Zincergic innervation from the anterior olfactory nucleus to the olfactory bulb displays plastic responses after mitral cell loss

Zinc ions are selectively accumulated in certain neurons (zinc-enriched neurons). The mouse olfactory bulb is richly innervated by zinc-enriched terminals. Here, the plasticity of the zincergic system was studied in the olfactory bulb of the Purkinje Cell Degeneration mutant mouse, an animal with specific postnatal neurodegeneration of the main projection neurons of the olfactory bulb. The analysis focused particularly on the anterior olfactory nucleus since most centrifugal afferents coming to the olfactory bulb arise from this structure. Zinc-enriched terminals in the olfactory bulb and zinc-enriched somata in the anterior olfactory nucleus were visualized after selenite injections. Immunohistochemistry against the vesicular zinc transporter was also carried out to confirm the distribution pattern of zinc-enriched terminals in the olfactory bulb. The mutant mice showed a clear reorganization of zincergic centrifugal projections from the anterior olfactory nucleus to the olfactory bulb. First, all zincergic contralateral neurons projecting to the olfactory bulb were absent in the mutant mice. Second, a significant increase in the number of stained somata was detected in the ipsilateral anterior olfactory nucleus. Since no noticeable changes were observed in the zinc-enriched terminals in the olfactory bulb, it is conceivable that mitral cell loss could induce a reorganization of zinc-enriched projections coming from the anterior olfactory nucleus, probably directed at balancing the global zincergic centrifugal modulation. These results show that zincergic anterior olfactory nucleus cells projecting to the olfactory bulb undergo plastic changes to adapt to the loss of mitral cells in the olfactory bulb of Purkinje Cell Degeneration mutant mice.     

Supraoptic nucleus afferents from the main olfactory bulb--I. Anatomical evidence from anterograde and retrograde tracers in rat

The morphological features of a putative connection between the main olfactory bulb and the supraoptic nucleus of the rat was studied using a combination of anatomical techniques. Immunocytochemistry of neurophysin-containing processes were employed to delineate morphological features of supraoptic dendrites. Main olfactory bulb efferents to the supraoptic nucleus were studied by injection of the anterogradely transported substances, wheatgerm agglutinin conjugated horseradish peroxidase or Phaseolus vulgaris leucoagglutinin, into the main olfactory bulb. To confirm the results of these studies, the distribution of retrogradely labeled cells within the main olfactory bulb was determined after injection of rhodamine-labeled latex microspheres or Fluoro-Gold into the supraoptic nucleus. Neurophysin immunocytochemistry revealed the supraoptic nucleus dendritic plexus which coursed anteroposteriorly beneath supraoptic somata. Additionally, a portion of this plexus also projected ventrolaterally into periamygdaloid areas, a feature of supraoptic architecture which is not generally appreciated. The anterograde tracers labeled main olfactory bulb efferents including a dense plexus of terminals and fibers ventrolateral to the ipsilateral supraoptic nucleus. The pattern of anterogradely labeled fibers and terminals appeared to overlap with the distribution of ventrolaterally projecting neurophysin-containing processes. Since the latter consists of dendritic processes of supraoptic origin, this suggests that the main olfactory bulb projects to the supraoptic nucleus. Injections of rhodamine-labeled latex microspheres or Fluoro-Gold resulted in retrogradely labeled mitral cells throughout the ipsilateral main olfactory bulb. Taken together, these anatomical studies demonstrate a direct projection from the main olfactory bulb to the supraoptic nucleus of the rat. A comparison electrophysiological study confirmed these results.     

Trajectory and terminal distribution of single centrifugal axons from olfactory cortical areas in the rat olfactory bulb

The olfactory bulb receives a large number of centrifugal fibers whose functions remain unclear. To gain insight into the function of the bulbar centrifugal system, the morphology of individual centrifugal axons from olfactory cortical areas was examined in detail. An anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into rat olfactory cortical areas, including the pars lateralis of the anterior olfactory nucleus (lAON) and the anterior part of the piriform cortex (aPC). Reconstruction from serial sections revealed that the extrabulbar segments of centrifugal axons from the lAON and those from the aPC had distinct trajectories: the former tended to innervate the pars externa of the AON before entering the olfactory bulb, while the latter had extrabulbar collaterals that extended to a variety of targets. In contrast to the extrabulbar segments, no clear differences were found between the intrabulbar segments of axons from the lAON and from the aPC. The intrabulbar segments of centrifugal axons were mainly found in the granule cell layer but a few axons extended into the external plexiform and glomerular layer. Approximately 40% of centrifugal axons innervated both the medial and lateral aspects of the olfactory bulb. The number of boutons found on single intrabulbar segments was typically less than 1000. Boutons tended to aggregate and form complex terminal tufts with short axonal branches. Terminal tufts, no more than 10 in single axons from ipsilateral cortical areas, were localized to the granule cell layer with varying intervals; some tufts formed patchy clusters and others were scattered over areas that extended for a few millimeters. The patchy, widespread distribution of terminals suggests that the centrifugal axons are able to couple the activity of specific subsets of bulbar neurons even when the subsets are spatially separated.     

Central olfactory connections in the microsmatic marmoset monkey (Callithrix jacchus)

The mammalian primary olfactory system consists of a set of different telencephalic structures, including paleo-, archi-, periarchi- and mesocortical components. We present the first characterisation of the normal and connectional anatomy of the primary olfactory cortex of the common marmoset, a microsmatic simian species increasingly used in primate research. The centrifugal and centripetal bulbar projections were determined by injections of the anterograde and retrograde tracer wheat germ agglutinin-conjugated horseradish peroxidase and fluorescent dyes into the ipsilateral main olfactory bulb. The efferent projections of the marmoset bulb are organised entirely ipsilaterally and are established via a rudimentary medial olfactory tract and the dominant lateral olfactory tract. Target areas are the anterior olfactory nucleus, the entire prepiriform cortex, ventral tenia tecta, periamygdaloid cortex and the rostral part of the entorhinal cortex. The bulbar axons predominantly terminate in the outer part of layer I. The anterior olfactory nucleus receives a weak additional input within layer II and III, which is not found in macrosmatic rodents. Further anterograde labelling was found in the endopiriform nucleus deep under the prepiriform cortex and within an anterolateral strip of the olfactory tubercle. However, control injections into the olfactory tubercle suggest that the marmoset olfactory tubercle receives a bisynaptic olfactory input only. Retrograde labelling after bulb injections revealed that, except for the olfactory tubercle, all primary olfactory cortices contributed to an ipsilateral bulbopetal feedback projection. Like in rodents, the only bulbopetal projection organised bilaterally in the marmoset is maintained by the anterior olfactory nucleus. With few exceptions, the projections of the marmoset olfactory brain are organised similarly to that of the macaque monkey or those of macrosmatic species.     

Direct pathways to the supraoptic nucleus from the brainstem and the main olfactory bulb are activated at parturition in the rat

Sensory input from female reproductive structures is paramount for the co-ordination of neuroendocrine changes at parturition. Using a retrograde tracer (fluorescent latex microspheres) in combination with Fos (as an indicator of neuronal activation) and tyrosine hydroxylase (to identify catecholaminergic neurons) immunocytochemistry we identified cells within the brainstem and main olfactory bulb that project to the supraoptic nucleus, and which become significantly activated at parturition (compared to virgin rats and rats on the day of expected parturition). Within the A2/C2 region in the nucleus tractus solitarii, 60% of the projecting activated cells were catecholaminergic, as were 59% of such cells in the A1/C1 region of the ventrolateral medulla. This suggests that oxytocin and vasopressin neurons within the supraoptic nucleus are stimulated at parturition via afferent inputs from the brainstem, but the input is not exclusively noradrenergic. Within the mitral layer of the main olfactory bulb, cells that projected to the supraoptic nucleus were significantly activated, suggesting that the olfactory system may regulate supraoptic nucleus cell firing at parturition. The preoptic area, organum vasculosum of the lamina terminalis and medial amygdala contained cells that projected to the supraoptic nucleus but these projections were not significantly activated at parturition, although non-projecting cells in these regions were. On the expected day of parturition, but before birth, projections from the organum vasculosum of the lamina terminalis to the supraoptic nucleus became significantly activated.These findings provide evidence of direct afferent pathways to the supraoptic nucleus from the brain stem and olfactory bulbs that are activated at parturition.     

Centrifugal innervation of the mammalian olfactory bulb

Although it has been known for decades that the mammalian olfactory bulb receives a substantial number of centrifugal inputs from other regions of the brain, relatively few data have been available on the function of the centrifugal olfactory system. Knowing the role of the centrifugal projection and how it works is of critical importance to fully understanding olfaction. The centrifugal fibers can be classified into two groups, a group that release neuromodulators, such as noradrenaiine, serotonin, or acetylcholine, and a group originating in the olfactory cortex. Accumulating evidence suggests that centrifugal neuromodulatory inputs are associated with acquisition of odor memory. Because the distribution of the terminals on these fibers is diffuse and widespread, the neuromodulatory inputs must affect diverse subsets of bulbar neurons at the same time. In contrast, knowledge of the role of centrifugal fibers from the olfactory cortical areas is limited. Judging from recent morphological evidence, these fibers may modify the activity of neurons located in sparse and discrete loci in the olfactory bulb. Given the modular organization of the olfactory bulb, centrifugal fibers from the olfactory cortex may help coordinate the activities of restricted subsets of neurons belonging to distinct functional modules in an odor-specific manner. Because the olfactory cortex receives inputs from limbic and neocortical areas in addition to inputs from the bulb, the centrifugal inputs from the cortex can modulate odor processing in the bulb in response to non-olfactory as well as olfactory cues.     

Chromogranin A in the olfactory system of the rat

The olfactory bulb of the rat contains chromogranin A at a similar level as the adrenal gland or the hypophysis as revealed by immunoblots. Olfactory chromogranin A also displays the same size as chromogranin A of endocrine cells. In the hippocampus and other brain regions, we could not detect chromogranin A by immunoblotting. In contrast, chromogranin A messenger ribonucleic acid (using S1 nuclease protection assays) was observed in all brain regions examined, including the olfactory bulb. By in situ hybridization histochemistry with a complementary ribonucleic acid probe (280 nucleotides), and by immunocytochemistry, chromogranin A synthesis could be localized to cell bodies of the mitral cell layer, of the external plexiform layer and of the periglomerular region of the olfactory bulb. Immunocytochemically, chromogranin A was also detected in the central projection areas of mitral and tufted cells in the primary olfactory cortex and the anterior amygdaloid area but not in the olfactory glomeruli, where the incoming olfactory nerve fibers of the primary olfactory neurons establish synaptic contacts. Taken together the data show that chromogranin A, following biosynthesis in the perikarya of the mitral and tufted cells, is specifically transported into their axonal terminals but not into their primary dendrites. We propose that the rat olfactory system could serve as a model for the study of chromogranin A regulation and function in neurons.     

Central connections of the olfactory bulb in the american opossum (didelphys virginiana): A light microscopic degeneration study

Destructive lesions were made in the right olfactory bulb of 16 adult opossums. Following postoperative survival periods of 4 to 31 days, the animals were sacrificed and perfused with 10% Formalin. Frozen sections of the brain were cut in either the coronal, horizontal, or sagittal plane and processed by the Fink-Heimer II method. Degenerating axons of olfactory bulb neurons were traced caudally in the ipsilateral lateral olfactory tract (LOT). Small lesions revealed a topographic representation of the olfactory bulb within the LOT. The dorsal, lateral, and ventral parts of the bulb were, respectively, represented in the dorsal, intermediate, and ventral parts of the LOT. Terminal degeneration was observed in the superficial half of the molecular layer ipsilaterally in the following structures: anterior olfactory nucleus, anterior hippocampal rudiment, olfactory tubercle, piriform cortex, ventrolateral frontal neocortex, lateral entorhinal cortex, nucleus of the LOT, and the lateral aspect of the cortical amygdaloid nucleus. No degeneration was observed in the anterior limb of the anterior commissure. Dorsal and lateral parts of the olfactory bulb projected to the anterolateral aspect of the olfactory tubercle, whereas the ventral part projected heavily to the entire tubercle. There was no evidence of topographic projections to other olfactory structures. The observations of the present investigation indicated that the olfactory bulb projections in the opossum, a primitive mammal, are essentially comparable with those of placental mammals.     

Neuronal gap junctions in the mouse main olfactory bulb: morphological analyses on transgenic mice

In the present study we analyzed the structural features of extraglomerular gap junction-forming processes in mouse olfactory bulb electron microscopically. This work complements a previous study in which we analyzed the structural features of neuronal gap junction-forming processes within the glomerulus itself. Furthermore we examined connexin 36 expressing cells in the mouse olfactory bulb by analyzing transgenic mice in which the connexin 36 coding sequence was replaced with histological reporters. In extraglomerular regions, the mitral/tufted cell somata, dendrites and axon hillocks made gap junctions and mixed synapses with interneuronal processes. These gap junctions and synapses were associated with various types of interneuronal processes, including a particular type of sheet-like or calyx-like process contacting the somata or large dendrites of mitral/tufted cells. In the olfactory bulbs of the transgenic mice, connexin 36 was expressed in mitral cells, tufted cells, presumed granule cells and periglomerular cells. Multiple immunofluorescent labelings further revealed that presumed interneurons expressing connexin 36 in the periglomerular region rarely expressed calbindin, calretinin or tyrosine hydroxylase and are likely to comprise a chemically uncharacterized class of neurons. Similarly, interneurons expressing connexin 36 in the granule cell layer were rarely positive for calretinin, which was expressed in numerous presumed granule cells in the mouse main olfactory bulb. In summary, these findings revealed that mitral/tufted cells make gap junctions with diverse types of neurons; in the glomeruli gap junction-forming interneuronal processes originated from some types of periglomerular cells but others from a hitherto uncharacterized neuron type(s), and in the extraglomerular region gap-junction forming processes originate mainly from a subset of cells within the granule cell layer.     

Distribution of orexins-containing fibers and contents of orexins in the rat olfactory bulb

Orexin-A and -B (identical to hypocretin-1 and -2) are hypothalamic neuropeptides that regulate appetite and arousal. Orexins-producing neurons project their axons to various brain regions, including the olfactory bulb. In the present study, to understand the relationship between orexins and olfaction, we investigated the distribution of the orexin-A- and -B-immunoreactive (ir) fibers in the rat olfactory bulb and the contents of orexin-A and -B in the rat olfactory bulb after food deprivation for 48 h by using immunohistochemistry and radioimmunoassay, respectively. Both orexin-A- and -B-ir fibers are similarly wide spread from the glomerular layer of the olfactory bulb where the terminals of the peripheral olfactory nerves make synapses with the mitral cells or the tufted cells, to the piriform cortex. Dense orexin-A- and -B-ir fibers were observed mainly in the granular cell layer and anterior olfactory nucleus. The contents of orexin-A and -B (pg/10 mg wet weight tissue) in fed rats (mean+/-S.E.M., n=6) were 2.72+/-0.24 and 6.31+/-0.63, respectively. Fasting for 48 h significantly reduced the contents of orexin-B, but not orexin-A. Orexins in the rat olfactory bulb may be involved in not only olfactory system but also energy balance.     

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals.

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.     

Effect of stimulating the nucleus of the horizontal limb of the diagonal band on single unit activity in the olfactory bulb

The effects of centrifugal afferents on single unit discharge in the main olfactory bulb were studied in anaesthetized rats. Recording with extracellular micropipettes revealed spontaneous firing in all bulb layers. Units were located to different laminae using evoked field-potential profiles and histological verification. Output neurons were identified by antidromic response to stimulation of the lateral olfactory tract. Single- or brief multiple-pulse stimulation in the nucleus of the horizontal limb of the diagonal band, but not in adjacent regions, facilitated 17 out of 27 mitral cells with no effect on 10, but inhibited 21 out of 33 granule cell layer units with no effect on 12. Of 13 presumed tufted cells, six were facilitated and the rest unaffected. In contrast, stimulation of olfactory cortex inhibited mitral cells and facilitated most granule layer cells. The results are consistent with an inhibition of tonic granule cell discharge by the horizontal diagonal band nucleus, with resultant disinhibition of mitral cells via the dendrodendritic synapses of granule cells on mitral cell secondary dendrites.     

Neuropeptide Y in the olfactory system, forebrain and pituitary of the teleost, Clarias batrachus

Distribution of neuropeptide Y (NPY)-like immunoreactivity in the forebrain of catfish Clarias batrachus was examined with immunocytochemistry. Conspicuous immunoreactivity was seen in the olfactory receptor neurons (ORNs), their projections in the olfactory nerve, fascicles of the olfactory nerve layer in the periphery of bulb and in the medial olfactory tracts as they extend to the telencephalic lobes. Ablation of the olfactory organ resulted in loss of immunoreactivity in the olfactory nerve layer of the bulb and also in the fascicles of the medial olfactory tracts. This evidence suggests that NPY may serve as a neurotransmitter in the ORNs and convey chemosensory information to the olfactory bulb, and also to the telencephalon over the extrabulbar projections. In addition, network of beaded immunoreactive fibers was noticed throughout the olfactory bulb, which did not respond to ablation experiment. These fibers may represent centrifugal innervation of the bulb. Strong immunoreactivity was encountered in some ganglion cells of nervus terminalis. Immunoreactive fibers and terminal fields were widely distributed in the telencephalon. Several neurons of nucleus entopeduncularis were moderately immunoreactive; and a small population of neurons in nucleus preopticus periventricularis was also labeled. Immunoreactive terminal fields were particularly conspicuous in the preoptic, the tuberal areas, and the periventricular zone around the third ventricle and inferior lobes. NPY immunoreactive cells and fibers were detected in all the lobes of the pituitary gland. Present results describing the localization of NPY in the forebrain of C. batrachus are in concurrence with the pattern of the immunoreactivity encountered in other teleosts. However, NPY in olfactory system of C. batrachus is a novel feature that suggests a role for the peptide in processing of chemosensory information.     

Projections of two subclasses of vomeronasal nerve fibers to the accessory olfactory bulb in the rabbit

The organization of the projections of subclasses of vomeronasal nerve fibers to the accessory olfactory bulb was analysed using monoclonal antibodies generated against a homogenate of the rabbit olfactory bulb. Monoclonal antibody R2D5 labels all the somata of vomeronasal receptor cells in the vomeronasal organ as well as all their axons (vomeronasal nerve fibers). Another monoclonal antibody (R4B12), which has been shown to selectively bind and thus identify a subclass of olfactory nerve fibers, also labels a subclass of vomeronasal nerve fibers. The R4B12-positive subclass of vomeronasal nerve fibers project to the glomeruli in the rostrolateral part of the accessory olfactory bulb. The third monoclonal antibody (R5A10) recognizes a complementary subclass of vomeronasal nerve fibers projecting to the glomeruli in the caudomedial part of the accessory bulb. In contrast to the clearly segregated terminations in the accessory bulb, the two subclasses of vomeronasal nerve fibers are intermingled with each other in the vomeronasal nerve bundles. Retrograde labeling of vomeronasal receptor cell somata following injection of horseradish peroxidase within the rostrolateral (R4B12-positive) part of the accessory bulb indicates that vomeronasal receptor cells of this subtype are widely distributed in the vomeronasal sensory epithelium. These results demonstrate the heterogeneity of vomeronasal receptor cells and the specificity of projections arising from subclasses of vomeronasal nerve fibers to the accessory olfactory bulb.     

Spread of the CVS strain of rabies virus and of the avirulent mutant AvO1 along the olfactory pathways of the mouse after intranasal inoculation

After intranasal instillation in the mouse, rabies virus (CVS strain) selectively infected olfactory receptor cells. In the main olfactory bulb (MOB), infection was observed in periglomerular, tufted, and mitral cells and in interneurons located in the internal plexiform layer. Beyond the MOB, CVS spread into the brain along the olfactory pathways. This infection is specific to chains of functionally related neurons but at the death of the animal some nuclei remain uninfected. CVS also penetrated the trigeminal system. The avirulent mutant AvO1, carrying a mutation in position 333 of the glycoprotein, infected the olfactory epithelium and the trigeminal nerve as efficiently as CVS. During the second cycle of infection, the mutant was able to infect efficiently periglomerular cells in the MOB and neurons of the horizontal limb of the diagonal band, which indicates that maturation of infective particles is not affected in primarily infected neuronal cells. On the other hand, other neuronal cells permissive for CVS, such as mitral cells or the anterior olfactory nucleus, are completely free of infection with the mutant, indicating that restriction is related to the ability of AvO1 to penetrate several categories of neurons. From these observations, we concluded that CVS should be able to bind several different receptors to penetrate neurons, while the mutant would be unable to recognize some of them.     

Patterns of expression of the immediate-early gene egr-1 in the accessory olfactory bulb of female mice exposed to pheromonal constituents of male urine

Male mice excrete large quantities of major urinary proteins that have been proposed to have an important pheromonal role either alone or by way of their bound ligands. We have found that these major urinary proteins are not only likely to mediate the pregnancy blocking effects of male urine, but that they also convey the strain recognition signal of the male pheromone. Recent molecular biological investigations have characterized two classes of pheromonal receptor in the vomeronasal organ that appear to project separately to anterior and posterior regions of the accessory olfactory bulb. However, it is not known whether these separate pathways handle fundamentally different types of pheromonal information. We have attempted to investigate this question using the expression of the immediate-early gene egr-1 as a marker for activity of neurons in the accessory olfactory bulb of female mice in response to putative pheromonal constituents. Exposure to 2,3 dihydro-exo-brevicomin and 2-sec-butyl-4,5-dihydro-thiazole, the main ligands bound to the major urinary proteins, elicited expression of egr-1 in clusters of presumed mitral neurons at the medial and lateral margins of the posterior accessory olfactory bulb. Whole male urine and a preparation of major urinary proteins that had been stripped of their ligands induced egr-1 expression in mitral cells of the anterior half of the accessory olfactory bulb in addition to the posterior clusters.

This would suggest that the anterior and posterior halves of the accessory olfactory bulb are processing different aspects of the male pheromone signal with the anterior region, which responds preferentially to major urinary proteins, being principally concerned with the strain recognition component.     

The bilateral bulbar projections of the primary olfactory neurons in the frog

Summary Whether or not the frog olfactory neuroreceptor cells project bilaterally to the olfactory bulb is still a debated question. We therefore decided to ascertain whether bilateral projections of the primary olfactory input exist and if so to investigate their extent. Reproducible extracellular bilateral bulbar potentials were recorded in the frog following electrical stimulation of dorsal or ventral olfactory nerve bundles. The general features of the contralateral evoked responses were very similar to those of the ipsilateral response. The contralateral response disappeared after transection of the rostral part of the olfactory interbulbar adhesion but not following transection of the habenular or anterior commissures. Horseradish peroxidase labelling showed that the fiber terminations of the olfactory nerve bundle was not restricted to the ipsilateral olfactory bulb but included the medial aspects of the contralateral bulb. The intertelencephalic sections increased the magnitude of the ipsilateral evoked responses. Olfactory bulb isopotential maps revealed a rough topographical correspondence between the olfactory neuroepithelium and bulb along the medio-lateral axis as well as along the dorso-ventral axis. In addition, a projection of the medial and central part of the olfactory sac to the medial part of the contralateral olfactory bulb through the interbulbar adhesion was confirmed. These findings suggest first, that the fibers from the neuro-receptors located in either the ventral or the dorsal olfactory mucosae project to both olfactory bulbs, and second, that the left and right bulbs exert a constant inhibition on each other via the habenular commissure.Abbreviations AON anterior olfactory nucleus - ax olfactory neuroreceptor axon - BA bulbar adhesion - D_I latero-dorsal olfactory nerve bundle - D_II centro-dorsal olfactory nerve bundle - D_III mediodorsal olfactory nerve bundle - EPL external plexiform layer - GL glomerular layer - gl glomerulus - GRL granular cell layer - MOB main olfactory bulb - m mitral cell - MBL mitral cell body layer - ON olfactory nerve - V lateral ventricule - V_I latero-ventral ol-factory nerve bundle - V_II centro-ventral olfactory nerve bundle - V_III medio-ventral olfactory nerve bundle - VN vomero-nasal nerve     

Long-distance fiber outgrowth from the heterotopically transplanted olfactory bulb in the rat

An embryonic olfactory bulb was heterotopically inserted and allowed to mature in young adult rat brains. The projection of the transplanted olfactory bulb to the host brain was examined by injections of peroxidase-labeled wheatgerm agglutinin into the host olfactory bulb (and anterior olfactory nucleus). Neurite elongation to the host olfactory area occurred most frequently from the transplant which had been inserted into the anterior horn of the lateral ventricle and fused medially with the lateral septum in host brains with no detectable damage of host olfactory connections. Transplants in the septum, olfactory tubercle, nucleus of the horizontal limb of the diagonal band, or anterior piriform cortex also showed the projection to the host olfactory area. These results indicate that the transplanted olfactory bulb projection neurons have potent abilities to detect the target and project to it even if there is a considerable distance (2-5 mm).     

The olfactory marker protein in the olfactory system of the mouse during development

The olfactory marker protein, a protein specific to the olfactory sensory neurons, has been studied in mouse during embryogenesis and in the postnatal period up to 30 days, with the unlabeled antibody enzyme method of immunohistochemistry. Olfactory neurons, which are morphologically detectable in 10-day-old embryos, do not contain olfactory marker protein. The protein is present in the olfactory neuroepithelium at embryonic day 14 and its appearance coincides with the establishment of sensory synapses in the olfactory bulb. Neurons containing the protein increase in number up to 30 days after birth. At 15 days of embryonic life, immunostaining was observed in sensory axons at the rostral tip of the olfactory bulb, and by embryonic day 17 a plexus of stained fibres has covered the bulbar surface. Between embryonic day 15 and postnatal day 1, olfactory axons have been observed to reach the mitral cell layer. In the vomeronasal system the olfactory marker protein is present at later stages and both the receptors' perikarya and their axons and axon terminals in the accessory olfactory bulb show a lower level of staining than the olfactory system proper.This study of the olfactory marker protein has allowed us to correlate its appearance with significant developmental phenomena.