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.     

Afferents to the rostral olfactory bulb in sheep with special emphasis on the cholinergic, noradrenergic and serotonergic connections

The olfactory bulb (OB) is involved in the processing of olfactory information particularly through the activation of its afferents. To localize their cell origin in sheep, a specific retrograde fluorescent tracer, Fluoro-Gold, was injected into the olfactory bulb of seven ewes. By using immunocytochemical techniques, retrogradely labeled neurons were colocalized with choline acetyltransferase, tyrosine hydroxylase, dopamine-β-hydroxylase and serotonin to characterize cholinergic, noradrenergic and serotonergic Fluoro-Gold-labeled neurons. Most afferents originated from the ipsilateral side of the injection site. The OB received major inputs from the anterior olfactory nucleus (AON), the piriform cortex (PC), the olfactory tubercle, the diagonal band of Broca (DBB) and the amygdala. Other retrogradely labeled neurons were observed in the taenia tecta, the septum, the nucleus of the lateral olfactory tract, the preoptic area, the lateral hypothalamic area, the mediobasal hypothalamus, the lateral part of the premammillary nucleus, the paraventricular nucleus of the hypothalamus, the paraventricular thalamic nucleus, the central grey, the substantia nigra (SN), the ventral tegmental area (VTA), the lateral nucleus to the interpeduncular nucleus (lIP), the raphe and the locus coeruleus (LC). Contralateral labeling was also found in the AON, the PC, the SN compacta, the VTA, the lIP and the LC. Cholinergic Fluoro-Gold-labeled neurons belonged to the horizontal and vertical branch of the DBB. Noradrenergic afferents came from the LC and serotoninergic afferents came from the medial raphe nuclei and the lIP. These data are discussed in relation with olfactory learning in the context of maternal behavior in sheep.     

Cortical and quasi-cortical regions innervate ventrostriopallidal structures in the rat: An electrophysiological analysis

Antidromic unit driving was utilized to demonstrate afferent projections from prefrontal cortical (PFC) and quasi-cortical structures (main olfactory bulb, MOB; anterior olfactory nucleus, AON; basolateral amygdaloid nucleus, BLA) to the ventrostriopallidal region (VSPR) of the rat. In all regions explored, a substantial number of antidromically invaded neurons were found following electrical stimulation of the VSPR. In addition, both the AON and the amygdalostriatal zone harbor cells with branched axons which innervate the MOB and the VSPR and the agranular insular cortex and the VSPR, respectively. These results support and extend previous neuroanatomical and neurophysiological data on afferent connections of the VSPR and emphasize the fact that several regions of the basal forebrain, which are actively involved in processing of olfactory information, and the VSPR, are more closely interrelated than hitherto suspected.     

Somatostatin-14-like immunoreactive neurons and fibres in the human olfactory bulb

Summary This study describes the morphological features and the distribution pattern of neurons in the human olfactory bulb which are immunoreactive for an antiserum against the neuropeptide somatostatin-14.Immunoreactive nerve cell bodies were mainly found in the white matter surrounding the cell clusters of the anterior olfactory nucleus. Some immunoreactive neurons were also found scattered throughout the anterior olfactory nucleus and the deeper parts of the inner granule cell layer. Only a few immunoreactive neurons were localized in the glomerular layer and the outer granule cell layer.Immunoreactive fibres were found in all layers of the olfactory bulb. In addition, an impressive number of coiled and kinked immunoreactive fibres were localized within the anterior olfactory nucleus forming a dense plexus. Accumulations of twisted and coiled branches of immunoreactive fibres were rarely found either surrounding or within the olfactory glomerula.The characteristics of somatostatin-14 immunoreactive neurons as seen in the combined pigment-Nissl preparation were studied after decolourizing the chromogen and restaining the preparations with aldehydefuchsin in order to demonstrate the lipofuscin pigment and gallocyanin chrome alum for Nissl material. About 90% of the immunoreactive neurons studied in this manner turned out to be devoid of lipofuscin granules. The remaining 10% displayed different patterns of pigmentation.These findings suggest the presence of different types of somatostatin-14-like immunoreactive neurons in the olfactory bulb of the human adult.     

Calbindin-D-28k-like immunoreactive structures in the olfactory bulb and anterior olfactory nucleus of the human adult: distribution and cell typology--partial complementarity with parvalbumin

Calbindin-D-28k and parvalbumin are calcium-binding proteins. The laminar distribution and morphological features of calbindin-D-28k-like immunoreactive structures were studied in 60-microns-thick sections of the human olfactory bulb. Except for the olfactory nerve layer, immunoreactive neurons were present in all layers of the olfactory bulb. They reached highest densities in the external plexiform layer and internal granule cell layer. Considerable numbers of calbindin-like nerve cells were also found in the olfactory tract and in distal portions of the anterior olfactory nucleus. When comparing the distribution of calbindin-positive structures to that of parvalbumin-positive ones a partially complementary distribution pattern was found. Calbindin-like immunoreactive portions of the anterior olfactory nucleus and olfactory tract were mirrored by immunonegative areas in adjacent sections stained for parvalbumin. Using the combined pigment-Nissl procedure we observed the presence of lipofuscin deposits in nearly 80% of all the calbindin-immunoreactive neurons analysed. Moreover, analysis of their lipofuscin deposits rendered the further differentiation of morphologically similar neuronal subpopulations possible. In contrast, all parvalbumin-like immunoreactive neurons remained free of lipofuscin granules.     

Age-related change of neuropeptide Y-immunoreactive neurons in the rat anterior olfactory nucleus

Neuropeptide Y (NPY) is located in the olfactory system, including the olfactory bulb, and is thought to be one of the main neurotransmitters for olfaction. Thus, we examined age-related changes of NPY-immunoreactive (IR) neurons in the rat anterior olfactory nucleus (AON) at various aging stages over a period of 2 years; postnatal months 1 (PM 1), PM 6, PM 12 and PM 24. NPY-IR neurons in the AON were present in the lateral and medial subdivisions at PM 1 and at PM 6 were distributed in all subdivisions of the AON. Prior to PM 12, the NPY-IR neurons showed a tendency to change from bipolar cells with short processes into multipolar cells with long processes. Moreover, the population of NPY-IR neurons and nerve fibers in the AON increased in proportion to age. In particular, the number of NPY-IR neurons increased about 6-fold between PM 1 and PM 3. At PM 24, the number of NPY-IR neurons was much smaller than that at PM 12 and somal size had decreased. It is therefore suggested that the dramatic increase in the number and size of the NPY-IR neurons between PM 1 and PM 3 may be associated with sexual maturation and that the decrease in the number and cell size of the NPY-IR neurons at PM 24 may underlie age-related changes in the olfactory process.     

NADPH-diaphorase-positive neurons and pathways in the brain of the frog Rana esculenta

We described the NADPH-diaphorase-containing neurons and fibres in the brain of the frog Rana esculenta. In the telencephalon stained cells occurred in the olfactory bulb, all subdivisions of the pallium, the diagonal band, the medial septum and the striatum. The olfactory glomeruli showed the most intense enzyme reaction. The neuropil of the accessory olfactory bulb was also heavily stained and this staining extended to the rostral diencephalon through the ventral lateral pallium. Fibre staining was less intense in the medial pallium and the medial septum. In the diencephalon, NADPH-diaphorase staining was concentrated in the middle third of this part of the brain. The stained cells were embedded in a dense network of thin, stained fibres and terminals in the lateral anterior and central thalamic nuclei. Faintly stained cells were present also in the posterior preoptic nucleus, anterior thalamic nucleus, nucleus of Bellonci, corpus geniculatum thalamicum and the suprachismatic nucleus. In the mesencephalon, heavily stained cells occurred in the nucleus profundus mesencephali, anterodorsal, anteroventral and especially in the posterodorsal tegmental nuclei. Neuronal staining was less intense in the optic tectum and the torus semicircularis. Thick, intensely stained fibres occupied the lateral part of the tegmentum and the 7th layer of the tectum. A loose network of thin fibres occupied the periventricular area and all tegmental nuclei. In the rhombencephalon, the reticular nuclei and the inferior raphe nucleus showed the most intense staining, while some cells in the nucleus of the solitary tract and the dorsal column nuclei were less intensely stained. Heavy staining of fibres was characteristic of the spinal trigeminal tract, the solitary tract and the reticulospinal pathway.     

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.     

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.     

Membrane and synaptic properties of pyramidal neurons in the anterior olfactory nucleus

The anterior olfactory nucleus (AON) is positioned to coordinate activity between the piriform cortex and olfactory bulbs, yet the physiology of AON principal neurons has been little explored. Here, we examined the membrane properties and excitatory synapses of AON principal neurons in brain slices of PND22-28 mice and compared their properties to principal cells in other olfactory cortical areas. AON principal neurons had firing rates, spike rate adaptation, spike widths, and I-V relationships that were generally similar to pyramidal neurons in piriform cortex, and typical of cerebral cortex, consistent with a role for AON in cortical processing. Principal neurons in AON had more hyperpolarized action potential thresholds, smaller afterhyperpolarizations, and tended to fire doublets of action potentials on depolarization compared with ventral anterior piriform cortex and the adjacent epileptogenic region preendopiriform nucleus (pEN). Thus, AON pyramidal neurons have enhanced membrane excitability compared with surrounding subregions. Interestingly, principal neurons in pEN were the least excitable, as measured by a larger input conductance, lower firing rates, and more inward rectification. Afferent and recurrent excitatory synapses onto AON pyramidal neurons had small amplitudes, paired pulse facilitation at afferent synapses, and GABA(B) modulation at recurrent synapses, a pattern similar to piriform cortex. The enhanced membrane excitability and recurrent synaptic excitation within the AON, together with its widespread outputs, suggest that the AON can boost and distribute activity in feedforward and feedback circuits throughout the olfactory system.     

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.     

Immunohistochemical changes in the anterior olfactory nucleus of the developing rat

The anterior olfactory nucleus (AON) is located caudal to the olfactory bulb in the olfactory peduncle. Although this important structure is involved in the bilateral coordination of olfactory information, relatively little is known about its development, structure, or function. The present report details results from an immunohistochemical examination of specific neuronal (microtubule-associated protein-2: MAP2, calbindin D28-k, neuropeptide-Y: NPY) and glial (astrocytes; glial fibrillary-associated protein, or GFAP, oligodendrocytes; RIP) populations in postnatal Days 10, 20, and 30 rats. MAP2-immunoreactivity (-ir), was present throughout the AON, although most dense in the outer plexiform layer. Increases in labeling occurred from Day 10 to Day 30, reflecting the maturation of dendritic processes. Both temporal and regional differences in expression were found with the two neuronal markers. For example, although substantial numbers of calbindin-ir cells were observed as early as Day 10, relatively few cells exhibited NPY-ir. An apparent decline in the number of stained figures was observed from Days 20–30 with both markers. Most cells exhibiting calbindin- or NPY-ir were found in the inner half of the cellular zone of the AON. GFAP-ir was localized mainly to the subependymal zone and the lateral olfactory tract (LOT) at Day 10, with successive increases in staining in the cellular and plexiform layers at Days 20 and 30. Oligodendrocyte-ir was restricted to the anterior commissure and the LOT at Day 10, with dramatic increases in labeling of the cellular and plexiform layers observed by Days 20 and 30. These results represent some of the first analyses of the maturation of specific cellular phenotypes within this large neural region. © 1997 John Wiley & Sons, Inc. Dev Psychobiol 31: 181–192, 1997     

NADPH-diaphorase-positive neurons and pathways in the brain of the frog Rana esculenta

We described the NADPH-diaphorase-containing neurons and fibres in the brain of the frog Rana esculenta. In the telencephalon stained cells occurred in the olfactory bulb, all subdivisions of the pallium, the diagonal band, the medial septum and the striatum. The olfactory glomeruli showed the most intense enzyme reaction. The neuropil of the accessory olfactory bulb was also heavily stained and this staining extended to the rostral diencephalon through the ventral lateral pallium. Fibre staining was less intense in the medial pallium and the medial septum. In the diencephalon, NADPH-diaphorase staining was concentrated in the middle third of this part of the brain. The stained cells were embedded in a dense network of thin, stained fibres and terminals in the lateral anterior and central thalamic nuclei. Faintly stained cells were present also in the posterior preoptic nucleus, anterior thalamic nucleus, nucleus of Bellonci, corpus geniculatum thalamicum and the suprachismatic nucleus. In the mesencephalon, heavily stained cells occurred in the nucleus profundus mesencephali, anterodorsal, anteroventral and especially in the posterodorsal tegmental nuclei. Neuronal staining was less intense in the optic tectum and the torus semicircularis. Thick, intensely stained fibres occupied the lateral part of the tegmentum and the 7th layer of the tectum. A loose network of thin fibres occupied the periventricular area and all tegmental nuclei. In the rhombencephalon, the reticular nuclei and the inferior raphe nucleus showed the most intense staining, while some cells in the nucleus of the solitary tract and the dorsal column nuclei were less intensely stained. Heavy staining of fibres was characteristic of the spinal trigeminal tract, the solitary tract and the reticulospinal pathway.     

猫蓝斑核向嗅球、嗅前核的投射——HRP逆轴突传递法研究

本研究用HRP逆轴突传递法观察了28只猫蓝斑核(LC)向嗅球(OB)、嗅前核(AON)的投射,结果如下: 1.于OB注射例中,LC内出现少到中等量的标记细胞,其分布状况在各例大致相同:全长分布,中间居多,两端渐少;各亚核的细胞数为蓝班主核多于蓝班α核外侧部,后者又多于其内侧部,即pLC>ILCa>mLCa。细胞以中型为主,少量小及大型细胞,多为卵圆、三角和梭形。同侧LC中的标记细胞明显多于对侧。 2.于AON注射例中,LC内出现不等量的标记细胞,其分布特点、细胞大小及形态与OB例基本相同,LC中出现的标记细胞数平均多于OB例。 3.注HRP于OB和AON例中,LC中出现的标记细胞显著多于OB或AON注射例。     

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).     

Transneuronal transport of peroxidase-conjugated wheat germ agglutinin (WGA-HRP) from the olfactory epithelium to the brain of the adult rat

Summary The sensory neurons of the olfactory epithelium, as a consequence of their odor detection function, contact both the external environment and the central nervous system. The possibility that substances applied to the epithelium might reach the central nervous system was investigated by the intranasal application of peroxidase-conjugated wheat germ agglutinin (WGA-HRP). WGA-HRP was transported through olfactory receptor axons to the glomerulus of the olfactory bulb. Reaction product was localized electron microscopically to tubulovesicular profiles and dense bodies in sensory axons. Evidence of transneuronal transport was indicated by reaction product localized in dense bodies in dendrites postsynaptic to receptor cell axons. Periglomerular, tufted and mitral cells in the olfactory bulb also were transneuronally labeled. Anterograde transneuronal labeling occured in the olfactory tubercle, piriform cortex and surrounding the lateral olfactory tract. Retrograde transneuronal label was found in neurons of the basal forebrain with the largest number of perikarya in the lateral nucleus of the horizontal limb of the diagonal band, a major source of cholinergic afferents to the olfactory bulb. These data suggest that substances, specifically those which bind to receptors, are transported from the olfactory receptor neurons in the nasal epithelium to the brain. Thus, the olfactory system may provide a route of entry for exogenous substances to the basal forebrain.Abbreviations AC anterior commissure - CC corpus callosum - CI internal capsule - CP caudate putamen - DBB diagonal band of Broca - FX fornix - GP globus pallidus - IC island of Callelae - LV lateral ventricle - MS medial septum - OC optic chiasm - PIR piriform cortex - RF rhinal fissure - SON supraoptic nucleus - SCN suprachiasmatic nucleus - SM stria medullaris - ST stria terminalis - TOL lateral olfactory tract - TUO olfactory tubercle - III third ventricle     

Cholecystokinin in rat olfactory bulb is mainly contained in intrinsic neurons

Of all lesions tested, only a knife cut at the border between the olfactory bulb and the anterior olfactory nucleus decreased cholecystokinin-like immunoreactivity in the bulb (by 50%). At the same time, the bulbar concentration of preprocholecystokinin-mRNA was reduced by 60%, indicating that reduced peptide synthesis was the cause of diminished cholecystokinin immunoreactivity. Thus, cholecystokinin immunoreactivity in rat olfactory bulb seems to be mainly contained in intrinsic neurons. The contribution of afferents seems to be minimal. In addition, the lesion induced a transient decrease in the concentration of preprocholecystokinin mRNA in the anterior olfactory nucleus of both sides, exemplifying the intricate connections between olfactory areas of both sides.     

Supraoptic nucleus afferents from the main olfactory bulb--II. Intracellularly recorded responses to lateral olfactory tract stimulation in rat brain slices

To establish the functional nature of the anatomically demonstrated main olfactory bulb inputs to the supraoptic nucleus, electrophysiological responses of intracellularly recorded supraoptic neurons to lateral olfactory tract stimulation were recorded in horizontal slices of basal forebrain and hypothalamus. A total of 71 synaptically influenced neurons were studied in slices from adult rats of both sexes. Of these, 60 cells (84%) were monosynaptically activated by olfactory tract stimulation; seven cells (10%) were activated via polysynaptic pathways; and four cells (6%) were characterized by long latency inhibitory responses. Lucifer Yellow was injected into 64 cells and subsequent immunocytochemical identification of 44 of these neurons showed that both oxytocin and vasopressin cells, in approximately equal numbers, were excited by olfactory stimulation. Polysynaptically mediated excitation, however, was only associated with oxytocin cells (six of the six identified cells). These results corroborate anatomical tract tracing data showing main olfactory bulb efferents to both supraotic neurons and to neurons of the perinuclear zone. Also supported are earlier speculations of olfactory participation in release of oxytocin and vasopressin during various physiological states.     

A direct anterior cingulate pathway to the primate primary olfactory cortex may control attention to olfaction

Behavioral and functional studies in humans suggest that attention plays a key role in activating the primary olfactory cortex through an unknown circuit mechanism. We report that a novel pathway from the anterior cingulate cortex, an area which has a key role in attention, projects directly to the primary olfactory cortex in rhesus monkeys, innervating mostly the anterior olfactory nucleus. Axons from the anterior cingulate cortex formed synapses mostly with spines of putative excitatory pyramidal neurons and with a small proportion of a neurochemical class of inhibitory neurons that are thought to have disinhibitory effect on excitatory neurons. This novel pathway from the anterior cingulate is poised to exert a powerful excitatory effect on the anterior olfactory nucleus, which is a critical hub for odorant processing via extensive bilateral connections with primary olfactory cortices and the olfactory bulb. Acting on the anterior olfactory nucleus, the anterior cingulate may activate the entire primary olfactory cortex to mediate the process of rapid attention to olfactory stimuli.