Cellular Composition of the Piriform Cortex of the Rat Brain in Experimental Epilepsy

The cellular composition of all layers of the anterior, central, and posterior parts of the piriform cortex of the rat brain was studied two weeks and one month after specific electrical stimulation (kindling) of the ventral hippocampus. Stereomicroscopic analysis at both two weeks and one month after kindling showed significant decreases in the numbers of pyramidal cells and interneurons in all layers of all parts of the piriform cortex. At two weeks, the numbers of pyramidal cells and interneurons in the central part of the piriform cortex also decreased in rats in which electrodes were inserted into the ventral hippocampus but without stimulation. These results, along with published data, led to a series of suggestions regarding the involvement of the piriform cortex in epileptogenesis. __________ Translated from Morfologiya, Vol. 127, No. 1, pp. 14–17, January–February, 2005. director Dr M. G. Zhvaniya     

Unilateral low-frequency stimulation of central piriform cortex delays seizure development induced by amygdaloid kindling in rats

Low-frequency stimulation of the kindling site interferes with the course of kindling epileptogenesis. The present study examined the effect of unilateral low-frequency stimulation of the central piriform cortex on seizure development induced by amygdaloid kindling in rats. The ipsilateral or contralateral central piriform cortex received low-frequency stimulation (15 min train of 0.1 ms pulses at 1 Hz and 50-150 muA) immediately after termination of once daily kindling stimulation (2 s train of 1 ms pulses at 60 Hz and 150-300 microA) in the right amygdala for 30 days. Low-frequency stimulation of either the ipsilateral or contralateral central piriform cortex significantly suppressed the progression of seizure stages and reduced afterdischarge duration throughout the course of amygdaloid kindling. The marked suppression induced by low-frequency stimulation of the central piriform cortex on either side was predominantly due to the significant retardation of progression from stage 0 to stage 1 and stage 3 to stage 4 seizures. In addition, the suppressive effect of low-frequency stimulation did not disappear when the stimulation was stopped; it could persist for at least 10 days. These findings indicate that brain areas other than the kindling focus, such as the central piriform cortex on both sides, can also be used as reasonable targets for low-frequency stimulation to retard seizure development induced by amygdaloid kindling. Secondly, like the ipsilateral central piriform cortex, the contralateral central piriform cortex may also participate in the progression and secondary generalization of focal seizures. The study suggests that unilateral low-frequency stimulation of the central piriform cortex may have a significant antiepileptogenic effect, and may be helpful for exploring effective and long-lasting therapies for human temporal lobe epilepsy.     

Kindling-induced hippocampal cell death in rats

Quantitative analysis of the main cells and interneurons in different layers and fields of Ammon’s horn, the hilus, and dentate gyrus was performed two weeks and one month after kindling, i.e., specific electrical stimulation of the ventral hippocampus. At both experimental time points, cells numbers were significantly decreased at all levels of the areas studied except the marginal layer of field CA4. Thus, in this model of experimental epilepsy, the process of epileptogenesis involved both interneurons and main cells in the hippocampus. The possible reorganization of intrahippocampal circuits and their involvement in the increased excitability characteristic of epilepsy are discussed. __________ Translated from Morfologiya, Vol. 131, No. 2, pp. 49–52, March–April, 2007.     

Kindling causes persistent in vivo changes in firing rates and glutamate sensitivity of central piriform cortex neurons in rats

The present experiments were undertaken to study whether amygdala kindling induces persistent alterations in the functional status of neurons of the central piriform cortex, a subregion of the piriform cortex identified previously as a site involved in the kindling process. Extracellular, single-unit recordings of piriform cortex neurons were made in anesthetized fully kindled rats at an interval of at least five weeks after the last seizure. Electrode implanted but not kindled rats served as sham controls. An additional group of non-implanted rats was used as naive controls. Spontaneously firing piriform cortex neurons were characterized in all groups by smooth, sharp, biphasic (i.e. positive/negative) action potentials with a duration of 0.8-1.8 ms, and were primarily located at the border between piriform cortex layers II and III. In kindled rats, neurons in the central piriform cortex exhibited a significantly higher firing rate compared to controls. Based on median group values, the increase in basal activity in kindled rats averaged about 90%. The responsiveness of piriform cortex neurons to neurotransmitters was tested by microiontophoretic application of glutamate, N-methyl-D-aspartate and GABA. Piriform cortex neurons of kindled rats exhibited a significantly lower responsiveness to the excitatory effect of glutamate than naive controls. A lowered glutamate responsiveness was also seen in sham controls. No significantly altered transmitter sensitivities of piriform cortex neurons from kindled rats were seen with N-methyl-D-aspartate or GABA.The data indicate that amygdala kindling causes persistent interictal changes in both basal activity and glutamate responsiveness of central piriform cortex neurons which could contribute to the abnormal hyperexcitability characteristic of kindling.     

Bilateral lesions of the central but not anterior or posterior parts of the piriform cortex retard amygdala kindling in rats

The piriform cortex is thought to be involved in temporal lobe seizure propagation, such as that occurring during kindling of the amygdala or hippocampus. A number of observations suggested that the circuits of the piriform cortex might act as a critical pathway for limbic seizure discharges to assess motor systems, but direct evidence for this suggestion is scarce. Furthermore, the piriform cortex is not a homogeneous structure, which complicates studies on its role in limbic epileptogenesis. We have previously reported data indicating that the central part of the piriform cortex might be particularly involved during amygdala kindling. In order to further evaluate the role of different parts of the piriform cortex during kindling development, we bilaterally destroyed either the central, anterior or posterior piriform cortex by microinjections of ibotenate two weeks before onset of amygdala kindling. Lesions of the anterior piriform cortex hardly affected kindling acquisition, except that fewer animals exhibited stage 3 (unilateral forelimb) seizures compared to sham controls. Lesions of the central piriform cortex significantly retarded kindling, which was due to a decreased progression from stage 3 to stage 4/5 seizures, i.e. the lesioned rats needed significantly longer for the acquisition of generalized clonic seizures in the late stages of kindling development. Lesions of the posterior piriform cortex did not significantly affect kindling development.The data demonstrate that different parts of the piriform cortex mediate qualitatively different effects on amygdala kindling. The central piriform cortex seems to be a neural substrate involved in the continuous development of kindling from stage 3 to stages 4/5, indicating that this part of the piriform cortex may have preferred access, either directly or indirectly, to structures capable of supporting generalized kindled seizure expression.     

Effect of acoustic stimulation on GABAergic neurons in limbic structures of Krushinskii-Molodkina rats

Quantitative study of GABAergic and main cells in the hippocampus and piriform cortex of Krushinskii-Molodkina rats was performed 1 month after the incidence of seizure activity evoked by acoustic stimulation. The number of neurons significantly decreased in both regions and, particularly, in the hippocampus and central area of the piriform cortex. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 145, No. 4, pp. 375–377, April, 2008     

Properties of a population of GABAergic cells in murine auditory cortex weakly excited by thalamic stimulation

Feedforward inhibition triggered by thalamocortical (TC) afferents sharpens onset responses and shapes receptive fields of pyramidal cells in auditory cortex (ACx). Previous studies focused only on interneurons located in and around layer IV in primary ACx, target of the dense thalamic projections from ventral medial geniculate. We investigated a population of feedforward interneurons located throughout layers I-V and activated by both afferents from primary and nonprimary thalamus using recordings from auditory TC brain slices obtained from mice expressing green fluorescent protein under control of the glutamic acid decarboxylase (GAD65) promoter in a subpopulation of cortical GABAergic cells. We studied the responses of these interneurons and of pyramidal cells in ACx to thalamic stimulation and to hyper- and depolarizing current pulses. Most interneurons exhibited monosynaptic responses to thalamic stimulation, but this excitation was weak and subthreshold. Interneurons had multipolar dendritic morphology with widespread and dense axonal projections extending several hundred micrometers from the soma. In pyramidal cells from layers II-IV, thalamic excitatory postsynaptic potentials were significantly larger than in interneurons and were superthreshold in 40% of cells, but in these cells, there was no evidence of feedforward inhibition. By contrast, feedforward inhibition was observed in 12 of 18 layer V pyramidal cells. Thus feedforward inhibition in supragranular layers of ACx is weak, and these interneurons require coincident excitation to be activated by thalamic inputs.     

Effects of pentylenetetrazole-induced kindling on thyrotropin-releasing hormone biosynthesis and receptors in rat brain

It has been postulated that changes in thyrotropin-releasing hormone biosynthesis may be involved in the mechanism of kindling--an animal model of epileptogenesis. To test this hypothesis, a time-course study was carried out to investigate the effects of pentylenetetrazole kindling (40 mg/kg i.p., daily for eight days) on the expression of gene coding for preprothyrotropin-releasing hormone, the thyrotropin-releasing hormone tissue level and thyrotropin-releasing hormone receptor parameters in rat brain. As shown by an in situ hybridization study, a single, convulsant dose of pentylenetetrazole (70 mg/kg i.p.) increased the preprothyrotropin-releasing hormone messenger RNA level in the dentate gyrus of the hippocampal formation and piriform cortex after 3 h and, to a greater extent, after 24 h. Those changes were accompanied with increases in the thyrotropin-releasing hormone level in the striatum, hippocampus, amygdala and piriform cortex. Seven days after single pentylenetetrazole administration, the thyrotropin-releasing hormone level was still significantly elevated in the piriform cortex and striatum. Acute pentylenetetrazole decreased the density (Bmax) of thyrotropin-releasing hormone receptors in the striatum after 3 and 24 h, and increased that density in the piriform cortex and amygdala after 24 h and seven days, respectively. The thyrotropin-releasing hormone receptor affinity (Kd) was decreased in the striatum and increased in the amygdala after only 3 h. Kindled rats showed a moderate increase in the preprothyrotropin-releasing hormone messenger RNA content in the dentate gyrus of the hippocampal formation and piriform cortex after 3 and 24 h; however, a significant decrease in those parameters was found after 14 days. After 3 and 24 h, pentylenetetrazole kindling also elevated the thyrotropin-releasing hormone content in the hippocampus, piriform cortex, and striatum (in the latter structure after 24 h only), whereas in the septum the thyrotropin-releasing hormone level was decreased. After seven days, the thyrotropin-releasing hormone level was still elevated in the hippocampus and piriform cortex of kindled rats, but after 14 days it was significantly lowered in the hippocampus. The kindled rats also showed a significant decrease in the density (Bmax) of thyrotropin-releasing hormone receptors in the striatum (after 24 h, seven and 14 days), and an increase in the piriform cortex (after seven days). The thyrotropin-releasing hormone receptor affinity (Kd) value was increased in the hippocampus after seven and 14 days, and in the piriform cortex after seven days. These results indicate that pentylenetetrazole kindling induces long-lasting alterations in the thyrotropin-releasing hormone biosynthesis and thyrotropin-releasing hormone receptor affinity in discrete regions of rat brain. These region-specific changes, in particular down-regulation of the thyrotropin-releasing hormone biosynthesis in the hippocampus, may be involved in chronic neuronal hyperexcitability associated with kindling.     

Neurons in the ventral subiculum, amygdala and entorhinal cortex which project to the nucleus accumbens: Their input from somatostatin-immunoreactive boutons

Neurons in the hippocampus, amygdala and entorhinal cortex which project to the nucleus accumbens were labelled retrogradely following injection of horseradish peroxidase. The injections were targetted on the medial part of the nucleus accumbens, but some injection sites included the whole nucleus. Projection neurons in all three areas were found to be spiny, and from the entorhinal cortex and ventral subiculum of the hippocampus they were pyramidal neurons.Somatostatin (S28_1–12)-immunoreactive neurons were found in all parts of the three limbic areas examined. They were found to have various morphologies, but in the electron microscope all had the ultrastructural characteristics of interneurons. In the hippocampus the stratum lacunosum was found to contain the most immunoreactive fibres while most cells lay in the stratum oriens. In the amygdala the densest staining for both cells and fibres was in the central nucleus. In the entorhinal cortex somatostatin-immunoreactive fibres and cells seemed to have no preferential distribution.Examination of somatostatin-immunoreactive profiles in the electron microscope revealed that the majority of synaptic contacts were made with dendrites, many of which were spine-bearing.In the light microscope somatostatin-immunoreactive fibres could be seen to lie near the somata and proximal dendrites of neurons that projected to the nucleus accumbens. In the electron microscope it was found that somatostatin-immunoreactive boutons were in symmetrical synaptic contact with the somata and proximal dendrites of neurons in the ventral subiculum, entorhinal cortex and amygdala which project to the nucleus accumbens.     

Limbic seizures cause pronounced changes in the expression of neurokinin B in the hippocampus of the rat.

Immunohistological and in situ hybridization techniques were used to study the influence of kainic acid-induced seizures and of pentylenetetrazol kindling on neurokinin B immunoreactivity and neurokinin B mRNA in the rat hippocampus. Pronounced increases in neurokinin B immunoreactivity were observed in the terminal field of mossy fibres 10-60 days after intraperitoneal injection of kainic acid. These slow but persistent increases in immunoreactivity were accompanied by markedly enhanced expression of neurokinin B mRNA in the granule cells and in hilar interneurons adjacent to the granule cell layer. These changes were preceded by transient increases in neurokinin B mRNA and immunoreactivity in CA1 pyramidal cell layer two and 10 days after kainic acid, which, however, subsided later on. Pentylenetetrazol kindling caused similar increases in neurokinin B mRNA expression in granule cells and in CA1 pyramidal cells, but not in hilar interneurons. In CA1, increased neurokinin B message was present two days after termination of the kindling procedure but not after 10 days. Sixty days after kainic acid injection, neurokinin B immunoreactivity extended to the inner-third of the molecular layer of the dentate gyrus. After pentylenetetrazol kindling, a neurokinin B-immunoreactive band was observed in the infrapyramidal region of CA3. Lesions of the dentate granule cells by local injection of colchicine in kainic acid-treated rats abolished the supragranular neurokinin B-positive staining, whereas it was almost unchanged after transection of the ventral hippocampal commissure. These observations suggest that neurokinin B immunoreactivity may be located in ipsilateral mossy fibres undergoing collateral sprouting to the inner molecular layer or to the infrapyramidal region in CA3, respectively. Preprotachykinin A mRNA, which encodes for neurokinin A and substance P, and substance P immunoreactivity were not changed in the hippocampus of epileptic rats compared with untreated animals. The observed changes in neurokinin B immunoreactivity and mRNA indicate that specific functional and morphological changes may be induced in hippocampal neurons by recurrent limbic seizures.     

Bilateral microinjections of vigabatrin in the central piriform cortex retard amygdala kindling in rats

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to limbic structures, including the amygdala, hippocampus, and entorhinal cortex. Various previous studies in rodents suggest that the PC might be very important in the development and maintenance of limbic kindling, i.e. a widely used model of temporal lobe epilepsy. GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, seems to be particularly involved in the processes leading to progression of kindled seizures. This prompted us to study whether elevation of GABA levels in this subregion of the PC by bilateral microinjection of vigabatrin is capable of suppressing amygdala kindling. Rats were stimulated once daily until fully kindled (stage 5) seizures had developed. Vigabatrin (10 microg) was injected 24 h before the first stimulation as well as 6 h before the 5th and 10th stimulation, which approximately doubled the number of stimulations required for kindling development compared with controls. This marked retardation of kindling acquisition was predominantly due to a significant inhibition of the progression from stage 1 to stage 2 and stage 3 to stage 4 seizures, demonstrating that microinjection of vigabatrin into the central PC markedly inhibits the progression and secondary generalization of focal seizures emanating from the amygdala.     

神经激肽B受体在小鼠中枢神经系统内的定位分布

目的　研究神经激肽B受体 (NK3)在小鼠中枢神经系统内的定位分布。　方法　免疫组织化学染色。　结果　在小鼠中枢神经系统的绝大部分区域 ,NK3受体样免疫反应产物位于胞体和树突上 ,少部分区域位于神经毡 (neuropil)内。大量NK3受体样免疫反应神经元出现于前嗅核、伏隔核、隔区、腹侧苍白球、苍白球、尾壳核、终纹床核、下丘脑前区、下丘脑结节区、下丘脑外侧区、穹隆周区、视上核、弓状核、乳头体、黑质、腹侧被盖区、红核后区、上丘和下丘、导水管周围灰质、孤束核、及延髓和脊髓背角浅层。大脑皮质的浅层、梨状皮质、背侧海马、杏仁核、脑干网状结构等核团内也含有一定数量的阳性神经元。在丘脑的中线核团和板内核、腹侧海马和脚间核等处 ,NK3受体免疫反应产物主要位于神经毡内。　结论　NK3受体广泛分布于小鼠中枢神经系统内 ,提示它可能具有重要的生理功能     

Analysis of association fiber system in piriform cortex with intracellular recording and staining techniques

The piriform cortex of the opossum has been studied with intracellular recording and staining techniques. The experiments were designed to investigate the association fiber system, but the results have also revealed new properties of the afferent fiber system from the olfactory bulb and the inhibitory systems within the piriform cortex. Following shock stimulation of the lateral olfactory tract (LOT), the response of pyramidal cells consists of an initial excitatory postsynaptic potential (EPSP) followed by a long-lasting inhibitory postsynaptic potential (IPSP). The LOT-evoked EPSP consists of two components: an initial monosynaptic followed by a disynaptic component. The monosynaptic EPSP can be isolated by the use of conditioning LOT shocks to block the IPSP and disynaptic EPSP. The disynaptic EPSP can be demonstrated by cutting LOT fibers at the surface of the cortex to eliminate the monosynaptic EPSP and by the use of bicuculline to block the IPSP. The latency of the IPSP is sufficiently brief so that the disynaptic EPSP is blocked at presumed intrasomatic recording sites unless these experimental manipulations are carried out. In all histologically verified pyramidal cells in both layers II and III in which the appropriate tests were carried out, both mono- and disynaptic EPSP components were present. It was concluded on the basis of anatomical considerations, however, that a small number of pyramidal cells would be expected to receive only a disynaptic EPSP. Evidence that the LOT-evoked disynaptic EPSP is mediated, at least in part, by association axons was provided by direct stimulation of these fibers in layer III and by demonstrating that the EPSP is present distal to cuts that sever LOT axons. Direct stimulation of association axons in layer III evokes both a monosynaptic EPSP and a disynaptic IPSP in pyramidal cells at similar latencies. This IPSP is indistinguishable in properties from that evoked by LOT stimulation. Indirect evidence indicates that it is mediated via both feedforward and feedback mechanisms. In most neurons the association fiber-evoked EPSP is masked by the IPSP in response to single deep shocks but can be demonstrated by blocking the IPSP with a preceding LOT shock or by application of bicuculline. Intracellular injection of horseradish peroxidase (HRP) revealed that pyramidal cell axons give rise to an extensive system of local collaterals with a large number of synaptic terminal-like swellings in layer III. It is postulated that these collaterals synapse on both pyramidal and nonpyramidal cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrical stimulation of the horizontal limb of the diagonal band of broca modulates population EPSPs in piriform cortex.

Electrical stimulation of the horizontal limb of the diagonal band of Broca (HDB) was coupled with recording of evoked potentials in the piriform cortex. Stimulation of the HDB caused an enhancement of the late, disynaptic component of the evoked potential elicited by stimulation of the lateral olfactory tract but caused a suppression of the synaptic potential elicited by stimulation of the posterior piriform cortex. The muscarinic antagonist scopolamine blocked both effects of HDB stimulation. The enhancement of disynaptic potentials could be due to cholinergic depolarization of pyramidal cells, whereas the suppression of potentials evoked by posterior piriform stimulation could be due to presynaptic inhibition of intrinsic fiber synaptic transmission by acetylcholine.     

Comparison of neuronal inositol 1,4,5-trisphosphate 3-kinase and receptor mRNA distributions in the adult rat brain using in situ hybridization histochemistry.

As a result of its interaction with a specific receptor, inositol 1,4,5-trisphosphate mobilizes intracellular calcium. The metabolism of inositol 1,4,5-trisphosphate is rather complex: inositol 1,4,5-trisphosphate 3-kinase produces inositol 1,3,4,5-tetrakisphosphate, a putative second messenger. In order to elucidate inositol 1,3,4,5-tetrakisphosphate function, a comparative in situ hybridization study of the distributions of inositol 1,4,5-trisphosphate 3-kinase and receptor mRNAs was performed in the adult rat brain using oligonucleotides derived from their cDNA sequences. The neuronal distributions of the mRNA for the receptor were larger than for the kinase. Highest levels of both mRNAs were found in the cerebellar Purkinje cells, where they were enriched in their neuronal perikarya and to a lesser extent in their dendrites. In addition to the cerebellum, mRNAs were mainly detected in the hippocampal pyramidal cells of the CA1 sector of the Ammon's horn and in the granule cells of the dentate gyrus, and also in a majority of the neurons in the cortical layers II-III and V, especially in the frontal cortex and cingulate cortex; caudate-putamen, accumbens, olfactory tubercle and Calleja islets; claustrum; anterior olfactory nucleus; taenia tecta; piriform cortex; dorsolateral septum; bed nucleus stria terminalis; amygdala; hippocampal CA2-4 sectors and subiculum. The inositol 1,4,5-trisphosphate receptor mRNA but not kinase mRNA was found in a majority of the neurons in the thalamus, especially in the parafascicular nucleus; hypothalamus, especially the medial hypothalamus; substantia nigra pars compacta and ventral tegmental area; superior colliculus; lateral interpeduncular nucleus and central gray. Taking into account the limitation in sensitivity of the technique, both mRNAs were not detected in glial cells and in the olfactory bulb; basal nucleus of Meynert, diagonal band nuclei; medial septal nucleus; substantia innominata; globus pallidus; entopeduncular nucleus; substantia nigra pars reticulata; ventral pallidum; subthalamic nucleus; spinal cord and dorsal root ganglia. In conclusion, cerebellum and hippocampus appear to contain almost similar levels of kinase mRNA. This is in contrast to receptor mRNA levels which were at much higher levels in the cerebellum when compared with the hippocampus. For this reason, we have chosen hippocampal CA1 pyramidal cells and dentate gyrus granule cells for studying inositol 1,4,5-trisphosphate 3-kinase function.     

Heterogeneity of phasic cholinergic signaling in neocortical neurons

Acetylcholine (ACh) is a neurotransmitter critical for normal cognition. Here we demonstrate heterogeneity of cholinergic signaling in neocortical neurons in the rat prefrontal, somatosensory, and visual cortex. Focal ACh application (100 muM) inhibited layer 5 pyramidal neurons in all cortical areas via activation of an apamin-sensitive SK-type calcium-activated potassium conductance. Cholinergic inhibition was most robust in prefrontal layer 5 neurons, where it relies on the same signal transduction mechanism (M1-like receptors, IP(3)-dependent calcium release, and SK-channels) as exists in somatosensory pyramidal neurons. Pyramidal neurons in layer 2/3 were less responsive to ACh, but substantial apamin-sensitive inhibitory responses occurred in deep layer 3 neurons of the visual cortex. ACh was only inhibitory when presented near the somata of layer 5 pyramidal neurons, where repetitive ACh applications generated discrete inhibitory events at frequencies of up to approximately 0.5 Hz. Fast-spiking (FS) nonpyramidal neurons in all cortical areas were unresponsive to ACh. When applied to non-FS interneurons in layers 2/3 and 5, ACh generated mecamylamine-sensitive nicotinic responses (38% of cells), apamin-insensitive hyperpolarizing responses, with or without initial nicotinic depolarization (7% of neurons), or no response at all (55% of cells). Responses in interneurons were similar across cortical layers and regions but were correlated with cellular physiology and the expression of biochemical markers associated with different classes of nonpyramidal neurons. Finally, ACh generated nicotinic responses in all layer 1 neurons tested. These data demonstrate that phasic cholinergic input can directly inhibit projection neurons throughout the cortex while sculpting intracortical processing, especially in superficial layers.     

Changes in excitatory and inhibitory circuits of the rat hippocampus 12-14 months after complete forebrain ischemia.

Changes in interneuron distribution and excitatory connectivity have been investigated in animals which had survived 12-14 months after complete forebrain ischemia, induced by four-vessel occlusion. Anterograde tracing with Phaseolus vulgaris leucoagglutinin revealed massive Schaffer collateral input even to those regions of the CA1 subfield where hardly any surviving pyramidal cells were found. Boutons of these Schaffer collaterals formed conventional synaptic contacts on dendritic spines and shafts, many of which likely belong to interneurons. Mossy fibres survived the ischemic challenge, however, large mossy terminals showed altered morphology, namely, the number of filopodiae on these terminals decreased significantly. The entorhinal input to the hippocampus did not show any morphological alterations. The distribution of interneurons was investigated by neurochemical markers known to label functionally distinct GABAergic cell populations. In the hilus, spiny interneurons showed a profound decrease in number. This phenomenon was not as obvious in CA3, but the spiny metabotropic glutamate receptor 1alpha-positive non-pyramidal cells, some of which contain calretinin or substance P receptor, disappeared from stratum lucidum of this area. In the CA1 region, somatostatin immunoreactivity disappeared from stratum oriens/lacunosum-moleculare-associated cells, while in metabotropic glutamate receptor 1alpha-stained sections these cells seemed unaffected in number. Other interneurons did not show an obvious decrease in number. In stratum radiatum of the CA1 subfield, some interneuron types had altered morphology: the substance P receptor-positive dendrites lost their characteristic radial orientation, and the metabotropic glutamate receptor 1alpha-expressing cells became extremely spiny. The loss of inhibitory interneurons at the first two stages of the trisynaptic loop coupled with a well-preserved excitatory connectivity among the subfields suggests that hyperexcitability in the surviving dentate gyrus and CA3 may persist even a year after the ischemic impact. The dorsal CA1 region is lost; nevertheless hyperactivity, if it occurs, may have a route to leave the hippocampus via the longitudinally extensive axon collaterals of CA3 pyramidal cells, which may activate the subiculum and entorhinal cortex with a relay in the surviving ventral hippocampal CA1 region.     

Consequences of prolonged afferent stimulation of the rat fascia dentata: epileptiform activity in area CA3 of hippocampus

Following prolonged stimulation of the perforant path input to the dentate gyrus, long-lasting changes occur in the synaptic responses and cell properties of cells in the fascia dentata. The present study describes the effects of sustained stimulation on the major population of cells innervated by the dentate granule cells: are CA3 pyramidal cells of hippocampus. In 46% of slices from rat, sustained stimulation of perforant path was followed by spontaneous, synchronized, rhythmic bursting activity in area CA3 pyramidal cells that was evident for several hours. These bursts could be recorded extracellularly in the pyramidal cell layer, throughout the hilar region, and even in the granule cell layer. With intracellular recording, all of the cells of the fascia dentata were found to be affected by the pyramidal cell bursts. Hyperpolarizing, inhibitory postsynaptic potential (IPSP)-like events occurred in all granule cells tested during the CA3 pyramidal cell burst. In contrast, spiny hilar "mossy" cells discharged synchronously with the pyramidal cells, as did some of the "fast spiking" interneurons. However, most interneurons only depolarized a few millivolts during the pyramidal cell burst. These results show that sustained stimulation of the perforant path is followed by a period of hyperexcitability in area CA3 of the hippocampus, and that hyperexcitability in area CA3 influences the activity of the cells in the fascia dentata.     

Kv3.1和Kv3.2钾离子通道蛋白在大鼠神经系统的分布

本研究用免疫组织化学方法观察了Kv3.1和Kv3.2钾离子通道蛋白在大鼠神经系统的分布状况。结果显示,两种蛋白在神经系统内具有区域性的分布特点。Kv3.1免疫阳性神经元主要见于大脑皮层、海马锥体细胞层、纹状体、丘脑网状核、下丘脑、腹侧耳蜗核、桥核等脑区;Kv3.2阳性神经元主要分布于大脑皮层深层、海马始层、Calleja岛、外侧丘系腹侧核、背侧耳蜗核、小脑皮层Purkinje细胞层、前庭核等处。Kv3.1和Kv3.2阳性胞体在某些脑区如大脑皮层、海马、耳蜗核、外侧丘系核等处呈现"互补"性的分布特点。另外,在一些脑区还观察到了Kv3.1和Kv3.2免疫阳性纤维和终末。:Kv3.1和Kv3.2在神经系统内广泛的分布,提示它们可能参与了多种生理功能。     

THE ROLE OF THE PIRIFORM CORTEX IN KINDLING

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC (“primary olfactory cortex”) is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) the PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neurons at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC comparable to abnormalities seen in primary foci. Taken together, these observations indicate that the PC might be part of an epileptic network which is pivotal in the genesis of kindling, facilitating and intensifying the spread of seizures from a focus in amygdala or hippocampus to cortical and subcortical regions along pathways that also are utilized in normal movements. Although direct evidence implicating the PC in the pathogenesis of human epilepsy is not yet available, the experimental data reviewed in this paper should initiate clinical studies on the potential role of this brain structure as a pacemaker or secondary focus in TLE and other types of epilepsy. Copyright © 1996 Elsevier Science Ltd.