Amygdala Kindling of Forebrain Seizures and the Occurrence of Brainstem Seizures in Genetically Epilepsy-Prone Rats

Forebrain seizures were kindled in rats by daily electrical stimulation of the amygdala. Genetically epilepsy-prone rats scoring 9 (GEPR-9s) on the seizure severity scale during audiogenic seizure (AGS) screening (“brainstem seizure-experienced”) required fewer stimulations to achieve fully kindled seizures (forelimb clonus with rearing and falling) than control rats. AGS-naive GEPR-9s required an intermediate number of stimulations, indicating a role for both genetic predisposition and previous acoustically evoked brainstem seizure experience. Other forebrain kindling indices such as afterdischarge thresholdlduration and seizure latencylduration also involved genetic as well as phenotypic (previous seizure experience) factors. In most GEPR-9s in both groups, severe brainstem seizures occurred after forebrain stimulation. The occurrence of brainstem seizures had a random nature and was not related to the sequence of kindling-dependent forebrain seizure progression. The lack of a difference in the occurrence of brainstem seizures between seizure-experienced and AGS-naïve GEPR-9s suggest that genetic predisposition is the major factor in forebrain seizure-induced activation of brainstem seizure circuitry. This brainstem seizure activity appears to model pertinent aspects of secondary generalization observed in human partial seizures.     

Deep Prepiriform Cortex Lesion and Development of Amygdala Kindling

The effects of a unilateral thermocoagulating lesion of the deep prepiriform cortex on the development of amygdala kindling were studied in adult rats. The lesion did not alter the local excitability of the ipsilateral amygdala. Furthermore, the number of stimulations required to produce generalized seizures did not significantly differ between lesioned and control animals. The results suggest that the deep prepiriform cortex may not be important in the development of amygdala kindling.     

Ketogenic Diet: Effects on Expression of Kindled Seizures and Behavior in Adult Rats

Summary: Purpose : Despite use of the ketogenic diet (KD) for >75 years its effectiveness or mechanism of action has been examined in few animal studies. Using the kindling model of epilepsy, we tested the anticonvulsant effectiveness and behavioral consequences of an experimental KD in adult rats.
Methods : Rats fully kindled from the amygdala were divided into KD-fed or standard rat diet-fed groups; diet treatment continued for 5 weeks. The KD approximated at 4:l ("classic") ketogenic diet and consisted (by weight) of 70% fat, 14% protein, no carbohydrate, and appropriate vitamins, minerals and fiber; 92% of energy provided was contributed by fat and 8% was contributed by protein. Afterdischarge threshold and duration (ADT, ADD) and stage 5 seizure threshold and duration (ST, SD) were assessed weekly for 5 weeks. During week 3, learning and memory were tested by the water maze and the behavioral response to a novel environment was assessed by the open field test.
Results : Rats receiving the KD became ketonemic and had weight gains similar to those of control rats. As compared with rats receiving a standard diet, those fed the KD had an elevated ADT and ST for the first 2 weeks of treatment. The control and KD-fed groups did not differ with regard to ADD or SD at any time during the study, and both groups performed similarly in the water maze and open field test.
Conclusions : In the kindling model, the KD afforded transient protection against the focal generation of kindled seizures but not seizure spread. Rats that received the KD did not perform differently from control-fed rats on spatial learning or exploratory behavior tasks. Our results provide a promising model for study of the anticonvulsant mechanisms of ketosis.     

Amygdala kindling effects on sleep and memory in rats

Sleep disturbances accompany the development of amygdaloid-kindled seizures in cats. Some of these sleep deficits resemble those seen in aged rats; these latter changes in sleep patterns are correlated with memory impairments in the aged animals. In the present study, we examined the hypothesis that sleep deficits after kindling may be related to memory impairments. Rats were kindled for 4 weeks (2-2.5 weeks after stage 5 seizures) and were then allowed a one week recovery period. Sleep patterns were assessed through-out the kindling and recovery periods. The animals were then trained on an inhibitory avoidance apparatus and tested for retention 24 h later. Only transient sleep changes occurred during the development of kindling (to stage 5 seizures). However, continued kindling resulted in significant reductions in several sleep measures which remained depressed for at least one week after the termination of the kindling trials. As a group, kindled rats were impaired in retention of the inhibitory avoidance learned response. In kindled animals, retention performance was significantly correlated with total paradoxical sleep, the ratio of paradoxical/total sleep, and paradoxical sleep, the ratio of paradoxical/total sleep, and paradoxical sleep bout duration. These correlations are consistent with the view that deficits in paradoxical sleep may be related to deficits in some forms of memory.     

Short bursts of weak pulses break postictal inhibition in the neocortex of Wistar rats

PURPOSE: Postictal inhibition (PI) is a decrease in excitability that follows an epileptic seizure and decreases probability of new seizure occurrence. PI may involve both increased inhibition and persisting elevated excitation. Our experiments tested whether shorter trains of weak stimuli are able to unmask this residual increase of excitability during the PI. METHODS: Four epileptic afterdischarges (ADs) were evoked by intense electrical stimulation (20 s, 8 Hz, current intensity at 5x threshold) of the neocortex in two groups (A, B) of Wistar rats. Before the first AD and during the 10-min interictal period, 8-Hz trains of four weak pulses (half of the intensity used for the AD triggering; 4P) were applied every 20 s in group B and a single pulse with similar parameters in group A. RESULTS: The number of interictal epileptiform events evoked by 4P in the group B was significantly higher than that in the group A (evoked by single pulses) except after the second AD. Epileptic events were triggered by 4P also immediately after the AD termination. CONCLUSIONS: It is apparent that weak stimulation can trigger epileptic phenomena during PI. Our results indicate that it is no longer possible to perceive PI only as persisting extreme and active inhibition. An appropriate stimulation can reveal more subtle (but important) excitatory events contributing to the functional status during the postictal period.     

Differential Effects of Adenosine Analogs on Amygdala, Hippocampus, and Caudate Nucleus Kindled Seizures

This study explored the anticonvulsant effects of adenosine analogs at the focus of seizures kindled from various brain structures. Chemitrodes were implanted in the amygdala (AM), hippocampus (HIPP), or caudate nucleus (CN) of Long-Evans rats and electrically stimulated once daily until fully generalized seizures appeared (i.e., kindled). Once kindled, various doses (0.001-0.5 microgram/0.5 microliter) of the adenosine analogs, L-phenylisopropyladenosine (L-PIA), N-ethylcarboximidoadenosine (NECA) or vehicle were injected into the seizure focus 5 min prior to electrical stimulation. The afterdischarge (AD) and behavioral seizure stages were measured. L-PIA had potent anticonvulsant effects when injected directly into the kindled seizure focus in the AM, HIPP, or CN. NECA effects were statistically significant only in CN-kindled seizures. The regional differences in efficacy of the two adenosine analogs suggest that L-PIA, an A1 adenosine subtype agonist, may exert its effects through A1 adenosine receptors in the AM, HIPP, and CN, where A1 binding has been demonstrated, whereas NECA, an A2 adenosine receptor agonist, may only be maximally effective in the CN where A2 adenosine binding sites are located.     

Kindling of the Interpeduncular Nucleus and Its Influence on Subsequent Amygdala Kindling in Rats

Summary We examined the effect of interpeduncular nucleus (IPN) kindling on subsequent amygdala (AM) kindling in rats (n = 9). Eleven to 15 daily IPN stimulations at an afterdischarge (AD)-inducing threshold (40CL1000 PA, biphasic sine waves, 1–3 s) produced progressive AD growth (9 of 9 rats) and recruitment of behavioral seizures (7 of 9 rats). The final form of the latter was generalized tonic-clonic seizures with or without a limbic seizure component. The latter was associated with ictal involvement of AM and sensorimotor cortex. Subsequent AM kindling resulted not only in more rapid kindling, but also in tonic seizure associated with a protracted loss of postural control (5–20 s) not observed in animals undergoing AM kindling without previous IPN kindling (n = 5).These findings indicate that the IPN can be kindled and that subsequent AM kindling utilizes the proconvulsant neuroplastic changes that have been already established by IPN kindling.     

Effects of Taurine on Kindled Amygdaloid Seizures in Rats, Cats, and Photosensitive Baboons

Acute administration of taurine produced a transient loss of susceptibility to photically induced seizures in photosensitive baboons, but failed to affect kindled amygdaloid convulsions in baboons, rats, and cats. In addition, it was totally ineffective in changing the course of spontaneous status epilepticus in kindled cats. These results suggest that a taurine-deficiency model of epilepsy applies only to certain types of seizure-generating conditions, apparently excluding kindled amygdaloid convulsions.     

Kindled Amygdaloid Seizures in Rats Cause Immediate and Transient Increase in Protein Kinase C Activity Followed by Transient Suppression of the Activity

Summary: Protein kinase C (PKC) activity in hippocampus and amygdala was measured during kindled seizures and 30 rnin, 3, 24, and 80 h, and 2 weeks after seizures in amygdaloid-kindled rats. Shamperated rats not subjected to kindling were used as controls. During kindled seizures, membrane-bound PKC activity in bilateral hippocampi was significantly increased, with a slight reduction in cytosolic PKC activity, but there was no change in either membrane-bound or cytosolic PKC activity in bilateral amygdaIa. Thirty minutes after seizures, PKC activity in both fractions was significantly increased in bilateral hippocampi and amygdala. Three hours after seizures, PKC activity in both fractions was markedly decreased in bilateral hippocampi. In bilateral amygdala, a similar and significant decrease in membrane-bound PKC activity was noted, with no marked change in the cytosolic fraction. Twenty–four hours after seizurs, a significant decrease in membrane–bound PKC activity in bilateral hippocampi and amygdala was again noted, although cytosolic PKC activity was unchanged. Fortyeight hours after the seizures, PKC activity in both fractions had returned to control levels. Two weeks after the last seizure, there was no significant change in PKC activity in either fraction in any region, except for a slightincrease in membrane–bound PKC activity in unilateral hippocampus contralateral to the kindled arnygdala. These results suggest that kindled amygdaloid seizures cause an immediate and transient increase in PKC activity in limbic structures, followed by suppression of enzyme activity, and that PKC in hippocampus responds to kindled seizures more readily and preferentially than it does in amygdala. Examination of the direct effect of kindled seizures on PKC activity is important for understanding of the relation between kindled phenomena and PKC in limbic structures.     

Effects of Seizures, Kindling, and Carbamazepine on Sleep Organization in Cats

We studied the relationships between epilepsy, sleep, and anticonvulsant drugs in kindled cats. No sleep alteration was present at midkindling. When the animals became fully kindled, a reduction in REM sleep percentage and the number of entries into REM sleep were observed compared to baseline. In addition, with further seizures, an increase in the percentage of wakefulness appeared, accompanied by a further reduction in the number of entries into REM sleep. It therefore seems that there is a progressive disruption of sleep, dependent on the increasing number of tonic-clonic generalized seizures. After a seizure-free interval, REM sleep and wakefulness returned to baseline values. A reduction in the percentage of stage II compared to baseline was found and remained as a long-term effect of the kindling process. Acute administration of carbamazepine (CBZ) reduced the REM sleep percentage. This effect, paralleled by a reduction in the number of entries into REM sleep, was evident both at baseline and when the animals were fully kindled. After a large number of seizures, however, CBZ administration did not cause a further reduction in the already low percentage of REM sleep. Results are discussed with reference to previous literature. We propose a hypothesis of competition between seizure and REM sleep in the elimination of epileptogenic and hypnogenic factors.     

Effects of Dextromethorphan, a Nonopioid Antitussive, on Development and Expression of Amygdaloid Kindled Seizures

The effects of dextromethorphan (DM), a nonopioid antitussive and a functional N-methyl-D-aspartate (NMDA) antagonist, on expression and development of amygdaloid kindled seizures were examined. The maximum anticonvulsant effect of DM (30 mg/kg) on fully kindled seizures appeared within 30 min of administration and lasted for at least 2 h. DM decreased, in a dose-dependent manner [10-70 mg/kg, intraperitoneally (i.p.)], the severity of kindled seizures 30 min after injection, but the estimated ED50 was 3 times higher than the previously reported value for maximal electroshock convulsions. Furthermore, the high dose (70 mg/kg), while suppressing kindled seizures, produced myoclonus which coincided with EEG spike activity in the amygdala and the cortex. When tested on the development of kindling, 30 mg/kg DM retarded the growth of afterdischarge in the amygdala and the cortex, but had no effect on the development of behavioral seizures. DM 60 mg/kg accelerated development of kindling and produced spontaneous seizures. These results indicate that DM, unlike other NMDA antagonists, has a narrow therapeutic window as an anticonvulsant on kindled seizures and that higher doses may potentiate the kindling process.     

Electrical Stimulation of the Centromedian Thalamic Nucleus in Control of Seizures: Long-Term Studies

Summary: Five patients with chronic incapacitating seizures averaging 15–5,000/month were selected for study. All patients had more than one seizure type and had received maximal doses of antiepileptic drugs (AEDs). The centromedian thalamic nucleus (CM) was stimulated electrically through bilateral multicontact platinum electrodes stereotaxically placed in CM and connected to internalized pulse generators. Electrophysiologic confirmation of electrode position included thalamically elicited recruiting responses and EEG desynchronization recorded at the scalp. Stimulation parameters were adjusted individually in the range of 450–800-μA intensity, 65 pps, 0.09 ms, in 1-min trains, alternating right and left side stimulation and with 4-min intervals delivered for 2 h/day. Quantitative evaluation included frequency of seizures/month, number of maximal interictal paroxysmal discharges, and frequency of background activities counted in selected scalp EEG samples, taken throughout the observation period (7–33 months). Significance of changes was evaluated by parametric Student's t test. Generalized tonic-clonic seizures (GTC) decreased dramatically, almost disappearing in all cases (p < 0.001), with a significant reduction in interictal paroxysmal discharges (p < 0.01) and a tendency toward an increase in EEG back-ground frequency. Other generalized seizures (atypical absences) decreased significantly, but there was no change in the number of complex partial seizures (CPS). CM stimulation is useful in control of GTC, but its beneficial effect on other seizure types has not been established.     

Facilitation of the jaw reflexes by stimulation of the red nucleus in the rat

The effects of the red nucleus (RN) stimulation on the jaw-opening reflex (JOR) and the masseteric monosynaptic reflex (MMR) were studied in anesthetized rats. The JOR was evoked by electrical stimulation of the inferior alveolar nerve. The MMR was evoked by electrical stimulation of the mesencephalic trigeminal nucleus. The JOR and the MMR were recorded as electromyographic responses of the anterior belly of the digastric and the masseter muscles, respectively. The conditioning electrical stimulation of the RN facilitated both the JOR and the MMR bilaterally. The facilitatory effect on the JOR was much larger than that on the MMR. Additionally, microinjection of monosodium glutamate into the RN also elicited facilitation of the JOR and the MMR. The results suggest the RN plays an important role in reflex control of jaw movements.