Postnatal development of electrogenic sodium pump activity in rat hippocampal pyramidal neurons

Small animal PET with spontaneous inhalation of 15O-labelled oxygen gases: Longitudinal assessment of cerebral oxygen metabolism in a rat model of neonatal hypoxic-ischaemic encephalopathy

Perinatal hypoxic-ischaemic encephalopathy (HIE) is the leading cause of irreversible brain damage resulting in serious neurological dysfunction among neonates. We evaluated the feasibility of positron emission tomography (PET) methodology with 15O-labelled gases without intravenous or tracheal cannulation for assessing temporal changes in cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) in a neonatal HIE rat model. Sequential PET scans with spontaneous inhalation of 15O-gases mixed with isoflurane were performed over 14 days after the hypoxic-ischaemic insult in HIE pups and age-matched controls. CBF and CMRO2 in the injured hemispheres of HIE pups remarkably decreased 2 days after the insult, gradually recovering over 14 days in line with their increase found in healthy controls according to their natural maturation process. The magnitude of hemispheric tissue loss histologically measured after the last PET scan was significantly correlated with the decreases in CBF and CMRO2.This fully non-invasive imaging strategy may be useful for monitoring damage progression in neonatal HIE and for evaluating potential therapeutic outcomes.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Development and regulation of chloride homeostasis in the central nervous system

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter of the mature central nervous system (CNS). The developmental switch of GABAergic transmission from excitation to inhibition is induced by changes in Cl⁻ gradients, which are generated by cation-Cl⁻ co-transporters. An accumulation of Cl⁻ by the Na⁺-K⁺-2Cl⁻ co-transporter (NKCC1) increases the intracellular Cl⁻ concentration ([Cl⁻]_i) such that GABA depolarizes neuronal precursors and immature neurons. The subsequent ontogenetic switch, i.e., upregulation of the Cl⁻-extruder KCC2, which is a neuron-specific K⁺-Cl⁻ co-transporter, with or without downregulation of NKCC1, results in low [Cl⁻]_i levels and the hyperpolarizing action of GABA in mature neurons. Development of Cl⁻ homeostasis depends on developmental changes in NKCC1 and KCC2 expression. Generally, developmental shifts (decreases) in [Cl⁻]_i parallel the maturation of the nervous system, e.g., early in the spinal cord, hypothalamus and thalamus, followed by the limbic system, and last in the neocortex. There are several regulators of KCC2 and/or NKCC1 expression, including brain-derived neurotrophic factor (BDNF), insulin-like growth factor (IGF), and cystic fibrosis transmembrane conductance regulator (CFTR). Therefore, regionally different expression of these regulators may also contribute to the regional developmental shifts of Cl⁻ homeostasis. KCC2 and NKCC1 functions are also regulated by phosphorylation by enzymes such as PKC, Src-family tyrosine kinases, and WNK1–4 and their downstream effectors STE20/SPS1-related proline/alanine-rich kinase (SPAK)-oxidative stress responsive kinase-1 (OSR1). In addition, activation of these kinases is modulated by humoral factors such as estrogen and taurine. Because these transporters use the electrochemical driving force of Na⁺ and K⁺ ions, topographical interaction with the Na⁺-K⁺ ATPase and its modulators such as creatine kinase (CK) should modulate functions of Cl⁻ transporters. Therefore, regional developmental regulation of these regulators and modulators of Cl⁻ transporters may also play a pivotal role in the development of Cl⁻ homeostasis.

A sodium-pump-mediated afterhyperpolarization in pyramidal neurons

The sodium-potassium ATPase (i.e., the "sodium pump") plays a central role in maintaining ionic homeostasis in all cells. Although the sodium pump is intrinsically electrogenic and responsive to dynamic changes in intracellular sodium concentration, its role in regulating neuronal excitability remains unclear. Here we describe a physiological role for the sodium pump in regulating the excitability of mouse neocortical layer 5 and hippocampal CA1 pyramidal neurons. Trains of action potentials produced long-lasting (∼20 s) afterhyperpolarizations (AHPs) that were insensitive to blockade of voltage-gated calcium channels or chelation of intracellular calcium, but were blocked by tetrodotoxin, ouabain, or the removal of extracellular potassium. Correspondingly, the AHP time course was similar to the decay of activity-induced increases in intracellular sodium, whereas intracellular calcium decayed at much faster rates. To determine whether physiological patterns of activity engage the sodium pump, we replayed in vitro a place-specific burst of 15 action potentials recorded originally in vivo in a CA1 "place cell" as the animal traversed the associated place field. In both layer 5 and CA1 pyramidal neurons, this "place cell train" generated small, long-lasting AHPs capable of reducing neuronal excitability for many seconds. Place-cell-train-induced AHPs were blocked by ouabain or removal of extracellular potassium, but not by intracellular calcium chelation. Finally, we found calcium contributions to the AHP to be temperature dependent: prominent at room temperature, but largely absent at 35°C. Our results demonstrate a previously unappreciated role for the sodium-potassium ATPase in regulating the excitability of neocortical and hippocampal pyramidal neurons.

Alterations of sodium and potassium channels of RGCs in RCS rat with the development of retinal degeneration

All know that retinitis pigmentosa (RP) is a group of hereditary retinal degenerative diseases characterized by progressive dysfunction of photoreceptors and associated with progressive cells loss; nevertheless, little is known about how rods and cones loss affects the surviving inner retinal neurons and networks. Retinal ganglion cells (RGCs) process and convey visual information from retina to visual centers in the brain. The healthy various ion channels determine the normal reception and projection of visual signals from RGCs. Previous work on the Royal College of Surgeons (RCS) rat, as a kind of classical RP animal model, indicated that, at late stages of retinal degeneration in RCS rat, RGCs were also morphologically and functionally affected. Here, retrograde labeling for RGCs with Fluorogold was performed to investigate the distribution, density, and morphological changes of RGCs during retinal degeneration. Then, patch clamp recording, western blot, and immunofluorescence staining were performed to study the channels of sodium and potassium properties of RGCs, so as to explore the molecular and proteinic basis for understanding the alterations of RGCs membrane properties and firing functions. We found that the resting membrane potential, input resistance, and capacitance of RGCs changed significantly at the late stage of retinal degeneration. Action potential could not be evoked in a part of RGCs. Inward sodium current and outward potassium current recording showed that sodium current was impaired severely but only slightly in potassium current. Expressions of sodium channel protein were impaired dramatically at the late stage of retinal degeneration. The results suggested that the density of RGCs decreased, process ramification impaired, and sodium ion channel proteins destructed, which led to the impairment of electrophysiological functions of RGCs and eventually resulted in the loss of visual function.

The role of central nervous system development in late-onset neurodegenerative disorders

The human brain is dependent upon successfully maintaining ionic, energetic and redox homeostasis within exceptionally narrow margins for proper function. The ability of neurons to adapt to genetic and environmental perturbations and evoke a 'new normal' can be most fully appreciated in the context of neurological disorders in which clinical impairments do not manifest until late in life, although dysfunctional proteins are expressed early in development. We now know that proteins controlling ATP generation, mitochondrial stability, and the redox environment are associated with neurological disorders such as Parkinson's disease and amyotrophic lateral sclerosis. Generally, focus is placed on the role that early or long-term environmental stress has in altering the survival of cells targeted by genetic dysfunctions; however, the central nervous system undergoes several periods of intense stress during normal maturation. One of the most profound periods of stress occurs when 50% of neurons are removed via programmed cell death. Unfortunately, we have virtually no understanding of how these events proceed in individuals who harbor mutations that are lethal later in life. Moreover, there is a profound lack of information on circuit formation, cell fate during development and neurochemical compensation in either humans or the animals used to model neurodegenerative diseases. In this review, we consider the current knowledge of how energetic and oxidative stress signaling differs between neurons in early versus late stages of life, the influence of a new group of proteins that can integrate cell stress signals at the mitochondrial level, and the growing body of evidence that suggests early development should be considered a critical period for the genesis of chronic neurodegenerative diseases.

Differential effects of Na+-K+ ATPase blockade on cortical layer V neurons

Sodium-potassium ATPase ('Na(+)-K(+) ATPase') contributes to the maintenance of the resting membrane potential and the transmembrane gradients for Na(+) and K(+) in neurons. Activation of Na(+)-K(+) ATPase may be important in controlling increases in intracellular sodium during periods of increased neuronal activity. Down-regulation of Na(+)-K(+) ATPase activity is implicated in numerous CNS disorders, including epilepsy. Although Na(+)-K(+) ATPase is present in all neurons, little is known about its activity in different subclasses of neocortical cells. We assessed the physiological properties of Na(+)-K(+) ATPase in fast-spiking (FS) interneurons and pyramidal (PYR) cells to test the hypothesis that Na(+)-K(+) ATPase activity would be relatively greater in neurons that generated high frequency action potentials (the FS cells). Whole-cell patch clamp recordings were made from FS and PYR neurons in layer V of rat sensorimotor cortical slices maintained in vitro using standard techniques. Bath perfusion of Na(+)-K(+) ATPase antagonists (ouabain or dihydro-ouabain) induced either a membrane depolarization in current clamp, or inward current under voltage clamp in both cell types. PYR neurons were divided into two subpopulations based on the amplitude of the voltage or current shift in response to Na(+)-K(+) ATPase blockade. The two PYR cell groups did not differ significantly in electrophysiological properties including resting membrane potential, firing pattern, input resistance and capacitance. Membrane voltage responses of FS cells to Na(+)-K(+) ATPase blockade were intermediate between the two PYR cell groups (P < 0.05). The resting Na(+)-K(+) ATPase current density in FS interneurons, assessed by application of blockers, was 3- to 7-fold larger than in either group of PYR neurons. Na(+)-K(+) ATPase activity was increased either through direct Na(+) loading via the patch pipette or by focal application of glutamate (20 mM puffs). Under these conditions FS interneurons exhibited the largest increase in Na(+)-K(+) ATPase activity. We conclude that resting Na(+)-K(+) ATPase activity and sensitivity to changes in internal Na(+) concentration vary between and within classes of cortical neurons. These differences may have important consequences in pathophysiological disorders associated with down-regulation of Na(+)-K(+) ATPase and hyperexcitability within cortical networks.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Temporal and topographic alterations in expression of the alpha3 isoform of Na+, K(+)-ATPase in the rat freeze lesion model of microgyria and epileptogenesis

Na(+),K(+)-ATPase contributes to the asymmetrical distribution of sodium and potassium ions across the plasma membrane and to maintenance of the membrane potential in many types of cells. Alterations in this protein may play a significant role in many human neurological disorders, including epilepsy. We studied expression of the alpha3 isoform of Na(+),K(+)-ATPase in the freeze lesion (FL) microgyrus model of developmental epileptogenesis to test the hypothesis that it is downregulated following neonatal cortical injury. FL and sham-operated rat brains were examined at postnatal day (P)7, P10, P14, P21-28 and P50-60 after placement of a transcranial freeze lesion at P0 or P1. Immunohistochemistry and in situ hybridization were used to assess the expression of the alpha3 isoform of Na(+),K(+)-ATPase (termed alpha3, or alpha3 subunit below) in neuropil and the perisomatic areas of pyramidal cells and parvalbumin-containing interneurons. There was a significant decrease (P<0.05) in alpha3 subunit immunoreactivity (IR) in the neuropil of FL cortical layer V of the P14 and P21-28 groups that extended up to 360 mum from the border of the microgyrus, an area that typically exhibits evoked epileptiform activity. Alpha-3 was decreased in the perisomatic area of pyramidal but not parvalbumin-containing cells in P21-28 FL animals. A reduction in alpha3 mRNA was observed in the neuropil of FL cortical layer V up to 1610 mum from the microgyral edge. The developmental time course for expression of the alpha3 subunit between P7 and P60 was examined in naive rat cortices and results showed that there was a significant increase in alpha3 IR between P7 and P10. The significant decreases in Na(+),K(+)-ATPase in the paramicrogyral cortex may contribute to epileptogenesis.

The neurobiology of epilepsy

Epilepsy is a complex disease with diverse clinical characteristics that preclude a singular mechanism. One way to gain insight into potential mechanisms is to reduce the features of epilepsy to its basic components: seizures, epileptogenesis, and the state of recurrent unprovoked seizures that defines epilepsy itself. A common way to explain seizures in a normal individual is that a disruption has occurred in the normal balance of excitation and inhibition. The fact that multiple mechanisms exist is not surprising given the varied ways the normal nervous system controls this balance. In contrast, understanding seizures in the brain of an individual with epilepsy is more difficult because seizures are typically superimposed on an altered nervous system. The different environment includes diverse changes, making mechanistic predictions a challenge. Understanding the mechanisms of seizures in an individual with epilepsy is also more complex than understanding the mechanisms of seizures in a normal individual because epilepsy is not necessarily a static condition but can continue to evolve over the lifespan. Using temporal lobe epilepsy as an example, it is clear that genes, developmental mechanisms, and neuronal plasticity play major roles in creating a state of underlying hyperexcitability. However, the critical control points for the emergence of chronic seizures in temporal lobe epilepsy, as well as their persistence, frequency, and severity, are questions that remain unresolved.

Rotenone potentiates NMDA currents in substantia nigra dopamine neurons

Rotenone is a pesticide that produces a rodent model of Parkinson's disease. Although much evidence suggests that oxidative stress mediates the toxicity of rotenone on dopamine neurons, rotenone can also potentiate glutamate excitotoxicity. We used whole-cell patch pipettes to investigate actions of rotenone on currents evoked by N-methyl-d-aspartate (NMDA) in dopamine neurons in slices of rat midbrain. After superfusing the slice for 20-30 min, rotenone (100 nM) caused a 162% increase in the average amplitude of inward current evoked by 30 microM NMDA. This effect of rotenone was mimicked by the sodium pump inhibitor strophanthidin (10 microM) and was abolished when pipettes contained an ATP regeneration solution. Although strophanthidin also significantly increased the amplitude of inward currents evoked by (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 10 microM), rotenone failed to potentiate AMPA currents. Because rotenone potentiated NMDA- but not AMPA-dependent currents, this suggests that rotenone acts selectively to augment NMDA receptor function. Furthermore, the failure of rotenone to mimic strophanthidin suggests that rotenone does not inhibit sodium pump activity. Our results suggest that an excitotoxic mechanism might contribute to rotenone neurotoxicity.

Postnatal development of a new type of epileptiform activity in the rat hippocampus

Long-term application of Cs(+) (5 mM) induces an epileptiform field potential (Cs-FP) in area CA1 of the rat hippocampus, which is independent of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors and gamma-aminobutyric acid (GABA)(A) receptors. To gain insight into possible mechanisms for the induction of the Cs-FP, we investigated the postnatal development of the response. In brain slices prepared from rats at different ages, the Cs-FP was evoked by stimulation of the Schaffer-collateral-commisural pathway. We found that expression of this potential was clearly dependent on the postnatal age. Thus, the Cs-FP was completely absent at 1 week of age. By 2 weeks, a reduced form of the response was observed, whereas slices taken from 3-week-old rats, displayed full Cs-FP, which were indistinguishable in size and shape from the adult form. In the presence of 4-aminopyridine, potentials resembling the Cs-FP were evoked. These potentials showed a similar age-dependency as the Cs-FP. The Na(+)/K(+) pump inhibitor dihydroouabain (DHO), when present during wash-in of Cs(+), gave a partial block of the Cs-FP in adult slices. This effect was not seen when DHO was applied after development of the Cs-FP. The data indicate that the processes necessary for expression of the Cs-PF are absent at birth and develop during the second postnatal week. We propose that the Cs-FP depends on Cs(+) entry into presynaptic neurons, and that the Na(+)/K(+) pump contributes to this transport of Cs(+). The observed age-dependency could therefore, in part, reflect the delayed development of the Na(+)/K(+) pump.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Actin-ATP hydrolysis is a major energy drain for neurons

In cultured chick ciliary neurons, when ATP synthesis is inhibited, ATP depletion is reduced approximately 50% by slowing actin filament turnover with jasplakinolide or latrunculin A. Jasplakinolide inhibits actin disassembly, and latrunculin A prevents actin assembly by sequestering actin monomers. Cytochalasin D, which allows assembly-disassembly, but only at pointed ends, is less effective in conserving ATP. Ouabain, an Na(+)-K(+)-ATPase inhibitor, and jasplakinolide both prevent approximately 50% of the ATP loss. When applied together, they completely prevent ATP loss over a period of 20 min, suggesting that filament stabilization reduces ATP consumption by decreasing actin-ATP hydrolysis directly rather than indirectly by modulating the activity of Na(+)-K(+)-ATPase, a major energy consumer.

Hypoglycemia enhances ionotropic but reduces metabotropic glutamate responses in substantia nigra dopaminergic neurons

It is widely accepted that energy deprivation causes a neuronal death that is mainly determined by an increase in the extracellular level of glutamate. Consequently an excessive membrane depolarization and a rise in the intracellular concentration of sodium and calcium are produced. In spite of this scenario, the function of excitatory and inhibitory amino acids during an episode of energy failure has not been studied yet at a cellular level. In a model of cerebral hypoglycemia in the rat substantia nigra pars compacta, we measured neuronal responses to excitatory amino acid agonists. Under single-electrode voltage-clamp mode at -60 mV, the application of the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, kainate, and the metabotropic group I agonist (S)-3,5-dihydroxyphenilglycine (DHPG) produced reversible inward currents in the dopaminergic cells. In addition, an outward current was caused by the superfusion of the metabotropic GABA(B) agonist baclofen. Glucose deprivation enhanced the inward responses caused by each ionotropic glutamate agonist. In contrast, hypoglycemia depressed the DHPG-induced inward current and the baclofen-induced outward current. These effects of hypoglycemia were reversible. To test whether a failure of the Na(+)/K(+) ATPase pump could account for the modification of the agonist-induced currents during hypoglycemia, we treated the midbrain slices with strophanthidin (1-3 microM). Strophanthidin enhanced the inward currents caused by glutamate agonists. However, it did not modify the GABA(B)-induced outward current. Our data suggest that glucose deprivation enhances the inward current caused by the stimulation of ionotropic glutamate receptors while it dampens the responses caused by the activation of metabotropic receptors. Thus a substantial component of the augmented neuronal response to glutamate, during energy deprivation, is very likely due to the failure of Na(+) and Ca(2+) extrusion and might ultimately favor excitotoxic processes in the dopaminergic cells.

Postnatal development of electrophysiological properties of nucleus accumbens neurons

We have studied the postnatal development of the physiological characteristics of nucleus accumbens (nAcb) neurons in slices from postnatal day 1 (P1) to P49 rats using the whole cell patch-clamp technique. The majority of neurons (102/108) were physiologically identified as medium spiny (MS) projection neurons, and only these were subjected to detailed analysis. The remaining neurons displayed characteristics suggesting that they were not MS neurons. Around the time of birth and during the first postnatal weeks, the membrane and firing characteristics of MS neurons were quite different from those observed later. These characteristics changed rapidly during the first 3 postnatal weeks, at which point they began to resemble those found in adults. Both whole cell membrane resistance and membrane time constant decreased more than fourfold during the period studied. The resting membrane potential (RMP) also changed significantly from an average of -50 mV around birth to less than -80 mV by the end of the third postnatal week. During the first postnatal week, the current-voltage relationship of all encountered MS neurons was linear over a wide range of membrane potentials above and below RMP. Through the second postnatal week, the proportion of neurons displaying inward rectification in the hyperpolarized range increased steadily and after P15, all recorded MS neurons displayed significant inward rectification. At all ages, inward rectification was blocked by extracellular cesium and tetra-ethyl ammonium and was not changed by 4-aminopyridine; this shows that inward rectification was mediated by the same currents in young and mature MS neurons. MS neurons fired single and repetitive Na(+)/K(+) action potentials as early as P1. Spike threshold and amplitude remained constant throughout development in contrast to spike duration, which decreased significantly over the same period. Depolarizing current pulses from rest showed that immature MS neurons fired action potentials more easily than their older counterparts. Taken together, the results from the present study suggest that young and adult nAcb MS neurons integrate excitatory synaptic inputs differently because of differences in their membrane and firing properties. These findings provide important insights into signal processing within nAcb during this critical period of development.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

In vivo electrical activity of brainstem neurons in fetal rats during asphyxia

To see changes in the activity and the sensitivity to glutamate of fetal brain neurons during asphyxia, the electrical activity of brainstem neurons was recorded extracellularly in fetal rats which were still connected with the dams by the intact umbilical cord. In urethan-anesthetized pregnant rats, fetal asphyxia (2-10 min) was induced by occluding the umbilical cord with a surgical clip, while reperfusion of the umbilical blood flow was performed by local application of a relaxant of blood vessels to the occlusion site. The spontaneous discharge of fetal brainstem neurons was suppressed for a long period of time by umbilical cord occlusion. The suppression of the firing occurred 48-150 (78+/-7) s after the start of umbilical cord occlusion, and lasted even after fetal cortical PO(2) recovered to control level after reperfusion. The changes occurred with a marked reduction in spike amplitude. A similar suppression was observed for the spikes induced by iontophoretic application of glutamate, although fetal brainstem neurons were extremely sensitive to glutamate before asphyxia. The suppression of the spontaneous spikes became more notable and longer when asphyxia was repeated. These findings suggest that the long-lasting suppression of fetal neurons during asphyxia may contribute to a reduction of cellular energy requirements in the fetal brain, thereby playing a role in the resistance of fetal neurons to brain damage caused by asphyxia. Furthermore, the reduced sensitivity of fetal neurons to glutamate during asphyxia may also contribute to prevent brain damage due to excitotoxity of glutamate.

Selective depolarization of interneurons in the early posttraumatic dentate gyrus: involvement of the Na(+)/K(+)-ATPase

Interneurons innervating dentate granule cells are potent regulators of the entorhino-hippocampal interplay. Traumatic brain injury, a leading cause of death and disability among young adults, is frequently associated with rapid neuropathological changes, seizures, and short-term memory deficits both in humans and experimental animals, indicating significant posttraumatic perturbations of hippocampal circuits. To determine the pathophysiological alterations that affect the posttraumatic functions of dentate neuronal networks within the important early (hours to days) posttraumatic period, whole cell patch-clamp recordings were performed from granule cells and interneurons situated in the granule cell layer of the dentate gyrus of head-injured and age-matched, sham-operated control rats. The data show that a single pressure wave-transient delivered to the neocortex of rats (mimicking moderate concussive head trauma) resulted in a characteristic ( approximately 10 mV), transient (<4 days), selective depolarizing shift in the resting membrane potential of dentate interneurons, but not in neighboring granule cells. The depolarization was not associated with significant changes in action potential characteristics or input resistance, and persisted in the presence of antagonists of ionotropic and metabotropic glutamate, and GABA(A) and muscarinic receptors, as well as blockers of voltage-dependent sodium channels and of the h-current. The differential action of the cardiac glycosides oubain and stophanthidin on interneurons from control versus head-injured rats indicated that the depolarization of interneurons was related to the trauma-induced decrease in the activity of the electrogenic Na(+)/K(+)-ATPase. In contrast, the Na(+)/K(+)-ATPase activity in granule cells did not change. Intracellular injection of Na(+), Ca(2+)-chelator and ATP, as well as ATP alone, abolished the difference between the resting membrane potentials of control and injured interneurons. The selective posttraumatic depolarization increased spontaneous firing in interneurons, enhanced the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in granule cells, and augmented the efficacy of depolarizing inputs to discharge interneurons. These results demonstrate that mechanical neurotrauma delivered to a remote site has highly selective effects on different cell types even within the same cell layer, and that the electrogenic Na(+)-pump plays a role in setting the excitability of hippocampal interneuronal networks after injury.

Membrane dysfunction induced by in vitro ischemia in immature rat hippocampal CA1 neurons

We investigated differences between immature and mature hippocampal neurons in their response to deprivation of oxygen and glucose (in vitro ischemia), using intracellular recording techniques from CA1 pyramidal neurons in rat brain slices. The membrane was more depolarized in immature hippocampal CA1 neurons (postnatal day 7, P7) compared with the adult neurons (P140), and the apparent input resistance in immature neurons was higher than that in adult neurons. In immature neurons, the threshold for action potential generation was high, and the peak amplitude of the action potential was low in comparison with adult neurons. A time-dependent inward rectification, at potentials negative than the resting potential, was prominent in neurons of P14 and P21. After P21, the resting membrane potential, the apparent input resistance, and the threshold and the peak amplitude of the action potential did not significantly change with increasing age. In adult neurons, application of ischemia-simulating medium caused irreversible changes in membrane potential consisting of an initial hyperpolarization followed by a slow depolarization and a rapid depolarization. Once the rapid depolarization occurred, reintroduction of oxygen and glucose failed to restore the membrane potential, a state referred to as irreversible membrane dysfunction. In neurons of ages P7 or P14, the initial hyperpolarization was not apparent, whereas a slow depolarization followed by a rapid depolarization was observed. With development of the neurons, the latency for onset of the rapid depolarization became shorter and its maximal slope increased. Moreover, neurons of ages P14 or P21 showed a partial or complete recovery after reintroduction of oxygen and glucose, unlike mature neurons. In summary, the present study has demonstrated that the initial hyperpolarization and rapid depolarization induced by in vitro ischemia is age dependent. The rapid depolarization is not readily produced in the neurons in age less than P21 during ischemic exposure.

A relative energy failure is associated with low-Mg2+ but not with 4-aminopyridine induced seizure-like events in entorhinal cortex

During seizure-like events (SLEs), intracellular Ca2+ concentration ([Ca2+]i) increases causing depolarization of the mitochondrial membrane and subsequent intramitochondrial accumulation of Ca2+. Mitochondrial depolarization results in an interruption of oxidative phosphorylation and increase in reactive oxygen species. Calcium activates enzymes of the citrate cycle. A characteristic feature of the low-Mg2+-induced SLEs is that they are transformed to a late activity refractory to anticonvulsant drugs, which may be regarded as a model system of difficult to treat status epilepticus. In contrast, 4-aminopyridine (4-AP)-induced activity rarely evolves to such late activity. The autofluorescence of NAD(P)H was used to monitor changes in cellular energy metabolism in the entorhinal cortex in two in vitro models of focal epilepsy. During repetitive 4-AP-induced SLEs there was a short decrease followed by a long-lasting overshoot of the NAD(P)H signal. This sequence remained unaltered during recurring SLEs. In contrast, during recurrent low-Mg2+-induced SLEs, the brief initial NADH signal reduction was unchanged but the following overshoot of NADH displayed a continuous decrease. This indicates a relative energy failure, which may contribute to the transformation to late activity in the low-Mg2+ model.

Phenytoin, phenobarbital, and midazolam fail to stop status epilepticus-like activity induced by low magnesium in rat entorhinal slices, but can prevent its development

OBJECTIVES

It was shown previously that low-Mg2+-induced epileptiform activity in rat entorhinal cortex slices changes with time from a pattern of serial seizure-like events (SLEs) to a state of continuously recurring epileptiform activity. Valproic acid blocked the early SLEs but not the late activity. It was proposed that the late activity is a model for pharmacoresistant status epilepticus since it was also refractory to phenytoin, carbamazepine, phenobarbital, and midazolam. In the present study, it is demonstrated that phenytoin (50 microM, n=6), phenobarbital (150 microM, n=7), and midazolam (50 microM, n=5) were able to block the early SLEs but not the late activity at the same concentrations. Carbamazepine (50 microM) reduced the duration of the SLEs from 21 +/-5 s to 4+/-3 s (P<0.01), the interictal interval from 123+/-27 s to 27+/-19 s (P<0.01), the SLE-associated rise of [K+]o from 7.7+/-0.5 mM to 5.7+/-0.8 mM (n=4, P<0.05), and the spread of the SLE between entorhinal cortex and neocortex from 4.0+/-0.6 s to 0.8+/-0.1 s (n=4, P<0.05). Lower concentrations of phenytoin (5 and 10 microM, n=5), carbamazepine (10 microM, n =6), and phenobarbital (50 microM, n = 4) had no effect. In conclusion, the hypothesis is supported that low-Mg2+-induced epileptiform activity in rat entorhinal cortex is an in vitro model for the transition from pharmacosensitive to pharmacoresistant status epilepticus.

Postnatal development of signal generation in auditory thalamic neurons

Using whole cell recording techniques, we distinguished immature from mature stages of development in auditory thalamic neurons of rats at ages P5 to P21. We compared voltage responses to injected currents and firing patterns of neurons in ventral partition of medial geniculate body (MGBv) in slices. Resting potential, input resistance and membrane time constant diminished to mature values between P5 and P14. Responses of young neurons to hyperpolarizing pulses showed delayed inward rectification; after P13, this was obscured by a rapid onset of another inward rectifier. All neurons possessed tetrodotoxin (TTX)-sensitive, depolarization-activated rectification, implying persistent Na+-current involvement. Despite a slightly higher voltage threshold for spiking, the current threshold was lower in younger neurons. Young neurons fired a short latency spike with afterhyperpolarization whereas older neurons exhibited a slow ramplike depolarization before tonic firing. Large currents caused continuous firing in all neurons. Before day P13, a high threshold Ca2+ spike (HTS) often was appended to action potentials. The low threshold Ca2+-spike (LTS) was too small in amplitude to evoke action potentials before P11 but produced a single spike at P12 and P13 and burst firing with HTS after P13. MGBv neurons have mature properties after P14, relevant for reactions to sound and the oscillations of slow-wave sleep.

In vivo whole-cell recording from neurons of the superior colliculus in fetal rats

In vivo whole-cell recording was made from neurons of the superior colliculus (SC) in rat fetuses which were connected with the dam by the umbilical cord. Fast action potentials could be generated by changing membrane potentials to depolarizing direction. The firing thresholds of fetal SC neurons appeared to be higher (between -50 and -35 mV) than those of brain neurons in later developmental stages. The action potentials of fetal SC neurons, which were smaller in amplitude and wider in duration as compared to mature brain neurons, revealed a small or no afterhyperpolarization. In addition to these fast action potentials, slow depolarizations of smaller amplitudes were evoked by intracellular injection of long depolarization pulses. Most neurons of the fetal SC (7/11) revealed linear current-voltage (I-V) relations, while the remaining neurons displayed marked rectification. In some fetal SC neurons, presumed EPSPs and IPSPs occurred spontaneously. These presumed postsynaptic potentials showed temporal summation. These results suggest that rat SC neurons are functionally active even before birth, though the membrane properties remain immature.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.

NMDA receptor-mediated differential laminar susceptibility to the intracellular Ca2+ accumulation induced by oxygen-glucose deprivation in rat neocortical slices

Slices of somatosensory cortex taken from immature rats on postnatal day (P)7-14 were labeled with fura-2. Intracellular Ca2+ concentration ([Ca2+]i) was monitored in identified pyramidal cells as the ratio of fluorescence intensities (RF340/F380) during oxygen-glucose deprivation. The RF340/F380 ([Ca2+]i) of individual pyramidal cells was monitored in each of the cortical layers II-VI simultaneously. Neurons in all neocortical layers exhibited significant increases in [Ca2+]i that varied with the duration of oxygen-glucose deprivation. Individual neurons responded to oxygen-glucose deprivation with abrupt increases in [Ca2+]i after various latencies. The ceiling level of the [Ca2+]i increase differed from cell to cell. Neurons in layer II/III showed significantly greater increases in [Ca2+]i than those in layers IV, V, or VI. Kynurenic acid, a nonselective glutamate receptor antagonist, and bicuculline, a selective gamma-aminobutyric acid (GABA)A receptor antagonist, suppressed the intracellular Ca2+ accumulation induced by oxygen-glucose deprivation in all neocortical layers examined. After kynurenic acid, but not after bicuculline, there was no longer a differential [Ca2+]i increases in layer II/III. Both 2-amino-5-phosphonopentanoic acid (AP5), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist, strongly suppressed the intracellular Ca2+ accumulation induced by oxygen-glucose deprivation in all layers. The laminar difference in terms of the [Ca2+]i increases was abolished by AP5, but not by CNQX. These results indicate that layer II/III cells are the most prone to oxygen-glucose deprivation-induced intracellular Ca2+ accumulation, and that this is primarily mediated by NMDA receptors. Thus, layer II/III neurons would be more likely to suffer cellular Ca2+ overload and excitotoxicity during ischemia than layer IV-VI cells. Such a differential laminar vulnerability might play an important role in determining the pathological characteristics of the immature cortex and its sequelae later in life.

Digoxin prevents MPTP-induced dopamine depletion in mouse striatum

Inhibition of Na+/K+ ATPase by cardiac glycosides has been shown to potentiate toxic effects of excitatory amino acids and mitochondrial poisons in neurons in vitro. The present study tested the hypothesis that the systemic administration of the cardiac glycoside, digoxin, potentiates effects of the dopamine neurotoxin 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) in vivo. Mice were injected with digoxin (1 mg/kg) or vehicle followed by MPTP (20 mg/kg) or saline 1 h later. After 1 or 8 days, mice were euthanized and dopamine levels in the striatum were measured by high-performance liquid chromatography with electrochemical detection. MPTP caused a significant 35-45% reduction in striatal dopamine levels compared to those in control mice. However, pretreatment with digoxin completely prevented the MPTP-induced dopamine depletion. This result was unexpected and suggests that cardiac glycosides may protect against MPTP neurotoxicity.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Postnatal development of ionic currents in rat hippocampal astrocytes in situ

Developmental changes in ion channel expression and cell morphology were studied in glial cells with the use of whole cell patch-clamp recordings in rat [postnatal day (P)5-P50] hippocampal slices. Recordings were obtained from 234 cells, presumed to be glia, in stratum radiatum and stratum lacunosum-moleculare of the CA1 region. Of 66 recorded cells filled with Lucifer yellow, 48 stained positive for glial fibrillary acidic protein, which identified them as astrocytes. All glial cells studied were of a stellate morphology, and developmental changes primarily comprised an increase in the length and number of cell processes associated with an overall increase in cell size and membrane capacitance. Two distinct outward potassium currents could be identified: a transient 4-aminopyridine-sensitive current (Ia) and a persistent outward current sensitive to tetraethylammonium (Id). Ia activated at -40 mV, and steady-state activation and inactivation midpoints were -16 and -74 mV, respectively. Decay time constants ranged from 7 ms at -30 mV to 19 ms at +80 mV. Id activated at -30 mV. A third K+ current sensitive to cesium activated with hyperpolarizing command voltages and showed strong inward rectification. Transient, voltage-activated sodium currents (I(Na)) were tetrodotoxin sensitive (100 nM) and activated at about -40 mV, peaked at about -10 mV, and reversed at +63 mV. I(Na) was half-inactivated at -49 mV and half-activated at -19 mV. During the first 2 wk of postnatal development, the percentage of cells showing inwardly rectifying K+ current (Ir), Ia, and I(Na) increased significantly from 40% (at P5) to 90% (at P20-P50). By contrast, almost all cells independent of age expressed Id. Specific conductances for Ir (g(ir)) and Ia increased significantly between P5 and P20, concomitant with a decrease in input resistance. By contrast, specific conductance of the outwardly rectifying K+ current (g(d)) decreased threefold between P5 and P20. Specific Na+ conductance was always <1/4 of the total potassium conductance. These results indicate that CA1 hippocampal astrocytes are characterized by expression of voltage-activated Na+ channels and three types of K+ channels showing changes in their relative expression during early postnatal development: 1) the number of cells expressing Ia, Ir, and I(Na) increases significantly and 2) their specific conductance changes such that g(d), predominant at P5-P20, is gradually replaced by g(ir), the predominant conductance in adult astrocytes. Adult morphological and electrophysiological phenotypes are established at about P20. These data suggest that previous studies in which cultured or acutely isolated cells from immature or embryonic rats were used were not adequately reflecting the properties of hippocampal astrocytes in situ.

Maturation of neuronal form and function in a mouse thalamo-cortical circuit

Postnatal development of physiological properties underlying slow intrathalamic oscillations was studied by whole-cell recording from synaptically coupled neurons of the reticular nucleus (RTN) and ventral posterior nucleus (VPN) of mouse brain slices in vitro and compared with the morphological development of dye-injected cells. Between postnatal days 3 and 11 (P3-P11), progressive changes in RTN and VPN neurons included shortening of the membrane time constant, decreasing input resistance, and lowering of the resting membrane potential (RMP). Low-threshold Ca2+ spikes (LTS) were present from P3, but their capacity to sustain multispike bursts was limited before P11. Synaptic responses were evoked in RTN and VPN neurons by electrical stimulation of the internal capsule from P3. Younger RTN neurons responded with a single spike, but their capacity to fire bursts gradually improved as the RMP reached levels below the LTS activation potential. Concomitantly, as the reversal potential of the inhibitory postsynaptic potential in VPN neurons became more negative, its capacity to deinactivate the LTS increased, and rebound bursts that could maintain oscillations were produced; sustained oscillations became the typical response to internal capsule stimulation at P12. The functional maturation of the intrathalamic circuitry, particularly between P10 and P14, occurs in parallel with the morphological maturation (size, dendritic growth, and dendritic field structure) of individual RTN and VPN neurons, as studied by confocal microscopy. Maturation of RTN cells led that of VPN cells by 2-3 d. The appearance of intrathalamic oscillations is probably correlated with the appearance of slow-wave sleep in postnatal animals.

Inhibition of the electrogenic Na pump underlies delayed depolarization of cortical neurons after mechanical injury or glutamate

We previously characterized the electrophysiological response of cortical neurons to a brief sublethal stretch-injury using an in vitro model of traumatic brain injury. This model revealed that cortical neurons undergo a stretch-induced delayed depolarization (SIDD) of their resting membrane potential (RMP) which is approximately 10 mV in magnitude. SIDD is dependent on N-methyl-D-aspartate (NMDA) receptor activation, neuronal firing, and extracellular calcium for its induction but not its maintenance. SIDD was maximal 1 h after the insult and required incubation at 37 degrees C. The present study examined the mechanism mediating SIDD and its relation to glutamate receptor activation. The Na pump inhibitor ouabain was used to assess the contribution of the Na pump to the RMP of control and stretched neurons using whole cell patch-clamp techniques. The nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine and a polyethylene glycol conjugate of superoxide dismutase were used to assess whether NO or superoxide anion, respectively, were involved in the induction of SIDD. Neurons were exposed to exogenous glutamate in the absence of cell stretch to determine whether glutamate alone can mimic SIDD. We report that SIDD is mediated by Na pump inhibition and is likely to result from reduced energy levels since the RMP of neurons dialyzed with a pipette solution containing 5 mM ATP were identical to controls. NO, but not superoxide anion, also may contribute to SIDD. A 3-min exposure to 10 microM glutamate produced a SIDD-like depolarization also associated with Na pump inhibition. The results suggest that Na pump inhibition secondary to alterations in cellular energetics underlies SIDD. Na pump inhibition due to glutamate exposure may contribute to traumatic brain injury or neurodegenerative diseases linked to glutamate receptor activation.

Serotonin blocks different patterns of low Mg2+-induced epileptiform activity in rat entorhinal cortex, but not hippocampus

Low Mg2+-induced epileptiform activity in the entorhinal cortex is characterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as the transition between them were blocked by serotonin. In contrast, serotonin had little effect upon the epileptiform activity in areas CA3 and CA1 of the hippocampus. Both forms of epileptiform activity in the entorhinal cortex are sensitive to N-methyl-D-aspartate receptor antagonists and it is shown here that serotonin blocked both types of epileptiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials in deep layer entorhinal cortex cells. Serotonin also prolonged or even prevented the transition between the two types of epileptiform activity and we suggest that this may be through activation of the Na+/K+-ATPase. The resistance of epileptiform activity in CA1 and CA3 to serotonin was most likely related to the inability of serotonin to reduce Schaffer collateral-evoked excitatory postsynaptic potentials. Given the strong serotonergic inputs to both the hippocampus and entorhinal cortex, the differential sensitivity of the two regions to serotonin suggests functional differences. In addition since the late recurrent discharges in the entorhinal cortex are resistant to all clinically used anticonvulsants, serotonin may open new avenues for the development of novel anticonvulsant compounds.

Developmental changes in NMDA-induced cell swelling and its transition to necrosis measured with 1H magnetic resonance imaging, impedance and histology

The vulnerability of the rat brain to intracerebrally injected N-methyl-D-aspartate (NMDA) drastically changes with age. We evaluated the developmental changes in the early and late responses to NMDA using 1H magnetic resonance imaging (MRI), cortical impedance and histology. NMDA, injected in the striatum of rats at postnatal days (P) 4, 7, 10, 14 and 21, induced a significant age-dependent reduction in the apparent diffusion coefficient (ADC) of tissue water in the striatum and the cerebral cortex monitored 1 h later using diffusion-weighted MRI. The reduction in ADC amounted 65% at P4 with lower values thereafter and was about 30% at P21. NMDA similarly induced a reduction in the cortical extracellular space (by 50% at P7 and 10% at P16) as measured with impedance recordings. The progressive decrease in the effect of NMDA with brain development was also indicated by a decrease in the volume of tissue in which the changes in ADC occurred (50 mm3 at P4 and 8 mm3 at P21). The diffusion of extracellular tracer molecules Mn2+ or [3H]-(R)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) injected in the striatum and monitored with T1-weighted MRI and autoradiography respectively showed a similar age dependence with the diffusion volume being twofold larger in P7 than in P21 brain. Thus restriction in diffusion during brain development may contribute to the decrease in NMDA-induced injury with age. The volume of tissue necrosis and gliosis, measured with T2-weighted MRI and histology 5 days after NMDA injection, was similar to that outlined by the ADC reduction detected soon after the insult at P4, P7 and P21. However, at P10 and P14 only 50% of the tissue showing a hyperintense signal in DW images displayed necrosis and gliosis 5 days later. This study shows that during development the early response to NMDA in terms of cytotoxic cell swelling (indicated both with impedance recordings and diffusion-weighted MRI) decreases with age. In addition, with maturation only part of the brain tissue acutely affected by NMDA does proceed into necrosis and gliosis, indicating an increased capacity of cells in the developing rat brain to survive NMDA-induced cell swelling.

Developmental changes of K(+)-dependent para-nitrophenylphosphatase (Na(+)-K(+)-ATPase) distribution in the synaptic regions in the cerebral cortex of rats

Using Mayahara's method, the distribution of K(+)-dependent p-nitrophenylphosphatase activity was examined electron microscopically in the synaptic regions of the cerebral cortex of 10, 15 and 60-day-old Wistar rats. The enzyme achieved gradually its characteristic localization and uniform distribution. The main developmental changes were associated with the establishment of the postsynaptic density's activity. The controls with ouabain revealed activity only on the postsynaptic densities.

Properties of low Mg2+ induced epileptiform activity in rat hippocampal and entorhinal cortex slices during adolescence

Properties of low Mg2+ induced epileptiform activity were studied in isolated rat hippocampal slices or in combined slices containing the entorhinal cortex and hippocampus. Slices were prepared from rats which were 1, 2, 3 or more weeks of age. Field potentials and often also changes in [K+]0, [Ca2+]0 and [Mg2+]0 were recorded with appropriate ion selective microelectrodes. In isolated hippocampal and entorhinal cortex/hippocampal combined slices the latency to onset of epileptiform activity upon lowering of extracellular Mg2+ was shortest in the youngest age group and approached adult levels at about the fourth postnatal week. Washout kinetics of Mg2+ were fastest in slices from 1-week-old rats. The onset of low Mg2+ induced epileptiform activity occurred at higher Mg2+ levels in slices from young compared with those from adult animals. In isolated hippocampal slices the epileptiform discharges varied in appearance during development. Short discharges lasting for 40 to 80 ms were observed in hippocampal slices prepared from 1-week-old and adult animals. Seizure-like events (SLE's) characterized by slow negative potential shifts and characteristic elevations in [K +]0 and decreases in [Ca2+]0 lasting for up to 30 s were observed in a proportion of hippocampal slices prepared after the first, second and third postnatal week. In slices from week 2 and 3 seizure-like events often progressed into spreading depressions (SD's). In entorhinal cortex/hippocampal combined slices seizure-like events were observed in all age groups. The seizure-like events spread readily into dentate gyrus (DG), area CA3 and CA1 after week 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Epileptogenesis in immature neocortical slices induced by 4-aminopyridine

The laminar site of onset of 4-aminopyridine (4AP)-induced epileptiform discharges in immature neocortical brain slices was localized to layer V using conventional extracellular field electrodes, current source-density analysis (CSD), and subdivided slices. Intracellular patch-electrode recordings in immature layer V neurons confirmed that intrinsically bursting (IB) neurons were not present at the ages studied or with bath application of 50-200 microM 4AP. IB properties were not being masked in the younger animals by the patch electrodes because typical IB neurons in layer V were seen in older rats when the same intracellular techniques were used. These results demonstrate that epileptiform activity can be initiated in the absence of IB neurons, and suggest that other factors are responsible for the preferential onset in layer V.

Developmental changes in the number, size, and orientation of GFAP-positive cells in the CA1 region of rat hippocampus

Changes in extracellular potassium concentration as measured with ion-selective microelectrodes revealed abnormally large accumulations in the hippocampus during postnatal development. While rises in [K+]o during stimulation of the Schaffer collaterals were limited to about 12 mM in adult animals, identical stimulations elicited rises to levels as large as 18 mM in juveniles. Since astrocytes are believed to play an important role in K+ homeostasis, we studied the postnatal development of astrocytes in the CA1 region of rat hippocampus in four age groups using a polyclonal antibody against glial fibrillary acidic protein (GFAP). The main proliferation of GFAP-positive cells (GFAPpc) occurred in all laminae between postnatal days 8 and 16. The number of GFAP-positive astrocytes per unit area was reached in stratum lacunosum-moleculare and stratum oriens at about 2 weeks and in stratum radiatum at about 3 weeks of age. During further development--at the age of 24 days--the orientation of individual astrocytes in stratum radiatum became polar with an orientation almost perpendicular to stratum pyramidale. This was revealed by an analysis based on determination of the quotients between the angular orientation of the processes of single individual GFAP-positive cells. When the crossing points of all glial processes over vertical and horizontal grid lines were determined and respective quotients evaluated, the same development towards a perpendicular orientation of astrocytes was noted in stratum radiatum. The same approach revealed a transient orientation parallel to the fissure in stratum lacunosum-moleculare around day 24. Camera lucida drawings of GFAPpc in stratum radiatum revealed that astrocytes became larger during the first three postnatal weeks, followed by a reduction of various parameters (e.g., cell extension, branching pattern) until adulthood. The observed developmental changes of astroglial cells may contribute to the known delayed maturation of potassium regulation in rat hippocampus.

Postnatal development of cholinergic presynaptic inhibition in rat hippocampus

Muscarinic depression of field potential excitatory postsynaptic potentials (fEPSPs) in striatum radiatum of area CA1 was compared in hippocampal slices from rats of different ages. Bath application of 4 microM muscarine reversibly depressed the fEPSP slope by 68.4% in slices from adult animals (P43-P60), but caused only a 32.2% depression in slices from P5-P7 animals. The magnitude of the depression increased with age during the first postnatal month. Reduced sensitivity of excitatory synaptic transmission to cholinergic depression during postnatal development could be one factor contributing to the hyperexcitability of immature hippocampus.