Control and modulation of action potential initiation in principal neurons of the medial superior olive
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The axon initial segment (AIS) serves as the site of action potential initiation in most neurons, but the technical difficulty in isolating the effects of voltage-gated ion channels in the AIS from those of the soma and dendrites has hampered understanding how AIS properties influence neural coding. Here we have combined confocal microscopy, patch-clamp recordings and light-sensitive channel blockers (“photoswitches”) in binaural auditory neurons to show that hyperpolarization and cyclic nucleotide-gated (HCN) channels are expressed in the AIS and decrease spike probability, distinct from the role of HCN channels in the soma and dendrites. We show further that control of spike threshold by HCN channels in the AIS can be altered through serotonin modulation of 5-HT₁ [subscript A] receptors, which in turn hyperpolarizes the activation range of HCN channels. As serotonin release signals differences in attention states and motivation in broad regions in the brain, the present results reveal a role for axonal HCN channels as a mechanism to translate these signals into changes in the threshold for sensory stimuli.