Comparison of direct influences of the perforant path on hippocampal CA1 and CA3 neurons in vitro

The efficiency of synapses of the perforant path located on terminals of apical dendrites of CA₁ and CA₃ neurons was investigated in sections of the guinea pig hippocampus in vitro. Neurons of both areas were shown to respond to stimulation of the perforant path by action potential generation. Responses of most CA₁ neurons appeared to repetitive stimulation with a frequency of up to 30–80/sec. Neurons in area CA₃ respond only to low-frequency stimulation (under 5/sec). Posttetanic potentiation of responses to stimulation of the perforant path was found in both areas of the hippocampus.Institute of Biophysics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 11, No. 4, pp. 303–310, July–August, 1979.     

Short- and long-term potentiation in afferent pathways of the neonatal rabbit hippocampus

Vocal potentials were recorded in hippocampal area CA₁ and dentate fascia in unanesthetized rabbits aged from 1 to 50 days during stimulation of Schaffer's collaterals and the perforant path, respectively, with paired (interval 15–100 msec) and repetitive (20–40 Hz for 3–5 sec) electric pulses. Short-term potentiation of focal potentials during paired stimulation and post-tetanic potentiation lasting from a few minutes to 3 h were shown to be reproduced in the hippocampus from the first days after birth, whereas in the dentate fascia, which matures later, reproduction began on the 8th–10th day, when neurons first began to respond to stimulation of the corresponding afferent pathways.     

Functional development of hippocampal mossy fiber synapses in rabbits

The course of functional maturation with age of mossy fiber synapses on pyramidal cells in areas CA_3,4 of the dorsal hippocampus was investigated by extracellular recording of focal potentials and single unit responses of the hippocampus to electrical stimulation of the dentate fascia in waking, unimmobilized rabbits aged from 1 to 14 days. After the 4th day of postnatal life focal potentials appeared in response to single stimulation, in the form of a biphasic short-latency wave, characteristic of responses of the mature hippocampus, accompanied by spike discharges with a latent period of 3 to 10 msec and inhibitory responses of the hippocampal neurons. During the next 10 days the amplitude of the focal potentials increased from several hundred millivolts, with the sharpest increase observed from the 4th through the 7th days. In early age periods global and unitary responses were shown to be capable of frequency potentiation and also of short-term after-potentiation.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 12, No. 3, pp. 246–254, May–June, 1980.     

Spatial distribution of hippocampal response to tooth pulp stimulation and acoustic stimulation

Evoked potentials (EP) and neuronal responses produced by tooth pulp stimulation and a clicking sound were recorded at different hippocampal sites using microelectrodes in unrestrained rats. Spatial distribution of EP was found to be the same for both types of stimulation. Averaged EP consisted of a high amplitude negative preceded by a low-amplitude positive component (N₁ and P₁, respectively). Latency of the N₁ wave reached its minimum (of 27 msec) at the middle third of the molecular layer of the dentate gyrus and the outer portion of the CA₃ apical dendrites. Latency of N₁ was considerably longer in the stratum radiatum layer of the CA₁. Laminar profiles of the amplitude of the N₁ componenent of EP produced in the dentate gyrus and the CA₃ by tooth pulp stimulation resemble those observed during perforant path stimulation; in the CA₁ they are similar to those evoked by stimulating the Schaffer collaterals. Maximum amplitude of the P₁ component was observed above the pyramidal layer of the CA₁ and the hilus. Neuronal discharge pattern changed in all hippocampal regions under the effects of both tooth pulp stimulation and the clicking sound. It is deduced that information can reach the hippocampus by two routes: via a "fast" (inhibitory) pathway through the fimbria and the fornix and a slower (excitatory) path through the entorhinal cortex.P. Flexig Institute for Brain Research, Karl Marx University, Leipzig, DR. Institute of Physiology, Pecs University Medical School, Pecs, Hungary. Translated from Neirofiziologiya, Vol. 19, No. 1, pp. 36–46, January–February, 1987.     

Formation of spontaneous and evoked unit activity in organotypical hippocampal tissue culture

Spontaneous and evoked single unit activity of Ammon's horn and the dentate fascia was investigated by extracellular recording in organotypical explants of neonatal mouse hippocampus during the first 2 weeks in culture. Three main types of spontaneous activity were observed: single, group, and volley. After the fifth to sixth day in culture the neurons were found to respond to stimulation of the entorhinal cortex and dentate fascia. Both short-latency (evidently monosynaptic) and long-latency responses were observed, and they usually varied if the stimulus was repeated. The appearance of evoked unit activity in the explants is considered to be due to maturation of synaptic contacts at axon endings of granule cells and perforant path, the formation of which continues during culture.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 257–266, May–June, 1977.     

Posttetanic changes in hippocampal minimal evoked potentials

Changes in the EEG induced by a single spike were recorded in the hippocampus of an unanesthetized rabbit. Summation of focal electrical activity synchronous with spontaneous single unit discharges at the symmetrical point of contralateral hemisphere revealed no stable potentials which could reflect these changes. In two cases discharges identified as activity of Shaffer's collaterals were recorded in area CA₁. Summation of post-spike changes in evoked activity recorded by the same microelectrode showed stable negative waves with an amplitute of 40–60 µV, which could have been evoked by single spikes. The curve of amplitude of the averaged evoked potentials versus near-threshold current strength stimulating the intrahippocampal pathways was not smooth in most experiments but stepwise in character. It is suggested that the minimal evoked potential corresponding to the first step (amplitude 40–80 µV) reflects a response to stimulation of one fiber. After above-threshold tetanization prolonged posttetanic potentiation of the minimal evoked potentials did not arise in CA₁ in response to stimulation of Shaffer's collaterals. Minimal evoked potentials recorded in area CA₃ in response to stimulation of the dentate fascia showed clear potentiation. The results are in agreement with the hypothesis of the synaptic localization of the mechanisms responsible for prolonged posttetanic potentiation.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 9, No. 2, pp. 124–134, March–April, 1977.     

Frequency potentiation in the neonatal rabbit hippocampus

The formation of properties of frequency potentiation in the entorhinal afferent pathway of the hippocampus was studied in unanesthetized rabbits aged from 1 to 15 days. In areas CA₁ and CA₃ of the dorsal hippocampus in newborn rabbits repetitive (1–20 Hz) electrical stimulation of the perforant path led to an increase in amplitude of the slow wave of the field potential by 20–100% compared with the control and to an increase in the probability of response discharges from the neurons from 0–0.5 in the control to 0.8–1.0 during tetanization. In rabbits aged 2–3 days potentiation was more marked at a frequency of 4–6 Hz, whereas depression of the responses developed rapidly to a higher frequency of stimulation. The frequency optimum of 4–15 Hz was established on the 5th day. Potentiation of the first component of the field potential was observed starting from the 8th–10th day of life. The experimental results show that the property of frequency potentiation in the cortical afferent connections of the hippocampus is found in rabbits actually at birth, and it acquires the adult form at the beginning of the second week of life.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 6, pp. 533–539, November–December, 1979.     

Increase in proportion of hippocampal spine synapses expressing neural cell adhesion molecule NCAM180 following long-term potentiation

Neural recognition molecules such as the neural cell adhesion molecule (NCAM) have been implicated in synaptic plasticity, including long-term potentiation (LTP), sensitization, and learning and memory. The major isoform of NCAM carrying the longest cytoplasmic domain of all NCAM isoforms (NCAM180) is predominantly localized in postsynaptic membranes and postsynaptic densities of hippocampal neurons, with only a proportion of synapses carrying detectable levels of NCAM180. To investigate whether this differential expression of NCAM180 may correlate with distinct states of synaptic activity, LTP was induced by high-frequency stimulation of the perforant path and the percentage of NCAM180 immunopositive spine synapses determined in the outer third of the dentate molecular layer of the dentate gyrus by immunoelectron microscopy. Twenty-four hours following induction of LTP by high-frequency stimulation, the percentage of spine synapses expressing NCAM180 increases from 37% (passive control) to 70%. This increase was inhibited by the noncompetitive N-methyl-D -aspartate receptor antagonist MK801. Following repeated LTP induction at 10 consecutive days with one tetanization each day, 60% of all spine synapses were NCAM180 immunoreactive. Compared to passive control animals, the percentage of NCAM180 expressing synapses in low-frequency stimulated animals decreased from 37% to 28%. Spine synapses in the inner part of the dentate molecular layer not contacted by the afferents of the perforant path did not change the percentage of NCAM180-expressing synapses. The results obtained by the postembedding immunogold staining technique confirmed the difference in NCAM180 expression of spine synapses between passive control and potentiated animals. These observations suggest a role for NCAM180 in synaptic remodeling accompanying LTP. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 359–372, 1998     

Effect of reticular formation on hippocampal neurons (field CA1)

The reaction of field CA₁ hippocampal neurons to stimulation of the reticular formation (RF) with impulses of different frequencies was investigated in experiments on unanesthetized rabbits. The effect of electrical and sensory stimuli was compared and the effect of reticular stimulation on the sensory responses was determined. With an increase in the frequency of RF stimulation, the number of neurons of field CA₁ responding with inhibition of the activity increases. Multimodal neurons of the hippocampus depend on the reticular input to a greater degree than unimodal neurons. Neurons whose activity does not change in response to the effect of sensory stimuli also do not respond to stimulation of the RF. Neurons responding with inhibitory reactions to sensory stimulation show a higher correlation with the effects of RF stimulation than neurons with activation reactions and, especially those with "complex" responses to the effect of sensory stimuli. In a considerable number of hippocampal neurons the responses to sensory stimuli change in the course of 10–15 min after stimulation of the RF. The role of the RF in the organization of the reactions of hippocampal neurons is discussed.Division of Memory Problems, Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oke. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 227–235, May–June, 1971.     

Conduction of excitation in the cat visual system

Unit responses in area 17 of the visual cortex to stimulation of the lateral geniculate body and optic tract were studied in experiments on unanesthetized cats immobilized with D-tubocurarine. Of the neurons tested, 53.6% responded to stimulation of the lateral geniculate body. In 92% of these cells the responses were orthodromic with latent periods of between 2 and 12.5 msec. Most cells responded with latent periods of 2.0–2.5, 3.0–3.5, and 4.0–4.5 msec, corresponding to latent periods of the components of the electropositive wave of the primary response. Antidromic responses to stimulation of the lateral geniculate body were given by 8% of neurons. The difference between the latent periods of responses of the same visual cortical neurons to stimulation of the optic tract and lateral geniculate body was 0.1–1.8 msec, but for most neurons (55.8%) it was 0.5–1 msec. The histograms of response latencies of visual cortical neurons to stimulation of the above-mentioned formations were found to be similar. It is concluded that the optic radiation contains three principal groups of fibers with conduction velocities of 28.5–16.6, 11.7–8.9, and 7.4–6.0 m/sec, respectively.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 6, pp. 589–596, November–December, 1975.     

Pyramidal influences on interneurons of spinal segmentary reflector arcs in cat

Microelectrode discharges of potentials have been realized from segmentary interneurons of the dorsal horn and intermediate nucleus of the spinal cord in cat at the L₆–L₇ level by electrical stimulation of the sensorimotor region of the brain cortex. It has been established that corticifugal influences on segmentary interneurons of the system of the flexor reflex and on neurons activated by high threshold muscle afferents (groups Ib, II, and III), or high threshold cutaneous afferents are predominantly excitatory. Interneurons activated by muscle afferents of group Ia or by the lowest threshold cutaneous fibers are weakly subjected to pyramidal influences. The mean latencies of excitatory postsynaptic potentials (EPSP's) and discharges evoked under the influence of pyramidal volley, for the neurons under study in the system of afferents of the flexor reflex are equal to 11.8±2.6 and 20.1±1.8 msec, respectively; for interneurons, excited only by high threshold muscle afferents, they are equal to 15.5±3.6 and 16.3±2.2 msec, respectively; and for interneurons, excited by high threshold cutaneous fibers they are equal to 11.8±2.6 and 18.3±1.4 msec, respectively. Possible pathways of activating segmentary interneurons from the lateral sensorimotor region of the brain cortex have been discussed.The A. A. Bogomolets Institute of Physiology, Academy of Sciences, Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 17–25, January–February, 1970.     

Excitatory postsynaptic potentials of hippocampal neurons and their extinction during repeated stimulation

A method of detecting "minimal" excitatory postsynaptic potentials (EPSP) in neurons of hippocampal area CA₃ of the unanesthetized rabbit during stimulation of the septo-fimbrial region and the dentate fascia is described. The method consists of presenting a strong (a current of up to 1 mA) conditioning stimulus, inducing a distinct inhibitory postsynaptic potential (IPSP), before a near-threshold (current of 0.03–0.35 mA) testing stimulus. The response to the testing stimulus, develoing after the previous conditioning IPSP, in most cases was purely depolarizing and, judging from the change in the latent period in some cases and the absence of correlation between its amplitude and that of the IPSP, it is a pure EPSP. If the testing stimuli are presented at low enough frequency (intervals of not less than 1 sec) the amplitude of the EPSP evoked by them gradually falls. This decrease exhibits some of the characteristic properties of extinction of behavioral responses (recovery after an interruption, a more rapid decrease during repeated series of stimuli, a slower decrease in amplitude during less frequent stimulation). The amplitude of the IPSP also fell or showed no significant change. The results are evidence in support of the hypothesis that extinction is based on a mechanism of homosynaptic depression.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 3–12, January–February, 1978.     

Quantitative histochemical study of monoamine oxidase in the rabbit hippocampus

The distribution of monoamine oxidase activity in the rabbit hippocampus was studied by quantitative histochemical analysis. The presence of monoamine oxidase activity was found in str. lacunosum-moleculare of areas CA₁ and CA₂ of the hippocampus and in str. moleculare of the dentate fascia. A strong positive reaction was found in layers containing many myelinated fibers (the alveus and Shaffer's collaterals). However, when the reaction was carried out without substrate, considerable deposition of diformazan was observed in these layers. These observations and the ability of myelin to reduce nitro-BT spontaneously, described in the literature, suggest that the positive Glenner's reaction in the alveus and Shaffer's collaterals is not specific. The deposition of diformazan in the layer of pyramidal and granular neurons likewise is nonspecific, as is confirmed by the results of experiments with preincubation in iproniazid and with incubation without substrate.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 9, No. 2, pp. 135–141, March–April, 1977.     

Switching off of granule cells after intrahippocampal injection of colchicine eliciting the disappearance of evoked potentials and a deterioration in the development of a conditioned avoidance reflex in rats

The aim of the work was to study the involvement in the process of learning of synapses between the perforant path and granular cells of the hippocampal dentate fascia for formation of conditioned reflex (CR) of active avoidance elaborated in response to electrostimulation of the perforant path. For solving this task, the property was used of the neurotoxin colchicine in a definite concentration to abolish selectively the fascia cells. One month after colchicine injection, destruction of granular cells was observed, disappearance of the dentate fascia focal evoked potentials and considerable deterioration of elaboration of active avoidance CRs. Administration of saline solution in control experiments did not elicit such changes. The obtained results show the decisive role of entorhinal input to the hippocampus in the used learning model.     

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.     

The excitation wave returning to the hippocampus through the entorhinal cortex can reactivate the populations of "trained" CA1 neurons during deep sleep

It is suggested that the information about a new stimulus from the neocortex is transferred to the hippocampus and forms there a transient trace in the form of a distributed pattern of modified synapses. During sleep, the neuronal populations which store this trace are reactivated and return to the neocortex the information necessary for consolidation of the permanent memory trace. A possible mechanism of the reactivation of the "learned" hippocampal neurons during memory consolidation is the reverberation of excitation in the neuronal circuits connecting the hippocampus and the entorhinal cortex. In rats, we recorded responses in hippocampal field CA1 to stimulation of the Schaffer collaterals with potentiated synapses during wakefulness and sleep. We showed that in the periods of deep sleep, after the discharge of CA1 neurons, the wave of excitation passes through the entorhinal cortex and via the perforant path fibers enters the hippocampus and the dentate gyrus, causing in the latter the discharge of neurons. The repeated discharge of the CA1 neurons develops as the result of interaction of the early wave which is returned directly via the perforant path fibers and the late wave which is returned via the Schaffer collaterals, but not through the dentate gyrus and hippocampal field CA3 (trisynaptic pathway), but, probably, through the field CA2.     

Corticofugal influences on postsynaptic processes in rubrospinal neurons in the cat brain

Composite and unitary EPSPs of red nucleus neurons evoked by stimulation of the sensomotor and association parietal cortex and nucleus interpositus of the cerebellum were studied in acute experiments on cats anesthetized with pentobarbital. A monosynaptic connection was shown to exist between not only the sensomotor, but also the association cortex, and rubrospinal neurons, in which unitary EPSPs appeared during stimulation of the association cortex after a latent period of 1.5–2.7 msec, with a peak rise time of 1.1–3.1 msec and an amplitude of 0.22–0.65 mV. Analysis of the temporal characteristics of the unitary EPSP suggested that synapses formed by fibers from the association cortex occupy a position nearer the soma than synapses formed by axons of sensomotor cortical cells.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 67–74, January–February, 1984.     

Possible pathogenic mechanisms on trimethyltin-induced lesions in the hippocampus of adult and neonatal rats

The extent of trimethyltin (TMT) induced lesions in the rat hippocampal formation was reviewed. Adult rats were treated with a single dose of 6.0 mg TMT/kg body wt and were sacrificed between 3–60 d following exposure. In the hippocampal formation, the granule cells of fascia dentata showed early changes, which subsided considerably at a later time of the intoxication. On the other hand, destruction of the pyramidal neurons in the Ammon’s horn became more pronounced with time, resulting in an extensive destruction of this structure. It is interesting to note that the CA₃ neurons in the septal portion of the Ammon’s horn were more vulnerable than those located more temporally, whereas the reverse pattern was observed for the dentate granule cells as well as for the CA_1,2 neurons of the Ammon’s horn. Special stain for zinc (Timm’s method) also revealed a progressive depletion of zinc in the mossy fibers. When neonatal rats were treated at various times with a single injection of TMT, rapid and progressive destruction of the Ammon’s horn was observed in animals injected between postnatal day (PND) 5–15. The progression of neuronal involvement was CA_3b →CA_{3a, b} →CA_3(a,b,c)→CA₃+CA₂→entire Ammon’s horn (CA_1,2,3). This pattern of pathological lesion was in good concert with morphological development and functional maturity of the hippocampal formation. Destruction of the Ammon’s horn neurons was proposed to be the result of hyperexcitation of the dentate granule neurons under the influence of TMT. Other possible mechanisms are also discussed. Author to whom all correspondence and reprint requests should be addressed.     

Effect of electrical stimulation of the septum on subicular neurons with different kinds of spontaneous activity in rabbits

The effect of electrical stimulation of the medial nucleus of the septum and of hippocampal area CA₁ on subicular neurons with three different types of spontaneous activity (with theta-modulation, with delta-modulation and complex spikes, and with irregular single-spike activity) was studied in unanesthetized rabbits by extracellular recording of unit activity. Cells with theta-activity were found to respond in a distinctive functional manner to stimulation of the medial nucleus of the septum: Their reactivity was higher but latent periods of their responses were much shorter than those of cells with delta-activity and irregular discharges. Stability of modulation of theta-cell activity increased both during and after stimulation of the medial septal nucleus. Electrical stimulation of hippocampal area CA₁, on the other hand, led to disappearance of rhythm modulation. Average response latencies of all three types of cells to stimulation of area CA₁ were equal. The results indicate special properties of the septal input to subicular cells with theta-modulation.Institute of Biological Physics, Academy of Sciences of the USSR, Pushchino-on-Oka. Translated from Neirofiziologiya, Vol. 17, No. 3, pp. 326–333, May–June, 1985.     

Responses of medial geniculate body neurons to auditory cortical stimulation

Extracellular and intracellular unit responses of thepars principalis of the medial geniculate body to stimulation of the first (AI), second (AII), and third (AIII) auditory cortical areas were studied in cats immobilized with D-tubocurarine. In response to auditory cortical stimulation both antidromic (45–50%) and orthodromic (50–55%) responses occurred in the geniculate neurons. The latent period of the antidromic responses was 0.3–2.5 msec and of the orthodromic 2.0–18.0 msec. Late responses had a latent period of 30–200 msec. Of all neurons responding antidromically to stimulation of AII, 63% responded antidromically to stimulation of AI also, confirming the hypothesis that many of the same neurons of the medial geniculate body have projections into both auditory areas. Orthodromic responses of geniculate neurons consisted either of 1 or 2 spikes or of volleys of 8–12 spikes with a frequency of 300–600/sec. It is suggested that the volleys of spikes were discharges of inhibitory neurons. Intracellular responses were recorded in the form of antidromic spikes, EPSPs, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP, and primary IPSP. Over 50% of primary IPSP had a latent period of 2.0–4.0 msec. It is suggested that they arose through the participation of inhibitory interneurons located in the medial geniculate body.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 5–12, January–February, 1976.