Do fits really beget fits? The effect of previous epileptic activity on the subsequent induction of the tetanus toxin model of limbic epilepsy in the rat.

The effect of pretreatment with either tetanus toxin (in ventral hippocampus) or kainic acid (into dorsal hippocampus, with or without suppression of seizures by phenobarbital) on the subsequent development of epilepsy in rats injected with tetanus toxin (into ventral hippocampus) has been studied. Both treatments advanced the timing of the development of the subsequent epilepsy by a few days but did not affect the severity of the syndrome. The fits stopped after 3 weeks in all the rats but recurred in 6 of 20 of those given kainic acid, with or without phenobarbital, but not in those given only tetanus toxin. It is concluded that while fits make the brain more sensitive to a further epileptogenic stimulus they do not themselves increase their severity or persistence. It is the destruction of the CA3/4 area of the hippocampus which results in this advance and in the predisposition to permanent epilepsy.

Gamma-vinyl GABA reduces paired pulse inhibition in the rat dentate gyrus in vivo and in vitro.

Gamma vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, has anticonvulsant effects. GVG increases GABA levels in the brain by blocking its degradation, and is presumed to enhance GABAergic inhibition, however, in some cases it exacerbates seizures. We investigated the effects of GVG in vivo and in vitro on paired pulse inhibition (PPI) recorded in the rat dentate gyrus (DG) evoked by perforant path stimulation. At 2.5 h and 24 h after administration of GVG (1 g/kg, i.p.), there was a loss of PPI at both 15- and 25-ms interpulse intervals (IPI). Activation of presynaptic GABA(B) autoreceptors could explain this in vivo effect. We therefore further investigated the effects of co-application of GVG with the GABA(B) antagonists 2-OH saclofen (saclofen) or CGP 35348 (CGP) on PPI in hippocampal slices by in vitro study. Bath application of GVG (400 and 500 microM) not only resulted in a loss of perforant path evoked PPI at a 15-ms IPI, but produced facilitation of the second population spike relative to the first. Co-application of saclofen (250 microM) with GVG (500 microM) prevented facilitation of the second response of a paired-pulse. The facilitation of the second stimulation response produced by GVG (400 microM) was converted to inhibition by bath application of CGP 35348 (400 microM). These results suggest that activation of presynaptic GABA(B) receptors by increased extracellular GABA may be one of the contributing factors to the apparent paradoxical effect of GVG on PPI in the DG.

The 'gender gap' in final examination results at Oxford University.

A lower proportion of women than men obtain first class degrees at British universities (the so-called gender gap). At Oxford University, this difference is not seen in all degree subjects but is found both in some Arts and in some Science subjects. We have used a questionnaire administered under supervision to undergraduates 2 to 3 months before their final examination to assess factors which might be expected to affect examination performance. These included measures of verbal and non-verbal reasoning (Alice Heim AH6 test), self-esteem, motivation, responses to stresses of examinations and of personal relationships, happiness, risk-taking and working patterns. We have also obtained a detailed breakdown of the marks the students were given in the examination. Women scored higher on negative emotions while men scored higher on self-esteem, their perception of their own academic efficacy and on risk-taking strategies, but none of these factors predicted outcome. Verbal reasoning ability did predict outcome but there was no gender difference. Hence, it is concluded that the gender gap is not due to any of these individual differences and is more likely to be related to the nature of the academic assessment system.

The effects of tetanus toxin-induced limbic epilepsy on the exploratory response to novelty in the rat.

PURPOSE: To investigate the exploratory response to novelty in rats that have recovered from experimental limbic epilepsy. METHODS: Epilepsy was induced in 12 male rats by injecting a minute amount of tetanus toxin into the ventral hippocampus (and buffer vehicle was injected into 12 controls). Eight weeks after the injection, when the animals appeared behaviourally normal (and previous work would indicate that their electroencephalograms also would have returned to normal), they were tested on the playground maze. In this, their exploratory response to a novel object introduced in the context of seven familiar objects is measured. Simultaneously, their locomotion and investigation of familiar objects is measured. RESULTS: Whereas the control animals showed a significant response to the novel object on both test days, in the toxin-injected rats the novelty response was not present. There was no difference between the groups on the locomotion measure, but the toxin rats explored the familiar as well as the novel objects less. CONCLUSIONS: The exploratory response to a novel object was abolished in the previously epileptic rats.

Forebrain ischaemia with CA1 cell loss impairs epileptogenesis in the tetanus toxin limbic seizure model.

There is a long-standing controversy as to whether Ammon's horn sclerosis is the result or the cause of severe limbic epilepsy. In the tetanus toxin model of limbic epilepsy, rats have intermittent spontaneous fits over a period of 3-6 weeks after injection of tetanus toxin into the hippocampus. The fits then usually remit and the EEG returns to normal. In a few rats, however, the fits recur some weeks to months later, and it was previously found that in these rats there was gross cell loss in area CA1 of the dorsal hippocampus (distant from the injection site in ventral hippocampus). Such cell loss might either promote recurrence of fits or be the result of the recurrence. In the present experiment, the effect of previous induction of CA1 cell loss by transient 4-vessel occlusion cerebral ischaemia on the subsequent development of the tetanus toxin-induced epilepsy was studied, using continuous time-lapse video monitoring to assess the number of fits. The hypothesis that the previous forebrain ischaemia would predispose rats to reoccurring fits was not supported: no rats in the ischaemia group had reoccurring fits and additionally fits were delayed and fewer occurred than in the control groups.

The effect of experimental epilepsy induced by injection of tetanus toxin into the amygdala of the rat on eating behaviour and response to novelty.

A minute dose of tetanus toxin injected into the amygdala of rats produced an apparently reversible epileptiform syndrome similar to that previously described after injection of the toxin into the hippocampus. During the active epilepsy the toxin-injected rats occasionally exhibited 'paroxysmal eating' and also sometimes ran round in circles attempting to bite their own tails. When presented with a novel but palatable food (chocolate buttons or harvest crunch) the toxin-injected rats showed less neophobia than their controls--they ate sooner and ate more. This was found both during the active epilepsy and several weeks later when they had recovered. A similar effect of amygdala injections was found in a second experiment, in which the effect was compared with that of toxin injection in the hippocampus. These rats were tested also on the playground maze on their approach response to a neutral novel object (in a familiar environment in the context of seven familiar objects). The amygdala rats did not show any increase in their novelty response; thus their reduction in neophobia was specific to an appetitive behaviour. In contrast, the hippocampally-injected rats did not exhibit a novelty response in the playground maze, but showed normal neophobia to a new food.

Heparin injection into the adult rat hippocampus induces seizures in the absence of macroscopic abnormalities.

The pathological hallmarks of Alzheimer's disease include neurofibrillary tangles, neuropil threads and neuritic plaques. Neurofibrillary tangles and neuropil threads are comprised of paired helical filaments which are themselves composed of a hyperphosphorylated form of the microtubule-associated protein tau. Neuritic plaques are extracellular deposits of aggregated beta amyloid associated with neurites containing hyperphosphorylated tau. The mechanisms by which the neurofibrillary tangles and neuritic plaques develop in Alzhemier's disease are not clear but it is hypothesized that sulphated glycosaminoglycans are important in their formation. This impression is based on the finding that the glycosaminoglycan, heparan sulphate, is found associated with neurofibrillary tangles, neuritic plaques and neuropil threads while dermatan sulphate, chondroitin sulphate and keratan sulphate immunoreactivity is found around neuritic plaques in brains of Alzheimer's disease patients. Furthermore, in vitro studies demonstrate that sulphated glycosaminoglycans such as heparan sulphate and the closely related molecule heparin interact with tau and potentiate its phosphorylation by a number of serine/threonine kinases, reduce its ability to bind to microtubules and induce paired helical filament formation, all properties associated with tau isolated from Alzheimer's disease brain. Thus, we were interested to learn whether intracerebral injection of the sulphated glycosaminoglycan heparin would give rise to alterations in the cytoskeletal protein tau in the rat brain. Although no cytoskeletal changes were observed, to our considerable surprise we found that the intrahippocampal injection of heparin gave rise to seizures. We have investigated this unexpected effect further in vivo and by using in vitro electrophysiological techniques.

Discrepantly poor verbal skills in poor readers: a failure of learning or ability?

Poor verbal skills in poor readers have long been reported in the literature. There have been many attempts to understand the interaction between poor verbal ability and poor verbal achievement. The methodological problems are considerable, including the measurement of verbal ability, which has been confounded by previous learning. A new reasoning test, the VESPAR, has been designed to measure novel problem solving and thus to be less reliant on acquired verbal skills. One hundred and seventy 14-year-olds completed the VESPAR, the Cognitive Abilities Test (CAT) and a single-word reading test. Overall, verbal scores were weaker than spatial scores. A subgroup of 38 pupils with particularly marked discrepancies between verbal and non-verbal CAT was identified. The especially discrepant pupils were matched with other non-discrepant pupils from the year group for either verbal or non-verbal CAT. The discrepant group's reading was at the same level as the matched verbal CAT group. However, the primary verbal ability of the discrepancy group, as measured on the VESPAR, was greater than the matched verbal CAT group. This raises the possibility that CAT- but not VESPAR-discrepant pupils may be at particular risk of under-achievement in the verbal domain.

A new protocol for the transmission of physiological signals by digital telemetry.

A method has been devised to transmit physiological signals from the brains of rats using a new digital telemetry transmission protocol. The chief advantage of the present system over existing systems is that the circuit only consumes power during the transmission of a brief pulse of information. This results in a very much extended battery life allowing the device to be implanted and recordings to be carried out over a longer period of time.

Chronic focal epilepsy induced by intracerebral tetanus toxin.

A single, minute dose of tetanus toxin injected into mammalian cerebral cortex induces a chronic epileptic syndrome. Seizures lasting up to 3 minutes occur spontaneously and intermittently for several weeks to months. The cellular mechanisms of this model have been studied in detail using brain slices in vitro. Initially the release of the inhibitory neurotransmitter, GABA, is blocked, but after 2-4 weeks, other mechanisms take over. Intrahippocampal tetanus toxin models human complex partial seizures (temporal lobe epilepsy). It results in consistent behavioural changes analogous with those seen clinically, in spite of the limited neuronal loss found in only 10-30% of rats. Treatment with carbamazepine ameliorates both the seizures and their behavioural consequences. Tetanus toxin provides a versatile and long-lasting model of focal epilepsies.

MHC class II expression by microglia in tetanus toxin-induced experimental epilepsy in the rat.

Minute amounts of tetanus toxin injected into the hippocampus of rats results in an epileptiform syndrome. When the toxin injection is made unilaterally or bilaterally into the ventral hippocampus, about one-third of animals with seizures show bilateral neuronal loss in dorsal CA1 of the hippocampus after 1 week. In animals with seizures, microglia in hippocampus are found to be activated. The present work shows that during the acute phase, microglia in the substantia nigra become activated and express MHC class II antigens in the majority of animals with seizures. After the animals have recovered from the acute phase at 8 weeks, the MHC class II expression has largely disappeared from the substantia nigra but MHC class II-expressing microglia are found in the dorsal hippocampus of those rats with loss of cells from CA1. These results show that microglia are responsive to abnormal electrical activity in the central nervous system in the absence of degenerative changes. Further studies are required to determine how microglia may contribute to the neuropathology of epilepsy.

The effect of Ro 15-4513, an inverse agonist at the benzodiazepine receptor, on the exploratory response to novelty in the playground maze.

The effects of chlordiazepoxide and the inverse agonist, Ro 15-4513, were compared on the exploratory response of rats to a novel object introduced into a familiar environment containing seven familiar objects. While chlordiazepoxide (5 mg/kg) increased the novelty response, Ro 15-4513 reduced the response in a dose-dependent manner (0.5-5.0 mg/kg). This action was specific to novelty since the response to the familiar objects was unaffected. Both drugs produced some reduction in ambulation. The effects of both drugs were blocked by flumazenil (10 mg/kg), which at this dose did not itself have any intrinsic effect on the response. Muscimol (0.001 mg/kg) had a weak chlordiazepoxide-like effect and baclofen (3 mg/kg) had a weak effect in the opposite direction.

Effect of experimental limbic epilepsy on the estrus cycle and reproductive success in rats.

Epilepsy was induced in female rats by the injection of tetanus toxin (5 mouse LD50) unilaterally into the ventral hippocampus under anesthesia. During the 2-4 weeks that the rats exhibited intermittent spontaneous convulsions, daily vaginal smear tests showed that their estrous cycle was interrupted. In control rats such interruption only occurred for just a few days after the operation. Investigation of mating behavior, on the first night of proestrus, which occurred after 7 weeks from the operation, showed that there were fewer mounts, intromissions or ejaculations from the males, which were caged with previously epileptic animals. These females produced slightly smaller litters than their controls and there was a marked failure of their young to thrive in comparison with those of the control females. This failure appeared to be related to relatively high "stress" levels in the general laboratory environment. The impairment of reproductive success only lasted about 3 months after the original induction of epilepsy since subsequent litters to the same animals thrived normally.

The playground maze: a new method for measuring directed exploration in the rat.

A new test, called the 'playground maze', is described. Rat exploratory responses to a single novel object are measured in the context of responses to 7 familiar objects in a familiar environment. Responses are measured as time spent in areas around the objects on a circular open field. These times are expressed as percentages of the total time spent exploring all the objects and a value which is significantly greater than the expected chance level (12.5%) indicates a novelty response. The paths traversed by the animals on the maze are also recorded and the lengths of these give a measure of locomotion. Preliminary experiments on the effects of chlordiazepoxide (CDP) (1-5 mg/kg) and amphetamine (1.5-4 mg/kg) are reported. CDP significantly increased the novelty response but had no effect on locomotion. Amphetamine treatment at 4 mg/kg abolished the response to novel objects while lower doses (1.5 and 2 mg/kg) did not affect it. All 3 doses of amphetamine significantly increased locomotion. This test provides a new way of measuring the exploratory response to novelty under low stress conditions and allows the separation of drug effects on directed exploration and locomotion.

Tetanus toxin-induced seizures cause microglial activation in rat hippocampus.

Tetanus toxin (about 20 mouse LD50) injected into the ventral hippocampus of rats leads to brief seizures occurring intermittently over a period of weeks. Toxin injection leads to the appearance of activated microglia (detected with OX42 immunohistochemistry) in the hippocampus. After 7-14 days, many activated microglia are visible in CA1 area of dorsal hippocampus aligned with the pyramidal cell dendrites and having the morphology characteristic of 'rod cells'. Extensive cell loss is found in dorsal CA1, but not at the injection site, in about one third of injected rats.

Tetanus toxin blocks inhibition of granule cells in the dentate gyrus of the urethane-anaesthetized rat.

Field potentials of dentate granule cells in response to stimulation of the perforant path have been studied before and after injecting tetanus toxin (200 mouse LD50; or phosphate-buffered saline in controls) into the hilus of the dentate gyrus of rats under urethane anaesthesia. Within 1 h of toxin injection, the population spike, but not the slope of the excitatory postsynaptic potential, had increased markedly in amplitude and double or treble population spikes appeared in response to perforant path stimulation. Both paired pulse inhibition (15-ms interval between conditioning and test stimuli) and commissural inhibition (10-ms interval) were substantially reduced by the toxin. Neither multiple spikes nor the reduction in inhibition were seen in controls. Apparent inhibition of the excitatory postsynaptic potential, seen with paired stimuli to the perforant path, was not affected by the toxin. At later times after the injections, a progressive increase in the size of the spikes was seen in the controls while in the toxin animals there was often a secondary decrease in size. It is concluded that tetanus toxin can block both feed-back and feed-forward inhibitory components acting on dentate granule cells. The results are discussed with respect to the role of inhibitory processes in the control of epileptogenesis.

Fragment A-B of tetanus toxin does not block neuromuscular transmission in the goldfish.

While 4 micrograms of Fragment A-B of tetanus toxin (which lacks the binding site for nervous tissue) causes flaccid paralysis and death in mice, 26 micrograms has no toxic effect in goldfish. Antibodies to either A-B or to fragment C (which contains the binding site) block the paralytic effect of whole toxin in goldfish. It is concluded that binding is necessary for the neuromuscular blocking action of the toxin in goldfish.

The effect of lanthanum on nerve terminals in goldfish muscle after paralysis with tetanus toxin.

Lanthanum (1.9 mM) has previously been shown to produce a massive increase in the frequency of spontaneous miniature junction potentials at the neuromuscular junctions of goldfish fin muscles. In fins where transmission has been blocked by previous injection of tetanus toxin and where there are few (if any) spontaneous miniature potentials, lanthanum treatment is able to restore a modest frequency. The results of parallel experiments in which the ultrastructure of the nerve endings has been investigated by electron microscopy are reported. In normal goldfish muscles, the lanthanum-induced increase in frequency is accompanied by depletion of synaptic vesicles. In contrast, there is no depletion in tetanus toxin-paralysed nerve endings subjected to lanthanum treatment, which parallels the relative insensitivity of the endings to activation by lanthanum. Of particular interest is the finding that the lanthanum treatment of the toxin muscles apparently causes accumulation of vesicles in a row just inside the terminal membrane, both at synaptic and non-synaptic positions. The results are discussed with respect to the mechanisms of transmitter release and to the actions of tetanus toxin and lanthanum.

Limbic epilepsy induced by tetanus toxin: a longitudinal electroencephalographic study.

An epileptiform syndrome was induced in rats by injection of tetanus toxin (approximately 10 mouse LD50) unilaterally into the hippocampus. Continuous EEG records were obtained from implanted hippocampal electrodes for periods of 4-7 weeks in 14 rats. In a pilot study of two of these rats, three more recording electrodes were placed in other brain areas. Six of the rats (including the latter two) were simultaneously filmed using time-lapse videorecording, and the relationships between EEG events and overt motor fits were assessed using a split-screen video monitoring system. Characteristic peaks and troughs in the numbers of overt fits occurring each day were noted in all the rats that were filmed, and less marked peaks occurred in the numbers of hippocampal seizure discharges. At the start of the syndrome, seizure discharges occurred without accompanying fits; then overt fits occurred with some of the discharges; later the animals stopped having fits but seizure discharges continued to occur alone for several weeks. Fits only occurred with longer seizure discharges (more than approximately 30 s), but not all longer seizures were associated with fits. Whether or not a hippocampal seizure discharge leads to a motor fit appears to depend not on the nature of the electrical activity in the hippocampus but probably on the properties of areas, such as the cingulate gyrus, to which the seizure activity may spread. Large epileptiform spikes occurred throughout the syndrome, and their frequency was often increased for some minutes after a seizure discharge. While there was an overall correlation between the number of fits and the number of seizure discharges occurring during the entire syndrome, within relatively brief periods this relation was not consistent. The occurrence of motor fits was often associated with a decrease in seizure discharge frequency. In the four rats with bilateral electrodes, some independent EEG activity was observed in the uninjected hippocampus.