Interpretation of voltage-clamp measurements in hippocampal neurons

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Abstract
The expected accuracy of voltage-clamp measurements of local and remote conductance changes in [guinea pig] hippocampal pyramidal neruons was assessed, with special emphasis on the mossy fiber synaptic input to these cells. The passive cable properties of these neurons and the actual performance characteristics of the best available single-microelectrode voltage-clamp (SEC) circuit were analyzed. The analog simulations were used to assess the performance of the SEC and the consequences of imperfect space clamp. Anatomical measurements were made of the CA3 hippocampal subfield from transverse sections that were stained by the rapid Golgi or Timm methods. The mossy fiber synapses were located in a discrete band in the stratum lucidum. The average distance from the cell soma or stratum pyramidale to the distal portion of this band was 0.15 mm. The average diameter of the primary apical dendrites of the pyramidal cells was 6.0 .mu.m. From these cells was 6.0 .mu.m. From these data and an equivalent-cylinder representation of the dendrites, it was calculated that the most distal mossy fiber synapses were located at an average electronic distance no greater than 0.07 from the pyramida cell bodies. Branched compartmental models were constructed that preserved the actual dendritic arborization. The average electronic distance from the soma to the end of the mossy fiber synaptic zone was 0.06. The SEC will yield accurate (flat frequency response) current measurements for conductance changes within a frequency band of 0-300 Hz, which includes most of the power spectrum for a mossy fiber synaptic-conductance waveform (which lies in the range of 0-200 Hz). The SEC system will attenuate frequency components much higher than .apprx. 300 Hz.