Elevated level of metabotropic glutamate receptor 2/3 in the prefrontal cortex in major depression

Clinical, postmortem and preclinical research strongly implicates dysregulation of glutamatergic neurotransmission in major depressive disorder (MDD). Recently, metabotropic glutamate receptors (mGluRs) have been proposed as attractive targets for the discovery of novel therapeutic approaches against depression. The aim of this study was to examine mGluR2/3 protein levels in the prefrontal cortex (PFC) from depressed subjects. In addition, to test whether antidepressants influence mGluR2/3 expression we also studied levels of mGluR2/3 in fluoxetine-treated monkeys. Postmortem human prefrontal samples containing Brodmann's area 10 (BA10) were obtained from 11 depressed and 11 psychiatrically healthy controls. Male rhesus monkeys were treated chronically with fluoxetine (dose escalated to 3mg/kg, p.o.; n=7) or placebo (n=6) for 39 weeks. The mGluR2/3 immunoreactivity was investigated using Western blot method. There was a robust (+67%) increase in the expression of the mGlu2/3 protein in the PFC of depressed subjects relative to healthy controls. The expression of mGlu2/3 was unchanged in the PFC of monkeys treated with fluoxetine. Our findings provide the first evidence that mGluR2/3 is elevated in the PFC in MDD. This observation is consistent with reports showing that mGluR2/3 antagonists exhibit antidepressant-like activity in animal models and demonstrates that these receptors are promising targets for the discovery of novel antidepressants.

Canine distemper virus infection of primary hippocampal cells induces increase in extracellular glutamate and neurodegeneration

The canine distemper virus (CDV) belongs to the Morbillivirus genus which includes important human pathogens like the closely related measles virus. CDV infection can reach the nervous system where it causes serious malfunctions. Although this pathology is well described, the molecular events in brain infection are still poorly understood. Here we studied infection in vitro by CDV using a model of dissociated cell cultures from newborn rat hippocampus. We used a recombinant CDV closely related to the neurovirulent A75/17 which also expresses the enhanced green fluorescent protein. We found that infected neurons and astrocytes could be clearly detected, and that infection spreads only slowly to neighboring cells. Interestingly, this infection causes a massive cell death of neurons, which includes also non-infected neurons. Antagonists of NMDA-type or alpha-amino-3-hydroxy-5-methylisoxazole-4-propinate (AMPA)-type glutamate receptors could slow down this neuron loss, indicating an involvement of the glutamatergic system in the induction of cell death in infected and non-infected cells. Finally, we show that, following CDV infection, there is a steady increase in extracellular glutamate in infected cultures. These results indicate that CDV infection induces excitotoxic insults on neurons via glutamatergic signaling.

The SLC1 high-affinity glutamate and neutral amino acid transporter family

Glutamate transporters play important roles in the termination of excitatory neurotransmission and in providing cells throughout the body with glutamate for metabolic purposes. The high-affinity glutamate transporters EAAC1 (SLC1A1), GLT1 (SLC1A2), GLAST (SLC1A3), EAAT4 (SLC1A6), and EAAT5 (SLC1A7) mediate the cellular uptake of glutamate by the co-transport of three sodium ions (Na(+)) and one proton (H(+)), with the counter-transport of one potassium ion (K(+)). Thereby, they protect the CNS from glutamate-induced neurotoxicity. Loss of function of glutamate transporters has been implicated in the pathogenesis of several diseases, including amyotrophic lateral sclerosis and Alzheimer's disease. In addition, glutamate transporters play a role in glutamate excitotoxicity following an ischemic stroke, due to reversed glutamate transport. Besides glutamate transporters, the SLC1 family encompasses two transporters of neutral amino acids, ASCT1 (SLC1A4) and ASCT2 (SLC1A5). Both transporters facilitate electroneutral exchange of amino acids in neurons and/or cells of the peripheral tissues. Some years ago, a high resolution structure of an archaeal homologue of the SLC1 family was determined, followed by the elucidation of its structure in the presence of the substrate aspartate and the inhibitor d,l-threo-benzyloxy aspartate (d,l-TBOA). Historically, the first few known inhibitors of SLC1 transporters were based on constrained glutamate analogs which were active in the high micromolar range but often also showed off-target activity at glutamate receptors. Further development led to the discovery of l-threo-β-hydroxyaspartate derivatives, some of which effectively inhibited SLC1 transporters at nanomolar concentrations. More recently, small molecule inhibitors have been identified whose structures are not based on amino acids. Activators of SLC1 family members have also been discovered but there are only a few examples known.

Spine Ca2+ signaling in spike-timing-dependent plasticity

Calcium is a second messenger, which can trigger the modification of synaptic efficacy. We investigated the question of whether a differential rise in postsynaptic Ca2+ ([Ca2+]i) alone is sufficient to account for the induction of long-term potentiation (LTP) and long-term depression (LTD) of EPSPs in the basal dendrites of layer 2/3 pyramidal neurons of the somatosensory cortex. Volume-averaged [Ca2+]i transients were measured in spines of the basal dendritic arbor for spike-timing-dependent plasticity induction protocols. The rise in [Ca2+]i was uncorrelated to the direction of the change in synaptic efficacy, because several pairing protocols evoked similar spine [Ca2+]i transients but resulted in either LTP or LTD. The sequence dependence of near-coincident presynaptic and postsynaptic activity on the direction of changes in synaptic strength suggested that LTP and LTD were induced by two processes, which were controlled separately by postsynaptic [Ca2+]i levels. Activation of voltage-dependent Ca2+ channels before metabotropic glutamate receptors (mGluRs) resulted in the phospholipase C-dependent (PLC-dependent) synthesis of endocannabinoids, which acted as a retrograde messenger to induce LTD. LTP required a large [Ca2+]i transient evoked by NMDA receptor activation. Blocking mGluRs abolished the induction of LTD and uncovered the Ca2+-dependent induction of LTP. We conclude that the volume-averaged peak elevation of [Ca2+]i in spines of layer 2/3 pyramids determines the magnitude of long-term changes in synaptic efficacy. The direction of the change is controlled, however, via a mGluR-coupled signaling cascade. mGluRs act in conjunction with PLC as sequence-sensitive coincidence detectors when postsynaptic precede presynaptic action potentials to induce LTD. Thus presumably two different Ca2+ sensors in spines control the induction of spike-timing-dependent synaptic plasticity.

Blockade of NMDA receptor subtype NR2B prevents seizures but not apoptosis of dentate gyrus neurons in bacterial meningitis in infant rats

BACKGROUND: Excitotoxic neuronal injury by action of the glutamate receptors of the N-methyl-d-aspartate (NMDA) subtype have been implicated in the pathogenesis of brain damage as a consequence of bacterial meningitis. The most potent and selective blocker of NMDA receptors containing the NR2B subunit is (R,S)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperid inepropanol (RO 25-6981). Here we evaluated the effect of RO 25-6981 on hippocampal neuronal apoptosis in an infant rat model of meningitis due to Streptococcus pneumoniae. Animals were randomized for treatment with RO 25-6981 at a dosage of either 0.375 mg (15 mg/kg; n = 28) or 3.75 mg (150 mg/kg; n = 15) every 3 h or an equal volume of sterile saline (250 microl; n = 40) starting at 12 h after infection. Eighteen hours after infection, animals were assessed clinically and seizures were observed for a period of 2 h. At 24 h after infection animals were sacrificed and brains were examined for apoptotic injury to the dentate granule cell layer of the hippocampus. RESULTS: Treatment with RO 25-6981 had no effect on clinical scores, but the incidence of seizures was reduced (P < 0.05 for all RO 25-6981 treated animals combined). The extent of apoptosis was not affected by low or high doses of RO 25-6981. Number of apoptotic cells (median [range]) was 12.76 [3.16-25.3] in animals treated with low dose RO 25-6981 (control animals 13.8 [2.60-31.8]; (P = NS) and 9.8 [1.7-27.3] (controls: 10.5 [2.4-21.75]) in animals treated with high dose RO 25-6981 (P = NS). CONCLUSIONS: Treatment with a highly selective blocker of NMDA receptors containing the NR2B subunit failed to protect hippocampal neurons from injury in this model of pneumococcal meningitis, while it had some beneficial effect on the incidence of seizures.

Activation of metabotropic glutamate 5 and NMDA receptors underlies the induction of persistent bursting and associated long-lasting changes in CA3 recurrent connections.

The aim of this study was to describe the induction and expression mechanisms of a persistent bursting activity in a horizontal slice preparation of the rat limbic system that includes the ventral part of the hippocampus and the entorhinal cortex. Disinhibition of this preparation by bicuculline led to interictal-like bursts in the CA3 region that triggered synchronous activity in the entorhinal cortex. Washout of bicuculline after a 1 hr application resulted in a maintained production of hippocampal bursts that continued to spread to the entorhinal cortex. Separation of CA3 from the entorhinal cortex caused the activity in the latter to become asynchronous with CA3 activity in the presence of bicuculline and disappear after washout; however, in CA3, neither the induction of bursting nor its persistence were affected. Associated with the CA3 persistent bursting, a strengthening of recurrent collateral excitatory input to CA3 pyramidal cells and a decreased input to CA3 interneurons was found. Both the induction of the persistent bursting and the changes in synaptic strength were prevented by antagonists of metabotropic glutamate 5 (mGlu5) or NMDA receptors or protein synthesis inhibitors and did not occur in slices from mGlu5 receptor knock-out mice. The above findings suggest potential synaptic mechanisms by which the hippocampus switches to a persistent interictal bursting mode that may support a spread of interictal-like bursting to surrounding temporal lobe regions.

Coreleased orexin and glutamate evoke nonredundant spike outputs and computations in histamine neurons.

Stable wakefulness requires orexin/hypocretin neurons (OHNs) and OHR2 receptors. OHNs sense diverse environmental cues and control arousal accordingly. For unknown reasons, OHNs contain multiple excitatory transmitters, including OH peptides and glutamate. To analyze their cotransmission within computational frameworks for control, we optogenetically stimulated OHNs and examined resulting outputs (spike patterns) in a downstream arousal regulator, the histamine neurons (HANs). OHR2s were essential for sustained HAN outputs. OHR2-dependent HAN output increased linearly during constant OHN input, suggesting that the OHN→HAN(OHR2) module may function as an integral controller. OHN stimulation evoked OHR2-dependent slow postsynaptic currents, similar to midnanomolar OH concentrations. Conversely, glutamate-dependent output transiently communicated OHN input onset, peaking rapidly then decaying alongside OHN→HAN glutamate currents. Blocking glutamate-driven spiking did not affect OH-driven spiking and vice versa, suggesting isolation (low cross-modulation) of outputs. Therefore, in arousal regulators, cotransmitters may translate distinct features of OHN activity into parallel, nonredundant control signals for downstream effectors.