Gadolinium reduces AMPA receptor desensitization and deactivation in hippocampal neurons

The actions of the trivalent cation Gd3+ on whole cell AMPA receptor-mediat ed currents were studied in isolated hippocampal neurons, in nucleated or o utside-out patches taken from cultured hippocampal neurons, and on miniatur e excitatory postsynaptic currents (mEPSCs) recorded in cultured hippocampa l neurons. Glutamate, AMPA, or kainate was employed to activate AMPA recept ors. Applications of relatively low concentrations of Gd3+ (0.1-10 muM) sub stantially enhanced steady-state whole cell glutamate and kainate-evoked cu rrents without altering peak currents, suggesting that desensitization was reduced. However, higher concentrations (>30 muM) depressed steady-state cu rrents, indicating an underlying inhibition of channel activity. Lower conc entrations of Gd3+ also increased the potency of peak glutamate-evoked curr ents without altering that of steady-state currents. An ultrafast perfusion system and nucleated patches were then used to better resolve peak glutama te-evoked currents. Low concentrations of Gd3+ reduced peak currents, enhan ced steady-state currents, and slowed the onset of desensitization, providi ng further evidence that this cation reduces desensitization. In the presen ce of cyclothiazide, a compound that blocks desensitization, a low concentr ation Gd3+ inhibited both peak and steady-state currents, indicating that G d3+ both reduces desensitization and inhibits these currents. Gd3+ reduced the probability of channel opening at the peak of the currents but did not alter the single channel conductance calculated using nonstationary varianc e analysis. Recovery from desensitization was enhanced, and glutamate-evoke d current activation and deactivation were slowed by Gd3+. The Gd3+ induced reduction in desensitization did not require the presence of the GluR2 sub unit as this effect was seen in hippocampal neurons from GluR2 null-mutant mice. Gd3+ reduced the time course of decay of mEPSCs perhaps as a conseque nce of its slowing of AMPA receptor deactivation although an increase in th e frequency of mEPSCs also suggested enhanced presynaptic release of transm itter. These results demonstrate that Gd3+ potently reduces AMPA receptor d esensitization and mimics a number of the properties of the positive modula tors of AMPA receptor desensitization such as cyclothiazide.