Olfactory Bulb Ablation Research Articles

Restoration of serotonin neuronal firing following long-term administration of bupropion but not paroxetine in olfactory bulbectomized rats.

Background:Olfactory bulbectomized rats generally manifest many of the neurochemical, physiological, and behavioral features of major depressive disorder in humans. Another interesting feature of this model is that it responds to chronic but not acute antidepressant treatments, including selective serotonin reuptake inhibitors. The purpose of the present study was first to characterize the firing activity of dorsal raphe serotonin neurons in olfactory bulbectomized rats and then examine the effects of 2 antidepressants, bupropion and paroxetine.Methods:Olfactory bulbectomy was performed by aspirating olfactory bulbs in anesthetized rats. Vehicle and drugs were delivered for 2 and 14 days via subcutaneously implanted minipumps. In vivo electrophysiological recordings were carried out in male anesthetized Sprague-Dawley rats.Results:Following ablation of olfactory bulbs, the firing rate of serotonin neurons was decreased by 36%, leaving those of norepinephrine and dopamine neurons unchanged. In olfactory bulbectomized rats, bupropion (30mg/kg/d) restored the firing rate of serotonin neurons to the control level following 2- and 14-day administration and also induced an increase in the tonic activation of serotonin1A receptors; paroxetine (10mg/kg/d) did not result in a return to normal of the attenuated firing of serotonin neurons in olfactory bulbectomized rats. In the hippocampus, although at a higher dose of WAY 100635 than that required in bupropion-treated animals, paroxetine administration also resulted in an increase in the tonic activation of serotonin1A receptors.Conclusions:The present results indicate that unlike paroxetine, bupropion administration normalized serotonin neuronal activity and increased tonic activation of the serotonin1A receptors in hippocampus.

Panax quinquefolium involves nitric oxide pathway in olfactory bulbectomy rat model

Olfactory bulbectomy (OBX) is a well known screening model for depression. Panax quinquefolium (PQ) is known for its therapeutic potential against several psychiatric disorders. Nitric oxide (NO), an intercellular messenger has been suggested to play a crucial role in the pathogenesis of depression. The present study was designed to explore the possible involvement of NO mechanism in the protective effect of PQ against olfactory bulbectomy induced depression. Wistar rats were bulbectomized surgically and kept for a rehabilitation period of two weeks. PQ (50, 100 and 200mg/kg; p.o.) alone and in combination with NO modulators like l-NAME (10mg/kg, i.p.) and l-arginine (100mg/kg; i.p.) were then administered daily for another two weeks. Ablation of olfactory bulbs caused depression-like symptoms as evidenced by increased immobility time in forced swim test, hyperactivity in open field arena, and anhedonic like response in sucrose preference test. Further, OBX caused elevation in serum corticosterone levels and increased oxidative–nitrosative damage. These deficits were integrated with increased levels of neuroinflammatory cytokines (TNF-α), apoptotic factor (caspase-3) and a marked reduction in neurogenesis factor (BDNF) in both cerebral cortex and hippocampal regions of bulbectomized rats. Treatment with PQ significantly and dose-dependently restored these behavioral, biochemical and molecular alterations associated with OBX. Further, pretreatment of l-NAME with subeffective dose of PQ (100mg/kg) significantly potentiated its protective effects; however l-arginine pretreatment reversed the beneficial effects. The present study suggests that protective effect of P. quinquefolium might involve nitric oxide modulatory pathway against olfactory bulbectomy-induced depression in rats.

Quercetin suppress microglial neuroinflammatory response and induce antidepressent-like effect in olfactory bulbectomized rats

In rodents, olfactory bulbectomy (OBX) results in several behavioral and biochemical changes, useful as a screening model for antidepressants. Recent evidences suggest that quercetin; a bioflavonoid exhibits a variety of behavioral effects including anxiolytic, antidepressant, etc. Since microglia are commonly implicated in the neuroinflammation cascade of depression, we hypothesized that quercetin might involve microglial inhibition pathway in its antidepressant-like effects. To support such a possibility, we investigated the interaction of quercetin with a known microglial inhibitor (minocycline) against OBX-induced depression in male Wistar rats. In our study, ablation of olfactory bulbs caused hyperactivity in open field arena and increased immobility time in forced swim test (FST) which was coupled with enhanced serum corticosterone levels. Additionally, there were increased oxidative–nitrosative stress markers, inflammatory mediators (tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6)) along with enhanced apoptotic factor (caspase-3) in both cerebral cortex and hippocampal brain regions of OBX animals. These results were further supported by reports from histopathological analysis. After a surgical recovery period of 2weeks, treatment with quercetin (40, 80mg/kg; per oral (p.o.) p.o., 14days) significantly prevented OBX-induced behavioral, biochemical, molecular and histopathological alterations. Further, combination of sub effective doses of quercetin (20, 40mg/kg; p.o.) with minocycline (25mg/kg; p.o.) significantly potentiated their protective effects as compared to their effects alone. Based on our results, we propose that microglial inhibitory pathway might be involved in the neuroprotective effects of quercetin and suppression of oxidative–nitrosative stress mediated neuroinflammation-apoptotic cascade associated with OBX rat model of depression.

Suppression of Neuroinflammatory and Apoptotic Signaling Cascade by Curcumin Alone and in Combination with Piperine in Rat Model of Olfactory Bulbectomy Induced Depression

ObjectivesBilateral destruction of the olfactory bulbs is known to cause behavioral changes analogous to symptoms of depression. Curcumin, a traditional Indian spice is currently being investigated in different psychiatric problems including depression. Dietary phytochemicals are currently used as an adjuvant therapy to accelerate their therapeutic efficacy. Therefore, the present study is an attempt to elucidate the neuroprotective mechanism of curcumin and its co-administration with piperine against olfactory bulbectomy induced depression in rats.MethodsRats undergone olfactory bulbs ablations were analyzed after post-surgical rehabilitation period of 2 weeks. Animals were then treated with different doses of curcumin (100, 200 and 400 mg/kg; p.o.), piperine (20 mg/kg; p.o.) and their combination daily for another 2 weeks. Imipramine (10 mg/kg; i.p.) served as a standard control. Various behavioral tests like forced swim test (FST), open field behaviour and sucrose preference test (SPT) were performed, followed by estimation of biochemical, mitochondrial, molecular and histopathological parameters in rat brain.ResultsAblation of olfactory bulbs caused depression-like symptoms as evidenced by increased immobility time in FST, hyperactivity in open field arena, and anhedonic like response in SPT along with alterations in mitochondrial enzyme complexes, increased serum corticosterone levels and oxidative damage. These deficits were integrated with increased inflammatory cytokines (TNF-α) and apoptotic factor (caspase-3) levels along with a marked reduction in neurogenesis factor (BDNF) in the brain of olfactory bulbectomized (OBX) rats. Curcumin treatment significantly and dose-dependently restored all these behavioral, biochemical, mitochondrial, molecular and histopathological alterations associated with OBX induced depression. Further, co-administration of piperine with curcumin significantly potentiated their neuroprotective effects as compared to their effects alone.ConclusionsThe present study highlights that curcumin along with piperine exhibits neuroprotection against olfactory bulbectomy induced depression possibly by modulating oxidative-nitrosative stress induced neuroinflammation and apoptosis.

An IP3R3- and NPY-Expressing Microvillous Cell Mediates Tissue Homeostasis and Regeneration in the Mouse Olfactory Epithelium

Calcium-dependent release of neurotrophic factors plays an important role in the maintenance of neurons, yet the release mechanisms are understudied. The inositol triphosphate (IP3) receptor is a calcium release channel that has a physiological role in cell growth, development, sensory perception, neuronal signaling and secretion. In the olfactory system, the IP3 receptor subtype 3 (IP3R3) is expressed exclusively in a microvillous cell subtype that is the predominant cell expressing neurotrophic factor neuropeptide Y (NPY). We hypothesized that IP3R3-expressing microvillous cells secrete sufficient NPY needed for both the continual maintenance of the neuronal population and for neuroregeneration following injury. We addressed this question by assessing the release of NPY and the regenerative capabilities of wild type, IP3R3+/−, and IP3R3−/− mice. Injury, simulated using extracellular ATP, induced IP3 receptor-mediated NPY release in wild-type mice. ATP-evoked NPY release was impaired in IP3R3−/− mice, suggesting that IP3R3 contributes to NPY release following injury. Under normal physiological conditions, both IP3R3−/− mice and explants from these mice had fewer progenitor cells that proliferate and differentiate into immature neurons. Although the number of mature neurons and the in vivo rate of proliferation were not altered, the proliferative response to the olfactotoxicant satratoxin G and olfactory bulb ablation injury was compromised in the olfactory epithelium of IP3R3−/− mice. The reductions in both NPY release and number of progenitor cells in IP3R3−/− mice point to a role of the IP3R3 in tissue homeostasis and neuroregeneration. Collectively, these data suggest that IP3R3 expressing microvillous cells are actively responsive to injury and promote recovery.

P.1.014 Intracerebroventricular administration of interleukin-1β elevates brain kynurenic acid and disrupts prepulse inhibition in C57BL/6 mice

Olfactory bulbectomy (OBX) is a well known screening model for depression. Panax quinquefolium (PQ) is known for its therapeutic potential against several psychiatric disorders. Nitric oxide (NO), an intercellular messenger has been suggested to play a crucial role in the pathogenesis of depression. The present study was designed to explore the possible involvement of NO mechanism in the protective effect of PQ against olfactory bulbectomy induced depression. Wistar rats were bulbectomized surgically and kept for a rehabilitation period of two weeks. PQ (50, 100 and 200 mg/kg; p.o.) alone and in combination with NO modulators like l-NAME (10 mg/kg, i.p.) and l-arginine (100 mg/kg; i.p.) were then administered daily for another two weeks. Ablation of olfactory bulbs caused depression-like symptoms as evidenced by increased immobility time in forced swim test, hyperactivity in open field arena, and anhedonic like response in sucrose preference test. Further, OBX caused elevation in serum corticosterone levels and increased oxidative–nitrosative damage. These deficits were integrated with increased levels of neuroinflammatory cytokines (TNF-α), apoptotic factor (caspase-3) and a marked reduction in neurogenesis factor (BDNF) in both cerebral cortex and hippocampal regions of bulbectomized rats. Treatment with PQ significantly and dose-dependently restored these behavioral, biochemical and molecular alterations associated with OBX. Further, pretreatment of l-NAME with subeffective dose of PQ (100 mg/kg) significantly potentiated its protective effects; however l-arginine pretreatment reversed the beneficial effects. The present study suggests that protective effect of P. quinquefolium might involve nitric oxide modulatory pathway against olfactory bulbectomy-induced depression in rats.

Global expression profiling of globose basal cells and neurogenic progression within the olfactory epithelium

Ongoing, lifelong neurogenesis maintains the neuronal population of the olfactory epithelium in the face of piecemeal neuronal turnover and restores it following wholesale loss. The molecular phenotypes corresponding to different stages along the progression from multipotent globose basal cell (GBC) progenitor to differentiated olfactory sensory neuron are poorly characterized. We used the transgenic expression of enhanced green fluorescent protein (eGFP) and cell surface markers to FACS-isolate ΔSox2-eGFP(+) GBCs, Neurog1-eGFP(+) GBCs and immature neurons, and ΔOMP-eGFP(+) mature neurons from normal adult mice. In addition, the latter two populations were also collected 3 weeks after olfactory bulb ablation, a lesion that results in persistently elevated neurogenesis. Global profiling of mRNA from the populations indicates that all stages of neurogenesis share a cohort of >2,100 genes that are upregulated compared to sustentacular cells. A further cohort of >1,200 genes are specifically upregulated in GBCs as compared to sustentacular cells and differentiated neurons. The increased rate of neurogenesis caused by olfactory bulbectomy had little effect on the transcriptional profile of the Neurog1-eGFP(+) population. In contrast, the abbreviated lifespan of ΔOMP-eGFP(+) neurons born in the absence of the bulb correlated with substantial differences in gene expression as compared to the mature neurons of the normal epithelium. Detailed examination of the specific genes upregulated in the different progenitor populations revealed that the chromatin modifying complex proteins LSD1 and coREST were expressed sequentially in upstream ΔSox2-eGFP(+) GBCs and Neurog1-eGFP(+) GBCs/immature neurons. The expression patterns of these proteins are dynamically regulated after activation of the epithelium by methyl bromide lesion.

P.2.017 Size does matter: environmental enrichment rescues cognitive and synaptic deficits following olfactory bulbectomy in mice

Environmental enrichment (EE) has been shown to improve cognitive performance and enhance synaptic plasticity in various neurodegenerative animal models. Antidepressant efficacy of environmental enrichment, however, remains unclear. To investigate the effect of EE on cognitive and depressive symptoms, and on structural plasticity, we used olfactory bulbectomy (OBX) in mice. OBX is primarily known as a model for predicting antidepressant activity, since its hyperactivity is normalized by chronic antidepressant administration; however, the ablation of olfactory bulbs in a rodent also results in progressive neurodegeneration, behavioral disturbances, and cognitive impairments − symptoms reminiscent of those of Alzheimer’s and depressed patients [1]. Four weeks after the bulbectomy, mice were exposed to EE, consisting of tunnel, toys and nesting material, for 4 weeks. The animals’ locomotor activity was observed, using the open-field paradigm and their cognitive ability was measured throughout the study using passive avoidance (assessing fear memory) and T-maze (assessing spatial memory). Compared to sham-operated controls, OBX mice showed significantly increased locomotor activity (F(3,20) = 4.38, p< 0.05), impaired shortand long-term fear memory in the passive avoidance test, and hindered spatial memory in the T-maze. EE-treated OBX mice demonstrated significantly increased fear (both short and long-term) ((H(4) = 14.11, p< 0.01 and (H(4) = 7.15, p< 0.05, respectively) and spatial memory when compared to the non-enriched bulbectomized mice (H(4) = 8.12, p< 0.05). There was no antidepressant effect of EE on the OBX mice. The brain weights of the EE-OBX mice were significantly larger in comparison to the standard housed bulbectomized mice (F(3,20) = 5.73, p< 0.05) and coincided with improved cognitive functioning. Furthermore, preliminary data demonstrated EE treatment reversed the bulbectomy-induced reduction in dendritic spine density in hippocampus (F(3,8) = 18.86, p< 0.01) without affecting the OBX-mediated decrease in the number of neurons. Thus, despite a substantial neuronal loss, EE induced a refinement of the synaptic network, which may underlie the improved cognitive functioning in the OBX mice following EE, suggesting that the beneficial effects of enrichment on memory stems mainly from enhanced synaptic plasticity rather than impediment of general brain atrophy. Interestingly, in rats, bulbectomy also results in a loss of dendritic spines in hippocampus [2], and although EE recovers the reduced hippocampal weights in the OBX rat, it is unable to alleviate OBX-induced cognitive deficits, yet it exerts antidepressant effects in rats [3]. The lack of EE efficacy on the OBX mediated cognitive impairments in the rat compared to the mouse, may be due to differences in the ability of these species to rewire their brain after severe neurodegeneration. Why EE induced antidepressant effect in the OBX rat but not in mice remains to be elucidated. In addition to possible implications for the development of novel therapeutic agents that enhance the beneficial effects of EE, our findings raise an important question of translational value in animal research: is the mouse or the rat a better animal model for human neurodegenerative disorders and for testing therapeutic interventions for their efficacy? It appears that, after all, size does matter both at the level of the animal strain, brain size and dendritic spines.

The cell surface carbohydrate blood group A regulates the selective fasciculation of regenerating accessory olfactory axons

Cell surface carbohydrates are differentially expressed by discrete subpopulations of primary sensory axons in the mammalian main and accessory olfactory systems. It has been proposed that these carbohydrates provide a glycocode which mediates the sorting of these sensory axons as they project from the olfactory neuroepithelium to their central targets in the main and accessory olfactory bulbs during development. As the differential expression of cell surface carbohydrates on olfactory axons persists in the adult we have now investigated their role during regeneration. We have recently generated a line of transgenic mice, BGAT-Tg, that mis-express the blood group A (BGA) carbohydrate on all primary olfactory axons rather than just on accessory olfactory axons as in wild-type mice. Following unilateral bulbectomy, accessory and main olfactory axons regenerate and grow into the frontal cortex where they fill the cavity which remains after the olfactory bulb ablation. In wild-type mice, the regenerating BGA-expressing accessory olfactory axons selectively aggregated with each other in large bundles but clearly separated from the BGA-negative main olfactory axons. In contrast, in the BGAT-Tg transgenic mice in which all main and accessory axons express the BGA carbohydrate, the accessory olfactory axons failed to correctly separate from the main olfactory axons. Instead, these axons formed numerous small bundles interspersed with main olfactory axons. These data provide strong evidence that the restricted expression of BGA is in part responsible for the selective segregation of accessory olfactory axons.

Factors affecting neuronal birth and death in the mammalian olfactory epithelium.

To identify factors regulating neurogenesis and neuronal death in mammals and to determine the mechanisms by which these factors act, we have studied mouse olfactory epithelium using two different experimental paradigms: tissue culture of olfactory epithelium purified from mouse embryos; and ablation of the olfactory bulb in adult mice, a procedure that induces olfactory receptor neuron (ORN) death and neurogenesis in vivo. Studies of olfactory epithelium cultures have allowed us to characterize the cellular stages in olfactory neurogenesis and to identify factors regulating proliferation and differentiation of precursor cells in the ORN lineage. Studies of adult olfactory epithelium have enabled us to determine that all cell types in this lineage-proliferating neuronal precursors, immature ORNs and mature ORNs-undergo cell death following olfactory bulb ablation and that this death has characteristics of programmed cell death or apoptosis. In vitro studies have confirmed that neuronal cells of the olfactory epithelium undergo apoptotic death and have permitted identification of several polypeptide growth factors that promote survival of a fraction of ORNs. Using this information, we have begun to explore whether these factors, as well as genes known to play crucial roles in cell death in other systems, function to regulate apoptosis and neuronal regeneration in the adult olfactory epithelium following lesion-induced ORN death.

Olfactory system modulation of hippocampal cell death

The hippocampal dentate gyrus is a major recipient of olfactory input in rodents, via connections from the olfactory (piriform) cortex and the olfactory bulb to the entorhinal cortex. Given this connectivity and the known role of activity in dentate gyrus granule cell survival, the present experiment examined the immediate effects of loss of olfactory input to the hippocampus on apoptosis. Adults rats underwent unilateral or bilateral olfactory bulb ablations (OBX), and allowed to recover 24-72 h before the piriform cortex and hippocampal dentate gyrus were processed for terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling [TUNEL] of apoptotic cells. OBX transiently increased TUNEL-positive cells in the ipsilateral piriform cortex and dentate gyrus. Increased TUNEL-labeling was apparent within 24h in both structures, but was more extensive and prolonged in piriform cortex. The results suggest a trans-synaptic regulation of cell survival through at least two synapses.

A Circadian Clock in the Olfactory Bulb Controls Olfactory Responsivity

Recently, it has been shown that multiple mammalian cell types express daily rhythms in vitro. Although the suprachiasmatic nucleus (SCN) of the hypothalamus is known to regulate a wide range of circadian behaviors, the role for intrinsic rhythmicity in other tissues is unknown. We tested whether the main olfactory bulb (OB) of mice mediates daily changes in olfaction. We found circadian rhythms in cedar oil-induced c-Fos, a protein marker of cellular excitation, in the mitral and granular layers of the OB and in the piriform cortex (PC). These oscillations persisted in constant darkness with a fourfold change in amplitude and a peak approximately 4 h after the onset of daily locomotor activity. Electrolytic lesions of the SCN abolished circadian locomotor rhythms, but not odor-induced c-Fos rhythms in the OB or PC. Furthermore, removal of the OB abolished spontaneous circadian cycling of c-Fos in the PC, shortened the free-running period of locomotor rhythms, and accelerated re-entrainment after a 6 h advance and slowed re-entrainment after a 6 h delay in the light schedule. OB ablation or odorant altered the amplitude of c-Fos rhythms in the SCN and ablation of one OB abolished c-Fos rhythms in the ipsilateral PC, but not in the contralateral OB and PC. We conclude that the OB comprises a master circadian pacemaker, which enhances olfactory responsivity each night, drives rhythms in the PC, and interacts with the SCN to coordinate other daily behaviors.

Time course of alterations after olfactory bulbectomy in mice

Olfactory bulbectomy in rodents causes behavioral alterations, which result in a model of depression, validated for pharmacological screening of antidepressant drugs. To unravel the appearance and time course of the major behavioral effects which follow surgery, mice underwent olfactory bulb ablation or sham operation, and were analyzed after 1, 2, or 4 weeks. Bulbectomized (BX) mice were anosmic, and hyperactive when tested under stressful situations in the forced swimming test. Predatory aggression was upregulated in a time-dependent way: only after 4 weeks BX mice were faster than controls in attacking prey. At the same time, they were less aggressive against intruders; they did not differ from controls in open field exploration, but displayed a cognitive impairment in water maze. Behavioral tests thus indicated a marked hyperreactivity, a dissociation among different aggressive behaviors, and also a cognitive impairment induced by bulbectomy. Histological confirmation of the damage revealed that major modifications took place in the rostral pole of frontal lobes, with a significant increase in the width of the rostral migratory stream, 2 weeks after surgery, and in the subventricular zone, 4 weeks after surgery. These results suggest a base for the time-course of appearance of behavioral symptoms in BX mice.

Pathogenic Role for Virus-Specific CD4 T Cells in Mice with Coronavirus-Induced Acute Encephalitis

Acute viral encephalitis is believed to result from direct virus destruction of infected cells and from virus-induced host immune response, but the relative contribution of each remains largely unknown. For example, C57BL/6 (B6) mice infected with mouse hepatitis virus (JHM strain, JHMV) develop severe encephalitis, with death occurring within 7 days. Here, we show that the host response to a single JHMV-specific immunodominant CD4 T-cell epitope is critical for severe disease. We engineered a recombinant JHMV with mutations in the immunodominant CD4 T-cell epitope (rJ.MY135Q). Infection of naïve B6 mice with this virus resulted in mild disease with no mortality. However, introduction of a CD4 T-cell epitope from Listeria monocytogenes into rJ.MY135Q generated a highly virulent virus. The decrease in disease severity was not due to a switch from Th1 to Th2 predominance in rJ.MY135Q-infected mice, an effect on CD8 T-cell function, or differential expression of tumor necrosis factor-α by JHMV-specific CD4 T cells. These results show that the response to a single virus-specific CD4 T-cell epitope may contribute to a pathogenic host response in the setting of acute viral disease and that abrogation of this response ameliorates clinical disease without diminishing virus clearance.

Ablação cirúrgica dos bulbos olfatórios em coelhos: modelo para estudos de patogenia de infecções por vírus neurotrópicos

Coelhos têm sido utilizados como modelo para o estudo da neuropatogenia da infecção pelo herpesvírus bovino tipo 5 (BHV-5), um importante agente de doença neurológica em bovinos. O sistema olfatório tem sido apontado como a principal via de acesso do vírus ao cérebro após replicação na cavidade nasal. Para investigar a importância da via olfatória na patogenia da infecção neurológica pelo BHV-5, foi elaborada e avaliada uma técnica operatória de craniotomia transfrontal para remoção dos bulbos olfatórios (BOs), definindo-se as órbitas como referência anatômica. Foram utilizados 45 coelhos com 30 dias de idade, sendo 23 submetidos à ablação cirúrgica dos BOs e posteriormente inoculados pela via intranasal (IN) ou no saco conjuntival (IC) com o BHV-5. Após incisões de pele, tecido subcutâneo e periósteo, a craniotomia foi realizada em um ponto eqüidistante entre os cantos mediais dos olhos, com uma broca sulcada de 3mm acoplada a uma perfuratriz elétrica de baixa rotação. A remoção dos BOs foi realizada com uma sonda uretral n°6 acoplada a um aspirador. O estudo macroscópico de três animais após a cirurgia comprovou que o procedimento foi eficiente na remoção total dos BOs. Isso também foi comprovado pela interrupção do acesso do vírus ao córtex cerebral: apenas um animal (1/11 ou 9,1%) no grupo submetido à ablação dos BOs com inoculação IN desenvolveu enfermidade neurológica, contra 100% (10/10) dos coelhos controle. Conclui-se que a técnica de craniotomia transfrontal utilizando a órbita como referência anatômica permite o acesso adequado para localização e remoção dos BOs e pode ser utilizada em estudos de patogenia de infecções por vírus neurotrópicos que exijam a interrupção completa da via olfatória.

Retinoic Acid Receptor-Dependent Survival of Olfactory Sensory Neurons in Postnatal and Adult Mice

To address the hypothesis that retinoids produced by synthesizing enzymes present in the primary olfactory system influence the mouse olfactory sensory map, we expressed a dominant-negative retinoic acid receptor selectively in olfactory sensory neurons. We show that neurons deficient in nuclear retinoid signaling are responsive to odors and form correct odorant receptor-specific axonal projections to target neurons in the olfactory bulb of the brain. Subsequent to the formation of the map, the neurons die prematurely by retrograde-driven caspase-3 activation, which resembles the previously described mechanism of neural death after olfactory bulb ablation. This neurodegenerative event is initiated the second postnatal week and occurs in the adult animal without a compensatory increase of progenitor cell proliferation. In addition, we find that nuclear retinoid signaling is required for the expression of a retinoic acid-degrading enzyme, Cyp26B1, in a small fraction of mature neurons. Collectively, the results provide evidence for a role of locally regulated retinoid metabolism in neuroprotection and in determining population size of neurons at a late stage of neural circuit formation.

Modification of Odor Investigation and Discrimination in Female Opossums (Monodelphis domestica) Following the Ablation of the Accessory Olfactory Bulbs.

To determine whether the vomeronasal system of the Brazilian short-tailed opossum (Monodelphis domestica) is important to the response to conspecific chemical signals, the authors tested female opossums with conspecific odors, before and after ablation of their accessory olfactory bulbs (AOBs). Anesthesia and sham treatments did not modify females' discrimination of conspecific odors when tested against water, between male and female odors, or between different odors from the same male donors. Odor investigation was partially diminished following partial ablation of the AOB, and complete ablation of the AOBs further impaired the ability of females to discriminate between certain odors. These findings provide the first evidence for the importance of the vomeronasal system in the detection of chemosignals of known origin in opossums.

Animal models of depression: olfactory lesions affect amygdala, subventricular zone, and aggression

Psychiatric or depressed patients show alterations in both olfactory projection areas and mucosa. In rodents, ablation of olfactory bulbs causes a depression-like syndrome, useful to test antidepressant agents. We studied in mice the behavioral symptoms and neuroanatomical correlates after mucosal damage or ablation of the olfactory bulb. Our results are based on a battery of tests exploiting anxious, aggressive, and depressive behavior, on morphological and immunohistochemical analysis. We found similar results in both sensory-damaged and bulbectomized animals, with a behavioral dissociation concerning different forms of aggression. These findings do not support a simple downregulation of social interactions in damaged mice. The most prominent modifications in the brains of sensory damaged and bulbectomized mice are detected in the subventricular zone (SVZ), the source area of neural stem cells, and in the content of cAMP-dependent protein kinase within the amygdala, suggesting a central role of this structure in the functional modulation of behavior.

Leukemia Inhibitory Factor Is a Key Signal for Injury-Induced Neurogenesis in the Adult Mouse Olfactory Epithelium

The mammalian olfactory epithelium (OE) is composed of primary olfactory sensory neurons (OSNs) that are renewed throughout adulthood by local, restricted neuronal progenitor cells. The molecular signals that control this neurogenesis in vivo are unknown. Using olfactory bulb ablation (OBX) in adult mice to trigger synchronous mitotic stimulation of neuronal progenitors in the OE, we show the in vivo involvement of a cytokine in the cellular events leading to the regeneration of the OE. We find that, of many potential mitogenic signals, only leukemia inhibitory factor (LIF) is induced before the onset of neuronal progenitor proliferation. The rise in LIF mRNA expression peaks at 8 hr after OBX, and in situ RT-PCR and immunocytochemistry indicate that LIF is upregulated, in part, in the injured neurons themselves. This rise in LIF is necessary for injury-induced neurogenesis, as OBX in the LIF knock-out mouse fails to stimulate cell proliferation in the OE. Moreover, delivery of exogenous LIF to the intact adult OE using an adenoviral vector stimulates BrdU labeling in the apical OE. Taken together, these results suggest that injured OSNs release LIF as a stimulus to initiate their own replacement.

Leukemia inhibitory factor, interleukin-6, and their receptors are expressed transiently in the olfactory mucosa after target ablation.

Removal of the synaptic targets of olfactory receptor neurons by olfactory bulb ablation results in apoptosis of olfactory receptor neurons and up-regulation of proliferation of their progenitors. This study focuses on the expression of the neuropoietic cytokines leukemia inhibitory factor (LIF) and its receptor (LIFR) and interleukin 6 (IL-6) and its receptor (IL-6R) in intercellular signaling pathways in the olfactory mucosa after target ablation. Olfactory bulbectomy (OBX) resulted in several transient, early-onset, temporally integrated events that were detected immunohistochemically. Macrophages infiltrated the olfactory epithelium (OE) by 16 hours post-OBX. LIF expression was up-regulated transiently at 2 days post-OBX, when up-regulated expression of LIFR also was detected on globose basal cells (GBCs), a subpopulation of which are immediate progenitors of olfactory receptor neurons. GBC proliferation peaked at 3--4 days post-OBX. In the olfactory nerve (ON), LIF-positive and IL-6-positive macrophage infiltration was followed by the transient up-regulation of expression of LIFR, IL-6, and IL-6R in ensheathing cells by 3 days post-OBX. The mRNAs for LIF/LIFR, IL-6/IL-6R, and their common signal-transduction molecule, gp130, in olfactory-nasal mucosa from control mice and from 3-day post-OBX mice were detected with reverse transcriptase-polymerase chain reaction (RT-PCR). Analysis of Northern blot and relative quantitative RT-PCR demonstrated similar temporal patterns of changes in relative mRNA levels for both LIF and IL-6, which were up-regulated by 16 hours post-OBX and peaked at 2--3 days post-OBX. These data indicate that LIF from infiltrating macrophages acts as a mitogen for GBCs and that LIF from infiltrating macrophages and IL-6 from infiltrating macrophages and ensheathing cells act as repair factors in the ON.