Surface roughness enhances the osseointegration of titanium headposts in non-human primates.

It is well recognized that micrometer and nanometer sized surface features enhance the skeletal attachment of implants within bone. However, little is known regarding the integration of implants placed outside the bone but in contact with the surface. Loosening of chronic skull anchored headposts in non-human primate based experiments can be a factor. The purpose of this study was to evaluate the effect of a simple and easily applied surface texture on bone apposition to titanium implants fixed to the periosteal surface of the skull. Implants possessed either a polished surface or a textured surface created by grit-basting followed by acid etching. The percent of bone in contact with the implant surface (bone apposition) to three polished and three textured implants was evaluated in one adult female monkey after 14 weeks. Upon harvest, implants were processed for undecalcified histology and regions of bone apposition were quantified using backscatter electron microscopy and digital image analysis. The bone apposition to textured implants was 62±20% and to polished implants was 42±21%. The application of a peak-and-pit like texture to the surface of titanium implants significantly increased bone apposition to titanium implants placed on the periosteal surface of the skull. This study demonstrates that titanium headposts can easily be modified to improve osseointegration using equipment and supplies available to most neurophysiological laboratories. In addition, implant texturing may have utility in areas including skeletal trauma and reconstruction where devices are placed in contact with the bone surface.

Somatic EPSP amplitude is independent of synapse location in hippocampal pyramidal neurons.

Most neurons receive thousands of synaptic inputs onto widely spread dendrites. Because of dendritic filtering, distant synapses should have less efficacy than proximal ones. To investigate this, we characterized the amplitude and kinetics of excitatory synaptic input across the apical dendrites of CA1 pyramidal neurons using dual whole-cell recordings. We found that dendritic EPSP amplitude increases with distance from the soma, counterbalancing the filtering effects of the dendrites and reducing the location dependence of somatic EPSP amplitude. Dendritic current injections and a multi-compartmental computer model demonstrated that dendritic membrane properties have only a minor role in elevating the local EPSP. Instead a progressive increase in synaptic conductance seems to be primarily responsible for normalizing the amplitudes of individual inputs.

Voltage-dependent properties of dendrites that eliminate location-dependent variability of synaptic input.

We examined the hypothesis that voltage-dependent properties of dendrites allow for the accurate transfer of synaptic information to the soma independent of synapse location. This hypothesis is motivated by experimental evidence that dendrites contain a complex array of voltage-gated channels. How these channels affect synaptic integration is unknown. One hypothesized role for dendritic voltage-gated channels is to counteract passive cable properties, rendering all synapses electrotonically equidistant from the soma. With dendrites modeled as passive cables, the effect a synapse exerts at the soma depends on dendritic location (referred to as location-dependent variability of the synaptic input). In this theoretical study we used a simplified three-compartment model of a neuron to determine the dendritic voltage-dependent properties required for accurate transfer of synaptic information to the soma independent of synapse location. A dendrite that eliminates location-dependent variability requires three components: 1) a steady-state, voltage-dependent inward current that together with the passive leak current provides a net outward current and a zero slope conductance at depolarized potentials, 2) a fast, transient, inward current that compensates for dendritic membrane capacitance, and 3) both alpha amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and N-methyl-D-aspartate-like synaptic conductances that together permit synapses to behave as ideal current sources. These components are consistent with the known properties of dendrites. In addition, these results indicate that a dendrite designed to eliminate location-dependent variability also actively back-propagates somatic action potentials.

Active dendrites reduce location-dependent variability of synaptic input trains.

We examined the hypothesis that dendritic voltage-gated channels can reduce the effect synaptic location has on somatic depolarization in response to patterns of short synaptic trains (referred to as location-dependent variability). Three computer models of a reconstructed hippocampal CA1 cell, each of increasing realism and complexity, were used. For each model, the goal was to identify the dendritic composition that best reduced the location-dependent variability. The first model was linear and a single parameter, dendritic membrane conductance (GDm, where Rm = 1/GDm), was varied. Surprisingly, a negative GDm minimized the location-dependent variability. Superposition of the synaptic inputs showed that, compared with passive dendrites, active dendrites increase the mean of the individual responses while decreasing the variance between synapses at different locations. Active dendrites compensate the three components of passive cable signal interference that increase with distance from the soma: the accumulation of charge on dendritic membrane capacitance, the escape of charge across synaptic and nonsynaptic dendritic membrane conductances, and the reduction in synaptic charge entry due to increased depolarization of dendrites located farther from the soma. We also found that the entire active dendritic tree contributes charge to any one active synapse. The second model contained an artificial voltage-dependent current (Iboost) added to passive apical dendrites. The optimal amount of Iboost that minimized location-dependent variability was found to be independent of the strength of individual synaptic inputs but inversely related to the synaptic duration. In the third model, realistic T-type Ca2+ and persistent Na+ channel models were added to passive dendrites and numerically fit to reproduce the effects of Iboost. Both realistic currents minimized synaptic variability. The densities for the realistic dendritic currents were not uniform but showed subtle variations and a slight reduction with distance from the soma. A heteroassociative memory network also was modeled to demonstrate the important relationship between location-dependent variability and memory recall performance. Compared with passive dendrites, active dendrites increased memory storage by reducing recall errors. These simulations demonstrate that active dendrites can minimize the cable properties of passive dendrites and enhance the soma's ability to determine the strength of the synaptic input. These models predict dendrites that minimize location-dependent variability will have an overall negative slope conductance I-V relationship that is tuned precisely.

Fluorescent indicators give biased estimates of intracellular free calcium change in aggregating platelets: implication for studies with human von Willebrand factor.

The ratiometric fluorescent indicators Fura-2 and Indo-1 are considered optimal probes for monitoring intracellular free calcium concentration ([Ca2+]i). Unique problems arise, however, in studying [Ca2+]i changes induced in platelets by von Willebrand factor (vWF). Binding of native multimeric vWF causes extensive platelet aggregation, and is reported to evoke a gradual [Ca2+]i increase. the present investigation examined the reliability of platelet [Ca2+]i measurements in these circumstances. Ristocetin-mediated binding of vWF to human platelets promoted a slow rise in Fura-2 fluorescence ratio. Fura-2 extrusion contributed substantially to this rise, unless blocked by probenecid. Despite this precaution, the platelets were invariably contaminated slightly with extracellular indicator. As aggregation progressively reduced the number of platelets in the spectrofluorometer beam, through settling of the larger aggregates, such extracellular Fura-2 contributed proportionately more to the observed fluorescence. This extraneous signal accounted completely for the fluorescence ratio increase, and apparent [Ca2+]i rise, in response to native multimeric vWF. The same problem arose with Indo-1, whereas the single wavelength indicator Fluo-3 showed the opposite pattern of apparent [Ca2+]i changes. Thus, none of these indicators provides reliable data on [Ca2+]i signals in aggregating platelets. Use of a dimeric form of vWF eliminated the problem of platelet aggregates settling out of suspension, but also virtually abolished the [Ca2+]i increase. These observations may explain some of the inconsistencies among previous investigations of vWF-induced calcium signaling. Moreover, similar problems may arise in studies with other adhesive proteins.

Computer simulations of morphologically reconstructed CA3 hippocampal neurons.

1. We tested several hypotheses with respect to the mechanisms and processes that control the firing characteristics and determine the spatial and temporal dynamics of intracellular Ca2+ in CA3 hippocampal neurons. In particular, we were interested to know 1) whether bursting and nonbursting behavior of CA3 neurons could be accounted for in a morphologically realistic model using a number of the known ionic conductances; 2) whether such a model is robust across different cell morphologies; 3) whether some particular nonuniform distribution of Ca2+ channels is required for bursting; and 4) whether such a model can reproduce the magnitude and spatial distribution of intracellular Ca2+ transients determined from fluorescence imaging studies and can predict reasonable intracellular Ca2+ concentration ([Ca2+]i) distribution for CA3 neurons. 2. For this purpose we have developed a highly detailed model of the distribution and densities of membrane ion channels in hippocampal CA3 bursting and nonbursting pyramidal neurons. This model reproduces both the experimentally observed firing modes and the dynamics of intracellular Ca2+. 3. The kinetics of the membrane ionic conductances are based on available experimental data. This model incorporates a single Na+ channel, three Ca2+ channels (CaN, CaL, and CaT), three Ca(2+)-independent K+ channels (KDR, KA, and KM), two Ca(2+)-dependent K+ channels (KC and KAHP), and intracellular Ca(2+)-related processes such as buffering, pumping, and radial diffusion. 4. To test the robustness of the model, we applied it to six different morphologically accurate reconstructions of CA3 hippocampal pyramidal neurons. In every neuron, Ca2+ channels, Ca(2+)-related processes, and Ca(2+)-dependent K+ channels were uniformly distributed over the entire cell. Ca(2+)-independent K+ channels were placed on the soma and the proximal apical dendrites. For each reconstructed cell we were able to reproduce bursting and nonbursting firing characteristics as well as Ca2+ transients and distributions for both somatic and synaptic stimulations. 5. Our simulation results suggest that CA3 pyramidal cell bursting behavior does not require any special distribution of Ca(2+)-dependent channels and mechanisms. Furthermore, a simple increase in the Ca(2+)-independent K+ conductances is sufficient to change the firing mode of our CA3 neurons from bursting to nonbursting. 6. The model also displays [Ca2+]i transients and distributions that are consistent with fluorescent imaging data. Peak [Ca2+]i distribution for synaptic stimulation of the nonbursting model is broader when compared with somatic stimulation. Somatic stimulation of the bursting model shows a broader distribution in [Ca2+]i when compared with the nonbursting model.(ABSTRACT TRUNCATED AT 400 WORDS)

Using paired-pulse facilitation to probe the mechanisms for long-term potentiation (LTP).

Paired-pulse facilitation (PPF) of excitatory synaptic transmission at Schaffer collateral synapses in the hippocampus was examined in relationship to long-term potentiation (LTP). PPF is a relatively simple-to-measure presynaptic form of synaptic plasticity. It is hypothesized that if the expression of LTP includes a presynaptic component, then PPF and LTP may interfere with one another. When averaged over more than 100 experiments, we observed no change in average PPF with LTP, as reported previously by a number of investigators. When individual experiments were analyzed, however, PPF significantly increased or decreased with LTP in direct relation to the initial value of PPF. There was also a linear relationship between the change in PPF and the magnitude of LTP. The PPF changes were specific to LTP and presynaptic in origin as they were input-specific and persisted with low concentrations of CNQX, GABAA and GABAB antagonists, different interstimulus intervals, and different Ca2+ concentrations. To understand the interaction between LTP and PPF, we constructed a simple model of LTP in which potential contributions by increases in three synaptic parameters were examined: the number of neurotransmitter release sites (n), the probability of release (p), and the postsynaptic unit potential (q). The data were fit by a model in which there were increases in n that changed the average p of the population, but not by a model that increased p or q alone. This is the first experimental evidence for an increase in the number of release sites with LTP, which could be due to pre- or postsynaptic mechanisms.

Changes in paired-pulse facilitation suggest presynaptic involvement in long-term potentiation.

Long-term potentiation (LTP) is a use-dependent form of synaptic plasticity that is of great interest as a potential cellular substrate underlying memory. It is important to determine the pre- and/or postsynaptic locus of LTP expression in order to study its underlying mechanisms. Despite intensive investigation, however, its locus of expression remains uncertain. It has been hypothesized that if LTP expression includes a presynaptic locus then it may alter the expression of another presynaptically mediated form of potentiation like paired-pulse facilitation (PPF), which is an increase in a second population excitatory postsynaptic potential when it is elicited shortly after a first. Previous authors have found no change in PPF in association with LTP. We re-examined the hypothesis, however, to reconcile the negative PPF data with other data that have suggested presynaptic involvement in LTP. Extracellular recordings were made in area CA1 of the rat hippocampal slice preparation. Surprisingly, PPF both increased and decreased significantly in association with LTP. The changes in PPF occurred in a predictable way, however. They correlated inversely with initial PPF magnitude so that a larger initial PPF was associated with a decrease in PPF with LTP while a smaller initial PPF was associated with an increase. Because PPF increased or decreased in individual slices in association with LTP, the average PPF of all slices did not change, in agreement with previous studies. The changes in PPF were also specific to LTP; that is, they were input specific, were not due to changes in inhibition or nonspecific effects of high-frequency stimulation, were not due to active postsynaptic currents or their nonlinear summation, and PPF changed with the same time course as LTP. We conclude that the mechanism of early LTP expression includes at least the presynaptic locus. Two hypotheses regarding the presynaptic mechanism underlying LTP expression, which are consistent with finding both increases and decreases in PPF with LTP, are (1) that there is an increase in the number of release sites with LTP or (2) that there is an increase in both the number of release sites and the probability of neurotransmitter release. Increases in the probability of neurotransmitter release alone would not appear to account for our findings since such increases have been associated only with decreases in PPF. Our findings do not exclude additional postsynaptic involvement.(ABSTRACT TRUNCATED AT 400 WORDS)

Interference with oxidative processes inhibits proliferation of human peripheral blood lymphocytes and murine B-lymphocytes.

A number of agents capable of interfering with oxidative events were found to inhibit, in a dose-dependent manner, DNA synthesis in isolated human peripheral blood lymphocytes stimulated with phytohaemagglutinin, or phorbol myristate acetate plus ionomycin. These inhibitory substances were: the iron chelators desferrioxamine and desferrithiocin; the electron acceptor ferricyanide; the anti-oxidant nordihydroguaiaretic acid; ebselen, an agent with glutathione peroxidase-like activity; and diphenylene iodonium, an inhibitor of NADPH-oxidase. The actions of desferrioxamine and desferrithiocin were abolished by prior saturation with iron. Ferrocyanide was much less active in inhibiting human lymphocyte DNA synthesis than its redox partner ferricyanide. Desferrioxamine, ferricyanide and nordihydroguaiaretic acid also inhibited lipopolysaccharide-initiated DNA synthesis in mouse splenocytes in vitro. The common property of these structurally dissimilar agents is their ability to prevent formation of, or detoxify, reactive oxygen species. Thus, the data are consistent with an obligatory role for reactive oxygen formation in human T-cell and mouse B-cell activation at a stage prior to DNA synthesis.

Neoplasia in adoptively immunosuppressed rats. A possible model for tumorigenesis in transplant recipients.

An extremely high incidence of malignant tumors was observed in groups of rats that had previously been exposed to whole body irradiation, grafted with allogeneic tissue, and injected with lymphocytes capable of specifically suppressing the rejection of the grafted tissue. Neoplasia in these adoptively immunosuppressed rats had features in common with that in therapeutically immunosuppressed transplant recipients. Increased tumor incidence could not be accounted for on the basis of the effects of whole body irradiation or failure of immune surveillance, nor could it be a direct effect of lymphoid tissue stimulation. It is suggested that cell mediated suppressor responses play a critical role in tumorigenesis. The mechanism of this is not simply direct stimulation of lymphoid tissue proliferation.