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Slow integration leads to persistent action potential firing in distal axons of coupled interneurons
BMC Neuroscience volume 12, Article number: O17 (2011)
The conventional view of neurons is that synaptic inputs are integrated on a timescale of milliseconds to seconds in the dendrites, with action potential initiation occurring in the axon initial segment. In a subset of rodent hippocampal and neocortical interneurons in acute slices prepared from serotonin 5b receptor (Htr5b) BAC transgenic mice , we found a much slower form of integration leading to action potential initiation in the distal axon. In approximately 80% of these interneurons (n=214 of 274), and in 23% of hippocampal interneurons in wild-type C57BL/6 mice (n=6 of 26), hundreds of spikes, evoked over a period of minutes, resulted in persistent firing that lasted for a similar duration.
Persistent firing was observed in response to step current injections, synaptic stimulation, sine wave current injections or in response to stimulation with natural spike trains . With all of these protocols, multiple stimuli were required to induce persistent firing. While axonal action potential firing was required to trigger persistent firing, somatic depolarization was not; antidromic stimulation of the axon while hyperpolarizing the soma with current injection produced persistent firing. In addition, phase plots of persistent firing revealed that spikes had two components: an initial component represented spiking in the axon and a second component that overlapped with the current-evoked spikes, indicative of a somato-dendritic spike following an initial, axonally initiated spike.
In some recordings (n = 11), partial spikes (spikelets) were observed during persistent firing. These spikelets overlapped the first component of the full-amplitude spikes, with the peak of the spikelets corresponding to an inflection on the rising phase seen in the full-amplitude spikes. These observations suggest that the first component of each action potential during persistent firing is an axonal spike, which sometimes fails to evoke a somato-dendritic spike. Furthermore, in some cells (n=3), spikelets were observed if the soma was hyperpolarized during persistent firing. These spikelets were smaller than those observed without hyperpolarization, suggesting that they are caused by propagation failures at a more distal axonal location.
Using a stylized computational model constructed with the NEURON simulation environment  of a branching axon attached to a soma, we simulated both small- and large amplitude spikelets, as well as full-amplitude spikes, by depolarizing a branch of the axon during somatic hyperpolarization. Large-amplitude spikelets corresponded to failure of the action potential to invade the soma, whereas small-amplitude spikelets corresponded to failures at the axon branches, 40 μm from the soma. Similar results were obtained with a full morphological model of a branching axonal arborization.
Additionally, in paired recordings, persistent firing was not restricted to the stimulated neuron; it could also be produced in the unstimulated cell (n=3). None of these pairs exhibited direct electrical coupling, and both glutamate and GABA receptors were blocked.
Consolidating these results suggests the existence of a previously unknown operational mode for some mammalian neurons. These interneurons can slowly integrate spiking, share the output across a coupled network of axons and respond with persistent firing even in the absence of input to the soma or dendrites.
Heintz N: Gene expression nervous system atlas (GENSAT). Nat. Neurosci. 2004, 7: 483-10.1038/nn0504-483.
Klausberger T, Marton LF, O’Neill J, Huck JH, Dalezios Y, Fuentealba P, Suen WY, Papp E, Kaneko T, Watanabe M: Complementary roles of cholecystokinin- and parvalbuminexpressing GABAergic neurons in hippocampal network oscillations. J. Neurosci. 2005, 25: 9782-9793. 10.1523/JNEUROSCI.3269-05.2005. unpublished data, cell T82e
Hines ML, Carnevale NT: The NEURON simulation environment. Neural Comput. 1997, 9: 1179-209. 10.1162/neco.1922.214.171.1249.
Grant support was provided by the US National Institutes of Health (NS-046064).
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Sheffield, M.E., Best, T.K., Mensh, B.D. et al. Slow integration leads to persistent action potential firing in distal axons of coupled interneurons. BMC Neurosci 12, O17 (2011). https://0-doi-org.brum.beds.ac.uk/10.1186/1471-2202-12-S1-O17
- Current Injection
- Axon Initial Segment
- Action Potential Firing
- Axonal Arborization
- Paired Recording