Adjusting neurophysiological computations in the adult olfactory bulb

The olfactory bulb receives signals from olfactory sensory neurons and conveys them to higher centers. The mapping of the sensory inputs generates a reproducible spatial pattern in the glomerular layer of the olfactory bulb for each odorant. Then, this restricted activation is transformed into highly distributed patterns by lateral interactions between relay neurons and local interneurons. Thus, odor information processing requires the spatial patterning of both sensory inputs and synaptic interactions. In other words, odor representation is highly dynamic and temporally orchestrated. Here, we describe how the local inhibitory network shapes the global oscillations and the precise synchronization of relay neurons. We discuss how local inhibitory interneurons transpose the spatial dimension into temporal patterning. Remarkably, this transposition is not fixed but highly flexible to continuously optimize olfactory information processing.     

Activity-induced remodeling of olfactory bulb microcircuits revealed by monosynaptic tracing

The continued addition of new neurons to mature olfactory circuits represents a remarkable mode of cellular and structural brain plasticity. However, the anatomical configuration of newly established circuits, the types and numbers of neurons that form new synaptic connections, and the effect of sensory experience on synaptic connectivity in the olfactory bulb remain poorly understood. Using in vivo electroporation and monosynaptic tracing, we show that postnatal-born granule cells form synaptic connections with centrifugal inputs and mitral/tufted cells in the mouse olfactory bulb. In addition, newly born granule cells receive extensive input from local inhibitory short axon cells, a poorly understood cell population. The connectivity of short axon cells shows clustered organization, and their synaptic input onto newborn granule cells dramatically and selectively expands with odor stimulation. Our findings suggest that sensory experience promotes the synaptic integration of new neurons into cell type-specific olfactory circuits.     

Recurrent circuitry dynamically shapes the activation of piriform cortex

In the piriform cortex, individual odorants activate a unique ensemble of neurons that are distributed without discernable spatial order. Piriform neurons receive convergent excitatory inputs from random collections of olfactory bulb glomeruli. Pyramidal cells also make extensive recurrent connections with other excitatory and inhibitory neurons. We introduced channelrhodopsin into the piriform cortex to characterize these intrinsic circuits and to examine their contribution to activity driven by afferent bulbar inputs. We demonstrated that individual pyramidal cells are sparsely interconnected by thousands of excitatory synaptic connections that extend, largely undiminished, across the piriform cortex, forming a large excitatory network that can dominate the bulbar input. Pyramidal cells also activate inhibitory interneurons that mediate strong, local feedback inhibition that scales with excitation. This recurrent network can enhance or suppress bulbar input, depending on whether the input arrives before or after the cortex is activated. This circuitry may shape the ensembles of piriform cells that encode odorant identity.     

Local interneurons and information processing in the olfactory glomeruli of the moth Manduca sexta

Intracellular recordings were made from the major neurites of local interneurons in the moth antennal lobe. Antennal nerve stimulation evoked 3 patterns of postsynaptic activity: (i) a short-latency compound excitatory postsynaptic potential that, based on electrical stimulation of the antennal nerve and stimulation of the antenna with odors, represents a monosynaptic input from olfactory afferent axons (71 out of 86 neurons), (ii) a delayed activation of firing in response to both electrical- and odor-driven input (11 neurons), and (iii) a delayed membrane hyperpolarization in response to antennal nerve input (4 neurons).Simultaneous intracellular recordings from a local interneuron with short-latency responses and a projection (output) neuron revealed unidirectional synaptic interactions between these two cell types. In 20% of the 30 pairs studied, spontaneous and current-induced spiking activity in a local interneuron correlated with hyperpolarization and suppression of firing in a projection neuron. No evidence for recurrent or feedback inhibition of projection neurons was found. Furthermore, suppression of firing in an inhibitory local interneuron led to an increase in firing in the normally quiescent projection neuron, suggesting that a disinhibitory pathway may mediate excitation in projection neurons. This is the first direct evidence of an inhibitory role for local interneurons in olfactory information processing in insects. Through different types of multisynaptic interactions with projection neurons, local interneurons help to generate and shape the output from olfactory glomeruli in the antennal lobe.Abbreviations AL antennal lobe - EPSP excitatory postsynaptic potential - GABA -aminobutyric acid - IPSP inhibitory postsynaptic potential - LN local interneuron - MGC macroglomerular complex - OB olfactory bulb - PN projection neuron - TES N-tris[hydroxymethyl]methyl-2-aminoethane-sulfonic acid     

Gaseous Oxides and Olfactory Computation

The gaseous neurotransmitters nitric oxide (NO) and carbon monoxide(CO) are prominent and universal components of the array ofneurotransmitters found in olfactory information processingsystems. These highly mobile communication compounds have effectson both second messenger signaling and directly on ion channelgating in olfactory receptors and central synaptic processingof receptor input. Olfactory systems are notable for the plasticityof their synaptic connections, revealed both in higher-orderassociative learning mechanisms using odor cues and developmentalplasticity operating to maintain function during addition ofnew olfactory receptors and new central olfactory interneurons.We use the macrosmatic terrestrial mollusk Limax maximus toinvestigate the role of NO and CO in the dynamics of centralodor processing and odor learning. The major central site ofodor processing in the Limax CNS is the procerebral (PC) lobeof the cerebral ganglion, which displays oscillatory dynamicsof its local field potential and periodic activity waves modulatedby odor input. The bursting neurons in the PC lobe are dependenton local NO synthesis for maintenance of bursting activity andwave propagation. New data show that these bursting PC interneuronsare also stimulated by carbon monoxide. The synthesizing enzymefor carbon monoxide, heme oxygenase 2, is present in the neuropilof the PC lobe. Since the PC lobe exhibits two forms of synapticplasticity related to both associative odor learning and continualconnection of new receptors and interneurons, the use of multiplegaseous neurotransmitters may be required to enable these multipleforms of synaptic plasticity.     

The sox gene Dichaete is expressed in local interneurons and functions in development of the Drosophila adult olfactory circuit

In insects, the primary sites of integration for olfactory sensory input are the glomeruli in the antennal lobes. Here, axons of olfactory receptor neurons synapse with dendrites of the projection neurons that relay olfactory input to higher brain centers, such as the mushroom bodies and lateral horn. Interactions between olfactory receptor neurons and projection neurons are modulated by excitatory and inhibitory input from a group of local interneurons. While significant insight has been gleaned into the differentiation of olfactory receptor and projection neurons, much less is known about the development and function of the local interneurons. We have found that Dichaete, a conserved Sox HMG box gene, is strongly expressed in a cluster of LAAL cells located adjacent to each antennal lobe in the adult brain. Within these clusters, Dichaete protein expression is detected in both cholinergic and GABAergic local interneurons. In contrast, Dichaete expression is not detected in mature or developing projection neurons, or developing olfactory receptor neurons. Analysis of novel viable Dichaete mutant alleles revealed misrouting of specific projection neuron dendrites and axons, and alterations in glomeruli organization. These results suggest noncell autonomous functions of Dichaete in projection neuron differentiation as well as a potential role for Dichaete‐expressing local interneurons in development of the adult olfactory circuitry. © 2012 Wiley Periodicals, Inc. Develop Neurobiol, 2013     

Excitatory local interneurons enhance tuning of sensory information

Neurons in the insect antennal lobe represent odors as spatiotemporal patterns of activity that unfold over multiple time scales. As these patterns unspool they decrease the overlap between odor representations and thereby increase the ability of the olfactory system to discriminate odors. Using a realistic model of the insect antennal lobe we examined two competing components of this process -lateral excitation from local excitatory interneurons, and slow inhibition from local inhibitory interneurons. We found that lateral excitation amplified differences between representations of similar odors by recruiting projection neurons that did not receive direct input from olfactory receptors. However, this increased sensitivity also amplified noisy variations in input and compromised the ability of the system to respond reliably to multiple presentations of the same odor. Slow inhibition curtailed the spread of projection neuron activity and increased response reliability. These competing influences must be finely balanced in order to decorrelate odor representations.     

Blanes cells mediate persistent feedforward inhibition onto granule cells in the olfactory bulb

Inhibitory local circuits in the olfactory bulb play a critical role in determining the firing patterns of output neurons. However, little is known about the circuitry in the major plexiform layers of the olfactory bulb that regulate this output. Here we report the first electrophysiological recordings from Blanes cells, large stellate-shaped interneurons located in the granule cell layer. We find that Blanes cells are GABAergic and generate large I(CAN)-mediated afterdepolarizations following bursts of action potentials. Using paired two-photon guided intracellular recordings, we show that Blanes cells have a presumptive axon and monosynaptically inhibit granule cells. Sensory axon stimulation evokes barrages of EPSPs in Blanes cells that trigger long epochs of persistent spiking; this firing mode was reset by hyperpolarizing membrane potential steps. Persistent firing in Blanes cells may represent a novel mechanism for encoding short-term olfactory information through modulation of tonic inhibitory synaptic input onto bulbar neurons.     

Pharmacological analysis of ionotropic glutamate receptor function in neuronal circuits of the zebrafish olfactory bulb

Although synaptic functions of ionotropic glutamate receptors in the olfactory bulb have been studied in vitro, their roles in pattern processing in the intact system remain controversial. We therefore examined the functions of ionotropic glutamate receptors during odor processing in the intact olfactory bulb of zebrafish using pharmacological manipulations. Odor responses of mitral cells and interneurons were recorded by electrophysiology and 2-photon Ca(2+) imaging. The combined blockade of AMPA/kainate and NMDA receptors abolished odor-evoked excitation of mitral cells. The blockade of AMPA/kainate receptors alone, in contrast, increased the mean response of mitral cells and decreased the mean response of interneurons. The blockade of NMDA receptors caused little or no change in the mean responses of mitral cells and interneurons. However, antagonists of both receptor types had diverse effects on the magnitude and time course of individual mitral cell and interneuron responses and, thus, changed spatio-temporal activity patterns across neuronal populations. Oscillatory synchronization was abolished or reduced by AMPA/kainate and NMDA receptor antagonists, respectively. These results indicate that (1) interneuron responses depend mainly on AMPA/kainate receptor input during an odor response, (2) interactions among mitral cells and interneurons regulate the total olfactory bulb output activity, (3) AMPA/kainate receptors participate in the synchronization of odor-dependent neuronal ensembles, and (4) ionotropic glutamate receptor-containing synaptic circuits shape odor-specific patterns of olfactory bulb output activity. These mechanisms are likely to be important for the processing of odor-encoding activity patterns in the olfactory bulb.     

Morphometric modeling of olfactory circuits in the insect antennal lobe: I. Simulations of spiking local interneurons.

Inhibitory local interneurons (LNs) play a critical role in shaping the output of olfactory glomeruli in both the olfactory bulb of vertebrates and the antennal lobe of insects and other invertebrates. In order to examine how the complex geometry of LNs may affect signaling in the antennal lobe, we constructed detailed multi-compartmental models of single LNs from the sphinx moth, Manduca sexta, using morphometric data from confocal-microscopic images. Simulations clearly revealed a directionality in LNs that impeded the propagation of injected currents from the sub-micron-diameter glomerular dendrites toward the much larger-diameter integrating segment (IS) in the coarse neuropil. Furthermore, the addition of randomly-firing synapses distributed across the LN dendrites (simulating the noisy baseline activity of afferent input recorded from LNs in the odor-free state) led to a significant depolarization of the LN. Thus the background activity typically recorded from LNs in vivo could influence synaptic integration and spike transformation in LNs through voltage-dependent mechanisms. Other model manipulations showed that active currents inserted into the IS can help synchronize the activation of inhibitory synapses in glomeruli across the antennal lobe. These data, therefore, support experimental findings suggesting that spiking inhibitory LNs can operate as multifunctional units under different ambient odor conditions. At low odor intensities, (i.e. subthreshold for IS spiking), they participate in local, mostly intra-glomerular processing. When activated by elevated odor concentrations, however, the same neurons will fire overshooting action potentials, resulting in the spread of inhibition more globally across the antennal lobe. Modulation of the passive and active properties of LNs may, therefore, be a deciding factor in defining the multi-glomerular representations of odors in the brain.     

Optimizing interneuron circuits for compartment-specific feedback inhibition

Cortical circuits process information by rich recurrent interactions between excitatory neurons and inhibitory interneurons. One of the prime functions of interneurons is to stabilize the circuit by feedback inhibition, but the level of specificity on which inhibitory feedback operates is not fully resolved. We hypothesized that inhibitory circuits could enable separate feedback control loops for different synaptic input streams, by means of specific feedback inhibition to different neuronal compartments. To investigate this hypothesis, we adopted an optimization approach. Leveraging recent advances in training spiking network models, we optimized the connectivity and short-term plasticity of interneuron circuits for compartment-specific feedback inhibition onto pyramidal neurons. Over the course of the optimization, the interneurons diversified into two classes that resembled parvalbumin (PV) and somatostatin (SST) expressing interneurons. Using simulations and mathematical analyses, we show that the resulting circuit can be understood as a neural decoder that inverts the nonlinear biophysical computations performed within the pyramidal cells. Our model provides a proof of concept for studying structure-function relations in cortical circuits by a combination of gradient-based optimization and biologically plausible phenomenological models.     

Olfactory map formation in the Drosophila brain: genetic specificity and neuronal variability

The development of the Drosophila olfactory system is a striking example of how genetic programs specify a large number of different neuron types and assemble them into functional circuits. To ensure precise odorant perception, each sensory neuron has to not only select a single olfactory receptor (OR) type out of a large genomic repertoire but also segregate its synaptic connections in the brain according to the OR class identity. Specification and patterning of second-order interneurons in the olfactory brain center occur largely independent of sensory input, followed by a precise point-to-point matching of sensory and relay neurons. Here we describe recent progress in the understanding of how cell-intrinsic differentiation programs and context-dependent cellular interactions generate a stereotyped sensory map in the Drosophila brain. Recent findings revealed an astonishing morphological diversity among members of the same interneuron class, suggesting an unexpected variability in local microcircuits involved in insect sensory processing.     

Oscillations and gaseous oxides in invertebrate olfaction

Olfactory systems combine an extraordinary molecular sensitivity with robust synaptic plasticity. Central neuronal circuits that perform pattern recognition in olfaction typically discriminate between hundreds of molecular species and form associations between odor onsets and behavioral contingencies that can last a lifetime. Two design features in the olfactory system of the terrestrial mollusk Limax maximus may be common elements of olfactory systems that display the twin features of broad molecular sensitivity and rapid odor learning: spatially coherent oscillations in the second-order circuitry that receives sensory input; and involvement of the interneuronal messengers nitric oxide (NO) and carbon monoxide (CO) in sensory responses and circuit dynamics of the oscillating olfactory network. The principal odor processing center in Limax, the procerebrum (PC) of the cerebral ganglion, contains on the order of 10⁵ local interneurons and receives both direct and processed input from olfactory receptors. Field potential recordings in the PC show an oscillation at approximately 0.7 Hz that is altered by odor input. Optical recordings of voltage changes in local regions of the PC show waves of depolarization that originate at the distal pole and propagate to the base of the PC. Weak odor stimulation transiently switches PC activity from a propagating mode to a spatially uniform mode. The field potential oscillation in the PC lobe depends on intercellular communication via NO, based on opposing effects of reagents that decrease or increase NO levels in the PC. Inhibition of NO synthase slows the field potential oscillation, while application of exogenous NO increases the oscillation frequency. A role for CO in PC dynamics is suggested by experiments in which CO liberation increases the PC oscillation frequency. These design features of the Limax PC lobe odor processing circuitry may relate to synaptic plasticity that subserves both connection of new receptors throughout the life of the slug and its highly developed odor learning ability. © 1996 John Wiley & Sons, Inc.     

Response properties of higher level neurons in the central olfactory pathway of the crayfish

We have examined the electrical activity of interneurons within the higher levels of the crayfish olfactory system. In unstimulated isolated crayfish head preparations, local protocerebral interneurons (LPI) of the hemiellipsoid bodies generate periodic, low-frequency membrane depolarizations. The most reasonable explanation for these baseline fluctuations, which were exhibited by all of the LPIs examined and which were reversibly abolished by either tetrodotoxin or low-calcium saline solution, is that they reflect periodic synaptic drive from the axon terminals of olfactory projection neurons. One-third of tested LPIs generated impulses in response to the odor stimuli we applied to the antennules. Those cells that did respond exhibited a brief excitatory postsynaptic potential and one or two action potentials, even during prolonged odor pulses. Many of the responding neurons also exhibited a delayed impulse burst 1 or 2 s following the stimulus pulse. Most of the responding cells recovered their sensitivity to odors very slowly, exhibiting disadaptation periods of several minutes. The apparent refractory nature of individual LPIs to olfactory stimulation is attributed in part to a hypothesized selectivity of connections between projection neurons and protocerebral targets and in part to the electrical isolation of the recording electrode from many regions of the extensive LPI dendritic tree. Accepted: 20 March 1997     

Local peptidergic signaling in the antennal lobe shapes olfactory behavior

Transfer and processing of olfactory information in the antennal lobe of Drosophila relies primarily on neurotransmitters such as acetylcholine and GABA, but novel studies also implicated a neuropeptide: the Drosophila tachykinin (DTK). DTK is expressed in local interneurons that innervate the glomeruli of the antennal lobe with varicose processes. Recently, DTK was shown to mediate presynaptic inhibition of olfactory sensory neurons by physiological and behavioral analysis (Ignell et al. 2009, PNAS). That study drew our attention to the issue of alternative targets of DTK in the antennal lobe. Hence, in the present study, we interfered with DTK peptide and DTK receptor (DTKR) expression in local interneurons of the antennal lobe and studied the behavioral outcome of these manipulations. We show that the DTKR is expressed not only in olfactory sensory neurons, but most likely also in local interneurons. The behavioral consequences of interfering with postsynaptic peptide receptors are different from presynaptic peptide receptor interference. We discuss the possibility that the sum of pre- and postsynaptic interactions may be to modulate the dynamic range in odor sensitivity.     

Inhibition of nitric oxide and soluble guanylyl cyclase signaling affects olfactory neuron activity in the moth, <Emphasis Type="Italic">Manduca sexta</Emphasis>

Nitric oxide is emerging as an important modulator of many physiological processes including olfaction, yet the function of this gas in the processing of olfactory information remains poorly understood. In the antennal lobe of the moth, Manduca sexta, nitric oxide is produced in response to odor stimulation, and many interneurons express soluble guanylyl cyclase, a well-characterized nitric oxide target. We used intracellular recording and staining coupled with pharmacological manipulation of nitric oxide and soluble guanylyl cyclase to test the hypothesis that nitric oxide modulates odor responsiveness in olfactory interneurons through soluble guanylyl cyclase-dependent pathways. Nitric oxide synthase inhibition resulted in pronounced effects on the resting level of firing and the responses to odor stimulation in most interneurons. Effects ranged from bursting to strong attenuation of activity and were often accompanied by membrane depolarization coupled with a change in input resistance. Blocking nitric oxide activation of soluble guanylyl cyclase signaling mimicked the effects of nitric oxide synthase inhibitors in a subset of olfactory neurons, while other cells were differentially affected by this treatment. Together, these results suggest that nitric oxide is required for proper olfactory function, and likely acts through soluble guanylyl cyclase-dependent and -independent mechanisms in different subsets of neurons.     

Functional organization of olfactory processing in the accessory lobe of the spiny lobster

An isolated brain preparation was used to characterize neurons innervating the accessory lobe (AL) of the spiny lobster (Panulirus argus). Four distinct classes of neurons responded to electrical stimulation of the olfactory (antennular) nerve. These cells responded to electrical stimulation with a long and variable latency; they also responded to odor stimulation in a nose-brain preparation. Neurons connecting the AL with the olfactory lobe branched in the central AL layer and selectively innervated olfactory lobe glomeruli. These cells had response latencies which were significantly shorter than those of other AL neurons. Intrinsic AL interneurons were heterogeneous as a population, and most arborized in irregular but circumscribed regions of either the lateral or medial layers. The final class of neurons branched ipsilaterally in the deutocerebral neuropil and bilaterally innervated only a few AL glomeruli. The physiology and morphology of these four classes of neurons confirm an olfactory function for the AL and identify the input and output regions of the lobe. Based on these findings, we propose that the AL processes odor information in the context of higher order multimodal input.Abbreviations AL accessory lobe - DCN deutocerebral neuropil - OGT olfactory-globular tract - OGTN olfactory-globular tract neuropil - OL olfactory lobe     

Synaptic activation of metabotropic glutamate receptors regulates dendritic outputs of thalamic interneurons

Information gating through the thalamus is dependent on the output of thalamic relay neurons. These relay neurons receive convergent innervation from a number of sources, including GABA-containing interneurons that provide feed-forward inhibition. These interneurons are unique in that they have two distinct outputs: axonal and dendritic. In addition to conventional axonal outputs, these interneurons have presynaptic dendrites that may provide localized inhibitory influences. Our study indicates that synaptic activation of metabotropic glutamate receptors (mGluRs) increases inhibitory activity in relay neurons by increasing output of presynaptic dendrites of interneurons. Optic tract stimulation increases inhibitory activity in thalamic relay neurons in a frequency- and intensity-dependent manner and is attenuated by mGluR antagonists. Our data suggest that synaptic activation of mGluRs selectively alters dendritic output but not axonal output of thalamic interneurons. This mechanism could serve an important role in focal, feed-forward information processing in addition to dynamic information processing in thalamocortical circuits.     

Genetic and functional subdivision of the Drosophila antennal lobe

Olfactory systems confer the recognition and discrimination of a large number of structurally distinct odor molecules. Recent molecular analysis of odorant receptor (OR) genes and circuits has led to a model of odor coding in which a population of olfactory sensory neurons (OSNs) expressing a single OR converges upon a unique olfactory glomerulus. Activation of the OR can thus be read out by the activation of its cognate glomerulus. Drosophila is a powerful system in which to test this model because the entire repertoire of 62 ORs can be manipulated genetically. However, a complete understanding of how fly olfactory circuits are organized is lacking. Here, we present a nearly complete map of OR projections from OSNs to the antennal lobe (AL) in the fly brain. Four populations of OSNs coexpress two ORs along with Or83b, and a fifth expresses one OR and one gustatory receptor (GR) along with Or83b. One glomerulus receives coconvergent input from two separate populations of OSNs. Three ORs label sexually dimorphic glomeruli implicated in sexual courtship and are thus candidate Drosophila pheromone receptors. This olfactory sensory map provides an experimental framework for relating ORs to glomeruli and ultimately behavior.     

Determination of cell fate within the telencephalon

The telencephalon (basal ganglia, septum, cerebral cortex and olfactory bulb) contains two general classes of neurons: those that project axons to distant targets and those that make only local connections. While projection neurons can be either excitatory (such as those in the olfactory bulb and cortex) or inhibitory (such as those in the striatum), local circuit neurons (interneurons) are usually inhibitory. Within these two general classes of neurons there are a myriad of cell subtypes based upon axonal and dendritic morphology, chemical markers, neurotransmitters, connectivity and physiology. A crucial issue regarding the development of the telencephalon is the molecular determination of neuronal subtypes. Since important aspects of neuronal fate determination occur within the proliferative zone, the consideration of determinants of a mature neuron's fate requires consideration of that cell's origin.