On the multiple‐conductance single channels activated by excitatory amino acids in large cerebellar neurones of the rat.

Abstract

3. From the relative areas under current amplitude histograms it was estimated that the percentage of openings with conductances greater than 30 pS was about 83 % with NMDA, 79 % with glutamate and 78 % with aspartate. In some patches, the majority of > 30 pS events evoked by these agonists were to the maximum conductance of 48 pS, whereas in other patches there were more 38 pS openings than 48 pS openings. Only 27 % of quisqualate openings, and about 10 % of kainate openings, were > 30 pS.4. Of the small amplitude (< 20 pS) events, 93 % of quisqualate openings were to the 8 pS level whereas -87 % of < 20 pS currents produced by NMDA, glutamate and aspartate were to the 18 pS level (the remainder being 8 pS). Direct transitions could occur between certain levels (including events above and below 30 pS) suggesting that these are sublevels of multiple-conductance channels. The most frequently occurring transitions were between the 48 and 38 pS levels, and the 38 and 18 pS levels.5. Channel openings occurred in bursts, within which individual openings were separated either by brief closures (gaps), or by direct transitions between the multiple conductance levels. The briefest of these gaps (< 200-400 7. XVhen applied to isolated patches, kainate (and to a lesser extent quisqualate) gave a noise increase, accompanied in some patches by discrete single-channel openings (similar in conductance to those produced by other agonists). Analysis of kainate currents in outside-out patches gave two component spectra with time constants of 0 9 + 0d and 16 4 + 2-4 ms (E -=70 mV). The mean single-channel conductance estimated from the spectra was y(kainate) = [03 +02 pS. The fast component contributed most power to spectra of kainate and quisqualate singlechannel currents.8. Our results may be explained by assuming that NMDA, quisqualate and kainate receptors are coupled to separate channels, and that some, or all, of these receptor channels possess multiple conductance levels. All the channels appear to show two kinetically distinct open states, and two (or three) shut states. Differences in the properties of glutamate channels present in large cerebellar neurones and in certain other neurones are discussed.

Pentobarbitone modulation of NMDA receptors in neurones isolated from the rat olfactory brain

“…Pentobarbitone did not alter the reversal potential. Noise analysis As previously reported by Ascher et al (1988), Cull-Candy et al (1988) and Cull-Candy & Usowicz (1989), application of NMDA and other glutamate agonists is accompanied by an increase in signal noise. The plots of noise variance against mean current were linear (not shown) suggesting that the open probability was low and the single channel conductance estimated from the variance of the current noise was 45 + 3 pS and it was not altered by co-application of the anaesthetic (100-400 gM).…”

“…Unlike nictonic receptors which have relatively simple kinetics, NMDA receptors have multiple open and closed states (Gibb & Colquhoun, 1992). Openings occur in bursts and clusters and they have multiple conductance levels (Cull-Candy & Usowicz, 1989). The effects of anaesthetics on such intricate gating poses a number of interesting problems: Is it possible to describe the effects of an anaesthetic on NMDA receptors in terms of a simple kinetic model?…”

Thiocyanate ions selectively antagonize AMPA‐evoked responses in Xenopus laevis oocytes microinjected with rat brain mRNA

“…In radioligand binding studies of non-NMDA receptors, chaotropic ions increased the specific binding of [3H]-AMPA in rat brain membranes by up to 800% of basal binding at concentrations ranging from 1-100mM (Honore & Nielsen, 1985;Murphy et al, 1987;Olsen et al, 1987;Honore & Drejer, 1988;Nielsen et al, 1988;Shahi & Baudry, 1992 (Shahi & Baudry, 1992 the AMPA response at a greater level of desensitization since it was markedly enhanced by Con-A treatment, whereas Ip was relatively insensitive . In addition, a study of the single channel currents activated by quisqualate on hippocampal neurones, revealed that Ip may be carried by rapidly-inactivating high conductance channels (single channel conductance (7) = 35pS; Tang et al, 1989), whereas the maintained current (approximating to ISS) was carried mostly by low conductance channels (y = 8pS; Jahr & Stevens, 1987;Cull-Candy & Usowicz, 1987;Ascher & Nowak, 1988 Ascher & Nowak, 1988).…”

Kinetic properties of NMDA receptor‐mediated synaptic currents in rat hippocampal pyramidal cells versus interneurones.

“…1990) and in cultures of synaptically connected hippocampal neurones (Lester, Clements, Westbrook & Jahr, 1990) (Cull-Candy & Usowicz, 1987;Jahr & Stevens, 1987;Ascher, Bregestowski & Nowak, 1988), the mean open time cannot be the 'elementary event' that determines the decay of NMDA EPSCs. However, the activity of NMDA receptor channels in outside-out patches tends to appear in clusters lasting hundreds of milliseconds (Jahr & Stevens, 1987;Howe, Colquhoun & Cull-Candy, 1988;Keller et al 1991) even after pulse-like applications of agonist (Lester et al 1990).…”