Mechanisms and consequences of action potential burst firing in raf neocortical pyramidal neurons

1. Electrophysiological recordings and pharmacological manipulations were u sed to investigate the mechanisms underlying the generation of action poten tial burst firing and its postsynaptic consequences in visually identified rat layer 5 pyramidal neurons in vitro. 2. Based upon repetitive firing properties and subthreshold membrane charac teristics, layer 5 pyramidal neurons were separated into three classes: reg ular firing and weak and strong intrinsically burst firing. 3. High frequency (330 +/- 10 Hz) action potential burst firing was abolish ed or greatly weakened by the removal of Ca2+ (n = 5) from, or by the addit ion of the Ca2+ channel antagonist Ni2+ (250-500 mu M; n = 8) to, the perfu sion medium. 4. The blockade of apical dendritic sodium channels by the local dendritic application of TTX (100 nM; n = 5) abolished or greatly weakened action pot ential burst firing, as did the local apical dendritic application of Ni2(1 mM; n = 5). 5. Apical dendritic depolarisation resulted in low frequency (157 +/- 26 Hz ; n = 6) action potential burst firing in regular firing neurons, as classi fied by somatic current injection. The intensity of action potential burst discharges in intrinsically burst firing neurons was facilitated bs dendrit ic depolarisation (n = 11). 6. Action potential amplitude decreased throughout a burst when recorded so matically, suggesting that later action potentials mag fail to propagate ax onally. Axonal recordings demonstrated that each action potential in a burs t is axonally initiated and that no decrement in action potential amplitude is apparent in the axon >30 mu m from the soma. 7. Paired recordings (n = 16) from synaptically coupled neurons indicated t hat each action potential in a burst could cause transmitter release. EPSPs or EPSCs evoked by a presynaptic burst of action potentials showed use-dep endent synaptic depression. 8. A postsynaptic, TTX-sensitive voltage-dependent amplification process en sured that later EPSPs in a burst were amplified when generated fm om membr ane potentials positive to -60 mV, providing a postsynaptic mechanism that counteracts use-dependent depression at synapses between layer 5 pyramidal neurons.