unusual biphasic response in the muscle receptor. 4. These results are discussed with respect to differences in the amino acid sequences of the M2 regions of the two receptors. 5. A third type of noncompetitive inhibitor, Mg2+, was also examined and it inhibited both receptors with a similar potency (IC50, 0.5-1.0 mM). However, Mg2+ appeared to act at sites other than the PCP site.

[3H]1-[2-(2-thienyl)cyclohexyl]piperidine labels two high-affinity binding sites in human cortex: further evidence for phencyclidine binding sites associated with the biogenic amine reuptake complex

Previous work demonstrated two high-affinity PCP binding sites in guinea pig brain labeled by [3H]TCP (1-(1-[2-thienyl]cyclohexyl)piperidine): site 1 (N-methyl-D-aspartate [NMDA]-associated) and site 2 (dopamine-reuptake complex associated). The present study examined brain membranes prepared from various species, including human, for the presence of site 2, defined as binding in the presence of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d]cyclohepten-5,10-imine maleate ((+)-MK801) minus binding in the presence of 10 microM TCP (nonspecific binding). Studies were conducted in absence of sodium which was found to be inhibitory to [3H]TCP binding. The results demonstrated detectable levels of site 2 in brain membranes of guinea pig, rabbit, pig, mouse, sheep, and human but not in the rat or chicken. Using human cortical membranes, site 2 was the predominant binding site. Detailed studies conducted with human cortical tissue showed that high-affinity dopamine (1-[2- [bis(4-fluorophenyl)-methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909)], [1,2]benzo(b)thiophenylcyclo-hexylpiperidine (BTCP), and serotonin (fluoxetine) uptake inhibitors produced a wash-resistant inhibition of [3H]TCP binding to site 2, but not site 1. Preincubation of guinea pig brain membranes with BTCP was shown to produce an increase in the dissociation rate of [3H]TCP from PCP site 2. Structure activity studies with various uptake inhibitors showed that GBR12909, benztropine, fluoxetine, and BTCP have higher affinity for site 2 than for site 1. (+)-MK801, ketamine, and tiletamine were very selective for site 1, whereas dexoxadrol and TCP were moderately selective for site 1. These results suggest that human cortex possesses high-affinity PCP binding sites associated with biogenic reuptake binding sites, and that guinea pig brain, but not rat brain, may be an appropriate animal model for studying PCP site 2 in human brain.

Binding sites for alpha-bungarotoxin and the noncompetitive inhibitor phencyclidine on a synthetic peptide comprising residues 172-227 of the alpha-subunit of the nicotinic acetylcholine receptor

The binding of the competitive antagonist alpha-bungarotoxin (alpha-Btx) and the noncompetitive inhibitor phencyclidine (PCP) to a synthetic peptide comprising residues 172-227 of the alpha-subunit of the Torpedo acetylcholine receptor has been characterized. 125I-alpha-Btx bound to the 172-227 peptide in a solid-phase assay and was competed by alpha-Btx (IC50 = 5.0 x 10(-8) M), d-tubocurarine (IC50 = 5.9 X 10(-5)M), and NaCl (IC50 = 7.9 x 10(-2)M). In the presence of 0.02% sodium dodecyl sulfate, 125I-alpha-Btx bound to the 56-residue peptide with a KD of 3.5 nM, as determined by equilibrium saturation binding studies. Because alpha-Btx binds to a peptide comprising residues 173-204 with the same affinity and does not bind to a peptide comprising residues 205-227, the competitive antagonist and hence agonist binding site lies between residues 173 and 204. After photoaffinity labeling, [3H]PCP was bound to the 172-227 peptide. [3H]PCP binding was inhibited by chlorpromazine (IC50 = 6.3 x 10(-5)M), tetracaine (IC50 = 4.2 x 10(-6)M), and dibucaine (IC50 = 2.7 x 10(-4)M). Equilibrium saturation binding studies in the presence of 0.02% sodium dodecyl sulfate showed that [3H]PCP bound at two sites, a major site of high affinity with an apparent KD of 0.4 microM and a minor low-affinity site with an apparent KD of 4.6 microM. High -affinity binding occurred at a single site on peptide 205-227 (KD = 0.27 microM) and was competed by chlorpromazine but not by alpha-Btx.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular environment of the phencyclidine binding site in the nicotinic acetylcholine receptor membrane

Phencyclidine is a highly specific noncompetitive inhibitor of the nicotinic acetylcholine receptor. In a novel approach to study this site, a spin-labeled analogue of phencyclidine, 4-phenyl-4-(1-piperidinyl)-2,2,6,6-tetramethylpiperidinoxyl (PPT) was synthesized. The binding of PPT inhibits 86Rb flux (IC50 = 6.6 microM), and [3H]phencyclidine binding to both resting and desensitized acetylcholine receptor (IC50 = 17 microM and 0.22 microM, respectively). From an indirect Hill plot of the inhibition of [3H]phencyclidine binding by PPT, a Hill coefficient of approximately one was obtained in the presence of carbamylcholine and 0.8 in alpha-bungarotoxin-treated preparations. Taken together, these results indicate that PPT mimics phencyclidine in its ability to bind to the noncompetitive inhibitor site and is functionally active in blocking ion flux across the acetylcholine receptor channel. Analysis of the electron spin resonance signal of the bound PPT suggests that the environment surrounding the probe within the ion channel is hydrophobic, with a hydrophobicity parameter of 1.09. A dielectric constant for the binding site was estimated to be in the range of 2-3 units.

Saturable binding of anesthetics to nicotinic acetylcholine receptors. A possible mechanism of anesthetic action

Recent controversies in the existence of saturable binding of general anesthetics in brain tissues prompted a careful examination of specific binding of anesthetics to neural receptors. We examined the binding of both local and general anesthetics using electron spin resonance and radioligand criteria. Our results suggested that the hydrophobic path, most probably through the lipid bilayer, figures importantly in the binding of the uncharged moieties of anesthetics. Competitive interactions by hydrophobic compounds for the high-affinity site in the nicotinic acetylcholine receptor led us to propose a hypothesis that includes a hydrophobic crevice of limited volume as part of the high-affinity site. Association of anesthetic at this crevice is in turn dependent on the anesthetic concentration in the lipid phase of the membrane. The hypothesis provides a mechanism for the saturable interaction of anesthetics with their protein target site in the membrane without violating the correlations expressed by the Meyer-Overton rule of anesthetic action.

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

(+/-)alpha-Phenyl-beta-(3,4-dimethoxy)- and (+/-)alpha-phenyl-beta -(3,4-dihydroxy)phenethylamines: potential probes for nicotinic acetylcholine receptor-ion channel molecule from torpedo electric organ

The synthesis of some N-methyl, N-alkyl derivatives of (+/-)alpha-phenyl-beta-(3,4-dimethoxy)- and (+/-)alpha-phenyl-beta-(3,4-dihydroxy)-phenethylamines was achieved. These compounds were shown to bear certain structural features of acetylcholine (ACh), as well as phencyclidine (PCP). The latter was reported to act as a specific probe for the nicotinic ACh receptor-ion channel molecule from Torpedo electric organ. Biochemical binding studies revealed that for the nicotinic ACh receptor, the 3,4-dimethoxy derivatives behaved as blockers for the binding interaction of [3H]ACh, whereas the 3,4-dihydroxy analogues stimulated such binding. On the other hand, all of the tested phenethylamines exhibited potent blockade towards [3H]PCP binding interactions. The results indicated that the tested compounds might be applied as potential probes for the ACh receptor-ion channel molecule.

Phencyclidine. Physiological actions, interactions with excitatory amino acids and endogenous ligands

Phenycyclidine (PCP) produces many profound effects in the central nervous system. PCP has numerous behavioral and neurochemical effects such as inhibiting the uptake and facilitating the release of dopamine, serotonin, and norepinephrine. PCP also interacts with sigma, mu opioid, muscarinic, and nicotinic receptors. However, the psychotomimetic effects induced by PCP are believed to be mediated by specific PCP receptors, where PCP binds with greater potency than sigma compounds. Electrophysiological, behavioral, and neuro-chemical evidence strongly suggests that at least some of the many PCP actions result from antagonism of excitatory amino acid-induced responses via PCP receptors. The recent isolation and partial characterization of the alpha and beta endopsychosins and the identification of other endogenous ligands for the PCP and sigma receptors, is another promising area of research in the elucidation of the physiological role of an endogenous PCP and sigma system.

Biochemical interactions of carbamates and ecothiophate with the activated conformation of nicotinic acetylcholine receptor

Purified Torpedo nobiliana electric organ acetylcholine receptor (AChR) was reconstituted into membranes containing natural phospholipids supplemented with cholesterol (25% w/w). The reconstituted system facilitates the study of the effects of drugs on the regulation of the AChR channel complex under both resting and carbachol (carb)-stimulated conditions. Neostigmine (Neo) was the only carbamate to induce activation of [3-H]-phencyclidine ([3-H]-PCP) binding to the channel sites, acting as a weak agonist. The activation of [3-H]-PCP binding is dependent upon the nature of the reconstituted systems, with carb/Neo activation ratios of 8, 3, and 1 for the intact purified AChR vesicles fraction (PVF), the PVF reconstituted in phospholipid/cholesterol (CRPVF), and the PVF reconstituted in phospholipid (RPVF), respectively. The carbamates Neo, physostigmine (Physo), and pyridostigmine (Pyrido) inhibited carb-activated [3-H]-PCP binding with Ki values of 10, 20, and 1,600 microM, respectively. The inhibition was mixed competitive-noncompetitive in nature. The characteristic response of CRPVF to carb-stimulated [22-Na] influx was inhibited by the three carbamates, with IC-50 values of 6, 50, and 1,000 microM for Neo, Physo, and Pyrido, respectively. The quaternary ammonium organophosphate ecothiophate (Eco) inhibited carb-stimulated [22-Na] influx with potency similar to that of Neo. Preincubation of AChR preparation with the carbamates and ecothiophate caused a reduction in the binding of [125-I]-alpha-bungarotoxin ([125-I]-alpha-BGT) with the following decreasing order of potency: Neo less than Physo less than Eco less than Pyrido. Calcium has a direct modulatory role on the time-course inhibition of [125-I]-alpha-BGT binding by these drugs. While we observed a high potency of Neo and Physo in inhibiting [125-I]-alpha-BGT binding, it was undetectable for the carbamate insecticide 2-methyl-2-(methylthio)propionaldehyde-O-(methylcarbamoyl)oxime (aldicarb). These data suggest that the potent anticholinesterase carbamate agents interact differently with the AChR and its ionic channel. Their interactions with the nicotinic AChR channel system can be described as (a) weakly agonist, (b) directly acting on the open conformation of the channel, and (c) blocking the AChR-binding sites.

The mechanism by which cyclopiazonic acid potentiates accumulation of tetraphenylphosphonium in cultured renal epithelial cells

Cyclopiazonic acid (CPA), a fungal metabolite produced by Aspergillus and Penicillium, potentiated the accumulation of the quaternary cation tetraphenylphosphonium (TPP+) in cultured pig renal epithelial cells. This is the first report of a natural product mediating the tight and apparently nonsaturable binding of a membrane potential probe to subcellular compartments. The potentiated TPP+ accumulation was dose dependent, nonsaturable, and not a result of hyperpolarization across the plasma membrane. Cyclopiazonic acid-potentiated accumulation was completely inhibited by the protonophore carbonylcyanide-m-chlorophenylhydrazone (CCCP). Dinitrophenol (DNP), tetrahexylammonium (THA), and n-ethylmaleimide (NEM) were also effective inhibitors of CPA-potentiated TPP+ accumulation. Although CPA-potentiated TPP+ uptake appeared to be energy dependent, TPP+ efflux (in the presence of CCCP) from CPA-treated cells was incomplete and most of the TPP+ accumulated in the presence of CPA was tightly bound. Dicyclohexylcarbodiimide (DCC), verapamil, and monensin also stimulated TPP+ accumulation, but the TPP+ which accumulated in the presence of these compounds was not tightly bound. As with controls, fractionation of cells which had accumulated TPP+ in the presence of DCC, verapamil, or monensin always resulted in near complete recovery (greater than 93%) of the TPP+ in the cytosolic fraction, whereas with CPA, greater than 88% of the TPP+ was recovered noncovalently bound in the plasma membrane and mitochondrial fractions. These results are consistent with the hypothesis that CPA-potentiated TPP+ accumulation is a result of potentiated partitioning of TPP+ into the plasma membranes and mitochondria of LLC-PK1 cells.

Block of single acetylcholine-activated channels in chick myotubes by alkylguanidines

The mechanism of block of single acetylcholine-activated channels by alkylguanidines was studied by means of the gigaohm seal, patch clamp technique on chick myotubes. The single channel current-voltage relationship in symmetrical Cs solutions was linear over membrane potentials from -100 mV to +100 mV with the reversal potential close to 0 mV. Single channel conductance was estimated to be 40 pS with 120 mM cesium in both external and internal solutions and 54 pS with 360 mM cesium in both solutions. Cs binds within the channel with an apparent KD of approximately 100 mM and the channel has a saturating current of 8 -9 pA at +100 mV. External application of 10 mM and 20 mM methylguanidine decreased single channel current amplitude in a manner dependent upon concentration and membrane potential. The block was greater for inward currents than for outward currents. Ethylguanidine was more potent when applied externally (2 mM) than when applied internally (2-20 mM) and reduced single channel current amplitude in a voltage dependent manner. Increasing internal cesium concentration from 60 to 360 mM decreased the block of the single channel by internally applied ethylguanidine. There appear to be at least two channel binding sites for methyl- and ethylguanidine located approximately 25% of the membrane electric field from either membrane surface. The acetylcholine-activated ionic channel is symmetric with respect to cesium permeation but not to block by ethylguanidine. Permeant ions and blocking ions appear to compete for channel occupancy.

Endplate channel actions of a hemicholinium-3 analog, DMAE

The effect of the hemicholinium-3 analog, DMAE, on endplate currents (EPC) was investigated in the transected cutaneous pectoris muscle of the frog using a conventional two-microelectrode voltage clamp. At a low concentration (5 microM), DMAE produced a long-lasting decrease in the rate constant of decay (alpha) and an increase in the peak current amplitude (Ip). At higher concentrations (10--100 microM), DMAE produced biphasic changes characterized by a transient, marked decrease of alpha and increase of Ip followed by a long-lasting marked increase of alpha and decrease of Ip. When DMAE was removed from the bath recovery from block was asymmetrical in that alpha recovered more quickly than did Ip. Pretreatment with neostigmine or collagenase partially antagonized the initial effects without affecting the steady state effects of DMAE, indicating that the initial effects of DMAE may be, at least in part, due to inhibition of the enzyme acetylcholinesterase. The drug reverses the normal voltage dependence of alpha without altering the single exponential nature of decay of the EPC. The inward EPC was more markedly blocked than outward EPC, resulting in a highly non-linear current-voltage relation with Ip decreasing with increasing hyperpolarization. This effect may indicate that DMAE causes a voltage-dependent block of closed acetylcholine-activated ion channels.

Effects of calcium on the binding of phencyclidine to acetylcholine receptor-rich membrane fragments from Torpedo californica electroplaque

The influence of calcium on the binding of phencyclidine (PCP) to acetylcholine (ACh) receptor-rich membrane fragments was investigated. Calcium decreased the equilibrium affinity for PCP in the presence, but not in the absence, of the cholinergic agonist carbamylcholine. The effect of calcium was rapidly reversible by EGTA, indicating that it was not attributable to a calcium-activated protease or a phospholipase. Following detergent solubilization of the nicotinic ACh receptor, the calcium effect on PCP remained, suggesting that calcium may interact directly with the receptor to exert its effect. Other divalent cations (Mn2+, La2+, Co2+, Mg2+) had similar effects. A correlate of "desensitization" of the ACh receptor can be observed using PCP binding, and a two-step "desensitization" process can be observed. Calcium seemed to increase the amplitude of a rapid component of receptor "desensitization." The results presented in this paper suggest that calcium may play a role in the modulation of the nicotinic ACh receptor.

Phencyclidine in nanomolar concentrations binds to synaptosomes and blocks certain potassium channels

Phencyclidine [1-(phenylcyclohexyl)piperidine; PCP], in low dose (approximately equal to 0.1-0.2 mg/kg of body weight), induces a schizophrenia-like behavioral syndrome in man; this effect has been attributed to block of neuronal K channels. We used a K-stimulated 86Rb efflux assay to demonstrate that low concentrations of PCP (10-50 nM) block a class of depolarization-activated K channels in rat brain synaptosomes--pinched-off presynaptic nerve terminals. The dose-response curve is biphasic, and much higher PCP concentrations (greater than 10 microM) are required to block the remainder of the K-stimulated 86Rb efflux. The [3H]PCP binding curve for synaptosomes is also biphasic: PCP binds to some components with high affinity (Kd approximately equal to 6.0 X 10(-8) M), and to other components with much lower affinity (Kd approximately equal to 1.15 X 10(4) M). PCP can be photoactivated with UV light to form covalent bonds: after UV irradiation, previously-bound [3H]PCP is no longer displaceable by a large excess of unlabeled PCP. Preliminary data from NaDodSO4/polyacrylamide gel electrophoresis studies after covalent binding of [3H]PCP to synaptosomes, suggest that the high-affinity binding site may be on a large protein (Mr approximately equal to 220,000). We conclude that the high-affinity PCP binding protein is associated with the K channels that are blocked by nanomolar concentrations of PCP. Block of these channels could, by prolonging action-potential duration in presynaptic nerve terminals, enhance calcium entry and neurotransmitter release, thereby altering transmission at central synapses involved in behavioral expression.

Triphenylmethylphosphonium is an ion channel ligand of the nicotinic acetylcholine receptor

The lipophilic cation triphenylmethylphosphonium (Ph3MeP+), which is widely used as a sensor for membrane potential with cells, organelles, and membrane vesicles, is shown also to accumulate in membranes rich in nicotinic acetylcholine receptor in a voltage-independent way. Evidence is presented that Ph3MeP+ in this system is bound to a cation-binding site of the ion channel that is part of the acetylcholine receptor complex. Binding is stimulated by cholinergic effectors (Kd = 13 microM in the absence of carbamoylcholine; Kd = 1.5 microM in the presence of 10 microM carbamoylcholine), and this stimulation is blocked by alpha-bungarotoxin. Ph3MeP+ blocks efflux of 22Na from receptor-rich microsacs and appears to compete with the channel ligand phencyclidine for a common binding site. In contrast to the binding of other proven channel ligands, Ph3MeP+-binding is not affected by desensitization.

Cocaine and phencyclidine inhibition of the acetylcholine receptor: analysis of the mechanisms of action based on measurements of ion flux in the millisecond-to-minute time region

The effects of cocaine and of phencyclidine and procaine on acetylcholine receptor-controlled ion flux were measured in the millisecond-to-minute time region. Chemical kinetic measurements of ion flux were made in membrane vesicles prepared from the electric organ of Electrophorus electricus and in PC-12 cells, a sympathetic neuronal cell line. A quench-flow technique was used to measure ion flux in the millisecond-to-second range in membrane vesicles. Cocaine and phencyclidine both inhibit acetylcholine receptor-controlled ion flux, but by different mechanisms. Both compounds decrease the initial rate of ion flux, an effect observed with the local anesthetic procaine. This inhibition cannot be prevented by saturating concentrations of acetylcholine (1 mM). These results from chemical kinetic experiments are consistent with electrophysiological measurements which indicate that local anesthetics act by interfering with the movement of ions through receptor-formed channels. The chemical kinetic experiments, however, give additional information about the action of phencyclidine. They indicate that phencyclidine also increases the rate of receptor inactivation (desensitization) and changes the equilibrium between active and inactive receptor conformations, effects not observed in the presence of cocaine or procaine.

Effects of phencyclidine and its derivatives on enteric neurones

The effects of N-(1-phenylcyclohexyl) piperidine (PCP) and related drugs on isolated intact segments of the guinea-pig ileum were determined. 1-1-(2-Thienyl) cyclohexylpiperidine (TCP), PCP and ketamine decreased the height of electrically induced contractions (0.1 Hz) of intact segments of isolated guinea-pig ileum. Thirty to forty percent of the inhibition of contraction height (0.1 Hz) was reversed by pretreatment with the pure narcotic antagonist, naloxone. This naloxone-reversible component showed cross-tolerance with morphine. PCP pretreatment caused a shift to the right in the dose-response curve to acetylcholine (ACh) that was not parallel with the control dose-response curve. Thus PCP does not interact with the muscarinic cholinoceptor in a strictly atropine-like competitive fashion. Binding sites for [3H]-PCP were detected in homogenates of the guinea-pig longitudinal muscle-myenteric plexus preparation. The affinity constants and the rank order of potencies of various PCP derivatives competing with [3H]-PCP for binding suggest that these binding sites are very similar to those found in the central nervous system. These data suggest that the guinea-pig isolated ileum may be used as an in vitro system for studying the mechanism of action of phencyclidines.

Selective labeling of the delta subunit of the acetylcholine receptor by a covalent local anesthetic

A radioactive photoaffinity derivative of the potent local anesthetic trimethisoquin, 5-azido[3H]trimethisoquin, was used to label the acetylcholine receptor from Torpedo marmorata electric organ. The product labeled the 66 000-dalton (delta) subunit of the receptor with the selectivity expected for an affinity label of the site for noncompetitive blockers. That is, the labeling was enhanced by cholinergic agonists and inhibited by other noncompetitive blockers. The 40 000-dalton (alpha)( subunit of the receptor was labeled in a manner consistent with the attachment of 5-azido[3H]trimethisoquin to an acetylcholine binding site as the incorporation of radioactivity into the alpha chain was inhibited by cholinergic agonists and antagonists, such as carbamylcholine, d-tubocurarine, and alpha-bungarotoxin. The reversible binding of [3H]phencyclidine, a potent noncompetitive blocker, to acetylcholine receptor rich membranes resembled qualitatively and quantitatively the 5-azido[3H]trimethisoquin labeling of the delta subunit and was inhibited by the prior covalent labeling of the membranes with nonradioactive 5-azidotrimethisoquin. Thus, 5-azido[3H]-trimethisoquin labels at least a portion of the binding site for noncompetitive blockers at the level of the delta subunit. The functional significance of this site and the use of 5-azidotrimethisoquin in the study of acetylcholine receptor structure and function are discussed.

The behavioral effects of phencyclidines may be due to their blockade of potassium channels

The action of phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] and its behaviorally active analog (m-amino-PCP) and of two behaviorally inactive analogs [m-nitro-PCP and 1-piperidinocyclohexanecarbonitrile (PCC)] were examined in this study. In a test of spatial alternation performance in rats, PCP and m-amino-PCP were much more potent behavior modifiers than were PCC and m-nitro-PCP. We studied the effects of the drugs on the ionic channels of the electrically excitable membrane and of the nicotinic acetylcholine (AcCho) receptors at the neuromuscular junction of frog skeletal muscle. All four compounds blocked the indirectly elicited muscle twitch and depressed the amplitude and rate of rise of directly elicited muscle action potentials. They also caused a voltage- and concentration-dependent decrease in the peak amplitude of the endplate current but did not react with the nicotinic AcCho receptor. These observations indicate that the four compounds have comparable blocking effects on the ionic channels associated with the nicotinic AcCho receptor. In contrast, the behaviorally active agents could be distinguished from behaviorally inactive ones by their effects on K+ conductance. PCP and m-amino-PCP blocked delayed rectification in frog sartorius muscles, prolonged the muscle action potential more than 2-fold, and markedly potentiated the directly elicited muscle twitch. The behaviorally active compound also blocked depolarization-induced 86Rb+ efflux from rat brain synaptosomes (presumably a measure of K+ conductance) and increased quantal content at the frog neuromuscular junction. In these actions, m-nitro-PCP was much less effective, and PCC was relatively ineffective. Because PCP and m-amino-PCP are much more potent behavior modifiers than PCC and m-nitro-PCP, we suggest that the behavioral effects of PCP and m-amino-PCP, may be due to a block of K+ conductance and enhancement of transmitter release at central neurons.

Phencyclidine (angel dust)/sigma "opiate" receptor: visualization by tritium-sensitive film

[3H]Phencyclidine ([3H]PCP) binds specifically to an apparently single class of binding sites on slide-mounted sections of rat olfactory bulb (Kd = 46 nM; Bmax = 10.5 fmol per slice). Bound [3H]PCP can be displaced by nonradioactive PCP and a series of its analogs with relative potencies that correlate closely (P less than 0.001) with values determined in a rat discrimination test that utilized PCP as a cue. Although morphine, naloxone, and opiate peptides do not displace bound [3H]PCP, psychotomimetic benzomorphans, classed as "sigma opiates," are quite potent displacers in vitro and have PCP-like behavioral properties in vivo. These results suggest that phencyclidine and the sigma opiates act at the same sites. [3H]PCP binding sites were visualized by using tritium-sensitive LKB film analyzed by computerized densitometry and color coding. The [3H]PCP binds most densely to cortical areas, diffusely in neocortex, and somewhat heterogeneously in the laminae of the hippocampal formation and dentate gyrus. Most of the brainstem and spinal cord show low specific [3H]PCP binding, with gray matter generally showing more binding than white.

Differentiation of the open and closed states of the ionic channels of nicotinic acetylcholine receptors by tricyclic antidepressants

The actions of two clinically important dibenzocycloheptane antidepressant drugs, amitriptyline and nortriptyline, were studied on ionic channels of nicotinic acetylcholine (AcCho) receptors at the neuromuscular junction of frog skeletal muscle. Amitriptyline (5-10 microM) and nortriptyline (1-2 microM), like imipramine (5-10 microM), did not react with the nicotinic AcCho receptor but caused a voltage- and time-dependent decrease in the peak amplitude of the endplate current (epc). The time constant of epc decay, however, retained its voltage sensitivity. The voltage- and time-dependent effect of amitriptyline was nonlinear with regard to the current/voltage (I/V) relationship. Nortriptyline also had a more pronounced voltage- and time-dependent effect evidenced by a hysteresis loop in the I/V relationship of the epc was eliminated by the use of 50-msec stepwise changes of the membrane potential. The nonlinearity and hysteresis were due to a time-dependent phenomenon and did not involve previous AcCho receptor activation. The rate constant of the voltage- and time-dependent decrease in epc amplitude was sensitive to the membrane electric field and varied linearly with the membrane potential. Iontophoretically elicited epcs were much more depressed by both drugs than were spontaneous miniature epcs. There was no effect on the time constant of miniature epc decay, single-channel lifetime, or conductance. Thus (as we have pointed out in our histrionicotoxin studies) the primary site of action of these agents presumably is the activated but nonconducting species of the ionic channel of the nicotinic AcCho receptor. These agents, particularly nortriptyline, point to several different binding sites of the ionic channel and are suitable tools for the separation of the effects on peak current amplitude from its time constant of decay.

Ultraviolet light-induced labeling by noncompetitive blockers of the acetylcholine receptor from Torpedo marmorata

Reversible ligands were attached covalently to membrane-bound acetylcholine receptor from Torpedo marmorata by a method which is generally applicable and does not require the synthesis of specially designed molecules. UV irradiation of the receptor in the presence of [3H]trimethisoquin, [3H]phencyclidine, or [3H]perhydrohistrionicotoxin resulted in the labeling of the binding site(s) for these noncompetitive blockers of the permeability response. The labeling of the delta chain was enhanced by carbamoylcholine, and this increase was blocked by snake alpha-toxins. The effect of carbamoylcholine on [3H]trimethisoquin binding was more pronounced than with the other two noncompetitive blockers; in all instances, the labeling was abolished by unlabeled histrionicotoxin. These three compounds therefore interact with the high-affinity site for noncompetitive blockers. Incorporation of radioactivity also occurred into the alpha chain but either was insensitive to cholinergic effectors or decreased in the presence of carbamoylcholine (or snake alpha-toxin), probably as a result of an interaction with the acetylcholine-binding site. In contrast to the other noncompetitive blockers tested, [3H]chlorpromazine heavily labeled the four receptor polypeptides (alpha, beta, gamma, delta), and this labeling also was enhanced by carbamoylcholine and decreased by histrionicotoxin. These data indicate a contribution of the delta chain to the binding site(s) of several well-characterized noncompetitive blockers and suggest that other receptor polypeptides may also contribute to this binding.

[3H]Phencyclidine: a probe for the ionic channel of the nicotinic receptor

To evaluate [3H]phencyclidine ([3H]PCP)as a probe for the ionic channel of the nicotinic receptor, the characteristics of its binding to electric organ membranes od Torpedo ocellata and its effects on frog sartorius muscle were studied. Similar to PCP, [3H]PCP depressed the peak amplitude of endplate current, caused nonlinearity in the voltage-current relationship at negative potentials, accelerated the decay time of the end-plate current, and shortened the channel lifetime. Thus, [3H]PCP interacted with the ionic channel of the nicotinic receptor, although there were a few differences between its effect and that of PCP. Binding of [3H]PCP to Torpedo membranes was to sites on the ionic channel of acetylcholine (AcCho) receptor because it was saturable, dependent upon protein concentration, and inhibited by drugs that interact with the ionic channel, and the initial rate of binding was potentiated by receptor agonists. Equilibrium binding of [3H]PCP to Torpedo membranes was with two affinities, but in the presence of AcCho, [3H]PCP binding was with a single affinity. The affinities of channel drugs obtained by inhibition of binding of [3H]PCP and [3H[perhydrohistrionicotoxin to Torpedo membranes were different, with correlation coefficients of 0.52 and 0.82 in the absence and presence of a receptor agonist, respectively; this suggests differences in their binding sites on the ionic channel of the AcCho receptor.