Synthesis, NMDA receptor antagonist activity, and anticonvulsant action of 1-aminocyclobutanecarboxylic acid derivatives

A range of cis- and trans-3-substituted 1-aminocyclobutane-1-carboxylic acids has been synthesized and evaluated for antagonism at excitatory amino acid receptor sites and for anticonvulsant activity. Potent and selective antagonist activity at N-methyl-D-aspartate (NMDA) receptor sites in neonatal rat motoneurones was shown by compounds in which the 3-substituent was, or contained, a 2'-carboxyethyl or 2'-phosphonoethyl moiety. Substances 4b, 24, 35, and 40 were more potent than the standard NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (D-AP5) as NMDA antagonists in this preparation, and about equipotent with [3-(+/-)-2-carboxypiperazin-4-yl)-1-propyl]phosphonate (CPP). Anticonvulsant activity, as assessed following intracerebroventricular injection into audiogenic DBA/2 mice, generally paralleled NMDA receptor antagonist activity.

The gamma-aminobutyric acid uptake inhibitor, tiagabine, is anticonvulsant in two animal models of reflex epilepsy

The effects of i.p. administration of the gamma-aminobutyric acid (GABA) uptake inhibitors R(-)N-(4,4-di(3-methylthien-2-yl)-but-3-enyl) nipecotic acid hydrochloride (tiagabine; molecular weight 412.0), (1-(2-(((diphenylmethylene)-amino)oxy)ethyl)-1,2,5,6-tetrahydro-3- pyridinecarboxylic acid hydrochloride (NNC-711; molecular weight 386.9), and (+/-)-nipecotic acid (molecular weight 128.2) are compared with those of carbamazepine (molecular weight 236.3) on sound-induced seizures and locomotor performance in genetically epilepsy-prone (GEP) rats. The ED50 value against clonic seizures (in mumol kg-1 at the time of maximal anticonvulsant effect) for tiagabine was 23 (0.5 h), and for NNC-711 was 72 (1 h), and for carbamazepine was 98 (2 h). (+/-)-Nipecotic acid (0.4-15.6 mmol kg-1) was not anticonvulsant. High doses of NNC-711 (207-310 mumol kg-1) and of (+/-)-nipecotic acid (39-78 mmol kg-1) induced ataxia and myoclonic seizures 0.25-1 h. Tiagabine and carbamazepine did not induce myoclonic seizures and had similar therapeutic indices (locomotor deficit ED50/anticonvulsant ED50) ranging from 0.4 to 1.9. In Papio papio, we observed a reduction in photically induced myoclonic seizures with tiagabine (2.4 mumol kg-1 i.v.) accompanied with neurological impairment. Tiagabine has comparable anticonvulsant action to carbamazepine in rats and has anticonvulsant effects in non-human primates supporting the potential use of inhibitors of GABA uptake as therapy for epilepsy.

Anticonvulsant activity of a novel NMDA/glycine site antagonist, MDL 104,653, against kindled and sound-induced seizures

MDL 104,653 (3-phenyl-4-hydroxy-7-chloro-quinolin-2(1H)-one), acts as an antagonist at the glycine site of the NMDA receptor. MDL 104,653 protects against sound-induced clonic seizures in DBA/2 mice following intracerebroventricular (ED50 = 19.1 nmol, 30 min), intraperitoneal (i.p.; ED50 = 6.1 mumol/kg, 45 min), or oral (ED50 = 23.0 mumol/kg, 2 h) administration. Optimal protection by MDL 104,653 was observed 15-60 min after i.p. administration, and the therapeutic index, as assessed by rotarod performance, was 4.0 at 45 min after i.p. administration. Fully amygdala-kindled motor seizures in rats were significantly reduced at 15, 30 and 60 min, and the duration of the after-discharge was significantly shortened at 30 min after the i.p. administration of 74 mumol/kg MDL 104,653. A lower dose of MDL 104,653 (37 mumol/kg) had no significant effect on either motor seizures or after-discharge duration. The rate of amygdala kindling was also significantly retarded following the daily administration of 56 mumol/kg MDL 104,653 (1 times daily for 6 days; i.p. 30 min before kindling stimulus).

Lytic effect of heparin on liposomes: possible mechanism of lysis of red blood cells by heparin

Heparin causes lysis of the multilamellar liposomes of all three charges, positive, neutral, and negative, and thus releases the entrapped [3H]glucose or chromate. The lytic effect of heparin is also observed in liposomes prepared from the lipids extracted from human red blood cells. Heparin is found to interact with the phospholipid bilayers, which suggests that the reported lytic effect of heparin on the red blood cells may be mediated through the membrane phospholipid components of these cells.

Probing soft fibrous materials by indentation

Indentation is often used to measure the stiffness of soft materials whose main structural component is a network of filaments, such as the cellular cytoskeleton, connective tissue, gels, and the extracellular matrix. For elastic materials, the typical procedure requires fitting the experimental force-displacement curve with the Hertz model, which predicts that f=kδ1.5 and k is proportional to the reduced modulus of the indented material, E/(1-ν2). Here we show using explicit models of fiber networks that the Hertz model applies to indentation in network materials provided the indenter radius is larger than approximately 12lc, where lc is the mean segment length of the network. Using smaller indenters leads to a relation between force and indentation displacement of the form f=kδq, where q is observed to increase with decreasing indenter radius. Using the Hertz model to interpret results of indentations in network materials using small indenters leads to an inferred modulus smaller than the real modulus of the material. The origin of this departure from the classical Hertz model is investigated. A compacted, stiff network region develops under the indenter, effectively increasing the indenter size and modifying its shape. This modification is marginal when large indenters are used. However, when the indenter radius is small, the effect of the compacted layer is pronounced as it changes the indenter profile from spherical towards conical. This entails an increase of exponent q above the value of 1.5 corresponding to spherical indenters. STATEMENT OF SIGNIFICANCE: The article presents a study of indentation in network biomaterials and demonstrates a size effect which precludes the use of the Hertz model to infer the elastic constants of the material. The size effect occurs once the indenter radius is smaller than approximately 12 times the mean segment length of the network. This result provides guidelines for the selection of indentation conditions that guarantee the applicability of the Hertz model. At the same time, the finding may be used to infer the mean segment length of the network based on indentations with indenters of various sizes. Hence, the method can be used to evaluate this structural parameter which is not easily accessible in experiments.

Investigation of Geometric Landscape and Structure-Property Relations for Colloidal Superstructures Using Genetic Algorithm

Over the past two decades, colloidal particles with a variety of shapes, sizes, and compositions have been synthesized and characterized successfully. One of the most important applications for colloidal building blocks is to engineer functional structures as mechanical, electrical, and optical metamaterials. However, complex interaction dynamics between the building blocks as well as sophisticated structure-property relationships make it challenging to design structures with predictable target properties. In this paper, we implement an inverse material design framework using Genetic Algorithm (GA)-based techniques to streamline the design of colloidal structures based on target properties. We investigate spherical particles as well as colloidal molecules of different sizes and shapes and evaluate a Geometric Landscape Accessibility parameter that identifies the size of feasible domains within the geometric phase space of each structure. Considering target photonic properties, our GA-assisted framework is further utilized to identify sets of building blocks and structures that lead to various target values for the size of the photonic band gaps. The proposed framework in this study will provide new insight for predictive computational material design approaches and help establish more efficient ways of understanding structure-property relations in sub-micrometer-scale materials.

Stiffening mechanisms in stochastic athermal fiber networks

Stochastic athermal networks composed of fibers that deform axially and in bending strain stiffen much faster than thermal networks of axial elements, such as elastomers. Here we investigate the physical origin of stiffening in athermal network materials. To this end, we use models of stochastic networks subjected to uniaxial deformation and identify the emergence of two subnetworks, the stress path subnetwork (SPSN) and the bending support subnetwork (BSSN), which carry most of the axial and bending energies, respectively. The BSSN controls lateral contraction and modulates the organization of the SPSN during deformation. The SPSN is preferentially oriented in the loading direction, while the BSSN's preferential orientation is orthogonal to the SPSN. In nonaffine networks stiffening is exponential, while in close-to-affine networks it is quadratic. The difference is due to a much more modest lateral contraction in the approximately affine case and to a stiffer BSSN. Exponential stiffening emerges from the interplay of the axial and bending deformation modes at the scale of individual or small groups of fibers undergoing large deformations and being subjected to the constraint of rigid cross-links, and it is not necessarily a result of complex interactions involving many connected fibers. An apparent third regime of quadratic stiffening may be evidenced in nonaffinely deforming networks provided the nominal stress is observed. This occurs at large stretches, when the BSSN contribution of stiffening vanishes. However, this regime is not present if the Cauchy stress is used, in which case stiffening is exponential throughout the entire deformation. These results shed light on the physical nature of stiffening in a broad class of materials including connective tissue, the extracellular matrix, nonwovens, felt, and other athermal network materials.

Effect of connectivity on the elasticity of athermal network materials

Network materials with stochastic structure are ubiquitous in biology and engineering, which drives the current interest in establishing relations between their structure and mechanical behavior. In this work we focus on the effect of connectivity defined by the number of fibers emerging from a crosslink, z, and compare networks with identical (z-homogeneous) and distinct (z-heterogeneous) z at the crosslinks. We observe that the functional form of strain stiffening is z-independent, and that the central z-dependent parameter is the small strain stiffness, E₀. We confirm previous results indicating that the functional form of E₀(z) is a power function with 3 regimes and observe that this applies to a broad range of z. However, the scaling exponents are different in the z-homogeneous and z-heterogeneous cases. We confirm that increasing z across the Maxwell's central force isostatic point leads to a transition from bending to axial energy storage. However, we observe that this does not necessarily imply that deformation becomes affine in the large z limit. In fact, networks of fibers with low bending stiffness retain a relaxation mode based on the rotational degree of freedom of the crosslinks which allows E₀ in the large z limit to be smaller than the affine model prediction. We also conclude that in the z-heterogeneous case, the mean connectivity z̄ is sufficient to evaluate the effect of connectivity on E₀ and that higher moments of the distribution of z are less important.

Nonlinear behavior of stochastic athermal fiber networks with elastic-plastic fibers

Stochastic fiber networks form the structural component of network materials, which are broadly encountered in engineering and biology. Apparent elastic-plastic behavior, characterized by a yield point and softening at larger strains, is observed in some of these materials. A range of mechanisms, some of which being unrelated to fiber plasticity, may cause this behavior. In this work we investigate network plasticity caused by the plastic deformation of fibers and develop a comprehensive perspective on its relationship with network structural parameters. We determine the scaling of the yield stress and yield strain with network parameters emphasizing differences between the affine and non-affine deformation regimes. The non-linear response of the network is more complex when fiber plasticity takes place than in the purely elastic case. We describe four non-linear regimes and their dependence on network parameters. Further, we evaluate the dissipation and residual strains resulting upon loading-unloading cycles for a variety of networks and discuss design strategies for maximizing energy dissipation. Finally, we provide guidelines for the interpretation of experimental results and discuss ways to distinguish between various mechanisms that may cause a yield point and apparent elastic-plastic behavior.