Although dendritic spikes are generally thought to be restricted to the distal apical dendrite, we know very little about the possible modulatory mechanisms that set the spatial limits of dendritic spikes. Our experiments demonstrated that high-frequency trains of backpropagating action potentials avoided filtering in the apical dendrite and initiated all-or-none dendritic Ca2+ transients associated with dendritic spikes in layer 5 pyramidal neurons of the prefrontal cortex. The block of hyperpolarization-activated currents (Ih) by ZD7288 could shift the frequency threshold and decreased the number of action potentials required to produce the all-or-none Ca2+ transient. Activation of α2-adrenergic receptors could also shift the frequency domain of spike induction to lower frequencies. Our data suggest that noradrenergic activity in the prefrontal cortex influences dendritic Ih and extends the zone of dendritic spikes in the apical dendrite via α2-adrenergic receptors. This mechanism might be one cellular correlate of the α2-receptor- mediated actions on working memory.
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