Frequency modulation of theta volleys of septal neurons during rhythmic oligosynaptic stimulation in the rabbit

Activity of the neurones with stable theta-bursts was recorded extracellularly in intact and hippocampectomized septum of unanaesthetized chronic rabbits during low-frequency (3-17 Hz) stimulation of horizontal limb of the diagonal band or the lateral septal nucleus. Gradual entrainment and phase-locking of the spontaneous theta-cycles occurred. Two types of entrainment were observed: "entrainment by pause", where interburst interval was reset by the stimuli; and "entrainment by burst", where bursts were time-locked to the stimuli. Such reorganization of the spontaneous bursts occurred in a narrow frequency range of stimulation (from 4 Hz up to 9-12 Hz), with the best resonance following in the range of "basic" theta frequencies of the awake rabbit (5-6 Hz). With stimulation beyond the theta-range three phenomena occurred: shift of the burst frequencies to higher or lower harmonics of stimulation frequencies; complex interactions of basic background frequency with the rhythm of stimulation ("beating"); escape from the influence of the stimuli with return to background theta-burst frequency.     

The theta modulation of rabbit hippocampal neurons and its correlation with other indices of spontaneous and evoked activity]

Reliability of the existing functional criteria for differentiation of pyramidal ("complex spike neurones") and inhibitory ("theta neurones") cells in the hippocampus of waking rabbit is evaluated on the basis of statistical analysis of neuronal spontaneous and evoked activity. The analysis shows, that the criteria of mean frequency, presence of theta modulation, neuronal behaviour in situations provoking EEG theta rhythm (e.g., excitation or inhibition during presentation of sensory stimuli), effects of medial septum and intrahippocampal stimulation do not permit reliable identification of the hippocampal neuronal types in the waking rabbit. The data on functional classification of the hippocampal neurones are discussed in connection with existing suggestions about their state in situations inducing theta rhythm generation.     

Glutamate-induced suppression of inhibitory synaptic transmission in cultivated hippocampal neurons

In a dissociated culture of rat hippocampal neurons (14 to 24 daysin vitro), modulation effects of glutamate on GABA_A-ergic inhibitory transmission were studied with the use of simultaneous patch-clamp whole-cell recording from monosynaptically connected neuron pairs. In all experiments (n=49), 1.5-min-long or longer extracellular application of 0.5 to 100 μM glutamate suppressed evoked inhibitory postsynaptic currents (IPSC). This suppression usually included fast (seconds) and slow (τ=1.3 min) phases. In 83.7% of the cases studied, IPSC did not return to the control values during the entire subsequent recording period (from 10 to 64 min). When glutamate was applied in the presence of blockers of glutamate ionotropic receptors, DL-APV or CNQX, the fast phase of the effect was removed, while some suppression of inhibitory neuronal responses, although weaker, was preserved (n=19); in most cases (73.3%) this residual suppression was slow and long-lasting. It is concluded that both types of glutamate receptors, ionotropic and metabotropic, are involved in modulation of GABA_A-ergic synaptic transmission. The first above receptor type provides fast and reversible suppression, while the effect provided by the second type is slow and long-lasting.     

Conditioned reactions of hippocampal neurons during amphetamine poisoning and its counteraction with haloperidol

Activity of 144 neurones of the dorsal part of the rabbits hippocamp was recorded during elaboration of motor conditioned reflex to time. Chronic amphetamine intoxication lowered the ability of hippocampal neurones to form conditioned reactions in response to pairings of sound stimuli with electrocutaneous reinforcement and fully suppressed mechanisms of reproduction by cells of engrams of previous pairings in series of their omissions Single administration of haloperidol to intact animals somewhat increased the number of neurones reacting to the pairing and their omissions in conditioned reflex to time without significantly influencing the intensity and dynamics of reproduction of endogenous cellular reactions in the series of consecutive omissions of pairing. Haloperidol administration during amphetamine intoxication elicited shifts towards normalization of conditioned activity of neurones, eliminating the suppressing action of amphetamine on mechanisms of reproduction of engrams of combined stimuli. Such "therapeutic" effect of haloperidol in many cases did not depend on the character of its psychotropic action. The properties of amphetamine and haloperidol action on the cells of the hippocamp are discussed as compared to their action on the neurones of other brain structures, previously studied in an analogous experimental situation.     

Serotonin reuptake blocker fluoxetine suppresses hippocampal theta oscillation in rabbits during EEG

In earlier studies it has been shown that stimulation of the median raphe nucleus (MR) in awake rabbits decreases the expression and frequency of oscillatory theta activity in the septohippocampal system, and the functional blockade of this nucleus evokes the regular and high-frequency theta rhythm. The present work was aimed at elucidation of serotoninergic influence of MR (which also contains cells of other chemical nature) to the septohippocampal system of theta activity. Serotonin reuptake blocker fluoxetine that increases brain serotonin level was applied. Hippocampal electroencephalogram was recorded in awake rabbits. Bilateral intracerebroventricular infusion of fluoxetine hydrochloride (Sigma, St. Louis; 15 micrograms in 5 microliters saline) in all cases reduced the rhythmic theta activity. In 15 of 18 (83.3%) of experiments the decrease in hippocampal theta oscillations was more than 50% of the control level. The theta band of the spectral density histogram decreased in the mean by 56 +/- 5.8% of the control level (from 10 to 93% in different experiments, p < 0.001). The mean latency of these changes was 3.5 +/- 0.11 minutes (2.9-4.1 min), the effect duration was 64 +/- 3.2 min (45.3-90 min). The mean frequency of the theta waves did not change as compared to the baseline and was equal to 5.25 +/- 0.5 Hz (4.5-6.5 Hz). The fluoxetine-induced reduction of the theta rhythm expression in hippocampus is the evidence of its inhibitory control by serotoninergic brain system. It is suggested that the increase of the frequency of hippocampal theta rhythm after the functional blockade of MR observed in our earlier experiments was the result of a release of the septohippocampal system from the influence of nonserotoninergic neurons (via glutamatergic reticular formation) and/or temporary cessation of the MR interaction with noradrenergic, dopaminergic and glutamate/aspartate systems.     

High-frequency component of hippocampal theta activity

The signal corresponding to the second frequency group in spectra of the theta activity of a rat (a twofold theta-frequency harmonic with adjoning frequency components) and its regulation were studied. The level of the signal of the second frequency group was high in the theta activity diring natural orienting behavior and in desynchronized EEG during waking immobility. In some rats, the EEGs segments recorded during orienting behavior evoked by reticular stimulation contained an enhanced signal of the second frequency group (as compared to the signal of the first frequency group). The second frequency group could be also observed in the absence of the first. Estimation of signal dimension revealed two independent components in the theta activity with the high-level signal of the second frequency group. Time correlation between variations of signals of the first and second frequency groups in the theta activity accompanying natural orienting behavior was absent. These signals changed in different ways during transition from awake immobility of an animal to orienting behavior. It was concluded that in addition to the main component of the hippocampal theta activity, there exists the independently regulated twofold theta-freduency component.     

Inhibition in hippocampal pyramidal neurons during peripheral stimulation

Responses of hippocampal pyramidal neurons were investigated intracellularly in unanesthetized rabbits immobilized with tubocurarine. A single stimulus, applied to the sciatic nerve, evoked prolonged (up to 2.5 sec) hyperpolarization of the cell membrane, accompanied by inhibition of action potentials. The latent period of the evoked hyperpolarization was 48±16.4 msec, and its amplitude 2.5±1.9 mV. In some neurons the development of hyperpolarization potentials was preceded by excitation. The suggestion is made that hyperpolarization of the membrane of pyramidal cells during peripheral stimulation is manifested as an inhibitory postsynaptic potential (IPSP), generated with the participation of hippocampal interneurons. The possibility of prolonged tonic action of interneurons from outside as a cause of prolonged inhibition of the pyramidal neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 278–284, November–December, 1969.     

Early presynaptic changes during plasticity in cultured hippocampal neurons

Long-lasting increase in synaptic strength is thought to underlie learning. An explosion of data has characterized changes in postsynaptic (pstS) AMPA receptor cycling during potentiation. However, changes occurring within the presynaptic (prS) terminal remain largely unknown. We show that appearance of new release sites during potentiation between cultured hippocampal neurons is due to (a) conversion of nonrecycling sites to recycling sites, (b) formation of new releasing sites from areas containing diffuse staining for the prS marker Vesicle-Associated Membrane Protein-2 and (c) budding of new recycling sites from previously existing recycling sites. In addition, potentiation is accompanied by a release probability increase in pre-existing boutons depending upon their individual probability. These prS changes precede and regulate fluorescence increase for pstS GFP-tagged-AMPA-receptor subunit GluR1. These results suggest that potentiation involves early changes in the prS terminal including remodeling and release probability increase of pre-existing synapses.     

Estradiol induces formation of dendritic spines in hippocampal neurons: functional correlates

Dissociated cultured rat hippocampal pyramidal neurons respond to estradiol with a time-dependent, twofold increase in density of their dendritic spines. This effect is mediated by an estrogen receptor, probably of the alpha nuclear receptor type. In searching for the molecular mechanisms leading from the initial activation of the estrogen receptor to the final formation of new dendritic spines, we found that estradiol acts on GABAergic interneurons expressing the estrogen receptor by decreasing their inhibitory tone. In culture, this is assumed to cause a shift in the balance between excitation and inhibition toward enhanced excitation, overactivation of the pyramidal neurons, and subsequent formation of novel dendritic spines. The action of estradiol on spine formation is mediated by phosphorylation of cyclic AMP response element binding protein in the pyramidal neurons and is blocked when inhibition is enhanced by diazepam and when excitation is blocked by tetrodotoxin. Progesterone blocks the effect of estradiol on dendritic spines through its conversion to tetrahydroprogesterone, which enhances GABAergic inhibition. Subsequent to formation of novel dendritic spines, there is an increase in the density of glutamatergic receptors in the affected cells, an increase in the cellular calcium response to glutamate, and an increase in network synaptic activity among the cultured neurons.     

Limited plasticity of the rabbit visual cortex and hippocampal neurons in the oddball test

The activity of 41 visual cortex and 20 hippocampal neurons from field CA1 was registered in experiments using oddball-stimulation with different color stimuli varied in intensity. 34% cortical and 37% hippocampal neurons demonstrated plasticity reactions. The significant increase of latest phases of neuronal activity (200-500 and 200-1000 ms after stimulation for cortical neurons and 300-550 ms for hippocampal neurons) was shown in responses to rare deviant stimuli, which had a less intensity than frequently standards. The quantity of the earliest neuronal phase of activity (40-120 ms after stimulation) was stabilized in responses to deviants and standards during the experiment. We propose that such increase of the latest phases of neuronal activity (the limited plasticity) may reflect the mechanisms of orienting reaction.     

Distribution of GAD67-expressing neurons and morphological changes in hippocampal structures during pubertal period after acute perinatal hypoxia in rats

We investigated structural changes in the Wistar rat hippocampal CA1 field and fascia dentate during the pubertal period (on P60) after perinatal hypoxic exposure as well as the distribution of GAD67-expressing neurons in these structures. It was established that in the granular layer of the fascia dentata and in the CA1 field acute perinatal hypoxia leads to irreversible homotypic abnormalities as expressed in the reduced number of neurons and their rows as well as injury of a considerable portion of cells, which exhibit the signs of chromatolysis and vacuolization of the cytoplasm. Both in experimental and control animals, GAD67-expressing neurons in the fascia dentata are scattered diffusely and share approximately the same size of their populations. In the CA1 field, immunoreactive neurons lie in the lower rows of the pyramidal layer, while neurons in the upper layers exhibit no immunolabeling and have less synaptic structures in experimental animals than in control. We suggest that neurons in the hippocampal structures are involved in the regulation of functions and formation of prenatal pathology.     

大鼠海马神经元膜离子通道随培养时间变化的特点

目的和方法:采用膜片钳全细胞记录技术观察新生大鼠海马神经元体外分散培养过程中,基本离子通道和膜参数随培养天数延长而变化的规律.结果:在7 d,14 d和21 d时电压依赖性钠电流(Voltage-dependent Na cur-rent,ⅠNa)和延迟整流性钾电流(Delayed rectifier K current,Ⅰk)的幅度无显著性差异.电压依赖性钙电流(Voltage-dependent Ca2 current,ⅠCa)和ⅠCa密度则持续增大,进一步研究表明,L型钙通道(L-type voltage-dependent Ca2 channel,L-VDCC)的增加是其主要原因.NMDA诱发电流随培养时间延长而明显增加.结论:钙通道和NMDA受体所介导的Ca2 内流是神经元易感于衰老和死亡的重要机制之一.     

Age-specific morphological-functional characteristics of the rabbit's hippocampal neurons during conditioning

Formation of trace rhythm recruitment (an analogue of conditioned time reflex) was studies in CA3 hippocampal neurons of alert young (less than one year), old (54-65 months), and very old rabbits after a prolonged (10-20 min) electro-cutaneous stimulation of a forelimb with the frequency of 0.5-1 Hz. Comparative analysis of neuronal spike activity in young and old rabbits showed that in the late ontogeny the number of spontaneously active neurons was significantly decreased, the proportion of slowly firing neurons increased, the interspike intervals and intervals between spike groups became longer, the number of spikes in a group reduced. The ability of hippocampal neurons to acquire and reproduce the rhythm of the previous stimulation declined with age. No appropriate rhythms were found in neurons of very old animals. A nonspecific increase in neuronal baseline activity was observed in old rabbits after the stimulation. Deterioration of morphological structures of hippocampal neurons and glial cells may explain the impairment of mnestic processes in late ontogeny.     

Effects of polymethylene derivatives of 4-aminopyridine on functional properties of hippocampal neurons

The effects of amiridin (9-amino-2,3,5,6,7,8-hexahydro-IH-cyclopenta(b) quinoline) and tacrine (1,2,3,4-tetrahydro-9-aminoacridine) on Schaffer collaterals--CAI field potentials were compared in rat hippocampal slice preparations. Similar dose-dependent increase in pop-spike amplitude was observed during slice perfusion with low concentrations of amiridin (5-50 microM) or tacrine (0.5-10 microM). This facilitation was not always fully reversible. The effect was accompanied by slight decrease in pop-EPSP amplitude suggesting membrane depolarization as a possible mechanism of pop-spike facilitation. Further increase in drug concentrations led to the depression and full blockade of pop-spike, that was associated with significant decrease in the pop-EPSP and fiber potential amplitudes. In contrast structurally related 4-aminopyridine evoked dose-dependent increase in both pop-EPSP and pop-spike amplitudes with all the concentrations tested (0.05-1000 microM), this facilitation was transformed into epileptiform response with 4-aminopyridine concentration about 500 microM. Possible mechanisms of drug actions on hippocampal neuron reactivity are discussed. It is suggested that amiridin might turn to be as effective as tacrine in symptomatic treatment of Alzheimer disease.     

The activity of hippocampal neurons during several types of behavior]

The activity of 87 hippocampal units and the EEG (field CAI) were studied in unrestrained rabbits during calm and active alertness and at different stages of sleep. Correlation has been established between the characteristics (mean frequency and the pattern) of unit firing, EEG and the animal's activity. For most of the neurones, fixed values of mean frequency and the discharge pattern corresponded to a definite functional state. With transition from sleep to alertness, 63.6% of the units became active, 29.1% were inhibited, and the rest of the units changed the firing pattern only. The cells which became activated during awakening, showed a reduced firing frequency during a more profound sleep and higher discharge frequency during the paradoxal phase of sleep, while the inhibitory cells revealed reverse dynamics of discharge frequency. In a state of alertness, the most pronounced shifts in firing activity were observed in 33.3% of the nerve cells at orienting investigating behaviour, in 27.1%, during attention reaction, in 22.9% at some kinds of movement, and in 16.7%, in the course of feeding and drinking. A conclusion has been drawn that the role of the hippocampus in achieving different behavioral reactions is probably to a great extent determined by its participation in setting up a level of the brain central tone, specific for each state.