title: Back-propagating action potentials mediate calcium signalling in dendrites of bitufted interneurons in layer 2/3 of rat somatosensory cortex.
abstract: 1. Bitufted interneurons in layer 2/3 of the rat (P14) somatosensory cortex have elongated apical and basal dendritic arbors that can span the entire depth of the cortex. Simultaneous dendritic and somatic whole-cell voltage recordings combined with Ca2+ fluorescence measurements were made to quantify voltage and Ca2+ signalling in dendritic arbors of bitufted neurons. 2. Action potentials (APs) initiated close to the soma by brief current injection back-propagated into the apical and basal dendritic arbors and evoked a transient increase in volume-averaged dendritic Ca2+ concentration (Delta[Ca(2+)](i)) of about 140 nM peak amplitude per AP. The AP evoked Ca2+ signal decayed with a time constant of about 200 ms. 3. A relatively high endogenous Ca(2+) binding ratio of approximately 285 determines the comparatively small rise in [Ca(2+)](i) of bitufted cell dendrites evoked by a back-propagating AP. 4. The [Ca(2+)](i) transient evoked by back-propagating dendritic APs decreased with distance (< or = 50 microm) from the soma in some neurons. At distances greater than 50 microm transients did not show a spatial gradient between the proximal and distal dendritic branches. 5. During trains of APs the mean amplitude of the steady-state increase in dendritic [Ca(2+)](i) encoded the AP frequency linearly up to 40 Hz with a slope of 20 nM Hz(-1). 6. The results suggest that APs initiated in the axon of bitufted neurons back-propagate and 'copy' the pattern of the axon's electrical activity also to the dendritic arbor. The AP pattern is transduced into a transient rise of dendritic [Ca(2+)](i) which, presumably, can regulate the receptive properties of the dendritic arbor for synaptic input. 7. Bitufted interneurons in layer 2/3 of the rat (P14) somatosensory cortex have elongated apical and basal dendritic arbors that can span the entire depth of the cortex. Simultaneous dendritic and somatic whole-cell voltage recordings combined with Ca2+ fluorescence measurements were made to quantify voltage and Ca2+ signalling in dendritic arbors of bitufted neurons. 8. Action potentials (APs) initiated close to the soma by brief current injection back-propagated into the apical and basal dendritic arbors and evoked a transient increase in volume-averaged dendritic Ca2+ concentration (Delta[Ca(2+)](i)) of about 140 nM peak amplitude per AP. The AP evoked Ca2+ signal decayed with a time constant of about 200 ms. 9. A relatively high endogenous Ca2+ binding ratio of approximately 285 determines the comparatively small rise in [Ca(2+)](i) of bitufted cell dendrites evoked by a back-propagating AP. 10. The [Ca(2+)](i) transient evoked by back-propagating dendritic APs decreased with distance (< or = 50 microm) from the soma in some neurons. At distances greater than 50 microm transients did not show a spatial gradient between the proximal and distal dendritic branches. 11. During trains of APs the mean amplitude of the steady-state increase in dendritic [Ca(2+)](i) encoded the AP frequency linearly up to 40 Hz with a slope of 20 nM Hz(-1). 12. The results suggest that APs initiated in the axon of bitufted neurons back-propagate and also 'copy' the pattern of the axon's electrical activity to the dendritic arbor. The AP pattern is transduced into a transient rise of dendritic [Ca(2+)](i) which, presumably, can regulate the receptive properties of the dendritic arbor for synaptic input.
authors.affiliation: Abteilung Zellphysiologie, Max-Planck-Institut für medizinische Forschung, Jahnstrasse 29, D-69120 Heidelberg, Germany. kaiser@mpimf-heidelberg.mpg.de
authors.forename: K M
authors.initials: KM
authors.lastname: Kaiser
authors.forename: Y
authors.initials: Y
authors.lastname: Zilberter
authors.forename: B
authors.initials: B
authors.lastname: Sakmann
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001369:Axons / Q000648:ultrastructure; D002021:Buffers; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D003712:Dendrites / Q000502:physiology* / Q000648:ultrastructure; D066298:In Vitro Techniques; D007395:Interneurons / Q000502:physiology* / Q000648:ultrastructure; D009994:Osmolar Concentration; D051381:Rats; D017208:Rats, Wistar; D026941:Sodium Channel Blockers; D013003:Somatosensory Cortex / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D013779:Tetrodotoxin / Q000494:pharmacology; D014018:Tissue Distribution
chemical_list: D002021:Buffers; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin; D002118:Calcium