Preferential reduction of synaptic efficacy in the dentate gyrus of hippocampal slices from aged rats during reduced glucose availability.

Glutamatergic synaptic activity entails a high energetic cost. During aging, a variety of neural metabolic changes have been reported that could compromise the capacity of neural circuits to maintain synaptic transmission during periods of reduced extracellular glucose. Indeed, a preferential compromise in evoked synaptic activity has been observed in hippocampal CA1 with age during exposure to low-glucose solutions. Whether this aging-related compromise in synaptic activity is regionally specific is unclear, however. Data suggest that the dentate gyrus (DG) preferentially exhibits hypometabolism with age and this region plays a critical role in spatial pattern separation, which is compromised with age. Therefore, we assessed whether synaptic activity is also preferentially affected in the DG with age. In vitro extracellular field potential recordings were used to monitor orthodromic and antidromic evoked activity in the DG granule cell layer in hippocampal slices from adult (8-12 months) and aged (22-27 months) rats in aCSF containing 10mM glucose, followed by a reduced glucose aCSF containing 1mM glucose. In 10mM glucose-aCSF, orthodromic- and antidromic-evoked field potential activity was comparable between age groups. However, orthodromic-evoked population spike amplitude and field excitatory post-synaptic potential (EPSP) slope were preferentially decreased in slices from aged rats during exposure to 1mM glucose-aCSF. Antidromic population spike amplitude was not differentially affected in slices from aged versus adult rats, however. These data suggest that synaptic efficacy is preferentially compromised with age under reduced glucose availability and, combined with a decreased capacity of the periphery to provide glucose to the central nervous system (CNS) during metabolically challenging conditions, could contribute to aging-related hippocampal dysfunction and cognitive decline.

GABA uptake and heterotransport are impaired in the dentate gyrus of epileptic rats and humans with temporal lobe sclerosis.

In vivo dialysis and in vitro electrophysiological studies suggest that GABA uptake is altered in the dentate gyrus of human temporal lobe epileptics characterized with mesial temporal sclerosis (MTLE). Concordantly, anatomical studies have shown that the pattern of GABA-transporter immunoreactivity is also altered in this region. This decrease in GABA uptake, presumably due to a change in the GABA transporter system, may help preserve inhibitory tone interictally. However, transporter reversal can also occur under several conditions, including elevations in [K(+)]o, which occurs during seizures. Thus GABA transporters could contribute to seizure termination and propagation through heterotransport. To test whether GABA transport is compromised in both the forward (uptake) and reverse (heterotransport) direction in the sclerotic epileptic dentate gyrus, the physiological effects of microapplied GABA and nipecotic acid (NPA; a compound that induces heterotransport) were examined in granule cells in hippocampal slices from kainate (KA)-induced epileptic rats and patients with temporal lobe epilepsy (TLE). GABA- and NPA-induced responses were prolonged in granule cells from epileptic rats versus controls (51.3 and 31.3% increase, respectively) while the conductance change evoked with NPA microapplication was reduced by 40%. Furthermore the ratio of GABA/NPA conductance, but not duration, was significantly >1 in epileptic rats but not controls, suggesting a compromise in transporter function in both directions. Similar changes were observed in tissue resected from epileptic patients with medial temporal sclerosis but not in those without the anatomical changes associated with MTLE. These data suggest that the GABA transporter system is functionally compromised in both the forward and reverse directions in the dentate gyrus of chronically epileptic tissue characterized by mesial temporal sclerosis. This alteration may enhance inhibitory tone interically yet be permissive for seizure propagation due to a decreased probability for GABA heterotransport during seizures.

Lateral hypothalamus: early developmental expression and response to hypocretin (orexin).

Hypocretin is a recently discovered peptide that is synthesized by neurons in the lateral hypothalamic area (LH) and is believed to play a role in sleep regulation, arousal, endocrine control, and food intake. These functions are critical for the development of independent survival. We investigated the developmental profile of the hypocretin system in rats. Northern blot analysis showed that the expression of hypocretin mRNA increased from postnatal day 1 to adulthood. Both of the identified hypocretin receptor mRNAs were strongly expressed very early in hypothalamic development, and expression subsequently decreased in the mature brain. Immunocytochemistry revealed hypocretin-2 peptide expression in the cell bodies of LH neurons and in axons in the brain and spinal cord as early as embryonic day 19. Whole-cell patch clamp recordings from postnatal P1-P14 LH slices demonstrated a robust increase in synaptic activity in all LH neurons tested (n = 20) with a 383% increase in the frequency of spontaneous activity upon hypocretin-2 (1.5 microM) application. A similar increase in activity was found with hypocretin-1 application to LH slices. Hypocretin-2 evoked a robust increase in synaptic activity even on the earliest day tested, the day of birth. Furthermore, voltage-clamp recordings and calcium digital imaging experiments using cultured LH cells revealed that both hypocretin-1 and -2 induced enhancement of neuronal activity occurred as early as synaptic activity was detected. Thus, as in the adult central nervous system, hypocretin exerts a profound excitatory influence on neuronal activity early in development, which might contribute to the development of arousal, sleep regulation, feeding, and endocrine control.

Assessment of inhibition and epileptiform activity in the septal dentate gyrus of freely behaving rats during the first week after kainate treatment.

Mossy fiber reorganization has been hypothesized to restore inhibition months after kainate-induced status epilepticus. The time course of recovery of inhibition after kainate treatment, however, is not well established. We tested the hypothesis that if inhibition is decreased after kainate treatment, it is restored within the first week when little or no mossy fiber reorganization has occurred. Chronic in vivo recordings of the septal dentate gyrus were performed in rats before and 1, 4, and 7-8 d after kainate (multiple injections of 5 mg/kg, i.p.; n = 17) or saline (n = 11) treatment. Single and paired-pulse stimuli were used to assess synaptic inhibition. The first day after kainate treatment, only a fraction of rats showed multiple population spikes (35%), prolonged field postsynaptic potentials (76%), and loss of paired-pulse inhibition (29%) to perforant path stimulation. Thus, inhibition was reduced in only some of the kainate-treated rats. By 7-8 d after treatment, nearly all kainate-treated rats showed partial or full recovery in these response characteristics. Histological analysis indicated that kainate-treated rats had a significant decrease in the number of hilar neurons compared to controls, but Timm staining showed little to no mossy fiber reorganization. These results suggest that a decrease in synaptic inhibition in the septal dentate gyrus is not a prerequisite for epileptogenesis and that most of the recovery of inhibition occurs before robust Timm staining in the inner molecular layer.

Abnormal responses to perforant path stimulation in the dentate gyrus of slices from rats with kainate-induced epilepsy and mossy fiber reorganization.

Previous electrophysiological studies have demonstrated that in a subset of hippocampal slices from tissue resected from patients with mesial temporal lobe epilepsy, perforant path stimulation can elicit prolonged negative field-potential shifts in the dentate granule cell layer (Masukawa et al., 1989. Brain Res. 493, 168-174; Isokawa and Fried, 1996. Neuroscience 72, 31-37). In this investigation, hippocampal slices were prepared from rats: (1) 2-4 days following kainate treatment, when little or no reorganization of the mossy fibers would be present and (2) 3-13 months after kainate treatment, when mossy fiber reorganization would have occurred. In saline-treated controls, perforant path stimulation typically evoked a single population spike. In contrast, perforant path stimulation could evoke 3-12 population spikes in nearly all slices from kainate-injected rats 2-4 days and 3-13 months after treatment. The majority of slices from kainate-injected rats 3-13 months after treatment had qualitatively similar responses to perforant path stimulation as that observed in slices from kainate-injected rats 2-4 days after treatment. However, in 17% of the slices from kainate-treated rats 3-13 months after treatment (29% of rats), the multiple population spikes were followed by a prolonged negative field-potential shift (duration: 140 ms-1.5 s) with variable superimposed population spike activity. This type of epileptiform activity was only observed in slices with robust Timm's staining in the inner molecular layer and similar responses could also be evoked in these slices with hilar stimulation. Furthermore, pharmacological depression of inhibition by adding the GABA(A) receptor antagonist bicuculline unmasked hilar-evoked prolonged negative field-potential shifts in most slices from kainate-treated rats 3-13 months following treatment, and these slices had robust Timm's staining in the inner molecular layer. Such events were not observed in slices from saline-treated controls or kainate-injected rats 2-4 days after treatment. In conclusion, the prolonged negative field-potential shifts evoked to perforant path stimulation in normal ACSF were associated with mossy fiber reorganization, but the relative contribution of altered inhibition, increased synaptic excitation, or even non-synaptic mechanisms is unknown.

Kainate acts at presynaptic receptors to increase GABA release from hypothalamic neurons.

Recent reports suggest that kainate acting at presynaptic receptors reduces the release of the inhibitory transmitter GABA from hippocampal neurons. In contrast, in the hypothalamus in the presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor antagonists [1-(4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466) and D,L-2-amino-5-phosphonopentanoic acid (AP5)], kainate increased GABA release. In the presence of tetrodotoxin, the frequency, but not the amplitude, of GABA-mediated miniature inhibitory postsynaptic currents (IPSCs) was enhanced by kainate, consistent with a presynaptic site of action. Postsynaptic activation of kainate receptors on cell bodies/dendrites was also found. In contrast to the hippocampus where kainate increases excitability by reducing GABA release, in the hypothalamus where a much higher number of GABAergic cells exist, kainate-mediated activation of transmitter release from inhibitory neurons may reduce the level of neuronal activity in the postsynaptic cell.

NPY inhibits glutamatergic excitation in the epileptic human dentate gyrus.

Neuropeptide Y (NPY) has been shown to depress hyperexcitable activity that has been acutely induced in the normal rat brain. To test the hypothesis that NPY can also reduce excitability in the chronically epileptic human brain, we recorded intracellularly from dentate granule cells in hippocampal slices from patients with hippocampal seizure onset. NPY had a potent and long-lasting inhibitory action on perforant path-evoked excitatory responses. In comparison, the group 3 metabotropic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4) evoked a mild and transient decrease. NPY-containing axons were found throughout the hippocampus, and in many epileptic patients were reorganized, particularly in the dentate molecular layer. NPY may therefore play a beneficial role in reducing granule cell excitability in chronically epileptic human tissue, and subsequently limit seizure severity.

Decrease in inhibition in dentate granule cells from patients with medial temporal lobe epilepsy.

Alterations in synaptic inhibition are associated with epileptiform activity in several acute animal models; however, it is not clear if there are changes in inhibition in chronically epileptic tissue. We have used intracellular recordings from granule cells of patients with temporal lobe epilepsy to determine whether synaptic inhibition is compromised. Two groups of patients with medial temporal lobe epilepsy were used, those with medial temporal lobe sclerosis (MTLE), and those with extrahippocampal masses (MaTLE) where the cell loss and synaptic reorganization that characterize MTLE are not seen. Although the level of tonic inhibition at the somata was not significantly different in the two patient groups, there was a reduction in the conductance of polysynaptic perforant path-evoked fast and slow inhibitory postsynaptic potentials (IPSPs) (53% and 66%, respectively). We found that there was a comparable decrease in the monosynaptic IPSP conductances examined in the presence of glutamatergic antagonists as that seen for the polysynaptically evoked IPSPs. These data suggest that the decrease in inhibition seen in normal artificial cerebrospinal fluid in MTLE granule cells cannot be solely explained by a decrease in excitatory input onto inhibitory interneurons and may reflect changes at the interneuron-granule cells synapse or in the number of specific inhibitory interneurons.

Glutamate inhibits GABA excitatory activity in developing neurons.

In contrast to the mature brain, in which GABA is the major inhibitory neurotransmitter, in the developing brain GABA can be excitatory, leading to depolarization, increased cytoplasmic calcium, and action potentials. We find in developing hypothalamic neurons that glutamate can inhibit the excitatory actions of GABA, as revealed with fura-2 digital imaging and whole-cell recording in cultures and brain slices. Several mechanisms for the inhibitory role of glutamate were identified. Glutamate reduced the amplitude of the cytoplasmic calcium rise evoked by GABA, in part by activation of group II metabotropic glutamate receptors (mGluRs). Presynaptically, activation of the group III mGluRs caused a striking inhibition of GABA release in early stages of synapse formation. Similar inhibitory actions of the group III mGluR agonist L-AP4 on depolarizing GABA activity were found in developing hypothalamic, cortical, and spinal cord neurons in vitro, suggesting this may be a widespread mechanism of inhibition in neurons throughout the developing brain. Antagonists of group III mGluRs increased GABA activity, suggesting an ongoing spontaneous glutamate-mediated inhibition of excitatory GABA actions in developing neurons. Northern blots revealed that many mGluRs were expressed early in brain development, including times of synaptogenesis. Together these data suggest that in developing neurons glutamate can inhibit the excitatory actions of GABA at both presynaptic and postsynaptic sites, and this may be one set of mechanisms whereby the actions of two excitatory transmitters, GABA and glutamate, do not lead to runaway excitation in the developing brain. In addition to its independent excitatory role that has been the subject of much attention, our data suggest that glutamate may also play an inhibitory role in modulating the calcium-elevating actions of GABA that may affect neuronal migration, synapse formation, neurite outgrowth, and growth cone guidance during early brain development.

Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy.

Human temporal lobe epilepsy is associated with complex partial seizures that can produce secondarily generalized seizures and motor convulsions. In some patients with temporal lobe epilepsy, the seizures and convulsions occur following a latent period after an initial injury and may progressively increase in frequency for much of the patient's life. Available animal models of temporal lobe epilepsy are produced by acute treatments that often have high mortality rates and/or are associated with a low proportion of animals developing spontaneous chronic motor seizures. In this study, rats were given multiple low-dose intraperitoneal (i.p.) injections of kainate in order to minimize the mortality rate usually associated with single high-dose injections. We tested the hypothesis that these kainate-treated rats consistently develop a chronic epileptic state (i.e. long-term occurrence of spontaneous, generalized seizures and motor convulsions) following a latent period after the initial treatment. Kainate (5 mg/kg per h, i.p.) was administered to rats every hour for several hours so that class III-V seizures were elicited for > or = 3 h, while control rats were treated similarly with saline. This treatment protocol had a relatively low mortality rate (15%). After acute treatment, rats were observed for the occurrence of motor seizures for 6-8 h/week. Nearly all of the kainate-treated rats (97%) had two or more spontaneous motor seizures months after treatment. With this observation protocol, the average latency for the first spontaneous motor seizure was 77+/-38 (+/-S.D.) days after treatment. Although variability was observed between rats, seizure frequency initially increased with time after treatment, and nearly all of the kainate-treated rats (91%) had spontaneous motor seizures until the time of euthanasia (i.e. 5-22 months after treatment). Therefore, multiple low-dose injections of kainate, which cause recurrent motor seizures for > or = 3 h, lead to the development of a chronic epileptic state that is characterized by (i) a latent period before the onset of chronic motor seizures, and (ii) a high but variable seizure frequency that initially increases with time after the first chronic seizure. This modification of the kainate-treatment protocol is efficient and relatively simple, and the properties of the chronic epileptic state appear similar to severe human temporal lobe epilepsy. Furthermore, the observation that seizure frequency initially increased as a function of time after kainate treatment supports the hypothesis that temporal lobe epilepsy can be a progressive syndrome.

Physiological unmasking of new glutamatergic pathways in the dentate gyrus of hippocampal slices from kainate-induced epileptic rats.

In humans with temporal lobe epilepsy and kainate-treated rats, the mossy fibers of the dentate granule cells send collateral axons into the inner molecular layer. Prior investigations on kainate-treated rats demonstrated that abnormal hilar-evoked events can occasionally be observed in slices with mossy fiber sprouting when gamma-aminobutyric acid-A (GABAA)-mediated inhibition is blocked with bicuculline. However, these abnormalities were observed infrequently, and it was unknown whether these rats were epileptic. Wuarin and Dudek reported that in slices from kainate-induced epileptic rats (3-13 mo after treatment), hilar stimulation evoked abnormal events in most slices with mossy fiber sprouting exposed simultaneously to bicuculline and elevated extracellular potassium concentration [K+]o. Using the same rats, extracellular recordings were obtained from granule cells in hippocampal slices to determine whether 1) hilar stimulation could evoke abnormal events in slices with sprouting in normal artificial cerebrospinal fluid (ACSF), 2) adding only bicuculline could unmask hilar-evoked abnormalities and glutamate-receptor antagonists could block these events, and 3) increasing only [K+]o could unmask these abnormalities. In normal ACSF, hilar stimulation evoked abnormal field potentials in 27% of slices with sprouting versus controls without sprouting (i.e., saline-treated or only 2-4 days after kainate treatment). In bicuculline (10 microM) alone, hilar stimulation triggered prolonged field potentials in 84% of slices with sprouting, but not in slices from the two control groups. Addition of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), either blocked the bursts or reduced their probability of occurrence. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), always eliminated the epileptiform bursts. In kainate-treated rats with sprouting, but not in saline-treated controls, abnormal hilar-evoked responses were also revealed in 6-9 mM [K+]o. Additionally, 63% of slices with sprouting generated spontaneous bursts lasting 1-40 s in ACSF containing 9 mm [K+]o; similar bursts were not observed in controls. These results indicate that 1) mossy fiber sprouting is associated with new glutamatergic pathways, and although NMDA receptors are important for propagation through these circuits, AMPA receptor activation is crucial, 2) modest elevations of [K+]o, in a range that would have relatively little effect on granule cells, can unmask these new excitatory circuits and generate epileptiform bursts, and 3) this new circuitry underlies an increased electrographic seizure susceptibility when inhibition is depressed or membrane excitability is increased.

Multiple-unit recordings during slow field-potential shifts in low-[Ca2+]0 solutions in rat hippocampal and cortical slices.

Multiple-unit and field-potential recordings in low-[Ca2+]0 solutions were used to study epileptiform bursts generated in hippocampal region CA1 and medial entorhinal cortex, a cortical region that is not as densely packed or highly laminated as the hippocampus. As expected in CA1, multiple-unit activity appeared as large spikes that corresponded one-to-one with population spikes in the field-potential recordings. During the negative field-potential shifts that lacked large population spikes, the multiple-unit recordings showed an increase in baseline activity. Initiation of the negative field-potential shift always coincided with increased multiple-unit activity. Slices displaying a post-burst positive overshoot showed a corresponding decrease in multiple-unit activity. In addition to the large ictal-like events in CA1, small-amplitude field-potential shifts were also observed, these events were associated with increases in baseline spike activity in the multiple-unit recording. These small-amplitude field-potential shifts appeared to precede the occurrence of the ictal-like events, but they decreased in frequency during low-[Ca2+]0 exposure. Recordings in normal artificial cerebrospinal fluid (nominally 1.3 mM [Ca2+]0) showed rhythmic, multiple-unit bursts of action potentials and corresponding negative small-amplitude field-potential shifts in the medial entorhinal cortex of immature rats (two-to three-weeks old), but not of adult rats. Rhythmic, spontaneous bursts of activity in low-[Ca2+]0 solution were found in both immature and adult medial entorhinal cortex, and were similar in amplitude to the small field-potential events generated in CA1. The probability of burst generation was higher in the immature than the adult medial entorhinal cortex, and the bursts in the immature cortex had more robust multiple-unit activity and an increased burst frequency compared with adult. These results indicate that the medial entorhinal cortex can also generate spontaneous synchronous bursts of activity in low-[Ca2+]0 solutions, and they suggest that the increased susceptibility of medial entorhinal cortex from immature versus adult rats to generate intense bursts of electrical activity does not require active chemical synaptic transmission. The various forms of epileptiform activity in low-[Ca2+]0 solutions probably arise from different contributions of electrical and ionic mechanisms of synchronization in these neuronal populations. The data suggest the hypothesis that ionic mechanisms (i.e. changes in [K+]0) may synchronize neurons in cortical regions (e.g. entorhinal cortex) that are not as densely packed and highly laminated as the hippocampus and dentate gyrus. The data also support the hypothesis that these mechanisms contribute significantly to the increased seizure susceptibility of the immature brain.

Potassium-dependent prolonged field bursts in the dentate gyrus: effects of extracellular calcium and amino acid receptor antagonists.

The dentate gyrus in rat hippocampal slices produces spontaneous, prolonged bursts of population spikes (i.e. prolonged field bursts) when [Ca2+]0 is lowered (0-0.5 mM) and [K+]0 is concurrently elevated (9-11 mM). In this investigation we examined whether the dentate gyrus could also generate spontaneous field bursts in relatively "normal" (i.e. nominal 1.3 mM) or only moderately decreased [Ca2+]0 (i.e. nominal 0.9 mM). In 1.3 mM [Ca2+]0, no prolonged field bursts occurred spontaneously in the dentate gyrus when [K+]0 was raised as high as 12 mM. Prolonged field bursts were generated, however, when [K+]0 was further increased to 13-15 mM. Similar bursts could be generated at [K+]0 within the "physiological ceiling level" observed in vivo during seizure activity (i.e. 11-12 mM) if: (i) the bath [Ca2+] was reduced to 0.9 mM; or (ii) the GABA type A-receptor antagonist bicuculline was added in the presence of "normal" (1.3 mM) [Ca2+]0. Adding both the N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonists, (+/-)-2-amino-5-phosphonopentanoic acid (50-100 microM) and 6,7-dinitroquinoxaline-2,3-dione (50-100 microM), respectively, did not block the occurrence of the field bursts. The bursts generated in 1.3 mM [Ca2+]0, 12 mM [K+]0, bicuculline, (+/-)-2-amino-5-phosphonopentanoic acid and 6,7-dinitroquinoxaline-2,3-dione could, however, be reversibly depressed or blocked if [Ca2+]0a was raised to 2.0 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology and distribution of astrocytes in the molecular layer of the dentate gyrus in NZB/BlNJ, Dreher, and C57BL/6J mice.

Numerous ectopic granule cells are found in the molecular layer of the dentate gyrus in NZB/BlNJ and dreher mutant mice. These ectopic neurons occur either singly or in small clusters. In contrast, few ectopic granule cells are seen in the dentate molecular layer of C57BL/6J mice and dreher control littermates. In this investigation we have examined the morphology, number, and distribution of molecular layer astrocytes in NZB/BlNJ, dreher homozygotes, dreher littermate controls, and C57BL/6J mice to determine the effect of the presence of ectopic granule cells. In the molecular layer of C57BL/6J mice and dreher control littermates, astrocytes have a typical stellate appearance with processes emanating in all directions. The arborization of astrocytes in areas devoid of ectopic granule cells in NZB/BlNJ mice and dreher homozygotes was similar to that in C57BL/6J mice and dreher control littermates. In contrast, the morphology of astrocytes in the immediate vicinity of ectopic granule cells or ectopic clusters was distinctly non-stellate. Furthermore, the somata and processes of these astrocytes occasionally appeared to make intimate contact with the ectopic granule cells. A quantitative analysis of the number and distribution of astrocytes in NZB/BlNJ vs C57BL/6J mice and dreher vs. control littermates indicated that these parameters were not altered by the presence of the ectopic neurons. We conclude that the trophic effects of ectopic neurons on glial cells can affect the growth and orientation of astrocytic processes without a concomitant effect on glial cell number.

Prolonged field bursts in the dentate gyrus: dependence on low calcium, high potassium, and nonsynaptic mechanisms.

1. The dentate gyrus has been proposed to be a gate for entry of neuronal activity into the hippocampus. This function would give it a critical role in the propagation of seizure activity in that region. The hallmark of epileptiform activity in the dentate itself, often referred to as "maximal dentate activation" (MDA), has not been reproduced previously in vitro. 2. With the use of rat hippocampal slices, bath [Ca2+] was decreased, and [K+] was increased concurrently to simulate conditions found during intense neuronal activity in vivo. Both evoked and spontaneous field bursts were observed in the dentate granule cell layer under these conditions. These bursts were similar to MDA, consisting of a prolonged negative shift in extracellular potential with large-amplitude population spikes. 3. In 0.5 mM bath [Ca2+], single stimuli applied to the perforant path could evoke prolonged field bursts in the dentate only when bath [K+] was > or = 9 mM. However, repetitive stimulation (10 Hz) of the perforant path could elicit similar dentate responses when bath [K+] was as low as 5 mM. 4. In 0.5 mM bath [Ca2+], interictal-type bursts appeared spontaneously in CA1 and CA3 when bath [K+] was > or = 5 mM but were lost when [K+] was > 9 mM. Spontaneous seizurelike activity in the dentate required a higher minimum bath [K+] (9 mM) and persisted at [K+] of 11 mM. 5. Stimulation-evoked field bursts in the dentate altered epileptiform activity in CA3. At bath [K+] insufficient to cause spontaneous CA3 bursts, CA3 was activated transiently when prolonged field bursts occurred in the dentate. At higher bath [K+] in which spontaneous CA3 bursts did occur, they were depressed during the dentate bursts. 6. Deletion of Ca2+ from the bath; the addition of 30 microM each of bicuculline methiodide, D,L-2-amino-5-phosphonopentanoate (AP-5), and 6,7-dinitroquinoxaline-2,3-dione (DNQX); or the combination of both manipulations did not block antidromically evoked or spontaneous prolonged field bursts in the dentate. Thus the mechanisms maintaining and propagating these events did not require fast amino acid-mediated synaptic transmission. 7. The concurrent alteration of [K+] and [Ca2+] required to produce prolonged field bursts in the dentate underscores the positive feedback relationship between neuronal excitation and extracellular ionic concentrations, whereas the ability of synaptic stimulation to trigger nonsynaptic seizurelike events such as these prolonged field bursts may be relevant to the transition from interictal to ictal activity in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterozygote effects in dreher mice.

The dreher mutation (gene symbol: dr) is an autosomal recessive mutation located on chromosome 1 of the mouse. Homozygous dreher mice (dr/dr) are ataxic, have a white belly spot, a short-tail, inner ear and skeletal malformations, and a variety of CNS abnormalities. Recently in our dreher colony (the drsst-J allele on a B6C3Fe background), we noticed mice with one or more white belly spots typical of drsst-J/drsst-J mice but which were non-ataxic and had a normal tail length; wild-type mice (+/+) of the same genetic background do not have simialr belly spots. Results of three breeding experiments indicate that a new mutation had not occurred, but rather that the spotted, non-ataxic mice are heterozygous dreher mice (drsst-J/+). Histological examination showed that drsst-J/+ mice have abnormalities in the hippocampal formation that are qualitatively similar to those found in drsst-J/drsst-J mice. Most frequently there is an increase in the number of pyramidal cells in CA3 and a marked thickening of the pyramidal cell layer. In contrast to dreher homozygotes the cerebellum appears to have a normal foliation pattern and no discernible laminar abnormalities. Thus, both breeding experiments and histological examination indicate that drsst-J is semidominant. We speculate that drsst-J is a "loss of function" mutation, but, in any event, the presence of phenotypic abnormalities in drsst-J/+ mice may be useful in identifying the primary developmental defect in dreher mice.