Hippocampal Dentate Granule Cells Research Articles

Altered morphology of hippocampal dentate granule cell presynaptic and postsynaptic terminals following conditional deletion of TrkB

Dentate granule cells play a critical role in the function of the entorhinal-hippocampal circuitry in health and disease. Dentate granule cells are situated to regulate the flow of information into the hippocampus, a structure required for normal learning and memory. Correspondingly, impaired granule cell function leads to memory deficits, and, interestingly, altered granule cell connectivity may contribute to the hyperexcitability of limbic epilepsy. It is important, therefore, to understand the molecular determinants of synaptic connectivity of these neurons. Brain-derived neurotrophic factor and its receptor TrkB are expressed at high levels in the dentate gyrus (DG) of the hippocampus, and are implicated in regulating neuronal development, neuronal plasticity, learning, and the development of epilepsy. Whether and how TrkB regulates granule cell structure, however, is incompletely understood. To begin to elucidate the role of TrkB in regulating granule cell morphology, here we examine conditional TrkB knockout mice crossed to mice expressing green fluorescent protein in subsets of dentate granule cells. In stratum lucidum, where granule cell mossy fiber axons project, the density of giant mossy fiber boutons was unchanged, suggesting similar output to CA3 pyramidal cell targets. However, filopodial extensions of giant boutons, which contact inhibitory interneurons, were increased in number in TrkB knockout mice relative to wildtype controls, predicting enhanced feedforward inhibition of CA3 pyramidal cells. In knockout animals, dentate granule cells possessed fewer primary dendrites and enlarged dendritic spines, indicative of disrupted excitatory synaptic input to the granule cells. Together, these findings demonstrate that TrkB is required for development and/or maintenance of normal synaptic connectivity of the granule cells, thereby implying an important role for TrkB in the function of the granule cells and hippocampal circuitry.

TDP‐43 Neuronal Cytoplasmic Inclusions in the Amygdala of Patients with Advanced Alzheimer Disease

TAR DNA binding protein‐43 (TDP‐43)‐immunoreactive neuronal cytoplasmic inclusions (NCIs), characteristically associated with frontotemporal lobar degenerations (FTLD‐U and FTLD‐MND), have been found in the hippocampal dentate granule cells of about 20% of cases with advanced Alzheimer disease (AD). The extent and distribution of TDP‐43 pathology in brain regions other than hippocampus in AD is not known, especially in AD cases without hippocampal NCIs.Sections of hippocampus, amygdala, neocortex, and medulla from patients with clinical dementia and pathologic features of AD were analyzed for the presence of TDP‐43‐immunoreactive NCIs.22 cases of advanced AD (Braak Stage V‐VI) without hippocampal NCIs and 23 cases of advanced AD with hippocampal NCIs were included. 6 hippocampal‐negative cases (27%) had TDP‐43 positive NCIs in the amygdala. All 23 cases of hippocampal‐positive cases had numerous TDP‐43 NCIs in the amygdala, with decreasing density of NCIs in the other brain regions.In summary, TDP‐43 immunoreactive NCIs are found predominantly in the medial temporal structures in advanced AD, and the amygdala appears to be the area most vulnerable.

Alterations of GABAA-Receptor Function and Allosteric Modulation During Development of Status Epilepticus

Partial limbic seizures in rodents induced by pilocarpine progress from stages I-II (mouth and facial movements; head nodding) to stage III (forelimb clonus) and then progress rapidly to stages IV-V (generalized limbic seizures; rearing, and rearing with falling) followed by status epilepticus (SE). Although limbic seizures in rodents are terminated by benzodiazepines, a group of gamma-aminobutyric acid type A (GABA(A))-receptor positive modulators, significant pharmacoresistance to benzodiazepines develops within minutes during SE. The alterations of GABA(A)-receptor function and allosteric modulation during development of SE are poorly understood. We induced seizures in juvenile rats by administration of lithium followed by pilocarpine, and whole cell recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained from hippocampal dentate granule cells in brain slices. Compared with a sham-treated group, mIPSC amplitude was reduced and decay was accelerated at onset of the first occurrence of stage III (S3) seizures [S3(0)], resulting in a reduction in the total charge transfer at S3(0). Recovery of mIPSC amplitude and prolongation of mIPSC decay occurred 30 min after onset of S3 seizures [S3(30)]. The mIPSC frequency was not altered for S3(0) and S3(30) neurons compared with sham neurons. The net enhancement of total charge transfer by diazepam was smaller for S3(30) than that for sham and S3(0) neurons; however, the net reduction of total charge transfer by zinc was greater for S3(30) than that for sham and S3(0) neurons. These findings suggest that substantial plastic changes of GABA(A)-receptor function and allosteric modulation occur rapidly in neurons from juvenile animals during development of SE.

The type 1 interleukin 1 receptor is not required for the death of murine hippocampal dentate granule cells and microglia activation

Alterations in inflammatory process, neuronal death, and glia response have been observed under manipulation of interleukin-1 (IL-1) and subsequent signaling through the type 1 IL-1 receptor (IL-1R1). To investigate the influence of IL-1R1 activation in the pathophysiology of a chemical-induced injury to the murine hippocampus, we examined the level and pattern of neuronal death and neuroinflammation in male weanling mice exposed to trimethyltin hydroxide (2.0 mg TMT/kg, i.p.). Dentate granule cell death occurred at 6 h post-TMT as detected by active caspase 3 immunostaining and presence of lectin positive microglia. The severity of neuronal death and microglia response increased by 12–24 h with elevations in mRNA levels for TNFα and IL-1α. In IL-1R1 null (IL-1R1−/−) mice, the pattern and severity of neuronal death at 24 or 72 h post-TMT was similar as compared to wildtype (WT) mice. In both groups, mRNA levels for TNFα and MIP-1α were elevated, no significant change was seen in either IL-1α or IL-1β, and the early activation of microglia, including their ability to progress to a phagocytic phenotype, was maintained. Compared to WT mice, IL-1R1−/− mice displayed a limited glial fibrillary acidic protein (GFAP) astrocytic response, as well as a preferential induction in mRNA levels of Fas signaling components. Cumulatively, these results indicate that IL-1R1 activation is not necessary for TMT-induced death of dentate granule neurons or local activation of microglia; however, IL-1R1 signaling is involved in mediating the structural response of astrocytes to injury and may regulate apoptotic mechanisms via Fas signaling components.

Disrupted Dentate Granule Cell Chloride Regulation Enhances Synaptic Excitability during Development of Temporal Lobe Epilepsy

GABA(A) receptor-mediated inhibition depends on the maintenance of intracellular Cl- concentration ([Cl-]in) at low levels. In neurons in the developing CNS, [Cl-]in is elevated, E(GABA) is depolarizing, and GABA consequently is excitatory. Depolarizing GABAergic synaptic responses may be recapitulated in various neuropathological conditions, including epilepsy. In the present study, rat hippocampal dentate granule cells were recorded using gramicidin perforated patch techniques at varying times (1-60 d) after an epileptogenic injury, pilocarpine-induced status epilepticus (STEP). In normal, non-epileptic animals, these strongly inhibited dentate granule cells act as a gate, regulating hippocampal excitation, controlling seizure initiation and/or propagation. For 2 weeks after STEP, we found that E(GABA) was positively shifted in granule cells. This shift in E(GABA) altered synaptic integration, increased granule cell excitability, and resulted in compromised "gate" function of the dentate gyrus. E(GABA) recovered to control values at longer latencies post-STEP (2-8 weeks), when animals had developed epilepsy. During this period of shifted E(GABA), expression of the Cl- extruding K+/Cl- cotransporter, KCC2 was decreased. Application of the KCC2 blocker, furosemide, to control neurons mimicked E(GABA) shifts evident in granule cells post-STEP. Furthermore, post-STEP and furosemide effects interacted occlusively, both on E(GABA) in granule cells, and on gatekeeper function of the dentate gyrus. This suggests a shared mechanism, reduced KCC2 function. These findings demonstrate that decreased expression of KCC2 persists for weeks after an epileptogenic injury, reducing inhibitory efficacy and enhancing dentate granule cell excitability. This pathophysiological process may constitute a significant mechanism linking injury to the subsequent development of epilepsy.

Impact of receptor changes on treatment of status epilepticus

Neurotransmitter receptors play a key role in the pathogenesis and current treatment strategies for status epilepticus (SE). Benzodiazepines are commonly used to treat SE exert their anticonvulsant action by stimulating γ-aminobutyric type A receptors (GABAA receptors). These drugs are quite effective in terminating SE in approximately 55–66% of patients, but there is a residual group of patients with seizures resistant to benzodiazepeines. The mechanisms underlying initiation and maintenance of prolonged SE were explored.

Bidirectional activity-dependent plasticity of membrane potential and the influence on spiking in rat hippocampal dentate granule cells

Granule cells of the dentate gyrus in the hippocampus generally fire at low frequencies but are known to respond to sensory cues by increasing their rate of firing. We have previously shown that a burst of action potentials in synaptically isolated granule cells can induce a long-term depolarisation (LTDepol) of the neuronal membrane potential. This form of excitability plasticity could be an important mechanism for learning and memory. Here we demonstrate that this depolarisation can be reversed by physiologically relevant firing patterns. At a basal action potential frequency of 0.1 Hz the membrane potential depolarises in response to brief high frequency stimulation (HFS) but this depolarisation is blocked or reversed by 1 Hz action potential firing. The depolarisation of the neurones did not, however, affect the input–output function of the dentate gyrus measured by field or single cell recordings. HFS or brief forskolin application that mimics LTDepol did not alter the excitatory input–output spike coupling measured using field potential recordings. Similarly, LTDepol did not change the probability of spike firing in response to a given input. The mechanism underlying the lack of change in input–output relationship was found to be a concurrent depolarisation of the threshold potential for the initiation of action potentials. We demonstrate that physiologically relevant patterns of activity regulate the membrane potential of dentate gyrus granule cells in a bidirectional manner but the changes in membrane potential do not alter the excitability of these neurones measured by their response to excitatory inputs.

Pilocarpine-Induced Seizures Cause Selective Time-Dependent Changes to Adult-Generated Hippocampal Dentate Granule Cells

Aberrantly interconnected granule cells are characteristic of temporal lobe epilepsy. By reducing network stability, these abnormal neurons may contribute directly to disease development. Only subsets of granule cells, however, exhibit abnormalities. Why this is the case is not known. Ongoing neurogenesis in the adult hippocampus may provide an explanation. Newly generated granule cells may be uniquely vulnerable to environmental disruptions relative to their mature neighbors. Here, we determine whether there is a critical period after neuronal birth during which neuronal integration can be disrupted by an epileptogenic insult. By bromodeoxyuridine birthdating cells in green fluorescent protein-expressing transgenic mice, we were able to noninvasively label granule cells born 8 weeks before (mature), 1 week before (immature), or 3 weeks after (newborn) pilocarpine-epileptogenesis. Neuronal morphology was examined 4 and 8 weeks after pilocarpine treatment. Strikingly, almost 50% of immature granule cells exposed to pilocarpine-epileptogenesis exhibited aberrant hilar basal dendrites. In contrast, only 9% of mature granule cells exposed to the identical insult possessed basal dendrites. Moreover, newborn cells were even more severely impacted than immature cells, with 40% exhibiting basal dendrites and an additional 20% exhibiting migration defects. In comparison, <5% of neurons from normal animals exhibited either abnormality, regardless of age. Together, these data demonstrate the existence of a critical period after the birth of adult-generated neurons during which they are vulnerable to being recruited into epileptogenic neuronal circuits. Pathological brain states therefore may pose a significant hurdle for the appropriate integration of newly born endogenous, and exogenous, neurons.

Coordinated Expression of HuD and GAP-43 in Hippocampal Dentate Granule Cells During Developmental and Adult Plasticity

Previous work from our laboratory demonstrated that the RNA-binding protein HuD binds to and stabilizes the GAP-43 mRNA. In this study, we characterized the expression of HuD and GAP-43 mRNA in the hippocampus during two forms of neuronal plasticity. During post-natal development, maximal expression of both molecules was found at P5 and their levels steadily decreased thereafter. At P5, HuD was also present in the subventricular zone, where it co-localized with doublecortin. In the adult hippocampus, the basal levels of HuD and GAP-43 were lower than during development but were significantly increased in the dentate gyrus after seizures. The function of HuD in GAP-43 gene expression was confirmed using HuD-KO mice, in which the GAP-43 mRNA was significantly less stable than in wild type mice. Altogether, these results demonstrate that HuD plays a role in the post-transcriptional control of GAP-43 mRNA in dentate granule cells during developmental and adult plasticity.

Distribution of Kv3.3 potassium channel subunits in distinct neuronal populations of mouse brain

Kv3.3 proteins are pore-forming subunits of voltage-dependent potassium channels, and mutations in the gene encoding for Kv3.3 have recently been linked to human disease, spinocerebellar ataxia 13, with cerebellar and extracerebellar symptoms. To understand better the functions of Kv3.3 subunits in brain, we developed highly specific antibodies to Kv3.3 and analyzed immunoreactivity throughout mouse brain. We found that Kv3.3 subunits are widely expressed, present in important forebrain structures but particularly prominent in brainstem and cerebellum. In forebrain and midbrain, Kv3.3 expression was often found colocalized with parvalbumin and other Kv3 subunits in inhibitory neurons. In brainstem, Kv3.3 was strongly expressed in auditory and other sensory nuclei. In cerebellar cortex, Kv3.3 expression was found in Purkinje and granule cells. Kv3.3 proteins were observed in axons, terminals, somas, and, unlike other Kv3 proteins, also in distal dendrites, although precise subcellular localization depended on cell type. For example, hippocampal dentate granule cells expressed Kv3.3 subunits specifically in their mossy fiber axons, whereas Purkinje cells of the cerebellar cortex strongly expressed Kv3.3 subunits in axons, somas, and proximal and distal, but not second- and third-order, dendrites. Expression in Purkinje cell dendrites was confirmed by immunoelectron microscopy. Kv3 channels have been demonstrated to rapidly repolarize action potentials and support high-frequency firing in various neuronal populations. In this study, we identified additional populations and subcellular compartments that are likely to sustain high-frequency firing because of the expression of Kv3.3 and other Kv3 subunits.

Selective loss of dentate hilar interneurons contributes to reduced synaptic inhibition of granule cells in an electrical stimulation‐based animal model of temporal lobe epilepsy

Neuropeptide-containing hippocampal interneurons and dentate granule cell inhibition were investigated at different periods following electrical stimulation-induced, self-sustaining status epilepticus (SE) in rats. Immunohistochemistry for somatostatin (SOM), neuropeptide Y (NPY), parvalbumin (PV), cholecystokinin (CCK), and Fluoro-Jade B was performed on sections from hippocampus contralateral to the stimulated side and studied by confocal laser scanning microscopy. Compared to paired age-matched control animals, there were fewer SOM and NPY-immunoreactive (IR) interneurons in the hilus of the dentate gyrus in animals with epilepsy (40-60 days after SE), and 1, 3, and 7 days following SE. In the hilus of animals that had recently undergone SE, some SOM-IR and NPY-IR interneurons also stained for Fluoro-Jade B. Furthermore, there was electron microscopic evidence of the degeneration of SOM-IR interneurons following SE. In contrast, the number of CCK and PV-IR basket cells in epileptic animals was similar to that in controls, although it was transiently diminished following SE; there was no evidence of degeneration of CCK or PV-IR interneurons. Patch-clamp recordings revealed a diminished frequency of inhibitory postsynaptic currents in dentate granule cells (DGCs) recorded from epileptic animals and animals that had recently undergone SE compared with controls. These results confirm the selective vulnerability of a particular subset of dentate hilar interneurons after prolonged SE. This loss may contribute to the reduced GABAergic synaptic inhibition of granule cells in epileptic animals.

Synapses formed by normal and abnormal hippocampal mossy fibers

The axon terminals (mossy fibers) of hippocampal dentate granule cells form characteristic synaptic connections with large spines or excrescences of both hilar mossy cells and CA3 pyramidal neurons. Interneurons of the hilar region and area CA3 are also prominent targets of mossy fibers. The tracing of biocytin-filled mossy fibers and immunolabeling of target cells with interneuron markers has revealed that the majority of mossy fiber synapses project to gamma aminobutyric acid (GABA)-ergic inhibitory interneurons rather than to excitatory principal cells, although the functional implications of these quantitative differences are unclear. Following a brief description of the "classical" mossy fiber synapse on excrescences of CA3 pyramidal cells, the present review focuses on the contacts formed between granule cells and GABAergic interneurons, both normally and after synaptic reorganization. In response to deafferentation of mossy cell target cells, which include both granule cells and interneurons, mossy fibers "sprout" new axon collaterals that form a band of supragranular mossy fibers in the inner molecular layer of the dentate gyrus. Although most newly formed recurrent mossy fibers establish synapses with granule cells, there is an apparently convergent input of new mossy fibers onto GABA-immunoreactive interneuron dendrites that traverse the inner molecular layer. These mossy fiber-interneuron synapses in the dentate gyrus are observed in chronically epileptic rats and may be the structural correlate of the granule cell hyperinhibition observed in these animals in vivo. Together, the findings reviewed here establish mossy fiber synapses as an important component of inhibitory circuits in the hippocampus.

The effects of combined exposure to the pyrethroids deltamethrin and S-bioallethrin on hippocampal inhibition and skeletal muscle hyperexcitability in rats

The default assumption that different pyrethroid insecticides, sharing a common mode of action, will show additivity of toxicity has not always been supported by in vitro measures, some of which have indicated antagonism. Our intention was to see whether the antagonism between pyrethroids of different classes seen in vitro could be reproduced in vivo. We therefore investigated the effects of single and combined exposures to two commonly used pyrethroids, deltamethrin (type II) and S-bioallethrin (type I) given intravenously to anaesthetised rats. We used two quantitative measures that are responsive to pyrethroids: the duration of prolongation of hippocampal dentate granule cell inhibition and the amplitude of the abnormal electromyogram discharge. At equi-toxic doses, S-bioallethrin extended the inter-stimulus interval evoking 50% inhibition in the hippocampus by 30+/-2.2 ms, and deltamethrin extended it by 199+/-21 ms. Combined administration of the same doses of deltamethrin and S-bioallethrin extended hippocampal inhibition by 164+/-14 ms, which did not differ significantly from the effect of deltamethrin alone. S-bioallethrin was without any effect on the electromyogram, and produced no significant change in the amplitude of the abnormal muscle discharges evoked by deltamethrin. The increase in arterial blood pressure evoked by the combination was significantly less than that evoked by either pyrethroid alone (p<0.001). In summary, although our electrophysiological indices provide no support for functional antagonism between these two pyrethroids, they also fail to indicate any summation of effect.

Changes in mIPSCs and sIPSCs after kainate treatment: possible actions mediated by the direct activation of kainate receptors

In a very interesting paper, [Shao and Dudek (2005)][1] described the changes in miniature inhibitory postsynaptic currents (mIPSCs) and spontaneous inhibitory synaptic currents (sIPSCs) that occur after kainate treatment recorded in hippocampal dentate granule cells. Basically, in acutely and

In vivo post‐transcriptional regulation of GAP‐43 mRNA by overexpression of the RNA‐binding protein HuD

HuD is a neuronal-specific RNA-binding protein that binds to and stabilizes the mRNAs of growth-associated protein-43 (GAP-43) and other neuronal proteins. HuD expression increases during brain development, nerve regeneration, and learning and memory, suggesting that this protein is important for controlling gene expression during developmental and adult plasticity. To examine the function of HuD in vivo, we generated transgenic mice overexpressing human HuD under the control of the calcium-calmodulin-dependent protein kinase IIalpha promoter. The transgene was expressed at high levels throughout the forebrain, including the hippocampal formation, amygdala and cerebral cortex. Using quantitative in situ hybridization, we found that HuD overexpression led to selective increases in GAP-43 mRNA in hippocampal dentate granule cells and neurons in the lateral amygdala and layer V of the neorcortex. In contrast, GAP-43 pre-mRNA levels were unchanged or decreased in the same neuronal populations. Comparison of the levels of mature GAP-43 mRNA and pre-mRNA in the same neurons of transgenic mice suggested that HuD increased the stability of the transcript. Confirming this, mRNA decay assays revealed that the GAP-43 mRNA was more stable in brain extracts from HuD transgenic mice than non-transgenic littermates. In conclusion, our results demonstrate that HuD overexpression is sufficient to increase GAP-43 mRNA stability in vivo.

S.23.05 New functions for presenilins: do they identify novel therapeutic targets?

Traumatic brain injury (TBI) can result in altered inhibitory neurotransmission, hippocampal dysfunction, and cognitive impairments. GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) and tonic (extrasynaptic) whole cell currents were recorded in control rat hippocampal dentate granule cells (DGCs) and at 90 days after controlled cortical impact (CCI). At 34 °C, in CCI DGCs, sIPSC frequency and amplitude were unchanged, whereas mIPSC frequency was decreased (3.10 ± 0.84 Hz, n = 16, and 2.44 ± 0.67 Hz, n = 7, p < 0.05). At 23 °C, 300 nM diazepam increased peak amplitude of mIPSCs in control and CCI DGCs, but the increase was 20% higher in control (26.81 ± 2.2 pA and 42.60 ± 1.22 pA, n = 9, p = 0.031) compared to CCI DGCs (33.46 ± 2.98 pA and 46.13 ± 1.09 pA, n = 10, p = 0.047). At 34 °C, diazepam did not prolong decay time constants (6.59 ± 0.12 ms and 6.62 ± 0.98 ms, n = 9, p = 0.12), the latter suggesting that CCI resulted in benzodiazepine-insensitive pharmacology in synaptic GABAA receptors (GABAARs). In CCI DGCs, peak amplitude of mIPSCs was inhibited by 100 μM furosemide (51.30 ± 0.80 pA at baseline and 43.50 ± 5.30 pA after furosemide, n = 5, p < 0.001), a noncompetitive antagonist of GABAARs with an enhanced affinity to α4 subunit-containing receptors. Potentiation of tonic current by the GABAAR δ subunit-preferring competitive agonist THIP (1 and 3 µM) was increased in CCI DGCs (47% and 198%) compared to control DGCs (13% and 162%), suggesting the presence of larger tonic current in CCI DGCs; THIP (1 µM) had no effect on mIPSCs. Taken together, these results demonstrate alterations in synaptic and extrasynaptic GABAARs in DGCs following CCI.

FoxG1 haploinsufficiency results in impaired neurogenesis in the postnatal hippocampus and contextual memory deficits

FoxG1 (formerly BF-1) encodes a transcription factor that regulates neurogenesis in the embryonic telencephalon. The current study suggests that FoxG1 also regulates neurogenesis in the postnatal hippocampus. FoxG1 continues to be strongly expressed in areas of known postnatal neurogenesis, including the subventricular zone of the lateral ventricle and the dentate gyrus (DG) of the hippocampus. Remarkably, FoxG1+/- mice have a 60% decrease in the total number of hippocampal dentate granule cells that is related to a loss of DG neurogenesis. Comparison of acute and chronic BrdU labeling, and PSA-NCAM staining suggests that the stage at which this loss of neurogenesis occurs progresses with age. Juvenile mice FoxG1+/- primarily show failed apparent survival of postnatally born DG neurons, whereas adult FoxG1+/- mice also show impairment of proliferation and initial DG neuron differentiation. Consistent with this process predominantly affecting postnatal hippocampal neurogenesis, BrdU pulses at embryonic days 16, 17, and 18 labels a higher percentage of DG cells in 6-week-old FoxG1+/- mice than in littermate controls. In contrast to the marked effect of FoxG1 haploinsufficiency on postnatal hippocampal neurogenesis, postnatal neurogenesis of olfactory bulb interneurons is grossly unaffected. Behaviorally, FoxG1+/- mice show hyperlocomotion and impaired habituation in the open field, and a severe deficit in contextual fear conditioning that are suggestive of impaired hippocampal function. Although mechanistic connections between FoxG1 haploinsufficiency and either failed postnatal DG neurogenesis or the behavioral deficits remain to be elucidated, these results present a new model system for impaired postnatal neurogenesis in the DG of adult mice.

Forebrain dopamine receptors in cognitive, memory and learning processes

Neurons that utilize dopamine (DA) as a neurotransmitter have attracted great interest because of their involvement in the behavioral, endocrine and descending control of major brain functions. DA is known to exert its physiological effects on target neurons through D1-like (D1 and D5) and D2-like (D2, D3 and D4) receptor subtypes. To better understand the DA modulation of brain functions, the distribution and cellular localization of D1 and D2 DA receptors in the rat forebrain is reviewed, and their possible role in cognitive aspects of behavior is discussed. In general, DA receptors are differentially expressed and mostly distributed in different target fields of DA neurons. Both D1 and D2 receptor gene products are found in the cerebral cortex, basal ganglia and hippocampal formation, albeit in different cell groups or neuronal subpopulations, whereas D3 receptors are mainly located in limbic regions. In the cortex, mRNA signals are seen in all the areas and cortical layers except layer I. In the striatum, the most intense signal is found in the caudate-putamen, nucleus accumbens and olfactory tubercle where a large number of cells are strongly labeled for D1 and D2. In the globus pallidus only scattered D2 mRNA-containing cells are present. In contrast, no D1 or D2 messages can be seen in the ventral pallidum. In the basal forebrain, mRNA encoding the D1 receptor is detected in the islands of Calleja. The medial and lateral septal nuclei show a low D2 signal. In the amygdaloid nuclear complex, the strongest D1 receptor message is observed in the basomedial and basolateral nuclei. Conversely, the highest density of D2 mRNA-expressing cells is revealed in the central nucleus. Moderately labeled for D1 and D2 cells are scattered throughout the anterior and posterior subdivisions of the bed nucleus of stria terminalis, and within all subfields of the hippocampal formation and dentate granule cell layer. Differential regional and cellular distribution of DA receptors in the forebrain provides anatomical evidence for an area-specific regulation of the DAergic neurotransmission. It can be inferred that DA facilitates learning, memory and cognition processes via activation of both the D1 and D2 receptors. Biomedical Reviews 2005; 16: 59-75.

Ketogenic diet decreases the level of proenkephalin mRNA induced by kainic acid in the mouse hippocampus

The ketogenic diet (KD) has been used to control medically refractory epilepsy in children for more than 80 years. Despite the clinical efficacy of the KD, its underlying bases are still obscure. Previous work from our laboratory has established that the KD has an antiepileptic and neuroprotective effect in the kainic acid (KA)-induced seizure model. The neuronal excitation caused by KA leads to increases in the expression of a variety of genes, including immediate-early genes and opioid peptides derived from proenkephalin (PENK) and prodynorphin (PDYN). In particular, the up-regulated PENK gene that is induced by KA in the hippocampal dentate granule cells has proconvulsant properties. PENK is regulated by the c-jun amino-terminal kinase (JNK) signaling pathway, the crucial role of which is involved in the regulation of transcription factors, such as Jun and Fos. In the present study, we examined the effect of the KD on the increase of PENK, Fos, Jun, AP-1 DNA-binding activity and JNK gene expression induced by KA in the mouse hippocampus. Using in situ hybridization and northern blot analysis, we found that the KD significantly decreases the level of PENK gene expression induced by KA of the granular cells in the hippocampus. In addition, we have also found that KD diminished KA-induced AP-1 DNA-binding activity, Fos and Jun expression, and phoshorylated form of the three types of JNKs. These results suggest that the KD suppresses KA-induced activation of JNK signaling pathways, followed by a decrease of PENK gene expression in the hippocampus, thereby resulting in antiepileptic effects.

High-Affinity, Slowly Desensitizing GABAA Receptors Mediate Tonic Inhibition in Hippocampal Dentate Granule Cells

The tonic form of GABA-mediated inhibition requires the presence of slowly desensitizing GABA(A) receptors with high affinity, which has not yet been directly demonstrated in hippocampal neurons. Low concentration of GABA (1 microM) persistently increased baseline noise, increased membrane slope conductance, but did not affect spontaneous inhibitory postsynaptic currents (sIPSCs) in dentate granule cells (DGCs). Higher concentrations of GABA (10-100 microM) desensitized synaptic currents quickly, and there was a large residual current. Saturating concentration of GABA (1 mM) completely desensitized synaptic currents and revealed a slowly desensitizing, persistent current. Penicillin (300 microM) inhibited baseline noise without affecting mean current and inhibited decay time of sIPSCs. GABA(A) receptors mediating baseline noise in DGCs were sensitive to allopregnanolone, furosemide, and loreclezole and insensitive to diazepam and zolpidem. These studies demonstrate persistently open GABA(A) receptors on DGCs with high affinity for GABA, slow desensitization rate, and pharmacological properties similar to those of recombinant receptors containing alpha(4), beta(1), and the delta subunits.