Glutamatergic Synapses within the Framework of the “Chemical Soup” Concept: Relation to the Cognitive Activity

Modern concepts on the dependence of the efficacy of glutamatergic synaptic transmission on the subunit structure of glutamate (Glu) receptors and their chemical surroundings are described and discussed in the review. The above surroundings can significantly modulate the affinity of these receptors and/or change their number. Peculiarities of the effects exerted by different components of such surroundings (allosteric modulators, redox-active substances, some ions, hormones, etc.) on Glu receptors are analyzed. In the light of the chemical soup concept, the effects of Glu receptor ligands on the cognitive activity of experimental animals and of humans are discussed.     

Altered Neuronal–Glial Signaling in Glutamatergic Transmission as a Unifying Mechanism in Chronic Pain and Mental Fatigue

Recent experimental results have demonstrated a glial activation during long-term pain that produces and releases cytokines, free oxygen radicals, nitric oxide, and other neuroactive substances in the spinal cord dorsal horns. Such activation might generate a vicious circle by increasing the neuronal excitability level due to a decreased astroglial glutamate uptake and thereby reinforce pain signals that travel up to the thalamus and further up into the parietal cortex for identification and interpretation. In this paper, we adapt new knowledge on neuronal-glial signaling in the CNS to develop tentative explanations at the cellular level for the maintenance of pain signals in the brain, for formation of "pain memory," and even for the increased pain sensitivity that persons with chronic pain often experience in body regions other than those originally affected. We also suggest a hypothetical mechanism at the cellular level underlying the mental fatigue from which persons with chronic pain may suffer. This hypothesis relies on the impaired astroglial glutamate uptake capacity due to the production of neuroactive substances, altered conditions in the chronic pain state, and the anxiety and stress reactions that may occur secondary to the pain. Neuronal activity over time in the dysfunctional state of the astroglial network leads to an increase in extracellular glutamate levels in the vicinity of glutamate synapses. In turn, this increase leads over time to less precision in glutamate transmission. The increased extracellular glutamate levels lead to increased excitability and increased energy requirements. When cellular energy decreases the glutamate transmission decreases, and according to our hypothesis, this is one cause of mental fatigue. New strategies for treatment of chronic pain and the associated mental fatigue are formulated and should be explored.     

Increase in Glutamatergic Terminals in the Striatum Following Dopamine Depletion in a Rat Model of Parkinson’s Disease

Dopaminergic neuron degeneration is known to give rise to dendrite injury and spine loss of striatal neurons, however, changes of intrastriatal glutamatergic terminals and their synapses after 6-hydroxydopamine (6OHDA)-induced dopamine (DA)-depletion remains controversial. To confirm the effect of striatal DA-depletion on the morphology and protein levels of corticostriatal and thalamostriatal glutamatergic terminals and synapses, immunohistochemistry, immuno-electron microscope (EM), western blotting techniques were performed on Parkinson’s disease rat models in this study. The experimental results of this study showed that: (1) 6OHDA-induced DA-depletion resulted in a remarkable increase of Vesicular glutamate transporter 1 (VGlut1) + and Vesicular glutamate transporter 2 (VGlut2)+ terminal densities at both the light microscope (LM) and EM levels, and VGlut1+ and VGlut2+ terminal sizes were shown to be enlarged by immuno-EM; (2) Striatal DA-depletion resulted in a decrease in both the total and axospinous terminal fractions of VGlut1+ terminals, but the axodendritic terminal fraction was not significantly different from the control group. However, total, axospinous and axodendritic terminal fractions for VGlut2+ terminals declined significantly after striatal DA-depletion. (3) Western blotting data showed that striatal DA-depletion up-regulated the expression levels of the VGlut1 and VGlut2 proteins. These results suggest that 6OHDA-induced DA-depletion affects corticostriatal and thalamostriatal glutamatergic synaptic inputs, which are involved in the pathological process of striatal neuron injury induced by DA-depletion.

     

Single Particle Tracking of α7 Nicotinic AChR in Hippocampal Neurons Reveals Regulated Confinement at Glutamatergic and GABAergic Perisynaptic Sites

α7 neuronal nicotinic acetylcholine receptors (α7-nAChR) form Ca²⁺-permeable homopentameric channels modulating cortical network activity and cognitive processing. They are located pre- and postsynaptically and are highly abundant in hippocampal GABAergic interneurons. It is unclear how α7-nAChRs are positioned in specific membrane microdomains, particularly in cultured neurons which are devoid of cholinergic synapses. To address this issue, we monitored by single particle tracking the lateral mobility of individual α7-nAChRs labeled with α-bungarotoxin linked to quantum dots in live rat cultured hippocampal interneurons. Quantitative analysis revealed different modes of lateral diffusion of α7-nAChR dependent on their subcellular localization. Confined receptors were found in the immediate vicinity of glutamatergic and GABAergic postsynaptic densities, as well as in extrasynaptic clusters of α-bungarotoxin labeling on dendrites. α7-nAChRs avoided entering postsynaptic densities, but exhibited reduced mobility and long dwell times at perisynaptic locations, indicative of regulated confinement. Their diffusion coefficient was lower, on average, at glutamatergic than at GABAergic perisynaptic sites, suggesting differential, synapse-specific tethering mechanisms. Disruption of the cytoskeleton affected α7-nAChR mobility and cell surface expression, but not their ability to form clusters. Finally, using tetrodotoxin to silence network activity, as well as exposure to a selective α7-nAChR agonist or antagonist, we observed that α7-nAChRs cell surface dynamics is modulated by chronic changes in neuronal activity. Altogether, given their high Ca²⁺-permeability, our results suggest a possible role of α7-nAChR on interneurons for activating Ca²⁺-dependent signaling in the vicinity of GABAergic and glutamatergic synapses.     

A 5-HT7 Heteroreceptor-Mediated Inhibition of [3H]Serotonin Release in Raphe Nuclei Slices of the Rat: Evidence for a Serotonergic–Glutamatergic Interaction

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain, loaded with [3H]serotonin ([3H]5-HT), superfused, and the electrically induced efflux of radioactivity was determined. The nonselective 5-HT receptor agonist 5-carboxamido-tryptamine (5-CT; 0.001 to 1 microM) inhibited the electrically stimulated [3H]5-HT overflow from raphe nuclei slices (IC50 of 3.34 +/- 0.37 nM). This effect of 5-CT on [3H]5-HT overflow was antagonized by the 5-HT7 receptor antagonist SB-258719 (10 microM) and the 5-HT(1B/1D) antagonist SB-216641 (1 microM), the IC50 values for 5-CT in the presence of SB-258719 and SB-216641 were 94.23 +/- 4.84 and 47.81 +/- 4.66 nM. The apparent pA2 values for SB-258719 and SB-216641 against 5-CT were 6.43 and 7.12, respectively. The inhibitory effect of 5-CT on [3H]5-HT overflow was weakly antagonized by 10 microM of WAY-100635, a 5-HT1A receptor antagonist (IC50 6.65 +/- 0.56 nM, apparent pA2 4.99). The antagonist effect of SB-258719 (10 microM) on 5-CT-evoked [3H]5-HT overflow inhibition was also determined in the presence of 1 microM SB-216641 or 1 microM SB-216641 and 10 microM WAY-100635, and additive interactions were found between the antagonists of 5-HT7 and 5-HT1 receptor subtypes. Addition of the Na+ channel blocker tetrodotoxin (1 microM) in the presence of SB-216641 (1 microM) and WAY-100635 (10 microM) attenuated the inhibitory effect of 5-CT on KCl-induced [3H]5-HT overflow. These findings indicate that 5-CT inhibits [3H]5-HT overflow from raphe nuclei slices of the rat by stimulation of 5-HT7 and 5-HT(1B/1D receptors, whereas the role of 5-HT1A receptors in this inhibition is less pronounced. They also suggest that 5-HT7 receptors are probably not located on serotonergic neurons and thus may serve as heteroreceptors in regulation of 5-HT release in the raphe nuclei. 5-CT (0.1 microM) also inhibited [3H]glutamate release, and SB-258719 (10 microLM) suspended this effect. We therefore speculated that the axon terminals of the glutamatergic cortico-raphe neurons may possess 5-HT7 receptors that inhibit glutamate release, which consequently leads to decreased activity of serotonergic neurons. The postulated glutamatergic-serotonergic interaction in the raphe nuclei was further evidenced by the finding that N-methyl-D-aspartate and AMPA enhanced [3H]5-HT release.     

Comparison of Effects of DL-Threo-β-Benzyloxyaspartate (DL-TBOA) and L-Trans-Pyrrolidine-2,4-Dicarboxylate (t-2,4-PDC) on Uptake and Release of [3H]D-Aspartate in Astrocytes and Glutamatergic Neurons

Uptake and release processes in cerebellar astrocytes and granule neurons (glutamatergic) for glutamate were investigated by the use of [³H]D-aspartate, a non-metabolizable glutamate analog. The effects of DL-threo--benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [³H]D-aspartate were studied. Both compounds inhibited potently uptake of [³H]D-aspartate in neurons and astrocytes (IC₅₀ values 10-100 M), DL-TBOA being slightly more potent than t-2,4-PDC. Release of preloaded [³H]D-aspartate from neurons or astrocytes could be stimulated by addition of excess t-2,4-PDC whereas addition of DL-TBOA had no effect on [³H]D-aspartate efflux. Moreover, DL-TBOA inhibited significantly the depolarization-induced (55 mM KCl) release of preloaded [³H]D-aspartate in the neurons. The results reflect the fact that DL-TBOA is not transported by the glutamate carriers while t-2,4-PDC is a substrate which may heteroexchange with [³H]D-aspartate. It is suggested that DL-TBOA may be used to selectively inhibit depolarization coupled glutamate release mediated by reversal of the carriers.