DIRECT MEASUREMENT OF COUPLING BETWEEN DENDRITIC SPINES AND SHAFTS

Characterization of the diffusional and electrotonic coupling of spine s to the dendritic shaft is crucial to understanding neuronal integrat ion and synaptic plasticity. Two-photon photobleaching and photoreleas e of fluorescein dextran were used to generate concentration gradients between spines and shafts in rat CAI pyramidal neurons. Diffusional r eequilibration was monitored with two-photon fluorescence imaging. The time course of reequilibration was exponential, with time constants i n the range of 20 to 100 milliseconds, demonstrating chemical compartm entalization on such time scales. These values imply that electrical s pine neck resistances are unlikely to exceed 150 megohms and more like ly range from 4 to 50 megohms.