Reciprocal functional connections of the olfactory bulbs and other olfactory related areas with the prefrontal cortex

Reciprocal putative connections of the prefrontal cortex (PFC) (agranular insular, ventral and lateral orbital region) with the ipsi and contralateral main olfactory bulb (IOB; COB), the mediodorsal thalamic nucleus (MD), the basolateral amygdaloid nucleus (BLA) and the piriform cortex (PC) were investigated with electrophysiological techniques. Evoked field responses and orthodromic unit driving, generated in PFC following electrical stimulation of the above mentioned structures, were abolished following topical application of KCl, except for COB evoked mass potentials. Thus, locally generated activity was elicited in agranular insular cortex following IOB activation, the same region where recently, the taste cortex in the rat was localized. Since gustatory-visceral afferent information reaches insular cortex via 2-3 synaptic relays, autonomic, olfactory and gustatory inputs may interact at this level, and, as suggested previously for the mouse, play a key integrative role in flavor perception. Antidromically invaded neurons, 47% of which were identified by the collision-extinction technique, were also found in PFC areas which overlapped to a considerable extent with those from which orthodromic unit responses were obtained. In particular, closely spaced neurons in ventrolateral orbital (VLO) and lateral orbital (LO) regions were antidromically invaded following IOB and PC shocks; some neurons antidromically discharged by IOB were also transsynaptically activated following PC stimulation. These findings are in agreement with recent neuroanatomical studies which demonstrate axonal projections from PFC neurons to the IOB and COB in the rat and South American armadillo. In addition, stimulation of PFC regions dorsal to the rhinal fissure mostly inhibited spontaneous unit discharges recorded at the mitral cell layer of the IOB, suggesting that this effect may be partially mediated by excitatory inputs of prefrontal axons onto granule cells. The conduction properties, antidromic thresholds and activity-dependent variations in conduction velocity (CV) of bulbopetal neurons in prefrontal cortex were found to be similar to those exhibited by cells projecting to the IOB from olfactory peduncle regions, but not to those present in bulbopetal neurons of the horizontal limb of diagonal band, indicating that the OB may be subjected to centrifugal control by at least two cell groups differing in both histochemical and electrophysiological properties.     

the connections of the mouse olfactory bulb: A study using orthograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase

The efferent and centrifugal afferent connections of the main olfactory bulb (MOB) of the mouse were studied by orthograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). MOB projects ipsilaterally to the anterior olfactory nucleus, taenia tecta, anterior hippocampal continuation, indusium grisium, olfactory tubercle, and the lateral and medial divisions of the entorhinal area. In the region of the anterior one-half to two-thirds of the posterior division of the insular cortex the projection from MOB extends into the insular cortex. The only efferent projection of MOB to the contralateral half of the brain was to the anterior olfactory nucleus. All efferent projections of MOB, thus, are to telencephalic structures. By contrast the centrifugal afferents to MOB originate from every major division of the neuraxis. Neurons projecting to the bulb were found ipsilaterally in all divisions of the anterior olfactory nucleus (AON). In some cases, labeling in the external division of AON was weak or absent. In the contralateral AON, pars externa was the most intensively labeled sub-division. Retrogradely labeled neurons were also present in all other subdivisions of the contralateral AON but were fewer in number and less heavily labeled than in the ipsilateral AON. Ipsilaterally, positive neurons were also present in taenia tecta, and the anterior hippocampal continuation. There was profuse retrograde labeling of neurons in the entire extent of the ipsilateral piriform cortex (PC). There was a rostral to caudal gradient of labeling in PC with more positive neurons in rostral than caudal parts. Labeled neurons were present in the lateral entorhinal cortex LEC and in the transitional cortex between LEC and PC. Very heavy retrograde labeling was present in the nuclei of the horizontal and vertical limbs of the diagonal band (HDB and VDB). More cells were labeled in HDB than in VDB. Neurons were labeled in the ipsilateral nucleus of the lateral olfactory tract (NLOT) and, when the injection spread into the accessory olfactory bulb, labeled neurons were present ventral to NLOT in accessory NLOT. A few lightly labeled neurons were always present in the posterolateral and medial cortical amygdaloid areas. Neurons were labeled in the zona inserta and scattered throughout several hypothalamic nuclei. There was massive retrograde labeling of neurons in the locus coeruleus and neurons were abundantly labeled in the dorsal and medial raphe nuclei and nucleus raphe pontis. In general, the labeling of MOB connections was more extensive than that which has been reported in closely related species.(ABSTRACT TRUNCATED AT 400 WORDS)     

Adenosine deaminase-containing neurons in the olfactory system of the rat during development

The development, distribution and olfactory bulb projections of neurons immunoreactive for the enzyme adenosine deaminase (ADA) were studied in olfactory systems of embryonic, early postnatal and young adult rats. On embryonic day (E) 12, ADA-immunoreactivity first appeared in the placode of the olfactory epithelium. On E15, ADA-immunoreactive olfactory receptor and precursor cells gave rise to immunostained axons projecting to the olfactory bulb. Numerous immunostained glomeruli were observed on postnatal day (P) 1. After P25, immunoreactivity within receptor cells and glomeruli decreased. In prenatal and early postnatal animals, ADA-immunoreactive neurons were observed in the anterior olfactory nucleus (AON), dorsal transition area, ventral taenia tecta, primary olfactory cortex (POC), entorhinal cortex and ventral agranular insular cortex. After P25 to P30, these neurons lost their immunoreactivity, except those in the medial AON where light immunostaining persisted. In contrast, ADA-immunostaining of neurons in the horizontal limb of the diagonal band (HDB) and olfactory tubercle increased throughout development. About 70 to 75% of the ADA-immunoreactive neurons in the AON, a small number of those in the POC and about 75% of the ADA-immunoreactive non-cholinergic neurons in the HDB were found to project to the olfactory bulb. The functions of ADA in the olfactory system may be related to the precocious development of, and/or purinergic neurotransmission within, this system.     

Quantitative determination of collateral anterior olfactory nucleus projections using a fluorescent tracer with an algebraic solution to the problem of double retrograde labeling

The bilateral projections of the rat anterior olfactory nucleus (AON) were evaluated using retrograde fluorescent tracers. Competitive effects of these tracers led to severe underestimation of bilaterally projecting neurons, when double-labeled cells were counted. The underestimate was corrected using a numerical approach, which is of general utility for problems in double labeling and requires only a single tracer. With this method we estimated that approximately 63% of AON neurons project bilaterally to the olfactory bulbs, except for the external part which projects exclusively to the contralateral olfactory bulb. No other AON neurons project only to the contralateral bulb.     

Collateral projections from the rat hippocampal formation to the lateral and medial prefrontal cortex

Projections from the hippocampal formation to the medial prefrontal cortex are well known. In this report we used two retrogradely transported tracers to show that a small but significant subpopulation of pyramidal neurons in area CA1 and subiculum of the hippocampal formation projects to the lateral prefrontal cortex. About half of these neurons also possess collateral projections to the medial prefrontal cortex. The neurons projecting only to the lateral PFC are found in the intermediate hippocampal formation and in the most ventral part of the temporal subdivision. On the other hand, most of the neurons projecting to the medial prefrontal cortex only are present in the temporal and ventral intermediate hippocampal formation, and their number decreases in the dorsal intermediate subdivision. The distribution of neurons having collateral projections is comparable to that of neurons projecting to the medial prefrontal cortex only. In view of proposed functional differences between the septal one-third and the temporal two-third of the hippocampal formation, it is of interest that the neurons projecting to the prefrontal cortex are only present in the temporal two-thirds. Hippocampus 7:397–402, 1997. © 1997 Wiley-Liss, Inc.     

Afferent and efferent connections of the olfactory bulbs in the lizard Podarcis hispanica.

The connections of the olfactory bulbs of Podarcis hispanica were studied by tract-tracing of injected horseradish peroxidase. Restricted injections into the main olfactory bulb (MOB) resulted in bilateral terminallike labeling in the medial part of the anterior olfactory nucleus (AON) and in the rostral septum, lateral cortex, nucleus of the lateral olfactory tract, and ventrolateral amygdaloid nucleus. Bilateral retrograde labeling was found in the rostral lateral cortex and in the medial and dorsolateral AON. Ipsilaterally the dorsal cortex, nucleus of the diagonal band, lateral preoptic area, and dorsolateral amygdala showed labeled cell bodies. Retrogradely labeled cells were also found in the midbrain raphe nucleus. Results from injections into the rostral lateral cortex and lateral olfactory tract indicate that the mitral cells are the origin of the centripetal projections of the MOB. Injections in the accessory olfactory bulb (AOB) produced ipsilateral terminallike labeling of the ventral AON, bed nucleus of the accessory olfactory tract, central and ventromedial amygdaloid nuclei, medial part of the bed nucleus of the stria terminalis, and nucleus sphericus. Retrograde labeling of neurons was observed ipsilaterally in the bed nucleus of the accessory olfactory tract and stria terminalis, in the central amygdaloid nucleus, dorsal cortex, and nucleus of the diagonal band. Bilateral labeling of somata was found in the ventral AON, the nucleus sphericus (hilus), and in the mesencephalic raphe nucleus and locus coeruleus. Injections into the dorsal amygdala showed that the mitral neurons are the cells of origin of the AOB centripetal projections. Reciprocal connections are present between AOB and MOB. To our knowledge, this is the first study to address the afferent connections of the olfactory bulbs in a reptile. On the basis of the available data, a discussion is provided of the similarities and differences between the reptilian and mammalian olfactory systems, as well as of the possible functional role of the main olfactory connections in reptiles.     

Efferents and centrifugal afferents of the main and accessory olfactory bulbs in the hamster

The efferents and centrifugal afferents of the hamster olfactory bulbs were studied using orthograde and retrograde tracing techniques. Following injections of tritiated amino acids which were restricted to the main olfactory bulb (MOB), autoradiographic grains were observed ipsilaterally over layer IA of the entire anterior olfactory nucleus (AON), the ventral portion of the hippocampal rudiment (HR), the entire prepyriform cortex and olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei and the lateral entorhinal cortex. An ipsilateral projection to the nucleus of the lateral olfactory tract (nLOT) was also indicated. No subcortical or contralateral projections were observed. Amino acid injections into the accessory olfactory bulb (AOB) revealed ipsilateral projections to the superficial plexiform layer of the medial and posteromedial cortical amygdaloid nuclei and to the bed nucleus of the accessory olfactory tract (nAOT) and the bed nucleus of the stria terminalis (nST). Following injections of HRP which were restricted to the MOB, contralateral HRP-positive neurons were found predominantly in pars externa and to a lesser extent in the other subdivisions of the AON. Centrifugal projections to the MOB were identified ipsilaterally from the entire AON, the ventral portion of the HR, the anterior portion of the prepyriform cortex, and the nLOT. No labelled neurons were found in the olfactory tubercle, the anterior and posterolateral cortical amygdaloid nuclei or the entorhinal cortex. Centrifugal projections to the MOB were also identified from subcortical structures of the ipsilateral basal forebrain and from midline structures of the midbrain. Labelling occurred in the fusiform neurons of the diagonal band near the medial base of the forebrain at the level of caudal olfactory tubercle. Heavy labelling was seen in a distinct group of large, predominantly multipolar neurons (magnocellular preoptic area) that continued from the level of caudal olfactory tubercle to the level of the nLOT. This band of HRP-positive neurons could be followed more caudally to a position dorsal and medial to the nLOT near the lateral margin of the lateral anterior hypothalamic area. The midbrain projections to the MOB originated in the dorsal and median raphe nuclei. After injections of HRP into the AOB, centrifugal projections were identified from the nAOT and the posteromedial cortical amygdaloid nucleus. In addition, isolated neurons were labelled in the medial cortical amygdaloid nucleus but no labelled neurons were found in the nST. These results support the notion of two anatomically distinct olfactory systems and demonstrate two previously unreported pathways through which the limbic system may modulate sensory processing in the olfactory bulb.     

The organization of centrifugal projections from the anterior olfactory nucleus, ventral hippocampal rudiment, and piriform cortex to the main olfactory bulb in the hamster: an autoradiographic study

The centrifugal projections from the various subdivisions of the anterior olfactory nucleus (AON) can be categorized into four groups based on the organization of terminal fields in the main olfactory bulb (MOB). Pars lateralis and dorsalis have bilaterally asymmetric laminar projections to the MOB. The ipsilateral projections terminate primarily in the superficial half of the granule cell layer and in the deep third of the glomerular layer, whereas the contralateral projections terminate primarily in the superficial half of the granule cell layer and do not extend into the glomerular layer. Pars ventralis and posterior have bilaterally symmetric laminar projections with heavy terminations both in the superficial half of the granule cell layer and in the deep third of the glomerular layer. Pars medialis sends predominantly ipsilateral projections to the deep half of the granule cell layer. Pars externa has predominantly contralateral projections with a very narrow terminal field immediately deep to the internal plexiform layer. The projections to the MOB from the ventral hippocampal rudiment (HR) and the piriform cortex (PC) are exclusively ipsilateral. The projections from the ventral HR terminate primarily in the deep half of the granule cell layer. The projections from the PC also terminate predominantly in the granule cell layer, but there is a progressive shifting of terminal fields from the superficial half of this layer toward deeper regions for centrifugal axons arising from progressively more caudal levels of the PC. The laminar termination patterns of cortical afferents to the ipsilateral MOB thus are correlated with the mediolateral axis of the olfactory peduncle and the rostrocaudal axis of the piriform cortex. The centrifugal axons from these various sources enter directly into the granule cell layer of the caudal MOB or pass through the internal plexiform layer of the accessory olfactory bulb to reach the middle and anterior part of the MOB. We have termed these two routes the final common bulb pathway. The centrifugal axons from the laterally situated sources join the anterior and bulbar limbs of the anterior commissure before entering the final common bulbar pathway. In contrast, the centrifugal axons from pars medialis and the ventral HR travel diffusely in the cellular layer of the ipsilateral olfactory peduncle. A small component of the centrifugal projections from the PC travels in association with the lateral olfactory tract.     

Axonal projection of anterior olfactory nuclear neurons to the olfactory bulb bilaterally.

The axonal projection of anterior olfactory nuclear (AON) neurons to the ipsilateral and contralateral olfactory bulbs and to the prepiriform cortex was analyzed electrophysiologically in the rabbit. Of 117 AON neurons which sent their axons to the anterior commissure, 46 cells (39%) and 55 cells (47%) were activated antidromically by ipsilateral and contralateral olfactory bulb stimulation, respectively, and 22 AON neurons (19%) were activated antidromically from both. The mean axonal conduction velocity of the AON neurons was 2.8 m/s in the AON—anterior commissure axonal segment, 1.6 m/s in the AON—contralateral offactory bulb segment, and 1.0 m/s in the AON—ipsilateral bulb segment. These results and the collision tests between the antidromically evoked spikes indicate that a number of AON neurons send their axons to the contralateral olfactory bulb via the anterior commissure and that the same neurons send thin axon collaterals to the ipsilateral bulb. These axonal projections are significant in relation to the synaptic influences of these axons upon olfactory bulb neurons.     

Forebrain GABAergic projections from the dorsal raphe nucleus identified by using GAD67–GFP knock‐in mice

The dorsal raphe nucleus (DR) contains serotonergic (5‐HT) neurons that project widely throughout the forebrain. These forebrain regions also receive innervation from non–5‐HT neurons in the DR. One of the main groups of non–5‐HT neurons in the DR is γ‐aminobutyric acid (GABA)ergic, but their projections are poorly understood due to the difficulty of labeling these neurons immunohistochemically. To identify GABAergic projection neurons within the DR in the current study, we used a knock‐in mouse line in which expression of green fluorescent protein (GFP) is controlled by the glutamic acid decarboxylase (GAD)67 promotor. Projections of GAD67–GFP neurons to the prefrontal cortex (PFC), nucleus accumbens (NAC), and lateral hypothalamus (LH) were evaluated by using retrograde tract tracing. The location of GAD67–GFP neurons projecting to each of these areas was mapped by rostrocaudal and dorsoventral location within the DR. Overall, 16% of DR neurons projecting to either the PFC or NAC were identified as GAD67–GFP neurons. GAD67–GFP neurons projecting to the PFC were most commonly found ventrally, in the rostral two‐thirds of the DR. NAC‐projecting GAD67–GFP neurons had an overlapping distribution that extended dorsally. GAD67–GFP neurons made a larger contribution to the projection of the DR to the LH, accounting for 36% of retrogradely labeled neurons, and were widespread throughout the DR. The current data indicate that DR GABAergic neurons not only may have the capacity to influence local network activity, but also make a notable contribution to DR output to multiple forebrain targets. J. Comp. Neurol. 520:4157–4167, 2012. © 2012 Wiley Periodicals, Inc.     

Central Connections of the Ovine Olfactory Bulb Formation Identified Using Wheat Germ Agglutinin-Conjugated Horseradish Peroxidase

Pheromonal stimuli elicit rapid behavioral and reproductive endocrine changes in the ewe. The neural pathways responsible for these effects in sheep are unknown, in part, because the olfactory bulb projections have not been examined in this species. Using the anterograde and retrograde neuronal tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), we describe the afferent and efferent olfactory bulb connections of the Suffolk ewe. Injections of WGA-HRP limited to the main olfactory bulb resulted in retrograde labeling of cells in numerous telencephalic, diencephalic, and metencephalic regions. Terminal labeling was limited to layer Ia of ipsilateral cortical structures extending rostrally from the anterior olfactory nucleus (AON), piriform cortex, anterior-, and posterolateral-cortical amygdaloid nuclei to lateral entorhinal cortex caudally. Injections involving the accessory olfactory bulb and AON produced additional labeling of cells within the bed nucleus of the stria terminalis (BNST), medial nucleus of the amygdala, and a few cells in the posteromedial cortical nucleus of the amygdala. Terminal labeling included a small dorsomedial quadrant of BNST and also extended to the far lateral portions of the supraoptic nucleus. A clearly defined accessory olfactory tract and nucleus was not evident, perhaps due to limitations in the sensitivity of the method. With this possible exception, the afferent and efferent olfactory connections in the sheep appear similar to those reported for other species.     

A horseradish peroxidase study of the olfactory system of the frog, Rana esculenta

The olfactory system of the frog Rana esculenta was studied by using horseradish peroxidase (HRP) tracing of axonal pathways. Injections of HRP were made in the main olfactory bulb (MOB), accessory olfactory bulb (AOB), anterior olfactory nucleus (AON), the amygdala (AMY), and in a zone of the leteral wall of the telencephalic hemisphere immediately posterior to the AOB. Projections from these sites are described and are generally similar to those obtained by degeneration methods. However, HRP reveals more extensive olfactory connections than previously reported. Ipsilateral, contralateral, and bilateral projections are described. The MOB, AOB, and AON have ipsilateral connections to each other. The MOB and AOB have very different projections. The MOB and AON project via the habenular commissure (HC) to the contralateral medial wall of the telencephalon. Ipsilateral MOB fibers also terminate in this cell-free zone where the medial forebrain bundle (MFB) originates. The AOB projects to the lateral cortex of the contralateral telencephalic hemisphere via the HC and also to the ipsilateral AMY and lateral forebrain bundle (LFB) from where some fibers project contralaterally. HRP injections in the AMY retrogradely fill cells in the ipsilateral AOB, two nuclei of the ipsilateral hypothalamus and a nucleus of cells caudal to the ipsilateral nucleus isthmi. Fibers are also labeled that project to the contralateral AMY. Few fibers were observed to decussate in the interpeduncular nucleus or optic chiasma. No olfactory fibers were found to project to the habenular nuclei, and no labeled neurons were found to project to the olfactory bulbs. No morphological asymmetry was observed qualitatively in the distribution of olfactory fibers in the two halves of the brain.     

Contralateral projections of the rat anterior olfactory nucleus

The anterior olfactory nucleus (AON) is a central olfactory cortical structure that has heavy reciprocal connections with both the olfactory bulb (OB) and piriform cortex. While it has been firmly established that the AON is a primary source of bilateral projections in the olfactory system through extensive connections with both the ipsilateral and contralateral OB, AON, and piriform cortex, few studies have examined this circuitry in detail. In the present study we used small injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and the retrograde tracer FluoroGold in specific subregions of the AON to explore the topography of the interconnections between the left and right AONs. Labeled fibers were found in the contralateral AON following injections in all areas. However, detailed quantitative analyses revealed that different regions of the AON have distinct patterns of interhemispheric innervation; contralateral fibers were most heavily targeted to dorsal and lateral AON subregions, while the medial and ventral areas received relatively light projections. These results demonstrate important features of the interhemispheric circuitry of the AON and suggest separate functional roles for subregions of the AON in olfactory information processing.     

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.     

Petit mal seizure spikes in olfactory bulb and cortex caused by runaway inhibition after exhaustion of excitation

The olfactory bulb (OB), anterior olfactory nucleus (AON) and prepyriform cortex (PC) maintain 3 kinds of feedback among their populations of excitatory and inhibitory neurons: negative feedback, mutual excitation, and mutual inhibition. At normal levels of synaptic input these are balanced and give rise to chaotic and near-sinusoidal oscillatory EEG activity. Under intense repetitive electrical stimulation of the lateral olfactory tract (LOT), there is failure of the afferent excitatory terminals, perhaps due to transmitter depletion. In this circumstance there is deficient excitatory input under the condition of a high level of sustained activity among mutually inhibitory neurons. An instability develops in which some inhibitory neurons become more disinhibited (excited) and others more inhibited (less active) to the point of a paroxysmal discharge that is manifested in a massive compound IPSP of the excitatory neurons. The paroxysm terminates abruptly, but by mechanisms still unclear repeats at a rate of about 3/s for 10–70 s. It is accompanied by simultaneous ipsilateral twitching of the eyelids and muzzle, salivation, tearing, arrest, and lack of responding to sensory stimuli but without loss of posture, resembling absence in humans. It does not result from runaway mutual excitation, and it rarely culminates in full-blown convulsions. Similar spikes usually also occur in the OB and AON; the sequences of spikes appear to entrain. These normal and seizure EEGs are simulated with a network of non-linear differential equations, that is designed in conformance with the anatomy and physiology of the olfactory system. The seizure appears as an emergent property of the OB, AON and PC interactive system, that is due to an induced asymmetry in the feedback network that controls normal background activity.     

Petit mal seizure spikes in olfactory bulb and cortex caused by runaway inhibition after exhaustion of excitation

The olfactory bulb (OB), anterior olfactory nucleus (AON) and prepyriform cortex (PC) maintain 3 kinds of feedback among their populations of excitatory and inhibitory neurons: negative feedback, mutual excitation, and mutual inhibition. At normal levels of synaptic input these are balanced and give rise to chaotic and near-sinusoidal oscillatory EEG activity. Under intense repetitive electrical stimulation of the lateral olfactory tract (LOT), there is failure of the afferent excitatory terminals, perhaps due to transmitter depletion. In this circumstance there is deficient excitatory input under the condition of a high level of sustained activity among mutually inhibitory neurons. An instability develops in which some inhibitory neurons become more disinhibited (excited) and others more inhibited (less active) to the point of a paroxysmal discharge that is manifested in a massive compound IPSP of the excitatory neurons. The paroxysm terminates abruptly, but by mechanisms still unclear repeats at a rate of about 3/s for 10-70 s. It is accompanied by simultaneous ipsilateral twitching of the eyelids and muzzle, salivation, tearing, arrest, and lack of responding to sensory stimuli but without loss of posture, resembling absence in humans. It does not result from runaway mutual excitation, and it rarely culminates in full-blown convulsions. Similar spikes usually also occur in the OB and AON; the sequences of spikes appear to entrain. These normal and seizure EEGs are simulated with a network of non-linear differential equations, that is designed in conformance with the anatomy and physiology of the olfactory system. The seizure appears as an emergent property of the OB, AON and PC interactive system, that is due to an induced asymmetry in the feedback network that controls normal background activity.     

Effects of lateral olfactory tract stimulation on Fos immunoreactivity in vasopressin neurones of the rat piriform cortex

In the main olfactory system, odours are registered at the main olfactory epithelium and are then processed at the main olfactory bulb (MOB) and, subsequently, by the anterior olfactory nucleus (AON), the piriform cortex (PC) and the cortical amygdala. Previously, we reported populations of vasopressin neurones in different areas of the rat olfactory system, including the MOB, accessory olfactory bulb (AOB) and the AON and showed that these are involved in the coding of social odour information. Utilising immunohistochemistry and a transgenic rat in which an enhanced green fluorescent protein reporter gene is expressed in vasopressin neurones (eGFP‐vasopressin), we now show a population of vasopressin neurones in the PC. The vasopressin neurones are predominantly located in the layer II of the PC and the majority co‐express the excitatory transmitter glutamate. Furthermore, there is no sex difference in the number of neurones expressing vasopressin. Electrical stimulation of the lateral olfactory tract leads to a significant increase in the number of Fos‐positive nuclei in the PC, MOB, AOB, dorsal AON and supraoptic nucleus (SON). However, there was only a significant increase in Fos expression in vasopressin cells of the PC and SON. Thus, functionally distinct populations of vasopressin cells are implicated in olfactory processing at multiple stages of the olfactory pathway.     

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.