Globose basal cells are neuronal progenitors in the olfactory epithelium: a lineage analysis using a replication-incompetent retrovirus.

We have used a replication-incompetent retrovirus to analyze the lineage of olfactory receptor neurons in young rats. At 5-40 days after infection, clusters of infected cells comprised two major types: one consisted of 1-2 horizontal basal cells, and a second consisted of variable numbers of globose basal cells and immature and mature sensory neurons. Olfactory nerve lesion (which enhances neuronal turnover) increased the frequency of the globose-sensory neuron clusters as well as the number of cells in such clusters. No clusters contained both horizontal and globose basal cells, and none contained sustentacular cells. These data suggest, at least in young rats, that horizontal basal cells are not precursors of olfactory neurons, that there is a lineage path from globose cells to mature neurons, and that sustentacular cells may arise from a separate lineage.

Olfactory processing: a time and place for everything.

The behavioral effects of pharmacologically desynchronizing neuronal firing in the brain of the honeybee provide new evidence that the oscillatory synchronization of neuronal activity plays an important role in fine olfactory discrimination.

Contributions of topography and parallel processing to odor coding in the vertebrate olfactory pathway.

Odor information appears to be encoded by activity distributed across many neurons at each level in the olfactory pathway. Thus olfactory circuits function as parallel distributed processors. New methods for observing distributed activity in such systems permit computer simulations to be constructed that are constrained by patterns of activity observed in the real system. Analysis of the system using a combination of physiological measurements and computational approaches might elucidate the principles by which odors are discriminated.

Characterizing complex chemosensors: information-theoretic analysis of olfactory systems.

The mechanisms that underlie a wine lover's ability to identify a favorite vintage and a dog's ability to track the scent of a lost child are still deep mysteries. Our understanding of these olfactory phenomena is confounded by the difficulty encountered when attempting to identify the parameters that define odor stimuli, by the broad tuning and variability of neurons in the olfactory pathway,and by the distributed nature of olfactory encoding. These issues pertain to both biological systems and to newly developed 'artificial noses' that seek to mimic these natural processes. Information theory, which quantifies explicitly the extent to which the state of one system (for example, the universe of all odors) relates to the state of another (for example, the responses of an odor-sensing device),can serve as a basis for analysing both natural and engineered odor sensors. This analytical approach can be used to explore the problems of defining stimulus dimensions, assessing strategies of neuronal processing, and examining the properties of biological systems that emerge from interactions among their complex components. It can also serve to optimize the design of artificial olfactory devices for a variety of applications, which include process control, medical diagnostics and the detection of explosives.

Current trends in 'artificial-nose' technology.

Basic principles derived from biological olfaction, such as combining semiselective sensor arrays with pattern recognition, have been used to develop instrumentation capable of broad-band chemical detection and quantification. Commercially available instruments are useful in areas including quality control in the food, beverage and fragrance industries, environmental monitoring, chemical-purity and -mixture analysis, and medical diagnostics. Ongoing research is aimed at the development of more-advanced instruments that are smaller, cheaper, faster and more stable and reliable. These second-generation instruments are likely to find an increasing number of applications, including the on-line monitoring of fermentation and other bioprocesses.

Functional organization of rat olfactory bulb analysed by the 2-deoxyglucose method.

The spatial patterns of activity elicited in the rat olfactory bulb under different odor conditions have been analysed using the 2-deoxyglucose (2DG) technique. Rats were injected with 14C-2DG, exposed to controlled environments of amyl acetate, camphor, cage air, dimethyl disulfide, and pure air and autoradiographs prepared by the method of Sokoloff. Amyl acetate was associated with regions of glomerular layer densities in the anterolateral and mid- to posteromedial parts of the bulbar circumference, as previously reported. The extents of the densities increased with increasing concentration. Camphor odor was associated with regions of increased density in the anterodorsal and mid- to posteromedial parts of the bulb. Exposure to cage air produced scattered densities in the posteromedial and posterolateral bulb. Exposure to dimethyl disulfide gave variable results. Pure air was associated with a minimal number of small dense foci. The results with amyl acetate, camphor and cage air suggest that patterns for different odors are distinguishably different but overlapping. The regions of activity are greatest in extent and density with the highest odor concentrations. These define the regions within which more restricted and isolated foci appear at lower concentrations. The results thus provide evidence for the specific role of spatial factors in the neural processing of odor quality and odor concentration.

New structure, the "olfactory pit," in human olfactory mucosa.

A whole-mount immunocytochemical method was devised to study the olfactory receptor neurons on the surface of the human olfactory mucosal sheet. Antibodies to neuron-specific tubulin and/or microtubule-associated protein 5 and phosphorylated neurofilament protein were used. Specimens taken at autopsy from 56 patients ranging in age from 2 days to 92 years revealed a structure not previously described, an olfactory pit. Round or oval openings with a diameter of 50 to 500 microns were observed on the surface of the olfactory epithelium in the whole-mount specimen. The morphology, number, and distribution of these openings varied among the different individuals. A detailed analysis of these structures was carried out by rehydrating and sectioning the whole-mount specimens. The olfactory pit (OP) is a blind pouch lined with olfactory epithelium (OE), which appears as an invagination of OE into the connective tissue, with a depth varying between 150 and 200 microns. In some sections through an OP, a thick axon bundle emerging from the bottom of the pouch was visible. The extension and termination of this axon bundle in the central nervous system has not been explored. We have found OPs in monkey olfactory mucosa, but none in rodents. The function of the pit specialization is unclear, but it appears to be a feature of normal, young epithelium. The configuration of the blind pouch may prolong odorant association with the olfactory receptor neurons, or the OP may contain specialized neurons that have not yet been recognized by morphological, biochemical, or functional techniques.

2-Deoxyglucose uptake in the cat spinal cord during sustained and habituated activity in the plantar cushion reflex pathway.

[14C]2-deoxy-D-glucose (2-DG) was administered intravenously to anesthetized cats during electrical stimulation of the plantar cushion (central foot pad). Afferent volleys and the efferent reflex were monitored by recording from the tibial nerve at the ankle. Plantar cushion stimulation at 3 HZ, 5 x threshold for 45 minutes led to a discrete region of increased 2-DG uptake dorsomedially in the dorsal horn, predominantly in Rexed's laminae III and IV, ipsilateral to the stimulation. A less marked increase in labeling was also sometimes observed in the medial portion of lamina V. No labeling specific to stimulation was observed in the ventral horns. A control preparation, to determine the effects of surgical manipulations alone, confirmed that the labeling was indeed specific to the plantar cushion stimulation. A pattern of labeling identical to that seen during 3 Hz, 5 x threshold stimulation occurred when 2-DG was administered during 10 Hz, 5 x threshold stimulation, after the reflex elicited by plantar cushion stimulation had completely habituated. The most straightforward interpretation of our results suggests that the increases in 2-DG labeling produced by stimulation of the plantar cushion probably occurred in regions with heavy concentrations of axon collaterals of the primary afferent A alpha beta fibers from the plantar cushion, at or near their sites of termination in the dorsal horn.

Computer-assisted three-dimensional reconstructions of [14C]-2-deoxy-D-glucose metabolism in cat lumbosacral spinal cord following cutaneous stimulation of the hindfoot.

We report on computer-assisted three-dimensional reconstruction of spinal cord activity associated with stimulation of the plantar cushion (PC) as revealed by [14C]-2-deoxy-D-glucose (2-DG) serial autoradiographs. Moderate PC stimulation in cats elicits a reflex phasic plantar flexion of the toes. Four cats were chronically spinalized at about T6 under barbiturate anesthesia. Four to 11 days later, the cats were injected (i.v.) with 2-DG (100 microCi/kg) and the PC was electrically stimulated with needle electrodes at 2-5 times threshold for eliciting a reflex. Following stimulation, the spinal cord was processed for autoradiography. Subsequently, autoradiographs, representing approximately 8-18 mm from spinal segments L6-S1, were digitized for computer analysis and 3-D reconstruction. Several strategies of analysis were employed: 1) Three-dimensional volume images were color-coded to represent different levels of functional activity. 2) On the reconstructed volumes, "virtual" sections were made in the horizontal, sagittal, and transverse planes to view regions of 2-DG activity. 3) In addition, we were able to sample different regions within the grey and white matter semi-quantitatively (i.e., pixel intensity) from section to section to reveal differences between ipsi- and contralateral activity, as well as possible variation between sections. These analyses revealed 2-DG activity associated with moderate PC stimulation, not only in the ipsilateral dorsal horn as we had previously demonstrated, but also in both the ipsilateral and contralateral ventral horns, as well as in the intermediate grey matter. The use of novel computer analysis techniques--combined with an unanesthetized preparation--enabled us to demonstrate that the increased metabolic activity in the lumbosacral spinal cord associated with PC stimulation was much more extensive than had heretofore been observed.

Day-night variation in prepro vasoactive intestinal peptide/peptide histidine isoleucine mRNA within the rat suprachiasmatic nucleus.

Neurons within the suprachiasmatic nuclei of the hypothalamus (SCN) appear to function as a circadian clock that controls the timing of many physiological systems. The SCN contain several chemically distinct neuronal subpopulations, including a large group of interneurons within the ventrolateral SCN that exhibit co-localizable immunoreactivity for both vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI). The purpose of the present study was to determine whether VIP/PHI neurons within the rat SCN exhibit rhythmicity in the cellular levels of the messenger RNA encoding the precursor from which both VIP and PHI are derived. Using both quantitative in situ and solution hybridization prepro-VIP/PHI mRNA levels early in the dark phase were demonstrated to be significantly higher than those 5 h after the onset of the daily light period. Since no statistically reliable (P greater than 0.05) day-night variation was observed in the levels of prepro-VIP/PHI mRNA within cortex, these data suggest that the rhythmicity in prepro-VIP/PHI mRNA is an intrinsic property of VIP/PHI-containing SCN neurons, or rhythmically driven by local synaptic events within the SCN.

Pathologic changes in olfactory neurons in Alzheimer's disease.

Olfactory deficits and degenerative changes in central olfactory pathways are prominent in patients with Alzheimer's disease (AD). We hypothesized that peripheral olfactory neurons that reside in the nasal epithelium would show degenerative changes similar to the characteristic pathologic features of AD brain. Immunohistochemical studies of nasal tissue taken at autopsy reveal extensive degeneration in the sensory epithelium as well as abnormal neurites that share immunoreactive epitopes with dystrophic neurites and neurofibrillary tangles of the AD brain. The neuritic masses are stained with well-characterized monoclonal antibodies that do not normally stain olfactory neurons but which are very reactive with dystrophic neuritic structures and neurofibrillary tangles in AD brain. These include antibodies to phosphorylated and nonphosphorylated neurofilament subunits, tau, and also ALZ50, which is characteristically reactive with AD but not with normal brains. Such changes are present in 81% of AD patients. Similar accumulations of ectopic neurites are found in the olfactory epithelium of about 22% of non-demented patients. Preliminary statistical analysis fails to reveal any age-linked association. It has been proposed that the aged monkey is a good model for AD inasmuch as amyloid accumulations similar to those of humans are found in monkey brain. We examined a series of 13 rhesus monkeys, including aged animals with behavioral deficits. Although the olfactory epithelium was very similar to that of humans, no abnormal olfactory structures were observed. Aged rhesus monkeys do not appear to be a good model for the neuritic abnormalities of AD.

Are there structural and functional modules in the vertebrate olfactory bulb?

A number of different recording methods have shown that odorants elicit patterns of neuronal activity widely distributed across cells of the olfactory receptor epithelium, olfactory bulb, and piriform cortex in the vertebrate olfactory system. These findings suggest that the physicochemical properties of odorant molecules are processed by distributed coding mechanisms activated in parallel in olfactory circuits in order to characterize a single, "monomolecular" odorant. These findings also suggest that the response patterns seen at higher levels are set up by differential responses in peripheral receptor cells of the olfactory epithelium. One requirement for understanding the details of this proposed encoding scheme is correlation of odor-generated patterns with the components of these circuits. In this paper, results from 2-deoxyglucose and voltage-sensitive dye studies suggest that certain components of these responses may relate to patterns established in reproducibly identifiable aggregates of bulbar cells. These findings are consistent with previous observations suggesting that columnar groups of periglomerular, mitral/tufted and granule cells, oriented perpendicular to the laminae of the bulb, are functionally related to one another. Such cell groups or modules, when activated in parallel, could serve as building block components of the complete ensemble response. According to this hypothesis, different sets of such modules would be activated with different odorant stimuli and modules could be shared to the degree to which the physicochemical properties of the different stimuli overlap.

A simplified method for computerized densitometry of complex shapes in 2-deoxyglucose autoradiographs.

A simple, computerized densitometer is described which can be used to measure densities of photographic prints by means of a fiber-optic reflectance densitometric probe coupled to the movable cursor of a digitizing tablet. The cursor, with its attached probe, is moved by hand along a scan trajectory determined by the operator. In its configuration for use with 2-deoxyglucose autoradiographs, the histological section from which the autoradiograph was derived provides architectonic landmarks for guiding the path of the scan. The X, Y and density values taken along a scan line are sequentially stored in the computer memory. Algorithms are presented for plotting densities along unfolded scan lines within layers of structures with complicated shapes, for normalizing non-linearities introduced during photographic processing, for standardizing the data sets with reference to the density of average gray matter in different animals, for calculating total integrated density within defined boundaries along the scan line, for generating averages of multiple scans, and for stacking sequential scans to form pseudo-3-dimensional plots. This system allows densitometric measures to be made from autoradiographs in anatomically defined regions, thereby permitting precise correlation between isotope concentration and histological structure.

Intracellular potentials of salamander mitral/tufted neurons in response to odor stimulation.

Intracellular responses of salamander mitral/tufted neurons to defined odor pulses are described. Responses to odor stimulation, which generally are more complex than responses to olfactory nerve or tract electrical stimulation, show periods of depolarization and hyperpolarization that are influenced by odor concentration and quality. The way these periods coincide with different types of spike patterns in individual cells supports the hypothesis that the temporal patterning of odor responses is generated by differential activation of bulbar circuits.

Metabolic mapping of 2-deoxyglucose uptake in the rat piriform cortex using computerized image processing.

Using a computerized image processing method, the metabolic activity of the unfolded molecular and pyramidal layers of the piriform cortex was studied in rats during odorous stimulation. Animals were either intact or had sustained a bilateral transection of the lateral olfactory tract. 2-DG labelling was observed in corresponding areas across these two layers. No spatial pattern of 2-DG uptake could be seen in correlation with the odorant quality. These results are discussed with respect to the known anatomical and functional properties of the piriform cortex.

Odor-elicited activity monitored simultaneously from 124 regions of the salamander olfactory bulb using a voltage-sensitive dye.

In response to controlled, odor pulse stimulation of the olfactory receptor mucosa, large fluorescence signals were recorded simultaneously from 124 contiguous anatomical regions of the salamander olfactory bulb using the potentiometric probe RH 414. The amplitudes and waveforms of the signals varied systematically across the bulbar surface in apparent correspondence with the laminae of the bulbar neurons. Qualitatively similar results were obtained using both intact and decorporate preparations, although fluorescence signals obtained from intact animals were distorted by optical noise generated by mechanical disturbances related to the functioning cardiovascular system. These results indicate that multiple site optical recording can be used to obtain information about spatio-temporal patterning of bulbar electrical activity evoked by physiological odor stimulation of the receptor mucosa. This is the first demonstration that activity elicited by a single, one second odor stimulus at physiological concentration and duration can be measured across many elements in the olfactory bulb. Information provided by this approach, in combination with complementary data derived from 2-deoxyglucose and single unit studies, may yield a better understanding of how the vertebrate central nervous system extracts quality and concentration information from olfactory afferent input.

Dopamine-sensitive adenylate cyclase within laminae of the olfactory tubercle.

There are three histological laminae within the rat olfactory tubercle: plexiform, pyramidal and polymorphic. We have assayed dopamine-sensitive adenylate cyclase in homogenates of frozen sections cut parallel to these laminae. Consecutive sections were cut of alternating thickness, 16 micron and 100 micron. The 16 micron sections were stained with toluidine blue to ascertain the depth and orientation of each section. The 100 micron sections were homogenized and assayed for dopamine-sensitive adenylate cyclase. Substantial levels of dopamine-sensitive adenylate cyclase were found within all three laminae. The results suggest that the enzyme occurs in cell processes, including dendrites, of the plexiform layer, and they are consistent with the localization of dopamine-sensitive adenylate cyclase in the pyramidal cells.

Laminar distributions of 2-deoxyglucose uptake in the rat spinal cord following electrical stimulation of the sciatic nerve.

The 2-deoxyglucose method was employed to study the functional organization of afferent axons in the dorsal horn of the rat spinal cord. Stimulation of axons in the sciatic nerve with conduction velocities greater than 17 m/s produced activity predominantly in the deep laminae of the dorsal horn. In contrast, stimulation of the entire population produced activity throughout the dorsal horn. The results are discussed in relation to the functional organization of the rat dorsal horn.

Topographical and laminar localization of 2-deoxyglucose uptake in rat olfactory bulb induced by electrical stimulation of olfactory nerves.

Experiments were carried out to examine the topographical projection of the olfactory nerves to the olfactory bulb in the rat, using the Sokoloff [14C]2-deoxyglucose (2-DG) technique. Electrical stimulation of a medially located bundle of olfactory nerves produced a discrete zone of 2-DG uptake at the rostral pole of the bulb. Increasing stimulus strength yielded a slightly larger focus at this site. In contrast, electrical stimulation of laterally situated bundles of olfactory nerves resulted in a broad zone of activity extending along the lateral wall of the bulb, and increasing stimulus intensity produced a more extensive area of uptake. Laminar analyses provided information on the relation between activity in the glomerular layer, where the olfactory nerves terminate, and activity in deeper layers. The results support previous studies of the topographical projections of the olfactory nerves to the olfactory bulb. They also support the hypothesis that odor-induced 2-DG uptake in the olfactory bulb represents activation of groups of receptors in the olfactory epithelium whose axons terminate in activated glomerular regions in the olfactory bulb.

Intracellular injection of vital dyes into single cells in the salamander olfactory epithelium.

Intracellular vital dye injection was used to examine the morphology of single sustentacular and receptor cells and the developmental fate of individual basal cells in the olfactory epithelium of the tiger salamander. In acute experiments, Lucifer yellow injections were used to identify single basal, receptor or sustentacular cells on the basis of their overall morphology. Dye-coupling between a number of the different epithelial cells was observed. Progeny of basal cells were examined by following labeled cells for up to 2 weeks using intracellular injection of rhodamine-labeled dextran. These experiments indicate that some olfactory epithelial cells are dye-coupled and that dye-filled basal cells can undergo division and migration.