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

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

Quantified distribution of the serotonin innervation in adult rat hippocampus

To quantify the serotonin innervation in adult rat hippocampus, serotonin axon terminals (varicosities) were uptake-labeled for light microscope radioautography in whole hemisphere slices incubated with 1 microM [3H]serotonin. The labeled varicosities were visualized as small aggregates of silver grains and counted with the aid of an image analysis system across all layers in representative sectors of subiculum, Ammon's horn (CA1, CA3-a, CA3-b) and dentate gyrus (medial blade, crest and lateral blade). Counts were obtained in six rats at three equidistant horizontal levels from the ventral two-thirds of the hippocampus. After double correction for duration of radioautographic exposure and section thickness, and measurement of the mean diameter of labeled varicosities in electron microscope radioautographs, the results were expressed in number of varicosities per mm3 of tissue. The overall density of hippocampal serotonin innervation was thus evaluated at 2.7 x 10(6) varicosities per mm3, and appeared significantly higher in subiculum (3.6 x 10(6)) and Ammon's horn (3.1 x 10(6)) than in dentate gyrus (2.2 x 10(6)). Subiculum and dentate gyrus-crest (2.0 x 10(6)) had the highest and lowest regional densities. There was a marked heterogeneity also in terms of laminar distribution. For example, the stratum moleculare of subiculum and CA1, and the stratum oriens of CA3 (5.2 x 10(6)) varicosities in CA3-a), showed much higher values than the pyramidal cell layer (0.7, 1.1 and 0.7 x 10(6) in CA1, CA3-a and CA3-b, respectively). Similarly, the granular layer of dentate gyrus had a much lower density (1.1 x 10(6)) than did the molecular (2.8 x 10(6)) and the polymorph layer (2.4 x 10(6)). From these data, it was possible to evaluate the mean endogenous amine content per hippocampal serotonin varicosity (0.05-0.07 fg), and the average number of serotonin varicosities per hippocampal neuron in both CA3 (130) and dentate gyrus (20-35). In the context of current data on the distribution of serotonin receptors and diverse actions of serotonin at the cellular level in hippocampus, such quantified information provides new insights on some basic properties of serotonin in this part of the brain.     

Characteristics of the functioning of the hippocampal formation in waking and paradoxical sleep

Analysis of published data identifying different neurotransmitter concentrations in the brain in paradoxical sleep and waking and data on the influences of neurotransmitters on the efficiency of the synaptic inputs to hippocampal neurons led to the conclusion that increases in the acetylcholine, cortisol, and dopamine concentrations during paradoxical sleep, with simultaneous reductions in serotonin and noradrenaline levels, may lead synergistically to a significant depression of transmission efficiency in polysynaptic pathways running through the hippocampus (i.e., the perforant path to neurons of the dentate gyrus, the pathway from the dentate gyrus to field CA3, from field CA3 to field CA1, and from field CA1 to the subiculum) but also to potentiation of the efficiency of the perforant path to pyramidal neurons of fields CA1 and CA3 and increases in the efficiency of associative connections between neurons in field CA3. This pattern of changes in the functioning of the hippocampal formation circuit may underlie differences in remembering and extracting information from memory in paradoxical sleep as compared with waking. Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 58, No. 3, pp. 261–275, May–June, 2008.     

Distribution of thyrotropin-releasing hormone (TRH) in the hippocampal formation as determined by radioimmunoassay

The distribution of thyrotropin-releasing hormone (TRH) in the hippocampal formation was determined using a radioimmunoassay (RIA) specific for TRH. RIA of hippocampal subregions revealed that the CA3 region of the hippocampal formation contained the highest amount of TRH, followed by intermediate levels in region CA1 and the dentate gyrus. The hilus and subiculum contained the lowest levels. The issue of whether hippocampal TRH is derived from extrinsic and/or intrinsic sources was evaluated by making lesions of the major subcortical afferent to the hippocampus, the fornix pathway. Analysis of the hippocampal formation by RIA revealed that the ventral hippocampus contains higher levels of TRH than the dorsal hippocampus (6.01 +/- 0.62 pg/mg tissue weight vs 1.11 +/- 0.19 pg/mg tissue weight). Lesions of the fornix produced significant decreases in ventral TRH to 52.9% of its control level and in dorsal TRH to 28.8% of its control level. The results from these studies suggest that (1) there is a differential distribution of TRH in the hippocampal formation, (2) the hippocampal formation might be composed of extrinsic and intrinsic sources of TRH, and (3) extrinsic sources of TRH might enter the hippocampus via the fornix pathway. In addition (4) the greater post-lesion decrement in ventral vs dorsal hippocampal TRH suggests that TRH fibers traversing the fornix innervate the ventral hippocampal formation in preference to its dorsal counterpart.     

Threefold Exposure to Moderate Hypobaric Hypoxia Decreases the Expression of Cu,Zn-Superoxide Dismutase in Some Regions of Rat Hippocampus

The effect of moderate hypobaric hypoxia on the expression of a peptide antioxidant Cu,Zn-superoxide dismutase in rat hippocampal neurons was evaluated in an immunocytochemical study. The expression of Cu,Zn-superoxide dismutase decreased significantly in the dorsal hippocampus (CA1 and CA2) and tended to decrease in ventral regions (CA3 and dentate gyrus) by the 24th hour after 3-fold exposure to hypoxia.     

The bite-raised condition enhances the aging process in the dorsal and ventral hippocampus

The bite raised condition decreases the number of neurons and increases the amount of glial fibrillary acidic protein in the hippocampus of aged SAMP8 mice. In the present study, we examined whether these effects differ between the dorsal and ventral hippocampus. In bite-raised SAMP8 mice, the number of neurons was significantly lower in the hippocampal CA1 and dentate gyrus (DG) subfields compared to control mice. In the bite raised condition, the number of neurons was significantly lower in both the dorsal and ventral CA3 subfields, and the number of glial fibrillary acidic protein-labeled astrocytes was increased in the CA1, CA3, and DG subfields, compared to control mice. These data suggest that in aged SAMP8 mice, the bite-raised condition enhanced aging processes in both the dorsal and ventral hippocampus.     

A 72 kDa heat shock protein is protective against the selective vulnerability of CA1 neurons and is essential for the tolerance exhibited by CA3 neurons in the hippocampus

The correlation between the expression of a 72 kDa heat shock protein and vulnerability of hippocampal CA1, CA3, and dentate gyrus regions to glutamate toxicity was investigated using a highly specific antisense oligonucleotide technique. Glutamate (1 mM, 15 min) caused region-dependent neuronal damage in cultured hippocampal slices 24 h after exposure and the most severe damage was observed in CA1. When slices were heat-shocked (43.5 degrees C, 30 min) before exposure to glutamate, neuronal damage in CA1 was attenuated. The strongest protection was observed when the interval between the heat shock and the exposure to glutamate was 3 days, which coincided with the maximal induction of a 72 kDa heat shock protein in neurons. When the expression of a 72 kDa heat shock protein was suppressed by the antisense oligonucleotide, the protective effect of the heat shock was completely inhibited. Glutamate itself also induced a 72 kDa heat shock protein in neurons, region-dependently, 24 h after the exposure. The signal of a 72 kDa heat shock protein in CA3 and dentate gyrus was significantly stronger than that in CA1. When the antisense oligonucleotide was applied, the damage in CA3 and dentate gyrus was exaggerated dose-dependently, and this effect was more remarkable in CA3 than in the dentate gyrus. Based on these data, we concluded that: (i) a 72 kDa heat shock protein has a protective effect against the selective vulnerability of CA1 neurons, (ii) a 72 kDa heat shock protein is an essential factor for the tolerance exhibited by CA3 neurons, and (iii) dentate gyrus tolerance is based on mechanisms other than those mediated through a 72 kDa heat shock protein.     

Distribution of neuropeptides in the limbic system of the rat: the hippocampus

The distribution of several neuropeptides (vasoactive intestinal polypeptide, cholecystokinin octapeptide, substance P, neurotensin, methionine-enkephalin and somatostatin) in the hippocampal formation has been studied with immunocytochemical techniques. Numerous vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin-positive cell bodies were found within the hippocampus and subiculum. Neurotensin-positive cell bodies were found within the subiculum, but no substance P or methionine-enkephalin-containing cell bodies were seen in either hippocampus proper or subiculum. Vasoactive intestinal polypeptide and cholecystokinin-octapeptide-positive cell bodies were predominantly located in the stratum moleculare and stratum radiatum of CA 1-2 regions and dentate gyrus, whilst somatostatin-containing cell bodies were found mainly in the stratum oriens. Nerve fibres containing each of the six peptides were found within the hippocampus. Large numbers of vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin fibres innervated the pyramidal and granule cell layers, with smaller numbers in the stratum radiatum and fewer still in the stratum moleculare and stratum oriens. Other than a moderately dense neurotensin-positive fibre plexus in the dorsal subiculum, fewer neurotensin, substance P and methionine-enkephalin fibres were present. However, when present, these three peptides had a distribution restricted to a region close to the pyramidal layer in the CA 2/3 region and to the stratum moleculare of the CA 1 region. Peptide-containing fibres were identified entering or leaving the hippocampus in three ways, via (i) the fornix (all six peptides), (ii) the dorsal subiculum (neurotensin-positive fibres projecting to the cingulate cortex: somatostatin, vasoactive intestinal polypeptide, and cholecystokinin-octapeptide present in fibres running between the dorsal subiculum and occipito-parietal cortex) and (iii) the ventral subiculum (vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin in fibres running between entorhinal cortex and hippocampus, and all six peptides in fibres crossing the amygdalo-hippocampal border). These findings indicate a major distinction between those peptides (vasoactive intestinal polypeptide, cholecystokinin-octapeptide, somatostatin, neurotensin) which are found in cell bodies intrinsic to the hippocampal formation and those peptides (substance P, methionine-enkephalin) which are found only in hippocampal afferents.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections.

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.     

Neurochemical characteristics of aluminum-induced neurofibrillary degeneration in rabbits

Aluminum-induced neurofibrillary degeneration in rabbits is known to affect particular populations of neurons. The neurotransmitter alterations which accompany aluminum neurofibrillary degeneration were examined in order to assess how closely they mimic those of Alzheimer's disease. There was a significant reduction in choline acetyltransferase activity in entorhinal cortex and hippocampus as well as significant reductions in cortical concentrations of serotonin and norepinephrine in the aluminum-treated rabbits. Significant reductions in glutamate, aspartate and taurine were found in frontoparietal and posterior parietal cortex. Concentrations of GABA were unchanged in cerebral cortex. Both substance P and cholecystokinin immunoreactivity were significantly reduced in entorhinal cortex but there were no significant changes in somatostatin, neuropeptide Y and vasoactive intestinal polypeptide. The five neuropeptides were unaffected in striatum, thalamus, cerebellum and brainstem. Neurochemical changes were found in the regions with the most neurofibrillary degeneration while regions with little or no neurofibrillary degeneration were unaffected. The reductions in choline acetyltransferase activity, serotinin and noradrenaline suggest that some neuronal populations preferentially affected in Alzheimer's disease are also affected by aluminum-induced neurofibrillary degeneration; however, the cortical somatostatin deficit which is a feature of Alzheimer's disease is not replicated in the aluminum model.     

Aging is accompanied by a subfield-specific reduction of serotonergic fibers in the tree shrew hippocampal formation

The hippocampal formation is a crucial structure for learning and memory, and serotonin together with other neurotransmitters is essential in these processes. Although the effects of aging on various neurotransmitter systems in the hippocampus have been extensively investigated, it is not entirely clear whether or how the hippocampal serotonergic innervation changes during aging. Rat studies, which have mostly focused on aging-related changes in the dentate gyrus, have implied a loss of hippocampal serotonergic fibers. We used the tree shrew (Tupaia belangeri), an intermediate between insectivores and primates, as a model of aging. We applied immunocytochemistry with an antibody against serotonin to assess serotonergic fiber densities in the various hippocampal subfields of adult (0.9–1.3 years) and old (5–7 years) tree shrews. Our results have revealed a reduction of serotonergic fiber densities in the stratum radiatum of CA1 and CA3, and in the stratum oriens of CA3. A partial depletion of serotonin in the hippocampal formation, as can be expected from our current observations, will probably have an impact on the functioning of hippocampal principal neurons. Our findings also indicate that the rat and the tree shrew hippocampal serotonergic innervation show some variations that seem to be differentially affected during aging.     

Aging is accompanied by a subfield-specific reduction of serotonergic fibers in the tree shrew hippocampal formation

The hippocampal formation is a crucial structure for learning and memory, and serotonin together with other neurotransmitters is essential in these processes. Although the effects of aging on various neurotransmitter systems in the hippocampus have been extensively investigated, it is not entirely clear whether or how the hippocampal serotonergic innervation changes during aging. Rat studies, which have mostly focused on aging-related changes in the dentate gyrus, have implied a loss of hippocampal serotonergic fibers. We used the tree shrew (Tupaia belangeri), an intermediate between insectivores and primates, as a model of aging. We applied immunocytochemistry with an antibody against serotonin to assess serotonergic fiber densities in the various hippocampal subfields of adult (0.9–1.3 years) and old (5–7 years) tree shrews. Our results have revealed a reduction of serotonergic fiber densities in the stratum radiatum of CA1 and CA3, and in the stratum oriens of CA3. A partial depletion of serotonin in the hippocampal formation, as can be expected from our current observations, will probably have an impact on the functioning of hippocampal principal neurons. Our findings also indicate that the rat and the tree shrew hippocampal serotonergic innervation show some variations that seem to be differentially affected during aging.     

Atrophy of pyramidal neurons and increased stress-induced glutamate levels in CA3 following chronic suppression of adult neurogenesis

Following their birth in the adult hippocampal dentate gyrus, newborn progenitor cells migrate into the granule cell layer where they differentiate, mature, and functionally integrate into existing circuitry. The hypothesis that adult hippocampal neurogenesis is physiologically important has gained traction, but the precise role of newborn neurons in hippocampal function remains unclear. We investigated whether loss of new neurons impacts dendrite morphology and glutamate levels in area CA3 of the hippocampus by utilizing a human GFAP promoter-driven thymidine kinase genetic mouse model to conditionally suppress adult neurogenesis. We found that chronic ablation of new neurons induces remodeling in CA3 pyramidal cells and increases stress-induced release of the neurotransmitter glutamate. The ability of persistent impairment of adult neurogenesis to influence hippocampal dendrite morphology and excitatory amino acid neurotransmission has important implications for elucidating newborn neuron function, and in particular, understanding the role of these cells in stress-related excitoxicity.     

Patterns of colocalization of neuronal nitric oxide synthase and somatostatin-like immunoreactivity in the mouse hippocampus: quantitative analysis with optical disector

In some brain regions, previous studies reported the frequent coexistence between neuronal nitric oxide synthase (nNOS) and somatostatin (SOM). In the hippocampus, nNOS and SOM were mainly expressed in GABAergic nonprincipal neurons. Here we estimated the immunocytochemical colocalization of nNOS and SOM in the mouse hippocampus using the optical disector. Both in the Ammon's horn and dentate gyrus, we encountered only a few nNOS-immunoreactive (IR)/SOM-like immunoreactive (LIR) neurons. They were mainly located in the stratum oriens of the Ammon's horn and in the dentate hilus. The nNOS-IR/SOM-LIR neurons usually showed characteristic large somata with thick dendrites, whereas the majority of nNOS-IR/SOM-negative neurons showed small somata with thin dendrites. Quantitative data revealed that the double-labeled cells represented only 4% and 7% of nNOS-IR neurons and SOM-LIR neurons, respectively, in the whole area of the hippocampus. We also found the laminar and dorsoventral differences in the degree of colocalization between nNOS and SOM. The percentages of nNOS-IR neurons containing SOM-like immunoreactivity were relatively high in the stratum oriens of the ventral CA1 region (24%), stratum lucidum of the dorsal CA3 region (29%) and dorsal dentate hilus (32%), but they were quite low in the other layers. On the other hand, the percentages of SOM-LIR neurons containing nNOS immunoreactivity were somewhat high in the stratum lucidum of the dorsal CA3 region (19%) and dorsal dentate hilus (28%), whereas they were very low in the other layers. Immunofluorescent triple labeling of axon terminals for nNOS, SOM and glutamic acid decarboxylase indicated that some nNOS-IR/SOM-LIR neurons might be dendritic inhibitory cells. The present results show the infrequent colocalization of nNOS and SOM in the mouse hippocampus, and also suggest that the double-labeled cells may be a particular subpopulation of hippocampal GABAergic nonprincipal neurons.     

Involvement of the trisynaptic hippocampal pathway in generating neural representations of object–place associations (an analytical review)

The possible mechanisms by which neural representations of object–place associations are generated in different parts of the network consisting of the hippocampus and the parahippocampal complex are analyzed. Spatial and non-spatial information arrives in the hippocampus via two streams from the parahippocampal complex, which consists of the perirhinal, postrhinal, and entorhinal areas of the cortex. It can be suggested that because there are no connections between the lateral and medial areas of the entorhinal cortex, these representations, as particular patterns of connected and discharging neurons, are generated mainly in the hippocampus, though they may also be generated in the entorhinal cortex because of the input from the postrhinal cortex. As both information streams converge on neurons in the dentate gyrus and field CA3, the trisynaptic pathway through the hippocampus may play a key role in generating these representations. As spatial information arrives in the neocortex and passes from there via the parahippocampal complex to the hippocampus about 20 msec earlier than non-spatial information, spatial information is processed first in the dentate gyrus and field CA3. Later, because of the return of excitation from field CA3c to the dentate gyrus, neural representations of object–place associations start to be generated in the dentate gyrus. Signals are transferred from the dentate gyrus to field CA3, where information arriving from the entorhinal cortex is superimposed on the neuronal patterns activated by these signals. As a result, more complex neural representations are generated in field CA3 and signals are sent to field CA1. In the dorsal (ventral) part of field CA1, non-spatial (spatial) information arriving from the lateral (medial) part of the entorhinal cortex is superimposed on the activated neuronal pattern. The result is that higher-order representations are generated in field CA1. In the parahippocampal cortex, the generation of neuronal representations of object–place associations can result from the transfer of activity from the dorsal part of hippocampal field CA1.     

Sex differences in the stereological parameters of the hippocampal dentate gyrus of the guinea-pig before puberty

Studies in rats and mice have shown several sex-dependent functional and structural differences in the hippocampal region, a brain structure playing a key role in learning and memory. The aim of the present study was to establish whether sex differences exist prior to puberty in the stereological parameters of the dentate gyrus in the guinea-pig, a long-gestation rodent, whose brain is at a more advanced stage of maturation at birth than the rat and mouse. The number of granule cells and volumes of the granule cell layer, molecular layer and hilus were evaluated in Nissl-stained brains of neonatal (15-16 days old) and peripubescent (45-46 days old) guinea-pigs. Based on a pilot study, the optical disector method was preferred to the optical fractionator method to estimate cell number. For volume (Vref) estimation with the Cavalieri principle, contour tracing was preferred to the point counting method, as the latter appeared to underestimate volumes. The results showed that neonatal males had more granule cells than females in both the dorsal and ventral dentate gyrus and a larger volume in all layers. Peripubescent males had a larger volume of the granule cell layer than females in both the dorsal and ventral dentate gyrus, more granule cells in the ventral dentate gyrus, a larger volume of the hilus in both the dorsal and ventral dentate gyrus and a larger volume of the molecular layer in the ventral dentate gyrus. The results show that sex differences are present in the guinea-pig dentate gyrus prior to puberty and go in the same direction at both investigated ages, with males exhibiting more granule cells and larger volumes than females. The widespread distribution of these sex differences suggests that in the guinea-pig, similarly to other rodents, hippocampus-dependent functions may be sexually dimorphic.     

NMDA receptor dependent paroxysmal discharges in the dentate gyrus

The N-methyl-D-aspartic acid subclass of glutamate receptors is thought to be involved in events that depend on repetitive activation of neurons. The present study shows that repetitive stimulation of CA3 can produce epileptiform discharges in the contralateral dentate gyrus. This distinctive paroxysmal response was blocked by the NMDA antagonists, ketamine and MK-801. The MK-801 blockade required previous activation of the dentate gyrus in the presence of the drug, thus demonstrating, in vivo, a use-dependent effect. These studies show that repetitive, low intensity stimulation of hippocampal circuits can produce NMDA-mediated epileptiform discharges in the dentate gyrus.     

Activity-dependent changes in synaptophysin immunoreactivity in hippocampus,piriform cortex,and entorhinal cortex of the rat

Synaptophysin, an integral membrane glycoprotein of synaptic vesicles, has been widely used to investigate synaptogenesis in both animal models and human patients. Kindling is an experimental model of complex partial seizures with secondary generalization, and a useful model for studying activation-induced neural growth in adult systems. Many studies using Timm staining have shown that kindling promotes sprouting in the mossy fiber pathway of the dentate gyrus. In the present study, we used synaptophysin immunohistochemistry to demonstrate activation-induced neural sprouting in non-mossy fiber cortical pathways in the adult rat. We found a significant kindling-induced increase in synaptophysin immunoreactivity in the stratum radiatum of CA1 and stratum lucidum/radiatum of CA3, the hilus, the inner molecular layer of the dentate gyrus, and layer II/III of the piriform cortex, but no significant change in layer II/III of the entorhinal cortex, 4 weeks after the last kindling stimulation. We also found that synaptophysin immunoreactivity was lowest in CA3 near the hilus and increased with increasing distance from the hilus, a reverse pattern to that seen with Timm stains in stratum oriens following kindling. Furthermore, synaptophysin immunoreactivity was lowest in dorsal and greatest in ventral sections of both CA3 and dentate gyrus in both kindled and non-kindled animals. This demonstrates that different populations of sprouting axons are labeled by these two techniques, and suggests that activation-induced sprouting extends well beyond the hippocampal mossy fiber system.     

Kainic acid-mediated increase of preprotachykinin-A messenger RNA expression in the rat hippocampus and a region-selective attenuation by dexamethasone.

The hippocampus contains the highest number of glucocorticoid-sensitive neurons in the rat brain and excessive exposure to glucocorticoids can cause damage to hippocampal neurons and impair the capacity of the hippocampus to survive neuronal insults. In this study in situ hybridization combined with quantitative image analysis was used to study preprotachykinin-A mRNA levels after administration of a toxic dose of kainic acid in animals pretreated with glucocorticoids. Kainic acid was injected into dorsal hippocampus CA3 region in animals pretreated with the synthetic glucocorticoid receptor agonist dexamethasone and in control animals. Preprotachykinin-A mRNA was not detected in the hippocampus of untreated animals or in animals analysed 30 min after a kainic acid injection. However, 4 h after injection of kainic acid, the level of preprotachykinin-A mRNA increased to 20-times above the detection limit both in the dentate gyrus and the CA3 region of the hippocampus. Treatment of kainic acid-injected animals with dexamethasone 30 min before and 2 h after the injection attenuated the increase in the granule cells of the dentate gyrus by 50%. In contrast, dexamethasone pretreatment had no significant effect on the kainic acid-induced increase of preprotachykinin-A mRNA in pyramidal cells in regions CA3 or CA1. These results show that an excitatory stimulus within the hippocampus causes a substantial increase in the level of preprotachykinin-A mRNA in hippocampal granule and pyramidal cells and suggest that in granule cells of the dentate gyrus this increase can be modulated by glucocorticoids.     

Dissociation between changes in glutamate receptor binding sites and their coupling to phosphatidylinositol metabolism following intrahippocampal colchicine injection

Intrahippocampal colchicine injection produces a rapid death of granule cells and pyramidal neurons in the hippocampus in the rat. Under the appropriate assay conditions, [3H]glutamate labels the N-methyl-D-aspartate type of glutamate receptors while [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate labels the quisqualate type. Unilateral injection of colchicine (15 micrograms) in the dorsal hippocampus did not produce any change in [3H]glutamate and [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding in membrane fractions from the dentate gyrus or CA1 field contralateral to the injection side, at least up to 12 days after the injection. However, it produced a progressive decrease in the binding of both ligands in dentate gyrus and CA1 of the injected hippocampus. In the dentate gyrus the changes in binding as a function of time after the injection were biphasic with a rapid exponential decrease (t1/2 about 8 days for both [3H]glutamate and [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) until 12 days after the injection followed by a much slower decrease afterwards. A similar pattern was observed in CA1 although the changes in binding were smaller and delayed by about three days as compared to the dentate gyrus. Kinetic analyses of the binding at equilibrium were performed seven days after the injection and indicated that the changes in [3H]glutamate binding were due to a change in the maximum number of sites but not in affinity for the ligand.(ABSTRACT TRUNCATED AT 250 WORDS)