Fig. 4.

Coincident EPSP-bAP trains evoked larger dendritic Ca²⁺ signals than bAPs alone, but only in BDNF-treated neurons. (A) The ratio of dendritic Ca²⁺ signals evoked by coincident EPSP-bAP to those evoked by bAPs alone is larger than 1 in BDNF-treated neurons. This ratio was used to estimate the contribution of synaptically originated Ca²⁺- presumably from individual spines - to Ca²⁺ signals in dendrites. Ten out of thirteen cells treated with BDNF exhibited an EPSP-bAP/bAP ratio larger than 1, whereas all 10 serum-free cells showed a less-than-1 ratio. Co-application of the receptor tyrosine kinase inhibitor k-252a prevented the effect of BDNF, without an effect of its own. (B) The ratio of the dendritic Ca²⁺ signal evoked by coincident EPSP-bAP to that evoked by bAPs alone before and after application of D, L-APV. NMDAR blockade equalized the ratio in BDNF-treated cells, without having an effect in control neurons, or in neurons exposed to k-252a alone or in combination with BDNF. These results suggest that the activation of NMDARs during paired EPSP-bAP contributes to dendritic Ca²⁺ signals only in BDNF-treated neurons.