MK-801 inhibition of nicotinic acetylcholine receptor channels

Gradients of neurotransmitter receptor expression in the macaque cortex

Dynamics and functions of neural circuits depend on interactions mediated by receptors. Therefore, a comprehensive map of receptor organization across cortical regions is needed. In this study, we used in vitro receptor autoradiography to measure the density of 14 neurotransmitter receptor types in 109 areas of macaque cortex. We integrated the receptor data with anatomical, genetic and functional connectivity data into a common cortical space. We uncovered a principal gradient of receptor expression per neuron. This aligns with the cortical hierarchy from sensory cortex to higher cognitive areas. A second gradient, driven by serotonin 5-HT1A receptors, peaks in the anterior cingulate, default mode and salience networks. We found a similar pattern of 5-HT1A expression in the human brain. Thus, the macaque may be a promising translational model of serotonergic processing and disorders. The receptor gradients may enable rapid, reliable information processing in sensory cortical areas and slow, flexible integration in higher cognitive areas.

Acute D-Serine Co-Agonism of β-Cell NMDA Receptors Potentiates Glucose-Stimulated Insulin Secretion and Excitatory β-Cell Membrane Activity

Insulin-secreting pancreatic β-cells express proteins characteristic of D-serine regulated synapses, but the acute effect of D-serine co-agonism on its presumptive β-cell target, N-methyl D-aspartate receptors (NMDARs), is unclear. We used multiple models to evaluate glucose homeostasis and insulin secretion in mice with a systemic increase in D-serine (intraperitoneal injection or DAAO mutants without D-serine catabolism) or tissue-specific loss of Grin1-encoded GluN1, the D-serine binding NMDAR subunit. We also investigated the effects of D-serine ± NMDA on glucose-stimulated insulin secretion (GSIS) and β-cell depolarizing membrane oscillations, using perforated patch electrophysiology, in β-cell-containing primary isolated mouse islets. In vivo models of elevated D-serine correlated to improved blood glucose and insulin levels. In vitro, D-serine potentiated GSIS and β-cell membrane excitation, dependent on NMDAR activating conditions including GluN1 expression (co-agonist target), simultaneous NMDA (agonist), and elevated glucose (depolarization). Pancreatic GluN1-loss females were glucose intolerant and GSIS was depressed in islets from younger, but not older, βGrin1 KO mice. Thus, D-serine is capable of acute antidiabetic effects in mice and potentiates insulin secretion through excitatory β-cell NMDAR co-agonism but strain-dependent shifts in potency and age/sex-specific Grin1-loss phenotypes suggest that context is critical to the interpretation of data on the role of D-serine and NMDARs in β-cell function.

Effects of 7.5% Carbon Dioxide and Nicotine Administration on Latent Inhibition

Stratified medicine approaches have potential to improve the efficacy of drug development for schizophrenia and other psychiatric conditions, as they have for oncology. Latent inhibition is a candidate biomarker as it demonstrates differential sensitivity to key symptoms and neurobiological abnormalities associated with schizophrenia. The aims of this research were to evaluate whether a novel latent inhibition task that is not confounded by alternative learning effects such as learned irrelevance, is sensitive to (1) an in-direct model relevant to psychosis [using 7.5% carbon dioxide (CO₂) inhalations to induce dopamine release via somatic anxiety] and (2) a pro-cognitive pharmacological manipulation (via nicotine administration) for the treatment of cognitive impairment associated with schizophrenia. Experiment 1 used a 7.5% CO₂ challenge as a model of anxiety-induced dopamine release to evaluate the sensitivity of latent inhibition during CO₂ gas inhalation, compared to the inhalation of medical air. Experiment 2 examined the effect of 2 mg nicotine administration vs. placebo on latent inhibition to evaluate its sensitivity to a potential pro-cognitive drug treatment. Inhalation of 7.5% CO₂ raised self-report and physiological measures of anxiety and impaired latent inhibition, relative to a medical air control; whereas administration of 2 mg nicotine, demonstrated increased latent inhibition relative to placebo control. Here, two complementary experimental studies suggest latent inhibition is modified by manipulations that are relevant to the detection and treatment of schizophrenia. These results suggest that this latent inhibition task merits further investigation in the context of neurobiological sub-groups suitable for novel treatment strategies.

Differential effects of glutamate N-methyl-D-aspartate receptor antagonists on risky choice as assessed in the risky decision task

RATIONALE

Risky choice can be measured using the risky decision task (RDT). In the RDT, animals choose between a large, risky option that is paired with probabilistic foot shock and a small, safe option that is never paired with shock. To date, studies examining the neurochemical basis of decision-making in the RDT have focused primarily on the dopaminergic system but have not focused on the glutamatergic system, which has been implicated in risky decision-making.

OBJECTIVES

Because glutamate is known to play a critical role in decision-making, we wanted to determine the contribution of the glutamatergic system to performance in the RDT.

METHODS

In the experiment, 32 rats (16 male; 16 female) were tested in the RDT. The probability of receiving a foot shock increased across the session (ascending schedule) for half of the rats but decreased across the session (descending schedule) for half of the rats. Following training, rats received injections of the N-methyl-D-aspartate (NMDA) receptor competitive antagonist CGS 19755 (0, 1.0, 2.5, 5.0 mg/kg; s.c.) and the GluN2B-selective antagonist Ro 63-1908 (0, 0.1, 0.3, 1.0 mg/kg; s.c.).

RESULTS

CGS 19755 (2.5 and 5.0 mg/kg) increased risky choice in males and females trained on the ascending schedule. Ro 63-1908 (1.0 mg/kg) decreased risky choice, but only in male rats trained on the ascending schedule.

CONCLUSIONS

Although NMDA receptor antagonists differentially alter risky choice in the RDT, the current results show that NMDA receptors are an important mediator of decision-making involving probabilistic delivery of positive punishment.

Tonic NMDA receptor signalling shapes endosomal organisation in mammalian cells

Calcium signalling through NMDA-type glutamate receptors (NMDARs) plays a key role in synaptic plasticity in the central nervous system (CNS). NMDAR expression has also been detected in other tissues and aberrant glutamate signalling has been linked to cancer; however, the significance of NMDAR function outside of the CNS remains unclear. Here, I show that removal of extracellular calcium rapidly decreases the size of early endosomes in primary human fibroblasts. This effect can be mimicked by blockade of NMDA-type glutamate receptors but not voltage-gated calcium channels (VGCCs), and can also be observed in primary hippocampal neurons and Jurkat T cells. Conversely, in a breast cancer cell line MDA-MB-231 NMDAR blockade results in an increase in endosomal size and decrease in number. These findings reveal that calcium signalling via glutamate receptors controls the structure of the endosomal system and suggest that aberrations in NMDAR-regulated membrane trafficking may be associated with cancer.

Influence of retinal NMDA receptor activity during autoimmune optic neuritis

Autoimmune optic neuritis (AON), a model of multiple sclerosis-associated optic neuritis, is accompanied by degeneration of retinal ganglion cells (RGCs) and optic nerve demyelination and axonal loss. In order to investigate the role of N-methyl-d-aspartate (NMDA) receptors in mediating RGC degeneration, upstream changes in the optic nerve actin cytoskeleton and associated deterioration in visual function, we induced AON in Brown Norway rats by immunization with myelin oligodendrocyte glycoprotein. Subsequently, visual acuity was assessed by recording visual evoked potentials and electroretinograms prior to extraction of optic nerves for western blot analysis and retinas for quantification of RGCs. As previously reported, in Brown Norway rats RGC degeneration is observed prior to onset of immune cell infiltration and demyelination of the optic nerves. However, within the optic nerve, destabilization of the actin cytoskeleton could be seen as indicated by an increase in the globular to filamentous actin ratio. Interestingly, these changes could be mimicked by intravitreal injection of glutamate, and similarly blocked by application of the NMDA receptor blocker MK-801, leading us to propose that prior to optic nerve lesion formation, NMDA receptor activation within the retina leads to retinal calcium accumulation, actin destabilization within the optic nerve as well as a deterioration of visual acuity during AON.

Antinociceptive activity of the Psidium brownianum Mart ex DC. leaf essential oil in mice

Chronic pain management has several adverse effects and research looking for new and effective pain management drugs posing lower undesirable effects is necessary. Given the above, the pharmacological investigation of medicinal plants significantly contributes to the dissemination of plant-derived therapeutics. The aim of this study was to evaluate the antinociceptive activity of the Psidium brownianum Mart ex DC. leaf essential oil (PBEO) and the participation of the opioid pathway in this effect in mice. Swiss Mus musculus male mice were tested using acute nociception models (acetic acid induced abdominal contortions, formalin, capsaicin and hot plate tests). The possible myorelaxant action of the PBEO was tested using the rotarod test. The essential oil reduced animal nociception in chemical and heat models, with this action being devoid of a myorelaxant effect. Naloxone (2 mg/kg, intraperitoneally - i.p.) partially antagonized the PBEO activity, possibly acting via opioid receptors. The results obtained provide evidence that the traditional Psidium brownianum use may be effective for pain treatment.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

Elucidating the mechanisms of fear extinction in developing animals: a special case of NMDA receptor-independent extinction in adolescent rats

NMDA receptors (NMDARs) are considered critical for the consolidation of extinction but recent work challenges this assumption. Namely, NMDARs are not required for extinction retention in infant rats as well as when extinction training occurs for a second time (i.e., reextinction) in adult rats. In this study, a possible third instance of NMDAR-independent extinction was tested. Although adolescents typically exhibit impaired extinction retention, rats that are conditioned as juveniles and then given extinction training as adolescents (JuvCond-AdolesExt) have good extinction retention. Unexpectedly, this good extinction retention is not associated with an up-regulation of a synaptic plasticity marker in the medial prefrontal cortex, a region implicated in extinction consolidation. In the current study, rats received either the noncompetitive NMDAR antagonist MK801 (0.1 mg/kg, s.c.) or saline before extinction training. In several experiments, rats conditioned and extinguished as juveniles, adolescents, or adults exhibited impaired extinction retention after MK801 compared to saline, but this effect was not observed in JuvCond-AdolesExt rats. Further experiments ruled out several alternative explanations for why NMDAR antagonism did not affect extinction retention in adolescents extinguishing fear learned as a juvenile. These results illustrate yet another circumstance in which NMDARs are not required for successful extinction retention and highlight the complexity of fear inhibition across development.

Attenuation of noise-induced hyperactivity in the dorsal cochlear nucleus by pre-treatment with MK-801

It has previously been hypothesized that hyperactivity of central auditory neurons following exposure to intense noise is a consequence of synaptic alterations. Recent studies suggest the involvement of NMDA receptors in the induction of this hyperactive state. NMDA receptors can mediate long term changes in the excitability of neurons through their involvement in excitotoxic injury and long term potentiation and depression. In this study, we examined the effect of administering an NMDA receptor blocker on the induction of hyperactivity in the dorsal cochlear nucleus (DCN) following intense sound exposure. Our prediction was that if hyperactivity induced by intense sound exposure is dependent on NMDA receptors, then blocking these receptors by administering an NMDA receptor antagonist just before animals are exposed to intense sound should reduce the degree of hyperactivity that subsequently emerges. We compared the levels of hyperactivity that develop in the DCN after intense sound exposure to activity recorded in control animals that were not sound exposed. One group of animals to be sound exposed received intraperitoneal injection of MK-801 twenty minutes preceding the sound exposure, while the other group received injection of saline. Recordings performed in the DCN 26-28 days post-exposure revealed increased response thresholds and widespread increases in spontaneous activity in the saline-treated animals that had been sound exposed, consistent with earlier studies. The animals treated with MK-801 preceding sound exposure showed similarly elevated thresholds but an attenuation of hyperactivity in the DCN; the attenuation was most robust in the high frequency half of the DCN, but lower levels of hyperactivity were also found in the low frequency half. These findings suggest that NMDA receptors are an important component of the hyperactivity-inducing mechanism following intense sound exposure. They further suggest that blockade of NMDA receptors may offer a useful therapeutic approach to preventing induction of noise-induced hyperactivity-related hearing disorders, such as tinnitus and hyperacusis.

Effects of NMDA receptor antagonists on probability discounting depend on the order of probability presentation

Risky decision making can be measured using a probability-discounting procedure, in which animals choose between a small, certain reinforcer and a large, uncertain reinforcer. Recent evidence has identified glutamate as a mediator of risky decision making, as blocking the N-methyl-d-aspartate (NMDA) receptor with MK-801 increases preference for a large, uncertain reinforcer. Because the order in which probabilities associated with the large reinforcer can modulate the effects of drugs on choice, the current study determined if NMDA receptor ligands alter probability discounting using ascending and descending schedules. Sixteen rats were trained in a probability-discounting procedure in which the odds against obtaining the large reinforcer increased (n=8) or decreased (n=8) across blocks of trials. Following behavioral training, rats received treatments of the NMDA receptor ligands MK-801 (uncompetitive antagonist; 0, 0.003, 0.01, or 0.03mg/kg), ketamine (uncompetitive antagonist; 0, 1.0, 5.0, or 10.0mg/kg), and ifenprodil (NR2B-selective non-competitive antagonist; 0, 1.0, 3.0, or 10.0mg/kg). Results showed discounting was steeper (indicating increased risk aversion) for rats on an ascending schedule relative to rats on the descending schedule. Furthermore, the effects of MK-801, ketamine, and ifenprodil on discounting were dependent on the schedule used. Specifically, the highest dose of each drug decreased risk taking in rats in the descending schedule, but only MK-801 (0.03mg/kg) increased risk taking in rats on an ascending schedule. These results show that probability presentation order modulates the effects of NMDA receptor ligands on risky decision making.

Role of Ryanodine and NMDA Receptors in Tetrabromobisphenol A-Induced Calcium Imbalance and Cytotoxicity in Primary Cultures of Rat Cerebellar Granule Cells

The study assessed the role of ryanodine receptors (RyRs) and NMDA receptors (NMDARs) in the Ca²⁺ transients and cytotoxicity induced in neurons by the brominated flame retardant tetrabromobisphenol A (TBBPA). Primary cultures of rat cerebellar granule cells (CGC) were exposed to 7.5, 10, or 25 µM TBBPA for 30 min, and cell viability was assessed after 24 h. Moreover, ⁴⁵Ca uptake was measured, and changes in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) were studied using the fluo-3 probe. The involvement of NMDARs and RyRs was verified using the pertinent receptor antagonists, 0.5 µM MK-801 and 2.5 µM bastadin 12, which was co-applied with 200 µM ryanodine, respectively. The results show that TBBPA concentration-dependently induces an increase in [Ca²⁺]_i. This effect was partly suppressed by the inhibitors of RyRs and NMDARs when administered separately, and completely abrogated by their combined application. A concentration-dependent activation of ⁴⁵Ca uptake by TBBPA was prevented by MK-801 but not by RyR inhibitors. Application of ≥10 µM TBBPA concentration-dependently reduced neuronal viability, and this effect was only partially and to an equal degree reduced by NMDAR and RyR antagonists given either separately or in combination. Our results directly demonstrate that both the RyR-mediated release of intracellular Ca²⁺ and the NMDAR-mediated influx of Ca²⁺ into neurons participate in the mechanism of TBBPA-induced Ca²⁺ imbalance in CGC and play a significant, albeit not exclusive, role in the mechanisms of TBBPA cytotoxicity.

Role of ionotropic glutamate receptors in delay and probability discounting in the rat

RATIONALE

Discounting of delayed and probabilistic reinforcement is linked to increased drug use and pathological gambling. Understanding the neurobiology of discounting is important for designing treatments for these disorders. Glutamate is considered to be involved in addiction-like behaviors; however, the role of ionotropic glutamate receptors (iGluRs) in discounting remains unclear.

OBJECTIVES

The current study examined the effects of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor blockade on performance in delay and probability discounting tasks.

METHODS

Following training in either delay or probability discounting, rats (n = 12, each task) received pretreatments of the NMDA receptor antagonists MK-801 (0, 0.01, 0.03, 0.1, or 0.3 mg/kg, s.c.) or ketamine (0, 1.0, 5.0, or 10.0 mg/kg, i.p.), as well as the AMPA receptor antagonist CNQX (0, 1.0, 3.0, or 5.6 mg/kg, i.p.). Hyperbolic discounting functions were used to estimate sensitivity to delayed/probabilistic reinforcement and sensitivity to reinforcer amount.

RESULTS

An intermediate dose of MK-801 (0.03 mg/kg) decreased sensitivity to both delayed and probabilistic reinforcement. In contrast, ketamine did not affect the rate of discounting in either task but decreased sensitivity to reinforcer amount. CNQX did not alter sensitivity to reinforcer amount or delayed/probabilistic reinforcement.

CONCLUSIONS

These results show that blockade of NMDA receptors, but not AMPA receptors, decreases sensitivity to delayed/probabilistic reinforcement (MK-801) and sensitivity to reinforcer amount (ketamine). The differential effects of MK-801 and ketamine demonstrate that sensitivities to delayed/probabilistic reinforcement and reinforcer amount are pharmacologically dissociable.

Unraveling mechanisms underlying partial agonism in 5-HT3A receptors

Partial agonists have emerged as attractive therapeutic molecules. 2-Me-5HT and tryptamine have been defined as partial agonists of 5-HT3 receptors on the basis of macroscopic measurements. Because several mechanisms may limit maximal responses, we took advantage of the high-conductance form of the mouse serotonin type 3A (5-HT3A) receptor to understand their molecular actions. Individual 5-HT-bound receptors activate in long episodes of high open probability, consisting of groups of openings in quick succession. The activation pattern is similar for 2-Me-5HT only at very low concentrations since profound channel blockade takes place within the activating concentration range. In contrast, activation episodes are significantly briefer in the presence of tryptamine. Generation of a full activation scheme reveals that the fully occupied receptor overcomes transitions to closed preopen states (primed states) before opening. Reduced priming explains the partial agonism of tryptamine. In contrast, 2-Me-5HT is not a genuine partial agonist since priming is not dramatically affected and its low apparent efficacy is mainly due to channel blockade. The analysis also shows that the first priming step is the rate-limiting step and partial agonists require an increased number of priming steps for activation. Molecular docking suggests that interactions are similar for 5-HT and 2-Me-5HT but slightly different for tryptamine. Our study contributes to understanding 5-HT3A receptor activation, extends the novel concept of partial agonism within the Cys-loop family, reveals novel aspects of partial agonism, and unmasks molecular actions of classically defined partial agonists. Unraveling mechanisms underlying partial responses has implications in the design of therapeutic compounds.

Poly(ethylene glycol) as a scaffold for high-affinity open-channel blockers of the mouse nicotinic acetylcholine receptor

High-affinity blockers for an ion channel often have complex molecular structures that are synthetically challenging and/or laborious. Here we show that high-affinity blockers for the mouse nicotinic acetylcholine receptor (AChR) can be prepared from a structurally simple material, poly(ethylene glycol) (PEG). The PEG-based blockers (PQ1–5), comprised of a flexible octa(ethylene glycol) scaffold and two terminal quaternary ammonium groups, exert low- to sub-micromolar affinities for the open AChR pore (measured via single-channel analysis of AChRs expressed in human embryonic kidney cells). PQ1–5 are comparable in pore-binding affinity to the strongest AChR open-channel blockers previously reported, which have complex molecular structures. These results suggest a general approach for designing potent open-channel blockers from a structurally flexible polymer. This design strategy involves simple synthetic procedures and does not require detailed information about the structure of an ion-channel pore.

Pharmacological or genetic orexin1 receptor inhibition attenuates MK-801 induced glutamate release in mouse cortex

The orexin/hypocretin neuropeptides are produced by a cluster of neurons within the lateral posterior hypothalamus and participate in neuronal regulation by activating their receptors (OX1 and OX2 receptors). The orexin system projects widely through the brain and functions as an interface between multiple regulatory systems including wakefulness, energy balance, stress, reward, and emotion. Recent studies have demonstrated that orexins and glutamate interact at the synaptic level and that orexins facilitate glutamate actions. We tested the hypothesis that orexins modulate glutamate signaling via OX1 receptors by monitoring levels of glutamate in frontal cortex of freely moving mice using enzyme coated biosensors under inhibited OX1 receptor conditions. MK-801, an NMDA receptor antagonist, was administered subcutaneously (0.178 mg/kg) to indirectly disinhibit pyramidal neurons and therefore increase cortical glutamate release. In wild-type mice, pretreatment with the OX1 receptor antagonist GSK-1059865 (10 mg/kg S.C.) which had no effect by itself, significantly attenuated the cortical glutamate release elicited by MK-801. OX1 receptor knockout mice had a blunted glutamate release response to MK-801 and exhibited about half of the glutamate release observed in wild-type mice in agreement with the data obtained with transient blockade of OX1 receptors. These results indicate that pharmacological (transient) or genetic (permanent) inhibition of the OX1 receptor similarly interfere with glutamatergic function in the cortex. Selectively targeting the OX1 receptor with an antagonist may normalize hyperglutamatergic states and thus may represent a novel therapeutic strategy for the treatment of various psychiatric disorders associated with hyperactive states.

A novel mechanism of modulation of 5-HT₃A receptors by hydrocortisone

Modulation of Cys-loop receptors by steroids is of physiological and therapeutical relevance. Nonetheless, its molecular mechanism has not been elucidated for serotonin (5-HT) type 3 receptors. We deciphered the mechanism of action of hydrocortisone (HC) at 5-HT type 3A receptors. Single-channel currents from the high-conductance form (∼4.7 pA, -70 mV) appear as a series of long opening events forming bursts, which group into long clusters. Although they are very infrequent, subconductance events (∼2.4 pA) are detected within clusters. HC produces a significant concentration-dependent reduction in open and burst durations, demonstrating open-channel block. In addition, it increases the appearance of subconductance levels in a concentration- and slightly voltage-dependent manner. The amplitude of the subconductance level does not change with HC concentration and its open duration is briefer than that of full amplitude events, indicating lower open-channel stability. Dual effects are distinguished from macroscopic responses: HC reduces amplitude by acting from either open or closed states, and it increases decay rates from the open state. Thus, HC acts as a negative modulator of 5-HT type 3A receptors by different mechanisms: It acts as an open-channel blocker and it favors opening to a preexisting subconductance level. The latter constitutes a novel, to our knowledge, mechanism of channel modulation, which might be applicable to other steroids and channels.

Nicotine ameliorates NMDA receptor antagonist-induced deficits in contextual fear conditioning through high-affinity nicotinic acetylcholine receptors in the hippocampus

NMDA glutamate receptors (NMDARs) and nicotinic acetylcholine receptors (nAChRs) are both involved in learning and synaptic plasticity. Increasing evidence suggests processes mediated by these receptors may interact to modulate learning; however, little is known about the neural substrates involved in these interactive processes. The present studies investigated the effects of nicotine on MK-801 hydrogen maleate (MK-801) and DL-2-Amino-5-phosphonovaleric acid (APV)-induced disruption of contextual fear conditioning in male C57BL/6J mice, using direct drug infusion and selective nAChR antagonists to define the brain regions and the nAChR subtypes involved. Mice treated with MK-801 showed a deficit in contextual fear conditioning that was ameliorated by nicotine. Direct drug infusion demonstrated that the NMDAR antagonists disrupted hippocampal function and that nicotine acted in the dorsal hippocampus to ameliorate the deficit in learning. The high-affinity nAChR antagonist Dihydro-β-erythroidine hydrobromide (DhβE) blocked the effects of nicotine on MK-801-induced deficits while the α7 nAChR antagonist methyllycaconitine citrate salt hydrate (MLA) did not. These results suggest that NMDARs and nAChRs may mediate similar hippocampal processes involved in contextual fear conditioning. Furthermore, these results may have implications for developing effective therapeutics for the cognitive deficits associated with schizophrenia because a large subset of patients with schizophrenia exhibit cognitive deficits that may be related to NMDAR dysfunction and smoke at much higher rates than the healthy population, which may be an attempt to ameliorate cognitive deficits.

Recurrent inhibitory network among striatal cholinergic interneurons

The striatum plays a central role in sensorimotor learning and action selection. Tonically active cholinergic interneurons in the striatum give rise to dense axonal arborizations and significantly shape striatal output. However, it is not clear how the activity of these neurons is regulated within the striatal microcircuitry. In this study, using rat brain slices, we find that stimulation of intrastriatal cholinergic fibers evokes polysynaptic GABA(A) IPSCs in cholinergic interneurons. These polysynaptic GABA(A) IPSCs were abolished by general nicotinic acetylcholine receptor antagonists and also by a specific antagonist of nicotinic receptors containing beta2 subunits. Dopamine receptor antagonists or dopamine depletion failed to block polysynaptic IPSCs, indicating that phasic dopamine release does not directly mediate the polysynaptic transmission. Dual recording from pairs of cholinergic interneurons revealed that activation of a single cholinergic interneuron is capable of eliciting polysynaptic GABA(A) IPSCs both in itself and in nearby cholinergic interneurons. Although polysynaptic transmission arising from a single cholinergic interneuron was depressed during repetitive 2 Hz firing, intrastriatal stimulation reliably evoked large polysynaptic IPSCs by recruiting many cholinergic fibers. We also show that polysynaptic GABAergic inhibition leads to a transient suppression of tonic cholinergic interneuron firing. We propose a novel microcircuit in the striatum, in which cholinergic interneurons are connected to one another through GABAergic interneurons. This may provide a mechanism to convert activation of cholinergic interneurons into widespread recurrent inhibition of these neurons via nicotinic excitation of striatal GABAergic neurons.

Spinal N-methyl-D-aspartate receptors and nociception-evoked release of primary afferent substance P

Dorsal horn N-methyl-D-aspartate (NMDA) receptors contribute significantly to spinal nociceptive processing through an effect postsynaptic to non-primary glutamatergic axons, and perhaps presynaptic to the primary afferent terminals. The present study sought to examine the regulatory effects of NMDA receptors on primary afferent release of substance P (SP), as measured by neurokinin 1 receptor (NK1r) internalization in the spinal dorsal horn of rats. The effects of intrathecal NMDA alone or in combination with D-serine (a glycine site agonist) were initially examined on basal levels of NK1r internalization. NMDA alone or when co-administered with D-serine failed to induce NK1r internalization, whereas activation of spinal TRPV1 receptors by capsaicin resulted in a notable NK1r internalization. To determine whether NMDA receptor activation could potentiate NK1r internalization or pain behavior induced by a peripheral noxious stimulus, intrathecal NMDA was given prior to an intraplantar injection of formalin. NMDA did not alter the formalin-induced NK1r internalization nor did it enhance the formalin paw flinching behavior. To further characterize the effects of presynaptic NMDA receptors, the NMDA antagonists DL-2-amino-5-phosphonopentanoic acid (AP-5) and MK-801 were intrathecally administered to assess their regulatory effects on formalin-induced NK1r internalization and pain behavior. AP-5 had no effect on formalin-induced NK1r internalization, whereas MK-801 produced only a modest reduction. Both antagonists, however, reduced the formalin paw flinching behavior. In subsequent in vitro experiments, perfusion of NMDA in spinal cord slice preparations did not evoke basal release of SP or calcitonin gene-related peptide (CGRP). Likewise, perfusion of NMDA did not enhance capsaicin-evoked release of the two peptides. These results suggest that presynaptic NMDA receptors in the spinal cord play little if any role on the primary afferent release of SP.

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus

In rats, the pharmacological (interoceptive) effects of 0.4 mg/kg nicotine can serve as a conditional stimulus in a Pavlovian conditioning task. Nicotine administration is paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Rats were trained on a nicotine dose ((-)-1-Methyl-2-(3-pyridyl)pyrrolidine; 0.1, 0.2, or 0.4 mg base/kg, s.c.). Generalization was examined using 0.025, 0.05, 0.1, 0.2, and 0.4 mg/kg nicotine and saline. Some behavioral effects of nicotine have been attributed to dopamine and glutamate. Accordingly, potential blockade of the nicotine cue via the dopamine system was examined by administering (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390; 0.005, 0.01, and 0.03 mg/kg), 3-Chloro-5-ethyl-N-[[(2S)-1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxy-benzamide hydrochloride (eticlopride; 0.01, 0.03, 0.1, and 0.3 mg/kg), or N-[(1-Butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphthalenecarboxamide (nafadotride; 0.03, 0.1, 0.3, 1, and 3 mg/kg) before nicotine. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 0.3, 1, and 3 mg/kg) and (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801; 0.01, 0.03, 0.1, and 0.2 mg/kg; dizocilpine) were used to examine possible glutamatergic components. Substitution tests were conducted with MPEP and nafadotride. Differential conditioned responding was acquired in the 3 groups. Conditioned responding generally decreased as the nicotine test dose moved away from the training dose; responding increased when 0.4 mg/kg trained rats were tested with 0.2 mg/kg. SCH-23390, eticlopride, nafadotride, and MPEP decreased conditioned responding on nicotine at doses that also decreased chamber activity. In contrast, MK-801 decreased goal tracking on nicotine without decreasing chamber activity, indicating a role for N-methyl-D-aspartate receptors in expression of nicotine-evoked conditioned responding.

Nicotine exposure refines visual map topography through an NMDA receptor-mediated pathway

The precise mapping of one surface onto another is fundamental to visual system organization and depends upon adequate stimulation of postsynaptic targets to stabilize correctly placed synapses. As exogenous nicotine alters neuronal activity, we investigated whether it would affect the visual map created by retinal ganglion cell terminals in the frog optic tectum. Chronic exposure of the tectum to nicotine decreased the retinal area from which cells project to a given tectal site. This map refinement was also produced by exposure to either the alpha-bungarotoxin sensitive nicotinic receptor agonist, anatoxin-a or the alpha-bungarotoxin-insensitive nicotinic receptor agonist epiboxidine. Immunocytochemical studies using mAb306 and mAb22 demonstrated that alpha-bungarotoxin-sensitive and -insensitive nicotinic receptors, respectively, occupied different tectal sites. Choline acetyltransferase immunoreactivity overlapped with mAb306, but not mAb22, staining. The developing optic tectum was more sensitive to nicotine than the adult tectum and nicotine induced both map refinements and map disruptions in a concentration-dependent manner. Blockade of the N-methyl-D-aspartate (NMDA) receptor with D(-)-2-amino-5-phosphonopentanoic acid (D-APV) prevented nicotine from refining the map in the adult tectum. Exposure to the use-dependent NMDA antagonist MK801 alone had no effect on retinotectal topography but in combination with either NMDA or nicotine it disrupted the map. Exposure to NMDA alone produced refinement. We conclude that the map refinement induced by chronic nicotine treatment has as its basis an increase in the level of NMDA receptor activity. The data are consistent with a model whereby map topography can be bidirectionally affected by either increasing or decreasing NMDA receptor activity.

Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.

Binding of [3H]MK-801 in subcellular fractions of Schistosoma mansoni: evidence for interaction with nicotinic receptors

Several studies have suggested that l-glutamate is a putative neurotransmitter in Schistosoma mansoni. Recently, we detected the presence of low-affinity binding sites for [(3)H]kainic acid in the heterogeneous (P(1)) subcellular fraction of S. mansoni. In an attempt to characterize N-methyl-d-aspartate (NMDA) receptors in this worm, we performed binding assays with [(3)H]MK-801, a NMDA non-competitive antagonist, in the P(1) fraction of adult S. mansoni. In competition experiments, MK-801 (IC(50) approximately 200 microM) and ketamine (IC(50) approximately 500 microM) exhibited a low affinity for the sites labeled with [(3)H]MK-801. Along with the lack of modulation of this binding by glutamatergic agonists and antagonists and the absence of stereoselectivity for MK-801 isomers, these results suggest that [(3)H]MK-801 could label a site different from the classical NMDA receptor in S. mansoni. Based on the evidences that MK-801 interacts with mammalian muscle and central nervous system nicotinic receptors as a low-affinity noncompetitive antagonist, we have investigated the effects of MK-801 on the nicotine-induced flaccid paralysis of the worm, in vivo. The motility of S. mansoni was quantified by image analysis through a measure of displacement of the worm's extremities. In the presence of (-)-nicotine (10-100 microM), we observed an immediate paralysis of the worms, that was inhibited by 1mM MK-801. Besides nicotine, choline (10-50mM) was also able to inhibit the worm's motility. As a conclusion, we suggest that [(3)H]MK-801 binds to nicotinic receptors, and not NMDA receptors, in subcellular fractions of S. mansoni.

Peripheral NMDA receptor modulation of jaw muscle electromyographic activity induced by capsaicin injection into the temporomandibular joint of rats

We have previously documented that peripheral N-methyl-d-aspartate (NMDA) receptor mechanisms are involved in nociceptive reflex increases in jaw muscle activity to injection of mustard oil or glutamate into the rat temporomandibular joint (TMJ). The aim of the present study was to determine whether peripheral NMDA receptor mechanisms are also involved in the nociceptive reflex responses in the jaw muscles evoked by injection of the inflammatory irritant and algesic chemical capsaicin into the TMJ. The effects of peripheral injection of NMDA receptor antagonists, MK-801 and APV, on the increases in electromyographic (EMG) activities of digastric and masseter muscles reflexly evoked by capsaicin injection into the TMJ were tested in halothane-anesthetized male rats. The capsaicin injection following pre-injection of vehicle evoked significant increases in EMG activity in both digastric and masseter muscles whereas pre-injection of MK-801 or APV into the TMJ resulted in a significant concentration-related reduction in the magnitude of capsaicin-evoked digastric and masseter EMG activity (ANOVA-on-ranks, P < 0.05). This finding indicates that capsaicin-evoked digastric and masseter EMG activity can be attenuated by pre-injection into the TMJ of NMDA receptor antagonists, and that the activation of peripheral NMDA receptors may be important in the mechanisms whereby capsaicin evokes nociceptive trigeminal responses.

Pronounced cell death in the absence of NMDA receptors in the developing somatosensory thalamus

Genetic deletion of NMDA glutamate receptors disrupts development of whisker-related neuronal patterns in the somatosensory system. Independent studies have shown that NMDA receptor antagonists increase cell death among developing neurons. Here, we report that a dramatic feature of the developing somatosensory system in newborn NMDA receptor 1 (NMDAR1) knock-out mice is increased cell death in the ventrobasal nucleus (VB) of the thalamus. Sections were subject to terminal deoxynucleotidyl transferase dUTP nick end labeling staining for apoptotic DNA fragmentation, thionine staining for pyknotic nuclei, silver staining for degenerating cells, and immunostaining for caspase-3. All four methods demonstrated that deletion of NMDAR1 causes a large (on the order of threefold to fivefold) increase in cell death in the VB. The NMDA receptor antagonists dizocilpine maleate (MK-801) and phencyclidine also increase cell death in this structure. The onset of increased cell death in the VB in the absence of NMDA receptor function is approximately the time of birth, overlaps with naturally occurring cell death and synaptogenesis, and displays some anatomical specificity. For example, there was no increase in cell death in the hippocampus or neocortex of NMDAR1 knock-out mice at any of the time points examined: embryonic day 15.5 (E15.5), E17.5, and postnatal day 0. We also report a significant reduction in the size of the VB that is evident starting at E17.5. The results indicate that NMDA receptors play a major role in cell survival during naturally occurring cell death in the VB and demonstrate that cell death is a consideration in NMDA receptor knock-out studies.

Bimodal effects of MK-801 on locomotion and stereotypy in C57BL/6 mice

RATIONALE

Systemic injection of the non-competitive NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 (dizocilpine maleate) causes both increased locomotion in rodents and various stereotypic behaviors that are proposed to model certain aspects of schizophrenic symptoms in humans.

OBJECTIVES

This study presents a comprehensive characterization of the bimodal effects of MK-801 on locomotion and stereotypy in the C57BL/6 mouse strain, a strain commonly used for genetically modified mice.

RESULTS

We found that it is important to analyze both locomotion and stereotypy in parallel, as MK-801-induced stereotypy results in abnormal movements that are recorded as locomotion by automated beam detection systems. Furthermore, it is important to analyze the bimodal effects of MK-801 over an extended time span, rather than the commonly used narrower time window, as at higher doses (e.g., above 0.3 mg/kg) the hyperlocomotion phase develops only after the stereotypic phase subsides. We also observed that the apparent dose-response curve is very sensitive to the particular time window chosen for analysis because MK-801 affects both the time course and maximum value of stimulated locomotion. We show that analyzing the absolute peak value of locomotion induced for each animal, rather than group-averaged time courses, provides a measure that is sensitive over a wider range of MK-801 doses. Interestingly, MK-801 even at a very low dose of 0.02 mg/kg suppressed rather than enhanced rearing behavior, differing in this regard from amphetamine.

CONCLUSIONS

The non-competitive NMDA receptor antagonist MK-801 induces a complex pattern of behavioral modification in mice with respect to both the time course and the dose-response relationship of behavioral changes. The results of this study provide a foundation and frame of reference for the growing interest in studying MK-801-induced behavior in mice.

Selection of 2?-Fluoro-modified RNA Aptamers for Alleviation of Cocaine and MK-801Inhibition of the Nicotinic Acetylcholine Receptor

The nicotinic acetylcholine receptor (nAChR) belongs to a group of five structurally related proteins that regulate signal transmission between approximately 10(12) cells of the mammalian nervous system. Many therapeutic agents and abused drugs inhibit the nAChR, including the anti-convulsant MK-801 and the abused drug cocaine. Many attempts have been made to find compounds that prevent inhibition by cocaine. Use of transient kinetic techniques to investigate the inhibition of the receptor by MK-801 and cocaine led to an inhibition mechanism not previously proposed. The mechanism led to the development of combinatorially synthesized RNA ligands that alleviate inhibition of the receptor. However, these ligands are relatively unstable. Here we determined whether much more stable 2'-fluoro-modified RNA ligands can be prepared and used to study the alleviation of receptor inhibition. Two classes of 2'-fluoro-modified RNA ligands were obtained: One class binds with higher affinity to the cocaine-binding site on the closed-channel form and, as predicted by the mechanism, inhibits the receptor. The second class binds with equal or higher affinity to the cocaine-binding site on the open-channel form and, as predicted by the mechanism, does not inhibit the receptor, and does alleviate cocaine and MK-801 inhibition of the nAChR. The stability of these 2'-fluoro-RNAs expands the utility of these ligands.

Calcium channel antagonists attenuate cross-sensitization to the rewarding and/or locomotor effects of nicotine, morphine and MK-801

The present study focused on the evaluation of behavioural cross-sensitization, particularly in locomotor activities and conditioned rewarding effects, between nicotine and morphine, cocaine, amphetamine or MK-801. Nicotine (0.5 mg kg(-1))-experienced mice manifested an enhanced locomotor response to morphine (5 mg kg(-1)) or MK-801 (0.3 mg kg(-1)). No cross-sensitization was observed between nicotine and amphetamine (2 mg kg(-1)) or cocaine (15 mg kg(-1)). Additionally, the L-type voltage-dependent calcium-channel antagonists, nimodipine and verapamil, but not diltiazem, at a dose of 20 mg kg(-1) injected before morphine or MK-801 challenge, blocked the expression of this cross-sensitization. In the second test, an enhancement of morphine place conditioning in rats pre-exposed to nicotine (0.5 mg kg(-1), injected daily for 5 days) was demonstrated. After two conditioning sessions, morphine (5 mg kg(-1)) induced a clear place preference only in animals that had previously received nicotine injections. The administration of nimodipine (10 and 20 mg kg(-1)), verapamil (10 and 20 mg kg(-1)) and diltiazem (10 and 20 mg kg(-1)) prior to nicotine dose-dependently prevented this sensitization to the rewarding effect of morphine produced by prior injections of nicotine. These findings support the hypothesis that similar neural calcium-dependent mechanisms are involved in the appetitive effects of nicotine and morphine and in the sensitized locomotor stimulant effects of nicotine and morphine or MK-801.

Molecular Basis of the Differential Sensitivity of Nematode and Mammalian Muscle to the Anthelmintic Agent Levamisole

Levamisole is an anthelmintic agent that exerts its therapeutic effect by acting as a full agonist of the nicotinic receptor (AChR) of nematode muscle. Its action at the mammalian muscle AChR has not been elucidated to date despite its wide use as an anthelmintic in humans and cattle. By single channel and macroscopic current recordings, we investigated the interaction of levamisole with the mammalian muscle AChR. Levamisole activates mammalian AChRs. However, single channel openings are briefer than those activated by acetylcholine (ACh) and do not appear in clusters at high concentrations. The peak current induced by levamisole is about 3% that activated by ACh. Thus, the anthelmintic acts as a weak agonist of the mammalian AChR. Levamisole also produces open channel blockade of the AChR. The apparent affinity for block (190 microm at -70 mV) is similar to that of the nematode AChR, suggesting that differences in channel activation kinetics govern the different sensitivity of nematode and mammalian muscle to anthelmintics. To identify the structural basis of this different sensitivity, we performed mutagenesis targeting residues in the alpha subunit that differ between vertebrates and nematodes. The replacement of the conserved alphaGly-153 with the homologous glutamic acid of nematode AChR significantly increases the efficacy of levamisole to activate channels. Channel activity takes place in clusters having two different kinetic modes. The kinetics of the high open probability mode are almost identical when the agonist is ACh or levamisole. It is concluded that alphaGly-153 is involved in the low efficacy of levamisole to activate mammalian muscle AChRs.

Modulation of cellular differentiation by N-methyl-D-aspartate receptors in osteoblasts

N-methyl-D-aspartate (NMDA) receptors for the central neurotransmitter l-glutamate (Glu) have been shown to be present in both osteoblasts and osteoclasts. Sustained exposure to the NMDA channel antagonist dizocilpine (MK-801) significantly prevented increases in both alkaline phosphatase activity and Ca2+ accumulation in a concentration-dependent manner in osteoblasts cultured for 7-28 days in vitro (DIV), without significantly affecting cell survivability. Osteocalcin expression was markedly reduced in the presence of MK-801 in osteoblasts cultured for 28 DIV. Both an NMDA domain antagonist and a glycine domain antagonist similarly prevented Ca2+ accumulation in osteoblasts exposed for 28 consecutive DIV. MK-801 was effective in significantly inhibiting Ca2+ accumulation determined at 28 DIV in osteoblasts exposed before 7 DIV but was ineffective in cells exposed after 11-21 DIV. Sustained exposure to MK-801 significantly inhibited DNA binding activity and expression of core binding factor alpha-1 (CBFA1) in osteoblasts exposed after 7 DIV up to 28 DIV, but not in those exposed before 7 DIV. These results suggest that heteromeric NMDA receptor channels may be functionally expressed to regulate mechanisms underlying cellular differentiation rather than proliferation and/or maturation through modulation of expression of CBFA1 in cultured rat calvarial osteoblasts.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

Are neuronal nicotinic receptors a target for antiepileptic drug development? Studies in different seizure models in mice and rats

Altered function of neuronal nicotinic acetylcholine receptors in the brain has recently been associated with an idiopathic form of partial epilepsy, suggesting that functional alterations of these receptors can be involved in the processes leading to epileptic seizures. Thus, nicotinic acetylcholine receptors may form a novel target for antiepileptic drug development. In the present study, various nicotinic acetylcholine receptor antagonists, including novel amino-alkyl-cyclohexane derivatives, were evaluated in two animal models, namely the maximal electroshock seizure test in mice and amygdala-kindling in rats. For comparison with these standard models of generalized and partial seizures, the effects against nicotine-induced seizures were examined. Because some of the agents tested showed an overlap between channel blocking at nicotinic acetylcholine receptors and NMDA receptors, the potency at these receptors was assessed by using patch clamp in a hippocampal cell preparation. Preferential nicotinic acetylcholine receptor antagonists were potent anticonvulsants in the maximal electroshock seizure test and against nicotine-induced seizures. The anticonvulsant potency in the maximal electroshock seizure test was decreased by administration of a subconvulsant dose of nicotine. Such a potency shift was also seen with selective NMDA receptor antagonists, which were also efficacious anticonvulsants against both maximal electroshock seizures and nicotine-induced seizures. Experiments with agents combining nicotinic acetylcholine receptor and NMDA receptor antagonistic effects suggested that both mechanisms contributed to the anticonvulsant effect of the respective agents in the maximal electroshock seizure test. This was not found in kindled rats, in which nicotinic acetylcholine receptor antagonists exerted less robust effects. In conclusion, it may be suggested that nicotinic acetylcholine receptor antagonism might be a valuable therapeutic approach to treat generalized epileptic seizures but rather not complex partial seizures.

Early postdenervation depolarization is controlled by acetylcholine and glutamate via nitric oxide regulation of the chloride transporter

Resting non-quantal acetylcholine (ACh) and probably glutamate (Glu) release from nerve endings activates M1- and NMDA receptor-mediated Ca2+ entry into the sarcoplasm with following activation of NOS and production of NO. This is a trophic message from motoneurons, which keeps the Cl- transport inactive in the innervated sarcolemma. After denervation, the secretion of ACh and Glu at the neuromuscular junction is eliminated within 3-4 h and the production of NO in the sarcoplasm is lowered. As a result, the Cl- influx is probably activated by dephosphorylation of the Cl- transporter with subsequent elevation of intracellular Cl- concentration. The equilibrium Cl- potential becomes more positive and the muscle membrane becomes depolarized.

Effects of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a selective ligand for nicotinic acetylcholine receptors, in tests of visual attention and distractibility in rats and monkeys

Nicotine, a nonselective ligand for nicotinic acetylcholine receptors (nAChRs), has been shown to improve attention and reduce distractibility in humans. Although the numerous side effects induced by nicotine prevent its use as a therapeutic agent, it is hypothesized that subtype-selective nAChR ligands may offer a potential therapeutic benefit to humans with attention deficits. In this study, we evaluated the attention-enhancing properties of (+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride (SIB-1553A), a ligand selective for neuronal nAChRs with predominant activity at the human beta 4 subtype. SIB-1553A was evaluated in a test of attention (i.e., five-choice serial reaction time task or SRTT) and distractibility (i.e., delayed matching to sample task with distractor or DMTS-D) in adult rats and monkeys, respectively. SIB-1553A did not improve SRTT performance in normal rats, but reversed deficits induced by the N-methyl-D-aspartate (NMDA) antagonist dizocilpine. In the DMTS-D, SIB-1553A improved accuracy across several doses at the short delay intervals, which were affected most by distracting stimuli in adult monkeys. Subsequent testing with optimal doses for each monkey was also associated with significant improvements in DMTS-D accuracy at short delays, indicating the reproducibility of the drug effect. In both species, SIB-1553A had no significant effects on latencies for sample or choice selection and was not associated with adverse effects at efficacious doses. Although it remains to be further demonstrated, SIB-1553A may act through combined nicotinic and non-nicotinic mechanisms. Collectively, the present data suggest that in specific conditions SIB-1553A may improve certain aspects of attentional function in young adult rats and nonhuman primates without adverse side effects.

Structural characteristics of ionotropic glutamate receptors as identified by channel blockade

The channels of four types of ionotropic glutamate receptor (NMDA receptors and Ca-permeable AMPA receptors of rat brain neurons, and cation-selective receptors from mollusk neurons and insect postsynaptic muscle membranes) and two subtypes of nicotinic cholinoreceptor (from frog neuromuscular junctions and cat sympathetic ganglia) were studied. The structural characteristics of channels determining their susceptibility to blockade by organic mono- and dications were identified. These studies used homologous series of adamantane and phenylcyclohexyl derivatives. These experiments showed that the receptors studied here could be divided into two groups. The first group included the AMPA receptor and the mollusk and insect receptors. These were characterized by the lack of effect on the part of monocations and a strong relationship between the activity of dications and the distance between nitrogen atoms. The second group included the NMDA receptor and both subtypes of the nicotinic cholinoreceptor (muscular and neuronal). Here, conversely, the activity of monocations and dications, regardless of their lengths, were essentially identical. A model for the binding sites of blockers in channels is proposed, which takes these observations into account.

The anthelmintic pyrantel acts as a low efficacious agonist and an open-channel blocker of mammalian acetylcholine receptors

Pyrantel is an anthelmintic which acts as an agonist of nicotinic receptors (AChRs) of nematodes and exerts its therapeutic effects by depolarizing their muscle membranes. Here we explore at the single-channel level the action of pyrantel at mammalian muscle AChR. AChR currents are elicited by pyrantel. However, openings do not appear in clearly identifiable clusters over a range of pyrantel concentrations (1-300 microM). The mean open time decreases as a function of concentration, indicating an additional open-channel block. Single-channel recordings in the presence of high ACh concentrations and pyrantel demonstrate that the anthelmintic acts as a high-affinity open-channel blocker. When analyzed in terms of a sequential blocking scheme, the calculated forward rate constant for the blocking process is 8x10(7) M(-1) x s(-1), the apparent dissociation constant is 8 microM at a membrane potential of -70 mV and the process is voltage dependent. Pyrantel displaces alpha-bungarotoxin binding but the concentration dependence of equilibrium binding is shifted towards higher concentrations with respect to that of ACh binding. Thus, by acting at the binding site pyrantel activates mammalian AChRs with low efficacy, and by sterical blockade of the pore, the activated channels are then rapidly inhibited.

Characterization of the dizocilpine binding site on the nicotinic acetylcholine receptor

Although the dissociative anesthetic dizocilpine [(+)-MK-801] inhibits nicotinic acetylcholine receptor (AChR) function in a noncompetitive manner, the location of the dizocilpine binding site(s) has yet to be clearly established. Thus, to characterize the binding site for dizocilpine on the AChR we examined 1) the dissociation constant (K(d)) and stoichiometry of [(3)H]dizocilpine binding; 2) the displacement of dizocilpine radioligand binding by noncompetitive inhibitors (NCIs) and conversely dizocilpine displacement of fluorescent and radiolabeled NCIs from their respective high-affinity binding sites on the AChR; and 3) photoaffinity labeling of the AChR using (125)I-dizocilpine. The results establish that one high-affinity (K(d) = 4.8 microM) and several (3-6) low-affinity (K(d) = approximately 140 microM) binding sites exist for dizocilpine on the desensitized and resting AChR, respectively. The binding of the fluorescent NCIs ethidium, quinacrine, and crystal violet as well as [(3)H]thienylcyclohexylpiperidine was inhibited by dizocilpine on desensitized AChRs. However, Schild-type analyses indicate that only the inhibition of quinacrine in the desensitized state seems to be mediated by a mutually exclusive action. Photoaffinity labeling of the AChR by (125)I-dizocilpine was primarily restricted to the alpha1 subunit and subsequent mapping revealed that the principal sites of labeling are localized to the M4 (approximately 70%) and M1 (30%) transmembrane domains. Collectively, the data indicate that the high-affinity dizocilpine binding site is not located in the lumen of the ion channel but probably near the quinacrine binding locus at a nonluminal domain in the AChR desensitized state.

Mechanism-based discovery of ligands that counteract inhibition of the nicotinic acetylcholine receptor by cocaine and MK-801

Nicotinic acetylcholine receptors (AChR) belong to a family of proteins that form ligand-gated transmembrane ion channels. They are involved in the fast transmission of signals between cells and the control of intercellular communication in the nervous system. A variety of therapeutic agents and abused drugs, including cocaine, inhibit the AChR and monoamine transporters and interfere with nervous system function. Here we describe a mechanism-based approach to prevent this inhibition. We had previously developed presteady-state kinetic (transient kinetic) techniques, with microsecond-to-millisecond time resolutions, for investigations of reactions on cell surfaces that allow one to determine the effects of inhibitors not only on the channel-opening probability but also on the opening and closing rates of the AChR channel. The transient kinetic measurements led to two predictions. (i) Ligands that bind to a regulatory site on the closed-channel conformation of the AChR with higher affinity than to the site on the open-channel form shift the equilibrium toward the closed-channel form, thereby inhibiting the receptor. (ii) Ligands that bind to a regulatory site with an affinity for the open conformation equal to or higher than their affinity for the closed conformations are expected not to inhibit the receptor and to displace inhibitors. The identification of such ligands in a combinatorial library of RNA ligands is reported. The implication of this approach to other protein-mediated reactions in which an inhibitor changes the equilibrium between active and inactive conformations is discussed.

Alteration of mitochondrial calcium homeostasis by ammonia-induced activation of NMDA receptors in rat brain in vivo

The aim of the present work was to assess the effects of activation of NMDA receptors in rat brain in vivo on calcium homeostasis in isolated non-synaptic brain mitochondria. We have shown recently that acute intoxication with large doses of ammonia leads to activation of NMDA receptors in rat brain in vivo. In the present work we injected rats with ammonium acetate to activate NMDA receptors in vivo and isolated non-synaptic mitochondria to assess calcium homeostasis. We also tested whether blocking NMDA receptors with MK-801 prevents effects on calcium homeostasis induced by ammonium injection. It is shown that activation of NMDA receptors in rat brain in vivo leads to a rapid increase in intramitochondrial calcium content followed by a reduction in the calcium capacity and calcium uptake rate in rat brain mitochondria. Activation of NMDA receptors resulted in increased spontaneous calcium efflux from rat brain mitochondria and in a strong inhibition of Na-induced and tert-butylhydroperoxide-induced calcium efflux. All these effects were prevented by previous blocking of NMDA receptors by injection of MK-801. Cyclosporin A did not affect any of the above parameters, indicating that the mitochondrial permeability transition pore does not play a role in calcium efflux under any of the conditions studied. The results reported indicate that ammonia-induced activation of NMDA receptors in rat brain in vivo alters mitochondrial calcium homeostasis at several different steps.

Prolonged exposure to cigarette smoke blocks the neurotoxicity induced by kainic acid in rats

We examined the effects of cigarette smoke (CS) on three parameters associated with kainic acid (KA)-induced neurotoxicity: seizure activity, cell loss in the hippocampus, and increased Fos-related antigen (FRA) expression. Animals were exposed to the main stream of CS from 15 Kentucky 2R1F research cigarettes containing 28.6 mg tar and 1.74 mg nicotine per cigarette, for 10 min a day, 6 days per week, for 4 weeks, using an automatic smoking machine. KA administration (10 mg/kg, i.p.) produced robust behavioral convulsions lasting 4-5 h. Pre-exposure to CS significantly reduced the seizures, mortality, and severe loss of cells in regions CA1 and CA3 of the hippocampus after KA administration. Consistently, pre-exposure to CS significantly attenuated the KA-induced increased FRA immunoreactivity in the hippocampus. In contrast, pretreatment with central nicotinic antagonist, mecamylamine (2 or 10 mg/kg, i.p.) blocked the neuroprotective effects mediated by CS in a dose-dependent manner. These results indicate that CS exposure provides neuroprotection against the KA insult via nicotinic receptor activation.

Amphetamine acts as a channel blocker of the acetylcholine receptor

Non-competitive inhibitors (NCIs) of the nicotinic receptors (AChR) comprise a wide range of compounds. The chemical scaffold of amphetamine is similar to those of some NCIs. We investigated the effects of amphetamine (1-100 microM) on the muscle AChR by recording single-channel currents. The drug reduces the duration of the open state in a concentration-dependent manner and causes the appearance of brief closings, resembling the action of open-channel blockers. The forward rate constant for the blocking process is of the order of 10(7) M(-1) s(-1) and the blocking process is voltage dependent. The results are consistent with the steric block of the open channel as the primary action of amphetamine. At high drug concentrations the mechanism of inhibition deviates from that of classical open-channel blockers.

Reduction in the MK-801 binding sites of the NMDA sub-type of glutamate receptor in a mouse model of congenital hyperammonemia: prevention by acetyl-L-carnitine

Our earlier studies on the pharmacotherapeutic effects of acetyl-L-carnitine (ALCAR), in sparse-fur (spf) mutant mice with X linked ornithine transcarbamylase deficiency, have shown a restoration of cerebral ATP, depleted by congenital hyperammonemia and hyperglutaminemia. The reduced cortical glutamate and increased quinolinate may cause a down-regulation of the N-methyl-D-aspartate (NMDA) receptors, observed by us in adult spf mice. We have now studied the kinetics of [3H]-MK-801 binding to NMDA receptors in spf mice of different ages to see the effect of chronic hyperammonemia on the glutamate neurotransmission. We have also studied the Ca2+-dependent and independent (4-aminopyridine (AP) and veratridine-mediated) release of glutamate and the uptake of [3H]-glutamate in synaptosomes isolated from mutant spf mice and normal CD-1 controls. All these studies were done with and without ALCAR treatment (4 mmol/kg wt i.p. daily for 2 weeks), to see if its effect on ATP repletion could correct the glutamate neurotransmitter abnormalities. Our results indicate a normal MK-801 binding in 12-day-old spf mice but a significant reduction immediately after weaning (21 day), continuing into the adult stage. The Ca2+-independent release of endogenous glutamate from synaptosomes was significantly elevated at 35 days, while the uptake of glutamate into synaptosomes was significantly reduced in spf mice. ALCAR treatment significantly enhanced the MK-801 binding, neutralized the increased glutamate release and restored the glutamate uptake into synaptosomes of spf mice. These studies point out that: (a) the developmental abnormalities of the NMDA sub-type of glutamate receptor in spf mice could be due to the effect of sustained hyperammonemia, causing a persistent release of excess glutamate and inhibition of the ATP-dependent glutamate transport, (b) the modulatory effects of ALCAR on the NMDA binding sites could be through a repletion of ATP, required by the transporters to efficiently remove extracellular glutamate.

In vitro selection of RNA molecules that displace cocaine from the membrane-bound nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) controls signal transmission between cells in the nervous system. Abused drugs such as cocaine inhibit this receptor. Transient kinetic investigations indicate that inhibitors decrease the channel-opening equilibrium constant [Hess, G. P. & Grewer, C. (1998) Methods Enzymol. 291, 443-473]. Can compounds be found that compete with inhibitors for their binding site but do not change the channel-opening equilibrium? The systematic evolution of RNA ligands by exponential enrichment methodology and the AChR in Torpedo californica electroplax membranes were used to find RNAs that can displace inhibitors from the receptor. The selection of RNA ligands was carried out in two consecutive steps: (i) a gel-shift selection of high-affinity ligands bound to the AChR in the electroplax membrane, and (ii) subsequent use of nitrocellulose filters to which both the membrane-bound receptor and RNAs bind strongly, but from which the desired RNA can be displaced from the receptor by a high-affinity AChR inhibitor, phencyclidine. After nine selection rounds, two classes of RNA molecules that bind to the AChR with nanomolar affinities were isolated and sequenced. Both classes of RNA molecules are displaced by phencyclidine and cocaine from their binding site on the AChR. Class I molecules are potent inhibitors of AChR activity in BC3H1 muscle cells, as determined by using the whole-cell current-recording technique. Class II molecules, although competing with AChR inhibitors, do not affect receptor activity in this assay; such compounds or derivatives may be useful for alleviating the toxicity experienced by millions of addicts.

NMDA receptor mechanisms contribute to neuroplasticity induced in caudalis nociceptive neurons by tooth pulp stimulation

We recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant and prolonged increases in jaw muscle electromyographic (EMG) activity that are suggestive of central neuroplasticity. Because small-fiber afferents, including pulp afferents, access nociceptive neurons in trigeminal (V) subnucleus caudalis, this study examined whether pulpal application of MO induces neuroplastic changes in caudalis nociceptive neurons (wide dynamic range and nociceptive specific) and whether central N-methyl--aspartate (NMDA) receptor mechanisms are involved in these MO-induced neuroplastic changes. After pretreatment with vehicle (saline, 10 microliter i.t.) to the surface of the medulla, the pulpal application of MO to the maxillary molar tooth pulp produced a significant increase in neuronal spontaneous activity, a significant expansion of the pinch and/or tactile mechanoreceptive field (RF), a significant decrease in mechanical threshold, and significant increases in neuronal responses to graded pinch stimuli. Compared with vehicle-treated rats, pretreatment with the NMDA receptor antagonist MK-801 (10 microgram/10 microliter i.t.) followed by MO application to the pulp in another group of rats significantly reduced or abolished these MO-induced neuroplastic changes in nociceptive neurons. In another group of rats pretreated with saline (intrathecally), mineral oil application to the pulp did not show any significant changes in spontaneous activity or RF properties over the 40-min observation period. The pulpal application of MO in other rats (pretreated with saline, intrathecally) did not produce any significant neuroplastic changes in caudalis low-threshold mechanoreceptive neurons. These results indicate that the MO-induced activation of molar pulpal afferents can produce profound NMDA receptor-related neuroplastic changes in caudalis nociceptive neurons. Such neuroplastic changes may contribute to the hyperalgesia and spread of pain that can be associated with pulpal inflammation.

Sensitization of the mesoaccumbens dopamine response to nicotine

This article reviews the evidence that pretreatment with nicotine causes a regionally selective sensitization of its stimulatory effects on a pathway, the mesoaccumbens dopamine (DA) system, which has been implicated in the locomotor stimulant response to nicotine and its ability to reinforce self-administration. The sensitization evoked by daily injections of nicotine is associated with a regionally selective downregulation of the control of mesoaccumbens DA neurons by inhibitory autoreceptors and depends upon co-stimulation of NMDA glutamatergic receptors. It is suggested that the sensitization is related to enhanced burst firing of mesoaccumbens neurons, which results in an enhancement of DA release into the extracellular space between the cells where it acts upon putative extrasynaptic dopamine receptors. The studies with NMDA receptor antagonists revealed a dissociation between the expression of sensitized mesoaccumbens DA and locomotor responses to nicotine. It is proposed, therefore, that the sensitized mesoaccumbens DA responses to nicotine may be implicated in psychopharmacological responses to drug concerned more closely with nicotine dependence.

The effect of glutamate and inhibitors of NMDA receptors on postdenervation decrease of membrane potential in rat diaphragm

The early postdenervation depolarization of rat diaphragm muscle fibers (8-10 mV within 3 h in vitro) is substantially smaller (3 mV) when muscles are bathed with 1 x 10(-3) M L-glutamate (Glu) or 1 x 10(-3) M N-methyl-D-aspartate (NMDA). The effects of Glu and NMDA are inhibited in a dose-dependent manner by competitive inhibitor 2-amino-5-phosphonovaleric acid (APV) with Ki 6.3 x 10(-4) M, by 2 x 10(-7) M MK-801, which acts as an open channel inhibitor, by 2-3 x 10(-4) Zn2+, which reacts with surface-located sites of the NMDA subtype of the glutamate receptor, and also by glycine-free solutions and 7-Cl-kynurenic acid, which inhibits the glycine binding sites on NMDA receptors. It follows that the effect of glutamate on early post-denervation depolarization is mediated by the NMDA subtype of glutamate receptor with similar pharmacological properties to those found in neurons. The only exception found was the glutamate-like action of 1 x 10(-7) M MK-801, which partially prevented the early postdenervation depolarization when present in the muscle bath during the first 3 h after nerve section.