Glomerular synaptic responses to olfactory nerve input in rat olfactory bulb slices

Primary and secondary olfactory centres in human ontogeny

The olfactory centres are the evolutionary oldest and most conservative area of the telencephalon. Olfactory deficiencies are involved in a large spectrum of neurologic disorders and neurodegenerative diseases. The growing interest in human olfaction has been also been driven by COVID-19-induced transitional anosmia. Nevertheless, recent data on the human olfactory centres concerning normal histology and morphogenesis are rare. Published data in the field are mainly restricted to classic studies with non-uniform nomenclature and varied definitions of certain olfactory areas. While the olfactory system in model animals (rats, mice, and more rarely non-human primates) has been extensively investigated, the developmental timetable of olfactory centres in both human prenatal and postnatal ontogeny are poorly understood and unsystemised, which complicates the process of analysing human material, including medical researches. The main purpose of this review is to provide and discuss relevant morphological data on the normal ontogeny of the human olfactory centres, with a focus on the timetable of maturation and developmental cytoarchitecture, and with special reference to the definitions and terminology of certain olfactory areas.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

Insight in taste alterations during treatment with protein kinase inhibitors

The role of Protein Kinase Inhibitors (PKI) in the treatment of various types of cancer is increasingly prominent. Their clinical application is accompanied by the development of side effects, among which patient-reported taste alterations. These alterations are missed frequently, but impair nutritional intake, are associated with weight loss and often result in significant morbidity, especially in the context of chronic administration. Accurate reporting of taste alterations is hampered by lack of modules for symptom objectification and inadequate understanding on the underlying mechanisms. In this review we initially describe the physiology of taste and smell and the mechanism of action of PKIs. We proceed to summarize taste related side effects as reported in major clinical trials and describe possible causal factors. Lastly, an in-depth analysis is given on potential molecular pathways responsible for the PKI-induced taste alterations. Objectification of patient-reported symptoms and universal reporting, along with a better understanding of the underlying mechanisms, will lead to early recognition and optimized treatment, ultimately improving patient adherence and quality of life.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Tunicamycin impairs olfactory learning and synaptic plasticity in the olfactory bulb

Tunicamycin (TM) induces endoplasmic reticulum (ER) stress and inhibits N-glycosylation in cells. ER stress is associated with neuronal death in neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease, and most patients complain of the impairment of olfactory recognition. Here we examined the effects of TM on aversive olfactory learning and the underlying synaptic plasticity in the main olfactory bulb (MOB). Behavioral experiments demonstrated that the intrabulbar infusion of TM disabled aversive olfactory learning without affecting short-term memory. Histological analyses revealed that TM infusion upregulated C/EBP homologous protein (CHOP), a marker of ER stress, in the mitral and granule cell layers of MOB. Electrophysiological data indicated that TM inhibited tetanus-induced long-term potentiation (LTP) at the dendrodendritic excitatory synapse from mitral to granule cells. A low dose of TM (250nM) abolished the late phase of LTP, and a high dose (1μM) inhibited the early and late phases of LTP. Further, high-dose, but not low-dose, TM reduced the paired-pulse facilitation ratio, suggesting that the inhibitory effects of TM on LTP are partially mediated through the presynaptic machinery. Thus, our results support the hypothesis that TM-induced ER stress impairs olfactory learning by inhibiting synaptic plasticity via presynaptic and postsynaptic mechanisms in MOB.

The effect of bilirubin on the excitability of mitral cells in the olfactory bulb of the rat

Olfactory dysfunction is a common clinical phenomenon observed in various liver diseases. Previous studies have shown a correlation between smell disorders and bilirubin levels in patients with hepatic diseases. Bilirubin is a well-known neurotoxin; however, its effect on neurons in the main olfactory bulb (MOB), the first relay in the olfactory system, has not been examined. We investigated the effect of bilirubin (>3 μM) on mitral cells (MCs), the principal output neurons of the MOB. Bilirubin increased the frequency of spontaneous firing and the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs). TTX completely blocked sEPSCs in almost all of the cells tested. Bilirubin activity was partially blocked by N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepro pionic acid (AMPA) receptor antagonists. Furthermore, we found that bilirubin increased the frequency of intrinsic firing independent of synaptic transmission in MCs. Our findings suggest that bilirubin enhances glutamatergic transmission and strengthens intrinsic firing independent of synaptic transmission, all of which cause hyperexcitability in MCs. Our findings provide the basis for further investigation into the mechanisms underlying olfactory dysfunction that are often observed in patients with severe liver disease.

Sensory-Evoked Intrinsic Imaging Signals in the Olfactory Bulb Are Independent of Neurovascular Coupling

Functional brain-imaging techniques used in humans and animals, such as functional MRI and intrinsic optical signal (IOS) imaging, are thought to largely rely on neurovascular coupling and hemodynamic responses. Here, taking advantage of the well-described micro-architecture of the mouse olfactory bulb, we dissected the nature of odor-evoked IOSs. Using in vivo pharmacology in transgenic mouse lines reporting activity in different cell types, we show that parenchymal IOSs are largely independent of neurotransmitter release and neurovascular coupling. Furthermore, our results suggest that odor-evoked parenchymal IOSs originate from changes in light scattering of olfactory sensory neuron axons, mostly due to water movement following action potential propagation. Our study sheds light on a direct correlate of neuronal activity, which may be used for large-scale functional brain imaging.

Mechanisms underlying early odor preference learning in rats

Early odor preference training in rat pups produces behavioral preferences that last from hours to lifetimes. Here, we discuss the molecular and circuitry changes we have observed in the olfactory bulb (OB) and in the anterior piriform cortex (aPC) following odor training. For normal preference learning, both structures are necessary, but learned behavior can be initiated by initiating local circuit change in either structure. Our evidence relates dynamic molecular and circuit changes to memory duration and storage localization. Results using this developmental model are consistent with biological memory theories implicating N-methyl-D-aspartate (NMDA) receptors and β-adrenoceptors, and their associated cascades, in memory induction and consolidation. Finally, our examination of the odor preference model reveals a primary role for increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor synaptic strength, and in network strength, in the creation and maintenance of preference memory in both olfactory structures.

Olfactory bulb glomerular NMDA receptors mediate olfactory nerve potentiation and odor preference learning in the neonate rat

Rat pup odor preference learning follows pairing of bulbar beta-adrenoceptor activation with olfactory input. We hypothesize that NMDA receptor (NMDAR)-mediated olfactory input to mitral cells is enhanced during training, such that increased calcium facilitates and shapes the critical cAMP pattern. Here, we demonstrate, in vitro, that olfactory nerve stimulation, at sniffing frequencies, paired with beta-adrenoceptor activation, potentiates olfactory nerve-evoked mitral cell firing. This potentiation is blocked by a NMDAR antagonist and by increased inhibition. Glomerular disinhibition also induces NMDAR-sensitive potentiation. In vivo, in parallel, behavioral learning is prevented by glomerular infusion of an NMDAR antagonist or a GABA(A) receptor agonist. A glomerular GABA(A) receptor antagonist paired with odor can induce NMDAR-dependent learning. The NMDA GluN1 subunit is phosphorylated in odor-specific glomeruli within 5 min of training suggesting early activation, and enhanced calcium entry, during acquisition. The GluN1 subunit is down-regulated 3 h after learning; and at 24 h post-training the GluN2B subunit is down-regulated. These events may assist memory stability. Ex vivo experiments using bulbs from trained rat pups reveal an increase in the AMPA/NMDA EPSC ratio post-training, consistent with an increase in AMPA receptor insertion and/or the decrease in NMDAR subunits. These results support a model of a cAMP/NMDA interaction in generating rat pup odor preference learning.

What a nostril knows: olfactory nerve-evoked AMPA responses increase while NMDA responses decrease at 24-h post-training for lateralized odor preference memory in neonate rat

Increased AMPA signaling is proposed to mediate long-term memory. Rat neonates acquire odor preferences in a single olfactory bulb if one nostril is occluded at training. Memory testing here confirmed that only trained bulbs support increased odor preference at 24 h. Olfactory nerve field potentials were tested at 24 h in slices from trained and untrained bulbs. A larger AMPA component and a smaller NMDA component characterized responses in the bulb receiving odor preference training. Field potential changes were not seen in a bulbar region separate from the lateral odor-encoding area. These results support models in which memory is mediated by increased olfactory nerve-mitral cell AMPA signaling, and memory stability is promoted by decreased NMDA-mediated signaling.

Role of astrocytes in neurovascular coupling

Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain.

The source of spontaneous activity in the main olfactory bulb of the rat

Introduction

In vivo, most neurons in the main olfactory bulb exhibit robust spontaneous activity. This paper tests the hypothesis that spontaneous activity in olfactory receptor neurons drives much of the spontaneous activity in mitral and tufted cells via excitatory synapses.

Methods

Single units were recorded in vivo from the main olfactory bulb of a rat before, during, and after application of lidocaine to the olfactory nerve. The effect of lidocaine on the conduction of action potentials from the olfactory epithelium to the olfactory bulb was assessed by electrically stimulating the olfactory nerve rostral to the application site and monitoring the field potential evoked in the bulb.

Results

Lidocaine caused a significant decrease in the amplitude of the olfactory nerve evoked field potential that was recorded in the olfactory bulb. By contrast, the lidocaine block did not significantly alter the spontaneous activity of single units in the bulb, nor did it alter the field potential evoked by electrical stimulation of the lateral olfactory tract. Lidocaine block also did not change the temporal patters of action potential or their synchronization with respiration.

Conclusions

Spontaneous activity in neurons of the main olfactory bulb is not driven mainly by activity in olfactory receptor neurons despite the extensive convergence onto mitral and tufted cells. These results suggest that spontaneous activity of mitral and tufted is either an inherent property of these cells or is driven by centrifugal inputs to the bulb.

An olfactory bulb slice-based biosensor for multi-site extracellular recording of neural networks

Multi-site recording is the important component for studies of the neural networks. In order to investigate the electrophysiological properties of the olfactory bulb neural networks, we developed a novel slice-based biosensor for synchronous measurement with multi-sites. In the present study, the horizontal olfactory bulb slices with legible layered structures were prepared as the sensing element to construct a tissue-based biosensor with the microelectrode array. This olfactory bulb slice-based biosensor was used to simultaneously record the extracellular potentials from multi-positions. Spike detection and cross-correlation analysis were applied to evaluate the electrophysiological activities. The spontaneous potentials as well as the induced responses by glutamic acid took on different electrophysiological characteristics and firing patterns at the different sites of the olfactory bulb slice. This slice-based biosensor can realize multi-site synchronous monitoring and is advantageous for searching after the firing patterns and synaptic connections in the olfactory bulb neural networks. It is also helpful for further probing into olfactory information encoding of the olfactory neural networks.

Correlated firing in tufted cells of mouse olfactory bulb

Temporally correlated spike discharges are proposed to be important for the coding of olfactory stimuli. In the olfactory bulb, correlated spiking is known in two classes of output neurons, the mitral cells and external tufted cells. We studied a third major class of bulb output neurons, the middle tufted cells, analyzing their bursting and spike timing correlations, and their relation to mitral cells. Using patch-clamp and fluorescent tracing, we recorded spontaneous spiking from tufted-tufted or mitral-tufted cell pairs with visualized dendritic projections in mouse olfactory bulb slices. We found peaks in spike cross-correlograms indicating correlated activity on both fast (peak width 1-50 ms) and slow (peak width>50 ms) time scales, only in pairs with convergent glomerular projections. Coupling appeared tighter in tufted-tufted pairs, which showed correlated firing patterns and smaller mean width and lag of narrow peaks. Some narrow peaks resolved into 2-3 sub-peaks (width 1-12 ms), indicating multiple modes of fast correlation. Slow correlations were related to bursting activity, while fast correlations were independent of slow correlations, occurring in both bursting and non-bursting cells. The AMPA receptor antagonist NBQX (20 microM) failed to abolish broad or narrow peaks in either tufted-tufted or mitral-tufted pairs, and changes of peak height and width in NBQX were not significantly different from spontaneous drift. Thus, AMPA-receptors are not required for fast and slow spike correlations. Electrical coupling was observed in all convergent tufted-tufted and mitral-tufted pairs tested, suggesting a potential role for gap junctions in concerted firing. Glomerulus-specific correlation of spiking offers a useful mechanism for binding the output signals of diverse neurons processing and transmitting different sensory information encoded by common olfactory receptors.

Synaptic activation of T-type Ca2+ channels via mGluR activation in the primary dendrite of mitral cells

Mitral cells are the primary output of the olfactory bulb, projecting to many higher brain areas. Understanding how mitral cells process and transmit information is key to understanding olfactory perception. Mitral dendrites possess high densities of voltage-gated channels, are able to initiate and propagate orthodromic and antidromic action potentials, and release neurotransmitter. We show that mitral cells also possess a low-voltage-activated T-type Ca(2+) current. Immunohistochemistry shows strong Cav3.3 labeling in the primary dendrite and apical tuft with weaker staining in basal dendrites and no staining in somata. A low-voltage-activated Ca(2+) current activates from -68 mV, is blocked by 500 microM Ni(2+) and 50 microM NNC 55-0396, but is insensitive to 50 microM Ni(2+) and 500 microM isradipine. 2-photon Ca(2+) imaging shows that T channels are functionally expressed in the primary dendrite where their activity determines the resting [Ca(2+)] and are responsible for subthreshold voltage-dependent Ca(2+) changes previously observed in vivo. Application of the group 1 mGluR agonist dihydroxyphenylglycine (DHPG) (50 microM) robustly upregulates T-channel current in the primary and apical tuft dendrite. Olfactory nerve stimulation generates a long-lasting depolarization, and we show that mGluRs recruit T channels to contribute approximately 36% of the voltage integral of this depolarization. The long-lasting depolarization results in sustained firing and block of T channels decreased action potential firing by 84.1 +/- 4.6%. Therefore upregulation of T channels by mGluRs is required for prolonged firing in response to olfactory nerve input.

Role of cortical feedback in regulating inhibitory microcircuits

The olfactory bulb contains an impressive array of specialized inhibitory local circuits. The most frequent inhibitory microcircuit in this brain region is the reciprocal dendrodendritic synapse formed between the lateral dendrites of mitral cells and distal dendritic spines of GABAergic granule cells. Recent work discussed in this review suggests that release of GABA from granule cell spines may reflect near-coincident activation of both mitral cell-to-granule cell synapses and proximal excitatory synapses on granule cells that originate from pyramidal cells in piriform cortex. Recent work using two-photon guided microstimulation demonstrated that proximal and distal excitatory synapses onto granule cells exhibit different forms of short-term plasticity, with feedback inputs from piriform cortex facilitating when tested with brief ( approximately 50 ms) interstimulus intervals. One consequence of this synaptic plasticity is that short duration, gamma-frequency, oscillatory discharges in piriform cortical cells evoke summating excitatory postsynaptic potentials (EPSPs) in granule cells that effectively trigger action potentials. Piriform cortex stimulation can gate dendrodendritic inhibition onto mitral cells, presumably through the ability of EPSP-driven action potentials in granule cells to temporarily relieve the tonic blockade of NMDA receptors by extracellular Mg(2+) ions. Feedback projections in other CNS systems also may target inhibitory neurons, such as the backprojection from CA3 pyramidal neurons to GABAergic hilar interneurons. The ability of downstream processing areas to rapidly and selectively activate inhibitory interneurons in other brain regions may provide an important mechanism to dynamically modulate biological information processing.

Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

Coupling of neural activity to blood flow in olfactory glomeruli is mediated by astrocytic pathways

Functional neuroimaging uses activity-dependent changes in cerebral blood flow to map brain activity, but the contributions of presynaptic and postsynaptic activity are incompletely understood, as are the underlying cellular pathways. Using intravital multiphoton microscopy, we measured presynaptic activity, postsynaptic neuronal and astrocytic calcium responses, and erythrocyte velocity and flux in olfactory glomeruli during odor stimulation in mice. Odor-evoked functional hyperemia in glomerular capillaries was highly correlated with glutamate release, but did not require local postsynaptic activity. Odor stimulation induced calcium transients in astrocyte endfeet and an associated dilation of upstream arterioles. Calcium elevations in astrocytes and functional hyperemia depended on astrocytic metabotropic glutamate receptor 5 and cyclooxygenase activation. Astrocytic glutamate transporters also contributed to functional hyperemia through mechanisms independent of calcium rises and cyclooxygenase activation. These local pathways initiated by glutamate account for a large part of the coupling between synaptic activity and functional hyperemia in the olfactory bulb.

Long-term enhancement of synchronized oscillations by adrenergic receptor activation in the olfactory bulb

The noradrenergic system is widely thought to be important for associative learning in the olfactory system through actions in the first processing structure, the main olfactory bulb (MOB). Here, we used extracellular local field potential (LFP) and patch-clamp recordings in rat MOB slices to examine norepinephrine (NE)-induced long-term changes in circuit properties that might underlie learning. During responses to patterned olfactory nerve stimulation mimicking the breathing cycle, NE induced a long-term increase in gamma frequency (30-70 Hz) synchronized oscillations. The enhancement persisted long after washout of NE (<or=70 min), depended on the combined actions of NE and neuronal stimulation, and seemed to be caused by enhanced excitatory drive on the mitral/granule cell network that underlies rapid gamma oscillations. The last effect, increased excitation, was manifested as an increase in evoked long-lasting depolarizations (LLDs) in mitral cells. From a functional perspective, the observed long-term cellular and network changes could promote associative learning by amplifying odor-specific signals.

Sensory deafferentation transsynaptically alters neuronal GluR1 expression in the external plexiform layer of the adult mouse main olfactory bulb

Altered distribution of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 has been linked to stimulation-dependent changes in synaptic efficacy, including long-term potentiation and depression. The main olfactory bulb (OB) remains plastic throughout life; how GluR1 may be involved in this plasticity is unknown. We have previously shown that neonatal naris occlusion reduces numbers of interneuron cell bodies that are immunoreactive for GluR1 in the external plexiform layer (EPL) of the adult mouse OB. Here, we show that immunoreactivity of mouse EPL interneurons for GluR1 is also dramatically reduced following olfactory deafferentation in adulthood. We further show that expression of glutamic acid decarboxylase (GAD) 65, 1 of 2 GAD isoforms expressed by adult gamma-aminobutyric acidergic interneurons, is reduced, but to a much smaller extent, and that in double-labeled cells, immunoreactivity for the Ca(2+)-binding protein parvalbumin (PV) is also reduced. In addition, GluR1 expression is reduced in presumptive tufted cells and interneurons that are negative for GAD65 and PV. Consistent with previous reports, sensory deafferentation resulted in little neuronal degeneration in the adult EPL, indicating that these differences were not likely due to death of EPL neurons. Together, these results suggest that olfactory input regulates expression of the GluR1 AMPA receptor subunit by tufted cells that may in turn regulate GluR1 expression by interneurons within the OB EPL.

Experience-dependent modification of primary sensory synapses in the mammalian olfactory bulb

Experience-dependent changes in neural circuits have traditionally been investigated several synapses downstream of sensory input. Whether experience can alter the strength of primary sensory synapses remains mostly unknown. To address this issue, we investigated the consequences of odor deprivation on synapses made by olfactory sensory axons in the olfactory bulb of rats. Odor deprivation triggered an increase in the probability of glutamate release from olfactory sensory neuron synapses. Deprivation also increased the amplitude of quantal synaptic currents mediated by AMPA- and NMDA-type glutamate receptors, as well as the abundance of these receptors in the glomerular region. Our results demonstrate that sensory experience is capable of modulating synaptic strength at the earliest stages of information transfer between the environment and an organism. Such compensatory experience-dependent changes may represent a mechanism of sensory gain control.

Activation of group I metabotropic glutamate receptors on main olfactory bulb granule cells and periglomerular cells enhances synaptic inhibition of mitral cells

Granule and periglomerular cells in the main olfactory bulb express group I metabotropic glutamate receptors (mGluRs). The group I mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) increases GABAergic spontaneous IPSCs (sIPSCs) in mitral cells, yet the presynaptic mechanism(s) involved and source(s) of the IPSCs are unknown. We investigated the actions of DHPG on sIPSCs and TTX-insensitive miniature IPSCs (mIPSCs) recorded in mitral and external tufted cells in rat olfactory bulb slices. DHPG, acting at mGluR1 and mGluR5, increased the rate but not amplitude of sIPSCs and mIPSCs in both cell types. The increase in mIPSCs depended on voltage-gated Ca2+ channels but persisted when ionotropic glutamate receptors and sodium spikes were blocked. Focal DHPG puffs onto granule cells or bath application after glomerular layer (GL) excision failed to increase mIPSCs in mitral cells. Additionally, GL excision reduced sIPSCs in mitral cells by 50%, suggesting that periglomerular cells exert strong tonic GABAergic inhibition of mitral cells. In contrast, GL DHPG puffs readily increased mIPSCs. These findings indicate that DHPG-evoked GABA release from granule cells requires spikes, whereas in the GL, DHPG facilitates periglomerular cell GABA release via both spike-dependent and spike-independent presynaptic mechanisms. We speculate that mGluRs amplify spike-driven lateral inhibition through the mitral-to-granule cell circuit, whereas GL mGluRs may play a more important role in amplifying intraglomerular inhibition after subthreshold input.

Chemical factors determine olfactory system beta oscillations in waking rats

Recent studies have pointed to olfactory system beta oscillations of the local field potential (15-30 Hz) and their roles both in learning and as specific responses to predator odors. To describe odorant physical properties, resultant behavioral responses and changes in the central olfactory system that may induce these oscillations without associative learning, we tested rats with 26 monomolecular odorants spanning 6 log units of theoretical vapor pressure (estimate of relative vapor phase concentration) and 10 different odor mixtures. We found odorant vapor phase concentration to be inversely correlated with investigation time on the first presentation, after which investigation times were brief and not different across odorants. Analysis of local field potentials from the olfactory bulb and anterior piriform cortex shows that beta oscillations in waking rats occur specifically in response to the class of volatile organic compounds with vapor pressures of 1-120 mmHg. Beta oscillations develop over the first three to four presentations and are weakly present for some odorants in anesthetized rats. Gamma oscillations show a smaller effect that is not restricted to the same range of odorants. Olfactory bulb theta oscillations were also examined as a measure of effective afferent input strength, and the power of these oscillations did not vary systematically with vapor pressure, suggesting that it is not olfactory bulb drive strength that determines the presence of beta oscillations. Theta band coherence analysis shows that coupling strength between the olfactory bulb and piriform cortex increases linearly with vapor phase concentration, which may facilitate beta oscillations above a threshold.

Disynaptic amplification of metabotropic glutamate receptor 1 responses in the olfactory bulb

Sensory systems often respond to rapid stimuli with high frequency and fidelity, as perhaps best exemplified in the auditory system. Fast synaptic responses are fundamental requirements to achieve this task. The importance of speed is less clear in the olfactory system. Moreover, olfactory bulb output mitral cells respond to a single stimulation of the sensory afferents with unusually long EPSPs, lasting several seconds. We examined the temporal characteristics, developmental regulation, and the mechanism generating these responses in mouse olfactory bulb slices. The slow EPSP appeared at postnatal days 10-11 and was mediated by metabotropic glutamate receptor 1 (mGluR1) and NMDA receptors. mGluR1 contribution was unexpected because its activation usually requires strong, high-frequency stimulation of inputs. However, dendritic release of glutamate from the intraglomerular network caused spillover-mediated recurrent activation of metabotropic glutamate receptors. We suggest that persistent responses in mitral cells amplify the incoming sensory information and, along with asynchronous inputs, drive odor-evoked slow temporal activity in the bulb.

Sensory-evoked intrinsic optical signals in the olfactory bulb are coupled to glutamate release and uptake

Functional imaging signals arise from metabolic and hemodynamic activity, but how these processes are related to the synaptic and electrical activity of neurons is not well understood. To provide insight into this issue, we used in vivo imaging and simultaneous local pharmacology to study how sensory-evoked neural activity leads to intrinsic optical signals (IOS) in the well-defined circuitry of the olfactory glomerulus. Odor-evoked IOS were tightly coupled to release of glutamate and were strongly modulated by activation of presynaptic dopamine and GABA-B receptors. Surprisingly, IOS were independent of postsynaptic transmission through ionotropic or metabotropic glutamate receptors, but instead were inhibited when uptake by astrocytic glutamate transporters was blocked. These data suggest that presynaptic glutamate release and uptake by astrocytes form a critical pathway through which neural activity is linked to metabolic processing and hence to functional imaging signals.

Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

Glomeruli are functional units of the olfactory bulb responsible for early processing of odor information encoded by single olfactory receptor genes. Glomerular neural circuitry includes numerous external tufted (ET) cells whose rhythmic burst firing may mediate synchronization of bulbar activity with the inhalation cycle. Bursting is entrained by glutamatergic input from olfactory nerve terminals, so specific properties of ionotropic glutamate receptors on ET cells are likely to be important determinants of olfactory processing. Particularly intriguing is recent evidence that AMPA receptors of juxta-glomerular neurons may permeate calcium. This could provide a novel pathway for regulating ET cell signaling. We tested the hypothesis that ET cells express functional calcium-permeable AMPA receptors. In rat olfactory bulb slices, excitatory postsynaptic currents (EPSCs) in ET cells were evoked by olfactory nerve shock, and by uncaging glutamate. We found attenuation of AMPA/kainate EPSCs by 1-naphthyl acetyl-spermine (NAS), an open-channel blocker specific for calcium permeable AMPA receptors. Cyclothiazide strongly potentiated EPSCs, indicating a major contribution from AMPA receptors. The current-voltage (I-V) relation of uncaging EPSCs showed weak inward rectification which was lost after > approximately 10 min of whole-cell dialysis, and was absent in NAS. In kainate-stimulated slices, Co(2+) ions permeated cells of the glomerular layer. Large AMPA EPSCs were accompanied by fluorescence signals in fluo-4 loaded cells, suggesting calcium permeation. Depolarizing pulses evoked slow tail currents with pharmacology consistent with involvement of calcium permeable AMPA autoreceptors. Tail currents were abolished by Cd(2+) and (+/-)-4-(4-aminophenyl)-2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), and were sensitive to NAS block. Glutamate autoreceptors were confirmed by uncaging intracellular calcium to evoke a large inward current. Our results provide evidence that calcium permeable AMPA receptors reside on ET cells, and are divided into at least two functionally distinct pools: postsynaptic receptors at olfactory nerve synaptic terminals, and autoreceptors sensitive to glutamate released from dendrodendritic synapses.

Spontaneous field potentials in the glomeruli of the olfactory bulb: the leading role of juxtaglomerular cells

Field potentials recorded in the olfactory bulb glomerular layer (GL) are thought to result mainly from activation of mitral and tufted cells. The contribution of juxtaglomerular cells (JG) is unknown. We tested the hypothesis that JG are the main driving force to novel spontaneous glomerular layer field potentials (sGLFPs), which were recorded in rat olfactory bulb slices maintained in an interface chamber. We found that sGLFPs have comparable magnitudes, durations and frequencies both in standard horizontal slices, where all layers with all cell types were present, and in isolated GL slices, where only JG cells were preserved. Hence, the impact of mitral and deep/medium tufted cells to sGLFPs turned out to be minor. Therefore, we propose that the main generators of sGLFPs are JG neurons. We further explored the mechanism of generation of sGLFPs using a neuronal ensemble model comprising all types of cells associated with a single glomerulus. Random orientation and homogenous distribution of dendrites in the glomerular neuropil along with surrounding shell of cell bodies of JG neurons resulted in substantial spatial restriction of the generated field potential. The model predicts that less than 20% of sGLFP can spread from one glomerulus to an adjacent one. The contribution of JG cells to the total field in the center of the glomerulus is estimated as approximately 50% ( approximately 34% periglomerular and approximately 16% external tufted cells), whereas deep/medium tufted cells provide approximately 39% and mitral cells only approximately 10%. Occasionally, some sGLFPs recorded in adjacent or remote glomeruli were cross-correlated, suggesting involvement of interglomerular communication in information coding. These results demonstrate a leading role of JG cells in activation of the main olfactory bulb (MOB) functional modules. Finally, we hypothesize that the GL is not a set of independent modules, but it represents a subsystem in the MOB network, which can perform initial processing of odors.

Effects of N-methyl-D-aspartate glutamate receptor antagonists on oscillatory signal propagation in the guinea-pig accessory olfactory bulb slice: characterization by optical, field potential and patch clamp recordings

To characterize the role of N-methyl-d-aspartate glutamate receptors in oscillations induced by a single electrical stimulation of the vomeronasal nerve layer, optical, field potential and patch clamp recordings were carried out in guinea-pig accessory olfactory bulb slice preparations. Bath application of the N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovaleric acid or MK-801, produced an increase in frequency of oscillating waves (oscillation) in external plexiform and mitral cell layers. The removal of Mg2+ from perfusate abolished oscillations, while subsequent application of 2-amino-5-phosphonovaleric acid or MK-801 restored oscillations. Vomeronasal nerve layer-evoked postsynaptic currents were analyzed by whole-cell clamp recordings from mitral and granule cells. A long-lasting excitatory postsynaptic current and periodic inhibitory postsynaptic currents, which were superimposed on the long excitatory postsynaptic current, were observed in mitral cells. The frequency of the periodic inhibitory postsynaptic currents correlated with the frequency of oscillations observed in the optical and field potential recordings. Furthermore, periodic inhibitory postsynaptic currents were blocked by puff application of bicuculline to the external plexiform layer/mitral cell layer, where mitral cells make dendrodendritic synapses with granule cells. In addition, puff application of the non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, to the external plexiform layer/mitral cell layer suppressed an early phase of periodic inhibitory postsynaptic currents (membrane oscillation), whereas 2-amino-5-phosphonovaleric acid suppressed the late phase of periodic inhibitory postsynaptic currents. These data indicate that periodic excitatory postsynaptic currents of granule cells induce relevantly periodic inhibitory postsynaptic currents in mitral cells via dendrodendritic synapses and suggest that feedback inhibition regulates generation of oscillation via activation of non-N-methyl-d-aspartate glutamate receptors and gradual attenuation of oscillation via activation of N-methyl-D-aspartate receptors on granule cells.

Olfactory nerve-evoked, metabotropic glutamate receptor-mediated synaptic responses in rat olfactory bulb mitral cells

The group I metabotropic glutamate receptor (mGluR) subtype, mGluR1, is highly expressed on the apical dendrites of olfactory bulb mitral cells and thus may be activated by glutamate released from olfactory nerve (ON) terminals. Previous studies have shown that mGluR1 agonists directly excite mitral cells. In the present study, we investigated the involvement of mGluR1 in ON-evoked responses in mitral cells in rat olfactory bulb slices using patch-clamp electrophysiology. In voltage-clamp recordings, the average EPSC evoked by single ON shocks or brief trains of ON stimulation (six pulses at 50 Hz) in normal physiological conditions were not significantly affected by the nonselective mGluR antagonist LY341495 (50-100 microM) or the mGluR1-specific antagonist LY367385 (100 microM); ON-evoked responses were attenuated, however, in a subset (36%) of cells. In the presence of blockers of ionotropic glutamate and GABA receptors, application of the glutamate uptake inhibitors THA (300 microM) and TBOA (100 microM) revealed large-amplitude, long-duration responses to ON stimulation, whereas responses elicited by antidromic activation of mitral/tufted cells were unaffected. Magnitudes of the ON-evoked responses elicited in the presence of THA-TBOA were dependent on stimulation intensity and frequency, and were maximal during high-frequency (50-Hz) bursts of ON spikes, which occur during odor stimulation. ON-evoked responses elicited in the presence of THA-TBOA were significantly reduced or completely blocked by LY341495 or LY367385 (100 microM). These results demonstrate that glutamate transporters tightly regulate access of synaptically evoked glutamate from ON terminals to postsynaptic mGluR1s on mitral cell apical dendrites. Taken together with other findings, the present results suggest that mGluR1s may not play a major role in phasic responses to ON input, but instead may play an important role in shaping slow oscillatory activity in mitral cells and/or activity-dependent regulation of plasticity at ON-mitral cell synapses.

In vivo preparation and identification of mitral cells in the main olfactory bulb of the mouse

The mouse main olfactory bulb (MOB) is commonly used as a mammalian model to study olfactory processing. The genetic techniques available with the mouse make its MOB a powerful model for analysis of neuronal circuitry. The mouse has been used as a mammalian model for all types of MOB neurons, but especially to study the activity of mitral cells. However, mouse mitral cell activity is most commonly studied in vitro. Therefore, we aimed to develop a protocol to record the activity of antidromically identified mitral cells in mouse in vivo. Currently, such a protocol does not exist. Using extracellular techniques, we report a protocol that is able to record neurons from all mouse MOB layers. Specifically, mitral cell single-units were identified by antidromic activation from the posterior piriform cortex, and their spontaneous activity was recorded for more than 30 min. This protocol is stable enough to record from single-units while buprenorphine was applied both topically to the surface of the MOB and injected systemically.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Regulation of main olfactory bulb mitral cell excitability by metabotropic glutamate receptor mGluR1

In the rodent main olfactory bulb (MOB), mitral cells (MCs) express high levels of the group I metabotropic glutamate receptor (mGluR) subtype, mGluR1. The significance of this receptor in modulating MC excitability is unknown. We investigated the physiological role of mGluR1 in regulating MC activity in rat and mouse MOB slices. The selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), but not group II or III agonists, induced potent, dose-dependent, and reversible depolarization and increased firing of MCs. These effects persisted in the presence of blockers of fast synaptic transmission, indicating that they are due to direct activation of mGluRs on MCs. Voltage-clamp recordings showed that DHPG elicited a voltage-dependent inward current consisting of multiple components sensitive to potassium and calcium channel blockade and intracellular calcium chelation. MC excitatory responses to DHPG were absent in mGluR1 knockout mice but persisted in mGluR5 knockout mice. Broad-spectrum LY341495, MCPG, as well as preferential mGluR1 LY367385 antagonists blocked the excitatory effects of DHPG and also potently modulated MC spontaneous and olfactory nerve-evoked excitability. mGluR antagonists altered spontaneous membrane potential bistability, increasing the duration of the up and down states. mGluR antagonists also substantially attenuated MC responses to sensory input, decreasing the probability and increasing the latency of olfactory nerve-evoked spikes. These findings suggest that endogenous glutamate tonically modulates MC excitability and responsiveness to olfactory nerve input, and hence the operation of the MOB circuitry, via activation of mGluR1.

Centre-surround inhibition among olfactory bulb glomeruli

Centre-surround inhibition--the suppression of activity of neighbouring cells by a central group of neurons--is a fundamental mechanism that increases contrast in patterned sensory processing. The initial stage of neural processing in olfaction occurs in olfactory bulb glomeruli, but evidence for functional interactions between glomeruli is fragmentary. Here we show that the so-called 'short axon' cells, contrary to their name, send interglomerular axons over long distances to form excitatory synapses with inhibitory periglomerular neurons up to 20-30 glomeruli away. Interglomerular excitation of these periglomerular cells potently inhibits mitral cells and forms an on-centre, off-surround circuit. This interglomerular centre-surround inhibitory network, along with the well-established mitral-granule-mitral inhibitory circuit, forms a serial, two-stage inhibitory circuit that could enhance spatiotemporal responses to odours.

Selective neuroinhibitory effects of taurine in slices of rat main olfactory bulb

Taurine is abundant in the main olfactory bulb, exceeding glutamate and GABA in concentration. In whole-cell patch-clamp recordings in rat olfactory bulb slices, taurine inhibited principal neurons, mitral and tufted cells. In these cells, taurine decreased the input resistance and caused a shift of the membrane potential toward the chloride equilibrium potential. The taurine actions were sustained under the blockade of transmitter release and were reversible and dose-dependent. At a concentration of 5 mM, typically used in this study, taurine showed 90% of its maximal effect. GABA(A) antagonists, bicuculline and picrotoxin, blocked the taurine actions, whereas the glycine receptor antagonist strychnine and GABA(B) antagonists, CGP 55845A and CGP 35348, were ineffective. These findings are consistent with taurine directly activating GABA(A) receptors and inducing chloride conductance. Taurine had no effect on periglomerular and granule interneurons. The subunit composition of GABA(A) receptors in these cells, differing from those in mitral and tufted cells, may account for taurine insensitivity of the interneurons. Taurine suppressed olfactory nerve-evoked monosynaptic responses of mitral and tufted cells while chloride conductance was blocked. This action was mimicked by the GABA(B) agonist baclofen and abolished by CGP 55845A; CGP 35348, which primarily blocks postsynaptic GABA(B) receptors, was ineffective. The taurine effect most likely was due to GABA(B) receptor-mediated inhibition of presynaptic glutamate release. Neither taurine nor baclofen affected responses of periglomerular cells. The lack of a baclofen effect implies that functional GABA(B) receptors are absent from olfactory nerve terminals that contact periglomerular cells. These results indicate that taurine decreases the excitability of mitral and tufted cells and their responses to olfactory nerve stimulation without influencing periglomerular and granule cells. Selective effects of taurine in the olfactory bulb may represent a physiologic mechanism that is involved in the inhibitory shaping of the activation pattern of principal neurons.

Neuronal gap junctions in the rat main olfactory bulb, with special reference to intraglomerular gap junctions

The structural features of neuronal gap junction-forming processes in the rat olfactory bulb were analyzed electron microscopically. Gap junctions were present in glomeruli and extraglomerular regions. In extraglomerular regions, mitral/tufted cell somata, dendrites and axon hillock-initial segments made gap junctions and mixed synapses with interneuronal processes, some of which were confirmed to be GABA positive. In glomeruli gap junctions were encountered mainly between mitral/tufted cell dendrites and diverse types of processes; a small population of them were conclusively identified as periglomerular cell dendrites. Gap junction-forming processes frequently received synapses from olfactory nerve terminals, suggesting that they could be type 1 periglomerular cells. However, the majority were GABA negative or only faintly positive and none were tyrosine hydroxylase positive, indicating that they were different from previously reported type 1 periglomerular cells. Furthermore serial sectioning analyses revealed that the majority of those processes forming gap junctions with mitral/tufted dendrites were smooth cylindrical and had few presynaptic sites, indicating that they were different from previously described periglomerular cells. These findings revealed that mitral/tufted cells make gap junctions with diverse types of neurons; and some of these gap junction-forming processes originated from some types of periglomerular cells but others from hitherto uncharacterized neuron type(s).

Pharmacological characterization of ionotropic glutamate receptors in the zebrafish olfactory bulb

The distribution of N-methyl-D-aspartate- (NMDA) and kainic acid- (KA) sensitive ionotropic glutamate receptors (iGluR) in the zebrafish olfactory bulb was assessed using an activity-dependent labeling method. Olfactory bulbs were incubated with an ion channel permeant probe, agmatine (AGB), and iGluR agonists in vitro, and the labeled neurons containing AGB were visualized immunocytochemically. Preparations exposed to 250 microM KA in the presence of a NMDA receptor antagonist (D-2-amino-5-phosphono-valeric acid) and an alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist (sym 2206), revealed KA receptor-mediated labeling of approximately 60-70% of mitral cells, juxtaglomerular cells, tyrosine hydroxylase-positive cells and granule cells. A higher proportion of ventral olfactory bulb neurons were KA-sensitive. Application of 333 microM NMDA in the presence of an AMPA/KA receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione) resulted in NMDA receptor-mediated labeling of almost all neurons. The concentrations eliciting 50% of the maximal response (effective concentration: EC(50)s) for NMDA-stimulated labeling of different cell types were not significantly different and ranged from 148 microM to 162 microM. These results suggest that while NMDA receptors with similar binding affinities are widely distributed in the neurons of the zebrafish olfactory bulb, KA receptors are heterogeneously expressed among these cells and may serve unique roles in different regions of the olfactory bulb.

Odor-stimulated glutamatergic neurotransmission in the zebrafish olfactory bulb

The role of glutamate as a neurotransmitter in the zebrafish olfactory bulb (OB) was established by examining neuronal activation following 1). glutamate receptor agonist stimulation of isolated olfactory bulbs and 2). odorant stimulation of intact fish. Four groups of neurons (mitral cells, projection neurons; granule cells, juxtaglomerular cells, and tyrosine hydroxylase-containing cells; interneurons) were identified on the basis of cell size, cell location, ionotropic glutamate receptor (iGluR) agonist/odorant sensitivity, and glutamate, gamma-aminobutyric acid (GABA), and tyrosine hydroxylase immunoreactivity. Immunoreactive glutamate levels were highest in olfactory sensory neurons (OSNs) and mitral cells, the putative glutamatergic neurons. The sensitivity of bulbar neurons to iGluR agonists and odorants was established using a cationic channel permeant probe, agmatine (AGB). Agmatine that permeated agonist- or odor-activated iGluRs was fixed in place with glutaraldehyde and detected immunohistochemically. N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA)/kainic acid (KA) iGluR agonists and odorants (glutamine, taurocholic acid) stimulated activity-dependent labeling of bulbar neurons, which was blocked with a mixture of the iGluR antagonists, D-2-amino-5-phosphono-valeric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The AMPA/KA antagonist CNQX completely blocked glutamine-stimulated AGB labeling of granule cells and tyrosine hydroxylase-containing cells, suggesting that, in these cell types, AMPA/KA receptor activation is essential for NMDA receptor activation. However, blocking AMPA/KA receptor activity failed to eliminate AGB labeling of mitral cells or juxtaglomerular cells. Collectively, these findings indicate that glutamate is the primary excitatory neurotransmitter in the zebrafish OB and that iGluR subtypes function heterogeneously in the bulbar neurons.

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.

Structure of intraglomerular dendritic tufts of mitral cells and their contacts with olfactory nerve terminals and calbindin-immunoreactive type 2 periglomerular neurons

Intraglomerular dendritic tufts of Golgi-impregnated and biotinylated dextran amine (BDA)-labeled mitral cells in the rat main olfactory bulb were analyzed in detail. In particular, the relationships of BDA-labeled tufts with olfactory nerve (ON) terminals and processes of calbindin D-28K-immunoreactive (CB-IR) cells were investigated with confocal laser-scanning light microscopic (CLSM) and electron microscopic (EM) analyses. CB-IR cells were type 2 periglomerular cells that restricted their processes in the ON-free (non-ON) zone of the glomerulus and received few synapses from ON terminals. The mitral tufts varied in complexity, but individual branches were rather simple, smooth processes that bore some branchlets and spines and extended more or less in a straight line or a gentle curve rather than winding tortuously within glomeruli as though they did not consider the compartmental organization, which consisted of ON and non-ON zones that interdigitated in a complex manner with one another. Conventional EM analysis revealed that both thin and thick, presumed proximal branches of mitral/tufted cell dendritic tufts received asymmetrical synapses from ON terminals. Correlated CLSM-EM analysis confirmed direct contacts between the BDA- and CB-labeled processes detected in the CLSM examinations, and synapses were recognized at some of those sites. Furthermore, ON terminals and CB-IR processes were distributed on both proximal and distal dendritic branches in a more or less mosaic pattern. These findings revealed that, on the mitral dendritic tufts, ON terminals and processes of type 2 periglomerular neurons were not clearly segregated proximodistally but, rather, were arranged in a mosaic pattern, which may be important in fine tuning the output from individual glomeruli.

Direct excitation of mitral cells via activation of alpha1-noradrenergic receptors in rat olfactory bulb slices

The main olfactory bulb receives a significant modulatory noradrenergic input from the locus coeruleus. Previous in vivo and in vitro studies showed that norepinephrine (NE) inputs increase the sensitivity of mitral cells to weak olfactory inputs. The cellular basis for this action of NE is not understood. The goal of this study was to investigate the effect of NE and noradrenergic agonists on the excitability of mitral cells, the main output cells of the olfactory bulb, using whole cell patch-clamp recording in vitro. The noradrenergic agonists, phenylephrine (PE, 10 microM), isoproterenol (Isop, 10 microM), and clonidine (3 microM), were used to test for the functional presence of alpha1-, beta-, and alpha2-receptors, respectively, on mitral cells. None of these agonists affected olfactory nerve (ON)-evoked field potentials recorded in the glomerular layer, or ON-evoked postsynaptic currents recorded in mitral cells. In whole cell voltage-clamp recordings, NE (30 microM) induced an inward current (54 +/- 7 pA, n = 16) with an EC(50) of 4.7 microM. Both PE and Isop also produced inward currents (22 +/- 4 pA, n = 19, and 29 +/- 9 pA, n = 8, respectively), while clonidine produced no effect (n = 6). In the presence of TTX (1 microM), and blockers of excitatory and inhibitory fast synaptic transmission [gabazine 5 microM, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) 10 microM, and (+/-)-2-amino-5-phosphonopentanoic acid (APV) 50 microM], the inward current induced by PE persisted (EC(50) = 9 microM), whereas that of Isop was absent. The effect of PE was also observed in the presence of the Ca(2+) channel blockers, cadmium (100 microM) and nickel (100 microM). The inward current caused by PE was blocked when the interior of the cell was perfused with the nonhydrolyzable GDP analogue, GDPbetaS, indicating that the alpha1 effect is mediated by G-protein coupling. The current-voltage relationship in the absence and presence of PE indicated that the current induced by PE decreased near the equilibrium potential for potassium ions. In current-clamp recordings from bistable mitral cells, PE shifted the membrane potential from the downstate (-52 mV) toward the upstate (-40 mV), and significantly increased spike generation in response to perithreshold ON input. These findings indicate that NE excites mitral cells directly via alpha1 receptors, an effect that may underlie, at least in part, increased mitral cell responses to weak ON input during locus coeruleus activation in vivo.

Odour-evoked [Ca2+] transients in mitral cell dendrites of frog olfactory glomeruli

We measured Ca2+ concentration, [Ca2+], transients in mitral cell distal apical dendritic tufts produced by physiological odour stimulation of the olfactory epithelium and electrical stimulation of the olfactory nerve (ON) using two-photon scanning and conventional wide-field microscopy of Ca2+-Green-1 dextran in an in vitro frog nose-brain preparation. Weak or strong ON shock-evoked fluorescence transients always had short latency with an onset 0-10 ms after the onset of the bulb local field potential, rapidly increasing to a peak of up to 25% fractional fluorescence change (DeltaF/F) in 10-30 ms, were blocked by 10 microM CNQX, decaying with a time constant of about 1 s. With stronger ON shocks that activated many receptor axons, an additional, delayed, sustained AP5-sensitive component (peak at approximately 0.5 s, up to 40% DeltaF/F maximum) could usually be produced. Odour-evoked [Ca2+] transients sometimes displayed a rapid onset phase that peaked within 50 ms but always had a sustained phase that peaked 0.5-1.5 s after onset, regardless of the strength of the odour or the amplitude of the response. These were considerably larger (up to 150% DeltaF/F) than those evoked by ON shock. Odour-evoked [Ca2+] transients were also distinguished from ON shock-evoked transients by tufts in different glomeruli responding with different delays (time to onset differed by up to 1.5 s between different tufts for the same odour). Odour-evoked [Ca2+] transients were increased by AMPA-kainate receptor blockade, but substantially blocked by AP5. Electrical stimulation of the lateral olfactory tract (5-6 stimuli at 10 Hz) that evoked granule cell feedback inhibition, blocked 60-100% of the odour-evoked [Ca2+] transient in tufts when delivered within about 0.5 s of the odour. LOT-mediated inhibition was blocked by 10 microM bicuculline.

Membrane bistability in olfactory bulb mitral cells

Whole-cell patch-clamp recordings were used to investigate the electrophysiological properties of mitral cells in rat main olfactory bulb brain slice preparations. The majority of mitral cells are bistable. These cells spontaneously alternate between two membrane potentials, separated by approximately 10 mV: a relatively depolarized potential (upstate), which is perithreshold for spike generation, and a relatively hyperpolarized potential (downstate), in which spikes do not occur. Bistability occurs spontaneously in the absence of ionotropic excitatory or inhibitory synaptic inputs. Bistability is voltage dependent; transition from the downstate to the upstate is a regenerative event activated by brief depolarization. A brief hyperpolarization can switch the membrane potential from the upstate to the downstate. In response to olfactory nerve (ON) stimulation, mitral cells in the upstate are more likely to fire an action potential than are those in the downstate. ON stimulation can switch the membrane potential from the downstate to the upstate, producing a prolonged and amplified depolarization in response to a brief synaptic input. We conclude that bistability is an intrinsic property of mitral cells that is a major determinant of their responses to ON input.

NMDA receptor regulation of cell death in the rat olfactory bulb

Cell death is widespread in the developing nervous system and is under complex regulation by numerous intra- and intercellular mechanisms. Blockade of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor has been shown to promote cell death in the developing brain (Ikonomidou et al., 1999), suggesting that afferent functional activation is an important regulator of cell survival. The olfactory bulb, the first central relay for olfactory information from the nose, is well suited for examining the role of afferent activity in neuronal development. Functional deprivation is easily performed by surgical blockade of airflow to one side of the nasal passage, which results in dramatic alterations in postnatal development of the bulb (Brunjes, 1994), including enhanced neuronal loss (Frazier and Brunjes, 1988; Najbauer and Leon, 1995). The present report examined the specific role of NMDA receptor activation in regulating cell survival within the rat bulb. Pharmacological blockade of receptors with the noncompetitive channel blocker MK-801 (3 x 0.5 mg/kg i.p.) resulted in profound increases in cell death within 24 h. Furthermore, in contrast to other regions, where the effects of receptor blockade were confined to the first 2 postnatal weeks (Ikonomidou et al., 1999), enhancement of cell death was seen in the deeper granule cell-containing regions of the bulb with injections as late as postnatal day 28. In addition, the effects of MK-801 were much more dramatic than those seen after unilateral naris closure, suggesting that NMDA receptor activation may mediate additional survival pathways in the bulb beyond that provided by first nerve input.

A dendrodendritic reciprocal synapse provides a recurrent excitatory connection in the olfactory bulb

Neuronal synchronization in the olfactory bulb has been proposed to arise from a diffuse action of glutamate released from mitral cells (MC, olfactory bulb relay neurons). According to this hypothesis, glutamate spills over from dendrodendritic synapses formed between MC and granule cells (GC, olfactory bulb interneurons) to activate neighboring MC. The excitation of MC is balanced by a strong inhibition from GC. Here we show that MC excitation is caused by glutamate released from bulbar interneurons located in the GC layer. These reciprocal synapses depend on an unusual, 2-amino-5-phosphonovaleric acid-resistant, N-methyl-d-aspartate receptor. This type of feedback excitation onto relay neurons may strengthen the original sensory input signal and further extend the function of the dendritic microcircuit within the main olfactory bulb.

Isoproterenol increases CREB phosphorylation and olfactory nerve-evoked potentials in normal and 5-HT-depleted olfactory bulbs in rat pups only at doses that produce odor preference learning

Norepinephrine (NE) and serotonin (5-HT) are important modulators of early odor preference learning. NE can act as an unconditioned stimulus (UCS), whereas 5-HT facilitates noradrenergic actions. In this study, we examined the phosphorylation of an important transcription factor, cAMP response element binding protein (CREB), which has been implicated in long-term-memory formation (McLean et al. 1999) during NE-induced odor preference learning in normal and olfactory bulb 5-HT-depleted rat pups. We also examined NE modulation of olfactory nerve-evoked field potentials (ON-EFPs) in anesthetized normal and bulbar 5-HT depleted pups. Systemic injection of 2 mg/kg isoproterenol (beta-adrenoceptor agonist) induced odor preference learning, enhanced pCREB expression in the olfactory bulbs at 10 min after odor pairing, and increased ON-EFPs in normal rat pups but not in bulbar 5-HT-depleted rat pups. A dose of 6 mg/kg isoproterenol, which was ineffective in modulating these measures in normal rat pups, induced odor preference learning, enhanced phosphorylated CREB (pCREB) expression, and increased ON-EFPs in bulbar 5-HT-depleted pups. These outcomes suggest that NE and 5-HT promote specific biochemical and electrophysiological changes that may critically underlie odor preference learning.

Tonic and synaptically evoked presynaptic inhibition of sensory input to the rat olfactory bulb via GABA(B) heteroreceptors

Olfactory receptor neurons of the nasal epithelium send their axons, via the olfactory nerve (ON), to the glomeruli of the olfactory bulb (OB), where the axon terminals form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the OB, and with juxtaglomerular (JG) interneurons. Many JG cells are GABAergic. Here we show that, despite the absence of conventional synapses, GABA released from JG cells activates GABA(B) receptors on ON terminals and inhibits glutamate release both tonically and in response to ON stimulation. Field potential recordings and current-source density analysis, as well as intracellular and whole cell recording techniques were used in rat OB slices. Baclofen (2-5 microM), a GABA(B) agonist, completely suppressed ON-evoked synaptic responses of both mitral/tufted cells and JG cells, with no evidence for postsynaptic effects. Baclofen (0.5-1 microM) also reversed paired-pulse depression (PPD) of mitral/tufted cell responses to paired-pulse facilitation (PPF), and reduced depression of JG cell excitatory postsynaptic currents (EPSCs) during repetitive ON stimulation. These results suggest that baclofen reduced the probability of glutamate release from ON terminals. The GABA(B) antagonists CGP35348 or CGP55845A increased mitral/tufted cell responses evoked by single-pulse ON stimulation, suggesting that glutamate release from ON terminals is tonically suppressed via GABA(B) receptors. The same antagonists reduced PPD of ON-evoked mitral/tufted cell responses at interstimulus intervals 50-400 ms. This finding suggests that a single ON impulse evokes sufficient GABA release, presumably from JG cells, to activate GABA(B) receptors on ON terminals. Thus GABA(B) heteroreceptors on ON terminals are activated by ambient levels of extrasynaptic GABA, and by ON input to the OB. The time course of ON-evoked, GABA(B) presynaptic inhibition suggests that neurotransmission to M/T cells and JG cells will be significantly suppressed when ON impulses arrive in glomeruli at 2.5-20 Hz. GABA(B) receptor-mediated presynaptic inhibition of sensory input to the OB may play an important role in shaping the activation pattern of the OB glomeruli during olfactory coding.

Functional role of NMDA autoreceptors in olfactory mitral cells

The output of the olfactory bulb is governed by the interaction of synaptic potentials with the intrinsic conductances of mitral cells. While mitral cells often are considered as simple relay neurons, conveying activity in olfactory receptor cells to the piriform cortex, there is strong physiological and behavioral evidence that local synaptic interactions within the olfactory bulb modulate mitral cell discharges and facilitate odorant discrimination. Understanding the circuitry of the olfactory bulb is complicated by the fact that most dendrites in this region are both pre- and postsynaptic. Feedback inhibition is mediated through reciprocal dendrodendritic synapses between the secondary dendrites of mitral cells and GABAergic granule cells. Here we show that glutamate released from mitral cell dendrites also activates local N-methyl-D-aspartate (NMDA) autoreceptors, generating an inward tail current following depolarizing voltage steps. Autoreceptor-mediated self-excitation is calcium dependent, can be evoked by single action potentials in the presence of magnesium, and is graded with the number of spikes in a train. We find that dendrodendritic inhibition also is evoked by single action potentials but saturates rapidly during repetitive discharges. Self-excitation also underlies the prolonged afterdischarges apparent in mitral cells following potassium channel blockade. Both afterdischarges and autoreceptor-mediated tail currents persist in TTX, suggesting that they are produced by local rather than polysynaptic actions of glutamate. Blockade of NMDA autoreceptors with 2-amino-5-phosphonovaleric acid (APV) reduces the firing frequency within action potential cluster. The rapid kinetics of self-excitation suggests a functional role of NMDA autoreceptors in prolonging periods of phasic firing in mitral cells.

Ultrastructural characteristics and conduction velocity of olfactory receptor neuron axons in the olfactory marker protein-null mouse

Olfactory receptor neuron (ORN) axon diameters and the conduction velocity of the compound action potential along ORN axons were studied in olfactory marker protein (OMP)-null mice and genotypically matched controls. The compound action potential was distinguished from postsynaptic field potentials by its shorter latency, its persistence following application of cobalt or kynurenic acid that blocked postsynaptic responses, and its ability to follow paired-pulse stimulation at 300 Hz. Blockade of the postsynaptic field responses by kynurenic acid indicates that in the mouse, as in the rat, glutamate is the olfactory nerve transmitter. The mean conduction velocity of ORNs in wild-type control mice was 0. 47+/-0.19 (S.E.M.) m/s (n=5), similar to the conduction velocity reported for other mammals. The mean diameter of ORN axons in control mice was 0.202+/-0.005 and 0.261+/-0.006 microm in the OMP-null mice. This increase in fiber diameter in the OMP-nulls predicts an increase in impulse conduction velocity. However, the mean conduction velocity of OMP-null mice, 0.38+/-0.03 m/s (n=6), was not significantly different from control (P>0.1). The conduction velocity predicted by the increase in fiber diameter in OMP-null mice was within the 95% confidence interval of the measured value. Thus, OMP-null ORNs are normal with respect to the conduction velocity of their axons. The number of axodendritic synapses in the glomeruli of OMP-null mice is higher than in congenic wild-type mice.

Zinc-enriched (ZEN) terminals in mouse olfactory bulb

The present study was designed to localize zinc-enriched (ZEN) terminals in mouse olfactory bulb by means of ZnT3 immunocytochemistry (ICC) and zinc autometallography (AMG). The immunocytochemical staining of ZnT3 was closely correlated with the AMG pattern. ZEN terminals were defined as terminals showing both ZnT3 immunoreactivities and AMG granules. At the light microscopic level, dense staining patterns for ZnT3 immunoreactivity were seen in the granule cell layer and the olfactory glomerular layer. At the ultrastructural level, ZEN terminals were restricted to presynaptic terminals with single or multiple postsynaptic thickenings. The postsynaptic profiles contacting ZEN terminals appeared to be dendrites or somata of granule cells in the granule cell layer and periglomerular cells and mitral/tufted (M/T) cells in the olfactory glomerular layer. This suggests that two main sources of ZEN terminals are present in mouse olfactory bulb: (1) centrifugal fibres making asymmetrical synapses with granule cells and periglomerular cells, and (2) olfactory receptor terminals contacting dendritic profiles of M/T cells or periglomerular cells. The close correlation between ZEN terminals and the glutamatergic system is discussed.