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PubMed2003

Two-photon calcium imaging reveals an odor-evoked map of activity in the fly brain.

An understanding of the logic of odor perception requires a functional analysis of odor-evoked patterns of activity in neural assemblies in the brain. We have developed a sensitive imaging system in the Drosophila brain that couples two-photon microscopy with the specific expression of the calcium-sensitive fluorescent protein, G-CaMP. At natural odor concentration, each odor elicits a distinct and sparse spatial pattern of activity in the antennal lobe that is conserved in different flies. Patterns of glomerular activity are similar upon imaging of sensory and projection neurons, suggesting the faithful transmission of sensory input to higher brain centers. Finally, we demonstrate that the response pattern of a given glomerulus is a function of the specificity of a single odorant receptor. The development of this imaging system affords an opportunity to monitor activity in defined neurons throughout the fly brain with high sensitivity and excellent spatial resolution.

Animals↗
PubMed1994

Honeybee olfactory sensilla behave as integrated processing units.

Honeybee placode sensilla contain 18-35 olfactory receptor neurons. In insects, such neurons are thought to not interact with one another before reaching the central nervous system. Extracellular, multiunit recordings were made from the placodes and separated into spike shape classes, termed subplacode units. An analysis of the response spectra of subplacode units demonstrated that subplacode units with similar response spectra were more likely to be found in different placodes than in the same placode. An analysis was made of the mean interspike intervals and its variation for whole placodes and subplacode units. The coefficient of variation for whole placodes was less than that for subplacode units. Whole placode spike trains are therefore more uniform than subplacode spike trains, indicating that neurons might not be firing independently of each other.

Animals↗
PubMed2001

Neurobiology of fish olfaction: a review.

The last decade saw important advances in our understanding of the olfactory system function. In some animals, we now have the basic knowledge necessary to investigate coding mechanisms employed in olfaction. So far, studies of the fish olfactory system have focused on odor detection and the early processing of olfactory information in the olfactory bulb. How this information is integrated in the forebrain is unknown. Here, we first describe the anatomy of the fish olfactory system. The problems faced when describing the anatomy of the terminal nerve complex and nucleus olfactoretinalis are highlighted. Olfactory sensory neurons are randomly distributed over the entire olfactory epithelium, a unique feature of the olfactory sense. These primary olfactory neurons converge upon their second-order targets in segregated areas of the olfactory bulb. Exchange of information occurs in the glomeruli and glomerular plexus, where primary neurons synapse on mitral cell dendrites. The spatial distribution of glomerular activity induced by odorants of different classes shows that distinct neuron populations of the olfactory bulb encode information related to different odorant groups. In most cases, these neuron populations synchronize their alternating sequences of firing and silence when stimulated by primary input. Synchronized oscillations of these second-order neurons could contain important coding information, or represent a mechanism by which learning is facilitated. Alternatively, oscillations could be solely used to shape the olfactory bulb response. The nature of the olfactory information that reaches the forebrain and decoding of this information by the central nervous system are discussed.

Animals↗
PubMed1996

Combining laser scanning confocal microscopy and electron microscopy in studies of the insect nervous system.

Experimentally determining the synaptic interconnections between neurons in the nervous system is laborious and difficult in any animal species, but especially so in many invertebrates, including insects, where neurons generally have large, finely branching neuritic trees that form both pre- and postsynaptic specializations in dense neuropils with other neuritic trees. Electron microscopy is needed to identify synapses, but correlation of synapse type and location with the overall branching patterns of neurons, which are visible readily only in the light microscope or through extensive reconstruction of serial electron-microscope sections, is very difficult. In this paper, we present a simple method that we have developed (Sun et al. (1995) J. Histochem. Cytochem., 43: 329-335) that combines laser scanning confocal microscopy and electron microscopy for the study of synaptic relationships of neurons in the antennal lobe, the first central neuropil in the olfactory pathway, of the moth Manduca sexta. Briefly, neurons are labeled by intracellular injection with neurobiotin or biocytin, and then processed with a gold-particle tag for electron microscopic study and a fluorescent tag for confocal microscopy, and embedded in plastic. The fluorescence of the labeled neuron in the plastic blocks is imaged in three dimensions with laser scanning confocal microscopy and then the neuron is thin-sectioned at precisely chosen depths for electron microscopic study. The fluorescence pattern can be monitored repeatedly between episodes of thin-sectioning, and subtraction of a fluorescence image from the previous fluorescence image reveals which fluorescent processes have been sectioned. In this way, electron microscopic detail can be mapped onto a three-dimensional light microscopic image of the neuron.

Animals↗
PubMed1997

Calretinin immunoreactivity in the developing olfactory system of the rainbow trout.

The distribution of calretinin immunoreactivity in the developing olfactory system of the rainbow trout was studied by using an indirect immunocytochemical method. Calretinin immunoreactivity was firstly detected at 150 day-degrees in the olfactory placode, where labeled primordial cells were observed. At 250 day-degrees, precursor cells of the olfactory receptor neurons located in the olfactory pit were calretinin-immunoreactive. At 300 day-degrees, recognizable olfactory receptor neurons displayed calretinin immunoreactivity in the olfactory epithelium, and calretinin-immunopositive olfactory axons reached the presumptive olfactory bulb. After hatching (400 day-degrees) and during the subsequent development and maturation of the olfactory system, the number of calretinin-immunopositive olfactory receptor cells increased and distributed homogeneously throughout the olfactory epithelium. Accordingly, new positive olfactory fibers arrived to the olfactory bulb arborizing in olfactory glomeruli distributed in nine different terminal fields. Six days after hatching, calretinin-immunopositive interneurons within the olfactory bulb were also observed. The size and number of calretinin-immunoreactive interneurons increased from this stage to adulthood. The adult pattern demonstrated both similarities and differences with the distribution of calretinin immunoreactivity previously described in the olfactory system of mammals.

Aging↗
PubMed1997

Development of vomeronasal receptor neuron subclasses and establishment of topographic projections to the accessory olfactory bulb.

Previous studies of the adult vomeronasal system have shown that vomeronasal receptor neurons in the middle layer (expressing Gi alpha 2) and deep layers (expressing Go alpha) of the sensory epithelium project to the anterior and posterior parts of the accessory olfactory bulb (AOB), respectively. In the present study, the development of the two populations of vomeronasal receptor neurons and their segregated projections were investigated in the opossum, Monodelphis domestica. Antibodies to G proteins Gi alpha 2 and Go alpha were used to identify the two subpopulations of receptor neurons. The Gi alpha 2-immunoreactive (ir) cells and Go alpha-ir cells appeared between postnatal day 0 (P0) and postnatal day 3 (P3) and both types of cells increased in number during later development. The differential localization of Gi alpha 2-ir cells in the middle layer and Go alpha-ir cells in the deep layer of the VNO could be seen as early as P3 and became more prominent at later stages. The AOB was clearly identified at P10, and at this stage segregated projections of Gi alpha 2-ir fibers to the anterior part and Go alpha-ir fibers to the posterior part of the AOB were seen. The segregation of the two types of fibers in the AOB resemble that in the adult after P21. These results suggest that Gi alpha 2-ir and Go alpha-ir subpopulations of receptor neurons in the VNO develop in parallel, and that segregation of the two populations of receptor neurons in the VNO and the topographic projection to the AOB are established at very early stages during development.

Animals↗
PubMed2003

Putting smell on the map.

The vertebrate olfactory system must cope with a staggering developmental problem: how to connect millions of olfactory neurons expressing different odorant receptors to appropriate targets in the brain. Recent studies demonstrate remarkable plasticity in integrating novel olfactory neurons into this circuitry.

Animals↗
PubMed2002

Amygdala kindling induces upregulation of mRNA for NKCC1, a Na(+), K(+)-2Cl(-) cotransporter, in the rat piriform cortex.

GABA, the main inhibitory neurotransmitter in the brain, elicits a hyperpolarizing response by activation of the GABA(A)-receptor/chloride-channel complex under conditions of normal Cl(-) homeostasis. Thus the pathogenesis of epilepsy could involve an impairment of GABA(A)-receptor-mediated inhibition due to a collapse of the Cl(-) gradient. We examined the expression patterns of Cl(-) transporters and a Cl(-) channel in a rat amygdala-kindling model. Activity-dependent increases were observed in the mRNA for NKCC1, an inwardly-directed Cl(-) transporter, in the piriform cortex. This suggests that an increase in [Cl(-)](i) and a resultant reduction in GABAergic inhibition may occur in the kindled piriform cortex.

Amygdala↗
PubMed2002

Glucuronidation of odorant molecules in the rat olfactory system: activity, expression and age-linked modifications of UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT2A1, and relation to mitral cell activity.

The aim of the present study was to examine the glucuronidation of a series of odorant molecules by homogenates prepared either with rat olfactory mucosa, olfactory bulb or brain. Most of the odorant molecules tested were efficiently conjugated by olfactory mucosa, whereas olfactory bulb and brain homogenates displayed lower activities and glucuronidated only a few molecules. Important age-related changes in glucuronidation efficiency were observed in olfactory mucosa and bulb. Therefore, we studied changes in expression of two UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT2A1, in 1-day, 1- and 2-week-, 3-, 12- and 24-month-old rats. UGT1A6 was expressed at the same transcriptional level in the olfactory mucosa, bulb and brain, throughout the life period studied. UGT2A1 mRNA was expressed in both olfactory mucosa and olfactory bulb, in accordance with previous results [Mol. Brain Res. 90 (2001) 83], but UGT2A1 transcriptional level was 400-4000 times higher than that of UGT1A6. Moreover, age-dependent variations in UGT2A1 mRNA expression were observed. As it has been suggested that drug metabolizing enzymes could participate in olfactory function, mitral cell electrical activity was recorded during exposure to different odorant molecules in young, adult and old animals. Age-related changes in the amplitude of response after stimulation with several odorant molecules were observed, and the highest responses were obtained with molecules that were not efficiently glucuronidated by olfactory mucosa. In conclusion, the present work presents new evidence of the involvement of UGT activity in some steps of the olfactory process.

Action Potentials↗
PubMed2001

The nose and paranasal sinuses physiology and anatomy.

The paranasal sinuses and nose are much more than two cavities behind a projection on the centre of the face. They humidify, filter, warm, and sense what we breathe. The anatomy and physiology interact forming a dynamic system. The anatomy, airflow, nasal resistance, its turbulence, the nasal cycle - a process by which the turbinates or cushions lining the nose alternatively swell and congest from side to side, can all potentially influence the nasal delivery of drugs. Along with these factors mucus rheology and mucociliary clearance influence the removal of substances delivered to the nose. The health of the nose and its immunological response to what is inhaled, be it pollutants, allergens, drugs or vaccines, all need to be considered. It is a fascinating sensor for the body, not only detecting the potentially harmful substances such as smoke, but its psychosexual aspects have far reaching implications and the olfactory pathway has potential as a pathway for the delivery of drugs.

Animals↗
PubMed1997

NO synthase-II is transiently expressed in embryonic mouse olfactory receptor neurons.

Among three NO synthase (NOS) isoforms only the inducible NOS-II was localized in developing olfactory receptor neurons of the mouse. First NOS-II immunoreactive receptor cells including their processes were detected by embryonic day 11 when the olfactory pit starts to invaginate. Cellular staining lasted until embryonic day 16, and was reduced during the next few days. At embryonic day 20 no reactivity was found in the olfactory epithelium, whereas centripetal nerve fibers remained positive. This transient expression of NOS-II implies a role for the differentiation of early olfactory receptor neurons and synaptic plasticity.

Animals↗
PubMed1998

A dopamine-synthesizing cell group demonstrated in the human basal forebrain by dual labeling immunohistochemical technique of tyrosine hydroxylase and aromatic L-amino acid decarboxylase.

The human basal forebrain has been known to contain many neurons immunoreactive (ir) to tyrosine hydroxylase (TH; the first dopamine-synthesizing enzyme). We examined whether these neurons might contain aromatic L-amino acid decarboxylase (AADC; the second step dopamine-synthesizing enzyme) by dual labeling immunohistochemistry and confocal laser-scanning microscopy. Neurons dually-labeled for TH and AADC were found in the anterior olfactory nucleus, olfactory tubercle and the ventral margin of the rostral nucleus accumbens. The examination in the basal forebrain of the macaque monkey also gave substantially the same results. These neurons appear to constitute an independent dopaminergic cell group in the primate basal forebrain.

Adult↗
PubMed1998

Identification of shyc, a novel gene expressed in the murine developing and adult nervous system.

The embryonal carcinoma cell line P19 responds to treatment with retinoid acid by differentiation into neuronal cell types [2]. Using radioactively labeled cDNA derived from differentiating P19 cells we screened an adult mouse brain cDNA library and isolated a gene named shyc for selective hybridizing clone. The encoded protein did not reveal homology to any known protein. We used in situ hybridization on mouse embryonic and adult brain sections to study shyc expression. The developing and embryonic nervous system showed the most prominent hybridization signals. In the adult brain the olfactory pathway was marked by shyc expression.

Adaptor Proteins, Signal Transducing↗
PubMed2000

GABA(B)-mediated action in the frog olfactory bulb makes odor responses more salient.

In the olfactory bulb, GABA(B) receptors are selectively located in the glomerular layer. A current hypothesis is that GABAergic inhibition mediated through these receptors would be, at least partly, presynaptic and would exerted by decreasing the release of the olfactory receptor neuron excitatory neurotransmitter. Here, we assessed, in the frog, the in vivo action of baclofen, a GABA(B) agonist, on single-unit mitral cell activity in response to odors. Local application of baclofen in the glomerular region of the olfactory bulb was shown to drastically affect mitral cell spontaneous activity, since they became totally silent. Moreover, under baclofen, mitral cells still responded to odors and still specified odor concentration increases through their temporal response patterns. The pharmacological specificity of the GABA(B) agonist action was confirmed by showing that saclofen, a GABA(B) antagonist, partly prevented the inhibitory action of baclofen and restored the initial rate of mitral cell spontaneous activity. The results show that GABA(B)-mimicked inhibition suppressed mitral cell spontaneous activity while odor responses were maintained. This suggests that olfactory receptor neurons partly drive spontaneous mitral cell activity. Moreover, the effect of GABA(B)-mediated inhibition was seen to be very close to that described previously for dopamine D(2) receptor-mediated inhibition. In conclusion, we propose that these two inhibitory mechanisms would offer the possibility to reduce or suppress mitral cell spontaneous activity so as to make their responses to odor especially salient.

Animals↗
PubMed2000

Bilateral lesions of the central but not anterior or posterior parts of the piriform cortex retard amygdala kindling in rats.

The piriform cortex is thought to be involved in temporal lobe seizure propagation, such as that occurring during kindling of the amygdala or hippocampus. A number of observations suggested that the circuits of the piriform cortex might act as a critical pathway for limbic seizure discharges to assess motor systems, but direct evidence for this suggestion is scarce. Furthermore, the piriform cortex is not a homogeneous structure, which complicates studies on its role in limbic epileptogenesis. We have previously reported data indicating that the central part of the piriform cortex might be particularly involved during amygdala kindling. In order to further evaluate the role of different parts of the piriform cortex during kindling development, we bilaterally destroyed either the central, anterior or posterior piriform cortex by microinjections of ibotenate two weeks before onset of amygdala kindling. Lesions of the anterior piriform cortex hardly affected kindling acquisition, except that fewer animals exhibited stage 3 (unilateral forelimb) seizures compared to sham controls. Lesions of the central piriform cortex significantly retarded kindling, which was due to a decreased progression from stage 3 to stage 4/5 seizures, i.e. the lesioned rats needed significantly longer for the acquisition of generalized clonic seizures in the late stages of kindling development. Lesions of the posterior piriform cortex did not significantly affect kindling development. The data demonstrate that different parts of the piriform cortex mediate qualitatively different effects on amygdala kindling. The central piriform cortex seems to be a neural substrate involved in the continuous development of kindling from stage 3 to stages 4/5, indicating that this part of the piriform cortex may have preferred access, either directly or indirectly, to structures capable of supporting generalized kindled seizure expression.

Amygdala↗
PubMed1996

Early olfactory fiber projections and cell migration into the rat telencephalon.

The formation and development of primary olfactory axons was studied in the rat embryo using acetylcholinesterase histochemistry, immunocytochemistry for neuron-specific beta-tubulin (TuJ1) and growth associated protein 43 (GAP43), and a fluorescent tracer DiI. Olfactory axons extend from the olfactory receptor neurons localized in the olfactory epithelium. These fibers grow to reach and enter the olfactory bulbs, where they form the first relay and integrative synaptic station in the olfactory system: the olfactory glomerulus. In this communication we address the development of primary olfactory fibers: first from the olfactory placode and later from the olfactory epithelium. Olfactory fibers enter the olfactory bulbs apparently in a disordered manner but soon arrange themselves in hook shaped aggregates of fibers, with many boutons (immature synaptic terminals), to form the glomeruli. We detected this kind of structure for the first time at embryonic day 16. The olfactory receptor cells are usually anchored in the basal lamina of the olfactory epithelium but some of them, after reaching their targets, lose their epithelial attachment, leave the olfactory epithelium and migrate to and enter the olfactory bulbs. The traffic of cells between the olfactory epithelium and the brain lasts late into embryonic development. We describe four types of migratory mechanism used by different populations of cells to reach their targets in the telencephalic vesicle and propose the existence of migrating cells that enter the telencephalon. These data were corroborated by injections into the olfactory epithelium a of murine retrovirus carrying the Escherichia coli lac-Z gene.

Acetylcholinesterase↗
PubMed1999

[A network capable of reading the temporal codes in the olfactory discrimination of insects].

Recent work on the insect olfactory system has shown that its mushroom bodies (one of its major components) are involved in the fine discrimination of odours and that the temporal organisation of spike discharges plays a fundamental role. We propose here a model of a network that is able to decode the temporal patterns which characterise an odour. This model has three fundamental properties that seem to exist in all mushroom bodies of insects studied so far: a) long lasting inhibitions with rebounds, able to facilitate delayed spike generation; b) synaptic plasticity, which allows the network to learn to recognise temporal patterns; c) above all a large interconnection, which allows this network to recognise intervals of various duration. This model thus appears suited to identify combinations of temporal patterns in the dendrites of Kenyon cells (the principal cells in the calyces of the mushroom bodies). Moreover, the mushroom bodies integrate multimodal inputs, suggesting that the detection of temporal patterns may be extended to the detection of a complex environment, combining in particular olfactive and visual inputs.

Animal Structures↗
PubMed2002

NADPH diaphorase activity in olfactory receptor neurons and their axons conforms to a rhinotopically-distinct dorsal zone of the hamster nasal cavity and main olfactory bulb.

NADPH diaphorase histochemical protocols were optimized for the histochemical labeling of olfactory receptor neurons (ORNs) in the nasal cavity and their axon terminals in glomeruli of the main olfactory bulb (MOB) in the Syrian hamster. This labeling was then used to map and quantify the spatial distribution of ORNs and their central projections. Diaphorase-positive ORNs were found to be rhinotopically restricted to dorsal-medially situated segments of sensory mucosa associated with central air channels in the nose, together constituting about 25% of the total receptor sheet. This topography closely resembles the zonal expression patterns of putative odorant receptor genes and cell surface glycoconjugates in the nose. Moreover, the projections of ORNs in the diaphorase-positive dorsal/central zone were found to expand onto the entire dorsal half of the MOB, consistent with spatial patterns discerned in retrograde tract-tracing studies. These boundaries indicate that dorsal/central zone ORNs project to a disproportionately larger region of the MOB than do those in the more ventral/peripheral zones. The demonstration of NADPH diaphorase activity in ORNs is inconsistent with the expression of the best-known NADPH-dependent enzymes, such as nitric oxide synthase (neuronal and endothelial isoforms) and NADPH cytochrome P450 oxidoreductase. Understanding the spatial patterning of histochemical labeling in ORNs should facilitate the biochemical identification of this diaphorase.

Animals↗