The N-methyl-D-aspartate antagonists aminophosphonovalerate and carboxypiperazinephosphonate retard the development and expression of kindled seizures.

To investigate the possible role of N-methyl-D-aspartate (NMDA) receptors in the development and expression of amygdaloid-kindled seizures, rats were either chronically infused with 2-amino-5-phosphonovalerate (APV, 20-40 mM) or pre-injected with carboxypiperazine-phosphonate (CPP, 1-10 mg/kg), both selective NMDA-receptor antagonists, and then kindled from the amygdala. At the higher dose (40 mM), APV blocked the induction of long-term potentiation in the dentate gyrus. APV also retarded clinical seizure development dose-dependently and increased seizure thresholds without affecting afterdischarge (AD) duration. These same doses of APV had only small anticonvulsant effects on established kindled seizures. Although CPP (1-10 mg/kg) had no effect when rats were kindled 45 min after injection it dose-dependently retarded focal and generalization stages at the 150 min injection-kindling interval. Once relieved of drug, animals proceeded to develop stage 5 seizures with shorter duration ADs than saline-control animals. When the previously-kindled, saline groups were crossed to CPP a small depressant effect on seizure expression was observed. These results suggest that NMDA receptors are primarily involved in kindling development rather than in maintaining the kindled state.

Maintenance of long-term potentiation in rat dentate gyrus requires protein synthesis but not messenger RNA synthesis immediately post-tetanization.

The involvement of new protein and messenger ribonucleic acid synthesis in long-term potentiation was studied in the anaesthetized rat dentate gyrus using several inhibitors of protein synthesis (anisomycin, emetine, cycloheximide and puromycin) and an inhibitor of messenger ribonucleic acid synthesis (actinomycin D). When injected for 1 h just prior to tetanization, the four inhibitors of protein synthesis produced a mild reduction of long-term potentiation of the excitatory postsynaptic potential measured 10 min after tetanization. Anisomycin produced a significantly faster decay of long-term potentiation, while the other inhibitors had more moderate effects. Actinomycin D failed to affect long-term potentiation. In a second experiment, the time-dependency of the anisomycin effect was examined. Anisomycin injected immediately after tetanization promoted decay of long-term potentiation, but when injected after a 15-min delay, the drug had no effect. Inhibition of protein synthesis for 4 h prior to tetanization did not have any more effect on long-term potentiation than inhibition for 1 h. In no experiment was long-term potentiation of the population spike affected by drug manipulation. These results suggest that for long-term potentiation of the excitatory postsynaptic potential to be maintained for at least 3 h proteins must be synthesized from already existing messenger ribonucleic acid, and that this synthesis is mostly completed within 15 min after tetanization.

Cognitive and motor dysfunction in Parkinson's disease. Clinical, performance, and computed tomographic correlations.

The neuropathologic and pathophysiological relationship of specific to more generalized cognitive dysfunction in Parkinson's disease (PD) remains incompletely understood. This issue was examined in a study of 39 patients with PD, utilizing standardized clinical measures, computerized neuropsychological tests, and quantitative computed tomography. Disorders of visuospatial discrimination and perceptual-motor function closely paralleled motor scores, suggesting a common neuropathologic basis. Caudate nuclear and mesocortical dopamine depletion play a role in this context. More generalized cognitive dysfunction occurred in older patients with a somewhat longer disease duration, more advanced parkinsonism, and computed tomographic evidence of subcortical and frontal cortical atrophy but without significant cerebral atrophy when compared with age-matched controls. Further prospective clinicopathologic studies will be required to clarify the relative contribution of the primary dopaminergic dysfunction, age-related changes, Alzheimer-type pathologic condition, and other coexisting neurotransmitter deficits to the dementia seen in PD.

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.

Kindling-induced changes in EEG recorded during stimulation from the site of stimulation. III. Direct pharmacological manipulations of the kindled amygdala.

In our previous studies, we hypothesized that activation and subsequent collapse of GABA-mediated inhibition during tetanus is an important seizure-triggering mechanism in the kindled epileptogenic focus. To examine this hypothesis, in the present study, we investigated the effects of pharmacological manipulations of the kindled amygdala with several drugs, and measured the kindled seizures as well as the EEG events during tetanus. The results obtained were: (i) The selective GABA-A agonist, muscimol (1 and 5 nM/1 microliter), suppressed kindled seizures in a dose-dependent fashion, and the 5 nM muscimol significantly prolonged EEG suppression and reduced the number of oscillations in the subsequent rhythmic synchronous discharge. Similar effects followed systemic injection of diazepam (2 mg/kg). (ii) The selective GABA-B agonist, baclofen (5 nM), had no effect on kindled seizures nor on the EEG events during tetanus. (iii) The NMDA antagonist, 2-amino-5-phosphonovaleric acid (80 nM), significantly reduced the afterdischarge duration and significantly delayed the appearance of the rhythmic synchronous discharge. However, these effects were not observed immediately, but 24 to 72 h after microinjection. (iv) The muscarinic cholinergic antagonist, atropine (40 and 80 nM), suppressed kindled seizures in a dose-dependent fashion, but the atropine caused marked synchronous discharge both in the awake resting EEG and during tetanic stimulation. We conclude that the GABA-A system, including the benzodiazepine system, is more involved in the seizure-triggering mechanism of amygdala kindling than the GABA-B system, that there is an interaction between the GABA-A and NMDA system, and that the cholinergic participation is independent of the primary seizure-triggering mechanisms.

Proconvulsant effects of theophylline on hippocampal afterdischarges.

We tested the effects of the adenosine antagonist, theophylline, on electrically induced afterdischarges in the hippocampus of rats. Theophylline did not significantly alter the threshold for afterdischarge initiation or the duration of the initial afterdischarge. However, theophylline greatly prolonged the duration of secondary afterdischarges, and permitted generalization to motor seizures. Because theophylline antagonizes adenosine, and the hippocampus is a region rich in adenosine binding sites, we interpret these results to mean that endogenous adenosine has little action on focal seizure initiation, but a strong action to suppress secondary seizures and to inhibit seizure generalization.

Alteration in dentate neuronal activities associated with perforant path kindling. I. Long-term potentiation of excitatory synaptic transmission.

One candidate for the neuronal mechanism of kindling is the facilitation of excitatory synaptic transmission. The population EPSP component of the perforant path-dentate field potential is strongly potentiated by the first few kindling stimulations applied to the perforant path. As kindling proceeds further, however, subsequent changes in transmission efficacy have been a source of controversy. The present study reexamines these changes in transmission efficacy of the perforant path-dentate granule cell synapses during and after perforant path kindling using improved methods of analysis of the field potentials recorded in freely moving rats. The slope of the regression line of the population EPSPs on a range of stimulus strength values was found to be enhanced by the first kindling stimulation and then continued to increase gradually with subsequent kindling stimulations, indicating a cumulative increase in synaptic transmission efficacy throughout the period of kindling. The potentiated excitatory synaptic transmission lasted for at least 1 month after the cessation of kindling. On the other hand, the kindling stimulations produced a progressive increase in the x-intercept of the regression line, indicating an increase in the minimal EPSP threshold. These two effects seem to account for the apparent discrepancy between previous studies, each of which measured the population EPSP at a fixed stimulus strength.

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.

Alteration in dentate neuronal activities associated with perforant path kindling. III. Enhancement of synaptic inhibition.

Changes in the hippocampal commissural inhibition of dentate granule cell firing following perforant path kindling were examined by analysis of field potentials recorded in freely moving rats. Prior stimulation of the contralateral dentate hilus inhibits the dentate population spike response to perforant path stimulation at interpulse intervals of less than 11 ms via GABAergic control. This measure of inhibition was gradually enhanced from 26.6% to 76.9% by kindling stimulations delivered to the perforant path once a day for 3 weeks. It then decreased from 76.9% to 54.6% within 2 weeks after the last kindling trial, and remained at approximately this value for another 18 days of observation. Similarly, the mean maximum duration of commissural inhibition significantly increased from 7.43 ms to 10.6 ms during the kindling period, and decreased to 8.29 ms 2 weeks later. These results indicate that perforant path kindling temporarily increased the synaptic inhibition of granule cells, in a close temporal relationship to the temporary decrease in the granule cell excitability observed in our preceding study.

Effects of acute and long-term treatment with amphetamine on evoked responses and long-term potentiation in the dentate gyrus of anesthetized rats.

We examined the effects of D-amphetamine (AMP) on evoked responses and long-term potentiation (LTP) in the dentate gyrus of rats in 4 different conditions: untreated + saline, untreated + acute AMP, long-term AMP-treated + saline, and long-term AMP-treated + acute AMP. Long-term AMP-treatment comprised once daily i.p. injections of 5 mg/kg AMP for 14 consecutive days followed by 7 drug-free days. Acute AMP administration consisted of a single i.p. injection of 5 mg/kg. Acute AMP significantly increased the slope of the synaptic component, the excitatory postsynaptic potential (EPSP), of the evoked response without changing the cell discharge component (population spike), resulting in a significant decrease in the population spike per unit EPSP in the evoked potentials of both untreated and long-term AMP-treated rats. The acute AMP slightly reduced LTP of the population spike in the untreated rats and significantly reduced it in the long-term AMP-treated rats. LTP of the EPSP did not differ between the 4 conditions. The two saline conditions did not differ significantly in their evoked responses or in LTP. It is suggested that AMP has 3 acute effects in the dentate gyrus, and that the effect of reducing the capacity for LTP of the population spike is selectively enhanced following long-term AMP treatment.

Septohippocampal and commissural pathways antagonistically control inhibitory interneurons in the dentate gyrus.

Available evidence suggests that a portion of the septohippocampal pathway may form inhibitory synapses on inhibitory interneurons in the dentate gyrus. In contrast, a portion of the commissural input from the contralateral hilus may form excitatory synapses on inhibitory interneurons. To ascertain whether these pathways synapse onto a common population of interneurons, a series of pulses were applied and their effects on perforant path evoked, granule cell population spikes were measured. The population spike was markedly reduced when the perforant path pulse was preceded by a pulse to the contralateral hilus. This inhibition was markedly reduced, however, when a medial septal pulse was applied either prior to, or within 3 ms after the commissural pulse. The disinhibition was critically dependent on the temporal relationship between the medial septal and commissural pulse, and not on the medial septal-perforant path relationship. This finding suggests that the septohippocampal pathway inhibits the interneurons through which the commissural pathway is able to inhibit granule cells.

The substantia nigra is an important site for the containment of seizure generalization in the kindling model of epilepsy.

To investigate the role of the substantia nigra (SN) in kindling, electrical stimulation of the SN was delivered at various times before or after stimulation of the amygdala (AM) or pyriform cortex during or after kindling in rats. The results were as follows: Ipsilateral SN stimulation delivered prior to each AM kindling stimulation for 14 days significantly retarded the appearance of Stage 4 and 5 seizures and shortened the afterdischarge (AD) duration. Bilateral SN prestimulation blocked seizure generalization in some AM- or pyriform cortex-kindled animals, prolonged the latency to bilateral forelimb clonus in others, and shortened the AD duration of the kindled seizure in a current intensity-dependent fashion. These effects were only partially antagonized by haloperidol, but were completely abolished by picrotoxin. The picrotoxin alone significantly reduced the latency. Almost no effect was found when the SN stimulation was delivered after the onset of bilateral forelimb clonus. We conclude that the SN might be an important mediator of the early aspects of seizure generalization from limbic epileptic foci. The relative involvements of GABAergic gamma-aminobutyric acid and dopaminergic systems of the SN in this inhibitory function are discussed.

Field potential evidence for long-term potentiation of feed-forward inhibition in the rat dentate gyrus.

Trains of high-frequency stimulation to the perforant path cause (i) long-term potentiation (LTP) of the population excitatory post-synaptic potential (EPSP), (ii) a lasting increase in the population spike, and (iii) a lasting alteration of the relationship between the EPSP and population spike (E-S relationship), consisting of a decreased x-intercept and decreased slope of the linear regression. To compare the thresholds of these changes, we applied a series of trains, increasing in duration from below LTP threshold. The EPSP potentiated with about the same low threshold as the reduction in E-S slope, whereas the reduction in E-S x-intercept required longer trains. In the second experiment, LTP of the EPSP was reduced by concurrent high-frequency stimulation of the commissural input and a lasting reduction of the population spike height was observed. In a third experiment, picrotoxin, an antagonist of gamma-aminobutyric acid (GABA)-mediated inhibition, blocked the decrease in slope of the E-S relationship which normally accompanies LTP. These results imply that perforant path/granule cell LTP is normally accompanied by long-term potentiation of a feed-forward inhibitory pathway which may involve interneurones.

Kindling induced changes in EEG recorded during stimulation from the site of stimulation: collapse of GABA-mediated inhibition and onset of rhythmic synchronous burst.

As a new approach to the functional alteration of kindled epileptogenic foci, we have demonstrated electroencephalographic (EEG) changes at the site of stimulation at the times when the kindling stimulation is applied in freely moving rats. In the kindled amygdala, three components of response were observed: an initial evoked potential, a subsequent EEG suppression, and an eventual burst of rhythmic spiking. The three components were a common feature also in the kindled hippocampus and prepyriform region. The duration of component 2, EEG suppression, progressively increased with each day of kindling (40 to 50 days), but dramatically shortened again after a rest period of 1 month. It was sensitive to pharmacological manipulations of gamma-aminobutyric acid (GABA). The amplitude of component 3, rhythmic spiking, increased during kindling, and was relatively insensitive to current intensities. The increased amplitude was long-lasting, independent of GABAergic, monoaminergic, and cholinergic manipulations, but sensitive to a benzodiazepine. It is suggested that component 3 is the outbreak of synchronous excitatory action which may be related to the basic mechanism of kindling.

Deep prepyriform cortex kindling and its relation to amygdala kindling in the rat.

Kindling of the deep prepyriform cortex (DPC), a recently identified sensitive site to chemoconvulsants, was compared to kindling of the amygdala in rats. Although the two kindled sites were very similar in their initial responses, electroencephalographic changes during kindling stimulation, kindling seizure development, and growth of afterdischarge, they differed in that the deep prepyriform cortex showed a significantly lower final threshold, a significantly higher frequency of afterdischarge and a significantly shorter latency to bilateral forelimb clonus than the amygdala. Significant bidirectional transfer was found during secondary kindling of either of the two sites, suggesting a close interrelation between them. Rhythmic synchronous discharge was directly induced in the deep prepyriform cortex by ipsilateral amygdala stimulation, but not in the amygdala by ipsilateral cortical stimulation. Local application of both muscimol (0.45 nM) and 2-amino-5-phosphonovaleric acid (25 nM) into the cortical site reduced the afterdischarge duration of amygdala-kindled seizures by approximately 20% without changing the type or duration of the motor seizure. We suggest that the deep prepyriform cortex is "downstream" from the amygdala in the preferential routes of afterdischarge propagation, and that, although the deep prepyriform cortical neurocircuits of GABA and excitatory amino acids may be involved in the maintenance of amygdala-kindled, self-sustained afterdischarge, they are not essential in the expression of amygdala-kindled motor convulsions.

Effects of a new thyrotropin-releasing hormone analogue (DN-1417) on evoked responses and long-term potentiation in the dentate gyrus of rat.

The effects of a recently synthesized analogue, thyrotropin-releasing hormone (TRH) analogue, DN-1417, on evoked responses and long-term potentiation (LTP) of the perforant path input to the dentate gyrus were examined in normal rats and after hypophysectomy. Eighty micrograms of DN-1417 injected i.c.v. reduced granule cell excitability with a small increase in the excitatory postsynaptic potential (EPSP) and reduced LTP of the EPSP. However, DN-1417 slightly increased the potentiation of the population spike. Similar results were obtained in a rat 7 days post-hypophysectomy. It is concluded that the effects of DN-1417 on LTP are similar to those of TRH and may be unrelated to its endocrine actions.

Medial septal facilitation of hippocampal granule cell activity is mediated by inhibition of inhibitory interneurones.

Rats under barbiturate anaesthesia were implanted with stimulating electrodes in the medial septal nucleus and the medial perforant path. A recording electrode and cannula were implanted in the hilus of the dentate gyrus. Electrodes were positioned so that a conditioning pulse to the medial septum, although eliciting no field potential of its own, facilitated the granule cell population spike evoked by medial perforant path stimulation. In the first experiment, the infusion into the hilus of the GABA antagonist picrotoxin was found to block the facilitation. In a second experiment, it was found that a medial septal conditioning pulse blocked recurrent inhibition of the granule cells, only if it was timed to coincide with their initial activation. We suggest that these effects are mediated through an inhibitory connection from the medial septum onto inhibitory interneurones in the dentate gyrus, and that this connection may utilize the neurotransmitter GABA.

Effects of thyrotropin-releasing hormone on evoked responses and long-term potentiation in dentate gyrus of rat.

Thyrotropin-releasing hormone (TRH), a neuropeptide with multiple neuromodulatory functions, has been receiving examination for clinical relevance in epilepsy. To investigate the neurophysiologic properties of the anticonvulsant action of TRH, its effects on the evoked responses and long-term potentiation (LTP) of the perforant path input to the dentate gyrus were examined in anesthetized rats. Intracerebroventricular microinjected 80 micrograms TRH increased the relationship between the amplitude of the population spike and the slope of the EPSP, and reduced the LTP of the EPSP by approximately 30% compared with saline-injected controls. The TRH, however, slightly increased the amplitude of the population spike. These effects of TRH on LTP were confirmed in a dose-related manner. These results indicate that TRH may have two independent effects in the dentate gyrus, an increase in cellular excitability and a decrease in synaptic plasticity. The latter may be related to the previously reported anticonvulsant effects on kindling.

Is adenosine an endogenous anticonvulsant?

The anticonvulsant properties of adenosine were tested pharmacologically on amygdala-kindled seizure activity in rats. The adenosine analogue 2-chloroadenosine and the adenosine uptake blocker papaverine both increased the latency to behavioral clonus as well as reduced the duration and severity of the clonic motor convulsion. Both drugs, however, failed to alter the postkindling afterdischarge (AD) threshold. Theophylline, an adenosine antagonist, had the opposite effects, prolonging the AD and motor seizure durations and facilitating partially kindled seizures, but again not altering the prekindling or postkindling AD thresholds of amygdala-elicited seizures. In contrast, carbamazepine raised AD thresholds, suggesting that it does not produce its anticonvulsant effects through adenosine systems. Since endogenous adenosine can impede seizure spread and seizure continuation, but does not affect seizure initiation from the amygdala, perhaps endogenous adenosine has the special property of being brought into play as an anticonvulsant only by the seizure itself.