Transporters in drug-refractory epilepsy: clinical significance

Drug resistance remains an unmet challenge in a variety of neurological disorders, but epilepsy is probably the refractory disease that has received most experimental, preclinical, and therapeutic attention. Although resective surgery continues to improve our ability to provide seizure relief, new discoveries have potential as alternative therapeutic approaches to multiple drug resistance. As discussed here, the field is replete with controversies and false starts, in particular as it concerns the existence of genetic predisposition to inadequate pharmacological seizure control.

Tobacco smoke: a critical etiological factor for vascular impairment at the blood-brain barrier

Active and passive tobacco smoke are associated with the dysfunction of endothelial physiology and vascular impairment. Studies correlating the effects of smoking and the brain microvasculature at the blood-brain barrier (BBB) level have been largely limited to few selective compounds that are present in the tobacco smoke (TS) yet the pathophysiology of smoking has not been unveiled. For this purpose, we characterized the physiological response of isolated human brain microvascular endothelial cells (HBMEC) and monocytes to the exposure of whole soluble TS extract. With the use of a well established humanized flow-based in vitro blood-brain barrier model (DIV-BBB) we have also investigated the BBB physiological response to TS under both normal and impaired hemodynamic conditions simulating ischemia. Our results showed that TS selectively decreased endothelial viability only at very high concentrations while not significantly affecting that of astrocytes and monocytes. At lower concentrations, despite the absence of cytotoxicity, TS induced a strong vascular pro-inflammatory response. This included the upregulation of endothelial pro-inflammatory genes, a significant increase of the levels of pro-inflammatory cytokines, activated matrix metalloproteinase, and the differentiation of monocytes into macrophages. When flow-cessation/reperfusion was paired with TS exposure, the inflammatory response and the loss of BBB viability were significantly increased in comparison to sham-smoke condition. In conclusion, TS is a strong vascular inflammatory primer that can facilitate the loss of BBB function and viability in pathological settings involving a local transient loss of cerebral blood flow such as during ischemic insults.

Combined effects of prenatal inhibition of vasculogenesis and neurogenesis on rat brain development

Malformations of cortical development (MCD) are one of the most common causes of neurological disabilities including autism and epilepsy. To disrupt cortical formation, methylazoxymethanol (MAM) or thalidomide (THAL) has been used to affect neurogenesis or vasculogenesis. Although previous models of MCD have been useful, these models primarily attack a single aspect of cortical development. We hypothesized that simultaneous prenatal exposure to MAM or THAL will lead to the development of a novel and specific type of brain maldevelopment. Rats were prenatally exposed to MAM and THAL. At early postnatal days, brains displayed abnormal ventricular size and hemispheric asymmetry due to altered brain water homeostasis. The postnatal brain was also characterized by gliosis in regions of focal leakage of the blood brain barrier. These morphological abnormalities gradually disappeared at adult stages. Although the adult MAM-THAL rats showed normal cortical morphology, abnormal hippocampal connectivity and mossy fiber sprouting persisted well into adulthood.

Prenatal exposure to thalidomide, altered vasculogenesis, and CNS malformations

Malformations of cortical development (MCD) result from abnormal neuronal positioning during corticogenesis. MCD are believed to be the morphological and perhaps physiological bases of several neurological diseases, spanning from mental retardation to autism and epilepsy. In view of the fact that during development, an appropriate blood supply is necessary to drive organogenesis in other organs, we hypothesized that vasculogenesis plays an important role in brain development and that E15 exposure in rats to the angiogenesis inhibitor thalidomide would cause postnatal MCD. Our results demonstrate that thalidomide inhibits angiogenesis in vitro at concentrations that result in significant morphological alterations in cortical and hippocampal regions of rats prenatally exposed to this vasculotoxin. Abnormal neuronal development was associated with vascular malformations and a leaky blood-brain barrier. Protein extravasation and uptake of fluorescent albumin by neurons, but not glia, was commonly associated with abnormal cortical development. Neuronal hyperexcitability was also a hallmark of these abnormal cortical regions. Our results suggest that prenatal vasculogenesis is required to support normal neuronal migration and maturation. Altering this process leads to failure of normal cerebrovascular development and may have a profound implication for CNS maturation.

Relationship between expression of multiple drug resistance proteins and p53 tumor suppressor gene proteins in human brain astrocytes

Multiple drug resistance occurs when cells fail to respond to chemotherapy. Although it has been established that the drug efflux protein P-glycoprotein protects the brain from xenobiotics, the mechanisms involved in the regulation of expression of multiple drug resistance genes and proteins are not fully understood. Re-entry into the cell cycle and integrity of the p53 signaling pathway have been proposed as triggers of multiple drug resistance expression in tumor cells. Whether this regulation occurs in non-tumor CNS tissue is not known. Since multiple drug resistance overexpression has been reported in glia and blood vessels from epileptic brain, we investigated the level of expression of multidrug resistance protein, multidrug resistance-associated proteins and lung resistance protein in endothelial cells and astrocytes isolated from epileptic patients or studied in situ in surgical tissue samples by double label immunocytochemistry. Reverse transcriptase-polymerase chain reaction and Western blot analyses revealed that multiple drug resistance, multidrug resistance protein, and lung resistance protein are expressed in these cells. Given that lung resistance proteins have been reported to be preferentially expressed by tumors, we investigated expression of tumor suppressor genes in epileptic cortices. The pro-apoptotic proteins p53 and p21 could not be detected in "epileptic" astrocytes, while endothelial cells from the same samples readily expressed these proteins, as did normal brain astroglia and normal endothelial cells. Other apoptotic markers were also absent in epileptic glia. Our results suggest a possible link between loss of p53 function and expression of multiple drug resistance in non-tumor CNS cells.

Serum S-100beta as a possible marker of blood-brain barrier disruption

Two brain-specific proteins, S-100beta and neuron-specific enolase (NSE), are released systemically after cerebral lesions, but S-100beta levels sometimes rise in the absence of neuronal damage. We hypothesized that S-100beta is a marker of blood-brain barrier (BBB) leakage rather than of neuronal damage. We measured both proteins in the plasma of patients undergoing iatrogenic BBB disruption with mannitol, followed by chemotherapy. Serum S-100beta increased significantly after mannitol infusion (P<0.05) while NSE did not. This suggests that S-100beta is an early marker of BBB opening that is not necessarily related to neuronal damage.

Overexpression of multiple drug resistance genes in endothelial cells from patients with refractory epilepsy

PURPOSE

It has been suggested that altered drug permeability across the blood-brain barrier (BBB) may be involved in pharmacoresistance to antiepileptic drugs (AEDs). To test this hypothesis further, we measured multiple drug resistance (MDR) gene expression in endothelial cells (ECs) isolated from temporal lobe blood vessels of patients with refractory epilepsy. ECs from umbilical cord or temporal lobe vessels obtained from aneurysm surgeries were used as comparison tissue.

METHODS

cDNA arrays were used to determine MDR expression. MDR protein (MRP1) immunocytochemistry and Western blot analysis were used to confirm cDNA array data.

RESULTS

We found overexpression of selected MDR and significantly higher P-glycoprotein levels in "epileptic" versus "control" ECs. Specifically, MDR1, cMRP/MRP2, and MRP5 were upregulated in epileptic tissue, whereas Pgp3/MDR3 levels were comparable to those measured in comparison tissue. The gene encoding cisplatin resistance--associated protein (hCRA-alpha) also was overexpressed in epileptic tissue. Immunocytochemical analysis revealed that MDR1 immunoreactivity was localized primarily in ECs; MRP1 protein levels also were significantly higher in epileptic tissue.

CONCLUSIONS

Complex MDR expression changes may play a role in AEDs pharmacoresistance by altering the permeability of AEDs across the BBB.

Blood-brain barrier preservation in the in vitro isolated guinea pig brain preparation

The morphofunctional preservation of the blood-brain barrier (BBB) was evaluated in the isolated guinea pig brain maintained in vitro by arterial perfusion. Electron microscopy evaluation after 5 hr in vitro demonstrated that cerebral capillaries and BBB specializations in this preparation retain features compatible with structural integrity. BBB-impermeable and -permeable atropine derivatives arterially perfused to antagonize carbachol-induced fast oscillatory activity confirmed the functional preservation of the BBB in vitro. To study BBB function further, changes in extracellular K+ concentration during arterial perfusion of a high-K+ solution were measured with K+-sensitive electrodes positioned in the cortex and, as control, at the brain venous outlet, where the solution perfused through the brain arterial system was collected. After 5 hr in vitro, the [K+](o) values measured during high-K+ perfusion in the piriform and entorhinal cortices were 5.02 +/- 0.17 mM (mean +/- SE) and 5.2 +/- 0.21 mM, respectively (n = 6). Coperfusion of the high-K+ solution with the Na+/K+ pump blocker ouabain (10 microM; n = 4) induced consistently spreading depression preceded by a rise in [K+](o). Finally, sporadic, isolated spots of extravasation of the fluorescent marker fluorescein isothiocyanate (FITC)-dextran preferentially circumscribed to deep cortical layers was observed in brains perfused with FITC-dextran after 5 hr in vitro. The study demonstrates that the in vitro isolated guinea pig brain is viable for studying cerebrovascular interactions and BBB permeability of compounds active in the central nervous system.

Induction of a senescence-like phenotype in bovine aortic endothelial cells by ionizing radiation

Treatment of confluent monolayers of bovine aortic endothelial cells (BAEC) with gamma rays resulted in the delayed appearance of cells with an enlarged surface area that were morphologically similar to senescent cells. The majority of these cells stained positively for senescence-associated beta-galactosidase (SA-beta-gal), indicating that these cells are biochemically similar to senescent cells. The incidence of the senescence-like phenotype increased with dose (5-15 Gy) and time after irradiation. Cells with a senescence-like phenotype began to appear in the monolayer several days after irradiation. The onset of the appearance of this phenotype was accelerated by subculturing 24 h after irradiation. This acceleration was not entirely due to stimulation of progression through the cell cycle, since a high percentage of the senescent-like cells that appeared after subculture were not labeled with BrdUrd during the period after subculture. Prolonged up-regulation of expression of CDKN1A (also known as p21(CIP1/WAF1)) after irradiation was noted by Western blot analysis, again suggesting a similarity to natural senescence. Phenotypically altered endothelial cells were present in the irradiated monolayers as long as 20 weeks after irradiation, suggesting that a subpopulation of altered endothelial cells that might be functionally deficient could persist in the vasculature of irradiated tissue for a prolonged period after irradiation.

Regional variation in brain capillary density and vascular response to ischemia

Differences in brain neuroarchitecture have been extensively studied and recent results demonstrated that regional differences in the physiological properties of glial cells are equally common. Relatively little is known on the topographic differences in vascular supply, distribution and density of brain capillaries in different CNS regions. We developed a simple method consisting of intravascular injection of fluorescent dyes coupled to immunocytochemical techniques that allows for simultaneous observation of glia-neuronal-vascular interactions in immersion-fixed brain specimens from small rodents. This technique permits quantitative evaluation of regional differences in glial/neuronal distribution and the study of their relationship to vascular densities. Variations of this technique also allow the detection of abnormal microvasculature (i.e. 'leaky' vessels), a useful feature for studies of blood-brain barrier function in health and disease. By use of quantitative confocal microscopy, the three-dimensional geometry of cortical and hippocampal structures revealed remarkable differences in vascularization between cortical gray/white matter junction, and hippocampal formation (CA1 and CA3 regions). Significant differences were also observed within the same investigative region: CA1 was characterized by low capillary density compared to neighboring CA3. Following an ischemic insult, CA1 vessels had more extensive blood-brain barrier leakage than CA3 vessels. We conclude that in addition to neuronal and glial heterogeneity, cortical structures are also endowed with region-specific vascular patterns characterized by distinct pathophysiological responses.

Mechanisms of glucose transport at the blood-brain barrier: an in vitro study

How the brain meets its continuous high metabolic demand in light of varying plasma glucose levels and a functional blood-brain barrier (BBB) is poorly understood. GLUT-1, found in high density at the BBB appears to maintain the continuous shuttling of glucose across the blood-brain barrier irrespective of the plasma concentration. We examined the process of glucose transport across a quasi-physiological in vitro blood-brain barrier model. Radiolabeled tracer permeability studies revealed a concentration ratio of abluminal to luminal glucose in this blood-brain barrier model of approximately 0.85. Under conditions where [glucose](lumen) was higher than [glucose](ablumen), influx of radiolabeled 2-deoxyglucose from lumen to the abluminal compartment was approximately 35% higher than efflux from the abluminal side to the lumen. However, when compartmental [glucose] were maintained equal, a reversal of this trend was seen (approximately 19% higher efflux towards the lumen), favoring establishment of a luminal to abluminal concentration gradient. Immunocytochemical experiments revealed that in addition to segregation of GLUT-1 (luminal>abluminal), the intracellular enzyme hexokinase was also asymmetrically distributed (abluminal>luminal). We conclude that glucose transport at the CNS/blood interface appears to be dependent on and regulated by a serial chain of membrane-bound and intracellular transporters and enzymes.

Mechanisms of epileptogenesis

The cellular mechanisms of epileptogenesis are reviewed as related to their role(s) in the expression of hyperexcitability and hypersynchrony. The data on the roles of the glutamate, GABA, acetylcholine, and adenosine receptors is discussed. The recent information on the role of glial cells in the expression of epileptogenicity is reviewed.

Adenosine permeation of a dynamic in vitro blood-brain barrier inhibited by dipyridamole

Adenosine is an inhibitory neuromodulator in the central nervous system and has been reported to have neuroprotective properties. Using a dynamic in vitro blood-brain barrier, we investigated the hypothesis that inhibition of adenosine transporters on the lumenal side of the blood-brain barrier may decrease the loss of adenosine from the brain. Our results indicate that lumenal administration of dipyridamole, a nucleoside transport inhibitor, can inhibit adenosine permeation from the extracapillary space into the lumen.

Do glia have heart? Expression and functional role for ether-a-go-go currents in hippocampal astrocytes

Potassium homeostasis plays an important role in the control of neuronal excitability, and diminished buffering of extracellular K results in neuronal Hyperexcitability and abnormal synchronization. Astrocytes are the cellular elements primarily involved in this process. Potassium uptake into astrocytes occurs, at least in part, through voltage-dependent channels, but the exact mechanisms involved are not fully understood. Although most glial recordings reveal expression of inward rectifier currents (K(IR)), it is not clear how spatial buffering consisting of accumulation and release of potassium may be mediated by exclusively inward potassium fluxes. We hypothesized that a combination of inward and outward rectifiers cooperate in the process of spatial buffering. Given the pharmacological properties of potassium homeostasis (sensitivity to Cs(+)), members of the ether-a-go-go (ERG) channel family widely expressed in the nervous system could underlie part of the process. We used electrophysiological recordings and pharmacological manipulations to demonstrate the expression of ERG-type currents in cultured and in situ hippocampal astrocytes. Specific ERG blockers (dofetilide and E 4031) inhibited hyperpolarization- and depolarization-activated glial currents, and ERG blockade impaired clearance of extracellular potassium with little direct effect on hippocampal neuron excitability. Immunocytochemical analysis revealed ERG protein mostly confined to astrocytes; ERG immunoreactivity was absent in presynaptic and postsynaptic elements, but pronounced in glia surrounding the synaptic cleft. Oligodendroglia did not reveal ERG immunoreactivity. Intense immunoreactivity was also found in perivascular astrocytic end feet at the blood-brain barrier. cDNA amplification showed that cortical astrocytes selectively express HERG1, but not HERG2-3 genes. This study provides insight into a possible physiological role of hippocampal ERG channels and links activation of ERG to control of potassium homeostasis.

ATP-sensitive potassium channels may participate in the coupling of neuronal activity and cerebrovascular tone

K(+) dilate and constrict cerebral vessels in a dose-dependent fashion. Modest elevations of abluminal K(+) cause vasodilatation, whereas larger extracellular K(+) concentration ([K(+)](out)) changes decrease cerebral blood flow. These dilations are believed to be mediated by opening of inward-rectifier potassium channels sensitive to Ba(2+). Because BaCl(2) also blocks ATP-sensitive K(+) channels (K(ATP)), we challenged K(+) dilations in penetrating, resistance-size (<60 mmu) rat neocortical vessels with the K(ATP) channel blocker glibenclamide (1 microM). Glibenclamide reduced K(+) responses from 138 +/- 8 to 110 +/- 0.8%. K(+) constrictions were not affected by glibenclamide. The Na(+)-K(+)-pump inhibitor ouabain (200 microM) did not significantly change resting vessel diameter but decreased K(+) dilations (from 153 +/- 9 to 99 +/- 2%). BaCl(2) blocked K(+) dilations with a half-maximal dissociation constant of 2.9 microM and reduced dilations to the specific K(ATP) agonist pinacidil with equal potency. We conclude that, in resistance vessels, K(+) dilations are mediated by K(ATP); we hypothesize that [K(+)](out) causes activation of Na(+)-K(+) pumps, depletion of intracellular ATP concentration, and subsequent opening of K(ATP). This latter hypothesis is supported by the blocking effect of ouabain.

A new model of the blood--brain barrier: co-culture of neuronal, endothelial and glial cells under dynamic conditions

Developing in vitro blood-brain barrier (BBB) models that closely mimic the natural state is important for theoretical and practical applications, including drug development. We previously developed an in vitro BBB model based on co-culturing endothelial cells with glia in the presence of flow on hollow fiber tube culture substrates. We now report that this dynamic in vitro BBB (DIV-BBB) can be successfully used to co-culture differentiated serotonergic neurons in the presence of a BBB. These neurons demonstrated fluoxetine-sensitive serotonin (5HT) uptake and depolarization-induced release of [3H]5HT. Our results demonstrate that the DIV-BBB is a suitable model for culturing of neurons in a quasi-physiological microenvironment and in the presence of a high-resistance, stereoselective BBB.

Blood-brain barrier, ion homeostatis and epilepsy: possible implications towards the understanding of ketogenic diet mechanisms

The finding that epileptic seizures alter blood-brain barrier (BBB) properties has stimulated interest into the possibility that phenotypic changes in brain endothelium may constitute a pathological initiator leading to seizures. Recent evidence obtained from epileptic patients undergoing cortical resection, demonstrated abnormal expression of glucose transporter molecules (GLUT1), while [18F]deoxyglucose PET studies demonstrated regions of decreased glucose uptake and hypometabolism in seizure foci. The properties of other 'nonexcitable CNS cells' are also altered in epileptic tissue, and glial cells from epileptic brain displayed diminished capacity for ionic homeostasis; voltage-dependent mechanisms were primarily affected, increasing reliance on energy-dependent mechanisms. Diminished ion homeostasis together with increased metabolic demand of hyperactive neurons may further aggravate the neuropathological consequences of BBB loss of glucose uptake mechanisms. Since ketone bodies can provide an alternative to glucose to support brain energy requirements, it is hypothesized that one of the mechanisms of the ketogenic diet in epilepsy may relate to increased availability of beta-hydroxybutyrate, a ketone body readily transported at the BBB. This hypothesis is supported by the fact that the ketogenic diet is the treatment of choice for the glucose transporter protein syndrome and pyruvate dehydrogenase deficiency, both associated with cerebral energy failure and seizures.

Impaired K(+) homeostasis and altered electrophysiological properties of post-traumatic hippocampal glia

Traumatic brain injury (TBI) can be associated with memory impairment, cognitive deficits, or seizures, all of which can reflect altered hippocampal function. Whereas previous studies have focused on the involvement of neuronal loss in post-traumatic hippocampus, there has been relatively little understanding of changes in ionic homeostasis, failure of which can result in neuronal hyperexcitability and abnormal synchronization. Because glia play a crucial role in the homeostasis of the brain microenvironment, we investigated the effects of TBI on rat hippocampal glia. Using a fluid percussion injury (FPI) model and patch-clamp recordings from hippocampal slices, we have found impaired glial physiology 2 d after FPI. Electrophysiologically, we observed reduction in transient outward and inward K(+) currents. To assess the functional consequences of these glial changes, field potentials and extracellular K(+) activity were recorded in area CA3 during antidromic stimulation. An abnormal extracellular K(+) accumulation was observed in the post-traumatic hippocampal slices, accompanied by the appearance of CA3 afterdischarges. After pharmacological blockade of excitatory synapses and of K(+) inward currents, uninjured slices showed the same altered K(+) accumulation in the absence of abnormal neuronal activity. We suggest that TBI causes loss of K(+) conductance in hippocampal glia that results in the failure of glial K(+) homeostasis, which in turn promotes abnormal neuronal function. These findings provide a new potential mechanistic link between traumatic brain injury and subsequent development of disorders such as memory loss, cognitive decline, seizures, and epilepsy.

Extracellular chloride and the maintenance of spontaneous epileptiform activity in rat hippocampal slices

Previous studies showed that furosemide blocks spontaneous epileptiform activity without diminishing synaptic transmission or reducing hyperexcited field responses to electrical stimuli. We now test the hypothesis that the antiepileptic effects of furosemide are mediated through its blockade of the Na+,K+,2Cl- cotransporter and thus should be mimicked by a reduction of extracellular chloride ([Cl-]o). In the first set of experiments, field recordings from the CA1 cell body layer of hippocampal slices showed that spontaneous bursting developed within 10-20 min in slices perfused with low-[Cl-]o (7 mM) medium but that this spontaneous epileptiform activity ceased after a further 10-20 min. Intracellular recordings from CA1 pyramidal cells showed that normal action potential discharge could be elicited by membrane depolarization, even after the tissue was perfused with low-[Cl-]o medium for >2 h. In a second set of experiments, spontaneous bursting activity was induced in slices by perfusion with high-[K+]o (10 mM), bicuculline (100 microM), or 4-aminopyridine (100 microM). In each case, recordings from the CA1 region showed that reduction of [Cl-]o to 21 mM reversibly blocked the bursting within 1 h. Similar to previous observations with furosemide treatment, low-[Cl-]o medium blocked spontaneous hypersynchronous discharges without reducing synaptic hyperexcitability (i.e., hyperexcitable field responses evoked by electrical stimulation). In a third set of experiments, prolonged exposure (>1 h after spontaneous bursting ceased) of slices to systematically varied [Cl-]o and [K+]o resulted in one of three types of events: 1) spontaneous, long-lasting, and repetitive negative field potential shifts (7 mM [Cl-]o; 3 mM [K+]o); 2) oscillations consisting of 5- to 10-mV negative shifts in the field potential, with a period of approximately 1 cycle/40 s (16 mM [Cl-]o; 12 mM [K+]o); and 3) shorter, infrequently occurring negative field shifts lasting 20-40 s (21 mM [Cl-]o; 3 mM [K+]o). Our observations indicate that the effects of low [Cl-]o on neuronal synchronization and spontaneous discharge are time dependent. Similar effects were seen with furosemide and low [Cl-]o, consistent with the hypothesis that the antiepileptic effect of furosemide is mediated by the drug's effect on chloride transporters. Finally, the results of altering extracellular potassium along with chloride suggest that blockade of the Na+, K+,2Cl- cotransporter, which normally transports chloride from the extracellular space into glial cells, is key to these antiepileptic effects.

Functional specialization and topographic segregation of hippocampal astrocytes

Astrocytes have been suggested to play several roles in the complex control of brain microenvironment. However, they have been generally considered to constitute a homogeneous population of cells. Here we show that at least three electrophysiologically distinct types of astrocytes can be found in the mature hippocampus. These subpopulations of glia were characterized by expression of different ion currents. In astrocytes exposed to elevated K+, Cs+ prevented influx of K+ only in cells with inwardly rectifying currents (IIR). The topographic distribution of glia with Cs+-sensitive inward rectifying currents (involved in K+ buffering) was nonuniform. Cs+-sensitive astrocytes were predominantly found in CA3 radiatum, whereas most CA1 astrocytes were Cs+-insensitive. Functional significance of the spatial segregation of glial cells with inward rectification was addressed in slices that were bathed in Cs+-containing media. Under these conditions, neuronal stimulation induced spontaneous epileptiform activity, which first appeared in CA3 and was then synaptically propagated to CA1. Intracellular labeling of astrocytes with biocytin revealed that CA1 astrocytes are characterized by a high degree of cell-to-cell coupling; in contrast, cell labeling in CA3 revealed smaller groups and occasionally individual cells. Three individual biocytin-labeled cells had electrophysiological properties indistinguishable from Cs+-sensitive astrocytes but had morphology typical of oligodendroglia. These results provide evidence for a role of K+ uptake via IIR into astrocytes. The segregated expression of potassium channels in a subpopulation of astrocytes suggests that functionally specialized cell types are involved in K+ homeostasis.

Impaired induction of blood-brain barrier properties in aortic endothelial cells by astrocytes from GFAP-deficient mice

Cell culture models have been extensively used for studies of blood-brain barrier (BBB) function. However, most in vitro models fail to reproduce the peculiar physiological and morphological properties of in situ brain microvascular endothelial cells. A recently developed, tridimensional and dynamic model of the BBB has permitted studies of glial-endothelial interactions in hollow fibers exposed to intraluminal flow. We have taken advantage of this technique and have investigated the ability of glial fibrillary acidic protein (GFAP)-deficient (GFAP-/-) astrocytes to induce BBB properties in aortic endothelial cells (BAEC) cultured in vitro. BAEC exposed to flow were seeded intraluminally in hollow fibers and co-cultured with extraluminally seeded mouse astrocytes. Under these conditions, astrocytes have been shown to induce blood-brain barrier properties in non-brain endothelial cells. We followed induction of a BBB phenotype by measuring the transendothelial resistance, as well as endothelial permeability to potassium, theophylline, 8-sulphophenyl-theophylline (8-SPT), sucrose, and Evans blue. Wild-type mouse astrocytes induced BBB properties in aortic endothelial cells following 3-4 weeks of co-culturing. Thus, these endothelial cells restricted passage of K+ ions into the extracapillary space and selectively excluded hydrophilic molecules, such as 8-SPT and 14C-sucrose. GFAP-/- astrocytes failed to induce a significant restriction to the passage of potassium and hydrophilic drugs (sucrose, 8-SPT), failed to induce transendothelial resistance values comparable to control co-cultures, but were capable of inducing exclusion of Evans blue by endothelial cells. These results suggest that GFAP (and intermediate filaments) may play a role in the induction of BBB properties in non-BBB endothelial cells.

Selective loss of hippocampal long-term potentiation, but not depression, following fluid percussion injury

We investigated the early effects of in vivo fluid percussion injury (FPI) on hippocampal synaptic potentials and excitability. In vitro field potential recordings and immunocytochemistry were performed in the CA1 region in slices from naïve, post-FPI, or sham-operated rats. The following electrophysiological and morphological parameters were affected following FPI: (1) threshold for population spike generation was increased suggesting that post-FPI neurons were hypoexcitable; (2) long-term potentiation (LTP) could not be induced in injured hippocampi; (3) GFAP and inducible NO synthase (iNOS) immunoreactivity were enhanced post-FPI; and (4) following injury, synaptophysin immunoreactivity was enhanced in CA1 stratum radiatum. The effects of FPI on synaptic plasticity were LTP-specific, since long-term depression (LTD) could be equally induced and maintained in post-FPI, sham-operated and control slices. Sham-operated slices were characterized by synaptic excitability indistinguishable from naïve controls, but displayed decreased ability for LTP production and expressed high levels of iNOS. We conclude that FPI causes a selective loss of LTP, possibly due to a previous potentiation induced by trauma as reflected by the increased expression of synaptic proteins. Sham surgical procedures were, however, not without effects on long-term potentiation itself; the latter effects appear to be mediated by an increased production of NO. Our study demonstrates for the first time that hippocampal slices can be used to investigate the correlates of in vivo FPI. Furthermore, we describe LTP-specific deficits in post-traumatic brain injury, suggesting that FPI can selectively erase one of the two main NMDA-dependent forms of synaptic plasticity in the hippocampus.

Preferential inhibition of Ih in rat trigeminal ganglion neurons by an organic blocker

The potency and specificity of a novel organic Ih current blocker DK-AH 268 (DK, Boehringer) was studied in cultured rat trigeminal ganglion neurons using whole-cell patch-clamp recording techniques. In neurons current-clamped at the resting potential, the application of 10 microM DK caused a slight hyperpolarization of the membrane potential and a small increase in the threshold for action potential discharge without any major change in the shape of the action potential. In voltage-clamped neurons, DK caused a reduction of a hyperpolarization-activated current. Current subtraction protocols revealed that the time-dependent, hyperpolarization-activated currents blocked by 10 microM DK or external Cs+ (3 mM) had virtually identical activation properties, suggesting that DK and Cs+ caused blockade of the same current, namely Ih. The block of Ih by DK was dose-dependent. At the intermediate and higher concentrations of DK (10 and 100 microM) a decrease in specificity was observed so that time-independent, inwardly rectifying and noninactivating, voltage-gated outward potassium currents were also reduced by DK but to a much lesser extent than the time-dependent, hyperpolarization-activated currents. Blockade of the time-dependent, hyperpolarization-activated currents by DK appeared to be use-dependent since it required hyperpolarization for the effect to take place. Relief of DK block was also aided by membrane hyperpolarization. Since both the time-dependent current blocked by DK and the Cs+-sensitive time-dependent current behaved as Ih, we conclude that 10 microM DK can preferentially reduce Ih without a major effect on other potassium currents. Thus, DK may be a useful agent in the investigation of the function of Ih in neurons.

Morphological and functional characterization of an in vitro blood-brain barrier model

Cell culture models have been extensively used for studies of blood-brain barrier (BBB) function. However, several in vitro models fail to reproduce some, if not most, of the physiological and morphological properties of in situ brain microvascular endothelial cells. We have recently developed a dynamic, tridimensional BBB model where endothelial cells exposed to intraluminal flow form a barrier to ions and proteins following prolonged co-culturing with glia. We have further characterized this cell culture model to determine whether these barrier properties were due to expression of a BBB phenotype. Endothelial cells of human, bovine or rodent origin were used. When co-cultured with glia, intraluminally grown endothelial cells developed features similar to in vivo endothelial cells, including tight junctional contacts at interdigitating processes and a high transendothelial resistance. This in vitro BBB was characterized by the expression of an abluminal, ouabain-sensitive Na/K pump, and thus favored passage of potassium ions towards the lumen while preventing K+ extravasation. Similarly, the in vitro BBB prevented the passage of blood-brain barrier-impermeant drugs (such as morphine, sucrose and mannitol) while allowing extraluminal accumulation of lipophylic substances such as theophylline. Finally, expression of stereo-selective transporters for Aspartate was revealed by tracer studies. We conclude that the in vitro dynamic BBB model may become an useful tool for the studies of BBB-function and for the testing of drug passage across the brain endothelial monolayer.

Heterogeneity of astrocyte resting membrane potentials and intercellular coupling revealed by whole-cell and gramicidin-perforated patch recordings from cultured neocortical and hippocampal slice astrocytes

Astrocytes are thought to regulate the extracellular potassium concentration by mechanisms involving both voltage-dependent and transport-mediated ion fluxes combined with intercellular communication via gap junctions. Mechanisms regulating resting membrane potential (RMP) play a fundamental role in determining glial contribution to buffering of extracellular potassium and uptake of potentially toxic neurotransmitters. We have investigated the passive electrophysiological properties of cultured neocortical astrocytes and astrocytes recorded in hippocampal slices from 18-25 d postnatal rats. These experiments revealed a wide range of astrocyte RMPs that were independent of developmental factors, length of culturing, cellular morphology, the electrophysiological techniques used (whole-cell vs perforated recording), cell-specific expression of Na+/2HCO3- co-transporters, or voltage-dependent Na+ channels. Exposure of cultured astrocytes to differentiation-inducing factors (such as cAMP) or inhibition of proliferation (by serum deprivation) did not significantly influence RMP. Expression of ATP-sensitive potassium channels was absent in these glia; thus, K(ATP)-related mechanisms did not contribute to cell resting potential. In both cultured and slice astrocytes, spontaneous electrophysiological changes were commonly observed. These reversible events, which resulted in differential sensitivity to potassium channel blockers (cesium and barium) and sudden current-voltage profile changes, were attributable to dynamic changes in cell-to-cell coupling, as confirmed by recordings from isolated pairs of cells. We conclude that the heterogeneity of astrocytic RMP and intercellular coupling both in culture and in situ are intrinsic properties of glia that may contribute to transcellular transport of potassium. We propose a model in which spatial buffering may be facilitated by heterogeneous mechanisms controlling glial RMP in combination with dynamic changes in intercellular coupling.

Endothelium-dependent regulation of cerebrovascular tone by extracellular and intracellular ATP

ATP receptors and ATP-sensitive potassium channels (KATP) are expressed in vascular smooth muscle (VSM) and endothelial cells (EC). In isolated penetrating vessels, ATP caused a dilatation when applied intraluminally but not extraluminally. The actions of ATP were blocked by the nitric oxide (NO) synthesis inhibitor N omega-nitro-L-arginine (0.1 mM) but were only reduced by N-monomethyl-L-arginine (0.1 mM); responses to intraluminal ATP were also prevented by thapsigargin. The KATP opener (KCO) nicorandil (1 microM) caused an NO-independent vasodilatation when applied extraluminally and an NO-dependent response when applied intraluminally. Both responses were blocked by glibenclamide. EC-mediated responses to nicroandil were prevented by blockade of guanylate cyclase by LY-83583 (10 microM). The effects of nicorandil were mimicked by pinacidil (1-10 microM). Exposure of the endothelium to 500 microM cyanide and 0 mM glucose ("in vitro ischemia") caused a vasodilatation that was reduced by exposure to glibenclamide (5 microM). Blockade of NO synthase produced similar effects, suggesting that the ischemic dilation is mediated by KATP and NO. Our results suggest that both VSM and EC mediate the vascular responses induced by KCOs, whereas the dilatation induced by intraluminal ATP is mediated by the endothelium. The endothelium-dependent component of the in vitro ischemic vasodilatation is mediated by opening of endothelial KATP and subsequent release of NO.

Neurotoxicity in organotypic hippocampal slices mediated by adenosine analogues and nitric oxide

Adenosine (ADO) and nitric oxide (NO) have been implicated in a variety of neurophysiological actions, including induction of long-term potentiation, regulation of cerebral blood flow, and neurotoxicity/neuroprotection. ADO has been shown to promote NO release from astrocytes by a direct effect on A1 and A2 receptors, thus providing a link between actions of NO and adenosine in the brain. However, while adenosine acts as an endogenous neuroprotectant, NO is believed to be the effector of glutamate neurotoxicity. To resolve this apparent paradox, we have further investigated the effects of adenosine and NO on neuronal viability in cultured organotypic hippocampal slices exposed to sub-lethal (20') in vitro ischemia. Up to a concentration of 500 microM ADO did not cause toxicity while exposures to 100 microM of the stable ADO analogue chloroadenosine (CADO) caused widespread neuronal damage when paired to anoxia/hypoglycemia. CADO effects were significantly prevented by the ADO receptor antagonist theophylline and blockade of NO production by L-NA (100 microM). Moreover, CADO effects were mimicked by the NO donor SIN-1 (100 microM). Application of 100 microM ADO following blockade of adenosine deaminase (with 10 microM EHNA) replicated the effects of CADO. CADO, ADO + EHNA but not ADO alone caused a prolonged and sustained release of nitric oxide as measured by direct amperometric detection. We conclude that at high concentrations and/or following blockade of its enzymatic catabolism, ADO may cause neurotoxicity by triggering NO release from astrocytes. These results demonstrate for the first time that activation of pathways other than those involving neuronal glutamate receptors can trigger NO-mediated neuronal cell death in the hippocampus.

Reduction of K+ uptake in glia prevents long-term depression maintenance and causes epileptiform activity

Extracellular cesium causes synchronous, interictal-like bursting and prevents maintenance of long-term depression (LTD) in the CA1 hippocampal region. We have investigated the cellular mechanisms underlying cesium actions. Whole-cell recordings showed that brief (2 min) bath exposures to cesium caused pyramidal cell hyperpolarization associated with decreased membrane conductance attributable to blockade of an inward h-type current. After prolonged (>2 min) exposures, a late depolarizing response was observed; this effect was not associated with changes in cell membrane conductance. Recordings from interneurons revealed that Ih is expressed in a subpopulation of cells and that cesium effects on interneurons expressing Ih are comparable to those observed in pyramidal cells. Consistent with this effect, cesium decreased the early component of the IPSP recorded in pyramidal cells. Interneurons lacking Ih were not affected by cesium but developed a depolarizing response when drug applications were paired to orthodromic stimulation. We concluded that cesium actions on LTD and cesium-induced epileptiform activity were not attributable exclusively to its direct effects on neurons. Recordings from hippocampal slice astrocytes revealed that cesium interfered with glial electrical responses during LTD induction. Cesium blocked glial inwardly rectifying potassium channels and increased the amplitude and duration of stimulation-evoked [K+]out increases. Thus, the effects of cesium on CA1 synchronization and synaptic plasticity appear to be mediated predominantly by blockade of glial voltage-dependent potassium uptake.

Adenosine-induced release of nitric oxide from cortical astrocytes

Nitric oxide (NO) and adenosine are involved in coincident CNS functions, including long-term potentiation, neuronal protection, neurotoxicity and cerebral blood flow. We tested the hypothesis that glia may act as a cellular link between the two, through adenosine-induced NO release from astrocytes. A direct NO measuring system was used, allowing the kinetics of NO release to be measured. Our results show that adenosine, acting through purinoceptors, causes NO release from cultured cortical astrocytes. Mobilization of calcium from intracellular stores rather than influx is involved in the adenosine-induced activation of NO synthase. These results demonstrate a possible interaction between adenosine and NO in cerebrovascular physiology and neurotoxicity.

Astrocytic inwardly rectifying potassium currents are dependent on external sodium ions

1. Two subtypes of astrocytes that expressed distinctly different ion channel complements were identified in primary cultures from rat spinal cord and hippocampus using whole cell patch-clamp techniques. One population of cells expressed voltage-activated Na+ currents and displayed outwardly rectifying I-V relationships; the other group of cells had no detectable Na+ currents and pronounced inwardly rectifying I-V curves. 2. Astrocytes expressing Na+ currents were hyperpolarized (by approximately 7 mV) upon removal of external sodium, suggesting a resting Na+ conductance in these cells. In contrast, cells expressing primarily inwardly rectifying K+ currents, Kir, depolarized (by approximately 4-6 mV) in low-sodium solutions. 3. Removal of external Na+ ions increased the input resistance (189% of control) and reduced the whole cell current amplitude (60% of control at -120 mV) of cells with Kir. The reduction in current amplitude was dose-dependent and became apparent after a 10% reduction of [Na+]0 in 7/7 cells tested. At -120 mV, the effect was near maximal in response to a 50% reduction of [Na+]0. 4. The outward potassium currents of cells expressing Na(+)-currents were unaffected by removal of bath Na+. 5. We conclude that the conductance of glial inwardly rectifying K+ channels is dependent on external sodium ions via a mechanism that does not involve sodium ion permeation or blockade of these channels.

Physiological properties of ATP-activated cation channels in rat brain microvascular endothelial cells

Endothelial cells mediate the actions of a variety of vasoactive substances, including ATP. ATP vasodilatatory actions have been shown to depend on a calcium-dependent release of endothelium-derived relaxing factor(s) (EDRF). ATP induced a vasodilatation of pial penetrating microvessels when applied intraluminally; these relaxations were mediated by the endothelium and followed release of nitric oxide (NO), since they were sensitive to blockade of NO-synthesizing enzymes by NG-nitro-L-arginine (1 mM) and NG-mono-methyl-L-arginine (0.1 mM). We have also investigated the electrophysiological actions of extracellular ATP on rat brain microvascular (RBMEC) and bovine aortic endothelial cells (BAEC) using the patch-clamp technique. While BAEC were hyperpolarized by ATP (10 microM), ATP caused the activation of a depolarizing nonselective cation current in brain endothelial cells. NO production measurements by [3H]citrulline assay and by direct amperometric determination also revealed that after exposure to 1-100 microM ATP, RBMEC released NO. NO release from RBMEC was abolished by removal of external calcium. We conclude that, in the brain, ATP exerts its vasoactive roles by altering the electrophysiological properties of endothelial cells by acting on receptor-operated ion channels, thus providing a mechanism for calcium entry and subsequent release of EDRF.

Hyperpolarization-activated ion currents in cultured rat cortical and spinal cord astrocytes

Hyperpolarization-activated currents were recorded from rat brain cortical and spinal cord astrocytes maintained in culture. Spinal cord astrocytes expressed primarily an inward rectifier potassium current characterized by time-dependent inactivation, a strong dependence on extracellular Na+ and insensitivity to intracellular GTP-gamma-S (0.2 mM). In cortical astrocytes voltage clamp protocols aimed to elicit currents activated at, or negative to cell membrane potentials led to the development of two distinct ion currents. The most prominent current resembled the inward rectifier potassium current. This component was sensitive to blockade by extracellular cesium and was greatly reduced during recordings performed with GTP-gamma-S (0.2 Mm) added to the pipette solutions. The remaining current component was similar to the endothelial I ha current. I ha conductance was enhanced by extracellular potassium and the current reversal potential behaved as expected for a mixed cation, Na+/K+ current. I ha was nearly abolished after removal of extracellular Na. These results are consistent with the expression of a novel mixed cation conductance in glial cells, possibly involved in extracellular potassium buffering.

Cesium prevents maintenance of long-term depression in rat hippocampal CA1 neurons

Long-term depression of field excitatory postsynaptic potentials (EPSP) in the CA1 region of hippocampal slices was evoked by delivering a 15 min train of pulses at 1 Hz to the Schaffer-commissural-CA1 pathway, and prevented by adding an N-methyl-D-aspartate (NMDA) receptor antagonist (AP-5, 50 microM) to the perfusing medium. Superfusion of the slices with Cs (2 mM) during the 1 Hz stimulation period could both inhibit the maintenance phase of the depression itself and elicit spontaneous rhythmic activity. Cs had no effect on the postsynaptic response to the GABA-B agonist, baclofen. As a major effect of Cs is a block of the hyperpolarization-activated current (Ih), these results suggest the possible involvement of Ih in the maintenance of long-term depression.

Regulation of blood-brain barrier endothelial cells by nitric oxide

Nitric oxide (NO) synthesized by vascular endothelial cells is a potent vasodilator substance. The actions of NO extend well beyond its vasodilatory properties, and increasingly, NO has been recognized as an important signal for intercellular and intracellular communication. Recently, NO has been implicated in the regulation of vascular and blood-brain barrier permeability. NO has also been shown to modulate ion channels in excitable cells, thus affecting neuronal firing. We report the results of patch-clamp experiments that show a modulatory action of NO as well as cGMP and cAMP on a hyperpolarization-activated current (Iha) carried by both Na+ and K+ ions in blood-brain barrier endothelial cells. Iha was recorded in cells dialyzed with 0.2 mmol/L GTP-gamma-S to inhibit a large inwardly rectifying potassium current. This ionic current and its modulation by NO may play a role in the regulation of the transport of ions, nutrients, and other molecules to the brain and serve as an integral part of the blood-brain barrier. The modulation of Iha by a cyclic guanosine nucleotide may also explain previous reports suggesting a role for NO in the regulation of blood-brain barrier function.

ATP-sensitive K+ channels in rat aorta and brain microvascular endothelial cells

The endothelium plays an important role in the modulation of vascular tone and blood cell activation. Extensive work has demonstrated that the release of endothelium-derived relaxing factor (EDRF) from the endothelium is evoked by a number of physical and chemical stimuli requiring Ca2+. Because endothelial cells do not express voltage-dependent Ca2+ channels, Ca2+ influxes following receptor activation may be facilitated by cell hyperpolarizations mediated by the activation of K+ conductances. There has been recent interest in the role of ATP-sensitive K+ channels (KATP) suggesting that KATP may play a role in the regulation of blood flow. We have investigated the electrophysiological properties of an ATP-sensitive K+ conductance in whole cell and membrane patches from rat aorta and brain microvascular endothelial cells. Whole cell as well as single-channel currents were increased by either intracellular dialysis of ATP or application of glucose-free/NaCN (2 mM) solutions. Both currents were reversibly blocked by glibenclamide (1-100 microM). The KATP channel opener pinacidil (30 microM) caused activation of an outward current in the presence of physiological intracellular ATP concentrations. In inside-out patches, 10 microM-1 mM ATP invariably caused a dramatic decrease in channel activity. We conclude that both rat aorta and brain microvascular endothelial cells express KATP channels. KATP may play a role in the regulation of endothelial cell resting potential during impaired energy supply and therefore modulate EDRF release and thus cerebral blood flow.

Block of the cardiac pacemaker current (If) in the rabbit sino-atrial node and in canine Purkinje fibres by 9-amino-1,2,3,4-tetrahydroacridine

We have investigated the action of 9-amino-1, 2, 3, 4-tetrahydroacridine (THA) on the pacemaker current If in rabbit sino-atrial node myocytes and in canine Purkinje fibers. THA at concentrations in the range 3-300 microM blocked If in a voltage-independent manner, as revealed by measurements on the fully activated I/V relation for If. The dose/response relationship of the If maximal slope conductance (Gf) can be fitted by assuming a cooperative binding reaction where two THA molecules are required to block one If channel. Half-maximal block occurred at 18.2 microM in the sino-atrial node and 36.6 microM in Purkinje fibers. THA also affected the If kinetic properties. This was examined in the sino-atrial node where the current activation curve was shifted in the negative direction on the voltage axis (- 21 mV at 30 microM THA). The delayed rectifier current, IK, was also reduced by THA in sino-atrial node myocytes: at - 40 mV the IK fully activated value was decreased to 37% of its control value by 30 microM THA, with only a minor modification of the position of the activation curve at the same potential. Thus, although THA blocks If at a lower concentration than other known If-channel blockers [DiFrancesco (1982) J Physiol (Lond) 329:485-507], its action on the pacemaker current is not specific.

Calcium channels in undifferentiated PC12 rat pheochromocytoma cells

Undifferentiated rat pheochromocytoma PC12 cells were voltage clamped using the whole cell technique. After blockade of outward currents, calcium currents were elicited from -40 and -100 mV. A subpopulation of cells displayed only one current component activated at -10 mV and slowly decaying. In other cells this current coexisted with a component activated around -40 mV and decaying with a faster time constant. We conclude that undifferentiated PC12 cells can express two types of calcium channels, L (long-lasting) and N (neuronal)-type channels.

Are undifferentiated PC12 rat pheochromocytoma cells electrically excitable?

Undifferentiated rat pheochromocytoma PC12 cells were current clamped using the whole cell technique. Measurements of cell membrane resting potentials (RMP) gave values in the -30 and -50 mV range. Cell input resistance was between 200 and 400 Mohm. After blockade of K+ currents with intracellular Cs+, cell membrane depolarization showed that PC12 cells are able to generate active responses (i.e., calcium action potentials followed by after-hyperpolarizations partially blocked by tetraethylammonium). Taken together, our results indicate that PC12 cells do not require exposure to nerve growth factor to become electrically excitable.

Effects of GABA on CA3 pyramidal cell dendrites in rabbit hippocampal slices

Using the in vitro rabbit hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) iontophoresis on CA3 pyramidal cell dendrites. The predominant response (70% of the cells tested) was a hyperpolarization associated with a 30% decrease in cell input resistance (Rm). These hyperpolarizations displayed a very pronounced voltage dependency: they were decreased by cell depolarization and flattened by hyperpolarization. Bicuculline methiodide (BMI, 50 microM) did not abolish this response, nor did intracellular iontophoresis of chloride ions. In 5% of the cells, an additional hyperpolarization was obtained with longer ejection times; it reversed close to the reversal potential of the early component of the IPSP. In 25% of the cells, dendritic GABA application produced a depolarization. This response was reversed with cell membrane depolarization and was associated with a large (80%) decrease in Rm. The depolarizations were abolished by BMI (50 microM) and greatly increased by increasing the intracellular chloride concentration. None of the responses to GABA were affected by blockade of synaptic transmission. We conclude that the predominant response of CA3 pyramidal cell dendrites to GABA application is a hyperpolarization mediated by GABAB receptors and probably carried by potassium ions. The depolarizing responses are mediated via GABAA receptors and depend on an increase in chloride permeability.

Effects of GABA and baclofen on pyramidal cells in the developing rabbit hippocampus: an 'in vitro' study

Using the in vitro hippocampal slice preparation, we have investigated the effects of gamma-aminobutyric acid (GABA) and its analogue beta-(p-chlorophenyl)-GABA (baclofen) on CA1 and CA3 pyramidal cells in the developing rabbit hippocampus. Somatic applications: both GABA and baclofen, when applied to CA1 pyramidal cells from immature tissue, led to cell depolarization from resting membrane potential; this baclofen depolarization may be indirectly mediated. In contrast, CA3 pyramidal cells at the same age were primarily hyperpolarized by both drugs. In mature tissue, both GABA and baclofen applied at the soma induce cell hyperpolarizations. Dendritic applications: immature CA1 cells responded to dendritic GABA and baclofen application with depolarizations associated with increased cell excitability; here, too, the baclofen depolarization may be due to indirect 'disinhibition'. Both depolarizing and hyperpolarizing responses were recorded in immature tissue when GABA was applied to CA3 pyramidal cell dendrites: baclofen produced only hyperpolarizations. In mature CA1 cells, dendritic GABA application produced membrane depolarization, but dendritic baclofen application produced hyperpolarizations. In mature CA3 cells, dendritic GABA and baclofen application produced predominant hyperpolarizations. Mature CA1 pyramidal cells appear to retain some of the GABA-induced depolarizations characteristic of immature tissue. In contrast, mature CA3 neurons show only hyperpolarizing responses to GABA and baclofen application. In all cases, responses to GABA and baclofen are associated with a decrease in cell input resistance. We conclude that the GABAergic receptor/channel complexes mature differently in the CA1 and CA3 regions of the hippocampus.

Dissociation of the IPSP and response to GABA during spreading depression-like depolarizations in hippocampal slices

We have compared the sensitivity of CA1 and CA3 hippocampal pyramidal cells, in mature and immature tissue, to spreading depression-like depolarization episodes. Using hippocampal slices from rabbit, we have found that mature and immature tissue, and CA1 and CA3 neurons, were differentially prone to depolarization episodes, depending on the method used to produce the depolarization. CA1 region was generally more sensitive than CA3. Spontaneous and stimulus-evoked depolarizations were seen more frequently in immature tissue than in mature slices, but anoxia-induced depolarizations were much more likely to occur in mature tissue. Synaptic transmission and responses to somatic gamma-aminobutyric acid (GABA) ejection were compared during anoxia-induced depolarizations in mature slices. The early component of the inhibitory postsynaptic potential (IPSP) normally had the same reversal potential as the GABA response. During anoxia-induced depolarization, both the drug response and the PSPs were lost. Synaptic transmission generally disappeared before the response to exogenous GABA application; the GABA response reappeared before synaptic function was restored. During the recovery of resting potential (RMP) following depolarization, the reversal potential of the early IPSP differed significantly from that of the GABA response; when the cell had recovered to RMP, the IPSP was depolarizing, whereas GABA application produced a 'normal' cell hyperpolarization. IPSPs and GABA-mediated responses attained their pre-depolarization form within a few minutes of RMP recovery. These observations suggest that, at least under special circumstances, the early component of the IPSP and GABA-mediated hyperpolarizations can be dissociated. Therefore, the early IPSP may be mediated by more complex mechanisms than a simple alteration in chloride conductance due to GABA-receptor interactions.

Effects of trimethyltin on granule cells excitability in the in vitro rat dentate gyrus

The effects of trimethyltin (TMT) on passive properties and synaptic activity of dentate granule cell (GC) have been investigated in hippocampal slices in vitro. Intracellular recordings from GC indicated that TMT (1 and 10 microM) increased input resistance from 34.1 +/- 3.6 Mohms to 45.6 +/- 4.1 and 64.7 +/- 14.7 Mohms, respectively, 15 min after its application. This was accompanied by a 10-20 mV depolarization. A decrease in IPSP amplitude was also observed, but developed with longer delays (2-4 hr) following TMT exposure. Extracellular recording from the GC layer during paired pulse stimulation of the perforant path showed a decrease in the ratio of the amplitude of the first to the second population spikes (at an interpulse interval of 9 msec), from 1.8 +/- 0.14 to 0.8 +/- 0.08 (p less than 0.05). The amplitude of the first (conditioning) pulse remained unchanged, suggesting that TMT produced a specific decrease of inhibitory efficacy. These results add evidence to the hypothesis that TMT neurotoxicity is mediated by a decrease of inhibitory synaptic functions.

Effects of tissue preincubation and hypoxia on CA3 hippocampal neurons in the in vitro slice preparation

Using the in vitro hippocampal slice preparation, we studied the electrophysiological properties of pyramidal cells in tissue that was 'preincubated' (2-6 h in a large, static volume of oxygenated bathing medium) before being placed in an interface chamber for study. Striking differences were found in 'preincubated' vs 'non-preincubated' CA3 cells. The preincubated cells had more negative resting potentials, higher input resistance, lower threshold for stimulus-evoked burst discharge and larger hyperpolarizing afterpotentials. Cells in the preincubated CA3 region were also more likely to show spontaneous synchronized burst discharge, but were relatively resistant to hypoxia-induced spreading depression. CA1 cells were less dramatically affected by preincubation, showing little difference from their non-preincubated counterparts. Possible mechanisms involved in the CA3 preincubation effect, including glial buffering alterations and changes in Na+, K+-ATPase activity, are discussed.

In vivo treatment with GM1 prevents the rapid decay of ATPase activities and mitochondrial damage in hippocampal slices

Slices from rat CA3 hippocampal area show a 30% decrement in ATPase activity after 35 min of 'in vitro' incubation. Such a drop is accompanied by an alteration of mitochondrial ultrastructure. However, if rats are treated daily with GM1 ganglioside (10 mg/kg during 3 days) both phenomena are fully prevented. These results would suggest a protective effect of gangliosides onto membrane structures under stress conditions.

GM1 ganglioside enhances regrowth of noradrenaline nerve terminals in rat cerebral cortex lesioned by the neurotoxin 6-hydroxydopamine

The effect of exogenous GM1 ganglioside on selectively noradrenaline-denervated rat cerebral cortex was investigated by measuring the spatial distribution of endogenous noradrenaline levels and by fluorescence histochemical analysis. A local noradrenaline denervation was produced by intracortical infusion of the selective catecholamine neurotoxin 6-hydroxydopamine for 3 or 7 days. The neurotoxin infusion caused an almost complete noradrenaline denervation in a restricted area around the infusion point as reflected by an almost complete long-term disappearance of noradrenaline nerve terminals and reduction of noradrenaline levels. There was with time a slow recovery of the levels, most likely related to a spontaneous noradrenaline nerve terminal regeneration. Post-treatment for 1 week with GM1 had very small effects on the 6-hydroxydopamine-induced reduction of the noradrenaline levels, while pretreatment with GM1 for 3 days before the neurotoxin infusion and continuing the GM1 administration for another 7-14 days significantly enhanced noradrenaline recovery, as observed both bio- and histochemically. GM1 had no effect on the 6-hydroxydopamine-induced noradrenaline depletion acutely, indicating that GM1 does not interfere with the direct neurotoxic actions of 6-hydroxydopamine. The present results thus indicate that exogenous GM1 enhances regrowth of noradrenaline nerve terminals which may be due to a regrowth stimulatory effect (regeneration/collateral sprouting) and/or related to protective actions of GM1 against retrograde degeneration of noradrenaline axons following the neurotoxin-induced lesion.