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J W Clark

Publications and source records attributed to J W Clark.

At least 55 records · Page 3Linked to original sources
PubMed1997

Phase response characteristics of model neurons determine which patterns are expressed in a ring circuit model of gait generation.

In order to assess the relative contributions to pattern-generation of the intrinsic properties of individual neurons and of their connectivity, we examined a ring circuit composed of four complex physiologically based oscillators. This circuit produced patterns that correspond to several quadrupedal gaits, including the walk, the bound, and the gallop. An analysis using the phase response curve (PRC) of an uncoupled oscillator accurately predicted all modes exhibited by this circuit and their phasic relationships--with the caveat that in certain parameter ranges, bistability in the individual oscillators added nongait patterns that were not amenable to PRC analysis, but further enriched the pattern-generating repertoire of the circuit. The key insights in the PRC analysis were that in a gait pattern, since all oscillators are entrained at the same frequency, the phase advance or delay caused by the action of each oscillator on its postsynaptic oscillator must be the same, and the sum of the normalized phase differences around the ring must equal to an integer. As suggested by several previous studies, our analysis showed that the capacity to exhibit a large number of patterns is inherent in the ring circuit configuration. In addition, our analysis revealed that the shape of the PRC for the individual oscillators determines which of the theoretically possible modes can be generated using these oscillators as circuit elements. PRCs that have a complex shape enable a circuit to produce a wider variety of patterns, and since complex neurons tend to have complex PRCs, enriching the repertoire of patterns exhibited by a circuit may be the function of some intrinsic neuronal complexity. Our analysis showed that gait transitions, or more generally, pattern transitions, in a ring circuit do not require rewiring the circuit or any changes in the strength of the connections. Instead, transitions can be achieved by using a control parameter, such as stimulus intensity, to sculpt the PRC so that it has the appropriate shape for the desired pattern(s). A transition can then be achieved simply by changing the value of the control parameter so that the first pattern either ceases to exist or loses stability, while a second pattern either comes into existence or gains stability. Our analysis illustrates the predictive value of PRCs in circuit analysis and can be extended to provide a design method for pattern-generating circuits.

Animals↗
PubMed1997

Phase sensitivity and entrainment in a modeled bursting neuron.

A model of neuron R15 in Aplysia was used to study the mechanisms determining the phase-response curve (PRC) of the cell in response to both extrinsic current pulses and modeled synaptic input and to compare entrainment predictions from PRCs with those from actual simulations. Over the range of stimulus parameters studied, the PRCs of the model exhibited minimal dependence upon stimulus amplitude, and a strong dependence upon stimulus duration. State-space analysis of the effect of transient current pulses provided several important insights into the relationship between the PRC and the underlying dynamics of the model, such as a correlation between the prestimulus concentration of Ca2+ and the poststimulus phase of the oscillation. The system nullclines were also found to provide well-defined limits upon the perturbatory extent of a hyperpolarizing input. These results demonstrated that experimentally applied current pulses are sufficient to determine the shape of the PRC in response to a synaptic input, provided that the duration of the current pulse is of a duration similar to that of the evoked synaptic current. Furthermore, we found that predictions of phase-locked 1:m entrainment from PRCs were valid, even when the duration of the periodically applied pulses were a significant portion of the control limit cycle.

Animals↗
PubMed1997

A dynamic model of ventricular interaction and pericardial influence.

A mathematical model describing the dynamic interaction between the left and the right ventricle over the complete cardiac cycle is presented. The pericardium-bound left and right ventricles are represented as two coupled chambers consisting of the left and right free walls and the interventricular septum. Time-varying pressure-volume relationships characterize the component compliances, and the interaction of these components produces the globally observed ventricular pump properties (total chamber pressure and volume). The model 1) permits the simulation of passive (diastolic) and active (systolic) ventricular interaction, 2) provides temporal profiles of hemodynamic variables (e.g., ventricular pressures, volumes, and flow) that agree well with reported observations, and 3) can be used to examine the effect of the pericardium on ventricular interaction and ventricular mechanics. It can be reduced to equivalency with models previously reported by invoking simplifying assumptions. Furthermore, model-generated "dynamic interaction gains" are employed to quantify the mode and degree of ventricular interaction. The model also yields qualitative predictions of septal and free wall displacements similar to those detected experimentally via M-mode echocardiography. Such analogies may be extended easily to the study of pathophysiological states via appropriate modifications to 1) the pressure-volume characteristics of the component walls (and/or pericardium) and/or 2) the specific time course of activation of the ventricular free wall or the septum. A limited number of examples are included to demonstrate the utility of the model, which may be used as an adjunct to new experimental investigations into ventricular interaction.

Blood Volume↗
PubMed1997

Perspectives on new chemotherapeutic agents in the treatment of colorectal cancer.

In patients with metastatic colorectal cancer (CRC), conventional chemotherapy with 5-fluorouracil (5-FU) plus leucovorin provides an overall response rate of approximately 25% but has had little effect on survival. Thus, alternate agents, new combinations of agents, and new treatment strategies are being investigated. Research efforts over the past decade have increased our understanding of how anticancer agents mediate their antitumor effects, and specific targets for inhibiting the survival, growth, or metastasis of CRC cells have been elucidated. Advances in our understanding have led not only to improvements in the application of currently available agents, but also to the discovery of new agents with activity in CRC. The following active areas of research and/or treatment approaches are discussed: (1) approaches for enhancing 5-FU/leucovorin activity; (2) novel delivery of 5-FU or 5-FU precursor agents; (3) new thymidylate synthase inhibitors; (4) new platinum analogues; (5) topoisomerase I inhibitors; (6) targeting specific proteins or pathways important for the growth, survival, or metastasis of CRC cells; (7) biologic response modifiers, including monoclonal antibodies; and (8) gene therapy. As the cellular mechanisms involved in CRC are further defined and chemotherapy or biologic agents more precisely targeted, response rates and ultimately survival will hopefully improve in this patient population.

Animals↗
PubMed1996

Systemic and adjuvant therapy for patients with pancreatic carcinoma.

BACKGROUND: Pancreatic cancer is a highly lethal disease with < or = 5% of patients surviving 5 years. There is no curative therapy for patients who cannot be surgically resected. Chemotherapy and radiation therapy can provide palliation but have not had a significant impact on 5-year survival. METHODS: Newer approaches for improving the survival of patients with pancreatic cancer integrating chemotherapy, radiation therapy, and surgery are being evaluated. New chemotherapeutic agents (e.g., gemcitabine, camptothecins, taxanes, thymydilate synthase inhibitors, and fluorouracil-related compounds) are being studied alone and in combination with each other or different agents (e.g., trimetrexate or platinum-related compounds). RESULTS: Increased knowledge about the biology of pancreatic cancer (including high frequency of ras and p53 mutations in neoplastic cells or the expression of a number of growth factor receptors on the cell surface) has lead to preclinical evaluation of novel approaches attempting to specifically target these. These novel approaches include gene therapy, vaccines, and antisense oligonucleotides targeted to genes important for proliferation or survival of pancreatic cancer cells. CONCLUSIONS: Continued development of new approaches is needed to improve the treatment and survival of patients with pancreatic cancer.

Chemotherapy, Adjuvant↗
PubMed1996

Effects of tyrosine kinase inhibitors on the proliferation of human breast cancer cell lines and proteins important in the ras signaling pathway.

Breast cancers frequently over-express a number of growth factor receptors. In addition, elevated src family kinase activity is present in a percentage of these neoplasms and has been implicated in signal transduction in these cells. Therefore, inhibiting tyrosine kinase activity is a potential approach for treating these tumors. Utilizing the SKBR3 and MCF-7 breast cancer cell lines, we evaluated the effects of broadly targeting growth factor receptor and cytoplasmic tyrosine kinases with tyrosine kinase inhibitors (herbimycin A and genistein) to inhibit proliferation. We also evaluated these inhibitor's effects on proteins that regulate ras function, which is a convergence point for signaling through both src family kinases and a number of growth factor receptors with tyrosine kinase activity (e.g., epidermal growth factor and erbB-2 receptors). We specifically evaluated whether these compounds affected 2 recently discovered proteins involved in controlling ras function: Shc, which is tyrosine-phosphorylated by src and activated growth factor receptors, and Grb-2, which mediates signal transduction from activated growth factor receptors through ras. We evaluated their effects on tyrosine phosphorylation of Shc, binding of Grb-2 to Shc and MAP kinase activity. Both cell lines were inhibited in a dose-dependent manner by each compound. This was accompanied by decreased Shc tyrosine phosphorylation, Shc's association with Grb-2 and MAP kinase activity. Thus, tyrosine kinase inhibitors can inhibit proliferation of breast cancer cells, accompanied by inhibition of signal transduction steps potentially mediated through ras. Tyrosine kinase inhibitors might, therefore, be useful for the treatment of breast cancer.

Adaptor Proteins, Signal Transducing↗
PubMed1996

Characterization of a hyperpolarization-activated inward current in cultured pacemaker cells from the sinoatrial node.

The perforated-patch, whole-cell voltage-clamp technique was used to characterize a hyperpolarization-activated inward current (if) in cultured pacemaker cells isolated from the rabbit sinoatrial (S-A) node. A modified Tyrode solution was used to block interfering currents. After correcting for uncompensated series resistance, leakage current and liquid-junction potentials between the pipette and bath, we obtained the following values for the maximum conductance and reversal potential of (i(f)): 16.2 +/- 2.8 nS or 0.26 +/- 0.07 nS/pF (0.37 +/- 0.10 mS/cm2); and -27.7 +/- 1.2 mV (n = 10). In the modified Tyrode solution, the threshold for activation of i(f) and the half-activation potential and slope factor for Boltzmann fits were -66 +/- 1 mV (n = 53), -77.2 +/- 2.0 mV and 4.2 +/- 0.4 mV (n = 16), respectively. The time course of i(f) was best fit by a sum of two exponentials. Between -110 and -5 mV, time constants ranged from 0.06 to 0.42 s for the rapid component and from 0.65 to 4.39 s for the slow component. Using these characteristics of i(f) and a short segment of spontaneous firing as the membrane potential, we computed the expected time course of i(f) during normal pacemaker activity. It reached a maximum of -3 pA during the diastolic depolarization and +4 pA during the action potential. We conclude that most of the characteristics of i(f) in cultured S-A node cells are similar to those measured previously in freshly isolated pacemaker cells.

Animals↗
PubMed1996

Dissection and reduction of a modeled bursting neuron.

An 11-variable Hodgkin-Huxley type model of a bursting neuron was investigated using numerical bifurcation analysis and computer simulations. The results were applied to develop a reduced model of the underlying subthreshold oscillations (slow-wave) in membrane potential. Two different low-order models were developed: one 3-variable model, which mimicked the slow-wave of the full model in the absence of action potentials and a second 4-variable model, which included expressions accounting for the perturbational effects of action potentials on the slow-wave. The 4-variable model predicted more accurately the activity mode (bursting, beating, or silence) in response to application of extrinsic stimulus current or modulatory agents. The 4-variable model also possessed a phase-response curve that was very similar to that of the original 11-variable model. The results suggest that low-order models of bursting cells that do not consider the effects of action potentials may erroneously predict modes of activity and transient responses of the full model on which the reductions are based. These results also show that it is possible to develop low-order models that retain many of the characteristics of the activity of the higher-order system.

Animals↗
PubMed1996

1alpha,25-dihydroxy-24-oxo-16-ene vitamin D3, a metabolite of a synthetic vitamin D3 analog, 1alpha,25-dihydroxy-16-ene vitamin D3, is equipotent to its parent in modulating growth and differentiation of human leukemic cells.

1alpha,25(OH)2-16-ene-D3, a synthetic analog of the steroid hormone, 1alpha,25(OH)2D3, has great potential to become a drug in the treatment of leukemia and other proliferative disorders, because of its minimal in vivo calcemic activity associated with a potent inhibitory effect on cell growth. However, at present, the mechanisms through which 1alpha,25(OH)2-16-ene-D3 expresses its biological activities are still not completely understood. Our previous in vitro study in a perfused rat kidney indicated for the first time that 1alpha,25(OH)2-16-ene-D3 and 1alpha,25(OH)2D3 are metabolized differently. 1alpha,25(OH)2-24-oxo-16-ene-D3, an intermediary metabolite of 1alpha,25(OH)2-16-ene-D3 formed through the C-24 oxidation pathway, accumulated significantly in the perfusate when compared to 1alpha,25(OH)2-24-oxo-D3, the corresponding intermediary metabolite of 1alpha,25(OH)2D3. In a subsequent in vivo study, we also reported that 1alpha,25(OH)2-24-oxo-16-ene-D3 exerted immunosuppressive activity equal to its parent, without causing significant hypercalcemia. In order to establish further the critical role of 1alpha,25(OH)2-24-oxo-16-ene-D3, in generating some of the key biological activities ascribed to its parent, we performed the present in vitro study using a human myeloid leukemic cell line (RWLeu-4) as a model. Comparative target tissue metabolism studies indicated that 1alpha,25(OH)2-16-ene-D3 and 1alpha,25(OH)2D3 are metabolized differently in RWLeu-4 cells, and the differences were similar to the ones we previously observed in the rat kidney. The significant finding was the accumulation of 1alpha,25(OH)2-24-oxo-16-ene-D3 in RWLeu-4 cells because of its resistance to further metabolism. Biological activity studies indicated that both 1alpha,25(OH)2-16-ene-D3 and its 24-oxo metabolite produced growth inhibition and promoted differentiation of RWLeu-4 cells to the same extent, and these activities were several fold higher than those exerted by 1alpha,25(OH)2D3. In addition, the genomic action of each vitamin D compound was assessed in a rat osteosarcoma cell line (ROS 17/2.8) by measuring its ability to transactivate a gene construct containing the vitamin D response element of the osteocalcin gene linked to the growth hormone reporter gene. In these studies, both 1alpha,25(OH)2-16-ene-D3 and its 24-oxo metabolite exerted similar but potent transactivation activity which was several fold greater than that exerted by 1alpha,25(OH)2D3 itself. In summary, our results indicate that the production and slow clearance of the bioactive intermediary metabolite, 1alpha,25(OH)2-24-oxo-16-ene-D3, in RWLeu-4 cells contributes significantly to the final expression of the enhanced biological activities ascribed to its parent analog, 1alpha,25(OH)2-16-ene-D3.

Calcitriol↗
PubMed1996

Substance P concentration and history of headache in relation to postlumbar puncture headache: towards prevention.

Medical treatment of postlumbar puncture headache (post-LP HA) is often difficult and ineffective. Prevention would be preferable to more invasive procedures, including blood patch. The aim was to determine the incidence of post-LP HA in two suspected high risk groups compared with the general outpatient population. Based on previous research, it was hypothesised that a low substance P concentration, or a history of chronic headache, or both would be associated with a higher risk of post-LP HA. A total of 310 randomly selected patients undergoing diagnostic lumbar puncture in the outpatient neurology clinic over 30 consecutive months were studied. Follow up was by headache questionnaire or phone survey after diagnostic lumbar puncture. Substance P was measured by radioimmunoassay on a subset of 102 samples of CSF. The overall incidence of post-LP HA was 38%. Patients with a measured substance P value < 1.3 pg/ml were three times as likely to have post-LP HA than those with a higher value. A history of chronic or recurrent headache was reported by 57% of those who developed post-LP HA. This group was also three times as likely to experience post-LP HA as those who did not have chronic headaches.

Adult↗
PubMed1996

A model of the action potential and underlying membrane currents in a rabbit atrial cell.

We have developed a mathematical model of the rabbit atrial myocyte and have used it in an examination of the ionic basis of the atrial action potential. Available biophysical data have been incorporated into the model to quantify the specific ultrastructural morphology, intracellular ion buffering, and time- and voltage-dependent currents and transport mechanisms of the rabbit atrial cell. When possible, mathematical expressions describing ionic currents identified in rabbit atrium are based on whole cell voltage-clamp data from enzymatically isolated rabbit atrial myocytes. This membrane model is coupled to equations describing Na+, K+, and Ca2+ homeostasis, including the uptake and release of Ca2+ by the sarcoplasmic reticulum and Ca2+ buffering. The resulting formulation can accurately simulate the whole cell voltage-clamp data on which it is based and provides fits to a family of rabbit atrial cell action potentials obtained at 35 degrees C over a range of stimulus rates (0.2-3.0 Hz). The model is utilized to provide a qualitative prediction of the intracellular Ca2+ concentration transient during the action potential and to illustrate the interactions between membrane currents that underlie repolarization in the rabbit atrial myocyte.

Action Potentials↗
PubMed1996

Bistability and its regulation by serotonin in the endogenously bursting neuron R15 in Aplysia.

1. Previous computational studies of models of neuron R15 in Aplysia have indicated that several distinct modes of electrical activity may coexist at a given set of parameters, that this multistability can be modulated by transmitters such as serotonin (5-HT) and that brief perturbations of the membrane potential can induce persistent changes in the mode of electrical activity. To test these predictions, the responses of R15 neurons to injections of brief (1.5 s) current pulses were recorded intracellularly in the absence and presence of 5-HT. 2. In the absence of 5-HT, brief perturbations induced abrupt transitions in the electrical activity from bursting to beating. Such transitions were observed in approximately 20% of the cases. The duration of beating activity varied from several seconds to tens of minutes. In the presence of low concentrations (1 microM) of 5-HT, both the probability of mode transitions and the duration of induced beating activity increased significantly. 3. These results indicate that at least two stable modes of electrical activity can coexist in R15 neurons and that this bistability can be regulated by 5-HT. In general, these conclusions agree with the results from analyses of mathematical models of R15. Although the function of these dynamic properties in R15 is speculative, our results, interpreted on the background of the model, support the notion that nonlinear dynamical properties of individual neurons can endow them with richer forms of information processing than has generally been appreciated.

Amino Acid Sequence↗
PubMed1995

Analysis of the effects of modulatory agents on a modeled bursting neuron: dynamic interactions between voltage and calcium dependent systems.

In a computational model of the bursting neuron R15, we have implemented proposed mechanisms for the modulation of two ionic currents (IR and ISI) that play key roles in regulating its spontaneous electrical activity. The model was sufficient to simulate a wide range of endogenous activity in the presence of various concentrations of serotonin (5-HT) or dopamine (DA). The model was also sufficient to simulate the responses of the neuron to extrinsic current pulses and the ways in which those responses were altered by 5-HT or DA. The results suggest that the actions of modulatory agents and second messengers on this neuron, and presumably other neurons, cannot be understood on the basis of their direct effects alone. It is also necessary to take into account the indirect effects of these agents on other unmodulated ion channels. These indirect effects occur through the dynamic interactions of voltage-dependent and calcium-dependent processes.

Animals↗
PubMed1995

Hypersensitivity to substance P in the etiology of postlumbar puncture headache.

OBJECTIVE: Postlumbar puncture headache may represent a model which could be used to test the hypothesis that headache pain is caused by the release of substance P in patients who are predisposed to headache due to hypersensitivity to substance P. METHODS: We measured substance P in CSF and plasma in 37 patients undergoing diagnostic lumbar puncture. In 9 patients, plasma samples were obtained before lumbar puncture, in 28 patients plasma was obtained after lumbar puncture. Patients were followed up by telephone to determine if they developed postlumbar puncture headache. Patients were also asked about a history of chronic or recurrent headaches. Substance P was determined by radioimmunoassay. RESULTS: The mean plasma substance P levels obtained before lumbar puncture was 1.0 +/- 0.1 pg/mL and 1.3 +/- 1.2 after lumbar puncture (P < 0.0005). The mean plasma substance P levels in subjects who developed postlumbar puncture headache was 0.6 +/- 0.6 pg/mL compared with 1.4 +/- 1.5 in subjects who remained headache-free (P < 0.05). The mean CSF substance P levels in subjects who developed postlumbar puncture headache was 0.7 +/- 0.5 pg/mL compared with 1.2 +/- 0.8 in subjects who remained headache-free (P < 0.05). There were no significant differences in substance P levels between chronic headache sufferers and nonheadache subjects. CONCLUSIONS: Postlumbar puncture headache may be mediated by the release of substance P triggered by lumbar puncture, in patients predisposed to headache by a hypersensitivity to substance P. Hypersensitivity to substance P may also represent a mechanism for headache pain in other headache disorders.

Adult↗
PubMed1995

Afferent synaptic drive of rat medial nucleus tractus solitarius neurons: dynamic simulation of graded vesicular mobilization, release, and non-NMDA receptor kinetics.

1. We have developed a comprehensive mathematical model of an afferent synaptic connection to the soma of a medial nucleus tractus solitarius (mNTS) neuron. Model development is based on numerical fits to quantitative data recorded in our laboratory. This work is part of a continuing collaborative effort aimed at identifying and characterizing the mechanisms responsible for the non-linear integrative properties of this first synapse in the baroreceptor reflex. 2. The complete model consists of three major parts: 1) a Hodgkin-Huxley (HH)-type membrane model of the prejunctional sensory terminal bouton; 2) a multistage model describing vesicular storage, adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca(2+)-dependent mobilization, release and recycling; and 3) a HH-type membrane model of the postjunctional mNTS cell that includes descriptions for a desensitizing non-N-methyl-D-aspartate (NMDA) ionic current that is responsible for the fast excitatory postsynaptic potentials (EPSPs) observed in mNTS cells. The membrane models for both the terminal bouton and the mNTS neuron are coupled to separate lumped fluid compartment models describing intracellular Ca2+ ion concentration dynamics. 3. Our modeling strategy is twofold. The first is to validate model performance by reproducing a wide variety of experimental data both from our laboratory and from the literature. The second is to explore the functional aspects of the model in order to gain a greater appreciation for the balance between presynaptic mechanisms (e.g., terminal membrane properties and vesicular dynamics) and postsynaptic mechanisms (e.g., non-NMDA receptor kinetics and neuronal dynamics) that underlie the afferent synaptic drive of mNTS neurons. 4. The model accurately reproduces EPSP dynamics recorded with the use of a wide range of stimulus protocols. The model can also mirror the unique pattern of graded frequency- and use-dependent reduction in peak EPSP magnitude observed experimentally through 60 s of constant, suprathreshold synaptic activation. We demonstrate how vesicular mobilization, recycling, and receptor kinetics can function synergistically in establishing synaptic transfer. Furthermore, we show that by allowing the aggregate rate of vesicle mobilization to respond in a use-dependent manner, it is possible to compensate for the attenuating affects of desensitization at elevated rates of stimulation. 5. Our simulations indicate that the low-frequency characteristics of this synapse are dominated by vesicular dynamics, whereas the high-frequency properties arise from a combination of Ca(2+)-dependent vesicular mobilization and the kinetics of the non-NMDA receptor. Desensitization can influence the peak magnitude and decay time of the EPSP, thereby affecting synaptic throughput. However, we demonstrate that, as the time course of neurotransmitter in the synaptic cleft decreases, the influence of desensitization should be somewhat diminished. As a result, the effective bandwidth of the synapse increases and becomes limited by the gating characteristics of the non-NMDA channel. 6. The model also includes a neuromodulatory aspect in that the frequency response of the synapse can be modulated by an adenylate cyclase-mediated regulatory mechanism. Although our simulations indicate the behavior of a limited number of possible neuromodulatory agents, the results demonstrate the pivotal role such agents could play in modifying synaptic transfer characteristics presynaptically. 7. Both continuous and burst-mode tract stimulation evoke patterns of action potentials in spontaneously active mNTS neurons that are mimicked very well by our model. Our simulations demonstrate that, as the rate of stimulation increases beyond approximately 20-30 Hz, the inherent low-pass frequency-response characteristics of the synapse limit the overall dynamic range of the mNTS neuron, causing the postsynaptic cell to "entrain" at frequencies within its normal operating range.

Afferent Pathways↗