Regional rat brain benzodiazepine receptor number and gamma-aminobutyric acid concentration following a convulsion

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

The epileptogenic effect of seizures induced by hypoxia: the role of NMDA and AMPA/KA antagonists

Hypoxia of the brain may alter further seizure susceptibility in a different way. In this study, we tried to answer the question how episode of convulsion induced by hypoxia (HS) changes further seizure susceptibility, and how N-methyl-D-aspartic acid (NMDA) and AMPA/KA receptor antagonists influence this process. Adult Albino Swiss mice exposed to hypoxia (5% O(2)) developed clonic/tonic convulsions after about 340 s. Mice which underwent 10 s but not 5 s seizures episode subsequently exhibited significantly increased seizure susceptibility to low doses (equal ED(16)) of bicuculline (BCC) and NMDA during a 3-week observation period. No morphological signs of brain tissue damage were seen in light microscope on the third day after a hypoxia-induced seizure (HS). Learning abilities assessed in passive avoidance test as well as spontaneous alternation were not disturbed after an HS episode. Pretreatment with AMPA/KA receptor antagonist NBQX effectively prolonged latency to HS and given immediately after seizure episode also attenuated subsequent convulsive susceptibility rise, however, NMDA receptor antagonist, MK-801, appeared to be ineffective. These results suggest that a seizure episode induced by hypoxia, depending on its duration, may play an epileptogenic role. The AMPA/KA receptor antagonist prolongs the latency to HS, and given after this episode, prevents the long-term epileptogenic effect.

Sedative and anxiolytic effects of zopiclone's enantiomers and metabolite

We evaluated racemic zopiclone, its (S)- and (R)-enantiomers and a metabolite, (S)-desmethylzopiclone, for their actions on locomotor activity, rotarod performance, the elevated plus maze and the Vogel conflict test of anxiety, and electroconvulsive shock-induced seizures duration. Zopiclone and its (R)- and (S)-enantiomers reduced locomotor activity, and zopiclone and its (S)-enantiomer disrupted rotarod performance at 10 mg/kg. (S)-desmethylzopiclone did not alter these measures at doses of less than 200 mg/kg. (S)-desmethylzopiclone altered plus maze performance at the lowest dose of all the zopiclone derivatives tested, caused a dose-related effect on the Vogel conflict test and caused a dose-related reduction of electroconvulsive shock-induced seizure durations. The data indicate that (S)-desmethylzopiclone can bring about an anxiolytic effect without a substantial degree of central nervous system depression, and suggest that the agent may be particularly useful clinically in the treatment of anxiety.

GABA potentiation: a logical pharmacological approach for the treatment of acute ischaemic stroke

It has been shown that enhancing the function of the major inhibitory neurotransmitter GABA decreases glutamatergic activity in the brain. Since increased glutamatergic activity is the major primary event that results in cell death following an acute hypoxic-ischaemic stroke, GABAmimetic drugs might therefore be expected to be neuroprotective. This review examines the evidence that GABAergic function is acutely depressed following an ischaemic insult, and also reviews the data that suggest that increasing cerebral GABA concentration has a neuroprotective effect, as does the administration of some (but not all) GABAmimetic agents. The GABA uptake inhibitor CI-966, the GABA(A) agonist muscimol and the GABA(A)mimetic clomethiazole have all been shown to be neuroprotective in animal models of stroke when given after the ischaemic insult. In contrast, benzodiazepines and particularly barbiturates, although potent GABA(A) potentiators, have shown little promise as neuroprotectants. The diversity of GABA(A) receptor subtypes and the in vivo efficacy of certain GABA(A) receptor ligands in animal models of stroke suggests that GABAmimetic drugs are an undervalued approach to stroke therapy.

Prenatal morphine exposure alters ovarian steroid hormonal regulation of seizure susceptibility

The present study examined the ovarian hormonal regulation of seizure susceptibility in prenatally morphine- and saline-exposed adult female rats in the flurothyl seizure model in vivo, and in low-magnesium-induced epileptiform activity in brain slices, in vitro. All females were ovariohysterectomized (OVX); some received either estrogen (E) or progesterone (P) replacement, while others were injected with E + P sequentially. In prenatally saline-treated control females, there was an increase in the flurothyl-induced clonic seizure threshold (anticonvulsant effect) in the presence of both hormones (E + P) compared to OVX controls. In morphine-exposed females, there was an increase in the flurothyl-induced clonic seizure threshold after an E injection alone while there was a reduced tonic--clonic seizure threshold in the presence of both hormones (E + P) compared to the hormone treatment-matched group of saline-exposed females. In control females, in low magnesium medium in vitro, the development of two types of epileptiform activity (seizure-like events and status of short discharges) was not affected by the different hormonal conditions. However, prenatal morphine exposure suppressed the development of both types of epileptiform activity in the E-injected females compared to the E-injected, control females. The present data demonstrate that the anticonvulsant effects of P on seizure susceptibility requires the presence of E. Furthermore, prenatal morphine exposure alters ovarian steroid hormone-regulated seizure susceptibility.

Convulsant activity of nonanesthetic gas combinations

Most nonanesthetics (inhaled compounds that neither cause anesthesia when given alone nor decrease the partial pressure of a known inhaled anesthetic required to produce anesthesia) and transitional compounds (inhaled compounds that are less potent than would be predicted by the Meyer-Overton hypothesis) cause convulsions. A possible exception is the perfluoroalkane series of nonanesthetics. The present study tested whether perfluoroalkanes do provide an exception. Further, we tested whether the convulsant effects of nonanesthetic and transitional compounds were additive. The nonanesthetic perfluoropropane caused convulsions at 7.5 +/- 0.7 atm (mean +/- SD). Convulsions also were produced by perfluorocyclobutane (0.976 +/- 0.002 atm), 1,2-dichlorotetrafluoroethane (0.358 +/- 0.011 atm), 2,3-dichlorooctafluorobutane (0.085 +/- 0.007 atm), 1,2-dichlorohexafluorocyclobutane (0.055 +/- 0.007 atm), and flurothyl (0.00156 +/- 0.00039 atm). Of these, 1,2-dichlorotetrafluoroethane is a transitional compound, the remainder being nonanesthetics. The combination of flurothyl plus 1,2-dichlorohexafluorocyclobutane gave evidence of antagonism (a 17% +/- 21% deviation from additivity; P < 0.05), whereas the combination of 1,2-dichlorotetrafluoroethane plus 2,3-dichlorooctafluorobutane gave evidence of synergy (a -13% +/- 8% deviation from additivity; P < 0.05). The combinations of perfluoropropane plus perfluorocyclobutane (-4% +/- 15%), and perfluoropropane plus 1,2-dichlorohexafluorocyclobutane (-1% +/- 26%) did not produce results that deviated significantly from additivity. We conclude that pairs of these compounds either produce convulsions in an additive manner, a finding consistent with (but not proving) a common mode of action; or deviate modestly from additivity, a finding suggesting that at least a portion of the mechanistic basis for convulsions might differ, particularly for flurothyl plus other nonanesthetics, or for the combination of non-anesthetics and transitional compounds.

Differential effects of phenytoin and sodium valproate on seizure-induced changes in gamma-aminobutyric acid and glutamate release in vivo

The effects of intraperitoneal administration of the anticonvulsants phenytoin and sodium valproate were compared with ethosuximide on maximal electroshock seizure-related changes in rat hippocampal gamma-aminobutyric acid (GABA) and glutamate release in vivo as measured by microdialysis. There were immediate increases in GABA and glutamate in the 5 min post-ictal period, followed by a sustained reduction in GABA levels. Glutamate levels, however, were subsequently reduced until 20 min post-ictal before gradually increasing above basal. All animals displayed tonic hind-limb extension that was blocked by phenytoin (20 mg/kg) and sodium valproate (400 mg/kg) but not ethosuximide (150 mg/kg). Phenytoin attenuated the immediate post-ictal increase observed in glutamate whilst sodium valproate enhanced GABA release and prevented its secondary post-ictal inhibition. Ethosuximide was without effect on the post-ictal changes. These are the first data to show detailed seizure-induced amino acid changes and the in vivo effects of anticonvulsants on them in the seizure model.

Electroconvulsive shock alters GABAA receptor subunit mRNAs: use of quantitative PCR methodology

Electroconvulsive shock (ECS) may affect several neurotransmitter systems in brain, including the GABAergic inhibitory system. We used a quantitative PCR-based assay to evaluate mRNAs for five GABAa receptor subunits at 2 to 24 h after ECS. mRNAs for the alpha 1 and beta 2 subunits were significantly increased in cerebellum at 4 and 8 h after ECS, and returned to control levels at 24 h. No changes were observed in alpha 2, beta 3, gamma 1, or gamma 2 subunits, and no changes in any subunit evaluated were observed in cortex or hippocampus. These data corroborate prior results obtained for the alpha 1 subunit using Northern hybridization, and illustrate the utility of the PCR assay in quantitating low-abundance mRNAs.

Platelet GABA-transaminase in epileptic children: influence of epilepsy and anticonvulsants

The relationship between platelet GABA-transaminase (GABA-T) activity and either epilepsy or its treatment has been studied in 281 epileptic children: 55 were newly diagnosed untreated patients and 226 were chronically receiving anticonvulsants (154 in monotherapy and 72 in polytherapy). Results were compared with those from 48 control children. Untreated children had a GABA-T activity of 9.1 +/- 3.7 pmol/min/mg protein, lower than the control group (10.6 +/- 3.8 pmol/min/mg, P < 0.05), whereas treated epileptic children had higher values (11.9 +/- 6.3 pmol/min/mg) than those untreated (P < 0.01). In untreated children, the seven with absences and the nine with simple partial seizures had a GABA-T activity of 6.9 +/- 3.3 and 7.8 +/- 3.2 pmol/min/mg, respectively, lower than the control group (P < 0.05). In treated patients, those receiving valproate (VPA) in monotherapy had a GABA-T activity of 15.3 +/- 7.5 pmol/min/mg, higher than both the control group and the untreated children (P < 0.001). All patients receiving VPA in mono- or polytherapy had a higher activity than those receiving other anticonvulsants (16.4 +/- 8.4 vs. 9.9 +/- 3.9 pmol/min/mg, P < 0.001), the activity in Lennox syndrome and myoclonic epilepsies being significantly higher than in those with absences and partial epilepsy. GABA-T activity did not correlate with doses or trough steady-state serum levels of VPA. Platelet GABA-T could be useful as a peripheral marker of GABAergic alterations and GABAergic effects of antiepileptic drugs in epileptic patients.

A rapid assay for neurotransmitter amino acids, aspartate, glutamate, glycine, taurine and ?-aminobutyric acid in the brain by high-performance liquid chromatography with electrochemical detection

For simultaneous assay of the five neurotransmitter amino acids, Asp, Glu, Gly, Tau, and GABA in brain tissues, a very rapid and simple chromatographic method using high-performance liquid chromatography with electrochemical detection in combination with o-phthalaldehyde derivatization is described. Because the present method permits the determination of these five amino acids within less than five minutes in one chromatographic run, up to 100 samples a working day can be analyzed using an autosampler. Within-run coefficients of variation for these five amino acids were less than 2% (n = 20). The quantitative detection limit was 2.5 pmol for the 5 amino acids. The present method has been applied to the measurement of the five amino acid neurotransmitter levels in several discrete brain regions of mice treated with and without electroconvulsive shock.

Apparent disappearance of postseizure inhibitions and intensity of seizures during the development of rapid kindling in rabbits

The evolution of seizures and postseizure inhibitions in the course of 'rapid kindling' and after the termination of stimulation were studied in rabbits with chronically implanted electrodes (neocortex, dorsal hippocampus, amygdala, caudate nucleus). The amygdala (n = 4) or hippocampus (n = 7) was electrically stimulated every 5 min. Generalized convulsions and wide-spread electrographic epileptic changes together with a striking shortening of postictal refractory periods were produced by this procedure within 2-6 h. In most cases, these epileptogenic effects continued their progression after the termination of stimulation for more than 2-4 weeks. The degree of reduction of postseizure inhibition durations was significantly greater than the degree of increase of generalized motor seizure durations. These may be mediated by mechanisms which facilitate the onset of seizure but do not significantly influence seizure expression.

GABA-like immunoreactivity in different cellular populations of cerebellar cortex of rats before and after treatment with amino-oxyacetic acid

The postembedding immunogold procedure was used to detect changes in the levels of gamma-aminobutyric acid (GABA)-like immunoreactivity at the ultrastructural level in the cerebellar cortex of control rats and rats treated with the GABA transaminase inhibitor, amino-oxyacetic acid (AOAA), in order to increase the levels of GABA. GABA-immunoreactive structures were labelled using an antiserum directed against GABA coupled to bovine serum albumin and a secondary antibody conjugated to colloidal gold. The density of gold particles per square micron of tissue was taken as a measure of GABA-like immunoreactivity. In separate groups of control and AOAA-treated animals, the levels of GABA were assessed biochemically in the cerebellum, the cortex, the ventral mesencephalon and the striatum. Six hours after treatment with AOAA the GABA levels in the cerebellum, the cortex, the ventral mesencephalon and the striatum. Six hours after treatment with GABA immunoreactivity of the Golgi and basket cell terminals was significantly greater than that of mossy fibres, granule cell dendrites and perikarya and glial cells. The value obtained for Golgi terminals was the highest of all the structures examined and was twice that of their perikarya. Six hours after treatment with AOAA the GABA immunoreactivity in Golgi and basket cell terminals and in glial cells was greatly enhanced. The drug treatment slightly enhanced the immunoreactivity in mossy fibres and granule cell dendrites but induced no change in granule cell bodies. Thus, in both control and treated rats, the highest GABA immunoreactivity was present in the terminals of GABAergic cells, and the lowest in putative glutamatergic cells. The results demonstrate that there is a high degree of selectivity in the changes in GABA levels following the inhibition of GABA transaminase in the cerebellum. They also confirm the potential of the use of postembedding methods for the quantification of endogenous amino acid at cellular and subcellular levels, in relative and possibly also absolute terms.

The modulation by chlormethiazole of the GABAA-receptor complex in rat brain

1. The interactions of chlormethiazole with gamma-aminobutyric acid (GABA) synthesis and release, and with ligand binding to sites associated with the GABAA-receptor complex and the GABAB-receptor have been studied in the rat. The GABAA-receptor was studied using [3H]-muscimol, [3H]-flunitrazepam was used to label the benzodiazepine modulatory site, and [35S]-butyl-bicyclophosphorothionate ([35S]-TBPS) to label the chloride channel. 2. Chlormethiazole had no effect on GABA synthesis in the cortex, hippocampus and striatum or on GABA release from cortical slices in vitro. Chlormethiazole did not displace [3H]-baclofen binding to the GABAB-receptor. 3. Chlormethiazole (IC50 = 140 microM) and pentobarbitone (IC50 = 95 microM) both inhibited [35S]-TBPS binding by increasing the rate of [35S]-TBPS dissociation. In addition, chlormethiazole caused an apparent decrease in the affinity of [35S]-TBPS binding. 4. Chlormethiazole enhanced the binding of [3H]-muscimol but had no effect on [3H]-flunitrazepam binding. In contrast, the sedative barbiturate pentobarbitone enhanced both [3H]-muscimol and [3H]-flunitrazepam binding. 5. It is concluded that the sedative and anticonvulsant effects of chlormethiazole are probably mediated through an action at the GABAA-receptor. However, chlormethiazole does not interact with the GABAA-receptor complex in an identical manner to the sedative barbiturate pentobarbitone.

GABA receptors: are cellular differences reflected in function?

The putative involvement of GABAA and GABAB receptors in various behavioral and physiological effects is summarized in Table III. A division of function among the two types of GABA receptors appears to exist. GABAA receptors mediate feeding, cardiovascular regulation, anxiolytic effects, and anticonvulsive activity. GABAB receptors, on the other hand, are involved in analgesia, cardiovascular regulation, and depression. Although there is some overlap and shared functions among the receptor types, it is evident that GABAA and GABAB receptors have different behavioral and physiological profiles. Feeding, anticonvulsive activity and anxiety, for example, primarily involve GABAA receptors. Analgesia and depression, on the other hand, are GABAB effects. In those cases where GABAA and GABAB receptors mediate similar functions (e.g. cardiovascular regulation), they do so by affecting different transmitter systems and cellular mechanisms. It is proposed, therefore, that GABAA and GABAB receptors differ not only at the cellular level, but that they also have different functions in the mammalian central nervous system. The association of different subtypes of a receptor with different functions and mechanisms of action is not unique to the GABA system. D1 and D2 receptors in the dopamine system, for example, also exhibit some separation of function as do the mu, delta and kappa types of opiate receptors. Different subtypes of neurotransmitter receptors, therefore, appear to be a general organizing principle used by the brain to transduce chemical signals into different functional responses. A better understanding of the exact processes through which cellular signals are transformed into functional responses is a goal of future research.

Long lasting effect of a single audiogenic seizure on GABA turnover rates and steady-state levels

GABA turnover rates (TOR) and steady-state levels (SSL) were determined, 16-18 h after a single acoustic stimulation, in 15 brain areas of 3 mouse sublines. Each subline differs in its response to an acoustic stimulation (Rb1 mice are clonic-tonic seizure-prone, Rb2: clonic seizure-prone, Rb3: seizure-resistant). TOR and SSL were compared to those of unstimulated control mice and to those of repeatedly stimulated mice of the same subline. Following a single acoustic stimulation long-lasting alterations of GABA metabolism, mainly large alterations of GABA TOR, are observed. Most of the effects elicited after repeated stimulations, either on SSL or TOR, are not those of the last stimulation and repeated seizures (and/or stimulations) strengthen the effect of a single one. It appears that, for each of the Rb sublines, a specific and quite simple profile of the alterations of GABA metabolism in response to a single or repeated audiogenic seizures (and/or stimulations) can be given. The global analysis through the correlation of GABA TOR and SSL gives an indication that the alterations of the parameters of the correlation observed are to be allocated to the audiogenic seizures. Furthermore the tonic and clonic components of the audiogenic seizures can be distinguished.

Binding pattern of [35S]t-butylbicyclophosphorothionate is not altered following electroconvulsive shock treatment in rats

Single or repeated electroconvulsive shock (ECS) treatment-induced changes in [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) binding patterns in specific regions, i.e., cerebral cortex, cerebellum, hippocampus, and striatum of rat brain were investigated. Specific [35S]TBPS binding in these brain regions was not altered following a single or repeated administration of ECS, nor was the inhibition of [35S]TBPS binding to GABA affected. These observations tend to suggest that the picrotoxin-site on the GABA receptor complex may not be directly involved in electroconvulsive shock.

The effects of single and repeated electroconvulsive shock administration on the release of 5-hydroxytryptamine and noradrenaline from cortical slices of rat brain

1 A method is described of measuring the K+-evoked release of endogenous 5-hydroxytryptamine (5-HT) and noradrenaline (NA) from slices prepared from rat cortex. 2 There was no difference in either the spontaneous (basal) or K+-evoked release of 5-HT or NA from cortical slices prepared from handled animals and those given a single electroconvulsive shock (ECS) either 30 min or 24 h earlier. 3 In chronic studies, rats were either handled or given an ECS 5 times over 10 days and cortical slices prepared. There was no difference in 5-HT or NA release between the groups 30 min after the last treatment other than a modest attentuation of spontaneous NA release following ECS treatment. However 24 h after the last treatment K+-evoked release (above basal release) of 5-HT and NA was inhibited by 84% and 48%, respectively. 4 These data demonstrate that following a single ECS, normal 5-HT and NA release is seen at a time when GABA release is markedly inhibited. After repeated ECS the release of both monoamines was markedly inhibited. These 5-HT changes may be involved in the enhanced 5-HT-receptor function seen after repeated ECS.

Inhibition of GABA release from slices prepared from several brain regions of rats at various times following a convulsion

1 A method is described for the measurement of the K+-evoked release of endogenous gamma-aminobutyric acid (GABA) from slices of rat cortex, hippocampus and striatum. 2 In tissue prepared 30 min following an electroconvulsive shock, K+-evoked GABA release (above basal release) was inhibited by 45% in cortex, 50% in hippocampus and 75% in striatum. A similar inhibition of release was observed with slices prepared from rats in which a convulsion had been induced by flurothyl. There was no change in spontaneous (basal) release following either procedure. 3 An inhibition of K+-evoked endogenous GABA release was also seen in tissue prepared 4 min postictally but not 2 h after the seizure. 4 No difference was observed in the release of [3H]-GABA from preloaded cortical slices prepared from rats given a single electroconvulsive shock. 5 It is proposed that a convulsion results in an inhibition of GABA release and that this inhibition may in turn inhibit GABA synthesis as described in the preceding paper. 6 It is also proposed that changes in the endogenous releasable pool of GABA may not be detected by preloading slices with [3H]-GABA.

Inhibition of the rate of GABA synthesis in regions of rat brain following a convulsion

1 The rate of synthesis of gamma-aminobutyric acid (GABA) in the cortex, hippocampus and striatum of rat brain was assessed by measuring the linear rate of accumulation of GABA following injection of amino-oxyacetic acid (AOAA). 2 Five min after a single electrically induced seizure there was a rise in GABA content in these brain regions and an almost total inhibition of the rate of synthesis. 3 Five min after seizure induced by the inhalant convulsant flurothyl there was no rise in GABA content in these brain regions but a similar marked degree of inhibition of GABA synthesis. 4 Two hours after the convulsion the rate of GABA synthesis had returned to control values in all three brain regions. 5 A single convulsion did not alter the glutamic acid decarboxylase activity in these brain regions either in the absence or presence of added co-factor (pyridoxal phosphate). 6 Evidence for an inhibition of GABA release following a convulsion which may be associated with the inhibition of GABA synthesis is presented in the following paper.

Maximal electroshock increases the density of [3H]Ro 5-4864 binding to mouse cerebral cortex

The effects of chemically and electrically-induced convulsions on the binding of [3H]Ro 5-4864 to peripheral benzodiazepine receptors (PBR) was studied in both peripheral tissues and the central nervous system (CNS). Acute, maximal electroshock (MES) increased the density of PBR in mouse cerebral cortex as evidenced by a 30% increase in the Bmax of this archetypic ligand. These values returned to control levels by 60 minutes after MES treatment. In contrast, thirty and sixty minutes after convulsions induced by Ro 5-4864, strychnine, or pentylenetetrazol, neither the Bmax nor Kd of [3H]Ro 5-4864 binding to mouse cerebral cortical membranes was altered. The increase in [3H]Ro 5-4864 binding to cortex observed 30 minutes after MES was blocked by anticonvulsant doses of phenobarbital, phenytoin and clonazepam. No changes in the characteristics of [3H]Ro 5-4864 binding was observed in cerebellar or hippocampal membranes 30 minutes following acute MES. Further, after long-term MES administration (1 treatment/day, 5 days), no change in PBR density could be detected 30 minutes after the last MES. Finally, while no change in PBR density was noted in the kidneys 30 minutes after the MES, a significant increase in PBR density was seen in the cardiac ventricles. These results demonstrate a selective modulation of PBR density by MES, suggesting that the PBR could be involved in either the generation of seizures or in postictal compensatory processes.

Acetylcholinesterase activity rises in rat cerebrospinal fluid post-ictally; effect of a substantia nigra lesion on this rise and on seizure threshold

The activity of acetylcholinesterase (AChE) in the cerebrospinal fluid (CSF) of rats increased by 53% following an electroconvulsive shock (ECS) while non-specific cholinesterase (nsChE) activity was unchanged. A flurothyl-induced seizure failed to elicit a change in the AChE activity of CSF. A bilateral lesion of the substantia nigra pars reticulata abolished the rise in AChE activity in the CSF but did not diminish the increase of seizure threshold which follows a convulsion. These data suggest that AChE is released from the substantia nigra following a seizure but indicate that the change is not associated with the rise in seizure threshold which occurs.

Acetylcholinesterase activity in regions of rat brain following repeated administration of electroconvulsive shock

It has previously been shown that 30 min after administration of a single elec troconvulsive shock (ECS) to rats there is a marked decrease in both soluble and total acetylcholinesterase (AChE) activity in the midbrain and hippocampus. In the current study it has been shown that AChE activity is unchanged in these regions (apart from a small rise in soluble AChE in the hippocampus) 30 min after the final ECS of a series of 10 (once daily for 10 days). Twenty-four hours after the last ECS there was a significant increase in total and membrane bound AChE activity in the striatum but not hippocampus, midbrain, cortex and amygdala. This change in the striatum may be associated with the change in GABA synthesis which has been shown to also occur in this region at the same time.

Acetylcholinesterase activity in regions of the rat brain following a convulsion

The effects of electroconvulsive shock on the levels of acetylcholinesterase in several brain regions of the rat were studied. Hippocampus, mesencephalon, cortex, and striatum exhibited rapid changes in acetylcholinesterase activity during the first few minutes following the convulsion, whereas brainstem and basal forebrain levels remained unchanged. In both hippocampus and midbrain there was a sustained decrease in activity: the total acetylcholinesterase activity was decreased by up to 40% within 2 min of the convulsion and did not return to control values for another 3 h. Thirty minutes after a flurothyl-induced convulsion there was a similar fall in acetylcholinesterase activity in both these regions, whereas a subconvulsive electric shock produced no change. It is concluded that a convulsion produces significant short-term decreases in acetylcholinesterase activity in areas of the rat brain that are involved in the generation and propagation of seizures, and the question is raised of whether this is related to the increase in seizure threshold that follows a convulsion.

Effects of pentylenetetrazol on GABA-A/benzodiazepine/picrotoxinin receptor complexes in rat brain regions

The effects of acute convulsive doses of pentylenetetrazol (PTZ) on [35S]t-butyl-bicyclophosphorothionate (TBPS), [3H]flunitrazepam (FNP), [3H]muscimol, and [3H]gamma-aminobutyric acid (GABA) binding sites were examined in well-washed homogenates of various brain regions of rat. Except for a significant increase in the number of striatal [35S]TBPS binding sites, no significant change in [35S]TBPS, [3H]FNP, [3H]muscimol, and [3H]GABA binding was found in various brain regions 30 min after subcutaneous injection of PTZ at 90 or 100 mg/kg. Similarly, there were no significant changes in [35S]TBPS and [3H]FNP binding to unwashed P2 membranes of cerebral cortices 30 min following administration of convulsive doses of PTZ. These experiments failed to demonstrate acute modulation of GABA-A/benzodiazepine/picrotoxinin receptor complex by PTZ in the various brain regions examined except striatum. The significance of the increased [35S]TBPS binding in striatum caused by PTZ remains unclear.

gamma-Aminobutyric acid, benzodiazepine binding sites and gamma-aminobutyric acid concentrations in epileptic E1 mouse brain

All E1 mice provoked by postural stimulation since the age of 4 weeks had convulsions between 22 and 24 weeks of age, the refractory period ranging from 20 to 30 min. As compared to ddY mice, the maximal number of high-affinity [3H]muscimol binding sites was larger and the affinity was lower in the brains of the E1 mice, which had or had not experienced repeated seizures caused by postural stimuli. The basal and gamma-aminobutyric acid (GABA)-stimulated [3H]flunitrazepam binding sites, and GABA concentration in the brains of the E1 mice did not differ from those of the ddY mice. In E1 mice following provoked convulsions, there were no temporary changes in [3H]muscimol binding, or [3H]flunitrazepam binding with or without exogenous GABA stimulation. The GABA concentration in the brains of the E1 mice increased immediately after seizures, and returned to the control values within 60 min.

Some anticonvulsant drugs alter monoamine-mediated behaviour in mice in ways similar to electroconvulsive shock; implications for antidepressant therapy

The effects in mice of administration of the anticonvulsants, progabide, sodium valproate, diazepam, carbamazepine and phenytoin on 5-hydroxytryptophan (5-HTP)-induced head-twitch, apomorphine-induced locomotion, clonidine-induced sedation, and beta-adrenoceptor and 5-HT2 receptor number have been examined. Repeated progabide administration (400 mg kg-1, i.p. twice daily for 14 days) enhanced the head-twitch response the effect lasting for over 8 days after the last dose, and also increased 5-HT2 receptor number in frontal cortex. Progabide (400 mg kg-1, i.p.) enhanced the head-twitch response when given once daily for 10 days and when given intermittently (5 times over 10 days) but not after 1 day of administration. Repeated Na valproate (400 mg kg-1, i.p.) also increased the 5-HTP-induced head-twitch response and 5-HT2 receptor number in the frontal cortex when given twice daily for 14 days, but no behavioural enhancement was seen after 10 days' treatment. Diazepam (1.25 mg kg-1, i.p.) twice daily for 14 days increased the head-twitch response and 5-HT2 receptor number. Repeated progabide and valproate (but not diazepam) administration attenuated the sedation response to the alpha 2-adrenoceptor agonist, clonidine (0.15 mg kg-1) but neither drug altered beta-adrenoceptor number in the cerebral cortex. No changes in apomorphine-induced locomotor behaviour were seen after progabide, valproate or diazepam. Repeated carbamazepine (20 mg kg-1) or phenytoin (40 mg kg-1) administration failed to alter any of the biochemical or behavioural parameters listed above. Like repeated electroconvulsive shock (ECS), progabide altered the head-twitch response, clonidine-induced sedation response and 5-HT2 receptor number. Unlike repeated ECS, it did not alter beta-adrenoceptor number or the apomorphine-induced locomotor response. These data suggest that ECS may produce some changes in monoamine function by altering GABA metabolism as has previously been postulated.

The effect of repeated electroconvulsive shock on the function of THIP, a GABA agonist

Repeated electroconvulsive shock did not alter the anticonvulsant effect of THIP in rats, although it did elevate basal rectal temperature and abolish the hypothermic response to THIP.

Seizure thresholds and their postictal changes in audiogenic seizure (AGS)-susceptible rats

The convulsive thresholds for bicuculline and electroshock seizures were studied in audiogenic seizure (AGS)-susceptible and control rats. Electroshock seizure thresholds, determined as the amperage necessary to cause tonic extension of the hindlegs in 50% of the rats (CC50 = convulsive current fifty) were markedly lowered in rats of two stocks, bred for AGS susceptibility. During the clonic phase of electroshock seizure the bicuculline threshold was slightly lowered but started to rise after the convulsion had ceased. After 5 min, the threshold was significantly elevated and the maximal increase was reached in 15 min. In control rats the level normalized curvilinearly within an hour, but in AGS rats it decreased more slowly and was still elevated after 90 min. After an audiogenic seizure, the threshold for bicuculline-induced seizures in AGS rats also rose significantly but declined rapidly after having reached a maximum at 15 min. This rise in seizure threshold for bicuculline might indicate a postictal change in GABAergic transmission.

A study on benzodiazepine receptor binding in audiogenic seizure-susceptible rats

Benzodiazepine receptors were investigated in the cerebral cortex, the hippocampus, the brainstem, and the cerebellum of audiogenic seizure (AGS)-susceptible and seizure-resistant (ER) control rats. In AGS-susceptible rats of Sprague-Dawley descent, muscimol (10-6 M and 3 x 10-5 M) activated the binding of 3H-diazepam (0.4 nM) significantly less than in ER-rats. This finding may be strain selective, since it was not observed in AGS-susceptible rats of Wistar descent. Specific binding of the convulsant benzodiazepine receptor ligand methyl 6,7-dimethoxy-4-ethyl carboline-3-carboxylate (3H-DMCM), the benzodiazepine receptor ligand 3H-diazepam and the chloride channel directed cage convulsant t-butylbicyclophosphorothionate 35S-TBPS were not significantly changed in AGS-susceptible as compared to control rats. Our findings indicate that a disturbance at the level of the benzodiazepine receptor/GABA receptor/chloride channel complex is not a likely general aetiological factor for audigenic seizures in rats.

Characteristics of an atypical benzodiazepine, Ro 5-4864

Ro 5-4864 is a 1,4 benzodiazepine which, atypically, does not bind to the classical CNS benzodiazepine receptors, but has high affinity for the peripheral type of binding site found both in the periphery and in the brain. Biochemical evidence for alternative sites of action for this compound is discussed. We review the behavioral profile of Ro 5-4864 (sedative, convulsant and anxiogenic in rodents) and also describe the behavioral effects of combining Ro 5-4864 treatment with benzodiazepines (e.g., diazepam, chlordiazepoxide) and with other drugs that modify the activity of benzodiazepines (Ro 15-1788, CGS 8216, picrotoxin, PK 11195, phenytoin). In the light of these interactions and electrophysiological evidence we conclude that the actions of Ro 5-4864 are most likely to be mediated at the GABA-benzodiazepine receptor complex in the CNS.

Behavioural actions of Ro 5-4864: a peripheral-type benzodiazepine?

Ro 5-4864 is a 1,4 benzodiazepine lacking typical benzodiazepine behavioural actions, and which has very low affinity for the "classical" CNS benzodiazepine binding sites. However, Ro 5-4864 has very high affinity for the peripheral type of binding site in the periphery and in the brain. Evidence is reviewed that Ro 5-4864 is sedative, convulsant and anxiogenic in rodents. We also describe the effects of combining Ro 5-4864 treatment with benzodiazepines (e.g. diazepam, chlordiazepoxide) and with other drugs that modify the activity of benzodiazepines (Ro 15-1788, CGS 8216, picrotoxin, PK 11195, phenytoin). The binding sites that might be mediating these behavioural actions of Ro 5-4864 are discussed.

On the convulsant action of Ro 5-4864 and the existence of a micromolar benzodiazepine binding site in rat brain

The benzodiazepine Ro 5-4864 (60 mg/kg) produced convulsions in mice that could be antagonised either by diazepam (2-4 mg/kg) or by Ro 15-1788 (10-20 mg/kg). In mice and rats subconvulsant doses of Ro 5-4864 were proconvulsant when combined with subconvulsant doses of picrotoxin or pentylenetetrazole. Ro 15-1788 antagonised the tonic convulsions triggered by the drug combinations when it was given at the same time as Ro 5-4864; this antagonism was not observed when the drugs were injected at different times. In contrast to a previous report, we could find no evidence that Ro 5-4864 antagonised seizures induced by electroshock. Using two different ligand-binding techniques, no evidence was seen for the existence in rat brain of the previously reported "micromolar" benzodiazepine receptor, a suggested site of action of Ro 5-4864.

The GABA hypothesis of the pathogenesis of hepatic encephalopathy: current status

Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter of the mammalian brain, can induce coma. Outside the central nervous system it is synthesized by gut bacteria and catabolized largely in the liver. GABA and its agonists, as well as benzodiazepines and barbiturates, induce neural inhibition as a consequence of their interaction with specific binding sites for each of these classes of neuroactive substances on the GABA receptor complex of postsynaptic neurons. In a rabbit model of acute liver failure: (i) the pattern of postsynaptic neuronal activity in hepatic coma, as assessed by visual evoked potentials, is identical to that associated with coma induced by drugs which activate the GABA neurotransmitter system (benzodiazepines, barbiturates, and GABA agonists); (ii) the levels of GABA-like activity in peripheral blood plasma increase appreciably before the onset of hepatic encephalopathy, due at least in part to impaired hepatic extraction of gut-derived GABA from portal venous blood; (iii) the blood-brain barrier becomes abnormally permeable to an isomer of GABA, alpha-amino-isobutyric acid, before the onset of hepatic encephalopathy; and (iv) hepatic coma is associated with an increase in the density of receptors for GABA and benzodiazepines in the brain. These findings are the bases of the following hypotheses: (i) when the liver fails, gut-derived GABA in plasma crosses an abnormally permeable blood-brain barrier and by mediating neural inhibition contributes to hepatic encephalopathy; (ii) an increased number of GABA receptors in the brain found in liver failure increases the sensitivity of the brain to GABA-ergic neural inhibition; and (iii) an increased number of drug binding sites mediates the increased sensitivity to benzodiazepines and barbiturates observed in liver failure by permitting increased drug effect.

Studies on [3H]diazepam and [3H]ethyl-beta-carboline carboxylate binding to rat brain in vivo. II. Effects of electroconvulsive shock

In vivo specific binding of [3H]diazepam was not altered by a single electroconvulsive shock given 5, 30, or 60 min, or 24 h previously, nor 24 h after the last of 10 daily shocks. Similarly, in vivo [3H]ethyl-beta-carboline carboxylate binding was not changed in the brains of animals that had been given a single electroconvulsive shock 30 min previously or a series of 10 daily shocks. Brain areas examined included cerebral cortex, hippocampus, cerebellum, and striatum. However, cortical binding of [3H]diazepam was increased by 32% in animals which were present in the same room while another was being injected and killed. This may represent a response to stress and/or anxiety.

Interaction of uridine with GABA binding sites in cerebellar membranes of the rat

The effect of uridine, a postulated anticonvulsant agent, on GABA receptors has been investigated. Uridine inhibits [3H]GABA binding to rat cerebellar buffer-washed membranes. Pretreatment of the membranes with Triton X-100 increases the effect of uridine on GABA-binding. The Scatchard analysis reveals that both high and low affinities of GABA for its receptors are affected by 1 mM uridine, while the apparent number of binding sites remains unchanged. The ability of uridine to interact competitively with GABA binding sites, also examined by the Lineweaver-Burk analysis, suggests a possible mechanism of action of this anticonvulsant agent, so including it among those compounds characterized by a GABAergic agonist activity.

Regional GABA concentration and [3H]-diazepam binding in rat brain following repeated electroconvulsive shock

It has been confirmed that 24 hours following a series of electroconvulsive shocks (ECS) given once daily for 10 days (ECS X 10) to rats there is an increase in GABA concentration in the corpus striatum. A similar change was seen after the ECS had been given to rats anaesthetised with halothane, or when 5 ECS were given spread out over 10 days, the rats being anaesthetised during the ECS. A daily convulsion for 10 days elicited by flurothyl exposure resulted in an increased striatal GABA concentration, but also increased the GABA concentration in the hypothalamus, hippocampus and cortex. The increase in striatal GABA concentration was present 24 hours after ECS daily for 5 days or 3 days after ECS daily for 10 days. No change in [3H]-diazepam binding was seen in hippocampus, cortex or corpus striatum 24 hours after the last of 10 once daily ECS. The increase in striatal GABA concentration was therefore seen at all times when enhanced monoamine-mediated behaviours have been demonstrated following seizures.