Action potentials reliably invade axonal arbors of rat neocortical neurons

Neocortical pyramidal neurons have extensive axonal arborizations that make thousands of synapses. Action potentials can invade these arbors and cause calcium influx that is required for neurotransmitter release and excitatio n of postsynaptic targets. Thus, the regulation of action potential invasio n in axonal branches might shape the spread of excitation in cortical neura l networks. To measure the reliability and extent of action potential invas ion into axonal arbors, we have used two-photon excitation laser scanning m icroscopy to directly image action-potential-mediated calcium influx in sin gle varicosities of layer 2/3 pyramidal neurons in acute brain slices. Our data show that single action potentials or bursts of action potentials reli ably invade axonal arbors over a range of developmental ages (postnatal 10- 24 days) and temperatures (24 degrees C-30 degrees C). Hyperpolarizing curr ent steps preceding action potential initiation, protocols that had previou sly been observed to produce failures of action potential propagation in cu ltured preparations, were ineffective in modulating the spread of action po tentials in acute slices. Our data shaw that action potentials reliably inv ade the axonal arbors of neocortical pyramidal neurons. Failures in synapti c transmission must therefore originate downstream of action potential inva sion. We also explored the function of modulators that inhibit presynaptic calcium influx. Consistent with previous studies, we find that adenosine re duces action-potential-mediated calcium influx in presynaptic terminals. Th is reduction was observed in all terminals tested, suggesting that some mod ulatory systems are expressed homogeneously in most terminals of the same n euron.