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

Anticonvulsive properties of F1933 (dulozafone) on kindled seizures in the rat

The anticonvulsive properties of a new compound: F1933 (Dulozafone) were investigated in the amygdala kindling model and compared with those of diazepam. Both drugs protected fully kindled rats against generalized seizures but failed to suppress partial ictal events (amygdala afterdischarges and limbic seizures). The anticonvulsive action of F1933, administered at the ED100, was nearly reversed by the specific antagonist of benzodiazepines receptors; R015-1788 (Flumazenil), suggesting that the effect of F1933 is mediated by this receptor. These results also emphasize the usefulness of kindling to test antiepileptic drugs and to confirm their supposed profile of action.

The effect of repeated seizures on anticonvulsant drug response in the kindling model

Clinical reports suggest that the occurrence of numerous seizures prior to the onset of anticonvulsant therapy is associated with a poor prognosis for drug control. This seems to imply that seizures become harder to suppress as they recur--a possibility which argues for the early initiation of anticonvulsant therapy. The present study attempted to determine whether this effect could be demonstrated under controlled laboratory conditions. The kindling preparation, an animal model involving the elicitation of repeated focal seizures, was used. Drug response (ED50) was measured in Royal Victoria hooded rats after either a small or a large number of pretreatment seizures, administered in the kindling paradigm. A variety of experiments were performed, involving: different drugs (phenobarbital, phenytoin, and carbamazepine), different seizure types (amygdala focal seizure, cortical focal seizure, generalized convulsion), and different stimulation parameters (0, 20, 40 or 100 pretreatment seizures: 'near-threshold' vs. 'standard' stimulation). In no case were seizures found to be harder to suppress following repeated pretreatment seizures. After large numbers of pretreatment seizures (40, 100), drug response was actually enhanced. These data indicate that the mere repetition of seizures does not automatically lead to a decrease in anticonvulsant effectiveness. They offer no particular rationale for the early initiation of anticonvulsant therapy in clinical situations.

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.

Cortical kindled seizures: modification by excitant and depressant drugs

The effects of several excitant-convulsants, cataleptic anesthetics, and gamma-acetylenic gamma-aminobutyric acid (GABA) were tested on seizures kindled by repetitive electrical stimulation of the motor cortex in the rat. A dose response was determined for each drug. For most of the drugs, the doses tested ranged from those causing some signs of behavioral excitation to those inducing epileptoid activity. None of the excitant-convulsants, including strychnine, physostigmine, amphetamine, bicuculline, or pentylenetetrazol, increased the afterdischarge duration (AD) or behavioral response (BR) of the partially developed (PD-KCS) or generalized fully developed (KCS) kindled cortical seizures. Whereas pentylenetetrazol had no effect on the PD-KCS, it has been previously shown to increase significantly the AD and BR of the developing or partially developed amygdaloid kindled seizures. Lidocaine, gamma-butyrolactone, gamma-acetylenic GABA, phencyclidine, and ketamine inhibited the AD of the KCS by greater than or equal to 80%. Lidocaine, phencyclidine, and ketamine decreased whereas gamma-butyrolactone and gamma-acetylenic GABA increased the AD of the PD-KCS. The ability of gamma-butyrolactone and gamma-acetylenic GABA to potentiate the PD-KCS while inhibiting the KCS presents a paradox not readily explained. Our results combined with previous reports of the effects of gamma-butyrolactone and gamma-acetylenic GABA on amygdaloid kindled seizures indicate that the KCS is more susceptible to GABAnergic and cataleptic inhibition than is the fully developed amygdaloid kindled seizures. The differences between the response of cortical kindled and that of amygdaloid kindled seizures to some of the drugs tested may indicate differences in the physiological and biochemical mechanisms involved in producing these seizures.

Modification of the anticonvulsant efficacy of diazepam by Ro-15-1788 in the kindled amygdaloid seizure model

The effects of various doses of diazepam and the new central benzodiazepine antagonist Ro-15-1788 were investigated in fully amygdaloid kindled rats. Diazepam had a pronounced dose-dependent anticonvulsant effect in this model. Ro-15-1788 dose-dependently reduced the behavioral ranks of the elicited kindled seizures to a maximum of 60% of control without consistently modifying the afterdischarge duration. No prestimulation convulsant effects were seen with Ro-15-1788. When 2 mg/kg i.p. of Ro-15-1788 was given after various doses of diazepam, the prestimulation sedation and ataxia anticonvulsant effects of diazepam (0.5-2.0 mg/kg) were attenuated by treatment with 2 mg/kg dose of Ro-15-1788. At the low dose of diazepam (0.25 mg/kg), increased reduction of behavioral rank and after discharge duration was seen after the 2 mg/kg dose of Ro-15-1788. Thus, Ro-15-1788 appears not to have proconvulsant properties in the kindled amygdaloid seizure model. Further, Ro-15-1788 appears to have some anticonvulsant properties of its own. Mixed agonist and antagonist effects were seen with Ro-15-1788 when given after various doses of diazepam in this model.