Excitation necessary to reach threshold varies with input strength through a mechanism involving feed forward inhibition

ANTONIA MARIN BURGIN; FREDERIC POIULLE; MASSIMO SCANZIANI

Washington DC

Congreso; Society for Neuroscience Annual meeting; 2008

Society for Neuroscience

We determined the peak excitatory conductance necessary to reach threshold for action potential (AP) generation in CA1 pyramidal cells (PCs) recorded in acute hippocampal slices obtained from 3-4 weeks old rats. PCs were initially recorded in the loose patch configuration and Schaffer collaterals stimulated at an intensity just sufficient to reach threshold for AP generation. We subsequently gained whole cell access to the neurons and determined the peak conductance of the underlying "threshold" excitatory postsynaptic current (EPSG) by holding the PC at the reversal potential for inhibition. We found that the threshold EPSG varied over a wide range, between 3 and 30 nS. The threshold EPSG correlated with the number of stimulated Schaffer collaterals. Both the variability in the threshold EPSG and the correlation with the number of activated Schaffer collaterals were abolished in the presence of the GABA A receptor antagonists gabazine. We determined the peak conductance of feed forward inhibitory postsynaptic currents (IPSGs) elicited in response to stimulation of Schaffer collaterals at threshold for AP generation and found that it strongly correlated with the evoked threshold EPSG.These data suggest that the threshold excitation in PCs is not fixed but varies as a function of input strength. This variability depends on the presence of feed-forward inhibition. By instantaneously adjusting pyramidal cell excitability to the strength of incoming activity, feed-forward inhibitory circuits allow the pyramidal cell population to be sensitive to weak stimuli yet not to saturate with stronger ones. This results in an expansion of the dynamic range of the CA1 region to incoming activity.