Presynaptic nanoscale components of retrograde synaptic signaling.

While our understanding of the nanoscale architecture of anterograde synaptic transmission is rapidly expanding, the qualitative and quantitative molecular principles underlying distinct mechanisms of retrograde synaptic communication remain elusive. We show that a particular form of tonic cannabinoid signaling is essential for setting target cell-dependent synaptic variability. It does not require the activity of the two major endocannabinoid-producing enzymes. Instead, by developing a workflow for physiological, anatomical, and molecular measurements at the same unitary synapse, we demonstrate that the nanoscale stoichiometric ratio of type 1 cannabinoid receptors (CB1Rs) to the release machinery is sufficient to predict synapse-specific release probability. Accordingly, selective decrease of extrasynaptic CB1Rs does not affect synaptic transmission, whereas in vivo exposure to the phytocannabinoid Δ9-tetrahydrocannabinol disrupts the intrasynaptic nanoscale stoichiometry and reduces synaptic variability. These findings imply that synapses leverage the nanoscale stoichiometry of presynaptic receptor coupling to the release machinery to establish synaptic strength in a target cell-dependent manner.

P2X7 purinergic receptor modulates dentate gyrus excitatory neurotransmission and alleviates schizophrenia-like symptoms in mouse.

ATP-gated P2X7 receptors (P2X7Rs) play a crucial role in brain disorders. However, how they affect normal and pathological synaptic transmission is still largely unclear. Here, by using whole-cell patch-clamp technique to record AMPA- and NMDA receptor-mediated excitatory postsynaptic currents (s/mEPSCs) in dentate gyrus granule cells (DG GCs), we revealed a modulation by P2X7Rs of presynaptic sites, especially originated from entorhinal cortex (EC)-GC path but not the mossy cell (MC)-GC path. The involvement of P2X7Rs was confirmed using a pharmacological approach. Additionally, the acute activation of P2X7Rs directly elevated calcium influx from EC-GC terminals. In postnatal phencyclidine (PCP)-induced mouse model of schizophrenia, we observed that P2X7R deficiency restored the EC-GC synapse alteration and alleviated PCP-induced symptoms. To summarize, P2X7Rs participate in the modulation of GC excitatory neurotransmission in the DG via EC-GC pathway, contributing to pathological alterations of neuronal functions leading to neurodevelopmental disorders.

The Pharmacological Effects of Phenylephrine are Indirect, Mediated by Noradrenaline Release from the Cytoplasm.

Phenylephrine (PE) is a canonical α1-adrenoceptor-selective agonist. However, unexpected effects of PE have been observed in preclinical and clinical studies, that cannot be easily explained by its actions on α1-adrenoceptors. The probability of the involvement of α2- and ß-adrenoceptors in the effect of PE has been raised. In addition, our earlier study observed that PE released noradrenaline (NA) in a [Ca2+]o-independent manner. To elucidate this issue, we have investigated the effects of PE on [3H]NA release and α1-mediated smooth muscle contractions in the mouse vas deferens (MVD) as ex vivo preparation. The release experiments were designed to assess the effects of PE at the presynaptic terminal, whereas smooth muscle isometric contractions in response to electrical field stimulation were used to measure PE effect postsynaptically. Our results show that PE at concentrations between 0.3 and 30 µM significantly enhanced the resting release of [3H]NA in a [Ca2+]o-independent manner. In addition, prazosin did not affect the release of NA evoked by PE. On the contrary, PE-evoked smooth muscle contractions were inhibited by prazosin administration indicating the α1-adrenoceptor-mediated effect. When the function of the NA transporter (NAT) was attenuated with nisoxetine, PE failed to release NA and the contractions were reduced by approximately 88%. The remaining part proved to be prazosin-sensitive. The present work supports the substantial indirect effect of PE which relays on the cytoplasmic release of NA, which might explain the reported side effects for PE.

Convergent cross-species pro-cognitive effects of RGH-235, a new potent and selective histamine H3 receptor antagonist/inverse agonist.

The histamine H3 receptor is a favourable target for the treatment of cognitive deficits. Here we report the in vitro and in vivo profile of RGH-235, a new potent, selective, and orally active H3 receptor antagonist/inverse agonist developed by Gedeon Richter Plc. Radioligand binding and functional assays were used for in vitro profiling. Procognitive efficacy was investigated in rodent cognitive tests, in models of attention deficit hyperactive disorder (ADHD) and in cognitive tests of high translational value (rat touch screen visual discrimination test, primate fixed-foreperiod visual reaction time task). Results were supported by pharmacokinetic studies, neurotransmitter release, sleep EEG and dipsogenia. RGH-235 displayed high affinity to H3 receptors (Ki = 3.0-9.2 nM, depending on species), without affinity to H1, H2 or H4 receptors and >100 other targets. RGH-235 was an inverse agonist ([35S] GTPγS binding) and antagonist (pERK1/2 ELISA), showing favourable kinetics, inhibition of the imetit-induced dipsogenia and moderate effects on sleep-wake EEG. RGH-235 stimulated neurotransmitter release both in vitro and in vivo. RGH-235 was active in spontaneously hypertensive rats (SHR), generally considered as a model of ADHD, and revealed a robust pro-cognitive profile both in rodent and primate tests (in 0.3-1 mg/kg) and in models of high translational value (e.g. in a rodent touch screen test and in non-human primates). The multiple and convergent procognitive effects of RGH-235 support the view that beneficial cognitive effects can be linked to antagonism/inverse agonism of H3 receptors.

Contribution of analog signaling to neurotransmitter interactions and behavior: Role of transporter-mediated nonquantal dopamine release.

Neuronal networks cause changes in behaviorally important information processing through the vesicular release of neurotransmitters governed by the rate and timing of action potentials (APs). Herein, we provide evidence that dopamine (DA), nonquantally released from the cytoplasm, may exert similar effects in vivo. In mouse slice preparations, (+/-)-3,4-methylenedioxy-methamphetamine (MDMA, or ecstasy) and ß-phenylethylamine (ß-PEA)-induced DA release in the striatum and nucleus accumbens (NAc), two regions of the brain involved in reward-driven and social behavior and inhibited the axonal stimulation-induced release of tritiated acetylcholine ([3 H]ACh) in the striatum. The DA transporter (DAT) inhibitor (GBR-12909) prevented MDMA and ß-PEA from causing DA release. GBR-12909 could also restore some of the stimulated acetylcholine release reduced by MDMA or ß-PEA in the striatum confirming the fundamental role of DAT. In addition, hypothermia could prevent the ß-PEA-induced release in the striatum and in the NAc. Sulpiride, a D2 receptor antagonist, also prevented the inhibitory effects of MDMA or ß-PEA on stimulated ACh release, suggesting they act indirectly via binding of DA. Reflecting the neurochemical interactions in brain slices at higher system level, MDMA altered the social behavior of rats by preferentially enhancing passive social behavior. Similar to the in vitro effects, GBR-12909 treatment reversed specific elements of the MDMA-induced changes in behavior, such as passive social behavior, while left others including social play unchanged. The changes in behavior by the high level of extracellular DA-- a significant amount originating from cytoplasmic release--suggest that in addition to digital computation through synapses, the brain also uses analog communication, such as DA signaling, to mediate some elements of complex behaviors, but in a much longer time scale.

P2X7 Receptor-Dependent Layer-Specific Changes in Neuron-Microglia Reactivity in the Prefrontal Cortex of a Phencyclidine Induced Mouse Model of Schizophrenia.

Background: It has been consistently reported that the deficiency of the adenosine triphosphate (ATP) sensitive purinergic receptor P2X7 (P2X7R) ameliorates symptoms in animal models of brain diseases. Objective: This study aimed to investigate the role of P2X7R in rodent models of acute and subchronic schizophrenia based on phencyclidine (PCP) delivery in animals lacking or overexpressing P2X7R, and to identify the underlying mechanisms involved. Methods: The psychotomimetic effects of acute i.p. PCP administration in C57Bl/6J wild-type, P2X7R knockout (P2rx7-/-) and overexpressing (P2X7-EGFP) young adult mice were quantified. The medial prefrontal cortex (mPFC) of P2rx7-/- and heterozygous P2X7-EGFP acutely treated animals was characterized through immunohistochemical staining. The prefrontal cortices of young adult P2rx7-/- and P2rx7tg/+ mice were examined with tritiated dopamine release experiments and the functional properties of the mPFC pyramidal neurons in layer V from P2rx7-/- mice were assessed by patch-clamp recordings. P2rx7-/- animals were subjected to a 7 days subchronic systemic PCP treatment. The animals working memory performance and PFC cytokine levels were assessed. Results: Our data strengthen the hypothesis that P2X7R modulates schizophrenia-like positive and cognitive symptoms in NMDA receptor antagonist models in a receptor expression level-dependent manner. P2X7R expression leads to higher medial PFC susceptibility to PCP-induced circuit hyperactivity. The mPFC of P2X7R knockout animals displayed distinct alterations in the neuronal activation pattern, microglial organization, specifically around hyperactive neurons, and were associated with lower intrinsic excitability of mPFC neurons. Conclusions: P2X7R expression exacerbated PCP-related effects in C57Bl/6J mice. Our findings suggest a pleiotropic role of P2X7R in the mPFC, consistent with the observed behavioral phenotype, regulating basal dopamine concentration, layer-specific neuronal activation, intrinsic excitability of neurons in the mPFC, and the interaction of microglia with hyperactive neurons. Direct measurements of P2X7R activity concerning microglial ramifications and dynamics could help to further elucidate the molecular mechanisms involved.

3,4-Methylenedioxymethamphetamine, mephedrone, and ß-phenylethylamine release dopamine from the cytoplasm by means of transporters and keep the concentration high and constant by blocking reuptake.

The addiction-related behavioural effects of drugs of abuse are mediated by the mesocorticolimbic monoamine systems. We investigated the effects of 3,4-methylenedioxymethamphetamine (MDMA), mephedrone, ß-phenylethylamine (ß-PEA) methylphenidate (MPH) on dopamine release from mouse perfused nucleus accumbens and prefrontal cortex slices. The fractional release of [3H]-dopamine was measured at rest and in response to field stimulation. The distributions of [3H]-dopamine and its metabolites were determined using high-pressure liquid chromatography. The effect of drugs on [3H]-dopamine uptake was measured in synaptosomal P2 preparations from the frontal cortex and striatum. Similar to MDMA, mephedrone ß-PEA increased the resting release of [3H]-dopamine from the nucleus accumbens and prefrontal cortex in a [Ca2+]o-independent manner, and the stimulation-evoked release was also augmented. In contrast, MPH failed to affect the resting release but potentiated the release in response to axonal activity. Similar to dopamine transporter antagonist GBR 12909, mephedrone, MDMA and MPH biphasically inhibited the [3H]-dopamine uptake. The administration of GBR 12909 and nisoxetine, or lowering the bath temperature prevented MDMA, mephedrone and ß-PEA from enhancing the resting, cytoplasmic release of [3H]-dopamine, indicating the role of transporters in the release process. We conclude that amphetamine-like drugs of abuse and the trace amine ß-PEA excessively increase the [Ca2+]o-independent, non-vesicular release of dopamine from the cytoplasm into the extrasynaptic space and inhibit the high-affinity transporters, thereby maintaining a high ambient, non-synaptic concentration of dopamine that may tonically control the activity of neurons equipped with dopamine receptors and is likely involved in the reinforcing effects and abusive potential of amphetamines.

Effect of rat spinal cord injury (hemisection) on the ex vivo uptake and release of [3H]noradrenaline from a slice preparation.

We measured the ex vivo uptake and release of [3H]noradrenaline ([3H]NA) from perfused rat spinal cord slice preparations at 1, 3 and 14days after unilateral hemisection-induced spinal cord injury (SCI) compared with control slice preparations. After surgical hemisection under anaesthesia, the rats showed characteristic signs of hemiplegia, with no movement of the ipsilateral hindlimb. After 3days, the electron microscopy images showed overall degeneration of neuronal organelles and the myelin sheath, but the synapses seemed to be intact. In ex vivo experiments, the spinal cord injury did not influence uptake but increased [3H]NA release at rest and in response to axonal stimulation. The effect of a selective noradrenaline reuptake inhibitor, nisoxetine, was studied to identify the mechanisms underlying the increase in NA release. Nisoxetine potentiated stimulation-evoked [3H]NA release from the non-injured tissue, but it gradually lost its effectiveness after injury, depending on the time (1 and 3days) elapsed after hemisection, indicating that the noradrenaline transporter binding sites of the terminals become impaired after decentralisation.

Correlation between the increased release of catecholamines evoked by local anesthetics and their analgesic and adverse effects: Role of K(+) channel inhibition.

Because local anesthetics are known to inhibit both sodium and potassium channels, and anesthetic properties have been attributed to the former effect, we compared their effects with those of tetrodotoxin (TTX), a selective Na(+) channel inhibitor with anesthetic activity, and 4-aminopyridine (4-AP), a selective potassium channel blocker with convulsive activity, on transmitter release during rest and in response to field (axonal) stimulation using the microvolume perfusion method and isolated prefrontal cortex and spinal cord slice preparations loaded with the radioactive transmitters [(3)H]dopamine ([(3)H]DA) and [(3)H]noradrenaline ([(3)H]NA). It is also known that local anesthetics may exert analgesic effect and, rarely, some adverse effects on the central nervous system (CNS). Neurochemical evidence demonstrated that local anesthetics administered at concentrations ranging from 0.5 to 5mM, which might have been intentionally or accidentally achieved in clinical practice (e.g., during spinal and epidural anesthesia or peripheral nerve block), led to presynaptic failures during neurochemical transmission, including inhibited transmitter release associated with axonal firing and markedly enhanced extraneuronal concentrations of transmitters due to increased resting, [Ca(2+)]o-independent release. Tetrodotoxin, a toxin with selective Na(+) channel-blocking properties, inhibited the stimulation-evoked release but failed to affect the resting release. In contrast, the potassium channel inhibitor 4-AP enhanced both the resting- and action potential-evoked transmitter releases. It is concluded that effects of local anesthetics on resting catecholamine release in the spinal cord may contribute to their action during neuropathic pain relief and spinal analgesia as well as to their side effects in the CNS.

Hypervulnerability to Sound Exposure through Impaired Adaptive Proliferation of Peroxisomes.

A deficiency in pejvakin, a protein of unknown function, causes a strikingly heterogeneous form of human deafness. Pejvakin-deficient (Pjvk(-/-)) mice also exhibit variable auditory phenotypes. Correlation between their hearing thresholds and the number of pups per cage suggest a possible harmful effect of pup vocalizations. Direct sound or electrical stimulation show that the cochlear sensory hair cells and auditory pathway neurons of Pjvk(-/-) mice and patients are exceptionally vulnerable to sound. Subcellular analysis revealed that pejvakin is associated with peroxisomes and required for their oxidative-stress-induced proliferation. Pjvk(-/-) cochleas display features of marked oxidative stress and impaired antioxidant defenses, and peroxisomes in Pjvk(-/-) hair cells show structural abnormalities after the onset of hearing. Noise exposure rapidly upregulates Pjvk cochlear transcription in wild-type mice and triggers peroxisome proliferation in hair cells and primary auditory neurons. Our results reveal that the antioxidant activity of peroxisomes protects the auditory system against noise-induced damage.

Tonic endocannabinoid-mediated modulation of GABA release is independent of the CB1 content of axon terminals.

The release of GABA from cholecystokinin-containing interneurons is modulated by type-1 cannabinoid receptors (CB1). Here we tested the hypothesis that the strength of CB1-mediated modulation of GABA release is related to the CB1 content of axon terminals. Basket cell boutons have on average 78% higher CB1 content than those of dendritic-layer-innervating (DLI) cells, a consequence of larger bouton surface and higher CB1 density. The CB1 antagonist AM251 caused a 54% increase in action potential-evoked [Ca(2+)] in boutons of basket cells, but not in DLI cells. However, the effect of AM251 did not correlate with CB1 immunoreactivity of individual boutons. Moreover, a CB1 agonist decreased [Ca(2+)] in a cell type- and CB1-content-independent manner. Replica immunogold labelling demonstrated the colocalization of CB1 with the Cav2.2 Ca(2+) channel subunit. Our data suggest that only a subpopulation of CB1s, within nanometre distances from their target Cav2.2 channels, are responsible for endocannabinoid-mediated modulation of GABA release.

Mode of action of the positive modulator PNU-120596 on α7 nicotinic acetylcholine receptors.

We investigated the mode of action of PNU-120596, a type II positive allosteric modulator of the rat α7 nicotinic acetylcholine receptor expressed by GH4C1 cells, using patch-clamp and fast solution exchange. We made two important observations: first, while PNU-120596 rapidly associated to desensitized receptors, it had at least hundredfold lower affinity to resting conformation, therefore at 10 µM concentration it dissociated from resting receptors; and second, binding of PNU-120596 slowed down dissociation of choline molecules from the receptor radically. We propose that when agonist concentration is transiently elevated in the continuous presence of the modulator (as upon the neuronal release of acetylcholine in a modulator-treated animal) these two elements together cause occurrence of a cycle of events: Binding of the modulator is limited in the absence of the agonist. When the agonist is released, it binds to the receptor, and induces desensitization, thereby enabling modulator binding. Modulator binding in turn traps the agonist within its binding site for a prolonged period of time. Once the agonist finally dissociated, the modulator can also dissociate without re-binding, and the receptor assumes its original resting conformation. In kinetic simulations this "trapped agonist cycle" mechanism did not require that the orthosteric and allosteric ligands symmetrically modify each other's affinity, only the modulator must decrease agonist accessibility, and the agonist must induce a conformation that is accessible to the modulator. This mechanism effectively prolongs and amplifies the effect of the agonist.

Kinetic properties and open probability of α7 nicotinic acetylcholine receptors.

The alpha7 nicotinic acetylcholine receptor (nAChR) has some peculiar kinetic properties. From the literature of α7 nAChR-mediated currents we concluded that experimentally measured kinetic properties reflected properties of the solution exchange system, rather than genuine kinetic properties of the receptors. We also concluded that all experimentally measured EC50 values for agonists must inherently be inaccurate. The aim of this study was to assess the undistorted kinetic properties of α7 nAChRs, and to construct an improved kinetic model, which can also serve as a basis of modeling the effect of the positive allosteric modulator PNU-120596, as it is described in the accompanying paper. Agonist-evoked currents were recorded from GH4C1 cells stably transfected with pCEP4/rat α7 nAChR using patch-clamp and fast solution exchange. We used two approaches to circumvent the problem of insufficient solution exchange rate: extrapolation and kinetic modeling. First, using different solution exchange rates we recorded evoked currents, and extrapolated their amplitude and kinetics to instantaneous solution exchange. Second, we constructed a kinetic model that reproduced concentration-dependence and solution exchange rate-dependence of receptors, and then we simulated receptor behavior at experimentally unattainably fast solution exchange. We also determined open probabilities during choline-evoked unmodulated and modulated currents using nonstationary fluctuation analysis. The peak open probability of 10 mM choline-evoked currents was 0.033 ± 0.006, while in the presence of choline (10 mM) and PNU-120596 (10 µM), it was increased to 0.599 ± 0.058. Our kinetic model could adequately reproduce low open probability, fast kinetics, fast recovery and solution exchange rate-dependent kinetics.

Comparison of Ca2+ transients and [Ca2+]i in the dendrites and boutons of non-fast-spiking GABAergic hippocampal interneurons using two-photon laser microscopy and high- and low-affinity dyes.

Using two-photon laser microscopy, high- and low-affinity dyes and patch clamp electrophysiology, we successfully measured somatic stimulation-evoked Ca(2+) transients simultaneously in the dendrites and axonal boutons of the same non-fast-spiking GABAergic interneurons in acute slice preparations obtained from hippocampal area CA1. The advantage of the acute preparation is that both neuronal connections and anatomy are maintained. Calculated as unperturbed values, the amplitudes of Ca(2+) transients and changes in [Ca(2+)]i in response to somatic single or burst stimulation were much higher in boutons (428 nM/AP) than in dendrites (49 nM/AP), leading to the conclusion that the much greater influx of Ca(2+) observed in terminals might be due to a higher density of N-type voltage-sensitive Ca(2+) channels compared to the L-type channels present in dendrites. Whereas the decay of Ca(2+) transients recorded in dendrites was primarily mono-exponential, the decay in boutons was bi-exponential, as indicated by an initial fast phase, followed by a much slower reduction in fluorescence intensity. The extrusion of Ca(2+) was much faster in boutons than in dendrites. To avoid saturation effects and the flawed conversion of fluorescence measures of [Ca(2+)]i, we assessed the limits of [Ca(2+)] measurements (which ranged between 6 and 82% of the applied dye saturation) when high- and low-affinity dyes were applied at different concentrations. When two APs were delivered at a high frequency (>3 Hz) of stimulation, the low-affinity indicators OGB-6F (KD = 3.0 µM) and OGB-5N (KD = 20 µM) were able to accurately reflect the changes in ΔF/F produced by the consecutive APs. There was no difference in the endogenous buffer capacity (κE), which can shape Ca(2+) signals, calculated in dendrites (κE = 354) or boutons (κE = 458).

Neurochemical Changes in the Mouse Hippocampus Underlying the Antidepressant Effect of Genetic Deletion of P2X7 Receptors.

Recent investigations have revealed that the genetic deletion of P2X7 receptors (P2rx7) results in an antidepressant phenotype in mice. However, the link between the deficiency of P2rx7 and changes in behavior has not yet been explored. In the present study, we studied the effect of genetic deletion of P2rx7 on neurochemical changes in the hippocampus that might underlie the antidepressant phenotype. P2X7 receptor deficient mice (P2rx7-/-) displayed decreased immobility in the tail suspension test (TST) and an attenuated anhedonia response in the sucrose preference test (SPT) following bacterial endotoxin (LPS) challenge. The attenuated anhedonia was reproduced through systemic treatments with P2rx7 antagonists. The activation of P2rx7 resulted in the concentration-dependent release of [(3)H]glutamate in P2rx7+/+ but not P2rx7-/- mice, and the NR2B subunit mRNA and protein was upregulated in the hippocampus of P2rx7-/- mice. The brain-derived neurotrophic factor (BDNF) expression was higher in saline but not LPS-treated P2rx7-/- mice; the P2rx7 antagonist Brilliant blue G elevated and the P2rx7 agonist benzoylbenzoyl ATP (BzATP) reduced BDNF level. This effect was dependent on the activation of NMDA and non-NMDA receptors but not on Group I metabotropic glutamate receptors (mGluR1,5). An increased 5-bromo-2-deoxyuridine (BrdU) incorporation was also observed in the dentate gyrus derived from P2rx7-/- mice. Basal level of 5-HT was increased, whereas the 5HIAA/5-HT ratio was lower in the hippocampus of P2rx7-/- mice, which accompanied the increased uptake of [(3)H]5-HT and an elevated number of [(3)H]citalopram binding sites. The LPS-induced elevation of 5-HT level was absent in P2rx7-/- mice. In conclusion there are several potential mechanisms for the antidepressant phenotype of P2rx7-/- mice, such as the absence of P2rx7-mediated glutamate release, elevated basal BDNF production, enhanced neurogenesis and increased 5-HT bioavailability in the hippocampus.

CD39 and CD73 in immunity and inflammation.

The enzymatic activities of CD39 and CD73 play strategic roles in calibrating the duration, magnitude, and chemical nature of purinergic signals delivered to immune cells through the conversion of ADP/ATP to AMP and AMP to adenosine, respectively. This drives a shift from an ATP-driven proinflammatory environment to an anti-inflammatory milieu induced by adenosine. The CD39/CD73 pathway changes dynamically with the pathophysiological context in which it is embedded. It is becoming increasingly appreciated that altering this catabolic machinery can change the course or dictate the outcome of several pathophysiological events, such as AIDS, autoimmune diseases, infections, atherosclerosis, ischemia-reperfusion injury, and cancer, suggesting these ectoenzymes are novel therapeutic targets for managing a variety of disorders.

Fast two-photon in vivo imaging with three-dimensional random-access scanning in large tissue volumes.

The understanding of brain computations requires methods that read out neural activity on different spatial and temporal scales. Following signal propagation and integration across a neuron and recording the concerted activity of hundreds of neurons pose distinct challenges, and the design of imaging systems has been mostly focused on tackling one of the two operations. We developed a high-resolution, acousto-optic two-photon microscope with continuous three-dimensional (3D) trajectory and random-access scanning modes that reaches near-cubic-millimeter scan range and can be adapted to imaging different spatial scales. We performed 3D calcium imaging of action potential backpropagation and dendritic spike forward propagation at sub-millisecond temporal resolution in mouse brain slices. We also performed volumetric random-access scanning calcium imaging of spontaneous and visual stimulation-evoked activity in hundreds of neurons of the mouse visual cortex in vivo. These experiments demonstrate the subcellular and network-scale imaging capabilities of our system.

GluN2B-containing NMDA receptors as possible targets for the neuroprotective and antidepressant effects of fluoxetine.

Accumulating evidence has indicated the involvement of glutamatergic neurotransmission in the pathophysiology of excitotoxicity and in the mechanism of action of antidepressants. We have previously shown that tricyclic desipramine and the selective serotonin reuptake inhibitor fluoxetine inhibit NMDA receptors (NMDARs) in the clinically relevant, low micromolar concentration range. As the different subtypes of NMDARs are markedly different in their physiological and pathological functions, our aim was to investigate whether the effect of antidepressants is subtype-specific. Using whole-cell patch-clamp recordings in rat cortical cell cultures, we studied the age-dependence of inhibition of NMDA-induced currents after treatment with desipramine and fluoxetine, as the expression profile of the NMDAR subtypes changes as a function of days in vitro. We also investigated the inhibitory effect of these antidepressants on NMDA-induced currents in HEK 293 cell lines that stably expressed rat recombinant NMDARs with GluN1a/GluN2A or GluN1a/GluN2B subunit compositions. The inhibitory effect of desipramine was not age-dependent, whereas fluoxetine displayed a continuously decreasing inhibitory profile, which was similar to the GluN1/GluN2B subtype-selective antagonist ifenprodil. In HEK 293 cells, desipramine equally inhibited NMDA currents in both cell lines, whereas fluoxetine showed an inhibitory effect only in cells that expressed the GluN1/GluN2B subtype. Our data show that fluoxetine is a selective inhibitor of GluN2B-containing NMDARs, whereas desipramine inhibits both GluN1/GluN2A and GluN1/GluN2B subtypes. As the clinical efficacy of these drugs is very similar, the putative NMDAR-associated therapeutic effect of antidepressants may be mediated only via inhibition of the GluN2B-containing subtype. The manifestation of the GluN1/GluN2B-selectivity of fluoxetine suggests the neuroprotective potential for this drug in both acute and chronic neurodegenerative disorders.