Studies on substances that induce long-term potentiation in guinea-pig hippocampal slices

The role of volume transmission of adaptogenic signals in forming the adaptive reactions of the brain

This review presents published data and results from our own studies providing evidence for the important role of volume, non-synaptic transmission of adaptogenic signals in the mechanisms forming the long-term adaptive reactions of the brain. The importance of chemical factors involved in volume transmission and secreted by cells in this process is discussed. Special attention is paid to peptides-possible mediators of volume transmission of adaptive-type signals. Evidence has been obtained for the presence of peptides and their role in the mechanism of development of adaptive brain reactions of different origins, especially those arising in response to tetanic stimulation of neurons and transient hypoxic stress. An original method for testing for the effects of neuromodulator factors released by cells in donor slices subjected to these treatments on recipient slices was used to show that these factors had pronounced effects on synaptic transmission and could induce long-term potentiation of synaptic transmission, protecting against functional derangements due to prolonged anoxia. Blockade of protein synthesis in donor slices subjected to adaptogenic treatments suppressed the appearance of these effects. The review concludes with a discussion of the mechanisms of interaction of the synaptic and volume transmission of signals involved in forming long-term adaptive brain reactions.

Synaptic potentiation induced by a protein factor in cultured cerebral neurons

1. We reported in a previous paper that long-lasting enhancement of spontaneous excitatory post synaptic currents (SEPSCs) in cultured chick cerebral neurons was induced by exposure to a conditioned medium (CM) prepared by Mg(2+)-free treatment of neurons. This suggested that the CM contained a diffusible factor(s) for the potentiation. 2. In this paper, the factor(s) was shown to be a protein(s) by heat and trypsin treatment of the CM. 3. The factor induced the potentiation within 5 min, but it was not required for maintenance of increased SEPSCs. 4. The factors in CM induced the potentiation without protein synthesis. 5. Protein synthesis at least in postsynaptic neurons, was indispensable to induce the potentiation by the Mg(2+)-free condition.

Endogenous regulators of long-term potentiation and depression in rat olfactory cortex slices

A perfusate collected from tetanized donor slices of rat olfactory cortex was separated by ultrafiltration into fractions containing substances with molecular weights of less than and greater than 50 kDal. Each fraction was separately tested on recipient slices. The fraction with substances of greater than 50 kDal elicited long-term depression of focal potentials in recipient slices. The fraction with substances of less than 50 kDal mainly induced activation. The chemical nature of the factors released during tetanization was studied by treating the high-molecular-weight fraction with immobilized trypsin; after proteolytic treatment this fraction produced a qualitatively different response in recipient slices, suggesting that the active factors were polypeptides. These data indicate that donor slice cells release a set of neurohumoral substances, probably polypeptides, during tetanization, which are involved in the modulation of synaptic plasticity.

Epileptiform activity increases the level of nerve growth factor in cerebrospinal fluid of epileptic patients and in hippocampal neurons in tissue culture

A neurite-stimulating effect was induced by both the cerebrospinal fluid of epileptic patients and the media of co-cultures of rat hippocampus and chick embryo sensory neurons after veratridine treatment. Cerebrospinal fluid from patients with epilepsy stimulated extensive neurite growth in the organotypic culture of chick embryo dorsal root ganglia. The anti-nerve growth factor antibody partly blocked the neurite-stimulating effect of the cerebrospinal fluid. Co-cultures of newborn rat hippocampus and chick embryo dorsal root ganglia were used to investigate the involvement of neurotrophic factors into the processes which are activated by neuronal activity. The data obtained suggest that veratridine, an epileptiform agent, gave rise to an elevation in the level of neurotrophic factors in the culture media and neurite outgrowth of dorsal root ganglia sensory neurons. The anti-nerve growth factor antibody was shown to block the neurite-stimulating effect mediated by veratridine. These results indicate that the epileptiform activity of neurons evokes the expression of neurotrophins.

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

Purification and characterization of a novel 70-kDa brain protein associated with seizure activities

Using ion exchange HPLC and ammonium sulfate precipitation, we have purified a 70-kDa protein (P70) specific to the cobalt-induced epileptogenic cortex of rat cerebrum and determined certain of its biochemical properties. P70 has a similar isoelectric point (pI; 4.6-4.8), amino acid composition and N-terminal amino acid sequence to rat serum albumin (RSA). Intracortical application of purified P70 to the motor area of normal rat cerebrum induces both ECoG seizure discharges and behavioral seizures. The data suggest that P70 is a novel albumin-like protein linked to the generation of seizure activities. However, it can be clearly distinguished from RSA, since it is able to produce seizure, is a glycoprotein and can be readily separated from RSA by 2-dimensional electrophoresis.

Influence of perfusates of tetanized donor slices on the induction of long-term potentiation in recipient slices

A perfusate collected from donor slices of the olfactory cortex of the brain of rats during tetanization of the lateral olfactory tract (100 imp/sec, 30 sec) induced prolonged changes in the amplitudes of various components of the focal potentials (FP) in recipient slices. When the perfusates are divided in relation to the reactions arising in the donor slices into three types, "potentiated," "depressive," and "nonpotentiated," the reactions of the recipient slices prove to be diverse. The potentiated perfusate induced the development of depression. The depressive induced potentiation. The reactions of the recipient slices to the nonpotentiated perfusate were indeterminate. The data obtained suggest the secretion of active substances during the electrical tetanization of donor slices. These factors are capable of inducing reactions in the recipient slices which the donor slices had "experienced."

Impairment of long-term potentiation in rats fed with vitamin E-deficient diet

Our previous results indicate that alpha-tocopherol induces a slowly developing long-term potentiation (LTP) of excitatory postsynaptic potentials (EPSP) in hippocampal slices. In the present study on hippocampal slices obtained from rats fed with vitamin E-deficient diet for 3 months, a tetanic stimulation of the stratum radiatum or applied alpha-tocopherol phosphate failed to induce LTP of CA1 neuronal EPSPs. However, LTP could be induced in rats fed on a matching control diet. These results indicate that vitamin E-deficiency leads to an impairment in LTP induction.

Acetylcholinesterase induces long-term potentiation in CA1 pyramidal cells by a mechanism dependent on metabotropic glutamate receptors

Acetylcholinesterase (AChE) has additional functions within the central nervous system that are unrelated to cholinergic transmission. In the cerebellar cortex AChE has been shown to potentiate synaptic responses evoked by excitatory amino acids. Because AChE is also secreted from the terminal regions of cholinergic nerves within the hippocampus, this study investigates the actions of AChE on synaptic transmission in guinea pig hippocampal slices. Application of AChE produced a long-lasting potentiation of both the field epsp and the resulting population spike evoked by stimulation of the Schaffer/commissural-CA1 pathway. This effect was independent of any cholinergic receptor stimulation since it persisted in the presence of the cholinergic antagonists atropine and mecamylamine. Furthermore, the effect was not mimicked by butyrylcholinesterase despite its cholinolytic activity. However, the effect of AChE was dependent on metabotropic glutamate receptor stimulation since it was prevented by the metabotropic receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine. Perfusion with AChE therefore induces a long-lasting potentiation of hippocampal synaptic transmission which is reminiscent of the classical LTP produced by tetanic stimulation. Consequently the secreted protein could play an important role in the molecular mechanisms of learning and memory in vertebrates.

Effects of brain tissue hydrolysate on synaptic transmission in the hippocampus

A mix of peptides and amino acids obtained from porcine brain tissue (Cerebrolysin) has been shown to affect passive avoidance behavior in neonatal rats. To identify the active components and mechanisms of action, Cerebrolysin effects were studied in in vitro hippocampal slices. Cerebrolysin induced dose-dependent suppression followed by a small rebound increase of synaptic responses in the CA1 but not dentate gyrus neurons. These actions may be due to peptides present in Cerebrolysin and may contribute to its reported behavioural effects.

Onset and stabilization of NMDA receptor-dependent hippocampal long-term potentiation

This article discusses recent data concerning the temporal development of N-methyl-D-aspartate (NMDA) receptor-dependent potentiation in the hippocampus. It argues against a mechanistic subdivision of NMDA receptor-dependent potentiation into an early short-term potentiation (STP) and a slowly developing long-term potentiation (LTP). Thus, the article proposes that LTP starts a few seconds after the induction event, that it is fully developed within a minute, and that its subsequent stabilization is controlled by the degree of NMDA receptor activation, and associated increase of calcium concentration in the spine, during the induction event. It is suggested that most biochemical interventions that have been reported to interfere with the LTP process, for example application of protein kinase inhibitors, might have acted through an impairment of the induction mechanism rather than through an impairment of specific stabilization or maintenance mechanisms.

Induction of hippocampal long-term potentiation by alpha-tocopherol

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is thought to be one of the cellular mechanisms underlying learning and memory. Recent evidence in literature suggests the involvement of free radicals in impeding LTP maintenance. In the present study, the effects of alpha-tocopherol, a major lipid-soluble antioxidant which could prevent lipid peroxidation, were examined on the excitatory post-synaptic potentials (EPSPs) of CA1 neurons in guinea pig hippocampal slices. alpha-Tocopherol phosphate disodium salt (0.2 mM applied for 5 min) induced a slowly developing long-lasting increase of the EPSP, without significantly changing the membrane potential, the input resistance and the ability to generate action potentials. No significant changes in the fast and the slow inhibitory post-synaptic potentials (IPSPs) were observed during the alpha-tocopherol-induced LTP of the EPSP. 2-Amino-5-phosphonovalerate (APV) did not block the induction of this LTP. L-Ascorbic acid (Na salt, 3-10 mM), a water-soluble antioxidant, failed to produce any significant enhancement in the EPSP. These results indicate that alpha-tocopherol can induce LTP of the EPSP in guinea pig hippocampal CA1 neurons. The activation of N-methyl-D-aspartate (NMDA) receptors does not appear to be necessary for this action of alpha-tocopherol. Whether the LTP-inducing action of this agent is related to its antioxidant property is unclear.

Biochemical correlates of long-term potentiation in hippocampal synapses

Figure 2 summarizes biochemical events which are currently known or hypothesized to participate in LTP induction/maintenance. Current evidence strongly suggests that postsynaptic Ca2+, both entered from the outside of cells and released from intracellular stores, is the initial key substance for the induction of LTP. A rise of [Ca2+]i triggers a variety of enzymatic reactions and initiates the enhancement of synaptic transmission. This first step may be achieved by direct/indirect phosphorylations of protein molecules in postsynaptic receptors/ion channels. This would result in an increase in receptor sensitivity. An immediate increase in the number of available postsynaptic receptors by modifications of spine morphology is another candidate. Such modifications may be accomplished by cytoskeleton rearrangements or changes in extracellular environments. A change in spine structure may also cause an increase in spine neck conductance. Although it is unknown to what extent the increase in [Ca2+]i affects cellular chemistry, Ca2+ probably also directly/indirectly stimulates cascades which exert effects more slowly. A delayed increase in metabotropic receptor sensitivity may occur. New synthesis of protein molecules may be involved in late periods of LTP by replacing turnovered molecules and/or by supplying new materials. Some of these chains of biochemical events may also apply to presynaptic terminals, although the existence of retrograde messenger substances must still be confirmed. In addition, interactions between different protein kinases and second messengers appear to occur to bring about final effects.

Actions of somatostatin on GABA-ergic synaptic transmission in the CA1 area of the hippocampus

Somatostatin and gamma-aminobutyric acid (GABA) are co-localized in some neurons in the CA1 area of the hippocampus. Since it is possible that the peptide and the amino acid are co-released, the interactions between the actions of somatostatin and GABA-ergic inhibitory post-synaptic potentials (IPSPs) in the CA1 pyramidal neurons of guinea pig hippocampal slices have been investigated. Somatostatin (2 microM) induced a hyperpolarization of the CA1 neurons associated with a reduction in the input resistance of the cells. These effects were not blocked by picrotoxinin (20 microM) or phaclofen (1 mM). Chelation of intracellular Ca2+ (Ca2+i) with BAPTA or the inhibition of protein kinase C (PKC) with sphingosine (30 microM) had no significant effects on the hyperpolarizing actions of somatostatin. The peptide suppressed the GABAA receptor-mediated fast IPSPs and the GABAB receptor-mediated slow IPSPs, but had no significant effect on the excitatory post-synaptic potentials (EPSPs). Somatostatin-induced depression of the IPSPs was not due to the hyperpolarization of the neurons. Baclofen (20 microM) suppressed the EPSP, as well as the fast and the slow IPSPs. The hyperpolarization of the CA1 neurons caused by somatostatin was greatly reduced in the presence of baclofen, an effect that was not due to the hyperpolarization of the cell by baclofen. The presence of QX-314 in the CA1 neurons, which suppressed the Na+ spikes and the slow IPSPs, prevented the hyperpolarization of the neurons by somatostatin and baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of hippocampal long-term potentiation by saccharin

Population spikes, population excitatory postsynaptic potentials and intracellular excitatory postsynaptic potentials were recorded in the CA1 area of guinea-pig hippocampal slices in response to low frequency stimulation of the stratum radiatum. Tetanic stimulation of the same afferents during an application of saccharin (10 mM, 10 min) failed to induced a long-term potentiation of the population spike, population excitatory postsynaptic potential and intracellularly recorded excitatory postsynaptic potential. A post-tetanic application of saccharin did not prevent long-term potentiation of the population spike from developing. Saccharin did not change the input resistance, the membrane potential or the ability to induce action potentials in the CA1 neurons. The slope of the intracellular excitatory postsynaptic potentials recorded in normal medium, in normal medium containing 2-amino-5-phosphonovalerate, or in Mg(2+)-free medium containing 6-cyano-7-nitroquinoxaline-2,3-dione was not significantly altered by saccharin. The depolarizations of CAI neurons produced by superfusion of N-methyl-D-aspartate or during a brief tetanic stimulation of the stratum radiation were also not altered by the drug. It therefore appears that saccharin blocks the induction of long-term potentiation by a mechanism that does not involve a blockade of N-methyl-D-aspartate receptors. Application of fluid samples collected from rabbit neocortical surface during a tetanic stimulation of the neocortex caused neurite growth in PC-12 cells, suggesting that growth-related substances were present in the collected samples. If these samples were superfused onto hippocampal slices, long-term potentiation developed. If however, the samples were co-applied with saccharin, neither neurite growth in PC-12 cells nor long-term potentiation in hippocampal slices was observed, raising the possibility that growth-related substances are involved in long-term potentiation.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.