SUBTHRESHOLD SYNAPTIC ACTIVATION OF VOLTAGE-GATED CA2+ CHANNELS MEDIATES A LOCALIZED CA2+ INFLUX INTO THE DENDRITES OF HIPPOCAMPAL PYRAMIDAL NEURONS

1. Whole cell recordings and high-speed fluorescence imaging were used to investigate the spatial and temporal characteristics of Ca2+ influ x during synaptic activity in hippocampal CA1 pyramidal neurons. Brief , subthreshold trains of synaptic potentials elicited by Schaffer coll ateral stimulation produced transient increases in [Ca2+](i) in the ap ical dendrites near the site of synaptic input. The rises in [Ca2+](i) were not due to Ca2+ entry through N-methyl-D-aspartate (NMDA)-activa ted or non-NMDA-activated glutamate channels, but were reduced by low concentrations of Ni2+. Hyperpolarizing prepulses caused an increase i n the synaptically evoked Ca2+ transients, whereas strong hyperpolariz ation during the train prevented the rise in [Ca2+](i). The data sugge st that subthreshold synaptic activity can open low-voltage-activated (T-type) Ca2+ channels and produce a local increase in intradendritic [Ca2+]. Such local increases in [Ca2+](i) may be important for modulat ing the strength of synaptic connections.