GABAB受体对大鼠海马CA1区锥体细胞突触传递的作用

目的研究激活GABA_B受体对大鼠海马CA1区锥体细胞突触传递的影响。方法对成年大鼠海马脑片CA1区锥体细胞采用“盲法”全细胞电压钳记录,分别检测和分析巴氯芬(10μmol/L)对自发性的兴奋性突触后电流(EPSCs)和抑制性突触后电流(IPSCs)的影响。结果巴氯芬可显著降低符氨酸能EPSCs和γ-氨基丁酸能IPSCs的频率(P＜0．01),各自达58%±7%(n=17)和42%±10%(n=15),而对它们的幅度无显著性影响。结论巴氯芬对海马CA1区锥体细胞EPSCs和IPSCs的抑制作用属于突触前抑制,推测GABA_B受体所介导的这种抑制作用对CA1区神经元兴奋性的传出具有抑制作用,从而对癫痫的产生有控制作用。     

氯化锂-匹罗卡品致痫大鼠海马EphA5及ephrinA3的表达及意义

目的探讨颞叶癫痫发作后海马EphA5及ephrinA3基因的表达变化和轴突出芽的关系。方法建立氯化锂-匹罗卡品颞叶癫痫大鼠模型,利用原位杂交方法检测致痫后12h、24h、7d、15d、30d、60d海马CA3区、CA1区EphA5及ephrinA3 mRNA的表达,快速Golgi染色观察CA1区的轴突出芽。结果致痫后,EphA5 mRNA在CA3区表达下调,ephrinA3 mRNA在CA1区表达下调,均在7d降至最低点,与对照组相比差异有显著意义(P<0.01),此后逐渐回升,但15d时仍低于对照组(P<0.05),在30d和60d与对照组相比差异无统计学意义(P>0.05)。快速Golgi染色显示,对照组大鼠CA1区轴突走行正常,匹罗卡品致大鼠SE后7dCA1区锥体细胞层出现显著增多的轴突染色。结论CA3区的EphA5和CA1区的ephrinA3的表达下调可能与CA1区的轴突出芽、突触重建有关。     

颅脑创伤后大鼠海马CA1锥体细胞内在电生理和兴奋性突触传递特性的变化

目的 在细胞及突触水平探讨外伤后癫痫的发病机制.方法 自由落体致伤法制备大鼠颅脑创伤模型,采用膜片钳技术监测海马CA1区锥体细胞内在电生理特性和局部突触兴奋性的变化.结果 颅脑创伤后,大鼠CA1锥体细胞膜输入阻抗和时间常数增加,动作电位的阈电流降低;给予配对刺激后,海马CA1区兴奋性突触后电流表现为配对脉冲比率的降低及配对脉冲易化向配对脉冲抑制的转变.结论 颅脑创伤后海马CA1区神经元内在兴奋性和突触传递功能增强,这些改变可能是外伤后癫痫发病的重要原因.     

颞叶癫痫大鼠海马CA1区突触可塑性与空间记忆能力关系的研究

目的探讨颞叶癫痫大鼠海马CA1区突触超微结构与空间记忆能力改变的关系。方法以海人酸杏仁核微量注射建立经典的雄性Wistar大鼠颞叶癫痫化学点燃模型,分别于点燃后11d、17d、21d测定大鼠的空间记忆能力,并观察第21d海马CA1区神经毡突触超微结构变化。结果颞叶癫痫大鼠空间记忆能力减低,同时伴有海马CA1区神经毡内突触数密度降低(P<0．05),突触活性区膜面积缩小(P<0．05),突触界面曲率降低(P< 0．05),比表面减小(P<0．05),突触小泡数密度降低(P<0．05)。结论颞叶癫痫大鼠海马CA1区神经毡内突触损害和活力可能是导致其空间记忆能力下降的重要机制。     

托吡酯对癫痫大鼠海马神经细胞凋亡的保护作用

目的　探讨托吡酯对癫痫发作大鼠海马神经元凋亡的影响及其可能的机制。方法　采用戊四氮致痫模型 ,大鼠癫痫发作后连续给予托吡酯 80mg/ (kg·d)和 4 0mg/ (kg·d) ) ,共 14d。以TUNEL方法标记DNA片段 ,原位检测海马CA1和CA3区的神经细胞凋亡。结果　各组大鼠海马CA1、CA3区均出现TUNEL阳性细胞。对照组 ,海马CA1、CA3区TUNEL阳性细胞数分别为 (35 .83± 4 .5 8)个和(36 .83± 3.87)个 ;4 0mg/ (kg·d)托吡酯组分别为 (31.5 2± 3.4 3)个和 (32 .35±4 .6 9)个 ;80mg/ (kg·d)托吡酯组为 (2 1.17± 3.0 6 )个和 (2 1.16± 3.87)个。 80mg/ (kg·d)托吡酯组与对照组比较存在显著差异(P<0 .0 0 1) ,4 0mg/ (kg·d)托吡酯组TPM组与对照组相比无显著差异 (P >0 .0 5 )。结论　TPM对癫痫发作后神经元凋亡具有一定的保护作用     

GAD67/GAD65在颞叶癫痫大鼠海马内源性促痫机制中的作用

目的探讨GAD67/GAD65在颞叶癫痫发生后大鼠海马内源性促痫机制中的作用.方法112只雄性SD大鼠随机分为实验组(n=70)与对照组(n=42),实验组大鼠选用海人酸腹腔注射法建立颞叶癫痫模型,对照组大鼠腹腔注射无菌生理盐水.选取腹腔注射后3 h、6 h、12 h、24 h、48 h、7 d、30 d为研究的时间点,颞叶海马的CA1区、CA3区、齿状回为研究部位.腹腔给药后每天观察大鼠的行为学变化,大鼠处死前进行EEG描记.免疫组织化学法检测GAD65、GAD67蛋白的表达.结果海人酸致痫后6 h,实验组大鼠海马CA1区、CA3区GAD67/GAD65的比率较对照组升高(P＜0.01);海人酸致痫后30 d,实验组大鼠海马齿状回GAD67/GAD65的比率较对照组降低(P＜0.05).结论颞叶癫痫急性期CA1区、CA3区GAD67/GAD65比率的增高及慢性期齿状回GAD67/GAD65比率的降低与颞叶癫痫发生及癫痫发生后机体的内源性抗痫机制密切相关.     

癫痫大鼠海马出芽苔藓纤维突触的超微结构特征

目的:探讨匹罗卡品颞叶癫痫大鼠海马出芽苔藓纤维突触的超微结构特征及其在颞叶癫痫发病机制中的作用。方法:采用Timm组化染色标记出芽苔藓纤维突触末端,在电镜下观察新生突触的类型、比例、定位、以及突触后靶成分。结果:颞叶癫痫大鼠齿状回内分子层可见到银标记的突触末端,出芽苔藓纤维突触主要是轴棘型非对称性突触,其次是轴树型非对称性突触,偶可看到出芽轴突和颗粒细胞体形成突触联系。结论:轴棘型非对称性突触是颞叶癫痫大鼠海马出芽苔藓纤维突触的主要类型,出芽苔藓纤维突触的超微结构特性支持重组突触形成重复的兴奋性环路,而且形成的新的兴奋性环路可能在颞叶癫痫的发生与发展中起重要作用。     

小白蛋白在新生大鼠缺氧后癫痫易感性升高中的作用机制

目的观察新生大鼠早期发育过程中及缺氧性痫性发作后海马小白蛋白(parvalbumin,PV)表达量和海马神经元内Ca2+荧光强度的改变,探讨PV在新生大鼠缺氧后癫痫易感性升高中的作用机制。方法采用出生后10 d的SD大鼠分为缺氧组与对照组(各64只),缺氧组建立改良Jensen缺氧诱导痫性发作模型,于1 d、3 d、7 d、14 d各组分别取16只大鼠取脑,免疫组化和免疫印迹检测海马PV的表达,流式细胞术检测海马神经元内Ca2+荧光强度。结果缺氧后7 d,海马CA1区、CA3区及DG区PV免疫阳性细胞数均明显减少,至缺氧后14 d,稍增加至对照组3 d的水平,而PV含量在缺氧后7 d、14 d则明显减少。对照组PV免疫阳性细胞数和PV含量在CA1区、CA3区及DG区均从3 d开始增加,并持续至14 d。与对照组比较,缺氧后海马神经元内Ca2+荧光强度增加。结论新生大鼠缺氧诱导痫性发作后海马PV表达减少可能导致其癫痫易感性升高。缺氧后细胞内游离Ca2+增加可能导致PV神经元损伤。     

红藻氨酸诱导癫痫发作大鼠海马EphA5受体及其配体ephrinA3基因表达变化的研究

目的研究在红藻氨酸(Kainic acid,KA)诱导的损伤型颞叶癫痫(Mesial temporal lobe epilepsy,MTLE)的大鼠海马中,轴突导向因子EphA5受体及其配体ephrinA3基因表达的变化,探讨EphA5/ephrinA3与癫痫后海马兴奋性神经网络形成的作用和关系。方法侧脑室内微量注射KA,建立KA诱导的成年大鼠MTLE模型,用原位杂交法检测癫痫发作1d、1周、2周、3周、4周大鼠海马内EphA5/ephrinA3 mRNA的表达,定量分析表达的动态变化。结果EphA5/ephrinA3 mRNA于癫痫发作后1周,在海马齿状回颗粒细胞层和CA_3区锥体细胞层开始增强,2周达到高峰,4周恢复接近对照组水平。结论在KA所致的癫痫持续状态(Status epilepsy,SE)中,海马神经元通过增强EphA5/ephrinA3 mRNA的表达。调控MTLE大鼠海马内苔藓纤维和突触的重建,是癫痫后海马新的稳定的异常兴奋性神经网络形成的可能机制。     

大鼠癫痫持续状态后海马组织各区脑红蛋白表达的实验研究

目的观察大鼠癫痫持续状态(Status epilepticus,SE)后海马组织脑红蛋白(Neuroglobin,NGB)表达动态变化,探讨NGB在癫痫发作中的作用。方法健康成年雄性SpragueDawley大鼠40只,随机分为对照组(n=5)、癫痫模型实验组(n=35);实验组再依据观察时间分为:0、1、3、12、24 h和10、30 d。应用锂-匹罗卡品(20~127 mg/kg)建立大鼠SE模型,观察大鼠致痫期间行为学变化;采用尼氏(Nissl)染色检测海马组织神经元损伤情况;SABC免疫组化法检测海马组织NGB表达水平。结果 SE后,海马组织各区均出现不同程度神经元细胞损伤坏死,随着发作时程进展,CA1、CA3区存活神经元呈近直线下降趋势。其中CA1区(12、24 h,10、30d)、CA3区(0、12、24 h,10、30 d)和(DG区12、24 h,10、30 d)神经元存活数较对照组明显减少(P0.05)。大鼠SE后,海马各区NGB表达水平均上调,CA1、DG区NGB表达均于SE后24 h达顶峰后轻度下降,但仍持续高于对照组,CA3区NGB表达呈持续升高趋势。其中CA1区(24 h,10、30 d)、CA3区(24 h,10、30 d)和DG区(12、24 h,10、30 d)NGB表达水平均较对照组显著升高(P0.05)。另外,海马CA1和CA3区神经元存活数与NGB表达水平呈正相关(R=0.206,P=0.015;R=0.306,P=0.011)。结论大鼠SE后海马各区NGB表达上调,且与CA1、CA3区神经元存活数呈正相关,提示NGB表达上调可能是癫痫发作所致缺血缺氧损害的一种代偿保护机制,参与癫痫相关神经元损害的保护。     

Alterations of Neuronal Connectivity in Area CA1 of Hippocampal Slices from Temporal Lobe Epilepsy Patients and from Pilocarpine-Treated Epileptic Rats

Summary: Purpose : Neuronal network reorganization might be involved in epileptogenesis in human and rat limbic epilepsy. Apart from aberrant mossy fiber sprouting, a more wide-spread fiber rearrangement in the hippocampal formation might occur. Therefore, we studied sprouting in area CA1 because this region is most affected in human temporal lobe epilepsy.
Methods : In slices from hippocampi of patients operated on for temporal lobe epilepsy (n = 134), from pilocarpine-treated rats (n = 74), and from control rats (n = 15), viable neurons were labeled with fluorescent dextran amines.
Results : In human hippocampi as well as in pilocarpine-treated rats, the degree of nerve cell loss varied. In 67 of 134 slices from human specimens with distinct Ammon's horn sclerosis and in 23 of 74 slices from pilocarpine-treated rats, a severe shrunken area CA1 presented with a similar picture: few damaged neurons were labeled, and aberrant fiber connections were not visible. This was in contrast to human resected hippocampi and hippocampi from pilocarpine-treated rats with no or moderate loss of neurons. In these cases, pyramidal cells remote from the injection site were labeled (human tissue, n = 59 of 134; pilocarpine-treated rats, n = 39 of 74). In human resected hippocampi without obvious pathology and in control animals, no pyramidal neurons were labeled apart from the injection site.
Conclusions : Axon collaterals of CA1 pyramidal cells are increased in human temporal lobe epilepsy and in pilocarpine-treated rats. Adjacent CA1 pyramidal cells project via aberrant collaterals to the stratum pyramidale and the stratum radiatum of area CA1. This network reorganization can contribute to hyperexcitability via increased backward excitation.     

Reciprocal anatomical connections between hippocampus and subiculum in the rabbit: Evidence for subicular innervation of regio superior

Anatomical connections between the dorsal hippocampus and subiculum were examined in the rabbit, using horseradish peroxidase (HRP) and autoradiographic methods. A previously undescribed pathway was found to project from the dorsal prosubicular-subicular region to dorsal hippocampal cell fields CA1 and CA2. Autoradiographic findings showed that subicular afferents travel via two routes. One pathway projected through the alveus and stratum oriens, with results suggesting collateral input to the basal dendritic pyramidal cell region. The other projection coursed through the stratum lacunosum-moleculare with apparent termination onto CA1 and CA2 apical dendrites. Regions of subiculum providing afferents to hippocampus were compared with subicular areas receiving efferent terminations from hippocampal CA1 and CA3 cell zones. Distribution of hippocampal-subicular terminations were regionally distinct from subicular retrograde cell fields in rostral areas of the subicular complex, extended over a much wider area of subiculum than was seen for retrograde-labeled cells, and was cytoarchitectonically organized. In total, findings indicated that a reciprocal anatomical relationship exists between dorsal hippocampus and subiculum in the rabbit.     

Cellular and network properties of the subiculum in the pilocarpine model of temporal lobe epilepsy

The subiculum was recently shown to be crucially involved in the generation of interictal activity in human temporal lobe epilepsy. Using the pilocarpine model of epilepsy, this study examines the anatomical substrates for network hyperexcitability recorded in the subiculum. Regular- and burst-spiking subicular pyramidal cells were stained with fluorescence dyes and reconstructed to analyze seizure-induced alterations of the dendritic and axonal system. In control animals burst-spiking cells outnumbered regular-spiking cells by about two to one. Regular- and burst-spiking cells were characterized by extensive axonal branching and autapse-like contacts, suggesting a high intrinsic connectivity. In addition, subicular axons projecting to CA1 indicate a CA1-subiculum-CA1 circuit. In the subiculum of pilocarpine-treated rats we found an enhanced network excitability characterized by spontaneous rhythmic activity, polysynaptic responses, and all-or-none evoked bursts of action potentials. In pilocarpine-treated rats the subiculum showed cell loss of about 30%. The ratio of regular- and burst-spiking cells was practically inverse as compared to control preparations. A reduced arborization and spine density in the proximal part of the apical dendrites suggests a partial deafferentiation from CA1. In pilocarpine-treated rats no increased axonal outgrowth of pyramidal cells was observed. Hence, axonal sprouting of subicular pyramidal cells is not mandatory for the development of the pathological events. We suggest that pilocarpine-induced seizures cause an unmasking or strengthening of synaptic contacts within the recurrent subicular network.     

Organization of CA1 projections to the subiculum: a PHA-L analysis in the rat.

The organization of CA1 projections to the rat subiculum was investigated with the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Discrete iontophoretic injections of PHA-L were placed into various transverse positions of the CA1 field at different septotemporal levels of the hippocampus. The distribution of CA1 projections was observed in dissected and extended hippocampal preparations. CA1 cells located proximally in the field, i.e., close to the CA2 field, gave rise to projections that terminated in the distal third of the subiculum, i.e., close to the presubiculum. CA1 cells located distally in the field, i.e., close to the subiculum, gave rise to projections that terminated proximally in the subiculum, i.e., just across the CA1/subiculum border. CA1 cells in the middle of the field projected to a midtransverse portion of the subiculum. The same general pattern of projections was observed at all septotemporal levels of the hippocampus. Varicose fibers from the CA1 neurons terminated among the basal dendrites of the subicular pyramidal cells, within the pyramidal cell layer, and in the deep portion of the molecular layer. In addition to the CA1 to subiculum projections, the discrete PHA-L injections provided the opportunity of examining the extent of local and associational connections within CA1. In general, associational connections in CA1 are far less extensive than in CA3. CA1 is not entirely without local connections, however. CA1 cells located close to the subicular border, for example, originated axons that first innervated the proximal subiculum and then reentered the CA1 field at the interface between stratum radiatum and stratum lacunosum-moleculare. In most of the experimental cases, there were collaterals located in stratum oriens of CA1 that branched from the fibers directed toward the subiculum. Thus, the basal dendrites of CA1 cells may receive associational inputs. The organization of the CA1 projections to the subiculum is discussed in relation to the organization of CA3 projections to CA1 and the differential output of transverse regions of the subiculum. The possibility is raised that information may be "channeled" through the hippocampal formation via the transverse organization of these connections and ultimately distributed to different recipients of hippocampal efferent projections.     

CA3 axonal sprouting in kainate-induced chronic epilepsy

Latency between an early neurological insult and development of spontaneous recurrent seizures suggests aberrant chronological reorganization in patients with mesial temporal sclerosis associated epilepsy. Kainate-induced status similarly results in delayed development of spontaneous recurrent seizures. Mossy fiber sprouting by the dentate granule cells is a well-characterized manifestation of such temporal structural reorganization in both patients and animal models. However, alterations in other components of hippocampal circuitry have not been evaluated. We present results from studies using precise anterograde and retrograde tract tracing methodologies to evaluate the reorganization of outflow of the CA3 pyramidal cells. Although septotemporal relationships of the normal CA3 outflow tract through the Schaffer collaterals are well known, their aberrant reorganization following kainate-induced spontaneous recurrent seizures is not known. We provide the first definitive evidence of widespread CA3 structural reorganization in the form of sprouting of CA3 axons to widespread areas throughout the hippocampus and entorhinal cortex. This includes an apparent increase in the density of projection to areas that normally receive CA3 outflow such as CA1 and subiculum as well as novel projections beyond the confines of the hippocampus to the pre and parasubiculum and medial and lateral entorhinal cortex. We provide the first evidence of novel CA3 Schaffer collateral projection to the entorhinal cortex. The sprouting of CA3 outflow to widespread regions of the hippocampus and the entorhinal cortex may provide insight into how the injured hippocampus propagates unconventional impulse excitation to cortical fields which have a critical role in providing excitatory inputs into the hippocampus possibly setting up reverberating excitatory circuits as well as widespread connections throughout the cortical mantle. Sprouting-related mechanisms may also explain the latency associated with development of spontaneous recurrent seizures, the hallmark of temporal lobe epilepsy.     

Increased excitatory synaptic input to granule cells from hilar and CA3 regions in a rat model of temporal lobe epilepsy

One potential mechanism of temporal lobe epilepsy is recurrent excitation of dentate granule cells through aberrant sprouting of their axons (mossy fibers), which is found in many patients and animal models. However, correlations between the extent of mossy fiber sprouting and seizure frequency are weak. Additional potential sources of granule cell recurrent excitation that would not have been detected by markers of mossy fiber sprouting in previous studies include surviving mossy cells and proximal CA3 pyramidal cells. To test those possibilities in hippocampal slices from epileptic pilocarpine-treated rats, laser-scanning glutamate uncaging was used to randomly and focally activate neurons in the granule cell layer, hilus, and proximal CA3 pyramidal cell layer while measuring evoked EPSCs in normotopic granule cells. Consistent with mossy fiber sprouting, a higher proportion of glutamate-uncaging spots in the granule cell layer evoked EPSCs in epileptic rats compared with controls. In addition, stimulation spots in the hilus and proximal CA3 pyramidal cell layer were more likely to evoke EPSCs in epileptic rats, despite significant neuron loss in those regions. Furthermore, synaptic strength of recurrent excitatory inputs to granule cells from CA3 pyramidal cells and other granule cells was increased in epileptic rats. These findings reveal substantial levels of excessive, recurrent, excitatory synaptic input to granule cells from neurons in the hilus and proximal CA3 field. The aberrant development of these additional positive-feedback circuits might contribute to epileptogenesis in temporal lobe epilepsy.     

Normal anatomy and neurophysiology of the hippocampal formation.

This article reviews the anatomy and neurophysiology of the normal hippocampal formation, with emphasis on the human hippocampus. The hippocampus receives inputs from numerous limbic, cortical, and subcortical areas, primarily via the entorhinal cortex and subiculum. The primary pathway of neural activity entering the hippocampus is from entorhinal cortex via the perforant path to the dentate granule cells, with collaterals to CA1 and CA3 pyramidal cells. Mossy fibers from granule cells excite CA3 pyramidal cells and hilar interneurons. CA3 pyramidal cells excite CA1 pyramidal cells, with local and commissural excitatory collaterals exciting other CA3 pyramidal cells and septum. CA1 pyramidal cells send efferent fibers to subiculum, entorhinal cortex, and several subcortical areas. The principal excitatory synapses are glutamatergic, with two important postsynaptic receptor types, alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid and N-methyl-D-aspartate. The primary inhibitory transmitter is gamma-aminobutyric acid (GABA), with two postsynaptic receptor types, GABAA and GABAB. A number of modulatory transmitters and neuropeptides are also present. Inhibitory local synaptic networks in the hippocampus are described. Membrane ion channels in hippocampal neurons, particularly Ca2+ channels and K+ channels, are responsible for the regulation and patterning of neural activity. Long-term potentiation and axon sprouting are two experimental paradigms of neural plasticity presumably involved in hippocampal memory function.     

Adverse effects of maternal ethanol consumption on development of dorsal hippocampus in rat offspring

We examined the laminar structure and distribution of mossy fiber terminal fields in the dorsal hippocampus, an important area for spatial learning, in rats exposed to ethanol during gestational days 10-21. Pyramidal cells in the CA3a subfield were loosely packed compared to control rats. Aberrant infra- and intrapyramidal mossy fibers were found in the CA3 region, especially in the CA3a subfield, throughout the dorsal hippocampus of ethanol-exposed rats. Aberrant mossy fiber terminals were observed more frequently in the rostral than the caudal level of the dorsal hippocampus. At the most caudal level of the dorsal hippocampus, disarrangement of pyramidal cells was seen in the CA3c subfield along with disturbed mossy fiber terminals. Immunohistochemical studies revealed that neural cell adhesion molecule (NCAM) was not related to aberrant distribution of mossy fiber terminals after prenatal exposure to ethanol. Parvalbumin immunoreactivity was increased in the dorsal hippocampus of ethanol-exposed rats compared with control rats. Abnormal development of the dorsal hippocampus induced by prenatal ethanol exposure may be associated with the defect of spatial memory seen in fetal alcohol syndrome children and their animal models.     

Commissurally projecting inhibitory interneurons of the rat hippocampal dentate gyrus: a colocalization study of neuronal markers and the retrograde tracer Fluoro-gold.

Improved methods for detecting neuronal markers and the retrograde tracer Fluoro-Gold (FG) were used to identify commissurally projecting neurons of the rat hippocampus. In addition to the dentate hilar mossy cells and CA3 pyramidal cells shown previously to transport retrograde tracers after injection into the dorsal hippocampus, FG-positive interneurons of the dentate granule cell layer and hilus were detected in numbers greater than previously reported. FG labeling of interneurons was variable among animals, but was as high as 96% of hilar somatostatin-positive interneurons, 84% of parvalbumin-positive cells of the granule cell layer and hilus combined, and 33% of hilar calretinin-positive cells. By comparison, interneurons of the dentate molecular layer and all hippocampal subregions were conspicuously FG-negative. Whereas hilar mossy cells and CA3 pyramidal cells were FG-labeled throughout the longitudinal axis, FG-positive interneurons exhibited a relatively homotopic distribution. "Control" injections of FG into the neocortex, septum, and ventral hippocampus demonstrated that the homotopic labeling of dentate interneurons was injection site-specific, and that the CA1-CA3 interneurons unlabeled by contralateral hippocampal FG injection were nonetheless able to transport FG from the septum. These data suggest a hippocampal organizing principle according to which virtually all commissurally projecting hippocampal neurons share the property of being monosynaptic targets of dentate granule cells. Because granule cells innervate their exclusively ipsilateral target cells in a highly lamellar pattern, these results suggest that focal granule cell excitation may result in commissural inhibition of the corresponding "twin" granule cell lamella, thereby lateralizing and amplifying the influence of the initiating discharge.     

Immunocytochemical investigation of L-glutamic acid decarboxylase in the rat hippocampal formation: the influence of transient cerebral ischemia

The hippocampus is especially vulnerable to ischemic damage. Neurons in the CA3c region and dentate hilus demonstrate fast progressive damage while CA1 pyramidal cells demonstrate delayed neuronal damage. The delayed CA1 pyramidal cell loss could be caused by postischemic neuronal hyperactivity if hippocampal interneurons are lost after ischemia. Therefore we have counted the L-glutamic acid decarboxylase (GAD)-immunoreactive neurons in the hippocampus from control rats and rats surviving 4 or 11 days after 20 minutes of cerebral ischemia. All rats were injected intraventricularly with colchicine before they were killed. The hippocampal cell counts showed an increase in GAD-immunoreactive somata visualized on the fourth postischemic day. Eleven days after ischemia, the number of GAD-immunoreactive neurons visualized in the hippocampus CA1 and CA3c region decreased. GAD-immunoreactive baskets were visualized in the pyramidal cell layer and the granule cell layer in controls and 4 days after ischemia, but not in the CA1 and CA3c pyramidal cell layer 11 days after ischemia. We suggest the number of GAD-immunoreactive neurons visualized on the fourth postischemic day increases because somatal GAD accumulation increases and, therefore, ischemia may enhance GAD production. Our previous counts of CA1 interneurons 21 days after ischemia in toluidine-stained semithin sections demonstrated no interneuron loss. Therefore we suggest that the decreased number of CA1 and CA3c GAD-immunoreactive neurons visualized 11 days after ischemia is related to a decreased GAD production. It is possible at this stage after ischemia that the interneurons have decreased their GAD production because they have lost their input and/or target cells. We conclude that our counts of GAD-immunoreactive neurons visualized after ischemia express changes in the content of somatal GAD rather than the actual number of GAD-immunoreactive somata. Finally, we conclude that the delayed loss of CA1 pyramidal cells seen 4 days after ischemia is not preceded by loss of hippocampal GAD-immunoreactive neurons.