Structure of the piriform cortex of the opossum. II. Fine structure of cell bodies and neuropil

The fine structure of cell bodies and neuropil in the piriform cortex of the opossum has been examined. A close similarity in ultrastructure of many features has been demonstrated between this pylogenetically old cortex in a primitive mammal and the neocortex of higher mammals. Cell bodies of pyramidal cells are very similar to those in the neocortex: The nucleus is pale with a smooth surface, the cytoplasm has a modest number of organelles, and the soma receives a small number of exclusively symmetrical synapses. Semilunar cells, which have apical but no basal den-drites, are very similar to pyramidal cells in ultrastructure of their cell bodies. Two populations of neurons with nonpyramidal ultrastructural features have been distinguished: (1) cells in layer III that closely resemble the well-known large multipolar cells in neocortex by virtue of a large number of symmetrical and asymmetrical somatic synapses and long cisterns of rough endoplasmic reticulum (ER); and (2) large cells in layer I with very few somatic synapses, a large number of mitochondria, and short cisterns of rough ER that may correspond to cells with somatic appendages described with the Golgi method. Large numbers of profiles are found in all layers that contain round vesicles and make asymmetrical synapses onto dendritic spines, and occasionally, dendritic shafts. Theseprofileshavedistinctly different morphological features in layer Ia, in which olfactory bulb afferents are concentrated, and in layers Ib, II, and III, which contain terminals of association and commis-sural fibers. A smaller number of profiles containing pleomorphic vesicles make symmetrical contacts onto initial segments, dendritic shafts, cell bodies, and occasionally, dendritic spines. Most dendritic spines in all layers are small to medium in size (0.3–1.2 μm) and presumably originate from pyramidal cells. In layer Ia, however, large, flattened spines are also present which appear to originate from semi-lunar cells. In layer III, and to a lesser extent other layers, large irregular spines are present that may be branched appendages on dendrites of complex appendage cells (Haberly, 1983).     

Structure of the piriform cortex of the opossum. III. Ultrastructural characterization of synaptic terminals of association and olfactory bulb afferent fibers

Terminals of olfactory bulb afferent (OB) and association (ASSN) fibers within the piriform cortex were characterized ultrastructurally. Identification was by electron microscopic (EM) autoradiography following injections of tritiated amino acids into the olfactory bulb and anterior piriform cortex. The results show that terminals of both fiber systems contain round vesicles and make asymmetrical synaptic contacts predominantly onto dendritic spines. Profiles with pleomorphic vesicles do not appear to be labeled from either site. Since there is strong evidence that both fiber systems generate excitatory postsynaptic potentials (EPSPs) in pyramidal cells, these results provide additional examples in the mammalian CNS of terminals with round vesicles and asymmetrical contacts that mediate an excitatory effect. Percentage density analysis and quantitative study of a large number of heavily labeled terminals revealed that while OB and ASSN terminals are similar in terms of vesicle shape and contact type, they differ in many morphological details including pre- and postsynaptic profile size, the packing density and distribution of synaptic vesicles, synaptic contact shape, and the presence of overlying neuroglial lamellae. However, large variations in appearance of different terminals of the same type are also present so that a small percentage of OB and ASSN terminals are indistinguishable morphologically in the absence of label. An important finding of the quantitative analysis is that spines contacted by lateral olfactory tract (LOT) terminals appear to be of two types based on a bimodal distribution in size and differences in morphology, while spines contacted by ASSN terminals appear to be of a single type. Comparison of these data with results from Golgi analysis indicates that ASSN terminals predominantly contact pyramidal cell spines while OB terminals contact both pyramidal and semilunar cell spines. Quantitative analysis of synaptic vesicles revealed that histograms of vesicle size for OB and ASSN terminals are virtually identical in shape, but peaks are slightly displaced (ASSN vesicles are 5% larger; significant with P less than .002). An analysis of the laminar distribution of OB and ASSN synaptic terminals revealed that while most OB terminals are segregated in layer Ia and most ASSN terminals in layer Ib, occasional OB terminals are observed up to approximately 50 micro deep to the Ia-Ib boundary and occasional ASSN terminals up to approximately 50 micro superficial to this boundary.     

Structure of the piriform cortex of the adult rat. A Golgi study

The piriform cortex (PC) was studied in the adult rat with anilines, rapid Golgi and Golgi-Colonnier techniques. As in other animals in the PC of the adult rat three layers can be distinguished: layer I or plexiform layer, layer II or superficial cellular layer and layer III or deep cellular layer. Golgi impregnations allowed to describe seven different types of cells. Pyramidal cells, in which it is possible to distinguish three subtypes (superficial, middle and deep pyramidal cells) by virtue of their morphology and location. Bipolar cells, a very little and scarce type of cell restricted to layer I and that has not been previously described. Fusiform cells, similar to those found in other species. Semilunar cells, also like those described in other animals but which are only found in the posterior PC. These cells resemble other type of cell, the ovoid cells that neither have been described in other animals and, on the contrary, are only located in the anterior PC. Stellate cells, which are a very wide population of cells with variable sizes, dendritic patterns and locations. Horizontal cells, similar to pyramidal cells but oriented in horizontal direction and polymorphic cells, whose the most striking feature is their smooth and beaded prolongations. Morphological features of these cells are compared with those described in the cells of the PC of other animals.     

Association and commissural fiber systems of the olfactory cortex of the rat. I. Systems originating in the piriform cortex and adjacent areas

The association and commissural fiber systems arising in the olfactory cortical areas caudal to the olfactory peduncle (the piriform cortex, nucleus of the lateral olfactory tract, anterior cortical nucleus of the amygdala, periamygdaloid cortex and entorhinal cortex) have been studied utilizing horseradish peroxidase as both an anterograde and a retrograde axonal tracer. In the piriform cortex two sublaminae within layer II (IIa and IIb) and layer III have been found to give rise to distinctly different projections. Retrograde cell labeling experiments indicate that the association fiber projection from layer IIb is predominantly caudally directed, while the projection from layer III is predominantly rostrally directed. Cells in layer IIa project heavily to areas both caudal and rostral to the piriform cortex. The commissural fibers from the piriform cortex are largely restricted in their origin to layer IIb of the anterior part of the piriform cortex and in their termination on the contralateral side to the posterior part of the piriform cortex and adjacent olfactory cortical areas. A projection to the olfactory bulb has also been found to arise from cells in layers IIb and III of the ipsilateral piriform cortex, but not in layer IIa. In addition to those from the piriform cortex, association projections have also been found from other olfactory cortical areas. The nucleus of the lateral olfactory tract has a heavy bilateral projection to the medial part of the anterior piriform cortex and the lateral part of the olfactory tubercle (as well as a lighter projection to the olfactory bulb); both the anterior cortical nucleus of the amygdala and the periamygdaloid cortex project ipsilaterally to several olfactory cortical areas. The entorhinal cortex has been found to project to the medial parts of the olfactory tubercle and the olfactory peduncle. The olfactory tubercle is the only olfactory cortical area from which no association fiber systems (instrinsic or extrinsic) have been found to originate. A broad topographic organization exists in the distribution of the fibers from several of the olfactory areas. This is most obvious in the anterior part of the olfactory cortex, in which fibers from the more rostral areas (the anterior olfactory nucleus and the anterior piriform cortex) terminate in regions near the lateral olfactory tract, while those from more caudal areas (the posterior piriform cortex and the entorhinal cortex) terminate in areas further removed, both laterally and medially, from the tract. Projections to olfactory areas from the hypothalamus, thalamus, diagonal band, and biogenic amine cell groups have been briefly described.     

Distribution and ultrastructure of neurons in opossum piriform cortex displaying immunoreactivity to GABA and GAD and high-affinity tritiated GABA uptake

GABAergic neurons have been identified in the piriform cortex of the opossum at light and electron microscopic levels by immunocytochemical localization of GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase and by autoradiographic visualization of high-affinity 3H-GABA uptake. Four major neuron populations have been distinguished on the basis of soma size, shape, and segregation at specific depths and locations: large horizontal cells in layer Ia of the anterior piriform cortex, small globular cells with thin dendrites concentrated in layers Ib and II of the posterior piriform cortex, and multipolar and fusiform cells concentrated in the deep part of layer III in anterior and posterior parts of the piriform cortex and the subjacent endopiriform nucleus. All four populations were well visualized with both antisera, but the large layer Ia horizontal cells displayed only very light 3H-GABA uptake, thus suggesting a lack of local axon collaterals or lack of high-affinity GABA uptake sites. The large, ultrastructurally distinctive somata of layer Ia horizontal cells receive a very small number of symmetrical synapses; the thin, axonlike dendrites of small globular cells are exclusively postsynaptic and receive large numbers of both symmetrical and asymmetrical synapses, in contrast to somata which receive a small number of both types; and the deep multipolar and fusiform cells receive a highly variable number of symmetrical and asymmetrical synapses on somata and proximal dendrites. Labeled puncta of axon terminal dimensions were found in large numbers in the neuropil surrounding pyramidal cell somata in layer II and in the endopiriform nucleus. Moderately large numbers of labeled puncta were found in layer I at the depth of pyramidal cell apical dendrites with greater numbers in layer Ia at the depth of distal apical segments than in layer Ib. High-affinity GABA uptake was demonstrated in the termination zone of the projection from the anterior olfactory nucleus to the anterior piriform cortex. Cell bodies of origin of this projection displayed heavy retrograde labeling with 3H-GABA. Matching neuropil and cellular labeling was demonstrated with the GABA-BSA antiserum but not with the GAD antiserum, thus suggesting that GABA is normally present in these cells but is taken up from the neuropil rather than synthesized. No comparable high-affinity GABA uptake was demonstrated in the association fiber systems that originate in the piriform cortex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.     

Urethane anesthesia produces selective damage in the piriform cortex of the developing brain.

The potential induction of neuronal death by neuroactive drugs at specific stages of embryonic or postnatal development is a serious concern in treating brain disease. Recent evidence indicates that NMDA antagonists, GABA agonists, ethanol and some anesthetics can all produce massive neuronal cell loss at critical times during development. We show here that the anesthetic urethane, once used clinically, produces a selective lesion of the piriform cortex, a region not previously implicated in such toxicity, in the developing brain. Young rats were injected with urethane at 1, 2, 3, and 4 weeks of age and brain damage was measured 1-4 days later. We found that urethane produces a large lesion in subfields of the piriform cortex and that the damage is most severe in 2 week-old animals. These data, together with other recent reports, show that there are multiple neuronal death-inducing pathways in the developing nervous system. It will be important to determine if anesthetics used in pregnant women and young children may have similar effects.     

Extent and organization of opossum prefrontal cortex defined by anterograde and retrograde transport methods

Prefrontal cortex is commonly defined as cortex which receives afferents from the thalamic mediodorsal nucleus (MD). The extent of opossum prefrontal cortex was mapped with anterograde and retrograde axonal transport methods. The prefrontal field was found to include not only cortex on the lateral convexity of the frontal lobe as reported in earlier studies, but, in addition, cortex within the rhinal fissure and cortex on the rostral medial wall of the hemisphere. The organization of the thalamic input to the medial wall was analyzed in some detail and compared with that of the rat. The reason for this emphasis stemmed from earlier observations which suggested that a lateral, nonolfactory segment of MD, prominent in the rat and other species, may not be present in opossum MD. In the rat, the lateral segment, which constitutes approximately one-third of MD, projects to a relatively large expanse of rostral medial cortex which is also projected upon by the anteromedial nucleus. The main projection field of the lateral one-third of opossum MD is to cortex on the lateral convexity of the frontal lobe which has no input from the anteromedial nucleus and has no counterpart in the rat. Only the most lateral edge of opossum MD projects to medial cortex, to a very small field, which is also projected upon by the anteromedial nucleus. In other respects, the organization of the rostral medial cortex is similar in rat and opossum. These results suggest that, rather than being absent, an equivalent of a nonolfactory segment may be present in opossum MD but is markedly reduced in size, compared to that in rat and other species.     

On the ictogenic properties of the piriform cortex in vitro

Purpose: The piriform cortex (PC) is known to be epileptic‐prone and it may be involved in the manifestation of limbic seizures. Herein, we have characterized some electrophysiologic and pharmacologic properties of the spontaneous epileptiform activity generated by PC networks maintained in vitro. Methods: We performed field potential recordings from the PC in coronal or sagittal rat brain slices along with pharmacologic manipulations of γ‐aminobutyric acid (GABA)ergic and glutamatergic signaling during application of the convulsant drug 4‐aminopyridine (4AP, 50 μm ). Key Findings: Coronal and sagittal preparations generated interictal‐like and ictal‐like epileptiform discharges with similar duration and frequency. Ictal‐like discharges in sagittal slices were initiated mostly in the PC anterior subregion, whereas interictal activity did not have any preferential site of origin. In sagittal slices, high frequency oscillations (HFOs) at 80–200 Hz were detected mainly at the beginning of the ictal discharge in both posterior and anterior subregions. N‐Methyl‐d ‐aspartate (NMDA) receptor antagonism abolished ictal discharges, but failed to influence interictal activity. In the absence of ionotropic glutamatergic transmission, PC networks generated slow, GABA receptor–dependent events. Finally, GABA_A receptor antagonism during application of 4AP only, abolished ictal discharges and disclosed recurrent interictal activity. Significance: Our findings demonstrate that PC networks can sustain in vitro epileptiform activity induced by 4AP. HFOs, which emerge at the onset of ictal activity, may be involved in PC ictogenesis. As reported in several cortical structures, ionotropic glutamatergic neurotransmission is necessary but not sufficient for ictal discharge generation, a process that also requires operative GABA_A receptor–mediated signaling.     

The striatum of the opossum, Didelphis virginiana. Description and experimental studies

The relationships and cytoarchitecture of the putamen, the caudate, the claustrum, the globus pallidus and the entopeduncular nuclei have been described for the opossum. The neocortical projections to these nuclei have ben studied by employing the Nauta-Gygax technique ('54) and the Swank Davenport modification of the Marchi technique ('34) on animals in which neocortical lesions were previously placed. Degenerating fibers from every cortical lesion were observed to terminate in both the putamen and the caudate with the Nauta-Gygax technique, whereas such connections were traced only to putamen woth the Marchi method. Terminations were present within the claustrum, but equivocal in the globus pallidus. In general, fibers from the more rostral cortices terminate in the rostral parts of both striatal nuclei, whereas fibers from more caudal neocortical areas project to more caudal parts of these same nuclei. In addition, the more dorsal or dorsomedial neocortical areas distribute more fibers to the caudate than to the putamen, whereas the opposite is true for the ventral or ventrolateral neocortical areas. Neocortical fibers did not project to the ventral, medial part of the head of the caudate which was cytoarchitectually different from the rest of the nucleus. A few fascicles of frontal, orbital and parietal origin terminated in the contralateral putamen and caudate after having decussated in the anterior commissure.     

Horizontal cells of the turtle retina. I. Light microscopy of golgi preparations

In Golgi preparations of turtle retina, four types of horizontal cells were observed and their morphological characteristics determined in vertical thick sections, whole mount preparations, and reconstructions from serial 1-μm sections. H1 consists of a nucleated, stellate cell body (H1CB) and an irregular, tuberous axon terminal (H1AT) connected by a slender axon. Both parts of these cells make contact with receptor cells. H1CB's appear to correspond to “L2-type cells” while H1AT's correspond to “L1-type cells” described in the physiological literature. H2 and H3 are axonless stellate cells which are similar to one another in vertical profile and may occasionally appear similar in horizontal view. In general, the dendritic tree is more densely branched and the density of receptor cell contacts is higher for H2 than for H3. H2-type cells may correspond to “R/G C-type cells.” H4 is also an axonless stellate cell type which is smaller than H2 or H3 at equivalent retinal locations. The dendritic fields of H1CB's vary widely, but systematically, in size and shape over the retina. Their size is inversely related to receptor cell density, and the shape of the dendritic tree varies from roughly circular in the central area to elliptical in the periphery of the retina.     

Characterization of axo-axonic synapses in the piriform cortex of Mus musculus

Previous anatomical and physiological studies have established major glutamatergic and GABAergic neuronal subtypes within the piriform cortical circuits. However, quantitative information regarding axo-axonic inhibitory synapses mediated by chandelier cells across major cortical subdivisions of piriform cortex is lacking. Therefore, we examined the properties of these synapses across the entire piriform cortex. Our results show the following. 1) γ-Aminobutyric acid membrane transporter 1-positive varicosities, whose appearance resembles chandelier cartridges, are found around the initial segments of axons of glutamatergic cells across layers II and III. 2) Both the density of axo-axonic cartridges and the degree of γ-aminobutyric acid membrane transporter 1 innervation in each axo-axonic synapse are significantly higher in the piriform cortex than in the neocortex. 3) Glutamate decarboxylase 67, vesicular GABA transporter, and parvalbumin, but not calbindin, are colocalized with the presynaptic varicosities, whereas gephyrin, Na-K-2Cl cotransporter 1, and GABA(A) receptor α1 subunit, but not K-Cl cotransporter 2, are colocalized at the presumed postsynaptic sites. 4) The axo-axonic cartridges innervate the majority of excitatory neurons and are distributed more frequently in putative centrifugal cells and posterior piriform cortex. We further describe the morphology of chandelier cells by using parvalbumin-immunoreactivity and single-cell labeling. In summary, our results demonstrate that a small population of chandelier cells mediates abundant axo-axonic synapses across the entire piriform cortex. Because of the critical location of these inhibitory synapses in relation to action potential regulation, our results highlight a critical role of axo-axonic synapses in regulating information flow and olfactory-related oscillations within the piriform cortex in vivo.     

Reticulospinal fibers of the opossum, Didelphis virginiana. I. Origin

Complete and partial cervical cord hemisections were placed at different levels according to a modification of the Gudden method (Brodal, '40), as applied to the opossum. Nissl staining techniques were employed and either acute retrograde cellular change of cell loss was studied following 7to 65 day survival periods. The results of a previous cytoarchitectonic study which followed the terminology of Meessen and Olszewski ('49) and Olszewski and Baxter ('54) served as controls. Chromatolysis or cells loss was found in medium-sized (20–29 μ), large (30–44 μ) and giant (45–60 μ) cells in the pontine and medial medullary reticular formation. Retrograde changes in the pontine reticular formation were found in the nucleus pontis centralis oralis, the nucleus pontis centralis caudalis and the rostral nucleus gigantocellularis. Within the pons, chromatolysis was particularly evident in the large and giant cells. Following cervical cord lesions, the medullary reticular formation contained chromatolytic neurons within the nucleus gigantocellularis, the nucleus paragigantocellularis dorsalis, the paramedian reticular nucleus and the nucleus interfascicularis hypoglossi. At pontine levels, chromatolytic cells were present mainly on the side of the lesion. However, cells were found to have undergone retrograde changes in specific areas of the contralateral side. Although the greatest number of reactive cells within the medullary reticular formation were also located on the side of the lesion, there was more contralateral involvement than was the case in the pons.     

Induction of Bdnf from promoter I following electroconvulsive seizures contributes to structural plasticity in neurons of the piriform cortex

BackgroundElectroconvulsive therapy (ECT) efficacy is hypothesized to depend on induction of molecular and cellular events that trigger neuronal plasticity. Investigating how electroconvulsive seizures (ECS) impact plasticity in animal models can help inform our understanding of basic mechanisms by which ECT relieves symptoms of depression. ECS-induced plasticity is associated with differential expression of unique isoforms encoding the neurotrophin, brain-derived neurotrophic factor (BDNF).HypothesisWe hypothesized that cells expressing the Bdnf exon 1-containing isoform are important for ECS-induced structural plasticity in the piriform cortex, a highly epileptogenic region that is responsive to ECS.MethodsWe selectively labeled Bdnf exon 1-expressing neurons in mouse piriform cortex using Cre recombinase dependent on GFP technology (CRE-DOG). We then quantified changes in dendrite morphology and density of Bdnf exon 1-expressing neurons.ResultsLoss of promoter I-derived BDNF caused changes in spine density and morphology in Bdnf exon 1-expressing neurons following ECS.ConclusionsPromoter I-derived Bdnf is required for ECS-induced dendritic structural plasticity in Bdnf exon 1-expressing neurons.     

PSA-NCAM expression in the piriform cortex of the adult rat. Modulation by NMDA receptor antagonist administration

Uptake of biocytin and biotin was investigated in cultured transformed variants of neuronal (NB2a neuroblastoma) and glial (C6 astrocytoma) CNS cells. NB2a cells took up both compounds but biocytin was transported more efficiently than biotin in the nanomolar concentration range. In NB2a cells a single transport mechanism was found for biocytin with different kinetic parameters in the presence of high extracellular Na+ (Km 0.4 microM, Vmax 20 pmol/min/mg), K+ (Km 1.7 microM, Vmax 32 pmol/min/mg), or choline+ (Km 0.1 microM, Vmax 5 pmol/min/mg). Two transport systems (Km1 17 microM, Vmax1 53 pmol/min/mg; Km2 314 microM, Vmax2 360 pmol/min/mg) were identified for biotin with only system 1 being Na+-dependent. Biocytin uptake was competitively inhibited by excess biotin but not vice versa. Inhibition studies with structural analogs indicated different specificities for biotin and biocytin uptake. Biocytin uptake into C6 cells was hardly detectable whereas biotin was taken up by diffusion (kD 0.6 microl/min/mg) and a single saturable mechanism (Km 70 microM, Vmax 119 pmol/min/mg) at high extracellular Na+. High extracellular K+ enhanced biotin diffusion into C6 cells. Inhibition studies with structural analogs revealed a less specific biotin uptake mechanism in C6 than in NB2a cells. Biocytin normalized deficient biotin-dependent propionyl-CoA carboxylase activity within 4 h in biotin-deficient NB2a cells whereas in C6 cells reactivation was <20% thereby confirming that biocytin is only poorly transported into C6 cells. Specific biocytin uptake into NB2a cells is to our knowledge the first demonstration of a carrier-mediated transport mechanism for this compound. Neuronal biocytin uptake might contribute to the pathogenesis of biotinidase deficiency where biocytin is present in elevated levels.     

Light-induced Egr-1 expression in the striate cortex of the opossum

In the present study, immunocytochemistry was used to assess the expression of Egr-1 nuclear protein across selected regions of the opossum visual system. In light-deprived (LD) animals, only a few scattered cell nuclei were found throughout the striate cortex (V1). Exposure to light promoted a significant increase in the density of Egr-1 labeled nuclei in V1. Laminar distribution of immunoreactive nuclei in light-stimulated animals (LS) tended to vary with topography: the lateral region, which corresponds to the central representation of the visual field, appeared to have higher density of cells expressing protein in the supragranular layers, as compared to the medial region, which corresponds to the representation of the peripheral field of vision. Finally, LS animals displayed a narrow band of labeled cell nuclei in the intergeniculate leaflet (IGL) and throughout the anteroposterior extent of the superior colliculus (SC). In contrast, almost no Egr-1 immunolabeling was found in the IGL and SC of LD animals.Our report is the first demonstration of light-regulated expression of the Egr-1 gene in the opossum visual system and provides evidence that the expression of an activity-dependent gene related to neural plasticity is evolutionarily conserved in the visual cortex of the mammalian lineage.     

Neuronal processes that underlie expression of kindled epileptiform events in the piriform cortex in vivo.

Recent studies with kindling and convulsant drug models of epilepsy suggest that the piriform (primary olfactory) cortex may be particularly susceptible to generation of epileptiform activity. The present study has examined the generation of interictal epileptiform events in the piriform cortex of kindled rats in vivo, taking advantage of special features of this system that facilitate physiological analysis. The investigation included analysis of extracellular and intracellular potentials, and membrane currents computed by current source density (CSD) analysis. In pyramidal cells, epileptiform events consisted of an initial EPSP that occurred in all-or-none fashion and a long-lasting IPSP with Cl(-)- and K(+)-mediated components. Onset of the IPSP was sufficiently fast that firing evoked by the EPSP was consistently limited to single action potentials. CSD analysis revealed the presence of two distinctly different excitatory epileptiform currents: an initial inward current of unknown origin that is widely distributed over depth, and a second much larger inward current at the depths of proximal apical and basal dendrites of pyramidal cells. It was concluded that this second component is mediated by the associational projections of pyramidal cells excited by the first component. Since these heavy associational projections also extend to neighboring areas including the amygdala, entorhinal cortex, and insular and orbitofrontal areas of neocortex, this second component could be widely propagated within the basal forebrain. An important finding was that the EPSP generated by this associational pathway was completely blocked in cell bodies of pyramidal cells in piriform cortex by the IPSP during most events. This IPSP may therefore play a critical role in limiting seizure activity by preventing reverberating positive feedback in the pyramidal cell population. It can be speculated that compromise of this IPSP, as by repetitive activation by the shock trains used for kindling, leads to prolonged epileptic activity in the piriform cortex and the many limbic structures to which it projects.