Somatostatin-immunoreactive interneurons contribute to lateral inhibitory circuits in the dentate gyrus of control and epileptic rats

GEVI cell-type specific labelling and a manifold learning approach provide evidence for lateral inhibition at the population level in the mouse hippocampal CA1 area

The CA1 area in the mammalian hippocampus is essential for spatial learning. Pyramidal cells are the hippocampus output neurons and their activities are regulated by inhibition exerted by a diversified population of interneurons. Lateral inhibition has been suggested as the mechanism enabling the reconfiguration of pyramidal cell assembly activity observed during spatial learning tasks in rodents. However, lateral inhibition in the CA1 lacks the overwhelming evidence reported in other hippocampal areas such as the CA3 and the dentate gyrus. The use of genetically encoded voltage indicators and fast optical recordings permits the construction of cell-type specific response maps of neuronal activity. Here, we labelled mouse CA1 pyramidal neurons with the genetically encoded voltage indicator ArcLight and optically recorded their response to Schaffer Collaterals stimulation in vitro. By undertaking a manifold learning approach, we report a hyperpolarization-dominated area focused in the perisomatic region of pyramidal cells receiving late excitatory synaptic input. Functional network organization metrics revealed that information transfer was higher in this area. The localized hyperpolarization disappeared when GABA_A receptors were pharmacologically blocked. This is the first report where the spatiotemporal pattern of lateral inhibition is visualized in the CA1 by expressing a genetically encoded voltage indicator selectively in principal neurons. Our analysis suggests a fundamental role of lateral inhibition in CA1 information processing.

Voluntary running improves depressive behaviours and the structure of the hippocampus in rats: A possible impact of myokines

This study investigated the impact of voluntary exercise on depressive behaviours, serum and hippocampal levels of myokines, and histopathological features of hippocampal formation in rats. Depressed rats were allowed to voluntarily run on a wheel for 3weeks. Locomotor activity was assessed by a forced swimming test and the myokine levels in sera and hippocampal homogenates were measured using Enzyme-linked Immunosorbent Assay. Brain sections were analysed for hippocampal structure and neuronal counts. Voluntary running produced significant increase in the distance moved by rats and significant decrease in immobility duration. After voluntary running, there were significant increases in serum and hippocampal brain-derived neurotrophic factor (BDNF) and macrophage migration inhibitory factor (MIF), significant increase in hippocampal vascular endothelial growth factor (VEGF), and significant decrease in serum interleukin-6 (IL-6). Significant correlation was detected between the serum levels of BDNF and MIF (r=0.276) as well as IL-6 (r=-0.340). In addition, significant correlation was observed between hippocampal BDNF levels and MIF (r=0.500) and VEGF levels (r=0.279). After voluntary running, there was significant decrease in number degenerated neurons in hippocampal areas and significant increase in number of healthy neurons in the upper limb of the dentate gyrus, but not in its lower limb, compared to depression group. This study showed the relation of myokines to the development and/or relief of depression, as well as the correlation between serum and hippocampal myokine levels. Attention should be paid to studying the biological effects of myokines on different hippocampal areas that could respond differently to treatments.

The influence of ectopic migration of granule cells into the hilus on dentate gyrus-CA3 function

Postnatal neurogenesis of granule cells (GCs) in the dentate gyrus (DG) produces GCs that normally migrate from the subgranular zone to the GC layer. However, GCs can mismigrate into the hilus, the opposite direction. Previous descriptions of these hilar ectopic GCs (hEGCs) suggest that they are rare unless there are severe seizures. However, it is not clear if severe seizures are required, and it also is unclear if severe seizures are responsible for the abnormalities of hEGCs, which include atypical dendrites and electrophysiological properties. Here we show that large numbers of hEGCs develop in a transgenic mouse without severe seizures. The mice have a deletion of BAX, which normally regulates apoptosis. Surprisingly, we show that hEGCs in the BAX^-/- mouse have similar abnormalities as hEGCs that arise after severe seizures. We next asked if there are selective effects of hEGCs, i.e., whether a robust population of hEGCs would have any effect on the DG if they were induced without severe seizures. Indeed, this appears to be true, because it has been reported that BAX^-/- mice have defects in a behavior that tests pattern separation, which depends on the DG. However, inferring functional effects of hEGCs is difficult in mice with a constitutive BAX deletion because there is decreased apoptosis in and outside the DG. Therefore, a computational model of the normal DG and hippocampal subfield CA3 was used. Adding a small population of hEGCs (5% of all GCs), with characteristics defined empirically, was sufficient to disrupt a simulation of pattern separation and completion. Modeling results also showed that effects of hEGCs were due primarily to “backprojections” of CA3 pyramidal cell axons to the hilus. The results suggest that hEGCs can develop for diverse reasons, do not depend on severe seizures, and a small population of hEGCs may impair DG-dependent function.

Epilepsy and epileptic syndrome

Epilepsy is one of the most common neurological disorders. In most patients with epilepsy, seizures respond to available medications. However, a significant number of patients, especially in the setting of medically-intractable epilepsies, may experience different degrees of memory or cognitive impairment, behavioral abnormalities or psychiatric symptoms, which may limit their daily functioning. As a result, in many patients, epilepsy may resemble a neurodegenerative disease. Epileptic seizures and their potential impact on brain development, the progressive nature of epileptogenesis that may functionally alter brain regions involved in cognitive processing, neurodegenerative processes that relate to the underlying etiology, comorbid conditions or epigenetic factors, such as stress, medications, social factors, may all contribute to the progressive nature of epilepsy. Clinical and experimental studies have addressed the pathogenetic mechanisms underlying epileptogenesis and neurodegeneration.We will primarily focus on the findings derived from studies on one of the most common causes of focal onset epilepsy, the temporal lobe epilepsy, which indicate that both processes are progressive and utilize common or interacting pathways. In this chapter we will discuss some of these studies, the potential candidate targets for neuroprotective therapies as well as the attempts to identify early biomarkers of progression and epileptogenesis, so as to implement therapies with early-onset disease-modifying effects.

Rapamycin suppresses axon sprouting by somatostatin interneurons in a mouse model of temporal lobe epilepsy

PURPOSE

In temporal lobe epilepsy many somatostatin interneurons in the dentate gyrus die. However, some survive and sprout axon collaterals that form new synapses with granule cells. The functional consequences of γ-aminobutyric acid (GABA)ergic synaptic reorganization are unclear. Development of new methods to suppress epilepsy-related interneuron axon sprouting might be useful experimentally.

METHODS

Status epilepticus was induced by systemic pilocarpine treatment in green fluorescent protein (GFP)-expressing inhibitory nerurons (GIN) mice in which a subset of somatostatin interneurons expresses GFP. Beginning 24 h later, mice were treated with vehicle or rapamycin (3 mg/kg intraperitoneally) every day for 2 months. Stereologic methods were then used to estimate numbers of GFP-positive hilar neurons per dentate gyrus and total length of GFP-positive axon in the molecular layer plus granule cell layer. GFP-positive axon density was calculated. The number of GFP-positive axon crossings of the granule cell layer was measured. Regression analyses were performed to test for correlations between GFP-positive axon length versus number of granule cells and dentate gyrus volume.

KEY FINDINGS

After pilocarpine-induced status epilepticus, rapamycin- and vehicle-treated mice had approximately half as many GFP-positive hilar neurons as did control animals. Despite neuron loss, vehicle-treated mice had over twice the GFP-positive axon length per dentate gyrus as controls, consistent with GABAergic axon sprouting. In contrast, total GFP-positive axon length was similar in rapamycin-treated mice and controls. GFP-positive axon length correlated most closely with dentate gyrus volume.

SIGNIFICANCE

These findings suggest that rapamycin suppressed axon sprouting by surviving somatostatin/GFP-positive interneurons after pilocarpine-induced status epilepticus in GIN mice. It is unclear whether the effect of rapamycin on axon length was on interneurons directly or secondary, for example, by suppressing growth of granule cell dendrites, which are synaptic targets of interneuron axons. The mammalian target of rapamycin (mTOR) signaling pathway might be a useful drug target for influencing GABAergic synaptic reorganization after epileptogenic treatments, but additional side effects of rapamycin treatment must be considered carefully.

Altered intrinsic properties of neuronal subtypes in malformed epileptogenic cortex

Neuronal intrinsic properties control action potential firing rates and serve to define particular neuronal subtypes. Changes in intrinsic properties have previously been shown to contribute to hyperexcitability in a number of epilepsy models. Here we examined whether a developmental insult producing the cortical malformation of microgyria altered the identity or firing properties of layer V pyramidal neurons and two interneuron subtypes. Trains of action potentials were elicited with a series of current injection steps during whole cell patch clamp recordings. Cells in malformed cortex identified as having an apical dendrite had firing patterns similar to control pyramidal neurons. The duration of the second action potential in the train was increased in paramicrogyral (PMG) pyramidal cells, suggesting that these cells may be in an immature state, as was previously found for layer II/III pyramidal neurons. Based on stereotypical firing patterns and other intrinsic properties, fast-spiking (FS) and low threshold-spiking (LTS) interneuron subpopulations were clearly identified in both control and malformed cortex. Most intrinsic properties measured in malformed cortex were unchanged, suggesting that subtype identity is maintained. However, LTS interneurons in lesioned cortex had increased maximum firing frequency, decreased initial afterhyperpolarization duration, and increased total adaptation ratio compared to control LTS cells. FS interneurons demonstrated decreased maximum firing frequencies in malformed cortex compared to control FS cells. These changes may increase the efficacy of LTS while decreasing the effectiveness of FS interneurons. These data indicate that differential alterations of individual neuronal subpopulations may endow them with specific characteristics that promote epileptogenesis.

Excitatory input onto hilar somatostatin interneurons is increased in a chronic model of epilepsy

The density of somatostatin (SOM)-containing GABAergic interneurons in the hilus of the dentate gyrus is significantly decreased in both human and experimental temporal lobe epilepsy. We used the pilocarpine model of status epilepticus and temporal lobe epilepsy in mice to study anatomical and electrophysiological properties of surviving somatostatin interneurons and determine whether compensatory functional changes occur that might offset loss of other inhibitory neurons. Using standard patch-clamp techniques and pipettes containing biocytin, whole cell recordings were obtained in hippocampal slices maintained in vitro. Hilar SOM cells containing enhanced green fluorescent protein (EGFP) were identified with fluorescent and infrared differential interference contrast video microscopy in epileptic and control GIN (EGFP-expressing Inhibitory Neurons) mice. Results showed that SOM cells from epileptic mice had 1) significant increases in somatic area and dendritic length; 2) changes in membrane properties, including a small but significant decrease in resting membrane potential, and increases in time constant and whole cell capacitance; 3) increased frequency of slowly rising spontaneous excitatory postsynaptic currents (sEPSCs) due primarily to increased mEPSC frequency, without changes in the probability of release; 4) increased evoked EPSC amplitude; and 5) increased spontaneous action potential generation in cell-attached recordings. Results suggest an increase in excitatory innervation, perhaps on distal dendrites, considering the slower rising EPSCs and increased output of hilar SOM cells in this model of epilepsy. In sum, these changes would be expected to increase the inhibitory output of surviving SOM interneurons and in part compensate for interneuronal loss in the epileptogenic hippocampus.

Initial loss but later excess of GABAergic synapses with dentate granule cells in a rat model of temporal lobe epilepsy

Many patients with temporal lobe epilepsy display neuron loss in the dentate gyrus. One potential epileptogenic mechanism is loss of GABAergic interneurons and inhibitory synapses with granule cells. Stereological techniques were used to estimate numbers of gephyrin-positive punctae in the dentate gyrus, which were reduced short-term (5 days after pilocarpine-induced status epilepticus) but later rebounded beyond controls in epileptic rats. Stereological techniques were used to estimate numbers of synapses in electron micrographs of serial sections processed for postembedding GABA-immunoreactivity. Adjacent sections were used to estimate numbers of granule cells and glutamic acid decarboxylase-positive neurons per dentate gyrus. GABAergic neurons were reduced to 70% of control levels short-term, where they remained in epileptic rats. Integrating synapse and cell counts yielded average numbers of GABAergic synapses per granule cell, which decreased short-term and rebounded in epileptic animals beyond control levels. Axo-shaft and axo-spinous GABAergic synapse numbers in the outer molecular layer changed most. These findings suggest interneuron loss initially reduces numbers of GABAergic synapses with granule cells, but later, synaptogenesis by surviving interneurons overshoots control levels. In contrast, the average number of excitatory synapses per granule cell decreased short-term but recovered only toward control levels, although in epileptic rats excitatory synapses in the inner molecular layer were larger than in controls. These findings reveal a relative excess of GABAergic synapses and suggest that reports of reduced functional inhibitory synaptic input to granule cells in epilepsy might be attributable not to fewer but instead to abundant but dysfunctional GABAergic synapses.

Surviving hilar somatostatin interneurons enlarge, sprout axons, and form new synapses with granule cells in a mouse model of temporal lobe epilepsy

In temporal lobe epilepsy, seizures initiate in or near the hippocampus, which frequently displays loss of neurons, including inhibitory interneurons. It is unclear whether surviving interneurons function normally, are impaired, or develop compensatory mechanisms. We evaluated GABAergic interneurons in the hilus of the dentate gyrus of epileptic pilocarpine-treated GIN mice, specifically a subpopulation of somatostatin interneurons that expresses enhanced green fluorescence protein (GFP). GFP-immunocytochemistry and stereological analyses revealed substantial loss of GFP-positive hilar neurons (GPHNs) but increased GFP-positive axon length per dentate gyrus in epileptic mice. Individual biocytin-labeled GPHNs in hippocampal slices from epileptic mice also had larger somata, more axon in the molecular layer, and longer dendrites than controls. Dual whole-cell patch recording was used to test for monosynaptic connections from hilar GPHNs to granule cells. Unitary IPSCs (uIPSCs) recorded in control and epileptic mice had similar average rise times, amplitudes, charge transfers, and decay times. However, the probability of finding monosynaptically connected pairs and evoking uIPSCs was 2.6 times higher in epileptic mice compared to controls. Together, these findings suggest that surviving hilar somatostatin interneurons enlarge, extend dendrites, sprout axon collaterals in the molecular layer, and form new synapses with granule cells. These epilepsy-related changes in cellular morphology and connectivity may be mechanisms for surviving hilar interneurons to inhibit more granule cells and compensate for the loss of vulnerable interneurons.

The dentate gyrus: fundamental neuroanatomical organization (dentate gyrus for dummies)

The dentate gyrus is a simple cortical region that is an integral portion of the larger functional brain system called the hippocampal formation. In this review, the fundamental neuroanatomical organization of the dentate gyrus is described, including principal cell types and their connectivity, and a summary of the major extrinsic inputs of the dentate gyrus is provided. Together, this information provides essential information that can serve as an introduction to the dentate gyrus--a "dentate gyrus for dummies."

The perforant path: projections from the entorhinal cortex to the dentate gyrus

This paper provides a comprehensive description of the organization of projections from the entorhinal cortex to the dentate gyrus, which together with projections to other subfields of the hippocampal formation form the so-called perforant pathway. To this end, data that are primarily from anatomical studies in the rat will be summarized, complimented with comparative data from other species. The analysis of the organization of any of the connections of the hippocampus, including that of the entorhinal cortex to the dentate gyrus, is severely hampered because of the complex three-dimensional shape of the hippocampus. In particular in rodents, but to a lesser extent also in primates, all traditional planes of sectioning will result in sections that at some point or another do not cut through the hippocampus at an angle that is perpendicular to its long axis. To amend this, we will describe own unpublished tracing data obtained in the rat with the use of the so-called extended preparation. A number of issues will be addressed. First, data will be summarized which will clarify the laminar origin of the perforant pathway within the entorhinal cortex. Second, we will discuss whether or not a radial organization, along the proximo-distal dendritic axis of granule cells, characterizes the entorhinal-dentate projection. Third, we will discuss whether this projection is governed by any transverse organization, and fourth, we will focus on the organization along the longitudinal axis. Finally, the synaptic organization and the contralateral entorhinal-dentate projection will be described briefly. Taken together, the available data suggest that the projection from the entorhinal cortex to the dentate gyrus is a fairly well conserved connection, present in all species studied, exhibiting a grossly similar organization.

The role of synaptic reorganization in mesial temporal lobe epilepsy

The mechanisms underlying mesial temporal lobe epilepsy (MTLE) remain uncertain. Putative mechanisms should account for several features characteristic of the clinical presentation and the neurophysiological and neuropathological abnormalities observed in patients with intractable MTLE. Synaptic reorganization of the mossy fiber pathway has received considerable attention over the past two decades as a potential mechanism that increases the excitability of the hippocampal network through the formation of new recurrent excitatory collaterals. Morphological plasticity beyond the mossy fiber pathway has not been as thoroughly investigated. Recently, plasticity of the CA1 pyramidal axons has been demonstrated in acute and chronic experimental models of MTLE. As the hippocampal formation is topographically organized in stacks of slices (lamellae), synaptic reorganization of CA1 axons projecting to subiculum appears to increase the connectivity between lamellae, providing a mechanism for translamellar synchronization of cellular hyperexcitability, leading to pharmacologically intractable seizures.

Patterns of expression of neuropeptides in GABAergic nonprincipal neurons in the mouse hippocampus: Quantitative analysis with optical disector

Neuropeptides are widely distributed in the central nervous system and are considered to play important roles in the regulation of neuronal activity. This study shows the patterns of expression of four neuropeptides [neuropeptide Y (NPY), somatostatin (SOM), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP)] in gamma-aminobutyric acid (GABA)-ergic neurons of the mouse hippocampus, with particular reference to the areal and dorsoventral difference. First, we estimated the numerical densities (NDs) of GABAergic neurons containing these neuropeptides using the optical disector. The NDs of NPY- and SOM-positive GABAergic neurons were generally higher than those of CCK- and VIP-positive GABAergic neurons. In the whole area of the hippocampus, the ND of NPY-positive GABAergic neurons showed no significant dorsoventral difference (1.90 x 10(3)/mm(3) in the dorsal level, 2.09 x 10(3)/mm(3) in the ventral level), whereas the ND of SOM-positive GABAergic neurons was higher in the ventral level (1.44 x 10(3)/mm(3)) than in the dorsal level (0.80 x 10(3)/mm(3)). The ND of CCK-positive GABAergic neurons was also higher in the ventral level (0.57 x 10(3)/mm(3)) than in the dorsal level (0.33 x 10(3)/mm(3)). Similarly, the ND of VIP-positive GABAergic neurons was higher in the ventral level (0.61 x 10(3)/mm(3)) than in the dorsal level (0.43 x 10(3)/mm(3)). Next, we calculated the proportions of GABAergic neurons containing these neuropeptides among the total GABAergic neurons. In the whole area of the hippocampus, NPY-, SOM-, CCK-, and VIP-positive neurons accounted for about 31%, 17%, 7%, and 8% of GABAergic neurons, respectively. The present data establish a baseline for examining potential roles of GABAergic neurons in the hippocampal network activity in mice.

Axon arbors and synaptic connections of a vulnerable population of interneurons in the dentate gyrus in vivo

The predominant gamma-aminobutyric acid (GABA)ergic neuron class in the hilus of the dentate gyrus consists of spiny somatostatinergic interneurons. We examined the axon projections and synaptic connections made by spiny hilar interneurons labeled with biocytin in gerbils in vivo. Axon length was 152-497 mm/neuron. Sixty to 85% of the axon concentrated in the outer two thirds of the molecular layer of the dentate gyrus. The septotemporal span of the axon arbor extended over 48-82% of the total hippocampal length, which far exceeds the septotemporal span of axons of granule cells whose complete axon arbors extended over 15-29%. A three-dimensionally reconstructed 216-microm-long spiny hilar interneuron axon segment in the outer third of the molecular layer formed an average of 1 synapse every 5.1 microm. Of the 42 symmetric (inhibitory) synapses formed by the reconstructed segment, 88% were with spiny dendrites of presumed granule cells, and 67% were with dendritic spines that also receive an asymmetric (excitatory) contact from an unlabeled axon terminal. Postembedding GABA-immunocytochemistry revealed that 55% of the GABAergic synapses in the outer third of the molecular layer were with spines. Therefore, in the outer molecular layer, spiny hilar interneurons form synaptic contacts that appear to be positioned to exert inhibitory control near sites of excitatory synaptic input from the entorhinal cortex to granule cell dendritic spines. These findings demonstrate far-reaching, yet highly specific, connectivity of individual interneurons and suggest that the loss of spiny hilar interneurons, as occurs in temporal lobe epilepsy, may contribute to hyperexcitability in the hippocampus.

Kappa opioid receptor is expressed by somatostatin- and neuropeptide Y-containing interneurons in the rat hippocampus

In our previous studies (J. Chem. Neuroanat. 2000;19:233-241), kappa opioid receptors were immunocytochemically identified in inhibitory interneurons of the dentate hilus and CA1 area of the rat hippocampus. From among the known interneuron subtypes, somatostatin- (SOM) and neuropeptide Y- (NPY) immunoreactive (IR) hippocampal interneurons show morphology and distribution similar to the kappa opioid receptor (KOR) immunopositive cells. In the present study, with the help of double immunocytochemical labelling, we provide direct evidence that the majority of the interneurons immunoreactive for SOM and/or NPY also express the kappa opioid receptor. The receptor was localized on the perikaryal and proximal dendritic region of the SOM- and NPY-immunopositive neurons in the dentate hilus and the CA1 region. From among the SOM-immunoreactive cells, 77% in the dentate hilus and 51% in the CA1 stratum oriens was double labelled. In the case of NPY-immunoreactive neurons this proportion was 56 and 65%, respectively. The co-expression of KOR and SOM/NPY suggests that hippocampal interneurons can selectively be activated by the different opioids under different physiological circumstances.