Newly generated dentate granule cells from epileptic rats exhibit elongated hilar basal dendrites that align along GFAP-immunolabeled processes.

Previous studies showed that neurogenesis occurs in the dentate gyrus of the adult rodent. Recent evidence suggests that the resulting newly born neurons integrate into pre-existing hippocampal circuitry. Newly born neurons in the developing and adult dentate gyrus exhibit a transient basal dendrite. In adult pilocarpine-induced epileptic rats, basal dendrites persist and are ectopically located in the hilus where they receive synaptic input from mossy fiber axons. We hypothesize that these hilar basal dendrites are derived from newly born neurons that are born after the pilocarpine-induced seizures. To test this hypothesis, the length of basal dendrites from epileptic rats was compared with that from control rats using doublecortin immunocytochemistry, which labels newly born neurons and their processes for up to 3 weeks after their genesis. The data on hilar basal dendrites in pilocarpine animals indicate that those from newly born neurons are significantly longer than those found in the control rats. We also demonstrate that 20% of newly born neurons in the epileptic rat have a basal dendrite that enters the hilus at an angle greater than 30 degrees from its cell body as compared with <2% in the control rats. Lastly, we provide evidence that the hilar basal dendrites in the epileptic rats are adjacent to glial fibrillary acidic protein-labeled astrocytic processes in the hilus and suggest that an ectopic glial scaffold in the hilus is involved with the formation of hilar basal dendrites. In conclusion, the data show that newly born neurons from epileptic rats have longer hilar basal dendrites and their formation might relate to gliosis which occurs as a result of hilar neuronal cell loss after status epilepticus.

Suppression of hippocampal neurogenesis is associated with developmental stage, number of perinatal seizure episodes, and glucocorticosteroid level.

Seizures increase dentate granule cell proliferation in adult rats but decrease proliferation in young pups. The particular period and number of perinatal seizures required to cause newborn granule cell suppression in development are unknown. Therefore, we examined cell proliferation with bromodeoxyuridine (BrdU) immunohistochemistry during the peak of neurogenesis (e.g., P6 and P9) and at later postnatal ages (e.g., P13, P20, or P30) following single and multiple episodes of perinatal status epilepticus induced by kainate (KA). Because an inverse relationship exists between glucocorticosteroids (CORT) levels and granule cell proliferation, plasma CORT levels and electroencephalographic (EEG) activity were simultaneously monitored to elucidate underlying mechanisms that inhibit cell proliferation. In control animals, the number of BrdU-labeled cells increased then declined with maturation. After 1x KA or 2x KA administered on P6 and P9, the numbers of BrdU-labeled cells were not different from age-matched controls. However, rat pups with 3x KA (on P6, P9, and P13) had marked suppression of BrdU-labeled cells 48-72 h after the last seizure (43 +/- 6.5% of control). Cell proliferation was also significantly inhibited on P20 after 2x KA (to 56 +/- 6.9%) or 3x KA (to 54 +/- 7.9%) and on P30 with 3x KA (to 74.5 +/- 8.2% of age-matched controls). Cell death was not apparent as chromatin stains showed increased basophilia of only inner cells lining the granule cell layers, in the absence of eosinophilia, argyrophilia, or terminal deoxynucleotidyl dUTP nick endlabeling (TUNEL) labeling at times examined. In P13 pups with 3x KA, electron microscopy revealed an increased number of immature granule cells and putative stem cells with irregular shape, condensed cytoplasm, and electron dense nuclei, and they were also BrdU positive. The EEG showed no relationship between neurogenesis and duration of high-synchronous ictal activity. However, endocrine studies showed a correlation with BrdU number and age, sustained increases in circulating CORT levels following 1x KA on P6 (0.7 +/- 0.1 to 2.40 +/- 0.86 microg/dl), and cumulative increases that exceeded 10 microg/dl at 4-8 h after 3x KA on P13 or P20. In conclusion, a history of only one or two perinatal seizure(s) can suppress neurogenesis if a second or third seizure recurs after a critical developmental period associated with a marked surge in CORT. During the first 2 weeks of postnatal life sustained increases in postictal circulating CORT levels but not duration or intensity of ictal activity has long-term consequences on neurogenesis. The occurrence of an increased proportion of immature granule cells and putative stem cells with irregular morphology in the absence of neurodegeneration suggests that progenitors may not differentiate properly and remain in an immature state.

Rapid induction of intraneuronal neurofibrillary tangles in apolipoprotein E-deficient mice.

Cultured hippocampal slices prepared from apolipoprotein E-deficient mice were exposed to an inhibitor of cathepsins B and L and then processed for immunocytochemistry using antibodies against human paired helical filaments. Dense, AT8-immunopositive deposits were found in the subiculum, stratum oriens of hippocampal field CA1, and the hilus of the dentate gyrus. This distribution agrees with that described for tangles in Alzheimer's disease. The appearance of the labeled structures fell into categories that correspond to previously proposed stages in the progression of intraneuronal neurofibrillary tangles in human hippocampus. Electron microscopic analyses confirmed that microtubule disruption and twisted bundles of filaments were present in neurons in the affected areas. These results support the hypothesis that partial lysosomal dysfunction is a contributor to Alzheimer's disease and suggest a simple model for studying an important component of the disease.

Expression of active caspase-3 in mitotic and postmitotic cells of the rat forebrain.

Active caspase-3 immunoreactivity was detected in the rat forebrain proliferative regions at birth and remained high in these areas for about 2 weeks, during which period labeled cells were present centroperipherally across the olfactory bulb. By the end of the third postnatal week, only a small number of immunolabeled cells remained in these forebrain structures. Active caspase-3 immunolabeling was localized mostly to cell nuclei and co-localized partially with TuJ1 and NeuN immunoreactivity, but not with glial fibrially acidic protein, OX-42, gamma-aminobutyric acid, or terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL)-positive labeling. Active caspase-3 and 5-bromo-2'-deoxyuridine (BrdU) double-labeled nuclei were seen in the proliferative regions after 2 hours and in the periglomerular region of the bulb after 7 days following BrdU injections. Examination of the cells with electron microscopy confirmed that the active caspase-3-containing nuclei in the proliferative regions often had infoldings and appeared to be undergoing division. Some of the cells with active caspase-3-labeled nuclei in the bulb had synapses on their somata or dendrites. Labeled dendritic spines and a few axon terminals were also observed in the olfactory bulb. Taken together, it appears that a wave of active caspase-3-positive cells are dividing in the proliferative zones and then migrating to the bulb as they differentiate into neurons. Therefore, active caspase-3 may play a role in cellular processes such as neuronal differentiation, migration, and plasticity, in addition to its role in cell death.

Atypical features of rat dentate granule cells: recurrent basal dendrites and apical axons.

The stereotyped morphology of dentate granule cells in rodents consists of apical dendrites arborizing in the molecular layer and an axon arising from the opposite pole of the soma. Recently, we showed that epilepsy induces the formation of basal dendrites on granule cells and that these dendrites extend into the hilus of the dentate gyrus. The present Golgi study of granule cells from adult rats shows two atypical features for granule cells in control rats. One is the occurrence of recurrent basal dendrites (RBDs) that are defined as basal dendrites arising at or near the hilar pole of the soma and then curving back to the molecular layer. The frequency of granule cells with RBDs was 3.8% in control rats. The second is apical axons of granule cells that were observed to originate from either the apical pole of the soma or an apical dendrite. The incidence of these "apical" axons was about 1%. These morphological findings in the present study suggest that rat granule cells are more heterogeneous than previously indicated. Furthermore, their frequency was not increased in epileptic rats.

Ultrastructural features and synaptic connections of hilar ectopic granule cells in the rat dentate gyrus are different from those of granule cells in the granule cell layer.

Several investigators have shown the existence of dentate granule cells in ectopic locations within the hilus and molecular layer using both Golgi and retrograde tracing studies but the ultrastructural features and synaptic connections of ectopic granule cells were not previously examined. In the present study, the biocytin retrograde tracing technique was used to label ectopic granule cells following injections into stratum lucidum of CA3b of hippocampal slices obtained from epileptic rats. Electron microscopy was used to study hilar ectopic granule cells that were located 20-40 microm from the granule cell layer (GCL). They had ultrastructural features similar to those of granule cells in the GCL but showed differences, including nuclei that often displayed infoldings and thicker apical dendrites. At their origin, these dendrites were 6 microm in diameter and they tapered down to 2 microm at the border with the GCL. Both biocytin-labeled and unlabeled axon terminals formed exclusively asymmetric synapses with the somata and proximal dendrites of hilar ectopic granule cells. The mean number of axosomatic synapses for these cells was three times that for granule cells in the GCL. Together, these data indicate that hilar ectopic granule cells are postsynaptic to mossy fibers and have less inhibitory input on their somata and proximal dendrites than granule cells in the GCL. This finding is consistent with recent physiological results showing that hilar ectopic granule cells from epileptic rats are more hyperexcitable than granule cells in the GCL.

Increased expression of gamma-aminobutyric acid transporter-1 in the forebrain of infant rats with corticotropin-releasing hormone-induced seizures but not in those with hyperthermia-induced seizures.

High affinity, gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) influence the availability of GABA, the main inhibitory neurotransmitter in the brain. Recent studies suggest a crucial role for GATs in maintaining levels of synaptic GABA in normal as well as abnormal (i.e., epileptic) adult brain. However, the role of GATs during development and specifically changes in their expression in response to developmental seizures are unknown. The present study examined GAT-1-immunolabeling in infant rats with two types of developmental seizures, one induced by corticotropin-releasing hormone (CRH) lasting about 2 h and the other by hyperthermia (a model of febrile seizures) lasting only 20 min. The number of GAT-1-immunoreactive (ir) neurons was increased in several forebrain regions 24 h after induction of seizures by CRH as compared to the control group. Increased numbers of detectable GAT-1-ir cell bodies were found in the hippocampal formation including the dentate gyrus and CA1, and in the neocortex, piriform cortex and amygdala. In contrast, hyperthermia-induced seizures did not cause significant changes in the number of detectable GAT-1-ir somata. The increase in GAT-1-ir somata in the CRH model and not in the hyperthermia model may reflect the difference in the duration of seizures. The brain regions where this increase occurs correlate with the occurrence of argyrophyllic neurons in the CRH model.

Status epilepticus-induced hilar basal dendrites on rodent granule cells contribute to recurrent excitatory circuitry.

Mossy fiber sprouting into the inner molecular layer of the dentate gyrus is an important neuroplastic change found in animal models of temporal lobe epilepsy and in humans with this type of epilepsy. Recently, we reported in the perforant path stimulation model another neuroplastic change for dentate granule cells following seizures: hilar basal dendrites (HBDs). The present study determined whether status epilepticus-induced HBDs on dentate granule cells occur in the pilocarpine model of temporal lobe epilepsy and whether these dendrites are targeted by mossy fibers. Retrograde transport of biocytin following its ejection into stratum lucidum of CA3 was used to label granule cells for both light and electron microscopy. Granule cells with a heterogeneous morphology, including recurrent basal dendrites, and locations outside the granule cell layer were observed in control preparations. Preparations from both pilocarpine and kainate models of temporal lobe epilepsy also showed granule cells with HBDs. These dendrites branched and extended into the hilus of the dentate gyrus and were shown to be present on 5% of the granule cells in pilocarpine-treated rats with status epilepticus, whereas control rats had virtually none. Electron microscopy was used to determine whether HBDs were postsynaptic to axon terminals in the hilus, a site where mossy fiber collaterals are prevalent. Labeled granule cell axon terminals were found to form asymmetric synapses with labeled HBDs. Also, unlabeled, large mossy fiber boutons were presynaptic to HBDs of granule cells. These results indicate that HBDs are present in the pilocarpine model of temporal lobe epilepsy, confirm the presence of HBDs in the kainate model, and show that HBDs are postsynaptic to mossy fibers. These new mossy fiber synapses with HBDs may contribute to additional recurrent excitatory circuitry for granule cells.

Immunocytochemical mapping of Fos protein following seizures in gerbils indicates the activation of hippocampal neurons.

The expression of the proto-oncogene, c-fos, and its protein, Fos, has been shown to be a useful marker for elevated levels of neuronal activity generated in the brain following different stimuli, including seizures. Since previous studies indicated hippocampal involvement in seizure activity in gerbils, Fos immunocytochemistry was used to determine whether hippocampal neurons become activated following environmentally induced seizures in this animal. Gerbils with maximal seizures showed many Fos-immunolabeled neurons in the granule cell layer and hilus of the dentate gyrus, as well as in CA3 and CA1 of the hippocampus. These gerbils had significantly greater numbers of Fos-immunolabeled dentate granule cells than gerbils with less severe seizures or no seizures. The number of dentate granule cells and CA3 pyramidal cells with Fos immunolabeling increased in an exponential manner with increased seizure severity. Many Fos-immunolabeled neurons were found in several regions of the neo- and paleocortex and in other limbic structures including the piriform cortex, cortical amygdaloid nucleus, and arcuate nucleus of the hypothalamus. These results indicate that hippocampal neurons are activated following seizures in a genetic model, and provide further proof that the hippocampal formation is involved in the circuitry for seizures in gerbils.

Synaptic and neurochemical features of calcitonin gene-related peptide containing neurons in the rat accessory optic nuclei.

Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), including the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neurons are involved in visual circuitry, electron microscopic preparations were analyzed from normal rats and rats with optic nerve transections. A co-localization analysis was also made because CGRP-labeled neurons had features of GABAergic neurons. Thus, sections were prepared for light microscopy to determine whether CGRP-containing neurons also had glutamate decarboxylase (GAD) and other markers for GABAergic neurons, such as calcium binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron microscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that were postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating that retinal ganglion cells form synapses with CGRP-labeled neurons in the AON. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric synapses with unlabeled somata as well as dendritic shafts and spines. Consistent with this type of synapse being GABAergic were the co-localization data showing that about 90% of the CGRP-labeled neurons co-localized GAD in the AON. Many CGRP-labeled neurons showed immunostaining for CR (40%) whereas only a few had labeling for CB (5%). No CGRP-labeled neurons had PV. These data show that CGRP-containing neurons receive direct retinal input and represent a subpopulation of GABAergic neurons which differentially co-express calcium-binding proteins.

Lysosomal dysfunction results in lamina-specific meganeurite formation but not apoptosis in frontal cortex.

An inhibitor of cathepsins B and L was used to test if lysosomal dysfunction in cultured slices of rat frontal cortex induces pathological features that develop in the human cortex during aging and Alzheimer's disease (AD). Incubation for 6 days with N-CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD) resulted in a massive proliferation of endosomes-lysosomes in all cortical layers. Slices additionally exposed to a washout of 4 days had numerous meganeurites, blister-like structures in the region of the axon hillock, in layer III but not in other cortical laminae. Meganeurites are a characteristic feature of the human frontal cortex after age 50 and are largely restricted to layer III. Tests for apoptosis were carried out at two intervals following meganeurite formation. TUNEL-labeled neurons were confined to layers II/III on the surface of the slices but there was no evidence for a ZPAD effect. In all, 6 days of lysosomal dysfunction reproduces characteristic effects of normal aging in neocortex without generating some key features of AD.

Alteration of GABA transporter expression in the rat cerebral cortex following needle puncture and colchicine injection.

In the adult cerebral cortex, GABA transporters (GATs) are expressed by both neurons and astrocytes. GAT-1 immunoreactivity is found in axon terminals of GABAergic neurons and astrocytes, while GAT-3 immunolabeling occurs only in the latter. The present study was designed to determine whether the expression of GAT-1 and GAT-3 in the adult rat cerebrum changes after needle lesion and colchicine infusion. Following a needle puncture or a saline injection, immunolabeling for GAT-1 and GAT-3 was slightly increased in an area around the needle track. Not only was the neuropil labeling for both GATs increased, but also a few neuronal somata were found to be immunoreactive for GAT-1. Colchicine injections induced a striking increase in immunolabeling for both GATs in the neuropil in an area adjacent to the needle path and surrounding it. A homologous region of the contralateral hemisphere also showed a moderate increase of immunoreactivity in the neuropil for both GATs. Furthermore, this contralateral site showed many neuronal somata immunolabeled for GAT-1. These changes were mainly detected during the first 5 days following intracortical lesions. These results indicate that (1) the upregulation of GAT-1 and GAT-3 in cortical interneurons and astrocytes is caused by both mechanical and chemical factors associated with the injections; (2) increased GAT-1 and GAT-3 expression contralateral to the site of colchicine injection is mediated by transcellular signaling across the corpus callosum; and (3) the lesion-induced GAT expression may play a protective role by helping to balance excitatory and inhibitory neuronal activities.

Co-localization of corticotropin-releasing hormone with glutamate decarboxylase and calcium-binding proteins in infant rat neocortical interneurons.

Corticotropin releasing hormone (CRH) has been localized to interneurons of the mammalian cerebral cortex, but these neurons have not been fully characterized. The present study determined the extent of co-localization of CRH with glutamate decarboxylase (GAD) and calcium-binding proteins in the infant rat neocortex using immunocytochemistry. CRH-immunoreactive (ir) neurons were classified into two major groups. The first group was larger and consisted of densely CRH-immunostained small bipolar cells, predominantly localized to layers II and III. The second group of CRH-ir cells was lightly labeled and included multipolar neurons mainly found in deep cortical layers. Co-localization studies indicated that the vast majority of CRH-ir neurons, including both bipolar and multipolar types, was co-immunolabeled for GAD-65 and GAD-67. Most multipolar, but only some bipolar, CRH-ir neurons also contained parvalbumin, while CRH-ir neurons rarely contained calbindin or calretinin. These results indicate that virtually all CRH-ir neurons in the rat cerebral cortex are GABAergic. Furthermore, since parvalbumin is expressed by cortical basket and chandelier cells, the co-localization of CRH and parvalbumin suggests that some cortical CRH-ir neurons may belong to these two cell types.

Developmental expression of gamma-aminobutyric acid transporters (GAT-1 and GAT-3) in the rat cerebellum: evidence for a transient presence of GAT-1 in Purkinje cells.

The cerebellar cortex contains several classes of GABAergic neurons. Previous studies have shown that most GABAergic neurons in this region possess the capacity for gamma-aminobutyric acid (GABA) uptake. The present study determined the postnatal expression of two GABA transporters, GAT-1 and GAT-3, in the cerebellar cortex and deep nuclei of the rat by using immunocytochemistry. Immunoreactivity for GAT-1 and GAT-3 appears at postnatal day 7 (P7), emerges centroperipherally across the cerebellum during the following 2 weeks and reaches an adult-like pattern by P30. The mature patterns are fully established by P45, which for GAT-1 is characterized by immunolabeled profiles localized exclusively to neuropil, mostly in the molecular layer and the pinceaux deep to the Purkinje cell bodies, and for GAT-3 as immunoreactivity distributed in the neuropil of mainly the granular layer. Before the adult patterns are completed, GAT-1 immunoreactivity is present in the somata of Purkinje, Golgi, basket and stellate cells between P7 and P21, while GAT-3 immunoreactivity is distinct in astrocytic somata which are organized in regularly spaced clusters. During this period, there is also a banding pattern in the sagittal plane of GAT-1 immunoreactivity in developing Purkinje cells. The postnatal development of GAT-1 and GAT-3 in the rat cerebellar cortex shares a similar spatiotemporal pattern with other GABAergic parameters, including the GABA synthesizing enzyme, GABA content and uptake. Specifically, the transient expression of GAT-1 in the somata and dendrites of cerebellar GABAergic neurons appears to correlate with the supra-adult levels of whole-tissue GABA uptake capability during development. Further, GAT-1 expression in immature Purkinje cells may play a unique role in regulating GABA's function during development, since mature Purkinje cells do not express GAT-1 or take up GABA.

Alumina gel injections into the temporal lobe of rhesus monkeys cause complex partial seizures and morphological changes found in human temporal lobe epilepsy.

The goal of the present study was to determine whether alumina gel injections into temporal lobe structures cause complex partial seizures (CPS) and pathological changes observed in human temporal lobe epilepsy. Rhesus monkeys with alumina gel injections in the amygdala, perirhinal and entorhinal cortices, or Ammon's horn and dentate gyrus all initially displayed focal pathological electroencephalographic (EEG) slowing limited to the site of injection. After clinical seizures developed, they also displayed widespread pathological EEG slowing over both hemispheres, interictal and ictal epileptiform EEG abnormalities limited to the mesial-inferior temporal lobe on the side of injection, and different degrees of spread to other ipsilateral and contralateral structures. Noninjected control and nonepileptic monkeys with injections into the middle and inferior temporal gyri displayed no hippocampal neuronal loss or mossy fiber sprouting. When alumina gel was injected into the amygdala, CPS began within 3-6 weeks and degeneration of neurons and gliosis occurred in the perirhinal cortex or the hippocampus, with consequent sprouting of mossy fibers in the dentate gyrus. Dispersion of the granule cell layer was also observed. Other monkeys with alumina gel in the perirhinal and entorhinal cortices developed CPS within 2-3 weeks after the injections and displayed mossy fiber sprouting only after 4 weeks after the injections. Alumina gel in Ammon's horn and the dentate gyrus also induced CPS, but mossy fiber sprouting was limited to sites immediately adjacent to the injection, probably because none survived more than 4 weeks after the injections. This nonhuman primate model of CPS displayed similar anatomical, behavioral, and EEG features as observed in human temporal lobe epilepsy and provides opportunities to analyze the chronological sequence of epileptogenesis and to test potential therapies.

Dentate granule cells form novel basal dendrites in a rat model of temporal lobe epilepsy.

Mossy fibre sprouting and re-organization in the inner molecular layer of the dentate gyrus is a characteristic of many models of temporal lobe epilepsy including that induced by perforant-path stimulation. However, neuroplastic changes on the dendrites of granule cells have been less-well studied. Basal dendrites are a transient morphological feature of rodent granule cells during development. The goal of the present study was to examine whether granule cell basal dendrites are generated in rats with epilepsy induced by perforant-path stimulation. Adult Wistar rats were stimulated for 24 h at 2 Hz and with intermittent (1/min) trains (10 s duration) of single stimuli at 20 Hz (20 V, 0.1 ms) delivered 1/min via an electrode placed in the angular bundle. The brains of these experimental rats and age- and litter-matched control animals were processed for the rapid Golgi method. All rats with perforant-path stimulation displayed basal dendrites on many Golgi-impregnated granule cells. These basal dendrites mainly originated from their somata at the hilar side and then extended into the hilus. Quantitative analysis of more than 800 granule cells in the experimental and matched control brains showed that 6-15% (mean=8.7%) of the impregnated granule cells have spiny basal dendrites on the stimulated side, as well as the contralateral side (mean=3.1%, range=2.9-3.9%) of experimental rats, whereas no basal dendrites were observed in the dentate gyrus from control animals. The formation of basal dendrites appears to be an adaptive morphological change for granule cells in addition to the previously described mossy fibre sprouting, as well as dendritic and somatic spine formation observed in the dentate gyrus of animal and human epileptic brains. The presence of these dendrites in the subgranular region of the hilus suggests that they may be postsynaptic targets of the mossy fibre collaterals.

Corticotropin-releasing hormone (CRH)-containing neurons in the immature rat hippocampal formation: light and electron microscopic features and colocalization with glutamate decarboxylase and parvalbumin.

Corticotropin-releasing hormone (CRH) excites hippocampal neurons and induces death of selected CA3 pyramidal cells in immature rats. These actions of CRH require activation of specific receptors that are abundant in CA3 during early postnatal development. Given the dramatic effects of CRH on hippocampal neurons and the absence of CRH-containing afferents to this region, we hypothesized that a significant population of CRHergic neurons exists in developing rat hippocampus. This study defined and characterized hippocampal CRH-containing cells by using immunocytochemistry, ultrastructural examination, and colocalization with gamma-aminobutyric acid (GABA)-synthesizing enzyme and calcium-binding proteins. Numerous, large CRH-immunoreactive (ir) neurons were demonstrated in CA3 strata pyramidale and oriens, fewer were observed in the corresponding layers of CA1, and smaller CRH-ir cells were found in stratum lacunosum-moleculare of Ammon's horn. In the dentate gyrus, CRH-ir somata resided in the granule cell layer and hilus. Ultrastructurally, CRH-ir neurons had aspiny dendrites and were postsynaptic to both asymmetric and symmetric synapses. CRH-ir axon terminals formed axosomatic and axodendritic symmetric synapses with pyramidal and granule cells. Other CRH-ir terminals synapsed on axon initial segments of principal neurons. Most CRH-ir neurons were coimmunolabeled for glutamate decarboxylase (GAD)-65 and GAD-67 and the majority also contained parvalbumin, but none were labeled for calbindin. These results confirm the identity of hippocampal CRH-ir cells as GABAergic interneurons. Further, a subpopulation of neurons immunoreactive for both CRH and parvalbumin and located within and adjacent to the principal cell layers consists of basket and chandelier cells. Thus, axon terminals of CRH-ir interneurons are strategically positioned to influence the excitability of the principal hippocampal neurons via release of both CRH and GABA.