Castration decreases olfactory bulb norepinephrine in male rats but not hamsters

Norepinephrine (NE) concentrations were measured in the olfactory bulbs and olfactory cortex of male albino rats and golden hamsters sacrificed 13–16 days after castration or sham surgery. Castration significantly decreased NE concentrations in the olfactory bulbs of rats but not hamsters. Castration had no significant influence on NE levels in the olfactory cortex of either species. Regional brain weights were not influenced by surgery, and previous olfactory exposure had no significant effect on any variable measured. The decrease in rat olfactory bulb NE levels may be involved in castration-induced changes in preferences for conspecific odors.     

Sniffing shapes the dynamics of olfactory bulb gamma oscillations in awake behaving rats

Mammals actively sample the environment for relevant olfactory objects. This active sampling is revealed by rapid changes in respiratory rate that influence the olfactory input. Yet the role of sniffing in shaping the neural responses to odorants has not been elucidated. In the olfactory bulb (OB), odorant-evoked gamma oscillations reflect the synchronous activity of mitral/tufted cells, a proposed mechanism for odorant representation. Here we examined the effect of sniffing frequency on the odorant-evoked gamma oscillations in the OB. We simultaneously recorded the respiratory rate and the local field potential while rats performed a lick/no-lick olfactory discrimination task with low odorant concentrations. High-frequency sniffing (HFS) augmented the power of gamma oscillations, suggesting an increase in the sensitivity to odorants. By contrast, coupling of the gamma oscillations to the sniff cycle and the amplitude of individual bursts were not modified by the respiratory rate. However, HFS prolonged the overall response to odorants and increased the frequency of the gamma oscillations, indicating that HFS reduces the adaptation to continuous odorant stimulation. Therefore, the increase in gamma power during HFS is the result of more frequent gamma bursts and the extended response to odorants. As odorant discrimination can be performed in a single sniff, a reduction in the adaptation mediated by HFS of novel odorants may facilitate odorant memory formation for subsequent odorant identification. Finally, these results corroborate that olfactory sampling should be integrated to the study of odorant coding in behaving animals.     

The topographical organization of neurons in the rat medial frontal, insular and olfactory cortex projecting to the solitary nucleus, olfactory bulb, periaqueductal gray and superior colliculus

In 19 rats two different retrograde tracers (Fast Blue, Diamidino Yellow, Rhodamine-labeled latex microspheres, or wheat germ agglutinin conjugated with HRP) were injected into the solitary nucleus (NTS) and either the olfactory bulb (OB), periaqueductal gray (PAG) or superior colliculus (SC). The pattern of retrogradely labeled neurons in the medial frontal, insular and olfactory cortices was examined to determine the topographical organization of the cell populations projecting to these subcortical targets and the extent to which they overlapped. In the medial frontal cortex (MFC) SC projections originated most dorsally, while NTS and OB projections originated most ventrally and exhibited slight overlap. PAG projections originated from virtually the entire MFC and overlapped with cells projecting to the OB, NTS and SC. These results are consistent with the role of dorsal MFC as the rat's frontal eye field and the ventral MFC as a visceral motor area. Laterally, in the insular cortex there was virtually complete overlap between cells projecting to the NTS and PAG. The extensive overlap of PAG projections with NTS projections medially and laterally and with SC projections medially suggests the PAG is involved in a variety of brain visceral and somatic functions. In the piriform cortex there was overlap between cells projecting to the OB and cells projecting to the SC; the cells projecting to the SC were located in the endopiriform nucleus, and may provide a substrate for orienting responses to odors.     

The laminar distribution of intracortical fibers originating in the olfactory cortex of the rat

In this study, the autoradiographic method for tracing axonal connections was used to identify the laminar distribution of intracortical fibers originating in the olfactory cortical areas of the rat. Most of the projections can be divided into two major fiber systems with different laminar patterns of termination. The first of these, termed the layer Ib fiber system, arises in the anterior olfactory nucleus, the anterior and posterior piriform cortex, and the lateral entorhinal cortex, and terminates predominantly in layer Ib and, in many cases, layer III of the entire olfactory cortex. The second system, termed the layer II-deep Ib fiber system, originates in three relatively small olfactory cortical areas-the dorsal peduncular cortex, the ventral tenia tecta, and the periamygdaloid cortex and terminates in and around the cells of layer II in most parts of the olfactory cortex. There is significant overlap in the laminar distribution of the two systems, although the distinction between them is readily apparent. Within the layer Ib fiber system there are relatively slight but consistent differences in the lamination of fibers from different areas. The fibers from the anterior olfactory nucleus are concentrated in the deep part of layer Ib while those from the anterior piriform cortex are concentrated in the superficial part of this layer. The fibers from the posterior piriform cortex tend to be densest in the middle of layer Ib. These differences are maintained in all areas of termination of each set of fibers, both ipsilaterally and contra-laterally. In addition, intracortical fibers from the anterior cortical nucleus of the amygdala are distributed throughout layer I, including layer la and Ib. Fibers from the nucleus of the lateral olfactory tract terminate bilaterally around the cells of the islands of Callej a and the medial edge of the anterior piriform cortex.     

The development of axonal connections in the central olfactory system of rats

The development of the cytoarchitecture and axonal connections of the central olfactory system were studied in fetal and neonatal rats from E16. In contrast to neocortical development, the olfactory cortex lacks a distinct cortical plate. In the piriform cortex and the olfactory tubercle the cellular laminae emerge simultaneously, while in the anterior olfactory nucleus, there are morphogenetic gradients from superficial to deep as well as from caudal to rostral which parallel the known cytogenetic gradients. Parallel morphogenetic and cytogenetic gradients are also present in the lateral to medial axis of the olfactory tubercle. The projection from the olfactory bulb and the associational projections from the piriform cortex begin to develop well before birth. At E17 fibers from the bulb are limited to the lateral olfactory tract (LOT) and the molecular layer just deep to it, and then spread out caudally, laterally, and medially away from the LOT. This sequence of innervation parallels and predicts the density of innervation in the adult: those areas which are innervated first (such as the piriform cortex deep to the LOT) ultimately receive the heaviest innervation; conversely, those areas which are innervated very late (such as the medial olfactory tubercle) receive the lightest projection. The intracortical projections from the anterior and posterior piriform cortex extend into layer I ipsilaterally by E20 and obtain their adult distribution by the middle of the first postnatal week. On the other hand, fibers from the anterior olfactory nucleus and the entorhinal area do not reach their full adult extent until the second postnatal week. Similarly, the crossed projection of the anterior piriform cortex to the contralateral posterior piriform cortex does not grow into layer I until this later time. The timing of fiber ingrowth showed no relation to the trajectory or eventual areal or laminar termination of fibers. As with the olfactory bulb projection, the timing may influence the density of termination. Centrifugal fibers to the bulb are demonstrable around the time of birth both by the retrograde transport of horseradish peroxidase (HRP) and by the anterograde transport of 3H-leucine. The arrival of additional fibers during the remainder of the first postnatal week parallels the known cytogenetic and morphogenetic gradients in the areas in which they arise. The projections of the olfactory cortex to the lateral hypothalamic area and the mediodorsal thalamic nucleus are evident before birth. This correlates with the early generation of the cells which give rise to these projections.     

Localization of corticotropin-releasing factor-like immunoreactivity in monkey olfactory bulb and secondary olfactory areas.

Electrophysiological and anatomical observations suggest that terminals of olfactory bulb mitral cells ending in rat primary olfactory cortex exert certain postsynaptic effects via an excitatory amino acid neurotransmitter. Recent anatomical studies have shown that several peptides, most notably corticotropin-releasing factor (CRF) (Imaki et al., '89) Brain Res., 496: 35-44), are also localized within rat olfactory bulb projection neurons, thus raising the possibility that there is a peptide cotransmitter in this system. In contrast to the availability of data for rodents, very little is known about the distribution of peptides and other putative transmitters in the olfactory systems of primate species. In the present study, sections through the olfactory bulb and its target areas were obtained from two monkey species (Saimiri sciureus and Macaca fascicularis) and processed for immunohistochemistry with a well-characterized polyclonal antiserum directed against the human form of CRF. Virtually identical results were obtained in the two species. Within the olfactory bulb, nearly all mitral and many tufted cells contained CRF-like immunoreactivity. CRF-positive fibers were seen within the olfactory tract and olfactory stria, which contain the axons of mitral and tufted cells. Within the anterior olfactory nucleus and layer Ia of the olfactory tubercle and piriform cortex, immunoreactivity was seen within fine processes, as well as in coarse, varicose fibers and isolated puncta. CRF-positive cells were seen within layer III of the olfactory tubercle and piriform cortex. Immunoreactive fibers and varicosities were also seen within olfactory-recipient regions of the amygdala and entorhinal cortex. These observations suggest that CRF may act as a transmitter and/or neuromodulator in primate olfactory system.     

Progressive tauopathy in P301S tau transgenic mice is associated with a functional deficit of the olfactory system

Multiple neurodegenerative disorders with tau pathology are characterised by the loss of memory and cognitive decline that can be associated with other symptoms including olfactory alterations that are often regarded as an early symptom of the diseases. Here, we have investigated whether olfactory dysfunction is present in the P301S human tau transgenic mice and if it is associated to tau pathology. Progressive tauopathy and neurodegeneration were noticeable in the olfactory bulb and piriform cortex at early age in the P301S human tau transgenic mice and olfactory sensitivity for social or non‐social odours was significantly impaired at 3 months of age, when the piriform cortex‐dependent odour‐cross habituation was also disrupted. The olfactory alterations in the P301S tau transgenic mouse line provide an in vivo system where to test the mechanism‐based therapies for the common and yet untreatable tauopathies.     

Selective retrograde transport of tritiated d-aspartate from the olfactory bulb to the anterior olfactory nucleus, pyriform cortex and nucleus of the lateral olfactory tract in the rat

After an injection of [³H]d-aspartate into the olfactory bulb of the rat, retrogradely labeled cells were detected bilaterally in the anterior olfactory nucleus (AON), and ipsilaterally in the pyriform cortex (PC) and nucleus of the lateral olfactory tract (NLOT). These results suggest a certain selective retrograde transport of this amino acid, and are discussed in relation to transmitter candidates in the olfactory bulb.     

The development of lamination of afferent fibers to the olfactory cortex in rats, with additional observations in the adult

The complementary distribution of the fibers from the olfactory bulb and the intracortical associational fibers to layers Ia and Ib, respectively, of the olfactory cortex has been examined in both adult and neonatal rats, using horseradish peroxidase (HRP) and 3H-leucine as double tracers in the same animal. The observations presented here confirm and extend the previous demonstration (Price, '73) that in the adult the two projections are essentially nonoverlapping throughout the olfactory cortex. Indeed, when the distribution of axons from the olfactory bulb (labeled by HRP inserted into a cut in the LOT) is compared on the same section with that of associational fibers (labeled by 3H-leucine injected into the cortex), the overlap between the two projections is limited to a zone only 5-10 micron in width in both the piriform cortex and olfactory tubercle. In contrast, at P1 the two projections overlap throughout layer I, although the bulbar and associational fibers are slightly concentrated superficially and deeply in layer I, respectively. This overlap is especially prominent in the part of the anterior piriform cortex deep to the LOT. During the remainder of the first postnatal week, this overlap resolves and by P7 the segregation of the two sets of afferent fibers is nearly equivalent to that seen in the adult. However, there are several instances in adults where the segregation of these afferents does not develop. First, a relatively small population of aberrant axons derived from the LOT may be traced from layer Ia into layer Ib and then back to layer Ia. Most of these axons are large in diameter and lack the boutonlike varicosities found on smaller axons in layer Ia. They are most prominent in areas where the cortex is highly curved. Second, in layer I of the nucleus of the lateral olfactory tract, bulbar and associational fibers are extensively intermingled. In this case also, the bulbar fibers are large in diameter with only a few boutonlike varicosities. The developmental emergence of afferent segregation and its breakdown in cases where the fibers from the olfactory bulb do not form boutons suggest that an interaction between the two distinct sets of fibers and the dendritic field is responsible for the normal development of this segregation and that this interaction depends on the process of synaptogenesis.     

Spatial distribution of neural activity in the anterior olfactory nucleus evoked by odor and electrical stimulation

Several lines of evidence indicate that complex odorant stimuli are parsed into separate data streams in the glomeruli of the olfactory bulb, yielding a combinatorial "odotopic map." However, this pattern does not appear to be maintained in the piriform cortex, where stimuli appear to be coded in a distributed fashion. The anterior olfactory nucleus (AON) is intermediate and reciprocally interconnected between these two structures, and also provides a route for the interhemispheric transfer of olfactory information. The present study examined potential coding strategies used by the AON. Rats were exposed to either caproic acid, butyric acid, limonene, or purified air and the spatial distribution of Fos-immunolabeled cells was quantified. The two major subregions of the AON exhibited different results. Distinct odor-specific spatial patterns of activity were observed in pars externa, suggesting that it employs a topographic strategy for odor representation similar to the olfactory bulb. A spatially distributed pattern that did not appear to depend on odor identity was observed in pars principalis, suggesting that it employs a distributed representation of odors more similar to that seen in the piriform cortex.     

Rhythm sequence through the olfactory bulb layers during the time window of a respiratory cycle

The mammalian olfactory bulb is characterized by prominent oscillatory activity of its local field potentials. Breathing imposes the most important rhythm. Other rhythms have been described in the beta- and gamma-frequency ranges. We recorded unitary activities in different bulbar layers simultaneously with local field potentials in order to examine the different relationships existing between (i) breathing and field potential oscillations, and (ii) breathing and spiking activity of different cell types. We show that, whatever the layer, odour-induced gamma oscillations always occur around the transition point between inhalation and exhalation while beta oscillations appear during early exhalation and may extend up to the end of inhalation. By contrast, unitary activities exhibit different characteristics according to the layer. They vary in (i) their temporal relationship with respect to the respiratory cycle; (ii) their spike rates; (iii) their temporal patterns defined according to the respiratory cycle. The time window of a respiratory cycle might thus be split into three main epochs based on the deceleration of field potential rhythms (from gamma to beta oscillations) and a simultaneous gradient of spike discharge frequencies ranging from 180 to 30 Hz. We discuss the possibility that each rhythm could serve different functions as priming, gating or tuning for the bulbar network.     

Vagus nerve stimulation modifies the electrical activity of the olfactory bulb

Evoked potential and unit activity recording techniques were used to study the effects of the vagus nerve stimulation on the olfactory bulb. A biphasic potential was evoked in the olfactory bulb by a single pulse delivered to the vagus nerve. Half of the neurons studied decreased discharge frequency after single pulse or train stimulation. The interval during which neurons ceased activity corresponded to the duration of the negative wave of the evoked potential. Responsive neurons were marked with horseradish peroxidase applied iontophoretically. Responsive neurons were located in the periglomerular layer of the olfactory bulb. These results suggest the existence of a vagus nerve-olfactory bulb pathway. The functional significance of this pathway is discussed.     

Labelling of olfactory bulb glomeruli following horseradish peroxidase lavage of the nasal cavity

The transport of horseradish peroxidase (HRP) from the nasal cavity to the olfactory bulb was examined in rat. HRP was present primarily in the olfactory nerve and glomerular layer. In some animals the glomeruli were densely filled with product while in others there was considerable interglomerular variation in density. Examination of the decalcified noses revealed a restricted distribution of HRP in those rats with partially labelled olfactory bulbs. The presence of small groups of densely labelled glomeruli was also noted using the 2-deoxyglucose method to examine odor-induced metabolic activity.     

Surprisingly rich projection from locus coeruleus to the olfactory bulb in the rat

The brainstem nucleus, locus coeruleus (LC), is the major, if not the sole, source of noradrenergic (NE) innervation of the telencephalon. It is generally held that LC neurons project diffusely to the entire neuroaxis and this had been the basis for theories that postulate 'general' functions (sleep, attention, learning, etc.) for LC. We report that at least 40% of all LC neurons project to the olfactory bulb; the projection is almost 10 times greater than to any other part of the cerebral cortex. This unsuspectedly rich LC-olfactory bulb connection is consistent with current theories which implicate LC neurons in heightened sensory vigilance and trophic regulation of connectional development and plasticity.     

Cholinergic modulation in the olfactory bulb influences spontaneous olfactory discrimination in adult rats

Cholinergic neuromodulation in the olfactory bulb has been hypothesized to regulate mitral cell molecular receptive ranges and the behavioral discrimination of similar odorants. We tested the effects of cholinergic modulation in the olfactory bulb of cannulated rats by bilaterally infusing cholinergic agents into the olfactory bulbs and measuring the rats' performances on separate spontaneous and motivated odor-discrimination tasks. Specifically, 6 microL/bulb infusions of vehicle (0.9% saline), the muscarinic antagonist scopolamine (7.6 mM and 38 mM), the nicotinic antagonist mecamylamine hydrochloride (3.8 mM and 19 mM), a combination of both antagonists, or the acetylcholinesterase inhibitor neostigmine (8.7 mM) were made 20 min prior to testing on an olfactory cross-habituation task or a rewarded, forced-choice odor-discrimination task. Spontaneous discrimination between chemically related odorants was abolished when nicotinic receptors were blocked in the olfactory bulb, and enhanced when the efficacy of cholinergic inputs was increased with neostigmine. Blocking muscarinic receptors reduced but did not abolish odor discrimination. Interestingly, no behavioral effects of modulating either nicotinic or muscarinic receptors were observed when rats were trained on a reward-motivated odor-discrimination task. Computational modeling of glomerular circuitry demonstrates that known nicotinic cholinergic effects on bulbar neurons suffice to explain these results.     

Topographical and laminar localization of 2-deoxyglucose uptake in rat olfactory bulb induced by electrical stimulation of olfactory nerves

Experiments were carried out to examine the topographical projection of the olfactory nerves to the olfactory bulb in the rat, using the Sokoloff [14C]2-deoxyglucose (2-DG) technique. Electrical stimulation of a medially located bundle of olfactory nerves produced a discrete zone of 2-DG uptake at the rostral pole of the bulb. Increasing stimulus strength yielded a slightly larger focus at this site. In contrast, electrical stimulation of laterally situated bundles of olfactory nerves resulted in a broad zone of activity extending along the lateral wall of the bulb, and increasing stimulus intensity produced a more extensive area of uptake. Laminar analyses provided information on the relation between activity in the glomerular layer, where the olfactory nerves terminate, and activity in deeper layers. The results support previous studies of the topographical projections of the olfactory nerves to the olfactory bulb. They also support the hypothesis that odor-induced 2-DG uptake in the olfactory bulb represents activation of groups of receptors in the olfactory epithelium whose axons terminate in activated glomerular regions in the olfactory bulb.     

Adjacent laminar terminations of two centrifugal afferent pathways to the accessory olfactory bulb in the mouse

Anterograde and retrograde axonal tracing methods have been combined with transection of the stria terminalis to investigate the centrifugal afferent connections of the accessory olfactory bulb in the mouse.Injection of tritiated proline into the postero-medial cortical amygdaloid nucleus (C₃) gives rise to anterograde autoradiographic labelling of a pathway terminating in the internal granular layer of the accessory olfactory bulb (AOB). Transection of the ipsilateral stria terminalis completely abolishes labelling of this pathway. Injections further rostral, in the bed nucleus of the accessory olfactory tract (bnAOT) and medial amygdaloid nucleus (M), give rise to labelling of a second ipsilateral afferent pathway to the AOB which terminates in the internal plexiform layer (IPL) and is unaffected by strial transection.Injections of wheat germ lectin-HRP conjugate into the AOB confirm that it receives afferents from the ipsilateral bnAOT, M and C₃, and from a few cells in the contralateral C₃. Transection of the ipsilateral stria terminalis prevents retrograde labelling of any cells in the ipsilateral C₃, but does not affect labelling of cells in M or bnAOT (or contralateral C₃). The conjugate is also transported anterogradely in this system, labelling the efferent projections of the AOB to bnAOT, M and C₃.It is concluded that the AOB receives at least two sets of ipsilateral afferents: one set from C₃, via the stria terminalis, terminating in the internal granular layer, and a second set from M and/or bnAOT terminating in the IPL and probably running in the accessory olfactory tract.     

Fasting induces astroglial plasticity in the olfactory bulb glomeruli of rats

The detection of food odors by the olfactory system, which plays a key role in regulating food intake and elaborating the hedonic value of food, is reciprocally influenced by the metabolic state. Fasting increases olfactory performance, notably by increasing the activity of olfactory bulb (OB) neurons. The glutamatergic synapses between olfactory sensory neurons and mitral cells in the OB glomeruli are regulated by astrocytes, periglomerular neurons, and centrifugal afferents. We compared the expansion of astroglial processes by quantifying GFAP‐labeled areas in fed and fasted rats to see whether OB glomerular astrocytes are involved in the metabolic sensing and adaptation of the olfactory system. Glomerular astroglial spreading was much greater in all OB regions of rats fasted for 17 hr than in controls. Intra‐peritoneal administration of the anorexigenic peptide PYY_3‐36 or glucose in 17 hr‐fasted rats respectively decreased their food intake or restored their glycemia, and reversed the fasting‐induced astroglial spreading. Direct application of the orexigenic peptides ghrelin or NPY to OB slices increased astroglial spreading, whereas PYY_3‐36 resulted in astroglial retraction, in agreement with the in vivo effects of fasting and satiety on glomerular astrocytes. Thus the morphological plasticity of OB glomerular astrocytes depends on the metabolic state of the rats and is influenced by peptides that regulate food intake. This plasticity may be part of the mechanism by which the olfactory system adapts to food intake.     

Potentiation of late components in olfactory bulb and piriform cortex requires activation of cortical association fibers

Previous research has demonstrated that repeated high-frequency stimulation of the granule cell layer of the olfactory bulb (OB) produces an enduring potentiation of late components (PLC) in potentials evoked in the OB and piriform cortex (PC), while leaving the monosynaptic EPSP produced by OB mitral cells in PC pyramidal cells unaltered. Two experiments were conducted using male Long-Evans rats with chronically implanted electrodes to assess the relative contribution to this potentiation of the two main fiber systems that interconnect the OB and PC: the lateral olfactory tract (LOT), which contains mitral cell axons that synapse on PC pyramidal cells, and the PC association fiber system, which consists of the axons of PC pyramidal cells that synapse on several cell populations within the PC and on granule cells in the OB. The results indicate that stimulation of PC association fibers is both necessary and sufficient to duplicate the pattern of potentiation seen following OB stimulation in previous experiments. LOT stimulation had no consistent effect, and coactivation of the LOT and PC association fibers was no more effective than activation of PC association fibers alone. Possible mechanisms underlying this effect are discussed, including (1) long-term potentiation (LTP) at synapses made by the axons of PC pyramidal cells on neurons in the OB and PC; and (2) repetitive firing in PC pyramidal cells due to regenerative excitation in a population of deep cells in the PC and endopiriform nucleus.