Hippocampal EEG Research Articles

Current source density analysis of the hippocampal theta rhythm: associated sustained potentials and candidate synaptic generators

Single-electrode depth profiles of the hippocampal EEG were made in urethane-anesthetized rats and rats trained in an alternating running/drinking task. Current source density (CSD) was computed from the voltage as a function of depth. A problem inherent to AC-coupled profiles was eliminated by incorporating sustained potential components of the EEG. ‘AC’ profiles force phasic current sinks to alternate with current sources at each lamina, changing the magnitude and even the sign of the computed membrane current. It was possible to include DC potentials in the profiles from anesthetized rats by using glass micropipettes for recording. A method of ‘subtracting’ profiles of the non-theta EEG from theta profiles was developed as an approach to including sustained potentials in recordings from freely-moving animals implanted with platinum electrodes. ‘DC’ profiles are superior to ‘AC’ profiles for analysis of EEG activity because ‘DC’-CSD values can be considered correct in sign and more closely represent the actual membrane current magnitudes. Since hippocampal inputs are laminated, CSD analysis leads to straightforward predictions of the afferents involved. Theta-related activity in afferents from entorhinal neurons, hippocampal interneurons and ipsi- and contralateral hippocampal pyramids all appear to contribute to sources and sinks in CA1 and the dentate area. The largest theta-related generator was a sink at the fissure, having both phasic and tonic components. This sink may reflect activity in afferents from the lateral entorhinal cortex. The phase of the dentate mid-molecular sink suggests that medial entorhinal afferents drive the theta-related granule and pyramidal cell firing. The sustained components may be simply due to different average rates of firing during theta rhythm than during non-theta EEG in afferents whose firing rates are also phasically modulated.

Intracarotid amobarbital injection produces hippocampal EEG changes in patients with temporal lobe epilepsy

EEGs of hippocampi of both sides were analyzed during intracarotid amobarbital tests in 12 patients with temporal lobe epilepsy. The number of paroxysms occuring 1 min before the injection ipsilaterally and contralaterally was compared with that occuring 1 min after the injection. The hippocampal EEG ipsilateral to the injection showed an increase in paroxysms irrespective of the laterality of the epileptogenic side. Contralateral to the amobarbital injection an increase in paroxysms was observed in the epileptogenic hippocampus only.

A cortical EEG frequency with a REM-specific increase in amplitude

1. We used period/amplitude (PA) analysis to survey electroencephalography (EEG) across vigilance states in the Sprague-Dawley rat. We found a fast cortical EEG frequency band, which we designate rho, whose wave amplitude in rapid eye movement (REM) was elevated above the levels in both waking and nonrapid eye movement (NREM). Rho wave amplitudes in NREM and waking were similar. 2. Rho occurred between 20 and 30 Hz, which appears to be a transitional range. In the 5-Hz band below rho, EEG wave amplitudes are higher in both NREM and REM than in waking; above 30 Hz, amplitudes are higher in waking and REM than in NREM. The augmented amplitude of rho in REM may result from a unique point of interaction of EEG mechanisms of sleep and waking. 3. In addition to its theoretical interest, rho is of practical value for scoring REM sleep. It could obviate the need to record hippocampal theta EEG, eliminating electrode penetration of the brain and reducing the number of leads required to classify rat vigilance states.

Abnormal Excitability of Hippocampal CA3 Pyramidal Neurons of Spontaneously Epileptic Rats (SER), a Double Mutant

The spontaneously epileptic rat (SER: zi/zi, tm/tm), a double mutant, shows both tonic convulsions and absence-like seizures characterized by low-voltage fast waves and by 5-7 Hz spike and wave-like complexes in the cerebral cortical and hippocampal EEG, respectively. Characteristics of hippocampal CA3 pyramidal neurons were examined to determine whether these neurons are abnormally excitable. When a single stimulus was given to the mossy fiber, there was repetitive firing and a depolarization shift in neurons of mature SER (over 12 weeks old), in which epileptic seizures had fully developed. However, in young SER (7-8 weeks old) and littermates ( zi/zi, tm/+), which did not show any seizures, only a single spike was elicited with each single stimulation of the mossy fiber. Intracellular recording showed that the resting membrane potential was not significantly different among young and mature SER and littermates, but a long-lasting (100-200 ms) depolarizing shift accompanied by repetitive firing was observed following a single stimulation of the mossy fiber in half of the CA3 neurons of mature SER. Furthermore, the input impedance of the CA3 neurons in mature SER was lower than that in young SER and in littermates. These results indicate that SER hippocampal CA3 neurons become abnormally excitable in conjunction with the development of epileptic seizures.

Effect of deltorphin on behavior and hippocampal electrical activity in rabbits

To clarify the role of δ-opioid receptors on modulation of hippocampal electrical activity and behavior, deltorphin (DT), a naturally occurring heptapeptide that selectively binds to δ-opioid receptors, was intravenously (IV) administered to rabbits. For this purpose, at 8-day intervals, the effects of IV administration of normal saline and IV infusion of synthetic DT (1 mg/kg b.wt. for 2 min) on the spontaneous behavior in neutral environment, both in absence of any external stimulus and after the introduction of a stuffed predator, were examined in seven adult male rabbits, on separate and successive occasions. During each session of experimental procedure, hippocampal EEG was also recorded by telemetry. Behavioral activity showed an increase in alert and reactive immobility after the peptide injection in comparison with that observed during control period (saline administration). Under DT treatment the frequency of hippocampal electrical activity decreased, and a reduction in rhythmicity of electrical pattern was also observed in presence of stressful stimulus. These findings show that DT may affect neural and behavioral elements related to the control of attentional and emotional processes, suggesting a modulating role of δ-opioid receptors.

Nonlinear analysis of the hippocampal subfields of CA1 and the dentate gyrus.

The paper discusses the use of nonlinear bispectral analysis in examining the hippocampal EEG collected at subfields of CA1 and the dentate gyrus during the vigilance state of REM sleep. The cross-bispectrum and its unique capabilities of detecting and quantifying quadratic nonlinear interactions occurring between these two hippocampal subfields are explained and demonstrated with simulation examples and EEG data. It was found in this study that quadratic nonlinear interactions exist between CA1 and the dentate gyrus in the 6-8 frequency band which dominates the theta (theta) rhythm observed in the hippocampal EEG during REM sleep. As a result, energy components around the frequency band of the second-order harmonics of theta rhythm are not totally spontaneous, but generated largely due to quadratic nonlinear interactions.

Effect of dermorphin on behavior and hippocampal electrical activity in rabbits

Dermorphin, a naturally occurring heptapeptide that selectively binds to μ-opioid receptors, was injected intravenously 0.4 mg/kg in male rabbits. Eight days before injection the spontaneous behavior of the animals was observed in a neutral environment in the absence of external stimuli. At the same time, hippocampal EEG was recorded by telemetry. After dermorphin injection, the same experimental procedure was repeated. Behavior showed a strong increase in quiet immobility and a concomitant reduction in voluntary activities as compared to control periods. Treatment did not affect either the pattern or the frequency of hippocampal electrical activity.

SDZ ENS 163 a novel pilocarpine like drug: Pharmacological in vitro and in vivo profile

The thiolactone analogue of pilocarpine, SDZ ENS 163, acts in vitro and in vivo as a partial agonist at M 1 M 3 and as an antagonist at M 2 muscarinic receptors. In vitro, the properties of SDZ ENZ 163 have been investigated in several functional models for muscarinic receptors: it is full agonist at M 1 (rat, superior cervical ganglion, carbachol = 100%) and a partial agonist at M 3 receptors (guinea pig ileum). However, the drug shows antagonistics properties at M 2 receptors (rat atria). Radiologand binding studies with 3H-N-methylscopolamine ( 3H-NMS) using CHO cells expressing m 1 or m 3 receptors indicate that SDZ ENS 163 does not discriminate between m 1 and m 3 receptors (K i 1.5 and 2.4 μM respectively). Regarding phosphoinositide (PI) turnover in A9L cells, SDZ ENS 163 is a partial agonist at m 1 receptors. In ex vivo neurochemical studies in rats SDZ ENS 163 displays effects characteristic of muscarinic antagonists regarding the turnover of ACh which is increased in the brain. At a similar dose-range SDZ ENS 163 accelerates PI metabolism in the rat brain in vivo and increases the energy of the low frequency band (2–5 Hz) in the rat hippocampal EEG. These effects observed in vivo are consistent with postsynaptic M 1 agonistic and presynaptic M 2 antagonistic activities. Since SDZ ENS 163 at centrally active doses exerts no peripheral cholinergic effects, it may be useful for the symptomatic treatment of Alzheimer's disease.

Alpha 2 modulation of type 1 and type 2 hippocampal theta in the freely moving rat

In order to determine the brain sites involved in the noradrenergic modulation of hippocampal theta, the alpha2 agonist detomidine was infused into one of three brain sites in the freely moving animal: hippocampus, median raphe nucleus (MRN), or locus coeruleus (LC). Bilateral hippocampal recording electrodes were implanted in 45 animals. Infusions of detomidine into the hippocampus produced an attenuation of type 1 (movement) theta. Detomidine infused into the MRN produced a release of type 2 (immobility) theta, although having no effect on type 1 theta. No significant changes were noted in the hippocampal EEG activity following infusions of detomidine into the LC. Infusions of Xylocaine into each of the above brain sites were ineffective in eliciting changes in hippocampal EEG activity. No behavioral effects were noted following infusions of detomidine or Xylocaine. The present results suggest that the attenuation of type 1 theta occurs at the hippocampus, while the release of type 2 theta occurs at the MRN.

Intracerebroventricular application of competitive and non-competitive NMDA antagonists induce similar effects upon rat hippocampal electroencephalogram and local cerebral glucose utilization

In this study we have used electrophysiological and metabolic markers to investigate the effects of competitive and non-competitive NMDA antagonists in rats after central or peripheral administration. The non-competitive antagonist, MK-801, induced dose-dependent suppression of rat hippocampal EEG energy both after intraperitoneal (i.p.) and intracerebroventricular (i.c.v.) application. Similar effects were observed after i.p. and i.c.v. application of the competitive antagonist, dl-CPP-ene. Whereas the MK-801 was more potent after i.p. application, dl-CPP-ene was more potent after i.c.v. administration. Intracerebroventricular administration of MK-801 and dl-CPP-ene resulted in similar changes in the pattern of local cerebral glucose utilization in the olfactory tubercle and regions of the limbic system such as the anteroventral thalamus, hippocampus and entorhinal cortex. Intravenous (i.v.) administration of MK-801 induced increases in glucose metabolism similar to those observed after i.c.v. application. In contrast, i.v. administration of dl-CPP-ene induced only small decreases of glucose utilization in several regions of the central sensory system. Thus the blockade of glutametergic (NMDA) transmission results in decreased hippocampal EEG activity which is paralleled by increased metabolic activity in this area. We conclude from EEG recordings and [ 14C]2-deoxyglucose uptake experiments that both non-competitive and competitive NMDA antagonists produce the same pattern of alterations after i.c.v. administration. Apparent differences in efficacy after peripheral administration may be largely due to differences in bioavaliability.

The combined EEG-intracerebral microdialysis technique: a new tool for neuropharmacological studies on freely behaving animals

In this study we combined EEG and intracerebral microdialysis techniques in freely behaving rats. Various drugs were delivered into the hippocampus and cerebral cortex by means of microdialysis and, simultaneously, the EEG activity of the dialyzed area was monitored. The microdialysis procedure itself, when artificial cerebrospinal fluid was perfused, did not change the normal hippocampal or cortical EEG pattern. Drug inclusions into the microdialysis fluid, however, caused marked changes in the electrical activity of the dialyzed sites. In this report we present the following examples: (1) the dose-dependent spike-provoking effect of NMDA in hippocampus, (2) the potentiation of this NMDA effect in hippocampus by dibutyryl cyclic AMP, and (3) the EEG depressant effect of high concentration of K + in the cerebral cortex. The artificial cerebrospinal fluid and drug solutions were alternated in the microdialysis system with a 2-way valve placed outside the test chamber. As a consequence, the drugs were delivered into the brain without interrupting the ongoing behavior, including sleep, of the examined animals. This study shows that the combined EEG-intracerebral microdialysis technique is a useful tool, with many unique advantages, for in vivo neuropharmacological studies.

SDZ ENS 163 is a selective M1 agonist and induces release of acetylcholine.

In the present study some pharmacological properties of the new muscarinic agonist SDZ ENS 163; (+)-(3S,cis)-3-ethyldihydro-4-[(1-methyl-1H-imidazol-5-yl) methyl-2(3H)-thiophenonedihydrogenphosphate] have been investigated. In the rat superior cervical ganglion, a model for M1 muscarinic receptors, SDZ ENS 163 induced concentration-dependent depolarizations (pD2 = 6.5 +/- 0.3; efficacy = 128 +/- 4.2% compared to carbachol). SDZ ENS 163 was a very weak partial agonist with respect to M2 receptor-induced decrease in contractile force in rat left atria (efficacy = 14 +/- 2.9%). In addition, SDZ ENS 163 competitively antagonized the effect of carbachol in rat left atria (pA2 = 5.8 +/- 0.2). In the guinea-pig ileum SDZ ENS 163 was a partial agonist with respect to force of contraction mediated by M3 receptors (pD2 = 5.3 +/- 0.1; efficacy = 72 +/- 4.2%). The oxotremorine-induced inhibition of the electrically stimulated release of acetylcholine (ACh) in rat hippocampal slices was reversed by SDZ ENS 163 (pA2 = 5.5 +/- 0.1). In addition after oral administration SDZ ENS 163 (3-10 mumol/kg) reduced brain ACh levels, which is indicative of increased ACh turnover. Finally, increases in energy of the low frequency band (2-5 Hz) were observed in rat hippocampal EEG after intraperitoneal administration of SDZ ENS 163 (0.3-30 mumol/kg). We conclude that SDZ ENS 163 is a selective M1 agonist in vitro with an additional M2 antagonistic effect. The in vivo effects of SDZ ENS 163 may result both from postsynaptic M1 agonistic as well as M2 receptor antagonistic activity. The unique pharmacological profile of SDZ ENS 163 may prove clinically favourable for treatment of cognitive deficits.

Accumbens-caudate-septal circuit as a system for hippocampal regulation: involvement of a GABAergic neurotransmission

Hippocampal-based epileptiform activity may reach the basal ganglia via the nucleus accumbens. Previous data suggested that caudate nucleus is able to influence hippocampal epilepsy, probably sending a projection to the septum. In order to test the hypothesis of a retrograde activation of accumbens-caudate pathway in hippocampal regulation, we electrically stimulated both caudate nucleus and nucleus accumbens and studied modifications of hippocampal EEG in the feline focal epilepsy model. We also performed bilateral electrolytic lesion of nucleus accumbens and repeated caudate stimulation. Results showed that nucleus accumbens stimulation was ineffective in modifying hippocampal epilepsy; on the contrary, caudate stimulation caused a statistically significant decrease of hippocampal spike frequency and amplitude. On the other hand, in accumbens-lesioned animals caudate activation consistently reduced hippocampal epilepsy to a significant degree. As the caudate nucleus influences hippocampal activity and the septum may constitute a relay station of this functional relation, a possibility was tested concerning a GABAergic mediation. To this end, after a stable caudate-induced effect was reached, an intraseptal microinjection of picrotoxin (GABA receptor antagonist) was made and caudate stimulation repeated at the same parameters. Such a study showed that after intraseptal picrotoxin, caudate stimulation failed to elicit any type of modification of hippocampal activity. Experimental findings support the notion that the striatal modulation on hippocampus is mediated by an anterograde rather than a retrograde pathway, and underline the possibility of a GABAergic caudate-septal influence.

Pharmacological Application of Spontaneously Epileptic Rat(SER).

The spontaneously epileptic rat (SER) is a double mutant (zi/zi, tm/tm) obtained by mating zitter homozygote (zi/zi) with tremor heterozygote (tm/ + ) . This animal exhibits both tonic convulsion and absence-like seizures characterized by a sudden appearance of 5-7 Hz spikes and wave complex in cortical and hippocampal EEG. This study was performed to elucidate the usefulness of SER in evaluating of acute and long-term effects of antiepileptic drugs. Moreover, activities of the central inhibitory system and abnormal excitability of neurons related to the epileptic seizures were investigated. (1) From recordings of implanted electrode in cerebral cortex and hippocampus, phenytoin and carbamazepine (agents for tonic-clonic grand mal seizure) inhibited tonic convulsion without affecting absence-like seizures, whereas trimethadione and ethosuximide (agents effective for absence seizure, petit mal epilepsy) suppressed absence-like seizures without any effects on tonic convulsion. Both seizures were inhibited by phenobarbital, valproate and diazepam, agents which are effective for grand and petit mal epilepsy. This antiepileptic profile suggests that tonic and absence-like seizures in SER correspond to human grand and petit mal seizures. Therefore, SER is a useful model for evaluating the acute effects of novel antiepileptics. (2) The group of SER, which were orally administered with 1 % phenobarbital-containing pellet displayed a marked increase in body weight and longer a life span than the control group with a plasma concentration of 35-50 μg/ml. Similar to the futile results of chronic administration of phenobarbital in human absence epilepsy, absence-like seizures in SER were not affected by this drug, although tonic convulsion was inhibited. Long-term effects of valproate (1 and 5 %) were similar to those of phenobarbital except for its efficacy on absence-like seizures. Therefore, SER is also useful for the evaluation of long-term effects of antiepileptics. (3) Both tonic and absence-like seizures in SER were inhibited by TRH and its analog, CNK-602A, and dopamine agonists, methamphetamine and apomorphine. These results suggest that dopaminergic pathways act as an inhibitory system on the seizure-producing mechanism. Further, SER can be used for investigations of the central inhibitory system involved in the regulation of seizures. (4) In hippocampal slice preparations, a single stimulus given to the mossy fiber induced a longlasting depolarization shift with repetitive firings in CA3 field pyramidal neurons of the SER but not zitter or wistar rats. This abnormal excitation of the neurons were completely blocked by verapamil, a calcium channel blocker. These findings suggest that abnormal calcium channel activities in the CA3 pyramidal neurons are involved in the seizures of SER. Thus, SER is a useful animal model for investigating abnormalities related to epileptic seizures.

Genetic Study on the Spontaneously Epileptic Rat(SER).

Spontaneously epileptic rats (SER, the double mutants homozygous for zitter and tremor genes), which originated from cross mating between tremor rat strain and zitter rat strain, spontaneously exhibit both absence-like and tonic seizures. In addition, wild jumping or running episodes sometimes occur after the tonic seizures. SER can, therefore, be used as an animal model for human epilepsy. We are going to develop congenic zitter strain (WTC-zi/zi) and new SER (WTC-tm/tm, zi/zi), since genetically defined control strains for SER and ZI (an inbred strain of zitter rats) are not available at present. Then, WTC strain, which derived from TRM (WTC-tm/tm, an inbred strain of tremor rats) will be able to be used as a control strain for the three mutant strains. Development of the absence-like seizure (appearance of 5-7 Hz spike and wave complexes in cortical and hippocampal EEG) and the mode of inheritance were examined in tremor rats and the backcross progeny. The results suggested that the absence-like seizure is semidominantly expressed. Zitter gene has been mapped on the rat chromosome 3 by linkage analysis using biochemical markers, RFLPs and microsatellites. Precise mapping of the epileptic seizurerelated genes should be the first step for gene cloning.

Hippocampal mobility-related theta activity is not diminished by vigabatrin, a GABAmimetic antiepileptic drug, in normal rats

Lesions causing loss of hippocampal theta activity have been shown to result in spatial memory deficits in rats. On the other hand, hippocampal theta activity is thought to be associated only with motor activity, and its role in learning/memory is not clear. Vigabatrin, an inhibitor of GABA-transaminase, causes elevation of brain GABA levels. Previously, we have found that subchronic administration of vigabatrin did not impair spatial learning/memory in a water maze task. This experiment was carried out to further examine the hippocampal effects of vigabatrin by studying whether vigabatrin at antiepileptic doses affects mobility-related hippocampal EEG. Administration of vigabatrin (100 mg/kg or 500 mg/kg, IP) to nonepileptic rats caused no significant changes in mobility-related rhythmic theta activity, and the relative spectral power of theta frequency had a slight increasing tendency. These results suggest that the vigabatrin-induced enhanced GABAergic inhibition does not disturb normal mobility-related hippocampal theta activity.

Analysis of rat EEG using autoregressive power spectra

The main objective of this paper is to make the auto regressive (AR) power spectrum estimation method accessible to electrophysiologists and present some typical applications. The AR method is explained, choices of various parameters are explored and examples from the analysis of rat hippocampal EEG are given. We also provide the pseudocode for the computation of the AR coefficients and the AR spectrum. We compare the results from the AR method to the FFT-based power spectrum method and demonstrate the superiority of the AR method. We also show the differences in the spectra of the EEG of alcohol-preferring (P) and non-preferring (NP) rats in the baseline condition when no alcohol is infused. We found a statistically significant difference in peak theta frequency which was at 6.96 Hz for the P rats and at 7.74 Hz for the NP rats. There were also other observable differences in the shape of the spectra of the EEG of the P and NP rats.

Hippocampal and cortical EEG activity in rats with transected hypothalamus

The brain was transected in eight rats: the transection passed through the posterior pole of the superior colliculi and ended down at midhypothalamic level. The EEG activity in the dorsal hippocampus and cortex showed continuously slow, high amplitude waves. Thus the posterior hypothalamus is critical for the previously described hippocampal theta rhythm found in rats transected at the posthypothalamic level.

Supramammillary cell firing and hippocampal rhythmical slow activity

Extracellular multi-unit activity was recorded from sites in the supramammillary area of urethane anaesthetized rats, while simultaneously recording the hippocampal EEG. The pattern of supramammillary area cell discharge was found to be rhythmic and in phase with concurrent hippocampal rhythmical slow activity (RSA). Septal application of procaine (20%, 1-5 microliters), that abolished hippocampal RSA, did not abolish the rhythmicity of supramammillary cell firing. These data suggest that the frequency of hippocampal RSA may be determined in the supramammillary area rather than in the medial septum as has previously been supposed.

Hippocampal grafts into the intact brain induce epileptic patterns

Spontaneous hippocampal EEG activity and evoked field potentials were investigated in intact rats and in animals with fetal hippocampal grafts. Pieces of hippocampal grafts, derived from 15- to 16-day-old fetuses, were used to prepare cell suspensions and grafted directly into the intact hippocampus. Control animals received suspension grafts of the cerebellum derived from fetuses of identical age. Host hippocampal electrical patterns were monitored with chronic single electrodes or with a 16-microelectrode probe from 7 to 10 months after grafting. In contrast to previously reported high survival rates of fetal grafts in studies with damage to the host brain prior to grafting, survival of both hippocampal (60%) and cerebellar grafts (20%) was very poor in the intact hippocampus. In animals with cerebellar transplants or without surviving grafted neurons the electrical activity of the host hippocampus was indistinguishable from normal controls. In rats with hippocampal grafts short duration, large amplitude EEG spikes (up to 10 mV) were recorded, predominantly during immobility. When the EEG spikes (putative interictal spikes) were of large amplitude and contained population spikes, test evoked responses delivered to the perforant path were suppressed after the spontaneous events. In contrast, evoked responses were facilitated by interictal spikes without population spikes. The threshold of electrically induced afterdischarges did not differ significantly between groups of intact rats and animals with or without hippocampal grafts. However, in three rats with hippocampal grafts the evoked afterdischarges were associated with behavioral seizures. In two of these rats spontaneously occurring seizures were also observed. Synaptophysin-immunoreactivity demonstrated growth of the host mossy fibers into the graft.(ABSTRACT TRUNCATED AT 250 WORDS)