Combination of hypothyroidism and stress abolishes early LTP in the CA1 but not dentate gyrus of hippocampus of adult rats.

Clinical experience suggests that both hypothyroidism and stress interfere with mental concentration and memory. This electrophysiological study examined the effect of hypothyroidism and stress, separately or combined, on long-term potentiation (LTP), a widely accepted cellular model for learning and memory. Measurements of early LTP (E-LTP) were carried out in the hippocampus of urethane-anesthetized adult Wistar rats. Hypothyroidism was achieved by thyroidectomy, and the 'intruder' stress was used as a model of chronic psychosocial stress. Stimulating electrodes were placed in the left CA3 region and right angular bundle and a recording electrode was placed in the right CA1 or the dentate gyrus (DG). The results showed that in the CA1 region of the hippocampus, hypothyroid or stress partially blocked E-LTP. However, when hypothyroidism and stress were combined, they eliminated E-LTP. In contrast, no significant change in E-LTP was seen in the DG of the three groups of rats. These results suggest that impaired memory because of hypothyroidism or stress may be related to impairment of the E-LTP in the Schaffer collateral synapses but not of that of the perforant path synapses.

Prolonged bursts occur in normal calcium in hippocampal slices after raising excitability and blocking synaptic transmission.

This study examined the conditions that are required for the appearance of the long-duration seizure-like activity that can be recorded in hippocampal slices. Spontaneous interictal activity was induced in CA1 and CA3 by perfusing hippocampal slices with high potassium, cesium, 4-aminopyridine, or tetraethylammonium chloride, in normal levels of calcium. Synaptic transmission was then blocked by the addition of neurotransmitter receptor blockers (6-cyano-7-nitroquinoxaline-2,3-dione, D,L-2-amino-5-phosphonopentanoic acid, and bicuculline) or the calcium channel blocker cadmium, resulting in complete blockade of the interictal discharges and the appearance of spontaneous seizure-like events (ictal-like discharges) primarily in CA1 and the dentate gyrus. Blocking synaptic transmission in normal artificial cerebrospinal fluid did not induce ictal-like discharges in any region. The results demonstrate that ictal-like discharges can appear in normal levels of extracellular calcium when chemical synaptic transmission is blocked pharmacologically. The results suggest that an increase in neuronal excitability and absence of interictal activity promote the appearance of the longer ictal-like discharges.

In vivo role of NAD(P)H:quinone oxidoreductase 1 (NQO1) in the regulation of intracellular redox state and accumulation of abdominal adipose tissue.

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a flavoprotein that utilizes NAD(P)H as an electron donor, catalyzing the two-electron reduction and detoxification of quinones and their derivatives. NQO1-/- mice deficient in NQO1 activity and protein were generated in our laboratory (Rajendirane, V., Joseph, P., Lee, Y. H., Kimura, S., Klein-Szanto, A. J. P., Gonzalez, F. J., and Jaiswal, A. K. (1998) J. Biol. Chem. 273, 7382-7389). Mice lacking a functional NQO1 gene (NQO1-/-) were born normal and reproduced adeptly as the wild-type NQO1+/+ mice. In the present report, we show that NQO1-/- mice exhibit significantly lower levels of abdominal adipose tissue as compared with the wild-type mice. The NQO1-/- mice showed lower blood levels of glucose, no change in insulin, and higher levels of triglycerides, beta-hydroxy butyrate, pyruvate, lactate, and glucagon as compared with wild-type mice. Insulin tolerance test demonstrated that the NQO1-/- mice are insulin resistant. The NQO1-/- mice livers also showed significantly higher levels of triglycerides, lactate, pyruvate, and glucose. The liver glycogen reserve was found decreased in NQO1-/- mice as compared with wild-type mice. The livers and kidneys from NQO1-/- mice also showed significantly lower levels of pyridine nucleotides but an increase in the reduced/oxidized NAD(P)H:NAD(P) ratio. These results suggested that loss of NQO1 activity alters the intracellular redox status by increasing the concentration of NAD(P)H. This leads to a reduction in pyridine nucleotide synthesis and reduced glucose and fatty acid metabolism. The alterations in metabolism due to redox changes result in a significant reduction in the amount of abdominal adipose tissue.

Effects of postsynaptic GABA(B) receptor activation on epileptiform activity in hippocampal slices.

GABA(B) receptor agonists have been reported to have both pro- and antiepileptic properties. Here, the effects of a GABA(B) receptor agonist, baclofen, were studied on epileptiform activity induced in the absence of synaptic transmission - to focus on the postsynaptic effects. Perfusion of hippocampal slices with 0-added calcium and high potassium induced field bursts in CA1 and the dentate gyrus. Addition of baclofen caused a transient suppression of the field bursts in CA1 and the dentate gyrus. The duration of the suppression was dependent on the concentration of baclofen and when the bursts reappeared they had a larger amplitude than before baclofen. Baclofen also suppressed the multiple population spikes evoked by antidromic stimulation in the dentate gyrus. This effect also decreased with continued baclofen perfusion. The effects of baclofen on the amplitude of the spontaneous field bursts and on the stimulation-induced multiple population spikes were blocked by the GABA(B) receptor antagonist SCH 50911, suggesting that these effects of baclofen are mediated by GABA(B) receptor activation. Baclofen significantly increased the peak extracellular K(+) concentration during each field burst in the dentate gyrus but did not change the baseline level of K(+) between field bursts. The results suggest that postsynaptic GABA(B) receptor activation by baclofen has transient antiepileptic effects followed by a rebound increase in excitability.

pH Sensitivity of non-synaptic field bursts in the dentate gyrus.

Under conditions of low [Ca(2+)](o) and high [K(+)](o), the rat dentate granule cell layer in vitro develops recurrent spontaneous prolonged field bursts that resemble an in vivo phenomenon called maximal dentate activation. To understand how pH changes in vivo might affect this phenomenon, the slices were exposed to different extracellular pH environments in vitro. The field bursts were highly sensitive to extracellular pH over the range 7.0-7.6 and were suppressed at low pH and enhanced at high pH. Granule cell resting membrane potential, action potentials, and postsynaptic potentials were not significantly altered by pH changes within the range that suppressed the bursts. The pH sensitivity of the bursts was not altered by pharmacologic blockade of N-methyl-D-aspartate (NMDA), non-NMDA, and GABA(A) receptors at concentrations of these agents sufficient to eliminate both spontaneous and evoked synaptic potentials. Gap junction patency is known to be sensitive to pH, and agents that block gap junctions, including octanol, oleamide, and carbenoxolone, blocked the prolonged field bursts in a manner similar to low pH. Perfusion with gap junction blockers or acidic pH suppressed field bursts but did not block spontaneous firing of single and multiple units, including burst firing. These data suggest that the pH sensitivity of seizures and epileptiform phenomena in vivo may be mediated in large part through mechanisms other than suppression of NMDA-mediated or other excitatory synaptic transmission. Alterations in electrotonic coupling via gap junctions, affecting field synchronization, may be one such process.

Regulation of extracellular pH in the developing hippocampus.

The increased propensity of the immature brain to have seizures may be due, in part, to an inability to regulate the extracellular ionic environment. Here, we tested the regulation of extracellular pH in the developing hippocampus during and after intense neuronal activity induced by 20 Hz electrical stimulation in vivo. In CA1, stimulus trains to the CA3 region elicited an early alkaline shift associated with a later slow acid shift in all ages tested (2 weeks old, 3 weeks old and adult). In the dentate gyrus in adults, stimulation only elicited acidification. An initial alkalinization was only observed in the dentate gyrus in 2-week-old animals. The rate of recovery of the extracellular pH after termination of the stimulation was slower in the younger animals compared to adults in both CA1 and the dentate gyrus. The results indicate that the extracellular pH is regulated by mechanisms that undergo developmental changes that parallel the development of the dentate gyrus and development of the regulation of extracellular potassium.

The effects of gabapentin in the rat hippocampus are mimicked by two structural analogs, but not by nimodipine.

Gabapentin has been shown to reduce paired-pulse inhibition in the dentate gyrus of the urethane-anesthetized rat and has been shown to block calcium channels, but its not known how these possible mechanisms relate to its antiepileptic effect. Here, we tested two structural analogs of gabapentin for the ability to reduce seizure duration and to alter paired-pulse inhibition in the dentate gyrus in urethane anesthetized adult Sprague-Dawley rats. We compared with our results to those with diazepam, an anxiolytic and GABA(A) positive modulator and with nimodipine, a specific blocker of L-type Ca2+ channels. Both structural analogs of gabapentin caused a dose-dependent loss of paired-pulse inhibition and blocked the lengthening of the duration of the seizure discharge. Nimodipine also blocked the increase in duration of the seizure discharge, but increased paired-pulse inhibition. The effects of the GABA derivatives on paired-pulse inhibition and on seizure duration may have a common mechanism. Furthermore, our results indicate that gabapentin's postulated block of L-type calcium channels is not responsible for reducing paired-pulse inhibition. However, calcium channel block could still be the basis for the antiepileptic effect of gabapentin and its analogs.

A comparison of topiramate and acetazolamide on seizure duration and paired-pulse inhibition in the dentate gyrus of the rat.

Topiramate is a relatively new antiepileptic drug with several putative anticonvulsant mechanisms. Among them is the ability to inhibit carbonic anhydrase, a property in common with the anticonvulsant acetazolamide. This study examined the effects of topiramate and acetazolamide on the duration of epileptiform activity and on paired-pulse inhibition in the dentate gyrus in urethane anesthetized adult Sprague-Dawley rats. Neither topiramate nor acetazolamide altered excitability in the dentate gyrus, as measured with input-output curves or induction of long-term potentiation. Topiramate increased paired-pulse inhibition, whereas acetazolamide had no effect. Both drugs dose-dependently blocked the lengthening of the duration of epileptiform activity compared to vehicle controls. These results indicate that topiramate has an anticonvulsant-related effect (increase in paired-pulse inhibition), which may contribute to its antiepileptic effect, that is not dependent on its ability to inhibit carbonic anhydrase.

Extracellular pH responses in CA1 and the dentate gyrus during electrical stimulation, seizure discharges, and spreading depression.

Since neuronal excitability is sensitive to changes in extracellular pH and there is regional diversity in the changes in extracellular pH during neuronal activity, we examined the activity-dependent extracellular pH changes in the CA1 region and the dentate gyrus. In vivo, in the CA1 region, recurrent epileptiform activity induced by stimulus trains, bicuculline, and kainic acid resulted in biphasic pH shifts, consisting of an initial extracellular alkalinization followed by a slower acidification. In vitro, stimulus trains also evoked biphasic pH shifts in the CA1 region. However, in CA1, seizure activity in vitro induced in the absence of synaptic transmission, by perfusing with 0 Ca(2+)/5 mM K(+) medium, was only associated with extracellular acidification. In the dentate gyrus in vivo, seizure activity induced by stimulation to the angular bundle or by injection of either bicuculline or kainic acid was only associated with extracellular acidification. In vitro, stimulus trains evoked only acidification. In the dentate gyrus in vitro, recurrent epileptiform activity induced in the absence of synaptic transmission by perfusion with 0 Ca(2+)/8 mM K(+) medium was associated with extracellular acidification. To test whether glial cell depolarization plays a role in the regulation of the extracellular pH, slices were perfused with 1 mM barium. Barium increased the amplitude of the initial alkalinization in CA1 and caused the appearance of alkalinization in the dentate gyrus. In both CA1 and the dentate gyrus in vitro, spreading depression was associated with biphasic pH shifts. These results demonstrate that activity-dependent extracellular pH shifts differ between CA1 and dentate gyrus both in vivo and in vitro. The differences in pH fluctuations with neuronal activity might be a marker for the basis of the regional differences in seizure susceptibility between CA1 and the dentate gyrus.

Sodium pump activity, not glial spatial buffering, clears potassium after epileptiform activity induced in the dentate gyrus.

A number of mechanisms have been proposed to play a role in the regulation of activity-dependent variations in extracellular potassium concentration ([K(+)](o)). We tested possible regulatory mechanisms for [K(+)](o) during spontaneous recurrent epileptiform activity induced in the dentate gyrus of hippocampal slices from adult rats by perfusion with 8 mM potassium and 0-added calcium medium in an interface chamber. Local application of tetrodotoxin blocked local [K(+)](o) changes, suggesting that potassium is released and taken up locally. Perfusion with barium or cesium, blockers of the inward rectifying potassium channel, did not alter the baseline [K(+)](o), the ceiling level of [K(+)](o) reached during the burst, or the rate of [K(+)](o) recovery after termination of the bursts. Decreasing gap junctional conductance did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did cause a decrease in the ceiling level of [K(+)](o). Perfusion with furosemide, which will block cation/chloride cotransporters, or perfusion with low chloride did not change the baseline [K(+)](o) or the half-time of recovery of the [K(+)](o) after the bursts but did increase the ceiling level of [K(+)](o). Bath or local application of ouabain, a Na(+)/K(+)-ATPase inhibitor, increased the baseline [K(+)](o), slowed the rate of [K(+)](o) recovery, and induced spreading depression. These findings suggest that potassium redistribution by glia only plays a minor role in the regulation of [K(+)](o) in this model. The major regulator of [K(+)](o) in this model appears to be uptake via a Na(+)/K(+)-ATPase, most likely neuronal.

Activity-dependent intracellular acidification correlates with the duration of seizure activity.

Synchronized neuronal activity (seizures) can appear in the presence or absence of synaptic transmission. Mechanisms of seizure initiation in each of these conditions have been studied, but relatively few studies have addressed seizure termination. In particular, how are seizures terminated in the absence of synaptic activity where there is no loss of excitatory drive or augmentation of inhibitory inputs? We have studied dynamic activity-dependent changes of intracellular pH in the absence of synaptic transmission using the fluorescent pH indicator carboxylseminaphthorhodafluo-1. During epileptiform activity we observed intracellular acidification, whereas between seizures the intracellular pH recovered. Experimental conditions that shortened the epileptiform discharge correlated with more rapid intracellular acidification. On the other hand, experimental manipulation of intracellular pH altered the duration of the seizure discharge, with acidification resulting in early termination of the epileptiform activity. These data show a direct relationship between the level of intracellular acidification and the duration of the seizures, suggesting that an intracellular pH-dependent process can terminate nonsynaptic neuronal synchronization.

Cesium induces spontaneous epileptiform activity without changing extracellular potassium regulation in rat hippocampus.

Cesium has been widely used to study the roles of the hyperpolarization-activated (I(h)) and inwardly rectifying potassium (K(IR)) channels in many neuronal and nonneuronal cell types. Recently, extracellular application of cesium has been shown to produce epileptiform activity in brain slices, but the mechanisms for this are not known. It has been proposed that cesium blocks the K(IR) in glia, resulting in an abnormal accumulation of potassium in the extracellular space and inducing epileptiform activity. This hypothesis has been tested in hippocampal slices and cultured hippocampal neurons using potassium-sensitive microelectrodes. In the present study, application of cesium produced spontaneous epileptiform discharges at physiological extracellular potassium concentration ([K(+)](o)) in the CA1 and CA3 regions of hippocampal slices. This epileptiform activity was not mimicked by increasing the [K(+)](o). The epileptiform discharges induced by cesium were not blocked by the N-methyl-D- aspartate (NMDA) receptor antagonist AP-5, but were blocked by the non-NMDA receptor antagonist CNQX. In the dentate gyrus, cesium induced the appearance of spontaneous nonsynaptic field bursts in 0 added calcium and 3 mM potassium. Moreover, cesium increased the frequency of field bursts already present. In contrast, ZD-7288, a specific I(h) blocker, did not cause spontaneous epileptiform activity in CA1 and CA3, nor did it affect the field bursts in the dentate gyrus, suggesting that cesium induced epileptiform activity is not directly related to blockade of the I(h). When potassium-sensitive microelectrodes were used to measure [K(+)](o), there was no significant increase in [K(+)](o) in CA1 and CA3 after cesium application. In the dentate gyrus, cesium did not change the baseline level of [K(+)](o) or the rate of [K(+)](o) clearance after the field bursts. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also produced cellular burst activity without significantly changing the resting membrane potential, which might indicate an increase in [K(+)](o). Our results suggest that cesium causes epileptiform activity by a mechanism unrelated to an alteration in [K(+)](o) regulation.

Astrocytic regulation of the recovery of extracellular potassium after seizures in vivo.

Glial cells are believed to play a major role in the regulation of the extracellular potassium concentration ([K+]o), particularly when the [K+]o is increased. Using ion-selective electrodes, we compared the [K+]o changes in the dentate gyrus of urethane-anaesthetized adult rats in the presence of reactive astrocytes and after reduction of glial function. The regulation of [K+]o in the dentate gyrus was determined by measuring the ceiling level of [K+]o and the half-time of recovery of [K+]o during and after seizures produced by 20 Hz trains of stimulation to the angular bundle. Reactive astrocytes were induced by repeated seizures and their presence was confirmed by a qualitative increase in glial fibrillary acidic protein (GFAP) and vimentin immunoreactivity. To inhibit glial function, fluorocitrate (FC), a reversible metabolic inhibitor, or alpha-aminoadipate (alpha-AA), an irreversible toxin, was injected into the dentate gyrus region, and the regulation of [K+]o was monitored for 8 h or 2 days later, respectively. After alpha-aminoadipate, loss of astrocytes in the dentate gyrus was demonstrated by loss of staining for GFAP. In the presence of reactive astrocytes there was no significant change in the peak [K+]o during seizures or the half-time of recovery of [K+]o after seizures compared to control animals. alpha-Aminoadipate significantly slowed the rate of recovery of [K+]o, but did not change the ceiling [K+]o. Fluorocitrate reversibly decreased the ceiling [K+]o, but also slowed the rate of recovery of [K+]o. Overall our results suggest that normal glial function is required for the recovery of elevated [K+]o after seizures in vivo.

The reduction in paired-pulse inhibition in the rat hippocampus by gabapentin is independent of GABA(B) receptor receptor activation.

Previously we have shown that gabapentin causes a reduction of paired-pulse inhibition in the dentate gyrus of the urethane-anesthetized rat, which looks very much like the effect of baclofen on paired-pulse inhibition. In addition, it has been proposed that gabapentin increases release of GABA from non-vesicular stores and may, therefore, interact with GABA(B) mechanisms. Here we tested the ability of a GABA(B) agonist, baclofen, and a GABA(B) antagonist, CGP35348, to block the effect of gabapentin on paired-pulse inhibition in the dentate gyrus in urethane-anesthetized adult Sprague-Dawley rats. Both baclofen (6 mg/kg) and gabapentin (100 mg/kg) caused a long-lasting reduction of paired-pulse inhibition in the dentate gyrus when given alone or in combination. CGP35348 (45 mg/kg) blocked the effect of baclofen on paired-pulse inhibition, but did not alter the effect of gabapentin. Gabapentin also caused a reduction of inhibition in the CA1 region, indicating that its effect is not specific for the dentate gyrus. These results suggest that gabapentin produces its effect on paired-pulse inhibition independent from the effect of baclofen and not through non-vesicular release of GABA interacting with the GABA(B) receptor system.

Regulation of extracellular potassium in the developing hippocampus.

It has long been felt that one of the reasons that the immature brain has an increased propensity to seize is due to an inability to regulate the extracellular potassium ([K+]o). However, the data supporting this hypothesis is controversial. Here, we tested the regulation of [K+]o in urethane-anesthetized juvenile and adult rats during and after synchronized epileptiform activity induced by 20 Hz stimulus trains to the CA3 region. The regulation of [K+]o in CA3 was compared to the same measurements in CA1. Across all age groups, there was no difference between CA1 and CA3. There was a slight decrease in the peak level of [K+]o reached during stimulation only in the youngest age groups tested (PN9-11). There was an age-dependent change in the rate of recovery of [K+]o from the elevated levels. This recovery was slowest in the PN9-11 age group. In all the animals in the PN9-11 group and 3/7 animals in the PN14-15 group, there was a secondary increase in the [K+]o during the afterdischarge. This secondary rise was never observed in animals over 15 days of age. These data confirm that there is altered regulation of [K+]o in the developing brain that takes two forms: (1) the ability to recover from elevated [K+]o levels, suggesting maturation of regulatory, or uptake, mechanisms; and (2) that which is related to mechanisms of synchronization and initiation of afterdischarges and the level of [K+]o that is produced during the discharge.

Developmental changes in expression of myotonic dystrophy protein kinase in the rat central nervous system.

Myotonic dystrophy protein kinase (DMPK) is the protein product of the genetic locus associated with myotonic dystrophy, in which alterations of muscle excitability, cardiac conduction defects, mental retardation, and cognitive deficiencies are inherited as an autosomal dominant trait. DMPK belongs to a novel protein serine/threonine kinase family, but its regulation and physiological functions have not been specified. In a first step toward understanding the functions of DMPK in the central nervous system, we have characterized its localization and developmental pattern of expression in rat brain and spinal cord by using a monospecific rabbit antiserum produced against bacterially expressed DMPK. Expression of DMPK begins after birth and increases gradually to peak at postnatal day 21 with antibody labeling of neuronal cell types in many regions. After postnatal day 21 and proceeding to the adult, the pattern of expression becomes more restricted, with localization to certain regions or cell groups in the central nervous system. Electron microscopy reveals localization within adult spinal motor neurons to the endoplasmic reticulum and dendritic microtubules. The adult localizations suggest that DMPK may function in membrane trafficking and secretion within neurons associated with cognition, memory, and motor control.

Effects of felbamate, gabapentin and lamotrigine on seizure parameters and excitability in the rat hippocampus.

The effects of three new antiepileptic drugs (felbamate, lamotrigine and gabapentin) on the parameters of seizure initiation and termination in one model of reverberatory seizures in the hippocampal-parahippocampal loop in urethane-anesthetized rats were determined. All three of the drugs caused a dose-dependent decrease in the duration of the seizure discharge. Only very high doses of felbamate had a significant effect on seizure initiation. In addition to the seizure model, we tested the three new anti-epileptic drugs for an effect on excitability, on paired-pulse inhibition and on long-term potentiation (LTP) in the dentate gyrus of the urethane-anesthetized rat. None of the drugs altered LTP or excitability. However, gabapentin altered paired-pulse inhibition, causing a loss of inhibition. For felbamate and lamotrigine, the effects of seizure duration are consistent with their clinical effects as antiepileptic drugs. However, the effect of gabapentin on paired-pulse inhibition suggests a proconvulsant effect and the effect on seizure duration suggests an antiepileptic effect.

Role of potassium and calcium in the generation of cellular bursts in the dentate gyrus.

Epileptiform activity, which appears to be endogenous, has been recorded in the granule cells of the dentate gyrus before the onset of synchronized seizure activity and has been termed cellular bursts. It has been postulated that an increase in input to the dentate gyrus causes a local increase in extracellular K+ concentration ([K+]o) and a decrease in [Ca2+]o that results in this cellular bursting. The first test of this hypothesis is to determine whether the cellular bursts appear in ionic conditions that occur in vivo before the onset of synchronized epileptic activity. This hypothesis was tested in vitro by varying the ionic concentrations in the perfusing solution and recording changes in the granule cells of the dentate gyrus. Intra- and extracellular recordings were made in the dentate gyri of hippocampal slices prepared from anesthetized adult Sprague-Dawley rats. Increasing the extracellular potassium or decreasing the extracellular calcium of the perfusing solution caused three forms of spontaneous activity to appear: depolarizing potentials, action potentials, and cellular bursts. Increasing potassium or decreasing calcium also caused the granule cells to depolarize and reduced their input resistance. No synchronized extracellular field activity was detected. Simultaneously increasing potassium and decreasing calcium caused cellular bursts to appear at concentrations recorded in vivo before the onset of synchronized reverberatory seizure activity.

Is cell death necessary for hippocampal mossy fiber sprouting?

To examine the relationship between cell death and sprouting of the mossy fibers, repeated seizures of the hippocampal-parahippocampal circuit were elicited in anesthetized rats. The presence of mossy fiber growth was assessed with the Timm's stain for zinc. At 4 weeks, after 18 repeated seizures, there was a significant increase in the degree of zinc containing granules in the inner molecular layer of the dentate gyrus. The amount of sprouting was less than that seen four weeks after a single injection of kainic acid. A silver impregnation stain and an assay for damaged DNA were used to detect damaged or dying neurons and immunohistochemistry for a 72 kDa heat shock protein was used to detect any neurons that had suffered potentially injurious stress. The same number of repeated seizures that caused sprouting of the mossy fibers did not cause detectable cell death or severe stress in any cells within the hippocampus, subicular region or adjacent entorhinal cortex. These experiments demonstrate that repeated seizures of the hippocampal-parahippocampal circuits can cause sprouting of mossy fibers in the absence of evidence of cell death. This supports the hypothesis that alterations in intrinsic neural excitability and impulse activity from the dentate gyrus can result in growth of axonal processes in the adult rat brain.

Effect of seizures and diuretics on the osmolality of the cerebrospinal fluid.

There was a significant increase in the osmolality of the cerebrospinal fluid (CSF) of the anesthetized rats after treatment with 80 mg/kg (but not 39 mg/kg) furosemide and 1 g/kg of mannitol, but not during seizures induced by kainic acid. Furosemide (10 mg/kg) blocked seizure activity by kainic acid, while mannitol (1 g/kg) did not. The results suggest that the antiepileptic effect of furosemide is due to a direct CNS effect not related to a change in CSF osmolality.