Biomedical subjects

E W Lothman

Publications and source records attributed to E W Lothman.

At least 73 records · Page 4Linked to original sources

PubMed1991

Functional anatomy: a challenge for the decade of the brain.

At the beginning of the "Decade of the Brain," a number of aims for epilepsy research can be identified. These range from fundamental questions about the mechanisms of seizure etiology to the ways that seizures disturb brain function and structure. Information from such research will help in the development of new treatment and rehabilitative strategies for patients with epilepsy. Current tools from molecular biology have greatly expanded the promise and challenges for epilepsy research. As these investigative efforts are undertaken, however, it is important to recall the biological environment in which epilepsy occurs. Three major axes can be identified: the organizational axis (spanning from molecules through synapses to neuronal circuits to behavior), the time axis (relating to the chronobiology of epilepsy and diversities of seizure expression as a function of age), and the functional-anatomy axis (relating to the rich heterogeneity of the brain). Focusing on functional anatomy, certain principles can be developed from work with experimental models of seizures in the hippocampus and related structures. Three levels of functional anatomy are examined--network, local circuit, and individual neuron. This information can serve as a basis for directing epilepsy research in the future.

Brain↗

PubMed1990

A1 adenosinergic modulation alters the duration of maximal dentate activation.

The role of adenosine systems in the initiation and termination of seizures was examined using a unique marker for limbic seizures--maximal dentate activation (MDA). At 10 mg/kg 2-chloroadenosine shortened the duration of MDA, a measure of seizure terminating processes. The selective A1 agonist, cyclopentyladenosine, at 3 mg/kg, blocked the increase in duration of MDA, while the A1 antagonist, 1,3-dipropyl-8-cyclopentylxanthine (0.05 mg/kg) had the opposite effect. None of the compounds tested altered the time to onset of MDA, a measure of processes that initiate seizures. Therefore, modulation of A1 adenosine systems appears to alter seizure termination much more than seizure initiation.

2-Chloroadenosine↗

PubMed1990

Autoradiographic evidence that NMDA receptor-coupled channels are located postsynaptically and not presynaptically in the perforant path-dentate granule cell system of the rat hippocampal formation.

In vitro quantitative autoradiography with [3H]MK-801 was used to determine Kd and Bmax values for the NMDA receptor-coupled channel in subregions of the rat hippocampal formation. A single form of the channel with an apparent Kd in the 15-20 nM range was found for [3H]MK-801 binding in the presence of both 1 microM glutamate and 1 microM glycine. Specific binding was highest in the molecular layer of the dentate gyrus, followed by CA1 stratum radiatum and CA1 stratum oriens. Fewer binding sites were observed in the hilus of the dentate gyrus, cerebral cortex, CA1 stratum pyramidale, CA3 subregion (stratum oriens, stratum pyramidale, stratum radiatum), and thalamus. Selective destruction of dentate granule cells by colchicine microinjections reduced the amount of specific [3H]MK-801 binding by half in the molecular layer of the dentate, compared to intact tissue. [3H]MK-801 binding did not change in other hippocampal subregions as a consequence of colchicine injection. Electrolytic entorhinal cortical lesions produced no changes in regional MK-801 binding site density in any of the regions under study. To address the tissue shrinkage following entorhinal cortex lesions, detailed analysis of the binding site density per fixed (16 microns) length of granule cell dendrite, and of the aggregate density across the entire molecular layer revealed no change in the number of MK-801 binding sites per unit length of dendrite in the molecular layer of the dentate gyrus. These findings indicate that NMDA receptor-coupled channels are confined to a postsynaptic location in the perforant path-dentate granule cell system of the adult rat.

Animals↗

PubMed1990

The hippocampus in experimental chronic epilepsy: a morphometric analysis.

The effect of intermittent seizures on the pyramidal neurons of the hippocampus is largely unknown. To determine whether recurrent seizures centered in the hippocampus can produce neuronal loss in this region, a morphometric analysis was performed from standardized sections of hippocampus using 5 groups of animals: (1) surgical control subjects, (2) rats kindled by the rapidly recurring hippocampal seizure (RRHS) paradigm, (3) kindled rats with a few additional limbic seizures (528 +/- 66 seizures), (4) kindled rats with many limbic seizures (1,523 +/- 130 seizures), and (5) rats experiencing limbic status epilepticus (SE) induced by "continuous" hippocampal stimulation. The RRHS and SE protocols induced significant neuronal loss in the CA1 region, but no evidence was found for additional cell loss with increasing numbers of intermittent seizures. These intermittent seizures were, however, associated with a significant thickening of the basal and apical dendritic fields of the CA1 region. These findings indicate that intermittent seizures produce no significant hippocampal neuronal loss and may result in a hypertrophy of CA1 dendritic fields.

Animals↗

PubMed1990

Pharmacological evidence indicating a role of GABAergic systems in termination of limbic seizures.

The role of inhibitory systems in the initiation and termination of seizures in limbic circuits of the rat was tested by measuring the time to onset and duration of maximal dentate activation before and after the administration of GABAergic agents. Both 0.1 and 0.3 mg/kg of the GABAA receptor antagonist bicuculline lengthened maximal dentate activation while 0.3 mg/kg was needed to reversibly decrease the time to onset. Picrotoxin, an antagonist at the GABAA receptor/channel complex, lengthened maximal dentate activation, but did not alter the time to onset. Muscimol, a GABAA receptor agonist, shortened maximal dentate activation, but did not lengthen the time to onset. Baclofen, a GABAB receptor agonist (3 and 10 mg/kg), had no effect on the time to onset of maximal dentate activation, while 10 mg/kg baclofen shortened maximal dentate activation. These results demonstrate that agents that have selective action at both GABAA and GABAB synapses alter the duration of maximal dentate activation more than they influence the time to onset of maximal dentate activation and suggest that GABAergic mechanisms are critical in the termination of seizures.

Animals↗

PubMed1990

NMDA receptor activation mediates the loss of GABAergic inhibition induced by recurrent seizures.

Previously we have shown that delivery of rapidly recurring hippocampal seizures (RRHS) to awake rats causes a rapid kindling and that RRHS in urethane-anesthetized rats leads to a progressive lengthening of afterdischarges and diminution of paired pulse inhibition. The present experiments examined the relationship between the changes in afterdischarge durations and inhibition. Pre-treatment before RRHS with the non-competitive NMDA receptor antagonists MK-801 and ketamine blocked afterdischarge lengthening. MK-801 also prevented RRHS-induced changes in paired pulse inhibition. For pharmacodynamic and pharmacokinetic reasons the ability of ketamine to counteract RRHS-induced changes of paired pulse inhibition was not examined. MK-801 also blocked the rightward shift of stimulus intensity vs. population spike curves which RRHS caused. We suggest that RRHS leads to an enduring diminution of GABAergic inhibition and that this accounts, at least in part, for the lengthening of afterdischarges seen with recurrent hippocampal seizures. In addition, NMDA receptor activation appears to play a role in this decrease of the potency of GABAergic inhibition. However, mechanisms which are not dependent on NMDA receptor activation also play a critical role in hippocampal epileptogenesis.

Animals↗

PubMed1990

Maximal dentate activation: a tool to screen compounds for activity against limbic seizures.

A model of partial complex (limbic) seizures in the anesthetized rat based on the phenomenon of maximal dentate activation was used to study the effect of various known antiepileptic drugs. Maximal dentate activation consists of a distinct triad of large amplitude population spikes, associated with a rise in [K+]0 to 10 mM and a negative shift of the DC potential. Repeated stimulation produces lengthening of the duration of maximal dentate activation (mimicking the lengthening of afterdischarges that occurs during kindling) and a decrease in the time to onset of maximal dentate activation. Diazepam (3 mg/kg), carbamazepine (50 mg/kg) and phenobarbital (60 mg/kg) caused a reversible reduction in the duration of maximal dentate activation. Carbamazepine (30 mg/kg) and phenobarbital (20 mg/kg) prevented the lengthening of maximal dentate activation that occurs with repeated elicitation while phenytoin (80 mg/kg), ethosuximide (300 mg/kg) and valproic acid (300 mg/kg) had no effect. The doses of these known antiepileptic drugs agree with the doses efficacious against limbic seizures in awake rats. This suggests that the ability to shorten the duration of maximal dentate activation provides a useful means to screen compounds for activity against partial complex seizures.

Animals↗

PubMed1990

NMDA receptor antagonists and limbic status epilepticus: a comparison with standard anticonvulsants.

Status epilepticus (SE) evolves through several stages when untreated. The later stages of SE are less responsive to standard anticonvulsants and may require general anesthesia to suppress seizures. Antagonists acting at the N-methyl-D-aspartate (NMDA) subclass of glutamate (excitatory) receptors have been demonstrated to exert antiepileptic activity in some seizure models. We report experiments performed to determine if NMDA receptor antagonists are effective in stopping seizures in the late stages of SE. A model of limbic SE induced by 90 min of 'continuous' electrical stimulation of the hippocampus in rats was employed. Three NMDA receptor antagonists, one 'competitive' (CPP) and two 'non-competitive' (ketamine and MK-801), were compared to 3 standard antiepileptic drugs (diazepam, phenobarbital, and phenytoin) for their ability to suppress seizures at a physiologically defined stage of SE. All NMDA receptor antagonists, diazepam and phenobarbital were effective in suppressing behavioral and electrographic seizures for varying periods of time. Phenytoin had no effect on SE. Ketamine and MK-801 induced a paradoxical enhancement of electrographic seizures that preceded SE suppression. We believe that NMDA-receptor antagonists offer a novel approach for treating the late stages of SE.

Animals↗

PubMed1990

Use of maximal dentate activation to study the effect of drugs on kindling and kindled responses.

Maximal dentate activation is a paroxysmal event characterized by the appearance of bursts of large amplitude population spikes in the dentate gyrus that has been shown to be associated with the production of afterdischarges in limbic circuits. Repeated elicitation of maximal dentate activation in the anesthetized rat produces a progressive increase in afterdischarge duration. The present work examined the role of NMDA receptor activation in this process. The effects of MK-801 (a systemically active NMDA receptor complex antagonist) and imipramine and desipramine (tricyclic antidepressants recently reported to have NMDA receptor-coupled channel blocking properties) were studied. MK-801, at 0.5 mg/kg, prevented the lengthening of the duration of maximal dentate activation. At 2 mg/kg, MK-801 also shortened the duration of maximal dentate activation already established. Imipramine showed a similar dose-dependent response pattern with 15 mg/kg and 50 mg/kg respectively. Desipramine (50 mg/kg) was similar to imipramine (50 mg/kg) in its effect. Thus, the drugs studied were effective at shortening limbic epileptiform discharges. These results support the use of this model to examine drug effects and strengthen the hypothesis that NMDA receptors play a critical role in seizure generation in the limbic system and, specifically, in the lengthening of the duration of maximal dentate activation.

Animals↗

PubMed1990

Recurrent spontaneous hippocampal seizures in the rat as a chronic sequela to limbic status epilepticus.

A period of continuous hippocampal stimulation (CHS) establishes an acute condition of self-sustaining limbic status epilepticus (SSLSE) which is followed by chronic neuropathological changes reminiscent of hippocampal sclerosis encountered in epileptic patients. In the chronic (greater than or equal to 1 month) condition following CHS-induced SSLSE, extended electrographic monitoring in the hippocampus revealed spontaneous recurrent paroxysmal discharges. All 6 animals studied had persistent interictal spiking; 3 had multiple fully developed electrographic seizures. There was a marked diminution of paired pulse inhibition, demonstrated by a protocol known to reflect the potency of inhibition mediated by GABAA receptors. Hippocampal slices from animals that had previously experienced CHS-induced SSLSE demonstrated an increased excitability relative to slices from control animals as evidenced by epileptiform bursting in increased extracellular potassium ([K+]0) and decreased extracellular calcium ([Ca2+]0). These studies establish that CHS-induced SSLSE in rats provides an experimental model with recurrent spontaneous hippocampal seizures. Based on electrophysiological data we suggest that a decrease in GABA-mediated inhibition and/or altered sensitivity to extracellular ions may play roles in the development of such seizures.

Action Potentials↗

PubMed1989

Repetitive seizures cause an increase in paired-pulse inhibition in the dentate gyrus.

A paired-pulse stimulus protocol was used to measure angular bundle to dentate gyrus paired-pulse inhibition in rats before and after the occurrence of 36 or 72 seizures elicited from the contralateral CA3 region. The seizures showed progressive lengthening. There was a moderate increase in paired-pulse depression 1 h after 36 seizures and a further increase after 72 seizures. These data demonstrate that the same experimental protocol which produced a decrease in paired-pulse inhibition in the CA1 region caused the opposite effect in the dentate gyrus.

Anesthesia↗

PubMed1989

An in vivo electrophysiological study of the ontogeny of excitatory and inhibitory processes in the rat hippocampus.

Although several studies have compared hippocampal slices from young vs adult rats, a systematic in vivo characterization of the ontogeny of electrophysiologic responses in this structure has not been done. The current report describes the postnatal development of excitatory and inhibitory responses in the CA1 region of the hippocampus in rats 7-65 days of age. Under urethane anesthesia, a recording electrode was placed in one CA1 region to measure extracellular population spikes elicited by stimulation of the contralateral CA3 region. Age-related changes in the maximal population spike amplitude, the voltage required to elicit a half-maximal amplitude spike, the width at half-maximal spike amplitude, and latency to onset of the population spike ('conduction velocity') were monitored as parameters describing excitatory processes in the hippocampus. A paired-pulse paradigm was used to quantify the ontogeny of inhibitory processes. In younger animals, population spikes were broader, smaller in amplitude, and required higher stimulus intensities to be elicited. After postnatal (PN) day 14, excitability (voltage to elicit half-maximal population spike) and spike width were at fully mature levels. Maximal spike amplitudes were also smaller in rats younger than PN14, but not thereafter. The conduction velocity parameter steadily increased during development. In contrast, no evidence of inhibition was found prior to PN18, after which it steadily increased to reach adult levels by PN28. These results indicate that, in the rat hippocampus, excitatory processes are well established or fully mature within 2 weeks following birth, whereas the maturation of inhibitory processes to adult levels is not achieved until several weeks later.

Action Potentials↗

PubMed1989

The effect of MK-801 on kindled seizures: implications for use and limitations as an antiepileptic drug.

MK-801 is a new drug that produces a noncompetitive blockade at the subclass of glutamate receptors activated by N-methyl-D-asparate (NMDA). The antiepileptic properties of MK-801 were studied using kindled seizures as a model of complex partial seizures with secondary generalization. A test protocol was employed that allowed: (1) examination of the efficacy of MK-801 against several parameters that gauge different aspects of epileptogenesis; (2) determination of the time-action profile of these effects; and (3) examination of the toxicity of MK-801 in animals experiencing seizures. The drug was found to be potent against the spread of seizures but less effective against parameters linked to partial seizures. At the higher doses of the drug required to truncate hippocampal afterdischarges, considerable neurotoxicity was encountered. In addition, the antiepileptic effects of MK-801 showed a use dependence so that, at a given time after the drug was administered, a greater suppression of seizures was noted if there had been preceding seizures in the presence of the drug than if there had not been. These findings indicate that there may be limitations to the clinical utility of MK-801 as an antiepileptic agent and that the drug may provide greatest benefit when used for the suppression of seizure generalization and when seizures are closely spaced.

Animals↗

PubMed1989

Reduction of paired pulse inhibition in the CA1 region of the hippocampus by pilocarpine in naive and in amygdala-kindled rats.

Subconvulsant doses (20 mg/kg) of pilocarpine administered to a kindled rat convert a kindled seizure to status epilepticus. The hippocampus is involved in such status epilepticus. Furthermore, evidence is accumulating that GABA-mediated inhibition in the hippocampus is chronically diminished by kindling. The studies presented here compared the electrophysiologic effects of pilocarpine in vivo in the CA1 region of the hippocampus in naive and amygdala-kindled rats. A paired pulse paradigm previously shown to quantify the potency of GABAergic inhibition was employed. Stimuli were delivered in the CA3 region of urethane-anesthetized rats and population spikes were recorded in the contralateral CA1 region. In naive rats, pilocarpine (6-60 mg/kg) caused a left shift in the input-output curve measuring stimulus intensity vs population spike amplitudes, indicating an increase in neuronal excitability. In addition, paired pulse inhibition was reduced for interpulse intervals less than 70 ms. In amygdala-kindled rats, neuronal excitability was also enhanced following pilocarpine administration. The potency of baseline paired pulse inhibition was decreased in kindled rats compared to naive controls. Following pilocarpine, inhibition for interpulse intervals less than 70 ms was further reduced, but to a lesser extent than in naive rats. These findings suggest that the ability of subconvulsive doses of pilocarpine to change a discrete kindled seizure triggered by one stimulus to status epilepticus depends on the suppression of GABAergic inhibition below a critical level.

Amygdala↗

PubMed1989

In vitro and in vivo transplantation of fetal rat brain cells following incubation with various anatomic tracing substances.

Implantation of fetal brain regional anlage into host brains ('brain transplantation') holds promise as a plausible treatment for certain human neurodegenerative disorders. Improvements in experimental brain transplantation techniques include: (1) utilization of brain cells in tissue culture as opposed to freshly prepared cell suspensions as a transplantation source, (2) prelabeling of fetal brain cells with inert, non-toxic tracer substances to allow subsequent (a) unequivocal identification of those cells as being fetally derived, and (b) anatomical and immunohistochemical identification of transplanted neurons, and (3) development of in vitro models for transplantation to allow physiological studies of connections formed between fetal neurons and host brain tissue. We examined the ability of brain cell suspensions derived from rat fetuses 15-17 gestational days old to accumulate and retain anatomic tracing substances, including Phaseolus vulgaris leucoagglutinin (PHA-L), rhodamine-labeled latex microspheres (RLM) and fluorogold (FG). All tracers were rapidly accumulated by fetal brain cells, but only PHA-L and RLM were retained following implantation into adult hosts or in tissue culture in vitro. PHA-L-labeled fetal brain cells transplanted in vivo showed morphological characteristics similar to fetal neurons kept in tissue culture in vitro. RLM- or PHA-L-labeled fetal brain cells can be co-cultured with rat brain slices maintained in long-term roller culture. This in vitro system will allow identification and physiological or immunohistochemical study of interactions between fetally derived and host brain neurons.

Animals↗

PubMed1989

Effect of volatile anesthetics on synaptic transmission in the rat hippocampus.

The synaptic effects of halothane, isoflurane, and enflurane were examined in the rat hippocampus in vivo and compared with the effects of ketamine and urethane. Actions of the agents on excitatory amino acid-mediated neurotransmission were studied by observing evoked responses and long-term potentiation in the stratum pyramidale of CA1 with stimulation of the contralateral CA3 region. Long-term potentiation is a long-lasting increase in synaptic efficacy, which follows a brief stimulus train. It has been shown to be established through activation of the NMDA subclass of excitatory amino acid receptors and is thought to be involved in memory processing. Volatile anesthetics had no effect on evoked excitatory responses or on long-term potentiation. Actions of the anesthetics on inhibitory processes in the hippocampus were studied by pairing stimuli at a range of interpulse intervals. The first stimulus activated inhibitory processes that caused the response to the second stimulus to be smaller than the initial response, a phenomenon termed paired pulse depression. Paired pulse depression was significantly prolonged by the volatile anesthetics compared with that under urethane or ketamine. These results indicate that the mechanism of action of the volatile anesthetics at the hippocampal CA1 synapse does not involve amino acid-mediated excitation but does involve enhancement of inhibition.

Animals↗

PubMed1989

Evidence that repetitive seizures in the hippocampus cause a lasting reduction of GABAergic inhibition.

1. A method was developed to quantify paired-pulse depression of population spikes in the CA1 region of the hippocampus of urethane-anesthetized rats with paired stimuli to the contralateral CA3 region at various states of excitability of pyramidal cells. This method was applied to measure changes following recurrent seizures, a single seizure, or long-term potentiation (LTP). 2. In naive animals paired-pulse depression was highly variable at low stimulus intensities, but constant above a certain "threshold" stimulus intensity. The potency of paired-pulse depression also depended on the time between paired stimuli, being maximal at an interpulse interval of 20 ms. The general relationships of paired-pulse depression to stimulus intensity and to interpulse interval were unaltered after LTP, after a single seizure, and after recurrent seizures, but there were quantitative changes in the last two cases. 3. A variety of pharmacologic agents known to interact with GABAergic inhibition were studied for their effect on paired-pulse depression. These agents affected earlier phases of paired-pulse depression (interpulse intervals less than or equal to 100 ms). The GABA agonist muscimol and the benzodiazepine diazepam enhanced paired-pulse depression whereas the GABA antagonist bicuculline decreased it. 4. Repeated seizures elicited by trains (50-Hz, 10-s durations every 5 min) of electrical stimuli to the hippocampus were associated with progressive lengthening of afterdischarges. 5. Recurrent seizures caused a statistically significant reduction in the potency of earlier phases of paired-pulse depression. There was an increase in the potency of later phases of paired-pulse depression after recurrent seizures, but this was not statistically significant. These changes were present for at least 2 h after the last seizure. 6. An antidromic-orthdromic paired-pulse protocol was used to exclude slow conductance changes as the cause of paired-pulse depression. Paired-pulse depression measured with this method was also decreased by recurrent seizures. 7. A single seizure caused a small reduction in paired-pulse depression that dissipated in less than an hour. 8. A single seizure caused LTP of stimulus intensity versus population spike curves whereas recurrent seizures attenuated or even reversed the potentiation, leading to a rightward shift of the curves relative to control curves. When LTP was produced by a less intense stimulus train (50-Hz, 400-ms duration), there were no associated seizures nor was there any change in paired-pulse depression.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Loss of inhibition precedes delayed spontaneous seizures in the hippocampus after tetanic electrical stimulation.

1. The electrophysiological consequences of a "continuous" hippocampal stimulation protocol consisting of 10-s-long stimulus trains administered every 11 s for 90 min to the hippocampus of urethane-anesthetized rats were studied. During the period of stimulation, there was an initial phase with tonic discharges occurring every 1-2 min followed by a secondary phase in which there were well-demarcated recurrent clonic seizures. 2. At the end of the 90 min stimulation period, 7 out of a total of 14 animals studied show epileptiform activity that lasted one-half hour. The other half show disappearance of epileptiform activity within 2 min after terminating the stimulus. In all the animals a quiescent period then followed. In 11 of the animals studied, spontaneous seizures reappeared after the quiescent period had been present for several hours. 3. A paired-pulse method that in previous studies had been shown to reflect the functional state of GABAergic inhibition was employed. Paired-pulse depression in the CA1 region of the hippocampus was measured before administration of stimulus trains, in the quiescent period following the trains, and during and after spontaneous seizures. By comparing indices of maximal inhibition under the various experimental conditions, estimates of the relative amount of inhibition lost could be made. 4. All animals showed reduction of paired-pulse depression in the quiescent period following stimulation when compared with prestimulation values.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗