Blockade of NMDA-activated channels by magnesium in the immature rat hippocampus

Abstract

1. N-methyl-D-aspartate (NMDA)-receptor single-channel currents were examined in outside-out patches from thin slices of rat hippocampus to examine possible age dependence in the Mg(2+)-block of NMDA-mediated responses. The kinetics of Mg2+ block, among other channel properties, were compared between CA1 pyramidal cells from neonatal and 2-wk-old animals and dentate gyrus granule cells from neonatal and 4-wk-old animals. 2. Two distinct conductance states were seen consistently in nominally Mg(2+)-free solutions. The main conductance state was 50 and 56 pS in pyramidal cells and granule cells, respectively. The difference in NMDA-receptor channel conductance between the two cell types was statistically significant at all ages examined. Both cell types exhibited a less frequent 35- to 40-pS conductance state. 3. Channel closing rates showed no intrinsic voltage dependence in Mg(2+)-free solutions in any group. Open times became strongly voltage dependent when Mg2+ was added. The rate of block by Mg2+ was similar in all groups. 4. Voltage and Mg2+ influenced primarily the shortest time constant of shut-time distributions. Longer components varied relatively little with voltage or [Mg2+]. The effects of voltage and [Mg2+] suggest that brief shuttings represent closed states in the absence of Mg2+ and primarily a Mg(2+)-blocked state in the presence of Mg2+. 5. The rate of unblocking by Mg2+ was similar in all groups. Thus the dissociation constant for Mg2+ binding (i.e., the ratio of the unblocking and blocking rates) also showed little variation. 6. NMDA-receptor channels in two regions of the hippocampus behaved in a qualitatively similar fashion both in neonates and 2-or 4-wk-old rats. These observations are inconsistent with previous studies obtained using other methods, which suggested that block of NMDA receptor channels by Mg2+ increases substantially with age. Nevertheless, subtle developmental and regional differences in other aspects of NMDA-receptor channel behavior were detected, perhaps reflecting variations in molecular structure tailored to specific functional requirements.

Direct Effects of Calmodulin on NMDA Receptor Single-Channel Gating in Rat Hippocampal Granule Cells

“…These data are similar to those from previous experiments on granule cell NMDA receptors (Strecker et al, 1994).…”

Synaptic NMDA receptors in developing mouse hippocampal neurones: functional properties and sensitivity to ifenprodil.

“…The apparent Kd values for Mg2+ binding to synaptic NMDA receptors in very young, intermediate and adult neurones (7-8 + 6-4, 10-4 + 14-1 and 6 5 + 4-7 mm, respectively, at 0 mV) were not significantly different from each other. In a recent study using single NMDA receptor-channel analysis (Strecker et al 1994), the Kd value for Mg2+ binding to extrasynaptic NMDA receptors in 2-week-old rat CAl PCs (6-8 mm, at 0 mV) was very similar to those reported here. The Kd for…”

“…Mg2+ binding in neonates also was the same (4 9 mm at O mV ;Strecker et al 1994). Taken together, these data indicate that the affinity of NMDA receptors for Mg2+ in rodent CAl PCs may not change significantly after birth.…”

“…contradictory results have been reported (Strecker, Jackson & Dudek, 1994). These developmental changes may underlie age-related differences in functional properties of synaptic NMDA receptors.…”

Molecular determinants of NMDA receptor function in GABAergic neurones of rat forebrain.

“…(Strecker, Jackson & Dudek, 1994;Spruston, Jonas & Sakmann, 1995), and dentate gyrus granule cells (Keller, Konnerth & Yaari, 1991;Strecker et al 1994) and basket cells (Koh, Geiger, Jonas & Sakmann, 1995) under similar conditions, and in cells expressing some combinations of recombinant receptors. In recombinant systems, expression of NR1 (;1) with NR2A or NR2B (e2) resulted in receptors with respective main conductance levels of 50 and 51 pS and lower levels of 38 and 39 pS (Stern et al 1992;Tsuzuki et al 1994).…”