EFFECT OF THIOCYANATE ON AMPA RECEPTOR-MEDIATED RESPONSES IN EXCISED PATCHES AND HIPPOCAMPAL SLICES

The binding affinity of alpha-amino-3-hydroxy-5-methyl-4-isoxazo acid (AMPA) receptors for [H-3]AMPA is increased 10-30-fold by the chaotrop ic anion thiocyanate. The present experiments tested if thiocyanate al ters AMPA receptor mediated current fluxes and if any such effects are reflected in the waveform of synaptic responses. Currents were measur ed after a step application of glutamate or AMPA to patches excised fr om pyramidal cells of hippocampal slice cultures. Application of 1 mM AMPA produced responses with an average peak amplitude of 86 pA at -50 mV and a 10-90% rise lime of 1.7 +/- 0.1 ms; the responses desensitiz ed to a steady-state level below 10% of the peak current with a time c onstant of 11.1 +/- 0.7 ms. Glutamate in presence of D-amino-phosphono pentanoate produced similar responses which were inhibited by 6-cyano- 7-nitro-quinoxaline-dione and enhanced by aniracetam or cyclothiazide and thus are characteristic for AMPA receptors. Thiocyanate accelerate d the decay of AMPA responses two-fold and reduced the peak current by 30-50% with an EC(50) of 3.2 mM which is comparable to its EC(50), fo r enhancing binding. Effects on the desensitization of glutamate induc ed responses were much smaller and only evident at the highest thiocya nate concentration; no effect was seen on response amplitude. Binding and physiological effects can be adequately explained by assuming that thiocyanate enhances conversion from the sensitive to the desensitize d state of the receptor and reduces ligand dissociation from the desen sitized state. Synaptic responses were measured in disinhibited hippoc ampal slices. Perfusion with 20 mM sodium thiocyanate increased the sl ope of the field excitatory postsynaptic potential by 44.9 +/- 4.2% an d reduced its decay time by 10.4 +/- 4.3%. The former effect appears t o result at least in part from an increase in transmitter release sinc e it was accompanied by a decrease in paired-pulse facilitation and wa s reduced in magnitude after enhancing transmitter release. The decrea se in the decay time constant points to an effect of thiocyanate on AM PA receptors in situ which is similar to that seen in excised patches. These results demonstrate that an increase in binding affinity may be indicative of reduced rather than enhanced current flow through AMPA receptors. In addition, the results provide further evidence that the kinetics of the AMPA receptor channel contribute significantly to at l east the decay phase of fast excitatory synaptic responses.