Reciprocal intraglomerular excitation and intra- and interglomerular lateral inhibition between mouse olfactory bulb mitral cells

Neuronal gap junctions between intraglomerular mitral/tufted cell dendrites in the mouse main olfactory bulb

In the mouse main olfactory bulb (MOB) gap junction-forming processes in glomeruli were analyzed by means of the serial electron microscopical reconstruction. Gap junctions were encountered between diverse types of dendritic processes and thus confirming our previous study on gap junctions in the rat MOB. Importantly, among more than 30 gap junctions examined in serial sections, we encountered 3 gap junctions made between mitral/tufted cell dendrites in the glomerulus. Then we must consider both direct coupling between mitral/tufted cells via gap junctions and indirect coupling between mitral/tufted cells via intervening interneuronal processes as suggested previously.

Phasic stimuli evoke precisely timed spikes in intermittently discharging mitral cells

Mitral cells, the principal cells of the olfactory bulb, respond to sensory stimulation with precisely timed patterns of action potentials. By contrast, the same neurons generate intermittent spike clusters with variable timing in response to simple step depolarizations. We made whole cell recordings from mitral cells in rat olfactory bulb slices to examine the mechanisms by which normal sensory stimuli could generate precisely timed spike clusters. We found that individual mitral cells fired clusters of action potentials at 20-40 Hz, interspersed with periods of subthreshold membrane potential oscillations in response to depolarizing current steps. TTX (1 microM) blocked a sustained depolarizing current and fast subthreshold oscillations in mitral cells. Phasic stimuli that mimic trains of slow excitatory postsynaptic potentials (EPSPs) that occur during sniffing evoked precisely timed spike clusters in repeated trials. The amplitude of the first simulated EPSP in a train gated the generation of spikes on subsequent EPSPs. 4-aminopyridine (4-AP)-sensitive K(+) channels are critical to the generation of spike clusters and reproducible spike timing in response to phasic stimuli. Based on these results, we propose that spike clustering is a process that depends on the interaction between a 4-AP-sensitive K(+) current and a subthreshold TTX-sensitive Na(+) current; interactions between these currents may allow mitral cells to respond selectively to stimuli in the theta frequency range. These intrinsic properties of mitral cells may be important for precisely timing spikes evoked by phasic stimuli that occur in response to odor presentation in vivo.

Interplay between local GABAergic interneurons and relay neurons generates gamma oscillations in the rat olfactory bulb

Olfactory stimuli have been known for a long time to elicit oscillations in olfactory brain areas. In the olfactory bulb (OB), odors trigger synchronous oscillatory activity that is believed to arise from the coherent and rhythmic discharges of large numbers of neurons. These oscillations are known to take part in encoding of sensory information before their transfer to higher subcortical and cortical areas. To characterize the cellular mechanisms underlying gamma (30-80 Hz) local field potential (LFP) oscillations, we simultaneously recorded multiunit discharges, intracellular responses, and LFP in rat OB slices. We showed that a single and brief electrical stimulation of olfactory nerve elicited LFP oscillations in the mitral cell body layer lasting >1 sec. Both action potentials and subthreshold oscillations of mitral/tufted cells, the bulbar output neurons, were precisely synchronized with LFP oscillations. This synchronization arises from the interaction between output neurons and granule cells, the main population of local circuit inhibitory interneurons, through dendrodendritic synapses. Interestingly enough, the synchronization exerted by reciprocal synaptic interactions did not require action potentials initiated in granule cell somata. Finally, local application of a GABA(A) receptor antagonist at the mitral cell and external plexiform layers confirmed the exclusive role of the granule cell reciprocal synapses in generating the evoked oscillations. We concluded that interneurons located in the granule cell layer generate synaptic activity capable of synchronizing activity of output neurons by interacting with both their subthreshold and spiking activity.

Voltage imaging from dendrites of mitral cells: EPSP attenuation and spike trigger zones

To obtain a more complete description of individual neurons, it is necessary to complement the electrical patch pipette measurements with technologies that permit a massive parallel recording from many sites on neuronal processes. This can be achieved by using voltage imaging with intracellular dyes. With this approach, we investigated the functional structure of a mitral cell, the principal output neuron in the rat olfactory bulb. The most significant finding concerns the characteristics of EPSPs at the synaptic sites and surprisingly small attenuation along the trunk of the primary dendrite. Also, the experiments were performed to determine the number, location, and stability of spike trigger zones, the excitability of terminal dendritic branches, and the pattern and nature of spike initiation and propagation in the primary and secondary dendrites. The results show that optical data can be used to deduce the amplitude and shape of the EPSPs evoked by olfactory nerve stimulation at the site of origin (glomerular tuft) and to determine its attenuation along the entire length of the primary dendrite. This attenuation corresponds to an unusually large mean apparent "length constant" of the primary dendrite. Furthermore, the images of spike trigger zones showed that an action potential can be initiated in three different compartments of the mitral cell: the soma-axon region, the primary dendrite trunk, and the terminal dendritic tuft, which appears to be fully excitable. Finally, secondary dendrites clearly support the active propagation of action potentials.

Functional organization of sensory input to the olfactory bulb glomerulus analyzed by two-photon calcium imaging

Glomeruli in the olfactory bulb are anatomically discrete modules receiving input from idiotypic olfactory sensory neurons. To examine the functional organization of sensory inputs to individual glomeruli, we loaded olfactory sensory neurons with a Ca(2+) indicator and measured odorant-evoked presynaptic Ca(2+) signals within single glomeruli by using two-photon microscopy in anaesthetized mice. Odorants evoked patterns of discrete Ca(2+) signals throughout the neuropil of a glomerulus. Across glomeruli, Ca(2+) signals occurred with equal probability in all glomerular regions. Within single glomeruli, the pattern of intraglomerular Ca(2+) signals was indistinguishable for stimuli of different duration, identity, and concentration. Moreover, the response time course of the signals was similar throughout the glomerulus. Hence, sensory inputs to individual glomeruli are spatially heterogeneous but seem to be functionally indiscriminate. These results support the view of olfactory glomeruli as functional units in representing sensory information.

Dynamics of olfactory bulb input and output activity during odor stimulation in zebrafish

The processing of odor-evoked activity in the olfactory bulb (OB) of zebrafish was studied by extracellular single unit recordings from the input and output neurons, i.e., olfactory receptor neurons (ORNs) and mitral cells (MCs), respectively. A panel of 16 natural amino acid odors was used as stimuli. Responses of MCs, but not ORNs, changed profoundly during the first few hundred milliseconds after response onset. In MCs, but not ORNs, the total evoked excitatory activity in the population was initially odor-dependent but subsequently converged to a common level. Hence, the overall population activity is regulated by network interactions in the OB. The tuning widths of both ORN and MC response profiles were similar and, on average, stable over time. However, when analyzed for individual neurons, MC response profiles could sharpen (excitatory response to fewer odors) or broaden (excitatory response to more odors), whereas ORN response profiles remained nearly unchanged. Several observations indicate that dynamic inhibition plays an important role in this remodeling. Finally, the reliability of odor identification based on MC population activity patterns improved over time, whereas odor identification based on ORN activity patterns was most reliable early in the odor response. These results demonstrate that several properties of MC, but not ORN, activity change during the initial phase of the odor response with important consequences for odor-encoding activity patterns. Furthermore, our data indicate that inhibitory interactions in the OB are important in dynamically shaping the activity of OB output neurons.

Mechanisms of lateral inhibition in the olfactory bulb: efficiency and modulation of spike-evoked calcium influx into granule cells

Granule cells are axonless local interneurons that mediate lateral inhibitory interactions between the principal neurons of the olfactory bulb via dendrodendritic reciprocal synapses. This unusual arrangement may give rise to functional properties different from conventional lateral inhibition. Although granule cells spike, little is known about the role of the action potential with respect to their synaptic output. To investigate the signals that underlie dendritic release in these cells, two-photon microscopy in rat brain slices was used to image calcium transients in granule cell dendrites and spines. Action potentials evoked calcium transients throughout the dendrites, with amplitudes increasing with distance from soma and attaining a plateau level within the external plexiform layer, the zone of granule cell synaptic output. Transient amplitudes were, on average, equal in size in spines and adjacent dendrites. Surprisingly, both spine and dendritic amplitudes were strongly dependent on membrane potential, decreasing with depolarization and increasing with hyperpolarization from rest. Both the current-voltage relationship and the time course of inactivation were consistent with the known properties of T-type calcium channels, and the voltage dependence was blocked by application of the T-type calcium channel antagonists Ni2+ and mibefradil. In addition, mibefradil reduced action potential-mediated synaptic transmission from granule to mitral cells. The implication of a transiently inactivating calcium channel in synaptic release from granule cells suggests novel mechanisms for the regulation of lateral inhibition in the olfactory bulb.

Dendritic processing within olfactory bulb circuits

Odors elicit a well-organized pattern of activation in glomeruli across the surface of the olfactory bulb. However, the mechanisms by which this map is transformed into an odor code by the bulb circuitry remain unclear. Recent physiological studies in bulb slices have identified several synaptic processes that could be involved in sharpening odorant signals. Mitral cells within a single odorant receptor-specific network can be synchronized by dendrodendritic excitatory interactions in a glomerulus, whereas mitral cells in different networks engage in long-lasting lateral inhibition mediated by dendrodendritic synapses with interneurons. The emerging picture is one in which groups of mitral cells use a unique set of mechanisms to accomplish computational functions similar to those performed by analogous modular structures in other sensory systems.

Mechanisms of lateral inhibition in the olfactory bulb: efficiency and modulation of spike-evoked calcium influx into granule cells

Granule cells are axonless local interneurons that mediate lateral inhibitory interactions between the principal neurons of the olfactory bulb via dendrodendritic reciprocal synapses. This unusual arrangement may give rise to functional properties different from conventional lateral inhibition. Although granule cells spike, little is known about the role of the action potential with respect to their synaptic output. To investigate the signals that underlie dendritic release in these cells, two-photon microscopy in rat brain slices was used to image calcium transients in granule cell dendrites and spines. Action potentials evoked calcium transients throughout the dendrites, with amplitudes increasing with distance from soma and attaining a plateau level within the external plexiform layer, the zone of granule cell synaptic output. Transient amplitudes were, on average, equal in size in spines and adjacent dendrites. Surprisingly, both spine and dendritic amplitudes were strongly dependent on membrane potential, decreasing with depolarization and increasing with hyperpolarization from rest. Both the current-voltage relationship and the time course of inactivation were consistent with the known properties of T-type calcium channels, and the voltage dependence was blocked by application of the T-type calcium channel antagonists Ni2+ and mibefradil. In addition, mibefradil reduced action potential-mediated synaptic transmission from granule to mitral cells. The implication of a transiently inactivating calcium channel in synaptic release from granule cells suggests novel mechanisms for the regulation of lateral inhibition in the olfactory bulb.

Electrical signaling in the olfactory bulb

The olfactory bulb employs lateral and feedback inhibitory pathways to distribute odor information across parallel assemblies of mitral and granule cells. The pathways involve dendritic action potentials that can interact with a variety of voltage-dependent conductances and synaptic transmission to produce complex and dynamic patterns of activity. Electrical coupling also helps to ensure proper coordination and synchronization of these patterns. These mechanisms provide numerous options for dynamic modulation and control of signaling in the olfactory bulb.

Dendrites: bug or feature?

The integrative properties of dendrites are determined by a complex mixture of factors, including their morphology, the spatio-temporal patterning of synaptic inputs, the balance of excitation and inhibition, and neuromodulatory influences, all of which interact with the many voltage-gated conductances present in the dendritic membrane. Recent efforts to grapple with this complexity have focused on identifying functional compartments in the dendritic tree, the number and size of which depend on the aspect of dendritic function being considered. We discuss how dendritic compartments and the interactions between them help to enhance the computational power of the neuron and define the rules for the induction of synaptic plasticity.

GABA(B) receptors inhibit dendrodendritic transmission in the rat olfactory bulb

In the mammalian olfactory bulb, mitral cell dendrites release glutamate onto the dendritic spines of granule cells, which in turn release GABA back onto mitral dendrites. This local synaptic circuit forms the basis for reciprocal dendrodendritic inhibition mediated by ionotropic GABA(A) receptors in mitral cells. Surprisingly little is known about neurotransmitter modulation of dendrodendritic signaling in the olfactory bulb. In this study, we examine whether metabotropic GABA(B) receptors modulate dendrodendritic signaling between mitral and granule cells. We find that the selective GABA(B) agonist baclofen reduces mitral cell recurrent inhibition mediated by dendrodendritic synapses. GABA(B) receptor activation causes only a weak inhibition of field EPSCs in the external plexiform layer and only slightly reduces glutamate-mediated mitral cell self-excitation. Although GABA(B) receptors depress mitral cell glutamate release only weakly, baclofen causes a marked reduction in the amplitude of granule-cell-evoked, GABA(A)-mediated IPSCs in mitral cells. In addition to reducing the amplitude of granule-cell-evoked IPSCs, baclofen causes a change from paired-pulse depression to paired-pulse facilitation, suggesting that GABA(B) receptors modulate GABA release from granule cells. To explore the mechanism of action of GABA(B) receptors further, we show that baclofen inhibits high-voltage-activated calcium currents in granule cells. Together, these findings suggest that GABA(B) receptors modulate dendrodendritic inhibition primarily by inhibiting granule cell calcium channels and reducing the release of GABA. Furthermore, we show that endogenous GABA regulates the strength of dendrodendritic inhibition via the activation of GABA(B) autoreceptors.

GABA(B) receptors inhibit dendrodendritic transmission in the rat olfactory bulb

In the mammalian olfactory bulb, mitral cell dendrites release glutamate onto the dendritic spines of granule cells, which in turn release GABA back onto mitral dendrites. This local synaptic circuit forms the basis for reciprocal dendrodendritic inhibition mediated by ionotropic GABA(A) receptors in mitral cells. Surprisingly little is known about neurotransmitter modulation of dendrodendritic signaling in the olfactory bulb. In this study, we examine whether metabotropic GABA(B) receptors modulate dendrodendritic signaling between mitral and granule cells. We find that the selective GABA(B) agonist baclofen reduces mitral cell recurrent inhibition mediated by dendrodendritic synapses. GABA(B) receptor activation causes only a weak inhibition of field EPSCs in the external plexiform layer and only slightly reduces glutamate-mediated mitral cell self-excitation. Although GABA(B) receptors depress mitral cell glutamate release only weakly, baclofen causes a marked reduction in the amplitude of granule-cell-evoked, GABA(A)-mediated IPSCs in mitral cells. In addition to reducing the amplitude of granule-cell-evoked IPSCs, baclofen causes a change from paired-pulse depression to paired-pulse facilitation, suggesting that GABA(B) receptors modulate GABA release from granule cells. To explore the mechanism of action of GABA(B) receptors further, we show that baclofen inhibits high-voltage-activated calcium currents in granule cells. Together, these findings suggest that GABA(B) receptors modulate dendrodendritic inhibition primarily by inhibiting granule cell calcium channels and reducing the release of GABA. Furthermore, we show that endogenous GABA regulates the strength of dendrodendritic inhibition via the activation of GABA(B) autoreceptors.

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).

Theta oscillation coupled spike latencies yield computational vigour in a mammalian sensory system

Theoretical work carried out almost a decade ago proposed that subthreshold oscillations in membrane potential could be used to convert synaptic current strength into a code reliant on action potential (AP) latencies. Using whole-cell recordings we present experimental evidence for the occurrence of prominent network-driven subthreshold theta oscillations in mitral cells of the mouse olfactory bulb. Activity induced by both injected current and sensory input was accurately reflected in initial AP latency from the beginning of each oscillation cycle. In a network model we found that an AP latency code rather than AP number or instantaneous firing rate provided computational speed and high resolution, and was easily implemented. This coding strategy was also found to be invariant to the total input current as long as the relative input intensities to glomeruli remained constant. However, it was highly sensitive to changes in the ratios of the input currents and improved by lateral inhibitory mechanisms. Since the AP latency-based coding scheme was dependent on the subthreshold oscillation we conclude that the theta rhythm serves a functional role in temporally reformatting the strengths and patterns of synaptic input in this sensory system.

AMPA autoreceptors drive correlated spiking in olfactory bulb glomeruli

Information processing in the brain may rely on temporal correlations in spike activity between neurons. Within the olfactory bulb, correlated spiking in output mitral cells could affect the odor code by either binding or amplifying signals from individual odorant receptors. We examined the timing of spike trains in mitral cells of rat olfactory bulb slices. Depolarization of mitral cell pairs elicited spikes that were correlated on a rapid timescale (< or =10 ms) for cells whose primary dendrites projected to the same glomerulus. Correlated spiking was driven by a novel mechanism that depended on electrical coupling at mitral cell primary dendrites; the specific synchronizing signal was a coupled depolarization ( approximately 20 ms) that was mediated by dendritic AMPA autoreceptors. We suggest that glomerulus-specific correlated spiking in mitral cells helps to preserve the fidelity of odor signals that are delivered to the olfactory cortex.