Unilateral odor deprivation: effects on the development of staining for olfactory bulb succinate dehydrogenase

Olfactory experience modulates immature neuron development in postnatal and adult guinea pig piriform cortex

Immature neurons expressing doublecortin (DCX+) are present around cortical layer II in various mammals including guinea pigs and humans, especially enriched in the paleocortex. However, little is known whether and how functional experience affects the development of this population of neurons. We attempted to explore a modulation by experience to layer II DCX+ cells in the primary olfactory cortex in postnatal and adult guinea pigs. Neonatal and 1-year-old guinea pigs were subjected to unilateral naris-occlusion, followed 1 and 2months later by morphometry of DCX+ cells in the piriform cortex. DCX+ somata and processes were reduced in the deprived relative to the non-deprived piriform cortex in both age groups at the two surviving time points. The number of DCX+ cells was decreased in the deprived side relative to internal control at 1 and 2months in the youths and at 2months in the adults post-occlusion. The mean somal area of DCX+ cells showed a trend of decrease in the deprived side relative to the internal control in the youths. In addition, DCX+ cells in the deprived side exhibited a lower frequency of colocalization with the neuron-specific nuclear antigen (NeuN) relative to counterparts. These results suggest that normal olfactory experience is required for the maintenance and development of DCX+ immature neurons in postnatal and adult guinea pig piriform cortex.

Functional deprivation promotes amyloid plaque pathogenesis in Tg2576 mouse olfactory bulb and piriform cortex

Cerebral hypometabolism and amyloid accumulation are principal neuropathological manifestations of Alzheimer's disease (AD). Whether and how brain/neuronal activity might modulate certain pathological processes of AD are interesting topics of recent clinical and basic research in the field, and may be of potential medical relevance in regard to both the disease etiology and intervention. Using the Tg2576 transgenic mouse model of AD, this study characterized a promotive effect of neuronal hypoactivity associated with functional deprivation on amyloid plaque pathogenesis in the olfactory pathway. Unilateral naris-occlusion caused beta-secretase-1 (BACE1) elevation in neuronal terminals in the deprived relative to the non-deprived bulb and piriform cortex in young adult mice. In parallel with the overall age-related plaque development in the forebrain, locally increased BACE1 immunoreactivity co-occurred with amyloid deposition first in the piriform cortex then within the bulb, more prominent on the deprived relative to the non-deprived side. Biochemical analyses confirmed elevated BACE1 protein levels, enzymatic activity and products in the deprived relative to non-deprived bulbs. Plaque-associated BACE1 immunoreactivity in the bulb and piriform cortex was localized preferentially to swollen/sprouting glutamatergic axonal terminals, with Abeta immunoreactivity occurring inside as well as around these terminals. Together, these findings suggest that functional deprivation or neuronal hypoactivity facilitates amyloid plaque formation in the forebrain in a transgenic model of AD, which operates synergistically with age effect. The data also implicate an intrinsic association of amyloid accumulation and plaque formation with progressive axonal pathology.

Naris occlusion alters the electro-olfactogram: evidence for compensatory plasticity in the olfactory system

Unilateral naris occlusion (NO) has been widely used as a method of olfactory deprivation to study the role of stimulus-driven activity in olfactory development [P.C. Brunjes, Unilateral naris closure and olfactory system development, Brain Res. Rev. 19 (1994) 146-160]. Recent immunochemical studies of the olfactory epithelium (OE) following NO provide evidence for a previously unknown compensatory response to deprivation [D.M. Coppola, A. Waguespack, M. Reems, M.L. Butman, J. Cherry, Naris occlusion alters transductory protein immunoreactivity in olfactory epithelium, Histol. Histopathol. 21 (2006) 487-501; A. Waguespack, M. Reems, M.L. Butman, J. Cherry, D.M. Coppola, Olfactory receptor neurons have enhanced olfactory marker protein immunoreactivity in the nasal cavity ipsilateral to naris occlusion, Brain Res. 1044 (2005) 1-7]. To further investigate this phenomenon we measured electro-olfactograms (EOG) from the open and occluded OE of adult mice that had undergone NO as newborns. EOG waveforms from the open side OE of NO mice were indistinguishable from those obtained from untreated animals. However, amplitudes of EOGs from the occluded OE of NO mice were greater, on average, than those recorded at matched locations from the open side. This result was consistent across turbinates, odors, and all but the highest odor concentration. In addition, EOGs recorded from the occluded OE had significantly slower onset and recovery kinetics. Responses in a double-pulse protocol confirmed that the kinetics of the cellular or population processes that underlie the EOG are slowed by NO. These results provide the most direct support, to date, for compensatory plasticity in the olfactory system.

beta-Secretase expression in normal and functionally deprived rat olfactory bulbs: inverse correlation with oxidative metabolic activity

Cerebral hypometabolism, mitochondrial dysfunction, and beta-amyloid peptide (Abeta) accumulation are well-characterized manifestations of Alzheimer's disease (AD). beta-Secretase (BACE) is a prerequisite for amyloidogenesis, and it is up-regulated in sporadic AD. To explore a potential in vivo mechanism by which Abeta production is modulated by neuronal activity and/or oxidative metabolism, we compared BACE expression with cytochrome c oxidase (CO) or succinic dehydrogenase (SDH) activity in normal and functionally deprived adult rat olfactory bulb. In normal bulb, BACE was expressed predominantly in the glomerular layer, but labeling intensity within individual glomeruli varied substantially. A strong negative correlation existed between BACE labeling intensity and CO or SDH activity among individual glomeruli. Unilateral naris occlusion resulted in elevated glomerular BACE labeling in the deprived bulbs relative to the nondeprived counterparts, which was correlated with decreased CO activity in the same anatomic location. Enhanced BACE labeling was confirmed by measurements of elevated protein levels, enzymatic activity, and beta-site cleavage products of amyloid precursor protein in bulb extracts. Our findings reveal a negative regulation of BACE expression by physiological neuronal activity and an intrinsic inverse correlation between BACE expression and oxidative metabolism at the first synapse on the olfactory pathway. The results point to a biological role of BACE in synapse function and plasticity as well as a potential mechanism whereby reduced neuronal activity or metabolism could lead to amyloid overproduction in synaptic terminals.

Bilateral olfactory deprivation reveals a selective noradrenergic regulatory input to the olfactory bulb

Unilateral olfactory deprivation in the rat induces changes in the catecholaminergic system of the olfactory bulb. Nevertheless, evidence suggests that unilateral deprivation does not fully prevent stimulation of the deprived bulb. The present report analyses the response of the catecholaminergic system of the olfactory bulb in fully deprived rats obtained by bilateral naris occlusion. The complete deprivation produces more rapid and dramatic changes in both the intrinsic and extrinsic catecholaminergic systems of the olfactory bulb. Intrinsic responses involve a rapid decrease in dopamine-containing cells to about 25% of controls, correlated with a decreased Fos expression in juxtaglomerular cells of all olfactory glomeruli, with the only exception of those of the atypical glomeruli which maintain unaltered expression of both markers. In parallel with these events, there is a progressive increase in the density of extrinsic noradrenergic axons arising from neurons in the locus coeruleus, which shows, in parallel, a progressive increase in Fos expression. This model demonstrates plastic changes in the catecholaminergic system of the olfactory bulb forming a valid morphological substrate for lowering thresholds in the processing of olfactory information. In addition to this generalized response, there is another one, directed to a specific subset of olfactory glomeruli (atypical glomeruli) involved in the processing of odor pheromone-like cues related to behavioral responses, that could be responsible for keeping active this reduced and selected group of glomeruli carrying crucial olfactory information. These results indicate the existence of adaptive changes in the catecholaminergic system of the olfactory bulb as a response to the lack of afferent peripheral stimulation. These changes involve dopamine- and noradrenaline-immunoreactive elements, in a strategy presumably directed at maintaining to the highest possible level the ability to detect olfactory signals.

Effects of unilateral deprivation in postnatal development of the olfactory bulb in an altricial rodent, the gerbil (Meriones unguiculatus)

To establish if olfactory bulb sensitivity to functional deprivation is related to the degree of development at birth, we studied the effects of surgical closure of one naris in the gerbil olfactory bulb development. The naris closure was performed at three different ages: at birth, P7 and P14 and maintained for 30 or 60 days. In coronal sections we measured total bulbar surface area and surface area of the different bulbar layers establishing an estimate multiple regression model for the percentage of surface area decrease in the deprived bulb related to non deprived one. The internal and external plexiform layers are the most sensitive layers to deprivation and age and duration of deprivation were factors in their mathematical models. The glomerular layer showed a surface reduction of about 25% without dependence either on age or duration. The deprived glomerular layer showed a much lower tyrosine hydroxylase-immunoreactivity and immunoreactive cell density than those in the non deprived one. However, differences in calbindin-immunoreactive and NADPH-diaphorase positive cell density between deprived and non deprived glomerular layer were not significant. Our results indicate that olfactory bulb sensitivity to functional deprivation is not related to the degree of precocity and changes in age and duration of deprivation cause different effects on the olfactory bulb layers.

Effects of 14 days of spaceflight and nine days of recovery on cell body size and succinate dehydrogenase activity of rat dorsal root ganglion neurons

The cross-sectional areas and succinate dehydrogenase activities of L5 dorsal root ganglion neurons in rats were determined after 14 days of spaceflight and after nine days of recovery. The mean and distribution of the cross-sectional areas were similar to age-matched, ground-based controls for both the spaceflight and for the spaceflight plus recovery groups. The mean succinate dehydrogenase activity was significantly lower in spaceflight compared to aged-matched control rats, whereas the mean succinate dehydrogenase activity was similar in age-matched control and spaceflight plus recovery rats. The mean succinate dehydrogenase activity of neurons with cross-sectional areas between 1000 and 2000 microns2 was lower (between 7 and 10%) in both the spaceflight and the spaceflight plus recovery groups compared to the appropriate control groups. The reduction in the oxidative capacity of a subpopulation of sensory neurons having relatively large cross-sectional areas immediately following spaceflight and the sustained depression for nine days after returning to 1 g suggest that the 0 g environment induced significant alterations in proprioceptive function.

Development of 5'-nucleotidase staining in the olfactory bulbs of normal and naris-occluded rats

The distribution of the adenosine-producing ecto-enzyme 5'-nucleotidase was investigated histochemically in the developing rat olfactory bulb. Rat pups underwent either unilateral surgical occlusion of the right external naris or sham surgery on postnatal day 1. At 10, 20, or 30 days postpartum, horizontal sections of the olfactory bulb were reacted histochemically to reveal the locus and intensity of 5'-nucleotidase activity. Relative staining levels were determined by optical densitometry in standardized bulb regions. A marked, age-related increase in staining density was observed. Reaction product was found primarily in neuropil areas. The P10 and P20 control animals did not exhibit right/left differences in bulb staining; however, some laterality was observed in P30 animals. Inter-glomerular and regional variations were observed throughout the developmental period, including (1) differences between neighboring glomeruli; (2) a gradient in the dorsal-ventral axis of the bulb; and (3) a higher staining density in the medial-caudal portion of the bulb. In subjects with occluded nares, asymmetries in right/left bulb 5'-nucleotidase staining patterns were detected throughout development. Bulbs ipsilateral to the blocked nares exhibited increased staining density, suggesting that the procedure enhanced enzymatic activity. Understanding these variations in 5'-nucleotidase staining may be important for a complete understanding of the mechanisms of olfactory bulb maturation and may give insight into the possible role of this enzyme in synaptic malleability during nervous system development and regeneration.

Rapid down-regulation of tyrosine hydroxylase expression in the olfactory bulb of naris-occluded adult rats

In most sensory systems, afferent innervation regulates morphological and biochemical characteristics of target cells for a limited time during development. Sensory deprivation experiments in adult rats also have suggested a critical period for afferent influences on olfactory bulb structure and function. Previous odorant deprivation studies that employed unilateral naris closure in neonatal rats demonstrated down-regulation of the catecholamine biosynthetic enzyme tyrosine hydroxylase (TH) in dopamine neurons intrinsic to the olfactory bulb. Accompanying the altered biochemical parameters was a decrease in bulb size. To distinguish between deprivation-induced alterations in TH expression secondary to developmental sequelae and those occurring in mature neurons, the consequences of unilateral naris closure were assessed in young adult rats. In agreement with previous studies significant postnatal increases occurred in TH expression and total protein, an indication of bulb size. At 30 days post-closure, total protein was unaltered in the ipsilateral olfactory bulb but showed a small (12.9%), significant decline at 60 days. In contrast to the limited morphological consequences of odor deprivation, profound reductions occurred in TH expression. TH activity ipsilateral to the closure decreased significantly by 14 days post-closure and remained depressed for up to 6 months. In parallel with enzyme activity, TH immunoreactivity did not decline in the first few days post-closure. In situ hybridization revealed that TH mRNA levels decreased rapidly, i.e., by 2 days post-closure, reached a nadir at 1 month, and remained depressed for at least 6 months. The capacity of odor deprivation in the adult rat olfactory system to down-regulate TH expression suggests that this phenotypic alteration occurs independently of a presumed critical period.

Fos protein immunoreactivity in the developing olfactory bulbs of normal and naris-occluded rats

Immediate early genes such as c-fos may be a route through which extracellular events affect genomic expression. Expression of immediate early genes is important in the transcriptional regulation necessary for the normal development of the nervous system. Developmental patterns of Fos protein (the product of c-fos immediate early gene expression) were studied in the main olfactory bulb of the rat using immunocytochemistry. Embryonic Day 21 (E21, the last prenatal day), as well as Postnatal Day 0 (P0), P1, P5, P10, P15, P20 and P30 subjects were examined. Although staining was absent in the E21 bulb, there was a rapid onset of Fos synthesis within hours after birth. Distribution of Fos-immunoreactive (Fos-ir) nuclei corresponded to the sequence of bulb maturation: numerous mitral/tufted and granule cells were labeled on P0, followed by the appearance of Fos-ir in the nuclei of periglomerular cells and an increase in the number of stained granule cells with development. Surgical closure of an external naris on P1 resulted in a 70% reduction in the number of Fos-ir granule cell nuclei as early as 2 h after the manipulation. During the next 30 days, levels of Fos staining further diminished in experimental bulbs when compared to their contralateral controls. Nevertheless, electrical stimulation of the contralateral bulb in P20 pups resulted in a robust increase of Fos labeling in most main and accessory olfactory bulb mitral cells and in many granule and periglomerular neurons, suggesting that the experimental bulbs remain competent to express Fos protein.

Expression of tyrosine hydroxylase in the aging, rodent olfactory system

Tyrosine hydroxylase (TH) mRNA, immunoreactivity, and activity were examined as a reflection of dopamine expression in juxtaglomerular neurons intrinsic to the olfactory bulbs of young (6-month-old), middle aged (18-month-old), and aged (25- to 29-month-old) rats and mice. TH expression was maintained at levels observed in young animals in the olfactory bulbs of aged animals from two mouse strains, C57Bl/6JNia and C57Bl/6NNia, and one rat strain, an F1 hybrid between F344 and Brown Norway strains. The parental F344 rat strain exhibited reductions in TH expression of about 20% in 26- to 29-month-old animals as compared to 6- and 18 month-old rats. However, there was significant inter-animal variability. Some aged F344 rats had TH levels that were similar and others had activity levels that were 50% of those in young and middle aged animals. Neither the general condition of the animals nor the presence of adrenal tumors predicted the individuals with reduced TH expression. Olfactory bulb size, estimated from protein content, did not differ between rats and mice of different ages. In addition, expression of olfactory marker protein, a protein found primarily in mature olfactory receptor neurons, also was unchanged indicating the maintenance of afferent innervation. These data suggest that, in contrast to other brain dopamine systems, the expression of the dopamine phenotype is maintained in the aging olfactory bulb.

Odor deprivation leads to reduced neurogenesis and reduced neuronal survival in the olfactory bulb of the adult mouse

Neurogenesis persists in the olfactory bulbs of adult mice, with new cells being generated in the proliferative subependymal layer. Our previous work has shown that unilateral odor deprivation through naris closure leads to a net loss of granule neurons in the ipsilateral (odor-deprived) olfactory bulb, while not affecting the contralateral bulb. Here we used several experimental approaches to determine if this loss of neurons results from reduced neurogenesis, reduced neuronal survival, or both. First, bromodeoxyuridine immunohistochemistry was used to determine the number of S-phase cells in the subependymal layer eight weeks after naris closure. Proliferation was reduced within and just caudal to the odor-deprived bulb compared to the open-side (control) bulb. Second, counts of pyknotic nuclei four weeks after naris closure were used to document a higher rate of cell death on the deprived side. Third, 3H-thymidine autoradiography was used to assess differences in granule cell survival on the two sides. Granule cell precursors were labeled by a single injection of 3H-thymidine eight weeks after naris closure, and the number of surviving labeled granule cells assessed four and 16 weeks later. Granule cell survival was significantly reduced within the odor-deprived bulbs. These data indicate that the loss of granule cells which follows odor deprivation is caused, at least in part, by reduced neurogenesis and reduced survival of these adult-generated neurons.

Unilateral naris closure and olfactory system development

In most animals there is bilateral access of odorants to the olfactory sensory epithelium. Air enters the nose through two external nares and passes back through the nasal cavity, which is divided down the midline by a cartilaginous nasal septum. The olfactory mucosa, a sheet of ciliated bipolar receptor cells, is found in the caudal two thirds of the nasal cavity. Axons from the sensory cells project to an ipsilateral extension of the telencephalon known as the olfactory bulb. If a single external naris of a rat pup is surgically closed (usually via brief cauterization) on the day after the day of birth (P1) and the subject is examined on P30, the size of the ipsilateral olfactory bulb is reduced by approximately 25%. The large reduction in size, coupled with the clear lamination and other features of the olfactory system, indicates that the manipulation is an ideal preparation for examining the regulation of early growth. We know that both olfactory bulbs are of equal size at the time of occlusion, but that 30 days later there is a large discrepancy. What series of events produces the changes? The present paper outlines what is known about the anatomical, biochemical and physiological changes introduced by naris occlusion in order to lay a framework for further work.

Adult naris closure profoundly reduces tyrosine hydroxylase expression in mouse olfactory bulb

Peripheral afferent innervation appears to be required for the expression of the dopamine phenotype in the rodent main olfactory bulb. Experiments utilizing neonatal naris closure as a means of sensory deprivation also suggest that odor-induced afferent activity is required for the expression of the phenotype. These experiments are confounded, however, by the significant postnatal maturation of the dopamine system. The current experiments utilized adult unilateral naris closure to address this issue. As with neonatal closure, adult deprivation produces a profound reduction in the expression of tyrosine hydroxylase (TH), the first enzyme in the dopamine biosynthetic pathway. By 4 days a small decrease is observed in TH activity and immunoreactivity. Activity reaches a nadir of 12% of control levels at about 1 month. TH mRNA is reduced similarly when analyzed at about 2 months post-closure. Glutamic acid decarboxylase protein and mRNA expression, which are co-localized with TH, remain at close to control levels indicating the continued presence of the dopamine neurons. The time-course of the loss of TH is identical to that for zinc sulphate-induced denervation of the olfactory bulb. These data support the hypothesis that odor modulated afferent activity is required for expression of the dopamine phenotype and that, if a trophic factor is involved, its release is also activity dependent.

Regulation of protein kinase C activity by sensory deprivation in the olfactory and visual systems

Environmental regulation of sensory function has provided an important model of plastic mechanisms mediating neural information processing. To define potential commonalities in information processing in different systems, we investigated molecular changes elicited by sensory deprivation in the developing rat olfactory and visual systems. Protein kinase C (PKC), an intracellular messenger implicated in synaptic plasticity and memory, was analyzed. Initial, developmental studies indicated that PKC activity in the soluble and particulate fractions of the olfactory bulb increased three- to fourfold from birth to 3 months of age. Unilateral olfactory deprivation prevented the developmental increase in both soluble and particulate PKC activities in the ipsilateral olfactory bulb and piriform cortex, the second-order relay. Phorbol ester binding localized PKC to intrinsic neuronal populations and their dendrites in the control and deprived bulbs. Moreover, PKC was similarly lower in the visual cortex of dark-reared rats than in light-reared controls. The changes in PKC were region specific, as activity was unchanged by either treatment in the parietal cortex, a control area that does not process primary olfactory or visual information. Our results suggest that the important intracellular messenger, PKC, is similarly regulated in entirely different sensory systems by different environmental stimuli. Consequently, different sensory systems may use common molecular mechanisms to process information.

Functional consequences of unilateral olfactory deprivation: Time-course and age sensitivity

Unilateral olfactory deprivation in the rat profoundly modifies olfactory bulb anatomy, chemistry and function. The present report examined the time-course of the functional effects of unilateral deprivation on inhibition in the olfactory bulb using paired-pulse stimulation of the lateral olfactory tract and olfactory nerve. In addition, an attempt was made to correlate these physiological measures with olfactory bulb dopamine and norepinephrine levels and tyrosine hydroxylase immunoreactivity. Deprivation from postnatal day 1 to postnatal day 20 or postnatal day 40 significantly enhanced lateral olfactory tract paired-pulse depression, while late onset deprivation (postnatal day 20) had no effect. Olfactory nerve paired-pulse depression was enhanced by 40 days of deprivation regardless of the age at onset. The time-course of these deprivation-induced physiological changes did not correlate well with reductions in dopamine. Dopamine levels were reduced in all deprivation conditions by 70-80% compared with control bulbs. Norepinephrine content was slightly elevated in deprived bulbs. These results suggest that early olfactory deprivation modifies olfactory bulb synaptic activity and further, as with other sensory systems, these effects are age and duration dependent.

Unilateral naris closure and vascular development in the rat olfactory bulb

The blood supply to the brain has been linked closely to nervous system function and metabolism, thereby possibly playing a direct role in brain maturation. Previously, we demonstrated that closure of an external naris early in life results in large changes within the olfactory bulb, including reductions in laminar volume and cell number and a rapid decline in metabolism and protein synthesis. To understand the role of the blood supply in the dramatic changes following naris closure, the present study examines the development of olfactory bulb vasculature in unilaterally odor-deprived and control rats. On post-partum day 1 (P1; the day after birth), littermate rat pups underwent either unilateral naris occlusion or sham surgery. On P5, P10, P15, P20, P30 and P60, animals were perfused with an india ink-gelatin mixture to assess blood vessel amount and complexity. Densitometric analyses were performed to obtain values of blood vessel area ratios (vessel area/tissue area), branch point number and branch point density. Considerable vessel development in all bulbs occurred over the first two to three weeks post-partum. By P20, large reductions in vessel area ratios were observed in all constituent laminae of deprived bulbs. While similar reductions in number of vessel branch points/tissue area were seen, few changes were noted in the number of branch points/vessel area. The effects were primarily confined to early developmental periods: bulb vasculature in animals deprived at older ages (P40) appeared normal. The results indicate that the vasculature responds to alterations in sensory stimulation early in life, therefore potentially playing an important regulative role in neural development.

Adaptability of the oxidative capacity of motoneurons

Previous studies have demonstrated that a chronic change in neuronal activation can produce a change in soma oxidative capacity, suggesting that: (i) these 2 variables are directly related in neurons and (ii) ion pumping is an important energy requiring activity of a neuron. Most of these studies, however, have focused on reduced activation levels of sensory systems. In the present study the effect of a chronic increase or decrease in motoneuronal activity on motoneuron oxidative capacity and soma size was studied. In addition, the effect of chronic axotomy was studied as an indicator of whether cytoplasmic volume may also be related to the oxidative capacity of motoneurons. A quantitative histochemical assay for succinate dehydrogenase activity was used as a measure of motoneuron oxidative capacity in experimental models in which chronic electromyography has been used to verify neuronal activity levels. Spinal transection reduced, and spinal isolation virtually eliminated lumbar motoneuron electrical activity. Functional overload of the plantaris by removal of its major synergists was used to chronically increase neural activity of the plantaris motor pool. No change in oxidative capacity or soma size resulted from either a chronic increase or decrease in neuronal activity level. These data indicate that the chronic modulation of ionic transport and neurotransmitter turnover associated with action potentials do not induce compensatory metabolic responses in the metabolic capacity of the soma of lumbar motoneurons. Soma oxidative capacity was reduced in the axotomized motoneurons, suggesting that a combination of axoplasmic transport, intracellular biosynthesis and perhaps neurotransmitter turnover represent the major energy demands on a motoneuron. While soma oxidative capacity may be closely related to neural activity in some neural systems, e.g. visual and auditory, lumbar motoneurons appear to be much less sensitive to modulations in chronic activity levels.

Quantification of the effects of long-term unilateral naris closure on the olfactory bulbs of adult mice

The effects of unilateral naris closure on the olfactory bulbs of adult mice were assessed quantitatively by measuring four parameters. Naris closures were performed when animals were at least 5 months of age and lasted for 4-8 months. The first parameter measured was mitral cell number, which revealed that there was no significant effect of closure on numbers of these cells. The next parameter measured was the area of the external plexiform layer (EPL). The area of the EPL was 41% smaller in the closed-side olfactory bulbs than in the open-side olfactory bulbs (P less than 0.01). Comparisons of the areas of the granule cell layers (GCL) showed that the closed-side GCL was 25% smaller than the open-side GCL (P less than 0.01). Finally, the number of cells in the open- and closed-side GCLs were compared. The number of cells in the closed-side GCL was 30% lower than the number of cells in the open-side GCL (P less than 0.01). These data show that the shrinkage of closed-side olfactory bulb after naris closure in adult mice is due, at least in part, to the loss of granule cells and not to the loss of the main output neurons, the mitral cells.

Unilateral olfactory deprivation: effects on succinate dehydrogenase histochemistry and [3H]leucine incorporation in the olfactory mucosa

Surgically closing one external naris reduces airflow through one half of the nasal cavity, decreasing the access of odors to the receptor sheet. In rats, unilateral naris occlusion performed near birth results in large reductions in the size of the olfactory bulb, the primary central relay, when examined 30 days later. Previous research has demonstrated that there is a rapid reduction in [3H]2-deoxyglucose (2-DG) and [3H]leucine uptake in the bulb within hours after naris closure. The present study examined whether similar rapid changes could be observed in the sensory periphery. Pups occluded on P1 and examined on P3 with succinate dehydrogenase histochemistry exhibited reduced staining on the closed side of the nasal cavity, suggesting occlusion results in reductions in mucosal metabolism. Larger differences in staining were observed in pups examined at P6. [3H]Leucine incorporation was quite similar on both sides of the nasal septum as late as 30 days post occlusion, suggesting less dramatic changes in protein synthesis. The results suggest that naris closure does indeed have rapid effects on mucosal function, but indicate that the changes are different than those observed in the bulb.

Olfactory deprivation increases dopamine D2 receptor density in the rat olfactory bulb

Unilateral olfactory deprivation during postnatal development results in significant anatomical and neurochemical changes in the deprived olfactory bulb. Perhaps the most dramatic neurochemical change is the loss of dopaminergic expression by neurons of the glomerular region. We describe here the effects of early olfactory deprivation on other elements of the bulb dopaminergic system, namely the dopamine receptors of the olfactory bulb. Rat pups had a single naris occluded on postnatal day 2 (PN2). On PN20 or PN60, animals were sacrificed and the bulbs were examined for catecholamine levels or D2 and D1 dopamine receptor binding. Receptor densities were quantified by in vitro autoradiography using the tritiated antagonists spiperone (D2) and SCH23390 (D1). Dopamine uptake sites were similarly examined using tritiated mazindol. No significant specific labeling of D1 or mazindol sites was observed in the olfactory bulbs of control or experimental animals at either age. Normal animals displayed prominent labeling of D2 sites in the glomerular and nerve layers. After 60 days of deprivation, deprived bulbs exhibited an average increase in D2 receptor density of 32%. As determined by Scatchard analysis, the mean values for Kd and Bmax were 0.134 nM and 293 fmol/mg protein in normal bulbs, and 0.136 nM and 403 fmol/mg protein in deprived bulbs. The results suggest that, as in the neostriatum, dopamine depletion in the olfactory bulb leads to an upregulation of D2 receptor sites. This change may represent an attempt by the system to adapt neurochemically to reduced dopaminergic activity and thereby maintain bulb function.

Development of the anterior olfactory nucleus in normal and unilaterally odor deprived rats

The development of a second order structure in the olfactory pathway, the anterior olfactory nucleus, was examined in both normal rat pups and in subjects which underwent unilateral naris closure on postnatal day 1 (P1). Naris occlusion in neonatal rats produces a constellation of changes within the first relay in the pathway, the olfactory bulb, including a 25% reduction in total volume. Such large changes suggest that higher order structures might also be affected. Anterior olfactory nucleus development was quantified in several ways. Laminar volumes were computed by using serial section planimetry. In control animals differential development was observed, with regions extending most rostrally (e.g., pars externa and pars lateralis) exhibiting the least growth. The anterior olfactory nucleus on the "deprived" side of subjects with a single naris occluded was identical in size to that observed in controls, development within the pars lateralis was examined in control animals at P10, P20, P30, and adults. Developmental increases in numbers of both branches per cell and spines were noted, but mean branch length remained relatively constant. Finally, the effects of naris occlusion on histological patterns of succinate dehydrogenase (SDH) staining and 2-deoxyglucose uptake within pars lateralis were examined at P20 to test for more subtle effects of naris occlusion. SDH staining was quite similar in deprived and control rats at P20. However, 3H-2-DG uptake was decreased in rostral areas of the anterior olfactory nucleus ipsilateral to the deprived olfactory bulb, suggesting that naris closure does affect the structure.

Decrease in tyrosine hydroxylase, but not aromatic L-amino acid decarboxylase, messenger RNA in rat olfactory bulb following neonatal, unilateral odor deprivation

Unilateral naris cauterization in rats results in occlusion of the affected naris and blockade of odorant access to ipsilateral olfactory receptor cells in the olfactory epithelium. These receptor cells project exclusively to the olfactory bulb (OB) and appear to regulate expression of the dopaminergic phenotype in a population of OB juxtaglomerular neurons. Unilateral odor deprivation results in a loss of normal stimulatory input to the OB and a marked and specific decrease in ipsilateral OB tyrosine hydroxylase (TH) expression. The expression of co-localized aromatic L-amino acid decarboxylase (AADC) is not similarly affected. We have used this procedure in neonatal rats to examine the effect of stimulus deprivation on OB TH and AADC mRNA levels. Both Northern blot and in situ hybridization analyses revealed a pronounced decrease in ipsilateral as compared to contralateral OB TH mRNA levels 40 days after naris closure. In contrast, the levels of OB AADC mRNA were unaltered by naris closure. By in situ hybridization histochemistry, both TH and AADC mRNAs were localized to OB juxtaglomerular neurons. Odor deprivation was associated with an apparent region-specific reduction in TH mRNA within the ipsilateral OB glomerular layer. By densitometric analysis, the loss of TH-specific message was quantitatively consistent with the decrease in TH activity, suggesting that the observed plasticity of OB dopaminergic neurons following functional deafferentation can be attributed to a selective, transneuronally-mediated down regulation of TH gene transcription.

Modification of olfactory bulb synaptic inhibition by early unilateral olfactory deprivation

Early unilateral olfactory deprivation produces large structural and neurochemical changes in the olfactory bulb, the first central relay for olfactory information. The functioning of deprived bulbs was examined in the present report by using paired-pulse stimulation of the lateral olfactory tract. Paired-pulse stimulation reflects interactions between mitral/tufted cells and granule cells, as well as the modulatory effects of centrifugal and intra-bulbar association fibers. Paired-pulse stimulation produced inhibition of mitral/tufted cells in control animals at PN20-PN22. This inhibition was significantly enhanced in littermates deprived of olfactory input from PN1 to PN20-PN22. Suppression of mitral/tufted cell single-unit spontaneous activity following single-pulse stimulation of the lateral olfactory tract (LOT) was similarly enhanced in deprived bulbs. These results suggest that early olfactory deprivation significantly modifies subsequent olfactory system function.

Cytochrome oxidase response to cochlea removal in chicken auditory brainstem neurons

Changes in cytochrome oxidase (CO) activity were studied in the chick brainstem auditory nuclei, n. magnocellularis (NM) and n. laminaris (NL), following unilateral cochlea removal. Chickens aged 10 days or 56 weeks underwent unilateral cochlea removal. Following survival periods of 30 minutes to 14 days for the 10-day-old birds and 6 hours or 14 days for the 56-week-old birds, the animals were perfused with paraformaldehyde/glutaraldehyde fixative. Cryostat sections of the brainstem were then prepared for CO histochemistry. Microdensitometry was used to quantify the difference in CO staining in NM and NL ipsilateral and contralateral to the cochlea removal. Since the cochlea projects to the ipsilateral NM, the contralateral NM was used as a within-animal control. In normal chickens, NM cell bodies and the cell bodies and dendrites of NL neurons stain darkly for CO in both young and adult birds. In 10-day-old birds, there is no significant change in CO staining in NM from 30 minutes to 3 hours after cochlea removal. Then, a rapid biphasic change in CO staining was found in the ipsilateral NM. An increase in staining was observed 6 to 24 hours postoperatively, followed by a decrease in CO staining at 3- to 14-day survival times. In the 56-week-old birds, no increases in CO staining were observed 6 hours after cochlea removal, but a decrease in CO staining was found 14 days postoperatively. In NL, no changes were observed until 3 days (10-day-old birds) or 14 days (56-week-old birds) after cochlea removal. Then a decrease in CO staining was observed in the dendritic and glial/fiber regions of NL containing axons from the deafferented NM. Thus it appears that afferent input has a regulatory effect on the oxidative metabolism of neurons in the chicken auditory brainstem nuclei, an effect that differs with the age of the animal at the time of afferent manipulation.

Rapid changes in 2-deoxyglucose uptake and amino acid incorporation following unilateral odor deprivation: a laminar analysis

Unilateral naris occlusion in neonatal rats results in large alterations in the olfactory bulb, including substantial changes in laminar volume and enhanced cell death. These gross changes are undoubtedly the result of a cascade of more basic cellular regulatory events. The present study assesses the possibility of rapid post-deprivation changes in two such processes: glucose metabolism and protein synthesis. On the day after the day of birth rat pups underwent unilateral naris occlusion or sham surgery. In one study, either 1, 12, 24 or 48 h following surgery [3H]2-deoxyglucose [( 3H]2-DG) was administered to gauge patterns of glucose uptake. In a second study, [3H]leucine was injected to assess patterns of protein synthesis. Autoradiographs were then subjected to quantitative analyses. As early as 1 h following occlusion reduced 2-DG uptake was observed in many bulb regions. By 24 h, leucine incorporation was also uniformly diminished. While 2-DG uptake remained suppressed 48 h after deprivation, levels of amino acid incorporation returned to normal patterns in most laminae, with the exception of the mitral cell layer, where increased uptake was encountered. To evaluate whether the effects were developmental by nature a group of P40-P45 animals treated similarly were also examined. While 24 h of deprivation impaired 2-DG uptake in older animals, no alterations in amino acid incorporation were observed. The results indicate that early odor deprivation has rapid and specific effects on cellular functioning within the developing olfactory bulb.

Unilateral, neonatal olfactory deprivation alters tyrosine hydroxylase expression but not aromatic amino acid decarboxylase or GABA immunoreactivity

Recent publications have demonstrated an important role for olfactory afferent innervation in maintenance of the dopamine phenotype of olfactory bulb target neurons. The mechanisms underlying the control of phenotypic expression in this system are not known. These studies employed the model of unilateral neonatal olfactory deprivation to investigate the effects of lack of odorant stimulation on dopamine expression in the rat. Immunoreactivity of tyrosine hydroxylase, the first and rate-limiting enzyme in dopamine biosynthesis, used as a marker of the dopamine system, exhibited a large decrease both 40 and 70 days following olfactory deprivation. The losses were region specific suggesting that the deprivation was not complete. The number of immunoreactive GABAergic neurons was not reduced. The number of neurons containing aromatic L-amino acid decarboxylase (the second enzyme in the dopamine biosynthetic pathway) was also not decreased. Olfactory marker protein immunoreactivity in the glomeruli, a marker for afferent innervation, was not significantly altered indicating that the olfactory bulb was not denervated. These data demonstrate that neonatal deprivation, and the resulting lack of odorant stimulation, produces a transneuronal alteration in dopamine expression without neuronal loss. The studies also suggest that neuronal activity or the activity-dependent release of a trophic factor is necessary for the expression of the dopamine phenotype.

Morphometric modifications associated with early sensory experience in the rat olfactory bulb: II. Stereological study of the population of olfactory glomeruli

The present study explores the local variations of size and number of olfactory glomeruli induced by the exposure of young rats to long-term stimulation with a single odor. Three groups of 5 rats were used that were either: (1) stimulated with ethyl acetoacetate from birth to 1 month of age, (2) unilaterally deprived following early occlusion of one nare, or (3) normal animals of the same age. Areas and coordinates of all glomerular profiles were measured in 14 coronal sections uniformly distributed along the rostrocaudal axis of the olfactory bulb. A distribution-free stereological method was applied to compute the size and number of glomeruli either along the bulbar rostrocaudal extent or in the bulbar coronal plane. Following complete sensory deprivation or long-term stimulation with ethyl acetoacetate, the mean diameter of glomeruli was significantly reduced everywhere, except in the ventrolateral and ventromedial regions of the posterior olfactory bulb in rats reared with a single odor. In both of these areas, the number of glomeruli was either significantly increased following long duration exposure or significantly reduced following unilateral deprivation. Thus these results show that selective modifications of the olfactory environment during postnatal maturation induce morphometric variations in specific areas of the glomerular layer. These data are discussed with respect to the concept of the topographical coding of odor quality at the level of the glomeruli and plasticity of the olfactory system during postnatal development.

Early postnatal cellular proliferation and survival in the olfactory bulb and rostral migratory stream of normal and unilaterally odor-deprived rats

Unilateral naris occlusion in rats on postnatal Day 1 results in dramatic decreases in the size of specific olfactory bulb cell populations when pups are examined 30 days later (Frazier and Brunjes: J. Comp. Neurol. 269: 355-370, '88). The observed reductions must result from alterations in cell proliferation and/or survival, alternatives examined in the present study. During early postnatal development, most cells destined for the bulb are produced in regions caudal to the structure and migrate to the bulb in the massive rostral migratory stream. The dynamics of the stream were examined in both normal rats and pups with a single naris closed on Day 1. 3H-thymidine injections were made on postnatal Days 2, 5, 10, 20, and 30. Groups of pups were killed 2 hours later to assess patterns of proliferation and 24 hours later to gauge initial stages of migration. A gradient of labeled cells was observed in the stream, with higher levels occurring at more caudal locations. The supply of cells to the bulb peaked on Day 5 and was still substantial as late as Day 30. The deprivation procedure did not affect patterns of cell labeling at any stage tested, indicating the procedure does not affect early cellular proliferation. A third group of pups was examined 30 days after thymidine injection to assess both time of cell origin and survival rates. Dark granule cells and glia in the granule cell layer were produced at a consistent rate until Day 20 with cells added during the period evenly spread throughout the layer. Light granule and periglomerular cell production decreased dramatically after P5. Thirty days after injections on P2, fewer labeled dark granule cells and their associated glia were found in deprived bulbs, indicating that enhanced cell death plays a major role in the deprivation-induced decrease in cell number.

Early postnatal differentiation of granule cell dendrites in the olfactory bulbs of normal and unilaterally odor-deprived rats

Unilateral naris closure in rat pups on the day after birth (P1) results in a substantial loss of synaptic targets for the late developing granule cell population, suggesting granule cell differentiation may be altered. However, dramatic reductions in granule cell numbers also occur in deprived bulbs. A loss of 'competitors' may therefore balance losses in synaptic availability. The issue was quantitatively addressed by analyzing granule cell dendritic development in both sham-operated and unilaterally deprived pups. The period between P10 and P30 was marked by significant developmental increases in dendritic height, length and numbers of branches in control subjects. Continued growth was virtually confined to the most distal processes. Spine number and density also increased, but subsequently decreased in all by sixth-order processes. Growth curves for normal and deprived conditions were virtually identical at all ages suggesting that deprivation-induced changes do not involve alterations in initial phases of granule cell dendritic elaboration.

NADPH diaphorase staining within the developing olfactory bulbs of normal and unilaterally odor-deprived rats

Littermate rat pups underwent either unilateral surgical occlusion of the right external naris or sham surgery on postnatal day 1. At 10, 20 or 30 days postpartum olfactory bulbs were sectioned and stained using nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry. Two types of staining were observed and analyzed. The reaction produced a Golgi-like filling of short-axon cells in both deep and superficial bulb areas. No differences in the number, morphology or distribution of these cells were found either across ages or treatment conditions, indicating that the cells are resistant to the effects of the deprivation paradigm. Large regional variations in glomerular and olfactory nerve layer staining density were also observed at each age, reinforcing notions of functional or structural differences between glomeruli at very early ages.

Unilateral odor deprivation: early postnatal changes in olfactory bulb cell density and number

Surgical closure of an external naris of the rat from postnatal day 1 to day 30 results in a 25% decrease in the size of the ipsilateral olfactory bulb. Decreases in size must result from changes in either the number of neurons and/or glia, or their size or both. The present study was designed to quantify cell density (Nv) and number at various early postnatal ages in order to examine both normal patterns of maturation and sequences of change resulting from deprivation. Data from control subjects indicated that numbers of mitral cells remained constant while numbers of external tufted cells increased. Both relay cell populations exhibited increases in nuclear size suggestive of substantial postnatal differentiation. All interneuronal and glial populations increased in number, although differences in maturational patterns were observed between cell species. For example, light and dark subpopulations of granule cells differed in the timing of peak cellular density, and an inside-to-outside gradient of maturation was found for dark granule cells. Growth curves were generally similar in occluded and control pups until approximately day 20, when deprivation resulted in decreased number and nuclear area of external tufted cells and density and number of granule cells and their associated glia. Light granule cells were affected earlier than dark cells, perhaps because of their earlier arrival in the granule cell layer. The affected cell groups represent the last relay and interneuronal populations to be generated, perhaps explaining their particular susceptibility to the effects of experience. Most of the changes emerged late, thus suggesting that they represent the culmination of a series of experience-induced changes within the maturing bulb. The observed effects may result from either altered cellular proliferation or death patterns (or both), alternatives now under investigation.

Precocity and plasticity: odor deprivation and brain development in the precocial mouse Acomys cahirinus

Altering the early olfactory environment of animals can have dramatic consequences on the development of brain regions which subserve olfaction. The present study indicates that early odor deprivation has a more severe effect on a species which is born relatively mature than it does on related species which are not. The results call into question prevailing notions about the developmental continuity between animals born in divergent ontogenetic states.