Olfactory bulb: odor signals put into context

In this issue, Doucette and colleagues demonstrate that information related to whether an odor is currently linked to reward can be observed uniquely in population activity in the olfactory bulb, changing our understanding both of what is coded by the first olfactory relay in the CNS and of how this coding is instantiated.     

Olfactory receptors: G protein-coupled receptors and beyond

Sensing the chemical environment is critical for all organisms. Diverse animals from insects to mammals utilize highly organized olfactory system to detect, encode, and process chemostimuli that may carry important information critical for health, survival, social interactions and reproduction. Therefore, for animals to properly interpret and react to their environment it is imperative that the olfactory system recognizes chemical stimuli with appropriate selectivity and sensitivity. Because olfactory receptor proteins play such an essential role in the specific recognition of diverse stimuli, understanding how they interact with and transduce their cognate ligands is a high priority. In the nearly two decades since the discovery that the mammalian odorant receptor gene family constitutes the largest group of G protein-coupled receptor (GPCR) genes, much attention has been focused on the roles of GPCRs in vertebrate and invertebrate olfaction. However, is has become clear that the 'family' of olfactory receptors is highly diverse, with roles for enzymes and ligand-gated ion channels as well as GPCRs in the primary detection of olfactory stimuli.     

Olfactory bulb axonal outgrowth is inhibited by draxin

Olfactory bulb (OB) projection neurons receive sensory input from olfactory receptor neurons and precisely relay it through their axons to the olfactory cortex. Thus, olfactory bulb axonal tracts play an important role in relaying information to the higher order of olfactory structures in the brain. Several classes of axon guidance molecules influence the pathfinding of the olfactory bulb axons. Draxin, a recently identified novel class of repulsive axon guidance protein, is essential for the formation of forebrain commissures and can mediate repulsion of diverse classes of neurons from chickens and mice. In this study, we have investigated the draxin expression pattern in the mouse telencephalon and its guidance functions for OB axonal projection to the telencephalon. We have found that draxin is expressed in the neocortex and septum at E13 and E17.5 when OB projection neurons form the lateral olfactory tract (LOT) rostrocaudally along the ventrolateral side of the telencephalon. Draxin inhibits axonal outgrowth from olfactory bulb explants in vitro and draxin-binding activity in the LOT axons in vivo is detected. The LOT develops normally in draxin−/− mice despite subtle defasciculation in the tract of these mutants. These results suggest that draxin functions as an inhibitory guidance cue for OB axons and indicate its contribution to the formation of the LOT.     

Olfactory bulb serotonin level modulates olfactory recognition in the neonate rat

Role of serotonin in olfactory recognition was tested by depleting the olfactory bulb serotonin during postnatal day (PND) 1 - 4 following administration of 5,7-dihydroxytryptamine. Significant difference in the olfactory recognition test was observed during PND5-7; control pups successfully recognized and oriented towards their mother; whereas treated pups failed to recognize their mother odour. Later on, during PND12-14, both group of pups responded equally in the recognition test. Levels of olfactory bulb serotonin were depleted (53.3%) in the treated pups on PND-8, which was restored on PND-14 with only 15% variation. Further analysis demonstrated that depletion of serotonin in olfactory bulb did not affect the normal suckling and weight gain, it only modulates olfactory recognition.     

Olfactory performance of rats after selective deafferentation of the olfactory bulb by 3-methyl indole

Rats trained to detect propyl acetate and valeric acid and to discriminate between propyl acetate and amyl acetate and between valeric acid and butyric acid were injected with a low dose of 3-methyl indole, a treatment that produces well-defined and selective deafferentation of the olfactory bulbs. Treatment completely deafferented most but not all bulbar loci for aliphatic acids and at least disrupted those for propyl and amyl acetate. In posttreatment tests, experimental rats performed somewhat but not significantly more poorly than controls and about as well on the acid detection and discrimination tasks as on the corresponding acetate tests.     

Olfactory bulb glomerular NMDA receptors mediate olfactory nerve potentiation and odor preference learning in the neonate rat

Rat pup odor preference learning follows pairing of bulbar beta-adrenoceptor activation with olfactory input. We hypothesize that NMDA receptor (NMDAR)-mediated olfactory input to mitral cells is enhanced during training, such that increased calcium facilitates and shapes the critical cAMP pattern. Here, we demonstrate, in vitro, that olfactory nerve stimulation, at sniffing frequencies, paired with beta-adrenoceptor activation, potentiates olfactory nerve-evoked mitral cell firing. This potentiation is blocked by a NMDAR antagonist and by increased inhibition. Glomerular disinhibition also induces NMDAR-sensitive potentiation. In vivo, in parallel, behavioral learning is prevented by glomerular infusion of an NMDAR antagonist or a GABA(A) receptor agonist. A glomerular GABA(A) receptor antagonist paired with odor can induce NMDAR-dependent learning. The NMDA GluN1 subunit is phosphorylated in odor-specific glomeruli within 5 min of training suggesting early activation, and enhanced calcium entry, during acquisition. The GluN1 subunit is down-regulated 3 h after learning; and at 24 h post-training the GluN2B subunit is down-regulated. These events may assist memory stability. Ex vivo experiments using bulbs from trained rat pups reveal an increase in the AMPA/NMDA EPSC ratio post-training, consistent with an increase in AMPA receptor insertion and/or the decrease in NMDAR subunits. These results support a model of a cAMP/NMDA interaction in generating rat pup odor preference learning.     

Olfactory bulb proteins linked to olfactory memory in C57BL/6J mice

Information on systematic analysis of olfactory memory-related proteins is poor. In this study, the odor discrimination task to investigate olfactory recognition memory of adult male C57BL/6J mice was used. Subsequently, olfactory bulbs (OBs) were taken, proteins extracted, and run on two-dimensional gel electrophoresis with in-gel-protein digestion, followed by mass spectrometry and quantification of differentially expressed proteins. Dual specificity mitogen-activated protein kinase kinase 1 (MEK1), dihydropyrimidinase-related protein 1 (DRP1), and fascin are related with Lemon odor memory. Microtubule-associated protein RP/EB family member 3 is related to Rose odor memory. Hypoxanthine-guanine phosphoribosyltransferase is related with both Lemon and Rose odors memory. MEK1 and DRP1 levels were increased, while microtubule-associated protein RP/EB family member 3, fascin and hypoxanthine-guanine phosphoribosyltransferase levels were decreased during olfactory memory. In summary, neurogenesis, signal transduction, cytoskeleton, and nucleotide metabolism are involved in olfactory memory formation and storage of C57BL/6J mice.     

Olfactory bulb output cell temporal response patterns to increasing odor concentrations in freely breathing rats

This study compares the single-unit responses of 74 mitral/tufted cells recorded in freely breathing rats to step increases of the intensity of five odorants from 2 x 10(-4) to 10(-1) of saturated vapor pressure. It reveals a stability of the responses of these olfactory bulb output cells. Olfactory stimulation has frequently been shown to produce a strong patterning of mitral/tufted cell discharges highly correlated with respiration. In this study, cells were generally found to show the same response type to two consecutive concentrations, and only a few cells switched their response from excitation to suppression or vice versa. Their firing peak and/or trough occupied the same position in a high proportion of respiratory cycles recorded during a stimulation, and they remained significantly time-locked to the same respiratory epoch for the next higher concentration. Increasing odor concentration did not cause the mean firing frequency of individual cells during a peak to change appreciably between successive or extreme concentrations. By contrast, it tended to shift their maximum frequency during this peak towards an earlier respiratory cycle after stimulation onset. These results are compared with data reported in other electrophysiological studies and with results given by olfactory bulb models before being discussed for their implications in odor coding.      

Auditory neuroscience: filling in the gaps

Our sensory systems fill in information obscured by other, competing signals to maintain a stable representation of the world. A correlate of the continuity illusion, in which sounds are perceived to continue despite being interrupted by other sounds, has now been found in the auditory cortex.     

Visual neuroscience: illuminating the dark corners

Recent experiments suggest that our perception of lightness involves a sophisticated interpretation of illumination and shadow. This finding challenges common notions about hierarchical processing and the neural basis of perception.     

Visual neuroscience: revealing the motion-detecting circuitry

Molecular approaches in Drosophila that allow the removal of specific classes of neurons are breaking new ground in determining the neural implementation of a computational algorithm for visual motion detection.     

Olfactory bulb cells generated in adult male golden hamsters are specifically activated by exposure to estrous females

Two experiments were carried out to test whether cells which are born in adulthood and migrate to the olfactory bulb of adult male golden hamsters are activated during sexual behaviors, to determine the time course over which such responsiveness appears, and to ask whether activation is specific to sexual cues. In the first experiment, adult male hamsters were injected with 5'-bromodeoxyuridine (BrdU, 50mg/kg b.w.) 3 times over the course of one week in order to mark dividing cells. Ten days, three weeks, or seven weeks after the first BrdU injection, the animals were allowed to mate with an estrous female for half an hour before being sacrificed. Confocal analysis of fluorescent immunostaining of BrdU and c-Fos first revealed dual labeled cells in the olfactory bulb 3 weeks after injection of the thymidine analog. In order to determine whether the activation of these newly generated cells is specific to sexual cues, we next compared the incidence of c-Fos expression in newborn (BrdU positive) cells among male hamsters exposed to an estrous female, an aggressive male, a cotton swab containing vaginal secretion from an estrous female hamster (FHVS), a cotton swab containing peppermint, or a cotton swab containing distilled water. In the mitral and glomerular layers of the accessory olfactory bulb, animals exposed to an estrous female had significantly more double labeled cells than did those given other treatments (p < 0.01). In the mitral layer of the main bulb, animals exposed to an estrous female had a significantly higher percentage of double labeled cells than those of other groups, except those exposed to an aggressive male (p < 0.05). No double labeled cells were seen in medial preoptic area (MPOA), medial nucleus of the amygdala (Me), the bed nucleus of the stria terminalis (BNST), or the hypothalamus. Our results indicate that cells born in adulthood are more responsive to cues arising from estrous females than other stimuli, and thus may participate in sociosexual behaviors.     

Cognitive neuroscience: numerate neurons

Whether the neuronal encoding of number is linear or logarithmic divides cognitive neuroscientists working on mathematical cognition. Recordings from the prefrontal cortex of the monkey support the logarithmic hypothesis. Similarities between number and the coding of other quantities are also beginning to become apparent.     

Cognitive neuroscience: Web alert

A selection of World Wide Web sites relevant to the reviews published in this issue of Current Opinion in Neurobiology.     

Affective neuroscience: tracing the trace of fear

The trace of fear has been elusive and difficult to discern in the human brain. Researchers have come up with a clever new way to track it down.     

Cognitive neuroscience: swapping bodies in the brain

A recent study has found that activity in multisensory brain areas, namely the premotor cortex, intraparietal cortex and the putamen, mirrors the vividness of?ownership over a mannequin, induced by the body-swap illusion.