Distribution of spontaneous currents along the somato-dendritic axis of rat hippocampal CA1 pyramidal neurons

Excitatory and inhibitory pathways have specific patterns of innervation al ong the somato-dendritic axis of neurons. We have investigated whether this morphological diversity was associated with variations in the frequencies of spontaneous and miniature GABAergic and glutamatergic synaptic currents along the somato-dendritic axis of rat hippocampal CA1 pyramidal neurons. U sing in vitro whole cell recordings from somata, apical dendrites and basal dendrites (for which we provide the first recordings) of CA1 pyramidal neu rons, we report that over 90% of the spontaneous currents were GABAergic, < 10% being glutamatergic. The frequency of spontaneous GABAergic currents wa s comparable in the soma and in the dendrites. In both somata and dendrites . the Na+ channel blocker tetrodotoxin abolished more than 80% of the spont aneous glutamatergic currents. In contrast, tetrodotoxin abolished most den dritic (>90%) but not somatic (<40%) spontaneous GABAergic currents. Comput er simulations suggest that in our experimental conditions, events below 40 pA are electrotonically filtered to such a degree that they are lost in th e recording noise. We conclude that, in vitro, inhibition is massively pred ominant over excitation and quantitatively evenly distributed throughout th e cell. However, inhibition appears to be mainly activity-dependent in the dendrites whereas it can occur in the absence of interneuron firing in the soma. These results can be used as a benchmark to compare values obtained i n pathological tissue, such as epilepsies, where changes in the balance bet ween excitation and inhibition would dramatically alter cell behaviour, (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.