Bidirectional flow of action potentials in axons drives activity dynamics in neuronal cultures

Objective. Recent technological advances are revealing the complex physiology of the axon and challenging long-standing assumptions. Namely, while most action potential (AP) initiation occurs at the axon initial segment in central nervous system neurons, initiation in distal parts of the axon has been reported to occur in both physiological and pathological conditions. The functional role of these ectopic APs, if exists, is still not clear, nor its impact on network activity dynamics.Approach. Using an electrophysiology platform specifically designed for assessing axonal conduction we show here for the first time regular and effective bidirectional axonal conduction in hippocampal and dorsal root ganglia cultures. We investigate and characterize this bidirectional propagation both in physiological conditions and after distal axotomy.Main results.A significant fraction of APs are not coming from the canonical synapse-dendrite-soma signal flow, but instead from signals originating at the distal axon. Importantly, antidromic APs may carry information and can have a functional impact on the neuron, as they consistently depolarize the soma. Thus, plasticity or gene transduction mechanisms triggered by soma depolarization can also be affected by these antidromic APs. Conduction velocity is asymmetrical, with antidromic conduction being slower than orthodromic.Significance.Altogether these findings have important implications for the study of neuronal functionin vitro, reshaping our understanding on how information flows in neuronal cultures.

Synaptic entrainment of ectopic action potential generation in hippocampal pyramidal neurons

KEY POINTS

Ectopic action potentials (EAPs) arise at distal locations in axonal fibres and are often associated with neuronal pathologies such as epilepsy or nerve injury, but they also occur during physiological network conditions. This study investigates whether initiation of such EAPs is modulated by subthreshold synaptic activity. Somatic subthreshold potentials invade the axonal compartment to considerable distances (>350 μm), whereas spread of axonal subthreshold potentials to the soma is inefficient. Ectopic spike generation is entrained by conventional synaptic signalling mechanisms. Excitatory synaptic potentials promote EAPs, whereas inhibitory synaptic potentials block EAPs. The modulation of ectopic excitability depends on propagation of somatic voltage deflections to the axonal EAP initiation site. Synaptic modulation of EAP initiation challenges the view of the distal axon being independent of synaptic activity and may contribute to mechanisms underlying fast network oscillations and pathological network activity.

ABSTRACT

While most action potentials are generated at the axon initial segment, they can also be triggered at more distal sites along the axon. Such ectopic action potentials (EAPs) occur during several neuronal pathologies such as epilepsy, nerve injuries and inflammation but have also been observed during physiological network activity. EAPs propagate antidromically towards the somato-dendritic compartment where they modulate synaptic plasticity. Here we investigate the converse signal direction: do somato-dendritic synaptic potentials affect the generation of ectopic spikes? We measured anti- and orthodromic spikes in the soma and axon of mouse hippocampal CA1 pyramidal cells. We found that synaptic potentials propagate reliably through the axon, causing significant voltage transients at distances >350 μm. At these sites, excitatory input efficiently facilitated EAP initiation in distal axons and, conversely, inhibitory input suppressed EAP initiation. Our data reveal a new mechanism by which ectopically generated spikes can be entrained by conventional synaptic signalling during normal and pathological network activity.

Persistent sodium current modulates axonal excitability in CA1 pyramidal neurons

Axonal excitability is an important determinant for the accuracy, direction, and velocity of neuronal signaling. The mechanisms underlying spike generation in the axonal initial segment and transmitter release from presynaptic terminals have been intensely studied and revealed a role for several specific ionic conductances, including the persistent sodium current (I_NaP ). Recent evidence indicates that action potentials can also be generated at remote locations along the axonal fiber, giving rise to ectopic action potentials during physiological states (e.g., fast network oscillations) or in pathological situations (e.g., following demyelination). Here, we investigated how ectopic axonal excitability of mouse hippocampal CA1 pyramidal neurons is regulated by I_NaP . Recordings of field potentials and intracellular voltage in brain slices revealed that electrically evoked antidromic spikes were readily suppressed by two different blockers of I_NaP , riluzole and phenytoin. The effect was mediated by a reduction of the probability of ectopic spike generation while latency was unaffected. Interestingly, the contribution of I_NaP to excitability was much more pronounced in axonal branches heading toward the entorhinal cortex compared with the opposite fiber direction toward fimbria. Thus, excitability of distal CA1 pyramidal cell axons is affected by persistent sodium currents in a direction-selective manner. This mechanism may be of importance for ectopic spike generation in oscillating network states as well as in pathological situations.