An autoradiographic study of complementary laminar patterns of termination of afferent fibers to the olfactory cortex.
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Amygdala↗
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Changes in the electrical activity of the olfactory cortex induced by radiation and drugs.
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Animals↗
Establishment of normal synaptic density in deafferented olfactory cortex.
The piriform cortex has been examined in rats following neonatal deafferentation produced by olfactory bulb removal. The denervated molecular layer of the piriform cortex grows to nearly its full adult thickness and the removal olfactory bulb axons are replaced by intracortical axons. The synaptic density of the molecular layer following this rearrangement is the same as that in the normal cortex.
Aging↗
Functional heterogeneity in human olfactory cortex: an event-related functional magnetic resonance imaging study.
Studies of patients with focal brain injury indicate that smell perception involves caudal orbitofrontal and medial temporal cortices, but a more precise functional organization has not been characterized. In addition, although it is believed that odors are potent triggers of emotion, support for an anatomical association is scant. We sought to define the neural substrates of human olfactory information processing and determine how these are modulated by affective properties of odors. We used event-related functional magnetic resonance imaging (fMRI) in an olfactory version of a classical conditioning paradigm, whereby neutral faces were paired with pleasant, neutral, or unpleasant odors, under 50% reinforcement. By comparing paired (odor/face) and unpaired (face only) conditions, odor-evoked neural activations could be isolated specifically. In primary olfactory (piriform) cortex, spatially and temporally dissociable responses were identified along a rostrocaudal axis. A nonhabituating response in posterior piriform cortex was tuned to all odors, whereas activity in anterior piriform cortex reflected sensitivity to odor affect. Bilateral amygdala activation was elicited by all odors, regardless of valence. In posterior orbitofrontal cortex, neural responses evoked by pleasant and unpleasant odors were segregated within medial and lateral segments, respectively. The results indicate functional heterogeneity in areas critical to human olfaction. They also show that brain regions mediating emotional processing are differentially activated by odor valence, providing evidence for a close anatomical coupling between olfactory and emotional processes.
Adolescent↗
Calcium-dependent action potentials in guinea-pig olfactory cortex neurones.
Ca2+-dependent action potentials were recorded in guinea pig olfactory neurones in vitro (23 degrees--25 degrees C). In most cells (in the presence of tetrodotoxin: TTX) the current-voltage relationship displayed 'anomalous' rectification (apparent high slope resistance) at potentials approximately 20 mV depolarized to the resting membrane potential (approximately -80 mV) and strong outward rectification at more positive potentials. Intracellular Cs+-loading blocked outward rectification and increased action potential duration. Such spikes were TTX-insensitive and were further prolonged by external addition of tetraethylammonium (TEA) or Ba2+. Spikes recorded from Cs+-loaded, TTX/TEA-treated neurones displayed a prolonged plateau and an after-depolarization. They persisted when Ba2+ or Sr2+ were substituted for external Ca2+, but not when Mg2+ was the sole extracellular divalent cation. The spikes were blocked in the presence of Cd2+ but persisted when 82% of the extracellular Na+ was substituted by choline. A TTX-insensitive, slowly inactivating inward current at depolarized potentials is believed to account for the subthreshold 'anomalous' rectification and prolonged spike plateau.
Acetates↗
Sox1-deficient mice suffer from epilepsy associated with abnormal ventral forebrain development and olfactory cortex hyperexcitability.
Mutations in several classes of embryonically-expressed transcription factor genes are associated with behavioral disorders and epilepsies. However, there is little known about how such genetic and neurodevelopmental defects lead to brain dysfunction. Here we present the characterization of an epilepsy syndrome caused by the absence of the transcription factor SOX1 in mice. In vivo electroencephalographic recordings from SOX1 mutants established a correlation between behavioral changes and cortical output that was consistent with a seizure origin in the limbic forebrain. In vitro intracellular recordings from three major forebrain regions, neocortex, hippocampus and olfactory (piriform) cortex (OC) showed that only the OC exhibits abnormal enhanced synaptic excitability and spontaneous epileptiform discharges. Furthermore, the hyperexcitability of the OC neurons was present in mutants prior to the onset of seizures but was completely absent from both the hippocampus and neocortex of the same animals. The local inhibitory GABAergic neurotransmission remained normal in the OC of SOX1-deficient brains, but there was a severe developmental deficit of OC postsynaptic target neurons, mainly GABAergic projection neurons within the olfactory tubercle and the nucleus accumbens shell. Our data show that SOX1 is essential for ventral telencephalic development and suggest that the neurodevelopmental defect disrupts local neuronal circuits leading to epilepsy in the SOX1-deficient mice.
Animals↗
Muscarinic receptor activation induces a prolonged post-stimulus afterdepolarization with a conductance decrease in guinea-pig olfactory cortex neurones in vitro.
The persistent excitation of guinea-pig olfactory cortical neurones in vitro by the muscarinic agonist oxotremorine-M (OXO-M) was investigated. In OXO-M (10-20 microM), a slowly-decaying afterdepolarization (sADP) accompanied by sustained repetitive firing was induced following a long depolarizing stimulus. The corresponding slow inward current (IADP) revealed under voltage clamp behaved like a K(+)-mediated tail current, but was associated with a decreased membrane conductance. IADP was insensitive to tetrodotoxin (TTX), Ba2+, Cs+, or 4-aminopyridine (4-AP), but was blocked by 500 microM TEA or TBA (tetrabutylammonium). The OXO-M response and IADP were also reduced by Cd2+ or Ca(2+)-free solution, suggesting a dependence on Ca(2+)-entry. We propose that OXO-M induces a novel outward K+ current that can be slowly de-activated by Ca(2+)-entry during a depolarizing stimulus. Summation of IADP tail currents could contribute to the sustained muscarinic excitation of mammalian cortical neurones.
4-Aminopyridine↗
Laminar analysis of 2-deoxyglucose uptake in olfactory bulb and olfactory cortex of rabbit and rat.
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Animals↗
Muscarinic receptors mediating suppression of the M-current in guinea-pig olfactory cortex neurones may be of the M2-subtype.
Guinea-pig olfactory cortical neurones in vitro were voltage clamped by means of a single intracellular microelectrode technique. Hyperpolarizing voltage commands from holding potentials between -40 to -50 mV produced slow inward current relaxations reflecting deactivation of the M-current (IM). IM was reversibly suppressed by 30 microM muscarine or carbachol; this suppression was insensitive to pirenzepine (up to 300 nM) but was inhibited by gallamine (10-20 microM) or 4-diphenyl-acetoxy-N-methylpiperidine (100, 500 nM), suggesting the involvement of the M2-type muscarinic receptor.
Animals↗
Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat. I. Anatomical observations.
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.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Independent development of presynaptic specializations in olfactory cortex of the fetal rat.
Electron microscopy was used to study synaptogenesis in prepyriform cortex of fetal rat pups during early stages of synapse formation. Of special interest is the frequent occurrence of unapposed, developing synaptic specializations in axon and growth cone profiles. The location and morphology of the unapposed specializations suggests that they are presynaptic in nature. These presumably immature presynaptic specializations are found in the lateral olfactory tract and subjacent cortex. Intermediate forms between uncontacted presynaptic specializations and definitive synapses suggest a synaptogenic sequence in which initial development of an immature presynaptic specialization begins without apposition of a postsynaptic element at that location. This implies that initiation of presynaptic development is not dependent upon postsynaptic contact and also raises the question of whether synaptic contacts could be established via presynaptic induction of postsynaptic formation.
Animals↗
Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat. II. Physiological studies.
Field potentials and unit activity elicited by electrical stimulation of the olfactory bulb (OB) and anterior and posterior prepiriform cortex (PPCa and PPCp) were measured extracellularly in the entorhinal cortex (EC) of the cat. Different topographic distributions of the amplitude and peak latency of average evoked potentials (AEPs) were obtained depending on the stimulated area. The maximal evoked activity in the EC showed a gradient in a latero-medial direction with the extremes corresponding to the stimulation of OB and PPCp respectively. Analysis of firing patterns of units in the EC in response to stimulation of the OB, PPCa and PPCp showed that an appreciable number of units responded to stimulation of different areas, mainly PPCa and PPCp. It was found that the pathways being stimulated differed in conduction velocities with the PPCp-EC being the slowest. Most responding units lay in layer I and II of the EC. The AEPs to PPCa- and PPCp-stimulation presented different types of depth profiles. Stimulation of the PPCa evoked an initial surface-negative depth-positive potential whereas the PPCp evoked a different type of AEP with an initial positive component at the surface and negative in depth. It is assumed that the stimulated fibres have their active synapses at different levels within the superficial layers of the EC. The possibility of direct influence of olfactory inputs on the hippocampus mediated by one synapse in the EC is discussed.
Animals↗
[Analysis of the formation of evoked potentials in neurons of the rat olfactory cortex].
Evoked focal potentials which were induced in vitro in a slice of olfactory tract by stimulation of the lateral olfactory tract (LOT) have been studied. The potential consisted of an initial biphasic wave, the compound action potential of LOT, population synaptic responses, and population spike. Functional significance and possible mechanisms of changes of different focal potential waves have been discussed.
Animals↗
Sniffing and smelling: separate subsystems in the human olfactory cortex.
The sensation and perception of smell (olfaction) are largely dependent on sniffing, which is an active stage of stimulus transport and therefore an integral component of mammalian olfaction. Electrophysiological data obtained from study of the hedgehog, rat, rabbit, dog and monkey indicate that sniffing (whether or not an odorant is present) induces an oscillation of activity in the olfactory bulb, driving the piriform cortex in the temporal lobe, in other words, the piriform is driven by the olfactory bulb at the frequency of sniffing. Here we use functional magnetic resonance imaging (fMRI) that is dependent on the level of oxygen in the blood to determine whether sniffing can induce activation in the piriform of humans, and whether this activation can be differentiated from activation induced by an odorant. We find that sniffing, whether odorant is present or absent, induces activation primarily in the piriform cortex of the temporal lobe and in the medial and posterior orbito-frontal gyri of the frontal lobe. The source of the sniff-induced activation is the somatosensory stimulation that is induced by air flow through the nostrils. In contrast, a smell, regardless of sniffing, induces activation mainly in the lateral and anterior orbito-frontal gyri of the frontal lobe. The dissociation between regions activated by olfactory exploration (sniffing) and regions activated by olfactory content (smell) shows a distinction in brain organization in terms of human olfaction.
Adult↗
Functional localization and lateralization of human olfactory cortex.
Anatomical and physiological investigations in monkeys indicate that olfaction is subserved by several cortical regions. But the areas implicated in the human olfactory system have not been definitively identified by functional criteria. Behavioural evidence has suggested that laterally specialized mechanisms for odour processing may exist, but the neuroanatomical substrate remains unknown. We used positron emission tomography to study the cortical representation of human olfactory processing by comparing cerebral blood flow changes evoked during olfactory stimulation with those of a control task. We report here significant cerebral blood flow increases at the junction of the inferior frontal and temporal lobes bilaterally, corresponding to the piriform cortex, and unilaterally, in the right orbitofrontal cortex. The results complement and extend previous data implicating these regions in olfactory processing, and indicate that a functional asymmetry exists in the human brain favouring the right orbitofrontal area in olfaction.
Cerebrovascular Circulation↗
Scopolamine enhances generalization between odor representations in rat olfactory cortex.
Acetylcholine (ACh) has a critical, modulatory role in plasticity in many sensory systems. In the rat olfactory system, both behavioral and physiological data indicate that ACh may be required for normal odor memory and synaptic plasticity. Based on these data, neural network models have hypothesized that ACh muscarinic receptors reduce interference between learned cortical representations of odors within the piriform cortex. In this study, odor receptive fields of rat anterior piriform cortex (aPCX) single-units for alkane odors were mapped before and after either a systemic injection of the muscarinic receptor antagonist scopolamine (0.5 mg/kg) or aPCX surface application of 500 microM scopolamine (or saline/ACSF controls). Cross-habituation between alkanes differing by two to four carbons was then examined following a 50-sec habituating stimulus. The results demonstrate that neither aPCX spontaneous activity nor odor-evoked activity (receptive field) was affected by scopolamine, but that cross-habituation in aPCX neurons was enhanced significantly by either systemic or cortical scopolamine. These results indicate that scopolamine selectively enhances generalization between odor representations in aPCX in a simple memory task. Given that ACh primarily affects intracortical association fibers in the aPCX, the results support a role for the association system in odor memory and discrimination and indicate an important ACh modulatory control over this basic sensory process.
Animals↗
[Endogenous regulators of long-term potentiation and depression in slices of the olfactory cortex in rats].
Two fractions of the perfusate collected during tetanic stimulation of the rat lateral olfactory tract, were studied. The > 50 kDa fractions induced a long-term depression of focal potentials, whereas the < 50 fractions mainly induced an activation. The findings suggest a release of a number of neurohumoral substances by the donor cells during tetanisation, the substances being polypeptides, most probably, modulating the synaptic plasticity.
Animals↗
Dissociated representations of irritation and valence in human primary olfactory cortex.
Irritation and negative valence are closely associated in perception. However, these perceptual aspects can be dissociated in olfaction where irritation can accompany both pleasant and unpleasant odorants. Whereas the sensation of odor reflects transduction at olfactory receptors, irritation reflects concurrent transduction of the odorant at trigeminal receptors. Thus a stimulus can be either a pure olfactant activating the olfactory receptors only or a bimodal odorant activating both types of receptors. Using event-related functional magnetic resonance imaging and a 2 x 2 experimental design contrasting odorant valence (pleasant/unpleasant) and odorant type (pure olfactant/bimodal) we found activity in piriform cortex to be associated with valence, and not type, of odors. In contrast, activity in the olfactory tubercle was associated with type, and not valence, of odors. Importantly, this was found when perceived intensity was held equal across odorants. These findings suggest that dissociable neural substrates subserve the encoding of irritation and valence in olfaction.
Adult↗