Mechanisms underlying burst and regular spiking evoked by dendritic depolarization in layer 5 cortical pyramidal neurons

Apical dendrites of layer 5 pyramidal cells in a slice preparation of rat s ensorimotor cortex were depolarized focally by long-lasting glutamate ionto phoresis while recording intracellularly from their soma; In most cells the firing pattern evoked by the smallest dendritic depolarization that evoked spikes consisted of repetitive bursts of action potentials. During larger dendritic depolarizations initial burst firing was followed by regular spik ing. As dendritic depolarization was increased further the duration (but no t the firing rate) of the regular spiking increased, and the duration of bu rst firing decreased. Depolarization of the soma in most of the same cells evoked only regular spiking. When the dendrite was depolarized to a critica l level below spike threshold, intrasomatic current pulses or excitatory po stsynaptic potentials also triggered bursts instead of single spikes. The b ursts were driven by a delayed depolarization (DD) that was triggered in an all-or-none manner along with the first Na+ spike of the burst. Somatic vo ltage-clamp experiments indicated that the action current underlying the DD was generated in the dendrite and was Ca2+ dependent. Thus the burst firin g was caused by a Na+ spike-linked dendritic Ca2+ spike, a mechanism that w as available only when the dendrite was adequately depolarized. Larger dend ritic depolarization that evoked late, constant-frequency regular spiking a lso evoked a long-lasting, Ca2+-dependent action potential (a "plateau"). T he duration of the plateau but not its amplitude was increased by stronger dendritic depolarization. Burst-generating dendritic Ca2+ spikes could not be elicited during this plateau. Thus plateau initiation was responsible fo r the termination of burst firing and the generation of the constant-freque ncy regular spiking. We conclude that somatic and dendritic depolarization can elicit quite different firing patterns in the same pyramidal neuron. Th e burst and regular spiking observed during dendritic depolarization are ca used by two types of Ca2+-dependent dendritic action potentials. We discuss some functional implications of these observations.