. Thin, unmyelinated axons densely populate the mammalian hippocampus and cortex. However, the location and dynamics of spike initiation in thin axons remain unclear. We investigated basic properties of spike initiation and propagation in CA3 neurons of juvenile rat hippocampus. Sodium channel alpha subunit distribution and local applications of tetrodotoxin demonstrate that the site of first threshold crossing in CA3 neurons is ϳ35 m distal to the soma, somewhat more proximal than our previous estimates. This discrepancy can be explained by the finding, obtained with simultaneous whole cell somatic and extracellular axonal recordings, that a zone of axon stretching to ϳ100 m distal to the soma reaches a maximum rate of depolarization nearly synchronously by the influx of sodium from the high-density channels. Models of the proximal axon incorporating observed distributions of sodium channel staining recapitulated salient features of somatic and axonal spike waveforms, including the predicted initiation zone, characteristic spike latencies, and conduction velocity. The preferred initiation zone was unaltered by stimulus strength or repetitive spiking, but repetitive spiking increased threshold and significantly slowed initial segment recruitment time and conduction velocity. Our work defines the dynamics of initiation and propagation in hippocampal principal cell axons and may help reconcile recent controversies over initiation site in other axons. I N T R O D U C T I O NAction potential initiation and downstream axonal propagation have been topics of intense scrutiny over several decades and are critical to nervous system information processing (Debanne 2004). However, much of what we know about spike initiation and propagation comes from studies of larger, myelinated axons. Thin, unmyelinated fibers are found throughout the hippocampus and cortex and serve the important task of local information processing. Nevertheless, critical details pertaining to spike initiation in these fibers and their potential to modulate downstream spike propagation are poorly understood because axons remain the most experimentally intractable neuronal elements.We studied spike initiation and propagation in unmyelinated fibers of CA3 pyramidal neurons, which give rise to intensively studied synapses of the hippocampal Schaffer collateral pathway. Several critical features of spike initiation are still controversial even in myelinated fibers. For instance, in rodent cerebellar Purkinje cells, some data suggest distal initiation at the first node of Ranvier, which typically coincides with the first axon branch point (Clark et al. 2005). Other studies suggest somatic/axon hillock initiation (Khaliq and Raman correlate with a high density of sodium channels, which modeling predicts is necessary for axonal initiation (Dodge and Cooley 1973;Mainen et al. 1995;Rapp et al. 1996).These issues are further compounded in small axons such as those of CA3 pyramidal neurons, which are unmyelinated over large portions, including the first several ...

In mice, nonvolatile social cues are detected and analyzed by the accessory olfactory system (AOS). Here we provide a first view of information processing in the AOS with respect to individual chemical cues. 12 sulfated steroids, recently-discovered mouse AOS ligands, caused widespread activity among vomeronasal sensory neurons (VSNs), yet VSN responses clustered into a small number of repeated functional patterns or processing streams. Downstream neurons in the accessory olfactory bulb (AOB) responded to these ligands with enhanced signal/noise compared to VSNs. Whereas the dendritic connectivity of AOB mitral cells suggests the capacity for broad integration, most sulfated steroid responses were well-modeled by linear excitatory drive from just one VSN processing stream. However, a significant minority demonstrated multi-stream integration. Most VSN excitation patterns were also observed in the AOB, but excitation by estradiol sulfate processing streams was rare, suggesting AOB circuit organization is specific to the biological relevance of sensed cues.

To examine persisting effects of depolarizing rises in extracellular potassium concentration ([K+](o)) on synapses, we depolarized cells to simulate ischemia-like rises in [K+](o). Elevated [K+](o) for 1-16 hr severely depressed glutamate signaling, while mildly depressing GABA transmission. The glutamate-specific changes were plastic over several hours and involved a decrease in the size of the pool of releasable vesicles. Rather than a reduction of the number of vesicles per release site, the change involved functional elimination of release sites. This change was clearly dissociable from a second effect, depressed probability of transmitter release, which was common to both glutamate and GABA transmission. Thus, while other recent evidence links alteration of the releasable pool size with changes in p(r), our results suggest the two can be independently manipulated. Selective depression of glutamate release may provide an adaptive mechanism by which neurons limit excitotoxicity.

• C, spiking superimposed on sustained somatic depolarization, but not spiking alone, produced similar axonal changes as the epileptiform activity. These results highlight the likely importance of steady-state inactivation of axonal channels in maintaining action potential fidelity. Such changes in axonal propagation properties could encode information and/or serve as an endogenous brake on seizure propagation.

We present cleared tissue Axially Swept Light-Sheet Microscopy (ctASLM), which enables isotropic, subcellular resolution, high optical sectioning capability, and large field of view imaging over a broad range of immersion media. ctASLM can image live, expanded, and both aqueous and organic chemically cleared tissue preparations. Depending on the optical configuration, ctASLM provides up to 260 nm axial resolution, an improvement over confocal and other reported cleared tissue light-sheet microscopes by a factor 3-10. We image millimeter-scale tissues with subcellular 3D resolution, which enabled us to automatically detect with computer vision multicellular tissue architectures, individual cells, synaptic spines, and rare cell-cell interactions.Human tissues are composed of multiple polarized cell types organized in well-defined three-dimensional architectures. Interestingly, it has been shown that rare subsets of cells play a crucial role in disease progression, 1 and interdisciplinary efforts now aim to generate comprehensive atlases of human cells in diverse tissue types. To date, this has largely relied on massively parallel sequencing and machine learning-based analyses to identify unique sub-populations of cells. Combined with advanced imaging, such efforts could not only shed light on the diversity of cell types, but the biological context in which each population operates. However, imaging large tissues with subcellular resolution remains challenging due to the heterogeneous refractive index and composition of tissues, which results in complex aberrations and an increased scattering coefficient, both of which decrease spatial resolution and limit imaging depth. 2