Graham V. Goddard

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.

Is Adenosine an Endogenous Anticonvulsant

Summary: 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.

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.

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.

Adenosine modulation of amygdala kindling

To test the hypothesis that kindling is restrained by the inhibitory neuromodulator, adenosine, the adenosine uptake blocker, papaverine, or the adenosine antagonist, aminophylline, were injected systemically into rats 20 min before each daily electrical stimulation of the amygdala. The effects on amygdala-triggered seizures of papaverine, adenosine, 2-chloroadenosine, and the adenosine antagonists, isobutylmethylxanthine and caffeine, were also investigated at seizure threshold. Papaverine inhibited kindling, whereas aminophylline accelerated kindling. The adenosine agonists had anticonvulsant effects on seizures, and the antagonists had proconvulsant effects which involved, primarily, the lengthening of afterdischarge duration. Aminophylline injected repeatedly, in the absence of electrical stimulation, induced seizures. These results support the hypothesis that adenosine can modulate kindling and affect the seizure process.

TheN-methyl-D-aspartate antagonists aminophosphonovalerate and car☐ypiperazinephosphonate 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.

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.

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.

The Substantia Nigra is an Important Site for the Containment of Seizure Generalization in the Kindling Model of Epilepsy

Summary: 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: (a) 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, (b) 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, (c) These effects were only partially antagonized by haloperidol, but were completely abolished by picrotoxin. The picrotoxin alone significantly reduced the latency, (d) Almost no effect was found when the SN stimulation was delivered after the onset of bilateral fore‐limb 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 7‐aminobutyric acid and dopaminergic systems of the SN in this inhibitory function are discussed.

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.