Cellular Analog Research Articles

Book Reviews

Book Reviews

The Na,K pump regulates decreases in the cholinosensitivity of neurons in the common snail to a cellular analog of habituation: the role of cellular calcium.

The effects of the Na,K pump inhibitor ouabain on the depth of depression of cholinosensitivity of defensive behavior command neurons LPa2, LPa3, RPa3, and RPa2 were studied in the common snail using a cellular analog of habituation; the role of intracellular Ca2+ in these effects was analyzed. Integral acetylcholine-evoked transmembrane currents (ACh currents) were recorded by two-electrode membrane voltage clamping. In one group of neurons, extracellular application of ouabain (0.1 mM) by addition to the bathing solution evoked increases in the depression of the ACh current evoked by rhythmic application of mediator (with interstimulus intervals of 1-3 min) while neurons of the other groups responded with decreases in depression. After spontaneous diffusion of the Ca2+ ion chelator BAPTA (1 mM) from the intracellular microelectrode for 60-150 min, ouabain only increased the level of depression of the ACh current. After intracellular injection of CaCl2 (100 mM), ouabain only decreased the level of depression of the ACh current. It was concluded that inhibition of the Na,K pump modifies depression of the cholinosensitivity of neurons in the cellular model of habituation. The direction of the effect depends on the basal concentration of intracellular Ca2+.

Identification of HIV-1 integrase inhibitors via three-dimensional database searching using ASV and HIV-1 integrases as targets

Integration of viral DNA into the host cell genome is a critical step in the life cycle of HIV. This essential reaction is catalyzed by integrase (IN) through two steps, 3′-processing and DNA strand transfer. Integrase is an attractive target for drug design because there is no known cellular analogue and integration is essential for successful replication of HIV. A computational three-dimensional (3-D) database search was used to identify novel HIV-1 integrase inhibitors. Starting from the previously identified Y3 (4-acetylamino-5-hydroxynaphthalene-2,7-disulfonic acid) binding site on the avian sarcoma virus integrase (ASV IN), a preliminary search of all compounds in the nonproprietary, open part of the National Cancer Institute 3-D database yielded a collection of 3100 compounds. A more rigorous scoring method was used to rescreen the 3100 compounds against both ASV IN and HIV-1 IN. Twenty-two of those compounds were selected for inhibition assays against HIV-1 IN. Thirteen of the 22 showed inhibitory activity against HIV-1 IN at concentrations less than 200 μM and three of them showed antiviral activities in HIV-1 infected CEM cells with effective concentrations (EC 50) ranging from 0.8 to 200 μM. Analysis of the computer-generated binding modes of the active compounds to HIV-1 IN showed that simultaneous interaction with the Y3 site and the catalytic site is possible. In addition, interactions between the active compounds and the flexible loop involved in the binding of DNA by IN are indicated to occur. The structural details and the unique binding motif between the HIV-1 IN and its inhibitors identified in the present work may contribute to the future development of IN inhibitors.

Cellular Analog of Differential Classical Conditioning inAplysia: Disruption by the NMDA Receptor Antagonistdl-2-Amino-5-Phosphonovalerate

We previously showed that the associative enhancement of Aplysia siphon sensorimotor synapses in a cellular analog of classical conditioning is disrupted by infusing the Ca(2+) chelator 1, 2-bis(2-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid into the postsynaptic motor neuron before training or by training in the presence of the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV). Our earlier experiments with APV used a nondifferential training protocol, in which different preparations were used for associative and nonassociative training. In the present experiments we extended our investigation of the role of NMDA receptor type potentiation in learning in Aplysia to differential conditioning. A cellular analog of differential conditioning was performed with a reduced preparation that consisted of the CNS plus two pedal nerves. A siphon motor neuron and two siphon sensory neurons, both of which were presynaptically connected to the motor neuron, were impaled with sharp microelectrodes. One sensorimotor synapse received paired stimulation with a conditioned stimulus (brief activation of a single sensory neuron) and an unconditioned stimulus (pedal nerve shock), whereas the other sensorimotor synapse received unpaired stimulation. Training in normal artificial seawater (ASW) resulted in significant differential enhancement of synapses that received the paired stimulation. Training in APV blocked this differential synaptic enhancement. A comparison of the present data with the data from earlier experiments that used nondifferential training is consistent with the possibility that differential training comprises competition between the presynaptic sensory neurons. Synaptic competition may contribute significantly to the associative effect of paired stimulation in the differential training paradigm.

Activity-dependent regulation of receptive field properties of cat area 17 by supervised Hebbian learning

Most algorithms currently used to model synaptic plasticity in self-organizing cortical networks suppose that the change in synaptic efficacy is governed by the same structuring factor, i.e., the temporal correlation of activity between pre- and postsynaptic neurons. Functional predictions generated by such algorithms have been tested electrophysiologically in the visual cortex of anesthetized and paralyzed cats. Supervised learning procedures were applied at the cellular level to change receptive field (RF) properties during the time of recording of an individual functionally identified cell. The protocols were devised as cellular analogs of the plasticity of RF properties, which is normally expressed during a critical period of postnatal development. We summarize here evidence demonstrating that changes in covariance between afferent input and postsynaptic response imposed during extracellular and intracellular conditioning can acutely induce selective long-lasting up- and down-regulations of visual responses. The functional properties that could be modified in 40% of cells submitted to differential pairing protocols include ocular dominance, orientation selectivity and orientation preference, interocular orientation disparity, and the relative dominance of ON and OFF responses. Since changes in RF properties can be induced in the adult as well, our findings also suggest that similar activity-dependent processes may occur during development and during active phases of learning under the supervision of behavioral attention or contextual signals. Such potential for plasticity in primary visual cortical neurons suggests the existence of a hidden connectivity expressing a wider functional competence than the one revealed at the spiking level. In particular, in the spatial domain the sensory synaptic integration field is larger than the classical discharge field. It can be shaped by supervised learning and its subthreshold extent can be unmasked by the pharmacological blockade of intracortical inhibition.

THE ROLE OF MEMBRANE ELECTROSTATICS IN THE REGULATION OF CELL VOLUME AND ION CONCENTRATIONS

We present a model to study how membrane surface negative charges can affect the electro-osmotic regulation properties of a cell. This model is based on the cellular analog proposed by Jakobsson, which includes passive and active ion transports; we further introduce the effect of membrane surface charges, using a generalized formulation of the Gouy–Chapman theory. We derive a system of nonlinear differential-algebraic equations (DAEs) which describes the dynamics of the cellular analog. The system admits a unique asymptotically stable stationary state, in which the Na-pump rate, which is crucial for electro-osmotic regulation, is inversely related to the Ca2+level in the extracellular milieu; numerical integration shows that this apparent inhibition of the Na-pump by external Ca2+results from a decrease in the electrostatic field produced by surface charges at the external side of the membrane. Furthermore, the degree of stability of the stationary state dramatically depends on the amount of negative charges on the membrane; a maximal stability is obtained for densities around - e /500 Å2, where the Na-pump is maximally activated by an increase in the Na content of the cytoplasm.

The genome of cowpox virus contains a gene related to those encoding the epidermal growth factor, transforming growth factor alpha and vaccinia growth factor.

Cowpox virus (CPV) is a member of the Orthopoxvirus genus and has the genetic capacity to encode a multitude of genes that interfere with the host inflammatory and immune response or modulate the physiological state of infected and non-infected cells. Among these CPV factors are receptors homologous to interferon and tumor necrosis factor receptors and also a viral cellular serine-proteinase analog. Here we describe the detection of a CPV gene that encodes a protein homologous to epidermal growth factor, transforming growth factor alpha and poxvirus growth factors, such as the vaccinia growth factor (VGF). The VGF and other poxvirus growth factors are produced early in the infection and are secreted into the medium where they bind to the EGF receptors, generating mytotic responses. The cowpox growth factor (CGF) gene was detected in three copies on the virus genome by PCR, and by northern and southern blot hybridization using VGF nucleotide sequences as primers and probes. The CPV gene has a strong nucleotide and predicted amino acid similarity with VGF, and is also produced early in the infection.

A cellular analog network for MRF-based video motion detection

The implementation of a visual motion detection algorithm on an analog network is presented. The algorithm is based on Markov random field (MRF) modeling. Robust motion detection is achieved by using a spatiotemporal neighborhood for modeling pixel interactions. Not only are the moving edges detected, but also the inner part of moving regions. Moreover, the motion echo is eliminated. For hardware implementation, the algorithm is mapped onto a resistive network incorporating, at each pixel-node, light acquisition and on-chip processing. Elementary Markov cells are made of resistors and nonlinear current sources driven by observations, derived from luminance information acquired by photoreceptors. Electrical simulations of the cell are reported. Experimental results with synthetic and real-world image sequences exhibit the performance of the network. For circuit design, the switched-current technique is used. The circuit is intended for submicronic complementary metal-oxide-semiconductor (CMOS) technology.

Involvement of Presynaptic and Postsynaptic Mechanisms in a Cellular Analog of Classical Conditioning atAplysiaSensory-Motor Neuron Synapses in Isolated Cell Culture

Temporal pairing of presynaptic activity and serotonin produces enhanced facilitation at Aplysia sensory-motor neuron synapses (pairing-specific facilitation), which may contribute to classical conditioning of the gill and siphon withdrawal reflex. This cellular analog of conditioning is thought to involve Ca2+ priming of the cAMP pathway in the sensory neurons. Consistent with that idea, we have found that pairing-specific facilitation by serotonin is greatly reduced by presynaptic injection of a slow Ca2+ chelator or a specific inhibitor of cAMP-dependent protein kinase and is accompanied by a transient increase in the frequency but by no change in the amplitude of spontaneous, miniature EPSPs. However, like post-tetanic potentiation (PTP) and long-term potentiation (LTP) at these synapses, pairing-specific facilitation is also greatly reduced by postsynaptic injection of a rapid Ca2+ chelator or by postsynaptic hyperpolarization during training, although postsynaptic hyperpolarization has no effect on the increase in frequency or on the amplitude of spontaneous EPSPs. These results suggest that pairing-specific facilitation by serotonin involves Hebbian postsynaptic as well as non-Hebbian presynaptic components that interact in some way, perhaps via retrograde signaling, to specifically enhance evoked, synchronized release of transmitter.

Mediation of classical conditioning in Aplysia californica by long-term potentiation of sensorimotor synapses.

Long-term potentiation (LTP) is considered an important neuronal mechanism of learning and memory. Currently, however, there is no direct experimental link between LTP of an identified synapse and learning. A cellular analog of classical conditioning in Aplysia was used to determine whether this form of invertebrate learning involves N-methyl-D-aspartate (NMDA)-type LTP. The NMDA receptor-antagonist dl-2-amino-5-phosphonovalerate significantly disrupted synaptic enhancement after associative training but did not disrupt synaptic enhancement after nonassociative training. Thus, classical conditioning in Aplysia appears to be mediated, in part, by LTP due to activation of NMDA-related receptors.

Poxvirus Genomes Encode a Secreted, Soluble Protein That Preferentially Inhibits β Chemokine Activity yet Lacks Sequence Homology to Known Chemokine Receptors

Poxvirus genomes encode several proteins which inhibit specific elements of the host immune response. We show the “35K” virulence gene in variola and cowpox viruses, whose vaccinia and Shope fibroma virus equivalents are strongly conserved in sequence, actually encodes a secreted soluble protein with high-affinity binding to virtually all known β chemokines, but only weak or no affinity to the α and γ classes. The viral protein completely inhibits the biological activity of monocyte chemotactic protein-1 (MCP-1) by competitive inhibition of chemokine binding to cellular receptors. As all β chemokines are also shown to cross-compete with MCP1 binding to the viral protein, we conclude that this viral chemokine inhibitor (vCCI) not only interacts through a common binding site, but is likely a potent general inhibitor of β chemokine activity. Unlike many poxvirus virulence genes to date, which are clearly altered forms of acquired cellular genes of the vertebrate immune system, this viral chemokine inhibitor (vCCI) shares no sequence homology with known proteins, including known cellular chemokine receptors, all of which are multiple membrane-spanning proteins. Thus, vCCI presumably has no cellular analogue and instead may be the product of unrelenting sequence variations which gave rise to a completely new protein with similar binding properties to native chemokine receptors. The proposed function of vCCI is inhibition of the proinflammatory (antiviral) activities of β chemokines.

Intact spatial learning and memory in transgenic mice with reduced BDNF.

Long-term potentiation (LTP) in hippocampal CA1 pyramidal cells is considered to be a cellular analogue of learning and may be useful in studying the molecular foundations of learning and memory. Because brain-derived neurotrophic factor (BDNF) had been shown to have a role in activity-dependent neuroplasticity in the hippocampus we studied spatial learning in mice with BDNF deficiency produced by gene-targeted mutation. Heterozygous BDNF knockout mice reportedly underexpress BDNF and have reduced LTP, but their spatial memory and search strategy assessed with Morris water maze (distally cued version) as well as their performance on the elevated plus maze were indistinguishable from that of controls. This indicates that extrapolation from LTP in a single brain structure to complex behaviours such as learning and memory may not be justified.

Cellular digital packet data network

Cellular digital packet data (CDPD) is a data network designed to meet growing mobile data transmission needs. CDPD networks can stand alone or overlay on the cellular voice system by drawing radio resources from the pool of unused or free cellular analog voice channels. Detection as to whether a channel is free or occupied is mechanized through channel sniffers. Once the CDPD network seizes a channel, it continues to use the channel as long as it is free and leaves the channel within 40 ms of initiation of any voice activity on the channel-the CDPD network then hops to another free channel, if available. CDPD follows slotted nonpersistent digital-sense multiple access with collision detection (DSMA/CD). In addition, it performs channel-error detection and channel-error recovery. We investigate several aspects of the CDPD network, namely, the: (1) effect of CDPD on cellular voice and (2) the delay-throughput performance due to DSMA/CD with error detection.

Enhancement of sensorimotor connections by conditioning-related stimulation in Aplysia depends upon postsynaptic Ca2+.

Classical conditioning of Aplysia's siphon-withdrawal reflex is thought to be due to a presynaptic mechanism-activity-dependent presynaptic facilitation of sensorimotor connections. Recent experiments with sensorimotor synapses in dissociated cell culture, however, provide an alternative cellular mechanism for classical conditioning-Hebbian long-term potentiation (LTP) of sensorimotor connections. Induction of Hebbian LTP of these connections is mediated by activation of N-methyl-D-aspartate-related receptors and requires the postsynaptic elevation of intracellular Ca2+. To determine whether the enhancement of sensorimotor synapses during classical conditioning in Aplysia-like LTP of sensorimotor synapses in culture-also depends upon the elevation of postsynaptic Ca2+, we carried out experiments involving the cellular analog of classical conditioning of siphon withdrawal. We examined changes in the strength of monosynaptic siphon sensorimotor connections in the abdominal ganglion of Aplysia following paired presentations of sensory neuron activation and tail nerve shock. This training regimen resulted in significant enhancement of the monosynaptic sensorimotor excitatory postsynaptic potential, as compared with the sensorimotor excitatory postsynaptic potential in preparations that received only test stimulation. Infusing the motor neuron with 1,2-bis(2-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid, a specific chelator of intracellular Ca2+, prior to paired stimulation training blocked this synaptic enhancement. Our results implicate a postsynaptic, possibly Hebbian, mechanism in classical conditioning in Aplysia.

Special issue on analog implementations of cellular neural networks and analog VLSI

Special issue on analog implementations of cellular neural networks and analog VLSI

Synaptic plasticity in the hippocampus: LTP and LTD

Perhaps the most fundamental and remarkable feature of the mammalian central nervous system is its ability to process and store large amounts of information. For many decades it has been postulated that the brain uses Ionglasting modifications of synaptic strength in critical neural circuits to accomplish this feat. One such activitydependent modification is long-term potentiation (LTP) in the hippocampus, a sustained increase in synaptic strength that is elicited by brief high frequency stimulation of excitatory afferents. Recent excitement about the phenomenon of LTP has arisen from three major sources. First, compelling evidence from lesion studies in higher primates, including humans, shows that the hippocampus is a critical component of a neural system that is required for the initial storage of certain forms of long-term memory (Squire and Zola-Morgan, 1991). Second, several properties of LTP make it an attractive cellular mechanism for information storage or memory (Bliss and Collingridge, 1993). Like memories, LTP can be generated rapidly and is strengthened with repetition. It exhibits input specificity; that is, LTP occurs only at synapses stimulated by afferent activity but not at adjacent synapses on the same postsynaptic cell. Input specificity presumbably dramatically increases the storage capacity within a neural circuit. Most importantly, LTP is associative; temporally pairing activity in a “weak” input (incapable of generating LTP by itself) with activation of a strong input (capable of eliciting LTP at adjacent synapses on the same postsynaptic cell) results in LTP of the weak input. This associative property, which is reminiscent of classical conditioning, can be considered a cellular analog of associative learning. Third, LTP is readily elicited in in vitro preparations of the hippocampus, and this makes it amenable to rigorous experimental manipulations. There are several different forms of LTP, but the majority of experimental work has focused on the LTP observed in hippocampal CA1 pyramidal cells. This minireview will provide an update on the cellular mechanisms of LTP and distinguish those mechanisms that are firmly established from those that remain contentious. The evidence connecting LTP to real learning and memory also will be reviewed briefly. The Induction of LTP: NMDA Receptors and Cap+ It is well accepted that the induction of LTP requires activation of postsynaptic N-methyl-o-aspartic acid (NMDA) receptors (a subtype of glutamate receptor) during postsynaptic depolarization, which is normally generated by high frequency afferent activity. This results in a rise in CaH concentration ([Ca%],), a necessary trigger for LTP. Figure 1 shows that during normal low frequency synaptic transmission, the excitatory neurotransmitter glutamate is

Habitat Destruction and Competitive Coexistence: A Cellular Model

In a recent issue of Journal of Animal Ecology, Nee & May (1992) presented a two-species model which described how a competitively inferior species would increase in both frequency and absolute numbers as habitat patches were destroyed. Their model was very simple: the inferior competitor, B, could only colonize patches which were empty, but not destroyed, while the superior competitor, A, could additionally colonize patches occupied by B, extinguishing B in the process. The species were assigned extinction rates, eA and eB, and colonization rates CA and CB. Three equations described the rate of change in number of patches which are either empty or occupied by A or B. Nee & May stated that an inferior competitor would persist if cB/eB > cA/eA and that neither species could persist when the proportion of habitable patches is below e/c. The Nee-May model outlined above describes a situation where there are a very large number of patches which are all equally accessible to colonizers and therefore takes no account of any spatial variation in metapopulations across the patchy landscape. Here I describe the results of a cellular analogue of the NeeMay model which does take into account the spatial variation in species distribution and shows both similarities and difference in results.

Effects of caloric restriction in animals on cellular function, oncogene expression, and DNA methylation in vitro.

While the life-extending and disease-modulating effects of caloric restriction (CR) are well documented in whole animal studies and in correlative experiments using cells taken from CR animals, very few studies have used cells in culture after their removal from the CR-fed animal. In using this in vivo-->in vitro approach we have attempted to examine the proposition that the effects of CR can be transferred to individual cells by analyzing the cellular functions of proliferation and transformation, the activation of oncogenes, and the methylation of DNA as a function only of diet. Pancreatic acinar cells excised from CR-fed Brown-Norway rats and placed in rich medium showed different responses compared to cells from ad libitum (AL)-fed controls. CR had the effect of slowing growth rate and protecting against spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced transformation over 14 passages of cells in culture. At the molecular level, cells from the CR animals showed reduced c-Ha-ras oncogene expression and mutation as well as reduced mutation of the p53 suppressor gene. CR also increased genomic methylation of ras DNA. We conclude that the effects of CR treatment of the animal are transferred to individual cells and note that these responses (decreased proliferation and transformation; depressed oncogene expression and mutation and decreased suppressor gene mutation; and increased oncogene methylation) are cellular and molecular analogs of in vivo weight loss, life extension, and carcinogenesis modulation, which are hallmarks of CR in the whole animal. The fact that these responses are seen generations after the cells are removed from the CR-treated animal indicates that CR causes a permanent predisposition of pancreatic acinar cells to these modulated responses and shows the value of the in vivo-->in vitro protocol in studies that relate diet to cellular and molecular function.

Cellular analogs of conditioning: A neural network model to learn sensori-motor programs

Cellular analogs of conditioning: A neural network model to learn sensori-motor programs

Effect of ageing on tissue levels of amino acids involved in the nitric oxide pathway in rat brain.

Nitric oxide (NO) and citrulline are produced from L-arginine by the action of NO synthase after activation of excitatory amino acid receptors. In addition to its role in neurodegeneration, there is convincing evidence that NO is also involved in long-term potentiation, a cellular analog of learning and memory in the mammalian nervous system. In the present study, concentrations of L-arginine, citrulline, aspartic acid and glutamic acid were determined in various brain regions of young and old rats. The aim was to examine whether changes in brain concentrations of these amino acids might be indicative of a possible decrease in NO production with ageing, in relation with the well-established decline of cognitive function. Brain aspartic acid, citrulline and L-arginine concentrations were found to be lower in old rats compared to young animals, although the decrease did not always reach statistical significance. In contrast, no change in glutamic acid levels was found. In all brain structures of young and old rats, concentrations of L-arginine were higher than the concentration for NO synthase to function at maximum velocity in the rat brain. Therefore, the decrease in citrulline concentrations found in some brain regions of old rats might be seen, at least partly, as a reflection of a lower production of NO with ageing, although further work is clearly needed to ascertain a decrease in rat brain NO synthase activity with age.