Mechanoelectric transduction in nematocytes of a hydropolyp (Corynidae)

In sensitivity and ultrastructure of their cnidocil apparatus (CA), the nematocytes (stinging cells) of hydrozoans are analogous to hair cells of vertebrates and epidermal mechanoreceptors of insects. Intracellular recordings using current and voltage clamp in the capitate tentacles of the marine hydropolyp Stauridiosarsia producta (Corynidae) now revealed that depolarizing receptor potentials and receptor currents are generated in nematocytes (stenotele type) in response to mechanical stimulation of the CA. The responsive cells were identified by injection of Lucifer Yellow. For recording, the tentacles were isolated from the polyp and held by a suction capillary. Stimuli were applied by a glass probe moved electromagnetically or piezoelectrically.The mechanosensitivity of the nematocytes was found to be strictly limited to the CA. The characteristics of the mechanoelectric transduction were those typical of mechanoreceptor cells: phasic-tonic time course of an increase in membrane conductance; latency between stimulus and receptor response < 50 s; sigmoid relationship between receptor-response amplitude and stimulus amplitude; maximal increase in conductance of 15 nS; reversal potential between + 35 mV and — 10 mV; unspecific cation dependence and reversible blocking by streptomycin. The results suggest a direct mechanical control of unspecific cation channels such as has been found for mechanoreceptor cells.Suprathreshold receptor potentials elicit two forms of regenerative depolarization: non-inactivating, steplike potentials and action potentials. The latter can trigger discharge of the nematocyst.The discharge of nematocysts in the intact animal (without recording) in response to adequate stimuli was blocked by streptomycin and Na⁺ depletion in the same way as the receptor potential.Mechanoreceptor potentials are thus the beginning of a stimulus-induced electrical reaction cascade that ends in nematocyst discharge.     

Independent origins of neurons and synapses: insights from ctenophores

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes. Genomics and metabolomics data from basal metazoans suggest that neural signalling evolved independently in ctenophores and cnidarians/bilaterians. This polygenesis hypothesis explains the lack of pan-neuronal and pan-synaptic genes across metazoans, including remarkable examples of lineage-specific evolution of neurogenic and signalling molecules as well as synaptic components. Sponges and placozoans are two lineages without neural and muscular systems. The possibility of secondary loss of neurons and synapses in the Porifera/Placozoa clades is a highly unlikely and less parsimonious scenario. We conclude that acetylcholine, serotonin, histamine, dopamine, octopamine and gamma-aminobutyric acid (GABA) were recruited as transmitters in the neural systems in cnidarian and bilaterian lineages. By contrast, ctenophores independently evolved numerous secretory peptides, indicating extensive adaptations within the clade and suggesting that early neural systems might be peptidergic. Comparative analysis of glutamate signalling also shows numerous lineage-specific innovations, implying the extensive use of this ubiquitous metabolite and intercellular messenger over the course of convergent and parallel evolution of mechanisms of intercellular communication. Therefore: (i) we view a neuron as a functional character but not a genetic character, and (ii) any given neural system cannot be considered as a single character because it is composed of different cell lineages with distinct genealogies, origins and evolutionary histories. Thus, when reconstructing the evolution of nervous systems, we ought to start with the identification of particular cell lineages by establishing distant neural homologies or examples of convergent evolution. In a corollary of the hypothesis of the independent origins of neurons, our analyses suggest that both electrical and chemical synapses evolved more than once.     

Ultrastructure of pheromone-producing eversible vesicles in males of Chrysopa perla L. (Insecta,Neuroptera)

Summary During courtship, males of Chrysopa perla extrude from the genital aperture two eversible vesicles that bear approximately 50 sclerotized tubercles with a long, apical hair. Ultrastructural study reveals within each tubercle two juxtaposed subunits: a sensory subunit formed by a mechanoreceptor neuron, a glial cell, two enveloping cells, and a glandular subunit consisting of a glandular cell provided with an extracellular reservoir, a duct cell and an enveloping cell. The glandular-cell cytoplasm is characterized by extensive RER, a large amount of SER, probably involved in lipid elaboration, and voluminous myeloid bodies encircling the reservoir. The secretion is released to the outside by a cuticular duct that opens into the side of the tubercle. The function of the mechanoreceptor associated with the glandular subunit is discussed.     

Ultrastructure of epidermal sensory receptors in Amphibolurus barbatus (Lacertilis: Agamidae)

Summary The histological and ultrastructural organisation of the epidermal sensory organs in Amphibolurus barbatus has been described with respect to their position and possible functions. The sensory organs, located at the scale's edge, are most numerous in scales of the dorsal surface of the head. Most other scales of the body surface have two receptors located laterally to the spine or keel of the scale. In the imbricate scales of the ventral body region, the receptors lie just beneath the reinforced scale lip. Scanning electron microscopy has revealed the surface of the organ to be a crater lacking any surface projections. These sensory organs have a dermal papilla consisting of a nerve plexus and loose connective tissue. The nerve fibres arising from the plexus, pass to the epidermal columnar cells, where some form nerve terminals at the base of the cells, while others pass between them to form nerve terminals embedded in a superficial layer of cuboidal cells. The superficial terminals are held against the overlying keratin by masses of tonofilaments. The keratin is thickened to form a collar around the periphery of the organ but is only about 0.5 m thick immediately above it. Mechanical deformation of the scale's spine or reinforced scale lip may initiate stimulation of the nerve terminals described.     

Structure of a putative sodium channel from the sea anemoneAiptasia pallida

A cDNA encoding a full length putative sodium channel has been cloned from the sea anemoneAiptasia pallida. The deduced protein, named AiNal, has a predicted molecular weight of 205 000 Da. It shows high structural similarity to other sodium channels from both invertebrates and vertebrates, and its structure is consistent with the four domain, six transmembrane segment motif of all known voltage-gated sodium channels. In the region purported to constitute the tetrodotoxin (TTX) receptor of sodium channels, AiNal differs from the TTX-sensitive motif, suggesting that currents carried by this channel would be insensitive to TTX. The presence of a conventional sodium channel protein in anemones indicates, for the first time, that neurons in sea anemones are likely to be capable of producing fast, overshooting action, potentials.     

Interaction between arthropod filiform hairs in a fluid environment

Many arthropods use filiform hairs as mechanoreceptors to detect air motion. In common house crickets (Acheta domestica) the hairs cover two antenna-like appendages called cerci at the rear of the abdomen. The biomechanical stimulus-response properties of individual filiform hairs have been investigated and modeled extensively in several earlier studies. However, only a few previous studies have considered viscosity-mediated coupling between pairs of hairs, and only in particular configurations. Here, we present a model capable of calculating hair-to-hair coupling in arbitrary configurations. We simulate the coupled motion of a small group of mechanosensory hairs on a cylindrical section of cercus. We have found that the coupling effects are non-negligible, and likely constrain the operational characteristics of the cercal sensory array.     

Activity of lateral line efferents in the axolotl (Ambystoma mexicanum)

Summary Activity of efferent fibers was recorded from the ramus ophthalmicus superficialis of the head lateral line nerve and the ramus medialis of the trunk lateral line nerve of the axolotl Ambystoma mexicanum. Baseline activity and activity evoked by sensory stimuli were examined. Electrical stimulation of selected branches was used to determine the conduction velocity and the branching pattern of efferent fibers. The influence of lesions at different levels in the CNS on efferent activity was studied.Up to 5 units with baseline activity were found in a single ramus of the lateral line nerve. Discharge rates were variable and highly irregular; they differed between units of the same branch. Bursting activity occurred in 62% of the units. Movements of the animal were accompanied by activity in up to 8 efferent units in a single nerve.Efferent activity could be elicited or modified by stimulation of visual, labyrinthine, somatosensory, and lateral line systems. Stimulation of the electrosensory system had no effect. Individual efferent neurons innervated different fields in the lateral line periphery. Conduction velocities of efferent fibers ranged from 5 to 12 m/s.Efferent units received input from various sources at different brain levels up to the diencephalon. These in puts determined the baseline activity. The mechanosensory input was mediated at the medullary level.Abbreviations r.m. ramus medialis - r.o.s. ramus ophthalmicus superficialis - r.s. ramus superior     

Morphology and neurophysiology of tarsal vibration receptors in the water strider Aquarius paludum (Heteroptera: Gerridae)

Substrate vibratory information receptors are extensively studied in insects and spiders, however for water surface dwelling species little data is available. We studied the vibration receptive organs in tarsi of the water strider Aquarius paludum, using light, transmission and scanning electron microscopes, and recorded the neural activity of the organs in response to vibrational stimuli, which were afterwards analysed with a custom made spike sorting program.We found that the tarsal chordotonal organ has one set of three scoloparia: one in the tarsomere I and two in the tarsomere II, all of which consisted of a few scolopidia. The chordotonal organ clearly responded to vibratory stimulation. Furthermore, we found that a pair of large subapical emergent dorsal setae, which had been deemed mechanosensory by previous authors, are not so. In turn, four ventral subapical trichobothria that are in direct contact with the water surface during locomotion, proved to be mechanosensory. The anatomical and ultrastructural observations support these electro-physiological results.     

Numerical simulations of the effects of retinoids on pattern formation in a hydroid

Summary Retinoids have been shown to influence pattern formation in hydroid polyps (Hydractinia echinata) at various levels. These effects are counteracted by an inhibitor isolated from Hydra. The present study provides a theoretical attempt to elucidate the role of retinoids by computer simulations based on a simple model of pattern formation in Hydractinia. The elements of this model are morphogens of the Gierer-Meinhardt type, namely a long-range inhibitor and a short-range activator. From the calcualtions, a reducing effect of retinoids on the propagation of the inhibitor seems most probable.     

Repair of hair cells following mild trauma may involve extracellular chaperones

Sea anemones were subjected to mild trauma consisting of a 2 min immersion in calcium-depleted seawater. The trauma caused a loss of vibration sensitivity that spontaneously recovered within 50 min of returning the anemones to calcium containing seawater. Apparently, recovery is conferred by proteins contained in fraction gamma, a chromatographic fraction of homogenized mucus collected at the base of anemones allowed to recover from similar trauma. On silver stained SDS-PAGE gels, fraction gamma consists of a single band having an estimated mass of 55 kDa. Fraction gamma is alone sufficient to repair hair bundle mechanoreceptors in anemones. Its biological activity is enhanced in the presence of exogenously supplied ATP, but not GTP nor ADP-ribose. Biotinylated fraction gamma binds to hair bundles. The hypothesis that fraction gamma consists of Hsp60 proteins was tested. Commercial antibodies to Hsp60 label a band at 55 kDa in western blots. Hsp60 antibodies label hair bundles in traumatized anemones but not in untreated controls. Dilute Hsp60 antiserum (but not nonimmune serum) delays the spontaneous recovery of vibration sensitivity in anemones subjected to mild trauma. Thus, fraction gamma likely consists of Hsp60, or a Hsp60-like protein, that functions on the extracellular face of the plasma membrane to restore function to traumatized hair bundles.     

Vertebrate and invertebrate TRPV-like mechanoreceptors

Our senses of touch, hearing, and balance are mediated by mechanosensitive ion channels. In vertebrates, little is known about the molecular composition of these mechanoreceptors, an example of which is the transduction channel of the inner ear's receptor cells, hair cells. Members of the TRP family of ion channels are considered candidates for the vertebrate hair cell's mechanosensitive transduction channel and here we review the evidence for this candidacy. We start by examining the results of genetic screens in invertebrates that identified members of the TRP gene family as core components of mechanoreceptors. In particular, we discuss the Caenorhabditis elegans OSM-9 channel, an invertebrate TRPV channel, and the Drosophila melanogaster TRP channel NOMPC. We then evaluate basic features of TRPV4, a vertebrate member of the TRPV subfamily, which is gated by a variety of physical and chemical stimuli including temperature, osmotic pressure, and ligands. Finally, we compare the characteristics of all discussed mechanoreceptive TRP channels with the biophysical characteristics of hair cell mechanotransduction, speculating about the possible make-up of the elusive inner ear mechanoreceptor.     

Evolution and dynamics of branching colonial form in marine modular cnidarians: gorgonian octocorals

Multi-branched arborescent networks are common patterns for many sessile marine modular organisms but no clear understanding of their development is yet available. This paper reviews new findings in the theoretical and comparative biology of branching modular organisms (e.g. Octocorallia Cnidaria) and new hypotheses on the evolution of form are discussed. A particular characteristic of branching Caribbean gorgonian octocorals is a morphologic integration at two levels of colonial organization based on whether the traits are at the module or colony level. This revealed an emergent level of integration and modularity produced by the branching process itself and not entirely by the module replication. In essence, not just a few changes at the module level could generate changes in colony architecture, suggesting uncoupled developmental patterning for the polyp and branch level traits. Therefore, the evolution of colony form in octocorals seems to be related to the changes affecting the process of branching. Branching in these organisms is sub-apical, coming from mother branches, and the highly self-organized form is the product of a dynamic process maintaining a constant ratio between mother and daughter branches. Colony growth preserves shape but is a logistic growth-like event due to branch interference and/or allometry. The qualitative branching patterns in octocorals (e.g. sea feathers, fans, sausages, and candelabra) occurred multiple times when compared with recent molecular phylogenies, suggesting independence of common ancestry to achieve these forms. A number of species with different colony forms, particularly alternate species (e.g. sea candelabrum), shared the same value for an important branching parameter (the ratio of mother to total branches). According to the way gorgonians branch and achieve form, it is hypothesized that the diversity of alternate species sharing the same narrow variance in that critical parameter for growth might be the product of canalization (or a developmental constraint), where uniform change in growth rates and maximum colony size might explain colony differences among species. If the parameter preserving shape in the colonies is fixed but colonies differ in their growth rates and maximum sizes, heterochrony could be responsible for the evolution among some gorgonian corals with alternate branching.     

Algal genotype and photoacclimatory responses of the symbiotic alga Symbiodinium in natural populations of the sea anemone Anemonia viridis

As an approach to investigate the impact of solar radiation on an alga–invertebrate symbiosis, the genetic variation and photosynthetic responses of the dinoflagellate algal symbiosis in an intertidal and a subtidal population of the sea anemone Anemonia viridis were explored. Allozyme analysis of the anemones indicated that the two populations were genetically very similar, with a Nei''s index value of genetic identity (I) of 0.998. The algae in all animals examined were identified as Symbiodinium of clade a by PCR-RFLP analysis of the small subunit ribosomal RNA gene. The symbiosis in the two populations did not differ significantly in algal population density, chlorophyll a content per algal cell or any photosynthetic parameter obtained from studies of the relationship between photosynthesis and irradiance. We conclude that there is not necessarily genetic variation or photosynthetic plasticity of the symbiotic algae in Anemonia viridis inhabiting environments characterized by the different solar irradiances of the subtidal and intertidal habitats.     

The functions of antennal mechanoreceptors and antennal joints in tactile discrimination of the honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.)

Honeybees learn and discriminate excellently between different surface structures and different forms of objects, which they scan with their antennae. The sensory plate on the antennal tip plays a key role in the perception of mechanosensory and gustatory information. It is densely covered with small tactile hairs and carries a few large taste hairs. Both types of sensilla contain a mechanoreceptor, which is involved in the antennal scanning of an object. Our experiments test the roles of the mechanoreceptors on the antennal tip in tactile antennal learning and discrimination. Joints between head capsule and scapus and between scapus and pedicellus enable the bee to perform three-dimensional movements when they scan an object. The role of these joints in tactile antennal learning and discrimination is studied in separate experiments. The mechanoreceptors on the antennal tip were decisive for surface discrimination, but not for tactile acquisition or discrimination of shapes. When the scapus–pedicellus joint or the headcapsule–scapus joint was fixed on both antennae, tactile learning was still apparent but surface discrimination was abolished. Fixing both scapi to the head capsule reduced tactile acquisition.     

Development of Morphological Polarity in Embryogenesis of Cnidaria

A brief review dedicated to the relationship between the egg animal-vegetal axis and polarity of future larva in Cnidaria. Possible variants of changes in polarity of the embryo during development are discussed.     

FMRFamide modulates the action of phase shifting agents on the ocular circadian pacemakers of Aplysia and Bulla

Summary The eye of the marine mollusk Aplysia californica contains a photo-entrainable circadian pacemaker that drives an overt circadian rhythm of spontaneous compound action potentials in the optic nerve. Both light and serotonin are known to influence the phase of this ocular rhythm. The current study evaluated the effect of FMRFamide on both light and serotonin induced phase shifts of this rhythm. The application of FMRFamide was found to block serotonin induced phase shifts but, by itself, FMRFamide did not cause significant phase shifts. Furthermore, the effects of FMRFamide on light-induced phase shifts appeared to be phase dependent (i.e., the application of FMRFamide inhibited light-induced phase delays but actually enhanced the magnitude of phase advances). As in Aplysia, the eye of Bulla gouldiana also contains a circadian pacemaker. In Bulla, FMRFamide prevented light-induced phase advances and delays. Although FMRFamide alone generated phase dependent phase shifts, it did not cause phase shifts at the phases where it blocked the effects of light. These data demonstrate that FMRFamide can have pronounced modulatory effects on phase shifting inputs to the ocular pacemakers of both Aplysia and Bulla.Abbreviations ASW artificial seawater - CAP compound action potential - CT circadian time - 5-HT serotonin