The biochemistry of memory: a new and specific hypothesis

The neurobiology of learning and memory

Study of the neurobiology of learning and memory is in a most exciting phase. Behavioral studies in animals are characterizing the categories and properties of learning and memory; essential memory trace circuits in the brain are being defined and localized in mammalian models; work on human memory and the brain is identifying neuronal systems involved in memory; the neuronal, neurochemical, molecular, and biophysical substrates of memory are beginning to be understood in both invertebrate and vertebrate systems; and theoretical and mathematical analysis of basic associative learning and of neuronal networks in proceeding apace. Likely applications of this new understanding of the neural bases of learning and memory range from education to the treatment of learning disabilities to the design of new artificial intelligence systems.

Neurotransmitters in the cerebral cortex

This article surveys the conventional neurotransmitters and modulatory neuropeptides that are found in the cerebral cortex and attempts to place them into the perspective of both intracortical circuitry and cortical disease. The distribution of these substances is related, where possible, to particular types of cortical neuron or to afferent or efferent fibers. Their physiological actions, where known, on cortical neurons are surveyed, and their potential roles in disease states such as the dementias, epilepsy, and stroke are assessed. Conventional transmitters that occur in afferent fibers to the cortex from brain-stem and basal forebrain sites are: serotonin, noradrenaline, dopamine, and acetylcholine. All of these except dopamine are distributed to all cortical areas: dopamine is distributed to frontal and cingulate areas only. The transmitter in thalamic afferent systems is unknown. Gamma aminobutyric acid (GABA) is the transmitter used by the majority of cortical interneurons and has a profound effect upon the shaping of receptive field properties. The vast majority of the known cortical peptides are found in GABAergic neurons, and the possibility exists that they may act as trophic substances for other neurons. Levels of certain neuropeptides decline in cases of dementia of cortical origin. Acetylcholine is the only other known transmitter of cortical neurons. It, too, is contained in neurons that also contain a neuropeptide. The transmitter(s) used by excitatory cortical interneurons and by the efferent pyramidal cells is unknown, but it may be glutamate or aspartate. It is possible that excitotoxins released in anoxic disease of the cortex may produce damage by acting on receptors for these or related transmitter agents.

The long and the short of long-term memory--a molecular framework

A single learning event initiates several memory processes with different time courses of retention. While short term memory involves covalent modification of pre-existing proteins, the finding that long-term memory requires the expression, during learning, of additional genes, makes it possible to analyse in molecular terms the induction and retention of long-term memory.

Enhanced long-term potentiation induced in rat dentate gyrus by coactivation of septal and entorhinal inputs: temporal constraints

High-frequency activation of the entorhinal cortical (perforant path) inputs to the rat dentate gyrus can produce a long-term potentiation (LTP) of perforant path-dentate evoked responses. In this paper we examined the enhanced LTP effects produced by coactivation of septal and entorhinal inputs to the dentate gyrus. Trains of electrical stimulation applied to the two inputs were found to increase the magnitude of LTP to a level above that produced by trains applied to the perforant path alone. The largest LTP increments were observed when the septal trains were applied less than 100 ms prior to the perforant path trains. If the septal trains followed the perforant path trains there was no additional increment in LTP magnitude, regardless of the intertrain interval. The relationship of this cooperativity effect to mechanisms of associative learning is discussed.

Protein kinase C in human brain and its inhibition by calmodulin

Protein kinase C is a ubiquitous enzyme which is especially concentrated in brain tissue and which has been reported to have a key role in the regulation of neuronal activity. Substrates for this kinase were studied in frozen postmortem human cortex. Two major protein substrates with mol. wts. of 86,000 and 67,000 Da were found in the cytosol the cytosol fraction. Calmodulin was found to inhibit phosphorylation of these two proteins.

Involvement of mitochondria in the age-dependent decrease in calcium uptake of rat brain synaptosomes

Calcium uptake in rat brain synaptosomes decreases during ageing. The possible involvement of mitochondria in altered calcium homeostasis has been investigated. Mitochondria isolated from old rat brain showed decreased calcium uptake rates. Since neither the mitochondrial membrane potential nor the delta pCa decreases with age, it was concluded that variations in the driving force for calcium uptake were not the cause for impaired calcium transport in mitochondria from aged rat brain. The steady state calcium distribution in isolated aged rat brain mitochondria was achieved at higher extramitochondrial calcium concentrations than that of adults. Studying the effects of the selective release of calcium from the mitochondrial pool by the addition of an uncoupler to 45Ca loaded synaptosomes incubated in high-potassium media, it was found that the intrasynaptic mitochondrial pool and the intra/extramitochondrial 45Ca distribution also decreased considerably in 24-month-old rats. Steady state fluorescence anisotropy (rs) of diphenylhexatriene-labelled mitoplasts from 'free' brain mitochondria increased with ageing. However, since no changes in rs from synaptosomal mitochondria were found in 24-month-old rats, it is suggested that alterations in lipid dynamics are not involved in the impaired calcium uptake observed in brain mitochondria from aged rats. The implications of these findings in the calcium homeostasis of brain endings are discussed.

Mechanisms of memory

Recent studies of animals with complex nervous systems, including humans and other primates, have improved our understanding of how the brain accomplishes learning and memory. Major themes of recent work include the locus of memory storage, the taxonomy of memory, the distinction between declarative and procedural knowledge, and the question of how memory changes with time, that is, the concepts of forgetting and consolidation. An important recent advance is the development of an animal model of human amnesia in the monkey. The animal model, together with newly available neuropathological information from a well-studied human patient, has permitted the identification of brain structures and connections involved in memory functions.

Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events

Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity.

Changes in free calcium ion concentration recorded inside hippocampal pyramidal cells in situ

In rats under urethane or pentobarbitone anesthesia, Ca2+ -sensitive microelectrodes were inserted into CA3 and CA1 hippocampal cells. In 23 neurons with a mean resting membrane potential (Vm) of -56.9 mV, the Ca potential (VCa) fell below Vm by an average of -22.1 mV (S.D. +/- 19.1 mV), indicating a mean intracellular free Ca2+ concentration ([Ca]i) of 9.7 microM (S.D. 14.9 microM). In spite of their better and more stable Vm (mean -67.1 mV), unresponsive cells (probably neuroglia) had a higher and more variable [Ca]i (mean 37.0 +/- 51.2 microM). In 21 of the neurons, repetitive stimulation of the fimbria--at 5-20 Hz for 30s, which is sufficient to elicit bursts of population spikes--evoked substantial increases in [Ca]i: the mean increase observed during or just after 29 such tetani was +27.1 +/- 54.5 microM. Typically [Ca]i reached a peak near the end of the tetanus and then decayed with a half-time of 5-10 s, though not necessarily to the initial level. In 7 cells, a large increase in [Ca] (mean +239 +/- 367 microM) appeared as a late event, 20-30 s after the end of the tetanus. In 5 cells, [Ca]i could thus be raised transiently to 10(-4) M or higher. All these increases in [Ca]i are far greater than can be evoked by tetanic activation in spinal motoneurons; their possible significance for long term potentiation or cell necrosis in the hippocampus is discussed.

Kindling enhances the stimulation of inositol phospholipid hydrolysis elicited by ibotenic acid in rat hippocampal slices

The increment of inositol phospholipid hydrolysis elicited by ibotenic acid (IBO) is greater in hippocampal slices prepared from brain of rats receiving single or repeated hippocampal electrical stimulation or electrically induced amygdala kindling. In the latter group of rats, a potentiation of IBO stimulation of inositol phospholipid hydrolysis was associated with stage 3-4 of kindling according to Racine's scale. This increment returned to normal within one month after withdrawal from electrical stimulations. In both control and stimulated animals, 2-amino-4-phosphonobutyric acid antagonized the increment of inositol phospholipid metabolism elicited by IBO. The stimulation of inositol phospholipid hydrolysis elicited by carbamylcholine and norepinephrine was virtually unaffected by amygdala kindling.

Excitatory amino acid-releasing and cholinergic neurones in Alzheimer's disease

Brains of normal controls and patients with primary degenerative dementia were investigated for choline acetyltransferase (ChAT) activity in the frontal, temporal and parietal cortex, hippocampus, amygdala and thalamus. A few patients with Alzheimer's disease were unusual as the cholinergic marker was unaffected, except in the amygdala. Other patients with dementia and undiagnosed neurodegenerative disorder had elevated cortical ChAT activity. The interpretations offered are: (a) the syndrome of dementia and Alzheimer pathologic change precedes significant loss of cortical cholinergic innervation; (b) denervation in dementia can occur early in olfactory areas, exemplified here by the amygdala; (c) dementia is associated with the loss of non-cholinergic structure. An indication of structures involved was given by loss of a marker of excitatory amino acid-releasing neurones.

Lithium mechanisms in bipolar illness and altered intracellular calcium functions

Calcium functions as an intracellular second messenger, transducing a variety of hormonal, electrical, and mechanical stimuli by activating a wide range of enzymes. There is evidence, ranging from definitive to strongly presumptive in quality, that lithium can alter many calcium-dependent processes. The list of enzyme systems dependent on calcium and altered by lithium includes adenylate cyclase, glycogen synthase, inositol-1-phosphatase, and calcium adenosine triphosphatase (ATPase). Lithium also interferes with calcium regulation of receptor sensitivity, parathyroid hormone release, microtubule structure, and other systems. All of the neural mechanisms that are hypothesized to explain various psychopharmacological treatments of bipolar illness involve functions that are critically controlled by calcium. Moreover, in every instance, a known action of lithium on calcium function could account for lithium's therapeutic or prophylactic results. From these considerations the dual hypotheses emerge that bipolar illnesses arise from disorders in calcium-regulated functions and that lithium acts by reversing or counterbalancing the effects of these calcium dysfunctions.

Proteolysis of the calcium-dependent protease inhibitor by myocardial calcium-dependent protease

Bovine heart peak II calcium-dependent protease was capable of hydrolyzing its specific inhibitor protein at high molar ratios of protease to inhibitor. The proteolysis was inhibited by leupeptin and required millimolar calcium. Thus, it appeared to be attributable to the calcium-dependent protease and not to possible contaminating proteases in the purified preparations of inhibitor or calcium-dependent protease. Incubation of the purified inhibitor with the calcium-dependent protease produced a discrete pattern of inhibitor fragments on Western blots developed with an inhibitor-specific monoclonal antibody. Traces of similar or identical lower molecular weight immunoreactive material could be observed in Western blots of bovine heart extracts, and the immunoreactivity present as these lower molecular weight forms could be increased by incubation of the extracts with calcium ion. These results suggest that the inhibitor can be proteolyzed to low molecular weight forms which can be detected in cardiac tissue extracts, and that calcium-dependent protease(s) may be responsible for this phenomenon.

Regional distribution of amino acid transmitters in postmortem brains of presenile and senile dementia of Alzheimer type

We measured the concentrations of glutamate, aspartate, and gamma-aminobutyrate (GABA) in cortical and subcortical areas by an enzymatic fluorometric method in 9 patients with histologically verified Alzheimer's disease and senile dementia of the Alzheimer type (AD/SDAT), and in 10 age-matched normal control subjects. When compared with controls, the concentration of glutamate in cortical and subcortical areas of AD/SDAT brains was significantly reduced. The concentrations of aspartate and GABA were also reduced in several areas, but the magnitude of reduction was less than that of glutamate. This apparent reduction of glutamate in postmortem brains might have a bearing on pathophysiological mechanisms in AD/SDAT.

Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5

Recent work has shown that the hippocampus contains a class of receptors for the excitatory amino acid glutamate that are activated by N-methyl-D-aspartate (NMDA) and that exhibit a peculiar dependency on membrane voltage in becoming active only on depolarization. Blockade of these sites with the drug aminophosphonovaleric acid (AP5) does not detectably affect synaptic transmission in the hippocampus, but prevents the induction of hippocampal long-term potentiation (LTP) following brief high-frequency stimulation. We now report that chronic intraventricular infusion of D,L-AP5 causes a selective impairment of place learning, which is highly sensitive to hippocampal damage, without affecting visual discrimination learning, which is not. The L-isomer of AP5 did not produce behavioural effects. AP5 treatment also suppressed LTP in vivo. These results suggest that NMDA receptors are involved in spatial learning, and add support to the hypothesis that LTP is involved in some, but not all, forms of learning.

Translocation of protein kinase C activity may mediate hippocampal long-term potentiation

Protein kinase C activity in rat hippocampal membranes and cytosol was determined 1 minute and 1 hour after induction of the synaptic plasticity of long-term potentiation. At 1 hour after long-term potentiation, but not at 1 minute, protein kinase C activity was increased twofold in membranes and decreased proportionately in cytosol, suggesting translocation of the activity. This time-dependent redistribution of enzyme activity was directly related to the persistence of synaptic plasticity, suggesting a novel mechanism regulating the strength of synaptic transmission.

Glutamate and the pathophysiology of hypoxic--ischemic brain damage

Information obtained over the past 25 years indicates that the amino acid glutamate functions as a fast excitatory transmitter in the mammalian brain. Studies completed during the last 15 years have also demonstrated that glutamate is a powerful neurotoxin, capable of killing neurons in the central nervous system when its extracellular concentration is sufficiently high. Recent experiments in a variety of preparations have shown that either blockade of synaptic transmission or the specific antagonism of postsynaptic glutamate receptors greatly diminishes the sensitivity of central neurons to hypoxia and ischemia. These experiments suggest that glutamate plays a key role in ischemic brain damage, and that drugs which decrease the accumulation of glutamate or block its postsynaptic effects may be a rational therapy for stroke.

Coupling of inositol phospholipid metabolism with excitatory amino acid recognition sites in rat hippocampus

Ibotenate, a rigid structural analogue of glutamate, markedly enhances the hydrolysis of membrane inositol phospholipids, as reflected by the stimulation of [3H]inositol monophosphate formation in rat hippocampal slices prelabeled with [3H]inositol and treated with Li+. Quisqualate, homocysteate, L-glutamate, and L-aspartate also induce a significant (albeit weaker) increase in [3H]inositol monophosphate formation, whereas N-methyl-D-aspartate, kainate, quinolinate, and N-acetylaspartylglutamate are inactive. The increase in [3H]inositol monophosphate formation elicited by the above-mentioned excitatory amino acids is potently and selectively antagonized by DL-2-amino-4-phosphonobutyric acid, a dicarboxylic amino acid receptor antagonist. These results suggest that, in the hippocampus, a class of dicarboxylic amino acid recognition sites is coupled with phospholipase C, the enzyme that catalyzes the hydrolysis of membrane inositol phospholipids.

Modification of excitation and inhibition evoked in dentate gyrus by perforant path stimulation: effects of aminophylline and kindling

Rats were implanted with chronic electrodes to stimulate the perforant path and record the elicited monosynaptic evoked potentials from the dentate gyrus of the hippocampal formation. Dentate responses were examined in awake and anesthetized animals after exposure to saline and aminophylline (100 mg/kg, IP). In the awake animal, aminophylline treatment did not significantly alter the threshold or elicited amplitude of either the excitatory post-synaptic potential (EPSP) or the population spike (PS). Aminophylline pretreatment markedly enhanced the length and severity of elicited seizures from hippocampal (dentate gyrus) or perforant pathway stimulation. After daily perforant pathway stimulations which established "kindled" seizures, aminophylline significantly increased only the amplitude of the evoked PS in awake animals. In animals anesthetized with chloropent, aminophylline increased significantly before kindling the amplitude of both the EPSP and PS without effecting thresholds for each. After perforant pathway kindling, only the PS amplitude was increased significantly by aminophylline. Inhibition, thought to be from GABA-mediated recurrent collaterals, was found to be increased rather than decreased by kindling. Further, aminophylline treatment did not result in reduction of this inhibition before or after kindling. These data suggest that at this dose of aminophylline neither enhanced transmitter release at this synapse as measured by the amplitude of the EPSP, nor reduced recurrent collateral inhibition significantly contributed to the prolongation of elicited seizure afterdischarge. The increase in PS amplitude reflecting an increased number of granule cells excited to discharge with perforant path stimulation after aminophylline was noted in awake animals but was greatest in the anesthetized animals. Although the number of granule cells excited to discharge was increased by aminophylline, the small increase in amplitude seen compared to the effects of other neurotoxins on this synapse makes this an unlikely explanation for the profound increased seizure response seen after aminophylline.

Melanocortins, neural plasticity and aging

Peptides derived from ACHT and alpha-MSH are known to exert trophic influences on peripheral and central nervous structures. Age-related brain diseases may in part be related to loss of neural plasticity. Melanocortins improve adaptional abilities of the nervous system. Chronic treatment with melanocortins may counteract age-related brain pathology.

ESR studies of the erythrocyte membrane skeletal protein network: influence of the state of aggregation of spectrin on the physical state of membrane proteins, bilayer lipids, and cell surface carbohydrates

The stability of the human erythrocyte membrane skeletal network is reported to be dependent on the state of aggregation of spectrin and decreased or increased by polyphosphate anions or the polyamine, spermine, respectively. We have employed polyacrylamide gel electrophoresis and electron spin resonance (ESR) utilizing spin labels specific for membrane proteins, bilayer lipids, or cell-surface sialic acid in order to gain insight into these observations and into the reliability of the ESR spectra of the protein-specific spin label used to correctly report the interactions of the skeletal protein network. The major findings are: (1) We confirm previous reports that the preferred state of spectrin aggregation in the skeletal network is tetrameric and that spectrin can be reversibly transformed to dimeric spectrin and back to tetrameric spectrin on the membrane. (2) The ESR spectra of the protein specific maleimide spin label employed accurately reflect the state of aggregation of spectrin. (3) As dimeric spectrin is increased on the membrane or when 2,3-bisphosphoglycerate was added to spin-labeled membranes, increased segmental motion of protein spin label binding sites reflecting decreased protein-protein interactions in the skeletal network is observed (P less than 0.002 and P less than 0.005, respectively). (4) Conversely, as protein-protein interactions between skeletal proteins or between skeletal proteins and the bilayer are increased by spermine (reflected in the total inability to extract spectrin from the membrane in contrast to control membranes), highly decreased segmental motion of the protein specific spin label binding site is observed (P less than 0.005). (5) The dimeric-tetrameric state of spectrin aggregation on the membrane does not have influence on the order or motion of bilayer lipids nor on the rotational rate of spin-labeled, cell-surface sialic acid, a result also observed when protein-protein interactions were decreased by 2,3-bisphosphoglycerate. In contrast, increased protein-protein interactions by addition of spermine produced a small, but significant, increase in order and decrease in motion of bilayer lipids near the membrane surface as well as a nearly 40% decrease in the apparent rotational correlation time of spin labeled, cell surface sialic acid (P less than 0.002). These latter observations are discussed with reference to possible associations of phospholipids and the major, transmembrane sialoglycoprotein with the skeletal protein network.

The role of monosialoganglioside GM1 in the synaptic plasticity: in vitro study on rat hippocampal slices

Rat hippocampal slices were incubated with neuraminidase from Vibrio Cholerae. This enzyme liberates sialic acid from polysialogangliosides converting them into monosialoganglioside GM1. Thus, the tissue is enriched in GM1 content. Another set of slices was incubated with GM1 itself. Both treatments increased the magnitude of potentiation of synaptic response recorded from pyramidal cell layer following high frequency stimulation of Schaffer collateral-commissural fibers. It is concluded that enrichment of synaptic membranes in GM1 enhances the ability of these nerve endings to be potentiated.

Degradation of rat brain cholinergic muscarinic receptors in vitro: enhancement by agonists and inhibition by antagonists

Cholinergic muscarinic receptors undergo proteolytic degradation in vitro under physiological conditions as shown by a loss in [3H]quinuclidinylbenzilate binding activity. The serine protease inhibitor phenylmethylsulfonyl fluoride was very effective in diminishing the receptor loss. Soybean trypsin inhibitor was less effective. Both EDTA and EGTA were also effective in abolishing receptor degradation, suggesting the involvement of metallopeptidases in the process. Calcium-dependent neutral proteases requiring sulfhydryl reducing agents did not seem to be involved in receptor degradation. Dithiothreitol failed to enhance receptor degradation and iodoacetamide, leupeptin, and antipain, inhibitors of this enzyme class, failed to alter receptor loss as measured by radioligand binding. Most of the proteolytic activity occurred in the cytosol and was readily resolved from the receptor in the membrane fraction. We found that [3H]quinuclidinylbenzilate, an antagonist, inhibited the rate of receptor loss. On the other hand, agonists (acetylcholine, methacholine, and muscarine) appeared to enhance the rate of receptor loss. We postulate that these opposite effects are due to differences in receptor conformation in response to ligand binding. Susceptibility to proteolysis may therefore serve as a probe for receptor conformation.

Long-term depression of parallel fibre synapses following stimulation of climbing fibres

The results from several investigations suggest that climbing fibres heterosynaptically depress parallel fibre responses in Purkinje cells. In the present investigation the mechanism behind the depression has been studied by extracellular recording of responses in single Purkinje cells, evoked by electrical stimulation of parallel fibres and climbing fibres. The results show that a short time of conjunctive stimulation of climbing fibres and parallel fibres results in a long-lasting depression of the parallel fibre responses and that this depression can be prevented if the Purkinje cells are inhibited during the conjunctive stimulation. Since inhibition has been shown to shorten or abolish the long-lasting plateau potentials which are evoked by climbing fibre impulses this finding supports the assumption that the climbing fibre evoked plateau potentials mediate the heterosynaptic depression of parallel fibre responses.

Immunocytochemical localization of actin in dendritic spines of the cerebral cortex using colloidal gold as a probe

Immunocytochemical localization of actin in rat cerebral cortex embedded in the resin LR White was performed using 5 nm colloidal gold as a probe. Antigenicity is maintained throughout the embedding procedure and the low electron opacity of LR White permits fine filamentous structures to be visualized. Control experiments included incubating the sections with normal goat serum or mouse IgG instead of the primary antibody, preadsorbing the antibody with actin from bovine muscle or liver acetone powder, and heat treating the primary antibody. Immunoreactive actin was identified primarily in dendritic spines, particularly in the postsynaptic density (PSD), the subsynaptic web, and the spine apparatus and endothelial and smooth muscle cells of blood vessels. Within dendritic spines, actin which is labeled in the PSD is in continuity with the filaments of the subsynaptic web. These filaments, in turn, are in continuity with the spine apparatus and/or the spine membranes adjacent to the PSD. The PSD may therefore function like other submembranous filamentous arrays which communicate events occurring at the membrane, in this case, the postsynaptic membrane, to the underlying cytoskeletal network, i.e., the subsynaptic web of the spine. It is also suggested that the actin present in the spine may play a role in changes in spine shape and synaptic curvature. Some actin was also seen in the presynaptic process in association with synaptic vesicles, the filamentous network that is contiguous with the synaptic vesicle membrane, and the presynaptic dense projections. Actin may be involved in dynamic processes in the presynaptic ending which include vesicle translocation.

Sequence-related changes in sensory-evoked potentials in the dentate gyrus: a mechanism for item-specific short-term information storage in the hippocampus

Sensory-evoked potentials were recorded from the dentate gyrus of the rat hippocampus during performance of a differential auditory discrimination task. The short latency (20 ms) component (N1) of the sensory-evoked potential showed systematic amplitude fluctuations dependent upon the sequence of positive and negative trials preceding the presentation of a given trial and did not depend on the associated reward values of the individual tone stimuli which evoked the potential. The amplitude fluctuations could be accurately depicted by a model which retained the sequence for the five preceding trials in a "buffer" with exponentially decaying influence as a function of time of trial occurrence within the sequence. The results provide evidence that the hippocampus encodes accurate short-lasting representations of sensory events which can provide the basis for storage of information pertaining to past experiences.

The function of dendritic spines: a review of theoretical issues

The discovery of dendritic spines in the late nineteenth century has prompted nearly 90 years of speculation about their physiological importance. Early observations that bulbous spine heads had very close approximations with the axon terminals of other neurons, confirmed later by ultrastructural study, led to ideas that spines enhance dendritic surface areas for making synaptic contacts. More recent application of cable and core-conductor theory to the anatomical study of spines has raised a number of new ideas about spine function. One important issue was derived from the theoretical treatment of spines as tiny dendrites with much higher input resistances than those of the larger parent dendrites. The high spine-stem resistance results in relative electrical isolation of the spine head; this causes large local depolarizations in the spine head. Several theoretical studies have also shown that if the spine-head input resistances are substantially higher than those of the parent dendrites, spines have the potential for modulating a host of biochemical and biophysical processes that might regulate synaptic efficacy. Empirical studies have documented that spine heads increase rapidly in size after afferent projections have been stimulated electrically and after animals have engaged in a single bout of ecologically important behavioral activity. Such spine head enlargement dilates the portion of the spine stem adjacent to the spine head and this process shortens the spine stem without appreciably altering overall spine length. Theoretical study shows that spine-stem shortening lowers the spine-head input resistance relative to the branch input resistance. This reduction in input resistance can enhance the transfer of electrical charge from the spine head to the parent dendrite, especially when the synaptic conductance is large relative to the spine-head input conductance. Spine-stem shortening also lowers the peak transient membrane potential in the spine head and this factor could delimit Ca2+ influx into the spine head via voltage-dependent Ca2+ channels. The modulation of Ca2+ influx by spine-stem shortening has the potential for regulating Ca2+-sensitive enzymatic activity in the spine head that could affect phosphorylation of cytoskeletal proteins maintaining spine shape and phosphorylation of proteins in the postsynaptic density. Finally, theoretical findings are described that examine the effects of voltage-dependent inward-current channels in the spine head and their ability to amplify the charge transfer due to transmitter-dependent synaptic conductances.

The duration of long-term potentiation in the CA1 region of the hippocampal slice preparation

The duration of long-term potentiation (LTP) of the monosynaptic excitatory Schaffer collateral-commissural input to hippocampal neurons of the CA1 region was examined in the in vitro slice. Relatively stable evoked potentials were obtained under conventional perfusion conditions at least for 10 hours. Tetanic stimulation (100 Hz, 1 sec) increased the population spike (pop-spike) amplitude by about 150% and the slope of the field-EPSP by about 30% over the pre-LTP baseline, whereas the latency and peak latency of the pop-spike decreased. In comparison to control experiments (same number of stimuli at 0.2 Hz) the differences were statistically significant for 2 hr (field-EPSP) and for greater than or equal to 10 hr (pop-spike), respectively. Repeated tetanization (3 X 100 Hz/1 sec), however, substantially prolongs EPSP-LTP (greater than or equal to 10 hr) and doubles the approximated half-life of pop-spike LTP. The threshold current intensity to elicit pop-spike responses decreased after the induction of LTP. Furthermore, the smaller field-EPSP values necessary to evoke near-threshold pop-spikes demonstrate an E-S potentiation (left-shift) at least in the low-intensity range. While the total duration of potentiation of the different parameters has not been determined, all the above mentioned effects could be observed at least 10 hr following the repeated tetanization. It is proposed that the slice preparation is suitable for the investigation of mechanisms of a postulated late phase of LTP if appropriate conditions are used.

Protein kinase C activation leading to protein F1 phosphorylation may regulate synaptic plasticity by presynaptic terminal growth

It has recently been proposed by the author that protein kinase C regulates the expression of synaptic plasticity. In the present review it is suggested that this regulation involves a growth of presynaptic terminals. This proposal was based on the discovery that one of the substrates of protein kinase C, protein F1 (molecular mass = 47 kDa, pI = 4.5) is increased in its phosphorylation 5 min, 1 hr, and 3 days following long-term potentiation (LTP) in the intact hippocampal formation. No other phosphoprotein studied was altered by LTP. The amplitude or persistence of synaptic plasticity was directly related to the extent of protein F1 phosphorylation. As a critical control, it was shown that protein F1 was unaltered following synaptic activation that did not alter synaptic strength. Protein F1 in the hippocampus was also altered in its phosphorylation after an experience involving memory of a spatial environment. Phosphorylation F1 may thus participate in both neurophysiological and behavioral events that evoke plasticity. The identification of the F1 substrate has recently been sought. The physical characteristics of protein F1 (mol wt., isoelectric point) indicate that it is the same as the B-50 protein and the growth protein, GAP-43. Protein F1 is then a brain-specific, synaptically enriched phosphoprotein. Recent evidence indicates that protein F1 is present in high concentration in growth cones of late embryonic rat brain in which postsynaptic specializations are not detected, suggesting a presynaptic locus. With respect to the identity of the F1 kinase, we have shown that protein F1, like B-50, is a substrate for protein kinase C, a Ca2+/phospholipid-dependent kinase. Activation of this enzyme by tumor-promoting phorbol esters can trigger cell growth and neurite extension. Recent evidence indicates a presynaptic localization of the enzyme. On the basis of the colocalization of enzyme and substrate in the presynaptic terminal it is proposed that protein kinase C control of the phosphorylation state of protein F1 may regulate the expression of synaptic plasticity via presynaptic terminal growth.

Mechanism of cytoskeletal regulation (I): functional differences correlate with antigenic dissimilarity in human brain and erythrocyte spectrin

Human erythrocyte and brain spectrin (fodrin, calspectin) have been compared quantitatively with respect to the extent and sites of antigenic and functional similarity. Brain spectrin cross-reacts strongly with approx. 1% of the epitopes in erythrocyte spectrin, but weakly with at least 50%. The distribution of shared determinants is not uniform. Brain spectrin is most deficient in epitopes characteristic of the 80 kDa and 52 kDa domains of the alpha-subunit (alpha-I and alpha-III) and of terminal portions of the 28 kDa and 74 kDa domains of the beta-subunit (beta-I and beta-IV). The functions associated with these domains also differ between the two proteins. Brain spectrin does not undergo extensive polymerization and binds calmodulin at a different site. The unique ability of erythrocyte spectrin to oligomerize beyond the tetramer reflects its role in the membrane skeleton. Non-erythroid spectrins probably function as specific linkers between membrane receptors and the filamentous cytoskeleton. In this sense, they may act as regulated transducers of information flow between the membrane and the cytoplasmic matrix.

Evidence for an increased rate of choline efflux across erythrocyte membranes in Alzheimer's disease

Alzheimer's disease (AD), the major dementing disorder of the elderly, is associated with cholinergic neuronal loss and decreased activity of choline acetyltransferase (CAT). Previous biophysical studies had suggested an altered conformation of membrane proteins in AD erythrocyte ghosts. Since erythrocytes have a choline transport system and cholinergic neurons are implicated in AD, the present experiments were undertaken to determine if the efflux rate of [14C]choline was altered in AD erythrocytes. The mean efflux rate constant was highly significantly increased (P less than 0.01) by greater than 25% in 9 drug-free AD patients compared to 9 sex-matched, drug-free controls of similar age. These results are discussed in terms of potential molecular mechanisms to account for cholinergic neuronal loss in AD.

Identification of low-Ca2+- and high-Ca2+-requiring neutral proteases in rat retina

Two low-Ca2+-requiring proteases (calpain I) and one high-Ca2+-requiring protease (calpain II) have been separated from the cytosol of rat retina by DEAE-cellulose column chromatography. Calpain I was half-maximally activated at 3 microM free Ca2+ and fully activated at 10 microM free Ca2+. Half-maximal activation of calpain II was at 0.4 mM free Ca2+ while full activation was observed at 0.8 mM free Ca2+. Calpain activity has also been demonstrated in sealed rod outer segments. The soluble fraction of the rod outer segments contained 89% of the enzyme activity. The possible role of calpain in retina is discussed.

Release of plasminogen activator and a calcium-dependent metalloprotease from cultured sympathetic and sensory neurons

Cultures of neurons from neonatal rat superior cervical, dorsal root, and trigeminal ganglia were grown in the absence of nonneuronal cells in serum-free defined medium. Proteins metabolically labeled with radioactive amino acids and spontaneously released into the culture medium were studied using two-dimensional gel electrophoresis and photofluorography. All three populations of neurons released 12-15 major proteins into the culture medium. Four proteins were released selectively by sympathetic neurons and two proteins were consistently released by both populations of sensory neurons but not by sympathetic neurons. Enzymatic activities are associated with at least two of the released proteins. One is a calcium-dependent metalloprotease, and the other a plasminogen activator. The calcium-dependent metalloprotease has a MW of 62 kDa, requires millimolar calcium for maximum activity, and has a restricted substrate specificity. It degraded native and denatured collagen more readily than casein, albumin, or fibronectin and denatured collagen (gelatin) was a better substrate than native collagen. The plasminogen activator released by neurons has a MW of 51 kDa and is converted to an active 32 kDa form. Its physiochemical properties are similar to urokinase and it was precipitated by a rabbit antiserum produced against human urokinase. A large fraction of both proteases was released by distal processes and/or growth cones suggesting that these proteases could be involved in growth cone functions.

Olfactory discrimination learning is blocked by leupeptin, a thiol protease inhibitor

Rats were trained on successive two-odor discriminations with the cues randomly located in an 8-arm radial maze. After several days of training using different odor pairs, the thiol protease inhibitor leupeptin was infused into the ventricles and testing continued. Leupeptin caused a pronounced, dose-dependent and reversible deficit in performance in this task. Previous studies have shown that these drug concentrations do not influence spontaneous activity, feeding and drinking, or the acquisition and retention of avoidance conditioning. The results are interpreted as supporting the hypothesis that a calcium-sensitive proteinase is involved in certain forms of memory that require modification of telencephalic circuitries.

A possible mechanism of morphometric changes in dendritic spines induced by stimulation

A number of experimental procedures which induce increased electrical activity (including long-term potentiation) were shown to be accompanied by morphometric changes in dendritic spines. These changes include an enlargement of the spine head, shortening and widening of the spine stalk, and an increase in the length of synaptic apposition. A possible mechanism is suggested which takes into account specific cytological features of the spine and the existence of contractile proteins in neurons. Dendritic spines are defined as special domains of the neuron which have a unique organization of the cytoplasm. Actin filaments form a very dense network in the spine head, and they are longitudinally organized within the spine stalk. Spines were also shown to contain myosin and other actin-regulatory proteins. The high density of the actin network could explain the characteristic absence of the cytoplasmic organelles from dendritic spines. In analogy with other cells, such an actin organization indicates low levels of free cytosolic calcium. Even in the resting state, calcium levels may be unevenly distributed through the neuron, being lowest within the subplasmalemmal region. Due to the high surface-to-volume ratio in spines, the cytoplasm is formed mostly by the subplasmalemmal region. The spine apparatus or the smooth endoplasmic reticulum, which is recognized as a calcium-sequestering site in spines, may also contribute to the low calcium levels there. However, when in the stimulated spine the voltage-dependent calcium channels open, then, given the spine's high surface-to-volume ratio, the concentration of calcium may very quickly attain levels that will activate the actin-regulatory proteins and myosin and thus trigger the chain of events leading to the enlargement of the spine head and to the contraction (i.e., widening and shortening) of the spine stalk. The increased free cytosolic calcium may also activate the protein-producing system localized at the base of the spine, which, under certain conditions, could stabilize the morphometric changes of the spine.

Adaptive plasticity in the spinal stretch reflex: an accessible substrate of memory?

The study of the substrates of memory in higher vertebrates is one of the major problems of neurobiology. A simple and technically accessible experimental model is needed. Recent studies have demonstrated long-term adaptive plasticity, a form of memory, in the spinal stretch reflex (SSR). The SSR is due largely to a two-neuron monosynaptic arc, the simplest, best-defined, and most accessible pathway in the primate central nervous system (CNS). Monkeys can slowly change SSR amplitude without a change in initial muscle length or alpha motoneuron tone, when reward is made contingent on amplitude. Change occurs over weeks and months and persists for long periods. It is relatively specific to the agonist muscle and affects movement. The salient features of SSR adaptive plasticity, combined with clinical and laboratory evidence indicating spinal cord capacity for intrinsic change, suggest that SSR change eventually involves persistent segmental alteration. If this is the case, SSR plasticity should be a powerful model for studying the neuronal and synaptic substrates of memory in a primate.