Sensory information processing in the frog olfactory pathways. Experimental basis for modeling studies

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. By comparing spontaneous and odor-evoked activities of neuroreceptor, mitral and cortical cells, we have collected fundamental data relating to coding abilities of the three olfactory levels, the olfactory mucosa, the bulb and the cortex. Based on a synthesis of our experimental data related to GABAergic and dopaminergic involvement in the olfactory bulb, this paper aims to match this information with computational data and to discuss some questions on bulbar processing. This paper is also devoted to further analyze original results on coding properties of two functionally evidenced neuron subpopulations in the olfactory cortex. Thus, the assumption according to which some cortical neurons may work as temporal integrators while others as coincidence detectors is presented. Moreover, the pertinence that the neural code may be carried by a single spike with varying latency was demonstrated.     

Polarity of axoplasmic microtubules in the olfactory nerve of the frog

Pieces of olfactory nerve of the bullfrog were extracted in a tubulin assembly buffer medium containing detergents. With incubation at 37 degrees C in such medium containing soluble tubulin, ribbons of protofilaments are formed on the surfaces of microtubules, with the ribbons curving in a clockwise or counterclockwise direction. The direction of hooking reflects the polarity of the microtubule. In nerve pieces oriented such that cross sections could be viewed toward the perikarya of the axons, over 90% of the ribbons on microtubules showed a clockwise orientation. When observers were looking toward the axonal terminals, most ribbons on microtubules showed a counterclockwise direction. In single axons in which ribbons appeared on all the contained microtubules, the ribbons showed a single directionality. The evidence suggests that microtubules in axons have a single polarity, probably reflecting their assembly from the perikarya outward through the axoplasm. If bidirectional transport is assumed in these axons, it is not reflected by the polarity of their microtubules, which may mean that the directionality of transport is provided by components other than microtubules.     

Excitatory synapses in the glomerular triad of frog olfactory bulb in vitro

Whole-cell patch clamp recording techniques were applied to periglomerular (PG) cells in slices of the frog olfactory bulb (OB) to study the properties of the excitatory synapses in the triad formed by the olfactory nerve (ON) and the dendrites of mitral/tufted (MT) cells and PG cells. The postsynaptic response evoked by ON stimulation was glutamatergic and could be dissected into NMDA and non-NMDA components of equivalent amplitudes. The dendro-dendritic synapse between MT and PG cells could be activated following antidromic stimulation of the lateral and medial olfactory tract (LOT and MOT). In this case the postsynaptic potentials had amplitudes and durations comparable to those obtained by ON stimulation, the neurotransmitter was glutamate, but the synapse was largely dominated by the slow NMDA component.     

Bidirectional processing in the olfactory-limbic axis during olfactory behavior

Field potentials were recorded simultaneously from the olfactory bulb (OB), prepyriform cortex (PPC), entorhinal cortex (EC), and dentate gyrus (DG) of rats trained to respond to appetitively reinforced odors. Preafferent anticipatory events in the beta band (12-35 Hz) suggest transmission from EC to OB before the odorant stimulus. Gamma band (35-120 Hz) power in olfactory regions is significantly reduced during stimulus presentation as compared with high values during preafferent expectation. High coherence of OB and PPC gamma activity during the preodorant control period is interrupted before the stimulus and is followed by increased gamma coherence among OB, EC, and DG. These results suggest that olfactory perceptual processing is bidirectional and covers a wide frequency range.     

Study of glomeruli in the frog olfactory bulb using a fluorescent potential-sensitive dye

Localized irradiation of the skin and subcutaneous tissues with large single doses of gamma rays can induce immediate effects characterized by erythema, desquamation, and necrosis. Correlations between the evolution of the lesions and dosimetry studies have to be established by biophysical methods. NMR studies of the effects of an irradiated Fricke solution might be a means of controlling the delivered irradiation doses. After exposition to ionizing radiations, ferrous ions are transformed into ferric ions. Both are paramagnetic ions, and proton spin-lattice relaxation is accelerated depending on the oxidation reaction. In this study, solution of ammonium ferrous sulfate in an acid environment was incorporated into a gelling substance made with agarose, so that T1 weighted image contrast could be used to detect ferric ion formation. Experiments with 192Ir and 60Co gamma rays with doses in the 0 to 100 Gy range were conducted with Fe2+ concentrations of 0.5, 1, 1.5, and 2 mM in a gelling substance containing 4% agarose. A relationship was established between the amount of Fe3+ created and the spin-lattice proton relaxation rate, which led to a straightforward dose-effect relation. The use of such high doses allowed us to reproduce realistic conditions of accidental overexposure. A linear relationship was obtained between the doses absorbed and the NMR parameters measured (T1 and relative image intensity).     

Glycobiology of the olfactory system

The olfactory system is a highly plastic region of the nervous system. Continuous remodeling of neuronal circuits in the olfactory bulb takes place throughout life as a result of constant turnover of primary sensory olfactory neurons in the periphery. Glycoconjugates are very important in olfactory development, regeneration and function. This article deals with different aspects of glycobiology relevant for the olfactory system. Various anatomical, developmental and functional subdivisions of the olfactory system have been labeled with exogenous lectins. The application of reverse lectin histochemistry resulted in the visualization of endogenous lectins, involved in fasciculation of olfactory axons. Numerous glycoproteins, among them members of the immunoglobulin superfamily, the cadherins and integrins as well as different glycolipids and proteoglycans can act as surface adhesion molecules in the olfactory system. The olfactory-specific form of the sialoglycoprotein neural cell adhesion molecule is implicated in olfactory neuronal and axonal guidance. Glycoconjugates including laminin, fibronectin and proteoglycans are abundant components of the olfactory extracellular matrix, influencing neurite outgrowth and cellular migration. Immunohistochemical labeling has revealed occurrence of the carbohydrate differentiation antigen, playing a role in neurulation and morphogenesis of the very early olfactory system. The synaptic vesicle glycoprotein, appearing also early in olfactory development, is used as a marker of olfactory tumors. Finally, membrane and transmembrane glycoconjugates as well as secreted glycoconjugates may act as olfactory receptor molecules.     

Expression of galectin-1 in the mouse olfactory system

Mast cells hold a key position in the defensive mechanisms against exogenous intruders. In this study, we investigated whether human mast cells express functional major histocompatibility complex (MHC) class II molecules that can transduce endogenous signals and present staphylococcal enterotoxin A (SEA) to T cells. Similar to HMC-1 human mast cell line, umbilical cord blood-derived mast cells express HLA-DR, -DP and -DQ molecules on their surface. MHC class II molecules expressed on HMC-1 cells bind significantly the SEA (a natural MHC class II ligand), and their ligation with specific mAbs or with SEA, leads ultrastructural changes, suggesting their degranulation. Recognition of SEA-bound MHC class II molecules on HMC-1 mast cells by the T cell receptor of K25 cells, an SEA-specific murine T cell hybridoma, triggers significant IL-2 secretion by these T cell hybridomas. Hence, our data point out the expression of functional MHC class II molecules on human mast cells, reinforcing the implication of these cells in the defense mechanisms of acquired immunity.     

"Novelty" neurons in the frog auditory system

Some neurones in the torus semicircularis of paralyzed lake frogs (Rana ridibunda) exhibited strong response habituation to tonal bursts following with interstimuli intervals of 1 to 5 sec. The habituation became less evident with a devrease of the repetition rate or increase of the stimulus intensity. These units revealed the phenomenon of "postzero" habituation. The frequency selectivity of the units exhibited by their responses to the first stimulus presentation was very poor. Usually the habituation occurred within a wide frequency band, although there were some exceptions. Afther the habituation was completed, the unit's response could be elicited by tones of a different frequency. The more distant on the frequency scale the tones were, the more vigorous was the reaction. Some properties of these "novelty" units can be explained by assuming that they are multipolar neurones located in the nucleus magnocellularis of the torus.     

Subdivisions of the olfactory system in the sterlet Acipenser ruthenus

Subdivisions of the olfactory system of the sterlet Acipenser ruthenus were investigated by means of horseradish peroxidase (HRP) injections into the nose, and by soybean agglutinin binding studies. With both methods primary olfactory fibers were labeled which projected to the ventral part of the glomerular layer of the olfactory bulb. The dorsal part of the olfactory bulb did not bind soybean agglutinin, however, even though HRP tracing showed primary olfactory fibers in that area. This confirms earlier morphological studies which claim the existence of distinct subdivisions of the olfactory system in the sturgeon. The lack of soybean agglutinin binding in the dorsal part of the olfactory bulb suggests, however, that this part is not homologous with the accessory olfactory system of tetrapods.     

Calretinin immunoreactivity in the developing olfactory system of the rainbow trout

OBJECTIVE: The objective of this study was to calculate and compare the effective dose and to estimate risk from the use of intraoral position-indicating devices of differing geometries. STUDY DESIGN: Thermoluminescent dosimeters were placed at selected sites in the upper portion of a tissue-equivalent human phantom to record the equivalent dose to weighted tissues and organs. The phantom was exposed to simulated complete mouth surveys with either a long (29.8 cm) or short (19.6 cm) round open-end position-indicating device, a long (35.3 cm) or short (23.3 cm) rectangular open-end position-indicating device, or a pointed (29.6 cm) closed-end position-indicating device. RESULTS: The effective dose was calculated as the sum of the equivalent doses to each organ or tissue multiplied by that organ or tissue's weighting factor. The salivary glands were included as part of the remainder. The effective dose ranged from 362 micro Sv for the pointed position-indicating device, to 63 micro Sv for both the long and the short rectangular position-indicating devices. CONCLUSIONS: These effective doses were calculated to represent a probability for stochastic effects that range in magnitude from 26 x 10(-6) to 4.6 x 10(-6).     

Purification and molecular cloning of a novel calcium-binding protein, p26olf, in the frog olfactory epithelium

The pharmacological profile and the acute and chronic aquaretic effects of OPC-41061, a novel nonpeptide human arginine vasopressin (AVP) V2-receptor antagonist, were respectively characterized in HeLa cells expressing cloned human AVP receptors and in conscious male rats. OPC-41061 antagonized [3H]-AVP binding to human V2-receptors (Ki = 0.43 +/- 0.06 nM) more potently than AVP (Ki = 0. 78 +/- 0.08 nM) or OPC-31260 (Ki = 9.42 +/- 0.90 nM). OPC-41061 also inhibited [3H]-AVP binding to human V1a-receptors (Ki = 12.3 +/- 0.8 nM) but not to human V1b-receptors, indicating that OPC-41061 was 29 times more selective for V2-receptors than for V1a-receptors. OPC-41061 inhibited cAMP production induced by AVP with no intrinsic agonist activity. In rats, OPC-41061 inhibited [3H]-AVP binding to V1a-receptors (Ki = 325 +/- 41 nM) and V2-receptors (Ki = 1.33 +/- 0. 30 nM), showing higher receptor selectivity (V1a/V2 = 244) than with human receptors. A single oral administration of OPC-41061 in rats clearly produced dose-dependent aquaresis. In treatment by multiple OPC-41061 dosing for 28 days at 1 and 10 mg/kg p.o. in rats, significant aquaretic effects were seen throughout the study period. As the result of aquaresis, hemoconcentration was seen at 4 hr postdosing although, no differences were seen in serum osmolality, sodium, creatinine and urea nitrogen concentrations at 24 hr postdosing. Furthermore, there was no difference in serum AVP concentration, pituitary AVP content or the number and affinity of AVP receptors in the kidney and liver at trough throughout the study period. These results demonstrate that OPC-41061 is a highly potent human AVP V2-receptor antagonist and produces clear aquaresis after single and multiple dosing, suggesting the usefulness in the treatment of various water retaining states.     

Multitasking in the olfactory system: context-dependent responses to odors reveal dual GABA-regulated coding mechanisms in single olfactory projection neurons

Studies of olfaction have focused mainly on neural processing of information about the chemistry of odors, but olfactory stimuli have other properties that also affect central responses and thus influence behavior. In moths, continuous and intermittent stimulation with the same odor evokes two distinct flight behaviors, but the neural basis of this differential response is unknown. Here we show that certain projection neurons (PNs) in the primary olfactory center in the brain give context-dependent responses to a specific odor blend, and these responses are shaped in several ways by a bicuculline-sensitive GABA receptor. Pharmacological dissection of PN responses reveals that bicuculline blocks GABAA-type receptors/chloride channels in PNs, and that these receptors play a critical role in shaping the responses of these glomerular output neurons. The firing patterns of PNs are not odor-specific but are strongly modulated by the temporal pattern of the odor stimulus. Brief repetitive odor pulses evoke fast inhibitory potentials, followed by discrete bursts of action potentials that are phase-locked to the pulses. In contrast, the response to a single prolonged stimulus with the same odor is a series of slow oscillations underlying irregular firing. Bicuculline disrupts the timing of both types of responses, suggesting that GABAA-like receptors underlie both coding mechanisms. These results suggest that glomerular output neurons could use more than one coding scheme to represent a single olfactory stimulus. Moreover, these context-dependent odor responses encode information about both the chemical composition and the temporal pattern of the odor signal. Together with behavioral evidence, these findings suggest that context-dependent odor responses evoke different perceptions in the brain that provide the animal with important information about the spatiotemporal variations that occur in natural odor plumes.     

Dopaminergic modulation of olfactory bulb processing affects odor discrimination learning in rats.

Olfactory behavioral studies have shown that, when modulated through systemic injections, D1 and D2 receptors have opposing effects on odor discrimination learning. In the present study, cannulated male Sprague–Dawley rats were used to investigate how the modulation of these 2 types of dopaminergic receptors through direct infusion of D1/D2 agonists and antagonists into the olfactory bulb affect olfactory perception. Dopaminergic modulation was locally altered by manipulations of D1 (agonist SKF 82958: 14.6, 43.8, & 143.6 mM; antagonist SCH-23390: 13.4, 40.1, & 60.1 mM) and D2 (agonists quinpirole: 78.2, 117.3, & 156.4 mM; antagonist sulpiride: 0.3, 0.9, & 2.9 mM) receptors during a simultaneous odor discrimination task. The authors found that modulation of D2, but not D1, receptors significantly affected rats’ odor discrimination performance. A significant positive correlation between blockade of D2 receptors and discrimination performance, as well as a significant negative correlation between D2 receptor activation and discrimination performance, was observed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Relationship between afferent and central temporal patterns in the locust olfactory system

Odors evoke synchronized oscillations and slow temporal patterns in antennal lobe neurons and fast oscillations in the mushroom body local field potential (LFP) of the locust. What is the contribution of primary afferents in the generation of these dynamics? We addressed this question in two ways. First, we recorded odor-evoked afferent activity in both isolated antennae and intact preparations. Odor-evoked population activity in the antenna and the antennal nerve consisted of a slow potential deflection, similar for many odors. This deflection contained neither oscillatory nor odor-specific slow temporal patterns, whereas simultaneously recorded mushroom body LFPs exhibited clear 20-30 Hz oscillations. This suggests that the temporal patterning of antennal lobe and mushroom body neurons is generated downstream of the olfactory receptor axons. Second, we electrically stimulated arrays of primary afferents in vivo. A brief shock to the antennal nerve produced compound PSPs in antennal lobe projection neurons, with two peaks at an approximately 50 msec interval. Prolonged afferent stimulation with step, ramp, or slow sine-shaped voltage waveforms evoked sustained 20-30 Hz oscillations in projection neuron membrane potential and in the mushroom body LFP. Projection neuron and mushroom body oscillations were phase-locked and reliable across trials. Synchronization of projection neurons was seen directly in paired intracellular recordings. Pressure injection of picrotoxin into the antennal lobe eliminated the oscillations evoked by electrical stimulation. Different projection neurons could express different temporal patterns in response to the same electrical stimulus, as seen for odor-evoked responses. Conversely, individual projection neurons could express different temporal patterns of activity in response to step stimulation of different spatial arrays of olfactory afferents. These patterns were reliable and remained distinct across different stimulus intensities. We conclude that oscillatory synchronization of olfactory neurons originates in the antennal lobe and that slow temporal patterns in projection neurons can arise in the absence of temporal patterning of the afferent input.     

Peripheral olfactory deafferentation of the primary olfactory system in rats using ZnSO4 nasal spray with special reference to maternal behavior

A modified method of applying ZnSO4 to the olfactory mucosa is described. Treated rats experienced severe nasal congestion that cleared within 24 h; more persistent morbidity did not occur. Nonpregnant females observed with male intruders 24 h following ZnSO4 showed no alterations in behavior other than a reduction in anogenital sniffing, indicating that they were not hypoactive or irritable. In other experiments, lactating females were observed in a hole-board apparatus; 2 days posttreatment anosmia was confirmed in 80% of bilaterally ZnSO4-treated females by the absence of preference for pup odors. After bilateral but not unilateral ZnSO4 treatment, initially activity scores and nose pokes were equivalent in all groups, but later they both were lower than in controls, probably due to a more rapid habituation to the novel apparatus. We conclude that intranasal ZnSO4 by small-volume spray is a useful experimental tool.     

Spatio-temporal patterns of ensheathing cell differentiation in the rat olfactory system during development

An immunocytochemical approach with specific glial markers was used to investigate the temporal and spatial patterns of differentiation of ensheathing glia wrapping axon fascicles along the primary olfactory pathway of the rat during development. The two glial markers tested, the proteins S-100 and glial fibrillary acidic protein, are known to be expressed at different stages of maturation in glial cells. The S-100 protein was first weakly expressed in cells accompanying the olfactory axons at embryonic day 14 (E14), while a first faint glial fibrillary acidic protein staining was detected along the olfactory axons at E15 and along the vomeronasal nerves at E16. A strong S-100 immunoreactivity was already present from E16 onwards along the axon fascicles through their course in both the nasal mesenchyme and the subarachnoid space before entering the olfactory nerve layer of the olfactory bulb. A gradual increase in glial fibrillary acidic protein expression was observed along this part of the developing olfactory pathway from E16 up to E20, when an adult-like pattern of staining intensity was seen. By contrast, most of the ensheathing cells residing in the olfactory nerve layer exhibited some delay in their differentiation timing and also a noticeable delayed maturation. It was only from E20 onwards that a weak to moderate S-100 expression was detected in an increasing number of cells throughout this layer, and only few of them appeared weakly glial fibrillary acidic protein positive at postnatal days 1 and 5. The immunocytochemical data indicate that there is a proximodistal gradient of differentiation of ensheathing cells along the developing olfactory pathway. The prolonged immaturity of ensheathing cells in the olfactory nerve layer, which coincides with the formation of the first glomeruli, might facilitate the sorting out of olfactory axons leading to a radical reorganization of afferents before they end in specific glomeruli.     

Changes in gene expression during post-eclosional development in the olfactory system of Drosophila melanogaster

We have found that the expression of some genes in Drosophila melanogaster changes during the life of the adult fly. These changes can be illustrated by the use of enhancer trap lines which mark the expression of particular genes in the adult fly. Although the fly is considered able to perform most necessary adult functions within the first 72 h after eclosion from the pupal case, we find changes in expression over the first 10 days of life in the antennae of several of the genes we have examined. Some genes change by increasing from an initially low level of expression of the marked gene, while other lines, which we have termed 'late-onset' genes, show no expression of the marked gene until 4-5 days following eclosion. In contrast, some genes decrease their expression during the first 10 days of life. The changes in gene expression seen over the first 10 days of the fly's adult life provides molecular evidence of the many maturational changes occurring during the early life of the adult fly.