Activity of new receptors after transection of the primary olfactory nerve in pigeons

Chronic odorant exposure upregulates acquisition of functional properties in cultured embryonic chick olfactory sensory neurons

Neuronal development and differentiation is modulated by activity-dependent mechanisms that stimulate endogenous neurogenesis and differentiation to promote adaptive survival of the organism. Studies on bird odor imprinting have shown how sensory stimuli or environmental influences can affect neonatal behavior, presumably by remodeling the developing nervous system. It is unclear whether these changes originate from the sensory neurons themselves or from the brain. Thus, we attempted to address this by using an in vitro system to separate the peripheral neurons from their central connections. Olfactory neurons from embryonic day 17 Gallus domesticus chicks were isolated, cultured, and exposed to 100 µM amyl acetate or phenethyl alcohol in 12-hr bouts, alternated with periods of no-odor exposure. On days 4 and 5 in vitro, cells were immunostained for olfactory marker protein, neuron-specific tubulin, and olfactory GTP-binding protein, and tested for odorant sensitivity using calcium imaging. While odorant exposure did not result in a significant increase in the overall number of neurons, it promoted neuron differentiation: a larger proportion of odorant-exposed cells expressed olfactory marker protein and the olfactory GTP-binding protein. When cell responsiveness was tested using calcium imaging, a greater proportion of odorant-exposed cells responded to stimulation with 100 µM amyl acetate or phenethyl alcohol. Thus, odorant exposure during development modulated the developmental trajectories of individual neurons, resulting in changes in protein expression associated with odorant signaling. This suggests that the neuronal changes in the periphery have an important contribution to the overall long-term functional changes associated with odor imprinting. © 2016 Wiley Periodicals, Inc.

The peripheral olfactory system of the domestic chicken: physiology and development

Olfaction is a ubiquitous sensory system found in all terrestrial vertebrates. Birds use olfaction for several important activities such as feeding and mating; thus, understanding bird biology would also require the systematic study olfaction. In addition, the olfactory system has several unique features that are useful for the study of nervous system function and development, including a large multigene family for olfactory receptor expression, peripheral neurons that regenerate, and a complex system for sensory innervation of the olfactory bulb. We focused on physiological, anatomical and behavioral approaches to study the chick olfactory neurons and the olfactory bulb. Chick olfactory neurons displayed some properties similar to those found in mature neurons of other vertebrate species, and other properties that were unique. Since information from these neurons is initially processed in the olfactory bulb, we also conducted preliminary studies on the developmental timeline of this structure and showed that glomerular structures are organized in ovo during a critical time period, during which embryonic chicks can form behavioral associations with odorants introduced in ovo. Lastly, we have shown that chick olfactory neurons can grow and mature in vitro, allowing their use in cell culture studies. These results collectively demonstrate some of the features of the olfactory system that are common to all vertebrates, and some that are unique to birds. These highlight the potential for the use of the physiology and development of the olfactory system as a model system for avian brain neurobiology.

Regeneration of olfactory receptor cells

The vertebrate olfactory system has become an important model for the study of neural regeneration. The most remarkable feature of this system is its unique capacity for neurogenesis and replacement of degenerating receptor neurons. This replacement is made possible by a persistent neurogenesis among basal cells. Basal cells differentiate, develop into sensory neurons and grow axon processes. Receptor cell axons project back to the olfactory bulb where they reestablish connections with the central nervous system. When mature receptors reach a critical age, are damaged by nerve injury, or are exposed to environmental agents that enter the nasal cavity, they degenerate and are subsequently replaced by newly regenerated receptor cells. Recent experiments demonstrate that olfactory neurogenesis is not simply an extension of growth and development but is a unique capacity for cell replacement that persists beyond maturity and well into old age. Even more remarkable is the finding that replacement receptor cells re-establish connections with the CNS and restore sensory function. It is expected that further studies of olfactory neurogenesis using cell and tissue culture methods will provide important advances for the field of neural regeneration.

Characteristics of odorant elicited calcium fluxes in acutely-isolated chick olfactory neurons

To understand avian olfaction, it is important to characterize the peripheral olfactory system of a representative bird species. This study determined the functional properties of olfactory receptor neurons of the chicken olfactory epithelium. Individual neurons were acutely isolated from embryonic day-18 to newborn chicks by dissection and enzymatic dissociation. We tested single olfactory neurons with behaviorally relevant odorant mixtures and measured their responses using ratiometric calcium imaging; techniques used in this study were identical to those used in other studies of olfaction in other vertebrate species. Chick olfactory neurons displayed properties similar to those found in other vertebrates: they responded to odorant stimuli with either decreases or increases in intracellular calcium, calcium increases were mediated by a calcium influx, and responses were reversibly inhibited by 100 microM L: -cis-diltiazem, 1 mM Neomycin, and 20 microM U73122, which are biochemical inhibitors of second messenger signaling. In addition, some cells showed a complex pattern of responses, with different odorant mixtures eliciting increases or decreases in calcium in the same cell. It appears that there are common features of odorant signaling shared by a variety of vertebrate species, as well as features that may be peculiar to chickens.

Differential responses of olfactory neurons to axotomy at embryonic and postnatal stages

In the nervous system, apoptosis is a major process during embryonic and postnatal stages. In chick, experimental cell death can be obtained by axotomy. We have compared the responses of olfactory neurons to axotomy at embryonic stage E17 and postnatal stages.Forty-three chicken embryos and 32 young chickens less than 3 weeks old were used. We combined optic microscopy, electronic microscopy, terminal (TdT)-mediated dUTP-nick end labeling (TUNEL) method and gel electrophoresis of genomic DNA to analyze cell death. Cells in synthetic phase were labeled by bromodeoxyuridine injected i.p. and detected by immunohistochemistry. Apoptotic index and 5-bromo-2'-deoxyuridine (BrdU) labeling index were calculated for each stage. The Wilcoxon test was used for statistical analysis. A P value <0.05 was considered significant. Within 3 days following axotomy in E17 chicken embryos, there was no significant increase of apoptosis on the lesion side analyzed 3, 6, 12, 18, 24, 48 or 56 h later. A slight increase of bromodeoxyuridine incorporation appeared at 3 h, was weakly significant at 24 h (P=0.045) and the return to basal rate took place at 48 h. In postnatal stages, an apoptotic wave appeared 12 h after axotomy, reached a maximum at 24 h, and then decreased between 48 h and 72 h. A significant elevation of bromodeoxyuridine incorporation occurred on 48 h with a 24 h delay after the peak of apoptotic death. This differential response to axotomy in embryos and postnatal chickens might be due to a less complete maturation and higher plasticity of embryonic olfactory neurons corresponding to different requirement for survival and differentiation factors. Thus the embryonic or immature neurons would depend more on local epithelial environment and mature or postnatal neurons would require target-derived survival factors and die from apoptosis after their shortage resulting from axotomy.

Ultrastructure of the olfactory epithelium in intact, axotomized, and bulbectomized goldfish, Carassius auratus

The ultrastructure of the olfactory epithelium in intact, axotomized, and bulbectomized goldfish was studied by scanning and transmission electron microscopy. A total of 58 adult goldfish of various survival times were examined to determine whether the different types of surgery--either olfactory nerve transection or bulbectomy--yielded differences in the extent or time course of cellular degeneration and renewal. Control animals were also examined in detail to elucidate previous controversial findings concerning the types of olfactory receptor neurons present in goldfish. We found that the intact olfactory epithelium of unoperated control goldfish contains the previously observed ciliated and microvillous receptor neurons, and the crypt cell, a cell type not yet seen in the goldfish but recently reported in other species of teleosts. Following either olfactory nerve transection or bulbectomy, the olfactory receptor neurons showed similar signs of degeneration and subsequent cell death, but, surprisingly, the thickness of the olfactory epithelium did not change significantly with either treatment. The time course of receptor cell renewal was different in axotomized and bulbectomized goldfish. In axotomized goldfish, the amount of receptor cells decreased continuously until 8-13 days after surgery, followed by rapid cell renewal. For bulbectomized goldfish, cell replacement began almost immediately after surgery, with degeneration and cell renewal occurring simultaneously. Six weeks after bulbectomy, cell death and cell proliferation reached a "steady state," and the epithelia did not further improve.

Role of olfaction in the formation of preference for high-fat foods in mice

Male albino mice were given access to both high- and low-fat food mixtures in the home cage for 4 days. Mice were then divided into three groups and given a choice test in which all groups demonstrated a significant preference for the high-fat food mixture. One group was then bilaterally olfactory nerve sectioned. Seven days following surgery, all groups were given a second choice test. Olfactory nerve-sectioned mice (while anosmic) showed no preference, whereas high-fat food preference in the two control groups strengthened. However, high-fat food preference returned to recovered nerve-sectioned mice by 21 days postsurgery. It appears that preference for many high-fat foods in mice requires olfaction. This is in good agreement with earlier work that reported the loss of the novel food effect in olfactory nerve-sectioned mice while anosmic, and points to an important role for olfaction in the formation of preference for many high-fat foods.

Olfactory toxicity of methyl iodide in the rat

The monohalomethanes (methyl iodide, methyl bromide and methyl chloride) are widely used industrial methylating agents with pronounced acute and chronic toxicity in both experimental animals and man. Recently inhalation exposure of rats to methyl bromide has been shown to result in severe olfactory toxicity. This study examined the effects on the rat nasal cavity of inhalation of methyl iodide (100 ppm for 0.5-6 h), and demonstrated that methyl iodide is a more potent olfactory toxin than methyl bromide. Within the nasal cavity the olfactory epithelium was the principle target tissue, and it was only at high doses (600 ppm.h) that limited damage to transitional epithelium occurred. The squamous and respiratory epithelia were consistently unaffected. Within olfactory epithelium the sustentacular cells were the primary cellular target and damage to sensory cells appeared to be a secondary event. Methyl iodide induced olfactory damage was reversible, and 2 weeks after exposure almost complete repair had taken place.

Time course of reinnervation of the olfactory bulb after transection of the primary olfactory nerve in the pigeon

Horseradish peroxidase (HRP) histochemistry was used to study the time course of reinnervation of the pigeon olfactory bulb after simple transection of the primary olfactory nerve. At selected intervals (9, 13, 29, 61 and 93 days) after transection of the right olfactory nerve, a concentrated solution of HRP was instilled in both nasal cavities. Intracarotid perfusion was performed 3 days after nasal instillation of HRP and 40-microns sections of olfactory bulb processed with the tetramethylbenzidine (TMB)-HRP histochemical protocol to visualize olfactory receptor axon terminals reinnervating the glomerular layer of the bulb. The total area of reinnervation of four representative regions in the bulb of the transected side were compared with that on the control bulb. The area of innervation by newly reconstituted olfactory axons was approximately 17% of control values at the 12-day posttransection time interval. A progressive increase in the area of reinnervation was observed over time. Reinnervation of the right bulb was approximately 70% complete at the 32-day posttransection time interval and indistinguishable from the left control bulb at the 64- and 96-day posttransection time intervals. A uniform pattern of reinnervation of different bulb regions was observed at all time intervals. These results indicate that the peripheral olfactory system of the pigeon is capable of complete reconstitution after nerve transection. Our findings should be useful in guiding functional comparisons of normal and newly reconstituted peripheral olfactory systems in the pigeon.

Motion-induced aversions during and after recovery from olfactory nerve section in mice

Bilaterally nerve-sectioned male albino mice were given access to a novel food (almond) and then body rotated for 50 min. Two days later the mice were given a second access to almond. The procedures were repeated after recovery of olfactory function. Mice formed a presumably taste-mediated aversion while anosmic and a second flavor-mediated aversion (to the same food) after the return of olfactory function. Rotation control subjects formed only the standard flavor aversion subsequent to the first pairing of novel food and motion. It appears that olfaction is required for recognition of a food as novel.

The effects of neonatal capsaicin administration on trigeminal nerve chemoreceptors in the rat nasal cavity

Trigeminal nerve fibers in the nasal cavity respond to a variety of volatile chemical stimuli. Some of these trigeminal nerve fibers have been suggested to be capsaicin-sensitive and thus belong to a class of pain receptor rather than constituting a separate class of chemoreceptor. Our current results confirm this suggestion. Trigeminal nerve responses to volatile chemical stimuli were eliminated in rats which were injected with capsaicin on the second day of life. Animals whose nerves were unresponsive to chemical stimuli also exhibited a loss of intraepithelial peptide-immunoreactive fibers in their nasal cavities. The results of this study suggest that trigeminal nerve fibers in the nasal cavity which respond to chemical stimuli may be polymodal nociceptors which contain substance P, calcitonin gene-related peptide, or perhaps other neuropeptides.

A comparison of the discriminatory ability and sensitivity of the trigeminal and olfactory systems to chemical stimuli in the tiger salamander

Trigeminal receptors can respond to a wide variety of chemical stimuli, but it is unknown whether these receptors mediate discrimination between chemical stimuli matched for equal perceptual intensity. The present electrophysiological and behavioral experiments address this issue using tiger salamanders, Ambystoma tigrinum, and four compounds (amyl acetate, cyclohexanone, butanol, and d-limonene). In addition, the relative sensitivities of the trigeminal and olfactory systems to these compounds are compared. In electrophysiological cross-adaptation experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), there was complete cross adaptation such that only concentrations above the background (cross-adapting) stimulus concentration elicited responses, suggesting that chemical stimuli may stimulate trigeminal receptors nonspecifically. In behavioral experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), only animals with intact olfactory nerves could discriminate between perceptually equivalent concentrations, that is concentrations that elicited the same level of responding. Both electrophysiologically and behaviorally, the trigeminal system exhibited higher thresholds than the olfactory system. We conclude that trigeminal chemoreceptors, at least in salamanders, are unable to discriminate between these two pairs of compounds when matched for equal perceptual intensity, and that trigeminal chemoreceptors are less sensitive than olfactory receptors.

Degeneration and regeneration of the olfactory epithelium following inhalation exposure to methyl bromide: pathology, cell kinetics, and olfactory function

The effects of acute inhalation exposure to methyl bromide (MeBr) on the olfactory epithelium of male F-344 rats was investigated by morphologic examination of animals killed at varying timepoints during and following exposure to 200 ppm MeBr 6 hr/day for 5 days. Cell replication rate and histopathology were used to assess the kinetics of repair. In addition, olfactory function, using the buried food pellet test, was assessed and the result compared with morphological recovery. Extensive destruction of the olfactory epithelium was evident in animals killed directly after a single 6-hr exposure to MeBr. Histologic features of these lesions indicate that the primary, or most severe, effect of MeBr exposure was on the sustentacular cells and mature sensory cells; basal cells were generally unaffected. By Day 3, despite continued exposure, there was replacement of the olfactory epithelium by a squamous cell layer that increased in thickness and basophilic cytoplasmic staining over the next 2 days of exposure. One week postexposure, the epithelial region was covered by a layer of polyhedral, basophilic cells, and from 2 to 10 weeks postexposure, the epithelium exhibited progressive reorganization to reform the original olfactory epithelium pattern. By Week 10, 75-80% of the olfactory epithelium appeared morphologically normal. Cell replication showed a single peak of olfactory epithelial cell proliferation at Day 3 of exposure, with a labeling index of 14.5% compared to 0.7% in controls. Cell replication rates returned gradually to control levels by Week 10 postexposure. Behavioral tests of olfactory function in animals after a single 6-hr exposure to 200 ppm MeBr demonstrated a loss of the sense of smell, with recovery of this function by Day 6. Exposure to 90 ppm caused no observable effect on olfactory function or morphology. These findings demonstrate that the olfactory mucosa is highly sensitive to the toxic effects of MeBr and that olfactory epithelial cell proliferation, and possible regeneration, begins and occurs rapidly even in the face of continued exposure. Cell replication was most prominent in the layer of basal cells adjacent to the basal lamina, supporting proposals by other workers that the progenitors of both sustentacular cells and neurons reside in this location. Of interest is the fact that functional recovery occurs prior to complete morphological reorganization, indicating the shortcoming of utilizing olfactory morphology as an index of functional integrity.

Olfactory deprivation in pigeons: examination of methods applied in homing experiments

1. Effectiveness of three methods of olfactory deprivation or impairment was tested by means of unconditioned cardiac acceleration in response to odorous stimuli. 2. Occlusion of nostrils reduced stimulus intensity to approx. 20-30% of the level in unimpeded state. Capability of stimulus quantification remained unimpaired. 3. Bilateral olfactory nerve section and backward bending of the nerve stumps irreversibly eliminated responses to weaker stimuli. With higher concentrations of the odorant, pigeons responded at a reduced level, most likely due to trigeminal reception. 4. Spraying the nasal cavities with an anaesthetic largely abolished sensitivity to odorous stimuli. However, its effect was quite variable depending on the kind of application (which cannot be fully standardized). The time course of effectiveness is shown. 5. The relevance of these findings to experiments on pigeon homing is discussed. Conclusions on involvement of unspecified non-olfactory stimuli can only be drawn if methods are applied that reliably isolate the birds from airborne environmental odours.

Olfactory and nonolfactory odor detection in pigeons: elucidation by a cardiac acceleration paradigm

A technique for the Pavlovian conditioning of cardiac acceleration in response to odorants was developed and used to compare the sensitivity of pigeons to four odorants before and after resection of the olfactory nerves. This method provided quite reliable psychophysical functions yet required relatively little training time. Thresholds of normal pigeons to n-amyl acetate, n-butyl acetate, benzaldehyde and butanol were approximately 10(-4), 10(-5), 10(-3.3) and 10(-4.3) of vapor saturation, respectively. Following resection surgery, sensitivity decreased by 2 to 4 log units. When transection of the ophthalmic branch of the trigeminal nerve was combined with olfactory nerve resection, little evidence of a further decline in odor sensitivity was seen. Based on these results each of these compounds could be used, at concentrations below the postoperative threshold, to study, in isolation, both normal and reconstituting olfactory systems in the pigeon.

Nasal trigeminal chemoreception: responses to n-aliphatic alcohols

Odorant molecules can stimulate nasal trigeminal receptors, but the properties of such molecules which make them effective stimuli are largely unknown. In the present study, we obtained integrated multiunit responses from the ethmoid branch of the rat trigeminal nerve to a homologous series of aliphatic alcohols. Our aim was to determine whether lipid solubility might correlate with stimulus efficacy. Response thresholds (ranging from 3000 ppm for methanol to 3 ppm for octanol) decreased with increasing carbon chain length, suggesting that lipid solubility is important for stimulus effectiveness. One plausible explanation for the importance of lipophilicity is that the more lipid soluble a substance, the more easily it can penetrate epithelial layers to reach chemoreceptive trigeminal nerve endings. Since all stimuli at vapor saturation elicited responses within 0.5 s, and because diffusion of stimulus molecules through epithelium is slow, we speculate that trigeminal nerve endings lie closer to the epithelial surface than previously thought.

Neural regeneration and functional reconnection following olfactory nerve transection in hamster

The olfactory sensory neurons in the vertebrate nervous system are unique in that they undergo continuous neurogenesis and replacement. Anatomical studies have shown that transection of the olfactory nerves leads to a degeneration of sensory neurons followed by a neurogenesis and replacement with newly formed cells. Replacement neurons grow axonal processes that are capable of reestablishing morphological connections with cells in the olfactory bulb. To determine the functional capacity of these anatomical reconnections, single unit responses to odor stimuli were recorded from cells in the olfactory bulb following recovery from unilateral olfactory nerve transection. A total of 56 cells were studied, taken from hamsters with recovery times of 4,35,60,90,120,180 and 270 days. At day 4, although there was spontaneous activity recorded from cells on the experimental side (n = 10), they did not respond to stimulation of the olfactory epithelium with odors. Control cells (n = 9) from the unoperated side of the same animals showed normal odor responses. By day 35, some of the cells tested on the experimental side responded to odor stimulation, indicating that connections had been reestablished with sensory neurons. With longer recovery times, an increasing percentage of cells responded to odor stimuli. In addition, concentration response functions showed that cells were capable of signaling differences in stimulus intensity. The response of cells to four odors (amyl acetate, 1-butanol, ethyl acetate and ethyl butyrate) showed differences in odor selectivity, suggesting their ability to discriminate among odors.(ABSTRACT TRUNCATED AT 250 WORDS)

An operant procedure for testing olfactory capacities in restrained pigeons

A psychophysical procedure was developed to test the olfactory capacities of the pigeon. The procedure employs symmetrical response contingencies, involves the confinement of the pigeon's head in the stimulus air stream and relies solely on positive reinforcement for the development and maintenance of discriminative responding. The absolute sensitivity of three pigeons to amyl acetate in air was measured. Thresholds for all three pigeons were between 10(-3.4) and 10(-3.6) of vapor saturation.

Comparison of olfactory receptor (EOG) and bulbar (EEG) responses to amino acids in the catfish, Ictalurus punctatus

The olfactory bulb electroencephalogram (EEG) has been used as a method to imply receptor events12,13. However, experiments to correlate olfactory receptor and bulbar EEG activity in the same species of fish has not been performed. Reported here is the comparison between the simultaneously recorded receptor electroolfactogram (EOG) and the bulbar EEG in the channel catfish, Ictalurus punctatus. With amino acid stimulation of the olfactory mucosa, both the EOG and EEG exhibited an initial phasic response followed by a tonic level maintained throughout the stimulus duration. The relative magnitude of the tonic EEG activity (tonic level/phasic response), however, was significantly less than that for the EOG. Both EOG and integrated EEG responses increased exponentially with logarithmic increase in stimulus concentration from threshold to 10(-3) M. Estimated electrophysiological thresholds for 5 amino acids tested determined by both recording methods did not differ significantly and averaged 10(-9.3) +/- 0.2 M for the EOG and 10(-9.1) +/- 0.2 M for the EEG. There was also a significant correlation between the order of relative effectiveness for 11 amino acids determined by EOG and EEG recordings. These results indicate that in the catfish the olfactory bulb EEG is an indicator of olfactory receptor activity.

Chemosensitivity of rat nasal trigeminal receptors

Electrophysiological responses to odorants delivered via an air dilution olfactometer were recorded from the ethmoid branch of the trigeminal nerve innervating the nasal cavity. Thresholds were obtained for nine compounds with those for heptanol (21-137 ppm) and propionic acid (39-49) ppm consistently being the lowest. Not all odorants e.g., phenethyl alcohol, elicited responses in all rats even at vapor saturation. A striking degree of correlation was present between the rat whole-nerve electrophysiological response magnitudes of this study and the human anosmic intensity ratings established in the work of Doty et al. [9] to vapor saturated stimuli. These results suggest that the rat is an excellent model for assessing the stimulatory effectiveness of odorants on human trigeminal receptors. The possible role of the trigeminal system in the perception of odors as well as the physiologic effects of odorants due to trigeminal stimulation are discussed.

Degeneration and regeneration of olfactory cells induced by ZnSO4 and other chemicals

The necrotic effect of various salt solutions was tested on the catfish olfactory mucosa. Only zinc cations were able to induce an extensive degeneration of the olfactory cells. Two different modes of irrigation of the mucosa with zinc sulfate were investigated. (1) The olfactory cavity is flushed with the chemical for not more than a few seconds. At concentrations above 30 mM, the resulting damage is very reproducible, largely concentration independent and almost completely specific for the olfactory receptor cells. The non-sensory respiratory cells are unaffected, the sustentacular cells surrounding the receptor cells are affected mainly by a loss of microvilli. The olfactory receptor cells, on the contrary, start to degenerate within a few hours and by day 4 only 20% of the original receptor population remains. Division of the mucosal basal cells increases during days 3 and 4 on and day 6 olfactory receptor cells reach the bare surface of the lamella. After day 7, the receptor population reaches a level of more than 80% of its original value. Because of the absence of sustentacular processes covering the olfactory cell's knobs on day 6, it has been possible to confirm that each of the two types of olfactory receptor cells previously characterized are concentrated on each half of the mucosa (2) The salt is maintained in contact with the tissue for several days. After this treatment most of the lamellae are irreversibly destroyed, some regeneration occurs in limited areas of the mucosa. In these small areas, indifferent respiratory cells reappear first between 20 and 35 days. It is only when the structure of the olfactory tissue is completely reorganized that the new receptor cells reappear between days 45 and 55. Regeneration is not completed before 60-65 days.

Chemical deafferentation of the olfactory bulb: plasticity of the levels of tyrosine hydroxylase, dopamine and norepinephrine

The laminar distribution of tyrosine hydroxylase activity, dopamine and norepinephrine was determined in the dog olfactory bulb. The levels of tyrosine hydroxylase activity and dopamine were highest in the glomerular layer, whereas norepinephrine appeared to be more uniformly distributed across the layers. A similar distribution was observed within the mouse olfactory bulb. Following deafferentation of the mouse olfactory bulb, the levels of tyrosine hydroxylase activity and dopamine declined, while norepinephrine levels showed a transient increase. Subsequent to regeneration of the olfactory nerve, these levels returned to control values. The levels of tyrosine hydroxylase activity and of dopamine were very low or non-detectable in the olfactory epithelium, which contains the olfactory receptor neuron perikarya. The data obtained indicate that tyrosine hydroxylase activity and dopamine content in the bulb are more tightly coupled to each other than either is to norepinephrine content. Since the two catecholamines are in two different classes of neurons, this implies that the bulk of the tyrosine hydroxylase activity in the bulb is associated with the dopamine-containing neurons. Finally, our data are consistent with a transsynaptic control mechanism of the tyrosine hydroxylase activity and dopamine level in the olfactory bulb.

Ultrastructural changes in olfactory receptor neurons following olfactory nerve section

Unilateral olfactory nerve section was performed in the salamander, Ambystoma tigrinum. An ultrastructural study was performed to investigate the changes occurring during degeneration and replacement of the mature olfactory receptor neurons. Experimental and contralateral control tissues were examined following postoperative survival periods ranging from 12 hours to 90 days. Normal bipolar receptor neurons have a fusiform cell body containing a thin rim of cytoplasm and an ovoid nucleus with a characteristic "checkerboard" chromatin pattern. A single apical dendrite projects to the surface of the epithelium, where numerous cilia extend from its apex into the overlying mucus. A single, unmyelinated, unbranching axon originates at the basal pole of the cell. After nerve section, retrograde degeneration of the mature neurons occurs. Early degenerative changes include pronounced condensation of the nuclear chromatin, increased number of nuclear membrane infoldings, and dilation of the space between the membranes of the nuclear envelope. At a later stage, the cytoplasm of the cell increases in volume and its organelle systems break down, resulting in accumulation of various forms of cell inclusions. Subsequently, proliferation of cells in the basal region of the epithelium occurs. Between 3 week and 2 months following nerve section, these cells differentiate into mature neurons. By 3 months, neurons within the epithelium have resumed their normal ultrastructure. Correlation of the time course of the ultrastructural changes with previously reported neurophysiological studies indicates that neuronal activity of the epithelium is dependent upon the presence of fully differentiated olfactory receptor neurons.

The response of ornithine decarboxylase during neuronal degeneration and regeneration in olfactory epithelium

Mature olfactory neurons are continually replaced from a population of progenitor cells. Olfactory nerve section, bulbectomy, or treatment with certain chemicals induces degeneration of olfactory neurons followed in some cases by regeneration. Ornithine decarboxylase (ODC) activity was measured in mouse olfactory tissues as an indicator of cellular regeneration. ODC activity in olfactory tissue (0.2-0.4 nmol/mg protein/h) is 10-30 times higher than in a variety of other cerebral tissues. Within 3 h after unilateral olfactory nerve section, ODC activity in the epithelium declines to 50% of control followed by a slow return to basal activity by 6 days. In the same animals, ODC activity increases severalfold in bulb (1 day) with a gradual decline to normal (9 days). Except for an early transient increase, the effects of unilateral bulbectomy on epithelial ODC activity are similar to those seen after nerve section. The changes in ODC activity following intranasal irrigation with 10 mM-colchicine also closely mimic those seen after nerve section. The effects of intranasal irrigation on ODC activity with 0.5% Triton X-100 or 0.17 M-ZnSO4 are more complex. Thus, when the mature neuronal population is degenerating after surgery or chemical treatments, ODC activity decreases in the epithelium. The subsequent increase of ODC activity prior to reconstitution of the mature neuronal population probably reflects the regeneration mechanism of the olfactory epithelium. The increase of ODC activity in the olfactory bulb after nerve section is best interpreted as a cellular injury response. These alterations in ODC activity in olfactory tissues after chemical and surgical treatments constitute the earliest biochemical events observed in these tissues in response to cellular damage.