N-methyl-D-aspartate and aspartate raise the cytosolic free calcium concentration by acting upon receptors transiently expressed on immature cerebellar Purkinje cells

Functional NMDA receptors are transiently active and support the survival of Purkinje cells in culture

Conflicting evidence exists concerning the activity of NMDA receptors (NMDARs) in cerebellar Purkinje cells and their possible functions. To investigate the activity of NMDARS, we used whole-cell recording on immunocytochemically identified Purkinje cells in primary culture. In addition, we used mice with a disrupted NMDAR1 gene that lack functional NMDARs (NR1-/-) to assess the physiological role of NMDARs. In cultures from normal mice, NMDA-medicated currents were detected in all identified Purkinje cells at 4 d in vitro (div). After 14 d, however, NMDA responses were reduced in amplitude, whereas the responses to kainate and glutamate increased steadily in amplitude. In addition, the NMDA-induced current displayed a pronounced desensitization at these later stages; peak current declined to zero during steady application of NMDA. At 7 div, the number of surviving Purkinje cells was less in cultures treated with NMDA antagonists, and their survival was dose-dependent. Purkinje cell survival was correspondingly poorer in cultures from the NR1-/- mice than in wild-type controls, suggesting that NMDAR activity enhances the survival of Purkinje cells in vitro. The addition of moderate doses of NMDA promoted the survival of wild-type Purkinje cells in the presence of tetrodotoxin. Feeder layers of cerebellar granule cells derived from wild-type or NR1-/- mice promoted survival of Purkinje cells to a similar degree, suggesting that the NMDAR in Purkinje cells, but not in other cells, is directly involved in Purkinje cell viability. The results demonstrate that NMDARs transiently produce membrane current in Purkinje cells and may serve as one of the epigenetic factors that support the survival of Purkinje cells in vitro.

Excitatory amino acid receptors in membranes from cultured human retinal pigment epithelium

The presence of specific, saturable receptor sites for excitatory amino acids (EAA) in membranes from cultured human retinal pigment epithelium (RPE) was established through the binding of [3H]L-glutamate (L-Glu). The age of the donors ranged from 6 days to 33 years. The affinity of the binding (KB) sites was between 1.2 and 1.5 microM, and did not change with the age of the donor, whereas the Bmax was slightly increased (8.6 to 13.0 pmol/mg) in membranes from the 33 year-old compared to the 29 day-old donor. The efficacy profile of agonists and antagonists acting at EAA receptors for displacing [3H]L-Glu was L-Glu = L-Aspartate > 2-amino-4-phosphonovalerate (AP5) > N-methyl-D-Aspartate (NMDA) > 1-aminocyclopentane-1,3 dicarboxylate (trans-ACPD) > 2-amino-3-phosphonopropionate (AP3). These data suggest the presence of either an NMDA-receptor sensitive to the metabotropic agonist trans-ACPD or alternatively, the presence of two different populations of receptors with similar affinity for the agonist: NMDA and metabotropic. Glycine highly stimulated Glu-binding; this effect was inversely related to the age of the donor. Taurine and to a lesser extent GABA, mimicked this effect. Stimulation by glycine was dose-dependent, insensitive to strychnine and 80% inhibited by 7-chlorokynurenate. This effect was also present in human RPE-derived fibroblasts, human scleral fibroblasts and the human lymphoblastoid cell line NB76, all continuously dividing cells. The results further support the possibility of the participation of EAA receptors in the regulation of phagocytosis in RPE.

Action potentials, calcium transients and the control of differentiation of excitable cells

Calcium influx via action potentials in differentiating nerve and muscle is regulated principally by the expression of potassium currents. Transient elevations of intracellular calcium in spontaneously active cells are necessary for normal neuronal development. The mechanisms that connect calcium elevations to long term developmental change are likely to be utilized in the mature nervous system.

Glutamate increases the [Ca2+]i but stimulates Ca(2+)-independent release of [3H]GABA in cultured chick retina cells

The effect of glutamate on [Ca2+]i and on [3H] gamma-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 microM) increases the [Ca2+]i by Ca2+ influx through Ca2+ channels sensitive to nitrendipine, but not to omega-conotoxin GVIA (omega-Cg Tx) (50%), and by other channels insensitive to either Ca2+ channel blocker. Mobilization of Ca2+ by glutamate required the presence of external Na+, suggesting that Na+ mobilization through the ionotropic glutamate receptors is necessary for the Ca2+ channels to open. The increase in [Ca2+]i was not related to the release of [3H]GABA induced by glutamate, suggesting that the pathway for the entry of Ca2+ triggered by glutamate does not lead to exocytosis. In fact, the glutamate-induced release of [3H]GABA was significantly depressed by Ca(2+)o, but it was dependent on Na(+)o, just as was observed for the [3H]GABA release induced by veratridine (50 microM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [3H]GABA release. Both veratridine- and glutamate-induced [3H]GABA release were inhibited by 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid (NNC-711), a blocker of the GABA carrier. Blockade of the NMDA and non-NMDA glutamate receptors with MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively, almost completely blocked the release of [3H]GABA evoked by glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium permeability of non-N-methyl-D-aspartate receptor channels in immature cerebellar Purkinje cells: studies using fura-2 microfluorometry

Using fura-2 microfluorometry, I investigated the mechanism by which non-N-methyl-D-aspartate (NMDA) receptor agonists increase the cytosolic free calcium concentration ([Ca]in) in single cerebellar Purkinje cells isolated from 3-10-day-old rats. Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate dose-dependently increased the cytosolic free Na+ concentration, which was measured using sodium-binding benzofuran isophthalate microfluorometry, confirming the Na+ influx through ion channels linked to non-NMDA receptors. The [Ca2+] increases induced by relatively lower concentrations of agonists were entirely dependent on external Ca2+ and were reduced by removal of external Na+ or by addition of a Ca2+ channel blocker, D600. The results indicate that the non-NMDA agonist-induced [Ca]in increase was due mainly to Ca2+ influx through voltage-dependent Ca2+ channels, which were activated by a massive Na+ influx. On the other hand, higher concentrations of agonists dose-dependently increased [Ca]in under conditions in which activation of voltage-dependent Ca2+ channels were blocked by a combination of Na+ removal with D600. These [Ca]in increases were Ca2+ dependent and little affected by adding a competitive NMDA antagonist. Non-NMDA agonists also stimulated influxes of Mn2+ and Co2+, both of which can be monitored by quenching fura-2 fluorescence under the same conditions. These results suggest that ion channels linked to non-NMDA receptors on immature Purkinje cells are permeable to Ca2+, Mn2+, and Co2+.