Beta-frequency (15-35 Hz) electroencephalogram activities elicited by toluene and electrical stimulation in the behaving rat

Bursts of beta-frequency (15-35 Hz) electroencephalogram activity occur in the olfactory system during odour sampling, but their mode of propagation within the olfactory system and potential contribution to the mechanisms of learning and memory are unclear. We have elicited large-amplitude beta activity in the rat olfactory system by applying noxious olfactory stimuli (toluene), and have monitored the bursts via chronically-implanted electrodes. Following exposure to toluene, coherent bursts with a peak frequency of 19.8 +/- 0.9 Hz were observed in the olfactory bulb, piriform cortex, entorhinal cortex and dentate gyrus. The timing of the bursts and the phases of electroencephalogram cross-spectra indicate that beta bursts propagate in a caudal direction from the olfactory bulb to the entorhinal cortex. The time delays between peaks of bursts in these structures were similar to latency differences for field potentials evoked by olfactory bulb or piriform cortex test-pulses. Peaks of burst cycles in the dentate region, however, were observed just prior to those in the entorhinal cortex. Surprisingly, power in toluene-induced beta-frequency oscillations was not increased following long-term potentiation induced by tetanic stimulation of the olfactory bulb, piriform cortex and entorhinal cortex. The activity of local inhibitory mechanisms may therefore counteract the effects of synaptic enhancements in afferent pathways during beta bursts. Low-frequency electrical stimulation of the piriform cortex was most effective in inducing coherent oscillatory responses in the entorhinal cortex and dentate gyrus at stimulation frequencies between 12 and 16 Hz. The results show that repetitive polysynaptic volleys at frequencies in the beta band induced by either toluene or electrical stimulation are transmitted readily within the olfactory system. The propagation of neural activity within this frequency range may therefore contribute to the transmission of olfactory signals to the hippocampal formation, particularly for those odours which induce high-amplitude bursts of beta activity.

Time course for kindling-induced changes in the hilar area of the dentate gyrus: reactive gliosis as a potential mechanism

Recurrent seizure activity induced during kindling has been reported to cause an increase in the hilar area of the dentate gyrus of the hippocampus. To date, very little is known about the mechanism of this increase. This study investigated the time course for kindling-induced changes in the hilar area of the dentate gyrus at seven days, one month, and two months post-kindling. Hilar area of the dentate gyrus was significantly increased by approximately 46% at seven days and remained elevated at one month, but declined back to control levels by two months. Glial fibrillary acidic protein (GFAP) immunostaining was also evaluated at the same time points to determine whether kindling-induced changes in the hilar area of the dentate gyrus are related to kindling-induced glial cell changes. Increases in hilar GFAP immunostaining by approximately 57% were observed at seven days and at one month post-kindling, but not at two months post-kindling. These findings indicate that kindling-induced changes in the hilar area of the dentate gyrus and kindling-induced glial cell changes follow a similar time course, and that kindling-induced glial cell changes may mediate the observed changes in the hilar area of the dentate gyrus.

The degree of potentiation is associated with synaptic number during the maintenance of long-term potentiation in the rat dentate gyrus

There is a considerable degree of variation in the amount of potentiation induced in different animals following the induction of long-term potentiation (LTP). This variation provided us with the opportunity to determine what types of synaptic changes were dependent upon the degree of induced potentiation. To examine possible 'degree of potentiation' effects on synapses, we conducted a multiple regression analysis examining the relationship between the degree of potentiation in LTP animals and a series of synaptic structural measures. We examined synapses in the middle third of the molecular layer (MML) of the rat dentate gyrus following repeated high frequency tetanization of the perforant path. LTP was induced over a 4 h period, and the animals were sacrificed 24 h after the final stimulation. Synapses from the ipsilateral inner third of the dentate molecular layer (IML) and from implanted only animals were also examined for comparison. Ultrastructural quantification included the total number of synapses per neuron, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic apposition. The only structural change that was significantly associated with the degree of potentiation was a positive correlation between the degree of LTP and the number of synapses per neuron. Therefore, synaptic number, while not appearing to be significantly associated with the induction of LTP, appears to be important for the degree of LTP expressed.

Development of spontaneous seizures over extended electrical kindling. I. Electrographic, behavioral, and transfer kindling correlates

The present study was aimed at evaluating an extended kindling model of spontaneous epilepsy. Behavioral and electrographic responses to repeated kindling of either the perforant path or amygdala were monitored for up to 300 trials. Kindling initially led to generalized convulsions equivalent to the level 5 seizure on the rating scale developed by Racine. The evoked seizures became progressively more complex with additional kindling, which was described by a 10-stage classification system. The highest stage (stage 10) was achieved when the kindling stimulation evoked two or more bouts of level 5 seizures combined with running and jumping fits. These more complex seizures developed over the course of amygdala, but not perforant path kindling. Electrographic seizures from both the amygdala and dentate gyrus increased in duration and amplitude during the early phase of kindling, but did not correlate with motor seizure development beyond level 5. During the late phase of kindling, the dentate gyrus afterdischarge amplitude decreased and became dissociated from the behavioral seizures. Manifestations of spontaneously recurring seizures were seen in the majority of animals, but spontaneous seizures of level 4 or greater were observed in only five rats. The second part of this study examined kindling transfer effects, the efficacy of kindling a new site after the completion of the initial (in this case extended) kindling protocol. The effect depended on both primary and secondary site location. When the amygdala served as primary site, perforant path transfer was complete in some animals but absent in others. No transfer occurred in the opposite direction, from the perforant path to the amygdala. Finally, transfer effects in the dentate gyrus, which was tested as tertiary site, were complete. Previous studies have found weaker transfer effects in the dentate when kindling to the standard stage 5 level.

Effects of extended electrical kindling on exploratory behavior and spatial learning

Short-term electrical kindling, a widely used experimental model of epilepsy, appears to have little effect on behavior. The effects of extended kindling are largely unknown. Rats implanted with kindling electrodes in amygdala (AM) or perforant path (PP) received 300 kindling trials over approximately 7 months, and were tested in the Morris watermaze after a 7-10 day recovery period. Kindled animals were impaired during the initial training on hidden-platform acquisition, but not in retention of platform location. No deficits were found in acquiring a new hidden-platform location, latency to reach a visible-platform, or in swim speed. Open-field activity showed a sustained increase when tested during kindling, but only a transient increase when tested following suspension of kindling. Similar results were obtained for both AM and PP kindled animals. Hence, long-term kindling of both of these sites produced behavioral changes that were transient in nature. Further, these results also indicate that propagation of seizure activity from remote sites can alter hippocampally-mediated or related behavior.

Long-term potentiation trains induce mossy fiber sprouting

It has been shown that both amygdaloid and hippocampal kindling induce sprouting of the mossy fibers in the dentate gyrus. In this study, we investigated whether non-epileptogenic stimulation could also induce mossy fiber sprouting. Long-term potentiation (LTP) was induced in the dentate gyrus by the application of brief, high frequency trains to the perforant path. The potentiating stimulation was applied each day for 10 days, and the tissue was prepared for Timm labelling 7 days later. Sprouting was significantly increased in the LTP group compared to the implanted control rats. These results suggest that mossy fiber sprouting is not damage-induced and is dependent on neuronal activation.

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis. J. Neurophysiol. 78: 2602-2615, 1997. The entorhinal cortex receives sensory inputs from the piriform cortex and modulatory inputs from the medial septum. To examine short-term synaptic facilitation effects in these pathways, current source density (CSD) analysis was used first to localize the entorhinal cortex membrane currents, which generate field potentials evoked by stimulation of these afferents. Field potentials were recorded at 50-micron intervals through the medial entorhinal cortex in urethan-anesthetized rats and the one-dimensional CSD was calculated. Piriform cortex stimulation evoked a surface-negative, deep-positive field potential component in the entorhinal cortex with mean onset and peak latencies of 10.4 and 18.4 ms. The component followed brief 100-Hz stimulation, consistent with a monosynaptic response. CSD analysis linked the component to a current sink, which often began in layer I before peaking in layer II. A later, surface-positive field potential component peaked at latencies near 45 ms and was associated with a current source in layer II. Medial septal stimulation evoked positive and negative field potential components which peaked at latencies near 7 and 16 ms, respectively. A weaker and more prolonged surface-negative, deep-positive component peaked at latencies near 25 ms. The early components were generated by currents in the hippocampal formation, and the late surface-negative component was generated by currents in layers II to IV of the entorhinal cortex. Short-term facilitation effects in conscious animals were examined using electrodes chronically implanted near layer II of the entorhinal cortex. Paired-pulse stimulation of the piriform cortex at interpulse intervals of 30 and 40 ms caused the largest facilitation (248%) of responses evoked by the second pulse. Responses evoked by medial septal stimulation also were facilitated maximally (59%) by a piriform cortex conditioning pulse delivered 30-40 ms earlier. Paired pulse stimulation of the medial septum caused the largest facilitation (149%) at intervals of 70 ms, but piriform cortex evoked responses were facilitated maximally (46%) by a septal conditioning pulse 100-200 ms earlier. Frequency potentiation effects were maximal during 12- to 18-Hz stimulation of either the piriform cortex or medial septum. Occlusion tests suggested that piriform cortex and medial septal efferents activate the same neurons. The CSD analysis results show that evoked field potential methods can be used effectively in chronically prepared animals to examine synaptic responses in the converging inputs from the piriform cortex and medial septum to the entorhinal cortex. The short-term potentiation phenomena observed here suggest that low-frequency activity in these pathways during endogenous oscillatory states may enhance entorhinal cortex responsivity to olfactory inputs.

Long-term enhancement of entorhinal-dentate evoked potentials following 'modified' ECS in the rat

Electroconvulsive therapy (ECT) is widely used as a treatment for drug-resistant depression. The animal analogue of ECT is electroconvulsive shock (ECS) seizures. We have recently shown that repeated ECS seizures cause a long-lasting, perhaps permanent, enhancement in entorhinal-dentate evoked potentials in the rat. Our study, however, involved 'unmodified' ECS, whereas in clinical practice ECT is now usually given in its 'modified' form (with near-threshold currents, a short-acting barbiturate, muscle relaxant and oxygen). We have therefore repeated our experiments using modified ECS. Entorhinal-dentate evoked potentials were measured in Long-Evans rats before and after: (1) eight modified ECS seizures; or (2) eight sham modified ECS trials. Despite the use of the modified procedure, a significant and long-lasting enhancement in population spike amplitude was seen in the ECS group. We conclude that the modified procedure does not protect rats against the long-lasting enhancement of evoked potentials. Similar changes may be occurring in the brains of patients subjected to modified ECT.

Nerve growth factor accelerates seizure development, enhances mossy fiber sprouting, and attenuates seizure-induced decreases in neuronal density in the kindling model of epilepsy

Recurrent seizure activity induced during kindling has been reported to produce a functional synaptic reorganization of the mossy fibers in the hippocampus. To date, it is unclear whether this kindling-induced growth is secondary to decreases in hilar neuron density, which are presumed to reflect hilar neuronal cell loss, or whether it is related specifically to an activation-dependent plasticity. We recently demonstrated that blocking nerve growth factor (NGF) biological activity retards seizure development and inhibits the sprouting of mossy fibers. We now demonstrate that intraventricular administration of NGF itself accelerates the progression of kindling epileptogenesis, increases mossy fiber sprouting in the CA3 region and in the inner molecular layer (IML), but reduces seizure-induced decreases in hilar cell density. These findings provide support for a role of NGF in kindling and kindling-induced mossy fiber sprouting. In addition, the results dissociate this form of epileptogenesis from hilar cell loss or decreases in hilar cell density attributable to increases in hilar area, thereby supporting seizure-induced mossy fiber sprouting as being primarily attributable to the combined effects of neuronal activation and the activation-induced upregulation of growth factors.

Piriform cortex efferents to the entorhinal cortex in vivo: kindling-induced potentiation and the enhancement of long-term potentiation by low-frequency piriform cortex or medial septal stimulation

The entorhinal cortex receives input from many cortical areas and mediates the flow of information between these sites and the hippocampal formation. Long-term synaptic plasticity in cortical efferents to the entorhinal cortex may contribute to the transmission of neural activity to the hippocampus, as well as the storage of information, but little is known about plasticity in these pathways. We describe here the use of evoked field potential recordings from chronically implanted electrodes in the rat entorhinal cortex to investigate synaptic plasticity in the large piriform (olfactory) cortex projection to the superficial layers of the entorhinal cortex. Both kindling-induced potentiation and long-term potentiation (LTP) were tested. In addition, we attempted to modulate LTP induction by the co-induction of frequency potentiation and by the co-activation of the medial septum. Epileptogenic kindling stimulations of the piriform cortex (1-s, 60-Hz trains 3 times/day for 5 days) were found to result in a reliable potentiation of field responses evoked by piriform cortex test pulses. Non-epileptogenic tetanization of the piriform cortex with 400-Hz 16-pulse trains reliably resulted in LTP effects. These effects could be augmented by embedding brief LTP induction stimuli within 11-pulse, 15-Hz trains that alone produce only frequency potentiation. Co-activating the medial septum with 10-Hz trains, just prior to tetanization of the piriform cortex, augmented LTP of piriform cortex inputs to the entorhinal cortex in an input-specific manner. All potentiation effects were found to last for periods of weeks. These findings demonstrate that both epileptogenic and non-epileptogenic piriform cortex stimulation induces lasting potentiation of population field responses in the entorhinal cortex of the awake rat. The LTP effects were inducible in a graded manner and were sensitive to the temporal context of stimulation. The finding that low-frequency activation of the septum can enhance plasticity in the entorhinal cortex adds to a body of data indicating a role for the medial septum in contributing to theta activity and plasticity in both the entorhinal cortex and hippocampal formation.

Post-activation potentiation in the neocortex. III. Kindling-induced potentiation in the chronic preparation

Previous experiments have shown the neocortex to be very resistant to the induction of long-term potentiation in chronic preparations. We show here that kindling-induced potentiation effects can be reliably produced in the neocortex of awake, freely moving rats. These effects develop rather slowly. In sites contralateral to the stimulation electrode, potentiation effects did not become clear until the animals had received about 5 days or more of stimulation. Ipsilateral sites required even longer (approximately 10 days), and both sites required more than 13 days to reach asymptotic levels of potentiation. Both monosynaptic and polysynaptic components were present in the neocortical field potentials. When population spikes were absent, the surface negative monosynaptic EPSP component tended to show a potentiation effect. If population spikes were present, they were generally enhanced while the monosynaptic population EPSP tended to be depressed. Consequently, the apparent depression may have been due to competing field currents. The later polysynaptic components (15-28 ms latency to peak) always showed a potentiation effect with 5 or more kindling stimulations and is presumed to result from activation of cortico-cortical associational fibers. All of these effects were long-lasting, showing little decay over a period of several weeks.

Post-activation potentiation in the neocortex. IV. Multiple sessions required for induction of long-term potentiation in the chronic preparation

The neocortex in chronically prepared rats is very resistant to the induction of long-term potentiation (LTP). In the first of two experiments described in this paper, we tried unsuccessfully to induce neocortical LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application of repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KIP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpus callosum for 25 days. LTP in callosal-neocortical field potentials became clear after about 5 days of stimulation and reached asymptotic levels by about 15 days. After the termination of treatment, LTP persisted for at least 4 weeks, the duration of our post-stimulation test period. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike components and in a late (probably disynaptic) component. The monosynaptic EPSP component was often depressed, but this may have been due to competing field currents generated by the enhanced population spike activity. We discuss these results in the context of theories emphasizing slower but more permanent memory storage in neocortex compared to the hippocampus.

Long-term changes in entorhinal-dentate evoked potentials induced by electroconvulsive shock seizures in rats

Entorhinal-dentate evoked potentials were measured in rats before and after: (1) eight electroconvulsive shock (ECS) seizures, or (2) matched handling. In animals that received ECS, evoked potentials were significantly enhanced, as evidenced by a long-lasting increase in the amplitude of the population spike. This increase in population-spike amplitude lasted for at least 3 months after the last ECS trial. No evoked-potential changes were observed in the subjects that received matched handling. These data suggest that ECS seizures produce long-lasting, perhaps permanent, changes in the brain.

A nerve growth factor peptide retards seizure development and inhibits neuronal sprouting in a rat model of epilepsy

Kindling, an animal model of epilepsy wherein seizures are induced by subcortical electrical stimulation, results in the upregulation of neurotrophin mRNA and protein in the adult rat forebrain and causes mossy fiber sprouting in the hippocampus. Intraventricular infusion of a synthetic peptide mimic of a nerve growth factor domain that interferes with the binding of neurotrophins to their receptors resulted in significant retardation of kindling and inhibition of mossy fiber sprouting. These findings suggest a critical role for neurotrophins in both kindling and kindling-induced synaptic reorganization.

Rhythm perception and differences in accent weights for musicians and nonmusicians

In order to investigate the contribution of harmonic-temporal and structural features to the perception of musical rhythm, three experiments were conducted in which a harmonic and a temporal accent were pitted against each other in such a way as to form five possible patterns. In three experiments, the temporal structure of various chord progressions was manipulated in an effort to determine the harmonic contributions to the inference of meter. The final experiment differed from the first two in the use of nondiatonic progressions that implied an unlikely key modulation. In all experiments, musicians and nonmusicians were requested to report perceived rhythm patterns in an attempt to determine the relative salience of various accents. Results indicated that changes in the temporal structure led to predictable change in an inferred meter, and that all diatonic chord progressions led to similar patterns of responses in which coincidences of harmonic, temporal, and metrical accents were perceptually salient events. Unusual progressions implying key modulations resulted in a qualitatively distinct pattern of results, and, in all experiments, amount of formal musical training was found to be a good predictor of the use of harmonic cues.

Intraventricular administration of antibodies to nerve growth factor retards kindling and blocks mossy fiber sprouting in adult rats

Repeated subconvulsive electrical stimulation of certain areas of the forebrain leads to kindling, a progressive and permanent amplification of evoked epileptiform activity, which is a model for human temporal lobe epilepsy. Recent studies have shown that kindling induces synthesis of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) but not neurotrophin-3 (NT-3) in the hippocampus and cortex. Kindling also elicits mossy fiber sprouting and functional synaptogenesis in the supragranular layer, the hilus, and the CA3 region of the hippocampus. Intraventricular administration of antibodies to NGF has been shown to effectively block septohippocampal sprouting in the adult rat, and has been reported to retard amygdaloid kindling. In the present study, we have investigated the possible role of NGF in both kindling and kindling-associated sprouting. We have confirmed a kindling-induced sprouting of the mossy fibers into the stratum oriens of the CA3 region of the hippocampus, utilizing a new semiquantitative method of analysis based on Timm staining. Previous studies found no overt signs of hippocampal damage with this kindling paradigm, indicating that the increased Timm staining likely reflects a purely activity-induced sprouting. Intraventricular infusion of affinity-purified anti-NGF IgGs (which cross-react with NT-3 but not BDNF) resulted in both significant retardation of kindling and inhibition of the kindling-induced mossy fiber sprouting. The findings suggest a role for NGF in both these phenomena.

Activation of astrocytes during epileptogenesis in the absence of neuronal degeneration

The issue of whether neuronal degeneration is a primary factor in activation of astrocytes during epileptogenesis was addressed using the kindling model of epilepsy. No degenerative changes specific to the kindling process were observed in brain sections from kindled animals, sampled from the olfactory bulbs through to cerebellum and processed with the degeneration-sensitive cupric silver stain. Also, examination of lectin-stained sections did not reveal any reactive microglia. At the same time, reactive astrocytes, as judged by an increase in glial fibrillary acidic protein immunoreactivity and a de novo vimentin immunoreactivity, were prominent in amygdala, piriform cortex, entorhinal cortex and hippocampus. These results suggest that loss of neurones is not a prerequisite for establishment of epilepsy-prone state, that seizures of short duration do not necessarily result in neuronal death, and that in kindling, astrocytes are activated by factors that are not related to neuronal degeneration, but which are likely associated with abnormal neuronal activity.

Post-activation potentiation and depression in the neocortex of the rat: II. Chronic preparations

Although long-term potentiation (LTP) has been demonstrated in a number of subcortical sites in chronic preparations, there have been no demonstrations of LTP in the neocortex of chronic preparations. Even neocortical slice and acute preparations often require a drug-induced suppression of inhibition before LTP effects can be reliably induced. We have attempted to induce LTP in neocortical sites in 7 different experiments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following manipulations were tested: (1) standard stimulation train parameters that have been shown to be highly effective in subcortical and hippocampal sites; (2) a 10-fold increase in the intra-train pulse durations; (3) variations in train pulse frequency (1 Hz to 300 Hz) and train duration (100 ms to 15 min); (4) co-activation of multiple inputs by stimulation of combinations of cortical sites or cortical and thalamic sites; (5) reduction of inhibition by administration of picrotoxin; 5) Housing of animals in an enriched environment; (6) utilization of the neocortical stimulation trains as a cue in a learning task; (7) application of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did facilitate the induction of a long-lasting depression effect. These findings contrast with the results obtained from anesthetized rats and from studies using brain slices, where LTP can be reliably induced. These results are discussed in light of other recent findings with respect to LTP and LTD effects.

Post-activation potentiation in the neocortex: I. Acute preparations

Long-term potentiation is widely studied as a memory model, and has been demonstrated in a number of subcortical sites in both acute and chronic preparations. In the neocortex, however, most of the demonstrations of LTP have been in neocortical slice or acute preparations, and even these have often required a drug-induced attenuation of inhibition before the LTP could be reliably expressed. In this paper we show that LTP can be reliably expressed in adult rats in a number of neocortical sites, both ipsilateral and contralateral to the site of callosal stimulation. We also show that, when recording field potentials, LTP is expressed roughly equally at all cortical depths. In a third experiment, we monitored input/output (I/O), paired-pulse inhibition and short-term potentiation effects over the course of LTP induction. The ipsilateral responses were, as expected, of shorter latency and larger amplitude than contralateral responses. They also showed small spike-like components that correlated with cell discharge. Nevertheless, the contralateral responses tended to show the largest LTP effects. The paired pulse effect was mainly depression, lasting for up to 3000 ms, at both ipsilateral and control sites. The short-term potentiation components were best fit by two summed exponentials with time constants of about 70 s and 12 min. The LTP effect lasted at least two h which was the longest period monitored in these experiments.

Transfer kindling between sites in the entorhinal cortex-perforant path-dentate gyrus system

Repeated electrical stimulation of forebrain sites can lead to an enhanced epileptogenic response (the kindling phenomenon). When stimulation is subsequently applied to related sites, these developments occur more rapidly (the transfer effect). Axonal pathways do not support epileptiform discharge, so it is generally assumed that the kindling develops, when these pathways are stimulated, in the target structures of those pathways. Consequently, transfer kindling should be immediate, or at least very rapid. Primary and 'transfer' kindling effects were examined in the excitatory, monosynaptically coupled, entorhinal cortex-dentate gyrus system. Transfer kindling was begun following either a 24 h delay or a 4 week delay between the last primary site convulsion and the start of transfer kindling in the secondary site. A 4 week delay between kindling sites was chosen to minimize the effects of a previously reported transient suppression of transfer kindling and the increased inhibition that has been shown to develop in the dentate gyrus as a result of kindling the perforant path. Although there was a significant transfer to the dentate gyrus following perforant path stimulation, the dentate gyrus still required a mean of 18.5 (24 h delay) and 20.3 (4 week delay) stimulations to reach criterion. In the entorhinal cortex, there was a significant positive transfer following primary kindling of the perforant path only in the group in which transfer kindling was begun after a 4 week delay. When the perforant path itself was the transfer site, there was a significant savings in number of afterdischarges needed to reach criterion following dentate gyrus kindling but no savings following entorhinal cortex kindling.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of kindling beyond the 'stage 5' criterion on paired-pulse depression and hilar cell counts in the dentate gyrus

Recent experiments have indicated that recurrent inhibition in the dentate gyrus, as measured with paired-pulse tests, is reduced following the induction of status epilepticus. Also, a loss of cells in the hilus has been reported, and it has been suggested that the two effect might be related. In this experiment, we have monitored paired-pulse depression and counted cells in the hilus in animals that have been kindled well beyond the typical stage 5 criterion. Responses evoked in the dentate gyrus by paired-pulse stimulation of the perforant path were monitored before and after kindling of the perforant path. One group of animals served as controls and received no kindling stimulations. Another group was kindled to 4 stage 5 seizures and then allowed to recover for 2 months. A third group was kindled to 44 stage 5 seizures and then allowed to recover for at least 5 weeks. Paired-pulse tests were taken at 1 week intervals during the kindling and recovery phases. Paired-pulse inhibition increased during kindling, peaked after 4 stage 5 seizures, remained enhanced throughout the additional 40 stage 5 seizures, and recovered towards baseline over a period of about 5 weeks. Upon completion of this phase of the experiment, cell counts were taken in the hilar regions of the Nissl stained brain sections. There was a significant reduction in number of cells in the tissue from kindled animals, compared to controls, but there was no significant difference between the 2 kindled groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Harmonic accents in inference of metrical structure and perception of rhythm patterns

Research on rhythmic structure is somewhat fragmented, due in part to differential use of terminology and a lack of research on the timing of harmonic accents. In this study, a harmonic and a temporal accent were pitted against each other in such a way as to form different rhythm patterns. In addition, two harmonic conditions that varied in the frequency of chord presentations (i.e., the composite rhythm) but not in the frequency of chord changes (i.e., the harmonic rhythm) were presented. Musicians and nonmusicians were requested to report perceived rhythm patterns in an attempt to determine the relative salience of the harmonic and temporal accents. In addition, a behavioral measure of the perceived meter was taken. Results indicated that the location of chord changes was the main determinant of subjects' rhythmic perceptions and the perceived onset of a measure. Furthermore, although subjects primarily inferred different meters based on the composite rhythm, an interaction of metrical and rhythmic choices was found, indicating that perception of rhythm patterns and inference of metrical structure may not always be independent.

Increased spontaneous unit discharge rates following electrical kindling in the rat

Changes in neuronal excitability that persist after seizures may play a key role in epilepsy. In this study, extracellular single unit recording techniques were used to investigate the changes in cell discharge patterns that result from kindling in vivo. The spontaneous activity of piriform and perirhinal cortical units in chronically implanted adult rats was recorded before, during and after kindling the amygdala. We observed that kindling-induced seizures resulted in a prolonged and dramatic increase in the rate of spontaneous background neuronal discharge in both the piriform and perirhinal regions.

Kindling causes changes in the composition of the astrocytic cytoskeleton

Changes in the astrocytic cytoskeleton were examined in amygdala kindled rats using immunocytochemical techniques. One week following kindling, there was a dramatic increase in immunoreactivity to glial fibrillary acidic protein and vimentin in astrocytes throughout amygdala, pyriform cortex and hippocampus bilaterally. Since these changes occurred in anatomical sites involved in the propagation of kindled seizures, the observed cytoskeletal reorganization in astrocytes may signify important functional alterations in the kindled brain.

Kindling-induced potentiation in the piriform cortex

At intensities sufficient to induce epileptiform afterdischarges, repeated electrical stimulation of limbic structures can lead to the development of permanent increases in the strength of the epileptiform response (kindling). Field potentials evoked by pulse stimulation are also increased in amplitude in a number of forebrain pathways following kindling. This kindling-induced potentiation effect is similar in many respects to the 'long-term potentiation' (LTP) effect which is produced by non-epileptogenic stimulation. There are, however, some interesting differences. For example, kindling-induced potentiation can far outlast LTP. In these experiments, we attempted to determine the longevity of the kindling-induced potentiation of the response evoked in the piriform cortex by olfactory bulb stimulation, following olfactory bulb kindling. This system was targeted because both the olfactory bulb and the piriform cortex are highly reactive kindling sites. In addition, we used the paired pulse technique to monitor facilitation and inhibition in this system. Kindling was found to induce a potentiation in the piriform field potential that lasted for at least 3 months (the period of the experiment) with little or no decay. Kindling also produced a decrease in paired pulse facilitation. In some animals the net facilitation was changed to a net depression. These results are consistent with the interpretation that kindling produces an increase in recurrent inhibition in the piriform cortex. The paired pulse measures, however, returned to near baseline levels over the 3-month test period.