Acute high-dose glycine attenuates mismatch negativity (MMN) in healthy human controls

Rationale Schizophrenia is commonly associated with impairments in pre-attentive change detection as represented by reduced mismatch negativity (MMN). The neurochemical basis of MMN has been linked to N-methyl-d-aspartate (NMDA) receptor function. Glycine augments NMDA receptor function via stimulation of the glycine modulatory site of the NMDA receptor and has been shown to effectively reduce negative symptoms in schizophrenia. However, no study has investigated the possible effects of high-dose glycine on MMN. Further, the physiological consequences of administering high-dose glycine in subjects with normal NMDA receptor function are unknown. Objectives The aim of the present project was to investigate the acute effects of a single large dose of glycine on the human MMN in healthy subjects. Materials and methods Sixteen healthy male subjects participated in a double blind, placebo-controlled, crossover design in which each subject was tested under two acute treatment conditions separated by a 1-week washout period; placebo and 0.8 g/kg glycine. The subjects were exposed to a duration-MMN paradigm with 50-ms standard tones (91%) and 100-ms deviant tones (9%). Results The results showed that glycine significantly attenuated duration MMN amplitude at frontal electrodes. There was no effect of glycine on MMN latencies or on amplitudes or latencies of N1, N2 and P3a. Conclusions These findings suggest that an acute high dosage of glycine attenuates MMN in healthy controls, raising the possibility that optimal effects of glycine and other glycine agonists may depend on the integrity of the NMDA receptor system.     

D-serine modulates the NMDA receptor/nitric oxide/cGMP pathway in the rat cerebellum during in vivo microdialysis

Several lines of investigation indicate that D-serine may be an endogenous ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in some CNS regions. We here studied the in vivo effects of D-serine on the NMDA receptor/nitric oxide/cGMP pathway by monitoring extracellular cGMP in the cerebellum of freely-moving rats subjected to transcerebral microdialysis. Local application of NMDA (200, 500 μM) through the dialysis probe for 20 min evoked transient, concentration-dependent cGMP responses which peaked in the fraction of drug administration, the nucleotide levels returning to basal values after 40 min. The NMDA-induced elevation of the extracellular nucleotide was completely inhibited by the selective receptor channel blocker dizocilpine (MK-801) locally co-perfused at the concentration of 10 μM. The non-competitive antagonist had no effect on its own suggesting that endogenous glutamic acid does not tonically activate NMDA receptors. The effect of 200 μM NMDA was largely attenuated by 30 μM 7-chloro-kynurenic acid and completely abrogated when the concentration of the strychnine-insensitive glycine receptor antagonist was raised to 100 μM. D-serine (300 μM), perfused in the presence of 7-chloro-kynurenate (30 μM), was able to fully restore the NMDA (200 μM)-induced increase of cGMP extracellular levels. On the other hand, the D-amino acid directly potentiated in a concentration-dependent manner (0.3, 1 and 10 mM) the NMDA (200 μM)-evoked cGMP production whereas it was inactive on its own. These data show that in vivo the activation of the strychnine-insensitive glycine site is essential for the functioning of the NMDA receptor complex and can be activated by the selective agonist D-serine. They also confirm that cerebellar NMDA receptors do not have their glycine sites saturated. Received: 25 June 1996 / Accepted: 23 September 1996     

Direct evidence that acutely enhancing serotonin with the selective serotonin reuptake inhibitor citalopram modulates the loudness dependence of the auditory evoked potential (LDAEP) marker of central serotonin function

The loudness dependence of the auditory evoked potential (LDAEP) has been suggested as a reliable measure of central serotonin function in humans. The most convincing evidence for a direct relationship between serotonergic function and LDAEP to date has come from animal studies, while evidence in humans has been circumstantial and inconsistent. In the current study, we examine the direct effect of serotonergic modulation with the selective serotonin reuptake inhibitor (SSRI) citalopram on the LDAEP. The study was a double-blind placebo controlled design in which healthy participants were tested under two acute treatment conditions: placebo and citalopram (20 mg). Enhancement of serotonin function with citalopram in comparison to placebo decreased the slope of the LDAEP (i.e. weaker LDAEP). The findings provide direct evidence in humans, of a relationship between central serotonin function and the LDAEP, supporting findings previously observed in animals and clinical populations. Together the results provide further support for the validity of the LDAEP as a non-invasive in vivo measure of central serotonin function in humans.     

Positive allosteric modulatory effects of ajulemic acid at strychnine-sensitive glycine α<Subscript>1</Subscript>- and α<Subscript>1</Subscript>β-receptors

The synthetic cannabinoid ajulemic acid (CT-3) is a potent cannabinoid receptor agonist which was found to reduce pain scores in neuropathic pain patients in the absence of cannabis-like psychotropic adverse effects. The reduced psychotropic activity of ajulemic acid has been attributed to a greater contribution of peripheral CB receptors to its mechanism of action as well as to non-CB receptor mechanisms. Loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord plays a key role in the development of chronic pain following inflammation or nerve injury. Inhibitory postsynaptic transmission in the adult spinal cord involves mainly glycine. As we hypothesised that additional non-CB receptor mechanisms of ajulemic acid might contribute to its effect in neuropathic pain, we investigated the interaction of ajulemic acid with strychnine-sensitive α₁- and α₁β-glycine receptors by using the whole-cell patch clamp technique. Ajulemic acid showed a positive allosteric modulating effect in a concentration range which can be considered close to clinically relevant concentrations (EC₅₀ values: α₁ = 9.7 ± 2.6 μM and α₁β = 12.4 ± 3.4 μM). Direct activation of glycine receptors was observed at higher concentrations above 100 μM (EC₅₀ values: α₁ = 140.9 ± 21.5 μM and α₁β = 154.3 ± 32.1 μM). These in vitro results demonstrate that ajulemic acid modulates strychnine-sensitive glycine receptors in clinically relevant concentrations.     

The cognitive effects of modulating the glycine site of the NMDA receptor with high-dose glycine in healthy controls

N-methyl-D-aspartate (NMDA) receptors play an important role in learning and memory. Targeting the glycine modulatory site of the NMDA receptor has been suggested as a therapeutic strategy to improve cognition, although findings have not been convincing. We used the Cognitive Drug Research computerised assessment system to examine the effects of high-dose glycine on a number of cognitive processes in healthy young subjects. The study was a randomised placebo controlled repeated measures design in which each subject received acute placebo or glycine (0.8 g/kg) orally, with treatment conditions separated by a 5-day washout period. No significant effects of glycine were found on measures of working memory, declarative memory, attention or perceptual processing. These findings, together with those of previous studies, cast doubt over the ability of acute high-dose glycine to improve cognitive function in healthy subjects and suggest that the optimum dose of glycine for improving cognition may vary between different populations, possibly due to differences in endogenous glycine levels and the functional status of NMDA receptors.     

A rapid method for evaluating the behavioral effects of phencyclidine-like dissociative anesthetics in mice

A simple and rapid method for detecting the behavioral effects of phencyclidine and related dissociative anesthetics is described. Dissociative anesthetics such as phencyclidine (PCP) and dizolcipine, which bind with high affinities at N-methyl-D-aspartate (NMDA) coupled cation channels (“PCP receptors”), produced a dose-related increase in the percentage of mice that fell from a 1.5 cm deep circular arena mounted on a 60 cm platform. A similar behavior was not manifest by other classes of compounds examined including competitive NMDA antagonists, an antagonist at strychnine-insensitive glycine receptors, and σ-receptor ligands with moderate to low affinities for PCP receptors. Pretreatment of mice with glycine reduced in a dose-dependent manner the percentage of falls elicited by a maximally effective dose of dizolcipine. This simple procedure may prove useful for both the rapid detection of dissociative anesthetics and evaluation of putative PCP antagonists.     

Dopamine receptor stimulation does not modulate the loudness dependence of the auditory evoked potential in humans

Rationale The Loudness Dependence of the Auditory Evoked Potential (LDAEP) has been suggested as a reliable measure of central serotonin function in humans; however, its specificity for the serotonin system remains a topic of debate, with possible modulation of this purported serotonin marker by other neurotransmitters, including dopamine.Objectives We examined the effect of dopaminergic modulation on the LDAEP using the D₁/D₂/D₃ dopamine receptor agonist pergolide and the D₂/D₃ agonist bromocriptine.Methods The study was a double-blind, placebo-controlled repeated-measures design in which healthy participants were tested under three acute treatment conditions: placebo, bromocriptine (2.5 mg), and pergolide (0.1 mg). Changes in the amplitude of the N1/P2 at intensities (60, 70, 80, 90, and 100 dB) were examined at C _Z.Results Acute stimulation of D₁/D₂/D₃ receptors with pergolide and D₂/D₃ receptors with bromocriptine in comparison with placebo had no effect on the LDAEP.Conclusion These findings indicate that acute stimulation of dopamine D₁, D₂, and D₃ receptors does not modulate the LDAEP in humans. Although the findings suggest that the LDAEP may not be modulated by acute changes in dopamine neurotransmission, further studies are needed to fully characterize its dopaminergic sensitivity.     

The incidence of unpleasant dreams after sub-anaesthetic ketamine

Rationale  Ketamine is an N-methyl-d-aspartate (NMDA) receptor antagonist with psychotogenic effects and for which there are diverse reports of whether pleasant or unpleasant dreams result during anaesthesia, post-operatively or after sub-anaesthetic use. Objective  To assess in healthy volunteers the incidence of unpleasant dreams over the three nights after receiving a sub-anaesthetic dose of ketamine, in comparison to placebo, and with retrospective home nightmare frequency as a covariate. Method  Thirty healthy volunteers completed questionnaires about retrospective home dream recall and were then given either ketamine (n = 19, males = 9, mean age = 23.5 years; mean ketamine blood plasma = 175.29 ng/mL) or placebo (n = 11, males = 5, mean age = 25.4 years). Dream recall and pleasantness/unpleasantness of dream content were recorded by questionnaire at home for the three nights after infusion. Results  Ketamine resulted in significantly more mean dream unpleasantness relative to placebo and caused a threefold increase in the odds ratio for the incidence of an unpleasant dream. The number of dreams reported over the three nights did not differ between the groups. The incidence of unpleasant dreams after ketamine use was predicted by retrospectively assessed nightmare frequency at home. Conclusions  Ketamine causes unpleasant dreams over the three post-administration nights. This may be evidence of a residual psychotogenic effect that is not found on standard self-report symptomatology measures or a result of disturbed sleep electrophysiology.     

Glycyldodecylamide, a phencyclidine behavioral antagonist, blocks cortical glycine uptake: implications for schizophrenia and substance abuse

  N-Methyl-d-aspartate (NMDA) antagonists induce psychotomimetic effects in humans that closely resemble negative and cognitive symptoms of schizophrenia. NMDA agonists, in contrast, may significantly ameliorate such symptoms. In rodents, phencyclidine (PCP) and other NMDA antagonists induce a hyperlocomotory syndrome that is reversed by NMDA agonists. The present study investigates the mechanism of action of glycyldodecylamide (GDA), a drug that is 80-fold more potent than glycine in reversing PCP-induced hyperactivity in rodents. At concentrations relevant to its behavioral actions, GDA significantly inhibits forebrain glycine uptake, indicating that glycine uptake inhibition may provide effective treatment for PCP psychosis and PCP psychosis-like symptoms of schizophrenia. Received: 17 June 1996 / Final version: 16 September 1996     

Effects of 2-phenoxyethanol on N-methyl-d-aspartate (NMDA) receptor-mediated ion currents

The actions were examined of 17 frequently used glycol ether compounds on the glutamate receptor-mediated ion currents. The receptors were expressed in Xenopus oocytes by injection of rat brain mRNA. Most of the 17 glycol ethers exerted no effects on the glutamate subreceptors activated by kainate and N-methyl-d-aspartate (NMDA), whereas 2-phenoxyethanol (ethylene glycol monophenyl ether) caused a considerable reduction of NMDA-induced membrane currents in a reversible and concentration-dependent manner. The threshold concentration of the ethylene glycol monophenyl ether effect was <10 μmol/l. The concentration for a 50% inhibition (IC₅₀) was ∼360 μmol/l. The results indicate a neurotoxic potential for 2-phenoxyethanol. Received: 13 October 1998 / Accepted: 24 November 1998     

Anxiolytic effect of serotonin depletion in the novelty-induced hypophagia test

Rationale Relatively little is known about the neural mechanisms underlying anxiety in the novelty-induced hypophagia test, the only known anxiety test that is responsive to chronic but not acute or subchronic antidepressant treatment. Objectives The goal of the present experiment was to characterize the role of serotonin in the ability of novelty to suppress feeding. Materials and methods Pair-housed male Sprague–Dawley rats were trained to eat graham cracker crumbs individually in their home cage (15 min/day). After stable daily intakes were obtained, the animals were depleted of serotonin using 4-chloro-dl-phenylalanine (150 mg kg⁻¹ day⁻¹ × 2 days). Forty-eight hours later, central serotonin was restored by the administration of the peripheral l-aromatic amino acid decarboxylase inhibitor, benserazide (10 mg/kg), followed 15 min later with the immediate precursor of serotonin, 5-hydroxy-l-tryptophan (30 mg/kg). Thirty minutes later, the animals were given access to graham cracker crumbs in a novel environment. Results The animals demonstrated increased latencies to approach the food and reduced food intake in the novel environment. This effect was attenuated by serotonin depletion. Repletion of central serotonin restored the inhibitory response to novelty. The analysis of serotonin content in different brain regions confirmed that serotonin was depleted by greater than 90%, whereas the repletion treatment resulted in serotonin levels similar to nondepleted animals. Conclusions Acute depletion of serotonin acts to reduce anxiety behavior as measured by an inhibitory anxiety response during exposure to novel stimuli. These findings are in agreement with the proposed general role for serotonin in behavioral inhibition and that reductions of serotonin facilitate the adoption of more active coping responses to stress.     

Examination of the effect of acute levodopa administration on the loudness dependence of auditory evoked potentials (LDAEP) in humans

Rationale

The loudness dependence of the auditory evoked potential (LDAEP) is considered a noninvasive in vivo marker of central serotonergic functioning in humans. Nevertheless, results of genetic association studies point towards a modulation of this biomarker by dopaminergic neurotransmission.

Objective

We examined the effect of dopaminergic modulation on the LDAEP using L-3,4-dihydroxyphenylalanine (levodopa)/benserazide (Madopar?) as a challenge agent in healthy volunteers.

Methods

A double-blind placebo-controlled challenge design was chosen. Forty-two healthy participants (21 females and 21 males) underwent two LDAEP measurements, following a baseline LDAEP measurement either placebo or levodopa (levodopa 200?mg/benserazide 50?mg) were given orally. Changes in the amplitude and dipole source activity of the N1/P2 intensities (60, 70, 80, 90, and 100?dB) were analyzed.

Results

The participants of neither the levodopa nor the placebo group showed any significant LDAEP alterations compared to the baseline measurement. The test?Cretest reliability (Cronbachs Alpha) between baseline and intervention was 0.966 in the verum group and 0.759 in the placebo group, respectively.

Conclusions

The administration of levodopa showed no effect on the LDAEP. These findings are in line with other trials using dopamine receptor agonists.     

Is the loudness dependence of auditory evoked potentials modulated by the selective serotonin reuptake inhibitor citalopram in healthy subjects?

The loudness dependence of auditory evoked potentials (LDAEP) has been discussed as a non-invasive in vivo marker of central serotonergic function. Evidence for this has been found in animal studies, but studies in humans provide less consistent results. In this study, the relationship between LDAEP and directly modulated central serotonergic activity in healthy subjects was investigated. In a single-blind cross-over design, the LDAEP of female participants (age: 24.0 +/- 2.3 years) was measured under two conditions: (1) infusion of 20 mg citalopram diluted in 250 ml 0.9% saline and (2) infusion of 250 ml 0.9% saline as placebo. LDAEP was measured at five different time points before, during and up to 60 min after drug/placebo administration and dipole source analysis was performed. The increase of the central serotonin activity in response to citalopram was not accompanied by a significant change of the LDAEP compared to the placebo condition. The result underlines that the acceptance of LDAEP as a marker of central serotonergic function still needs further discussion.     

Effect of NMDA receptor ligands on mast cell histamine release, a reappraisal

Natural polyamines have been proposed to induce histamine release from mast cells through a direct interaction with G proteins. Alternatively, the polyamine binding site of ionotropic N-methyl-d-aspartate (NMDA) receptors has been suggested as a target for spermine on mast cells. We reexamined both hypotheses. Incubation of rat peritoneal mast cells with spermine resulted in a concentration-dependent histamine release (EC₅₀ 270 μM). Incubation with NMDA receptor agonists, glutamate or NMDA, associated to the co-agonist glycine, did not induce secretion. Western blot experiments did not reveal NMDA R1, R2a, R2b or R2c subunit expression in rat peritoneal mast cell membranes. The NMDA receptor antagonist at the glycine site, L-689,560, did not modify, at relevant concentrations, the spermine-induced secretion. The NMDA receptor antagonists, ifenprodil and LY 235959, and the NMDA channel blocker, MK801, slightly inhibited, at high concentrations, the secretory effect of spermine. The polyamine arcaine, an antagonist of the NMDA receptor polyamine binding site, induced histamine secretion (EC₅₀ 350 μM). Both spermine- and arcaine-induced effects were independent upon extracellular calcium and were largely inhibited by treatment of mast cells with pertussis toxin or benzalkonium chloride. The response to spermine and arcaine was prevented by the hydrolysis of sialic acid residues of the cell surface by neuraminidase, and was restored by permeabilization of the plasma membrane with streptolysine-O, indicating that polyamines act intracellularly. These results confirm the involvement of pertussis toxin-sensitive G proteins in the secretory effect of polyamines and demonstrate the absence of NMDA receptors on rat peritoneal mast cells. Nonselective effects of some NMDA receptor ligands on mast cells cannot be excluded. Received: 28 December 1998 / Accepted: 19 March 1999     

d-Serine and a glycine transporter-1 inhibitor enhance social memory in rats

Rationale  

Glutamatergic abnormalities are involved in the etiology of schizophrenia. Clinical evidence demonstrates that positive modulation of “glycine modulatory sites” on N-methyl-d-aspartic acid (NMDA) receptors improve cognitive deficits as well as positive and negative symptoms in schizophrenic patients.     

Relationship between ketamine-induced psychotic symptoms and NMDA receptor occupancy—a [123I]CNS-1261 SPET study

Rationale Ketamine induces effects resembling both positive and negative psychotic symptoms of schizophrenia. These are thought to arise through its action as an uncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. Objectives We used [¹²³I]CNS-1261 to study ketamine binding to NMDA receptors in healthy human controls in vivo and its relationship to positive and negative psychotic symptom induction. Materials and methods Ten healthy controls underwent two single-photon emission tomography scans with [¹²³I]CNS-1261. On each occasion, they received a bolus infusion of either ketamine or saline. The Brief Psychiatric Rating Scale (BPRS) was administered at the end of each scan. Predefined regions of interest were used to estimate change in volume of distribution of [¹²³I]CNS-1261 following ketamine administration. Two normalised-to-cortex binding indices were also used in order to study effects of ketamine on NMDA receptor availability by region, after correction for global and nonspecific effects. Results Ketamine-induced reduction in [¹²³I]CNS-1261 volume of distribution in all regions showed the strongest correlation with BPRS negative subscale (p < 0.01). With the normalised-to-cortex measures, NMDA receptor binding in middle inferior frontal cortex showed a significant correlation with BPRS negative subscale (BI1 r = 0.88, BI2 r = 95.9, p < 0.001). Conclusions [¹²³I]CNS-1261 binding was modulated by ketamine, a drug known to compete for the same site on the NMDA receptor in vitro. Ketamine may induce negative symptoms through direct inhibition of the NMDA receptor, and positive symptoms may arise through a different neurochemical pathway.     

The antiepileptic agent gabapentin (Neurontin) possesses anxiolytic-like and antinociceptive actions that are reversed byd-serine

This report describes the activity of the antiepileptic agent gabapentin (Neurontin) in animal models predictive of anxiolysis and analgesia. Gabapentin displayed anxiolytic-like action in the rat conflict test, the mouse light/dark box and the rat elevated X-maze with respective minimum effective doses (MEDs) of 3, 10 and 30 mg/kg. Furthermore, gabapentin also induced behavioural changes suggestive of anxiolysis in the marmoset human threat test with a MED of 30 mg/kg. In the rat formalin test of tonic nociception, gabapentin dose-dependently (30–300 mg/kg) and selectively blocked the late phase with a MED of 100 mg/kg. However, it failed to block carrageenan-induced paw oedema. The intracerebroventricular (ICV) administration of the glycine/NMDA receptor agonistd-Serine, dose-dependently (10–100 μg/animal) reversed the antinociceptive action of gabapentin (200 mg/kg, SC).d-Serine (30 μg/animal, ICV) also reversed the anxiolytic-like effects (in the light/dark box and the rat elevated X-maze) of gabapentin (30 mg/kg). In contrast,l-Serine (100 μg, ICV) failed to block the antinociceptive action of gabapentin. The antinociceptive action of (+)-HA-966 (25 mg/kg, SC), a partial agonist at the glycine/NMDA receptor, was reversed byd-Serine (100 μg/animal, ICV). However,d-Serine (100 μg/animal, ICV) failed to affect the antinociceptive action of a competitive NMDA receptor antagonist CGS 19755 (3 mg/kg, SC). Gabapentin has negligible affinity for the strychnine insensitive [³H]glycine binding site. This indicates that the interaction between gabapentin andd-Serine may not involve the NMDA receptor complex. Gabapentin may represent a novel type of anxiolytic and analgesic agent.     

Effect of NMDA- and strychnine-insensitive glycine site antagonists on NMDA-mediated convulsions and learning

Intracerebroventricular (ICV) injection of N-methyl-d-aspartate (NMDA) was shown to induce generalized seizures in mice. The competitive NMDA antagonistsdl-2-amino-5-phosphonovaleroate (dl-AP7) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), the NMDA channel blocker antagonist (+)-5-methyl-10,11-dihydro 5H-dibenzo-[a,d] cycloheptan-5,10-imine maleate (MK-801) and the strychnine-insensitive glycine antagonists kynurenic acid (KYNA) and 7-chloro-kynurenic acid (7-Cl-KYNA), when co-administered (ICV) with NMDA, antagonized NMDA-induced generalized seizures. Administration (ICV) ofdl-AP7, CPP and MK-801 resulted in impared learning performance in a passive avoidance task in mice, with ED₅₀ close to the anticonvulsant dose. The glycine antagonists KYNA and 7-Cl-KYNA at high doses significantly failed to affect performance in the same model of learning. The results indicate that compounds acting at the strychnine-insensitive glycine site may have a larger therapeutic window as anticonvulsants than antagonists of the NMDA receptor and channel.     

Acute or subchronic clozapine treatment does not ameliorate prepulse inhibition (PPI) deficits in CPB-K mice with low levels of hippocampal NMDA receptor density

Introduction The hypo-glutamatergic hypothesis of schizophrenia is based on clinical similarities between schizophrenia and phencyclidine (PCP)-induced psychosis in mentally healthy humans. Sensorimotor gating, as measured by prepulse inhibition (PPI) of the acoustic startle response (ASR), is impaired in schizophrenic patients. In animals, noncompetitive N-methyl-d-aspartate (NMDA) antagonists such as PCP disrupt PPI in a way that resembles the defect seen in schizophrenia. In a previous study with inbred mouse strains, low PPI levels have been demonstrated in CPB-K mice possessing low levels of hippocampal NMDA receptor densities. The present study was performed to test whether the low magnitude of PPI in CPB-K mice can be reversed by the atypical antipsychotic drug clozapine (CLZ). Results Before any treatment, CPB-K mice displayed a significant (p < 0.001) lower level in PPI and a significant (p < 0.001) higher ASR when compared to BALB/cJ mice known to have high hippocampal NMDA receptor densities. Acute and subchronic effects of a 2-week treatment with CLZ at daily doses of 5 and 10 mg/kg intraperitoneally, respectively, did not reveal any significant alteration of PPI levels in CPB-K mice. Nevertheless, the examination of motor behavior during nonstimulus trials provided a positive control for the drug’s effectiveness. Conclusion In summary, (1) this study confirmed our working hypothesis: Lower levels of hippocampal glutamatergic receptor densities correspond to lower sensorimotor gating in CPB-K mice, and (2) acute or subchronic treatment with CLZ did not elevate low PPI levels in CPB-K mice. Thus, further experiments will concentrate on other antipsychotic drugs to prove the predictive validity of this animal model.     

Electrophysiological characterisation of the actions of kynurenic acid at ligand-gated ion channels

To better understand the effects of the tryptophan metabolite kynurenic acid (kynA) in the brain, we characterised its actions at five ligand-gated ion channels: NMDA, AMPA, GABA_A, glycine and α7 nicotinic acetylcholine receptors. Using whole-cell patch-clamp recordings, we found that kynA was a more potent antagonist at human NR1a/NR2A compared with NR1a/NR2B receptors (IC₅₀: 158 μM and 681 μM, respectively; in 30 μM glycine). KynA inhibited AMPA-evoked currents to a similar degree in cultured hippocampal neurons and a human GluR2(flip/unedited) cell line (IC₅₀: 433 and 596 μM, respectively) and at higher concentrations, kynA also inhibited the strychnine-sensitive glycine receptor (∼35% inhibition by 3 mM kynA). Interestingly, kynA inhibited the peak amplitude (IC₅₀: 2.9 mM for 10 μM GABA) and slowed the decay kinetics of GABA-evoked currents in cultured neurons. In contrast, we found that kynA (1-3 mM) had no effect on ACh-evoked, methyllycaconitine (MLA)-sensitive currents in a human α7 nicotinic receptor (nAChR) cell line, rat hippocampal neurons in primary culture or CA1 stratum radiatum interneurons in rat brain slices. However, DMSO (>1%) did inhibit α7 nAChR-mediated currents. In conclusion, kynA is an antagonist at NMDA, AMPA and glycine receptors and a modulator of GABA_A receptors, but we find no evidence for any effect of kynA at the α7 nAChR.