DENDRITIC CALCIUM TRANSIENTS EVOKED BY SINGLE BACK-PROPAGATING ACTION-POTENTIALS IN RAT NEOCORTICAL PYRAMIDAL NEURONS

1. Dendrites of rat neocortical layer V pyramidal neurons were loaded with the Ca2+ indicator dye Calcium Green-1 (CG-1) or fluo-3, and the mechanisms which govern action potential (AP)-evoked transient changes in dendritic cytosolic Ca2+ concentration ([Ca2+](i)) were examined. APs were initiated either by synaptic stimulation or by depolarizing t he soma or dendrite by current injection, and changes in fluorescence of the indicator dye were measured in the proximal 170 mu m of the api cal dendrite. 2. Simultaneous two-pipette recordings of APs from the s oma and apical dendrite, and dendritic fluorescence imaging indicated that a single AP propagating from the soma into the apical dendrite ev okes a rapid transient increase in fluorescence indicating a transient increase in [Ca2+](i). At 35-37 degrees C the decay time constant of the fluorescence transient following an AP was around 80 ms. 3. Voltag e-activated Ca2+ channels (VACCs) of several subtypes mediated the AP- evoked fluorescence transient in the proximal (100-170 mu m) apical de ndrite. The AP-evoked fluorescence transient resulted from Ca2+ entry through L-type (nifedipine sensitive; 25%), N-type (omega-conotoxin GV IA sensitive; 28%) and P-type (omega-agatoxin IVA sensitive; 10%) Ca2 channels and through Ca2+ channels (R-type) not sensitive to L-, N- a nd P-type Ca2+ channel blockers (cadmium ion sensitive; 37%). 4. The d ecay time course of the dendritic fluorescence transient was prolonged by the blockers of endoplasmic reticulum (ER) Ca2+-ATPase, cyclopiazo nic add and thapsigargin, suggesting that uptake of Ca2+ into the ER i n dendrites governs clearance of dendritic Ca2+. 5. The decay time cou rse of the fluorescence transient was slightly prolonged by benzamil, a blocker of plasma membrane Na+-Ca2+ exchange and by calmidazolium, a blocker of the calmodulin-dependent plasma membrane Ca2+-ATPase, sugg esting that these pathways are less important for dendritic Ca2+ clear ance following a single AP. Neither the mitochondrial uncoupler carbon yl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) nor the blocker of Ca2+ uptake into mitochondria, Ruthenium Red, had any measurable ef fect on the decay time course of the fluorescence transient. 6. Dendri tic fluorescence transients measured during trains of dendritic APs be gan to summate at impulse frequencies of 5 APs s(-1). At higher freque ncies APs caused a concerted and maintained elevation of dendritic flu orescence during the train. 7. It is suggested that dendritic [Ca2+](i ) transients evoked by back-propagating APs represent a retrograde sig nal sent to dendrites which may alter both the receptive and the integ rative properties of the dendritic compartment depending on the impuls e activity of the neuron.