Activity regulates functional connectivity from the vomeronasal organ to the accessory olfactory bulb.

The mammalian accessory olfactory system is specialized for the detection of chemicals that identify kin and conspecifics. Vomeronasal sensory neurons (VSNs) residing in the vomeronasal organ project axons to the accessory olfactory bulb (AOB), where they form synapses with principal neurons known as mitral cells. The organization of this projection is quite precise and is believed to be essential for appropriate function of this system. However, how this precise connectivity is established is unknown. We show here that in mice the vomeronasal duct is open at birth, allowing external chemical stimuli access to sensory neurons, and that these sensory neurons are capable of releasing neurotransmitter to downstream neurons as early as the first postnatal day (P). Using major histocompatibility complex class I peptides to activate a selective subset of VSNs during the first few postnatal days of development, we show that increased activity results in exuberant VSN axonal projections and a delay in axonal coalescence into well defined glomeruli in the AOB. Finally, we show that mitral cell dendritic refinement occurs just after the coalescence of presynaptic axons. Such a mechanism may allow the formation of precise connectivity with specific glomeruli that receive input from sensory neurons expressing the same receptor type.

Recurrent dendrodendritic inhibition of accessory olfactory bulb mitral cells requires activation of group I metabotropic glutamate receptors.

Metabotropic glutamate receptors (mGluRs) modulate neural excitability and network tone in many brain regions. Expression of mGluRs is particularly high in the accessory olfactory bulb (AOB), a CNS structure critical for detecting chemicals that identify kin and conspecifics. Because of its relative simplicity and its direct projection to the hypothalamus, the AOB provides a model system for studying how mGluRs affect the flow of encoded sensory information to downstream areas. We investigated the role of group I mGluRs in synaptic processing in AOB slices and found that under control conditions, recurrent inhibition of principal neurons (mitral cells) was completely eliminated by the mGluR1 antagonist LY367385 [(S)-(+)-alpha-amino-4-carboxy-2 methylbenzeneacetic acid]. In addition, the group I mGluR agonist DHPG [(S)-3,5-dihydroxyphenylglycine; 20 microM] induced a dramatic increase in the rate of spontaneous IPSCs. This increase was dependent on voltage-gated calcium channels but persisted even after blockade of ionotropic glutamatergic transmission and sodium channels. Together, these results indicate that mGluR1 plays a critical role in controlling information flow through the AOB and suggest that mGluR1 may be an important locus for experience-dependent changes in synaptic function.

A simple method of in vitro electroporation allows visualization, recording, and calcium imaging of local neuronal circuits.

Since Cajal's early drawings, the characterization of neuronal architecture has been paramount in understanding neuronal function. With the development of electrophysiological techniques that provide unprecedented access to the physiology of these cells, experimental questions of neuronal function have also become more tractable. Fluorescent tracers that can label the anatomy of individual or populations of neurons have opened the door to linking anatomy with physiology. Experimentally however, current techniques for bulk labeling of cells in vitro often affect neuronal function creating a barrier for exploring structure-function questions. Here we describe a new technique for highly localized electroporation within a cell or cell population that enables the introduction of membrane impermeable charged dyes including dextran-conjugated fluorophores, hydrazide tracers, and calcium indicator dyes in vitro. We demonstrate that this technique is highly versatile, allowing for labeling of large or small areas of tissue, allowing for the investigation of both cellular morphology and physiological activity in identified neuronal circuits in acute brain slices. Furthermore, this approach allows subsequent targeted whole-cell patch recording based on well-defined connectivity as well as assessment of physiological activity in targeted circuits on a fast time scale.