Odor-induced fos-like immunoreactivity in the rat olfactory bulb

mTOR signaling in VIP neurons regulates circadian clock synchrony and olfaction

The mammalian/mechanistic target of rapamycin (mTOR) kinase resides at the crux of an intracellular signaling network that controls fundamental biological processes. Dysregulation of mTOR signaling is linked to neurological and psychiatric diseases. However, the physiological functions of mTOR signaling in the adult brain are not fully understood. In the current study, we discovered that mTOR in vasoactive intestinal peptide (VIP) neurons plays a key role in regulating neurophysiology in the brain circadian clock and the olfactory system. The conditional mTOR knockout mouse will be a useful model for future investigations of mTOR and/or VIP.

Mammalian/mechanistic target of rapamycin (mTOR) signaling controls cell growth, proliferation, and metabolism in dividing cells. Less is known regarding its function in postmitotic neurons in the adult brain. Here we created a conditional mTOR knockout mouse model to address this question. Using the Cre-LoxP system, the mTOR gene was specifically knocked out in cells expressing Vip (vasoactive intestinal peptide), which represent a major population of interneurons widely distributed in the neocortex, suprachiasmatic nucleus (SCN), olfactory bulb (OB), and other brain regions. Using a combination of biochemical, behavioral, and imaging approaches, we found that mice lacking mTOR in VIP neurons displayed erratic circadian behavior and weakened synchronization among cells in the SCN, the master circadian pacemaker in mammals. Furthermore, we have discovered a critical role for mTOR signaling in mediating olfaction. Odor stimulated mTOR activation in the OB, anterior olfactory nucleus, as well as piriform cortex. Odor-evoked c-Fos responses along the olfactory pathway were abolished in mice lacking mTOR in VIP neurons, which is consistent with reduced olfactory sensitivity in these animals. Together, these results demonstrate that mTOR is a key regulator of SCN circadian clock synchrony and olfaction.

Daily oscillation of odorant detection in rat olfactory epithelium

Most of biological variables follow a daily rhythm. It holds true as well for sensory capacities as two decades of research have demonstrated that the odorant induced activity in the olfactory bulbs oscillates during the day. Olfactory bulbs are the first central nervous system structures, which receive inputs from the olfactory neurons located in the nose olfactory epithelium in vertebrates. So far, data on variation in odorant detection in the olfactory epithelium throughout the day are missing. Using electroolfactogram recordings in rats housed under daily light and dark cycles, we found that the olfactory epithelium responsiveness varies during the day with a maximum in the beginning of the light phase. This fluctuation was consistent with cycling of transduction pathway gene expression in the olfactory epithelium examined by qPCR. It was also consistent with the levels of two transduction pathway proteins (olfactory-type G protein and adenylyl cyclase III) examined by western blot. Daily variations were also observed at the level of olfactory sensory neurons responses recorded by patch-clamp. To rule out a potential effect of the feeding status of the animal, we examined the variation in odorant response in starved animals during the day. We observed a similar pattern to ad libidum fed animals. Taken together, our results reveal that the olfactory epithelium sensitivity varies during the day in part due to modulation of the very first step of odorant detection.

Cellular registration without behavioral recall of olfactory sensory input under general anesthesia

BACKGROUND

Previous studies suggest that sensory information is "received" but not "perceived" under general anesthesia. Whether and to what extent the brain continues to process sensory inputs in a drug-induced unconscious state remain unclear.

METHODS

One hundred seven rats were randomly assigned to 12 different anesthesia and odor exposure paradigms. The immunoreactivities of the immediate early gene products c-Fos and Egr1 as neural activity markers were combined with behavioral tests to assess the integrity and relationship of cellular and behavioral responsiveness to olfactory stimuli under a surgical plane of ketamine-xylazine general anesthesia.

RESULTS

The olfactory sensory processing centers could distinguish the presence or absence of experimental odorants even when animals were fully anesthetized. In the anesthetized state, the c-Fos immunoreactivity in the higher olfactory cortices revealed a difference between novel and familiar odorants similar to that seen in the awake state, suggesting that the anesthetized brain functions beyond simply receiving external stimulation. Reexposing animals to odorants previously experienced only under anesthesia resulted in c-Fos immunoreactivity, which was similar to that elicited by familiar odorants, indicating that previous registration had occurred in the anesthetized brain. Despite the "cellular memory," however, odor discrimination and forced-choice odor-recognition tests showed absence of behavioral recall of the registered sensations, except for a longer latency in odor recognition tests.

CONCLUSIONS

Histologically distinguishable registration of sensory processing continues to occur at the cellular level under ketamine-xylazine general anesthesia despite the absence of behavioral recognition, consistent with the notion that general anesthesia causes disintegration of information processing without completely blocking cellular communications.

Olfactory receptor and neural pathway responsible for highly selective sensing of musk odors

Musk odorants are used widely in cosmetic industries because of their fascinating animalic scent. However, how this aroma is perceived in the mammalian olfactory system remains a great mystery. Here, we show that muscone, one musk odor secreted by various animals from stink glands, activates a few glomeruli clustered in a neuroanatomically unique anteromedial olfactory bulb. The muscone-responsive glomeruli are highly specific to macrocyclic ketones; interestingly, other synthetic musk odorants with nitro or polycyclic moieties or ester bonds activate distinct but nearby glomeruli. Anterodorsal bulbar lesions cause muscone anosmia, suggesting that this region is involved in muscone perception. Finally, we identified the mouse olfactory receptor, MOR215-1, that was a specific muscone receptor expressed by neurons innervating the muscone-responsive anteromedial glomeruli and also the human muscone receptor, OR5AN1. The current study documents the olfactory neural pathway in mice that senses and transmits musk signals from receptor to brain.

Dynamic sensory representations in the olfactory bulb: modulation by wakefulness and experience

How are sensory representations in the brain influenced by the state of an animal? Here we use chronic two-photon calcium imaging to explore how wakefulness and experience shape odor representations in the mouse olfactory bulb. Comparing the awake and anesthetized state, we show that wakefulness greatly enhances the activity of inhibitory granule cells and makes principal mitral cell odor responses more sparse and temporally dynamic. In awake mice, brief repeated odor experience leads to a gradual and long-lasting (months) weakening of mitral cell odor representations. This mitral cell plasticity is odor specific, recovers gradually over months, and can be repeated with different odors. Furthermore, the expression of this experience-dependent plasticity is prevented by anesthesia. Together, our results demonstrate the dynamic nature of mitral cell odor representations in awake animals, which is constantly shaped by recent odor experience.

Chemical stress induces the unfolded protein response in olfactory sensory neurons

More than any other neuron, olfactory sensory neurons are exposed to environmental insults. Surprisingly, their only documented response to damaging stress is apoptosis and subsequent replacement by new neurons. However, they expressed unfolded protein response genes, a transcriptionally regulated defense mechanism activated by many types of insults. The unfolded protein response transcripts Xbp1, spliced Xbp1, Chop (Ddit3), and BiP (Hspa5) were decreased when external access of stressors was reduced by blocking a nostril (naris occlusion). These transcripts and Nrf2 (Nfe2l2) were increased by systemic application of tunicamycin or the selective olfactotoxic chemical methimazole. Methimazole's effects overcame naris occlusion, and the unfolded protein response was independent of odor-evoked neuronal activity. Chemical stress is therefore a major and chronic activator of the unfolded protein response in olfactory sensory neurons. Stress-dependent repression of the antiapoptotic gene Bcl2 was absent, however, suggesting a mechanism for disconnecting the UPR from apoptosis and tolerating a chronic unfolded protein response. Environmental stressors also affect both the sustentacular cells that support the neurons and the respiratory epithelia, because naris occlusion decreased expression of the xenobiotic chemical transformation enzyme Cyp2a5 in sustentacular cells, and both naris occlusion and methimazole altered the abundance of the antibacterial lectin Reg3g in respiratory epithelia.

Differential microglial activation between acute stress and lipopolysaccharide treatment

Acute stress was demonstrated to induce morphological microglial activation in several brain regions including the midbrain periaqueductal gray (PAG), an area that plays important roles in behavioral responses to uncontrollable stress, threat, anxiety, and pain. To determine whether neuronal activation may be involved in the stress-induced microglial activation, the present study investigated the correlation between neuronal activity measured as c-Fos expression and morphological microglial activation in the PAG. Acute stress was followed by morphological activation of microglia and increased c-Fos expression in the PAG but not in the surrounding midbrain. Double immunohistochemistry and topological analysis demonstrated that microglial activation occurred adjacent to responsive neurons. By contrast, lipopolysaccharide (LPS) treatment induced microglial activation even in the absence of neuronal responses in the PGA as well as in the rest of the midbrain. These findings suggest that the mechanism of microglial activation during stress may differ from those of infection or inflammation. It also indicates that the neuronal cells expressing c-Fos protein may play some roles to trigger microglial activation.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

Temporal and Spatial Disparity in cFOS Expression and Dopamine Phenotypic Differentiation in the Neonatal Mouse Olfactory Bulb

The mammalian olfactory bulb (OB) is among the few regions in adult brain which generates interneurons. A subpopulation of these phenotypically diverse interneurons is dopaminergic (DA) periglomerular cells. Full phenotypic development as indicated by expression of tyrosine hydroxylase (TH), the first enzyme in DA biosynthesis, requires afferent activity or equivalent depolarizing conditions. To investigate the hypothesis that cFOS regulates TH expression, this study analyzed OB slice cultures obtained from neonatal transgenic mice expressing 9 kb of TH promoter directing expression of green fluorescent protein (TH/GFP). Cultures were depolarized with 50 mM potassium chloride (KCl), the calcium channel blocker, nifedipine (10 microM) with KCl, or an equimolar concentration of sodium chloride (NaCl). Depolarization increased cFOS expression 6-fold peaking at about 3 h. Staining decreased rapidly returning to control, NaCl, levels by 48 h post-stimulation when TH/GFP expression was highest. Nifedipine blocked the increase in TH and cFOS suggesting that similar signal transduction pathways mediate both responses.

Maps of odorant molecular features in the Mammalian olfactory bulb

The olfactory bulb (OB) is the first relay station of the central olfactory system in the mammalian brain and contains a few thousand glomeruli on its surface. Because individual glomeruli represent a single odorant receptor, the glomerular sheet of the OB forms odorant receptor maps. This review summarizes the emerging view of the spatial organization of the odorant receptor maps. Recent studies suggest that individual odorant receptors are molecular-feature detecting units, and so are individual glomeruli in the OB. How are the molecular-feature detecting units spatially arranged in the glomerular sheet? To characterize the molecular-feature specificity of an individual glomerulus, it is necessary to determine the molecular receptive range (MRR) of the glomerulus and to compare the molecular structure of odorants within the MRR. Studies of the MRR mapping show that 1) individual glomeruli typically respond to a range of odorants that share a specific combination of molecular features, 2) each glomerulus appears to be unique in its MRR property, and 3) glomeruli with similar MRR properties gather together in proximity and form molecular-feature clusters. The molecular-feature clusters are located at stereotypical positions in the OB and might be part of the neural representation of basic odor quality. Detailed studies suggest that the glomerular sheet represents the characteristic molecular features in a systematic, gradual, and multidimensional fashion. The molecular-feature maps provide a basis for understanding how the olfactory cortex reads the odor maps of the OB.

Lesion of the lateral entorhinal cortex amplifies odor-induced expression of c-fos, junB, and zif 268 mRNA in rat brain

Paradoxical facilitation of olfactory learning following entorhinal cortex (EC) lesion has been described, which may result from widespread functional alterations taking place within the olfactory system. To test this hypothesis, expression of the immediate early genes c-fos, junB, and zif 268 was studied in response to an olfactory stimulation in several brain areas in control and in EC-lesioned rats. Olfactory stimulation in control rats induced the expression of the three genes in the granular/mitral and glomerular layers of the olfactory bulb, as well as c-fos and junB expression in the piriform cortex. However EC lesion was devoid of effects in nonstimulated animals; it significantly amplified the odor-induced expression of the three genes in these areas, as well as in the amygdala, hippocampus, and parietal-temporal cortices. The data suggest that EC lesion modifies the neural processing of odor by suppressing an inhibitory influence on brain areas connected to this cortex.

Anatomical contributions to odorant sampling and representation in rodents: zoning in on sniffing behavior

Odorant sampling behaviors such as sniffing bring odorant molecules into contact with olfactory receptor neurons (ORNs) to initiate the sensory mechanisms of olfaction. In rodents, inspiratory airflow through the nose is structured and laminar; consequently, the spatial distribution of adsorbed odorant molecules during inspiration is predictable. Physicochemical properties such as water solubility and volatility, collectively called sorptiveness, interact with behaviorally regulable variables such as inspiratory flow rate to determine the pattern of odorant deposition along the inspiratory path. Populations of ORNs expressing the same odorant receptor are distributed in strictly delimited regions along this inspiratory path, enabling different deposition patterns of the same odorant to evoke different patterns of neuronal activation across the olfactory epithelium and in the olfactory bulb. We propose that both odorant sorptive properties and the regulation of sniffing behavior may contribute to rodents' olfactory capacities by this mechanism. In particular, we suggest that the motor regulation of sniffing behavior is substantially utilized for purposes of "zonation" or the direction of odorant molecules to defined intranasal regions and hence toward distinct populations of receptor neurons, pursuant to animals' sensory goals.

Odor-concentration coding in the guinea-pig piriform cortex

By optical imaging of intrinsic signals, we demonstrated a possible code for odor concentration in the anterior piriform cortex of the guinea-pig. Odor-induced cortical activation, which primarily originated in layer II, appeared in a narrow band beneath the rhinal sulcus over the lateral olfactory tract, corresponding to the dorsal part of the anterior piriform cortex. Lower concentrations activated the rostral region of the band, whereas higher ones generated caudally spreading activation, and the site at which neural activation reached its maximum extent depended upon odor concentration. Different odors with low concentrations generated distinct but somewhat overlapping patterns in the rostral region of the band; the limited extent of cortical activity may be one focal domain for each odor. It was hard to judge, however, that odor-specific domains appeared in the anterior piriform cortex, because the strong stimuli induced largely overlapping patterns. Furthermore, the total area activated increased in proportion to concentrations raised to a power of 0.5-0.9. Importantly, these imaging results were confirmed with unit recordings which indicated a rostro-caudal gradient in odor-sensitivity among cortical neurons. Our results suggest that the dorsal part of the anterior piriform cortex may be associated with odor concentration. Therefore, in addition to recruitment of more olfactory sensory cells and glomeruli in response to stronger stimuli, a rostro-caudal gradient in axonal projections from mitral/tufted cells and/or in association fibers may play an important role in odor-concentration coding in the anterior piriform cortex.

Experience-dependent activation of extracellular signal-related kinase (ERK) in the olfactory bulb

Protein kinase-mediated signaling cascades play a fundamental role in translating extracellular signals into cellular responses in CNS neurons. The mitogen-activated protein kinase / extracellular signal-regulated kinase (MAPK/ERK) pathway participates in regulating diverse neuronal processes such as proliferation, differentiation, survival, synaptic efficacy, and long-term potentiation by inducing cAMP-response element (CRE)-mediated gene transcription. Central olfactory structures show plasticity throughout the lifespan, but the role of the MAPK/ERK pathway in odor-evoked activity has yet to be determined. Therefore, we examined the effect of odorant exposure and early postnatal deprivation on ERK activity. We found that odor stimulation induced ERK phosphorylation, that activation of the ERK pathway was decreased with early postnatal deprivation, and that ERK phosphorylation was subsequently increased by restoring stimulation. Further, locations of ERK activation in bulbar neurons after exposure to single odorants corresponded to odor-evoked activity patterns found with other measures of activity in the bulb. Finally, due to the cytoplasmic location of pERK, activated dendrites belonging to the primary excitatory output neurons of the bulb were observed following a single odor exposure. The results indicate that the MAPK/ERK pathway is activated by odorant stimulation and may play an important role in developmental sensory plasticity in the olfactory bulb.

Centre-surround inhibition among olfactory bulb glomeruli

Centre-surround inhibition--the suppression of activity of neighbouring cells by a central group of neurons--is a fundamental mechanism that increases contrast in patterned sensory processing. The initial stage of neural processing in olfaction occurs in olfactory bulb glomeruli, but evidence for functional interactions between glomeruli is fragmentary. Here we show that the so-called 'short axon' cells, contrary to their name, send interglomerular axons over long distances to form excitatory synapses with inhibitory periglomerular neurons up to 20-30 glomeruli away. Interglomerular excitation of these periglomerular cells potently inhibits mitral cells and forms an on-centre, off-surround circuit. This interglomerular centre-surround inhibitory network, along with the well-established mitral-granule-mitral inhibitory circuit, forms a serial, two-stage inhibitory circuit that could enhance spatiotemporal responses to odours.

c-Fos expression in the rat cerebral cortex during systemic GvH reaction

OBJECTIVE

It is becoming clear that the CNS receives signals from the peripheral immune system. In order to identify the areas of the brain that receive information about a specific immune response to allogeneic antigens, we studied the expression of c-Fos, a neural activation marker, in the cerebral cortex following the induction of a graft-vs.-host reaction (GvHR) in rats.

METHODS

C-Fos expression in the brain was studied by immunohistochemistry. GvHR was induced in (WKY x PVG)F(1) rats by injecting 5 x 10(8) spleen cells from PVG rats. Control rats received syngeneic cells.

RESULTS

No c-Fos immunoreactivity (IR) was observed in animals undergoing GvHR in the nucleus tractus solitarii (NTS), the locus coeruleus (LC), the organum vasculosum of lamina terminalis (OVLT), the paraventricular nucleus (PVN) or the central amygdaloid nucleus (Ce). In contrast, 3 days after GvH induction c-Fos IR was observed in the piriform cortex and several other olfactory-related regions indicating the stimulation of the olfactory pathway during GvHR. Strong c-Fos IR was also observed in the occipital visual cortex of animals undergoing a GvHR, suggesting that GvHR can affect visual functions. In addition, GvHR induced c-Fos IR in the prefrontal cortex (Cg3, orbital cortex), a region that has interconnections with most sensory modalities. Double-staining studies indicate that the cells that express the c-Fos signal are neurons.

CONCLUSION

We have defined the distribution of brain neurons that are affected during the induction phase of GvHR. Our results also indicate that the integration and processing of information from the immune system at CNS levels involve different areas during different types of immune responses.

Molecular-feature domains with posterodorsal-anteroventral polarity in the symmetrical sensory maps of the mouse olfactory bulb: mapping of odourant-induced Zif268 expression

Individual glomeruli in the mammalian olfactory bulb presumably represent a single type of odourant receptor. Thus, the glomerular sheet provides odourant receptor maps at the surface of the olfactory bulb. To understand the basic spatial organization of the olfactory sensory maps, we first compared the spatial distribution of odourant-induced responses measured by the optical imaging of intrinsic signals with that detected immunohistochemically by expressions of Zif268, one of the immediate early gene products in juxtaglomerular cells. In the dorsal surface of the bulb, we detected a clear correlation in the spatial pattern between these responses. In addition, the molecular-feature domains and their polarities (spatial shifts of responses with an increase in carbon chain length) that were defined by the optical imaging method could be also detected by the Zif268 mapping method. We then mapped the Zif268 signals over the entire olfactory bulb using a homologous series of fatty acids and aliphatic alcohols as stimulus odourants. We superimposed the Zif268 signals onto the standard unrolled map with the help of cell adhesion molecule compartments. Each odourant typically elicited two pairs of clusters of dense Zif268 signals. The results showed that molecular-feature domains and their polarities were arranged symmetrically at stereotypical positions in a mirror-image fashion between the lateral and the medial sensory maps. The polarity of each domain was roughly in parallel with the posterodorsal-anteroventral axis that was defined by the cell adhesion molecule compartments. These results suggest that the molecular-feature domain with its fixed polarity is one of the basic structural units in the spatial organization of the odourant receptor maps in the olfactory bulb.

Evaluation of the T&T olfactometer by mapping c-fos protein in an olfactory bulb

OBJECTIVES

Although in Japan the T&T olfactometer is most commonly used in patients suffering from smelling disorders, no scientific analysis has been performed so far. The objective of this study was to clarify whether the five odorants used in the T&T olfactometer are suitable or not. We mapped the glomeruli activated by the five test odorants employed in the T&T olfactometer. The expression of c-fos protein as a marker of neuronal excitation was monitored using an immunohistochemical technique.

STUDY DESIGN

The expression of c-fos protein in the rat olfactory bulb was investigated to determine what part of the olfactory system is activated by five odorants used in the T&T olfactometer.

METHODS

Each rat was isolated in a clean cage for 120 min to reduce the basal expression of c-fos protein. Each rat was fixed in the cage and exposed to one of five test odorants for 90 min. The expression of c-fos protein was measured using an immunohistochemical technique.

RESULTS

Each odorant activated numerous glomeruli and the patterns of distribution of activated glomeruli were specific to each odorant. Glomeruli in most regions of the bulb were activated by all five test odorants.

CONCLUSION

We assume that in the T&T olfactometer the optimal ((five)) odorants are employed, because the glomeruli activated by those odorants showed unique patterns in the immunohistochemical assay.

Electrophysiological evidence for a chemotopy of biologically relevant odors in the olfactory bulb of the channel catfish

Extracellular electrophysiological recordings from single olfactory bulb (OB) neurons in the channel catfish, Ictalurus punctatus, indicated that the OB is divided into different functional zones, each processing a specific class of biologically relevant odor. Different OB regions responded preferentially at slightly above threshold to either a mixture of 1) bile salts (10(-7) to 10(-5) M Na(+) salts of taurocholic, lithocholic, and taurolithocholic acids), 2) nucleotides [10(-6) to 10(-4) M adenosine-5'-triphosphate (ATP), inosine-5'-monophosphate (IMP), and inosine-5'-triphosphate (ITP)], or 3) amino acids (10(-6) to 10(-4)M L-alanine, L-methionine, L-arginine, and L-glutamate). Excitatory responses to bile salts were observed primarily in a thin, medial strip in both the dorsal (100-450 microm) and ventral (900-1,200 microm) OB. Excitatory responses to nucleotides were obtained primarily from dorsal, caudolateral OB, whereas excitatory responses to amino acids occurred more rostrally in the dorsolateral OB, but continued more medially in the ventral OB. The chemotopy within the channel catfish OB is more comparable to that previously described by optical imaging studies in zebrafish than by field potential studies in salmonids. The present results are consistent with recent studies, suggesting that the specific spatial organization of output neurons in the OB is necessary for the quality coding/decoding of olfactory information.

Symmetry, stereotypy, and topography of odorant representations in mouse olfactory bulbs

The molecular basis of vertebrate odorant representations has been derived extensively from mice. The functional correlates of these molecular features were visualized using optical imaging of intrinsic signals in mouse olfactory bulbs. Single odorants activated clusters of glomeruli in consistent, restricted portions of the bulb. Patterns of activated glomeruli were clearly bilaterally symmetric and consistent in different individual mice, but the precise number, position, and intensity of activated glomeruli in the two bulbs of the same individual and between individuals varied considerably. Representations of aliphatic aldehydes of different carbon chain length shifted systematically along a rostral-caudal strip of the dorsal bulb, indicating a functional topography of odorant representations. Binary mixtures of individual aldehydes elicited patterns of glomerular activation that were topographic combinations of the maps for each individual odor. Thus the principles derived from the molecular organization of a small subset of murine olfactory receptor neuron projection patterns-bilateral symmetry, local clustering, and local variability-are reliable guides to the initial functional representation of odorant molecules.

Maternal odours induce Fos in the main but not the accessory olfactory bulbs of neonatal male and female ferrets

Previous research demonstrated that exposing gonadectomized adult ferrets to odours in oestrous female bedding induced nuclear Fos-immunoreactivity (Fos-IR; a marker of neuronal activity) in the main as opposed to the accessory olfactory system in a sexually dimorphic fashion, which was further augmented in both sexes by treatment with testosterone propionate. Ferrets are born in an altricial state and presumably use maternal odour cues to locate the nipples until the eyes open after postnatal (P) day 23. We investigated whether maternal odours augment neuronal Fos preferentially in the main versus accessory olfactory system of neonatal male and female ferret kits. Circulating testosterone levels peak in male ferrets on postnatal day P15, and mothers provide maximal anogenital stimulation (AGS) to males at this same age. Therefore, we assessed the ability of maternal odours to augment Fos-IR in the accessory olfactory bulb (AOB), the main olfactory bulb (MOB) and other forebrain regions of male and female ferret kits on P15 and investigated whether artificial AGS (provided with a paintbrush) would further enhance any effects of maternal odours. After separation from their mothers for 4 h, groups of male and female kits that were placed for 1.5 h with their anaesthetized mother had significantly more Fos-IR cells in the MOB granule cell layer and in the anterior-cortical amygdala, but not in the AOB cell layer, compared to control kits that were left on the heating pad. Artificial AGS failed to amplify these effects of maternal odours. Maternal odours (with or without concurrent AGS) failed to augment neuronal Fos-IR in medial amygdaloid and hypothalamic regions that are activated in adult ferrets by social odours. In neonatal ferrets of both sexes, as in adults, socially relevant odours are detected by the main olfactory epithelium and initially processed by the MOB and the anterior-cortical amygdala. In neonates, unlike adults, medial amygdaloid and hypothalamic neurones either do not respond to these inputs or respond in a manner that fails to induce Fos expression.

Mapping of brain stem neuronal circuitry active during swallowing

A poorly understood neural circuit in the brain stem controls swallowing. This experiment studied the swallowing circuit in the rat brain stem by means of fos immunocytochemistry. The fos protein is a marker of activated neurons, and under experimental conditions, repetition of a behavior causes the fos protein to be produced in the neurons involved in that behavior. The fos technique has been successfully used to delineate neural circuits involved in reflex glottic closure, cough, and vocalization; however, the technique has not been used to map the swallowing circuit. Nine rats were used in this study. Swallows were evoked in anesthetized rats for 1 hour, then, after a 4-hour delay to allow maximum fos production, the rats were painlessly sacrificed by perfusion. The brain stems were removed and sectioned in the frontal plane, and every fourth section was immunoreacted for fos protein. All sections were examined by light microscopy, and cells positive for fos were marked on drawings of brain stem structures for different levels throughout the brain stem. Control animals underwent sham experiments. After subtraction of the areas of fos labeling seen in controls, all experimental rats showed fos-labeled neurons in very discrete and localized areas, including practically all regions implicated by prior neurophysiology studies of swallowing. The distribution of labeled neurons was more dispersed through the brain stem than current theories of swallowing would suggest. Specifically, recent studies of swallowing control have focused on the nucleus of the solitary tract (NST) and the region surrounding the nucleus ambiguus (periambigual area) just rostral to the obex. These areas contained fos-labeled neurons, but unexpectedly, heavy labeling was found in the same areas caudal to the obex. Areas containing the heaviest labeling were specific subnuclei of the NST and surrounding reticular formation; the periambigual area; and the intermediate reticular zone in the pons and caudal medulla. Interestingly, none of these anatomic structures had uniform fos labeling; this finding suggests that the unlabeled areas are involved in other oromotor behaviors, or that the specific protocol did not activate the full population of swallowing-related neurons. A notable finding of this study is a candidate for the central pattern generator (CPG) of swallowing. Careful lesioning studies in cats strongly suggest that a region in the rostral-medial medulla contains the CPG for swallowing, although the exact location of the CPG was never pinpointed. In the homologous region of the rat brain stem, fos labeling was only found in a small group of neurons within the gigantocellular reticular formation that may be a candidate for the CPG. In summary, correlation with prior physiology experiments suggests that this experiment appears to have delineated many, if not all, of the components of the swallowing circuit for the first time in any mammal. In addition, other areas were found that might also be swallowing-related. One notable example is a small group of fos-labeled cells that may be the CPG for swallowing. Further studies are required to clarify the specific roles of the fos-labeled neurons seen in this study.

c-Fos-related antigens in the central nervous system of an insect, Acheta domesticus

Fos-related antigens (Fra) were detected in the nuclei of neurones in young adult Acheta domesticus female crickets by immunohistochemical analysis, using an antibody that recognizes the amino-acid sequence 127-152 of c-Fos protein. Specificity of Fra immunoreactivity was confirmed by Western blot analysis of nuclear extracts from neural tissues. A major immunoreactive doublet with an apparent molecular mass of 52,000/54,000 Da was detected in nuclear extracts. Immunostaining of the 52,000/54,000 Da doublet showed variations in intensity during the first 5 days following the imaginal molt. Staining was more intense between day 2 and day 4 when ecdysteroid titers were high. Expression of Fra was low in allatectomized (i.e., deprived of juvenile hormone and ecdysteroids) and ovariectomized (i.e., deprived of ecdysteroids) females as compared to control females. These results show the involvement of hormone-regulated process in expression of Fra. The effect of nociceptive stimulation on Fra expression was tested. Twenty minutes after removal of the ovipositor, a supplementary band with an apparent molecular mass of 70,000 Da appeared in the nuclear extracts, then decreased and disappeared totally after 45 min. Several other Fos-related antigens with different temporal patterns of expression were also detected.

Modular representations of odorants in the glomerular layer of the rat olfactory bulb and the effects of stimulus concentration

To study the mechanism whereby odorants are encoded in the nervous system, we studied the glomerular-layer activity patterns in the rat olfactory bulb evoked by closely related odorants from different chemical families. These odorants had a common straight-chain hydrocarbon structure, but differed systematically in their functional groups. Neural activity was mapped across the entire glomerular layer by using the ¿(14)C2-deoxyglucose method. Group responses were averaged and compared by using data matrices. The glomerular activity patterns that resulted from this analysis were comprised of modules. Unique combinations of modules were activated by each odorant, demonstrating what may be part of the neural code for odorants. Most of the modules were clustered together in the bulb, perhaps providing for enhanced contrast between related chemicals by means of lateral inhibition. We also determined whether changes in odorant concentration would affect spatial patterns of glomerular activity. Two odorants, pentanal and 2-hexanone, evoked different patterns at increased concentrations, with additional glomeruli being recruited at a great distance from glomeruli in which activity was evoked at lower concentrations. Humans report that both of these odorants change in perceived odor with increasing concentration. Three other odorants (pentanoic acid, methyl pentanoate, and pentanol) did not recruit new areas of glomerular activation with increasing concentration, and humans do not report a changed odor across concentrations of these odorants. The results suggest that changes in modular glomerular activity patterns could underlie altered odor perception across odorant concentrations, and they provide additional support for a combinatorial, spatially based code in the olfactory system.

Experience-dependent decrease in synaptically localized Fra-1

The Fos family of transcription factors has been repeatedly shown to participate in the long-term neural responses associated with a variety of physiological stimuli, including activity-dependent plastic processes. Quite recently, several transcription factors have been found in synaptic regions, localized in dendrites and presynaptic terminals. Here we show that the transcription factor Fos-related antigen-1 (Fra-1) was detected in synaptosomes (Syn) and synaptic plasma membrane (SPM) fractions from the rat cerebral cortex and hippocampus as a single band migrating with M(r) 42-43 kDa. The 55-kDa c-Fos protein was also detected in syn and SPM fractions. Conversely, the inducible 62-65-kDa c-Fos is present in nuclear fractions from metrazole-treated animals (positive control), but not in Syn or SPM fractions. Furthermore, no Fra-2, Fos B or c-Jun immunoreactivities were detected in these same synaptic regions. DNA-mobility shift assays showed the presence of specific AP-1 binding activity in synaptic protein extracts. Immunoelectronmicroscopic analysis of cortical and hippocampal tissues revealed that Fra-1 and Fos-like immunoreactivities are localized in association with presynaptic plasma membranes. One trial inhibitory avoidance training, a hippocampal-dependent task, is associated with a time-dependent decrease (-31%) in Fra-1, but not in 55-kDa c-Fos, levels in hippocampal SPM fractions. In hippocampal homogenates, we do not detect significant changes in Fra-1 immunoreactivity, suggesting that this behavioural experience is probably accompanied by a subcellular redistribution of Fra-1 protein. These results suggest that Fra-1 may participate in the communication between synapse and the nucleus and in experience-dependent hippocampal plasticity.

The olfactory bulb: coding and processing of odor molecule information

Olfactory sensory neurons detect a large variety of odor molecules and send information through their axons to the olfactory bulb, the first site for the processing of olfactory information in the brain. The axonal connection is precisely organized so that signals from 1000 different types of odorant receptors are sorted out in 1800 glomeruli in the mouse olfactory bulb. Individual glomerular modules presumably represent a single type of receptor and are thus tuned to specific molecular features of odorants. Local neuronal circuits in the bulb mediate lateral inhibition among glomerular modules to sharpen the tuning specificity of output neurons. They also mediate synchronized oscillatory discharges among specific combinations of output neurons and may contribute to the integration of signals from distinct odorant receptors in the olfactory cortex.

Expression of c-fos in the main olfactory bulb of neonatal rabbits in response to garlic as a novel and conditioned odour

Expression of c-Fos was examined in the olfactory bulbs of 3-day-old rabbits after they had been presented with the odour of garlic as a novel stimulus, as a learned odour, or during conditioning, and this expression compared with baseline levels in non-stimulated controls. Exposure to garlic odour resulted in substantial and widespread increases in c-Fos expression in the olfactory bulbs of all animals. However, although conditioned pups showed a specific behavioural response to the learned garlic odour, neither the amount nor pattern of c-Fos expression differed compared to pups exposed to garlic as a novel odour. The odour-induced expression of c-Fos was not well localised, although there was a significant increase in the number of granule cells expressing c-Fos in the ventrolateral region of the bulb. These results support previous reports that the response to odours in the olfactory bulb of new-born animals is not as spatially distinct as that in adults. Nevertheless, the immature olfactory system of these young animals is clearly capable of very specific odour learning.

In rats, odor-induced Fos in the olfactory pathways depends on the phase of the circadian clock

We used immunostaining for Fos to study the effect of circadian clock phase on odor-induced neuronal activation in the olfactory system in rats. Brief presentation of cedar odor to rats housed in constant darkness stimulated Fos expression in the main olfactory bulb, anterior olfactory nucleus, piriform cortex, and several other odor-responsive structures, both in the subjective day and subjective night phases of the cycle. Fos expression in response to odor, but not basal expression, was greatly enhanced in the subjective night in all structures examined. These findings are consistent with the idea that odor-induced neuronal activation in the olfactory pathways is modulated by the phase of the circadian clock.

Optical imaging of odorant representations in the mammalian olfactory bulb

We adapted the technique of intrinsic signal imaging to visualize how odorant concentration and structure are represented spatially in the rat olfactory bulb. Most odorants activated one or more glomeruli in the imaged region of the bulb; these optically imaged responses reflected the excitation of underlying neurons. Odorant-evoked patterns were similar across animals and symmetrical in the two bulbs of the same animal. The variable sensitivity of individual glomeruli produced distinct maps for different odorant concentrations. Using a series of homologous aldehydes, we found that glomeruli were tuned to detect particular molecular features and that maps of similar molecules were highly correlated. These characteristics suggest that odorants and their concentrations can be encoded by distinct spatial patterns of glomerular activation.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Odor detection in rats with 3-methylindole-induced reduction of sensory input

Rats were tested on odor-detection tasks after treatment with 400 mg/kg of 3-methyl-indole. As revealed by anterograde transport of horseradish peroxidase from the olfactory epithelium to the olfactory bulb glomeruli, treatment produced a severe (>97%) loss in sensory input relative to untreated controls. In almost all cases, only glomeruli in a restricted ventromedial segment of the bulb contained control levels of reaction product. In Experiment 1, five of nine experimental rats were anosmic or severely hyposmic, but the remaining four rats were able to detect amyl acetate vapor. In Experiment 2, four of seven experimental rats were anosmic, but the remaining three were able to detect each of four different odors. Among all experimental rats, those that were anosmic had significantly fewer glomeruli with dense anterograde transport than did those that could smell. Among rats that could smell, performance accuracy was related to the number of glomeruli with reaction product.

Olfaction in rats with extensive lesions of the olfactory bulbs: implications for odor coding

Rats were initially trained on a series of odor detection tasks and then received a unilateral olfactory bulbectomy and removal of different parts of the contralateral bulb. After postoperative recovery they were tested for detection of different concentrations of four odors, on a series of odor discrimination tasks and for their ability to acquire a relatively easy and a more difficult odor mixture discrimination task. Groups were formed based on which region of the bulb was intact (regional savings score) and on amount of bulb intact (bulbar savings score). In general, only rats with bulbar savings scores of less than 21% had deficits in detection or discrimination tasks but most performed as well as controls in most tasks. Correlations between bulbar savings scores and error scores were relatively low across all rats but, within the subgroup with the largest lesions (bulbar savings scores <21%), high correlations between these variables were obtained. There was no evidence for a specific anosmia in any group or individual rat and, except for the more difficult odor mixture discrimination, no one task proved difficult for any subgroup. The present results demonstrate that rats with relatively small remnants of one olfactory bulb can perform a variety of odor detection and discrimination tasks as well or nearly as well as controls. These outcomes provide no support for localization of function within the olfactory bulb but are in accord with recent proposals that odors may be coded by a highly distributed pattern of bulbar input.

AP-1, CREB and CBP transcription factors differentially regulate the tyrosine hydroxylase gene

The tyrosine hydroxylase (TH) gene encodes the rate-limiting enzyme in the biosynthesis of catecholamines. We have investigated the roles of two elements of the TH promoter, the TH-'Fat Specific Element' (TH-FSE) which binds the Fos-Jun complex, and the cAMP Response Element (CRE), which binds CREB and the co-activator protein, CREB Binding Protein (CBP) in regulating TH gene transcription. In PC12 cells, the TH-FSE was required for induction by NGF while the CRE was required for induction by cAMP. We show that both elements can function independently and contribute strongly to TH promoter basal activity in PC12 cells. We employed transient expression in the F9 teratocarcinoma cell line to vary experimentally the levels of the nuclear regulators implicated in TH control by the PC12 studies. In F9 cells, the TH promoter was strongly activated by Fos and Jun, and by PKA-stimulated CREB protein. In F9 and NIH3T3 cells, CBP, a co-activator which targets Fos-Jun and PKA-stimulated CREB, also induced the TH promoter. Immunohistochemical studies in rat brain regions enriched in dopaminergic neurons, including the midbrain and olfactory bulb (OB), suggest that Fos-Jun and CREB make differential contributions to TH gene activity in different tissues. Whereas changes in Fos protein levels parallel decreases in TH protein upon olfactory deprivation, CBP levels remain unchanged. This suggests that CRE-associated factors, including CBP, are not major regulators in the OB. In contrast, the presence of CREB and the absence of Fos immunoreactivity in midbrain dopaminergic cells suggests that the CRE is the primary regulator in this region.

Odor exposure reveals non-uniform expression profiles of c-Jun protein in rat olfactory bulb neurons

In the main olfactory bulb, neurons are arranged strategically in distinct layers among which translaminar synaptic transmission can be made from the superficial, sensory to the deep, output layers that account for the processing of olfactory information. To search for stimulus-transcription coupling thought to be operated differentially in several cell types, c-Jun expression was examined immunohistochemically in rat olfactory bulb following 30-min odor stimulation with acetic acid and 1-butanol. c-Jun was rapidly induced in neuronal cell nuclei belonging to periglomerular, tufted, mitral and granule cells. The disappearance of c-Jun, however, differed between each cell type. In the glomerular layer, the glomeruli composed of c-Jun-expressing periglomerular cells were seen. Different odors led to labeling of different sets of glomeruli. The labeled periglomerular cells disappeared within 2 h. In all the deeper layers, however, a rather homogeneous label was noted for the tufted, mitral and granule cells present throughout the olfactory bulb, regardless of the difference in odor. In tufted and mitral cells, the c-Jun expression persisted for 4 days after odor stimulation. In the granule cell layer, numerous granule cells increased c-Jun immunoreactivity which lasted for 1 day following odor application. In control rats which were given clean air, the basal amount of c-Jun expression was seen confined to scattered granule cells. The results suggest that c-Jun is expressed in a variety of odorant-stimulated bulb neurons with a time course being dependent on cell type.

Detection and discrimination of propionic acid after removal of its 2-DG identified major focus in the olfactory bulb: a psychophysical analysis

Prior 2-deoxyglucose and c-fos studies have demonstrated increased metabolic activity in a rostral dorsomedial area of the olfactory bulb in response to the vapor of propionic acid. We used psychophysical tests to assess the effect of removing this area of the bulb on odor sensitivity and discrimination. Normal rats, those with lesions of the rostral dorsomedial bulb or with control lesions of the lateral olfactory bulb were tested for propionic acid absolute detection and intensity difference thresholds and ability to discriminate propionic acid from other odors. There were no differences among groups for absolute or intensity difference threshold or on simple 2-odor discrimination tests but both groups with bulbar lesions made more errors than controls on a relatively difficult odor-mixture task. The results demonstrate that removal of an area of the bulb identified as responsive to propionic acid is essentially without effect on sensitivity to that odor or ability to discriminate it from other odors.

Distribution of fos-like immunoreactivity in the medullary reticular formation of the rat after gustatory elicited ingestion and rejection behaviors

The distribution of neurons in the medullary reticular formation (RF) activated by the ingestion of sucrose or rejection of quinine was examined using standard immunohistochemical techniques to detect the expression of the Fos protein product of the immediate-early gene c-fos. Double-labeling techniques were used to gain further insight into the possible functional significance of RF neurons exhibiting Fos-like immunoreactivity (FLI). Compared with sucrose and unstimulated controls, quinine elicited significantly more FLI neurons in three specific RF subdivisions: parvocellular reticular nucleus (PCRt), intermediate reticular nucleus (IRt), and dorsal medullary reticular nucleus (MdD). Moreover, the number of FLI neurons in the RF of quinine-stimulated animals was significantly correlated with the degree of oromotor activity. Thus, the distinct distribution of FLI neurons throughout the RF after quinine may reflect the activation of a specific oral rejection circuit. The double-labeling results indicated a high degree of segregation between FLI neurons and premotor projection neurons to the hypoglossal nucleus (mXII) retrogradely labeled with Fluorogold. Thus, although there were a significant number of double-labeled neurons in the RF, the major concentration of premotor projection neurons to mXII in IRt were medial to the preponderance of FLI neurons in the PCRt. In contrast, there was substantial overlap between FLI neurons in the RF and labeled fibers after injections of the anterograde tracer, biotinylated dextran into the rostral (gustatory) portion of the nucleus of the solitary tract. These results support a medial (premotor)/lateral (sensory) functional topography of the medullary RF.

Fos induction in central structures after afferent renal nerve stimulation

Experiments were done in the conscious and unrestrained rat to identify central structures activated by electrical stimulation of afferent renal nerves (ARN) using the immunohistochemical detection of Fos-like proteins. Fos-labelled neurons were found in a number of forebrain and brainstem structures bilaterally, but with a contralateral predominance. Additionally, Fos-labelled neurons were found in the lower thoracolumbar spinal cord predominantly ipsilateral to the side of ARN stimulation. Within the forebrain, neurons containing Fos-like immunoreactivity after ARN stimulation were primarily found along the outer edge of the rostral organum vasculosum of the laminae terminalis, in the medial regions of the subfornical organ, in the median preoptic nucleus, in the ventral subdivision of the bed nucleus of the stria terminalis, along the lateral part of the central nucleus of the amygdala, throughout the deeper layers of the dysgranular insular cortex, in the parvocellular component of the paraventricular nucleus of the hypothalamus (PVH), and in the paraventricular nucleus of the thalamus. Additionally, a smaller number of Fos-labelled neurons was observed in the supraoptic nucleus, in the magnocellular component of the PVH and along the lateral border of the arcuate nucleus. Within the brainstem, Fos-labelled neurons were found predominantly in the commissural and medial subnuclei of the nucleus of the solitary tract and in the external subnucleus of the lateral parabrachial nucleus. A smaller number were observed near the caudal pole of the locus coeruleus, and scattered throughout the ventrolateral medullary and pontine reticular formation in the regions known to contain the A1, C1 and A5 catecholamine cell groups. The final area observed to contain Fos-labelled neurons in the central nervous system was the thoracolumbar spinal cord (T9-L1) which contained cells in laminae I-V of the dorsal horn ipsilateral to side of stimulation and in the intermediolateral cell column at the same levels bilaterally, but with an ipsilateral predominance. Few, if any Fos-labelled neurons were observed in the same structures of control animals in which the ARN were stimulated, but the renal nerves proximal to the site of stimulation were transected, or in the sham operated animals. These data indicate that ARN information originating in renal receptors is conveyed to a number of central areas known to be involved in the regulation of body fluid balance and arterial pressure, and suggest that this afferent information is an important component of central mechanisms regulating these homeostatic functions.

Granule and mitral cell densities are unchanged following early olfactory preference training

Early olfactory preference training causes both an increased number of juxtaglomerular cells and an increased number of such cells expressing Fos protein. In contrast, there are fewer cells expressing Fos protein in the granule cell layer after training. Here, we report no change in the number or size of granule and mitral cells as a consequence of these early olfactory experiences.

Age-related changes in c-fos mRNA induction after open-field exposure in the rat brain

In the present study, functional activation of brain regions was measured by the induction of c-fos and c-jun mRNA following exposure to a novel open field. Fischer 344 rats at 5 months, 14 months, and 21 months were exposed to a square open field for 20 min. Rats were then immediately sacrificed and their brains were examined for c-fos and c-jun mRNA using in situ hybridization. Control rats were sacrificed directly from their home cage. Results showed no significant age-related changes in locomotor activity. Autoradiogram analyses showed that open-field exposure induced c-fos mRNA throughout the brain, while c-jun mRNA was induced in a few brain regions. Aged rats showed a lower elevation of c-fos mRNA in the prelimbic cortex compared to 5-month rats. In addition, grain analyses revealed age-related decreases in c-fos mRNA induction in the medial prefrontal cortex, caudate, and ventral lateral septum. These findings indicate age-related changes in the induction of c-fos mRNA in certain brain regions following exploration of a novel environment.

Mechanisms of olfactory discrimination: converging evidence for common principles across phyla

Olfaction begins with the transduction of the information carried by odor molecules into electrical signals in sensory neurons. The activation of different subsets of sensory neurons to different degrees is the basis for neural encoding and further processing of the odor information by higher centers in the olfactory pathway. Recent evidence has converged on a set of transduction mechanisms, involving G-protein-coupled second-messenger systems, and neural processing mechanisms, involving modules called glomeruli, that appear to be adapted for the requirements of different species. The evidence is highlighted in this review by focusing on studies in selected vertebrates and in insects and crustaceans among invertebrates. The findings support the hypothesis that olfactory transduction and neural processing in the peripheral olfactory pathway involve basic mechanisms that are universal across most species in most phyla.

Regulation of c-Fos mRNA and fos protein expression in olfactory bulbs from unilaterally odor-deprived adult mice

Odorant deprivation, produced by unilateral naris closure, profoundly reduces tyrosine hydroxylase (TH) expression within intrinsic olfactory bulb dopamine neurons. The TH gene contains an AP-1 site, which interacts with the product of the immediate early gene, c-fos. c-Fos exhibits activity dependent regulation in the CNS. The hypothesis that odorant stimulation and deprivation might modify c-fos expression in TH neurons was tested in adult CD-1 mice, subjected to unilateral naris closure. After 2 months, naris closed and control mice were exposed to either clean air for 60 min or clean air for 60 min followed by 30 min of alternating exposure to 10% isoamyl acetate (1 min) and air (4 min). A parallel reduction occurred in TH and fos expression (both c-fos mRNA and fos-like immunoreactivity) in the glomerular layer of the odorant-deprived olfactory bulb. Odor stimulation induced a short-lived increase in c-fos mRNA and fos-like immunoreactivity in olfactory bulbs contralateral to naris closure. The increase in fos expression was region-specific in the glomerular layer but more diffuse in mitral and granule cell layers. In olfactory bulbs ipsilateral to naris closure, odor stimulation also induced c-fos mRNA expression in the mitral and granule cell layers and sparsely within limited periglomerular regions. Odor induced expression in mitral and granule cell layers may represent increased centrifugal activity acting on as yet unknown genes. These results suggest a correlation between c-fos mRNA expression and increased neuronal activity in the olfactory bulb which, in turn, acts to regulate TH expression in periglomerular neurons.

NMDA-receptor modulation of lateral inhibition and c-fos expression in olfactory bulb

Olfactory bulb primary output neurons, mitral/tufted cells, are glutamatergic and excite inhibitory interneurons, granule cells, by activation of both alpha-amino-3-hydroxy-5-methyl-ioxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) glutamate receptors. The data presented here demonstrate that the NMDA antagonists MK-801 and CGP39551, but not ketamine, significantly enhanced expression of c-fos mRNA by mitral cells as measured by in situ hybridization. All three antagonists significantly reduced mitral/tufted cell excitation of granule cells as measured with extracellular field potentials following antidromic stimulation of the lateral olfactory tract (LOT). In turn, the NMDA antagonists significantly reduced granule cell mediated feedback inhibition of mitral/tufted cells, as measured with field potential recordings of paired-pulse LOT stimulation, suppression of mitral/tufted cell single-unit spontaneous activity following LOT stimulation, and intracellularly recorded IPSP amplitude in mitral/tufted cells following LOT stimulation. While there was not a perfect correlation between the effects of the NMDA antagonists on c-fos mRNA expression and on inhibition, the results suggest that disinhibition of mitral/tufted cells accounts for the observed enhancement in c-fos mRNA expression induced by NMDA receptor antagonists.

Information processing in mammalian olfactory system

In recent years, considerable progress has been made in understanding how the olfactory system uses neural space to encode sensory information. In this review, we focus on recent studies aimed at understanding the organizational strategies used by the mammalian olfactory system to encode information. The odorant receptor gene family is discussed in the context of its genomic organization as well as the specificity of olfactory sensory neurons. These data have important consequences for the mechanisms of odorant receptor gene choice by a given sensory neuron. Division of the olfactory epithelium into zones that express different sets of odorant receptors is the first level of input organization. The topographical relationship between periphery and olfactory bulb represents a further level of processing of information and results in the formation of a highly organized spatial map of information in the olfactory bulb. There, local circuitry refines the sensory input through various lateral interactions. Finally, the factors that may drive the development of such a spatial map are discussed. The onset of expression and the establishment of the zonal organization of odorant receptor genes in the epithelium are not dependent upon the presence of the olfactory bulb, suggesting that the functional identity of olfactory sensory neurons is determined independently of target selection.