Interaction among excitatory and inhibitory neuronal circuits in the representation of afferent inputs in the hippocampus.

ANTONIA MARIN BURGIN; FREDERIC POIULLE; MASSIMO SCANZIANI

Los Cardales

Congreso; Biology in Balance; 2009

Cell Press

Cortical neuronal structures are subject to large moment to moment variation in afferent activity. They need to be sensitive to weak stimuli yet not saturate to stronger ones; in other words they need to operate over a wide dynamic range.   What mechanisms control this wide dynamic range?   We have addressed the question in rat hippocampal slices where we monitored the progressive recruitment of CA1 pyramidal cells (PCs) in response to stimuli of increasing strength delivered to the Shaffer collaterals, their main excitatory afferent input.   We recorded first the input strength necessary to evoke spikes in PCs and subsequently the inhibitory (IPSC) and excitatory (EPSC) post synaptic currents that underlay that spiking. We found that the amount of excitation required to fire CA1 PCs is not fixed but varies as a function of input strength. This variability is accomplished by a global feedforward inhibition that increases proportionally with the input strength, so that weak inputs produce weak inhibition and therefore PCs necessitate less excitation to spike, while strong inputs recruit a larger inhibition and therefore PC necessitate more excitation to spike. 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 balance between input specific excitation with global inhibition enables the cortical neuronal circuit to expand its dynamic range of operation.