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At least 433 records · Page 24Linked to original sources
PubMed1982

Ultrastructure of the water-movement-sensitive sensilla in the medicinal leech.

Behavioral and physiological experiments have shown that medicinal leeches are able to detect low amplitude surface waves, and further, that the transduction of this stimulus modality occurs primarily, if not exclusively, at the annular sensilla (Young, Dedwylder, and Friesen, 1981; Friesen, 1981). Here we examine the morphology of these specialized sensory structures using light, scanning electron, and transmission electron microscopes. We found that three types of ciliated sensory cells occur at the sensilla: (1) a uniciliate cell, with an axial cilium that projects at least 12 micrometers beyond the cuticle; (2) a multiciliate cell with from two to four grouped cilia that extend 1--3 micrometers beyond the cuticle; and (3) a second multiciliate cell, whose cilia project parallel to the body surface but remain within the cuticle. The cilia of all three cell types arise from the cuplike depressions which form the apices of slender, elongated cells (approximately 2 micrometers diameter X 50 micrometers length). A complexly interconnected ring of microvilli surrounds the cilium of the uniciliate cells. The morphology of the uniciliate cells closely resembles the structure of vibration-sensitive sensory neurons found in other species. We propose, based on previous results and our new findings, that the uniciliate receptor cells are the sensillar movement receptors which mediate leech sensitivity to water movements.

Animals↗
PubMed1984

Photoreceptors and visual interneurons in the medicinal leech.

The medicinal leech has five pairs of eyes, each with about 50 photoreceptors. Receptors produce propagating impulses which constitute their output to second order neurons in the CNS. Within the eye, receptors have diverse thresholds, and thus the aggregate output of the eye is graded with light intensity. By having many receptors in parallel, the eye may achieve better intensity discrimination and temporal response than would be predicted from the relatively poor characteristics of individual receptors. Receptors in eyes 3-5 on one side of the animal excite the ipsilateral LV (lateral visual) cell, an interneuron in the first segmental ganglion. By physiological tests the receptor axons are electrically coupled to the LV cell. Moreover, the LV cell is Lucifer Yellow dye-coupled to many fine fibers that appear to be receptor axons of the ipsilateral eyes 3-5. The receptors of the contralateral eyes 3-5, and those of the photosensitive sensilla lining the body inhibit the LV cell via polysynaptic pathways. Thus, the LV cells are central elements of the neural circuit processing input from the leech's spatially distributed visual system.

Animals↗
PubMed1987

Extension and retraction of axonal projections by some developing neurons in the leech depends upon the existence of neighboring homologues. II. The AP and AE neurons.

To assess the generality of our previous finding (Gao and Macagno, 1987) that segmental homologues play a role in the establishment of the pattern of axonal projections of the heart accessory HA neurons, we have extended our studies to two other identified leech neurons: the anterior pagoda (AP) neurons and the annulus erector (AE) motor neurons. Bilateral pairs of AP neurons are found in the first through the twentieth segmental ganglia (SG1 through SG20) of the leech ventral nerve cord. All AP neurons initially extend axonal projections to the contralateral periphery as well as longitudinal projections along the contralateral interganglionic connective nerves toward anterior and posterior neighboring ganglia. Although the peripheral projections are maintained by all AP neurons throughout the life of the animal, the longitudinal projections disappear in all but two segments: the AP neurons in SG1 maintain their anterior projections and extend them into the head ganglion, and those in SG20 maintain their posterior projections and extend them into SG21 and the tail ganglion. When single AP neurons are deleted anywhere along the nerve cord before processes begin to atrophy, however, the longitudinal projections are retained by their ipsilateral homologues in adjacent ganglia. The rescued processes appear to take over the projections of the deleted neurons. In cases where two or more AP neurons on the same side of the nerve cord are deleted from adjacent ganglia, a contralateral homologue sometimes extends projections to the periphery ipsilaterally or on both sides. We obtained similar results when we deleted single AE neurons from midbody ganglia. Thus, our experiments with three different identified neurons consistently show that the initial pattern of projections is the same in all ganglia, but that the existence of homologues in adjacent ganglia leads to the pruning of some of the initial projections. A consequence of this homologue-dependent process retraction is that neurons normally lacking neighboring homologues will have patterns of projections different from those neurons that do have such neighbors. Process loss by the HA, AP, and AE neurons may be the result either of competition for targets, inputs, or growth factors or of direct interactions among homologous cells.

Animals↗
PubMed1988

An identified cell is required for the formation of a major nerve during embryogenesis in the leech.

Investigations of the cues by which axonal growth cones navigate long distances to their targets have revealed the use of a rich and complex diversity of cellular and extracellular information. In the present study we describe one of the most conceptually simple pathfinding cues: a single identified cell in the leech, Hirudo medicinalis, that may guide axons several hundred micrometers to innervate a particular target. One of the stereotyped nerves of H. medicinalis is a "sex nerve" that projects from the anterior root of ganglion 6 [SNA (6)] to the male reproductive structures in the adjacent anterior segment. The pathway for SNA (6) is completely underlain by a single peripheral cell, here called the axonal runway cell (ARC), before axons enter the pathway. The ARC is apparently a nonneuronal cell that stains with a monoclonal antibody that recognizes leech muscle cells. The importance of the ARC for establishing SNA(6) was tested by ablating it before axons entered the pathway. When the ARC was killed either by physical disruption with a microelectrode, or by photoablation after filling it with the fluorescent dye Lucifer yellow, SNA(6) always failed to form, whereas all other nerves formed normally. Killing other peripheral cells in proximity to the ARC did not interfere with SNA(6) formation. Ablation of possible "pioneer neurons" for SNA(6) also did not prevent its formation. These results show that formation of a particular nerve requires only a single cell to serve as a guide for outgrowing processes.

Animals↗
PubMed1993

Different forms of 130 kD connective tissue protein are specific for boundaries in the nervous system and basement membrane of muscle cells in leech.

The nervous system and muscle tissue of the leech express two different organ-specific forms of connective tissue protein. The nervous system-specific form appears in regional boundaries separating cell bodies, axonal tracts and areas of the neuropile during late embryogenesis. In contrast, the muscle-specific form appears earlier during development in the basement membrane of muscle cells. In extraction experiments both forms behave like extracellular matrix proteins and because of their molecular weight, are considered members of a group of cell type-specific 130 kD proteins (leech gp130s). However, the two forms differ in their posttranslational modification. As determined by Con A and lentil lectin affinity chromatography, only the nervous system-specific, but not the muscle-specific form, has fucosylated and high mannose N-linked carbohydrates. These differences in the developmental onset and glycosylation suggest that nervous system-specific and muscle-specific connective tissue proteins are regulated differently and participate in different molecular interactions.

Animals↗
PubMed1994

Identification of a 70 kD protein with sequence homology to squid neurofilament protein in glial cells of the leech CNS.

A monoclonal antibody G39, generated against a protein extract of leech central nervous system, labels specific cell types in adult, embryonic, and regenerating preparations. The antibody stained glial cells, microglial cells, and connective tissue cells, but not neurons or muscle on cryosections. The staining pattern resembled that of an intracellular network. Affinity purification of the antigen revealed a 70 kD protein. Peptide sequencing showed significant homology of a stretch of 15 amino acids to squid neural filament protein. The same mAb G39 delineated glial cells as they formed during development of the CNS and showed that the giant neuropil glial cells appear before those in the packets. The antigen recognized by mAb G39 represents a nonneuronal intermediate filament of the leech Hirudo medicinalis found in various cell-types such as glia, microglia, and some cells of the connective tissue.

Amino Acid Sequence↗
PubMed1994

Cell-cell interactions that modulate neuronal development in the leech.

Mitotic lineage has been found to determine the cellular identity of leech neurons (reviewed in Stent et al., 1992), Int. Rev. Neurobiol. 33:109-133. However, the details of the adult phenotype of many neurons in the central nervous system of the leech have been shown to be shaped by interactions either with other neurons or with non-neuronal tissues in the environment. Four effects of cell-cell interactions will be considered in this article: stimulation of mitosis that generates new neurons, modulation of cell death or axonal retraction, modification of neurotransmitter metabolism, and modification of other physiological properties. In all cases, the interactions that modify development are thought to occur at a location distant from the soma, requiring that signals be transmitted a significant distance from the site of interaction to the metabolic machinery in the soma.

Animals↗
PubMed1994

Neuronal responses to purinoceptor agonists in the leech central nervous system.

Extracellular nucleotides like ATP and its derivatives are possible chemical messengers in vertebrate nervous systems. In invertebrate nervous system, however, little is known about their role in neurotransmission. We have studied the response of identified neurones of the leech Hirudo medicinalis to the purinoceptor agonist ATP, ADP, AMP, and adenosine using conventional intracellular microelectrodes and whole-cell patch-clamp recording. Bath application of the agonists depolarized the different neurons, but not the neuropil glial cells. The most effective responses (up to 10 mV) were observed with ATP (100 microM) or ADP (100 microM) in the noxious and touch cells. In most neurons the nonhydrolyzable ATP derivative ATP-gamma-S (5 microM) induced larger depolarizations than 100 microM ATP, indicating that most of the potency of ATP is lost presumably due to its degradation by ectonucleotidases. In medial noxious cells, ATP (100 microM) induced an inward current of 1.7 +/- 1.1 nA at a holding potential of -60 mV. The ATP-induced current-voltage relationship showed an inward rectification and a reversal potential close to 0 mV. In a Na+-free extracellular solution, the ATP-induced inward current decreased and in a Na+- and Ca(2+)-free saline only a small residual current persisted. The possible P2 purinoceptor antagonist suramin did not antagonize the ATP-induced current, but itself evoked an inward current and a conductance increase. We conclude that ATP activates nonselective cation channels in medial noxious cells of the leech with the order of potency of purinoceptor agonists ATP > or = ADP > AMP. The results suggest that these cells express purinoceptors of the P2 type.

Adenosine Triphosphate↗
PubMed1995

Synapse formation and function: insights from identified leech neurons in culture.

Identified leech neurons in culture are providing novel insights to the signals underlying synapse formation and function. Identified neurons from the central nervous system of the leech can be removed individually and plated in culture, where they retain their characteristic physiological properties, grow neurites, and form specific synapses that are directly accessible by a variety of approaches. Synapses between cultured neurons can be chemical or electrical (either rectifying or not) or may not form, depending on the neuronal identities. Furthermore, the characteristics of these synapses depend on the regions of the cells that come into contact. The formation and physiology of synapses between the Retzius cell and its partners have been well characterized. Retzius cells form purely chemical, inhibitory synapses with pressure-sensitive (P) cells where serotonin (5-HT) is the transmitter. Retzius cells synthesize 5-HT, which is stored in vesicles that recycle after 5-HT is secreted on stimulation. The release of 5-HT is quantal, calcium-dependent, and shows activity-dependent facilitation and depression. Anterograde and retrograde signals during synapse formation modify calcium currents, responses to 5-HT, and neurite outgrowth. The nature of these synaptogenic signals is being elucidated. For example, contact specifically with Retzius cells induces a localized selection of transmitter responses in postsynaptic P cells. This effect is signaled by tyrosine phosphorylation prior to synapse formation.

Animals↗
PubMed1995

Heartbeat control in the medicinal leech: a model system for understanding the origin, coordination, and modulation of rhythmic motor patterns.

We have analyzed in detail the neuronal network that generates heartbeat in the leech. Reciprocally inhibitory pairs of heart interneurons form oscillators that pace the heartbeat rhythm. Other heart interneurons coordinate these oscillators. These coordinating interneurons, along with the oscillators interneurons, form an eight-cell timing oscillator network for heartbeat. Still other interneurons, along with the oscillator interneurons, inhibit heart motor neurons, sculpting their activity into rhythmic bursts. Critical switch interneurons interface between the oscillator interneurons and the other premotor interneurons to produce two alternating coordination states of the motor neurons. The periods of the oscillator interneurons are modulated by endogenous RFamide neuropeptides. We have explored the ionic currents and graded and spike-mediated synaptic transmission that promote oscillation in the oscillator interneurons and have incorporated these data into a conductance-based computer model. This model has been of considerable predictive value and has led to new insights into how reciprocally inhibitory neurons produce oscillation. We are now in a strong position to expand this model upward, to encompass the entire heartbeat network, horizontally, to elucidate the mechanisms of FMRFamide modulation, and downward, to incorporate cellular morphology. By studying the mechanisms of motor pattern formation in the leech, using modeling studies in conjunction with parallel physiological experiments, we can contribute to a deeper understanding of how rhythmic motor acts are generated, coordinated, modulated, and reconfigured at the level of networks, cells, ionic currents, and synapses.

Animals↗
PubMed1998

Expression of the head gene Lox22-Otx in the leech Helobdella and the origin of the bilaterian body plan.

The orthodenticle orthologue Lox22-Otx was isolated from an annelid worm, the leech Helobdella triserialis. In situ hybridization reveals that embryonic expression of Lox22-Otx RNA is primarily restricted to an unsegmented head domain, including tissues in the foregut, surface ectoderm, and the head ganglion of the central nervous system. The patterns of head expression form concentric rings about the stomadeum and mark tissue domains that exhibit discrete behaviors during later morphogenesis and differentiation. Expression was also observed in one to two bilateral pairs of neurons in each segmental ganglion or neuromere of the body trunk. The largely head-specific expression of Lox22-Otx in this annelid species supports data from two other bilaterian phyla in suggesting the existence of a genetically defined head/trunk distinction. We suggest here that this head/trunk distinction is a synapomorphy of the Bilateria as a whole, and that it reflects the body plan of an early bilaterian ancestor. In addition, we discuss the possibility that the radial organization of gene expression and cell lineages in the leech's head domain may reflect the symmetry properties of a prebilaterian ancestor that had a radially symmetric body plan.

Amino Acid Sequence↗
PubMed1998

Mannose-specific recognition mediates two aspects of synaptic growth of leech sensory afferents: collateral branching and proliferation of synaptic vesicle clusters.

The developmental role of carbohydrate markers in the genesis of neuronal networks was studied using leech sensory afferents as a model. Leech sensory afferents express a mannose-containing epitope on their cell surface that is recognized by monoclonal antibody Lan3-2. Previously, the elaboration of sensory arbors in the synaptic neuropil of CNS ganglia was experimentally shown to depend on this mannose marker. Sensory arbors were abolished by perturbing sensory afferents in the intact nervous system with Lan3-2 Fab fragments, a glycosidase, or mannose-BSA. To understand the cytological mechanisms underlying mannose-specific recognition for synaptogenesis, we have now studied the effects of antibody perturbation at the ultrastructural level in the sensory afferent target region. A characteristic signature of a normal sensory afferent is its profuse collateral branching, which, with ongoing development, is replaced by a single widened process, the sensory trunk, which possesses numerous synaptic vesicle clusters. The inhibition of mannose-specific recognition leads to a rapid, major reorganization of different stages of sensory afferent growth. Collateral branches at the distal growing region are reduced three- to fourfold. The pruned axons grow at an accelerated rate. Developmentally older sensory trunks experience a threefold reduction in synaptic vesicle clusters. These responses suggest that depriving sensory afferents of mannose-specific recognition aborts their synaptogenesis and causes them to resume behavior typical of tracking through axonal tracts. The current findings also suggest that the mannose marker, by promoting both collateral branching andthe proliferation of synaptic vesicle clusters, plays a critical role in two stages of sensory afferent synaptogenesis.

Animals↗
PubMed2000

Leech segmental repeats develop normally in the absence of signals from either anterior or posterior segments.

We have investigated whether the development of segmental repeats is autonomous in the embryo of the leech Helobdella robusta. The segmental tissues of the germinal band arise from progeny of five stem cells called teloblasts. Asymmetric divisions of the teloblasts form chains of segment founder cells (called primary blast cells) that divide in a stereotypical manner to produce differentiated descendants. Using two distinct techniques, we have looked for potential interactions between neighboring blast cell clones along the anterior-posterior axis. In one technique, we prevented the birth of primary blast cells by injection of DNase I into the teloblast, thereby depriving the last blast cell produced before the ablation of its normal posterior neighbors. We also ablated single blast cells with a laser microbeam, which allowed us to assess potential signals acting on either more anterior or more posterior primary blast cell clones. Our results suggest that interactions along the anterior-posterior axis between neighboring primary blast cell clones are not required for development of normal segmental organization within the blast cell clone. We also examined the possibility that blast cells receive redundant signals from both anterior and posterior neighboring clones and that either is sufficient for normal development. Using double blast cell laser ablations to isolate a primary blast cell clone by removal of both its anterior and its posterior neighbor, we found that the isolated clone still develops normally. These results reveal that the fundamental segmental repeat in the leech embryo, the primary blast cell clone, can develop normally in the apparent absence of signals from adjacent repeats along the anterior-posterior axis.

Animals↗
PubMed2000

The complex dynamic network of microtubule and microfilament cytasters of the leech zygote.

The organization of the cytoskeleton in the early first interphase zygote and its involvement in organelle redistribution were studied in the glossiphoniid leech Theromyzon trizonare by confocal and electron microscopy, immunofluorescence, and time-lapse video imaging after microinjection of labeled tubulin and/or actin and loading with a mitotracker. The cytoskeleton consists of an inner or endoplasmic and an outer or ectoplasmic domain. The inner domain consists of a monaster whose fibers retract from the zygote periphery by the end of the early first interphase. The outer domain is built upon a network of microtubules and microfilaments cytasters. Short pulses of microinjected labeled actin or tubulin and Taxol treatment demonstrate that cytasters are centers of microtubule and microfilament nucleation. Immunostaining with anti-centrophilin, anti-BX-63, and anti-AH-6 indicates that the network of cytasters includes centrosomal antigens. Cytasters move in an orderly fashion at speeds of 0.5-2 micrometer/min, in an energy-dependent process retarded and finally blocked by the ATP analogue AMP-PNP and high concentrations of Taxol. Colliding cytasters fuse and form larger cytoskeletal nucleation centers. The leech zygote is a highly compartmentalized cell whose cytasters function as articulated components of a very dynamic cytoskeletal system engaged in bulk transportation of organelles during ooplasmic segregation.

Actin Cytoskeleton↗
PubMed1998

Phylogeny of leeches (Hirudinea) based on mitochondrial cytochrome c oxidase subunit I.

The phylogenetic relationships of leeches were investigated for the first time using molecular data. Twenty-one species were examined representing 7 of the 10 conventionally recognized euhirudinean families. In addition, Acanthobdella peledina, a branchiobdellid, four oligochaetes, and two polychaetes were included. Cladistic analysis of the mitochondrial cytochrome c oxidase subunit I gene yielded one most-parsimonious tree. Contemporary taxonomic groupings of leeches into higher categories were found to be largely consistent with monophyletic groups identified in the analysis. Unusual relationships for which there is some precedent include a sister-group relationship between the piscicolids and Arhynchobdellida, as well as the grouping of the haemopids within Hirudinidae.

Animals↗
PubMed1999

Higher level relationships of leeches (Annelida: Clitellata: Euhirudinea) based on morphology and gene sequences.

The evolutionary patterns of divergence of seven euhirudinean families were investigated by cladistic analysis of 33 euhirudinean species. Oligochaetes, Acanthobdella peledina, and branchiobdellidans were included as outgroup taxa. Cladistic analysis employed 1.8 kb of nuclear 18S ribosomal DNA and 651 bp of mitochondrial cytochrome c oxidase subunit I in addition to morphological data. The use of two molecular data sets, one nuclear gene and one mitochondrial gene, as well as morphological data combined historical information evolving under a variety of different constraints and therefore was less susceptible to the biases that could confound the use of only one type of data. Results suggest that the nuclear 18S rDNA gene yields a meaningful historical signal for determining higher level relationships. The more rapidly evolving CO-I gene was informative for recent or local areas of the evolutionary hypothesis, such as within-family relationships. Analyses combining all data from the three character sets yielded one most-parsimonious tree. Most of the higher taxa in recent leech systematics were well corroborated in the resulting topology. However, these results suggested paraphyly of the order Rhynchobdellida, which contradicts the presence of a proboscis as a synapomorphy. The medicinal leech family Hirudinidae was polyphyletic because Haemadipsidae and Haemopidae each have a hirudinid ancestor. In addition, all but one of the genera within the family Erpobdellidae must be either abandoned or renamed. Unusual findings included compelling evidence of historical plasticity in bloodfeeding behavior, having been lost at least four times in the course of euhirudinean evolution. Biogeographic patterns supported a New World origin for Arhynchobdellida.

Animals↗
PubMed1990

Action of FMRFamide on longitudinal muscle of the leech, Hirudo medicinalis.

1. Nerve terminals associated with longitudinal muscle in the leech show FMRFamide-like immunoreactivity. 2. Structure-activity studies using FMRFamide analogs show that the C-terminal RFamide portion of the molecule is crucial for biological activity on leech longitudinal muscle. 3. The putative protease inhibitor FA (Phe-Ala) increases the peak tension produced by longitudinal muscle in response to superfused FMRFamide and the majority of its analogs, suggesting the presence of peripheral proteases capable of degrading RFamide peptides. 4. FMRFamide decreases the relaxation rate of neurally evoked contractions of longitudinal muscle. FA also decreases the relaxation rate of neurally evoked contractions. 5. Intact and isolated muscle cells respond to superfused FMRFamide with a conductance increase, that leads to depolarization and often with a delayed conductance decrease as the membrane potential is restored to resting levels. 6. The depolarizing response of isolated muscle cells to FMRFamide is dependent on external calcium.

Animals↗
PubMed1990

Segmental specialization of neuronal connectivity in the leech.

1. Every segmental ganglion of the leech Hirudo medicinalis contains two serotonergic Retzius cells. However, Retzius cells in the two segmental ganglia associated with reproductive function are morphologically distinct from Retzius cells elsewhere. This suggested that these Retzius cells might be physiologically distinct as well. 2. The degree of electrical coupling between Retzius cells distinguishes the reproductive Retzius cells; all Retzius cells are coupled in a non-rectifying manner, but reproductive Retzius cells are less strongly coupled. 3. Retzius cells in standard ganglia depolarize following swim motor pattern initiation or mechanosensory stimulation while Retzius cells in reproductive ganglia either do not respond or hyperpolarize. 4. In standard Retzius cells the depolarizing response caused by pressure mechanosensory neurons has fixed latency and one-to-one correspondence between the mechanosensory neuron action potentials and Retzius cell EPSPs. However, the latency is longer than for most known monosynaptic connections in the leech. 5. Raising the concentration of divalent cations in the bathing solution to increase thresholds abolishes the mechanosensory neuron-evoked EPSP in standard Retzius cells. This suggests that generation of action potentials in an interneuron is required for production of the EPSP, and therefore that the pathway from mechanosensory neuron to Retzius cell is polysynaptic. 6. P cells in reproductive segments have opposite effects on reproductive Retzius cells and standard Retzius cells in adjacent ganglia. Thus the difference in the pathway from P to Retzius is not localized specifically in the P cell, but elsewhere in the pathway, possibly in the type of receptor expressed by the Retzius cells.

Animals↗