Olfactory Local Interneurons Research Articles

Adult expression of the cell adhesion protein Fasciclin 3 is required for the maintenance of adult olfactory interneurons.

The proper functioning of the nervous system is dependent on the establishment and maintenance of intricate networks of neurons that form functional neural circuits. Once neural circuits are assembled during development, a distinct set of molecular programs is likely required to maintain their connectivity throughout the lifetime of the organism. Here, we demonstrate that Fasciclin 3 (Fas3), an axon guidance cell adhesion protein, is necessary for the maintenance of the olfactory circuit in adult Drosophila. We utilized the TARGET system to spatiotemporally knockdown Fas3 in selected populations of adult neurons. Our findings show that Fas3 knockdown results in the death of olfactory circuit neurons and reduced survival of adults. We also demonstrated that Fas3 knockdown activates caspase-3-mediated cell death in olfactory local interneurons, which can be rescued by overexpressing baculovirus p35, an anti-apoptotic protein. This work adds to the growing set of evidence indicating a crucial role for axon guidance proteins in the maintenance of neuronal circuits in adults.

Local olfactory interneurons provide the basis for neurochemical regionalization of olfactory glomeruli in crustaceans.

The primary olfactory centers of metazoans as diverse as arthropods and mammals consist of an array of fields of dense synaptic neuropil, the olfactory glomeruli. However, the neurochemical structure of crustacean olfactory glomeruli is largely understudied when compared to the insects. We analyzed the glomerular architecture in selected species of hermit crabs using immunohistochemistry against presynaptic proteins, the neuropeptides orcokinin, RFamide and allatostatin, and the biogenic amine serotonin. Our study reveals an unexpected level of structural complexity, unmatched by what is found in the insect olfactory glomeruli. Peptidergic and aminergic interneurons provide the structural basis for a regionalization of the crustacean glomeruli into longitudinal and concentric compartments. Our data suggest that local olfactory interneurons take a central computational role in modulating the information transfer from olfactory sensory neurons to projection neurons within the glomeruli. Furthermore, we found yet unknown neuronal elements mediating lateral inhibitory interactions across the glomerular array that may play a central role in modulating the transfer of sensory input to the output neurons through presynaptic inhibition. Our study is another step in understanding the function of crustacean olfactory glomeruli as highly complex units of local olfactory processing.

Localization of neurons expressing choline acetyltransferase, serotonin and/or FMRFamide in the central nervous system of the decapod shore crab Hemigrapsus sanguineus.

Although it is now established that neurons in crustacea contain multiple transmitter substances, little is know about patterns of expression and co-expression or about the functional effects of such co-transmission. The present study was designed to characterize the distributions and potential colocalization of choline acetyltransferase (ChAT), serotonin (5-HT) and neuropeptide H-Phe-Met-Arg-Phe-NH2 (FMRFamide) in the central nervous system (CNS) of the Asian shore crab, Hemigrapsus sanguineus using immunohistochemical analyses in combination with laser scanning confocal microscopy. ChAT was found to be expressed by small, medium-sized, and large neurons in all regions of the brain and ventral nerve cord (VNC). For the most part, ChAT, FMRFamide, and 5-HT are expressed in different neurons, although some colocalization of ChAT- with FMRFamide- or 5-HT-LIR is observed in small and medium-sized cells, mostly neurons that immunostain only weakly. In the brain, such double immunolabeling is observed primarily in neurons of the protocerebrum and, to a particularly great extent, in local olfactory interneurons of the deutocerebrum. The clusters of neurons in the VNC that stain most intensely for ChAT, FMRFamide, and 5-HT, with colocalization in some cases, are located in the subesophageal ganglia. This colocalization appears to be related to function, since it is present in regions of the CNS characterized by multiple afferent projections and outputs to a variety of functionally related centers involved in various physiological and behavioral processes. Further elucidation of the functional significance of these neurons and of the widespread process of co-transmission in the crustaceans should provide fascinating new insights.

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae)

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

Interneuron Diversity: Toward a Better Understanding of Interneuron Development In the Olfactory System

The Drosophila olfactory system is an attractive model for exploring the wiring logic of complex neural circuits. Remarkably, olfactory local interneurons exhibit high diversity and variability in their morphologies and intrinsic properties. Although olfactory sensory and projection neurons have been extensively studied of development and wiring; the development, mechanisms for establishing diversity, and integration of olfactory local interneurons into the developing circuit remain largely undescribed. In this review, we discuss some challenges and recent advances in the study of Drosophila olfactory interneurons.

Systematic Analysis of Transmitter Coexpression Reveals Organizing Principles of Local Interneuron Heterogeneity.

Broad neuronal classes are surprisingly heterogeneous across many parameters, and subclasses often exhibit partially overlapping traits including transmitter coexpression. However, the extent to which transmitter coexpression occurs in predictable, consistent patterns is unknown. Here, we demonstrate that pairwise coexpression of GABA and multiple neuropeptide families by olfactory local interneurons (LNs) of the moth Manduca sexta is highly heterogeneous, with a single LN capable of expressing neuropeptides from at least four peptide families and few instances in which neuropeptides are consistently coexpressed. Using computational modeling, we demonstrate that observed coexpression patterns cannot be explained by independent probabilities of expression of each neuropeptide. Our analyses point to three organizing principles that, once taken into consideration, allow replication of overall coexpression structure: (1) peptidergic neurons are highly likely to coexpress GABA; (2) expression probability of allatotropin depends on myoinhibitory peptide expression; and (3) the all-or-none coexpression patterns of tachykinin neurons with several other neuropeptides. For other peptide pairs, the presence of one peptide was not predictive of the presence of the other, and coexpression probability could be replicated by independent probabilities. The stochastic nature of these coexpression patterns highlights the heterogeneity of transmitter content among LNs and argues against clear-cut definition of subpopulation types based on the presence of single neuropeptides. Furthermore, the receptors for all neuropeptides and GABA were expressed within each population of principal neuron type in the antennal lobe (AL). Thus, activation of any given LN results in a dynamic cocktail of modulators that have the potential to influence every level of olfactory processing within the AL.

Diverse populations of local interneurons integrate into the Drosophila adult olfactory circuit

Drosophila olfactory local interneurons (LNs) in the antennal lobe are highly diverse and variable. How and when distinct types of LNs emerge, differentiate, and integrate into the olfactory circuit is unknown. Through systematic developmental analyses, we found that LNs are recruited to the adult olfactory circuit in three groups. Group 1 LNs are residual larval LNs. Group 2 are adult-specific LNs that emerge before cognate sensory and projection neurons establish synaptic specificity, and Group 3 LNs emerge after synaptic specificity is established. Group 1 larval LNs are selectively reintegrated into the adult circuit through pruning and re-extension of processes to distinct regions of the antennal lobe, while others die during metamorphosis. Precise temporal control of this pruning and cell death shapes the global organization of the adult antennal lobe. Our findings provide a road map to understand how LNs develop and contribute to constructing the olfactory circuit.

Colocalization of allatotropin and tachykinin-related peptides with classical transmitters in physiologically distinct subtypes of olfactory local interneurons in the cockroach (Periplaneta americana).

In the insect antennal lobe different types of local interneurons mediate complex excitatory and inhibitory interactions between the glomerular pathways to structure the spatiotemporal representation of odors. Mass spectrometric and immunohistochemical studies have shown that in local interneurons classical neurotransmitters are likely to colocalize with a variety of substances that can potentially act as cotransmitters or neuromodulators. In the antennal lobe of the cockroach Periplaneta americana, gamma-aminobutyric acid (GABA) has been identified as the potential inhibitory transmitter of spiking type I local interneurons, whereas acetylcholine is most likely the excitatory transmitter of nonspiking type IIa1 local interneurons. This study used whole-cell patch clamp recordings combined with single-cell labeling and immunohistochemistry to test if the GABAergic type I local interneurons and the cholinergic type IIa1 local interneurons express allatotropin and tachykinin-related neuropeptides (TKRPs). These are two of the most abundant types of peptides in the insect antennal lobe. GABA-like and choline acetyltransferase (ChAT)-like immunoreactivity were used as markers for GABAergic and cholinergic neurons, respectively. About 50% of the GABA-like immunoreactive (-lir) spiking type I local interneurons were allatotropin-lir, and ∼ 40% of these neurons were TKRP-lir. About 20% of nonspiking ChAT-lir type IIa1 local interneurons were TKRP-lir. Our results suggest that in subpopulations of GABAergic and cholinergic local interneurons, allatotropin and TKRPs might act as cotransmitters or neuromodulators. To unequivocally assign neurotransmitters, cotransmitters, and neuromodulators to identified classes of antennal lobe neurons is an important step to deepen our understanding of information processing in the insect olfactory system.

Neuropeptide complexity in the crustacean central olfactory pathway: immunolocalization of A-type allatostatins and RFamide-like peptides in the brain of a terrestrial hermit crab

BackgroundIn the olfactory system of malacostracan crustaceans, axonal input from olfactory receptor neurons associated with aesthetascs on the animal’s first pair of antennae target primary processing centers in the median brain, the olfactory lobes. The olfactory lobes are divided into cone-shaped synaptic areas, the olfactory glomeruli where afferents interact with local olfactory interneurons and olfactory projection neurons. The local olfactory interneurons display a large diversity of neurotransmitter phenotypes including biogenic amines and neuropeptides. Furthermore, the malacostracan olfactory glomeruli are regionalized into cap, subcap, and base regions and these compartments are defined by the projection patterns of the afferent olfactory receptor neurons, the local olfactory interneurons, and the olfactory projection neurons. We wanted to know how neurons expressing A-type allatostatins (A-ASTs; synonym dip-allatostatins) integrate into this system, a large family of neuropeptides that share the C-terminal motif –YXFGLamide.ResultsWe used an antiserum that was raised against the A-type Diploptera punctata (Dip)-allatostatin I to analyse the distribution of this peptide in the brain of a terrestrial hermit crab, Coenobita clypeatus (Anomura, Coenobitidae). Allatostatin A-like immunoreactivity (ASTir) was widely distributed in the animal’s brain, including the visual system, central complex and olfactory system. We focussed our analysis on the central olfactory pathway in which ASTir was abundant in the primary processing centers, the olfactory lobes, and also in the secondary centers, the hemiellipsoid bodies. In the olfactory lobes, we further explored the spatial relationship of olfactory interneurons with ASTir to interneurons that synthesize RFamide-like peptides. We found that these two peptides are present in distinct populations of local olfactory interneurons and that their synaptic fields within the olfactory glomeruli are also mostly distinct.ConclusionsWe discuss our findings against the background of the known neurotransmitter complexity in the crustacean olfactory pathway and summarize what is now about the neuronal connectivity in the olfactory glomeruli. A-type allatostatins, in addition to their localization in protocerebral brain areas, seem to be involved in modulating the olfactory signal at the level of the deutocerebrum. They contribute to the complex local circuits within the crustacean olfactory glomeruli the connectivity within which as yet is completely unclear. Because the glomeruli of C. clypeatus display a distinct pattern of regionalization, their olfactory systems form an ideal model to explore the functional relevance of glomerular compartments and diversity of local olfactory interneurons for olfactory processing in crustaceans.

Distinct Electrophysiological Properties in Subtypes of Nonspiking Olfactory Local Interneurons Correlate With Their Cell Type–Specific Ca2+Current Profiles

A diverse population of local interneurons (LNs) helps to process, structure, and spatially represent olfactory information in the insect antennal lobe. In Periplaneta americana, we identified two subtypes of nonspiking local interneurons (type II LNs) by their distinct morphological and intrinsic electrophysiological properties. As an important step toward a better understanding of the cellular mechanisms that mediate odor information processing, we present a detailed analysis of their distinct voltage-activated Ca(2+) currents, which clearly correlated with their distinct intrinsic electrophysiological properties. Both type II LNs did not posses voltage-activated Na(+) currents and apparently innervated all glomeruli including the macroglomerulus. Type IIa LNs had significant longer and thicker low-order neurites and innervated each glomerulus entirely and homogeneously, whereas type IIb LNs innervated only parts of each glomerulus. All type II LNs were broadly tuned and responded to odorants of many chemical classes with graded changes in the membrane potential. Type IIa LNs responded with odor-specific elaborate patterns of excitation that could also include "spikelets" riding on the depolarizations and periods of inhibition. In contrast, type IIb LNs responded mostly with sustained, relatively smooth depolarizations. Consistent with the strong active membrane properties of type IIa LNs versus type IIb LNs, the voltage-activated Ca(2+) current of type IIa LNs activated at more hyperpolarized membrane potentials and had a larger transient component.

Nitric oxide as a putative messenger molecule in the crayfish olfactory midbrain

NADPH-d histochemistry was used to investigate presumptive nitric oxide synthase (NOS)-containing neurons in the crayfish olfactory midbrain. Three anatomically different types of local olfactory interneurons exhibiting NADPH-d activity were observed: two pairs of large interneurons as well as positively stained globuli cells. Branches derived from the large interneurons were confined to the ipsilateral olfactory lobe and accessory lobe, but only a few branches innervated the olfactory lobe glomeruli. Local field potential recordings on the olfactory lobe showed that administration of SNP or SIN-1 (10 −4 M) into the brain had reversible inhibitory effects on electrically-evoked responses of unidentified neuronal cell populations.

Descending neurons with dopamine-like or with substance P/FMRFamide-like immunoreactivity target the somata of olfactory interneurons in the brain of the spiny lobster, Panulirus argus.

Two sets of descending neurons primarily target the somata of neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus. Hundreds to thousands of dopamine-like immunoreactive fibers originate in the lateral protocerebrum and terminate among the clustered somata of the olfactory deutocerebrum projection neurons (lateral soma cluster) and those of the olfactory deutocerebrum local interneurons (medial soma cluster). A pair of giant neurons with substance P- and FMRFamide-like immunoreactivity from the median protocerebrum terminate primarily in the lateral soma cluster, but also branch in the core of the olfactory lobe itself. Neurons of both types terminate in numerous bouton-like swellings. The terminals in the lateral cluster at least contain numerous, large, dense-core and small, clear vesicles. The terminals contact the somata and the primary neurites through both traditional chemical synapses and large zones of direct membrane appositions. In most instances, a vesicle-containing profile forms a triadic arrangement with a neurite and a soma the latter being frequently connected via large gap-junction-like structures. Rosette-like arrangements formed by a vesicle-containing profile surrounded by up to eight neurites are also common. Dissociated lateral cluster somata support both fast inward and sustained outward voltage-activated currents. Substance P, but not dopamine or FMRFamide-related peptides, alters the fast inward current. The somata of the olfactory projection neurons, and possibly those of the olfactory local interneurons, appear to serve an integrative, and not merely a supportive role in these invertebrate central neurons.

Dynamics of propagating waves in the olfactory network of a terrestrial mollusk: an electrical and optical study

1. The procerebral (PC) lobe of the terrestrial mollusk Limax maximus contains a highly interconnected network of local olfactory interneurons that receives ipsilateral axonal projections from superior and inferior noses. This network exhibits an approximately 0.7-Hz intrinsic oscillation in its local field potential (LFP). 2. Intracellular recordings show that the lobe contains at least two classes of neurons with activity phase locked to the oscillation. Neurons in one class produce periodic bursts of spikes, followed by a period of hyperpolarization and subsequently a depolarizing afterpotential. There is a small but significant chance for a second burst to occur during the depolarizing afterpotential; this leads to a double event in the LFP. Bursting neurons constitute approximately 10% of the neurons in the lobe. 3. Neurons in the other class fire infrequently and do not produce periodic bursts of action potentials. However, they receive strong, periodic inhibitory input during every event in the LFP. These nonbursting cells constitute the major fraction of neurons in the lobe. There is a clear correlation between the periodic burst of action potentials in the bursting neurons and the hyperpolarization seen in nonbursting neurons. 4. Optical techniques are used to image the spatially averaged transmembrane potentials in preparations stained with voltage-sensitive dyes. The results of simultaneous optical and electrical measurements show that the major part of the optical signal can be interpreted as a superposition of the intracellular signals arising from the bursting and nonbursting neurons. 5. Successive images of the entire PC lobe show waves of electrical activity that span the width of the lobe and travel its full length along a longitudinal axis. The direction of propagation in the unperturbed lobe is always from the distal to the proximal end. The wavelength varies between preparations but is on the order of the length of the preparation. 6. One-dimensional images along the longitudinal axis of the lobe are used to construct a space-time map of the optical activity, from which we calculate the absolute contribution of bursting and nonbursting neurons to the optical signal. The contribution of the intracellular signals from the two cell types appears to vary systematically across the lobe; bursting cells dominate at middle and proximal locations, and nonbursting cells dominate at distal locations. 7. The direction and form of the waves can be perturbed either by microsurgical manipulation of the preparation or by chemical modulation of its synaptic and neuronal properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Coherent network oscillations by olfactory interneurons: modulation by endogenous amines.

1. The procerebral (PC) lobe of the terrestrial mollusk Limax maximus contains a highly interconnected network of local olfactory interneurons that receives direct axonal projections from the two pairs of noses. This olfactory processing network generates a 0.7-Hz oscillation in its local field potential (LFP) that is coherent throughout the network. The oscillating LFP is modulated by natural odorants applied to the neuroepithelium of the superior nose. 2. Two amines known to be present in the PC lobe, dopamine and serotonin, increase the frequency of the PC lobe oscillation and alter its waveform. 3. Glutamate, another putative neurotransmitter known to be present in the lobe, suppresses the PC lobe oscillation by a quisqualate-type receptor and appears to be used by one of the two classes of neurons in the PC lobe to generate the basic LFP oscillation. 4. The known activation of second messengers in Limax PC lobe by dopamine and serotonin together with their effects on the oscillatory rhythm suggest the hypothesis that these amines augment mechanisms mediating synaptic plasticity in the olfactory network, similar to hypothesized effects of amines in vertebrate olfactory systems. 5. The use of a distributed network of interneurons showing coherent oscillations may relate to the highly developed odor recognition and odor learning ability of Limax.