Transfer properties of branching dendrites with tonically activated inputs

In the mathematical model of a neuron, properties of the active membrane during action of tonically activated synaptic inputs of an N-methyl-D-aspartate (NMDA) type were studied. In the dendrites with these properties, the impact of geometry on the somatopetal current transfer was investigated. The conditions were considered when the distal sites are more effective, as compared with the proximal ones, in their contribution to the somatopetal current. Contributions of “sister” branches to the net current transferred to the soma were compared at different levels of depolarization caused by action of homogeneously distributed NMDA synaptic inputs.     

A role for synaptic inputs at distal dendrites: instructive signals for hippocampal long-term plasticity

Synaptic potentials originating at distal dendritic locations are severely attenuated when they reach the soma and, thus, are poor at driving somatic spikes. Nonetheless, distal inputs convey essential information, suggesting that such inputs may be important for compartmentalized dendritic signaling. Here we report a new plasticity rule in which stimulation of distal perforant path inputs to hippocampal CA1 pyramidal neurons induces long-term potentiation at the CA1 proximal Schaffer collateral synapses when the two inputs are paired at a precise interval. This subthreshold form of heterosynaptic plasticity occurs in the absence of somatic spiking but requires activation of both NMDA receptors and IP(3) receptor-dependent release of Ca(2+) from internal stores. Our results suggest that direct sensory information arriving at distal CA1 synapses through the perforant path provide compartmentalized, instructive signals that assess the saliency of mnemonic information propagated through the hippocampal circuit to proximal synapses.     

Activity-dependent clustering of functional synaptic inputs on developing hippocampal dendrites

During brain development, before sensory systems become functional, neuronal networks spontaneously generate repetitive bursts of neuronal activity, which are typically synchronized across many neurons. Such activity patterns have been described on the level of networks and cells, but the fine-structure of inputs received by an individual neuron during spontaneous network activity has not been studied. Here, we used calcium imaging to record activity at many synapses of hippocampal pyramidal neurons simultaneously to establish the activity patterns in the majority of synapses of an entire cell. Analysis of the spatiotemporal patterns of synaptic activity revealed a fine-scale connectivity rule: neighboring synapses (<16?μm intersynapse distance) are more likely to be coactive than synapses that are farther away from each other. Blocking spiking activity or NMDA receptor activation revealed that the clustering of synaptic inputs required neuronal activity, demonstrating a role of developmentally expressed spontaneous activity for connecting neurons with subcellular precision.     

Barriers to diffusion in dendrites and estimation of calcium spread following synaptic inputs

The motion of ions, molecules or proteins in dendrites is restricted by cytoplasmic obstacles such as organelles, microtubules and actin network. To account for molecular crowding, we study the effect of diffusion barriers on local calcium spread in a dendrite. We first present a model based on a dimension reduction approach to approximate a three dimensional diffusion in a cylindrical dendrite by a one-dimensional effective diffusion process. By comparing uncaging experiments of an inert dye in a spiny dendrite and in a thin glass tube, we quantify the change in diffusion constants due to molecular crowding as D_cyto/D_water = 1/20. We validate our approach by reconstructing the uncaging experiments using Brownian simulations in a realistic 3D model dendrite. Finally, we construct a reduced reaction-diffusion equation to model calcium spread in a dendrite under the presence of additional buffers, pumps and synaptic input. We find that for moderate crowding, calcium dynamics is mainly regulated by the buffer concentration, but not by the cytoplasmic crowding, dendritic spines or synaptic inputs. Following high frequency stimulations, we predict that calcium spread in dendrites is limited to small microdomains of the order of a few microns (<5 μm).     

A behavioral role for dendritic integration: HCN1 channels constrain spatial memory and plasticity at inputs to distal dendrites of CA1 pyramidal neurons

The importance of long-term synaptic plasticity as a cellular substrate for learning and memory is well established. By contrast, little is known about how learning and memory are regulated by voltage-gated ion channels that integrate synaptic information. We investigated this question using mice with general or forebrain-restricted knockout of the HCN1 gene, which we find encodes a major component of the hyperpolarization-activated inward current (Ih) and is an important determinant of dendritic integration in hippocampal CA1 pyramidal cells. Deletion of HCN1 from forebrain neurons enhances hippocampal-dependent learning and memory, augments the power of theta oscillations, and enhances long-term potentiation (LTP) at the direct perforant path input to the distal dendrites of CA1 pyramidal neurons, but has little effect on LTP at the more proximal Schaffer collateral inputs. We suggest that HCN1 channels constrain learning and memory by regulating dendritic integration of distal synaptic inputs to pyramidal cells.     

Comparison of active transport in neuronal axons and dendrites

This paper presents a theoretical study, based on modified Smith-Simmons equations, that compares transport of intracellular organelles in two different neurite outgrowths, dendrites and axons. It is demonstrated that the difference in microtubule polarity orientations in dendrites and axons has significant implications on motor-assisted transport in these neurite outgrowths. The developed approach presents a qualitative theoretical basis for understanding important questions such as why axons exhibit almost an unlimited grows potential in vitro while dendrites remain relatively short. It is shown that the difference in a microtubule polarity arrangement between axons and dendrites may be a regulatory mechanism for limiting dendritic growth. Other biological implications of the developed theory as well as other possible reasons for the difference in microtubule structure between axons and dendrites are discussed.     

Contribution to sympathetic vasomotor tone of tonic glutamatergic inputs to neurons in the RVLM

The role of excitatory amino acid (EAA) receptors in the rostral ventrolateral medulla (RVLM) in maintaining resting sympathetic vasomotor tone remains unclear. It has been proposed that EAA receptors in the RVLM mediate excitatory inputs both to presympathetic neurons and to interneurons in the caudal ventrolateral medulla (CVLM), which then provide a counterbalancing inhibition of RVLM presympathetic neurons. In this study, we tested this hypothesis by determining the effect of blockade of EAA receptors in the RVLM on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), after inhibition of CVLM neurons. In anesthetized rats, bilateral injections of muscimol in the CVLM increased MAP, HR, and RSNA. Subsequent bilateral injections of kynurenic acid (Kyn, 2.7 nmol) in the RVLM caused a modest reduction of approximately 20 mmHg in the MAP but had no effect, when compared with the effect of vehicle injection alone, on HR or RSNA. By approximately 50 min after the injections of Kyn or vehicle in the RVLM, the MAP had stabilized at a level close to its original baseline level, but the HR and RSNA stabilized at levels above baseline. The results indicate that removal of tonic EAA drive to RVLM neurons has little effect on the tonic activity of RVLM presympathetic neurons, even when inputs from the CVLM are blocked. Thus the tonic activity of RVLM presympathetic neurons under these conditions is dependent on excitatory synaptic inputs mediated by non-EAA receptors and/or the autoactivity of these neurons.     

Plasticity of enzyme active sites

The expectation is that any similarity in reaction chemistry shared by enzyme homologues is mediated by common functional groups conserved through evolution. However, detailed enzyme studies have revealed the flexibility of many active sites, in that different functional groups, unconserved with respect to position in the primary sequence, mediate the same mechanistic role. Nevertheless, the catalytic atoms might be spatially equivalent. More rarely, the active sites have completely different locations in the protein scaffold. This variability could result from: (1) the hopping of functional groups from one position to another to optimize catalysis; (2) the independent specialization of a low-activity primordial enzyme in different phylogenetic lineages; (3) functional convergence after evolutionary divergence; or (4) circular permutation events.     

Dendritic analysis of lobster stretch receptor neurons: Electrotonic properties with single and distributed inputs

Using steady-state cable analysis as derived by Rall, electrotonic properties of the dendritic trees of the tonic stretch receptor neuron of the spiny lobster, Panulirus interruptus,have been examined. By directly measuring the somatic input resistance and by visualizing the dendritic trees of this neuron by backfilling the axon with cobalt, the electrotonic properties of the dendritic trees have been derived. The calculated membrane resistivity is 800-3600 -cm ². Voltage and current transfer functions were calculated for (a) single dendritic tips the size observed in the cobalt preparations and (b) for processes 2 µm or smaller, as observed in electron microscopy. Current transfer to the soma was high in both cases (greater than 80%). Voltage transfer was 22% for large and 4% for small dendrites. When a more natural simultaneous conductance change at the tips of all major dendrites was modeled, voltage transfer was 84% and current transfer 56%. But the dynamic range of the cell (rheobase to saturation) is well-predicted by varying the simultaneous inputs, not by scaling up a single input, thus illustrating that convenient indices of electrotonic properties may not prove useful in appreciating the integrative properties of a neuron.     

Parameter sensitivity of distributed transfer properties of neuronal dendrites: a passive cable approximation

Geometry and membrane properties of the dendrites crucially determine input–output relations in neurons. Unlike geometry often available in detail from computer reconstruction, the membrane resistivity is fragmentarily known if at all. Moreover, it varies during ongoing activity. In this study we address the question: what is the impact of the variation in membrane resistivity on the transfer properties of dendrites? Following a standard approach of the control system theory, we derive and explore the sensitivity functions complementary to the transfer functions of the passive dendrites with arbitrary geometrical parameters (length and diameter) and boundary conditions. We use the location-dependent somatopetal current transfer ratio (the reciprocal of the somatofugal voltage) as the transfer function, and its membrane resistivity derivatives, as the sensitivity functions. In the dendrites, at every path distance from the origin, the sensitivity function in a common form relates the transfer function, membrane resistivity, characteristic input conductance of semi-infinite cable and directional somatofugal input conductances at the given internal site and origin, and the length. Plotted in membrane resistivity versus path distance coordinates, the sensitivity functions display common features: along any coordinate there are low and high ranges, in which the sensitivity, respectively, increases and decreases. The ranges and corresponding rates depend on morphology and boundary conditions in a characteristic manner. These features predict existence of the geometry-dependent range of membrane resistivity (the earlier unattended mid-conductance state), such that the dendrites with a given metrical asymmetry are most distinguished in their transfer properties and electrical states if membrane resistivity is within the range and are not otherwise.     

The response of a nerve cylinder to spatially distributed white noise inputs

The response of a passive nerve cylinder (or dendritic tree in the equivalent cylinder representation) to random white noise input currents is determined. Results for the mean, variance and covariance of the depolarization are obtained for an arbitrary number of independent spatially distributed inputs. The case of a cylinder with sealed ends is considered in detail. The differences that arise when the input currents are distributed over a small but finite region of space instead of concentrated at a point are investigated. In the case of distributed inputs, the expectation is smoother near the stimulus and the variance becomes finite over the entire cable length including the region of the applied stimulus. Away from the stimulus, there are no appreciable differences between the responses for the two cases. The interaction between an excitatory input and an inhibitory input at various locations is examined and one case of more than two inputs is also analysed to study effects which could not have been discerned from point models for a neuron with random inputs.     

Prediction of location of active sites in biologically active peptides

In a previous paper we demonstrated that the short-range compact regions in atrial natriuretic factor (-hANF) predicted by the average distance map (ADM) correspond to its active sites [Kikuchi,J. Protein Chem.11, 579–581 (1992)]. In the present paper we apply the same method to other bioactive peptides and peptidic enzyme inhibitors. We again observe that active sites in each peptide are contained in short-range compact regions predicted by the ADM for the peptide. This demonstrates that the ADM method predicts the possible location of active sites in biologically active peptides in general. The possibility of practical application of the present method to rational drug design is also discussed.     

活性部位的柔性

比较酶在变性过程中构象和活力变化,发现在活性完全丧失时尚无可察 觉的整体构象变化。排除变性剂抑制和寡聚酶解聚等可能性之后,提出了酶活性部位柔性假说。随后用多种实验方法直接证实了活性部位的构象变化先于分子整体构象变化,并与活性丧失同步,根据催化过程中活性部位构旬变化,以及限制活性部位构象变化对酶活性的影响,提出了酶活性部位柔性为酶充分表现其催化活性所必需的设想。     

Effects of polarization induced by non-weak electric fields on the excitability of elongated neurons with active dendrites

An externally-applied electric field can polarize a neuron, especially a neuron with elongated dendrites, and thus modify its excitability. Here we use a computational model to examine, predict, and explain these effects. We use a two-compartment Pinsky-Rinzel model neuron polarized by an electric potential difference imposed between its compartments, and we apply an injected ramp current. We vary three model parameters: the magnitude of the applied potential difference, the extracellular potassium concentration, and the rate of current injection. A study of the Time-To-First-Spike (TTFS) as a function of polarization leads to the identification of three regions of polarization strength that have different effects. In the weak region, the TTFS increases linearly with polarization. In the intermediate region, the TTFS increases either sub- or super-linearly, depending on the current injection rate and the extracellular potassium concentration. In the strong region, the TTFS decreases. Our results in the weak and strong region are consistent with experimental observations, and in the intermediate region, we predict novel effects that depend on experimentally-accessible parameters. We find that active channels in the dendrite play a key role in these effects. Our qualitative results were found to be robust over a wide range of inter-compartment conductances and the ratio of somatic to dendritic membrane areas. In addition, we discuss preliminary results where synaptic inputs replace the ramp injection protocol. The insights and conclusions were found to extend from our polarized PR model to a polarized PR model with I _h dendritic currents. Finally, we discuss the degree to which our results may be generalized.     

Highly active heparin species with multiple binding sites for antithrombin

Porcine heparin has been fractionated by Sephadex G-100 gel filtration and affinity chromatography into mucopolysaccharide species with approximate molecular sizes of 20,000 daltons, and 7000 daltons, respectively. The larger component has a specific anticoagulant activity of 738 USP units/mg and contains two binding regions for antithrombin. The smaller component has a specific anticoagulant activity of 363 USP units/mg and possesses only a single interaction site for the inhibitor. These results provide the first demonstration that heparin molecules may bear multiple binding sites for antithrombin.     

Glutamatergic inputs to the CVLM independent of the NTS promote tonic inhibition of sympathetic vasomotor tone in rats

GABAergic neurons in the caudal ventrolateral medulla (CVLM) are driven by baroreceptor inputs relayed via the nucleus tractus solitarius (NTS), and they inhibit neurons in rostral ventrolateral medulla to reduce sympathetic nerve activity (SNA) and arterial pressure (AP). After arterial baroreceptor denervation or lesions of the NTS, inhibition of the CVLM continues to increase AP, suggesting additional inputs also tonically activate the CVLM. This study examined whether the NTS contributes to baroreceptor-independent drive to the CVLM and whether glutamate promotes baroreceptor- and NTS-independent activation of the CVLM to tonically reduce SNA. In addition, we evaluated whether altering central respiratory drive, a baroreceptor-independent regulator of CVLM neurons, influences glutamatergic inputs to the CVLM. Splanchnic SNA and AP were measured in chloralose-anesthetized, ventilated, paralyzed rats. The infusion of nitroprusside decreased AP below threshold for baroreceptor afferent firing (<50 mmHg) and increased SNA to 209+/-22% (P<0.05), but the subsequent inhibition of the NTS by microinjection of the GABA(A) agonist muscimol did not further increase SNA. In contrast, after inhibition of the NTS, blockade of glutamatergic inputs to CVLM by microinjection of kynurenate increased SNA (274+/-54%; P<0.05; n=7). In vagotomized rats with baroreceptors unloaded, inhibition of glutamatergic inputs to CVLM evoked a larger rise in SNA when central respiratory drive was increased (219+/-16% vs. 271+/-17%; n=5; P<0.05). These data suggest that baroreceptor inputs provide the major drive for the NTS-mediated excitation of the CVLM. Furthermore, glutamate tonically activates the CVLM to reduce SNA independent of the NTS, and this excitatory input appears to be affected by the strength of central respiratory drive.     

Structural identity of active sites of RNases

It is known that the affinity of all nucleoside monophosphate isomers for RNAase active sites increases in the following order: 5'-NMP----3'-NMP----2'-NMP, irrespective of the RNAase type (pyrimidine-specific, guanine-specific or non-specific) and stage of activity (transferase, hydrolase). It is known also that the nucleotides with the same degree of isomerism have substantially the same conformation. It was thus supposed that the structure of active sites of RNAase has common features with a closer similarity in case of pyrimidine-specific (EC 3.1.4.22), guanine-specific (EC 3.1.4.8) and non-specific (EC 3.1.4.23) RNAases.