Differential recruitment of pyramidal cells and interneurons in area CA1 of the hippocampus by afferent excitation.

ANTONIA MARIN BURGIN; FREDERIC POIULLE; MASSIMO SCANZIANI

San Diego

Congreso; Meeting of the society for neuroscience; 2007

Society for Neuroscience

Area CA1 of the hippocampus contains excitatory pyramidal neurons (PYR) as well as a heterogeneous population of inhibitory interneurons. All of these cell types receive afferent excitation from the Schaffer collaterals. In order to understand if different levels of afferent excitation engage different populations of excitatory and inhibitory neurons, we compared the responsiveness of these types of neurons to afferent stimulation. To address this question, we stimulated Schaffer collaterals while recording from PYR and interneurons in the loose patch configuration. We set the stimulus intensity to reach threshold for action potential in the recorded neuron. To normalize intensity of stimulation across slices, an extracellular electrode was placed in the PYR layer to record the population spike. Subsequently, neurons were recorded in the whole cell configuration to determine their electrophysiological characteristics and to be dialyzed with biocytin for post hoc morphological identification. Neurons were divided into three groups according to their electrophysiological properties: PYR, fast spiking interneurons (FS) and regular spiking interneurons (RS). We found that different classes of neurons were sequentially recruited as a function of the stimulus intensity. FS were most sensitive to low stimulus intensities, followed by RS and PYR. In fact, stimulation intensities sufficient to recruit ~10% of PYR, triggered spikes in ~70% of FS. In contrast, at that same intensity, only ~25% of RS were recruited. These results suggest that different types of interneurons are recruited at different levels of afferent activity. To compare the activity of PYR and FS in the same slice, we combined imaging and electrophysiological recordings. PYR activity was monitored by loading the PYR layer of CA1 with a calcium dye to image action potential evoked calcium transients at a single cell resolution. FS activity was recorded simultaneously using loose patch configuration. Two stimulating electrodes were placed in the stratum radiatum to recruit two independent Schaffer collaterals inputs. We stimulated each input at an intensity to recruit around 10% of the imaged PYR. Under these conditions, the majority of PYR responded to only one of the two stimuli, however most FS were equally recruited by both stimuli. This indicates that, while independent inputs tend to recruit different populations of PYR, they recruit the same population FS cells.