Dj. Waters et Tgj. Allen, CA2-PERMEABLE NON-NMDA GLUTAMATE RECEPTORS IN RAT MAGNOCELLULAR BASALFOREBRAIN NEURONS(), Journal of physiology, 508(2), 1998, pp. 453-469
1. Ionotropic glutamate receptor-mediated responses were recorded from
rat magnocellular basal forebrain neurones under voltage clamp from a
somatically located patch-clamp pipette. Currents were recorded from
both acutely dissociated neurones and neurones maintained in culture f
or up to 6 weeks. 2. Non-NMDA and NMDA receptor-mediated events could
be distinguished pharmacologically using the selective agonists -alpha
-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA), kainate and
N-methyl-D-aspartate (NMDA), and antagonists 6-cyano-7-nitroquinoxalin
e-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonopentanoic acid (AP5). 3
. Responses to rapid application of AMPA displayed pronounced and rapi
d desensitization. Responses to kainate showed no desensitization. Ste
ady-state EC50 values for AMPA and kainate were 2.7 +/- 0.4 mu M (n =
5) and 138 +/- 25 mu M (n = 10), respectively. Cyclothiazide markedly
increased current amplitude of responses to both agonists, whereas con
canavalin A had no clear effect on either response. The selective AMPA
receptor antagonist GYKI 53655 inhibited responses to kainate with an
IC50 of 1.2 +/- 0.08 mu M (n = 5) at -70 mV. These data strongly sugg
est that AMPA receptors are the predominant non-NMDA receptors express
ed by basal forebrain neurones. 4. At -70 mV, approximately 6% of cont
rol current amplitude remained, at a maximally effective concentration
of GYKI 53655. This residual response displayed desensitization, was
insensitive to cyclothiazide and was potentiated by concanavalin A, su
ggesting that it was mediated by a kainate receptor. 5. Current-voltag
e relationships for non-NMDA receptor-mediated currents were obtained
from both nucleated patches pulled from neurones in culture and from a
cutely dissociated neurones. With 30 mu M spermine in the recording pi
pette, currents frequently displayed double-rectification characterist
ic of non-NMDA receptors with high Ca2+ permeabilities. Ca2+ permeabil
ity, relative to Na+ and Cs+, was investigated using constant field th
eory. The measured Ca2+ to Na+ permeability coefficient ratio was 0.26
-3.6; median, 1.27 (n = 15). 6. Current flow through non-NMDA receptor
s was inhibited by Ca2+, Cd2+ and Co2+ ions. At a holding potential of
-70 mV, a maximally effective concentration of Cd2+ (>30 mM) reduced
current amplitude by approximately 90%, with an IC50 of 44 mu M. In si
x out of seven cells tested, block by Cd2+ was voltage sensitive. 7. C
a2+ permeability of many of the non-NMDA receptors expressed by magnoc
ellular basal forebrain neurones may underlie the unusual sensitivity
of cholinergic basal forebrain neurones to non-NMDA receptor-mediated
excitotoxicity.