The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

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Glutamate Receptor Ion Channels: Structure, Regulation, and Function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.

https://science.sciencemag.org/content/sci/235/4784/46.full.pdf

Multiple calcium channels and neuronal function

The Analysis of Variance.

Exocytotic Ca2+ channels in mammalian central neurons.

Glutathione (GSH) plays an important role in a multitude of cellular processes, including cell differentiation, proliferation, and apoptosis, and as a result, disturbances in GSH homeostasis are implicated in the etiology and/or progression of a number of human diseases, including cancer, diseases of aging, cystic fibrosis, and cardiovascular, inflammatory, immune, metabolic, and neurodegenerative diseases. Owing to the pleiotropic effects of GSH on cell functions, it has been quite difficult to define the role of GSH in the onset and/or the expression of human diseases, although significant progress is being made. GSH levels, turnover rates, and/or oxidation state can be compromised by inherited or acquired defects in the enzymes, transporters, signaling molecules, or transcription factors that are involved in its homeostasis, or from exposure to reactive chemicals or metabolic intermediates. GSH deficiency or a decrease in the GSH/glutathione disulfide ratio manifests itself largely through an increased susceptibility to oxidative stress, and the resulting damage is thought to be involved in diseases, such as cancer, Parkinson's disease, and Alzheimer's disease. In addition, imbalances in GSH levels affect immune system function, and are thought to play a role in the aging process. Just as low intracellular GSH levels decrease cellular antioxidant capacity, elevated GSH levels generally increase antioxidant capacity and resistance to oxidative stress, and this is observed in many cancer cells. The higher GSH levels in some tumor cells are also typically associated with higher levels of GSH-related enzymes and transporters. Although neither the mechanism nor the implications of these changes are well defined, the high GSH content makes cancer cells chemoresistant, which is a major factor that limits drug treatment. The present report highlights and integrates the growing connections between imbalances in GSH homeostasis and a multitude of human diseases.

/pdf/glutathione-dysregulation-and-the-etiology-and-progression-6hzggqm9ks.pdf

Glutathione dysregulation and the etiology and progression of human diseases.

Ethanol inhibits NMDA-induced increases in free intracellular Ca2+ in dissociated brain cells.

Uptake of 45Ca++ by synaptosomes isolated from cerebral cortex, cerebellum, midbrain and brain stem of male, Sprague-Dawley rats was measured at 1-, 3-, 5-, 15-, 30- and 60-sec time periods. At 1 sec, the Ca++ uptake rate by cerebrocortical synaptosomes was 1.45 mumol/sec/g of protein, whereas the 60-sec rate was 0.03 mumol/sec/g of protein. In vitro addition of ethanol, 80 mM, inhibited depolarization-dependent (65 mM KCl) 45Ca++ uptake by synaptosomes but the time-response relationships varied depending upon the brain region studied. In cerebrocortical synaptosomes, ethanol significantly inhibited only the fast-phase component of 45Ca++ uptake (1 and 3 sec). Ethanol inhibited 45Ca++ uptake by midbrain synaptosomes at all measurement times studied (1, 3, 5 and 15 sec), whereas in cerebellum and brain stem ethanol inhibited 45Ca++ uptake at 3- and 5-sec time periods. Ethanol at concentrations of 25, 50, 100 and 150 mM inhibited 45Ca++ uptake by 9.0, 15.9, 24.8 and 30.7%, respectively, in cerebrocortical synaptosomes. In vitro ethanol, 80 mM, added to cerebrocortical synaptosomes isolated from rats fed a nutritionally adequate liquid ethanol diet did not significantly inhibit depolarization-dependent 45Ca++ uptake. The results of this study show that pharmacologically relevant ethanol concentrations inhibit voltage-dependent 45Ca++ uptake into synaptosomes. This inhibitory action may, at least in part, underlie some of the intoxicating effects of ethanol. In addition, chronic administration of ethanol resulted in an apparent adaptive response such that addition of ethanol no longer blocked 45Ca++ uptake. This adaptive response involving the calcium channel may represent a cellular mechanism for functional tolerance development.

Inhibition of fast- and slow-phase depolarization-dependent synaptosomal calcium uptake by ethanol.

The medium spiny neurons of the nucleus accumbens receive both an excitatory glutamatergic input from forebrain and a dopaminergic input from the ventral tegmental area. This integration point may constitute a locus whereby the N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors promotes drug reinforcement. Here we investigate how dopaminergic inputs alter the ethanol sensitivity of NMDA receptors in rats and mice and report that previous dopamine receptor-1 (D1) activation, culminating in dopamine and cAMP-regulated phosphoprotein-32 kD (DARPP-32) and NMDA receptor subunit-1 (NR1)-NMDA receptor phosphorylation, strongly decreases ethanol inhibition of NMDA responses. The regulation of ethanol sensitivity of NMDA receptors by D1 receptors was absent in DARPP-32 knockout mice. We propose that DARPP-32 mediated blunting of the response to ethanol subsequent to activation of ventral tegmental area dopaminergic neurons initiates molecular alterations that influence synaptic plasticity in this circuit, thereby promoting the development of ethanol reinforcement.

DARPP-32 and regulation of the ethanol sensitivity of NMDA receptors in the nucleus accumbens.

Dissociated brain cells were isolated from newborn rat pups and loaded with fura-2. These cells were sensitive to low N-methyl-d-aspartate (NMDA) concentrations with EC50 values for NMDA-induced intracellular Ca2+ concentration ([Ca2+]i) increases of approximately 7–16 μM measured in the absence of Mg2+. NMDA-stimulated [Ca2+]j increases could be observed in buffer with Mg2+ when the cells were predepolarized with 15 mMKQ prior to NMDA addition. Under these predepolarized conditions, 100 mMethanol inhibited 25 μM NMDA responses by approximately 50%, which was similar to the ethanol inhibition observed in buffer without added Mg2+. Ethanol did not alter [Ca2+]i prior to NMDA addition. In the absence of Mg2+, 50 and 100 mM ethanol did not significantly alter the EC50 value for NMDA, but did inhibit NMDA-induced increases in [Ca2+]i in a concentration-dependent manner at 4, 16, 64, and 256 μM NMDA. Whereas NMDA-induced increases in [Ca2+]i were dependent on extracellular Ca2+ and were inhibited by Mg2+, the ability of 100 mM ethanol to inhibit 25 μM NMDA responses was independent of the external Ca2+ or Mg2+ concentrations. Glycine (1, 10, and 100 μM) enhanced 25 μM NMDA-induced increases in [Ca2+]i by approximately 50%. Glycine (1-100 μM) prevented the 100 mM ethanol inhibition of NMDA-stimulated [Ca2+]i observed in the absence of exogenous glycine. MK-801 (25-400 μM) inhibited 25 μM NMDA-stimulated rises in [Ca2+]i in a concentration-dependent manner. Unlike the additive inhibition observed with Mg2+ plus ethanol, as the concentration of MK-801 increased above 50 μM, ethanol (at 100 mM) did not produce further inhibition of NMDA responses compared with MK-801 alone. These results suggest that ethanol may produce a noncompetitive inhibition of NMDA-stimulated Ca2+ influx in dissociated brain cells, with interactions at the glycine and possibly the phencyclidine site on the NMDA-receptor complex.

Mechanism of inhibition of N-methyl-D-aspartate-stimulated increases in free intracellular Ca2+ concentration by ethanol.

Voltage-dependent 45Ca2+ uptake into rat whole brain synaptosomes was measured after 3-s KCl-induced depolarization to investigate possible inhibitory effects of calcium antagonists, nitrendipine, nimodipine, and nisoldipine. At a Ca2+ concentration of 1.2 mM, nitrendipine, in concentrations ranging from 0.1 nM to 10 microM, had no effect on 45Ca2+ uptake. When the Ca2+ concentration was lowered to 0.06 and 0.12 mM, nitrendipine, 10 microM, inhibited 45Ca2+ uptake in response to 109 mM KCl depolarization. However, in a separate concentration response study, nitrendipine, nimodipine, and nisoldipine, 0.1 nM to 10 microM, failed to alter the uptake of 45Ca2+ (0.06 mM Ca2+) into 30 mM KCl-depolarized synaptosomes. The high concentrations of these agents required to depress 45Ca2+ uptake indicate that the dihydropyridine calcium antagonists are considerably less potent in brain tissue than in peripheral tissue.

Steven W. Leslie

Papers

Ethanol inhibits NMDA-induced increases in free intracellular Ca2+ in dissociated brain cells.

Inhibition of fast- and slow-phase depolarization-dependent synaptosomal calcium uptake by ethanol.

DARPP-32 and regulation of the ethanol sensitivity of NMDA receptors in the nucleus accumbens.

Mechanism of inhibition of N-methyl-D-aspartate-stimulated increases in free intracellular Ca2+ concentration by ethanol.

45Ca2+ uptake into rat whole brain synaptosomes unaltered by dihydropyridine calcium antagonists.