Functional characteristics of Schaffer's collaterals studied in vivo and in vitro

A comparative analysis was made of the functional characteristics of connections between hippocampal areas CA₃ and CA₁ (Schaffer's collaterals) in experiments in vivo on unanesthetized rabbits and in vitro on surviving slices of guinea pig hippocampus, with extracellular recording in area CA₁. In the case of electrical stimulation of the collaterals in vitro, post-activational inhibition was weak, responses of inhibition of spontaneous discharges were absent, and low frequencies of stimulation were more effective than in vivo. Posttetanic changes were found more frequently in experiments in vitro and they lasted longer than in vivo. The predominant effect of tetanization under normal conditions was depression, but during incubation it was facilitation of responses. The possible causes of these differences are discussed.Institute of Biophysics, Academy of Sciences of the USSR, Puschino-on-Oka. Translated from Neirofiziologiya, Vol. 11, No. 3, pp. 208–217, May–June, 1979.     

Functional characteristics of the principal intrinsic systems of hippocampal connections

By extracellular recording of unit activity and electrical stimulation of unanesthetized rabbits the character of the following principal intrinsic hippocampal connections was investigated: fascia dentata with area CA₃ (series FD-CA₃) and area CA₃ with area CA₁ (series CA₃-CA₁). Differences between the functional characteristics of these two systems of connections were shown. The CA₃-CA₁ system was highly efficient, with a sharply defined boundary between subthreshold and threshold intensities of stimulation and with a wide range of active frequencies; recruiting was negligible and no sign of prolonged potentiation was present. The FD-CA₃ system was characterized by low efficiency, a wide threshold zone, and narrow range of active frequencies, slow recruiting, and long preservation of incoming influences. The probability that these functional differences depend on the morphological characteristics of the systems of connections and their possible role in the function of the hippocampus are discussed.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 259–266, May–June, 1976.     

Modulation of theta-activity in the septo-hippocampal system by the alpha-2-adrenoreceptor agonist clonidine

The influence of the alpha 2-adrenoreceptors agonist clonidine on the neuronal activity of the medial septal area (MS) and hippocampal EEG was studied in unanaesthetized rabbits. A slight and short-term decrease in the theta-rhythm modulation in the MS neuronal activity and/or EEG was revealed in 30.4% of tests after the bilateral intraventricular injection of a small dose of clonidine (0.5 microgram/5 microliters of water). On the contrary, a high dose of clonidine (5 micrograms/5 microliters) substantially enhanced the theta modulation in 100% of tests. The frequency of the theta bursts in the MS increased, on average, by 1.6 +/- 0.18 Hz (from 5.25 +/- 0.19 to 6.8 +/- 0.17 Hz). The regularity of the theta modulation became almost twice higher: the time constant of damping increased from 0.34 +/- 0.04 to 0.60 +/- 0.08 s. Increase in the neuronal activity in the MS produced by the high dose of clonidine was always accompanied by appearance of continuous stable theta waves in the EEG; the spectral power in the theta range increased, on average, by 480 +/- 98%. This strong effect arose suddenly but was relatively short-lasting (12 +/- 3.5 min) and usually abruptly terminated. It is concluded that the noradrenergic system has a double control over the theta oscillations through the alpha 2-adrenoreceptors agonist. In low concentrations the agonist clonidine acts on the high affinity inhibitory presynaptic autoreceptors reducing the noradrenaline release and suppressing the theta activity. In a high dose clonidine predominantly acts on postsynaptic (low affinity) adrenoreceptors substantially increasing the frequency and regularity of the theta bursts in the activity of septal neurons. Presumably, different types of alpha 2-adrenoreceptors participate in regulation of the theta oscillations in different functional states. It is suggested that the noradrenergic locus coeruleus is a functional synergist of the activating reticular formation participating in the urgent phasic activation of the septohippocampal system during the action of sudden strong stimuli.     

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.     

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.     

Aluminum exclusion from root zone and maintenance of nutrient uptake are principal mechanisms of Al tolerance in <Emphasis Type="Italic">Pisum sativum</Emphasis> L.

Our study aimed to evaluate intraspecific variability of pea (Pisum sativum L.) in Al tolerance and to reveal mechanisms underlying genotypic differences in this trait. At the first stage, 106 pea genotypes were screened for Al tolerance using root re-elongation assay based on staining with eriochrome cyanine R. The root re-elongation zone varied from 0.5 mm to 14 mm and relationships between Al tolerance and provenance or phenotypic traits of genotypes were found. Tolerance index (TI), calculated as a biomass ratio of Al-treated and non-treated contrasting genotypes grown in hydroponics for 10 days, varied from 30% to 92% for roots and from 38% to 90% for shoots. TI did not correlate with root or shoot Al content, but correlated positively with increasing pH and negatively with residual Al concentration in nutrient solution in the end of experiments. Root exudation of organic acid anions (mostly acetate, citrate, lactate, pyroglutamate, pyruvate and succinate) significantly increased in several Al-treated genotypes, but did not correlate with TI. Al-treatment decreased Ca, Co, Cu, K, Mg, Mn, Mo, Ni, S and Zn contents in roots and/or shoots, whereas contents of several elements (P, B, Fe and Mo in roots and B and Fe in shoots) increased, suggesting that Al toxicity induced substantial disturbances in uptake and translocation of nutrients. Nutritional disturbances were more pronounced in Al sensitive genotypes. In conclusion, pea has a high intraspecific variability in Al tolerance and this trait is associated with provenance and phenotypic properties of plants. Transformation of Al to unavailable (insoluble) forms in the root zone and the ability to maintain nutrient uptake are considered to be important mechanisms of Al tolerance in this plant species.     

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.     

Effect of elimination of brain stem-septal connections on prolonged posttetanic changes in the hippocampus

Comparative analysis of long-lasting effects of Schaffer's collaterals tetanization (15 Hz, 15 s) was performed in 61 extracellularly recorded units of the intact hippocampus (field CA1) and in 51 units after complete septo-hippocampal disconnection in chronic alert rabbits. The number of units with long-lasting potentiation (LLP) was significantly greater in the hippocampus deprived of brain stem-septal afferents (63%) as compared to the intact hippocampus (21%); the number of units with posttetanic depression was reduced (21% versus 34%). The mean level of LLP in the deafferentated hippocampus was significantly higher than in the intact hippocampus (mean 220% and 180% of control level; p less than 0,001), and its duration was greater. Conversely, the mean level of depression was lower in the deafferentated hippocampus than in the normal one (140% and 170% of control level; p less than 0,01). This increase of LLP may result from elimination of inhibitory influences of the brain stem-septal afferent input, and/or from sprouting of Schaffer's collaterals after deafferentation with increase in number of functional synapses.     

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.