Ronald J. Racine

Short-term potentiation phenomena in the rat limbic forebrain

Several types of short-term postactivation potentiation (PAP) effect were examined in limbic forebrain pathways in the chronic rat. We tested 9 different stimulation sites and 1-3 target sites for each stimulation site. All pathways showed PAP effects following activation by single electrical pulses or pulse trains. Using exponential curve fitting procedures, we found that the decay curves could be best fitted by one, or a sum of 2-3, exponential curves. On the basis of time constants, these curves fell into 4 different categories: facilitation (tau = 80 ms), augmentation (tau = 7s), potentiation 1 (tau = 70s), and potentiation 2(tau = 6.5 min). The latter component was the one most reliably generated in the chronic preparation. Frequency facilitation (facilitation during a stimulation train) was also examined and it appeared to be based upon a mechanism similar to that underlying paired pulse facilitation. Evidence is presented that facilitation and augmentation may be based on the the same mechanism. The possibility that the remaining components involve different mechanisms is discussed.

KindlingThe First Decade

The kindling phenomenon is a progressive increase in the strength of epileptiform activity evoked by spaced (in time) and repeated electrical stimulation of certain brain structures. The work that has been done on the kindling phenomenon is reviewed, with an emphasis on those studies that deal with underlying mechanisms. Based on the work that has been done thus far, it is clear that the kindling effect is not due to any type of gross tissue damage. It is also clear that at least some of the effects are due to changes at the synapse and that these changes are widely distributed in the brain. The changes might be due to an increasing efficacy at excitatory synapses or a decreased effectiveness at inhibitory synapses, or both. The long term post-tetanic potentiation data and some preliminary electron microscopic studies support the former mechanism, whereas the depletions of catecholamines in kindled tissue support the latter. In addition to these transynaptic changes, there may be other changes that occur at the site of the stimulating electrode, and these changes may be based on a different mechanism. These ideas and the relevant data are discussed.

The effects of kindling on GABA-Mediated inhibition in the dentate gyrus of the rat. I. Paired-pulse depression

Double-pulse stimulation of the perforant path input to the dentate gyrus was used in the following experiments to produce paired pulse depression in that site. This effect provided an estimate of GABA-mediated recurrent inhibition. The depression was enhanced by drugs that facilitate or mimic GABA action and attenuated by drugs which block GABA transmission. Paired-pulse depression was significantly increased following amygdala kindling and was further enhanced to near maximal levels by subsequent kindling in the dentate. In addition, kindling did not increase the rate at which inhibition failed under conditions of excessive activation. Trains of 5 Hz stimulation, applied to the perforant path, caused paired-pulse depression to disappear and elicited a brief AD. Following kindling, the latency to AD onset tended to be increased rather than shortened, suggesting an enhanced resistance to inhibitory failure. These results indicated that kindling increased, rather than reduced, inhibition in the dentate gyrus.

Modification of seizure activity by electrical stimulation: III. Mechanisms

Abstract Daily electrical stimulation of the amygdala or hippocampus via implanted electrodes results in the development of a potentiality to trigger motor seizures in those areas. Arguments against several possible mechanisms underlying this development are discussed. Experiments are described which are intended to decide between increases in the strength of limbic-“motor system” connections or limbic-limbic connections as events underlying motor seizure development. It was found that stimulation of more than one area increases the rate of seizure development, whereas disrupting inter-limbic connections retards seizure development. It is concluded that motor seizure development is dependent upon the increased strength of limbic-limbic connections.

Kindling mechanisms: current progress on an experimental epilepsy model.

For many years a major goal of epileptologists has been to discover the mechanisms of seizure generation. The experimental approaches to the problem have been numerous, addressing acute and chronic determinants in both focal and non-focal models. Perhaps the best controlled model of chronic focal epilepsy is that of kindling (Goddard et al., 1969). The latter refers to the progressive development of electroencephalographic and behavioral seizure resulting from the repeated application of a low-intensity electrical stimulus to one of many discrete forebrain structures. The increase in manifestations of the motor seizure proceeds through several stages (Racine, 1972). With subcortical stimulation, there is an initial ictus of the ipsilateral facial and neck muscles (stages 1 and 2) which is associated with focal EEG seizure. Further stimulations result in secondary generalization of the seizure to involve the forelimbs and trunk (stages 3 and 4), with an additional loss of balance during strong ictus (stage 5). The neurological changes which underlie these behavioral alterations seem to be permanent since animals which have not been stimulated for many months after stage 5 kindling often respond with a full seizure immediately upon reexposure to the original kindling stimulus (Goddard et al., 1969; Wada and Sato, 1974). In this paper, we review selectively some of the more obvious hypotheses concerning the nature of the mechanisms underlying kindling and conclude with a brief summary of the current working hypothesis. For more general or extensive reviews of the kindling literature see Racine (1978), McNamara et al. (1980), Kalichman (1982), and Peterson and Albertson (1982).

Modification of seizure activity by electrical stimulation: cortical areas.

1. Three different cortical areas were found with respect to the development, by repeated electrical stimulation, of electrographic and motor seizures in rats. Paleocortical areas responded similarly to subcorticallimbic structures. Initial after-discharges (ADs) were accompanied by little or no behavioral response. With spaced reptition ADs became longer, propagated more strongly, and were eventually accompanied by a convulsive response in which the dominant components were forelimb clonus and rearing. Anterior neocortical placements, on the other hand, yielded convulsive responses from the first AD. These convulsive responses became stronger with repeated stimulation until a mild form of extension was triggered. ADs remained relatively short. Posterior neocortical areas showed electrographic developments very similar to those found in anterior neocortical areas but convulsive responses never developed. 2. All areas tested showed similar reductions in AD thresholds when repeatedly stimulated. Initial thresholds were lower in paleocortical than in neocortical areas. 3. Motor seizures developed more rapidly with stimulation of contralateral homologous sites after seizures had been developed in the primary (initially stimulated) focus. AD thresholds, on the other hand, were not significantly affected in contralateral sites after being lowered in primary sites.

Maintenance on L-deprenyl prolongs life in aged male rats

The effect of l-deprenyl on longevity was examined in male Fischer rats. Subcutaneous injections of either l-deprenyl (0.25 mg/kg) or saline were given every other day starting at 23 to 25 months of age. The deprenyl-treated animals showed a significant increase in both mean and maximum survival. The differences were largest in the longest surviving animals, suggesting that an earlier onset for treatment may be beneficial. Analysis of body weights ruled out deprenyl-induced dietary restriction as an explanation for the group differences in survival. To the contrary, after about four months of treatment, the animals of l-deprenyl showed a slower rate of decrease in body weight than the controls.

Kindling, Unit Discharge Patterns and Neural Plasticity

Two approaches to the study of the kindling phenomenon were discussed: 1) an attempt to identify the pattern of neural activity required to produce the changes underlying kindling and 2) an investigation into the nature of those changes. Three experiments were reported that used the neocortical transcallosal system as a monosynaptic model system in which to study possible synaptic mechanisms of the kindling effect. Experiment I showed an increase in the transcallosal evoked potential following neocortical kindling. Experiment II showed an increase in the strength of the transcallosal evoked cell discharge following neocortical kindling. Experiment III reported the results of an histological examination of neocortical tissue in kindled and non-kindled animals using the Golgi-Cox technique. Spine density, spine dimension and branching were measured for pyramidal cell apical dendrites. No differences were found between primary and secondary (contralateral) foci or between kindled and non-kindled animals.

The development and decay of kindling-induced increases in paired-pulse depression in the dentate gyrus

Epileptogenic stimulation (kindling) leads to an increase in recurrent inhibition in the dentate gyrus, possibly due to an increase in benzodiazepine receptors. A second, late inhibitory component also potentiates as a result of kindling. In the present experiments, the time course of the development and decay of this kindling-induced increase in inhibition was studied. Rats were kindled by stimulation of the perforant path or by direct stimulation of the dentate gyrus. Paired-pulse stimulation was applied to the perforant path and field potential measures were taken within the dentate gyrus. These procedures allowed the monitoring of inhibitory events in chronic preparations over prolonged periods. Input/output measures of the baseline responses were also monitored during the course of, and after the completion of kindling. The increase in the early component (about 20-50+ ms) of paired-pulse depression was seen after the first kindling stimulation. The increase in the late component of depression (about 100-500+ ms) did not develop until after about 10 stimulations had been delivered. The late component then decayed more rapidly than the early component after the completion of kindling. The baseline response also showed some indication of depression, particularly in the dentate gyrus kindled group, raising the possibility that feedforward inhibition had also been potentiated.

Development of kindling-prone and kindling-resistant rats: selective breeding and electrophysiological studies

Because of the growing need for an animal model of complex partial seizures based on a genetic predisposition, we combined the kindling model of epilepsy with selective-breeding procedures to develop two new lines (or strains) of rats that are kindling-prone or kindling-resistant. The selection of these strains was based on their rates of amygdala kindling. From a parent population of Long Evans hooded and Wistar rats, the males and females that showed the fastest and slowest amygdala kindling rates were selected and bred. Similar selection procedures continued through F11, although there was little or no overlap in the distribution of kindling rates for the two new strains (FAST and SLOW) by F6. Examination of both local and propagating seizure profiles of the new strains from F6 to F10 revealed that the FAST and SLOW rats had similar amygdala afterdischarge (AD) thresholds and associated AD durations. Also, the convulsion profiles of the stage-5 responses were similar, although the severity was greater in the FAST rats. Clearly the selection was not based on local mechanisms controlling the threshold for amygdala AD evocation, but rather for the spread of AD from the focus and the recruitment of other structures, ultimately triggering convulsive seizures. Although evoked potentials and potentiation effects were similar between the strains, the SLOW rats showed a greater paired-pulse depression, raising the possibility that they differ in inhibitory mechanisms. The specificity of strain differences for the amygdala and its associated networks is described in our accompanying paper (McIntyre et al., 1999. FAST and SLOW amygdala kindling rat strains: Comparison of amygdala, hippocampal, piriform and perirhinal cortex kindling. Epilepsy Res. 35, 197-209). These strains should provide many clues to the dispositional differences between individuals for the development of epilepsy originating in temporal lobe structures.