Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 1982"

Predictive Coding: A Fresh View of Inhibition in the Retina

Abstract: Interneurons exhibiting centre--surround antagonism within their receptive fields are commonly found in peripheral visual pathways. We propose that this organization enables the visual system to encode spatial detail in a manner that minimizes the deleterious effects of intrinsic noise, by exploiting the spatial correlation that exists within natural scenes. The antagonistic surround takes a weighted mean of the signals in neighbouring receptors to generate a statistical prediction of the signal at the centre. The predicted value is subtracted from the actual centre signal, thus minimizing the range of outputs transmitted by the centre. In this way the entire dynamic range of the interneuron can be devoted to encoding a small range of intensities, thus rendering fine detail detectable against intrinsic noise injected at later stages in processing. This predictive encoding scheme also reduces spatial redundancy, thereby enabling the array of interneurons to transmit a larger number of distinguishable images, taking into account the expected structure of the visual world. The profile of the required inhibitory field is derived from statistical estimation theory. This profile depends strongly upon the signal: noise ratio and weakly upon the extent of lateral spatial correlation. The receptive fields that are quantitatively predicted by the theory resemble those of X-type retinal ganglion cells and show that the inhibitory surround should become weaker and more diffuse at low intensities. The latter property is unequivocally demonstrated in the first-order interneurons of the fly's compound eye. The theory is extended to the time domain to account for the phasic responses of fly interneurons. These comparisons suggest that, in the early stages of processing, the visual system is concerned primarily with coding the visual image to protect against subsequent intrinsic noise, rather than with reconstructing the scene or extracting specific features from it. The treatment emphasizes that a neuron's dynamic range should be matched to both its receptive field and the statistical properties of the visual pattern expected within this field. Finally, the analysis is synthetic because it is an extension of the background suppression hypothesis (Barlow & Levick 1976), satisfies the redundancy reduction hypothesis (Barlow 1961 a, b) and is equivalent to deblurring under certain conditions (Ratliff 1965).

Nuclear DNA Amounts in Angiosperms

Abstract: Nuclear DNA amounts have been estimated for more than 200 angiosperm species since the last collected list of such values for about 750 species was published by Bennett & Smith in 1976 (Phil. Trans. R. Soc. Lond. B 274, 227-274). These new estimates are either scattered in a wide range of scientific journals or, in many cases, unpublished; so they are not readily accessible. A publication, collecting these data in a single list is required. This paper contains a supplementary list of absolute DNA values, including estimates for 240 angiosperm species not listed by Bennett & Smith in 1976, as well as additional estimates for 41 species already listed by them. These data are assembled primarily for reference purposes. Consequently, the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists, who it is anticipated will be among the major users.

A calcium-dependent transient outward current in Xenopus laevis oocytes

Abstract: Membrane currents were investigated in Xenopus laevis oocytes under voltage clamp. Depolarizing pulses, given from a holding potential of about –100 mV, elicited a transient outward current when the membrane potential was made more positive than about –20 mV. As the potential was made increasingly positive the transient outward current first increased and then decreased. The amplitude of the transient current increased when the external Ca 2+ concentration was raised; and the current was abolished by Mn 2+ . It appears that when the membrane is depolarized Ca 2+ ions enter the oocyte and trigger an outward current, possibly by opening Cl – channels.

Functional implications of cross-orientation inhibition of cortical visual cells. I. Neurophysiological evidence

Abstract: Simple and complex cells of striate cortex of anaesthetized and paralysed cats were stimulated with two superimposed one-dimensional grating stimuli of different orientations to investigate inhibitory effects of non-optimally oriented stimuli. We confirmed that a stimulus of orientation orthogonal to a cell's long axis significantly reduces the cell's discharge rate. Further experiments revealed the following. (i) The inhibition was typically stronger for simple than for complex cells. (ii) It is very broadly tuned for orientation, all orientations outside the cell's tuning band having a comparable inhibitory effect. (iii) Similarly, it is broadly tuned for spatial frequency. These last two results suggest that the inhibition arises not from a single cell but from a pool of cells. (iv) The pattern of the discharge of the inhibition in response to stimulation by phase-reversed sinusoidal gratings is consistent with the notion that the inhibition arises from complex cells. A second series of recordings of stimulation by visual noise patterns demonstrated how 'cross-orientation inhibition' prevents simple cells from responding to two-dimensional visual noise while allowing them to respond to comparable one-dimensional noise patterns. We suggest that this mechanism may serve to render simple cells selectively sensitive to one-dimensional stimuli, such as the contours or borders of visual objects.

Profiles of water content and pore size in Sphagnum and peat, and their relation to peat bog ecology

Abstract: The bog mosses, Sphagnum, form a significant part of the total mass of plants in the world. Their rate of growth depends to a considerable extent on the supply of water to them, and different species occupy characteristic habitats which differ in their ability to supply water. We used the profiles of water content in almost undisturbed cores of two species to infer the size and distribution of spaces around the plants in an attempt to account for the observation that S. papillosum is usually found not far above the water table, while S. capilliforlium is usually found on hummocks well above the water table. Profiles of water content were recorded non-destructively from 30 cm diameter cores of Sphagnum and underlying peat, with use of the absorbance of the soft gamma radiation of $^{241}$Am. The distribution of water-fillable spaces of different size was inferred from profiles with the water table at different distances, to a maximum of 150 cm, below the surface. The larger spaces, which are the main path of water transport, are outside the plant cell walls: between leaves and between pendent branches and stems. The mean radius of such spaces around the hummock species S. capillifolium is smaller than that around S. papillosum. For a given depth of water table the water content of the apical tuft of branches, where growth occurs, is greater in the hummock species than it is in the lawn species. Of ecological importance is that, for a given water content in the apex, the water table is at a greater depth below the hummock species than it is below the lawn species. As the water table rises and falls, so the water content of both species shows hysteresis as large as the difference between them. The ecological significance of this and the need for measurements while water is flowing are discussed.

Aspects of the biology of the parasitic copepods Lepeophtheirus salmonis and Caligus elongatus on farmed salmonids, and their treatment

Abstract: The general biology and pathology of Lepeophtheirus salmonis and Caligus elongatus and the prevention and treatment of such ‘sea-lice’ infestations on farmed salmonids are described from the literature and original observations. The life-cycle of L. salmonis and probably also that of C. elongatus comprises the egg and 10 stages separated by moults, namely, two nauplius, one infective copepodite, four chalimus, two pre-adult and the adult (male and female) stages. Water temperature greatly affects the rate of development, especially for early larval stages. Heavy infestations of wild fish seem rare, and lice are lost fairly rapidly in freshwater. In Scotland at least L. salmonis shows a succession of generations on farmed salmonids; generation time is about six weeks at 9–12 C. Post-chalimus stages of C. elongatus may exchange between farmed salmonids and wild fish (especially gadoids). Epizootics (particularly with L. salmonis) cause great damage to salmonids in Norwegian and Scottish farms largely through feeding on host skin. The dermis is oedematous and haemorrhaged where lice feed, and blood seeps between scales; deaths probably result from osmoregulatory failure. Whilst prevention of infestation is difficult, a bath treatment for 1 h with 1 ppm of the organophosphorus compound Dichlorvos is effective against post-chalimus stages of L. salmonis on caged salmonids. Side effects are minimal and clearance rates from fish tissues satisfactory, but treatment may be required every 3–4 weeks.

Translation of Exogenous Messenger RNA Coding for Nicotinic Acetylcholine Receptors Produces Functional Receptors in Xenopus Oocytes

Abstract: Messenger RNA extracted from the electric organ of Torpedo was injected into Xenopus oocytes. This led to the synthesis and incorporation of functional acetylcholine receptors into the membrane of the oocyte. When activated by acetylcholine these Torpedo acetylcholine receptors in the oocyte membrane opened channels whose ionic permeability resembled that of nicotinic receptors in other cells.

Representation and recognition of the movements of shapes.

Abstract: The problems posed by the representation and recognition of the movements of 3-D shapes are analysed. A representation is proposed for the movements of shapes that lie within the scope of the Marr & Nishihara (1978) 3-D model representation of static shapes. The basic problem is how to segment a stream of movement into pieces, each of which can be described separately. The representation proposed here is based upon segmenting a movement at moments when a component axis, e.g. an arm, starts to move relative to its local coordinate frame (here the torso). For example, walking is divided into a segment of the stationary states between each swing of the arms and legs, and the actual motions between the stationary points (relative to the torso, not the ground). This representation is called the state-motion-state (SMS) moving shape representation, and several examples of its application are given.

Carotenoid pigments: their possible role in protecting against photooxidation in eyes and photoreceptor cells

Abstract: The effect of light on animal tissues is ambivalent. Light is necessary for many functions, e. g. for vision and, as in the flagellate halobacterium, to gain energy. But light is potentially dangerous: it is capable of destroying cells or their components by photooxidation, especially in the presence of sensitizing pigments such as haems and cytochromes, which are ubiquitous in aerobic cells. Several different examples are discussed to show how a compromise is achieved in animal tissues that for functional reasons receive high exposure to light. Carotenoid pigments, present in many eyes and photoreceptors, seem especially suited to protect against the deleterious effects of light because they absorb the dangerous short wavelength part of the light spectrum. In plant tissue, carotenoids are also well known to be capable of ‘quenching’ photoexcited states of sensitizing pigments and of oxygen, a function that they might have also in animal tissues. A consequence of the considerations is that whenever animal tissues are exposed to higher than usual light levels and/or oxygen pressures cellular damage might occur. Examples are discussed; strategies to circumvent the deleterious effects by photooxidation follow directly from the arguments.

A model for the formation of orientation columns.

Abstract: A mathematical model is proposed to describe the formation of orientation columns in mammalian visual cortex. The model is similar in concept to that proposed for ocular dominance column formation (Swindale 1980), the essential difference being that orientation is a vector rather than a scalar variable. It is assumed that initially orientation selectivity is weak and randomly distributed, and that selectivity develops in such a way that the orientation preferences of neurons less than about 200 microns apart tend to change in a similar direction, whereas the preferences of cells further apart tend to develop in opposite directions. No hypotheses are made about the anatomical or physiological basis of these interactions, and it is not necessary to assume that they are the result of environmental stimulation, as with existing models for the development of orientation selectivity (see, for example, von der Malsburg, 1973). The model reproduces the experimental data on orientation columns: roughly linear sequences of orientation change are produced, and these alternate unpredictably between clockwise and anticlockwise directions of change. Continuous sequences may span several 180 degrees cycles of rotation. The sequences are generally smooth, but abrupt discontinuities of up to 90 degrees also occur. The iso-orientation domains for large orientation ranges (60-90 degrees) are periodically spaced branching stripes that resemble those demonstrated in animals by the 2-deoxyglucose technique. The domains for narrower orientation ranges are periodically spaced but are more irregular in shape, though sometimes thin and elongated. The model makes a number of predictions that can be tested experimentally. Of particular interest are the discontinuities in the orientation sequences: these should be distributed with a spacing roughly equal to, or half, that of the iso-orientation domains. Each should be surrounded by one or two complete sets of iso-orientation domains, and each may be associated with regions where cells are not orientation selective. These regions may be more extensive in younger animals, when the columns are at an intermediate stage of formation, and less numerous where the columns run parallel and unbranched over large areas.

Photoperiodism in Japanese Quail: The Termination of Seasonal Breeding by Photorefractoriness

Abstract: The photoperiodic mechanisms underlying seasonal breeding in male Japanese quail change their characteristics annually, with the result that reproduction is terminated in the late summer rather than the autumn. The reproductive cycle was studied in 4 successive years to determine the critical daylengths for reproductive induction in spring, and for its termination in late summer. In spring luteinizing hormone levels first rise when the daylength has reached 11.9 h but in late summer the first signs of a decrease are detectable when the photoperiod has decreased to 14.7 h and levels are basal when it has reached about 14 h. Thus, quail appear to have evolved a refractoriness to daylengths in late summer that were maximally stimulatory earlier in the year. However, quail are not photorefractory in the `classic9 sense as they remain continuously in breeding if held on unchanging long days and can be photostimulated at any time of the year by daylengths in excess of 16 h light: 8 h darkness. Their particular form of refractoriness seems to involve a seasonal shift in the critical daylength. Refractoriness can be induced in quail held in the laboratory under a simulated annual photocycle, and, more importantly, it develops in birds exposed to long daylengths of a fixed duration. For example, quail exposed to 20 h light: 4 h darkness show a shift in the critical daylength from approximately 12 to 15 h over a period of about 2 months. The time course over which the shift occurs is intriguing and can explain the responses under natural photoperiods. Under fixed daylengths the critical daylength that induces regression depends upon the duration of the stimulatory photoperiod. Female quail also develop refractoriness. The evolution of refractoriness does not depend upon the activity of the hypothalamo-pituitary axis and develops in castrated quail as well as in birds implanted with androgens. Castrated quail show annual cycles in gonadotrophin secretion indistinguishable in timing from those in intact birds. Such results suggest that the physiological basis of refractoriness lies either in the neural circuits regulating neurohormone secretion or in the photoperiodic clock mechanism itself.

On the relation between fibre diameter and conduction velocity in myelinated nerve fibres

Abstract: Myelinated fibres less than 1 micrometer in diameter are rare in the peripheral nervous system; but fibres down to 0.2 micrometer in diameter exist in the central nervous system. These observations are consistent with Rushton's theory on the effects of fibre size on conduction in myelinated nerve when the different processes of myelination in the peripheral and central nervous systems are taken into account.

Regenerating Mammalian Nerve Fibres: Changes in Action Potential Waveform and Firing Characteristics Following Blockage of Potassium Conductance

Abstract: Extracellular application of potassium channel blocking agents is known to increase the amplitude and duration of the compound action potential in non-myelinated and demyelinated axons, but not in mature mammalian myelinated fibres. In the present study we used intra-axonal and whole nerve recording techniques to study the effects of the potassium channel blocking agent 4-aminopyridine (4-AP) on regenerating rat nerve fibres. Our results indicate that early regenerating (premyelinated) axons show considerable broadening of the action potential after 4-AP application and late regenerating (myelinated) axons give rise to burst activity following a single stimulus after 4-AP application. 4-AP did not affect spike waveform or firing properties of normal mature sciatic nerve fibres. These results demonstrate the importance of potassium conductance in stabilizing firing properties of myelinated regenerating axons.

What Does $^{14}$C Uptake by Phytoplankton Really Measure? A Theoretical Modelling Approach

Abstract: The patterns of photosynthetic $^{14}$C uptake predicted by four models, each of which incorporates different assumptions about the fate of carbon within a few hours of its fixation by phytoplankton, have been compared with the patterns of net $^{12}$C uptake predicted by the same models. According to all models tested, $^{14}$C will accumulate in the cells more rapidly (relative to its abundance) than $^{12}$C, until an equilibrium is established between the $^{14}$C:$^{12}$C ratio inside the cells and that in the external medium. Since several hours are required for cells to approach this equilibrium, all models predict that measurements of photosynthesis based on $^{14}$C uptake overestimate net photosynthesis in incubations of up to 6-12 h, and that this overestimation will be especially severe close to the compensation point, or at depth in a water column. Experiments showed approximately linear uptake of $^{14}$C for up to 4-5 h. The only model predicting this pattern indicated that $^{14}$C uptake was proportional to gross photosynthesis, and could not be used to estimate either respiration or net photosynthesis. Other physiological and ecological evidence suggests that the conventional assumption that the radiocarbon technique measures net photosynthesis should be re-examined.

A Binding-Site Model for Calcium Channel Inactivation that Depends on Calcium Entry

Abstract: We describe a model for the mechanism by which Ca channel inactivation might depend on calcium entry. Ca is assumed to bind to a site at the internal membrane surface to cause inactivation of Ca channels. We assume that Ca that enters through the membrane accumulates in a submembrane compartment and also make simplifying assumptions about Ca buffering and removal. Our model predicts the results of single- and double-pulse voltage-clamp experiments well. The predicted turn-off of Ca current is non-exponential. The model also predicts that procedures that slow inactivation will increase peak Ca current and suggests that both two-phase turn-off of currents and failure of normalized current to recover to 1.0 in two-pulse experiments may be explained without assuming a voltage-dependent component of inactivation or two populations of Ca channels.

The Peripheral and Central Changes Resulting from Cutting or Crushing the Afferent Nerve Supply to the Whiskers

Abstract: In neonatal rats, crushing or cutting the infraorbital nerve, the sensory nerve supply to the whiskers, has been found to prevent cortical barrel formation. However, both procedures are followed by regeneration of one-third to one-half of the nerve fibres and reinnervation of the whiskers. By counting fibres in individual whisker follicle nerves, it has been shown that 29-67% (mean 45%) of the myelinated fibres regenerate to the whiskers after a crush compared to 24-56% (mean 39%) after a cut. Further differences between the crush and cut lesions were indicated by studies on the time course of regeneration. Counts of the regenerating fibres at various ages as well as recordings of cortical evoked potentials in normal, nerve-crushed and nerve-cut animals showed that recovery was 3-4 days earlier in the nerve-crushed, compared with the nerve-cut animals. In normal and nerve-crushed animals the evoked potential was first detectable 2-3 days after birth while the response after nerve cut could not be recorded until day 7. Even after 60 days the amplitude of responses on both crushed and cut pathways was only about one-third of normal, while the latency was prolonged (normal 5.8 +/- 0.25 ms, crush 6.5 +/- 0.26 ms, cut 7.7 +/- 0.67 ms). Central changes occurring as a result of nerve cut or crush have been studied by microelectrode recordings from the trigeminal nucleus (the first synaptic level) and the somatosensory cortex. These also indicate clearly the greater severity of the cut lesion. Thus, in crushed animals, all levels of the trigeminal nucleus as well as the cortex show only minor modifications. The whiskers occupy the same total area and responses from all whiskers are present at their normal sites. However, after nerve cut, the responses from both the trigeminal nucleus and cortex show clear abnormalities. The total whisker area is reduced with a concomitant expansion of responses from the nose, check, lower jaw, and whiskers by the eye and ear. In addition, only one-third to one-half of the whiskers give responses. The site of these abnormalities is localized to the trigeminal nucleus since all whiskers show innervation in the peripheral nerve. It is suggested that the longer recovery time as well as the reduced accuracy of reinnervation may contribute to the poorer central recovery after a nerve cut.

Sodium and Potassium Channels in Regenerating and Developing Mammalian Myelinated Nerves

Abstract: A study has been made of how the normal complementary distribution of sodium and potassium channels in mammalian myelinated nerve fibres (all the sodium channels being in the node with all the potassium channels in the internode) is altered in regenerating and in developing rabbit sciatic nerves. In regenerating nerve fibres, where a marked increase in the number of nodes per unit length occurs, there is a corresponding increase in the sodium channel content (determined from the maximum saturable binding of labelled saxitoxin), consistent with the idea that the number of sodium channels per node remains roughly constant. The use of 4-aminopyridine, which by blocking potassium channels prolongs the action potential, has shown that both in regenerating nerve fibres and in developing nerve fibres potassium currents contribute to the mammalian action potential. In both cases, with the passage of time, the sensitivity to 4-aminopyridine progressively decreases.

The antagonism between botulinum toxin and calcium in motor nerve terminals.

Abstract: The effects of tetraethylammonium and manganese, which modify calcium entry into motor nerve terminals, have been studied during advanced stages of botulinum paralysis. Evidence has been obtained that the voltage-activated calcium current in the nerve endings is not significantly reduced by botulinum toxin. The depression of transmitter release that the toxin produces must arise at a later stage, at an intracellular site of the release mechanism.

Primary Production of Phytoplankton off Signy Island, South Orkneys, the Antarctic

Abstract: Marine planktonic primary production was studied in Borge Bay, Signy Island, the Antarctic, during the period January 1972-February 1974. Productivity was measured in situ by the radiocarbon uptake method. In 1972-1973 and 1973-1974, respectively, 86 and 289 g C m$^{-2}$ a$^{-1}$ was fixed in 16.5 m of water. The seasonal production cycle consisted of a rapid increase of a diatom standing crop dominated by Thalassiosira antarctica Comber, to a peak crop of 0.38 l$^{-1}$ m$^{-2}$ standing cell volume, 605 mg chl a m$^{-2}$ and 23.5 g C m$^{-2}$, by the first week of January. Mid-summer productivity was high, 4.8 g C m$^{-2}$ d$^{-1}$ being fixed on occasion but specific fixation rates were relatively low (maximum 1.12 mg C mg$^{-1}$ chl a h$^{-1}$). In late February the standing crop declined abruptly and during the winter period productivity and standing crop was small. Productivity and standing crop was greatly influenced by light climate and water stability.

Synthesis of chick brain GABA receptors by frog oocytes.

Abstract: Poly(A)-mRNA, extracted from the optic lobe of chick embryos, directs the synthesis of $\gamma $-aminobutyric acid (GABA) receptors in Xenopus laevis oocytes. The receptors are inserted into the oocyte membrane, where they form receptor--channel complexes. When activated by GABA, and related agonists, the chick brain receptors open membrane channels that are permeable to chloride ions. Thus, Xenopus oocytes provide a novel and useful approach to the study of brain receptors.

Mast cells in spotted mutant mice (W Ph mi)

Abstract: Mast cells derived from haematopoietic tissue are deficient in numbers in spleen, stomach and skin of Harwell mice doubly mutant at the spotting W locus: seven viable combinations of four mutants. Most combinations have variably impaired viability, anaemia and infertility; but homozygous W$^{sh}$W$^{sh}$ are normal in these respects yet still lack mast cells. The effect of the W gene on mast cells acts in recessive fashion. Effects of doubly mutant W genes on mast cells and coat colour, the latter usually regarded as dominant, appear more closely related than other pleiotropic effects. The spotting gene Ph, closely linked to W, has but marginal effects on mast cells, whereas mi, another spotting gene, quite unrelated to W affects mast cells in the spleen in a dominant way. Thus, splenic mast cells may be a special category of a heterogeneous population. Peptic ulceration, recorded in W/W$^{v}$ mice of Jackson stock, was not seen in Harwell mice. We suggest that this lesion is due to genetic complementation or environmental causes.

A Novel Effect of Zinc on the Lobster Muscle GABA Receptor

Abstract: The effect of zinc (and copper) was investigated on the lobster muscle γ-aminobutyric acid (GABA) receptor Zinc (10 μm-1 mM) depressed the GABA-evoked conductance increase in a fully reversible manner by possibly binding to an imidazole group, suggested from pH titration studies on the evoked-chloride conductance Other transition metal (period 4) divalent cations (up to 500 μM) were inactive in antagonizing GABA responses Variation of external chloride or anion substitution did not perturb the zinc antagonism ; however, decreasing the pH markedly decreased the potency of zinc A possible explanation for these results is discussed Although the zinc antagonism resembled that produced by picrotoxinin, combination of these two agents depressed the GABA dose-conductance curve in a manner expected for two antagonists acting on independent sites The zinc binding site was also discrete from the GABA recognition site; the results are interpreted in terms of a distinct binding site for zinc and H + The distortion of an agonist dose-response curve by formation of an inactive agonist-divalent cation complex is discussed; however, complexation of GABA did not explain the observed antagonism by zinc By comparison, zinc had no effect on the GABA responses of rat ganglionic neurons It is concluded that the zinc binding site, on lobster muscle, may be an important modulatory site for the GABA-evoked chloride conductance

Pollen-Pistil Interaction in Lilium longiflorum: The Role of the Pistil in Controlling Pollen Tube Growth Following Cross- and Self-Pollinations

Abstract: By cytophysiological methods, the self-incompatibility mechanism of the breeding system in Lilium longiflorum has been examined with particular reference to the synthesis, location and nature of the stylar factors involved in the control of pollen tube development. A \`bioassay' has been developed by which the effect of stylar extracts on pollen tube elongation may be investigated. With use of this system, a crude fraction of proteins from the stylar fluid has been shown to inhibit pollen tube growth only when protein fractions from \`self' styles are used. The proteins of this fraction have been analysed by thin-layer gel electrofocusing. Changes in the profiles thus obtained following selfing and a heat treatment known to inactivate the self-incompatibility response indicate a highly polarized glycoprotein to be an active component of the system. The various ways by which such a glycoprotein could control pollen tube elongation are considered in detail, and these events in Lilium are discussed in the light of our knowledge of other self-incompatibility systems operating in angiosperms.

Efferent Regulation of Hair Cells in the Turtle Cochlea

Abstract: Intracellular recordings were made from hair cells in the isolated cochlea of the turtle to characterize the inhibition achieved by the cochlea’s efferent innervation. A short train of shocks delivered to the efferent axons produced in the hair cells slow hyperpolarizing synaptic potentials which could be reversed by shifting the membrane potential more negative than about - 80 mV. Throughout the efferent hyperpolarization, there was a reduction of up to 25-fold in the amplitude of the receptor potential for tones presented at the hair cell’s characteristic frequency. Efferent stimulation also was shown to degrade the cell’s tuning properties.It is argued that the combined effects of the hyperpolarization and the loss in hair cell sensitivity could account for a threshold elevation of at least 70 dB in the auditory nerve fibres.

Plasma protein concentrations in interstitial fluid from human aortas.

Abstract: The concentration of plasma proteins was examined in interstitial fluid collected from human aortas, obtained at autopsy, from patients from whom a blood sample had been taken for routine analysis shortly before death. The interstitial fluid was absorbed onto small, preweighed pieces of filter paper which were inserted into natural strip planes in the tunica intima and inner tunica media. After equilibration the papers were removed and weighed to measure the amount of interstitial fluid collected, then analysed by quantitative immunoelectrophoresis for three plasma proteins covering a range of molecular masses ( M r ): low density lipoprotein (LDL) with molecular mass 2.4 x 10 6 , α 2 -macroglobulin ( M r 720 000) and serum albumin ( M r 68 000). In interstitial fluid obtained from normal intima of 20 men and women, the mean levels of the proteins were LDL 215%, α 2 -macroglobulin 115% and albumin 54% of the concentration in the patient’s own plasma. Concentrations were not influenced by depth within the intima, or by distance down the aorta. The patients covered the age range 31-96 years; relative concentrations of LDL, α 2 -macroglobulin and albumin increased in parallel by about 10% per decade. Large samples (up to 7 μl) of interstitial fluid were collected from inner media but they contained no measurable LDL; the levels of α 2 -macroglobulin and albumin were respectively 11 and 18% of plasma concentration. Analysis of interstitial fluid from one sample of normal intima obtained at vascular surgery, and from two freshly killed pigs, suggested that results were not invalidated by the use of autopsy material. Direct comparison of whole intimal tissue and interstitial fluid provided no evidence of preferential binding of LDL in tissue. There was no evidence of preferential adsorbtion of LDL by the filter paper. Interstitial fluid of six samples of normal intima from four patients was compared with normal plasma by two-dimensional immunoelectrophoresis. The peaks produced in the antiserum to LDL were identical in mobility, shape and staining properties. Thus the concentration of LDL found in interstitial fluid from normal aortic intima was more than twice the plasma concentration, and the relation between concentration and molecular mass was the inverse of that reported for peripheral interstitial fluid and lymph. Endothelium appears to trap LDL in the intima instead of keeping it out, and this raises fundamental questions about the function of arterial endothelium.

Discrepancies between Spontaneous and Evoked Synaptic Potentials at Normal, Regenerating and Botulinum Toxin Poisoned Mammalian Neuromuscular Junctions

Abstract: Amplitudes and times to peak of spontaneous miniature endplate potentials (m.e.p.ps) and evoked quantal endplate potentials (e.p.ps) were compared at normal, regenerating and botulinum toxin poisoned neuromuscular junctions of the extensor digitorum longus muscle of the rat. At normal junctions the mean time to peak of m.e.p.ps was longer and more variable than that of similar-sized e.p.ps. At endplates where nerve regeneration was induced by mechanical crushing of the motor nerve the frequency of m.e.p.ps was reduced and their amplitude distribution was broader than normal. The distribution of times to peak of m.e.p.ps was considerably broader than that of quantal e.p.ps recorded at the same endplates. At neuromuscular junctions poisoned with botulinum toxin type A, spontaneous and evoked transmitter release were greatly reduced. The amplitude distribution of m.e.p.ps was wider than that of e.p.ps and the time to peak of e.p.ps was about twice as fast as and less variable than that of m.e.p.ps. To explain the observed differences in time to peak among m.e.p.ps and between m.e.p.ps and quantal e.p.ps we suggest that some m.e.p.ps, but not e.p.ps, originate from transmitter quanta released from sites at a greater distance from postsynaptic receptors or that the release or diffusion process for acetylcholine is more prolonged when producing some of the m.e.p.ps. Such mechanisms produce at normal junctions a small population of m.e.p.ps with prolonged times to peak, at regenerating junctions a greater proportion of such m.e.p.ps and in botulinum toxin poisoning a majority.

Dissection of stages in exocytosis in the adrenal chromaffin cell with use of trifluoperazine.

Abstract: Stimulation of isolated chromaffin cells with carbamylcholine led to a number of morphological changes, indicative of exocytosis, apparently resulting from translocation of secretory granules to the plasma membrane and their subsequent fusion with the plasma membrane to release their contents. However, stimulation in the presence of trifluoperazine resulted only in the accumulation of secretory granules close to the plasma membrane. Thus exocytosis could be divided into two stages: a trifluoperazine-insensitive stage involving translocation of secretory granules to the plasma membrane and a second trifluoperazine-sensitive stage resulting in granule-plasma membrane fusion.

4-Aminoquinoline-induced ‘giant’ miniature endplate potentials at mammalian neuromuscular junctions

Abstract: 4-Aminoquinoline (4-AQ) in concentrations around 200 μM induces, within minutes of its application to isolated mouse or rat neuromuscular junctions, the appearance of a population of miniature endplate potentials (m. e. p. ps) with a larger than normal amplitude, so-called giant m. e. p. ps (g. m. e. p. ps). With amplitudes 21-22 times the modal value of m. e. p. p. amplitude, the population of g. m. e. p. ps. varied between 15 and 45% of the total population of m. e. p. ps. There was no increase in the frequency of m. e. p. ps. but a positive correlation between the frequency of g. m. e. p. ps and the total frequency of m. e. p. ps. In many instances the rise time and decay time of g. m. e. p. ps were prolonged compared to normal. Elevated extracellular calcium concentrations increased the frequency of m. e. p. ps but had no effect on g. m. e. p. p. frequency. High extracellular potassium concentrations markedly increased m. e. p. p. frequency but failed to influence g. m. e. p. p. frequency. Similar observations were made with ethanol 0.1 M, ouabain 200 μM or black widow spider venom. Botulinum toxin type A markedly reduced total m. e. p. p. frequency but 4-AQ still induced g. m. e. p. ps. Nerve stimulation failed to release quanta corresponding to the g. m. e. p. ps. G. m. e. p. ps seemed to originate from quantal acetylcholine release from the nerve terminal since they were abolished by surgical denervation and by the addition of d -tubocurarine to the medium. Blockade of voltage-sensitive calcium or sodium channels by, respectively, manganese ions or tetrodotoxin failed to affect the appear­ance and the frequency of g. m. e. p. ps. The electrophysiological findings and a statistical analysis of the characteristicsofthe m. e. p. psindicatethat they belong to two populations. One population is accelerated by the depolarization–release coupling mechanism responsible for evoked transmitter release and is characterized by an amplitude distribution and a process in time that indicate that they correspond to releases occurring at 9 active zones ’ in the nerve terminal. The second population of m. e. p. ps is uninfluenced by nerve terminal depolarization and transmembrane calcium fluxes. This population apparently originates from sites dispersed in the nerve terminal membrane and outside the ‘ active zone ’. 4-AQ increases the frequency of this second m. e. p. p. population without affecting the first population.

Tidal Micro-Growth Bands in Intertidal Gastropod Shells, with an Evaluation of Band-Dating Techniques

Abstract: Four methods were used in dating micro-growth bands in the intertidal gastropods Littorina littorea, Patella vulgata and Nucella lapillus. One of the methods employed, immersion in 2.25 x 10 -4 M acetazolamide, was always successful in producing a stress band by inhibiting shell growth, but in Littorina littorea and Nucella lapillus the inhibition persisted for several days after treatment and reduced the number of bands subse­quently laid down. A surface check mark associated with an internal stress band could be made in a percentage of all three species if they were kept at 3°C for 3–4 days. However, this treatment was damaging to Nucella lapillus . The most reliable method for dating the bands without appreciably interrupting subsequent shell deposition was to remove mechanically a thin layer from the outer lip of the shell or to keep the animals in sea water with ample food for 36 h to produce an abnormally wide shell increment. When the last two methods were used and the number of bands from the check band to the growing edge were counted in peels of sectioned and etched shells, the number agreed within 1% with the number of low waters that the animals had experienced.

`Yellow Lens' Eyes of a Stomiatoid Deep-Sea Fish, Malacosteus niger

Abstract: Bright yellow lenses were found in the eyes of the stomiatoid deep-sea fish, Malacosteus niger Ayres. The optical properties of the yellow lens and the retinal specializations in the eyes were examined. Absorption spectra of the yellow lens revealed two peaks at wavelengths 425 and 460 nm. The photoreceptors were all rods and were arranged in two superimposed layers. An astaxanthin-type retinal tapetum was observed in the pigment epithelium. Some chemical evidence is presented showing that the tapetal material is an astaxanthin ester. The ecological significance of the yellow lens is discussed in connection with that of Malacosteus' orbital light organ which has a reddish filter.