Dr. Nace Golding published an article in Nature Neuroscience entitled "Serotonin modulates spike probability in the axon initial segment through HCN".
In most neurons the Axon Initial Segment (AIS) represents the final stage of synaptic integration, where the complex interplay between excitatory and inhibitory synaptic signals and voltage-gated ion channels determines whether the neuron generates an all-or-none action potential and successfully forwards its signal to its network targets. Action potential initiation sometimes has been regarded as static, exhibiting a discrete and constant threshold. In the report the Golding lab shows that hyperpolarization and cyclic-nucleotide-gated (HCN) channels are expressed in the AIS of auditory neurons in the sound localization circuitry and, by their proximity to the voltage-gated sodium and potassium channels mediating action potentials, regulate spike threshold through their influence on the local resting potential. In addition, this research reveals that modulation of Ih by 5-HT can modify spike threshold through activation of 5-HT1A receptors. Thus HCN channels in the AIS are in a position to adjust spike sensitivity in response to changes in motivation or state of attention.
Nat Neurosci. 2016 Jun;19(6):826-34. doi: 10.1038/nn.4293. Epub 2016 Apr 25.