Neocortex in the hippocampus: an anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats

Migration disorders cause neurons to differentiate in an abnormal heterotopic position. Although significant insights have been gained into the etiology of these disorders, very little is known about the anatomy of heterotopias. We have studied heterotopic masses arising in the hippocampal CA1 region after prenatal treatment with methylazoxymethanol (MAM) in rats. Heterotopic cells were phenotypically similar to neocortical supragranular neurons and exhibited the same temporal profile of migration and neurogenesis. However, they did not express molecules characteristic of CA1 neurons such as the limbic-associated membrane protein. Horseradish peroxidase injections in heterotopia demonstrated labeled fibers not only in the neocortex and white matter but also in the CA1 stratum radiatum and stratum lacunosum. To study the pathophysiological consequences of this connectivity, we compared the effects of neocortical and limbic seizures on the expression of Fos protein and on cell death in MAM animals. After metrazol-induced seizures, Fos-positive cells were present in CA1 heterotopias, the only hippocampal region to be activated with the neocortex. By contrast, kainic acid-induced seizures caused a prominent delayed cell death in limbic regions and in CA1 heterotopias. Together, these results suggest that neocortical heterotopias in the CA1 region are integrated in both the hippocampal and neocortical circuitry.     

Multiple actions of methohexital on hippocampal CA1 and cortical neurons of rat brain slices

To explore the mechanism by which methohexital (MTH) activates epileptiform activity in patients with epilepsy, we examined the effects of MTH on hippocampal CA1 and neocortical neurons via extracellular and whole-cell patch-clamp recordings in rat brain slices. Perfusion of slices with 10 to 100 microM MTH caused no significant change in glutamatergic transmission in the hippocampal CA1 region, but enhanced gamma-aminobutyric acid (GABA)A-mediated inhibitory postsynaptic currents and induced spontaneous inhibitory postsynaptic currents in neocortical and hippocampal CA1 neurons. In addition, MTH induced a tonic, bicuculline-sensitive hyperpolarization in association with increases in membrane conductance, suggesting a direct stimulation of GABAA receptors by MTH. Spontaneous epileptiform activity was not observed in the neocortex and hippocampus after exposure of slices to MTH, neither in the standard in vitro condition nor in the presence of 4-aminopyridine, which promotes rhythmic synaptic activities. We suggest that the activation of epileptiform activity in vivo by MTH may result from increased neuronal synchrony via the potentiation of GABAA-mediated synaptic inhibition.     

Spike-wave complexes and fast components of cortically generated seizures. I. Role of neocortex and thalamus

We explored the relative contributions of cortical and thalamic neuronal networks in the generation of electrical seizures that include spike-wave (SW) and polyspike-wave (PSW) complexes. Seizures were induced by systemic or local cortical injections of bicuculline, a gamma-aminobutyric acid-A (GABAA) antagonist, in cats under barbiturate anesthesia. Field potentials and extracellular neuronal discharges were recorded through arrays of eight tungsten electrodes (0.4 or 1 mm apart) placed over the cortical suprasylvian gyrus and within the thalamus. 1) Systemic injections of bicuculline induced SW/PSW seizures in cortex, whereas spindle sequences continued to be present in the thalamus. 2) Cortical suprasylvian injection of bicuculline induced focal paroxysmal single spikes that developed into full-blown seizures throughout the suprasylvian cortex. The seizures were characterized by highly synchronized SW or PSW complexes at 2-4 Hz, interspersed with runs of fast (10-15 Hz) activity. The intracellular aspects of this complex pattern in different types of neocortical neurons are described in the following paper. Complete decortication abolished the seizure, leaving intact thalamic spindles. Injections of bicuculline in the cortex of athalamic cats resulted in similar components as those occurring with an intact thalamus. 3) Injection of bicuculline in the thalamus decreased the frequency of barbiturate spindles and increased the synchrony of spike bursts fired by thalamocortical and thalamic reticular cells but did not induce seizures. Decortication did not modify the effects of bicuculline injection in the thalamus. Our results indicate that the minimal substrate that is necessary for the production of seizures consisting of SW/PSW complexes and runs of fast activity is the neocortex.     

Functional and pharmacological properties of human neocortical neurons maintained in vitro

The availability of neocortical tissue obtained during brain surgery has allowed for detailed studies of the membrane and synaptic properties of neurons maintained in vitro in a slice preparation. Many of the findings obtained in these studies are summarized here. The majority of the basic electrophysiological properties appear to be similar when human and rodent neurons are compared. However, some notable exceptions regarding specific membrane properties have been reported. Since the majority of the material used in these studies is obtained from epileptic patients, several neuroscientists have tried to determine whether this tissue retains any sign of epileptogenicity when analyzed in vitro. Abnormal synaptic activity was only seen in a fraction of neurons near identified anatomical foci, including tumors, or within neocortical areas that displayed abnormal electrographic activity in situ. This cellular activity included both the presence of all-or-none and graded synaptic bursts. Epileptiform activity comparable to that seen in rodent tissue has been obtained in vitro using several pharmacological procedures including the disinhibition and the Mg(2+)-free model. In conclusion, electrophysiological and pharmacological studies of the human neocortex obtained during surgery have so far been unsuccessful in isolating any definite cellular mechanism that may account for the expression of the epileptiform activity in situ. Nevertheless, these studies have provided valuable information on the cellular and synaptic properties of human neocortex under normal conditions, and following experimental procedures capable of increasing neuronal excitability.     

Monaural sound localization: acute versus chronic unilateral impairment

We report that choline acetyltransferase (ChAT) activity and neuronal survival were enhanced in rat septal neurons cocultured with hippocampal neurons. The enhancement of ChAT activity also occurred as a result of the addition of hippocampal conditioned medium (HpCM). When septal neurons from embryonic day 17 (E17) rats were cocultured with hippocampal neurons, ChAT activity was increased 2-fold compared with homogeneous culture of septal neurons. By contrast, no increase in ChAT activity was observed in coculture of septal and neocortical neurons. Treatment with HpCM obtained from cultured E19 rat hippocampal neurons enhanced the ChAT activity of E17 rat septal neurons. The enhancement of ChAT activity caused by coculture with hippocampal neurons and that caused by the addition of HpCM were not blocked by the addition of anti-nerve growth factor (NGF) antibody, suggesting that NGF, which is known to increase the ChAT activity of septal neurons both in vivo and in vitro, did not participate in the increase of ChAT activity. These findings indicate that possible target-derived neurotrophic factor(s), other than NGF, from hippocampal neurons enhance(s) the ChAT activity of septal neurons.     

Impairments in conditioned stimulus processing and conditioned responding after combined selective removal of hippocampal and neocortical cholinergic input.

Previous studies indicated that changes in attentional processing of conditioned stimuli (CSs) are regulated by the basal forebrain (BF) cholinergic system. In those studies, destruction of BF innervation of the neocortex interfered with enhancements in CS processing, and destruction of BF innervation of the hippocampus prevented reductions in CS processing. In the current experiments, the performance of rats with 192 IgG-saporin lesions of both hippocampal and neocortical cholinergic input was examined. These combined lesions disrupted both enhancements and reductions in CS processing. Lesioned rats also showed more general impairments in conditioned responding. These results indicate that, although the neural systems for increasing and decreasing attentional processing may be largely independent, combined loss of hippocampal and neocortical cholinergic input may produce behavioral impairments that are not apparent after either lesion alone. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Why there are complementary learning systems in the hippocampus and neocortex: Insights from the successes and failures of connectionist models of learning and memory.

Damage to the hippocampal system disrupts recent memory but leaves remote memory intact. The account presented here suggests that memories are first stored via synaptic changes in the hippocampal system, that these changes support reinstatement of recent memories in the neocortex, that neocortical synapses change a little on each reinstatement, and that remote memory is based on accumulated neocortical changes. Models that learn via changes to connections help explain this organization. These models discover the structure in ensembles of items if learning of each item is gradual and interleaved with learning about other items. This suggests that the neocortex learns slowly to discover the structure in ensembles of experiences. The hippocampal system permits rapid learning of new items without disrupting this structure, and reinstatement of new memories interleaves them with others to integrate them into structured neocortical memory systems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Imaging of hippocampal and neocortical neural activity following intravenous cocaine administration in freely behaving cats

We examined spatial-temporal patterns of neural activity, as inferred from 700 nm light reflectance, from the dorsal hippocampus and surrounding neocortex in seven freely behaving cats following 1.5, 2.5, 3.5 and 5.0 mg/kg intravenous cocaine administration. Images were acquired using a new technique which gathered reflected light from cortical and subcortical structures. Cardiac and respiratory patterning, collected simultaneously with optical images, revealed increased rates and diminished variation after intravenous cocaine administration. Cocaine increased reflectance correlates of hippocampal neural activity in a dose-dependent fashion over a 120 min period, with a lengthening time-to-peak effect (22-76 min). The largest dose resulted in an initial decrease, followed by the greatest enhancement in neuronal activity. Correlates of neural activation in the neocortex displayed an inverse dose-response curve to that found in the hippocampus; the time-to-peak effect was shorter (6-43 min) and the maximal change was reduced. Regional patches and bands of activation occurred during the period of the cocaine response, and were more pronounced in the hippocampus than the neocortex. Procaine, administered in a similar dose, slightly increased neural activity for 10 min in both the hippocampus and neocortex, and elicited a small increase in respiration. Cocaine induces a pronounced enhancement of neural activation in the neocortex and dorsal hippocampus; the time course of activation in the hippocampus parallels an increased respiratory pattern and outlasts the neocortical response. We speculate that hippocampal activation may be related to the profound respiratory acceleration found in response to cocaine.     

Intraventricular anti-cholinergics do not block cholinergic hippocampal RSA or neocortical desynchronization in the rabbit or rat

Electroencephalographic (EEG) electrodes and ventricular cannulae were implanted in 8 rabbits and 12 rats. Two anti-cholinergic agents, atropine sulfate and scopolamine hydrobromide, were given systemically (1-50 mg/kg) and intraventricularly (5-800 mug). Systemic but not intraventricular injections blocked sensory stimulation-induced or eserine-induced neocortical desynchronization and hippocampal RSA in rats and rabbits which were immobile and either undrugged or ethanol intoxicated. Systemic injections also blocked hippocampal RSA but not neocortical desynchronization in rats given sensory stimulation under urethane anaesthesia, while intraventricular injections only reduced RSA amplitude. Neither systemic nor intraventricular injections blocked neocortical desynchronization or hippocampal RSA recorded from animals when they walked in a motor driven wheel. These experiments support the hypothesis that there are two types of neocortical desynchronization and hippocampal RSA, one cholinergic and one non-cholinergic. They also suggest that atropine and scopolamine pass more readily to the neural system responsible for cholinergic EEG activity from the capillary bed than from the ventricular fluid.     

Hippocampal contributions to cortical plasticity

The hippocampal complex and neocortex are both integrally important in memory function, in particular as regards memory for episodes and knowledge about the world that is derived from them. It is traditionally assumed that the role of the hippocampus is time-limited, after which retrieval of episodic memory depends only upon neocortical stores. A number of lines of evidence indicate that this traditional view is incorrect. We propose that the hippocampal complex is always necessary for retrieval of episodes and their contextual frame, and that hippocampal-neocortical interactions contribute instead to the extraction of semantic information to be stored in the neocortex.     

Spike-wave complexes and fast components of cortically generated seizures. IV. Paroxysmal fast runs in cortical and thalamic neurons

In the preceding papers of this series, we have analyzed the cellular patterns and synchronization of neocortical seizures occurring spontaneously or induced by electrical stimulation or cortical infusion of bicuculline under a variety of experimental conditions, including natural states of vigilance in behaving animals and acute preparations under different anesthetics. The seizures consisted of two distinct components: spike-wave (SW) or polyspike-wave (PSW) at 2-3 Hz and fast runs at 10-15 Hz. Because the thalamus is an input source and target of cortical neurons, we investigated here the seizure behavior of thalamic reticular (RE) and thalamocortical (TC) neurons, two major cellular classes that have often been implicated in the generation of paroxysmal episodes. We performed single and dual simultaneous intracellular recordings, in conjunction with multisite field potential and extracellular unit recordings, from neocortical areas and RE and/or dorsal thalamic nuclei under ketamine-xylazine and barbiturate anesthesia. Both components of seizures were analyzed, but emphasis was placed on the fast runs because of their recent investigation at the cellular level. 1) The fast runs occurred at slightly different frequencies and, therefore, were asynchronous in various cortical neuronal pools. Consequently, dorsal thalamic nuclei, although receiving convergent inputs from different neocortical areas involved in seizure, did not express strongly synchronized fast runs. 2) Both RE and TC cells were hyperpolarized during seizure episodes with SW/PSW complexes and relatively depolarized during the fast runs. As known, hyperpolarization of thalamic neurons deinactivates a low-threshold conductance that generates high-frequency spike bursts. Accordingly, RE neurons discharged prolonged high-frequency spike bursts in close time relation with the spiky component of cortical SW/PSW complexes, whereas they fired single action potentials, spike doublets, or triplets during the fast runs. In TC cells, the cortical fast runs were reflected as excitatory postsynaptic potentials appearing after short latencies that were compatible with monosynaptic activation through corticothalamic pathways. 3) The above data suggested the cortical origin of these seizures. To further test this hypothesis, we performed experiments on completely isolated cortical slabs from suprasylvian areas 5 or 7 and demonstrated that electrical stimulation within the slab induces seizures with fast runs and SW/PSW complexes, virtually identical to those elicited in intact-brain animals. The conclusion of all papers in this series is that complex seizure patterns, resembling those described at the electroencephalogram level in different forms of clinical seizures with SW/PSW complexes and, particularly, in the Lennox-Gastaut syndrome of humans, are generated in neocortex. Thalamic neurons reflect cortical events as a function of membrane potential in RE/TC cells and degree of synchronization in cortical neuronal networks.     

A comparative morphological study of homo- and heterotopic neural transplants

A comparative study of the development of neocortex embryonal anlage was carried out using light and electron microscopy in 15 days Wistar-line rat embryo after its transplantation into the brain and disturbed sciatic nerve of mature rats. Homotopic transplants contained twice more neurons than heterotopic ones 30 days later. Unlike to intracerebral transplants, in grafts developing in the nerve, there forms a multilayer lining from ependymocytes on the border with recipient tissue. Microenvironment was suggested to influence the realization of the transferred cells precursors histoblastic properties.     

Concentration of mercury in hair of indigenous mothers and infants from the Amazon basin

HSV-1 mutants in the RL-1 gene encoding the ICP34.5 protein have been demonstrated to have diminished neurovirulence in brain yet replicate as efficiently as parental virus in transformed tissue culture cells. Thus they have been proposed as candidates viruses for human brain tumor therapies. Evaluation of their replicative properties and pathogenesis within the nervous system has been limited. As most patients undergoing therapies for brain tumors are likely to be immunocompromised, it will be important to understand the pathogenesis of these viruses in immunocompromised hosts. To this end, the lateral ventricle of nude mice was injected with high (2.5 x 10(7) PFU), medium (10(5) PFU), or low dose (10(3) PFU) HSV-1 variant-1716, which has a deletion in the RL-1 gene. Ten of 10 mice died within 2-3 days following the high titer infection. Six of 19 animals with medium titer infection died within 9 days, and viral antigens were seen in ependymal cells as well as neurons within the brainstem and thalamus. Although only two of 19 animals became moribund 18 days after medium titer viral infection, many neocortical and hippocampal neurons were positive for HSV-1 antigens. However, plaque-purified viral isolates recovered from brain homogenates of these animals demonstrated no increase in pathogenicity. Nine of 20 animals died following low dose infection; six of these animals, from which tissue was analyzed, all had many HSV antigen-positive neurons in the neocortex and hippocampus. These data imply that if this type of virus is used for human brain tumor therapy immunosuppressed patients may suffer from significant viral pathogenesis outside the tumor.     

Site accessibility and the pH dependence of the saturation capacity of a highly cross-linked matrix. Immobilized metal affinity chromatography of bovine serum albumin on chelating Superose

Striatal and cortical neurons containing nitric oxide synthase (NOS) were studied in adult rats subjected to different periods of perinatal asphyxia (PA) using immunohistochemistry at both light microscopy (LM) and electron microscopy (EM). Another group was subjected to PA + hypothermia to study its neuroprotective effect. Quantitative image analysis was performed on the striatum and neocortex in order to count the number of immunoreactive neurons and to compare the pattern of staining between the different groups. Six-month-old rats that suffered subsevere and severe PA demonstrated, at LM, cytomegaly of the striatal and neocortical neurons containing NOS. Control and hypothermic neurons were more weakly immunostained than PA neurons. Subsevere and severe asphyctic rats showed an important neuronal loss that was reduced by hypothermic treatment. The PA group disclosed, at EM, dense electronic bodies distributed in terminals surrounding synaptic vesicles and in dendrites. Non-NOS-containing neurons showed signs of degeneration, such as dark cytoplasm and shrunken nuclei. Surrounding the blood vessels, we observed a clear edema. The immunolabeling in hypothermic rats resembled that observed in controls. These data suggest that subsevere and severe PA induces chronic changes in the neuronal content of NOS in the striatum and neocortex. Degeneration observed in neurons surrounding cytomegalic NOS-containing cells may be due to the excess of NO in their environment. Moreover, the chronic alterations produced by PA seem to be prevented by hypothermia.     

Effects of small concentrations of volatile anesthetics on action potential firing of neocortical neurons in vitro

BACKGROUND: Volatile general anesthetics depress neuronal activity in the mammalian central nervous system and enhance inhibitory Cl- currents flowing across the gamma-aminobutyric acid A (GABA(A)) receptor-ion channel complex. The extent to which an increase in GABA(A)-mediated synaptic inhibition contributes to the decrease in neuronal firing must be determined, because many further effects of these agents have been reported on the molecular level. METHODS: The actions of halothane, isoflurane, and enflurane on the firing patterns of single neurons were investigated by extracellular recordings in organotypic slice cultures derived from the rat neocortex. RESULTS: Volatile anesthetics depressed spontaneous action potential firing of neocortical neurons in a concentration-dependent manner. The estimated median effective concentration (EC50) values were about one half the EC50 values for general anesthesia. In the presence of the GABA(A) antagonist bicuculline (20 microM), the effectiveness of halothane, isoflurane, and enflurane in reducing the discharge rates were diminished by 48-65%, indicating that these drugs act via the GABA(A) receptor. CONCLUSIONS: Together with recent investigations, our results provide evidence that halothane, isoflurane, and enflurane reduced spontaneous action potential firing of neocortical neurons in cultured brain slices mainly by increasing GABA(A)-mediated synaptic inhibition. At concentrations, approximately one half the EC50 for general anesthesia, volatile anesthetics increased overall GABA(A)-mediated synaptic inhibition about twofold, thus decreasing spontaneous action potential firing by half.     

Green fluorescent protein expressed by a recombinant vaccinia virus permits early detection of infected cells by flow cytometry

One of the most prominent effects of Alzheimer disease is the disruption of finely tuned neuronal circuitry of discrete brain regions associated with learning and memory. Results from the present study support a role for the intrinsic inhibitory component of neuronal circuitry in determining the magnitude of beta-amyloid peptide induced cell death in the highly vulnerable pyramidal neurons of the hippocampus. Previous efforts have mostly focused on direct effects on excitatory neurons. By contrast, less emphasis has been placed on addressing a role for the intrinsic inhibitory component of cell-cell interactions of neuronal networks in response to Abeta. The present study provides evidence demonstrating that blockage of the intrinsic inhibitory component between Abeta exposed neurons leads to destabilization of calcium homeostasis and exacerbated neuronal death compared to Abeta treated cultures. Neuronal electrical activity was first silenced by exposing cultures to tetrodotoxin (TTX; 100 nM) plus Abeta, followed by survival counts. Cell death, unexpectedly, did not significantly differ from Abeta-exposed neurons. The intrinsic inhibition in Abeta-exposed cultures was then pharmacologically removed with picrotoxin (40 microM) or bicuculline (25 microM) resulting in significantly greater death than Abeta-exposed neurons alone. From these observations, it is proposed that intrinsic functional inhibition in hippocampal circuits can reduce adverse effects of Abeta on the excitatory component. By considering not just the excitatory component of electrical activity, but the intrinsic balance between excitation and inhibition, new strategies for the treatment of Alzheimer disease may emerge.     

Coordinated interactions between hippocampal ripples and cortical spindles during slow-wave sleep

Sleep is characterized by a structured combination of neuronal oscillations. In the hippocampus, slow-wave sleep (SWS) is marked by high-frequency network oscillations (approximately 200 Hz "ripples"), whereas neocortical SWS activity is organized into low-frequency delta (1-4 Hz) and spindle (7-14 Hz) oscillations. While these types of hippocampal and cortical oscillations have been studied extensively in isolation, the relationships between them remain unknown. Here, we demonstrate the existence of temporal correlations between hippocampal ripples and cortical spindles that are also reflected in the correlated activity of single neurons within these brain structures. Spindle-ripple episodes may thus constitute an important mechanism of cortico-hippocampal communication during sleep. This coactivation of hippocampal and neocortical pathways may be important for the process of memory consolidation, during which memories are gradually translated from short-term hippocampal to longer-term neocortical stores.     

Thapsigargin inhibits bicuculline-induced epileptiform excitability in rat hippocampal slices

Evoked field potentials were recorded in the CA3 region of rat hippocampal slices to detect whether intracellular Ca2+ stores are involved in the epileptiform effects of the two prototypic GABA(A) antagonists, bicuculline methiodide (BMI) and gabazine (SR-95531; GBZ). Field population spikes gradually increased and became repetitive (epileptiform bursting) in the presence of either BMI (5 microM), or GBZ (5 microM). Thapsigargin (2 microM), a depletor of intracellular Ca2+ stores, reduced the epileptiform effect of BMI, but had no significant effect on the GBZ-induced hyperexcitability. These data suggest that Ca2+ release from intracellular stores participates in the epileptiform response of hippocampal CA3 neurons to BMI, but not in the response to GBZ.     

Changes in the mRNAs encoding subtypes I, II and III sodium channel alpha subunits following kainate-induced seizures in rat brain

Several lines of evidence underscore a possible role of voltage-gated Na+ channels (NaCH) in epilepsy. We compared the regional distribution of mRNAs coding for Na+ channel alpha subunit I, II and III in brains from control and kainate-treated rats using non-radioactive in situ hybridization with subtype-specific digoxigenin-labelled cRNA probes. Labelling intensity was evaluated by a densitometric analysis of digitized images. Heterogeneous distribution of the three Na+ channel mRNAs was demonstrated in brain from adult control rats, which confirmed previous studies. Subtype II mRNAs were shown to be abundant in cerebellum and hippocampus. Subtype I mRNAs were also detected in these areas. Subtype III mRNAs were absent in cerebellar cortex, but significantly expressed in neurons of the medulla oblongata and hippocampus. The three subtypes were differentially distributed in neocortical layers. Subtype II mRNAs were present in all of the layers, but mRNAs for subtypes I and III were concentrated in pyramidal cells of neocortex layers IV-V. During kainate-induced seizures, we observed an increase in Na+ channel II and III mRNA levels in hippocampus. In dentate gyrus, subtype III mRNAs increased 3 h after KA administration to a maximum at 6 h. At this latter time, a lower increase in NaCh III mRNAs was also recorded in areas CA1 and CA3. NaCh III overexpression in dentate gyrus persisted for at least 24 h. In the same area, NaCh II mRNAs were also increased with a peak 3 h after KA injection and a return to control levels by 24 h. No changes in NaCh I mRNAs were seen. The KA-induced up-regulation in NaCh mRNAs probably resulted in an increase in hippocampal neuronal excitability.