Electrophysiological connections of neurons in ventral pallidal regions of the olfactory tubercle with the main olfactory bulb and piriform cortex

Field potential and single unit recordings were used to assess the connections of the olfactory tubercle (OT) with the main olfactory bulb (MOB) and the piriform cortex (PC) in urethane-anesthetized rats. Current generators of depth profiles evoked in OT following MOB stimulation were localized 300 microns superficial to those elicited by PC shocks, suggesting that afferents from the MOB and PC end in different regions of the OT. Following MOB and PC stimulation antidromically invaded neurons were recorded in the ventral pallidal regions of the OT and in the vicinity of the islands of Calleja, respectively. These results demonstrate that the OT, which receives a monosynaptic input from the MOB, projects back to the bulb and that the PC seems to be also reciprocally linked with differentiated structures in the OT.     

Afferent projections to the different medial amygdala subdivisions: a retrograde tracing study in the mouse

The medial amygdaloid nucleus (Me) is a key node in the socio-sexual brain, composed of anterior (MeA), posteroventral (MePV) and posterodorsal (MePD) subdivisions. These subdivisions have been suggested to play a different role in reproductive and defensive behaviours. In the present work we analyse the afferents of the three Me subdivisions using restricted injections of fluorogold in female outbred CD1 mice. The results reveal that the MeA, MePV and MePD share a common pattern of afferents, with some differences in the density of retrograde labelling in several nuclei. Common afferents to Me subdivisions include: the accessory olfactory bulbs, piriform cortex and endopiriform nucleus, chemosensory amygdala (receiving direct inputs from the olfactory bulbs), posterior part of the medial bed nucleus of the stria terminalis (BSTM), CA1 in the ventral hippocampus and posterior intralaminar thalamus. Minor projections originate from the basolateral amygdala and amygdalo-hippocampal area, septum, ventral striatum, several allocortical and periallocortical areas, claustrum, several hypothalamic structures, raphe and parabrachial complex. MeA and MePV share minor inputs from the frontal cortex (medial orbital, prelimbic, infralimbic and dorsal peduncular cortices), but differ in the lack of main olfactory projections to the MePV. By contrast, the MePD receives preferential projections from the rostral accessory olfactory bulb, the posteromedial BSTM and the ventral premammillary nucleus. In summary, the common pattern of afferents to the Me subdivisions and their interconnections suggest that they play cooperative instead of differential roles in the various behaviours (e.g., sociosexual, defensive) in which the Me has been shown to be involved.     

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.     

Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat

Summary The spatial organization and laminar distribution of projections from the olfactory bulb and the anterior (PPCa) and posterior (PPCp) divisions of the prepiriform cortex to the entorhinal cortex were studied with anterograde (³H-leucine) and retrograde (WGA-HRP) tracing techniques. After ³H-leucine injections into the olfactory bulb transported labeling was seen over the lateral entorhinal area, except its most medial part, and over the rostral part of the medial entorhinal area. The labeling covers exclusively layer Ia. The lateral and medial entorhinal areas are also reached by fibers from the prepiriform cortex. The projection to the medial entorhinal area has not been described previously. Following injections of ³H-leucine into the PPCa transported labeling is present over the entire expanse of the entorhinal cortex and is located over layer Ib with the greatest density in its superficial part. Injections of ³H-leucine into the PPCp give rise to transported labeling over much of the entorhinal cortex. No labeling was found over the most medial parts of the medial subdivision (VMEA) of the lateral entorhinal area and the medial entorhinal area. Labeling occupies layer Ib, especially its middle part, and layers II and III. Both PPCa and PPCp appear to project most heavily to the dorsal (DLEA) and ventral (VLEA) subdivisions of the lateral entorhinal area. From the retrograde experiments it can be inferred that cells of layers II and III of the PPCa project predominantly to the DLEA, whereas those of the PPCp project predominantly to the VLEA. The MEA receives its heaviest projection from layer II of both PPCa and PPCp. In control experiments with ³H-leucine injections into the endopiriform nucleus it was found that this nucleus projects to the entire expanse of the entorhinal cortex. The fibers distribute to all layers with the exception of layer Ia.Abbreviations AI agranular insular cortex - AL lateral nucleus of the amygdala - BL basolateral nucleus of the amygdala - BM basomedial nucleus of the amygdala - C claustrum - CoA cortical nucleus of the amygdala - DLEA dorsal division of the lateral entorhinal cortex - END endopiriform nucleus - H hippocampus - I granular insular cortex - lot lateral olfactory tractus - MCL mitral cell layer of the olfactory bulb - MEA medial entorhinal area - OB olfactory bulb - PPCa anterior part of the prepiriform nucleus - PPCp posterior part of the prepiriform nucleus - VLEA ventral division of the lateral entorhinal cortex - VMEA ventromedial division of the lateral entorhinal cortex - 35 area 35 of the perirhinal cortex - 36 area 36 of the perirhinal cortex     

Immunocytochemical identification of luteinizing hormone-releasing hormone-positive fibres and terminals in the olfactory system of the rat

Luteinizing hormone-releasing hormone immunoreactivity was studied in the olfactory system of the rat in combination with acetylcholinesterase histochemistry. Neuronal perikarya containing luteinizing hormone-releasing hormone lie in the medial septal nucleus, the vertical limb of the diagonal band of Broca, the olfactory tubercule and the ganglionated plexus of the terminal nerve. Labelled fibres spread in the superficial layers of the main and accessory olfactory bulbs, some encompassing the strongly acetylcholinesterase-positive atypical glomeruli. Others are observed on the medial side of the bulb, running along the terminal nerve bundles and ganglia. These fibres join the vomeronasal nerve branches and proceed distally towards the nasal cavity. In the septal submucosa, immunoreactive fibres are partly associated with the terminal nerve network. Conspicuous endings filled with luteinizing hormone-releasing hormone are observed on blood vessels of the olfactory mucosa. Such well-differentiated terminals might be the neurosecretory afferents of a new neurohemal area. Immunoreactive terminals are also observed around the excretory ducts of the anterior medial glands. We have failed to observe any labelled fibres in the olfactory and vomeronasal epithelia. The results of the present study are discussed with respect to possible functional interpretations. It is suggested that significant amounts of luteinizing hormone-releasing hormone could be released in the submucosal capillaries in spite of the scarcity of immunoreactive fibres. Similar afferents could also modulate the secretory activity of some nasal glands. Synaptic events involving the neuropeptide might occur in the olfactory bulb, particularly in atypical glomerular areas previously characterized by their high acetylcholinesterase content. Finally, no anatomical support for a chemosensory function of fibres containing luteinizing hormone-releasing hormone has been brought out by our work.     

The piriform cortex is not a direct olfactory relay to the mediodorsal thalamic nucleus in cats and rabbits

Following injections of horseradish peroxidase into the mediodorsal thalamic nucleus (MD), retrogradely labeled cells were found in various areas in the cat and rabbit. Among these, olfactory-related areas to which the olfactory bulb projects directly or indirectly via the piriform cortex were the olfactory tubercle, amygdala and insular cortex, while no labeled cells were detected in the piriform cortex and endopiriform nucleus in both species. These results indicate that the piriform cortex and endopiriform nucleus do not send their axons directly to the MD.     

Delayed deficits in behavior after transection of the olfactory tracts in hamsters

This study compared the effects of transection of the lateral olfactory tracts (LOT) and the accessory olfactory tracts (AOT) in male hamsters on nest building, food piling, and sexual behavior. Autoradiographic tracing of amino acids injected into the olfactory bulbs allowed accurate determination of the location and extent of the transections. Animals with complete bilateral transections of the projections to the amygdaloid targets of the accessory olfactory bulbs and to the main olfactory targets posterior to the olfactory tubercle showed no sexual behavior postoperatively; they did not exhibit extensive genital investigation and did not mount females. In contrast, most of the animals with partial sparing of accessory olfactory bulb efferents to the amygdala did exhibit investigatory and copulatory behaviors postoperatively, although half of the animals with this partial sparing developed delayed deficits in these sexual behaviors. Almost all animals without detectable main olfactory bulb efferents to posterior targets showed delayed deficits in nest building and food piling. This was true whether or not there was partial sparing of accessory olfactory bulb efferents to the amygdala. The animals with LOT transections typically built nests and piled food during the first postoperative week, but stopped building nests and piling food by the fourth postoperative week. Cold stress enhanced these two behaviors in control animals but did not obviate the deficits in experimental animals. Caudally placed transections, which spared a larger portion of the main olfactory projections than rostally placed transections, did not spare more behavior. In fact, the caudally placed transections produced shorter delays in the appearance of deficits in nest building and food piling. These results indicate that the accessory olfactory bulb efferents to the amygdala are more important for sexual behavior than for nest building and food piling in male hamsters. Nest building and food piling are not directly dependent on normal ongoing or sensory evoked activity in the main or accessory olfactory bulb efferents which project through the LOT and AOT. The deficits in nest building and food piling may represent a deterioration in the ability of the animals to organize their living space. The observed delays in the appearance of deficits in behavior may also reflect slow degenerative processes or humoral changes associated with loss of input from the main olfactory bulbs to posterior olfactory target areas, and possibly with interruptions of projections to targets of the accessory olfactory system.     

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     

An electrophysiological study of the accessory olfactory bulb in the rabbit--I. Analysis of electrically evoked potential fields

Following electrical stimulation of the vomeronasal nerves, the primary olfactory nerves, the lateral olfactory tract and the corticomedial amygdala, we have made a study of evoked potentials in the rabbit accessory olfactory bulb. Vomeronasal nerve stimulation evoked a complex field potential consisting of a compound action potential followed by 4 negative waves (N1, N2, N3, N4). In contrast to the field potential elicited in the main olfactory bulb following primary olfactory nerve stimulation, there was either no evoked wave or only a weak positive component of the field in the accessory bulb. Amygdala stimulation caused a long latency, long duration negative-positive dipolar field potential in the accessory olfactory bulb. Both antidromic and orthodromic field potentials showed sign reversal when the electrode penetrated the bulb at a point corresponding to the lower border of the mitral cell band. Stimulation of the lateral olfactory tract elicited a weak, short-latency wave which did not show any sign reversal when the electrode was lowered into the accessory bulb. This wave was presumably due to fibres arising in the main bulb and projecting through the accessory bulb into the lateral olfactory tract. Electrical stimulation of the primary olfactory nerves did not induce any response in the accessory bulb neither did vomeronasal nerve stimulation evoke a response in the main olfactory bulb. The origin of these potential fields is discussed and it is concluded that the synaptic organization of the accessory olfactory bulb resembles that of the main olfactory bulb in lower vertebrates. There is no detectable communication between the two olfactory systems.     

Efferent connections of the "olfactostriatum": a specialized vomeronasal structure within the basal ganglia of snakes

The olfactostriatum is a portion of the basal ganglia of snakes that receives substantial vomeronasal afferents through projections from the nucleus sphericus. In a preceding article, the olfactostriatum of garter snakes (Thamnophis sirtalis) was characterized on the basis of chemoarchitecture (distribution of serotonin, neuropeptide Y and tyrosine hydroxylase) and pattern of afferent connections [Martinez-Marcos, A., Ubeda-Banon, I., Lanuza, E., Halpern, M., 2005. Chemoarchitecture and afferent connections of the "olfactostriatum": a specialized vomeronasal structure within the basal ganglia of snakes. J. Chem. Neuroanat. 29, 49-69]. In the present study, its efferent connections have been investigated. The olfactostriatum projects to the main and accessory olfactory bulbs, lateral cortex, septal complex, ventral pallidum, external, ventral anterior and dorsolateral amygdalae, bed nucleus of the stria terminalis, preoptic area, lateral posterior hypothalamic nucleus, ventral tegmental area, substantia nigra and raphe nuclei. Tracer injections in the nucleus accumbens proper, a structure closely associated with the olfactostriatum, result in a similar pattern of efferent connections with the exception of those reaching the main and accessory olfactory bulbs, lateral cortex, external, ventral anterior and dorsolateral amygdalae and bed nucleus of the stria terminalis. These data, therefore, help to characterize the olfactostriatum, an apparently specialized area of the nucleus accumbens. Double labeling experiments after tracer injections in the nucleus sphericus and the lateral posterior hypothalamic nucleus demonstrate a pathway between these two structures through the olfactostriatum. Injections in the olfactostriatum and in the medial amygdala show parallel projections to the lateral posterior hypothalamic nucleus. Since this hypothalamic nucleus has been previously described as projecting to the hypoglossal nucleus, both, the medial amygdala and the olfactostriatum may mediate vomeronasal influence on tongue-flick behavior.     

The main olfactory system mediates pheromone-induced fos expression in the extended amygdala and preoptic area of the male Syrian hamster

Copulation in male hamsters is stimulated by exposure to vaginal secretions of conspecifics. These pheromones also stimulate fos expression in neural areas that regulate copulation including: the medial nucleus of the amygdala, the bed nucleus of the stria terminalis, and the preoptic area. The pheromones in vaginal secretions are detected by both the main and accessory olfactory systems. However, the accessory system plays the greater role in the regulation of mating behavior and has direct connections with the medial nucleus of the amygdala and bed nucleus of the stria terminalis. The goal of the present study was to determine which system mediates the effect of pheromones on the stimulation of more central areas by deafferenting these systems in experienced male hamsters before exposure to vaginal secretions. Destruction of the receptors in the main olfactory system with zinc sulfate eliminated the increase in fos immunoreactivity in the amygdala, bed nucleus of the stria terminalis and preoptic area following exposure to sexually stimulating pheromones. Deafferentation of the accessory olfactory system by removing the vomeronasal organ had no effect on pheromone-induced fos expression in these areas.We conclude that neurons expressing fos following exposure to vaginal secretions are stimulated via the main olfactory system and are not associated with the expression of copulatory behavior.     

Some primary olfactory axons project to the contralateral olfactory bulb in Xenopus laevis

Cobaltous lysine and horseradish peroxidase (HRP) were used to trace primary olfactory axons to their terminations. The tracers were taken up in the olfactory mucosa and transported to the olfactory bulb. The results suggest that either HRP or cobaltous-lysine methods are useful and simple tools for studying such connections. In addition to the ipsilateral projection, we report here for the first time a projection to the medial part of the contralateral olfactory bulb. The overlaps of ipsi- and contralateral inputs to the medial bulb provide further evidence that its functions in frogs may be different from the lateral part, because other connections differ as well.     

Neural basis of olfactory memory in the context of pregnancy block

In mice, only strange male pheromones block pregnancy; pheromones of the familiar male with which the female has mated have the capacity to block pregnancy but are ineffective with the consort female. Hence, some form of recognition/memory to the stud male is formed at mating. By infusing lignocaine locally into the accessory olfactory bulb and second order olfactory synapses in the medial nucleus of the amygdala, this study localizes changes that occur in the accessory olfactory bulb at mating to be subsequently important in preventing the stud male's pheromones from blocking pregnancy. Further attention is focused on the dendrodendritic synapses between mitral and granule cells in the accessory olfactory bulb. Blockade of the GABA receptors (granule to mitral cell synapse) in the accessory bulb without mating, but in the presence of male pheromones, prevents any male from blocking pregnancy. Conversely inhibition of protein kinase C, a second messenger system activated by excitatory amino acids (mitral to granule cell synapse), in the accessory bulb during a 4-h period after mating permits all male pheromones including the stud's to activate pregnancy block. While blockade of protein kinase C activity during the critical exposure time for memory formation prevents memory formation, infusions of a protein synthesis inhibitor (anisomycin) are without effect. However, protein synthesis inhibition in the accessory olfactory bulb in the late phase of the critical exposure time (3-6 h after mating) does prevent memory formation. These studies show that changes in synaptic plasticity in the accessory olfactory bulb following mating are critical to recognition of the stud male's pheromones, hence preventing these from subsequently blocking pregnancy.     

Afferent and efferent connections of the cortical and medial nuclei of the amygdala in sheep

The cortical (CoA) and the medial (MeA) nuclei of the amygdala are involved in the processing of olfactory information relevant to social recognition in the ewe. To better understand the neural pathways responsible for these effects, the connections of both CoA and MeA with the telencephalic and diencephalic regions were studied by injecting an anterograde (Biotin-Dextran-Amine, BDA) or a retrograde (Fluorogold, FG) neuronal tracer into either the CoA or the MeA. Concerning the primary olfactory structures, the CoA receives inputs from both the main olfactory bulb and the accessory olfactory bulb (AOB), while the MeA is innervated by cells only from the AOB. Among the other olfactory structures, only the entorhinal cortex and the tenia tecta are connected with both the CoA and the MeA. With respect to the other secondary olfactory structures, the connections with the CoA and the MeA show segregating neuronal routes. The CoA is connected with the accessory olfactory nucleus, the piriform, the endopiriform and the orbitofrontal cortices while the MeA exhibited connections with the nucleus of the lateral olfactory tract, the perirhinal and the insular cortices. Concerning the diencephalic structures, only the MeA receives projections from the PVN and the MBH. On the other hand, we showed that the BNST is the major site of connection with both the CoA and the MeA. Reciprocal projections were observed between the CoA and the MeA and between both nuclei and the basal or the lateral nuclei of the amygdala with the exception of the CoA which does not send inputs to the lateral nucleus. These data are discussed in relation with olfactory learning in the context of sexual and maternal behavior in sheep.     

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

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

Amygdaloid projections to ventromedial striatal subterritories in the primate

The ventral striatum is the part of the striatum associated with reward and goal-directed behaviors, which are mediated in part by inputs from the amygdala. The ventral striatum is divided into 'shell' and 'core' subterritories which have different connectional, histochemical and pharmacological properties. Behavioral studies also indicate that subterritories of the ventral striatum are differentially involved in specific goal-directed behaviors. The amygdala is a heterogeneous structure which has multiple nuclei involved in processing emotional information. While the existence of an amygdalostriatal pathway has long been established, the relationship between amygdaloid afferents and specific subterritories of the ventral striatum is not known. In this study we operationally defined the ventromedial striatum as the region receiving cortical inputs primarily from the orbital and medial prefrontal cortex. We placed retrograde tracer injections into subregions of the ventromedial striatum of macaques monkeys to determine the relative contribution of specific amygdaloid inputs to each region. Calbindin-D28k immunostaining was used to further define the shell subterritory of the ventromedial striatum. Based on these definitions, the amygdala innervates the entire ventromedial striatum, and has few to no inputs to the central striatum. The basal and accessory basal nuclei are the major source of input to the ventromedial striatum, innervating both the shell and ventromedial striatum outside the shell. However, a restricted portion of the dorsomedial shell receives few basal nucleus inputs. Afferent inputs from the basal nucleus subdivisions are arranged such that the parvicellular subdivision projects mainly to the ventral shell and core, and the magnocellular subdivision targets the ventral shell and ventromedial putamen. In contrast, the intermediate subdivision of the basal nucleus projects broadly across the ventromedial striatum avoiding only the dorsomedial shell. The shell has a specific set of connections derived from the medial part of the central nucleus and periamygdaloid cortex. Within the shell, the dorsomedial region is distinguished by additional inputs from the medial nucleus. The ventromedial caudate nucleus forms a unique transition zone with the shell, based on inputs from the periamygdaloid cortex. Together, these results indicate that subterritories of the ventromedial striatum are differentially modulated by amygdaloid nuclei which play roles in processing olfactory, visual/gustatory, multimodal sensory, and 'drive'-related stimuli.     

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.     

Induction and maintenance of epileptiform activity in the rabbit olfactory bulb depends on centrifugal input

Summary A technique of cryogenic blockade was used in waking rabbits to produce complete and reversible isolation of the olfactory bulb from the rest of the brain. During cooling of the olfactory peduncle epileptiform activity occurred spontaneously in the pyriform cortex in 3 out of 20 sessions, but never in the bulb. Following removal of the cryoblockade, during the seizure state, epileptiform discharges appeared simultaneously in the bulb and pyriform cortex. In the control state, without cooling of the peduncle, epileptiform activity could be evoked in the bulb and cortex by intense electrical stimulation of either the bulb or the lateral olfactory tract. During the cryoblockade, however, intense stimulation of the bulb failed to evoke seizure-like discharges. The results demonstrate a dependency on more central olfactory structures for the induction and maintenance of epileptiform activity in the olfactory bulb.This project was supported by a grant no. HL31164 from NIH     

Entorhinal cortex stimulation modulates amygdala and piriform cortex responses to olfactory bulb inputs in the rat

The rodent olfactory bulb sends direct projections to the piriform cortex and to two structures intimately implicated in memory processes, the entorhinal cortex and the amygdala. The piriform cortex has monosynaptic projections with the amygdala and the piriform cortex and is therefore in a position to modulate olfactory input either directly in the piriform cortex, or via the amygdala. In order to investigate this hypothesis, field potential signals induced in anesthetized rats by electrical stimulation of the olfactory bulb or the entorhinal cortex were recorded simultaneously in the piriform cortex (anterior part and posterior part) and the amygdala (basolateral nucleus and cortical nucleus). Single-site paired-pulse stimulation was used to assess the time courses of short-term inhibition and facilitation in each recording site in response to electrical stimulation of the olfactory bulb and entorhinal cortex. Paired-pulse stimulation of the olfactory bulb induced homosynaptic inhibition for short interpulse interpulse intervals (20-30 ms) in all the recording sites, with a significantly lower degree of inhibition in the anterior piriform cortex than in the other structures. At longer intervals (40-80 ms), paired-pulse facilitation was observed in all the structures. Paired-pulse stimulation of the entorhinal cortex mainly resulted in inhibition for the shortest interval duration (20 ms) in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. Double-site paired-pulse stimulation was then applied to determine if stimulation of the entorhinal cortex can modulate responses to olfactory bulb stimulation. For short interpulse intervals (20 ms) heterosynaptic inhibition was observed in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. The level of inhibition was greater in the basolateral nucleus than in the other structures. Taken together these data suggest that the entorhinal cortex exerts a main inhibitory effect on the olfactory input via the amygdala basolateral nucleus and to a lesser extent the piriform cortex. The potential role of these effects on the processing of olfactory information is discussed.     

3H-thymidine-radiographic studies of neurogenesis in the rat olfactory bulb

Neurogenesis in the rat olfactory bulb was examined with 3H-thymidine-radiography. For the animals in the prenatal groups, the initial 3H-thymidine exposures were separated by 24 h; they were the offspring of pregnant females given two injections on consecutive embryonic (E) days (E12-E13, E13-E14, . . . E21-E22). For the animals in the postnatal (P) groups, the initial 3H-thymidine injections were separated by 48 h, each group receiving either four (PO-P3, P2-P4, . . . P6-P9) or two (P8-P9, P10-P11, . . . P20-P21) consecutive daily injections. On P60, the percentage of labeled cells and the proportion of cells added during either 24 h or 48 h periods were quantified at several anatomical levels for each neuronal population in the main olfactory bulb (mitral cells, tufted cells, granule cells, interneurons in the external plexiform layer, periglomerular granule cells) and accessory olfactory bulb (output neurons, granule cells, periglomerular granule cells). The total time span of neurogenesis extends from E12 to beyond P20. Output neurons are prenatally generated over 5-9 day periods (with most neurogenesis occurring over 2-4 days) in a strict sequential order beginning with the accessory bulb output neurons (E13-E14) and ending with the interstitial tufted cells lying between the glomeruli in the main bulb (E20-E22). These data are correlated with the main and accessory bulb projection fields in the amygdala and with the chronology of amygdala neurogenesis. With the exception of the granule cells in the accessory bulb (88% generated between E15-E22), the rest of the interneuronal populations are generated postnatally and nearly simultaneously. While most neurons (75-80%) originate during the first three weeks of life, all interneuronal populations, including accessory bulb granule cells, show some neurogenesis beyond P20. Injections of 3H-thymidine in juvenile and adult rats indicates neurogenesis up to P60 in the accessory bulb and up to P180 in the main bulb, especially in the main bulb granule cell population. There is circumstantial evidence for turnover of main bulb granule cells during adult life.