Antiepileptogenic effects of the novel synthetic neuroactive steroid,ganaxolone, against pentylenetetrazol-induced kindled seizures: Comparison with diazepam and valproate

Pharmacological treatment of epilepsy is often unsatisfactory due to side effects and the lack of drugs that control the progressive epileptogenic process. Modulation of inhibitory γ-aminobutyric acid (GABA)-ergic neurotransmission by synthetic agonists of the neuroactive steroid binding site on the GABA_A receptor complex is one approach toward the identification of improved antiepileptic agents. In this study, antiepileptogenic and anticonvulsive effects of the novel synthetic neuroactive steroid, ganaxolone (3α-hydroxy-3β-methyl-5α-pregnan-20-one), were evaluated in comparison with diazepam and valproate against pentylenetetrazol (PTZ)-induced kindled seizures in mice. Kindled seizures provide a model of the progressive epileptogenic process. Successive administration of 45 mg/kg PTZ on days 1, 3, 5, 8, and 10 resulted in the rapid development of kindled seizures and significant reductions in thresholds for clonic convulsions, tonic convulsions, and lethality induced by PTZ on day 10. Ganaxolone, diazepam, and valproate dose-dependently protected against clonic convulsions induced by acute submaximal dose of PTZ (70 mg/kg). The compounds also dose-dependently suppressed fully kindled seizures and blocked the expression of kindled seizures over successive treatments with PTZ (45 mg/kg). Relative to acute anticonvulsive potencies against 70 mg/kg PTZ, however, ganaxolone was more potent than valproate or diazepam against fully kindled seizures and in blocking the expression of kindled seizures over successive treatments with PTZ. Importantly, only ganaxolone demonstrated antiepileptogenic activity by blocking the development of kindling, as evidenced when PTZ was administered in the absence of anticonvulsant treatments. Both diazepam and valproate failed to prevent development of kindled seizures even at doses that fully suppressed motor expression of seizures during kindling acquisition. Unlike diazepam and valproate, ganaxolone did not impair ambulatory activity within the dose range used in this study. These data, taken in conjunction with other findings on the unique pharmacological actions of ganaxolone, predict an improvement in the pharmacological management of epilepsy with this synthetic neuroactive steroid. Drug Dev. Res. 44:21–33, 1998. © 1998 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.     

Acute and chronic effects of the neuroactive steroid pregnanolone on schedule-controlled responding in rhesus monkeys

This study used schedule-controlled responding to examine the acute and chronic effects of the neuroactive steroid and positive -aminobutyric acid A (GABA ) modulator pregnanolone. Pregnanolone, the positive GABA modulator triazolam, the GABA chloride channel site antagonist pentylenetetrazol (PTZ) and the -methyl-d-aspartate (NMDA) antagonist ketamine were administered to monkeys ( = 4) responding under a multiple fixed ratio (FR/FR) schedule of food presentation and stimulus shock termination (SST), before, during and after daily treatment with pregnanolone (3.2 mg/kg subcutaneously). Pregnanolone decreased responding in a dose- and time-related manner, with a duration of action of <2 h. Mutual antagonism occurred between pregnanolone and PTZ in the food component, and PTZ antagonized pregnanolone in the SST component. Daily treatment with pregnanolone increased the sensitivity to PTZ 24 h but not 2 h after daily pregnanolone administration, and daily pregnanolone treatment did not alter the sensitivity to pregnanolone, triazolam or ketamine. Baseline responding in the food component was decreased in some monkeys 24 h after daily pregnanolone administration and in all monkeys 48 h after discontinuation of daily pregnanolone treatment. These results suggest that positive GABA modulation is one mechanism by which pregnanolone decreases FR responding, and that dependence resulting from daily pregnanolone treatment is not necessarily accompanied by tolerance to pregnanolone. Failure of pregnanolone to confer tolerance under these conditions might suggest that neuroadaptations at the GABA receptor complex vary according to the site at which positive GABA modulation occurs.     

Acute neuroactive steroid withdrawal in withdrawal seizure-prone and withdrawal seizure-resistant mice

Allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one) is an endogenously derived metabolite of progesterone, and a potent positive modulator of gamma-aminobutyric acid(A) (GABA(A)) receptors. A withdrawal syndrome, characterized by central nervous system (CNS) hyperexcitability, has been demonstrated following abrupt discontinuation of high progesterone levels in rats, which was due in part to altered levels of allopregnanolone. The purpose of the present study was to determine if a single administration of pregnanolone or allopregnanolone could produce an acute withdrawal response in mice selected for susceptibility (Withdrawal Seizure-Prone, WSP) or resistance (Withdrawal Seizure-Resistant, WSR) to ethanol withdrawal convulsions. WSP mice administered 75 mg/kg pregnanolone showed a significant increase in handling-induced convulsion (HIC) scores over a 25-h testing period. In contrast, HIC scores in WSR mice were negligible after acute administration of 25, 50, 75, or 100 mg/kg pregnanolone. WSP mice also showed a similar increase in HIC after withdrawal from 75 mg/kg allopregnanolone. This effect was evident at both the 10-h and 25-h overall withdrawal severity assessment. These results demonstrate that neuroactive steroids can elicit an acute withdrawal response similar to that of other positive modulators of GABA(A) receptors in WSP mice, supporting the notion that a common set of genes underlie acute and chronic withdrawal severity from multiple agents with depressant effects on the central nervous system.     

Diphenylhydantoin potentiates the protective effect of diazepam against pentylenetetrazol but not against bicuculline and isoniazid-induced seizures in mice

The anticonvulsant activity of diazepam alone, or in combination with diphenylhydantoin was studied in pentylenetetrazol-, bicuculline- and isoniazid-induced Scizures. Alone, diphenylhydantoin did not influence the chemically-induced convulsions but enhanced the antipentylenetetrazol action of diazepam whilst failing to affect the protective effect of the benzodiazepine against bicuculline- and isoniazid-induced convulsions. It is suggested that a diphenylhydantoin-induced increase in the total number of specific benzodiazepine binding sites might be responsible for the enhancement of the antipentylenetetrazol activity of diazepam. The anticonvulsant action of diazepam against bicuculline and isoniazid-induced Scizures does not seem to involve an interaction with benzodiazepine receptors.     

Behavioral comparison of pentylenetetrazol,clonidine, chlordiazepoxide and diazepam in infant rats

The effects of various doses of pentylenetetrazol, clonidine, chlordiazepoxide and diazepam on limb and head movement and behavioral seizure signs were examined in 4-, 8- and 16-day old rats tested at ambient temperatures of either 25 or 35°C. All 4 drugs produced intense behavioral activation at the 2 younger ages but there were marked differences among in the effects of test temperature on this activation and in the relationship between age and their activating effect. A “paradoxical” and intense behavioral energization was observed after the administration of either of the 2 benzodiazepines at 4, 8 but not 16 days, particularly at the lower test temperature. Clonidine and pentylenetetrazol were activating at all 3 ages but while clonidine had greater effect at the low test temperature, the opposite was the case after pentylenetetrazol. The effects of the benzodiazepines and clonidine were clearly distinct from those of pentylenetetrazol and this was the only drug to substantially elicit seizure signs. It is uncertain whether or not the benzodiazepines cause brain seizures in young animals. If so, then their behavioral manifestation is clearly different from that observed after pentylenetetrazol.     

Anxiolytic effects of valproate and diazepam in mice are differentially sensitive to picrotoxin antagonism

Although it is widely believed that the anxiolytic effects of benzodiazepines are mediated through facilitation of GABA(A) receptor function, behavioural studies have to date provided rather weak support for this hypothesis. In particular, considerable inconsistency has been noted both for the effects of GABAergic manipulations in animal models of anxiety and the ability of GABA(A) receptor antagonists to block the anxiolytic effects of diazepam (DZ) and chlordiazepoxide. In view of the sensitivity of the murine plus-maze to the anxiety-modulating effects of GABAergic agents as well as classical benzodiazepines, the current study examined the extent to which the anxiolytic actions of valproic acid (VPA) and DZ in this test involve picrotoxin (PX)-sensitive receptor mechanisms. Subjects were male DBA/2 mice, test duration was 5 min, and ethological scoring methods were employed. Our results show that, while devoid of intrinsic behavioural effects under present test conditions, PX (0.25-0.5 mg/kg) selectively antagonised the anxiolytic-like (but not other) effects of VPA (400 mg/kg). In contrast, the same doses of PX failed to block any of the behavioural changes induced by DZ (1.5 mg/kg), including disinhibition of open arm exploration. These data suggest that the plus-maze anxiolytic effects of DZ in DBA/2 mice are not mediated through PX-sensitive GABA(A) receptors. Further studies will be required to assess the generality of present findings to other mouse strains, species and behavioural paradigms.     

A behavioral and electrocorticographic comparison of diazepam and pentylenetetrazol in rat pups

This experiment assessed the possibility suggested by previous research that benzodiazepines cause convulsions in infant rats. Seven-day-old Wistar rats were randomly assigned to receive either diazepam (DZP) (0, 0.5 or 2.5 mg/kg), the convulsogen pentylenetetrazol (PTZ) (50 mg/kg), or DZP followed 30 minutes later by PTZ. The amount of paddling and wall progression and head and body tremors was recorded for each group. Both DZP and PTZ elevated paddling and wall progression, but only PTZ elevated head and body tremor scores. DZP antagonized the PTZ-induced increases in head and body tremors. In a second experiment, seven-day-old pups were implanted with cortical electrodes. The following day, baseline electrocorticograms (ECoGs) were taken for each animal. Each pup subsequently received either DZP vehicle, 0.5 mg/kg DZP, 50 mg/kg PTZ, or 0.5 mg/kg DZP followed 30 minutes later by 50 mg/kg PTZ. Neither the vehicle nor the DZP injections altered ECoG activity. In contrast, PTZ-treated pups showed continuous, high-amplitude, spiking activity. Pretreatment with DZP eliminated these PTZ-induced alterations in ECoG activity. We conclude that in infant rats, the behavioral and electrophysiological effects of DZP and PTZ are distinct from one another. Furthermore, both the behavioral and the electrocorticographic effects of PTZ are blocked by DZP. It is unlikely that DZP causes seizures in neonatal rats.     

The effects of chronic valproate and diazepam in a mouse model of posttraumatic stress disorder

To better understand neurochemical and psychopharmacological aspects of post-traumatic stress disorder (PTSD), it is necessary establish an animal model of PTSD in which behavioral changes persist for a long time after the initial traumatization. The present study aimed to characterize long-term behavioral alterations in male ICR mice as an animal model of PTSD consisting of a 2-day foot shock (0.8 mA, 10 s) followed by 3 weekly situational reminders (SR), and to evaluate the effects of repeated administration of valproate and diazepam on behavioral deficits of this animal model. The results showed that the aversive procedure induced several long-term behavioral deficiencies: increased freezing behavior and anxiety level, reduced time spent in an aversive like context. Repeated treatment with valproate (100-400 mg/kg, i.p.) induced a dose-dependent reduction of these behavioral changes. In contrast, diazepam at a low dose (0.25 mg/kg) but not at a high dose (4 mg/kg) reduced the behavioral deficiencies. These results demonstrate that exposure to intense foot shock associated with repeated situational reminders elicits long-term disturbances that last about 4 weeks after the foot shock exposure. These behavioral deficits can be ameliorated by repeated administration of valproate or diazepam at some special dose ranges.     

The long-term effects of diazepam and pentylenetetrazol on the potentiated startle response

Stress desensitization is observed as a decrease in the disruptive effects of a stressor on behavior when the organism is repeatedly exposed to the stressor. For instrumental behavior, stress desensitization was also reported for rats preexposed to anxiogenic drugs; stress sensitization was reported for rats preexposed to an anxiolytic compound. The present study investigated whether similar effects are found in a noninstrumental task situation. First, rats received 12 daily injections of pentylenetetrazol (PTZ, 20 mg/kg, IP), diazepam (DZP, 5 mg/kg, IP) or saline. After each injection the effect of the drugs on the acoustic startle reflex was measured. No drugs were given during the remainder of the experiment. Eight days later rats were given 5 days of Pavlovian fear conditioning to establish a light as a shock signal. During the next 3 days, potentiation of the startle response by the light was measured. None of the drug treatments had an effect on startle potentiation, indicating that stress sensitivity is not affected by previous administration of PTZ and DZP in a noninstrumental task. An explanation for the different effects found for instrumental and noninstrumental tasks is suggested.     

A pooled, double-blind comparison of the effects of buspirone, diazepam and placebo in women with chronic anxiety

Pooled data were analyzed for 367 female patients enrolled in a double-blind, placebo-controlled, multi-centre trial comparing buspirone, a non-benzodiazepine anxiolytic, and diazepam in the treatment of generalized anxiety disorder. After a 4 to 7-day wash-out period, patients were allocated at random to receive one or other of the trial medications or placebo over a 4-week period. Mean daily dosages were 24.5 mg for buspirone and 20.8 mg for diazepam (range 10 mg to 60 mg for both drugs). Patients were assessed on entry and at weekly intervals using the Hamilton Anxiety Rating Scale, and at the end of treatment both patients and physicians gave an overall opinion of response to treatment. Details of adverse events were also recorded. The results showed that both buspirone and diazepam were approximately equal in efficacy and superior to placebo. Menstruation and the occurrence of premenstrual tension did not alter the anxiolytic activity of either drug. Patients taking diazepam had significantly more adverse effects, i.e. drowsiness, weakness, fatigue, inco-ordination and depression, than did those in the buspirone group. In a separate commentary, the anxiety disorder and the data from the study are reviewed to place them in the overall perspective of gynaecological care.     

Protection of mice against the lethal effects of sodium metavanadate: a quantitative comparison of a number of chelating agents

10 Chelating agents were administered to mice by intraperitoneal (i.p.) injection in order to compare their relative effectiveness in preventing death after a single i.p. injection of NaVO3. Ascorbic acid, Tiron (4,5-dihydroxy-1,3-benzene-disulfonic acid) and deferoxamine were the most effective antidotes for acute NaVO3-toxicity. The therapeutic index of ascorbic acid against 0.61 mmol/kg NaVO3 was found to be 95.2. This was 4.6 and 12.2 times greater than those of Tiron and Deferoxamine respectively. L-Cysteine greater than Na2CaEDTA greater than Na3CaDTPA reduced the acute mortality of mice following i.p. injection of NaVO3. Sodium salicylate, D,L-penicillamine, DMSA and DDC were not effective as antidotes for acute NaVO3-toxicity.     

Overlapping, but not identical, discriminative stimulus effects of the neuroactive steroid pregnanolone and ethanol

Many behavioral effects of neuroactive steroids are mediated by GABA(A) receptors; however, other receptors might be involved. Ethanol has a complex mechanism of action, and many of the same receptors have been implicated in the effects of neuroactive steroids and ethanol. The goal of this study was to determine whether actions of neuroactive steroids and ethanol at multiple receptors result in similar discriminative stimulus effects. Rats discriminated 5.6 mg/kg of pregnanolone while responding under a fixed-ratio 20 schedule of food presentation. Pregnanolone, flunitrazepam and pentobarbital produced >80% pregnanolone-lever responding. In contrast, neither morphine nor the negative GABA(A) modulator beta-CCE substituted for pregnanolone up to doses that markedly decreased response rates. Ethanol substituted only in some rats; in other rats, ethanol produced <20% pregnanolone-lever responding up to rate-decreasing doses. Thus, substitution of positive GABA(A) modulators, and not morphine or beta-CCE, for pregnanolone in all rats suggests that positive modulation of GABA(A) receptors is important in the discriminative stimulus effects of pregnanolone. Although pregnanolone might have actions at other receptors, in addition to actions at GABA(A) receptors, substitution of ethanol for pregnanolone only in some rats suggests that the mechanisms of action of pregnanolone and ethanol overlap, but are not identical.     

Effect of the neuroactive steroid α-THDOC on staircase test behavior in mice

This study examined the effect of the neuroactive steroid 3α, 5α-tetrahydrodeoxycorticosterone (α-THDOC) as compared to the benzodiazepines diazepam and midazolam and the barbiturate phenobarbital on the number of rearing events and the number of steps ascended in the mouse staircase test. The benzodiazepines, phenobarbital and α-THDOC all reduced rearing activity at doses that did not affect climbing. The rearing-suppression effect of the benzodiazepines and α-THDOC, but not of phenobarbital, was blocked by the benzodiazepine antagonist flumazenil. It appears that, although such neuroactive steroids, like barbiturates, bind to distinct sites within the chloride ion channel of the gamma-aminobutyric acid type A (GABA_A) receptor complex, α-THDOC behavioral activity is modulated by the benzodiazepine recognition site. Received: 19 October 1995 / Final version: 1 July 1996     

Tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats.

We assessed the development of tolerance to the anticonvulsant effects of carbamazepine (CBZ), diazepam (DZP), and sodium valproate (VPA) on convulsions elicited by amygdala stimulation in kindled rats in three similar experiments. In each experiment, amygdala-kindled rats were assigned to a drug group or to a corresponding vehicle control group. The rats in the three drug groups received a total of 10 bidaily (one every 48 h) IP injections of CBZ (70 mg/kg), DZP (2 mg/kg) or VPA (250 mg/kg) at a dose that initially blocked the forelimb clonus elicited by an amygdala stimulation (400 microA, 60 Hz, 1 s) administered 1 h after the injection. The rats in the three vehicle control groups were similarly treated except that they received injections of the saline vehicle. The drug tolerance test occurred 48 h after the final tolerance-development trial; the rats from each drug group and the corresponding vehicle control group received an injection of the appropriate drug followed 1 h later by the administration of a convulsive stimulation. The drug tolerance test revealed almost total tolerance in each of the three drug groups but no tolerance in any of the three vehicle control groups. Such large tolerance effects are inconsistent with the less dramatic effects reported in previous studies; possible reasons for this inconsistency were considered.     

Protection of mice against lethal effects of sodium arsenite--a quantitative comparison of a number of chelating agents

The LD50 of NaAsO₂ was found to be 0.129 mmol/kg, sc, using white mice. The ip administration of the sodium salt of 2,3-dimercapto-1-propanesulfonic acid (DMPS) or meso-dimercaptosuccinic acid (DMSA) (0.80 mmol/kg) immediately after and 90 min after NaAsO₂ increased the LD50 of NaAsO₂ about 4.2- and 4.4-fold, respectively. Neither d-penicillamine nor N-acetyl-dl-penicillamine affected the LD50 of NaAsO₂ under the same conditions. The LD50 of DMPS and DMSA in mice was found to be 5.22 and 13.58 mmol/kg, ip, respectively. The effective dose 50 for treating mice 10 min after receiving and LD100 of NaAsO₂ (0.15 mmol/kg) was 0.066 mmol/kg for DMPS and 0.065 mmol/kg for DMSA. The therapeutic index of DMSA against 0.15 mmol/kg NaAsO₂ was found to be 209. This was 2.6 times greater than that of DMPS. The explanation for this difference is that although DMSA was as effective as DMPS, it is less toxic. The LD50 of NaAsO₂ was not increased by sodium diethyldithiocarbamate, α-mercaptopropionylglycine, dl-N-acetylhomocysteinethiolactone, or monomercaptosuccinic acid. A series of polymercapto compounds, some having as many as four mercapto groups per molecule, also did not protect against the lethality of NaAsO₂. There is extensive experimental and clinical information about DMPS and DMSA available in the Soviet and Chinese literature where these agents are known as Unithiol or Unitiol and succimer, respectively. It would appear that DMPS and DMSA warrant further experimental studies and eventually clinical trials for the treatment of intoxication by arsenic as well as by certain other heavy metals.     

The effects of doxapram, diazepam, phenobarbital and pentylenetetrazol on suprathreshold and threshold stimulations in amygdaloid kindled rats

Amygdaloid kindled rats were utilized to determine the effects of the respiratory stimulant, doxapram (7.5-60 mg/kg) on seizures elicited by suprathreshold or threshold electrical stimulation. For comparison, a single dose of pentylenetetrazol (30 mg/kg), phenobarbital (40 mg/kg) and diazepam (2 mg/kg) were also tested in the same paradigm. With suprathreshold stimulation, doxapram did not increase the intensity of seizures. Neither the afterdischarge duration (AD) nor the behavioral rank (BR) were increased with doses high enough to cause systemic signs of toxicity. Similarly, the convulsant pentylenetetrazol did not increase AD or BR after suprathreshold stimulation. The anticonvulsants phenobarbital and diazepam significantly reduced both AD and BR after suprathreshold stimulation. Seizure thresholds tended to be higher after the various doses of doxapram with no consistent effect on elicited AD or BR. Pentylenetetrazol did not change threshold values, but was found to increase AD and BR at threshold. Both phenobarbital and diazepam raised thresholds with a significant decrease in elicited AD and BR. It would appear that the respiratory stimulant doxapram has little proconvulsant activity in this model of epilepsy.     

A comparison of buspirone, diazepam, and placebo in patients with chronic anxiety states

Buspirone is an antianxiety compound that has been extensively evaluated in clinical trials: it has proved superior to placebo and comparable to diazepam in the treatment of patients with generalized anxiety disorder. In this study, 33 outpatients with generalized anxiety disorder were entered into a crossover study of 3 weeks each of placebo, buspirone 10 to 30 mg daily, and diazepam 10 to 30 mg daily. Psychiatrist and patient ratings were made, together with psychological tests and EEG and skin conductance measures before and after each treatment. Of the nine dropouts, six were on buspirone at the time of dropout. For the remaining 24 patients, the mean daily doses attained of buspirone and diazepam were both 20 mg. On most clinical ratings diazepam was superior to buspirone and placebo, which did not differ. Diazepam produced minor psychomotor changes and the expected major effects on the EEG. Buspirone was without effect. Side effects on buspirone were mainly nausea and giddiness and on diazepam, drowsiness. The lack of efficacy of buspirone is discussed in terms of the previous benzodiazepine exposure--23/24 patients had had previous exposure and only 10 were able to tolerate a pretrial placebo washout period. The implications are considerable for the introduction of any new antianxiety agent not cross-tolerant with the benzodiazepines into a chronically anxious group of patients with previous long-term benzodiazepine therapy.