L.G. Stark

Anticonvulsant drugs and their antagonism of kindled amygdaloid seizures in rats

Abstract The kindling phenomenon has become a useful animal model for studying the processes in the central nervous system involved in one type of epileptiform seizure response. A systematic, multipledose evaluation of 20 drugs known to have anticonvulsant properties and two new compounds (WB-CPI and SHK-I-11) is reported using a standard protocol with rats kindled by amygdaloid stimulation. Drugs which have a wide anticonvulsant spectrum of activity against various animal models of epilepsy were the most effective. The barbiturates, the benzodiazepines, a piperazine, an acetate derivative and WB-CPI (a significantly modified phenacylurea), were found to be the most effective in attenuating the induced seizures. Kindled amygdaloid seizures in the rat can be characterized as a model useful for the detection of a wide variety of anticonvulsants likely to be effective in many types of experimental seizures. The compound WB-CPI merits further neuropharmacological and psychopharmacological testing due to its activity in rats.

The anticonvulsant properties of melatonin on kindled seizures in rats.

Abstract The literature suggesting that the pineal gland and the indolamine, melatonin, have a significant role in regulating and modulating brain electrical activity is reviewed. The anticonvulsant properties of melatonin were investigated by testing acute doses of melatonin against two types of kindled Scizures in the rat. Against the electrically kindled amygdaloid Scizure, melatonin significantly reduced afterdischarge length at a non-sedative dose, but failed to modify Scizure rank scores. With larger doses of melatonin, which were associated with some sedation and ataxia, the Scizure rank score was reduced, but there was no additional reduction of afterdischarge length. Naloxone treatment (20 mg/kg, i.p.) did not reverse the sedation, ataxia, nor the anticonvulsant effects seen with the large dose of melatonin (200 mg/kg, i.p.). Additional animals were kindled with pentylenetetrazol injections (30 mg/kg, i.p.) given at 2–3 day intervals. Melatonin significantly reduced Scizure rank scores in these kindled animals at one dose (150 mg/kg, i.p.). A larger dose, associated with sedation and ataxia, did not result in any further reduction of Scizure rank scores. For comparison, large and small doses of both phenobarbital and diazepam were also tested against the pentylenetetrazol kindled Scizures. Neither phenobarbital (30 mg/kg) nor diazepam (2.5 mg/kg) induced a neurological deficit; however, both agents reduced Scizure rank to approximately the same score as did melatonin (150 mg/kg). This is the first report of a substantial anticonvulsant property of parenteral melatonin in two animal models of epilepsy. It would appear tha melatonin is most effective against the kindled pentylenetetrazol Scizure, and somewhat less effective against the electrically kindled amygdaloid Scizure. Further testing in other Scizure models and species is needed to define better the anticonvulsant profile of melatonin.

A pharmacological study in the kindling model of epilepsy

The anticonvulsant properties of carbamazepine were evaluated in the kindled amygdaloid seizure model in rats. Carbamazepine significantly raised the threshold for seizures, reduced the duration of elicited afterdischarges and attenuated the severity of seizures in previously-kindled rats, at doses that did not cause sedation or ataxia. A similar reduction in the duration of elicited afterdischarges and severity of seizures was seen after suprathreshold stimulation (400 mu A) with doses of carbamazepine that were without obvious sedative or ataxic effects. After acute intraperitoneal injections (solvent = 2% Tween-80 and 70% propylene glycol), the maximum anticonvulsant effectiveness against suprathreshold stimulation was seen at 30 min. When administered daily (13 days) during acquisition or development of kindling, carbamazepine (25 and 50 mg/kg, i.p.) had variable effects on kindling. Neither dose consistently reduced the duration of elicited afterdischarges during the acquisition phase. Both groups tended to reduce the developing seizure, with the smaller dose of carbamazepine (25 mg/kg) resulting in a more consistent and significant reduction in severity of seizures. No significant differences in number of daily stimulations needed to reach fully kindled seizures were found. Previous studies have reported variable results with carbamazepine and the kindled amygdaloid seizure in rats. The present study provides a comprehensive evaluation of carbamazepine in this model of epilepsy and discusses the results with regard to the finding reported previously.

Aminophylline and kindled seizures.

The effects of aminophylline on amygdaloid and cortically kindled rats was studied. Rats implanted with chronic amygdaloid electrodes received either saline or 150 mg/kg, i.p., aminophylline 20 min prior to their first stimulation. On the first stimulation, aminophylline-treated rats had dramatically longer afterdischarge durations and more severe seizure ranks. When fully kindled, the animals were retested with saline or aminophylline. Again, the aminophylline-treated animals had longer afterdischarge durations than the saline-treated rats. In a second experiment, fully amygdaloid kindled rats were pretreated with various doses of aminophylline and stimulated 20 min later. With suprathreshold stimulation, a dose-dependent increase was noted in the afterdischarge duration. During seizure threshold determinations, aminophylline pretreatment markedly prolonged afterdischarge durations without significantly changing seizure severity or threshold. When animals were treated with the adenosine agonist, 2-chloroadenosine, prior to kindled amygdaloid stimulation, the elicited afterdischarge was shortened. The effect was antagonized where treatment with both 2-chloroadenosine and aminophylline occurred prior to amygdaloid stimulation. Rats with neocortical electrodes were also exposed to various doses of aminophylline while in a stable, partially developed kindled stage and again when fully kindled. At both stages, afterdischarge duration was increased by aminophylline in a dose-dependent manner. The partially developed, cortically kindled animals were more responsive to aminophylline than were those fully kindled and they tended to have greater increases in afterdischarge duration and seizure rank. These data demonstrate that aminophylline acts to prolong afterdischarges elicited at various stages of kindling from both amygdaloid and cortical sites.

Cumulative after-discharge as the principal factor in the acquisition of kindled seizures

Abstract The role of after-discharge in the development of behavioral and electrographic seizures (kindling) was evaluated. Rats were stimulated (amygdala) once daily for 10 days 30 min after an intraperitoneal injection of diazepam, phenobarbital, propranolol or vehicle. Following a 10 day period to allow for drug elimination, the daily stimulations (without drug) resumed until all animals responded with fully kindled seizures. Only the lowest dosage groups of diazepam (2 mg/kg) and phenobarbital (15 mg/kg) showed a savings in the number of stimulations to kindle in the non-drug state. This suggests that little or no kindling development had occurred in the drug state with the high doses (4 or 8 mg/kg diazepam, 30 mg/kg phenobarbital). Propranolol treated rats kindled with the same number of stimulations as their control group. As an alternative measure of kindling, the number of seconds of cortical after-discharge accumulated in the drug state, the non-drug state, and the grand total required to kindle were averaged and compared between the drug and control groups. The average grand total number of seconds of after-discharge accumulated by any of the drug or control 188 and 352 sec. None of the grand totals accumulated by any of the drug or control groups differed significantly. The lowest dosage groups treated with diazepam and phenobarbital accumulated significantly less afterdischarge in both the drug and non-drug state than the total for the control. These data suggest that a specific quantity of after-discharge must be elicited to kindle (200–300 sec) and that any after-discharge elicited in the presence of phenobarbital or diazepam will contribute to that quantity.

Caffeine modification of kindled amygdaloid seizures

Rats were kindled during exposure to caffeine (50 mg/kg) or saline given IP twenty minutes before daily electrical stimulation of the amygdala until 3 kindled amygdaloid seizures (KAS) occurred. They were then stimulated for 3 days without drug pretreatment followed by 5 additional days with drug pretreatment. There were no significant differences between the two groups in the number of daily stimulations or in the total seconds of cumulative afterdischarge (AD) needed to reach the first KAS. During kindling, the daily average AD tended to be longer in the caffeine treated group. This difference became significant (greater than 200% saline) when the KAS was reached. When KAS animals were stimulated without caffeine pretreatment, the average AD returned to control lengths. When put back on caffeine pretreatment, the average AD was again increased. Caffeine (6-50 mg/kg, IP) was also evaluated in previously kindled rats using suprathreshold (400 mu AMP) and threshold (20 microA increments) seizures. Caffeine had no consistent effect on threshold values. However, 12-50 mg/kg of caffeine increased seizure severity and AD durations after threshold stimulation. With suprathreshold stimulation, the length of the AD was significantly increased only after the highest dose of caffeine. It would appear that caffeine lengthens induced afterdischarges both during the acquisition phase of kindling and in the fully kindled subject. Caffeine does not appear to lower seizure thresholds or increase the rate of acquisition of the KAS in the doses tested in this model.(ABSTRACT TRUNCATED AT 250 WORDS)

An analysis of the actions of progabide, a specific GABA receptor agonist, on kindling and kindled seizures

The anticonvulsant properties of the specific GABA receptor agonist, progabide, were evaluated in the kindled amygdaloid seizure model in rats. Progabide attenuated afterdischarge durations and the severity of the accompanying convulsive responses in previously kindled rats. This effect occurred only at doses that also produced sedation and ataxia. When administered daily during kindling acquisition, progabide increased the number of trials necessary to complete kindling. The duration and severity of responses induced by stimulations during the acquisition period were reduced in a dose-dependent manner. In spite of these changes during the acquisition period, all subjects exhibited kindled behavior comparable to that of controls when they had accrued the same total afterdischarge experience. In light of these and other data regarding the GABA system and its influences on kindling, we conclude that GABA acts nonspecifically to attenuate various seizure states. It appears that GABA plays no major role in the mechanisms actually responsible for kindling development.

The effects of diphenylhydantoin, phenobarbital, and diazepam on the penicillin-induced epileptogenic focus in the rat

Abstract Techniques are described for the injection of minute quantities of penicillin into discrete intracortical loci in freely moving rats. The time course and quantification of the penicillin-induced cortical afterdischarges and seizure activity were determined. 14 C-penicillin was used to test assumptions related to the spread of this agent from the intracortical site of injection. The degree of spread was limited to the ipsilateral hemisphere along the cortical surface and was verified by both scintillation counting and autoradiography. The effects of diphenylhydantoin (30, 60 mg/kg, po), phenobarbital (15, 30 mg/kg, po) and diazepam (15, 30 mg/kg, po) on the penicillin-induced focal seizures were not marked. None of the agents produced a significant decrease in the spiking frequency, the number of afterdischarges, the total amount of afterdischarge activity or the average length of afterdischarge, even though the higher doses of all three compounds produced signs of neurotoxicity.

The anticonvulsant effects of diazepam and phenobarbital in prekindled and kindled seizures in rats

Abstract The effects of various doses of diazepam (0.5–4 mg/kg) and phenobarbital (7.5–60 mg/kg) were determined on prekindled and kindled amygdaloid seizures in the same rats. Diazepam was ineffective against the prekindled focal seizures, but demonstrated profound and statistically significant control of the kindled seizures. In the kindled state, diazepam reduced the afterdischarge duration and seizure rank score to prekindled levels. Only the largest sedating dose of phenobarbital produced a reduction of both prekindled afterdischarge duration and seizure rank score. Against the kindled seizure, phenobarbital showed a marked and statistically significant increase in effectiveness in all but the smallest dose tested. The afterdischarge duration of kindled seizures was reduced to prekindled levels by 15–60 mg/kg of phenobarbital, while seizure rank score was reduced to prekindled levels by 30 and 60 mg/kg phenobarbital. The effects of two doses of diazepam (0.5 and 2.5 mg/kg) and phenobarbital (7.5 and 30 mg/kg) were tested against prekindled and kindled pentylenetetrazol (PTZ)-induced seizures. Preliminary work with 3 doses of pentylenetetrazol (30, 40 and 60 mg/kg) demonstrated that repeated doses of 30 mg/kg readily kindled seizures without the significant mortality seen with larger doses. Both diazepam and phenobarbital were less effective against seizures kindled with 30 mg/kg pentylenetetrazol compared to prekindled seizures. The comparative lack of effect that was seen with diazepam and phenobarbital against the pentylenetetrazol kindled seizure at doses associated with control of the kindled amygdaloid seizure may reflect an underlying difference in the pathogenesis of kindling between these seizure models. Further, the lack of suppression of the prekindled amygdaloid afterdischarge duration by large doses of diazepam, in contrast to large doses of phenobarbital, may also reflect differences between the mechanisms of action of these two drugs. This paradigm provides a model for testing the effectiveness of anticonvulsants during the progressive development of various epileptogenic seizures.

Intertrial intervals and kindled seizures

Abstract The effects of nonuniform intertrial intervals on the development and retention of kindled amygdaloid seizures in rats were evaluated. Six consecutive grouped trial sessions (GTSs) consisting of five stimulations with 1-h intertrial intervals each day significantly reduced the average afterdischarge duration and behavioral seizure response of rats previously kindled by single, daily stimulations. Another group was treated with 32 consecutive stimulations at 1-h intervals. The development of kindling in this group was disrupted and only half the group exhibited occasional rank 5 seizures during the hourly stimulations. When the hourly group was subsequently stimulated once daily, kindled seizures were elicited in all animals. These data demonstrate that hourly stimulations or short, massed-trial paradigms disrupt kindled seizures and produce a convulsive response that is different from that produced by a single daily stimulation, suggesting that the pattern of stimulations must be taken into account when interpreting neurophysiologic, biochemical, or pharmacologic data from animals presumably kindled by the stimulations.