The apical shaft of CA1 pyramidal cells is under GABAergic interneuronal control

Dendrites of pyramidal cells perform complex amplification and integration (review ed in Refs 5, 9, 12 and 20), The presence of a large proximal apical dendrite has been shown to have functional implications for neuronal firing patterns(13) and under a variety of experimental conditions, the largest increases in intracellular Ca2+ occur in the apical shaft(4,8,15,16,19,21-23) An important step in understanding the functional role of the proximal apical dendrite is to describe the nature of synaptic input to this dendritic region. Using light and electron microscopic methods combined with in vivo labeling of rat hippocampal CAI pyramidal cells, we examined the total number of GABAergic and non-GABAergic inputs converging onto the first 200 mum of the apical trunk. The number of spines associated with excitatory terminals increased from <0.2 spines/m adjacent to the soma to 5.5 spines/mum at 200 pm from the soma, whereas the number of GABAergic, symmetric terminals decreased from 0.8/mum to 0.08/mum over the same anatomical region. GABAergic terminals were either parvalbumin-, cholecystokinin- or vasointestinal peptide-immunoreactive, These findings indicate that the apical dendritic trunk mainly receives synaptic input from GABAergic interneurons. GABAergic inhibition during network oscillation may serve to periodically isolate the dendritic compartments from the perisomatic action potential generating sites. (C) 2001 IBRO, Published by Elsevier Science Ltd. All rights reserved.