REDUCED PROBABILITY OF ORTHODROMICALLY EVOKED ACTION-POTENTIAL FIRINGIN CA1 PYRAMIDAL CELLS OF GUINEA-PIG HIPPOCAMPAL SLICES AFTER ACUTE THALLIUM EXPOSURE

We investigated the effect of thallium ions on extracellular field pot entials in the CA1 region of guinea pig hippocampal slices in a matche d-pair experimental setup. Somatic and dendritic responses evoked by p aired-pulse stimulation of the Schaffer collateral-commissural pathway were recorded before, during and after acute thallium exposure and co mpared to field potentials from nontreated control slices recorded sim ultaneously. Thallium reduced the orthodromically evoked population sp ike reversibly in a clear concentration-effect relationship. In contra st, the field excitatory postsynaptic potential fEPSP: as well as the presynaptic fiber volley of the afferent pathway, were not affected by thallium. Furthermore, the paired-pulse facilitation was reversibly r educed during thallium exposure. Input-output relations clearly demons trated that thallium did not interfere with the presynaptic transmitte r release mechanisms or the postsynaptic transmitter receptor sensitiv ity, but had a predominant postsynaptic target site. Additionally, any influence of thallium ions on the somatic and/or axonal membrane exci tability could be excluded, as the antidromically evoked responses aft er alvear stimulation were not diminished by thallium. Therefore, the main effect of thallium was a decoupling of the somatic from the dendr itic activity at the CA1 pyramidal cells. We conclude that the toxic i nfluence of thallium ions in the guinea pig hippocampus must be confin ed to intracellular somatic mechanisms. Interactions with intracellula r organelles and an impairment of their calcium storage capacity are s upposed.