In-situ monitoring of changes in ultrastructure and mechanical properties of flax cell walls during controlled heat treatment

Plant fibres are increasingly used as reinforcements, especially in thermoplastic composites. Understanding the impact of temperature on the properties of these fibres is an important issue for the manufacturing of high-performance materials with minimal defects. In this work, the structural evolution and mechanical behaviour of flax fibre cell walls were dynamically monitored by temperature-controlled X-ray diffraction and nanoindentation from 25 to 230 °C; detailed biochemical analysis was also conducted on fibre samples after each heating step. With increasing temperature up to 230 °C, a decrease in the local mechanical performance of the flax cell walls, of about -72 % for the indentation modulus and -35 % for the hardness, was measured. This was associated with a decrease in the packing of the cellulose crystal lattice (increase in d-spacing d200), as well as significant mass losses measured by thermogravimetric analysis and changes in the biochemical composition, i.e. non-cellulosic polysaccharides attributed to the middle lamellae but also to the cell walls. This work, which proposes for the first time an in-situ investigation of the dynamic temperature evolution of the flax cell wall properties, highlights the reversible behaviour of their crystalline structure (i.e. cellulose) and local mechanical properties after cooling to room temperature, even after exposure to high temperatures.

Ectopic callose deposition into woody biomass modulates the nano-architecture of macrofibrils

Plant biomass plays an increasingly important role in the circular bioeconomy, replacing non-renewable fossil resources. Genetic engineering of this lignocellulosic biomass could benefit biorefinery transformation chains by lowering economic and technological barriers to industrial processing. However, previous efforts have mostly targeted the major constituents of woody biomass: cellulose, hemicellulose and lignin. Here we report the engineering of wood structure through the introduction of callose, a polysaccharide novel to most secondary cell walls. Our multiscale analysis of genetically engineered poplar trees shows that callose deposition modulates cell wall porosity, water and lignin contents and increases the lignin-cellulose distance, ultimately resulting in substantially decreased biomass recalcitrance. We provide a model of the wood cell wall nano-architecture engineered to accommodate the hydrated callose inclusions. Ectopic polymer introduction into biomass manifests in new physico-chemical properties and offers new avenues when considering lignocellulose engineering.

In Situ Tensile Testing under High-Speed Optical Recording to Determine Hierarchical Damage Kinetics in Polymer Layers of Flax Fibre Elements

This study aims at better understanding the damage and fracture kinetics in flax fibre elements at both the unitary and bundle scales, using an experimental setup allowing optical observation at high recording rate in the course of tensile loading. Defects and issues from flax unitary fibre extraction are quantitated using polarized light microscopy. Tensile loading is conducted according to a particular setup, adapted to fibres of 10 to 20 µm in diameter and 10 mm in length. Optical recording using a high-speed camera is performed during loading up to the failure at acquisition, with speed ranging from 108,000 to 270,000 frames per second. Crack initiation in polymer layers of fibre elements, propagation as well as damage mechanisms are captured. The results show different failure scenarios depending on the fibre element's nature. In particular, fractured fibres underline either a fully transverse failure propagation or a combination of transverse and longitudinal cracking with different balances. Image recordings with high time resolution of down to 3.7 μs suggest an unstable system and transverse crack speed higher than 4 m/s and a slower propagation for longitudinal crack deviation. Failure propagation monitoring and fracture mechanism studies in individual natural fibre or bundles, using tensile load with optical observation, showed contrasted behaviour and the importance of the structural scale exanimated. This study can help in tailoring the eco-design of flax-based composites, in terms of toughness and mechanical performances, for both replacement of synthetic fibre materials and innovative composites with advanced properties.

A Circular Approach for the Valorization of Tomato By-Product in Biodegradable Injected Materials for Horticulture Sector

This study focuses on the use of tomato (Solanum lycopersicum L.) by-product biomass from industrial plants as reinforcement for designing a range of new degradable and biobased thermoplastic materials. As a novel technique, this fully circular approach enables a promising up-cycling of tomato wastes. After an in-depth morphological study of the degree of reinforcement through SEM and dynamic analysis, mechanical characterization was carried out. Our mechanical results demonstrate that this circular approach is of interest for composite applications. Despite their moderate aspect ratio values (between 1.5 and 2), the tomato by-product-reinforced materials can mechanically compete with existing formulations; PBS-Tomato fiber, for example, exhibits mechanical performance very close to that of PP-flax, especially regarding strength (+11%) and elongation at break (+6%). According to the matrix and particle morphology, a large range of products-biobased and/or degradable, depending on the targeted application-can be designed from tomato cultivation by-products.

Antibacterial Films Made of Bacterial Cellulose

Bacterial cellulose (BC) is naturally degradable, highly biocompatible, hydrophilic, and essentially non-toxic, making it potentially useful as a base for creating more sophisticated bio-based materials. BC is similar to plant-derived cellulose in terms of chemical composition and structure but has a number of important differences in microstructure that could provide some unique opportunities for use as a scaffold for other functions. In this study, bacterial cellulose was alkylated and then esterified to produce a carboxymethyl bacterial cellulose (CMBC) that was then used to produce six different composite films with potential antibacterial properties. The films were assessed for antibacterial activity against Staphylococcus aureus and Escherichia coli, pyrolysis characteristics using thermogravimetric analysis (TGA), microstructure using scanning electron microscopy (SEM), and mechanical properties. The addition of nano-silver (nano-Ag) markedly improved the antimicrobial activity of the films while also enhancing the physical and mechanical properties. The results indicate that the three-dimensional reticulated structure of the bacterial cellulose provides an excellent substrate for scaffolding other bioactive materials. Thus, the nano-BC was added into the CMBC/nano-Ag composites furthermore, and then the antibacterial and mechanical properties were improved 44% for E. coli, 59% for S. aureus, and 20% for tensile strength, respectively.

Bibliometric survey and network analysis of biomimetics and nature inspiration in engineering science

The field encompassing biomimetics, bioinspiration and nature inspiration in engineering science is growing steadily, pushed by exogenous factors like the search for potentially sustainable engineering solutions that might already exist in nature. With the help of information provided by a bibliometric database and further processed with a dynamic network and semantic analysis tool, we provide insight at two scales into the corpus of nature-inspired engineering field and its dynamics. At the macroscale, the Web of Science^®(WoS) categories, countries and institutions are ranked and ordered by thematic clusters and country networks, highlighting the leading countries and institutions and how they focus on specific topics. Such an insight provides an overview at the macroscale that can be valuable to orient scientific strategy at the country level. At the mesoscale, where science is incarnated by collaborative networks of authors and institutions that run across countries, we identify six semantic clusters and subclusters within them, and their dynamics. We also pinpoint leading academic collaborative networks and their activity in relation to the six semantic clusters. Trends and prospective are also discussed. Typically, one observes that the field is becoming mature since, starting by imitating nature, it proceeded with mimicking more complex natural structures and functions and now it investigates ways used in nature in response to changes in the environment and implements them in innovative and adaptive artefacts. The sophistication of devices, methods and tools has been increasing over the years as well as their functionalities and adaptability, whereas the size of devices has decreased at the same time.

Lessons on textile history and fibre durability from a 4,000-year-old Egyptian flax yarn

Flax has a long and fascinating history. This plant was domesticated around 8,000 BCE¹ in the Fertile Crescent area², first for its seeds and then for its fibres^1,3. Although its uses existed long before domestication, residues of flax yarn dated 30,000 years ago have been found in the Caucasus area⁴. However, Ancient Egypt laid the foundations for the cultivation of flax as a textile fibre crop⁵. Today flax fibres are used in high-value textiles and in natural actuators⁶ or reinforcements in composite materials⁷. Flax is therefore a bridge between ages and civilizations. For several decades, the development of non- or micro-destructive analysis techniques has led to numerous works on the conservation of ancient textiles. Non-destructive methods, such as optical microscopy⁸ or vibrational techniques^9,10, have been largely used to investigate archaeological textiles, principally to evaluate their degradation mechanisms and state of conservation. Vibrational spectroscopy studies can now benefit from synchrotron radiation¹¹ and X-ray diffraction measurement in the archaeometric study of historical textiles^12,13. Conservation of mechanical performance and the ultrastructural differences between ancient and modern flax varieties have not been examined thus far. Here we examine the morphological, ultrastructural and mechanical characteristics of a yarn from an Egyptian mortuary linen dating from the early Middle Kingdom (Eleventh Dynasty, ca. 2033-1963 BCE) and compare them with a modern flax yarn to assess the quality and durability of ancient flax fibres and relate these to their processing methods. Advanced microscopy techniques, such as nano-tomography, multiphoton excitation microscopy and atomic force microscopy were used. Our findings reveal the cultural know-how of this ancient civilization in producing high-fineness fibres, as well as the exceptional durability of flax, which is sometimes questioned, demonstrating their potential as reinforcements in high-technology composites.

The Middle Lamella of Plant Fibers Used as Composite Reinforcement: Investigation by Atomic Force Microscopy

Today, plant fibers are considered as an important new renewable resource that can compete with some synthetic fibers, such as glass, in fiber-reinforced composites. In previous works, it was noted that the pectin-enriched middle lamella (ML) is a weak point in the fiber bundles for plant fiber-reinforced composites. ML is strongly bonded to the primary walls of the cells to form a complex layer called the compound middle lamella (CML). In a composite, cracks preferentially propagate along and through this layer when a mechanical loading is applied. In this work, middle lamellae of several plant fibers of different origin (flax, hemp, jute, kenaf, nettle, and date palm leaf sheath), among the most used for composite reinforcement, are investigated by atomic force microscopy (AFM). The peak-force quantitative nanomechanical property mapping (PF-QNM) mode is used in order to estimate the indentation modulus of this layer. AFM PF-QNM confirmed its potential and suitability to mechanically characterize and compare the stiffness of small areas at the micro and nanoscale level, such as plant cell walls and middle lamellae. Our results suggest that the mean indentation modulus of ML is in the range from 6 GPa (date palm leaf sheath) to 16 GPa (hemp), depending on the plant considered. Moreover, local cell-wall layer architectures were finely evidenced and described.

Damage Kinetics at the Sub-micrometric Scale in Bast Fibers Using Finite Element Simulation and High-Resolution X-Ray Micro-Tomography

This study combines experimental testing and computation analysis to reveal the role of defects and sub-micrometric microstructure in tensile behavior of hemp bast fibers. In particular, these structural defects represent the footprint of the processes to which the fibers elements are subject along the whole transformation chain from the plant to the end use product. Tensile experiments performed on elementary fibers and bundles in a wide diameter range (40-200 μm) are simultaneously conducted with X-ray micro-tomography observation. 3D images of ultra-fine resolution (voxel size of 280 nm) are achieved at different deformation magnitudes up to the complete failure thanks to the use of synchrotron radiation (ESRF, Grenoble, France). A Finite element (FE) model is implemented based on the conversion of the tomograms into 3D meshes. High performance computing is used to simulate the tensile response of the hemp bast fibers. In particular, the effects of notching and sub-micrometric structure of the fibers are explored. Results show the presence of different types of diffuse damage kinetics, which are related to the variability in the fiber size, surface defects and the presence of the lumen space. The damage behavior is found to be sensitive to the type of stress criterion implemented in the FE computation. The predictive analysis demonstrates the relevance of using embedded microstructure simulations to reveal the extent of stress localization and predict the failure properties in bast fibers for innovative composite manufacturing for instance.

Evolution of flax cell wall ultrastructure and mechanical properties during the retting step

Flax retting is a major bioprocess in the cultivation and extraction cycle of flax fibres. The aim of the present study is to improve the understanding of the evolution of fibre properties and ultrastructure caused by this process at the plant cell wall scale. Initially, investigations of the mechanical performances of the flax cell walls by Atomic Force Microscopy (AFM) in Peak Force mode revealed a significant increase (+33%) in the cell wall indentation modulus with retting time. Two complementary structural studies are presented here, namely using X-Ray Diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of the cellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increase of 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. In addition, NMR investigations show a compaction of inaccessible cell wall polymers, combined with an increase in the relaxation times of the C4 carbon. This densification provides a structural explanation for the observed improvement in mechanical performance of the secondary wall of flax fibres during the field retting process.

Chemical composition of processed bamboo for structural applications

Natural materials are a focus for development of low carbon products for a variety of applications. To utilise these materials, processing is required to meet acceptable industry standards. Laminated bamboo is a commercial product that is currently being explored for structural applications, however there is a gap in knowledge about the effects of commercial processing on the chemical composition. The present study utilised interdisciplinary methods of analysis to investigate the effects of processing on the composition of bamboo. Two common commercial processing methods were investigated: bleaching (chemical treatment) and caramelisation (hygrothermal treatment). The study indicated that the bleaching process results in a more pronounced degradation of the lignin in comparison to the caramelised bamboo. This augments previous research, which has shown that the processing method (strip size) and treatment may affect the mechanical properties of the material in the form of overall strength, failure modes and crack propagation. The study provides additional understanding of the effects of processing on the properties of bamboo.

Syringaresinol: A Renewable and Safer Alternative to Bisphenol A for Epoxy-Amine Resins

A renewable bisepoxide, SYR-EPO, was prepared from syringaresinol, a naturally occurring bisphenol deriving from sinapic acid, by using a chemo-enzymatic synthetic pathway. Estrogenic activity tests revealed no endocrine disruption for syringaresinol. Its glycidylation afforded SYR-EPO with excellent yield and purity. This biobased, safe epoxy precursor was then cured with conventional and renewable diamines for the preparation of epoxy-amine resins. The resulting thermosets were thermally and mechanically characterized. Thermal analyses of these new resins showed excellent thermal stabilities (T_d5 % =279-309 °C) and T_g ranging from 73 to 126 °C, almost reaching the properties of those obtained with the diglycidylether of bisphenol A (DGEBA), extensively used in the polymer industry (T_d5 % =319 °C and T_g =150 °C for DGEBA/isophorone diamine resins). Degradation studies in NaOH and HCl aqueous solutions also highlighted the robustness of the syringaresinol-based resins, similar to bisphenol A (BPA). All these results undoubtedly confirmed the potential of syringaresinol as a greener and safer substitute for BPA.

Lignocellulosic fibers: a critical review of the extrusion process for enhancement of the properties of natural fiber composites

This review discusses the extrusion process parameters and their impact on the mechanical properties of composites reinforced with lignocellulosic fibers.

GH11 xylanases: Structure/function/properties relationships and applications

For technical, environmental and economical reasons, industrial demands for process-fitted enzymes have evolved drastically in the last decade. Therefore, continuous efforts are made in order to get insights into enzyme structure/function relationships to create improved biocatalysts. Xylanases are hemicellulolytic enzymes, which are responsible for the degradation of the heteroxylans constituting the lignocellulosic plant cell wall. Due to their variety, xylanases have been classified in glycoside hydrolase families GH5, GH8, GH10, GH11, GH30 and GH43 in the CAZy database. In this review, we focus on GH11 family, which is one of the best characterized GH families with bacterial and fungal members considered as true xylanases compared to the other families because of their high substrate specificity. Based on an exhaustive analysis of the sequences and 3D structures available so far, in relation with biochemical properties, we assess biochemical aspects of GH11 xylanases: structure, catalytic machinery, focus on their "thumb" loop of major importance in catalytic efficiency and substrate selectivity, inhibition, stability to pH and temperature. GH11 xylanases have for a long time been used as biotechnological tools in various industrial applications and represent in addition promising candidates for future other uses.

Algerian pearl millet ( Pennisetum glaucum L.) contains XIP but not TAXI and TLXI type xylanase inhibitors

An XIP (xylanase inhibiting protein) type xylanase inhibitor was purified from Algerian pearl millet ( Pennisetum glaucum L.) grains and characterized for the first time. Cation exchange and affinity chromatography with immobilized Trichoderma longibrachiatum glycoside hydrolase (GH) family 11 xylanase resulted in electrophoretically pure protein with a molecular mass of 27-29 kDa and a pI value of 6.7. The experimentally determined N-terminal amino acid sequence of the purified XIP protein is 87.5%, identical to that of sorghum ( Sorghum bicolor L.) XIP and 79.2% identical to that of wheat ( Triticum aestivum L.) XIP-I. The biochemical properties of pearl millet XIP are comparable to those described earlier for sorghum XIP, except for the higher specific activity toward a T. longibrachiatum GH family 11 xylanase. On the basis of immunoblot neither TAXI nor TLXI type xylanase inhibitors were detected in pearl millet grains.

Indirect enzyme-antibody sandwich enzyme-linked immunosorbent assay for quantification of TAXI and XIP type xylanase inhibitors in wheat and other cereals

To quantify Triticum aestivum xylanase inhibitor (TAXI) and xylanase inhibiting protein (XIP) type proteins in cereals in general and wheat ( T. aestivum) in particular, a robust enzyme-linked immunosorbent assay (ELISA) using an uncommon enzyme-antibody sandwich format was developed. Bacillus subtilis glycoside hydrolase family (GH) 11 and Aspergillus oryzae GH 10 xylanases were selected for coating ELISA plate wells to capture TAXI and XIP, respectively, prior to probing with antibodies. The detection threshold of the developed ELISA was much lower than that of the currently used xylanase inhibitor assay and the recently described Western blot approach. Because of its broad dynamic range (TAXI, 30-600 ng/mL, and XIP, 3-60 ng/mL), one proper standard extract dilution can be used for analyzing different wheat varieties, whereas for the currently used colorimetric assay, often different dilutions need to be analyzed. The TAXI ELISA for wheat was successfully adapted for barley ( Hordeum vulgare) and could also be used for other cereals.

TLXI, a novel type of xylanase inhibitor from wheat (Triticum aestivum) belonging to the thaumatin family

Wheat (Triticum aestivum) contains a previously unknown type of xylanase (EC 3.2.1.8) inhibitor, which is described in the present paper for the first time. Based on its >60% similarity to TLPs (thaumatin-like proteins) and the fact that it contains the Prosite PS00316 thaumatin family signature, it is referred to as TLXI (thaumatin-like xylanase inhibitor). TLXI is a basic (pI> or =9.3 in isoelectric focusing) protein with a molecular mass of approx. 18-kDa (determined by SDS/PAGE) and it occurs in wheat with varying extents of glycosylation. The TLXI gene sequence encodes a 26-amino-acid signal sequence followed by a 151-amino-acid mature protein with a calculated molecular mass of 15.6-kDa and pI of 8.38. The mature TLXI protein was expressed successfully in Pichia pastoris, resulting in a 21-kDa (determined by SDS/PAGE) recombinant protein (rTLXI). Polyclonal antibodies raised against TLXI purified from wheat react with epitopes of rTLXI as well as with those of thaumatin, demonstrating high structural similarity between these three proteins. TLXI has a unique inhibition specificity. It is a non-competitive inhibitor of a number of glycoside hydrolase family 11 xylanases, but it is inactive towards glycoside hydrolase family 10 xylanases. Progress curves show that TLXI is a slow tight-binding inhibitor, with a K(i) of approx. 60-nM. Except for zeamatin, an alpha-amylase/trypsin inhibitor from maize (Zea mays), no other enzyme inhibitor is currently known among the TLPs. TLXI thus represents a novel type of inhibitor within this group of proteins.

Unprocessed barley aleurone endo-β-1,4-xylanase X-I is an active enzyme

Endo-beta-1,4-xylanase X-I is a major hydrolase produced by the aleurone tissue of germinating barley grain. It was previously reported that this cytosolic enzyme is synthesized as an inactive precursor which is proteolytically processed to active forms upon its programmed cell death dependent release. We here demonstrate, however, that the precursor form of X-I is an active enzyme. Purified recombinant precursor X-I was characterised with respect to its molecular weight, iso-electric point and temperature and pH activity and stability. Analysis of the hydrolysis products showed that it is an endo-acting enzyme which has the striking ability to release xylose from both polymeric xylan as well as from small xylo-oligosaccharides. The implications of these findings in relation to the putative role of the N-terminal propeptide as a carbohydrate binding module and the possible consequences for the way X-I fulfils its role in the germination process, are discussed.

Probing the cell wall heterogeneity of micro-dissected wheat caryopsis using both active and inactive forms of a GH11 xylanase

The external envelope of wheat grain (Triticum aestivum L. cv. Isengrain) is a natural composite whose tissular and cellular heterogeneity constitute a significant barrier for enzymatic cell wall disassembly. To better understand the way in which the cell wall network and tissular organization hamper enzyme penetration, we have devised a strategy based on in situ visualization of an active and an inactive form of a xylanase in whole-wheat bran and in three micro-dissected layers (the outer bran, the inner bran and the aleurone layer). The main aims of this study were to (1) evaluate the role of cuticular layers as obstacles to enzyme diffusion, (2) assess the impact of the cell wall network on xylanase penetration, (3) highlight wall heterogeneity. To conduct this study, we created by in vitro mutagenesis a hydrolytically inactive xylanase that displayed full substrate binding ability, as demonstrated by the calculation of dissociation constants (K(d)) using fluorescence titration. To examine enzyme penetration and action, immunocytochemical localization of the xylanases and of feebly substituted arabinoxylans (AXs) was performed following incubation of the bran layers, or whole bran with active and inactive isoforms of the enzyme for different time periods. The data obtained showed that the micro-dissected layers provided an increased accessible surface for the xylanase and that the enzyme-targeted cell walls were penetrated more quickly than those in intact bran. Examination of immunolabelling of xylanase indicated that the cuticle layers constitute a barrier for enzyme penetration in bran. Moreover, our data indicated that the cell wall network by itself physically restricts enzyme penetration. Inactive xylanase penetration was much lower than that of the active form, whose penetration was facilitated by the concomitant depletion of AXs in enzyme-sensitive cell walls.

Structure, chemical composition, and xylanase degradation of external layers isolated from developing wheat grain

The external layers of wheat grain were investigated during maturation with respect to chemical and structural features and xylanase degradability. Cytochemical changes were observed in the isolated peripheral tissues of the wheat grain at four defined stages following anthesis. Marked chemical changes were highlighted at 11 days after anthesis, for which protein and lipid contents varied weakly. The profile of esterified ferulic acid showed large variation in the maturing peripheral layers of grain in contrast to the deposition of ferulate dimers, p-coumaric and sinapic acids. Lignin was monitored at the latest stages of ripening, which corresponds to the cessation of reserve accumulation in the grain. Arabinoxylans (AX) reached a maximum at 20 days and did not display any significant change in arabinosyl substitution proportion until ripeness. When submitted to xylanase, all outer layers were similarly altered in the proportion of soluble AX except for the peripheral tissues of the 11-day-aged wheat grain that had very little AX. Aleurone and nucellar layers were mostly degraded, whereas pericarp stayed intact at all stages of maturation. This degradation pattern was connected with the preferential immunolocalization of xylanase in aleurone and nucellar layers irrespective of the developmental stages. Further chemical examination of the enzyme-digested peripheral tissues of the grain supports the facts that ferulic ester is not a limiting factor in enzyme efficiency. Arabinose branching, ferulic dimers, and ether-linked monomers that are deposited early in the external layers would have more relevance to the in situ degradability of AX.

Impact and efficiency of GH10 and GH11 thermostable endoxylanases on wheat bran and alkali-extractable arabinoxylans

The results of a comparative study of two thermostable (1-->4)-beta-xylan endoxylanases using a multi-technical approach indicate that a GH11 xylanase is more useful than a GH10 xylanase for the upgrading of wheat bran into soluble oligosaccharides. Both enzymes liberated complex mixtures of xylooligosaccharides. 13C NMR analysis provided evidence that xylanases cause the co-solubilisation of beta-glucan, which is a result of cell-wall disassembly. The simultaneous use of both xylanases did not result in a synergistic action on wheat bran arabinoxylans, but instead led to the production of a product mixture whose profile resembled that produced by the action of the GH10 xylanase alone. Upon treatment with either xylanase, the diferulic acid levels in residual bran were unaltered, whereas content in ferulic and p-coumaric acids were unequally decreased. With regard to the major differences between the enzymes, the products resulting from the action of the GH10 xylanase were smaller in size than those produced by the GH11 xylanase, indicating a higher proportion of cleavage sites for the GH10 xylanase. The comparison of the kinetic parameters of each xylanase using various alkali-extractable arabinoxylans indicated that the GH10 xylanase was most active on soluble arabinoxylans. In contrast, probably because GH11 xylanase can better penetrate the cell-wall network, this enzyme was more efficient than the GH10 xylanase in the hydrolysis of wheat bran. Indeed the former enzyme displayed a nearly 2-fold higher affinity and a 6.8-fold higher turnover rate in the presence of this important by-product of the milling industry.

Effects of -methyl-p-tyrosine and L-DOPA on brain self-stimulation and motor activity in rats

Rats with electrodes implanted in the posterior lateral hypothalamus and ventromedial tegmentum were trained to self-stimulate. Animals were treated (i.p.) first with alpha-methyl-p-tyrosine (alpha-MPT) and then L-DOPA; their self-stimulation rates and spontaneous behaviour were recorded. alpha-MPT (100 mg/kg) had an immediate and long lasting suppressive effect on self-stimulation, within the first half hour. L-DOPA administration failed to reinstate self-stimulation. Hypoactivity also followed alpha-MPT injections. While the behavioural changes were minimal after L-DOPA 50 mg/kg, a decrease of spontaneous activity and moderate hyper-reactivity were observed following L-DOPA at a dose of 200 mg/kg. Theoretical implications are discussed.

Ambivalence in rats using single-lever Skinner boxes

Ambivalent behavior or ambivalence in rats was observed in a single-lever Skinner box situation. The behavioral features associated with ambivalence were filmed and described, and their intensities were graded. It was found that different behavioral patterns, depending on the site of stimulation, are involved in ambivalence. Moreover, the intensity of self-stimulation and withdrawal was found to be higher in subjects implanted in posterior brain areas than in rats implanted in anterior areas, suggesting the existence of a gradient of ambivalence. At a theoretical level, it was suggested that complex mechanisms, possibly involving an autonomous ambivalent system, could be responsible for ambivalence.