Compartmental organization of the olfactory bulb glomerulus

Keratan sulfate proteoglycan: putative template for neuroblast migratory and axonal fascicular pathways and fetal expression in globus pallidus, thalamus, and olfactory bulb

Keratan sulfate (KS) is a proteoglycan secreted in the fetal brain astrocytes and radial glia into extracellular parenchyma as granulofilamentous deposits. KS surrounds neurons except dendritic spines, repelling glutamatergic and facilitating GABAergic axons. The same genes are expressed in both neuroblast migration and axonal growth. This study examines timing of KS during morphogenesis of some normally developing human fetal forebrain structures. Twenty normal human fetal brains from 9-41 weeks gestational age were studied at autopsy. KS was examined by immunoreactivity in formalin-fixed paraffin sections, plus other markers including synaptophysin, S-100β protein, vimentin and nestin. Radial and tangential neuroblast migratory pathways from subventricular zone to cortical plate were marked by KS deposits as early as 9wk GA, shortly after neuroblast migration initiated. During later gestation this reactivity gradually diminished and disappeared by term. Long axonal fascicles of the internal capsule and short fascicles of intrinsic bundles of globus pallidus and corpus striatum also appeared as early as 9-12wk, as fascicular sleeves before axons even entered. Intense KS occurs in astrocytic cytoplasm and extracellular parenchyma at 9wk in globus pallidus, 15wk thalamus, 18wk corpus striatum, 22wk cortical plate, and hippocampus postnatally. Corpus callosum and anterior commissure do not exhibit KS at any age. Optic chiasm shows reactivity at the periphery but not around intrinsic subfasciculi. We postulate that KS forms a chemical template for many long and short axonal fascicles before axons enter and neuroblast migratory pathways at initiation of migration. Cross-immunoreactivity with aggrecan may render difficult molecular distinction.

α-Synuclein Pathology and Reduced Neurogenesis in the Olfactory System Affect Olfaction in a Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by multiple symptoms including olfactory dysfunction, whose underlying mechanisms remain unclear. Here, we explored pathologic changes in the olfactory pathway of transgenic (Tg) mice of both sexes expressing the human A30P mutant α-synuclein (α-syn; α-syn-Tg mice) at 6-7 and 12-14 months of age, representing early and late-stages of motor progression, respectively. α-Syn-Tg mice at late stages exhibited olfactory behavioral deficits, which correlated with severe α-syn pathology in projection neurons (PNs) of the olfactory pathway. In parallel, olfactory bulb (OB) neurogenesis in α-syn-Tg mice was reduced in the OB granule cells at six to seven months and OB periglomerular cells at 12-14 months, respectively, both of which could contribute to olfactory dysfunction. Proteomic analyses showed a disruption in endocytic and exocytic pathways in the OB during the early stages which appeared exacerbated at the synaptic terminals when the mice developed olfactory deficits at 12-14 months. Our data suggest that (1) the α-syn-Tg mice recapitulate the olfactory functional deficits seen in PD; (2) olfactory structures exhibit spatiotemporal disparities for vulnerability to α-syn pathology; (3) α-syn pathology is restricted to projection neurons in the olfactory pathway; (4) neurogenesis in adult α-syn-Tg mice is reduced in the OB; and (5) synaptic endocytosis and exocytosis defects in the OB may further explain olfactory deficits.SIGNIFICANCE STATEMENT Olfactory dysfunction is a characteristic symptom of Parkinson's disease (PD). Using the human A30P mutant α-synuclein (α-syn)-expressing mouse model, we demonstrated the appearance of olfactory deficits at late stages of the disease, which was accompanied by the accumulation of α-syn pathology in projection neurons (PNs) of the olfactory system. This dysfunction included a reduction in olfactory bulb (OB) neurogenesis as well as changes in synaptic vesicular transport affecting synaptic function, both of which are likely contributing to olfactory behavioral deficits.

The functional relevance of olfactory marker protein in the vertebrate olfactory system: a never-ending story

Olfactory marker protein (OMP) was first described as a protein expressed in olfactory receptor neurons (ORNs) in the nasal cavity. In particular, OMP, a small cytoplasmic protein, marks mature ORNs and is also expressed in the neurons of other nasal chemosensory systems: the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. While its expression pattern was more easily established, OMP's function remained relatively vague. To date, most of the work to understand OMP's role has been done using mice lacking OMP. This mostly phenomenological work has shown that OMP is involved in sharpening the odorant response profile and in quickening odorant response kinetics of ORNs and that it contributes to targeting of ORN axons to the olfactory bulb to refine the glomerular response map. Increasing evidence shows that OMP acts at the early stages of olfactory transduction by modulating the kinetics of cAMP, the second messenger of olfactory transduction. However, how this occurs at a mechanistic level is not understood, and it might also not be the only mechanism underlying all the changes observed in mice lacking OMP. Recently, OMP has been detected outside the nose, including the brain and other organs. Although no obvious logic has become apparent regarding the underlying commonality between nasal and extranasal expression of OMP, a broader approach to diverse cellular systems might help unravel OMP's functions and mechanisms of action inside and outside the nose.

Dopaminergic modulation of olfactory-evoked motor output in sea lampreys (Petromyzon marinus L.)

Detection of chemical cues is important to guide locomotion in association with feeding and sexual behavior. Two neural pathways responsible for odor-evoked locomotion have been characterized in the sea lamprey (Petromyzon marinus L.), a basal vertebrate. There is a medial pathway originating in the medial olfactory bulb (OB) and a lateral pathway originating from the rest of the OB. These olfactomotor pathways are present throughout the life cycle of lampreys, but olfactory-driven behaviors differ according to the developmental stage. Among possible mechanisms, dopaminergic (DA) modulation in the OB might explain the behavioral changes. Here, we examined DA modulation of olfactory transmission in lampreys. Immunofluorescence against DA revealed immunoreactivity in the OB that was denser in the medial part (medOB), where processes were observed close to primary olfactory afferents and projection neurons. Dopaminergic neurons labeled by tracer injections in the medOB were located in the OB, the posterior tuberculum, and the dorsal hypothalamic nucleus, suggesting the presence of both intrinsic and extrinsic DA innervation. Electrical stimulation of the olfactory nerve in an in vitro whole-brain preparation elicited synaptic responses in reticulospinal cells that were modulated by DA. Local injection of DA agonists in the medOB decreased the reticulospinal cell responses whereas the D2 receptor antagonist raclopride increased the response amplitude. These observations suggest that DA in the medOB could modulate odor-evoked locomotion. Altogether, these results show the presence of a DA innervation within the medOB that may play a role in modulating olfactory inputs to the motor command system of lampreys.

Matrix Metalloproteinase 9 and Osteopontin Interact to Support Synaptogenesis in the Olfactory Bulb after Mild Traumatic Brain Injury

Olfactory receptor axons reinnervate the olfactory bulb (OB) after chemical or transection lesion. Diffuse brain injury damages the same axons, but the time course and regulators of OB reinnervation are unknown. Gelatinases (matrix metalloproteinase [MMP]2, MMP9) and their substrate osteopontin (OPN) are candidate mediators of synaptogenesis after central nervous system (CNS) insult, including olfactory axon damage. Here, we examined the time course of MMP9, OPN, and OPN receptor CD44 response to diffuse OB injury. FVBV/NJ mice received mild midline fluid percussion insult (mFPI), after which MMP9 activity and both OPN and CD44 protein expression were measured. Diffuse mFPI induced time-dependent increase in OB MMP9 activity and elevated the cell signaling 48-kD OPN fragment. This response was bimodal at 1 and 7 days post-injury. MMP9 activity was also correlated with 7-day reduction in a second 32-kD OPN peptide. CD44 increase peaked at 3 days, delayed relative to MMP9/OPN response. MMP9 and OPN immunohistochemistry suggested that deafferented tufted and mitral neurons were the principal sites for these molecular interactions. Analysis of injured MMP9 knockout (KO) mice showed that 48-kD OPN production was dependent on OB MMP9 activity, but with no KO effect on CD44 induction. Olfactory marker protein (OMP), used to identify injured olfactory axons, revealed persistent axon damage in the absence of MMP9. MMP9 KO ultrastructure at 21 days post-injury indicated that persistent OMP reduction was paired with delayed removal of degenerated axons. These results provide evidence that diffuse, concussive brain trauma induces a post-injury interaction between MMP9, OPN, and CD44, which mediates synaptic plasticity and reinnervation within the OB.

Development of the human olfactory system

This chapter focuses on the development of the human olfactory system. In this system, function does not require full neuroanatomical maturity. Thus, discrimination of odorous molecules, including a number within the mother's diet, occurs in amniotic fluid after 28-30 weeks of gestation, at which time the olfactory bulbs are identifiable by MRI. Hypoplasia/aplasia of the bulbs is documented in the third trimester and postnatally. Interestingly, olfactory axons project from the nasal epithelium to the telencephalon before formation of the olfactory bulbs and lack a peripheral ganglion, but the synaptic glomeruli of the future olfactory bulb serves this function. Histologic lamination of the olfactory bulb is present by 14 weeks, but maturation remains incomplete at term for neuronal differentiation, synaptogenesis, myelination, and persistence of the normal transitory fetal ventricular recess. Myelination occurs postnatally. Although olfaction is the only sensory system without direct thalamic projections, the olfactory bulb and anterior olfactory nucleus are, in effect, thalamic surrogates. For example, many dendro-dendritic synapses occur within the bulb between GABAergic granular neurons and periglomerular neurons. Moreover, bulbar synaptic glomeruli are analogous to peripheral ganglia of other sensory cranial nerves. The olfactory tract contains much gray as well as white matter. The olfactory epithelium and bulb both incorporate progenitor cells at all ages. Diverse malformations of the olfactory bulb can be detected by clinical examination, imaging, and neuropathology; indeed, olfactory reflexes of the neonate can be reliably tested. We recommend that such testing be routine in the neonatal neurologic examination, especially in children with brain malformations, endocrinopathies, chromosomopathies, genetic/metabolic disorders, and perinatal hypoxic/ischemic encephalopathy.

Sema7A/PlxnCl signaling triggers activity-dependent olfactory synapse formation

In mammals, neural circuits are formed based on a genetic program and further refined by neuronal activity during the neonatal period. We report that in the mouse olfactory system, the glomerular map is not merely refined but newly connected to second-order neurons by odorant-receptor-derived neuronal activity. Here, we analyzed a pair of molecules, Sema7A, expressed in olfactory sensory neurons (OSNs) in an activity-dependent manner, and PlxnC1, localized to dendrites of mitral/tufted (M/T) cells in the first week after birth. In Sema7A or PlxnC1 knockout (KO) mice, initiation of synapse formation and dendrite selection of M/T cells were perturbed. Reconstitution and rescue experiments demonstrated that Sema7A-PlxnC1 interaction is essential to form the post-synaptic assembly. Pharmacological blocking experiments indicated that synaptic transmission triggers primary dendrite selection by synaptic competition. We conclude that Sema7A signaling is key to inducing activity-dependent post-synapse events and dendrite selection in M/T-cells during the neonatal period.

Cranial Pair I: The Olfactory Nerve

The olfactory nerve constitutes the first cranial pair. Compared with other cranial nerves, it depicts some atypical features. First, the olfactory nerve does not form a unique bundle. The olfactory axons join other axons and form several small bundles or fascicles: the fila olfactoria. These fascicles leave the nasal cavity, pass through the lamina cribrosa of the ethmoid bone and enter the brain. The whole of these fascicles is what is known as the olfactory nerve. Second, the olfactory sensory neurons, whose axons integrate the olfactory nerve, connect the nasal cavity and the brain without any relay. Third, the olfactory nerve is composed by unmyelinated axons. Fourth, the olfactory nerve contains neither Schwann cells nor oligodendrocytes wrapping its axons. But it contains olfactory ensheathing glia, which is a type of glia unique to this nerve. Fifth, the olfactory axons participate in the circuitry of certain spherical structures of neuropil that are unique in the brain: the olfactory glomeruli. Sixth, the axons of the olfactory nerve are continuously replaced and their connections in the central nervous system are remodeled continuously. Therefore, the olfactory nerve is subject to lifelong plasticity. Finally seventh, the olfactory nerve can be a gateway for the direct entrance of viruses, neurotoxins and other xenobiotics to the brain. In the same way, it can be used as a portal of entry to the brain for therapeutic substances, bypassing the blood-brain barrier. In this article, we analyze some features of the anatomy and physiology of the first cranial pair. Anat Rec, 302:405-427, 2019. © 2018 Wiley Periodicals, Inc.

Mild Fluid Percussion Injury Induces Diffuse Axonal Damage and Reactive Synaptic Plasticity in the Mouse Olfactory Bulb

Despite the regenerative capacity of the olfactory bulb (OB), head trauma causes olfactory disturbances in up to 30% of patients. While models of olfactory nerve transection, olfactory receptor neuron (ORN) ablation, or direct OB impact have been used to examine OB recovery, these models are severe and not ideal for study of OB synaptic repair. We posited that a mild fluid percussion brain injury (mFPI), delivered over mid-dorsal cortex, would produce diffuse OB deafferentation without confounding pathology. Wild type FVB/NJ mice were subjected to mFPI and OB probed for ORN axon degeneration and onset of reactive synaptogenesis. OB extracts revealed 3 d postinjury elevation of calpain-cleaved 150-kDa αII-spectrin, an indicator of axon damage, in tandem with reduced olfactory marker protein (OMP), a protein specific to intact ORN axons. Moreover, mFPI also produced a 3-d peak in GFAP+ astrocyte and IBA1+ microglial reactivity, consistent with postinjury inflammation. OB glomeruli showed disorganized ORN axons, presynaptic degeneration, and glial phagocytosis at 3 and 7 d postinjury, all indicative of deafferentation. At 21 d after mFPI, normal synaptic structure re-emerged along with OMP recovery, supporting ORN afferent reinnervation. Robust 21 d postinjury upregulation of GAP-43 was consistent with the time course of ORN axon sprouting and synapse regeneration reported after more severe olfactory insult. Together, these findings define a cycle of synaptic degeneration and recovery at a site remote to non-contusive brain injury. We show that mFPI models diffuse ORN axon damage, useful for the study of time-dependent reactive synaptogenesis in the deafferented OB.

G-protein coupled receptors Mc4r and Drd1a can serve as surrogate odorant receptors in mouse olfactory sensory neurons

In the mouse, most mature olfactory sensory neurons (OSNs) express one allele of one gene from the repertoire of ~1100 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express a given OR coalesce into homogeneous glomeruli, which reside at conserved positions in the olfactory bulb. ORs are intimately involved in ensuring the expression of one OR per OSN and the coalescence of OSN axons into glomeruli. But the mechanisms whereby ORs accomplish these diverse functions remain poorly understood. An experimental approach that has been informative is to substitute an OR genetically with another GPCR that is normally not expressed in OSNs, in order to determine in which aspects this GPCR can serve as surrogate OR in mouse OSNs. Thus far only the β2-adrenergic receptor (β2AR, Ardb2) has been shown to be able to serve as surrogate OR in OSNs; the β2AR could substitute for the M71 OR in all aspects examined. Can other non-olfactory GPCRs function equally well as surrogate ORs in OSNs? Here, we have generated and characterized two novel gene-targeted mouse strains in which the mouse melanocortin 4 receptor (Mc4r) or the mouse dopamine receptor D1 (Drd1a) is coexpressed with tauGFP in OSNs that express the OR locus M71. These alleles and strains are abbreviated as Mc4r → M71-GFP and Drd1a → M71-GFP. We detected strong Mc4r or Drd1a immunoreactivity in axons and dendritic knobs and cilia of OSNs that express Mc4r or Drd1a from the M71 locus. These OSNs responded physiologically to cognate agonists for Mc4r (Ro27-3225) or Drd1a (SKF81297), and not to the M71 ligand acetophenone. Axons of OSNs expressing Mc4r → M71-GFP coalesced into glomeruli. Axons of OSNs expressing Drd1a → M71-GFP converged onto restricted areas of the olfactory bulb but did not coalesce into glomeruli. Thus, OR functions in OSNs can be substituted by Mc4r or Drd1a, but not as well as by β2AR. We attribute the weak performance of Drd1a as surrogate OR to poor OSN maturation.

Expression of Nogo receptor 1 in the regeneration process of the mouse olfactory epithelium

Nogo receptor 1 (NgR1) is the most important Nogo-A receptor. By its interaction with myelin-associated inhibitory proteins, NgR1 inhibits the regeneration of axons and is extensively expressed in the central nervous system. However, the expression of NgR1 in regenerable neurons, such as olfactory neurons, and its expression in the regeneration progress of olfactory neurons have not been reported. In this study, we demonstrated that NgR1 was expressed in the cell bodies of certain mature olfactory receptor neurons (ORNs) but was not expressed in immature ORNs in the olfactory epithelium (OE) of normal adult mice. On day 21 after OE injury, NgR1 was expressed not only in the cell bodies of mature ORNs but also in the cell bodies of glial fibrillary acidic protein (GFAP)-positive cells in the top and submucosal layers of the OE. On day 48 after model establishment, NgR1 expression decreased in the cell bodies of the GFAP-positive cells. On day 56 after model establishment, no NgR1 expression was found in the cell bodies of the GFAP-positive cells, and NgR1 was again expressed only in the mature ORNs. Our results demonstrated that NgR1 expression is upregulated in the OE after injury, which suggests that NgR1 might be involved in the regeneration of the OE.

Tight temporal coupling between synaptic rewiring of olfactory glomeruli and the emergence of odor-guided behavior in Xenopus tadpoles

Olfactory sensory neurons (OSNs) are chemoreceptors that establish excitatory synapses within glomeruli of the olfactory bulb. OSNs undergo continuous turnover throughout life, causing the constant replacement of their synaptic contacts. Using Xenopus tadpoles as an experimental system to investigate rewiring of glomerular connectivity, we show that novel OSN synapses can transfer information immediately after formation, mediating olfactory-guided behavior. Tadpoles recover the ability to detect amino acids 4 days after bilateral olfactory nerve transection. Restoration of olfactory-guided behavior depends on the efficient reinsertion of OSNs to the olfactory bulb. Presynaptic terminals of incipient synaptic contacts generate calcium transients in response to odors, triggering long lasting depolarization of olfactory glomeruli. The functionality of reconnected terminals relies on well-defined readily releasable and cytoplasmic vesicle pools. The continuous growth of non-compartmentalized axonal processes provides a vesicle reservoir to nascent release sites, which contrasts to the gradual development of cytoplasmic vesicle pools in conventional excitatory synapses. The immediate availability of fully functional synapses upon formation supports an age-independent contribution of OSNs to the generation of odor maps.

Olfactory Development, Part 2: Neuroanatomic Maturation and Dysgeneses

Olfactory axons project from nasal epithelium to the primitive telencephalon before olfactory bulbs form. Olfactory bulb neurons do not differentiate in situ but arrive via the rostral migratory stream. Synaptic glomeruli and concentric laminar architecture are unlike other cortices. Fetal olfactory maturation of neuronal differentiation, synaptogenesis, and myelination remains incomplete at term and have a protracted course of postnatal development. The olfactory ventricular recess involutes postnatally but dilates in congenital hydrocephalus. Olfactory bulb, tract and epithelium are repositories of progenitor stem cells in fetal and adult life. Diverse malformations of the olfactory bulb can be diagnosed by clinical examination, imaging, and neuropathologically. Cellular markers of neuronal differentiation and synaptogenesis demonstrate immaturity of the olfactory system at birth, previously believed by histology alone to occur early in fetal life. Immaturity does not preclude function.

Dendritic Arborization Patterns of Small Juxtaglomerular Cell Subtypes within the Rodent Olfactory Bulb

Within the glomerular layer of the rodent olfactory bulb, numerous subtypes of local interneurons contribute to early processing of incoming sensory information. Here we have investigated dopaminergic and other small local juxtaglomerular cells in rats and mice and characterized their dendritic arborization pattern with respect to individual glomeruli by fluorescent labeling via patching and reconstruction of dendrites and glomerular contours from two-photon imaging data. Dopaminergic neurons were identified in a transgenic mouse line where the expression of dopamine transporter (DAT) was labeled with GFP. Among the DAT+ cells we found a small short-axon cell (SAC) subtype featuring hitherto undescribed dendritic specializations. These densely ramifying structures clasped mostly around somata of other juxtaglomerular neurons, which were also small, non-dopaminergic and to a large extent non-GABAergic. Clasping SACs were observed also in wild-type mice and juvenile rats. In DAT+ SAC dendrites, single backpropagating action potentials evoked robust calcium entry throughout both clasping and non-clasping compartments. Besides clasping SACs, most other small neurons either corresponded to the classical periglomerular cell type (PGCs), which was never DAT+, or were undersized cells with a small dendritic tree and low excitability. Aside from the presence of clasps in SAC dendrites, many descriptors of dendritic morphology such as the number of dendrites and the extent of branching were not significantly different between clasping SACs and PGCs. However, a detailed morphometric analysis in relation to glomerular contours revealed that the dendrites of clasping SACs arborized mostly in the juxtaglomerular space and never entered more than one glomerulus (if at all), whereas most PGC dendrites were restricted to their parent glomerulus, similar to the apical tufts of mitral cells. These complementary arborization patterns might underlie a highly complementary functional connectivity. The morphometric approach may serve to differentiate also other subtypes of juxtaglomerular neurons, help to identify putative synaptic partners and thus to establish a more refined picture of glomerular network interactions during odor sensing.

Presynaptic gain control by endogenous cotransmission of dopamine and GABA in the olfactory bulb

In the olfactory bulb, lateral inhibition mediated by local juxtaglomerular interneurons has been proposed as a gain control mechanism, important for decorrelating odorant responses. Among juxtaglomerular interneurons, short axon cells are unique as dual-transmitter neurons that release dopamine and GABA. To examine their intraglomerular function, we expressed channelrhodopsin under control of the DAT-cre promoter and activated olfactory afferents within individual glomeruli. Optical stimulation of labeled cells triggered endogenous dopamine release as measured by cyclic voltammetry and GABA release as measured by whole cell GABA_A receptor currents. Activation of short axon cells reduced the afferent presynaptic release probability via D₂ and GABA_B receptor activation, resulting in reduced spiking in both mitral and external tufted cells. Our results suggest that short axon cells influence glomerular activity not only by direct inhibition of external tufted cells but also by inhibition of afferent inputs to external tufted and mitral cells.NEW & NOTEWORTHY Sensory systems, including the olfactory system, encode information across a large dynamic range, making synaptic mechanisms of gain control critical to proper function. Here we demonstrate that a dual-transmitter interneuron in the olfactory bulb controls the gain of intraglomerular afferent input via two distinct mechanisms, presynaptic inhibition as well as inhibition of a principal neuron subtype, and thereby potently controls the synaptic gain of afferent inputs.

Interglomerular Connectivity within the Canonical and GC-D/Necklace Olfactory Subsystems

The mammalian main olfactory system contains several subsystems that differ not only in the receptors they express and the glomerular targets they innervate within the main olfactory bulb (MOB), but also in the strategies they use to process odor information. The canonical main olfactory system employs a combinatorial coding strategy that represents odorant identity as a pattern of glomerular activity. By contrast, the "GC-D/necklace" olfactory subsystem—formed by olfactory sensory neurons expressing the receptor guanylyl cyclase GC-D and their target necklace glomeruli (NGs) encircling the caudal MOB—is critical for the detection of a small number of semiochemicals that promote the acquisition of food preferences. The formation of these socially-transmitted food preferences requires the animal to integrate information about two types of olfactory stimuli: these specialized social chemosignals and the food odors themselves. However, the neural mechanisms with which the GC-D/necklace subsystem processes this information are unclear. We used stimulus-induced increases in intrinsic fluorescence signals to map functional circuitry associated with NGs and canonical glomeruli (CGs) in the MOB. As expected, CG-associated activity spread laterally through both the glomerular and external plexiform layers associated with activated glomeruli. Activation of CGs or NGs resulted in activity spread between the two types of glomeruli; there was no evidence of preferential connectivity between individual necklace glomeruli. These results support previous anatomical findings that suggest the canonical and GC-D/necklace subsystems are functionally connected and may integrate general odor and semiochemical information in the MOB.

Using Intrinsic Flavoprotein and NAD(P)H Imaging to Map Functional Circuitry in the Main Olfactory Bulb

Neurons exhibit strong coupling of electrochemical and metabolic activity. Increases in intrinsic fluorescence from either oxidized flavoproteins or reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] in the mitochondria have been used as an indicator of neuronal activity for the functional mapping of neural circuits. However, this technique has not been used to investigate the flow of olfactory information within the circuitry of the main olfactory bulb (MOB). We found that intrinsic flavoprotein fluorescence signals induced by electrical stimulation of single glomeruli displayed biphasic responses within both the glomerular (GL) and external plexiform layers (EPL) of the MOB. Pharmacological blockers of mitochondrial activity, voltage-gated Na⁺ channels, or ionotropic glutamate receptors abolished stimulus-dependent flavoprotein responses. Blockade of GABA_A receptors enhanced the amplitude and spatiotemporal spread of the flavoprotein signals, indicating an important role for inhibitory neurotransmission in shaping the spread of neural activity in the MOB. Stimulus-dependent spread of fluorescence across the GL and EPL displayed a spatial distribution consistent with that of individual glomerular microcircuits mapped by neuroanatomic tract tracing. These findings demonstrated the feasibility of intrinsic fluorescence imaging in the olfactory systems and provided a new tool to examine the functional circuitry of the MOB.

Three-dimensional synaptic analyses of mitral cell and external tufted cell dendrites in rat olfactory bulb glomeruli

Recent studies have suggested that the two excitatory cell classes of the mammalian olfactory bulb, the mitral cells (MCs) and tufted cells (TCs), differ markedly in physiological responses. For example, TCs are more sensitive and broadly tuned to odors than MCs and also are much more sensitive to stimulation of olfactory sensory neurons (OSNs) in bulb slices. To examine the morphological bases for these differences, we performed quantitative ultrastructural analyses of glomeruli in rat olfactory bulb under conditions in which specific cells were labeled with biocytin and 3,3'-diaminobenzidine. Comparisons were made between MCs and external TCs (eTCs), which are a TC subtype in the glomerular layer with large, direct OSN signals and capable of mediating feedforward excitation of MCs. Three-dimensional analysis of labeled apical dendrites under an electron microscope revealed that MCs and eTCs in fact have similar densities of several chemical synapse types, including OSN inputs. OSN synapses also were distributed similarly, favoring a distal localization on both cells. Analysis of unlabeled putative MC dendrites further revealed gap junctions distributed uniformly along the apical dendrite and, on average, proximally with respect to OSN synapses. Our results suggest that the greater sensitivity of eTCs vs. MCs is due not to OSN synapse number or absolute location but rather to a conductance in the MC dendrite that is well positioned to attenuate excitatory signals passing to the cell soma. Functionally, such a mechanism could allow rapid and dynamic control of OSN-driven action potential firing in MCs through changes in gap junction properties. J. Comp. Neurol. 525:592-609, 2017. © 2016 Wiley Periodicals, Inc.

Parallel processing of afferent olfactory sensory information

KEY POINTS

The functional synaptic connectivity between olfactory receptor neurons and principal cells within the olfactory bulb is not well understood. One view suggests that mitral cells, the primary output neuron of the olfactory bulb, are solely activated by feedforward excitation. Using focal, single glomerular stimulation, we demonstrate that mitral cells receive direct, monosynaptic input from olfactory receptor neurons. Compared to external tufted cells, mitral cells have a prolonged afferent-evoked EPSC, which serves to amplify the synaptic input. The properties of presynaptic glutamate release from olfactory receptor neurons are similar between mitral and external tufted cells. Our data suggest that afferent input enters the olfactory bulb in a parallel fashion.

ABSTRACT

Primary olfactory receptor neurons terminate in anatomically and functionally discrete cortical modules known as olfactory bulb glomeruli. The synaptic connectivity and postsynaptic responses of mitral and external tufted cells within the glomerulus may involve both direct and indirect components. For example, it has been suggested that sensory input to mitral cells is indirect through feedforward excitation from external tufted cells. We also observed feedforward excitation of mitral cells with weak stimulation of the olfactory nerve layer; however, focal stimulation of an axon bundle entering an individual glomerulus revealed that mitral cells receive monosynaptic afferent inputs. Although external tufted cells had a 4.1-fold larger peak EPSC amplitude, integration of the evoked currents showed that the synaptic charge was 5-fold larger in mitral cells, reflecting the prolonged response in mitral cells. Presynaptic afferents onto mitral and external tufted cells had similar quantal amplitude and release probability, suggesting that the larger peak EPSC in external tufted cells was the result of more synaptic contacts. The results of the present study indicate that the monosynaptic afferent input to mitral cells depends on the strength of odorant stimulation. The enhanced spiking that we observed in response to brief afferent input provides a mechanism for amplifying sensory information and contrasts with the transient response in external tufted cells. These parallel input paths may have discrete functions in processing olfactory sensory input.

Synaptic circuitry of identified neurons in the antennal lobe of Drosophila melanogaster

In Drosophila melanogaster olfactory sensory neurons (OSNs) establish synapses with projection neurons (PNs) and local interneurons within antennal lobe (AL) glomeruli. Substantial knowledge regarding this circuitry has been obtained by functional studies, whereas ultrastructural evidence of synaptic contacts is scarce. To fill this gap, we studied serial sections of three glomeruli using electron microscopy. Ectopic expression of a membrane-bound peroxidase allowed us to map synaptic sites along PN dendrites. Our data prove for the first time that each of the three major types of AL neurons is both pre- and postsynaptic to the other two types, as previously indicated by functional studies. PN dendrites carry a large proportion of output synapses, with approximately one output per every three input synapses. Detailed reconstructions of PN dendrites showed that these synapses are distributed unevenly, with input and output sites partially segregated along a proximal-distal gradient and the thinnest branches carrying solely input synapses. Moreover, our data indicate synapse clustering, as we found evidence of dendritic tiling of PN dendrites. PN output synapses exhibited T-shaped presynaptic densities, mostly arranged as tetrads. In contrast, output synapses from putative OSNs showed elongated presynaptic densities in which the T-bar platform was supported by several pedestals and contacted as many as 20 postsynaptic profiles. We also discovered synaptic contacts between the putative OSNs. The average synaptic density in the glomerular neuropil was about two synapses/µm(3) . These results are discussed with regard to current models of olfactory glomerular microcircuits across species.

Rapid and continuous activity-dependent plasticity of olfactory sensory input

Incorporation of new neurons enables plasticity and repair of circuits in the adult brain. Adult neurogenesis is a key feature of the mammalian olfactory system, with new olfactory sensory neurons (OSNs) wiring into highly organized olfactory bulb (OB) circuits throughout life. However, neither when new postnatally generated OSNs first form synapses nor whether OSNs retain the capacity for synaptogenesis once mature, is known. Therefore, how integration of adult-born OSNs may contribute to lifelong OB plasticity is unclear. Here, we use a combination of electron microscopy, optogenetic activation and in vivo time-lapse imaging to show that newly generated OSNs form highly dynamic synapses and are capable of eliciting robust stimulus-locked firing of neurons in the mouse OB. Furthermore, we demonstrate that mature OSN axons undergo continuous activity-dependent synaptic remodelling that persists into adulthood. OSN synaptogenesis, therefore, provides a sustained potential for OB plasticity and repair that is much faster than OSN replacement alone.

Serotonin increases synaptic activity in olfactory bulb glomeruli

Serotoninergic fibers densely innervate olfactory bulb glomeruli, the first sites of synaptic integration in the olfactory system. Acting through 5HT2A receptors, serotonin (5HT) directly excites external tufted cells (ETCs), key excitatory glomerular neurons, and depolarizes some mitral cells (MCs), the olfactory bulb's main output neurons. We further investigated 5HT action on MCs and determined its effects on the two major classes of glomerular interneurons: GABAergic/dopaminergic short axon cells (SACs) and GABAergic periglomerular cells (PGCs). In SACs, 5HT evoked a depolarizing current mediated by 5HT2C receptors but did not significantly impact spike rate. 5HT had no measurable direct effect in PGCs. Serotonin increased spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) in PGCs and SACs. Increased sEPSCs were mediated by 5HT2A receptors, suggesting that they are primarily due to enhanced excitatory drive from ETCs. Increased sIPSCs resulted from elevated excitatory drive onto GABAergic interneurons and augmented GABA release from SACs. Serotonin-mediated GABA release from SACs was action potential independent and significantly increased miniature IPSC frequency in glomerular neurons. When focally applied to a glomerulus, 5HT increased MC spontaneous firing greater than twofold but did not increase olfactory nerve-evoked responses. Taken together, 5HT modulates glomerular network activity in several ways: 1) it increases ETC-mediated feed-forward excitation onto MCs, SACs, and PGCs; 2) it increases inhibition of glomerular interneurons; 3) it directly triggers action potential-independent GABA release from SACs; and 4) these network actions increase spontaneous MC firing without enhancing responses to suprathreshold sensory input. This may enhance MC sensitivity while maintaining dynamic range.

Regional Specializations of the PAZ Proteomes Derived from Mouse Hippocampus, Olfactory Bulb and Cerebellum

Neurotransmitter release as well as structural and functional dynamics at the presynaptic active zone (PAZ) comprising synaptic vesicles attached to the presynaptic plasma membrane are mediated and controlled by its proteinaceous components. Here we describe a novel experimental design to immunopurify the native PAZ-complex from individual mouse brain regions such as olfactory bulb, hippocampus, and cerebellum with high purity that is essential for comparing their proteome composition. Interestingly, quantitative immunodetection demonstrates significant differences in the abundance of prominent calcium-dependent PAZ constituents. Furthermore, we characterized the proteomes of the immunoisolated PAZ derived from the three brain regions by mass spectrometry. The proteomes of the release sites from the respective regions exhibited remarkable differences in the abundance of a large variety of PAZ constituents involved in various functional aspects of the release sites such as calcium homeostasis, synaptic plasticity and neurogenesis. On the one hand, our data support an identical core architecture of the PAZ for all brain regions and, on the other hand, demonstrate that the proteinaceous composition of their presynaptic active zones vary, suggesting that changes in abundance of individual proteins strengthen the ability of the release sites to adapt to specific functional requirements.

Metabotropic glutamate receptors promote disinhibition of olfactory bulb glomeruli that scales with input strength

Increasing evidence indicates that the neural circuitry within glomeruli of the olfactory bulb plays a major role in affecting information flow between olfactory sensory neurons (OSNs) and output mitral cells (MCs). Glutamatergic external tufted (ET) cells, located at glomeruli, can act as intermediary cells in excitation between OSNs and MCs, whereas activation of MCs by OSNs is, in turn, suppressed by inhibitory synapses onto ET cells. In this study, we used patch-clamp recordings in rat olfactory bulb slices to examine the function of metabotropic glutamate receptors (mGluRs) in altering these glomerular signaling mechanisms. We found that activation of group II mGluRs profoundly reduced inhibition onto ET cells evoked by OSN stimulation. The mGluRs that mediated disinhibition were located on presynaptic GABAergic periglomerular cells and appeared to be activated by glutamate transients derived from dendrites in glomeruli. In terms of glomerular output, the mGluR-mediated reduction in GABA release led to a robust increase in the number of action potentials evoked by OSN stimulation in both ET cells and MCs. Importantly, however, the enhanced excitation was specific to when a glomerulus was strongly activated by OSN inputs. By being selective for strong vs. weak glomerular activation, mGluR-mediated disinhibition provides a mechanism to enhance the contrast in odor signals that activate OSN inputs into a single glomerulus at varying intensities.

Cellular and molecular cues of glucose sensing in the rat olfactory bulb

In the brain, glucose homeostasis of extracellular fluid is crucial to the point that systems specifically dedicated to glucose sensing are found in areas involved in energy regulation and feeding behavior. Olfaction is a major sensory modality regulating food consumption. Nutritional status in turn modulates olfactory detection. Recently it has been proposed that some olfactory bulb (OB) neurons respond to glucose similarly to hypothalamic neurons. However, the precise molecular cues governing glucose sensing in the OB are largely unknown. To decrypt these molecular mechanisms, we first used immunostaining to demonstrate a strong expression of two neuronal markers of glucose-sensitivity, insulin-dependent glucose transporter type 4 (GLUT4), and sodium glucose co-transporter type 1 (SGLT1) in specific OB layers. We showed that expression and mapping of GLUT4 but not SGLT1 were feeding state-dependent. In order to investigate the impact of metabolic status on the delivery of blood-borne glucose to the OB, we measured extracellular fluid glucose concentration using glucose biosensors simultaneously in the OB and cortex of anesthetized rats. We showed that glucose concentration in the OB is higher than in the cortex, that metabolic steady-state glucose concentration is independent of feeding state in the two brain areas, and that acute changes in glycemic conditions affect bulbar glucose concentration alone. These data provide new evidence of a direct relationship between the OB and peripheral metabolism, and emphasize the importance of glucose for the OB network, providing strong arguments toward establishing the OB as a glucose-sensing organ.

Effects of anticancer drug docetaxel on the structure and function of the rabbit olfactory mucosa

Docetaxel (DCT) is an anticancer drug which acts by disrupting microtubule dynamics in the highly mitotic cancer cells. Thus, this drug has a potential to affect function and organization of tissues exhibiting high cellular turnover. We investigated, in the rabbit, the effects of a single human equivalent dose (6.26 mg/kg, i.v.) of DCT on the olfactory mucosa (OM) through light and electron microscopy, morphometry, Ki-67 immunostaining, TUNEL assay and the buried food test for olfactory sensitivity. On post-exposure days (PED) 5 and 10, there was disarrangement of the normal cell layering in the olfactory epithelium (OE), apoptotic death of cells of the OE, Bowman's glands and axon bundles, and the presence (including on PED 3) of blood vessels in the bundle cores. A decrease in bundle diameters, olfactory cell densities and cilia numbers, which was most significant on PED 10 (49.3%, 63.4% and 50%, respectively), was also evident. Surprisingly by PED 15, the OM regained normal morphology. Furthermore, olfactory sensitivity decreased progressively until PED 10 when olfaction was markedly impaired, and with recovery from the impairment by PED 15. These observations show that DCT transiently alters the structure and function of the OM suggesting a high regenerative potential for this tissue.

Odorant response properties of individual neurons in an olfactory glomerular module

Neuronal networks that are directly associated with glomeruli in the olfactory bulb are thought to comprise functional modules. However, this has not yet been experimentally proven. In this study, we explored the anatomical and functional architecture of glomerular modules using in vivo two-photon calcium imaging. Surprisingly, the deep portions of the glomerular modules showed considerable spatial overlap with other modules. Juxtaglomerular cells showed similar excitatory odorant response profiles to presynaptic olfactory sensory neuron inputs. Mitral cells exhibited a more sharply tuned molecular receptive range compared to juxtaglomerular cells, and their odorant response profiles varied depending on their interneuronal horizontal distances. These data suggest that glomerular modules are composed of functionally distinct neurons, and that homogenous odor inputs to each glomerulus may be parsed and processed in different fashions within the modules before being sent to higher olfactory centers.

Transitory and activity-dependent expression of neurogranin in olfactory bulb tufted cells during mouse postnatal development

Neurogranin (Ng) is a brain-specific postsynaptic calmodulin-binding protein involved in synaptic activity-dependent plasticity. In the adult olfactory bulb (OB), Ng is expressed by a large population of GABAergic interneurons in the granule cell layer. We show here that, during postnatal development, Ng is also expressed by OB neurons in the superficial external plexiform layer (sEPL) and glomerular layer (GL). These Ng-positive neurons display morphological and neurochemical features of superficial and external tufted cells. Ng expression in these cells is transient during OB development: few elements express Ng at postnatal day (P) 5, increasing in number and reaching a peak at P10, then progressively decreasing. At P30, Ng is rarely detectable in these neurons. Ng expression in developing tufted cells is also modulated at the cellular level: at earlier stages, Ng labeling is distributed throughout the cell body and dendritic arborization in the GL, but, at P20, when the glomerular circuits are fully matured, Ng becomes restricted to the soma and proximal portion of tufted cell apical dendrites. We show that olfactory deprivation at early postnatal stages induces a strong increase in Ng-positive tufted cells from P10 to P20, whereas no changes have been observed following olfactory deprivation in adult mice. These findings demonstrate that Ng expression in sEPL-GL is restricted to developmental stages and indicate its activity-dependent regulation in a time window critical for glomerular circuit development, suggesting a role for Ng in maturation and dendritic remodeling of tufted cells.

Dishevelled proteins are associated with olfactory sensory neuron presynaptic terminals

Olfactory sensory neurons (OSNs) project their axons from the olfactory epithelium toward the olfactory bulb (OB) in a heterogeneous and unsorted arrangement. However, as the axons approach the glomerular layer of the OB, axons from OSNs expressing the same odorant receptor (OR) sort and converge to form molecularly homogeneous glomeruli. Axon guidance cues, cell adhesion molecules, and OR induced activity have been implicated in the final targeting of OSN axons to specific glomeruli. Less understood, and often controversial, are the mechanisms used by OSN axons to initially navigate from the OE toward the OB. We previously demonstrated a role for Wnt and Frizzled (Fz) molecules in OSN axon extension and organization within the olfactory nerve. Building on that we now turned our attention to the downstream signaling cascades from Wnt-Fz interactions. Dishevelled (Dvl) is a key molecule downstream of Fz receptors. Three isoforms of Dvl with specific as well as overlapping functions are found in mammals. Here, we show that Dvl-1 expression is restricted to OSNs in the dorsal recess of the nasal cavity, and labels a unique subpopulation of glomeruli. Dvl-2 and Dvl-3 have a widespread distribution in both the OE and OB. Both Dvl-1 and Dvl-2 are associated with intra-glomerular pre-synaptic OSN terminals, suggesting a role in synapse formation/stabilization. Moreover, because Dvl proteins were observed in all OSN axons, we hypothesize that they are important determinants of OSN cell differentiation and axon extension.

Age-dependent regional changes in the rostral migratory stream

Throughout life the subventricular zone (SVZ) is a source of new olfactory bulb (OB) interneurons. From the SVZ, neuroblasts migrate tangentially through the rostral migratory stream (RMS), a restricted route approximately 5 mm long in mice, reaching the OB within 10-14 days. Within the OB, neuroblasts migrate radially to the granule and glomerular layers where they differentiate into granule and periglomerular (PG) cells and integrate into existing synaptic circuits. SVZ neurogenesis decreases with age, and might be a factor in age-related olfactory deficits. However, the effect of aging on the RMS and on the differentiation of interneuron subpopulations remains poorly understood. Here, we examine RMS cytoarchitecture, neuroblast proliferation and clearance from the RMS, and PG cell subpopulations at 6, 12, 18, and 23 months of age. We find that aging affects the area occupied by newly generated cells within the RMS and regional proliferation, and the clearance of neuroblasts from the RMS and PG cell subpopulations and distribution remain stable.

Distribution and functional organization of glomeruli in the olfactory bulbs of zebrafish (Danio rerio)

Odor molecules are transduced by thousands of olfactory sensory neurons (OSNs) located in the nasal cavity. Each OSN expresses a single functional odorant receptor protein and projects an axon from the sensory epithelia to an olfactory bulb glomerulus, which is selectively innervated by only one or a few OSN types. We used whole-mount immunocytochemistry to study the neurochemistry and anatomical organization of glomeruli in the zebrafish olfactory system. By employing combinations of antibodies against G-protein α subunits, calcium-binding proteins, and general neuronal markers, we selectively labeled various OSN types, their axonal projections to glomeruli, and the detailed anatomical distributions of individual glomeruli in different regions of the olfactory bulb. In this way we identified ≈140 glomeruli in each olfactory bulb of mature zebrafish. A small subset (27) of these glomeruli was unambiguously identifiable in nearly all animals examined. These units were large and, located mainly in the medial olfactory bulbs. Most glomeruli, however, were comparatively small, anatomically indistinguishable, and located in coarsely circumscribed regions; almost all of these latter glomeruli were innervated by OSNs that were labeled with anti-G(α s/olf) and/or anti-calretinin antibodies. Collectively, our results provide a uniquely detailed description of a vertebrate olfactory system and highlight anatomically distinct parallel neural pathways that mediate early aspects of olfactory processing in the zebrafish.

Interaction of NMDA receptors and L-type calcium channels during early odor preference learning in rats

Early odor preference learning in rats provides a simple model for studying learning and memory. Learning results in an enhanced output from mitral cells, which carry odor information from the olfactory bulb to the olfactory cortex. Mitral cell NMDA receptors (NMDARs) are critically involved in plasticity at the olfactory nerve to mitral cell synapse during odor learning. Here we provide evidence that L-type calcium channels (LTCCs) provide an additional and necessary source of calcium for learning induction. LTCCs are thought to act downstream of NMDARs to bridge synaptic activation and the transcription of the plasticity-related proteins necessary for 24-h learning and memory. Using immunohistochemistry, we have demonstrated that LTCCs are present in the mitral cell and are primarily located on mitral cell proximal dendrites in neonate rats. Behavioral experiments demonstrate that inhibiting the function of LTCCs via intrabulbar infusion of nimidopine successfully blocks learning induced by pairing isoproterenol infusion with odor, while activation of LTCCs via an intrabulbar infusion of BayK-8644 rescues isoproterenol-induced learning from a D-APV block. Interestingly, the infusion of BayK-8644 paired with odor is by itself not sufficient to induce learning. Synaptoneurosome Western blot and immunohistochemistry measurement of synapsin I phosphorylation following BayK-8644 infusion suggest LTCCs are involved in synaptic release. Finally, odor preference can be induced by gabazine disinhibition of mitral cells, and NMDAR opening is sufficient for the gabazine-induced learning. These results provide the first evidence that NMDARs and LTCCs interact to permit calcium-dependent mitral cell plasticity during early odor preference learning.

Exuberant growth and synapse formation of olfactory sensory neuron axonal arborizations

Neural connections in the adult nervous system are established with a high degree of precision. Several examples throughout the nervous system indicate that this precision is achieved by first establishing an initial exuberant immature pattern of connectivity that is then sculpted into the adult pattern via pruning. This often emerges as an activity-dependent process. In the olfactory system, sensory axons from neurons expressing the same odorant receptor project with high precision to specific glomerular structures in the olfactory bulb. This process undergoes maturation-dependent refinements that are not fully understood. Due to technical impediments that have made it difficult to focus on single axons, it is unknown whether olfactory sensory projections are established in an exuberant fashion. Here we developed a novel technique of electroporation that allowed us to simultaneously label single olfactory sensory neuron (OSN) axonal arbors and their presynaptic specializations. Using this method we were able to incorporate plasmids into OSNs at an immature stage, thereby allowing a time-course study of axonal arbor development and synapse formation in single olfactory sensory axons. We observed that the number of branch points, the total branch length, and the number and density of presynaptic specializations peaked at postnatal day 8 and decreased afterwards. Our data demonstrate that olfactory sensory axons develop in an exuberant way, both in terms of branch growth and synaptic composition. We hypothesize that exuberant branches and synapses are eliminated to achieve the mature pattern in a process likely to be regulated by neural activity.

PKA increases in the olfactory bulb act as unconditioned stimuli and provide evidence for parallel memory systems: pairing odor with increased PKA creates intermediate- and long-term, but not short-term, memories

Neonatal odor-preference memory in rat pups is a well-defined associative mammalian memory model dependent on cAMP. Previous work from this laboratory demonstrates three phases of neonatal odor-preference memory: short-term (translation-independent), intermediate-term (translation-dependent), and long-term (transcription- and translation-dependent). Here, we use neonatal odor-preference learning to explore the role of olfactory bulb PKA in these three phases of mammalian memory. PKA activity increased normally in learning animals 10 min after a single training trial. Inhibition of PKA by Rp-cAMPs blocked intermediate-term and long-term memory, with no effect on short-term memory. PKA inhibition also prevented learning-associated CREB phosphorylation, a transcription factor implicated in long-term memory. When long-term memory was rescued through increased β-adrenoceptor activation, CREB phosphorylation was restored. Intermediate-term and long-term, but not short-term odor-preference memories were generated by pairing odor with direct PKA activation using intrabulbar Sp-cAMPs, which bypasses β-adrenoceptor activation. Higher levels of Sp-cAMPs enhanced memory by extending normal 24-h retention to 48-72 h. These results suggest that increased bulbar PKA is necessary and sufficient for the induction of intermediate-term and long-term odor-preference memory, and suggest that PKA activation levels also modulate memory duration. However, short-term memory appears to use molecular mechanisms other than the PKA/CREB pathway. These mechanisms, which are also recruited by β-adrenoceptor activation, must operate in parallel with PKA activation.

The influence of single bursts versus single spikes at excitatory dendrodendritic synapses

The synchronization of neuronal activity is thought to enhance information processing. There is much evidence supporting rhythmically bursting external tufted cells (ETCs) of the rodent olfactory bulb glomeruli coordinating the activation of glomerular interneurons and mitral cells via dendrodendritic excitation. However, as bursting has variable significance at axodendritic cortical synapses, it is not clear if ETC bursting imparts a specific functional advantage over the preliminary spike in dendrodendritic synaptic networks. To answer this question, we investigated the influence of single ETC bursts and spikes with the in vitro rat olfactory bulb preparation at different levels of processing, via calcium imaging of presynaptic ETC dendrites, dual electrical recording of ETC -interneuron synaptic pairs, and multicellular calcium imaging of ETC-induced population activity. Our findings supported single ETC bursts, versus single spikes, driving robust presynaptic calcium signaling, which in turn was associated with profound extension of the initial monosynaptic spike-driven dendrodendritic excitatory postsynaptic potential. This extension could be driven by either the spike-dependent or spike-independent components of the burst. At the population level, burst-induced excitation was more widespread and reliable compared with single spikes. This further supports the ETC network, in part due to a functional advantage of bursting at excitatory dendrodendritic synapses, coordinating synchronous activity at behaviorally relevant frequencies related to odor processing in vivo.

Normal mitral cell dendritic development in the setting of Mecp2 mutation

Rett syndrome (RTT) is an autism spectrum disorder caused by mutation in the gene encoding methyl CpG binding protein 2 (MECP2). Evidence to date suggests that these disorders display defects in synaptic organization and plasticity. A hallmark of the pathology in RTT has been identified as decreased dendritic arborization, which has been interpreted to represent abnormal dendritic formation and pruning during development. Our previous studies revealed that olfactory axons display defective pathfinding and targeting in the setting of Mecp2 mutation. In the present work, we use Mecp2 mutant mouse models and the olfactory system to investigate dendritic development. Here, we demonstrate that mitral cell dendritic development proceeds normally in mutant mice, resulting in typical dendritic morphology at early postnatal ages. We also failed to detect abnormalities in dendritic inputs at symptomatic stages when glomeruli from mutant mice appear smaller in area than the wild type (WT) (6 weeks postnatally). Collectively, these findings suggest that the initial defects in glomeruli impairment seen with Mecp2 mutation do not result from abnormal dendritic development. Our results using the olfactory system indicate that dendritic abnormalities are not an early feature in the abnormalities incurred by Mecp2 mutation.

Loss of olfactory cell adhesion molecule reduces the synchrony of mitral cell activity in olfactory glomeruli

Odours generate activity in olfactory receptor neurons, whose axons contact the dendritic tufts of mitral cells within olfactory bulb glomeruli. These axodendritic synapses are anatomically separated from dendrodendritic synapses within each glomerulus. Mitral cells within a glomerulus show highly synchronized activity as assessed with whole-cell recording from pairs of mitral cells. We examined glomerular activity in mice lacking the olfactory cell adhesion molecule (OCAM). Glomeruli in mice lacking OCAM show a redistribution of synaptic subcompartments, but the total area occupied by axonal inputs was similar to wild-type mice. Stimulation of olfactory nerve bundles showed that excitatory synaptic input to mitral cells as well as dendrodendritic inhibition was unaffected in the knockout. However, correlated spiking in mitral cells was significantly reduced, as was electrical coupling between apical dendrites. To analyse slow network dynamics we induced slow oscillations with a glutamate uptake blocker. Evoked and spontaneous slow oscillations in mitral cells and external tufted cells were broader and had multiple peaks in OCAM knockout mice, indicating that synchrony of slow glomerular activity was also reduced. To assess the degree of shared activity between mitral cells under physiological conditions, we analysed spontaneous sub-threshold voltage oscillations using coherence analysis. Coherent activity was markedly reduced in cells from OCAM knockout mice across a broad range of frequencies consistent with a decrease in tightly time-locked activity. We suggest that synchronous activity within each glomerulus is dependent on segregation of synaptic subcompartments.

Roles of glial cells in neural circuit formation: insights from research in insects

Investigators over the years have noted many striking similarities in the structural organization and function of neural circuits in higher invertebrates and vertebrates. In more recent years, the discovery of similarities in the cellular and molecular mechanisms that guide development of these circuits has driven a revolution in our understanding of neural development. Cellular mechanisms discovered to underlie axon pathfinding in grasshoppers have guided productive studies in mammals. Genes discovered to play key roles in the patterning of the fruitfly's central nervous system have subsequently been found to play key roles in mice. The diversity of invertebrate species offers to investigators numerous opportunities to conduct experiments that are harder or impossible to do in vertebrate species, but that are likely to shed light on mechanisms at play in developing vertebrate nervous systems. These experiments elucidate the broad suite of cellular and molecular interactions that have the potential to influence neural circuit formation across species. Here we focus on what is known about roles for glial cells in some of the important steps in neural circuit formation in experimentally advantageous insect species. These steps include axon pathfinding and matching to targets, dendritic patterning, and the sculpting of synaptic neuropils. A consistent theme is that glial cells interact with neurons in two-way, reciprocal interactions. We emphasize the impact of studies performed in insects and explore how insect nervous systems might best be exploited next as scientists seek to understand in yet deeper detail the full repertory of functions of glia in development.

Synaptic connectivity of serotonergic axons in the olfactory glomeruli of the rat olfactory bulb

Although the major mode of transmission for serotonin in the brain is volume transmission, previous anatomical studies have demonstrated that serotonergic axons do form synaptic contacts. The olfactory glomeruli of the olfactory bulb of mammals receive a strong serotonergic innervation from the dorsal and medial raphe nuclei. In the present report, we investigate the synaptic connectivity of these serotonergic axons in the glomerular neuropil of the rat olfactory bulb. Our study shows that serotonergic axons form asymmetrical synaptic contacts on dendrites within the glomerular neuropil. Analyzing the neurochemical nature of the synaptic targets, we have found that 55% of the synapses were on GABA-immunopositive profiles and 45% on GABA-immunonegative profiles. These data indicate that barely half of the contacts were found in GABA-immunonegative profiles and half of the synapses in GABA-positive dendrites belonging to type 1 periglomerular cells. Synaptic contacts from serotonergic axons on dendrites of principal cells cannot be excluded, since some of the GABA-immunonegative postsynaptic profiles contacted by serotonergic axons had the typical ultrastructural features of bulbar principal cell dendrites. Altogether, our results suggest a complex action of the serotonergic system in the modulation of the bulbar circuitry.

Systems approach to explore components and interactions in the presynapse

The application of proteomic techniques to neuroscientific research provides an opportunity for a greater understanding of nervous system structure and function. As increasing amounts of neuroproteomic data become available, it is necessary to formulate methods to integrate these data in a meaningful way to obtain a more comprehensive picture of neuronal subcompartments. Furthermore, computational methods can be used to make biologically relevant predictions from large proteomic data sets. Here, we applied an integrated proteomics and systems biology approach to characterize the presynaptic (PRE) nerve terminal. For this, we carried out proteomic analyses of presynaptically enriched fractions, and generated a PRE literature-based protein-protein interaction network. We combined these with other proteomic analyses to generate a core list of 117 PRE proteins, and used graph theory-inspired algorithms to predict 92 additional components and a PRE complex containing 17 proteins. Some of these predictions were validated experimentally, indicating that the computational analyses can identify novel proteins and complexes in a subcellular compartment. We conclude that the combination of techniques (proteomics, data integration, and computational analyses) used in this study are useful in obtaining a comprehensive understanding of functional components, especially low-abundance entities and/or interactions in the PRE nerve terminal.

Adult neurogenesis and the olfactory system

Though initially described in the early 1960s, it is only within the past decade that the concept of continuing adult neurogenesis has gained widespread acceptance. Neuroblasts from the subventricular zone (SVZ) migrate along the rostral migratory stream (RMS) into the olfactory bulb, where they differentiate into interneurons. Neuroblasts from the subgranular zone (SGZ) of the hippocampal formation show relatively little migratory behavior, and differentiate into dentate gyrus granule cells. In sharp contrast to embryonic and perinatal development, these newly differentiated neurons must integrate into a fully functional circuit, without disrupting ongoing performance. Here, after a brief historical overview and introduction to olfactory circuitry, we review recent advances in the biology of neural stem cells, mechanisms of migration in the RMS and olfactory bulb, differentiation and survival of new neurons, and finally mechanisms of synaptic integration. Our primary focus is on the olfactory system, but we also contrast the events occurring there with those in the hippocampal formation. Although both SVZ and SGZ neurogenesis are involved in some types of learning, their full functional significance remains unclear. Since both systems offer models of integration of new neuroblasts, there is immense interest in using neural stem cells to replace neurons lost in injury or disease. Though many questions remain unanswered, new insights appear daily about adult neurogenesis, regulatory mechanisms, and the fates of the progeny. We discuss here some of the central features of these advances, as well as speculate on future research directions.

Olfactory sensory axon growth and branching is influenced by sonic hedgehog

Olfactory sensory neuron (OSN) axons extend from the olfactory epithelium to the olfactory bulb without branching until they reach their target region, the glomerulus. In this report, we present evidence to support the involvement of sonic hedgehog in promoting rat olfactory sensory axons to branch and to enter into the glomerulus. Sonic hedgehog (Shh) protein is detected in the glomeruli of the olfactory bulb, whereas its transcript is expressed in the mitral and tufted cells, suggesting that Shh in the glomeruli is produced by mitral and tufted cells. In primary OSN cultures, Shh-N peptide promotes olfactory axon branching. When Shh function is neutralized in vivo by its antibody, growth of newly generated OSN axons into the glomeruli is vastly reduced.

Glial investment of the adult and developing antennal lobe of Drosophila

In recent years the Drosophila olfactory system, with its unparalleled opportunities for genetic dissection of development and functional organization, has been used to study the development of central olfactory neurons and the molecular basis of olfactory coding. The results of these studies have been interpreted in the absence of a detailed understanding of the steps in maturation of glial cells in the antennal lobe. Here we present a high-resolution study of the glia associated with olfactory glomeruli in adult and developing antennal lobes. The study provides a basis for comparison of findings in Drosophila with those in the moth Manduca sexta that indicate a critical role for glia in antennal lobe development. Using flies expressing GFP under a Nervana2 driver to visualize glia for confocal microscopy, and probing at higher resolution with the electron microscope, we find that glial development in Drosophila differs markedly from that in moths: glial cell bodies remain in a rind around the glomerular neuropil; glial processes ensheathe axon bundles in the nerve layer but likely contribute little to axonal sorting; their processes insinuate between glomeruli only very late and then form only a sparse, open network around each glomerulus; and glial processes invade the synaptic neuropil. Taking our results in the context of previous studies, we conclude that glial cells in the developing Drosophila antennal lobe are unlikely to play a strong role in either axonal sorting or glomerulus stabilization and that in the adult, glial processes do not electrically isolate glomeruli from their neighbors.

Synaptic organization of the olfactory bulb based on chemical coding of neurons

Olfaction is one of the chemical senses in both vertebrate and invertebrate animals essential for a variety of social behaviors. Recent molecular biological and physiological studies using optical recording have indicated elaborate mechanisms in the main olfactory bulb for processing input from olfactory receptor neurons and control of output to higher centers in the brain. The current challenge is to identify a structural basis for understanding such elaborate molecular and functional organization. Immunocytochemistry and other advanced technologies have enabled us to label bulbar neurons selectively, and they have shown that the olfactory bulb has much greater heterogeneity in chemical and structural neuronal organization and in synaptic connectivity than previously believed. This review describes the structural aspects of the main olfactory bulb of rats and summarizes the findings for its synaptic organization based on chemical coding of neurons. Current uncertainties and issues that need to be clarified in the future are also discussed.

STOP proteins contribute to the maturation of the olfactory system

Regulation of microtubule dynamics is crucial for axon growth and guidance as well as for the establishment of synaptic connections. STOPs (Stable Tubule Only Polypeptides) are microtubule-associated proteins that regulate microtubule stabilization but are also able to interact with actin or Golgi membranes. Here, we have investigated the involvement of STOPs during the development of the olfactory system. We first describe the spatio-temporal expression patterns of N- and E-STOP, the two neuronal-specific isoforms of STOP. E- and N-STOP are expressed in the axonal compartment of olfactory sensory neurons, but are differentially regulated during development. Interestingly, each neuronal isoform displays a specific gradient distribution within the olfactory nerve layer. Then, we have examined the development of the olfactory system in the absence of STOPs. Olfactory axons display a normal outgrowth and targeting in STOP-null mice, but maturation of the synapses in the glomerular neuropil is altered.

Two GABAergic intraglomerular circuits differentially regulate tonic and phasic presynaptic inhibition of olfactory nerve terminals

Olfactory nerve axons terminate in olfactory bulb glomeruli forming excitatory synapses onto the dendrites of mitral/tufted (M/T) and juxtaglomerular cells, including external tufted (ET) and periglomerular (PG) cells. PG cells are heterogeneous in neurochemical expression and synaptic organization. We used a line of mice expressing green fluorescent protein under the control of the glutamic acid decarboxylase 65-kDa gene (GAD65+) promoter to characterize a neurochemically identified subpopulation of PG cells by whole cell recording and subsequent morphological reconstruction. GAD65+ GABAergic PG cells form two functionally distinct populations: 33% are driven by monosynaptic olfactory nerve (ON) input (ON-driven PG cells), the remaining 67% receive their strongest drive from an ON-->ET-->PG circuit with no or weak monosynaptic ON input (ET-driven PG cells). In response to ON stimulation, ON-driven PG cells exhibit paired-pulse depression (PPD), which is partially reversed by GABA(B) receptor antagonists. The ON-->ET-->PG circuit exhibits phasic GABA(B)-R-independent PPD. ON input to both circuits is under tonic GABA(B)-R-dependent inhibition. We hypothesize that this tonic GABA(B)R-dependent presynaptic inhibition of olfactory nerve terminals is due to autonomous bursting of ET cells in the ON-->ET-->PG circuit, which drives tonic spontaneous GABA release from ET-driven PG cells. Both circuits likely produce tonic and phasic postsynaptic inhibition of other intraglomerular targets. Thus olfactory bulb glomeruli contain at least two functionally distinct GABAergic circuits that may play different roles in olfactory coding.