Ca2+-permeable non-NMDA glutamate receptors in rat magnocellular basal forebrain neurones

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 for up to 6 weeks. 2. Non-NMDA and NMDA receptor-mediated events could be distinguished pharmacologically using the selective agonists (S)-alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA), and antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonopentanoic acid (AP5). 3. Responses to rapid application of AMPA displayed pronounced and rapid desensitization. Responses to kainate showed no desensitization. Steady-state EC50 values for AMPA and kainate were 2.7 +/- 0.4 microM (n = 5) and 138 +/- 25 microM (n = 10), respectively. Cyclothiazide markedly increased current amplitude of responses to both agonists, whereas concanavalin 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 microM (n = 5) at -70 mV. These data strongly suggest that AMPA receptors are the predominant non-NMDA receptors expressed by basal forebrain neurones. 4. At -70 mV, approximately 6 % of control 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, suggesting that it was mediated by a kainate receptor. 5. Current-voltage relationships for non-NMDA receptor-mediated currents were obtained from both nucleated patches pulled from neurones in culture and from acutely dissociated neurones. With 30 microM spermine in the recording pipette, currents frequently displayed double-rectification characteristic of non-NMDA receptors with high Ca2+ permeabilities. Ca2+ permeability, relative to Na+ and Cs+, was investigated using constant field theory. The measured Ca2+ to Na+ permeability coefficient ratio was 0.26-3.6; median, 1.27 (n = 15). 6. Current flow through non-NMDA receptors 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 microM. In six out of seven cells tested, block by Cd2+ was voltage sensitive. 7. Ca2+ permeability of many of the non-NMDA receptors expressed by magnocellular basal forebrain neurones may underlie the unusual sensitivity of cholinergic basal forebrain neurones to non-NMDA receptor-mediated excitotoxicity.