EQUIVALENCE OF AMPLIFIED CURRENT FLOWING FROM DENDRITE TO SOMA MEASURED BY ALTERATION OF REPETITIVE FIRING AND BY VOLTAGE-CLAMP IN LAYER-5 PYRAMIDAL NEURONS

1. Plots of steady firing rate versus injected current (f-I relations) were constructed from intrasomatic injected current pulses applied al one (control relations) and together with dendritic glutamate iontopho resis (test relations) at sites on the distal apical dendrite 185-555 mu m from the soma in layer 5 pyramidal neurons from rat cortex studie d in a brain slice. The test f-I relations exhibited a parallel shift along the current axis, and the slopes of the control and test relatio ns differed by <10% in most neurons. This behavior indicates that cons tant injected current and steady glutaminergic dendritic input evoke e quivalent steady-state repetitive firing in a neuron with active dendr ites. The parallel shift of the f-I curves allowed us to compute the a mplitude of axial current arriving in the soma from the apical dendrit e during repetitive firing. 2. We compared the transmitted current com puted from the f-I curve shift with that measured by somatic voltage c lamp during the same iontophoresis. When measured during voltage clamp at different somatic membrane potentials, the transmitted current inc reased with somatic depolarization (was amplified) in most cells, an o bservation inconsistent with passive dendrites. This larger amplitude current closely predicted the transmitted current computed from the f- I curve shift, whereas the smaller transmitted current measured at res ting potential did not. A set of control experiments indicated that th ese different predictions were well within the measurement error assoc iated with computation of transmitted current based on f-I curve shift s. The action of blocking agents confirmed that the depolarizing ampli fication depended on tetrodotoxin (TTX)- and D-2-amino-5-phosphonopent oic acid (APV)-sensitive dendritic channels. 3. The agreement of two i ndependent measurements (somatic voltage clamp and f-I curve shift) of the axial current transmitted from dendrite to soma indicates that th e amplification of transmitted current observed in voltage clamp occur s physiologically. We discuss the usefulness of the effective current concept for determining synaptic weighting in network models of neuron s with active dendrites.