Action potential-induced dendritic calcium dynamics correlated with synaptic plasticity in developing hippocampal pyramidal cells

In hippocampal CA1 pyramidal cells, intracellular calcium increases are req uired for induction of long-term potentiation (LTP), an activity-dependent synaptic plasticity. LTP is known to develop in magnitude during the second and third postnatal weeks in the rats. Little is known, however, about dev elopment of intracellular calcium dynamics during the same postnatal weeks. We investigated postnatal development of intracellular calcium dynamics in the proximal apical dendrites of CA1 pyramidal cells by whole cell patch-c lamp recordings and calcium imaging with the Ca2+ indicator fura-2. Dendrit ic calcium increases induced by intrasomatically evoked action potentials w ere slight during the first postnatal week but gradually became robust 3 to B-fold during the second and third postnatal weeks. These calcium increase s were blocked by application of 250 mu M CdCl2, a nonspecific blocker for high-threshold voltage-dependent calcium channels (VDCCs). Under the voltag e-clamp condition, both calcium currents and dendritic calcium accumulation s induced by depolarization were larger at the late developmental stage (P1 5-18) than the early stage (P4-7), indicating developmental enhancement of calcium influx mediated by high-threshold VDCCs. Moreover, theta-burst stim ulation (TBS), a protocol for LTP induction, induced large intracellular ca lcium increases at the late developmental stage, in synchrony with maturati on of TBS-induced LTP. These results suggest that developmental enhancement of intracellular calcium increases induced by action potentials may underl ie maturation of calcium-dependent functions such as synaptic plasticity in hippocampal neurons.