Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 1980"

Theory of Edge Detection

Abstract: A theory of edge detection is presented. The analysis proceeds in two parts. (1) Intensity changes, which occur in a natural image over a wide range of scales, are detected separately at different scales. An appropriate filter for this purpose at a given scale is found to be the second derivative of a Gaussian, and it is shown that, provided some simple conditions are satisfied, these primary filters need not be orientation-dependent. Thus, intensity changes at a given scale are best detected by finding the zero values of delta 2G(x,y)*I(x,y) for image I, where G(x,y) is a two-dimensional Gaussian distribution and delta 2 is the Laplacian. The intensity changes thus discovered in each of the channels are then represented by oriented primitives called zero-crossing segments, and evidence is given that this representation is complete. (2) Intensity changes in images arise from surface discontinuities or from reflectance or illumination boundaries, and these all have the property that they are spatially. Because of this, the zero-crossing segments from the different channels are not independent, and rules are deduced for combining them into a description of the image. This description is called the raw primal sketch. The theory explains several basic psychophysical findings, and the operation of forming oriented zero-crossing segments from the output of centre-surround delta 2G filters acting on the image forms the basis for a physiological model of simple cells (see Marr & Ullman 1979).

The interpretation of a moving retinal image

Abstract: It is shown that from a monocular view of a rigid, textured, curved surface it is possible, in principle, to determine the gradient of the surface at any point, and the motion of the eye relative to it, from the velocity field of the changing retinal image, and its first and second spatial derivatives. The relevant equations are redundant, thus providing a test of the rigidity assumption. They involve, among other observable quantities, the components of shear of the retinal velocity field, suggesting that the visual system may possess specialized channels for computing these components.

The Croonian Lecture, 1978: The Crawling Movement of Metazoan Cells

Abstract: A great variety of metazoan cells have the ability to crawl across a surface. Students of inflammation, of wound healing, of developing embryos or of cell cultures have for many years investigated accessible aspects of the phenomenon, but largely in isolation. A general physiology of the movement hardly seemed possible. But the scene is beginning to change, and this lecture is an attempt to see what sort of a general picture may now be drawn. Present knowledge of the machinery of movement, which is undergoing a revolution as the muscle-like proteins of the cytoplasm emerge more clearly, is considered. The lecture then outlines the circumstances in which cell movement occurs in vivo, and suggests the functions that it performs. Finally, attention is turned to what may be called the behavioural aspects, to how the movement is guided in particular directions and started or stopped by the conditions around the cell.

A model for the formation of ocular dominance stripes

Abstract: The paper describes a model of competition that explains the formation of the ocular dominance stripes found in layer IVc of cat and monkey visual cortex. The main proposal is that synapses exert effects on the growth of other synapses, and that these effects extend over distances of at least 600 $\mu $m and vary in magnitude and sign within this distance. Interactions between like type synapses are assumed to be stimulating for distances up to about 200 $\mu $m, and inhibitory for distances of 200-600 $\mu $m. The reverse is true of interactions between synapses of opposite eye type, where the effects are inhibitory for distances up to about 200 $\mu $m and stimulating for longer ones. The interactions are assumed to be circularly symmetric. Growth of, for example, right eye synapses at one point will therefore (a) encourage local growth of right eye synapses and inhibit local growth of left eye synapses and (b) encourage growth of left eye synapses and inhibit growth of right eye synapses in an annular ring surrounding the point of initial increase. At the start of development, right and left eye synapses are assumed to be intermixed randomly within layer IVc. Computer simulations show that a wide variety of conditions incorporating these assumptions will lead to the formation of stripe patterns. These reproduce many of the morphological features of monkey ocular dominance stripes, including Y- and H-type branches and terminations, the tendency for stripes to run at right angles into the boundaries of the pattern, and to narrow at branch points. The model can explain the effects of monocular deprivation on stripe morphology if it is assumed that the effectiveness of deprived eye synapses in determining rates of growth locally is reduced. The existence of a critical period for these effects can be explained if it is assumed that lateral growth of terminals does not occur, and that some factor such as a limited availability of postsynaptic sites decreases the rate of growth of synapses as their density approaches a maximum. The model can be generalized to account for pattern formation in other systems, such as zebra or mackerel skin, where similar striped patterns occur. In this context, the simplest model based on diffusion that will produce a pattern of stripes requires that one cell type should secrete two substances, one of which stimulates growth or differentiation of its parent cell type and has a low rate of diffusion or is rapidly inactivated, and another that inhibits growth or differentiation and either has a higher rate of diffusion or is less rapidly inactivated. A preliminary account of some of these results has appeared elsewhere (Swindale 1979).

Genetic Variation in Symbiodinium (=Gymnodinium) microadriaticum Freudenthal, and Specificity in its Symbiosis with Marine Invertebrates. I. Isoenzyme and Soluble Protein Patterns of Axenic Cultures of Symbiodinium microadriaticum

Abstract: Axenically cultured Symbiodinium (= Gymnodinium) microadriaticum Freudenthal, isolated from 40 host individuals (or colonies) representing 17 species in two phyla were resolved into 12 strains, each strain having a unique combination of four isoenzyme patterns as revealed by starch gel electrophoresis. Each individual host appeared to contain an electrophoretically homogeneous population of algae. Each characteristic isoenzyme pattern was maintained after re-isolation of a cross-infecting strain (i.e. a strain that had been experimentally introduced into a host of a species other than that from which it was originally isolated), indicating a genetic basis for the protein patterns observed in each strain. The strains could be separated into three groups after constructing a dendrogram by the unweighted average linkage clustering method based on calculations of similarity coefficients. Soluble protein patterns obtained from the algae were also strainspecific. A dendrogram constructed from data obtained by analysis of protein patterns resolved by a polyacrylamide gel disc electrophoresis was similar to the dendrogram constructed from the data on isoenzyme patterns, but suggested that the \`phenetic' distances between strains were larger than indicated by the isoenzyme patterns. 1. When axenic cultures of the \`zooxanthella' Symbiodinium microadriaticum were analysed by starch gel electrophoresis for isoenzymes, the patterns obtained differed depending on the host from which the algae were originally isolated, but there was no 1:1 relation between algal strain and host species. 2. Patterns of soluble proteins on polyacrylamide gels corroborated the results obtained through isoenzyme studies. 3. Twelve \`electrophoretic strains' of Symbiodinium microadriaticum were recognized. These strains separated into three groups after construction of dendrograms by the unweighted average linkage clustering method based on calculations of similarity coefficients, of \`genetic identity' and of `genetic distance'. 4. Because so little is unambiguously known about the life history of Symbiodinium microadriaticum, and because our sample size was relatively small, it is not possible to discuss our data in the context of the population genetics of this symbiotic dinoflagellate. 5. The evidence presented indicates that the variation that we have observed in these algae is intrinsic, and is probably genetically based. Two possible mechanisms that may act as a selective force in promoting specificity between algal strains and their respective hosts are discussed.

A model for vein formation in higher plants

Abstract: Experiments on vein regeneration (Jost 1942; Jacobs 1952) suggest that a signal of some kind, which can cause the differentiation of veins, flows from a source in the growing tissues of leaves towards the root. Sachs (1969, 1978) has given evidence that the capacity of a given pathway to transport this signal increases with the flux it carries. He suggests that this progress could cause the canalization of signal flow into a pattern of discrete strands, which subsequently differentiate into veins. I formulate here a mathematical model based on these assumptions, and show that it can generate well defined strands. Given plausible estimates for diffusion constants and polar transport rates, it appears that vein formation could occur by this mechanism over an appropriate distance within an acceptably short time. I show that this model can simulate Sachs9s experiments on vein formation. I also show that, with suitable assumptions about the distribution of source activity, the model can generate elaborate networks, with branches and loops of the kind seen in the leaves of higher plants.

Genetic Variation in Symbiodinium (=Gymnodinium) microadriaticum Freudenthal, and Specificity in its Symbiosis with Marine Invertebrates. III. Specificity and Infectivity of Symbiodinium microadriaticum

Abstract: Nine of 12 tested strains of the zooxanthella Symbiodinium (= Gymnodinium) microadriaticum can initiate symbiotic associations with an aposymbiotic clone of the sea anemone Aiptasia tagetes. The growth of each strain during the first two months of association is generally related to the isoenzyme similarity coefficient of the strain relative to strain A, the strain originally isolated from Aiptasia tagetes. The growth of heterologous algae in Aiptasia is not enhanced by previous association with Aiptasia tagetes. An hypothesis on the nature of specificity, which involves a recognition mechanism between algae and potential host and competition between endosymbiont strains within hosts, is presented. 1. Strains of the symbiotic dinoflagellate Symbiodinium microadriaticum that were isolated and cultured axenically demonstrated differences in their respective capacity to infect and repopulate aposymbiotic Aiptasia tagetes. 2. Cultured algae from Aiptasia tagetes rapidly reinfected Aiptasia tagetes. Such homologous infections rapidly achieved high stable population densities similar to those found in freshly collected animals. 3. Algal strains isolated from other hosts infected Aiptasia tagetes less readily and produced lower stable population densities. Generally, those strains that were biochemically (with respect to isoenzyme patterns) similar to the algae from Aiptasia tagetes infected Aiptasia tagetes more readily than those that were dissimilar. Three strains did not infect Aiptasia tagetes. 4. An hypothesis is presented that postulates that a recognition process determines initial infectivity. Possible competition between algal strains for space in a host is visualized as a function of differences in interactions between hosts and algal strains, the net result of which is expressed as specificity in such associations.

The character of the muscarinic receptors in different regions of the rat brain.

Abstract: The binding characteristics of muscarinic receptors have been critically examined in six regions of the rat brain. The binding curves of antagonists are similar for all six areas but the binding curves of agonists show large differences. It is shown that in all regions there are three classes of receptors with similar binding characteristics but that these are present in different proportions. The binding constants to the three receptor types of a range of agonists were examined and evidence was produced in support of the theory that the subclasses of brain receptors are due to a single receptor subunit subject to different conformational constraints.

Amacrine Cells, Displaced Amacrine Cells and Interplexiform Cells in the Retina of the Rat

Abstract: The amacrine cells in the retina of the rat are described in Golgi-stained whole-mounted retinae. Nine morphologically distinct types of cell were found: one type of diffuse cell, five types of unistratified cell, two types of bistratified cell, and one type of stratified diffuse cell. Measurements show that the largest unistratified cells have a dendritic field 2 mm across. One type of interplexiform cell is also described. Wide-field diffuse amacrine cells and unistratified amacrine cells were found with their somata located in either the inner nuclear layer or the ganglion cell layer. It is clear that there may be an amacrine cell system in the ganglion cell layer of the rat retina.

Genetic Variation in Symbiodinium (=Gymnodinium) microadriaticum Freudenthal, and Specificity in its Symbiosis with Marine Invertebrates. II. Morphological Variation in Symbiodinium microadriaticum

Abstract: Morphological studies conducted on axenic cultures of the endozoic gymnodinioid dinoflagellate Symbiodinium (= Gymnodinium) microadriaticum demonstrated that there are intrinsic variations in the dimensions of the recently divided, but as yet unseparated, vegetative cells (the two-cell stage). Characteristic dimensions of a given strain are maintained when such a strain is artificially introduced into a host other than that from which it was originally isolated. Ultrastructural observations illustrated that, in contrast to the algae in their hosts' tissues, the algae in culture produced a fibrous or granular \`pellicle' approximately 0.2 $\mu $m thick. The algae in situ in Tridacna gigas also demonstrated the presence of a \`pellicle', but this structure was reduced when compared to that produced by the same cells in culture. Many of the stages in the described life cycle of the alga were observed, with the exception of those stages involving gametogenesis and sexual reproduction. Two distinct types of zoospores were observed; those that contained an \`accumulation body' in the epicone and those that did not. 1. Endozoic gymnodinioid dinoflagellates isolated from 17 different invertebrate hosts were cultured axenically in ASP-8A under identical conditions of light and temperature. 2. Analysis of the dimensions of the two-cell stages of these algae showed that they fall into two significantly distinct size categories. 3. Observations of the zoospores showed that those of \`small' strains possessed an \`accumulation body' in the epicone, while those of \`large' strains did not. 4. Examination of the ultrastructure of the amphiesma of the algae in situ illustrated that this structure is greatly reduced. By contrast, the cells in culture produced a `pellicle' approximately 0.2 $\mu $m thick, within which binary fission, or the production of tetraspores or of zoospores may occur. 5. Transmission and scanning electron microscopy of the cells in culture illustrated that the algal varied with respect to surface architecture. Most cells were smooth, while cells from Zoanthus sociatus uniquely possessed tufts of hair-like fibres projecting from the surface of the amphiesma.

Characterization of opioid receptors in nervous tissue.

Abstract: The concept that endogenous opioid peptides interact with at least two different receptor sites developed from several experimental approaches. First, when the peptides were assayed by their effects on two pharmacological and two binding models, the rank order of activity differed in these four systems. Secondly, naloxone had a smaller antagonist effect on $\delta $-receptors in the mouse vas deferens than on its $\mu $-receptors. Thirdly, the enkephalins and morphine each occupied less than half of the total number of sites available in brain homogenates. Fourthly, cold ligands of the $\delta $-type protected the binding of tritiated $\delta $-agonists better than that of $\mu $-agonists, and vice versa. Finally, tritiated ethylketazocine binds to $\kappa $-receptor sites in homogenates of guinea-pig brain. It is readily displaced by etorphine, which binds uniformly to $\mu $-, $\delta $- and $\kappa $-receptors, but only by very high concentrations of $\mu $- or $\delta $-agonists. An interesting phenomenon is the potentiation of activity when an enkephalin analogue is conjugated to tobacco mosaic virus by the group of R. Schwyzer.

The Croonian lecture, 1979: Regulation of muscle contraction.

Abstract: In this lecture I review briefly the history of the recognition of calcium ion as the sole regulatory factor of muscle contraction at the molecular level and how this led to the discovery of the troponin-tropomyosin system, which is the regulatory system of striated muscles of almost all deuterostomias and some protostomias. This is followed by a brief comment on the myosin-linked regulation, which plays a dominating role in many protostomian muscles. The regulatory mechanism in vertebrate smooth muscle is then discussed; the view is advanced that the leiotonin-tropomyosin system may be the only regulatory device for this muscle. Ca-binding components of troponin and smooth muscles of vertebrates are compared with modulator protein, an omnipresent Ca-binding protein of very conservative nature throughout evolution. Finally, the modes of action of Ca ion in different kinds of cell motility are discussed from an evolutionary point of view.

Electrical synapses between motoneurons in the spinal cord of the newborn rat

Abstract: Ventral roots of the newborn rat spinal cord were stimulated while recording intracellularly from motoneurons. In many cells, stimulation subthreshold for an antidromic action potential in the impaled cell produced a small, short-latency depolarization, which was unaffected by membrane polarization. This response (antidromic synaptic potential, a.s.p.) was also seen, in some cells, on stimulating the ventral root of an adjacent segment. Replacement of Ca$^{2+}$ (2 mM) with Mn$^{2+}$ (3 mM) or Mg$^{2+}$ (10 mM) completely abolished orthodromic synaptic potentials, but the a.s.p. persisted. These results strongly suggest that the a.s.p. is produced by an electrical interaction between motoneurons.

The Response Properties of Cells in the Middle Temporal Area (Area MT) of Owl Monkey Visual Cortex

Abstract: Recordings from 178 single cells in the middle temporal area (area MT) of owl monkey showed that most cells there are orientation- and direction-selective. They also revealed that a powerful range of binocular interactions occur in area MT, with 20% of the cells being responsive to binocular stimulation only, 5% to monocular stimulation only and about 50% of all cells showing some degree of interocular interaction.

Intracellular calcium and desensitization of acetylcholine receptors.

Abstract: Acetylcholine (ACh) was applied iontophoretically to voltage-clamped endplates in frog muscle. The current induced by prolonged application of ACh decreases progressively as the membrane becomes desensitized. Desensitization was sharply localized, and at a distance of 15 $\mu $m or less the ACh sensitivity of the membrane remained normal. Desensitization still occurred in muscles exposed to Ca$^{2+}$-free media for several hours. In these conditions the rate of desensitization was not greatly affected by altering the membrane potential. In normal Ringer (1.8 mM Ca$^{2+}$) desensitization was more pronounced and ACh application was frequently accompanied by localized contraction of the muscle fibre. Both the desensitization and the contraction were reduced after intracellular injection of EGTA, probably because this opposes the rise in internal Ca$^{2+}$ normally caused by ACh action.

Sexually Dimorphic Interneuron Arrangements in the Fly Visual System

Abstract: Silver-intensified cobalt preparations reveal sex-specific neurons in the optic lobes of flies. The most pronounced of these are found in the lobula neuropil and subserve projections from the dorsal and frontal retina specifically. Two main types of male neurons have been resolved. These are single unique elements, two in each lobula, and small groups of columnar neurons. The present account describes three main features of sex-specific neurons. These are, first their shapes and dispositions; secondly, their relations with visual nerve cells that are common to both sexes, and thirdly, their exact mapping within the retinotopic mosaic of the lobula. The possible functional significance of these arrangements are discussed with respect to sex-specific visual behaviour.

The Fracture Behaviour of Wood and the Relations between Toughness and Morphology

Abstract: Wood is a complex composite material that possesses an excellent combination of mechanical properties. In particular the toughness, or work of fracture, is very high, reaching a value in excess of 1.0 $\times $ 10$^{4}$ J/m$^{2}$, which, mass for mass, is roughly equal to the toughness of steel and higher than that of artificial composites. The high work of fracture of wood is shown to be due to the arrangement of the cellulose microfibrils in the secondary cell wall (S$_{2}$). The helically wound pattern of these fibrillae is such as to induce a novel form of buckling failure in tension, which produces an elastic behaviour analogous to the yield point of ductile metals. When glass-fibre composites were constructed with a similar morphology, a large increase in work of fracture was observed.

Molecular Evolution of Biologically Active Polypeptides

Abstract: Any biological function is at least bimolecular and involves primarily a specific recognition between the shapes (conformations) of the reacting molecules. The selective pressure of evolution therefore acted on the interaction so that coordinated changes probably occurred in two lines of molecules. Because the structure of the specific partner (receptor, macromolecular substrate, naturally occurring inhibitor, antigen, etc.) is rarely known, evolutionary speculations are often arbitrarily limited to the active polypeptide. During the life of a polypeptide chain, its conformation can be modified by ligands, by `conformers' or by morphogenic cleavages. Inactive preprohormones and prohormones (e.g. preproparathyrin, proopiocortin) are successively split by specific proteolytic enzymes. Several modulator- or activator-binding sites can be distinguished in addition to the active site, so that the chain can be regarded as the result of a multiple evolution. The conformation of an active polypeptide chain on the one hand displays a variable degree of flexibility, and on the other may include a hierarchy of organized substructures: secondary ($\alpha $-helix, $\beta $-pleated sheet, $\beta $-bend), super-secondary and domain. The amino acid sequence appears to program largely the organized substructures and the potential adaptability. However, general architectural rules on which selective pressure could primarily act remain unknown. Duplication seems to be a fundamental mechanism for increasing both the size and the number of polypeptide chains. Duplication without fusion may lead to parallel lines of peptides which differentiate functionally by subsequent mutations (e.g. neurohypophysical hormones and neurophysins). Duplication with fusion may give single-chain proteins with internal homology between two or several domains (e.g. somatotrophin). Repetitive duplication could involve fusion in the first steps and not in the last step so that several lines of homologous proteins, each with internal homology, could arise (e.g. somatotrophin, prolactin, choriomammotrophin). The assembly of polypeptide chains, whether covalent or not, is likely to represent a higher level of evolution. Each chain or subunit may have a distinct function, as in dimeric hormones (e.g. lutrophin, follitrophin, thyrotrophin, choriogonadotrophin), or the association may determine new cooperative properties (allostery). The integration of molecular evolution at the organelle, cellular and organismal levels raises the problem of the evolution of regulatory mechanisms.

The Dynamics of Auxin Transport

Abstract: The plant hormone auxin is transported with a well defined velocity through many tissues. To explain this, one type of theory proposes that a polar mechanism operates at the interface between two cells. I show that, if auxin diffuses freely through the interior of cells, then there is an upper limit to the velocity that can be achieved by such a mechanism. This is compatible with the observed velocities provided that the diffusion constant for auxin within a cell is not much less than that measured for auxin in aqueous media. Cytoplasmic streaming, unless specially organized, would not assist the movement of auxin. This is because rapid diffusion between streams will cancel out any directed motion. I also show that the permeability that characterizes the forward movement between cells must exceed a certain limit. If auxin moves mainly through the cytoplasm, which occupies only a small part of the volume of a cell, then the permeability per unit area of membrane needed to achieve a given velocity is much reduced. Transport would be channelled through the cytoplasm if the membrane bounding the vacuole were relatively impermeable to auxin. The theory that I develop leads to predictions about, for example, the route of auxin and its concentration gradients within cells, and the dependence of velocity on cell length.

Peptides as putative excitatory neurotransmitters: carnosine, enkephalin, substance P and TRH

Abstract: A number of in vitro preparations of the central nervous system have been used to characterize with intracellular recording the cellular actions of four neuropeptides. Carnosine, the putative excitatory neurotransmitter of olfactory nerves, was found to exert little or no effect in the turtle or the frog olfactory bulb, suggesting that this peptide may have other roles, e.g. neurotropic, in this system. Substance P and TRH were found to have some characteristics of a classical excitatory transmitter since they increase membrane conductance and depolarize frog motoneurons by a direct action. However, the slow time course and subthreshold nature of the depolarization may imply that these peptides function in a background manner to set the level of excitability of motoneurons. Finally, the effects of enkephalin on a variety of inhibitory systems have been examined. Enkephalin excites hippocampal pyramidal cells indirectly by blocking both spontaneous and evoked inhibitory potentials. In addition, both feedforward and feedback inhibitory pathways are depressed by enkephalin. All these effects are blocked by naloxone. Blockade of inhibitory pathways by enkephalin appears to be a general phenomenon, since similar depressant effects were seen for dendrodendritic inhibition in olfactory bulb mitral cells as well as for presynaptic inhibition of spinal primary afferents. These results indicate that neuroactive peptides can affect principal neurons by increasing their excitability via either subthreshold excitation or disinhibition.

Intracellular Analysis and Structural Correlates of the Organization of Inputs to Ganglion Cells in the Retina of the Turtle

Abstract: Intracellular recording from the ganglion cells of the retina of the turtle Pseudemys scripta elegans shows that the photoresponses are characterized by either of two reversal potentials. These have been related to the bipolar and amacrine cell inputs to ganglion cells. Of the recorded ganglion cells, 33 % (type A) are driven predominantly by one type of input, attributed to bipolar cells. Other ganglion cells (66%, type B) receive a mixed input from bipolar and amacrine cells. Type A ganglion cells show a higher conduction velocity (up to 3 m/s) and a higher firing frequency (up to 160 spikes per second) than type B cells. By injecting Procion yellow into bipolar, amacrine and ganglion cells it may be observed that type A ganglion cells ramify only in those regions of the inner plexiform layer that contain cell processes originating exclusively from the bipolar cell type predicted from the recordings. Type B ganglion cells ramify throughout the inner plexiform layer in either a multi-stratified or a diffuse fashion.

The Genus Partula on Moorea: Speciation in Progress

Abstract: Land snails of the genus Partula Fer. inhabiting the island of Moorea in French Polynesia show complex patterns of partial speciation. The late H. E. Crampton described eleven species from the island. Two have since been relegated to the genus Samoana . Of the remaining nine taxa, as many as four may coexist at a single locality without evidence of hybridization. Nevertheless an allele appearing in any of the taxa is potentially capable of spreading to any of the others, although in the process it may have to pass through several intervening taxa. Large morphological, ecological and behavioural differences have evolved despite this potential for gene flow. The biological differences are not reflected in 20 enzyme loci surveyed by conventional starch-gel electrophoresis, nor in the numbers of chromosomes. It is clear that a significant part of the morphological divergence between taxa has taken place of Moorea. Our observations are consistent with the view that all the Moorean taxa (excepting Samoana ) are descendants of a single ancestral invasion, although the possibility of more than one invasion cannot be excluded. The pattern of differentiation shows at least two cases of circular overlap (‘ring species’), with diameters of 2 and 3 km, and several apparent examples of character displacement. Coexisting taxa tend to partition the available habitat, although the separation is never complete.

Interpretation of the low angle X-ray diffraction from insect flight muscle in rigor

Abstract: The structure of insect flight muscle is formally described in terms of actin-based cross-bridges upon which successive symmetry operations are performed, in combination with a modulation function. The Fourier transform of the structure is generated by means of these steps. The model transform is fitted to the observed diffraction pattern from insect flight muscle in rigor and the position of the rigor cross-bridges deduced; they are found to lie across the long helix of actin monomers and to project away from the thin filament. The cross-bridges interact with approximately one-third of the actin monomers, and show a strong preference for a particular orientation between the thick and thin filaments.

Behavioural Actions of Neurohypophysial Peptides

Abstract: The neurohypophysial hormones vasopressin and oxytocin modulate memory processes. Vasopressin facilitates while oxytocin attenuates memory consolidation and retrieval. These influences are located in different regions of the molecules. Thus, the neurohypophysial hormones act as precursor molecules for neuropeptides involved in memory processes. The covalent ring structures of both vasopressin and oxytocin mainly affect consolidation, the linear parts, retrieval processes, while nearly the whole oxytocin or vasopressin molecule is needed for attenuation of consolidation and retrieval. Regional studies, by microdissection techniques in combination with a sensitive radioenzymatic catecholamine assay, indicate that vasopressin modulates memory processes by modulation of neurotransmission in distinct catecholamine systems. Recent experiments suggest that the influence of vasopressin on memory consolidation is mediated by the dorsal noradrenergic bundle via terminal regions of this bundle. Studies on the conversion of oxytocin in synaptosomal plasma membrane preparations of rat limbic brain suggest the possible generation of fragments with specific effects on memory processes. Regional differences in enzyme activity further substantiate the implication of oxytocin as a prohormone in this respect. Clinical studies support the evidence from laboratory findings that vasopressin is also involved in memory processes in man.

Cellular localization of peptides in neural structures.

Abstract: By means of the immunohistochemical technique of Coons and collaborators, numerous peptide-containing neurons have been observed in the brain, spinal cord and periphery. These neurons may contain peptides such as substance P, vasoactive intestinal polypeptide (VIP), enkephalin or somatostatin. Some systems are very extensive. For example, immunoreactive substance P has been observed in more than 30 cell groups in the central nervous system, in primary sensory neurons, in sensory neurons in the vagus nerve and in taste buds, and in intestinal neurons. Thus, one and the same peptide can be utilized at many different levels in the nervous system. Several examples are now known where a regulatory peptide occurs together with a classical transmitter, such as a catecholamine, in the same neuron, which suggests the possibility that a neuron can release more than one transmitter substance. Of particular interest is the occurrence of VIP in presumed cholinergic neurons innervating exocrine glands in the cat, and the coexistence of a cholecystokinin (CCK)-like peptide in dopamine neurons projecting mainly to limbic areas. In the former system VIP seems to be responsible mainly for vasodilation, whereas acetylcholine mainly causes secretion. Furthermore, combined infusion of both substances in very low doses results in a marked potentiation of the secretory and vasodilatory responses. Thus, we have an example where two putative transmitters, released from the same nerve endings, seem to cooperate to activate a physiological response (secretion). With regard to the central CCK/dopamine neurons the type of interaction between the two coexisting transmitter candidates is at present unclear. It is suggested that elucidation of different types of coexistence phenomena may advance our understanding of chemical transmission at synapses under normal and pathological conditions, and may lead to new approaches to the treatment of some nervous disorders.

Stereochemical mechanism of oxygen transport by haemoglobin.

Abstract: Spectroscopic and chemical evidence speak in favour of the iron--oxygen bond being polar. X-ray analysis shows that the oxygen molecule is inclined at an angle of about 115 degrees to the haem plane. Cooperative binding of oxygen by haemoglobin is due to an equilibrium between two alternative structures, which differ in oxygen affinity by the equivalent of 3-3.5 kcal/mol. I proposed that in the low affinity structure the globin opposes the movement of the iron atom from its five-coordinated pyramidal geometry in the haem of deoxyhaemoglobin to its six-coordinated planar geometry in the haem of oxyhaemoglobin, while in the high affinity structure this restraint is absent. Recent evidence supporting this mechanism is described.

The Green Hydra Symbiosis. III. The Biotrophic transport of Carbohydrate from Alga to Animal

Abstract: Pulse and pulse--chase experiments were used to study the movement of photosynthetically fixed $^{14}$C from alga to animal tissue in green hydra. The pool of mobile carbohydrate (maltose) was small, turned over rapidly and was preferentially fed with newly fixed carbon. Transport between the symbionts could also occur in darkness, and in this case the mobile maltose pool was fed from algal polysaccharide reserves. The specific activity of the maltose released by freshly isolated algae varied considerably according to experimental conditions. Cultured symbionts from European hydra do not release maltose and homogenates of animal tissue were ineffective in restoring this property. Contaminating host tissue could not be completely removed from freshly isolated symbionts during their isolation, and the possible effects that this may have on maltose release are considered. [$^{14}$C]maltose received from the alga is rapidly metabolized in animal tissue. All the major animal fractions incorporate $^{14}$C, with most accumulating in glycogen. Under standard conditions, green hydra have higher glycogen: protein ratios than aposymbionts and starvation causes an immediate utilization of glycogen in both forms. There was no good evidence that the animal can regulate carbon transport according to its requirements. Preliminary experiments suggest that the degree of transport is determined primarily by the amount of surplus photosynthate available to algal symbionts and not by any capacity of the animal to regulate algal release.

Reduced Reproductive Performance in Androgen-Resistant Tfm/Tfm Female Mice

Abstract: Androgen-resistant female mice (Tfm/Tfm) homozygous for the mutant gene Tfm were bred by making use of males chimaeric for the Tfm gene. All seven Tfm/Tfm females found were fertile, confirming that a normal level of androgen receptor protein is not essential for reproduction in female mice. However, when five of the seven were studied throughout their reproductive life they proved to have impaired reproductive performance and premature cessation of reproduction. No impairment of reproduction was seen in heterozygous Tfm/+ females. The ovarian histology suggested that in Tfm/Tfm the normal ageing processes were accelerated. This work is consistent with the work of others in that androgen is involved in the control of follicular maturation and atresia, and that the effect is mediated by the androgen receptor coded by the Tfm locus.

Characteristics and Ionic Processes Involved in Feedback Spikes of Turtle Cones

Abstract: In about 20% of the cones of untreated retinas of turtles, bright flash illumination of the periphery of their receptive field evokes a spike through the feedback mechanism from the L-horizontal cell. Such feed­back spikes, never observed with central stimulation, are labile, butafter they have disappeared they can be regained by depolarizing the cone. Feedback spikes are actual regenerative responses, since they show a critical threshold potential, are facilitated by cone depolarization and are blocked by hyperpolarization. They are associated with a membrane resistance decrease; tetrodotoxin (10 -5 M) does not block them. High Ca 2+ media facilitate their appearance, but their effect is transient because of the cone hyperpolarization and the light response block that Ca 2+ ions induce. Sr 2 + ions (4-10mM) facilitate the discharge of feedback spikes in response to peripheral illumination in every cone, whether or not it has previously shown feedback effects. In Sr 2+ media, feedback spikes are stable and can be evoked by dim lights. Ba 2+ (2-6 mM) also facilitates and stabilizes the discharge of feedback spikes. Co 2+ and D-600 block the feedback spikes. Pharmacological agents that depolarize the L-horizontal cells, such as GABA, glutamate or nicotine, also block the feedback spikes. Both Sr 2+ and Ba 2+ also induce the appearance of spontaneous and off spikes, which are also blocked by Co 2+ , but these are not related to the feedback mechanism. These results strongly suggest that every turtle cone receives a feedback input from the L-horizontal cells, which would be able to induce an increase of the cone Ca 2+ con­ductance, which may become regenerative.

Communication between Individuals in Salp Chains II. Physiology

Abstract: When stimulated, salp chains achieve rapid coordinated changes in locomotion by the spread of epithelial action potentials or outer skin pulses (o.s.ps) from one zooid to the next along the chain. This process involves alternating epithelioneural and neuroepithelial chemical synapses. Each zooid is linked to another in the chain by two asymmetric attachment plaques; these are polarized so that transmission of o.s.ps proceeds from one zooid to the next in one direction at one plaque, and in the reverse direction at the other plaque. Sensory cells at the plaques send axons to the brain; they are not electrically coupled to the conducting epithelium in which they lie. Input from the plaque sensory cells affects the swimming generator in the brain (causing locomotor changes) and evokes synaptic activity at neuroepithelial synapses around the brain. This gives rise to o.s.ps that are conducted around the whole of the outer epithelium of the zooid and are detected at the plaques by the sensory cells of adjacent zooids. Severe stimulation of a zooid in the chain induces all zooids to separate; possible mechanisms of separation are discussed.