Analysis of the Effect of Neuroprotectors That Reduce the Level of Degeneration of Neurons in the Rat Hippocampus Caused by Administration of Beta-Amyloid Peptide Aß25-35.

Deposition of beta-amyloid peptide in the brain observed in Alzheimer's disease contributes to the development of cognitive deficits. We studied the abilities of different neuroprotectors to prevent or reduce degeneration of hippocampal neurons in rat brain 14 and 45 days after single intrahippocampal injection of beta-amyloid peptide 25-35 (Aß25-35). Cytological analysis of the neurons of the hippocampal CA1 and CA3 fields showed predominant damage to CA1 neurons in 14 days and CA3 neurons in 45 days after Aß25-35 administration. Single preliminary administrations of neuroprotectors fullerene C60FWS (antioxidant), neuromedin (nonselective inhibitor of acetylcholinesterase), and AM404 (activator of the endocannabinoid system) largely prevented neurodegeneration of neurons. Fullerene produced the most pronounced protective effect, which can be explained by its ability to prevent aggregation of proteins and destroy Aß25-35 fibrils. The combined use of these neuroprotectors can provide the basis for the development of new approaches to prevention and treatment of Alzheimer's disease.

[Endogenous cannabinoid system in the brain: role in regulation of seizure activity].

In overview one can find up-today data on endogenous cannabinoids (EC), their role in brain functioning. Interest in EC in recent years has significantly increased. Despite the fact that existence of EC-system among mammals was identified in nineties of the twenties century, deciphering the mechanisms of its functioning both in healthy brain as well in various pathologies, is far from final stage. The main function of EC in brain is implementation of the retrograde synaptic function of communication and neuromodulation. In overview one can see data on localization and functions of cannabinoids receptors and its endogenous ligands in CMS, as well as on EC-system participation in epileptiform activity modulation. Special focus on the analysis of works, where the projection revealed the role of EC in experimental modeling of the temporal epilepsy with animals, as well as for diseases in humans epilepsy. Set out the estimated survival mechanisms of cells and their repair provided by cannabinoid system in the generation of seizure activity; also provides information about the neurotoxic effects of EC. Possible reasons of contradictions are being discussed, that exist in the literature regarding the functions of EC in the brain.

[Detection of c-fos expression in animal brain in a pilocarpine model of temporal lobe epilepsy].

The dynamics of the involvement of different brain structures in a pathological process is very important for decoding the mechanisms of temporal lobe epilepsy. In this work, the experimental model of temporal lobe epilepsy induced by lithium chloride and pilocarpine was used. The method of immunochemical detection of the immediate early gene c-fos was used as an indicator of functioning neurons in the brain. The c-fos expression was determined at different time points (30, 60 and 90 min) after the pilocarpine injection. An increase in the c-fos expression was observed in neuronal populations during the development of the status epilepticus, the time and degree of involvement of different brain structures being different. The expression of c-fos was first observed in the piriform cortex, the olfactory tubercle, thalamic nuclei, lateral habenular nuclei, and the caudate putamen. Then the hippocampus, the septal formation, the amygdala, and basal ganglia were involved in the activation process. In the hypothalamic areas, c-fos expression was observed latest. These data contribute to understanding the mechanisms of temporal lobe epilepsy and searching for the ways of its therapy.

[Protective effects of CB1 receptor agonist WIN 55.212-2 in seizure activity in the model of temporal lobe epilepsy].

The influence of a low dose (1 microM, 2 microl) of nonselective agonist of cannabinoid CB1 receptor WIN 55.212-2 on seizure activity in different brain structures was investigated in waking guinea pigs. Changes in spontaneous local field potentials and seizure afterdischarges evoked by the electrical stimulation of the perforant path were recorded simultaneously in the hippocampus, entorhinal cortex, medial septal region, and amygdala after a preliminary intraventricular injection of WIN 55.212-2. It was found that WIN 55.212-2 blocked the stimulation-elicited seizures in 80% of tests. A repeated injection of the agonist within 30 days caused an increase in the amplitude of local field potentials and the power of the theta rhythm in all the structures under study. The infusion of kainic acid provoked the onset of status epilepticus in control animals, whereas guinea pigs injected with the agonist (daily, during 25-30 days) did not develop the status. Possible mechanisms of the protective influence of WIN 55.212-2 are discussed.

[Changes in the electrical activity of the medial septal region, piriform cortex and amygdala during epileptogenesis in the model of temporal lobe epilepsy].

Local field potentials (EEGs) in the medial septal area, amygdala and piriform cortex were recorded in waking guinea pigs in the control and during epileptogenesis in the model of chronic temporal lobe epilepsy (lithium-pilocarpin model of status epilepticus). Analysis of changes in rhythmical activity and interstructural relations was carried out at different stages of epileptogenesis. Increased frequency of rhythmic activity in delta, theta, and alphabands was observed during epileptogenesis. Correlation relations between the activities of the medical septum with the piriform cortex and amygdala clearly decreased to 5 months after development of status epilepticus. Changes in the frequency of oscillations and structural correlations developed in time from two months on and reached a maximum 5 months after the status epilepticus development. It point to intensification of the pathological changes during formation of the epileptic focus. A possible role of the observed EEG changes in the formation of a pathological centre is discussed.

[Coherence and phase analysis of theta-oscillations in the septohippocampal system during seizures].

Interrelations of the hippocampus and medial septal area (MSA) in the theta band (4-8 Hz) were studied during seizures produced by electrical kindling in waking guinea pigs. Field activity (EEG) was analyzed using the wavelet transform. A decrease in coherence of theta-oscillations in the hippocampus and MSA was observed during seizures. Phase analysis showed that in the beginning of kindling the MSA led in phase, but after formation of the pathological focus, MSA lagged the hippocampus. The data may contribute to understanding mechanisms of temporal lobe epilepsy.

[Activation of glutamatergic system of the medial septal region facilitates the epileptogenesis].

The influence of L-glutamate injected into the medial septal region (MSR) on the epileptogenesis evoked by perforant path stimulation was investigated in waking guinea pigs. In animals injected with saline ("control") and L-glutamate ("glutamate"), field potentials (EEG) of the hippocampus and the MSR were recorded before the injections and during epileptogenesis. The delivery of L-glutamate to the MSR induced an increase in theta activity both in the MSR and the hippocampus. During kindling, the seizure behaviour and secondary epileptiform discharges were observed earlier in the glutamate group than in control animals. In the process of epileptogenesis, theta oscillations almost disappeared in all animals, but in the glutamate group this happened much earlier. The correlation between hippocampal and septal EEGs dramatically decreased in the animals with glutamate injections, and in the controls the decrease was not significant. These results contribute to understanding the mechanisms of temporal lobe epilepsy.

The nicotinic receptor blocker hexamethonium alters neuronal responses to glutamate in the medial septal area of the brain of the ground squirrel in vitro.

Despite extensive interest in studies of the medial septal area, the nature of the interactions of its various neurochemical systems remains largely unclear. The aim of the present work was to clarify the role of nicotinic receptors in mediating the interaction of the glutamatergic and cholinergic systems in this structure. Extracellular recording of neuron activity in living slices of ground squirrel brain was used to study the influences of L-glutamate (1 microM) during application of the nicotinic receptor blocker hexamethonium (1 mM). The responses of septal neurons to glutamate depended on the type of their initial activity and the presence of pacemaker properties. This study is the first to show that glutamate increases the frequency of volleys in rhythmic neurons in the septum. Hexamethonium induced changes in neuron activity similar to the influences of glutamate. After prior application of hexamethonium, the responses of neurons to glutamate changed: activatory responses were masked and inhibitory responses were enhanced. Cholinergic modulation of the responses of septal neurons to glutamate were shown to occur, as did modulation of the strength of the oscillatory properties of the septal network by nicotinic receptors.

Changes in oscillatory activity of neurons in the medial septal area in animals with a model of chronic temporal epilepsy.

Comparative studies of the activity of extracellularly recorded neurons in living slices of the medial septal area of healthy guinea pigs and animals with a model of chronic temporal epilepsy demonstrated differences between them in terms of the frequency and pattern of cell discharges. The brains of animals with experimental epilepsy showed a doubling of the total level of activity as compared with controls, due to increases in the discharge frequencies of irregular and regular non-volleying neurons. There was a sharp (three-fold) increase in the number of cells with rhythmic discharge volleys, along with changes in the parameters of volley activity - both in neurons with the endogenous (pacemaker) pattern and in cells with secondary involvement in rhythmic activity. The possible mechanisms for these changes are discussed. These data widen our understanding of the processes forming pathological synchronization in epilepsy and may assist in the creation of new approaches to the treatment of this disease.

[Relationships between the hippocampus and medial septal region and their alterations during epileptogenesis].

EEGs of the hippocampus and the medial septal region (MSR) in the control conditions and during repeated stimulation of the perforant path were simultaneously recorded in awake guinea pigs. Changes in correlation of activity of these structures during seizures provoked by the stimulation (model of acute epilepsy) and in the process of epileptogenesis induced by the kindling (model of chronic epilepsy) were analyzed. A high correlation of the baseline activities of the hippocampus and MSR observed in the control sharply decreased during acute and chronic seizures. Kindling led to emergence of the MSR capability of hippocampus-independent generation of the field seizure discharges. In the process of kindling the progressive disintegration of activities of the hippocampus and the MSR was revealed being indicative of disorders in functioning of the septohippocampal network during epileptogenesis.

[Hexamethonium, nicotinic receptor blocker, changes the neuronal reactions on glutamate in the medial septal area in vitro].

Despite the great interest in studying the medial septal area, the interactions of its neurochemical systems are not yet clearly understood. The aim of this study was to elucidate the role of nicotinic receptors in the interaction of glutamatergic and cholinergic systems of the medial septal area. The effect of L-glutamate (1 microM) on septal neurons was studied under the application of hexamethonium, nicotinic cholinoreceptor blocker by using the method of extracellular recording of neuronal activity in brain slices of ground squirrels. The response of septal neurons to glutamate depended on the type of their initial activity and on the presence of pacemaker properties. For the first time, the ability of septal neurons to respond to glutamate with an increase in burst frequency was shown. The influence of hexamethonium on the neuronal activity was similar to that of glutamate. After a preliminary application of hexamethonium, the reactions of neurons to glutamate changed. The excitatory reactions were masked, while the inhibitory reactions became stronger. It was found that nicotinic cholinergic receptors modulated the reactions of MS-DB cells to glutamate and the expression of the oscillatory properties of the septal neuronal network.

[Medial septal region as a target for modulation of seizure discharges in the hippocampus in a model of acute temporal lobe epilepsy].

Investigation of changes in the hippocampal EEG produced by GABAergic and cholinergic substances delivered into the medial septum region was performed in awake rabbits. Changes in the threshold of seizure discharges in the hippocampus evoked by perforant path stimulation (model of acute epilepsy) were also examined. Injections of GABAA receptor antagonist picrotoxin or agonist of cholinergic receptors carbacholine in low doses induced an increase in the power of delta- and theta modulation and appearance of 7-12-Hz oscillations. The threshold of hippocampal seizure afterdischarges decreased. In higher doses, these substances evoked 7-15-Hz oscillations followed by seizures. GABAA receptor agonist muscimol and muscarinic receptor antagonist scopolamine decreased the power of the theta rhythm and increased the seizure threshold. Picrotoxin or carbacholine injected after muscimol or scopolamine, respectively, did not evoke seizures. Thus, we have shown the possibility to control hippocampal activity by local changes in the GABAergic and cholinergic systems of the medial septum region.

[Alterations in the oscillatory activity of neurons in the medial septal area in animals with the chronic temporal lobe epilepsy model].

Comparative investigation of activity of medial septal neurons, which was performed by extracellular recordings in the brain slices of health guinea pigs and in such ones of animals with the model of chronic temporal lobe epilepsy, revealed the distinctions between them, concerning a neuronal frequency and pattern of discharges. In the epileptic brain, twofold increase of general level of activity was observed comparative to control one, owing to augmentation of frequency of discharges in non-regular and regular non-bursting neurons. Sharp increase (three times as much) of number of cells with rhythmic burst discharges and changing of parameters of burst activity were discovered, the letter both in the neurons with endogenous (pacemaker) pattern and in the cells secondary involved in the rhythmic activity. Possible mechanisms of these alterations are discussed. The present data advance the understanding of the processes of a pathological synchronization and may promote creation of new approaches to treatment this disease.

Sensory responses of neurons in the medial septal area in conditions of modulation of theta activity using the alpha-2-adrenoreceptor agonist clonidine.

Our previous studies on conscious rabbits showed that administration of the alpha-2-adrenoreceptor agonist clonidine induces dose-dependent changes in theta oscillations in the septohippocampal system. Low doses of clonidine suppressed theta activity, while high doses produced significant potentiation. It was suggested that the different effects of clonidine might be associated with differences in the sensitivities of pre- and postsynaptic alpha-2-adrenoreceptors to clonidine, this agent being a pure agonist of noradrenaline when used at high doses. It was suggested that functional synergism occurs between the activatory reticular formation and the noradrenergic system of the locus ceruleus in controlling the theta rhythm. The present study was performed to identify the nature of the responses of sensory neurons in the medial septal region in conditions of alterations in the magnitude of the theta rhythm induced by different doses of clonidine. Low and high doses of the agonist given bilaterally into the lateral ventricles were found to have different effects on the sensory responses of neurons in the medial septal region. Injection of small clonidine doses (0.5 microg in 5 microl into each lateral ventricle), which decrease theta activity, was found to lead to weakening of activatory processes and enhancement of inhibitory processes in the medial septal region. The number of activatory responses decreased significantly and persisting responses were significantly weakened; inhibitory responses, conversely, were seen more frequently and were significantly more marked. Administration of high clonidine doses (5 microg in 5 microl), which produce sharp increases in theta oscillations, led to significant reductions in the reactivity of cells in the medial septal region to sensory stimuli (from 76.8% in controls to 45% after clonidine), regardless of the nature of the initial responses. Persisting activatory and inhibitory responses were in most cases less marked than the initial responses. These results suggest that alpha-2-adrenoreceptors are involved in controlling the sensory reactivity of neurons in the medial septal region. The impairment of the normal processing of sensory stimuli seen during the continuous generation of rhythmic activity provoked by injection of large clonidine doses supports the role of the theta rhythm in the septohippocampal system as an active filter during the processing and recording of information.

[Neuronal septal activity during hippocampal seizure discharges generation in acute model of epilepsy].

The activity of the neurones of the medial septal region (MS) and the hippocampal EEG in control and during the appearance of seizure discharges provoked by electrical stimulation of the perforant path were investigated in the awake rabbit. During afterdischarge generation in the hippocampus the dense neuronal bursts separated by periods of inhibition were recorded in the MS. In one group of neurons the bursts of spikes coincided with the discharges in the hippocampus, in other group-occured during inhibitory periods. When the afterdischarge stopped, in the septal neurons with theta activity the disruption of theta pattern was recorded, which have been correlated with the occurrence of low amplitude high frequency (20-25 Hz) waves in the hippocampal EEG. As a rule, the neuronal activivity of the MS recovered much quickly than EEG of the hippocampus; in some cases the increasing of the theta regularity was observed. The definite accordance of the electrical activity of the hippocampus and MS during seizure discharges suggests that the septohippocampal system operate as integral nervous circuit in these conditions. Diverse in the temporal interrelations between the discharges of MS neurones and ictal discharges in the hippocampus in the different cells possible indicate that various groups of the septal nervous elements have different participation in the seizure development. Appearance of the high frequency bursts in the MS is a possible "precursor" of the seizure onsets.

Regulation of the septal pacemaker theta rhythm by the cervical nuclei of the midbrain.

Neuronal activity in the medial septal region (the medial nucleus and the diagonal band nucleus, MN-DBN) was recorded along with hippocampal EEG traces in conscious rabbits with stimulatory electrodes implanted in the median cervical nucleus (MCN) and the reticular formation (RF) of the midbrain and pons. In all animals with electrodes in the MCN, the background theta activity frequency was low (4.6 +/- 0.15 Hz) as compared with intact rabbits or those with electrodes implanted only in the RF (5.2 +/- 0.19 Hz, p < 0.5). Stimulation of the MCN with weak low-frequency impulses reduced theta volleys from MN-DBN cells, reducing their frequency and regularity and inducing the appearance or strengthening of low-frequency delta modulation. The number of spikes in a volley decreased, and the duration of inter-volley intervals increased. Stimulation of the MCN led to a gradual decrease in the frequency and amplitude of theta waves, induced irregular delta waves and spindles of 12 Hz in the hippocampal EEG. Stimulation of the RF produced the opposite changes in volley activity in the MN-DBN and hippocampal EEG, with increases in theta and decreases in delta components. These results support a role for the midbrain cervical nuclei as structures limiting the generation of theta activity by the reticular-septal system, but do not support the existence of an MN-DBN-independent high-frequency serotoninergic theta rhythm. It is proposed that the effect of the MCN may be important for suppression and switching of attention.

The spontaneous activity of hippocampal neurons during the modulation of theta rhythm by cholinergic substances.

A statistical analysis of the baseline activity of neurons, recorded intracellularly in the hippocampus of awake, nonimmobilized rabbits in three states, control and during the systemic administration of eserine and scopolamine, was carried out. Neurons of the hippocampus were additionally tested in a similar manner following the chronic basal undercutting of the septum, removing stem influences. The cholinergic substances regulate the number of neurons of the hippocampus having theta modulation and the degree of its stability, but do not influence its frequency. When the cholinergic theta rhythm is activated, regularization of the activity takes place with the suppression of delta modulation and of "complex spikes"; its blockade is accompanied by the opposite changes. Both substances stably alter the level of the baseline frequency of discharges of the majority of neurons, although the total average frequency remains constant. Regression analysis shows the predominance of a decrease in the activity in high-frequency (> 25 spikes/sec) and an increase in the low-frequency (< 25 spikes/sec) neurons during the effect of both substances. The constancy of the total average frequency and the unidirectionality of the shifts in the level of discharges of the neurons during the intensification (eserine) and blockade (scopolamine) of the cholinergic component of the theta rhythm points to the fact that the cholinergic septal input directly influences mainly the structure but not the level of the activity of the hippocampal neurons.

The serotonin reuptake inhibitor fluoxetine suppresses theta oscillations in the electroencephalogram of the rabbit hippocampus.

Our previous studies on conscious rabbits showed that stimulation of the median cervical nucleus (MCN) decreases the extent and frequency of oscillatory theta activity in the septohippocampal system, while functional blockade of the nucleus by administration of the anesthetic lidocaine produces a stage high-frequency theta rhythm. The present study addresses the nature of the serotoninergic influences of the MCN (which also contains cells of other chemical natures) on the septohippocampal system. Experiments on conscious rabbits involved recording of the hippocampal EEG in control conditions and after microinjection of fluoxetine, a serotonin reuptake blocker which increases the levels of this transmitter in the brain. In all experiments, bilateral intracerebroventricular administration of fluoxetine hydrochloride (Sigma, St. Louis, MO; 15 microg in 5 microl of physiological saline) induced decreases in the magnitude of the hippocampal theta rhythm. In 15 of 18 (83.3%) of experiments, suppression of the oscillator activity by at least 50% of control was seen. The amplitude of the theta band in the spectral density histogram decreased by an average of 56 +/- 5.8% compared with control values (decreases in different experiments were from 7% to 90% of control p < 0.001). The latent period of these changes averaged 3.5 +/- 0.11 min (range: 2.9-4.1 min). The effect lasted 64.8 +/- 3.2 min (varying from 45.3 to 90 min in different experiments). There were no significant changes in the theta rhythm frequency, as compared with controls; this averaged 5.25 +/- 0.5 Hz (range: 4.5-6.5 Hz). The decrease in the magnitude of theta oscillations in the hippocampus after administration of fluoxetine provided evidence of the inhibitory control of rhythmic theta activity by the serotoninergic system of the brain.

Background activity of rabbit hippocampal neurons in conditions of functional exclusion of structures which regulate the theta rhythm.

The functional importance of theta modulation in the activity of hippocampal neurons was further analyzed using a method consisting of controlled sequential short-term (25-30 min) inclusion or exclusion of the theta rhythm by local administration of lidocaine into the median cervical nucleus and medial septal region respectively. Studies were carried out using conscious rabbits with extracellular recording of hippocampal neuron activity in field CA1. Administration of lidocaine into the medial septal nucleus and diagonal tract nucleus (MS-DT) led to complete inhibition of theta modulation in neuronal and total hippocampus activity. The mean frequency of background discharges underwent no change in most neurons, but decreased significantly in a limited group of cells with high-frequency activity (presumptive inhibitory neurons). Administration of lidocaine into the median cervical nucleus (MCN), the source of serotoninergic pathways to the MS-DT and hippocampus, was accompanied by increases in the stability and frequency of theta modulation of neuronal activity, induction of theta modulation in an additional group of neurons, and expression of a continuous theta rhythm in the electrical activity (EA) of the hippocampus. The mean frequency and regularity of discharges increased in most cells. These data support the existence of tonic inhibitory effects on the part of the MCN on the septa-hippocampal system generating the theta rhythm; in this regard, the MCN can be regarded as an antagonist of the activating reticular formation.

Modulation of the reaction of hippocampal neurons to sensory stimuli by cholinergic substances.

The influences of increasing endogenous acetylcholine (eserine) and its blockade (scopolamine) on the effects of sensory stimuli were analyzed through the extracellular recording of the activity of individual hippocampal neurons of awake rabbits. An increase in the level of acetylcholine, accompanied by the appearance of stable theta rhythm, leads to a substantial decrease in the reactivity of neurons, the suppression, attenuation, and inversion of the majority of inhibitory reactions and of a substantial proportion of activational reactions including on-responses of a specific type. At the same time, a limited group of activational reactions is intensified and extended against the background of eserine. Scopolamine, which blocks theta rhythm, does not change or intensifies inhibitory and some activational reactions, including on-responses. Tonic reactions are shortened; however, their gradual extinction disappears. The effects described are preserved in the hippocampus in the presence of basal undercutting of the septum which eliminates ascending brainstem pathways. These data make it possible to draw the conclusion that, under normal conditions, a new (significant) sensory stimulus elicits in the hippocampus an initial stoppage (reset) of activity with the coordinated triggering of theta rhythm and the passage against this background of signals along the cortical input in a specific phase relationship to it. The period of theta modulation switched on by the signal fosters its recording and the limitation of the passage of subsequent, interfering signals. The septohippocampal influences may thus support the mechanism of selective attention, as a necessary precondition for memory.