Noradrenergic suppression of synaptic transmission may influence cortical signal-to-noise ratio

Norepinephrine has been proposed to influence signal-to-noise ratio within cortical structures, but the exact cellular mechanisms underlying this influence have not been described in detail. Here we present data on a cellular effect of norepinephrine that could contribute to the influence on signal-to-noise ratio. In brain slice preparations of the rat piriform (olfactory) cortex, perfusion of norepinephrine causes a dose-dependent suppression of excitatory synaptic potentials in the layer containing synapses among pyramidal cells in the cortex (layer Ib), while having a weaker effect on synaptic potentials in the afferent fiber layer (layer Ia). Effects of norepinephrine were similar in dose-response characteristics and laminar selectivity to the effects of the cholinergic agonist carbachol, and combined perfusion of both agonists caused effects similar to an equivalent concentration of a single agonist. In a computational model of the piriform cortex, we have analyzed the effect of noradrenergic suppression of synaptic transmission on signal-to-noise ratio. The selective suppression of excitatory intrinsic connectivity decreases the background activity of modeled neurons relative to the activity of neurons receiving direct afferent input. This can be interpreted as an increase in signal-to-noise ratio, but the term noise does not accurately characterize activity dependent on the intrinsic spread of excitation, which would more accurately be described as interpretation or retrieval. Increases in levels of norepinephrine mediated by locus coeruleus activity appear to enhance the influence of extrinsic input on cortical representations, allowing a pulse of norepinephrine in an arousing context to mediate formation of memories with a strong influence of environmental variables.     

Filopodia and growth cones in the vertically migrating granule cells of the postnatal mouse cerebellum

The significance of cholinergic modulation for associative memory performance in the piriform cortex was examined in a study combining cellular neurophysiology in brain slices with realistic biophysical network simulations. Three different physiological effects of acetylcholine were identified at the single-cell level: suppression of neuronal adaptation, suppression of synaptic transmission in the intrinsic fibers layer, and activity-dependent increase in synaptic strength. Biophysical simulations show how these three effects are joined together to enhance learning and recall performance of the cortical network. Furthermore, our data suggest that activity-dependent synaptic decay during learning is a crucial factor in determining learning capability of the cortical network. Accordingly, it is predicted that acetylcholine should also enhance long-term depression in the piriform cortex.     

Beta-frequency (15-35 Hz) electroencephalogram activities elicited by toluene and electrical stimulation in the behaving rat

Bursts of beta-frequency (15-35 Hz) electroencephalogram activity occur in the olfactory system during odour sampling, but their mode of propagation within the olfactory system and potential contribution to the mechanisms of learning and memory are unclear. We have elicited large-amplitude beta activity in the rat olfactory system by applying noxious olfactory stimuli (toluene), and have monitored the bursts via chronically-implanted electrodes. Following exposure to toluene, coherent bursts with a peak frequency of 19.8 +/- 0.9 Hz were observed in the olfactory bulb, piriform cortex, entorhinal cortex and dentate gyrus. The timing of the bursts and the phases of electroencephalogram cross-spectra indicate that beta bursts propagate in a caudal direction from the olfactory bulb to the entorhinal cortex. The time delays between peaks of bursts in these structures were similar to latency differences for field potentials evoked by olfactory bulb or piriform cortex test-pulses. Peaks of burst cycles in the dentate region, however, were observed just prior to those in the entorhinal cortex. Surprisingly, power in toluene-induced beta-frequency oscillations was not increased following long-term potentiation induced by tetanic stimulation of the olfactory bulb, piriform cortex and entorhinal cortex. The activity of local inhibitory mechanisms may therefore counteract the effects of synaptic enhancements in afferent pathways during beta bursts. Low-frequency electrical stimulation of the piriform cortex was most effective in inducing coherent oscillatory responses in the entorhinal cortex and dentate gyrus at stimulation frequencies between 12 and 16 Hz. The results show that repetitive polysynaptic volleys at frequencies in the beta band induced by either toluene or electrical stimulation are transmitted readily within the olfactory system. The propagation of neural activity within this frequency range may therefore contribute to the transmission of olfactory signals to the hippocampal formation, particularly for those odours which induce high-amplitude bursts of beta activity.     

Protein synthesis inhibitors delay transneuronal death in the piriform cortex of young adult rats

It has been demonstrated that apoptotic cell death is an active process that is dependent on RNA and protein synthesis. The question remains as to whether neuronal death in adult, mammalian brains can also be demonstrated in vivo to be dependent on protein synthesis. To address this question we have analysed transneuronal death in the piriform (olfactory) cortex. Following unilateral olfactory bulb ablation in young adult rats, layer IIa of the piriform cortex undergoes rapid degeneration, that commences 12 h after ablation and that is almost complete at 48 h. In order to block protein synthesis, three to six subcutaneous injections of the short acting protein synthesis inhibitor anisomycin, were given at 2 h intervals beginning just before the ablation of the olfactory bulb. In other cases a single injection of the long acting protein synthesis inhibitor emetine were made intracerebrally just before or after olfactory bulb ablation. The number of dying cells was then counted in sections through the rostrocaudal extent of the piriform cortex. Both anisomycin and emetine injections markedly reduced the number of pyknotic cells in layer IIa of the piriform cortex after olfactory bulb ablation. The effect of anisomycin was dose-dependent, near lethal doses leading to an almost complete absence of cell death (six injections of 100 mg/kg). As the doses of anisomycin were reduced, more dying cells were observed. Emetine was only effective at near lethal doses (10 mg/kg) and showed a greater capacity to reduce the levels of cell death when injected into structures near the piriform cortex (e.g., accumbens nucleus) than when injected into more distant structures. To further confirm that the cell death observed was due to apoptosis, we analysed sections by tunel staining to demonstrate DNA fragmentation. We found that tunel-positive cells were also always pyknotic, one of the landmarks of apoptosis. The appearance of pyknotic cells labelled by the tunel method demonstrated that the dying cells in the piriform cortex did indeed undergo apoptosis.     

ATP-sensitive voltage- and calcium-dependent chloride channels in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

In addition to its role in olfaction and as a primary epileptogenic site, the anterior piriform cortex has been suggested to play a role in neuroperception of deficiencies or imbalances in physiologically essential amino acids. In recent studies, amino acid deficient diets were shown to induce expression of c-fos in the anterior piriform cortex within the rapid time frame associated with the normal anorectic response to such diets. It became important to examine the neurocytochemical architecture of this region for clues as to how and more precisely where dietary amino acid deficiency or imbalance might be monitored. The relationships of neuropeptide Y-, somatostatin-, and cholecystokinin-containing neurons were of particular interest because ongoing studies indicate that those peptides administered to the anterior piriform cortex alter intake of diets deficient in essential amino acids. The neuropeptides were endogenous to intrinsic neurons only; none resembled pyramidal projection neurons. Peptidergic neurons and fibers were concentrated most heavily in layer III of the paleocortex. The cytoarchitecture suggests that neuropeptide Y-, somatostatin-, and cholecystokin-containing neurons of the anterior piriform cortex may relate synaptically or multisynaptically to local circuit neurons during electrical activity, modulation of olfactory information, and neuroperception of essential amino acids.     

A European inter-laboratory trial to evaluate the reliability of serological diagnosis of bovine herpesvirus 1 infections

Selective suppression of synaptic transmission during learning is proposed as a physiological mechanism for combining associative memory function at feedback synapses with self-organization of feedforward synapses in neocortical structures. A computational model demonstrates how selective suppression of feedback transmission allows this combination of synaptic function. During learning, sensory stimuli and the desired response are simultaneously presented as input to the network. Feedforward connections form self-organized representations of input, while suppressed feedback connections learn the transpose of the feedforward connectivity. During recall, suppression of transmission is removed, input activates the self-organized representation, and activity settles into a learned solution to the problem. This computational model can be used for learning of problems which are not linearly separable, including the negative patterning task (the XOR problem). Experiments in brain slice preparations of the rat somatosensory cortex tested whether the combination of self-organization and associative memory function could be provided by cholinergic suppression selective for feedback versus feedforward synapses. The cholinergic agonist carbachol selectively suppressed synaptic potentials elicited by stimulation of layer I (which contains a high percentage of feedback synapses), while having no effect on synaptic potentials elicited by stimulation of layer IV (with a high percentage of afferent and feedforward synapses).     

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex. J. Neurophysiol. 78: 2546-2559, 1997. Physiological analysis in the companion paper demonstrated that gamma-aminobutyric acid-A (GABAA)-mediated inhibition in piriform cortex is generated by circuits that are largely independent in apical dendritic and somatic regions of pyramidal cells and that GABAA-mediated inhibitory postsynaptic currents (IPSCs) in distal dendrites have a slower time course than those in the somatic region. This study used modeling methods to explore these characteristics of GABAA-mediated inhibition with respect to regulation of the N-methyl--aspartate (NMDA) component of excitatory postsynaptic potentials. Such regulation is relevant to understanding NMDA-dependent long-term potentiation (LTP) and the integration of repetitive synaptic inputs that can activate the NMDA component as well as pathological processes that can be activated by overexpression of the NMDA component. A working hypothesis was that the independence and differing properties of IPSCs in apical dendritic and somatic regions provide a means whereby the NMDA component and other dendritic processes can be controlled by way of GABAergic tone without substantially altering system excitability. The analysis was performed on a branched compartmental model of a pyramidal cell in piriform cortex constructed with physiological and anatomic data derived by whole cell patch recording. Simulations with the model revealed that NMDA expression is more effectively blocked by the slow GABAA component than the fast. Because the slow component is present in greater proportion in apical dendritic than somatic regions, this characteristic would increase the capacity of dendritic IPSCs to regulate NMDA-mediated processes. The simulations further revealed that somatic-region GABAergic inhibition can regulate the generation of action potentials with little effect on the NMDA component generated by afferent fibers in apical dendrites. As a result, if expression of the NMDA component or other dendritic processes were enabled by selective block of dendritic inhibition, for example, by centrifugal fiber systems that may regulate learning and memory, the somatic-region IPSC could preserve system stability through feedback regulation of firing without counteracting the effect of the dendritic-region block. Simulations with paired inputs revealed that the dendritic GABAA-mediated IPSC can regulate the extent to which a strong excitatory input facilitates the NMDA component of a concurrent weak input, providing a possible mechanism for control of "associative LTP" that has been demonstrated in this system. Postsynaptic GABAB-mediated inhibition had less effect on the NMDA component than either the fast or slow GABAA components. Depolarization from a concomitant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component also was found to have comparatively little effect on current through the NMDA channel because of its brief time course.     

CD28-costimulation activates cyclic AMP-responsive element-binding protein in T lymphocytes

The retrosplenial cortex (RSC) receives cholinergic afferent fibers from the medial septal nucleus and diagonal band of Broca (DBB) by way of the cingulate bundle and the fornix. Bilateral lesions of both the cingulate and fornix pathways result in a complete depletion of cholinergic input to the RSC. In the present study we have examined the effects of transplanting cholinergic neurons from fetal rat pups to the RSC of adult rats following lesions of the cingulate bundle and fornix. The animals with lesions exhibited severe spatial memory impairments with a complete loss of extrinsic cholinergic afferents to the RSC. Animals with intraretrosplenial cortical transplants exhibited significant improvements in learning and memory performance as revealed by decreased escape latencies in spatial reference memory tests, increased numbers of platform crossings in spatial navigation tests, and a higher percentage of correct choices in a spatial working memory task. These improvements appeared to be cholinergically mediated because atropine administration significantly disrupted spatial navigation performance. The survival of the transplanted cholinergic neurons and their innervation of the RSC were characterized using a monoclonal antibody to choline acetyltransferase (ChAT). The staining of graft-derived ChAT-positive fibers also revealed a pattern of innervation that mimicked that of the cholinergic input in normal animals. These results indicate that intraretrosplenial cortical transplants of cholinergic neurons can rectify spatial memory deficits produced by the loss of intrinsic cholinergic afferents from the medial septal nucleus.     

Synaptic correlates of odor habituation in the rat anterior piriform cortex

Responses of anterior piriform cortex layer II/III neurons to both odors and electrical stimulation of the lateral olfactory tract (LOT) were measured with intracellular recordings in urethan-anesthetized, freely breathing rats. Odor-evoked, respiration-entrained postsynaptic potentials (PSPs) rapidly habituated during a 50-s odor stimulus, then spontaneously recovered within 2 min of odor termination. Associated with the decrease in odor-evoked PSP amplitude was a decrease in the monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of the LOT. The decrement in LOT-evoked EPSPs recovered with a time course similar to the odor response recovery. These results demonstrate that odor habituation is associated with a decrease in afferent synaptic efficacy in the anterior piriform cortex.     

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.     

Anatomical localization and time course of Fos expression following soman-induced seizures

Soman (pinacolymethylphosphonofluoridate), a highly potent, irreversible inhibitor of cholinesterase, causes intense convulsions, neuropathology and, ultimately, death. There is evidence that certain brain structures are selectively vulnerable to the pathological consequences of soman-induced seizures. A working hypothesis is that central nervous system (CNS) structures with the earliest and most severe signs of neuropathology may be key sites for the initiation of the seizures. Fos, the immediate-early gene product, increases rapidly in several animal seizure models. Thus, we reasoned that the earliest brain regions to express Fos might be involved in the initiation and maintenance of soman-induced convulsions. To assess this, rats were injected with a single, convulsive dose of soman (77.7 micrograms/kg, i.m.). The animals were euthanized and processed for immunocytochemical analysis at several time points. Robust Fos expression was seen in layer II of the piriform cortex and the noradrenergic nucleus locus coeruleus within 30-45 minutes. One hour following soman injection, staining was more intense in the piriform cortex layer II and in the locus coeruleus. In addition, Fos was evident in the piriform cortex layer III, the entorhinal cortex, the endopiriform nucleus, the olfactory tubercle, the anterior olfactory nucleus and the main olfactory bulb. By 2 hours, Fos staining was present throughout the cerebral cortex, thalamus, caudate-putamen and the hippocampus. At 8 hours and beyond, Fos expression returned to control levels throughout the CNS except for the piriform cortex and the locus coeruleus which still had robust labeling. By 24 hours, neuropathology was evident throughout the rostral-caudal extent of layer II of the piriform cortex. The rapid induction of Fos in the piriform cortex and the locus coeruleus, taken together with previous anatomical, eletrophysiological and neurochemical studies, suggests that prolonged, excessive exposure to synaptically released acetylcholine and norepinephrine triggers the production of soman-induced seizures initially in the piriform cortex and subsequently in other cortical and subcortical structures.     

Deafferentation causes apoptosis in cortical sensory neurons in the adult rat

The present study provides an experimental model of the apoptotic death of pyramidal neurons in rat olfactory cortex after total bulbectomy. Terminal transferase (TdT)-mediated deoxyuridine triphosphate (d-UTP)-biotin nick end labeling (TUNEL), DNA electrophoresis, and neuronal ultrastructure were used to provide evidence of apoptosis; neurons in olfactory cortex were counted by stereology. Maximal TUNEL staining occurred in the piriform cortex between 18 and 26 hr postbulbectomy. Within the survival times used in the present study (up to 48 hr postlesion), cell death was observed exclusively in the piriform cortex; there was no evidence of cell death in any other areas connected with the olfactory bulb. Neurons undergoing apoptosis were pyramidal cells receiving inputs from, but not projecting to, the olfactory bulb. The apical dendrites of these neurons were contacted by large numbers of degenerating axonal terminals. Gel electrophoresis of DNA purified from lesioned olfactory cortex showed a ladder pattern of fragmentation. Inflammatory cells or phagocytes were absent in the environment of degenerating neurons in the early stages of the apoptotic process. The present model suggests that deafferentation injury in sensory systems can cause apoptosis. In addition, olfactory bulbectomy can be used for investigating molecular mechanisms that underlie apoptosis in mature mammalian cortical neurons and for evaluating strategies to prevent the degeneration of cortical neurons.     

Investigation of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone for in vivo and iIn vitro murine embryopathy and embryonic ras mutations

The evoked potential recorded in the rat piriform cortex in response to electrical stimulation of the olfactory bulb is composed of an early component occasionally followed by a late component (60-70 ms). We previously showed that the late component occurrence was enhanced following an olfactory learning. In the present study carried out in naive rats, we investigated the precise conditions of induction of this late component, and its spatiotemporal distribution along the olfactory pathways. In the anaesthetized rat, a stimulating electrode was implanted in the olfactory bulb. Four recording electrodes were positioned, respectively, in the olfactory bulb, the anterior and posterior parts of the piriform cortex, and the entorhinal cortex. Simultaneous recording of signals evoked in the four sampled structures in response to stimulation of the olfactory bulb revealed that the late component was detected in anterior and posterior piriform cortex as well as in entorhinal cortex, but not in the olfactory bulb. The late component occurred reliably for a narrow range of low intensities of stimulation delivered at frequencies not exceeding 1 Hz. Comparison of late component amplitude and latency across the different recorded sites showed that this component appeared first and with the greatest amplitude in the posterior piriform cortex. In addition to showing a functional dissociation between anterior and posterior parts of the piriform cortex, these data suggest that the posterior piriform cortex could be the locus of generation of this late high amplitude synchronized activity, which would then propagate to the neighbouring regions.     

Intrinsic innervation of a reptilian esophagus (Podarcis hispanica)

Localization of metabotropic glutamate receptor subtypes, mGluR1, mGluR1alpha, mGluR2/3, mGluR4a, mGluR5, mGluR7a, mGluR7b, and mGluR8, was examined in some of the target areas of projection fibers from the main and accessory olfactory bulbs (MOB and AOB) by using subtype-specific antibodies. The superficial layer of the olfactory tubercle and layer Ia of the piriform cortex, the target areas of MOB, showed marked mGluR1-, mGluR5-, mGluR7a-, and mGluR8-like immunoreactivities (-LI), and rather weak mGluR2/3-LI. The periamygdaloid cortical region including the target areas of both MOB and AOB showed intense mGluR2/3-LI as well as marked mGluR1-, mGluR5-, mGluR7a-, and mGluR8-LI. No significant mGluR1alpha-, mGluR4a-, or mGluR7b-LI was seen in these regions. After transection of the lateral olfactory tract, mGluR2/3-, mGluR7a-, and mGluR8-LI were reduced markedly in the target regions on the side ipsilateral to the transection; no significant changes were detected in mGluR1- or mGluR5-LI. Double labeling experiments indicated light and electron microscopically colocalization of mGluR7a- and mGluR8-LI in axon terminals on dendritic shafts of presumed interneurons in the superficial layer of the olfactory tubercle and layer Ia of the piriform cortex. Electron microscopically mGluR2/3-LI was seen in preterminal and terminal portions of axons, whereas mGluR7a- and mGluR8-LI were associated with presynaptic membrane specialization. Immunolabeled axon terminals were filled with round synaptic vesicles and constituted asymmetric synapses with dendritic profiles. The results suggest that glutamate release from axon terminals of projection fibers from MOB and AOB is regulated presynaptically and differentially through mGluR2/3, mGluR7a, and/or mGluR8.     

Olfactory-evoked regional cerebral blood flow in Alzheimer's disease.

Olfaction is impaired in Alzheimer's disease (AD). It was hypothesized that AD would reduce olfactory-evoked perfusion in mesial temporal olfactory (piriform) cortex, where neuropathology begins. Seven AD patients and 8 elderly controls (ECs) underwent olfactory threshold and identification tests and olfactory stimulation during positron emission tomography. Odor identification was impaired in AD, but threshold was not. Olfactory stimulation in ECs activated right and left piriform areas and right anterior ventral temporal cortex. AD patients had less activation in right piriform and anterior ventral temporal cortex but not in the left piriform area. Although orbital cortex did not activate in ECs, there was a significant between-groups difference in this area. Right piriform activation correlated with odor identification. Impaired odor identification likely reflects sensory cortex dysfunction rather than cognitive impairment. Given olfactory bulb projections to the mesial temporal lobe, olfactory stimulation during functional imaging might detect early dysfunction in this region. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis. J. Neurophysiol. 78: 2602-2615, 1997. The entorhinal cortex receives sensory inputs from the piriform cortex and modulatory inputs from the medial septum. To examine short-term synaptic facilitation effects in these pathways, current source density (CSD) analysis was used first to localize the entorhinal cortex membrane currents, which generate field potentials evoked by stimulation of these afferents. Field potentials were recorded at 50-micron intervals through the medial entorhinal cortex in urethan-anesthetized rats and the one-dimensional CSD was calculated. Piriform cortex stimulation evoked a surface-negative, deep-positive field potential component in the entorhinal cortex with mean onset and peak latencies of 10.4 and 18.4 ms. The component followed brief 100-Hz stimulation, consistent with a monosynaptic response. CSD analysis linked the component to a current sink, which often began in layer I before peaking in layer II. A later, surface-positive field potential component peaked at latencies near 45 ms and was associated with a current source in layer II. Medial septal stimulation evoked positive and negative field potential components which peaked at latencies near 7 and 16 ms, respectively. A weaker and more prolonged surface-negative, deep-positive component peaked at latencies near 25 ms. The early components were generated by currents in the hippocampal formation, and the late surface-negative component was generated by currents in layers II to IV of the entorhinal cortex. Short-term facilitation effects in conscious animals were examined using electrodes chronically implanted near layer II of the entorhinal cortex. Paired-pulse stimulation of the piriform cortex at interpulse intervals of 30 and 40 ms caused the largest facilitation (248%) of responses evoked by the second pulse. Responses evoked by medial septal stimulation also were facilitated maximally (59%) by a piriform cortex conditioning pulse delivered 30-40 ms earlier. Paired pulse stimulation of the medial septum caused the largest facilitation (149%) at intervals of 70 ms, but piriform cortex evoked responses were facilitated maximally (46%) by a septal conditioning pulse 100-200 ms earlier. Frequency potentiation effects were maximal during 12- to 18-Hz stimulation of either the piriform cortex or medial septum. Occlusion tests suggested that piriform cortex and medial septal efferents activate the same neurons. The CSD analysis results show that evoked field potential methods can be used effectively in chronically prepared animals to examine synaptic responses in the converging inputs from the piriform cortex and medial septum to the entorhinal cortex. The short-term potentiation phenomena observed here suggest that low-frequency activity in these pathways during endogenous oscillatory states may enhance entorhinal cortex responsivity to olfactory inputs.     

Cortical afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We have divided the cortical regions surrounding the rat hippocampus into three cytoarchitectonically discrete cortical regions, the perirhinal, the postrhinal, and the entorhinal cortices. These regions appear to be homologous to the monkey perirhinal, parahippocampal, and entorhinal cortices, respectively. The origin of cortical afferents to these regions is well-documented in the monkey but less is known about them in the rat. The present study investigated the origins of cortical input to the rat perirhinal (areas 35 and 36) and postrhinal cortices and the lateral and medial subdivisions of the entorhinal cortex (LEA and MEA) by placing injections of retrograde tracers at several locations within each region. For each experiment, the total numbers of retrogradely labeled cells (and cell densities) were estimated for 34 cortical regions. We found that the complement of cortical inputs differs for each of the five regions. Area 35 receives its heaviest input from entorhinal, piriform, and insular areas. Area 36 receives its heaviest projections from other temporal cortical regions such as ventral temporal association cortex. Area 36 also receives substantial input from insular and entorhinal areas. Whereas area 36 receives similar magnitudes of input from cortices subserving all sensory modalities, the heaviest projections to the postrhinal cortex originate in visual associational cortex and visuospatial areas such as the posterior parietal cortex. The cortical projections to the LEA are heavier than to the MEA and differ in origin. The LEA is primarily innervated by the perirhinal, insular, piriform, and postrhinal cortices. The MEA is primarily innervated by the piriform and postrhinal cortices, but also receives minor projections from retrosplenial, posterior parietal, and visual association areas.     

Innervation of cholinergic vestibular efferent system in vestibular periphery of rats

The innervation of cholinergic efferent fibers in the vestibular endorgans of the rats was investigated using a modified preembedding immunostaining technique of immunoelectron microscopy. A monoclonal antibody to choline acetyltransferase (ChAT) was used as a marker of cholinergic fibers. It was found that there were four types of cholinergic innervation in the vestibular endorgans of the rat: (1) cholinergic nerve endings formed axo-dendritic synapses with afferent chalice surrounding the type I sensory hair cells; (2) cholinergic nerve endings formed axo-somatic synapses with type II hair cells; (3) cholinergic fibers synapse with afferent nerve fibers and (4) a synaptic contact developed between cholinergic nerve endings. The results demonstrated that a multiform innervation of the cholinergic efferents exists in the rats vestibular periphery.     

MR cholangiography in children after liver transplantation from living related donors

A highly specific anti-glutamate monoclonal antibody, mAb2D7, was used together with light and electron microscopy to elucidate the role played by the amino acid glutamate in the projection from the olfactory bulb to the piriform cortex in the rat. By light microscopy, glutamate-like immunoreactivity was observed in neuronal cell bodies and in the neuropil of the piriform cortex. Double labelling experiments which involved injections of wheat germ agglutinin-horse--radish peroxidase into the olfactory bulb and a post-embedding immunogold method for electron microscopy revealed anterogradely labelled terminals making asymmetric synaptic contacts on dendrites in the piriform cortex which contained high levels of glutamate as assessed by quantification. These results further support a role for glutamate as a neurotransmitter in the efferent pathway of the rat olfactory bulb.     

Bcl-2 protein as a marker of neuronal immaturity in postnatal primate brain

The distribution of neurons expressing immunoreactivity for the protein Bcl-2 was studied in the brain of squirrel monkeys (Saimiri sciureus) of various ages. Several subsets of small and intensely immunoreactive neurons displaying an immature appearance were disclosed in the amygdala and piriform cortex. The piriform cortex exhibited clusters of various forms in which Bcl-2+ neurons appeared linked to one another by their own neurites. The subventricular zone, which is known to harbor the largest population of rapidly and constitutively proliferating cells in the adult rat brain, was intensely stained, particularly at the basis of the lateral ventricle. A long and dorsoventrally oriented Bcl-2+ fiber fascicle was seen to emerge from the subventricular zone, together with numerous Bcl-2+ cells that formed a densely packed column directed at the olfactory tubercle. In adult and aged monkeys, the small and intensely labeled neurons were progressively replaced by larger and more weakly stained neurons in the amygdala and piriform cortex. In contrast, Bcl-2 immunostaining did not change with age in the subventricular zone and olfactory tubercle, the islands of Calleja of which were markedly enriched with Bcl-2. The dentate gyrus contained only a few layers of intensely labeled granule cells in juvenile monkeys, but the number of these layers increased markedly in adult and aged monkeys. These findings suggest that Bcl-2 can serve as a marker of both proliferating and differentiating neurons and indicate that such immature neurons may be much more widespread than previously thought in postnatal primate brain.