The development of the interictal spike during kindling in the rat

Kindling-induced changes in the EEG recorded during stimulation from the site of stimulation. II. Comparison between spontaneous and evoked potentials

We previously described how the EEG activity recorded during kindling-inducing tetanic stimulation, from the site of stimulation, consists of three major components. Here, we investigate the nature and role of component 1, an initial evoked potential, seen at the beginning of tetanus, and we compare it with the potential evoked by single-pulse stimulation and with the spontaneous interictal discharges seen after kindling in both the amygdala and hippocampus. The amplitude of component 1 increased during kindling, and the waveform, latency, and stimulus-response curve of component 1 were all similar to those of potentials evoked by single-pulse stimulation. Furthermore, in acute experiments, in which the amygdala was kindled by tetanic stimulation at 1-h intervals, the evoked potentials in the amygdala and pyriform cortex were markedly potentiated, whereas no obvious potential was recorded at the same latency from the ventral or dorsal hippocampus, subiculum, nor the prepyriform region. Thus, we conclude that, in the case of amygdala kindling, the nature of component 1 is a local direct and monosynaptic response involving the amygdala and pyriform cortex, and that the increased amplitude of component 1 after kindling may be related to long-term potentiation. Although the single-pulse stimulation produced late burst-like components in the amygdala and hippocampus after kindling, it produced neither EEG suppression, well-formed rhythmic synchronous bursts, nor epileptiform discharge, suggesting that component 1 alone is not sufficient for the subsequent events that may be essential to the seizure-triggering mechanism.

Alteration in dentate neuronal activities associated with perforant path kindling. II. Decrease in granule cell excitability

Changes in the excitability of the dentate granule cells after perforant path kindling were examined by the analysis of perforant path-dentate gyrus field potentials recorded in freely moving rats. Using a range of test pulse intensities, the population spike heights (cellular output) were plotted against the magnitudes of the associated population EPSPs (synaptic input), and a linear regression (input/output) was estimated from the relative linear portion of the plot. The x-intercept of this regression defines the population spike threshold. With kindling, the overall amplitude of the population spikes decreased, in spite of an increase in the population EPSPs resulting in a flatter input/output regression. The x-intercept of the regression was unaffected by the first kindling stimulation, but consistently and significantly increased with subsequent kindling. These changes gradually reverted to near the prekindling control values during a postkindling rest period of 1 month, and were reinstated by rekindling after the period of rest. Similarly, the onset and peak latencies of the population spike were significantly retarded during the period of kindling. These results suggest that seizure activity causes a significant but temporary reduction in the excitability of dentate granule cells. This effect would tend to counteract the process of kindling and provide an explanation for some of the phenomena of postictal depression.

Postictal events in amygdala-kindled female rats with and without estradiol replacement

This study investigated the effect of estradiol on ictal and postictal events in ovariectomized female rats kindled by daily amygdala stimulation during the presence or absence of estradiol replacement. Although ictal components of kindled seizures were not altered by estradiol, several postictal events such as myoclonic jerks were significantly increased by estradiol. Twelve days after kindling, estradiol-replaced rats with electrodes in the basal, lateral, or cortical amygdala nuclei developed brief postictal "bursts" of behavioral and EEG activity which developed into a secondary seizure in some rats. Rats without estradiol never displayed these postictal events. Pretreatment of rats with pentylenetetrazol, 20 mg/kg, 15 days after kindling, resulted in a further increase in the incidence of bursts and secondary seizures in the presence of estradiol; these events were most prominent in rats kindled in the absence of estradiol. Pentylenetetrazol also caused a significant increase in myoclonic jerks in estradiol-replaced rats, and postictal spikes in all rats. A series of five consecutive generalized seizures at 48-h intervals caused an increase in bursts in rats with estradiol replacement, but not in rats without estradiol. The results indicate that estradiol exerts excitatory effects during the postictal period following amygdala-kindled seizures and suggest that multiple regions mediate the postictal effects of estradiol, perhaps depending on the intraamygdala site of kindling stimulations.

Deep prepiriform cortex kindling and amygdala interactions

The deep prepiriform cortex (DPC) has been recently suggested to be a crucial epileptogenic site in the rat brain. We investigated the susceptibility of the DPC to the development of electrical kindling as compared to that of the superficial prepiriform cortex (SPC) and amygdala as well as the transfer interactions between the two prepiriform sites and amygdala. Adult rats with electrodes implanted in the right prepiriform cortex (DPC or SPC) and left amygdala were divided into a DPC-amygdala and SPC-amygdala group while a third group consisted of rats with electrodes implanted in the ipsilateral DPC and amygdala. Within each group the rats were initially kindled from one site selected randomly and then rekindled from the other site. Both DPC and SPC were as sensitive to the development of kindling as the amygdala. The behavioral seizures elicited with DPC or SPC primary kindling were identical to those induced by amygdala kindling. Initial DPC kindling facilitated the development of kindling from either ipsilateral or contralateral amygdala with the ipsilateral transfer being significantly more potent than the contralateral. SPC kindling also facilitated the development of contralateral amygdala kindling but was less effective than DPC kindling. On the other hand, amygdala kindling did not facilitate contralateral SPC or DPC kindling although it transferred to the ipsilateral DPC. These results indicate that the prepiriform cortex can be readily kindled but not faster than the amygdala and that there are unequal kindling transfer interactions between prepiriform cortex and amygdala.

Modification of excitation and inhibition evoked in dentate gyrus by perforant path stimulation: effects of aminophylline and kindling

Rats were implanted with chronic electrodes to stimulate the perforant path and record the elicited monosynaptic evoked potentials from the dentate gyrus of the hippocampal formation. Dentate responses were examined in awake and anesthetized animals after exposure to saline and aminophylline (100 mg/kg, IP). In the awake animal, aminophylline treatment did not significantly alter the threshold or elicited amplitude of either the excitatory post-synaptic potential (EPSP) or the population spike (PS). Aminophylline pretreatment markedly enhanced the length and severity of elicited seizures from hippocampal (dentate gyrus) or perforant pathway stimulation. After daily perforant pathway stimulations which established "kindled" seizures, aminophylline significantly increased only the amplitude of the evoked PS in awake animals. In animals anesthetized with chloropent, aminophylline increased significantly before kindling the amplitude of both the EPSP and PS without effecting thresholds for each. After perforant pathway kindling, only the PS amplitude was increased significantly by aminophylline. Inhibition, thought to be from GABA-mediated recurrent collaterals, was found to be increased rather than decreased by kindling. Further, aminophylline treatment did not result in reduction of this inhibition before or after kindling. These data suggest that at this dose of aminophylline neither enhanced transmitter release at this synapse as measured by the amplitude of the EPSP, nor reduced recurrent collateral inhibition significantly contributed to the prolongation of elicited seizure afterdischarge. The increase in PS amplitude reflecting an increased number of granule cells excited to discharge with perforant path stimulation after aminophylline was noted in awake animals but was greatest in the anesthetized animals. Although the number of granule cells excited to discharge was increased by aminophylline, the small increase in amplitude seen compared to the effects of other neurotoxins on this synapse makes this an unlikely explanation for the profound increased seizure response seen after aminophylline.

Epileptiform burst responses in ventral vs dorsal hippocampal slices

Field potentials from area CA1 evoked by stimulation of the Schaffer collaterals were compared in dorsal and ventral hippocampal slices of rat brain. Responses were categorized into 5 response types on the basis of their morphology, ranging from simple (single spike component) to complex (multiple spike components). A higher percentage of ventral slices, compared to dorsal slices, responded with a complex morphology under normal and increased K+ concentrations. Thus, there was a significantly greater tendency for cells within the ventral hippocampus to generate burst responses.

Down-regulation of alpha 2- and beta-adrenoceptor binding sites in rat cortex caused by amygdalar kindling

The role of central noradrenergic neurons in kindled seizures was assessed by comparison of alpha 2- and beta-adrenoceptor binding in the cerebral cortex from kindled and control rats. To minimize handling, which may modify kindling-induced changes in binding, the kindling protocol involved stimulation of the amygdala every hour for a maximum of 26 h. Twenty-four hours after kindling, down-regulation of beta-adrenoceptors was found in both olfactory cortex and the remaining neocortex, whereas alpha 2 down-regulation was confined to the olfactory cortex. At 21 days after kindling, the only change found was a down-regulation of beta-adrenoceptors in the neocortex. The results support the view that functional changes in central noradrenergic transmission are associated with the reduction in seizure threshold induced by kindling.

Modification of local neuronal interactions by amygdala kindling examined in vitro

Stimulation of the amygdala in coronal slices of the amygdala-pyriform region elicited burst responses in the pyriform cortex cells. The burst responses, recorded intracellularly, consisted of a train of action potentials riding on a depolarizing envelope that lasted for an average of 0.4 s. When a similar stimulus was applied to slices prepared from rats previously subjected to amygdala kindling, burst responses were significantly prolonged (average of 6.6 s). Our results indicate that the increased excitability of the chronic epileptic site developed in vivo was retained in the excised amygdala-pyriform slice.