Dentate EEG spikes and associated interneuronal population bursts in the hippocampal hilar region of the rat.

1. This paper describes two novel population patterns in the dentate gyrus of the awake rat, termed type 1 and type 2 dentate spikes (DS1, DS2). Their cellular generation and spatial distribution were examined by simultaneous recording of field potentials and unit activity using multiple-site silicon probes and wire electrode arrays. 2. Dentate spikes were large amplitude (2-4 mV), short duration (< 30 ms) field potentials that occurred sparsely during behavioral immobility and slow-wave sleep. Current-source density analysis revealed large sinks in the outer (DS1) and middle (DS2) thirds of the dentate molecular layer, respectively. DS1 and DS2 had similar longitudinal, lateral, and interhemispheric synchrony. 3. Dentate spikes invariably were coupled to synchronous population bursts of putative hilar interneurons. CA3 pyramidal cells, on the other hand were suppressed during dentate spikes. 4. After bilateral removal of the entorhinal cortex, dentate spikes disappeared, whereas sharp wave-associated bursts, reflecting synchronous discharge of the CA3-CA1 network, increased several fold. 5. These physiological characteristics of the dentate spikes suggest that they are triggered by a population burst of layer II stellate cells of the lateral (DS1) and medial (DS2) entorhinal cortex. 6. We suggest that dentate spike-associated synchronized bursts of hilar-region interneurons provide a suppressive effect on the excitability of the CA3-CA1 network in the intact brain.

Genetic threshold hypothesis of neocortical spike-and-wave discharges in the rat: an animal model of petit mal epilepsy.

Neocortical high-voltage spike-and-wave discharges (HVS) in the rat are an animal model of petit mal epilepsy. Genetic analysis of total duration of HVS (s/12 hr) in reciprocal F1 and F2 hybrids of F344 and BN rats indicated that the phenotypic variability of HVS cannot be explained by a simple, monogenic Mendelian model. Biometrical analysis suggested the presence of additive, dominance, and sex-linked-epistatic effects, buffering maternal influence, and heterosis. High correlation was observed between average duration (s/episode) and frequency of occurrence of spike-and-wave episodes (n/12 hr) in parental and segregating generations, indicating that common genes affect both duration and frequency of the spike-and-wave pattern. We propose that both genetic and developmental-environmental factors control an underlying quantitative variable, which, above a certain threshold level, precipitates HVS discharges. These findings, together with the recent availability of rat DNA markers for total genome mapping, pave the way to the identification of genes that control the susceptibility of the brain to spike-and-wave discharges.

Sharp wave-associated high-frequency oscillation (200 Hz) in the intact hippocampus: network and intracellular mechanisms.

Sharp wave bursts, induced by a cooperative discharge of CA3 pyramidal cells, are the most synchronous physiological pattern in the hippocampus. In conjunction with sharp wave bursts, CA1 pyramidal cells display a high-frequency (200 Hz) network oscillation (ripple). In the present study extracellular field and unit activity was recorded simultaneously from 16 closely spaces sites in the awake rat and the intracellular activity of CA1 pyramidal cells during the network oscillation was studied under anesthesia. Current source density analysis of the high-frequency oscillation revealed circumscribed sinks and sources in the vicinity of the pyramidal layer. Single pyramidal cells discharged at a low frequency but were phase locked to the negative peak of the locally derived field oscillation. Approximately 10% of the simultaneously recorded pyramidal cells fired during a given oscillatory event. Putative interneurons increased their discharge rates during the field ripples severalfold and often maintained a 200 Hz frequency during the oscillatory event. Under urethane and ketamine anesthesia the frequency of ripples was slower (100-120 Hz) than in the awake rat (180-200 Hz). Halothane anesthesia prevented the occurrence of high-frequency field oscillations in the CA1 region. Both the amplitude (1-4 mV) and phase of the intracellular ripple, but not its frequency, were voltage dependent. The amplitude of intracellular ripple was smallest between -70 and -80 mV. The phase of intracellular oscillation relative to the extracellular ripple reversed when the membrane was hyperpolarized more than -80 mV. A histologically verified CA1 basket cell increased its firing rate during the network oscillation and discharged at the frequency of the extracellular ripple. These findings indicate that the intracellularly recorded fast oscillatory rhythm is not solely dependent on membrane currents intrinsic to the CA1 pyramidal cells but it is a network driven phenomenon dependent upon the participation of inhibitory interneurons. We hypothesize that fast field oscillation (200 Hz) in the CA1 region reflects summed IPSPs in pyramidal cells as a result of high-frequency barrage of interneurons. The sharp wave associated synchronous discharge of pyramidal cells in the millisecond range can exert a powerful influence on retrohippocampal targets and may facilitate the transfer of transiently stored memory traces from the hippocampus to the entorhinal cortex.

Gamma (40-100 Hz) oscillation in the hippocampus of the behaving rat.

The cellular generation and spatial distribution of gamma frequency (40-100 Hz) activity was examined in the hippocampus of the awake rat. Field potentials and unit activity were recorded by multiple site silicon probes (5- and 16-site shanks) and wire electrode arrays. Gamma waves were highly coherent along the long axis of the dentate hilus, but average coherence decreased rapidly in the CA3 and CA1 directions. Analysis of short epochs revealed large fluctuations in coherence values between the dentate and CA1 gamma waves. Current source density analysis revealed large sinks and sources in the dentate gyrus with spatial distribution similar to the dipoles evoked by stimulation of the perforant path. The frequency changes of gamma and theta waves positively correlated (40-100 Hz and 5-10 Hz, respectively). Putative interneurons in the dentate gyrus discharged at gamma frequency and were phase-locked to the ascending part of the gamma waves recorded from the hilus. Following bilateral lesion of the entorhinal cortex the power and frequency of hilar gamma activity significantly decreased or disappeared. Instead, a large amplitude but slower gamma pattern (25-50 Hz) emerged in the CA3-CA1 network. We suggest that gamma oscillation emerges from an interaction between intrinsic oscillatory properties of interneurons and the network properties of the dentate gyrus. We also hypothesize that under physiological conditions the hilar gamma oscillation may be entrained by the entorhinal rhythm and that gamma oscillation in the CA3-CA1 circuitry is suppressed by either the hilar region or the entorhinal cortex.

Spike-and-wave epilepsy in rats: sex differences and inheritance of physiological traits.

Spontaneously occurring spike-and-wave patterns were examined in seven to eight-month-old rats of the inbred Fischer 344 and Brown Norway strains and their F1 and F2 hybrids. Neocortical activity and movement were monitored for 12 night h. Spike-and-wave episodes were identified by a three-layer back-propagation neural network. The incidence, average duration and total duration of spike-and-wave episodes were significantly higher in F1 males and F2 hybrids than in the parental strains. Male rats of the Brown Norway strain had significantly more and longer episodes than females, whereas no sex differences were present in Fischer rats. The average intraepisodic frequency of spike-and-wave patterns was significantly lower in Fischer rats than in the other groups and significantly higher in males than females. Tremor (myoclonic movements) associated with spike-and-wave episodes was absent or of very small amplitude in Fischer rats but frequent and of large amplitude in Brown Norway rats and their F1 and F2 descendants. Most of the interstrain differences were limited to male rats. Spike-and-wave episodes recurred at predictable short-term (10-30 s) and long-term (15-30 min) periods. The long-term oscillation corresponded to a similar fluctuation of motor activity. The maximum probability of spike-and-wave patterns occurred at a relatively narrow range of delta power (0-3.1 Hz) of the background EEG activity. Systemic administration of the adrenergic alpha-2 agonist, clonidine, increased the incidence of spike-and-wave episodes several-fold. The total duration of spike-and-wave episodes in the clonidine sessions (15 min) and night sessions (12 h test) correlated significantly. We suggest that several genes interact with maturational, environmental and endocrine factors, resulting in sex differences, and produce the variety of EEG and behavioral findings encountered. In addition, we submit that the clonidine test may be useful in genetic investigations of human absence epilepsies. The findings of this work demonstrate that genetic manipulation of rodents is a promising method for producing analogous models for the various forms of human absence epilepsies.

Pattern recognition of the electroencephalogram by artificial neural networks.

A back-propagation network was trained to recognize high voltage spike-and-wave spindle (HVS) patterns in the rat, a rodent model of human petit mal epilepsy. The spontaneously occurring HVSs were examined in 137 rats of the Fisher 344 and Brown Norway strains and their F1, F2 and backcross hybrids. Neocortical EEG and movement of the rat were recorded for 12 night hours in each animal and analog data were filtered (low cut: 1 Hz; high cut: 50 Hz) and sampled at 100 Hz with 12 bit precision. A training data set was generated by manually marking durations of HVS epochs in 16 representative animals selected from each group. Training data were presented to back-propagation networks with variable numbers of input, hidden and output cells. The performance of different types of networks was first examined with the training samples and then the best configuration was tested on novel sets of the EEG data. FFT transformation of EEG significantly improved the pattern recognition ability of the network. With the most effective configuration (16 input; 19 hidden; 1 output cells) the summed squared error dropped by 80% as compared with that of the initial random weights. When testing the network with new patterns the manual and automatic evaluations were compared quantitatively. HVSs which were detected properly by the network reached 93-99% of the manually marked HVS patterns, while falsely detected events (non-HVS, artifacts) varied between 18% and 40%. These findings demonstrate the utility of back-propagation networks in automatic recognition of EEG patterns.

Computer analysis of single neuron activity during conditioned feeding task.

A computer controlled complex electrophysiological set-up employing the multibarrel micro-electrophoretic technique is reported in this paper. The laboratory equipped for this technique is used for recording single neuron activity from various sites of the central nervous system of rhesus monkeys during: 1) performing conditioned behavioral tasks, 2) intracerebral microelectrophoretic administration of chemicals, and 3) oral application of gustatory stimuli.

Feeding disturbances and EEG activity changes after amygdaloid kainate lesions in the rat.

Kainic acid (KA), in various concentrations, was applied iontophoretically into the central nucleus of the amygdala. Microlesions with this cell specific neurotoxin caused body weight loss, hypo- or aphagia and hypo- or adipsia in a dose-dependent manner. EEG-examinations proved that even low doses of KA produced seizure activity; however, these epileptiform symptoms disappeared within the first 48 h after the operations. Thus, the lasting feeding disturbances produced by iontophoretic KA applications to the central nucleus of the amygdala (i.e., even these fine microlesions) were not related causally to the pathological EEG activity changes. Our findings, along with previous data, indicated that the body weight loss and feeding deficits were due to the KA-induced impairment of complex regulatory mechanisms.

Microelectrophoretic application of kainic acid into the globus pallidus: disturbances in feeding behavior.

Body weight changes, food and water intake, and sensorimotor disturbances of male rats were studied after bilateral kainic acid-(KA) induced lesions of the globus pallidus (GP). To minimize the extent of damages, KA was applied electrophoretically by means of glass micropipettes (tip diameter of the pipettes was 10-15 microns). The neuron-specific damages of the GP resulted in aphagia and adipsia and rapid body weight decrease. Lesioned animals showed permanent motor disturbances but only temporary difficulties in the orientation toward sensory stimuli. Our data show that the selective destruction of the GP neurons results in a complex disorder that has motivational, (sensori)motor, and metabolic components.

Feeding and body weight regulation after 6-OHDA application into the preoptic area.

Our previous results showed that neurochemical destruction of the amygdaloid terminal field of the mesolimbic dopaminergic system caused disturbances in body weight regulation and feeding. In the present experiments, it was studied whether 6-hydroxydopamine (6-OHDA)-induced bilateral lesions of the mesolimbic dopaminergic system in the lateral preoptic area produce similar symptoms in rats. To enhance the selectivity of the neurotoxin, 6-OHDA was also used after desmethylimipramine (DMI) premedication. Both 6-OHDA and 6-OHDA + DMI treatments resulted in hypophagia, hypodipsia and body weight decrease. A significant increase of water intake was found in sham-operated controls and lesioned animals, in response to extracellular dehydration caused by polyethylene glycol. Intracellular dehydration induced by hypertonic saline resulted in increase of water intake of all animals; however, 6-OHDA- and 6-OHDA + DMI-treated rats drank less than the controls. Similar observation has been made when food intakes were compared after 2-deoxy-D-glucose treatment. Results show that mesolimbic dopaminergic elements play an essential role in the regulation of feeding.

Sex-dependent body weight changes after iontophoretic application of kainic acid into the LH or VMH.

Body weight changes and food and water intakes were studied in CFY male and female rats after kainic acid (KA)-induced destruction of the lateral hypothalamic area (LH) or the ventromedial hypothalamic nucleus (VMH). To minimize the extent of damages, KA was iontophoretically applied by means of glass micropipettes. KA was ejected in 50 or 80 mM concentrations with 5-15 microA current for 5 min. Tip diameter of pipettes varied between 10-20 microns. Lesions were restricted to the LH or VMH. Effects were sex-dependent. LH lesions resulted in hypophagia, hypodipsia and body weight loss only in male rats. On the other hand, only female animals exhibited hyperphagia and weight increase when the VMH was destroyed. The role of sex-dependence in hypothalamic body weight regulation is discussed.

Lateral hypothalamic feeding mechanisms: iontophoretic effects of kainic acid, ibotenic acid and 6-hydroxydopamine.

In order to study hunger motivated behavior kainic acid (KA), ibotenic acid (IB) and 6-hydroxydopamine (6-OHDA) were iontophoretically applied to the lateral hypothalamus of rats. Neurotoxins at concentrations between 20-120 mMol were applied with 5-20 microA current for 5-10 min. Tip diameter of glass micropipettes varied between 10-50 micron. Application of KA, IB and 6-OHDA caused temporary body weight loss, hypophagia and hypodipsia. Effects were dose dependent: correlation was found between current strength, tip diameter of pipettes, concentrations of neurotoxins and the extent of cellular damage. Aphagic and adipsic symptoms and the death of animals were only observed after extensive LH lesions. There was no significant difference in food consumption after 24 hr deprivation among groups. Water deprivation and extracellular or intracellular dehydration resulted in a considerable increase in water intake in all animals, however, consumption in lesioned rats was lower than that of controls. These water regulatory disturbances in 6-OHDA treated animals were somewhat less severe. Results show that the bilateral cellular microlesions of the LH and damage of dopaminergic (DA) elements with 6-OHDA both cause disturbances of feeding behavior. Although the severity of symptoms in some respects depends upon the nature of neurotoxic treatment, the basic consequences are essentially similar. It is suggested that the LH "feeding center" and the ascending DA pathways represent a single system involved in the organization of hunger motivated behavior.