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PubMed2006

Cortical contributions to olfaction: plasticity and perception.

In most sensory systems, the sensory cortex is the place where sensation approaches perception. As described in this review, olfaction is no different. The olfactory system includes both primary and higher order cortical regions. These cortical structures perform computations that take highly analytical afferent input and synthesize it into configural odor objects. Cortical plasticity plays an important role in this synthesis and may underlie olfactory perceptual learning. Olfactory cortex is also involved in odor memory and association of odors with multimodal input and contexts. Finally, the olfactory cortex serves as an important sensory gate, modulating information throughput based on recent experience and behavioral state.

Animals↗
PubMed2006

Vulnerability to Alzheimer's pathology in neocortex: the roles of plasticity and columnar organization.

Two principal findings in the Pearson et al. paper are commented on here. The first is the regional selectivity within the cerebrum of neurofibrillary tangle (NFT) formation in Alzheimer's disease (AD) which targets association cortex and the primary olfactory cortex alone among regions of primary sensory cortex. The second finding is the clustering of NFT in columns of supra- and infra-granular layers of association cortex. We review recent evidence confirming these findings and comment on their possible significance. We consider that the most attractive hypothesis to explain the vulnerability of the olfactory system and association cortex is the persistent neural plasticity of these regions. On this basis there would be no need to postulate a progressive spreading process. The columnar distribution of clustered NFT can be well understood in the context of recent concepts of columnar organization of the cerebral cortex. The original interpretation that this distribution of NFT reflects pathology in neurons subserving cortico-cortical and cortico-subcortical connections seems to us to have stood the test of time.

Alzheimer Disease↗
PubMed1999

Environmental control of collateral branching and target invasion of mitral cell axons during development.

During development, mitral cell axons, the major efferents of the olfactory bulb, exhibit a protracted waiting period in the lateral olfactory tract (LOT) before giving off collateral branches and innervating the target olfactory cortex. To investigate the target invasion mechanism, a series of heterochronic and heterotopic cocultures of olfactory bulbs with various olfactory cortical strips were conducted. These experiments indicated that development of collateral branches is triggered by environmental cues but not by intrinsic mechanisms in mitral cells. The collateral-inducing cues are apparently different from the cues directing outgrowth of primary mitral cell axons. Coculture experiments also indicated that the target olfactory cortex undergoes a developmental change to become accessible to mitral cell fibers. Primary mitral cell axons, however, still preferred the LOT position over such accessible piriform cortex when encountered both the locations. These results suggest that mitral cell projection comprises multiple steps which are controlled by various environmental cues.

Animals↗
PubMed1994

Mapping patterns of neuronal activity and seizure propagation by imaging intrinsic optical signals in the isolated whole brain of the guinea-pig.

Image analysis techniques were used to examine changes in the intrinsic optical properties in the isolated brain of the guinea-pig in order to map normal neuronal activity patterns and seizure propagation in the olfactory cortex. Electrical stimulation of the lateral olfactory tract decreased light reflectance in distant cortical areas where fibres of the tract are known to project. These areas included the amygdalar, anterior and posterior piriform, and entorhinal cortices, as well as the olfactory tubercle. Stimulation of the lateral entorhinal cortex decreased reflectance in a more circumscribed area in the lateral and medial entorhinal cortex. By imaging intrinsic signals in real-time, we also demonstrated that seizure activity elicited in the entorhinal cortex/hippocampus preferentially propagated to the posteromedial cortical amygdaloid nucleus. The magnitudes of the intrinsic optical signals were correlated with the amplitudes of field potentials recorded in laminae II or III of the olfactory cortex of the same preparations. These signals had onset times of approximately 3 s during 5 Hz stimulation, consistently recovered and were graded with stimulation frequency. The generation of the intrinsic signals required postsynaptic activation, since attenuating synaptic transmission with kynurenic acid (an excitatory amino acid antagonist) eliminated the signals. The intrinsic signals exhibited maxima at 425-450, 550 and 600 nm, suggesting that they arose from changes in light absorption by cytochromes. Intrinsic signals of relatively constant magnitude were also present at 400, 475-500 and 575 nm, and at wavelengths greater than 600 nm. This suggested that an additional component of the intrinsic signal arose from changes in light scattering, possibly due to cellular swelling.

Animals↗
PubMed1972

On the mechanism of barbiturate anaesthesia.

1. The effects of pentobarbitone (0.05-0.6 mM in saline solution) on the evoked field potentials of in vitro preparations of guinea-pig olfactory cortex were studied.2. The evoked field potentials comprised an initial diphasic wave - the lateral olfactory tract (l.o.t.) compound action potential - followed by a surface negative wave (e.p.s.p) of 1-3 mV amplitude and about 10 msec duration. Superimposed on the negative wave were a number of positive peaks (population spikes).3. Pentobarbitone depressed the e.p.s.p. but not the l.o.t. compound action potential. The number and size of the population spikes were progressively reduced as the e.p.s.p. became depressed, indicating a failure of transmission through the cortical relay. The e.p.s.p. depression increased with increasing concentrations of pentobarbitone.4. Pentobarbitone had no effect on the threshold to electrical stimulation of the l.o.t. fibres or on that of the post-synaptic cells to synaptic excitation.5. Post-tetanic potentiation and frequency potentiation were either of normal magnitude or were enhanced in the presence of 0.2-0.3 mM pentobarbitone.6. It is concluded that pentobarbitone probably reduces the output of transmitter from the presynaptic nerve terminals of the olfactory cortex and that this mechanism could be the basis of the depressant action of the barbiturates.

Action Potentials↗
PubMed1983

Neonatal 6-hydroxydopa alters conspecific odor investigation by male rats.

Odor-guided behavior was examined in male rats injected at birth and 48 h later with either the catecholaminergic neurotoxin 6-hydroxydopamine (60 micrograms/g, i.p.) or vehicle. In odor preference tests administered 8 or 74 days postnatally, drug-treated animals avoided novel odors which were neutral for controls and showed reduced preference for conspecific nest odors. In emergence tests administered 73-75 days postnatally, odors from an anesthetized female reduced approach latency and increased investigation of familiar conspecific odors for control but not drug-treated males. Neonatal drug treatment decreased adult olfactory cortex norepinephrine (NE) levels by 50%, but did not significantly influence either cardiac NE or olfactory cortex dopamine. The results imply that brain NE facilitates attraction to familiar conspecific odors.

Animals↗
PubMed1982

Effects of heating on electrical activities of guinea pig olfactory cortical slices.

We examined the effect of heating on electrical activity of neurons in the guinea pig olfactory cortex slice. At the control temperature (37 degree C) the potential evoked by stimulation of the lateral olfactory tract consisted of an initial spike (IS) potential and a negative (N) potential. The IS potential is considered to be presynaptic and the other transsynaptic. The IS potential decreased in amplitude on heating and completely disappeared at 49 degree C. However, it recovered when the temperature was lowered to 37 degree C after five minutes of incubation at 49 degree C. In contrast, the N potential increased in amplitude at 39 degree C, was completely suppressed at 47 degree C and did not recover when the temperature was dropped to the control temperature. The maximum temperature from which the N potential recovered was 43 degree C. Unit activity was extracellularly recorded from neurons in the slice. On heating the brain slice some neurons showed an increase in activity others a decrease, and the rest were unaffected. We conclude that neurons in the olfactory cortex have different thermal sensitivities.

Animals↗
PubMed2005

Dynamics of learning-induced spine redistribution along dendrites of pyramidal neurons in rats.

We have previously shown that olfactory-discrimination (OD) learning is accompanied by enhanced spine density along proximal apical dendrites of layer II pyramidal neurons in the piriform (olfactory) cortex. Here we studied the temporal dynamics of learning-induced modifications in dendritic spine density throughout the dendritic trees of these neurons. We observed a transient increase in proximal apical spine density after OD learning, suggesting a strengthening of intrinsic excitatory inputs interconnecting neurons within the olfactory cortex. By contrast, the afferent pathway receiving direct input from the olfactory bulb shows spine pruning, suggesting that the connectivity is weakened. The changes in spine density can be attributed to a net change in number of spines, as the morphometric parameters of the dendrites are unaffected by learning. We suggest that spine density changes may represent a mechanism of selective synaptic reorganization required for olfactory learning consolidation.

Animals↗
PubMed1980

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.

Animals↗
PubMed2005

Cortical metabotropic glutamate receptors contribute to habituation of a simple odor-evoked behavior.

Defining the circuits that are involved in production and cessation of specific behaviors is an ultimate goal of neuroscience. Short-term behavioral habituation is the response decrement observed in many behaviors that occurs during repeated presentation of non-reinforced stimuli. Within a number of invertebrate models of short-term behavioral habituation, depression of a defined synapse has been implicated as the mechanism. However, the synaptic mechanisms of short-term behavioral habituation have not been identified within mammals. We have shown previously that a presynaptic metabotropic glutamate receptor (mGluR)-dependent depression of synapses formed by olfactory bulb afferents to the piriform (olfactory) cortex significantly contributes to adaptation of cortical odor responses. Here we show that blockade of mGluRs within the olfactory cortex of awake, behaving rats diminishes habituation of a simple odor-induced behavior, strongly implicating a central mechanism for sensory gating in olfaction.

Animals↗
PubMed1998

Cholinergic agonist carbachol enables associative long-term potentiation in piriform cortex slices.

Pyramidal cells in piriform (olfactory) cortex receive afferent input from the olfactory bulb as well as intrinsic association input from piriform cortex and other cortical areas. These two functionally distinct inputs terminate on adjacent apical dendritic segments of the pyramidal cells located in layer Ia and layer Ib of piriform cortex. Studies with bath-applied cholinergic agonists have shown suppression of the fast component of the inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of the association fibers. It was previously demonstrated that an associative form of LTP can be induced by coactivation of the two fiber systems after blockade of the fast, gamma-aminobutyric acid-A-mediated IPSP. In this report, we demonstrate that an associative form of long-term potentiation can be induced by coactivation of afferent and intrinsic fibers in the presence of the cholinergic agonist carbachol.

Afferent Pathways↗
PubMed2003

Odor-evoked activity is spatially distributed in piriform cortex.

Much data on the olfactory bulb (OB) indicates that structural characteristics of odorant molecules are encoded as ordered, spatially consolidated sets of active cells. New results with "genetic tracing" (Zou et al. [2001] Nature 414:173-179) suggest that spatial order is also present in projections from the OB to the olfactory cortex. For the piriform cortex (PC), results with this technique indicate that afferents conveying input derived from single olfactory receptors (ORs) are distributed to well-defined patches in the anterior PC (APC) but that these patches are much larger than in the OB. We have used c-fos induction to examine how input patterning for single ORs is translated into patterns of odor-evoked cellular activity in the PC. The laminar distribution of labeled cells and dual-immunostaining for gamma-aminobutyric acid (GABA)ergic markers indicated that activity was detected largely in pyramidal cells. In odor-stimulated rats, labeled cells were present throughout the posterior PC (PPC) but were concentrated in prominent rostrocaudal bands in APC. Analysis of responses to different odorants and concentrations revealed that locations and shapes of bands conveyed no apparent information regarding odor quality, rather, they appeared to correspond to subregions of the APC distinguished by cytoarchitecture and connectivity. Small-scale variations in labeling density were observed within APC bands and throughout the PPC that could reflect the presence of a complex topographical order, but discrete patches at consistent locations as observed by genetic tracing were absent. This finding suggests that as a result of afferent overlap and intracortical processing, odor-quality information is represented by spatially distributed sets of cells. A distributed organization may be optimal for discriminating biologically relevant odorants that activate large numbers of ORs.

Animals↗
PubMed1985

Down-regulation of alpha 2- and beta-adrenoceptor binding sites in rat cortex caused by amygdalar kindling.

The role of central noradrenergic neurons in kindled seizures was assessed by comparison of alpha 2- and beta-adrenoceptor binding in the cerebral cortex from kindled and control rats. To minimize handling, which may modify kindling-induced changes in binding, the kindling protocol involved stimulation of the amygdala every hour for a maximum of 26 h. Twenty-four hours after kindling, down-regulation of beta-adrenoceptors was found in both olfactory cortex and the remaining neocortex, whereas alpha 2 down-regulation was confined to the olfactory cortex. At 21 days after kindling, the only change found was a down-regulation of beta-adrenoceptors in the neocortex. The results support the view that functional changes in central noradrenergic transmission are associated with the reduction in seizure threshold induced by kindling.

Adrenergic Fibers↗
PubMed2001

The effects of lesions to the rat hippocampus or rhinal cortex on olfactory and spatial memory: retrograde and anterograde findings.

The role of the hippocampal system in retrograde and anterograde amnesia was investigated by using a novel olfactory-guided paradigm and a traditional test of spatial learning. In the retrograde study, rats were trained on a sequence of two-choice olfactory discriminations in the weeks prior to receiving neurotoxic lesions of the hippocampus or aspiration lesions of the perirhinal-entorhinal cortex. Memory tests for preoperatively learned discriminations revealed no statistical impairment for subjects with damage to the hippocampus on a problem learned remote in time from surgery (i.e., 4 weeks +) or on the two recently learned discriminations (i.e., 1-3 weeks prior to surgery). The performance of subjects with perirhinal-entorhinal damage provided an important comparison for subjects with specific hippocampal lesions. Despite showing intact memory for the remotely learned problem, perirhinal-entorhinal damage resulted in numerically (although not significantly) weaker performance on postoperative tests of retention for the discriminations learned in the 3 weeks prior to surgery. In the anterograde portion of the study, long-term memory for newly acquired discriminations was spared in subjects with damage to the hippocampus, whereas subjects in the perirhinal-entorhinal lesion group again showed the weakest memory performance on these tests of 5-day retention. Postoperative water maze learning was uniformly impaired in subjects with damage to the hippocampus and perirhinal-entorhinal cortex, thus confirming the effect of these lesions and supporting the involvement of these brain areas in spatial processes. These findings further dissociate the specific involvement of the hippocampus in tasks of a spatial-relational nature versus nonrelational tasks, such as discrimination learning and recognition memory (e.g., Duva et al., 1997; Eichenbaum, 1997; Eichenbaum, Schoenbaum, Young, & Bunsey, 1996). Moreover, the results suggest that damage to the hippocampus itself does not contribute to retrograde or anterograde memory impairments for all types of information, whereas the data suggest a more important role for the perirhinal-entorhinal cortex in recognition memory, irrespective of modality.

Animals↗
PubMed1996

Concentration and storage of biotin in the amphibian brain.

Prominent displays of endogenous biotin reactivity can be observed at specific locations in histochemical preparations of the forebrain and midbrain in the northern leopard frog (Rana pipiens) and common American toad (Bufo americanus). At the light microscopic level, the biotin reactivity appears in clusters of darkly stained puncta of either spherical or rodlike shape in the olfactory cortex, nucleus isthmi, and hypothalamus. With the electron microscope, the biotin reactive spheres are identified as neuronal varicosities and synaptic boutons and the rods as short segments of axons. Appropriate controls demonstrate that the punctate biotin-reactive structures are sites of concentration of biotin or a biotin analog in the processes of certain neurons. These data represent the first observation on the selective concentration of a vitamin in vertebrate neurons and suggest that biotin may have specialized functions in anatomically delimited areas of the central nervous system. Localization of the densest concentration of the biotin-reactive puncta in the dorsolateral prominence of the olfactory cortex may have relevance to the functional organization of the olfactory system. The distributions of biotin-reactive puncta were observed in laboratory-housed frogs and in wild toads captured in the summer months but were sparse or absent in batches of commercially obtained frogs examined immediately upon arrival in the laboratory. Systemic administration of biotin or biocytin hydrochloride did not alter the appearance or numbers of the biotin-reactive structures either in newly received or laboratory-housed frogs. These findings suggest that the capacity of the biotin-storage mechanism in the amphibian brain may be set by environmental factors and may be readily saturable from natural dietary or enteric sources.

Animals↗
PubMedDate not supplied

[Protocortex versus protomap: a perspective from the olfactory bulb].

INTRODUCTION AND AIM: The olfactory sensory system is a unique model for the research of guidance and connectivity of growing axons. During development, the olfactory epithelium, the olfactory bulb and the olfactory cortex differentiate several cell types and extend projection axons. Because there is a close relationship between these three structures, we ask the question as to whether establishment of the olfactory bulb central projections can proceed independently of the arrival of the olfactory sensory afferents. This raises another more general question: is establishment of afferent connections necessary to awake a developmental program in target cells?. DEVELOPMENT: The initial establishment of the olfactory bulb central projections occurs independently of the arrival of the olfactory axons from the olfactory epithelium, which reinforces the idea that cortical regions are already patterned before migration of newborn neurons, at least for the olfactory bulb and maybe for the entire brain. This implies a strict intrinsic molecular control of the distinct olfactory structures, independent one of each other. CONCLUSIONS: How then, do axonal projections find their correct way within the brain? Contact-mediated mechanisms and chemotropic molecules cooperate to fix their position in the telencephalon, prevent bulbar axons from invading structures other than the olfactory cortex and, at the same time, stimulate axonal branching in an orchestra of both, attractive/promoting and repulsive/inhibiting signals. At later stages, the mature appearance of the olfactory bulb will be completed and refined.

Animals↗
PubMed1988

An anatomical study of cholinergic innervation in rat cerebral cortex.

The cholinergic innervation of rat cerebral cortex was studied by immunohistochemical localization of choline acetyltransferase. Stained bipolar cells, fibers and terminals were found in all areas of cortex. The density of cholinergic terminals was similar in all cortical areas with the exception of entorhinal and olfactory cortex, which showed a marked increase in the number of stained terminals. A laminar distribution of cholinergic terminals was found in many cortical areas. In motor and most sensory areas, terminal density was high in layer 1 and upper layer 5, and lowest in layer 4. Visual cortex, in contrast to other cortical areas, was characterized by a dense band of innervation in layer 4. It has been known that the majority of cortical cholinergic structures derive from a projection to cortex from large, multipolar neurons in the basal forebrain, which stain heavily for choline acetyltransferase. In this study, stained fibers were observed to take three different pathways from basal forebrain to cortex. The first, confined to medial aspects of forebrain and cortex, was observed to originate in the septal area, from where fibers formed a discrete bundle, swinging forward around the rostral end of the corpus callosum, then travelling caudally in the cingulate bundle. The second was found to consist of fibers fanning out laterally from the area of the globus pallidus, travelling through the caudate, then continuing for various distances in the corpus callosum before finally turning into the cortex. A third pathway appeared to innervate olfactory and entorhinal cortex. Ibotenic acid injections were made in the area of the globus pallidus to study the effect of lesioning the lateral pathway on the cholinergic innervation in cortex. A major loss of choline acetyltransferase positive terminals was observed in neocortex, but retrosplenial, cingulate, entorhinal and olfactory cortex showed a normal density of cholinergic innervation. The borders separating areas with lesioned cholinergic input from non-lesioned areas were precise. The distribution of stained terminals remaining in cortical areas with lesioned basal forebrain innervation suggests that the basal forebrain projection to cerebral cortex, and not the intrinsic cortical cholinergic neurons, give rise to the laminar distribution of cholinergic terminals observed in normal cortex. To compare the relative densities of different cholinergic cortical systems, the distribution of choline acetyltransferase staining was compared with that of vasoactive intestinal polypeptide and substance P, which are co-localized in some choline acetyltransferase-positive neurons innervating cortex.

Animals↗