Selective anxiolysis produced by ocinaplon, a GABA(A) receptor modulator

Benzodiazepines remain widely used for the treatment of anxiety disorders despite prominent, often limiting side effects including sedation, muscle relaxation, and ataxia. A compound producing a robust anxiolytic action comparable to benzodiazepines, but lacking these limiting side effects at therapeutic doses (an anxioselective agent), would represent an important advance in the treatment of generalized anxiety disorder, and perhaps other anxiety disorders. Here we report that the pyrazolo[1,5-a]-pyrimidine, ocinaplon, exhibits an anxioselective profile in both preclinical procedures and in patients with generalized anxiety disorder, the most common of the anxiety disorders. In rats, ocinaplon produces significant muscle relaxation, ataxia, and sedation only at doses >25-fold higher than the minimum effective dose (3.1 mg/kg) in the Vogel "conflict" test. This anticonflict effect is blocked by flumazenil (Ro 15-1788), indicating that like benzodiazepines, ocinaplon produces an anxiolytic action through allosteric modulation of GABA(A) receptors. Nonetheless, in eight recombinant GABA(A) receptor isoforms expressed in Xenopus oocytes, the potency and efficacy of ocinaplon to potentiate GABA responses varied with subunit composition not only in an absolute sense, but also relative to the prototypical benzodiazepine, diazepam. In a double blind, placebo controlled clinical trial, a 2-week regimen of ocinaplon (total daily dose of 180-240 mg) produced statistically significant reductions in the Hamilton rating scale for anxiety scores. In this study, the incidence of benzodiazepine-like side effects (e.g., sedation, dizziness) in ocinaplon-treated patients did not differ from placebo. These findings indicate that ocinaplon represents a unique approach both for the treatment and understanding of anxiety disorders.

Modulation of glutamate receptors: strategies for the development of novel antidepressants

On a biochemical level, conventional antidepressants have been shown to modulate synaptic levels of biogenic amines (i.e., serotonin, norepinephrine, and dopamine), most often by interfering with reuptake processes or inhibiting metabolism. Strategies directed at modulating glutamatergic transmission may overcome the principal limitations (i.e., delayed onset and low efficacy) that appear to be inherent to these conventional agents. In this brief overview, I summarize two glutamate-based approaches to develop novel antidepressants. These distinct and (on a cellular level) seemingly diametric strategies may converge on intracellular pathways that are also impacted upon by chronic treatment with biogenic amine based agents.

Intra- and interstrain differences in models of "behavioral despair"

In the present studies, base line and drug-induced performance of two mouse strains (C57Bl/6 and NIH-Swiss) was evaluated in the forced swim test (FST) and tail suspension test (TST). Intra- and interstrain comparisons indicate that the biological substrates mediating performance in these behavioral procedures are not identical. For example, in NIH-Swiss mice, a sevenfold difference in base line immobility was observed between the FST and TST. By contrast, the base line immobility in C57Bl/6 mice was similar in both procedures. Further, in C57Bl/6 mice, imipramine produced a "U-shaped" dose-response curve in the FST, whilst no evidence of a biphasic response was present in the TST at doses up to 45 mg/kg. In the FST, the AMPA receptor potentiator LY451646 produced a similar dose-response relationship in C57Bl/6 and NIH-Swiss mice, but the minimum effect dose (MED) was fivefold higher in NIH-Swiss mice. This potency difference appears due to both pharmacokinetic and pharmacodynamic factors. These intra- and interstrain differences in performance indicate that despite a face value similarity, the neurochemical pathways involved in mediating performance in these two widely used tests are not identical.

AMPA receptor function is altered in GLUR2-deficient mice

The GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor determines many of the biophysical properties of native AMPA receptors, including Ca++ permeability. Genetically engineered mice unable to edit the Q to R site of the GluR2 subunit die within 3 wk postpartum, presumably due to toxicity associated with enhanced Ca++ influx through AMPA receptors. In contrast, disruption of the gene encoding GluR2 is not necessarily lethal. The objective of this study was to explore potential mechanisms that permit survival of GluR2 (-/-) mice despite AMPA receptors that are highly Ca++ permeable. Whole-cell, patch-clamp recording of AMPAreceptor responses in cortical pyramidal cells revealed that the kinetics of recovery from desensitization were significantly slower for receptors from GluR2 (-/-) mice compared to receptors from GluR2 (+/+) mice. The recovery time constants for AMPA receptors from GluR2 (-/-) and GluR2 (+/+) mice were 109.8 +/- 17 ms and 54.4 +/- 7.1 ms, respectively. The slower recovery kinetics would be expected to reduce Ca++ influx during repetitive stimulation. Because both RNA editing at the R/G site and alternative splicing of the flip and flop module affect AMPA receptor desensitization recovery rates, the possibility that these mechanisms were changed in GluR2 (-/-) mice was investigated. On a macroscopic level, neither editing nor splicing of the GluR-1, 3 or 4 subunits were changed in GluR2 (-/-) mice compared to GluR2 (+/+) mice. In summary, an increase in the time constant for recovery from desensitization may contribute to the ability of GluR2 (-/-) to survive.

Antidepressant-like actions of an AMPA receptor potentiator (LY392098)

LY392098 is a member of a novel class of biarylpropylsulfonamides that potentiates AMPA receptor-mediated responses both in vitro and in vivo. In this study, the effects of LY392098 were evaluated in two "behavioral despair" models (the forced swim and tail suspension tests) commonly used to identify clinically useful antidepressants. LY392098 reduced immobility in the forced swim test in both rats and mice, with a minimum effective dose of 0.5 mg/kg (i.p.) in both species. LY392098 (0.1-10 mg/kg, i.p.) did not affect motor activity of rats, indicating that the ability of this compound to reduce immobility in the forced swim test is unrelated to a motor stimulant action. LY392098 also reduced immobility in the tail suspension test in a dose-dependent manner, with a minimum effective dose of 5 mg/kg (i.p). A non-competitive AMPA antagonist (LY300168) blocked the activity of LY392098 in the forced swim test, but did not affect imipramine-induced reductions in immobility. Thus, AMPA receptor activation appears to be required for the antidepressant-like effect of LY392098, but not imipramine. These findings indicate that biarylpropylsulfonamides, exemplified by LY392098, may represent a novel class of antidepressants.

Regulation of BDNF expression in primary neuron culture by LY392098, a novel AMPA receptor potentiator

The effects of a novel AMPA receptor potentiator (LY392098) on the expression of brain-derived neurotrophic factor (BDNF) were examined in primary neuron culture. The addition of either AMPA or LY392098 to cortical neurons elicited a time and concentration dependent increase in mRNA encoding BDNF. Moreover, co-addition of subeffective concentrations of AMPA (1 microM) and LY392098 (1 microM) resulted in dramatic increases in both BDNF mRNA (>25-fold) and protein ( approximately 7-fold) levels, whilst no changes in either NT-3 or NT-4 mRNA were detected. More modest ( approximately 1.5-2.5-fold) elevations in BDNF mRNA and protein expression were also produced by combinations of AMPA and LY392098 in cerebellar granule cell neurons. In contrast, AMPA and LY392098, either alone or in combination, did not elevate BDNF mRNA levels in primary astroglial cultures. Maximum elevations in BDNF mRNA and protein were produced by 6-12h of AMPA receptor activation 1-3h of AMPA receptor activation were required to elevate BDNF mRNA levels. AMPA receptor-mediated increases in BDNF mRNA and protein were abolished by the AMPA antagonist, NBQX, but were unaffected by the NMDA antagonist, MK-801. In cortical neuron cultures, activation of both L-type Ca(+2) channels and mitogen-activated protein (MAP) kinases contribute to AMPA receptor-mediated increases in BDNF mRNA. The ability of LY392098 to increase the expression of BDNF in primary neuron culture indicates this and related biarylpropylsulfonamides may be useful in the treatment of neuropsychiatric disorders.

Binding of an AMPA receptor potentiator ([3H]LY395153) to native and recombinant AMPA receptors

LY395153 is a member of a newly described class of arylpropylsulfonamide AMPA receptor potentiators. Here, we characterize and compare [(3)H]LY395153 binding to native AMPA receptors from rat cerebral cortex and recombinant human GluR4(flip) receptors expressed in HEK293 cells. L-Glutamate and AMPA increased [(3)H]LY395153 binding to both native and recombinant AMPA receptors in a concentration dependent and stereoselective manner; this effect of AMPA receptor agonists reflects an apparent increase in ligand affinity. In the presence of L-glutamate (500 microM), [(3)H]LY395153 binding is saturable; the affinity of this radioligand is slightly, albeit statistically significantly higher at human GluR4(flip) (K(d)=55.6+/-5.3nM) than rat cortical receptors (K(d)=110+/-15.1nM). NBQX competitively inhibited L-glutamate-induced increases in [(3)H]LY395153 binding in both native and recombinant receptors, whilst LY303070 (the active isomer of GYKI53655) noncompetitively inhibited this effect in native, but not recombinant receptors. The prototypic AMPA receptor potentiator cyclothiazide competitively inhibited [(3)H]LY395153 binding with a potency (K(i) approximately 7 microM) comparable to EC(50) values reported in electrophysiological studies. In contrast, the structurally unrelated AMPA receptor potentiator CX 516 did not inhibit [(3)H]LY395153 binding at concentrations of up to 600 microM. Further, at concentrations reported to facilitate AMPA receptor desensitization, thiocyanate acts as a competitive inhibitor of [(3)H]LY395153 binding. [(3)H]LY395153 binding was unaffected by a variety of structurally (and mechanistically) diverse compounds tested at a concentration of 10 microM. These data indicate [(3)H]LY395153 is a useful probe for labeling a unique modulatory site on both native and recombinant AMPA receptors.

Current perspectives on the development of non-biogenic amine-based antidepressants

Compounds that inhibit the re-uptake and/or metabolism of biogenic amines (i.e. serotonin, norepinephrine, and dopamine) have been used to treat depression for more than 40 years. Selective re-uptake inhibitors, currently the most widely prescribed class of biogenic amine-based agents, are certainly safe and relatively easy to use, but do not exhibit either a faster onset of action or greater efficacy than their predecessors. An approach to overcome the limitations that may be inherent to these 'conventional' therapies is to circumvent the monoaminergic synapse. In this review, two potential antidepressant strategies are discussed that may converge with intracellular pathways impacted by chronic treatment with biogenic amine-based agents. Drugs emerging from these strategies may offer significant advantages over currently used antidepressants.

Can 'differential display' methodologies make an impact on biological psychiatry?

The past decade has been marked by a dramatic increase in the availability of techniques to identify and clone genes that are differentially expressed in disease states and by drug treatments. The applications of such techniques to problems in biological psychiatry are manifold and the implications of discovering novel and/or known genes that are perturbed in neuropsychiatric disorders profound. While there are success stories, it is becoming ever more apparent that each of these techniques has its limitations, particularly when applied to the central nervous system. Given that these methods (e.g. differential display, RNA fingerprinting, suppression-subtractive hybridization, microarrays) are labour-intensive and potentially time-consuming, it is important to understand these limitations. For example, differential display is capable of detecting very small changes in the expression of mRNA species. Methods like suppression-subtractive hybridization are better suited to examine potential differences in rare transcripts, but only when their expression is changed substantially (currently ? 5-fold). Moreover, both the functional and morphological organization of the central nervous system present challenges that may not be encountered in other systems. In this overview, we will discuss the advantages and disadvantages of some of these approaches and their application to research in biological psychiatry.

A single amino acid residue on the alpha(5) subunit (Ile215) is essential for ligand selectivity at alpha(5)beta(3)gamma(2) gamma-aminobutyric acid(A) receptors

Imidazobenzodiazepines such as RY-80 have been reported to exhibit both high affinity and selectivity for GABA(A) receptors containing an alpha(5) subunit. A single amino acid residue (alpha(5)Ile215) has been identified that plays a critical role in the high-affinity, subtype-selective effects of RY-80 and structurally related ligands. Thus, substitution of alpha(5)Ile215 with the cognate amino acid contained in the alpha(1) subunit (Val211) reduced the selectivity of RY-80 for alpha(5)beta(3)gamma(2) receptors from approximately 135- to approximately 8-fold compared with alpha(1)beta(3)gamma(2) receptors. This mutation produced a comparable reduction in the selectivity of RY-24 (a structural analog of RY-80) for alpha(5)beta(3)gamma(2) receptors but did not markedly alter the affinities of ligands (e.g., flunitrazepam) that are not subtype-selective. Conversely, substitution of the alpha(1) subunit with the cognate amino acid contained in the alpha(5) subunit (i.e., alpha(1)V211I) increased the affinities of alpha(5)-selective ligands by a approximately 20-fold and reduced by 3-fold the affinity of an alpha(1)-selective agonist (zolpidem). Increasing the lipophilicity (e.g., by substitution of Phe) of alpha(5)215 did not significantly affect the affinities (and selectivities) of RY-80 and RY-24 for alpha(5)-containing GABA(A) receptors. However, the effect of introducing hydrophilic and or charged residues (e.g., Lys, Asp, Thr) at this position was no greater than that produced by the alpha(5)I215V mutation. These data indicate that residue alpha(5)215 may not participate in formation of the lipophilic L(2) pocket that has been proposed to contribute to the unique pharmacological properties of alpha(5)-containing GABA(A) receptors. RY-80 and RY-24 acted as inverse agonists in both wild-type alpha(5)beta(3)gamma(2) and mutant alpha(5)I215Kbeta(3)gamma(2) receptors expressed in Xenopus laevis oocytes. However, both RY-24 and RY-80 acted as antagonists at mutant alpha(5)I215Vbeta(3)gamma(2) and alpha(5)I215Tbeta(3)gamma(2) receptors, whereas the efficacy of flunitrazepam was similar at all three receptor isoforms. The data demonstrate that amino acid residue alpha(5)215 is a determinant of both ligand affinity and efficacy at GABA(A) receptors containing an alpha(5) subunit.

Glycine(B) receptor antagonists and partial agonists prevent memory deficits in inhibitory avoidance learning

Activation of N-methyl-d-aspartate (NMDA) receptors has been hypothesized to mediate certain forms of learning and memory. This hypothesis is based on the ability of competitive and uncompetitive NMDA receptor antagonists to disrupt learning. We investigated the effects of glycine site antagonists and partial agonists on deficits of acquisition (learning) and consolidation (memory) in a single trial inhibitory avoidance learning paradigm. Posttraining administration of either hypoxia (exposure to 7% oxygen) or the convulsant drug pentylenetetrazole (PTZ) (45 mg/kg) to mice impaired consolidation without producing neuronal cell death. Pretreatment with the competitive glycine antagonist 7-chlorokynurenic acid (7KYN) and the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+)HA-966 prevented memory deficits induced by hypoxia and PTZ, but did not affect scopolamine-induced learning impairment. In addition, ACPC prevented consolidation deficits evoked by a nonexcitotoxic concentration of l-trans-pyrrolidine-2, 4-dicarboxylate, a competitive inhibitor of glutamate transport that increases extracellular levels of glutamate. Moreover, (+)HA-966, 7KYN, and ACPC facilitated both acquisition and consolidation of inhibitory avoidance training, an effect that was dose-dependent and reversed by glycine. These results indicate that memory deficits induced by both hypoxia and PTZ involve NMDA receptor activation. Furthermore, the present findings demonstrate that glycine site antagonists and partial agonists prevent memory deficits of inhibitory avoidance learning by affecting consolidation, but not acquisition processes.

The antibacterial and NMDA receptor activating properties of aminoglycosides are dissociable

The use of aminoglycoside antibiotics is limited by side effects, the most critical of which are vestibular and cochlear toxicity. Recent evidence indicates that these effects result from an excitotoxic process mediated, at least in part, through a polyamine-like activation of NMDA receptors. This study investigated whether these positive modulatory effects of aminoglycosides at NMDA receptors are dissociable from their antibacterial properties. A group of structurally related apramycin derivatives was evaluated for the ability to enhance [3H]dizocilpine binding to rat brain membranes, and for the ability to augment agonist responses on recombinant (NR1A/2B) NMDA receptors expressed in Xenopus oocytes. Based on the antibacterial potencies of these derivatives against Staphylococcus aureus and Escherichia coli, it is concluded that there is no correlation between the ability of an aminoglycoside to produce a positive modulation of NMDA receptors and minimum inhibitory antibacterial concentrations. These findings indicate that it may be possible to develop an aminoglycoside antibiotic with reduced potential for ototoxicity.

Spermine-induced toxicity in cerebellar granule neurons is independent of its actions at NMDA receptors

The neurotoxic actions of polyamines such as spermine have been linked to their modulation of NMDA receptors, resulting in an excitotoxic cell death. Here, we demonstrate that chronic exposure to the polyamine spermine and acute exposure to the combination of spermine and glutamate result in significant toxicity to primary cultures of cerebellar granule neurons (CGNs). However, in both cases this cell death (a) lacks the characteristic cell swelling associated with the necrotic cell death induced by glutamate and (b) is characterized by the widespread formation of apoptotic nuclei. Whereas dizocilpine (MK-801) blocks the synergistic cell death resulting from acute exposure to spermine plus glutamate, neither MK-801 nor the calcium chelator EGTA appreciably attenuates CGN death resulting from chronic exposure to spermine. Consistent with previous reports, glutamate, both acute and chronic, causes CGN death that is characterized by cell swelling, sensitivity to MK-801 and EGTA, and only small numbers of apoptotic nuclei. Spermine-induced toxicity is not blocked by either the protein synthesis inhibitor cycloheximide or the pancaspase inhibitor tert-butoxycarbonyl-Asp-(O-methyl) fluoromethyl ketone. However, the antioxidant butylated hydroxyanisole is an effective blocker of spermine-induced CGN death, suggesting a free-radical component to this cell death. The intact spermine molecule, rather than a catabolic by-product, is required for cell death because the amine oxidase inhibitors N1,N2-bis(2,3-butadienyl)-1,4-butanediamine and aminoguanidine fail to block this toxicity. Thus, in CGNs, spermine-induced toxicity does not occur by its modulation of NMDA receptors, although, under some circumstances, NMDA receptor activation can modulate spermine-induced toxicity.

Putative partial agonist 1-aminocyclopropanecarboxylic acid acts concurrently as a glycine-site agonist and a glutamate-site antagonist at N-methyl-D-aspartate receptors

1-Aminocyclopropanecarboxylic acid (ACPC) has been shown to protect against neuronal cell death after ischemic insult in vivo. Such results can be correlated with in vitro assays in which ACPC protected neurons against glutamate-induced neurotoxicity by reducing the activity of N-methyl-D-aspartate (NMDA) channel activation. Electrophysiological studies have determined that ACPC inhibits NMDA receptor activity by acting as a glycine-binding site partial agonist. In this study, rapid drug perfusion combined with whole-cell voltage-clamp was used to elicit and measure the effects of ACPC on NMDA receptor-mediated responses from cultured hippocampal neurons and cerebellar granule cells. The ACPC steady-state dose-response curve had both stimulatory and inhibitory phases. Half-maximal activation by ACPC as a glycine-site agonist was 0.7 to 0.9 microM. Half-maximal inhibition by ACPC was dependent on NMDA concentration. Peak responses to a >100 microM ACPC pulse in the presence of 1 microM glutamate were similar to those of glycine but decayed to a steady-state amplitude below that of glycine. The removal of ACPC initially caused an increase in inward current followed by a subsequent decrease to baseline levels. This suggests that relief of low-affinity antagonism occurs before high-affinity agonist dissociation. Simulations of ACPC action by a two glutamate-binding site/two glycine-binding site model for NMDA channel activation in conjunction with the concurrent role of ACPC as a glycine-site full agonist and glutamate-site competitive antagonist were able to successfully approximate experimental results.

[3H]ATPA: a high affinity ligand for GluR5 kainate receptors

The pharmacological properties of [3H]ATPA ((RS)-2-amino-3(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid) are described. ATPA is a tert-butyl analogue of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid) that has been shown to possess high affinity for the GluR5 subunit of kainate receptors. [3H]ATPA exhibits saturable, high affinity binding to membranes expressing human GluR5 (GluR5) kainate receptors (Kd approximately 13 nM). No specific binding was observed in membranes expressing GluR2 and GluR6 receptors. Several compounds known to interact with the GluR5 kainate receptor inhibited [3H]ATPA binding with potencies similar to those obtained for competition of [3H]kainate binding to GluR5. Saturable, high affinity [3H]ATPA binding (Kd approximately 4 nM) was also observed in DRG neuron (DRG) membranes isolated from neonatal rats. The rank order potency of compounds to inhibit [3H]ATPA binding in rat DRG and GluR5 membranes were in agreement. These finding demonstrate that [3H]ATPA can be used as a radioligand to examine the pharmacological properties of GluR5 containing kainate receptors.

Polyamine-like actions of aminoglycosides at recombinant N-methyl-D-aspartate receptors

Recent pharmacological studies have led to the hypothesis that aminoglycoside-induced ototoxicity is an excitotoxic process mediated, at least in part, by a polyamine-like modulation of N-methyl-D-aspartate (NMDA) receptors. To explore this hypothesis, we compared the effects of several aminoglycosides (neomycin B, kanamycin A, streptomycin, and dihydrostreptomycin) with spermine on recombinant NMDA receptors of defined composition expressed in Xenopus oocytes. Like spermine, aminoglycosides potentiate agonist-induced responses in the presence of both saturating glycine ("glycine-independent") and subsaturating glycine ("glycine-dependent") concentrations on NR1A/2B receptors. Likewise, aminoglycosides and spermine potentiated the agonist responses under glycine-dependent conditions on NR1A/2A receptors. Despite these similarities, several prominent differences were observed between spermine and aminoglycosides as well as among individual aminoglycosides. For example, neomycin B, streptomycin, and kanamycin A, but neither spermine nor dihydrostreptomycin, potentiated glycine-dependent responses on NR1A/2C receptors. Differences between spermine and aminoglycosides also were observed with respect to the voltage dependence of polyamine-like actions. For example, under glycine-dependent conditions, potentiation at NR1A/2B receptors by spermine was voltage dependent, decreasing as the holding potential was changed from -35 to -85 mV; in contrast, potentiation induced by aminoglycosides at NR1A/2B receptors was voltage independent. No direct relationships emerged between the effect of an aminoglycoside at a specific NMDA receptor subtype and the ototoxicity of these antibiotics.

Antidepressants for the new millennium

Despite a remarkable structural diversity, most conventional antidepressants may be viewed as 'monoamine based', increasing the synaptic availability of serotonin, norepinephrine, and/or dopamine. Both preclinical and recent clinical studies indicate that compounds which reduce transmission at N-methyl-D-aspartate (NMDA) receptors are antidepressant. Moreover, chronic administration of antidepressants to mice alters both the mRNA levels encoding N-methyl-D-aspartate receptor subunits and radioligand binding to these receptors within circumscribed areas of the central nervous system. It is hypothesized that these two different treatment strategies converge to produce an identical functional endpoint: a region-specific dampening of NMDA receptor function. The pathways leading to this convergence provide a rudimentary framework for discovering novel antidepressants.

Dizocilpine attenuates streptomycin-induced vestibulotoxicity in rats

NMDA receptor mediated excitotoxicity contributes substantially to aminoglycoside antibiotic-induced cochlear damage. Since vestibular as well as cochlear hair cells have glutamatergic synapses, aminoglycoside-induced vestibulotoxicity may also have an excitotoxic component. This hypothesis was tested by examining the effects of the uncompetitive NMDA receptor antagonist dizocilpine on streptomycin-induced vestibulotoxicity. Streptomycin-treated rats exhibited almost complete destruction of sensory hair cells in the crista ampullaris, vestibular impairment in the drop test, and hyperkinesia. Concurrent treatment with dizocilpine not only rescued a substantial population of sensory hair cells in the cristae, but prevented the attendant hyperkinesis and vestibular impairments. These results indicate that excitotoxic mechanisms contribute to aminoglycoside-induced vestibulotoxicity and that NMDA antagonists may be useful in attenuating aminoglycoside ototoxicity.

Aminoglycoside neurotoxicity involves NMDA receptor activation

Previous studies have led to the hypothesis that the ototoxicity produced by aminoglycoside antibiotics involves the excitotoxic activation of cochlear NMDA receptors. If this hypothesis is correct, then these antibiotics should also injure neurons within the brain. Because aminoglycosides do not readily penetrate the blood brain barrier, we examined the effects of the aminoglycoside neomycin following intrastriatal injection. Neomycin (10-250 nmol) produced dose-dependent striatal damage manifested as an increased gliosis as measured by: (1) [3H]PK-11195 binding, (2) staining for the astrocytic marker glial fibrillary acidic protein (GFAP) and (3) staining for OX-6, an MHC class II antigen expressed by microglia and macrophages. Co-injection of subthreshhold doses of NMDA potentiates the striatal damage produced by neomycin (10 nmol). Moreover, neomycin-induced striatal damage is attenuated by a combination of the NMDA antagonists ifenprodil and 5, 7-dichlorokynurenic acid. Intrastriatal administration of compounds structurally related to neomycin, but devoid of modulatory actions at NMDA receptors (paromamine and 2-deoxystreptamine), fail to produce neuronal damage. These data support the hypothesis that aminoglycoside-induced ototoxicity is, in part, an excitotoxic process involving the activation of NMDA receptors. Moreover, aminoglycosides may damage the central nervous system in individuals with compromised blood brain barriers.

Distinct structural requirements for the direct and indirect actions of the anaesthetic etomidate at GABA(A) receptors

1. The intravenous anaesthetic etomidate augments GABA-gated chloride currents (indirect action) and, at higher concentrations, evokes chloride currents in the absence of GABA (direct action). 2. In order to identify amino acid residues essential for these actions, site directed mutagenesis was performed on the beta3 subunit. 3. Mutation of an asparagine to a serine residue at position 290 dramatically reduced both etomidate-induced chloride currents and its ability to enhance [3H]flunitrazepam binding in HEK293 cells expressing alpha1beta3gamma2 recombinant GABA(A) receptors. 4. In contrast, the indirect effect of etomidate was retained, though its potency was reduced. 5. These findings indicate that there are distinct requirements for these dual actions of etomidate at GABA(A) receptors.

Sustained exposure to a glycine receptor partial agonist differentially alters NMDA receptor agonist and antagonist potencies in cultured spinal cord neurons

Sustained (20 h) exposure to the glycine partial agonist 1-aminocyclopropanecarboxylic acid (ACPC) significantly reduced N-methyl-D-aspartate (NMDA)-induced neurotoxicity in cultured spinal cord neurons when the NMDA (25 and 100 microM) was added to the cultures 30 min after removal of the ACPC (1 mM). In contrast, ACPC preexposure failed to protect against kainate-induced neuronal injury. The magnitude of neuronal protection against NMDA (100 microM) was further enhanced if the neurons pretreated with ACPC were reexposed to this drug during the NMDA challenge. In addition, the potencies of both the competitive NMDA antagonist AP5 and the noncompetitive antagonist dizocilpine to block NMDA toxicity were significantly increased following ACPC preexposure, while the potency of the competitive glycine receptor antagonist 7-chlorokynurenate (7-CK) was unchanged. Analysis of Northern blots suggest that ACPC-induced changes in NMDA receptor function were not associated with alterations in the levels of the mRNAs encoding the NMDAR-1, -2A, -2B, or -2C subunits. These results indicate that sustained exposure to ACPC modifies NMDA receptors in a manner that diminishes NMDA receptor-mediated neurotoxicity while selectively enhancing the potencies of several NMDA receptor antagonists. These effects do not appear to be related to changes in expression of specific NMDA receptor subunits, and may instead involve a post-translational modification of one or more subunit proteins.

Embryonic rat hippocampal neurons and GABAA receptor subunit-transfected non-neuronal cells release GABA tonically

We used patch-clamp recording techniques to investigate the contribution of GABA to baseline membrane properties in cultured embryonic rat hippocampal neurons. Almost all of the neurons recorded with Cl--filled pipettes and clamped at negative potentials exhibited baselines that were noticeably noisy, with microscopic fluctuations superimposed on the macroscopic holding current. A gentle steam of saline applied to the neuronal surface rapidly and reversibly reduced the baseline current and fluctuations, both of which were completely eliminated by bicuculline. Fluctuation analysis showed that the variance in the baseline current signal was exponentially distributed with estimated kinetics comparable to those activated by submicromolar concentrations of exogenous GABA. The kinetics of Cl- channels activated by endogenous GABA displayed a potential sensitivity comparable to those activated by exogenous GABA. Non-neuronal cells stably transfected with alpha1 and gamma2 GABAA receptor subunits exhibited little baseline current variance when recorded with Cl--filled pipettes. Addition of micromolar GABA to the extracellular saline or to the pipette solution induced a saline- and bicuculline-sensitive baseline current signal comparable to that recorded in hippocampal neurons. Thus, both intra- and extracellular sources of GABA could contribute to the baseline properties recorded in these cultured neurons.

Synthesis and evaluation of analogues of the partial agonist 6-(propyloxy)-4-(methoxymethyl)-beta-carboline-3-carboxylic acid ethyl ester (6-PBC) and the full agonist 6-(benzyloxy)-4-(methoxymethyl)-beta-carboline-3-carboxylic acid ethyl ester (Zk 93423) at wild type and recombinant GABAA receptors

A pharmacophore and an alignment rule have previously been reported for BzR agonist ligands. The design and synthesis of 6-(propyloxy)-4-(methoxymethyl)-beta-carboline-3-carboxylic acid ethyl ester (6-PBC, 24, IC50 = 8.1 nM) was based on this pharmacophore. When evaluated in vivo this ligand exhibited anticonvulsant/anxiolytic activity but was devoid of the muscle relaxant/ataxic effects of "classical" 1,4-benzodiazepines (i.e., diazepam). Significantly, 6-PBC 24 also reversed diazepam-induced muscle relaxation in mice. The 3-substituted analogues 40-46 and 48 of 6-PBC 24 and Zk 93423 27(IC50 = 1 nM) were synthesized and evaluated in vitro to determine what affect these modifications would have on the binding affinity at recombinant BzR subtypes. With the exception of the 3-amino ligands 40 and 41, all the beta-carbolines were found to exhibit high binding affinity at BzR sites. The 3-propyl ether derivative 45 was also evaluated in vivo and found to be devoid of any proconvulsant or anticonvulsant activity at doses up to 40 mg/kg. The 6-(1-naphthylmethyloxy) and 6-octyloxy analogues 25, 26, 28, and 29 of 6-PBC 24 were synthesized to further evaluate the proposed alignment of agonists vs inverse agonists in the pharmacophore of the BzR. In addition, ligands 26 and 29 were designed to probe the dimensions of lipophilic pocket L3 at the agonist site. The activity of 29 was evaluated in vivo; however, this analogue elicited no pharmacological effects at doses up to 80 mg/kg. These and other related beta-carbolines were also examined in five recombinant GABAA receptor subtypes. Ligands 52-61 all exhibited moderate to high affinity at GABAA receptors containing alpha1 subunits. These ligands will be useful in further defining the pharmacophore at alpha1 beta3 gamma2 receptors.

Chronic administration of imipramine and citalopram alters the expression of NMDA receptor subunit mRNAs in mouse brain. A quantitative in situ hybridization study

Chronic administration of antidepressants produces region-specific adaptive changes in the radioligand binding properties of N-methyl-D-aspartate (NMDA) receptors. We hypothesized that this effect of chronic antidepressant administration was owing to an alteration in NMDA receptor subunit composition. This hypothesis was examined using in situ hybridization with [35S]-labeled riboprobes to quantify the impact of chronic (16 d) injection with either imipramine (15 mg/kg) or citalopram (20 mg/kg) on the levels of transcripts encoding NMDA receptor subunits in mouse brain. These antidepressants altered the levels of mRNA encoding the zeta-subunit in a parallel fashion, with both drugs either reducing transcript levels (e.g., in the cortex, cerebellum, thalamus, and striatum) or producing no substantial effects (e.g., hippocampus). In contrast, these antidepressants often produced distinct, region-specific effects on mRNA levels encoding the epsilon family of subunits. For example, citalopram treatment produced widespread reductions in epsilon 1-subunit mRNA levels (e.g., in frontal cortex, CA2 of hippocampus, and amygdala), whereas imipramine reduced levels of this transcript only in the amygdala. Conversely, imipramine treatment produced widespread reductions in epsilon 2-subunit mRNA levels (e.g., in cortex, CA1-4 of hippocampus, and amygdala), whereas the effects of citalopram on levels of this transcript were largely restricted to amygdala. These findings indicate that long-term antidepressant treatment produces region-specific changes in expression of transcripts for NMDA receptor subunits, presumably altering NMDA receptor composition. Because subunit composition determines the physiological and pharmacological properties of NMDA receptors, these changes may play a critical role in the therapeutic actions of structurally diverse antidepressants.

The encephalopathy associated with murine acquired immunodeficiency syndrome

Mice infected with the LP-BM5 murine leukemia virus (MuLV) develop an immune deficiency syndrome together with an encephalopathy characterized by impairments in spatial learning and memory. These cognitive deficits are evident before the appearance of neuron loss and lymphoid cell invasion of the brain. Nonetheless, a prominent gliosis and a variety of neurochemical changes precede the development of cognitive deficits. The neurochemical abnormalities include significant decreases in striatal Met-enkephalin and substance P (but not somatostatin), increases in concentrations of quinolinic acid and platelet-activating factor, and alterations in brain fyn kinase. At this stage of the infection, some of these neurochemical changes can be reversed by glutamate receptor antagonists, cytokine inhibitors, and anti-retroviral agents. In later stages of the infection, however, the infected mice develop irreversible neuronal loss, invasion of hematopoietic cells, and increased viral burden in the CNS. In addition, motor-neuron dysfunction (hindlimb paralysis, weakness, and ataxia) and seizures are sometimes observed during the late stages of infection. Thus, the LP-BM5 MuLV-infected mouse is a useful model for studying the chronology of neurodegenerative changes, ranging from reversible neuron dysfunction to irreversible neuron loss, that are associated with retrovirus-induced immunodeficiency.

Glycine site antagonists and partial agonists inhibit N-methyl-D-aspartate receptor-mediated [3H]arachidonic acid release in cerebellar granule cells

Activation of N-methyl-D-aspartate (NMDA) receptors is known to produce arachidonic acid release, which has been implicated in excitotoxicity. Antagonists and partial agonists at the glycine site of the NMDA receptor, despite exhibiting functional differences in electrophysiological studies, inhibit glutamate-induced neurotoxicity and ischemia-induced neurodegeneration. The objective of this study was to investigate the effects of both glycine site antagonists and partial agonists on NMDA receptor-mediated [3H]arachidonic acid (AA) release evoked by glutamate, NMDA or a competitive inhibitor of the glutamate/aspartate uptake carrier. The [3H]AA release evoked by a maximally effective concentration of glutamate (100 microM) was blocked by the glycine site antagonists 7-chlorokynurenic acid (7-CKYN) and 5,7-dichlorokynurenic acid (5,7-DCKYN) and by a low intrinsic efficacy glycine partial agonist (+)-1-hydroxy-3-aminopyrrolid-2-one [(+)-HA-966]. 1-Aminocyclopropanecarboxylic acid (ACPC), a high intrinsic efficacy glycine partial agonist, did not modify [3H]AA release evoked by 100 microM glutamate. However, ACPC blocked (in a glycine reversible manner) the [3H]AA release induced by NMDA (100 microM) with an IC50 of 131 +/- 2 microM. Furthermore, L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a competitive inhibitor of the glutamate transporter, also released [3H]AA (Emax and EC50 of 127 +/- 4% and 30 +/- 1 microM, respectively). ACPC, 7-CKYN and (+/-)-2-amino-7-phosphonoheptanoic acid (AP-7), a competitive NMDA receptor antagonist, inhibited [3H]AA release evoked by PDC. These results demonstrate that both glycine site antagonists and partial agonists can inhibit NMDA receptor-mediated [3H]AA release in cerebellar granule cells, an action consistent with the neuroprotective effects of these compounds.

Polyamine-like actions of aminoglycosides and aminoglycoside derivatives at NMDA receptors

Recent pharmacological studies indicate that aminoglycoside-induced hearing loss may be an excitotoxic process modulated by a polyamine-like activation of cochlear NMDA receptors. Aminoglycoside antibiotics are constituted by a series of glycosidically linked aminocyclitols and aminosugars. We report here on the actions of these individual aminocyclitols and aminosugars on wild type NMDA receptors from rat brain. Compared to the parent molecules, neither aminocyclitols (e.g., 2-deoxystreptamine, streptamine, and streptidine) nor aminosugars (e.g., D-glucosamine and kanosamine) were effective at enhancing [3H]dizocilpine ([3H]MK-801) binding or inhibiting [3H]ifenprodil at NMDA receptors. Moreover, the appropriate combinations of aminosugars and aminocyclitols did not reconstitute the activity of the parent aminoglycoside at NMDA receptors. These data indicate that the polyamine-like actions of aminoglycosides are attributable to the conformation of the parent molecule rather than a particular amine containing constituent. Thus, it may be possible to synthesize or isolate aminoglycoside antibiotics devoid of ototoxicity.

Anomalous rectifying properties of 'diazepam-insensitive' GABA(A) receptors

Studies using recombinant systems indicate that 'diazepam-insensitive' GABA(A) receptors in the central nervous system contain alpha4 and alpha6 subunits while 'diazepam-sensitive' GABA(A) receptors contain alpha1, alpha2, alpha3 and alpha5 subunits. Both native and recombinant diazepam-sensitive GABA(A) receptors typically exhibit large, outwardly rectifying currents. For example, in patch clamp studies, Human Embryonic Kidney (HEK) 293 cells transfected with cDNAs encoding alpha1beta2gamma2 subunits exhibit a rectification ratio (I +60 mV/I -60 mV) of 1.95 +/- 0.21. However, anomalous rectification was observed in recombinant diazepam-insensitive GABA(A) receptors composed of either alpha4beta2gamma2 (rectification ratio, 0.74 +/- 0.09) or alpha6beta2gamma2 (rectification ratio, 0.67 +/- 0.11) subunits. Based on sequence differences between diazepam-sensitive and -insensitive GABA(A) receptor alpha subunits in the vicinity of the putative channel lining, a point mutation was introduced at His273 on the alpha4 subunit. The rectification ratio in cells expressing a mutated alpha4(Asn273)beta2gamma2 receptor increased to 1.92 +/- 0.17. Moreover, mutation of the homologous residue in the alpha1 subunit to histidine reduced the rectification ratio of alpha1(His274)beta2gamma2 to 1.02 +/- 0.12. The affinities of benzodiazepine site ligands at diazepam-sensitive and -insensitive GABA(A) receptors were unaffected by these mutations. Thus, the electrophysiological properties of diazepam-sensitive and -insensitive GABA(A) receptors may be as divergent as their pharmacological characteristics.

Molecular features associated with polyamine modulation of NMDA receptors

The effect of 1,3-diamines on basal and spermine-stimulated [3H]MK-801 binding was investigated. Systematic variations in the molecular parameters revealed that, in general, lipophilic 1,3-diamines inhibited and hydrophilic 1,3-diamines enhanced [3H]MK-801 binding in the nominal absence of glutamate and glycine. Furthermore, 1,3-diamines which were highly monoprotonated at physiologic pH were more effective in modulating basal binding (at 100 microM 1,3-diamine) than analogues which were mostly diprotonated or unprotonated. Finally, the internuclear distance between the amino nitrogens and the extent of modulation of basal [3H]MK-801 binding were correlated. Similar, but more modest, effects were seen for spermine-enhanced [3H]MK-801 binding. These results are consistent with the existence of two polyamine binding sites associated with the NMDA receptor complex. One of the sites appears to preferentially recognize lipophilic substances while the other favors hydrophilic materials. Both sites appear to recognize polyamines with at least one charged (protonated) amino group and one uncharged amino group. The distance between amino groups is a determining factor as well.

Quinolinic acid protects rat cerebellar granule cells from glutamate-induced apoptosis

The effects of quinolinic acid (QUIN) on glutamate-induced excitotoxicity were examined in primary cultures of rat cerebellar granule neurons. Exposing these neurons to QUIN (< or =2.5 mM) in the presence of glucose and Mg2+ had no effect on their viability. Although pretreating neurons with QUIN (10 microM) for 6 h did not reduce necrotic death induced by glutamate exposure in the absence of glucose and Mg2+, QUIN pretreatment significantly suppressed glutamate-induced apoptosis by 68% (as indicated by DNA fragmentation) in cultures containing glucose and Mg2+. Furthermore, the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 reversed QUIN-induced neuroprotection, while the non-NMDA antagonist CNQX had no effect. This study demonstrates that pathophysiologically relevant concentrations of QUIN can protect neurons from apoptosis mediated via the NMDA receptor.

Adaptation of cortical NMDA receptors by chronic treatment with specific serotonin reuptake inhibitors

Glycine displaces [3H]CGP-39653 ([3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) binding to the glutamate recognition site with both high and low affinity. We reported previously that chronic treatment with antidepressants reduced the proportion of high to low affinity sites, or, even eliminated the high affinity sites in case of citalopram. Here, we compared the effects of citalopram with another serotonin specific reuptake inhibitor, fluoxetine on this measure. Chronic administration of citalopram or fluoxetine eliminated high affinity glycine-displaceable [3H]CGP-39653 binding to the mouse cortex in 78 and 56% of animals, respectively, indicating that selective serotonin reuptake inhibitors produce qualitatively similar adaptive changes at NMDA receptors, that differ from other antidepressants in this neurochemical measure.

Stereoselective actions of halothane at GABA(A) receptors

Isoflurane anesthesia exhibits stereoselectivity, and a corresponding stereoselectivity ((+)->(-)-isomer) has been reported at GABA(A) receptors in vitro. The objective of the present study was to determine if the positive modulatory actions of halothane at GABA(A) receptors exhibited a similar stereoselectivity. Both (R)- and (S)-halothane ((+)- and (-)- isomers, respectively) enhanced [3H]flunitrazepam binding to brain membranes in a concentration dependent manner without a significant difference in either potency (EC50) or efficacy (Emax). While both (R)- and (S)-halothane enhanced [3H]muscimol binding, the potency of the (+)-isomer was slightly greater than the corresponding (-)-isomer (0.91 +/- 0.17 versus 1.45 +/- 0.04% atmospheres, respectively (P < 0.02)). Thus, subtle structural differences between inhalational anesthetics can have a significant impact on the degree of stereoselectivity at the receptor level and may provide insights for the development of more specific drugs.

Diazepam enhancement of GABA-gated currents in binary and ternary GABAA receptors: relationship to benzodiazepine binding site density

Although the predominant GABAA receptor isoform in the adult rodent central nervous system is a ternary complex composed of alpha 1 beta 2/3 gamma 2-subunits, small populations of binary receptors lacking beta-subunits (i.e., complexes containing alpha gamma-subunits) have also been identified. When expressed in HEK 293 cells, recombinant GABAA receptors composed of either alpha 1 beta 2/3 gamma 2- or alpha 1 gamma 2-subunits form benzodiazepine-responsive, GABA-gated chloride channels. The objective of this study was to compare the ability of a prototypic benzodiazepine (diazepam) to augment GABA-gated chloride currents in these binary and ternary receptor isoforms. The potency of GABA was characteristically increased by diazepam (1 microM) in both receptor isoforms, but this increase was significantly greater (p < 0.05) in receptors composed of alpha 1 beta 2 gamma 2-subunits (approximately five- to sixfold) compared to alpha 1 gamma 2-subunits (approximately 2.2-fold). At GABA concentrations approximating its EC50 value (5 microM), the greater augmentation observed in ternary receptors was attributable to a higher efficacy of diazepam. Radioligand binding studies revealed that the Bmax of [3H]flunitrazepam was increased approximately 1.8- and 3.5-fold in cells expressing alpha 1 beta 2 gamma 2- and alpha 1 beta 3 gamma 2-subunits, respectively, compared to cells expressing alpha 1 gamma 2-subunits. A similar increase (approximately 3.8-fold) in the Bmax of [3H]Ro 15-4513 was observed in HEK 293 cells transiently transfected with cDNAs encoding alpha 6 beta 3 gamma 2-compared to alpha 6 gamma 2-subunits. The Kd values of these radioligands were not different in binary and ternary receptor isoforms. It is hypothesized that the greater efficacy of diazepam in alpha 1 beta 2 gamma 2 compared to alpha 1 gamma 2 GABAA receptors results from the higher benzodiazepine binding site density produced by the formation of a ternary complex.

Chronic administration of a glycine partial agonist alters the expression of N-methyl-D-aspartate receptor subunit mRNAs

Both acute and chronic treatments with the glycine partial agonist 1-aminocyclopropanecarboxylic acid (ACPC) are neuroprotective in animal models of focal, global and spinal ischemia. After a chronic regimen of ACPC, brain and plasma levels were undetectable at the time of ischemic insult, which suggests that the neuroprotective effects of acute and chronic ACPC are mediated by different mechanisms. To investigate the possibility that chronic administration of ACPC alters N-methyl-D-aspartate (NMDA) receptor composition, the levels of mRNAs encoding zeta and epsilon subunits were quantified by in situ hybridization histochemistry with 35S-labeled riboprobes. Chronic ACPC administered to mice (200 mg/kg for 14 days) increased the level of epsilon-1 mRNA in the hippocampus (particularly CA1 and CA2 regions) and cerebral cortex (frontal, parietal and occipital regions), without altering levels in cerebellum. In contrast, this regimen decreased epsilon-3 subunit mRNA levels in the hippocampus (especially CA1 and dentate gyrus) and frontal and occipital cortices. Decreases in epsilon-2 subunit mRNA levels in cerebral cortex (especially frontal and parietal cortices) were also observed without accompanying alterations in the cerebellum, hippocampus or dentate gyrus. The levels of zeta subunit mRNA (determined with a probe that detects all splice variants) were not altered in any brain areas examined. Based on studies in recombinant receptors, these region-specific changes in mRNAs produced by a chronic regimen of ACPC could result in NMDA receptors with reduced affinities for glycine and glutamate. It is hypothesized that such alterations in NMDA receptor subunit composition may explain the neuroprotective effects produced by chronic ACPC.

Chronic antidepressant treatments increase cytochrome b mRNA levels in mouse cerebral cortex

Antidepressant therapies include drugs with a remarkable structural diversity and non-pharmacological interventions, such as electroconvulsive shock. Although the primary neurochemical effects of these treatments may differ, the > or = 2- to 3-wk lag in therapeutic onset has led to the hypothesis that adaptive changes in a final common pathway are required for an antidepressant action. Based on this hypothesis, we sought to identify and characterize common changes in gene expression following chronic antidepressant treatments. We utilized a differential display strategy to identify genes that were differentially expressed in mice following chronic treatment with imipramine and electroconvulsive shock. Differential display PCR followed by subcloning, screening by reverse Northern blot, and confirmation by Northern blot revealed a significant increase in the expression of one gene candidate from mouse cortex following antidepressant treatments. The sequence of this 193-bp gene candidate was an exact match to the DNA sequence of mouse mitochondrial cytochrome b. In contrast to the increased mRNA levels of cytochrome b found in cortex, chronic treatment with these antidepressants did not alter mRNA levels in hippocampus, cerebellum, or liver. Moreover, no differences in cortical levels of cytochrome b mRNA were observed after either acute antidepressant treatments or chronic treatment with nonantidepressant drugs (haloperidol and morphine). The observation that chronic, but not acute treatment with imipramine and electroconvulsive shock produces a region-specific change in the levels of mRNA encoding cytochrome b suggests that this enzyme may be involved in the sequence of events resulting in an antidepressant action.

[3H]RY 80: A high-affinity, selective ligand for gamma-aminobutyric acidA receptors containing alpha-5 subunits

The radiochemical synthesis and pharmacological properties are described of [3H]RY 80 (ethyl-8-acetylene-5, 6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a][1, 4]benzodiazepine-3-carboxylate, [ethyl-3H]). This compound is one of a series of 8-substituted imidazobenzodiazepines that exhibits both high affinity and selectivity for gamma-aminobutyric acid (GABA)A receptors containing alpha-5 subunits. Saturable, high-affinity (Kd approximately 0.7 nM) binding of [3H]RY 80 was observed in hippocampal membranes. The maximum number (Bmax) of [3H]RY 80 binding sites was approximately 18% of that obtained with [3H]flunitrazepam, a radioligand that labels all "diazepam-sensitive" GABAA receptors. This value is consistent with previous estimates (10-20%) of the proportion of rat hippocampal GABAA receptors containing alpha-5 subunits determined by immunoprecipitation with selective antibodies and competition experiments using an alpha-5-selective ligand. In recombinant GABAA receptors composed of alpha-5 beta-3 gamma-2 subunits, the Kd of [3H]RY 80 (approximately 0.5 nM) was consistent with the value obtained in hippocampus, whereas the Bmax value was not significantly different from that obtained with [3H]flunitrazepam. The potencies of several benzodiazepine site ligands to inhibit [3H]RY 80 binding to hippocampal membranes were in agreement with the values obtained in recombinant (alpha-5 beta-3 gamma-2) GABAA receptors. [3H]RY 80 was used both in a "GABA shift" assay to correctly predict the in vivo actions of a novel, alpha-5-selective ligand and to characterize a population of GABAA receptors containing alpha-5 subunits in neonatal rat cortex. These findings demonstrate that [3H]RY 80 can be used as a radioligand to examine the properties of GABAA receptors containing alpha-5 subunits.

Distinct loci mediate the direct and indirect actions of the anesthetic etomidate at GABA(A) receptors

Most general anesthetics produce two distinct actions at GABA(A) receptors. Thus, these drugs augment GABA-gated chloride currents (referred to as an indirect action) and, at higher concentrations, elicit chloride currents in the absence of GABA (referred to as a direct action). Because a beta subunit appears to be required for the direct action of intravenous anesthetics in recombinant GABA(A) receptors, site-directed mutagenesis of the beta3 subunit was performed to identify amino acid residues that are critical for this action. In HEK293 cells expressing a prototypical GABA(A) receptor composed of alpha1beta3gamma2 subunits, mutation of amino acid 290 from Asn to Ser dramatically reduced both etomidate-induced chloride currents and its ability to stimulate [3H]flunitrazepam binding. By contrast, the ability of etomidate to augment GABA-gated chloride currents and GABA-enhanced [3H]flunitrazepam binding was retained. The demonstration that the direct, but not the indirect, actions of etomidate are dependent on beta3(Asn290) indicates that the dual actions of this intravenous anesthetic at GABA(A) receptors are mediated via distinct loci.

Chronic administration of a partial agonist at strychnine-insensitive glycine receptors: a novel experimental approach to the treatment of ischemias

During the past decade, converging lines of evidence have linked the abnormal release or leak of excitatory amino acids to the neurodegeneration associated with a wide range of pathologies including cerebral ischemias, Huntington's disease, and AIDS dementia (Coyle and Robinson, 1987; Lipton, 1994; Meldrum, 1994). Pharmacological studies indicate that activation of both ionotropic and metabotropic glutamate receptors can substantially contribute to excitotoxic cell damage (Choi, 1992; Pizzi et al., 1993; Sheardown et al., 1993; Xue et al., 1994). Based on these findings, therapeutic strategies based on blunting or blocking glutamatergic transmission may be useful in treating a variety of neurodegenerative disorders.

(2S,4R)-4-methylglutamic acid (SYM 2081): a selective, high-affinity ligand for kainate receptors

Glutamic acid activates ionotropic glutamate receptors that mediate excitatory transmission in the central nervous system. The introduction of a methyl group at position 4 of glutamic acid imparts selectivity for kainate receptors, relative to other (N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ionotropic glutamate receptors. Among the stereoisomers of 4-methylglutamic acid, the potency of the (2S,4R)-isomer (SYM 2081) to inhibit [3H]kainic acid binding to both wild-type (rat forebrain) and recombinant (GluR6) kainate receptors (IC50 values of approximately 32 and 19 nM, respectively) was comparable to that of kainic acid (IC50 values of approximately 13 and 28 nM, respectively). SYM 2081 was approximately 800- and 200-fold less potent as an inhibitor of radioligand binding to wild-type (rat forebrain) alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors, respectively. Preexposure of human embryonic kidney 293 cells stably expressing GluR6 receptors to low concentrations of SYM 2081 (30-300 nM) resulted in a reversible blockade of the rapidly desensitizing currents produced by kainate application. At higher concentrations, SYM 2081 (EC50 of approximately 1 microM) elicited kainate-like, rapidly desensitizing, inward currents. Pretreatment of recombinant GluR6 receptors with concanavalin A both abolished the effect of SYM 2081 to block kainate-induced currents and revealed nondesensitizing currents induced by SYM 2081 alone. The latter observations provide strong support for the hypothesis that SYM 2081 blocks kainate-induced currents through a process of agonist-induced desensitization. SYM 2081 also activated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor currents in primary cultures of cerebral cortex and, consistent with data obtained by radioligand binding, was approximately 5-fold less potent than kainate (EC50 values of 325 and 70 microM, respectively) in this measure. SYM 2081 is a high-affinity, selective, kainate agonist that may prove useful both as a probe to examine the physiological functions of kainate receptors and as the prototype of a novel class of therapeutic agents.

N-methyl-D-aspartate antagonists limit aminoglycoside antibiotic-induced hearing loss

The use of aminoglycoside antibiotics is limited by ototoxicity that can produce permanent hearing loss. We report that concurrent administration of N-methyl-D-aspartate (NMDA) antagonists markedly attenuates both the hearing loss and destruction of cochlear hair cells in guinea pigs treated with aminoglycoside antibiotics. These findings indicate that aminoglycoside-induced hearing loss is mediated, in part, through an excitotoxic process. The high correlation (Spearman correlation coefficient: 0.928; P < 0.01) obtained between the relative cochleotoxicities of a series of aminoglycosides in humans and the potencies of these compounds to produce a polyamine-like enhancement of [3H]dizocilpine binding to NMDA receptors is consistent with this hypothesis, and provides a simple in vitro assay that can predict this aspect of aminoglycoside-induced ototoxicity.

Stereoselective inhibition of natural killer activity by the sigma ligand (+)-pentazocine

The effect of sigma (sigma) receptor ligands on natural killer (NK) activity was examined both in vivo and in vitro. Following injection of mice with sigma receptor ligands such as (+)-pentazocine, (-)-pentazocine, BD 1073, BD 1165 and BD 737, NK activity was measured in poly-I.C.-stimulated mouse splenocytes. (+)-Pentazocine reduced NK cell activity in a dose-dependent fashion, while the (-) enantiomer was inactive in this measure. For example, at a dose of 50 mg/kg, (+)-pentazocine suppressed NK activity (using effector to target cell ratios of 200:1, 100:1 and 50:1) by > 70%, 24 h after injection while (-)-pentazocine was inactive. The other sigma ligands examined either slightly enhanced or had no effect on NK activity. Nonetheless, parenteral administration of the sigma receptor ligand BD 1165 blocked (+)-pentazocine-induced suppression of NK activity, while the opiate receptor antagonist naltrexone was ineffective. Addition of sigma receptor ligands (10(-11)-10(-5) M) to splenocyte cultures for 24 h did not affect NK activity. These findings indicate that while sigma receptor ligands are capable of modulating NK activity, this effect is not the result of an action on splenocyte sigma receptors, but may be mediated via sigma receptors either in the central or peripheral nervous systems.

Structurally modified ibogaine analogs exhibit differing affinities for NMDA receptors

Based on both preclinical findings and anecdotal evidence in man, the psychoactive indole alkaloid ibogaine has been suggested to have anti-addictive properties. Previous studies indicate that blockade of NMDA receptors may mediate at least some of the putative anti-addictive actions of ibogaine. The potencies of a series of ibogaine analogs to inhibit (+)-[3-3H]5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclohepten-5,10- imine ([3H]MK-801) binding to NMDA receptors were examined. This series of analogs included the putative ibogaine metabolite O-desmethylibogaine, its metabolism resistant analog O-t-butyl-O-desmethylibogaine, the iboga alkaloids (+/-)-ibogamine, (+/-)-coronaridine, tabernanthine, harmaline, and the indolotropanes endo-3-(1-methylindol-2-yl)-8-methyl-8-azabicyclo[3.2.1]loctane (RS 075194-190), exo-3-(1-methylindol-2-yl)-8-methyl-8-azabicyclo[3.2.1]octane (RS 075237-190), and endo-3-(indol-2-yl)-8-methyl-8-azabicyclo[3.2.1]octane (RS 025989-190). Among these compounds, ibogaine was the most potent inhibitor of [3H]MK-801 binding (Ki = approximately 1.2 microM), whilst the compounds with the greatest structural similarity to ibogaine, O-desmethylibogaine and O-t-butyl-O-desmethylibogaine were less potent (Ki = approximately 5.5 and 179.0 microL, respectively). In morphine-dependent mice, ibogaine, but not O-desmethylibogaine or O-t-butyl-O-desmethylibogaine, attenuated naloxone precipitated withdrawal jumping. These findings are consistent with the hypothesis that inhibition of the expression of morphine dependence by ibogaine is related to its NMDA receptor antagonist properties.

Synthesis and pharmacological properties of novel 8-substituted imidazobenzodiazepines: high-affinity, selective probes for alpha 5-containing GABAA receptors

The synthesis and pharmacological properties of imidazobenzodiazepines with both high affinity and selectivity for alpha 5-containing GABAA receptors are described. Four of these compounds (5, 6, 8, and 9) inhibited [3H]flunitrazepam binding to recombinant alpha 5 beta 2 gamma 2 GABAA receptors with IC50 values between approximately 0.4 and 5 nM. These compounds were > or = 24-75-fold more selective for recombinant receptors containing alpha 5 subunits compared to other, "diazepam-sensitive" GABAA receptors containing either alpha 1, alpha 2, or alpha 3 subunits. Imidazobenzodiazepine 9 (used as the prototypical alpha 5 selective ligand) inhibited [3H]flunitrazepam binding to hippocampal membranes with high- and low-affinity components (IC50 0.6 +/- 0.2 and 85.6 +/- 13.1 nM, respectively), representing approximately 16% and approximately 84% of the receptor pool. Inhibition of [3H]flunitrazepam binding to cerebellar membranes with imidazobenzodiazepine 9 was best fitted to a single population of sites with an IC50 of 79.8 +/- 18.3 nM. These imidazobenzodiazepines behaved as GABA negative ligands in recombinant GABAA receptors expressed in Xenopus oocytes and were convulsant in mice after parenteral administration. The relative potencies of flumazenil and zolpidem in blocking convulsions induced by 9 and DMCM, respectively, indicated that occupation of alpha 5-containing GABAA receptors substantially contributed to the convulsant properties of acetylene analog 9. These 8-substituted imidazobenzodiazepines (5, 6, 8 and 9) should prove useful in examining the physiological roles of GABAA receptors bearing an alpha 5 subunit and may also lead to the development of novel, subtype selective therapeutic agents.

The NMDA antagonist memantine blocks the expression and maintenance of morphine dependence

The ability of memantine (1-amino-3,5-dimethyl-adamantane) to block the expression and maintenance of morphine dependence was examined in mice. When administered to morphine-dependent mice 45 min prior to naloxone challenge, memantine (7.5-30 mg/kg IP) in a dose-dependent manner reduced jumping behavior (a manifestation of the expression of dependence). The ability of memantine to attenuate naloxone-precipitated jumping was reversed by administration of glycine, an observation consistent with electrophysiological studies indicating that memantine is a use-dependent (uncompetitive) N-methyl-D-aspartate (NMDA) antagonist. In an independent series of experiments, the effect of memantine on a preestablished morphine dependence was investigated. A residual dependence to morphine was present 3 days after cessation of morphine administration. Repeated administration of memantine (10 mg/kg, IP) or the competitive NMDA antagonist NPC 17742 [2R, 4R, 5S-(2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid)] (6 mg/kg, IP) during this 3-day period abolished subsequent naloxone-precipitated jumping. In contrast, when administered concurrently with morphine after dependence had already been well established, memantine (10 and 20 mg/kg, IP) did not affect the maintenance of morphine dependence. Based on these finding, NMDA antagonists appear to inhibit the maintenance of opioid dependence, an action distinct from their acute inhibitory effects on the expression of dependence. Nonetheless, these regimen-dependent effects of memantine indicate that the most efficacious use of NMDA antagonists would be in detoxified subjects, rather than in individuals with an established dependence who are currently abusing opioids.