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

Abstract Dendrites of pyramidal cells perform complex amplification and integration (reviewed in 5. , 9. , 12. , 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 Ca 2+ occur in the apical shaft. 4. , 8. , 15. , 16. , 19. , 21. , 22. , 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 CA1 pyramidal cells, we examined the total number of GABAergic and non-GABAergic inputs converging onto the first 200 μm of the apical trunk. The number of spines associated with excitatory terminals increased from