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 apica l dendrite has been shown to have functional implications for neuronal firi ng 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 dendrit ic region. Using light and electron microscopic methods combined with in vi vo labeling of rat hippocampal CAI pyramidal cells, we examined the total n umber of GABAergic and non-GABAergic inputs converging onto the first 200 m um of the apical trunk. The number of spines associated with excitatory ter minals increased from <0.2 spines/<mu>m adjacent to the soma to 5.5 spines/ mum at 200 pm from the soma, whereas the number of GABAergic, symmetric ter minals decreased from 0.8/mum to 0.08/mum over the same anatomical region. GABAergic terminals were either parvalbumin-, cholecystokinin- or vasointes tinal peptide-immunoreactive, These findings indicate that the apical dendr itic trunk mainly receives synaptic input from GABAergic interneurons. GABA ergic inhibition during network oscillation may serve to periodically isola te the dendritic compartments from the perisomatic action potential generat ing sites. (C) 2001 IBRO, Published by Elsevier Science Ltd. All rights res erved.