Number and regional distribution of GAD65 mRNA-expressing interneurons in the rat hippocampal formation

In rodent models for neuropsychiatric disorders reduced number of hippocampal interneurons have been reported, but the total number of GABAergic neurons in the normal rat hippocampus is yet unknown. We used in situ hybridization method to label the 65 isoform of glutamic acid decarboxylase (GAD65) and counted the number of GAD65 mRNA-expressing neurons along the entire septo-temporal axis of the hippocampus. We found that 2/3 of the interneurons were in Ammon's horn (61,590) and 1/3 in the dentate gyrus (28,000). We observed the following numbers in Ammon's horn: CA3 area 33,400, CA2 area 4,190, CA1 area 24,000 and in the dentate gyrus: 6,000 in the molecular and 9,000 in the granule cell layers and 13,000 in the hilus. GAD65 mRNA-expressing neurons were significantly more numerous in dorsal than in ventral hippocampus. The ratio between interneurons and principal cells was lowest in the granule cell layer (0.9%) and highest in hilus (21%). In Ammon's horn this ratio was constant being 13% in CA3 and 8% in CA1-2 areas. In the entire hippocampal formation, the interneuron/principal cell ratio was 6%, with a significant difference between Ammon's horn (9.5%) and the dentate gyrus (2.8%) including the hilus. Such low ratios could suggest that even a limited loss of GABAergic neurons in the hippocampus may have a considerable functional impact.

Morphological analysis of the connective tissue reaction in linear hypertrophic scars treated with intralesional steroid or silicone-gel sheeting. A light and electron microscopic study

The linear hypertrophic scar has become the most common type of pathologic scarring. Silicone-gel sheeting is the first line therapy while intralesional steroid is the second. A light and electron microscopic analysis was carried out to reveal differences in tissue reaction following the two different treatments. Two groups of 12 patients each were treated for 4 months. For the first group, diluted Triamcinolone acetonide was injected until an inactive state was achieved. The other group of patients was treated with silicone-gel sheeting. The scars were examined every two weeks and their appearance documented. After reaching the expected therapeutic response, inactive scars were removed. The excised scars were evaluated through light microscopic histopathology and electron microscopy. The light and electron microscopic observations revealed marked differences following treatments. The activity of fibroblasts and the numbers of collagen fibers forming bundles decreased and the orientation of the collagen fibers was more variable in the treated scars. The amount of elastic fibers increased after both steroid and silicone-gel sheeting treatment. Vascularization was also slightly changed, with more capillaries and fewer pre-capillary arteries detected in the treated scars. Both treatments resulted in the same decrease in score but steroid treatment was more rapid in onset. We suggest that the two different treatments work through different mechanisms, although the final functional outcome is similar.

Novel calretinin and reelin expressing neuronal population includes Cajal-Retzius-type cells in the neocortex of adult pigs

Cajal-Retzius cells and their secreted product reelin are essential for the lamination of the cerebral cortex. In all species studied to date Cajal-Retzius cells form a transient neuronal population that almost completely disappears from the neocortex postnatally. Recently, in the hippocampal formation of adult domestic pig, we have found a large calretinin- and reelin-immunoreactive cell population that morphologically corresponded to Cajal-Retzius cells. In the present study, we examined calretinin- and reelin-immunoreactive neurons in layer I of the prefrontal, temporal, parietal and occipital neocortical areas of newborn, young adult and adult domestic pigs. Large numbers of bipolar or fusiform calretinin-positive cells were found in the upper half of layer I in all examined age groups. The morphology of these neurons resembled that of the Cajal-Retzius cells. Layer I was occupied by a dense calretinin-positive axonal plexus that was similar to the previously described axons of Cajal-Retzius cells in other species. In a similar location, where calretinin-positive cells occurred in layer I, large numbers of reelin-immunoreactive cells were found in all examined age groups. In addition, reelin colocalized with calretinin in layer I neurons. The number of calretinin and reelin-positive neurons decreased from 1 day to one year, but calretinin-positive Cajal-Retzius-type cells still comprised a remarkable large population in 12-month-old animals. Correlated light and electron microscopic examination of calretinin-labeled Cajal-Retzius-type cells indicated that these cells are integrated in the synaptic circuitry of the neocortex. Our results suggest that Cajal-Retzius cells do not disappear inevitably from the mature neocortex in all mammalian species. The function of this cell type is not known, but late persisting Cajal-Retzius-type cells in the domestic pig provide an opportunity to study their neuronal connections and the possible role of reelin in plasticity and regeneration of neocortex.

Mossy cells and different subpopulations of pyramidal neurons are immunoreactive for cocaine- and amphetamine-regulated transcript peptide in the hippocampal formation of non-human primates and tree shrew (Tupaia belangeri)

Cocaine- and amphetamine-regulated transcript peptide mRNA was discovered in the rat striatum following cocaine and amphetamine administration. Since both psychostimulants elicit memory-related effects, localization of cocaine- and amphetamine-regulated transcript peptide in the hippocampal formation may have functional importance. Previous studies demonstrated different cellular localizations of cocaine- and amphetamine-regulated transcript peptide in humans and in rodents. Mossy cells were cocaine- and amphetamine-regulated transcript-positive in the human dentate gyrus, whereas granule cells contained this peptide in the rat. In the present study, the localization of cocaine- and amphetamine-regulated transcript peptide was examined using immunohistochemistry in the hippocampal formation of the rhesus monkey (Macaca mulatta), the common marmoset monkey (Callithrix jacchus) and in the tree shrew (Tupaia belangeri). In these species principal neurons of the hippocampal formation were cocaine- and amphetamine-regulated transcript-immunoreactive. In both monkeys and tree shrews, mossy cells of the hilus were cocaine- and amphetamine-regulated transcript-positive whereas granule cells of the dentate gyrus were cocaine- and amphetamine-regulated transcript-negative. The dense cocaine- and amphetamine-regulated transcript-immunoreactive axonal plexus of the associational pathway outlined the inner one-third of the dentate molecular layer. In the hippocampus of the tree shrew and marmoset monkey, a subset of CA3 pyramidal cells were cocaine- and amphetamine-regulated transcript-immunoreactive. In the marmoset monkey, cocaine- and amphetamine-regulated transcript labeling was found only in layer V pyramidal cells of the entorhinal cortex, while in the rhesus monkey, pyramidal cells of layers II and III were cocaine- and amphetamine-regulated transcript-immunopositive. Our results show that cocaine- and amphetamine-regulated transcript positive neurons in the dentate gyrus of non-human primates are similar to that of the human. Furthermore, in the hippocampal formation of the tree shrew similar cocaine- and amphetamine-regulated transcript-immunoreactive cell-types were observed as in monkeys, supporting their evolutionary relationship with primates. Mossy cells and granule cells are members of a mutual excitatory intrahippocampal circuitry, therefore cocaine- and amphetamine-regulated transcript-immunoreactivity of these neurons in primates and rodents suggests that psychostimulants cocaine and amphetamine may induce memory-related effects at different points of the same excitatory circuitry in the hippocampal formation.

NADPH-diaphorase positive neurons of the rat hippocampal formation: regional distribution, total number and colocalization with calcium binding proteins

The present study aimed to asses the total number and distribution of the NADPH-diaphorase-positive non-pyramidal neurons in Ammon's horn and dentate gyrus of rat hippocampal formation. Cell bodies were counted according to the "disector" principle. The total numbers varied from 27 000 to 32 400. In all strains, approximately one third of the NADPH-diaphorase-reactive non-principal cells were found in the dentate gyrus and the remaining two thirds were within the Ammon's horn. Analysis of the dorsoventral differences revealed that approximately 70% of NADPH-diaphorase-positive cells were in the dorsal and 30% in the ventral hippocampus. Distribution of NADPH-diaphorase-reactive cells in the different layers of the dentate gyrus and Ammon's horn was similar in all strains. Double-labelling studies revealed colocalization of NADPH-diaphorase with calretinin, but none with calbindin or parvalbumin. NADPH-diaphorase-positive neurons appear to form the largest chemically identified subpopulation of the GABAergic inhibitory cell population of the hippocampal formation.

Mitochondrial DNA4977 deletion in brain of newborns died after intensive care

Mitochondrial DNA (mtDNA) deletion affecting 4977 base pairs (mtDNA4977), the most common mtDNA mutation in humans, was analysed in brain specimens (frontal, temporal, and cerebellar cortices, caudate nucleus, thalamus, and hippocampus) and in other tissues (blood clot, liver, kidney, heart, and muscle) taken at autopsy of deceased neonates. mtDNA4977 deletion determined by polymerase chain reaction (PCR) could be demonstrated in each neonatal sample, however, quantity of mtDNA4977 deletion was less in the newborn samples than in those of the elderlies. Results obtained suggest that contrary to certain data mtDNA4977 deletion can be present in neonates. The mtDNA4977 deletion could be generated by perinatal hypoxia or temporary oxygen oversaturations during the intensive care of the neonates, as the mtDNA is sensitive to oxidative damage. In combination with other factors an additional causative role of mtDNA4977 deletion reported here cannot be ruled out in development of cerebral palsy or mental retardation of unknown origin often seen in neonates underwent neonatal intensive care procedures.

Cocaine- and amphetamine-regulated transcript peptide (CART) is a selective marker of rat granule cells and of human mossy cells in the hippocampal dentate gyrus

Cocaine- and amphetamine-regulated transcript (CART) peptide immunocytochemistry was used to reveal cellular localization in the dentate gyrus and in Ammon's horn of the rat and human hippocampal formations. In the rat dentate gyrus, only granule cells were labeled, whereas in humans, only mossy cells of the hilar region expressed CART peptide immunoreactivity. In the rat, CART-positive granule cells were located at the molecular layer border of the granule cell layer and had no features that would distinguish them from other granule cells. The mossy fiber bundle was labeled in the hilus as well as along the entire CA3 area of Ammon's horn. In the human, CART-immunoreactive mossy cells displayed the characteristic thorny excrescences both on their somata and their main dendrites. Axon collaterals of mossy cells could be seen in the hilus and the main axons formed a dense band in the inner molecular layer of the dentate gyrus, suggesting that mossy cells are the principal source of the associational pathway. Granule cells of the dentate gyrus and pyramidal neurons of the human hippocampal formation were devoid of CART peptide immunoreactivity. A few labeled non-pyramidal cells and a large group of strongly immunostained axons of unknown origin were present in all layers of CA1-3. Granule cells are the main excitatory cell population of the dentate gyrus while mossy cells are in a key position in controlling activity of granule cells. The specific location of CART peptide in the dentate granule cells of rodents and in the mossy cells of the human hippocampus may indicate involvement of neuronal circuitry of the dentate gyrus in the memory-related effects of cocaine and amphetamine. Independently of its functional role, CART peptide can be used as a specific marker of human mossy cells and of the dentate associational pathway. The sensitivity of CART peptide to postmortem autolysis may restrict the use of this marker in surgically removed hippocampi or in human brains removed and fixed shortly after death.

Axosomatic synapses on granule cells are preserved in human non-infiltrating tumour or lesion-related and mesial temporal sclerotic epilepsy, but markedly reduced in tumour-infiltrated dentate gyrus with or without epilepsy

Granule cells of the human hippocampal dentate gyrus were examined. In controls, granule cells displayed somatic spines and cell nuclei with small infoldings. In addition, the cytoplasm of human granule cells always displayed lipofuscin. Subsurface cisterns of endoplasmic reticulum were frequently observed in the human granule cells. Two types of axosomatic synapses were found; most frequently symmetric and less frequently asymmetric. Many of the axosomatic synapses were isolated by glial processes in tumour or lesion-related epileptic patients, but the ultrastructural characteristics of granule cells were not different from those of the control patients. Large bundles of reactive astroglial fibres appeared regularly in all layers of the dentate gyrus. In tumour infiltrated hippocampi, glial processes dominated the neuropil and the number of perisomatic synapses was markedly reduced. Reduction in the number of perisomatic synapses did not correlate with severity and duration of seizures but did correlate with the malignancy of the tumour. It is suggested that reduction of perisomatic inhibition may not be a characteristic of granule cells in the epileptic human dentate gyrus.

Neonatal anandamide treatment results in prolonged mitochondrial damage in the vanilloid receptor type 1-immunoreactive B-type neurons of the rat trigeminal ganglion

Capsaicin acting on the vanilloid type 1 receptor (VR1) excites a subset of primary sensory neurons. Systemic capsaicin treatment of adult or neonatal rats results in selective damage of the B-type neurons in the rat sensory ganglia by causing a long-lasting mitochondrial lesion that has been described in detail in previous studies. The endocannabinoid, anandamide, exhibits an agonist effect on VR1 receptors. The physiological role of anandamide as a VR1 agonist is still uncertain. This study addresses whether high doses of anandamide induce similar ultrastructural changes to those described for capsaicin. The effect of neonatally administered anandamide (1 mg/kg) on neurons of the trigeminal ganglia and the hippocampal formation was examined in the light and electron microscope from the first day after injections to the 20th week after treatment. Anandamide was found to cause mitochondrial damage of the B-type neurons of trigeminal ganglia similar to what has been described for capsaicin. The time course of damage was also comparable. In addition to the cells of the trigeminal ganglia, B-type cells of dorsal root ganglia were also damaged. A-type neurons and satellite glial cells were not affected either in the trigeminal or in the dorsal root ganglia. In the hippocampal formation, where a subpopulation of local circuit neurons is known to contain cannabinoid type 1 (CB1) but not VR1 receptors, anandamide did not cause morphological changes of mitochondria either in the dentate gyrus or in Ammon's horn. At 3 weeks of age, all VR1-immunoreactive neurons in the trigeminal ganglia of animals treated neonatally with anandamide displayed swollen mitochondria. The results suggest that anandamide, at pharmacologically relevant doses, acts on the VR1 receptor and causes prolonged and selective mitochondrial damage of B-type sensory neurons, as has previously been described for capsaicin.

Age-related mitochondrial damage in the B-type cells of the rat trigeminal ganglia

Aging is associated with signs of sensory impairment and neurological symptoms. Advancing age is characterized by increased thresholds of thermal, tactile and vibratory sensations. One important cause of the sensory disturbances has been stated to be the loss of neurons. Decreases have been observed in the number of peripheral nerve fibers and in the number of neurons in the spinal ganglia of rats. In the present study, the cytoplasmic organelles of the neurons of the trigeminal ganglia were examined in young and senescent rats in order to reveal the cause of cell loss during aging. Mitochondrial alterations, swelling and loss of internal cristae were observed from 23 week of age in the B-type neurons of the trigeminal ganglia. Other cytoplasmic elements were intact. Mitochondrial damage was never seen in A-type neurons and satellite glial cells. It was concluded that the ultrastructural changes in the mitochondria of the B-type cells may contribute to the nervous disturbances that occur in senescent individuals. The diminution of mitochondrial damage and the protection of B-type neurons through the use of nerve growth factors may prevent the sensory impairment late in life.

Neonatal capsaicin treatment results in prolonged mitochondrial damage and delayed cell death of B cells in the rat trigeminal ganglia

Capsaicin acts on the vanilloid receptor subtype 1, a noxious heat-gated cation channel located on a major subgroup of nociceptive primary afferent neurons. Following the systemic capsaicin treatment of neonatal rats, the loss of B-type sensory neurons in trigeminal ganglion of adult rats with chemoanalgesia and abolition of neurogenic inflammation was investigated. Our quantitative morphometric analysis revealed that in the trigeminal ganglion of neonatal rats treated with 50 mg/kg s.c. capsaicin, the total number of neurons, morphology of B-type cells and cell-size histograms did not differ from that of the controls 1 or 5 days after treatment. These observations indicate that early cell death does not play a significant part in the loss of B-type cells, which in our sample was 39.4% on the 19th day. However under the electron microscope pronounced selective mitochondrial swelling with disorganized cristae was observed in B-type neurons at 1-20 weeks after capsaicin treatment. Daily treatment with nerve growth factor (NGF, 10 x 100 microg/kg s.c.), started 1 day after capsaicin injection, prevented the loss of B-type cells but did not counteract the development of long-lasting mitochondrial damage. After NGF treatment, partial restitution of chemonociception to capsaicin instillation into the eye occurred but capsaicin-induced inhibition of neurogenic plasma extravasation in the hindpaw evoked by topical application of mustard oil remained unaltered. We conclude, that capsaicin treatment in neonatal rats, as in the adults, destroys terminal parts of the sensory neurons supplied by vanilloid receptors and induces long-lasting mitochondrial swelling in the soma. We hypothesize that loss of NGF uptake results in delayed cell death of B-type neurons in neonates.

Cell formation in the human hippocampal formation from mid-gestation to the late postnatal period

In the present study cell formation was studied in the human hippocampal formation from the 24th gestational week until the end of the first postnatal year. Proliferating cells were detected with the monoclonal antibody MIB-1. The cytoarchitectonic layers of Ammon's horn are formed before the 24th gestational week. In harmony with this observation, cell proliferation in the hippocampal ventricular zone is minimal after the 24th week. In addition, local cell multiplication in Ammon's horn is occasional and the proliferating cells are glial or endothelial cells. In contrast, cell formation continues in the hilar region of the dentate gyrus even after birth. Immature cells accumulate in the hilus, and at the border between the hilus and the granule cell layer throughout the first eight postnatal months. The subgranular zone of the dentate gyrus becomes a cell sparse area at about the 11th postnatal month, indicating that immature cells from the hilus have already migrated to the granule cell layer and differentiated into granule cells. There is an increase in glial cell proliferation both in Ammon's horn and the dentate gyrus at the 11.5th postnatal month suggesting the onset of myelination by the end of the first year. Our findings indicate that most pyramidal neurons of Ammon's horn are generated in the first half of pregnancy and no pyramidal neurons are formed after the 24th gestational week. In contrast, granule cells of the dentate gyrus proliferate in a decreasing rate during the second half of pregnancy and after birth. Proliferating neuronal precursors occur in a low percentage in the dentate gyrus of 3-, 5- and 11.5-month-old children.

Effect of neonatal dentate gyrus lesion on allothetic and idiothetic navigation in rats

Goal-directed navigation is believed to be the combined product of idiothetic and allothetic orientation. Although both navigation systems require the hippocampal formation, it is probable that different circuits implement them. Examination of Long-Evans rats with dentate gyrus lesions induced by neonatal X-ray irradiation may show the dissociation of these two components of navigation. Two recently developed place avoidance tasks on a rotating circular arena were used to test this hypothesis. In the first test, the position of the punished area is stable in the room frame but is permanently changing on the surface of the arena. This task requires the rat to use allothetic orientation and to disregard idiothetic orientation. In the second test, the prohibited area is fixed in the coordinate system of the arena and the experiment is conducted in complete darkness, forcing the rat to rely exclusively on idiothesis supported by substratal cues. The results suggest that the dentate gyrus lesion interferes less with idiothetic orientation than with allothetic orientation. In addition, an attempt was made to control the number of developing granule cells by exact timing of a single high dose of perinatal irradiation, and to measure the ensuing behavioral deficits. Rats irradiated at 6, 18, or 24 h after birth were tested as adults in the Morris water maze. Irradiated animals showed significant, but highly variable, learning deficit, but histological examination indicated that the granule cell loss did not correlate with the degree of behavioral impairment.

Granule cells are the main source of excitatory input to a subpopulation of GABAergic hippocampal neurons as revealed by electron microscopic double staining for zinc histochemistry and parvalbumin immunocytochemistry

Immunocytochemistry was combined with a recent modification of Timm's method to evaluate semiquantitatively the mossy fiber innervation of dendrites and somata of parvalbumin-containing neurons of the hilus of the dentate gyrus and the CA3 area of Ammon's horn. Using this electron microscopic double staining technique, it was found that (1) the overwhelming majority (95%) of terminals forming asymmetric synapses with parvalbumin-positive dendrites in the dentate hilus, and the strata pyramidale and lucidum of the CA3 area of Ammon's horn, originated from granule cells; (2) two-thirds of the asymmetric axosomatic terminals of parvalbumin-positive neurons contained zinc; and (3) no zinc-containing axon terminals formed synapses with somata or main dendritic shafts of the granule cells.

Cell formation in the cortical layers of the developing human cerebellum

Cell proliferation has been studied in the human cerebellar cortex between the 24th gestational week and the 12th postnatal month. Intensive cell formation has been found in the external granular layer (EGL) of the human cerebellum, where the highest cell proliferation rate occurs between the 28th and 34th gestational weeks. This is followed by a gradual decrease that lasts up to the eighth postnatal month. As late in development as the fifth postnatal month, still 30% of cells of the EGL are labeled with the monoclonal antibody Ki-67, which is specific for dividing cells. The width of the EGL remained unchanged from the 28th gestational week to the end of the first postnatal month, when it starts to decrease and completely disappears by the 11th postnatal month. Large number of Ki-67 labeled cells occurs in the internal granular layer (IGL) between the 24th and 28th gestational weeks. From the 36th week onwards, the labeling index is less than 1%, although a few labeled cells have always been found in this layer even in the late postnatal period. Labeled cells are distributed in the entire width of the IGL. However, from the 34th gestational week, almost all labeled cells are found among and directly below the Purkinje cells. Their position, the nuclear features, and their occasionally stained cell processes suggest that those are Bergmann glial cells. There are few Ki-67 labeled cells in the molecular layer (ML) and in the white matter (WM) of the cerebellum throughout the examined period. It is likely that most of these are glial cells. Pyknotic index has been found to be small in all layers of the cerebellum during the examined period.

The use of a sodium tungstate developer markedly improves the electron microscopic localization of zinc by the Timm method

The Timm's sulfide-silver method is frequently used for the demonstration of the mossy fiber bundle or sprouted mossy fibers in the normal or epileptic hippocampal dentate gyrus. Under the light microscope the results are excellent, but the ultrastructure is considerably impaired and the silver grains produced are too large as compared to the sizes of intra-synaptic structures. The present study was meant to test a series of physical developers containing, instead of gum arabic, sodium tungstate as protective colloid. One of them left the ultrastructure fairly intact and produced small, round silver grains, making it possible to precisely locate zinc in mossy terminals. With this method, it could be demonstrated that zinc is contained inside synaptic vesicles in the resting axon terminals of granule cells. As a consequence of prolonged sodium sulfide perfusion, zinc is released from synaptic vesicles and enters the synaptic cleft.

Electrophysiological characteristics and morphological properties of dentate granule--and CA3 pyramidal cells in slices cut from neonatally irradiated rats

Neonatal irradiation reduces the dentate granule cells by 60-80%, and consequently the mossy fiber projection toward the CA3 and hilar areas decreases. The number of hilar cells diminishes. Thorny excrescences on the dendrites of the CA3 pyramidal cells get smaller both in number (from 20-30 per neuron in normal to 1-6 per neuron after irradiation) and in size. In spite of these morphological changes functional efficacy of the mossy-fiber projection to CA3 pyramidal cells remains sufficient to generate monosynaptic action potentials when stimulated electrically. Inhibitory circuits activated by mossy fiber volleys seem to be unaffected by irradiation. Main biophysical properties of CA3 pyramidal and surviving granule cells remain within the normal range. Further work should determine if efficacy of the mossy fiber projection increases to compensate for the substantial decrease of presynaptic input, or the power of transmission far exceeds the level needed to fire postsynaptic cells in normal rats.

Muscle carnitine acetyltransferase and carnitine deficiency in a case of mitochondrial encephalomyopathy

Profound decrease of the carnitine acetyltransferase activity (0.08 U/g wet weight; 1.67% of control) and carnitine deficiency (total carnitine was 230 nmol/g wet weight in the patient vs 2730 in the controls) was detected in the skeletal muscle of a female paediatric patient. She died of her illness, which included cerebellar symptoms and slight muscle spasticity affecting mainly the lower extremities, at 1 year of age. Histological examination of the autopsy specimens revealed a selective Purkinje cell degeneration in the cerebellum: the cells had abnormal position, were shrunken and decreased in number, and displayed abnormal dendritic trees and fragmented, disorganized axons. Electron microscopy revealed mitochondrial abnormalities in skeletal and cardiac muscle and also in the Purkinje cells. Deletions of the mitochondrial DNA were detected in the muscle in heteroplasmic form (up to 7%). Mainly the ND4-ND4L region was affected, as evidenced by the PCR; however, other regions of the mitochondrial genome also showed deletions of varying size and extent, suggesting multiple deletions of the mitochondrial DNA.

Lateralized fascia dentata lesion and blockade of one hippocampus: effect on spatial memory in rats

Unilateral blockade of the dorsal hippocampus by tetrodotoxin makes it possible to form lateralized spatial memories, which rapidly transfer to the naive hippocampus when training continues with intact brain. Unilateral X-ray irradiation of newborn rats causes irreversible destruction of granule cells in the ipsilateral fascia dentata (FD). Possible compensation of poor learning in the lesioned hemisphere by commissural transfer of memories from the intact hippocampus was examined in seven rats with unilateral FD lesion, which were first trained in the Morris water maze to asymptotic performance (mean escape latency 6+/-1 s). Subsequent testing during functional ablation either of the intact or of the lesioned hippocampus by tetrodotoxin revealed escape latencies 35+/-8 s or 8+/-1 s, respectively. Probe trial tests during inactivation of the intact and lesioned hippocampus showed target quadrant preference of 32+/-2% or 54+/-3%, respectively. The results indicate: (a) that one intact hippocampus alone can support the water maze task, (b) that no, or only a very weak, memory trace is available in the lesioned hippocampus. It is concluded that the above results are due to the inability of the FD lesioned hippocampus to process the information received from the ipsilateral entorhinal cortex.

Residual granule cells can maintain susceptibility of CA3 pyramidal cells to kainate-induced epileptiform discharges

Slices of adult rat hippocampus made from animals exposed neonatally to X-ray irradiation were studied with electrophysiological techniques. A single dose of 6 Gy irradiation of the pup's head significantly but unevenly reduced the number of granule cells in the dentate gyrus. A larger reduction was detected in the septal than in the temporal hippocampus. The number of hilar cells decreased also. Effects of irradiation were confirmed with histological techniques. Field potential responses to mossy fiber stimulation in the pyramidal layer of the CA3 subfield was smaller in irradiated than in normal rats. Superfusion of the slices with kainic acid (KA, 300-500 nM) induced spontaneously recurrent paroxysmal activity (SRPA) in about 40% of irradiated slices in contrast with nearly 90% of slices cut from nonirradiated rats. Intracellular recordings from CA3 pyramidal cells in irradiated rats revealed recurrent bursts of action potentials on top of large depolarizing waves after KA application. Cells impaled in slices from the septal half of hippocampus of irradiated rats failed more often to respond with bursts to KA than cells in slices cut from the temporal half. Removal of mossy fiber input can therefore reduce KA induced hyperexcitability of CA3 pyramidal cells, but quantitative factors such as proportional loss of granule and hilar cells may explain the considerable differences found among cells and slices. Removal of 80% of granule cells reduces hyperexcitability consistently, while SRPA can be found in slices where as much as 50% of granule cells are missing. Intracellular findings suggest that failures of detection of SRPA following KA application to hippocampal slices of irradiated rats does not necessarily mean that CA3 pyramidal cells are no longer responding to KA with epileptiform bursting.

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.

Neuronal connections, cell formation and cell migration in the perinatal human hippocampal dentate gyrus

Jean Piaget's "stage theory" suggests that cognitive development proceeds in discrete steps, among which the first is the sensorimotor period that occupies the first two years. In recent years it became clear that an intact and mature hippocampus is necessary for memory formation both in experimental animals and in human. In the present experiments the perinatal morphological development of the human hippocampus was studied to describe structural changes that may correlate with the developmental changes of intellectual growth. Our results suggest that cell formation in the human hippocampus terminates several weeks before birth, but immature cells migrate to their final positions through the first six postnatal months. The newborn hippocampus contains all cell types and cell layers that are characteristic for the adult hippocampus. However, changes of the light microscopic features of the postsynaptic target neurons of hippocampal granule cells indicate that connections between granule cells and their target neurons are immature at birth and develop through an extended period of time that may last for three years. Since this neuronal connection is the first link in the chain of the main hippocampal synaptic circuitry, it may be suggested that human hippocampus is functionally impaired at birth. This period of light microscopic morphological maturation correlates well with the time period of Piaget's first stage of cognitive development. It can also be suggested that the prolonged postnatal development of some neuronal circuitries in the human hippocampus may be responsible for the psychological phenomenon of "infantile amnesia", that is the lack of memory traces from the early postnatal period.

Severe spatial navigation deficit in the Morris water maze after single high dose of neonatal x-ray irradiation in the rat

Ambiguous spatial behavior deficits induced in adult rats by different types of dentate gyrus lesions were examined by subjecting neonatal rats to x-ray irradiation, which reduces the granule cell population in fascia dentata without affecting the number of hilar neurons and pyramidal cells of Ammon's horn. Three- to six-month-old irradiated and intact male Long-Evans rats were tested in the Morris water maze. Four experiments were done. (i) Rats were trained to find an invisible escape platform, when started from any of four equidistant points at the circumference of the pool. (ii) The same rats then were trained to find a visible platform in the same pool. Poor performance of irradiated rats in both experiments suggested a visual deficit. (iii) Navigation in the absence of visual cues was studied in other rats trained in total darkness to find the escape platform under conditions of fixed start-fixed goal geometry. (iv) Contribution of nonvisual allocentric cues and egocentric path integration mechanisms to spatial performance of the above rats was tested in darkness after rotating both the start and goal positions by 90 degrees clockwise. Impairment of irradiated rats in Exp. 3 and 4 and histological examination of their brains support the conclusion that 60-70% reduction of granule cells in the dorsal hippocampus causes significant deterioration in both allocentric and egocentric orientation.

Distribution of substance P-immunoreactive neurons and fibers in the monkey hippocampal formation

Substance P containing neurons was visualized by immunocytochemistry in the monkey hippocampus, subicular complex, and entorhinal cortex. Immunoreactive neurons were found solely in the hilar region of the dentate gyrus, and in strata oriens and pyramidale of Ammon's horn. In the subicular complex, immunoreactive neurons were located in those layers which were close to the alveus, whereas in the entorhinal cortex most of the substance P-positive neurons appeared in the second and third layers above the lamina dissecans. The majority of substance P-containing neurons were large multipolar cells, but small bipolar and multipolar cells also occurred in Ammon's horn, subiculum and entorhinal cortex. Dendrites of immunoreactive cells were smooth and displayed a few small, faintly stained spines which were hard to identify in the light microscopic preparations, but were visible with electron microscopy. Substance P-positive dendrites were exclusively found in the hilar region and never observed in the upper two-thirds of the molecular layer of the dentate gyrus. Moreover, immunoreactive dendrites rarely penetrated the stratum lacunosum-moleculare of Ammon's horn. In the electron microscopic preparations, somal and dendritic features of substance P-positive neurons were similar to those observed for GABAergic local circuit neurons. Axons of the substance P-immunoreactive local circuit neurons were thin and richly arborized in the upper two-thirds of the molecular layer of the dentate gyrus, in the stratum lacunosum-moleculare of Ammon's horn as well as in the subpial layers of the subicular complex and entorhinal cortex. Their terminals formed exclusively symmetric synapses with dendrites and spines. However, substance P-immunoreactive boutons were not found to make symmetric, axosomatic synapses on the granule cells of the dentate gyrus and very few were present on the pyramidal neurons of Ammon's horn, subicular complex, and entorhinal cortex. Hippocampal neurons, which were immunoreactive for substance P, also contained the neuropeptide somatostatin. However, not all of the somatostatin-containing neurons were substance P-immunoreactive. Thus, substance P-positive neurons are a subpopulation of somatostatin immunoreactive, GABAergic neurons. In conclusion, substance P-immunoreactive neurons are ideally suited for feed-back dendritic inhibition which may control the effectiveness of the main excitatory cortical input to the granule cells of the dentate gyrus and pyramidal neurons of the Ammon's horn.

[Mitochondrial DNA deletion in hereditary cardio-encephalo-myopathy]

The case of a female patient with cardio-encephalo-myopathy who died of her illness at one year of age, similarly to her three sisters, is reported. In autopsy samples, like muscle, heart, liver and cerebellum activities of several mitochondrial enzymes were determined. In the skeletal muscle serious decrease of carnitine acetyltransferase was observed (from the normal 4.8 U/g to 0.08 U/g wet weight), while in other tissues this activity was normal. In the muscle activities of several other mitochondrial enzymes were also decreased (cytochrome oxidase, NADH cytochrome C oxidoreductase, citrate synthase), while in other tissues there were no similar changes. Serious distortion was observed in the structure of the majority of mitochondria of muscle and heart by electronmicroscopy. The number of the Purkinje-cells in the cerebellum decreased, and the cells were shrunken, their axons were fragmented and disoriented. Also the structure of the mitochondria was abnormal in the Purkinje-cells, while it was normal in other areas of the cerebrum. In te tissues of the patient normal and deleted mitochondrial DNA coexisted as which could explain the genetic background of this disease at molecular level.

Postnatal development and synaptic connections of hilar mossy cells in the hippocampal dentate gyrus of rhesus monkeys

Mossy cells of the hippocampal dentate gyrus were analyzed through postnatal development. At birth, a few thorny excrescences were found on the proximal dendrites of mossy cells, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number and complexity until the third month. After that age, the complexity of thorny excrescences is so great that an increase in spine density can be seen only in electron microscopic preparations. An increase in the number of pedunculate spines per unit length of distal dendrite was detected via light microscopy during the first 9 postnatal months. The somata and dendrites of mossy cells displayed adult-like characteristics after the ninth postnatal month. Mossy fiber terminals at birth frequently displayed immature ultrastructural characteristics and formed synapses with dendritic shafts and spines. At later postnatal ages and in adults, axospinous synapses were found almost exclusively. This is consistent with the postnatal development of the complex spines of the mossy cells. Axons of mossy cells were generally confined to the hilus in our 150-microns-thick sections, where they gave rise to several collaterals. The axon terminals from these collaterals formed asymmetric synapses with dendrites and dendritic spines in the hilar region of the dentate gyrus. These data provide the first anatomical evidence that hilar mossy cells of the primate dentate gyrus have excitatory projections similar to their equivalent cell type in subprimates. The present study indicates that mossy cells of the dentate gyrus are in a more advanced stage of development at birth and mature faster than similar neurons of the human hippocampus. This may represent a faster maturation of hippocampal circuitry in nonhuman primates compared to that in the human.

Postnatal development of CA3 pyramidal neurons and their afferents in the Ammon's horn of rhesus monkeys

Previous studies described the postnatal development of CA3 pyramidal neurons and their afferents in the rat. However, the postnatal development of the primate hippocampus was not previously studied. Thus, pyramidal neurons of the CA3 area of the monkey hippocampus were analyzed postnatally in the present study. At birth, a few thorny excrescences, the complex spines postsynaptic to mossy fibers, were found on the proximal segments of both apical and basal dendrites, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number until the third month. A continuous increase in the number of spines per unit length along the distal dendrites was observed during the first 12 months. The ultrastructural features of somata and dendrites of pyramidal cells in newborn monkeys were similar to those of adults. The analysis of the afferents to the CA3 pyramidal neurons was limited to the development of mossy fibers, the axons of granule cells, and myelinated axons in the alveus, stratum oriens, and stratum lacunosum-moleculare. At birth, most mossy fiber terminals were densely packed with synaptic vesicles and formed mainly axospinous synapses with CA3 pyramidal cells. By 1 month of age, the number of mitochondria and embedded spines increased to mature amounts. In the first postnatal month, degenerating axons and axon terminals were frequently observed in the mossy fiber bundles in stratum lucidum. The proportion of myelinated axons increased simultaneously in all three examined layers. At birth most axons were unmyelinated, whereas at 7 months of age the proportion of myelinated axons was similar to that found in adults. The present study indicates that most pyramidal neurons of the CA3 region in monkeys are in an advanced stage of development at the time of birth. Thus, mossy fibers from granule cells in the dentate gyrus have established mature-looking synapses, and the thorny excrescences of pyramidal cells that are postsynaptic to mossy fibers are also adult-like. Nevertheless, several of the adult features, such as the spine density of distal dendrites of pyramidal neurons and the myelination of afferent axons, develop during an extended period of time in the first year. The significance of this early anatomical maturation in a brain region involved in memory function is consistent with recent behavioral data that show a rapid postnatal maturation of limbic-dependent recognition memory in rhesus monkeys.

Distribution, morphological features, and synaptic connections of parvalbumin- and calbindin D28k-immunoreactive neurons in the human hippocampal formation

Calcium binding proteins calbindin D28k (CaBP) and parvalbumin (PV) are known to form distinct subpopulations of gamma-aminobutyric acid (GABA)ergic neurons in the rodent hippocampal formation. Light and electron microscopic morphology and connections of these protein-containing neurons are only partly known in the primate hippocampus. In this study, CaBP and PV were localized in neurons of the human hippocampal formation including the subicular complex (prosubiculum, subiculum, and presubiculum) in order to explore to what extent these subpopulations of hippocampal neurons differ in phylogenetically distant species. CaBP immunoreactivity was present in virtually all granule cells of the dentate gyrus and population of in a proportion of pyramidal neurons in the CA1 and CA2 regions. A distinct population of CaBP-positive local circuit neurons was found in all layers of the dentate gyrus and Ammon's horn. Most frequently they were located in the molecular layer of the dentate gyrus and the pyramidal layer of Ammon's horn. In the subicular complex pyramidal neurons were not immunoreactive for CaBP. In the prosubiculum and subiculum immunoreactive nonpyramidal neurons were equally distributed in all layers, whereas in the presubiculum they occurred mainly in the superficial layers. Electron microscopy showed typical somatic and dendritic features of the granule, pyramidal, and local circuit neurons. CaBP-positive mossy fiber terminals in the hilus of the dentate gyrus and terminals of presumed pyramidal neurons of Ammon's horn formed asymmetric synapses with dendrites and spines. CaBP-positive terminals of nonprincipal neurons formed symmetric synapses with dendrites and dendritic spines, but never with somata or axon initial segments. PV was exclusively present in local circuit neurons in both the hippocampal formation and subicular complex. Most of the PV-positive cell bodies were located among or close to the principal cell layers. However, large numbers of immunoreactive neurons were also found in the molecular layer of the dentate gyrus and in strata oriens of Ammon's horn. PV-positive cells were equally distributed in all layers of the subicular complex. Electron microscopy showed the characteristic somatic and dendritic features of local circuit neurons. PV-positive axon terminals formed exclusively symmetric synapses with somata, axon initial segments and dendritic shafts, and in a few cases with dendritic spines. The CaBP- and PV-containing neurons formed similar subpopulations in rodents, monkeys, and humans, although the human hippocampus displayed the largest variability of these immunoreactive neurons in their morphology and location. Calcium binding protein-containing neurons frequently occurred in the molecular layer of the human dentate gyrus and in the stratum lacunosum-moleculare of Ammon's horn.(ABSTRACT TRUNCATED AT 400 WORDS)

Calretinin immunoreactivity in the monkey hippocampal formation--I. Light and electron microscopic characteristics and co-localization with other calcium-binding proteins

Calretinin-containing neurons were visualized by immunocytochemistry in the monkey hippocampal formation, subicular complex, and entorhinal cortex. Calretinin-immunoreactivity was present exclusively in non-granule cells of the dentate gyrus and in non-pyramidal cells of Ammon's horn, subiculum and entorhinal cortex. Most frequently, calretinin-positive neurons were found at the hilar border of the dentate granule cell layer and in the stratum radiatum of CA1-3 areas. In the subicular complex, immunoreactive neurons were evenly distributed in all layers, whereas in the entorhinal cortex, they were accumulated in external layers above the lamina dissecans. Distinct bands of calretinin-positive fibers occupied the supragranular zone of the molecular layer in dentate gyrus, the pyramidal cell layer of the CA2 area in Ammon's horn and the upper two layers of presubiculum. The majority of calretinin-immunoreactive neurons were small, bipolar or fusiform neurons with a dendritic tree oriented parallel to the dendrites of principal cells (granule cells in dentate gyrus and pyramidal neurons elsewhere). Dendrites were smooth or sparsely spiny, displaying small spines of conventional type. Co-existence studies showed that these neurons were completely devoid of other calcium-binding proteins, parvalbumin and calbindin. Electron microscopic analysis revealed somata of immunoreactive neurons which contained a large nucleus and a small cytoplasmic rim, which contained only few organelles. The nucleus displayed deep infoldings and intranuclear rods. Input synapses of immunoreactive neurons were rare both on somata and dendrites and large surface areas were frequently apposed by glial processes. This was very prominent in the dentate gyrus and Ammon's horn. Axons of calretinin-positive neurons were thin, arborized in all layers and had small varicosities. Their terminals formed symmetric synaptic contacts mainly with dendrites and less frequently with somata of principal cells. Axon terminals of calretinin-immunoreactive fiber bundles in the supragranular layer, as well as in the pyramidal layer of the CA2 area, formed asymmetric synaptic contacts with dendritic shafts. In addition, they established asymmetric axospinous and axosomatic synaptic contacts with granule cells of the dentate gyrus. In the presubiculum, the calretinin-positive axon bundle included a large number of immunoreactive myelinated axons, as well as axon terminals. The characteristic location and features of synapses suggests that these fibers derive from extra-hippocampal afferents (Nitsch, R. and Leranth C. (1993) Neuroscience 55, 797-812) and not from the calretinin-immunoreactive neurons of the hippocampal formation.

Electron microscopic immunocytochemical study of the distribution of parvalbumin-containing neurons and axon terminals in the primate dentate gyrus and Ammon's horn

Five green monkeys were examined with light and electron microscopic preparations to explore the regional differences in the distribution of parvalbumin (PV)-positive neurons and axon terminals in the primate hippocampus. PV-positive neurons were mainly found in the hilus of the dentate gyrus and the strata oriens and pyramidale of Ammon's horn. In electron microscopic preparations, the PV-positive cells displayed nuclear infoldings, intranuclear rods, a large rim of perikaryal cytoplasm with numerous organelles and both asymmetric and symmetric axosomatic synapses. One prominent PV-positive cell type in CA1 was a large multipolar neuron that resembled the large basket cells of the neocortex. Although most PV-positive dendrites were aspiny and postsynaptic to numerous axon terminals, some PV-positive dendrites in the molecular layer of the dentate gyrus displayed filipodia-like appendages with no synapses or spines that were postsynaptic to multiple axon terminals. The PV-positive dendrites in the hilus and stratum oriens were apposed at specialized junctions that resembled gap junctions. PV-positive axons were concentrated in the principal cell layers, and formed axosomatic, axodendritic, and axon initial segment synapses. In cases where these axons were observed to appose the surface of granule cells for a long length, only one axosomatic symmetric synapse per cell was found. In the hilus, PV-positive axon terminals formed synapses onto thorny excrescences of spiny cells. Both semithin sections and electron microscopic preparations indicated that more PV-positive axon terminals formed symmetric axosomatic synapses with pyramidal cells in CA2 than in CA1 and CA3. Also, CA2 displayed a unique plexus of PV-positive axon terminals in stratum lacunosum moleculare. These results indicate that the PV-positive hippocampal cells form a subset of GABAergic local circuit neurons, including the basket and chandelier cells. The ubiquitous finding of PV-positive dendrites linked by gap junctions throughout the dentate gyrus and Ammon's horn adds further data to indicate that this subset of GABAergic neurons is linked electrotonically. The synaptic organization of PV-positive neurons in the hippocampus suggests their participation in both feedback and feedforward inhibition. The PV-positive neurons in the hippocampus are only a proportion of the basket and chandelier cells, whereas virtually all of these cells in neocortex are PV-positive.

Hippocampal morphology and spatially related behavior in Long-Evans and CFY rats

Behavioral responses to novelty in an open field and spatial learning in a radial maze with four arms out of eight reinforced were tested in male and female CFY and Long-Evans rats. Subsequently, the sizes of the total hippocampi and of various hippocampal cell layers and terminal fields at the midseptotemporal level were measured in Timm-stained sections. No strain differences were found in the open field (except for defecation). In the radial maze, Long-Evans rats showed better spatial reference memory capabilities than rats of the CFY strain. The relative sizes of the intra- and infrapyramidal mossy fiber (IIP-MF) projections did not differ between the strains. Within the more variable CFY strain, a positive correlation between the size of the IIP-MF projection and radial maze performance was found. The absolute sizes of the entire hippocampi and all hippocampal layers at the midseptotemporal level were larger in the CFY strain. The size of the suprapyramidal mossy fiber projection was related to the number of granule cells and to the ratio between granule and CA3 pyramidal cells. In contrast, the size of the IIP-MF projection did not correlate with either of these variables. The results indicate that the size of the mossy fiber projection may be determined mainly by the available postsynaptic surface on the dendrites of CA3 pyramidal neurons. Furthermore, an increased number of granule cells and their larger projection to the apical dendrites of pyramidal neurons does not appear to result in physiological changes with behavioral consequences.(ABSTRACT TRUNCATED AT 250 WORDS)

Pyramidal neurons are immunoreactive for calbindin D28k in the CA1 subfield of the human hippocampus

The calcium binding protein calbindin D28k (CaBP) is localized in the granule cells of the dentate gyrus, in pyramidal neurons of the CA1 and CA2 subfields of the Ammon's horn as well as in distinct groups of local circuit neurons in both parts of the human hippocampal formation. Immunostaining was performed on human brain perfused 2 h after death. The localization of CaBP in the human was found to be similar to that in the monkey hippocampus suggesting that there is no species difference in the cellular localization of CaBP among different primates.

Postnatal development of mossy cells in the human dentate gyrus: a light microscopic Golgi study

Mossy cells in the human dentate gyrus of adults and children of different ages were impregnated using the rapid-Golgi method. In every case the cause of death was verified by autopsy and the brains were used when neither the history of the patient nor autopsy revealed brain-related disease. Mossy cells in the human share common light microscopic features with the same cell type in rats and monkeys. Their most characteristic feature is the extremely large and complex excrescences on their proximal dendrites. Distal dendrites display pedunculate spines. Mossy cells have a few somal spines. The axon of mossy cells originates from the cell body and gives rise to several collaterals in the hilar region. The axons could be followed for several hundred microns, but in only one case did an axon collateral enter the granule cell layer of the adult dentate gyrus. In the newborn child, mossy cells display immature somal and dendritic features. The soma frequently bear spines. The dendrites are varicose and terminate in presumed growth cones. Both proximal and distal portions of the dendrites bear a few pedunculate spines and long-irregular filopodia. A few small excrescences are present on the proximal dendrites. The first large, complex excrescences on the proximal dendrites of mossy cells appeared in the 7-month-old child. Both somata and dendrites display adult-like characteristics in mossy cells from a 5-year-old child. However, not all mossy cells are alike and some dendrites still display long filopodia. The axons of immature mossy cells were similar to adults. The present results indicate that connections between granule cells and hilar mossy cells of the human dentate gyrus develop through an extended postnatal period of time that may last until the fifth year.

Ultrastructural features of primate granule cell bodies show important differences from those of rats: axosomatic synapses, somatic spines and infolded nuclei

Granule cells of the primate dentate gyrus were examined in the electron microscope where they displayed significantly less axosomatic synapses than granule cells in rodents. In addition, primate granule cells frequently had infolded cell nuclei and somal spines which are features that are both rare in rodents. Since the granule cell body is an important site for gamma-aminobutyric acid (GABA)ergic inhibitory control, the reduced number of axosomatic synapses in monkeys suggests that local inhibitory connections of primate granule cells are less than that of rodents. Together, these differences may indicate that the primate granule cells are physiologically more active than rat granule cells.

Morphological variability and developmental aspects of monkey and human granule cells: differences between the rodent and primate dentate gyrus

The postnatal generation, dendritic development and morphological variability of granule cells were studied in the monkey and human dentate gyrus. Granule cells are mainly formed prenatally in primates with an approximate 4- to 6-month postnatal generation time in humans. Dendritic development of individual granule cells appears to be prolonged over a long period of time. Immature granule cells were observed as late as in 15-month-old children. The morphological variability of granule cells is similar in monkeys and humans. Both display granule cells with basal dendrites as well as granule cells with different dendritic lengths and spine densities. The prolonged development of the spine structure of the human mossy cells suggests that synaptic connections between granule cells and their postsynaptic target neurons develop through a long postnatal period of time that may last as long as 5 years postnatally. The morphological variability of granule cells in primates should be considered when drawing conclusions about hippocampal neuropathology. The prolonged development of the neurons and neuronal circuitries in the human dentate gyrus may cause the lack of adult-like memory formation in early childhood resulting in the phenomenon of 'infantile amnesia'.

Parvalbumin- and calbindin D28k-immunoreactive neurons in the hippocampal formation of the macaque monkey

Calcium-binding proteins calbindin D28k (CaBP) and parvalbumin (PV) were localized in neurons of the monkey hippocampal formation. CaBP immunoreactivity is present in all granule cells and in a large proportion of CA1 and CA2 pyramidal neurons, as well as in a distinct population of local circuit neurons. In the dentate gyrus, CaBP-immunoreactive nongranule cells are present in the molecular layer and in the hilar region, but they do not include the pyramidal basket cells at the hilar border. In the Ammon's horn, CaBP-positive, nonpyramidal neurons are more frequent in the CA3 area than in any other parts of the hippocampal formation. They are concentrated in the strata oriens and pyramidale of areas CA1-3, whereas only a few small neurons were found in the strata lucidum and radiatum of CA3 and in the stratum moleculare of the CA1 area. PV is exclusively present in local circuit neurons both in the dentate gyrus and in Ammon's horn. In the dentate gyrus the presumed basket cells at the hilar border exhibit PV immunoreactivity. In the hilar region and molecular layer only a relatively small number of cells are immunoreactive for PV. Most of these PV-positive cell bodies are located in the inner half of the molecular layer, with occasional horizontal cells at the hippocampal fissure. In Ammon's horn, strata oriens and pyramidale of areas CA1-3 contain a large number of PV-positive cells. There are no PV-immunoreactive cells in the strata lucidum, radiatum, or lacunosum moleculare. The CaBP- and PV-containing neurons form different subpopulations of cells in the monkey hippocampal formation. With the exception of a basket cell type in the monkey dentate gyrus, the CaBP- and PV-positive cell types were found to be remarkably similar in rodents and primates.

Basket cells in the monkey fascia dentata: a Golgi/electron microscopic study

This study describes non-granule cells in the fascia dentata of rhesus monkeys and baboons. Their cell bodies are located in the molecular layer and at the hilar border of the granular layer. They are called basket cells since their axons give rise to collaterals that branch in the close vicinity of the parent cell body and form symmetric synapses with dendrites and cell bodies of granule cells. These neurons are further classified with regard to the shape and location of their cell bodies and the orientation of their dendrites. Basket cells in the molecular layer are mainly bipolar with dendrites oriented perpendicular to the granular layer. These dendrites are densely innervated by presynaptic boutons forming asymmetric synapses. We have rarely observed molecular layer basket cells with dendrites traversing the granular layer and invading the hilus. We thus conclude that these cells are mainly activated by extrinsic afferents terminating in the molecular layer. Basket cells at the hilar border display pyramidal, fusiform or multipolar cell bodies that give rise to apical dendrites traversing the molecular layer and basal dendrites invading the hilar region. Large boutons establish asymmetric synapses with identified basal dendrites of these neurons. The dendrites of all types of basket cell are smooth, i.e. they had few or no spines. Many of them display varicosities. Cell counts in Cresyl Violet-stained sections revealed a ratio of basket cells to granule cells of 1:500. Essentially, the types of basket cell in the monkey fascia dentata are similar to those described previously for the rat. This contrasts sharply to our recent findings for pyramidal neurons and granule cells of the monkey hippocampus which showed an increased complexity and variability when compared with rodents. These data do not support the hypothesis that only local circuit neurons evolve in phylogeny.

The mossy cells of the fascia dentata: a comparative study of their fine structure and synaptic connections in rodents and primates

In this study the fine structure and synaptic connections of mossy cells in the rat and monkey fascia dentata were analyzed. In order to study commissural connections of identified mossy cells in the rat, hilar neurons were retrogradely labeled by horseradish peroxidase (HRP) or Fast Blue (FB) injections into the contralateral hippocampus. Vibratome sections containing retrogradely HRP-labeled hilar neurons were Golgi-impregnated and gold-toned. Hilar commissural neurons identified by contralateral FB injection were intracellularly labeled with Lucifer Yellow (LY). Lucifer Yellow staining was made electron-dense by photoconversion thereby allowing for an electron microscopic analysis of the retrogradely labeled and intracellularly stained neurons. With these two different approaches, we succeeded in identifying rat mossy cells projecting to the contralateral hippocampus. Mossy cells in the fascia dentata of primates (Papio anubis, Macaca mulatta, Saimiri sciureus) were, like mossy cells of rats, either Golgi-impregnated and gold-toned or intracellularly injected with LY. No major differences were found between mossy cells of rats and monkeys. The mossy cell dendrites originated from the two sides of an ovoid cell body and were mainly oriented parallel to the granule cell layer. In contrast to the rat, dendrites of mossy cells in the primate did not respect the granule cell layer and penetrated frequently into the molecular layer. The occurrence of excrescences on proximal dendrites was a characteristic feature of all mossy cells. These large spines were more complex in the primate than in the rat. In both rats and primates they formed numerous asymmetric synapses with large boutons of mossy fibers. Peripheral dendrites were covered with small, simple spines. Interestingly, these peripheral dendrites lacking excrescences also established asymmetric synapses with mossy fiber boutons as well as asymmetric and symmetric contacts with smaller terminals of unknown origin. These findings indicate that in both rats and primates the thorny excrescences are not the only target of the mossy terminals. While the proximal portions of the mossy cell dendrites appear to be exclusively contacted by the granule cells, a larger number of neuron types may converge on the distal dendrites. The axons of mossy cells, in both rats and primates, although incompletely stained with the present methods, were seen to ramify in the hilar region. Our results demonstrate that, despite minor species differences, the mossy cells of the fascia dentata represent a cell type that is preserved in phylogenetically distant species.

Septal GABAergic neurons innervate inhibitory interneurons in the hippocampus of the macaque monkey

The septohippocampal projection was visualized in three Macaca mulatta monkeys by anterograde transport of Phaseolus vulgaris leucoagglutinin. Following injections of the lectin into the medial septal nucleus, P. vulgaris leucoagglutinin-labelled fibres were found in the hippocampal complex, mainly in stratum oriens of the CA1 subfield, throughout the CA3 subfield, and in the hilus and stratum moleculare of the dentate gyrus. The majority of labelled axons were varicose, and formed multiple contacts with cell bodies and dendrites of calbindin D28k- and parvalbumin-immunoreactive non-pyramidal cells. GABA immunoreactivity of P. vulgaris leucoagglutinin-labelled axons and their postsynaptic targets was investigated by sectioning varicose axon segments for correlated light and electron microscopy, and processing alternate ultrathin sections for postembedding immunogold staining for GABA. All P. vulgaris leucoagglutinin-labelled boutons examined were GABA-immunoreactive and the majority of them formed symmetrical synapses with GABA-immunoreactive cell bodies and dendrites. The results demonstrate that a GABAergic septohippocampal pathway exists in the monkey, and, similar to the rat, terminates on different types of GABAergic neurons, including the parvalbumin- and calbindin D28k-containing non-pyramidal cells.

Proportion of parvalbumin-positive basket cells in the GABAergic innervation of pyramidal and granule cells of the rat hippocampal formation

Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn contain immunoreactivity for the calcium-binding protein parvalbumin (PARV). Although the distribution of PARV-positive neurons has been previously described, detailed quantitative electron microscopic studies of the PARV-positive axon terminals in the hippocampal formation are lacking. In the present study, immunocytochemical methods were used to localize PARV-positive neurons and axon terminals to determine their similarity to GABAergic neurons. The PARV-positive cells and axon terminals are associated closely with the pyramidal and granule cell layers. In agreement with previous studies, the morphology of PARV-positive neurons is similar to that of GABAergic cells, including the basket cells of both the dentate gyrus and Ammon's horn. The PARV-positive axon terminals form exclusively symmetric synapses with somata, dendrites, dendritic spines, and axon initial segments. However, these terminals represent only a portion of the total number of terminals that form symmetric synapses. Quantitative results indicate that only 32-38% of the total number of terminals forming symmetric axosomatic synapses with principal cells of the dentate gyrus and Ammon's horn are PARV positive. Together with previous findings from light microscopic double-labeling studies, these data indicate that the PARV-positive terminals arise from a subpopulation of GABAergic hippocampal neurons. Finally, it is important to note that the terminal plexus of PARV-positive hippocampal axons overlaps at all postsynaptic sites with a plexus of PARV-negative axons.

The synaptic connections of basket cell axons in the developing rat hippocampal formation

Recent studies have indicated that hippocampal basket cells in both the dentate gyrus and Ammon's horn develop their somal and dendritic features during the first two postnatal weeks in rats. Their axon terminals form exclusively symmetric synapses that are found as early as 5 postnatal days in both regions. The present study used Golgi-electron microscopic material from 10 and 16 day old rats to demonstrate that the axon terminals of basket cells form synapses not only with somata, dendrites, and dendritic spines as reported for adult material but also with axon initial segments. However, the terminals forming synapses with axon initial segments and dendritic spines represent only a minor portion of the total number of basket cell terminals. Quantitative results indicate that 36-62% of the total number of these terminals form axosomatic synapses and 32-50% form axodendritic synapses depending on the analyzed cell. These data indicate that hippocampal basket cells have an axonal distribution similar to that found for cortical basket cells.

Postnatal development of the light and electron microscopic features of basket cells in the hippocampal dentate gyrus of the rat

Light and electron microscopic preparations were used to analyze the postnatal development of the basket cells of the rat dentate gyrus. The basket cells, located at the hilar border, were recognized in 2-day-old rats in Golgi preparations, where they displayed immature dendrites and a small axon arbor in the granule cell layer. At 5 days, the basket cells were found to have a large perikaryal cytoplasm, a round nucleus, an axon that forms symmetric synapses with granule cells, and dendrites and somata that are contacted by other axon terminals. The 10-day basket cells display more mature features, such as Nissl bodies and well-developed Golgi complexes. The basket cells from 16-day-old rats are mature in terms of their ultrastructural features, in that the nuclei are highly indented and display intranuclear rods or sheets, the perikaryal cytoplasm is packed with organelles, and the axon has developed an extensive arborization with the somata and dendrites of granule cells at the border with the molecular layer. This arborization will continue to expand as more granule cells are generated and added to the hilar border. These data correlate well with the immunocytochemical and biochemical development of GABAergic neurons in the dentate gyrus. Furthermore, the maturation of the structure of basket cells appears to precede the appearance of adult-like electrical activity in the hippocampus.

Local circuit neurons in both the dentate gyrus and Ammon's horn establish synaptic connections with principal neurons in five day old rats: a morphological basis for inhibition in early development

Glutamate decarboxylase (GAD)-positive and Golgi impregnated local circuit neurons of the hippocampal formation of five day old rats were examined in light and electron microscopic preparations. The ultrastructural features of these neurons were similar in both the dentate gyrus and CA1 area of Ammon's horn. Somata displayed a perikaryal cytoplasm rich in organelles but lacked organized Nissl bodies. Most nuclei showed intranuclear infoldings of varying degrees but no intranuclear sheets or rods were found. Somata and dendrites were contacted by relatively immature axon terminals that formed mainly symmetric synapses. The axons of local circuit neurons in both the dentate gyrus and Ammon's horn formed symmetric synapses with somata and dendrites of the principal neurons in these regions. Thus, both GAD-positive and Golgi-impregnated terminals of local circuit neurons were observed to form synapses with pyramidal and granule cells. These terminals were usually small and contained relatively few pleomorphic synaptic vesicles. The results show that a circuitry for inhibition is established in the 5 day old dentate gyrus and Ammon's horn, even though the local circuit neurons lack some of the typical adult ultrastructural features at this age.

Early onset of synapse formation in the human hippocampus: a correlation with Nissl-Golgi architectonics in 15- and 16.5-week-old fetuses

The developmental status of some potential components of hippocampal circuitry was studied at the time of the emergence of the hippocampal cytoarchitectonic subfields. The laminar distribution of synapses as seen with electron microscopy was correlated with Golgi architectonics in 15- and 16.5-week-old human fetuses. A systematic electron microscopic analysis of the distribution of synapses demonstrated that they were restricted to the two zones bordering the cortical plate, viz. the marginal and subplate zones, which contain dendritic branches of pyramidal and large polymorphous non-pyramidal neurons. The density of synapses (number per unit area) was higher in the marginal zone than in the subplate zone. Most synapses were of the asymmetric axodendritic type, although some were symmetric axodendritic synapses. The possible origins of the axons forming these synapses are discussed. This study demonstrates that the human hippocampus shows an early onset of synapse formation, with a characteristic distribution of synapses in restricted laminae. The finding of early synapse formation is consistent with observations made in other cortical areas during development. The prevalence of synaptogenesis at a superficial level of the cortex seems, however, to be specific to the "archicortex".

The development of GABAergic neurons in the rat hippocampal formation. An immunocytochemical study

Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn are generated prenatally. Although the adult distribution of GABAergic neurons has been previously described by numerous investigators, the early postnatal appearance of these neurons has not been described. In the present study, immunocytochemical methods were used to localize GABAergic neurons with antisera to both GABA and its synthesizing enzyme, glutamate decarboxylase (GAD). The GABA-positive neurons appeared at the earliest postnatal day (PND) examined, 4 PND. In contrast, GAD-positive cells were not observed until 6 PND, and the number of these neurons remained less than that of the GABA-positive neurons until 14 PND. These findings indicated that immunocytochemically detectable amounts of GAD were not present in many young GABAergic neurons. Both GABA- and GAD-positive hippocampal neurons showed two large increases in number during the 4-8 PND and 12-16 PND time periods, and they reached about 90% of adult levels before 18 PND. The regional distribution of GABA- and GAD-positive neurons throughout the hippocampal formation was homogeneous for all ages examined except 4 PND. At this age, the GABA-positive cells appeared in clusters in the proximal CA3 and the distal CA1 relative to the dentate gyrus. In addition, the number of hippocampal neurons immunostained in adult preparations for both antisera to GABA and GAD showed a similar number and distribution. The data on the developmental appearance of GABA and GAD immunoreactivities are consistent with biochemical data for the development of GABA concentration and GAD activity in the hippocampal formation. Together, these data provide important information about the functional maturation of the hippocampal GABAergic system in the first 3 weeks of rat brain development.

A Golgi-electron microscopic study of fusiform neurons in the hilar region of the dentate gyrus

The fusiform cells of the dentate gyrus are located in a portion of the hilus within 100 micron of the granule cell layer. They have ovoid somata and bipolar dendrites that generally run parallel to the granule cell layer. The dendrites of these cells are either spiny or sparsely spiny. The spiny fusiform cell has numerous spines along its dendrites, which are contacted by terminals with the features of granule cell axon collaterals. This cell type also displays somal spines that are contacted by similar terminals. In contrast, the sparsely spiny fusiform cell displays only a few spines, which are contacted by multiple small axon terminals that synapse with both the stalk and end bulb of the spine. Most synaptic input for this cell type is made with the smooth surfaces of the soma and dendrites. A variety of terminals form synapses with the sparsely spiny fusiform cell, including terminals that resemble the fine axon collaterals of mossy fibers. The somata of these two cell types also display differences in the amount of Nissl bodies and the degree of nuclear infolding. The results indicate that spiny fusiform cells are similar to mossy cells, another hilar cell type that receives its major synaptic input from axon collaterals of mossy fibers from granule cells. The distribution of the dendrites of spiny fusiform cells and the pattern of granule cell axon collaterals suggest a high degree of convergence from granule cells. In contrast, the variety of axodendritic synapses for sparsely spiny fusiform cells suggests that more diverse inputs affect this cell's activity. Therefore, the structure and circuitry of these two hilar cell types are probably different. This study adds further evidence to indicate that the hilus contains a large variety of cell types with different neuronal connections.

Electron microscopic localization of acetylcholinesterase in the dentate gyrus of young and adult rats

Acetylcholinesterase (AChE) histochemical staining occurred in neurons of the dentate gyrus at the day of birth and steadily increased in intensity and distribution during the first 3 postnatal weeks until the adult pattern was reached. Granule cells failed to display AChE staining; however, the somata of most non-principal cells in these regions showed AChE activity. It is interesting that most hilar neurons in the dentate gyrus were AChE-positive, but molecular layer local circuit neurons and pyramidal basket cells associated with the granule cell layer did not display AChE staining. AChE reaction product was localized to the nuclear envelope and cisternae of the granular endoplasmic reticulum in the labeled neuronal somata. In addition, the neuropil in the dentate gyrus displayed AChE staining associated with membranes. The possible cholinoceptive role of the AChE somata in the hilus is discussed.

Basal dendrites of granule cells are normal features of the fetal and adult dentate gyrus of both monkey and human hippocampal formations

The aim of this study was to analyze the granule cell population of the dentate gyrus both in healthy rhesus monkeys and in humans free of mental and neurological disorders. Brains of neonatal and adult rhesus monkeys as well as brains of fetal, neonatal and adult humans were impregnated with Golgi methods. The results show that a significant population of granule cells have basal dendrites in primates. Some basal dendrites curve up into the molecular layer where they have similar morphology as the apical dendrites. In contrast, other basal dendrites protrude into the hilus and they are shorter, thinner and have only a few side branches. The frequency of granule cells with basal dendrites in the human dentate gyrus is twice as much as that of the rhesus monkey. Most of these human granule cells have their basal dendrites in the hilus. This observation confirms the fact that discrepancies occur in the normal morphology of individual neurons between the rodent and primate hippocampal formations. The results indicate that basal dendrites of granule cells are not pathological as previously suggested.