Long-term inhibition of kindled seizures by brain stimulation

Kindled epileptic seizures, postictal refractoriness, status epilepticus, and electrical self-stimulation

A single stimulus applied once daily to the limbic system commonly leads to convulsive seizures yet seizures are relatively infrequent during intracranial self-stimulation (ICSS), a procedure that involves many hundreds of similar stimuli. The present study examined the possible role of electrode site, interstimulus interval, afterdischarge and reinforcement thresholds and postictal refractoriness in accounting for this paradox. Electrode location was an overriding factor: seizures were never seen with hypothalamic implants posterior to the level of the ventromedial nucleus but were elicited by the majority of more rostral reward sites. Frequent repeated stimulation by ICSS did not in itself prevent subsequent kindling or reverse the effects of earlier kindling; on the contrary, seizures induced by ICSS showed a progressive increase in severity similar to the progression produced by conventional kindling. Individual convulsive seizures, as in previous studies, conferred transient protection against further seizures whether from ICSS or from kindling. More prolonged protection occassionally developed after repeated convulsive seizures: protection was accompanied by continuous EEG slow-waves corresponding in presentation to clinical petit mal status. Prolonged resistance to seizures has also been reported after tonic-clonic status epilepticus causing temporal lobe damage. The relative infrequency of seizures during ICSS ordinarily appears to depend on the siting of the electrodes, on distinct short- and long-term postictal refractory states, and on the rat learning to restrict stimulus input to subseizural levels.

Effect of an adenosine A1 agonist injected into substantia nigra on kindling of epileptic seizures and convulsion duration

The substantia nigra pars reticulata (SNr) has been reported to be critically involved in the development and propagation of epileptic seizures, while extracellular adenosine appears to be important for making seizures stop. In the present study, an adenosine A1 receptor agonist [N6-cyclohexyladenosine (CHA); 2.0 nmol/side, or vehicle] was injected bilaterally into the SNr shortly before each of the first five of a series of daily kindling stimuli delivered to the rat amygdala. Injections did not affect the acquisition of kindled afterdischarges or the rate at which seizures developed over subsequent kindling sessions, but convulsions occurring 48-72 h after treatment were significantly shortened. Thus, purinergic mechanisms in the SNr do not appear to be specifically involved in the acquisition of kindled seizures but may contribute to a postictal inhibitory process that shortens the convulsive component.

Low frequency stimulation induces an increase in after-discharge thresholds in hippocampal and amygdaloid kindling

In rats with electrodes implanted in the hippocampus or amygdala, the effect was studied of a single episode of low frequency stimulation (125 msec, biphasic 4 Hz, for 1 min at a current 50% of threshold for after-discharge), LFS, on the after-discharge thresholds (ADTs) during the course of hippocampal and amygdaloid kindling. ADTs were significantly increased following the LFS. The effect lasted longer (hippocampus, 12 days; amygdala, 5 days) when the LFS was given on day 1 of kindling than when kindling was fully established (less than 24 h).

Decreased sensitivity to nicotine-induced seizures as a consequence of nicotine pretreatment in long-sleep and short-sleep mice

Male and female long-sleep (LS) and short-sleep (SS) mice were pretreated with a subseizure-producing dose of nicotine (2.0 mg/kg) 7.5, 15 and 30 minutes prior to challenge with seizure-producing doses of this drug. Nicotine pretreated animals were less susceptible to nicotine-induced seizures than were saline pretreated animals. The latency to seizure following nicotine challenge was greater in nicotine pretreated animals than in saline controls. Nicotine pretreated LS mice show a greater decrease in nicotine-induced seizure susceptibility than do nicotine pretreated SS mice. This decrease in seizure susceptibility is consistent with induction of nicotinic receptor desensitization via nicotine pretreatment. It is hypothesized that LS and SS mice might differ in sensitivity to nicotine in part because they differ in baseline levels of desensitized versus functional nicotinic receptors.

Modification of amygdala-kindled postictal inhibition by pentylenetetrazol and diazepam

When fully amygdala-kindled rats are electrically stimulated in grouped trials with intertrial stimulation intervals of less than 60 min, significant residual inhibition can be demonstrated. When these grouped trials of stimulation are repeated daily, additional cumulative inhibition is seen. The present study examined the effect of daily pretreatment with three doses of pentylenetetrazol (5, 10, and 20 mg/kg) and two doses of diazepam (0.5 and 2 mg/kg) on daily, grouped trial electrical stimulations of fully amygdala-kindled rats. Little or no reduction was seen in postictal inhibition by pentylenetetrazol pretreatment including the highest dose tested in which prestimulation bursts of spiking were associated with short episodes of forelimb clonus. Pretreatment with the benzodiazepine receptor agonist, diazepam, resulted in a dose-dependent reduction in the first elicited seizure response each day compared with control trials. Subsequent daily seizure trials of diazepam-treated animals demonstrated a relatively constant degree of dose-dependent seizure suppression without evidence of further postictal inhibition. The neural substrate that governs grouped trial postictal inhibition of amygdala-kindled seizures appears to be resistent to modification by near-convulsant doses of pentylenetetrazol, an agent with a presumed GABA-mediated mechanism of action. Diazepam, an anticonvulsant with a presumed GABA-related or associated mechanism of action, suppressed seizures in a dose-dependent manner nearly equally across all trials.

Suppression of amygdala kindling with short interstimulus intervals: effect of norepinephrine depletion

The rate of development of generalized kindled convulsions was profoundly influenced by the interval between amygdala stimulations. With stimulation every 10 min, nearly complete interference with the progression of kindling was observed in most rats, and hourly stimulation precipitated kindling rates three times longer than did once per day. Depletion of norepinephrine (NE), as a result of intracerebroventricular pretreatment with 6-hydroxydopamine, virtually eliminated the interference with kindling development seen in the vehicle control rats. Such depletion of NE, however, had little influence on the generalized responses once developed. At this stage, interference with seizure provocation was observed as truncated electroencephalographic seizures which were usually devoid of motor correlates. This interference was more profound in the shorter interstimulus intervals and was independent of NE depletion. Finally, when changing from the short kindling intervals of 10 min and 1 h to the longer interval of 24 h, an unexpected interference with seizure provocation was observed. The implication of these results for the biochemical basis of kindling and kindling as a model of learning are discussed.

Effect of aminophylline on amygdaloid-kindled postictal inhibition

Previous studies have shown that methylxanthines such as aminophylline increase the clinical severity and length of electrically elicited limbic afterdischarges in naive and kindled rats without lowering seizure threshold. When fully amygdaloid-kindled rats are electrically stimulated at intertrial stimulation intervals of less than 60 minutes, significant residual inhibition can be demonstrated. The present study examines the effect of three doses of aminophylline (25, 50 and 100 mg/kg) on repeated daily stimulations of fully amygdaloid-kindled rats. After 100 mg/kg aminophylline, the first elicited amygdaloid-kindled seizure afterdischarge was doubled in length compared to saline controls. The second elicited seizure 15 minutes later resulted in status epilepticus and hindlimb extension in the majority of the aminophylline-treated animals with death occurring in 28%. When 25 or 50 mg/kg of aminophylline was given daily for five days before the first of five daily stimulation trials, each separated by 15 minutes, no significant reduction in postictal inhibition was demonstrated compared to saline controls. The 50 mg/kg aminophylline dose consistently and significantly lengthened only the first afterdischarge of each day without affecting the postictal inhibition seen with repeated stimulations. The neural substrate that governs immediate postictal inhibition of amygdaloid-kindled seizures appears to be resistant to modification by aminophylline at low doses. At high doses of aminophylline (100 mg/kg), sustained epileptical activity occurred. The sustained seizure activity seen at the high dose of aminophylline may be secondary to blockade of the processes which normally terminate seizure activity, or it may represent actual inhibition of the immediate postictal inhibitory processes.

Amygdala-kindled postictal inhibition: effect of intertrial intervals on repeated days

Fully amygdala-kindled rats with stable afterdischarge durations and seizure ranks of five were evaluated with suprathreshold stimulations (400 microA) at variable interstimulus intervals. Subjects were stimulated either once a day or four times a day at one of five interstimulus intervals (5, 15, 30, 60, or 180 min) for 5 days. All subjects were tested under each of the six paradigms. Profound inhibition of seizure rank and afterdischarge duration occurred daily with intertrial stimulation times of 30 min or less. The initial stimulations each day at these intertrial intervals showed no significant residual inhibition from the stimulations of the previous day. The longer intertrial intervals of 60 and 180 min demonstrated little inhibition of elicited seizures on day 1 for trials 2 through 4; however, after the 1st day, increased inhibition was noted for trials 2 through 4 at the 60-min intertrial interval with relatively little inhibition noted for the first trial of each day at this interval. At the longest interval tested (180 min), a significant (P less than or equal to 0.05) reduction in both the seizure rank and afterdischarge duration was noted with the first elicited seizure of days 3 through 5. At the 180-min interval, seizures 2 through 4 on days 3 through 5 tended to increase in length and severity rather than decrease compared with the first seizure of the day. Significant interactions occurred between the length of intertrial interval used with multiple stimulation paradigms and repeated days of testing. The short-term inhibition seen with shorter intertrial interval testing interacted in a complex and poorly understood manner with the longer term inhibition associated with daily grouped stimulations in the amygdala-kindled model of epilepsy.