No abstract

The ventral midline of the embryonic neural tube, the floor plate, has a profound role in guiding axons during embryonic development. Floor plate-derived guidance cues attract or repel axons, depending on the neuronal subtype and developmental stage. Netrin-1 and its receptor, Deleted in Colon Carcinoma (DCC), are the key constituents of commissurral axons guidance cues toward the floor plate. Recent studies have implicated Down Syndrome Cell Adhesion Molecule (Dscam) as an additional Netrin-1 receptor. In this study, we examined the role of Dscam in guiding defined spinal dorsal interneuron populations. In vivo knockdown and ectopic expression of Dscam were performed in the dorsal dI1, dI2 and dI3 interneurons of chick embryos, by separately increasing or decreasing Dscam expression in each of these three specific interneuronal populations. Neuron-specific gain and loss of function of Dscam had no effect on the axonal trajectories of dI1-3 neurons. The commissural neurons, dI1c and dI2, crossed the midline, and the ipsilaterally projecting neurons, dI1i and dI3, projected ipsilaterally. However, the fasciculation of dI1 axons was diminished when Dscam expression was attenuated. Dscam is not required for either attraction to or repulsion from the floor plate. In contrast, Dscam is required for the fasciculation of axons, probably via homophilic interaction.

The vomeronasal system (VNS) is dedicated to processing ethologically relevant chemosensory cues from other organisms. Its first central processing stage is the accessory olfactory bulb (AOB), which sends information to downstream brain regions via spiking activity of its principal neurons, AOB mitral cells (AOB-MCs). Recent studies provided important insights about the functional properties of AOB-MCs and the stimulus aspects that they encode. However, very little is known about the principles that govern their coordinated activity and the manner by which AOB outputs are relayed to downstream regions. In many brain regions, network oscillations play a key role in orchestrating the activity of output neurons, and thereby facilitate and enhance transmission of information. A widely used readout of network processes and oscillations is the local field potential (LFP), which has been extensively studied in the main olfactory system (MOS) and, specifically, in the main olfactory bulb (MOB). To reveal population-level processes in the AOB, we analyzed LFP signals while presenting natural stimuli to anesthetized adult male and female mice. Our recordings reveal prominent LFP theta band oscillatory episodes with a characteristic spatial pattern across the AOB. Using a template matching approach, we find that network dynamics show varying degrees of similarity to this pattern throughout an experiment. Notably, both the similarity to this pattern, and the intensity of the pattern, are modulated by stimulus features. Analysis of LFP signal polarity and its spatial relationship to single unit activity, indicate that oscillatory episodes are generated locally within the AOB, likely representing a reciprocal interaction between AOB-MCs and granule cells (GCs). Co-examination of single unit and LFP activity suggests that oscillations appear when sensory activation surpasses a certain threshold. Importantly, spike times of many AOB-MCs are constrained to the negative LFP oscillation phase, in a manner that can drastically effect integration by downstream processing stages. Based on these observations, we propose that these LFP oscillations can gate, bind, and potentially also multiplex outgoing signals from individual AOB neurons to downstream processing stages.