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R A Webb

Publications and source records attributed to R A Webb.

At least 37 records · Page 2Linked to original sources
PubMed1989

Distribution of histamine in the lumen contents of the small intestine of uninfected and Hymenolepis diminuta-infected rats.

Using a specific radioenzymatic assay, histamine was detected and measured in the lumen contents of six different segments of the small intestine of uninfected rats and rats infected with Hymenolepis diminuta as well as in worm tissues. The distribution of histamine in the lumen of the small intestine of uninfected rats was found to range from 1.4 +/- 0.1 microM in the first (anterior) segment to 0.59 +/- 0.13 mM in the sixth (posterior) segment. There were no significant differences between these concentrations and those found in the lumen contents of intestine from rats infected with H. diminuta. On the other hand, although most H. diminuta was confined to the second and third segments, the concentration of histamine associated with the worm tissues (5.4 +/- 0.4 microM) was significantly lower than that in Hymenolepis-containing intestinal segments. The data suggest that established infections of H. diminuta do not cause a significant increase in histamine levels in host's intestinal lumen, nor do they affect the spatial gradient in the lumen.

Animals↗
PubMed1989

Demonstration of intense glutamate-like immunoreactivity in the longitudinal nerve cords of the cestode Hymenolepis diminuta.

Glutamate-like immunoreactivity in the cestode Hymenolepis diminuta was studied at the light microscopic level using an antibody specifically directed against a reduced conjugate of glutamate linked to protein by glutaraldehyde. Relatively low levels of glutamate-like immunoreactivity were present in the subtegumental region, which was presumed to reflect an active metabolic pool. Relatively high levels of this reactivity were observed in the longitudinal nerve cords and associated cells, and somewhat lower levels were occasionally noted in ring commissures. The present data in conjunction with previous studies on glutamate in H. diminuta, support the hypothesis that glutamate is an excitatory neurotransmitter in the platyhelminths.

Animals↗
PubMed1988

Release of exogenously supplied [3H]glutamate and endogenous glutamate from tissue slices of the cestode Hymenolepis diminuta.

The effect of high potassium depolarization on the release of exogenously supplied [3H]glutamate and endogenous glutamate from tissue slices of the cestode Hymenolepis diminuta was examined. Increasing concentrations of potassium stimulated the release of radiolabel from tissues preloaded with [3H]glutamate. This release was by a partially calcium-dependent, magnesium-antagonized process. In the presence of tetrodotoxin, or absence of sodium, release of radiolabel was depressed, presumably by blockade of sodium-dependent neuronal potentials. The release of glutamate of both exogenous and endogenous origin was specifically and significantly elevated by high potassium; glutamate release was significantly depressed in calcium-free saline. The release of other amino acids of endogenous origin, including aspartate, was not elevated by high potassium. Collectively the data provide strong evidence for glutamate to be viewed as the only acidic amino acid neurotransmitter candidate in the cestodes.

Animals↗
PubMed1987

Characterization of a serotonin transporter and an adenylate cyclase-linked serotonin receptor in the cestode Hymenolepis diminuta.

[3H]5-HT exhibited specific binding in membrane preparations of Hymenolepis diminuta. The specific binding was saturable, reversible and temperature dependent. A non-linear Scatchard plot was obtained in a concentration range of 11 nM - 1000 nM [3H]5-HT, which could be resolved into sites having apparent dissociation constants (KD) of 0.10 microM and 6.25 microM for the high-affinity and low-affinity components, respectively. The latter could be selectively eliminated by binding [3H]5-HT to H. diminuta membranes in the presence of 10(-3) M nitroimipramine. Drug displacement studies, using 0.20 microM and 2.0 microM [3H]5-HT, revealed that while low-affinity [3H]5-HT binding was displaced by unlabelled 5-HT and inhibitors of 5-HT uptake, high affinity [3H]5-HT binding was affected only by tryptamine derivatives and, to a lesser extent, methysergide. In addition, high-affinity binding was stimulated by MgCl2 while low-affinity binding showed sodium-dependency. The data implicate the low-affinity site as a putative 5-HT transporter and the high-affinity site as a putative 5-HT 1 receptor. Exposure of H. diminuta membranes to 5-HT resulted in a 3-4 fold stimulation of cAMP levels. The EC 50 for the 5-HT-induced activation of adenylate cyclase (0.76 microM) was of the same order of magnitude as the apparent KD for high-affinity binding. Furthermore, the order of drug potency for the elevation of cAMP levels by 5-HT agonists and reversal by 5-HT antagonists was identical to the order of drug potency for the inhibition of high-affinity binding, suggesting linkage of the putative 5-HT 1 receptor to adenylate cyclase in H. diminuta.

Adenylyl Cyclases↗
PubMed1986

The uptake and metabolism of L-glutamate by tissue slices of the cestode Hymenolepis diminuta.

The in vitro uptake of L-[3H]glutamate by tissue slices of the cestode Hymenolepis diminuta, denuded of tegument, was investigated. Two sodium concentration-dependent mechanisms, one of high affinity (Kt 1.8 X 10(-5) M; Vmax 4.76 pMoles/min/mg wet weight) and another of low affinity (Kt 2.2 X 10(-4) M; Vmax 50.7 pMoles/min/mg wet weight), were identified, in addition to a sodium insensitive component. Exchange of preloaded [3H]glutamate did not occur in tissue slices incubated in dilute unlabelled glutamate. Acidic amino acids, imipramine and fluoxetine were effective inhibitors of high and low affinity uptake, while glutamate receptor ligands, neurotransmitters and some antihelminthics generally were not. The concentrations present in, and the metabolism of glutamate by, tissue slices was examined by HPLC. The significance of the three modes of glutamate uptake and their possible role in the physiology of H. diminuta are discussed.

Amino Acids↗
PubMed1986

Demonstration of specific high-affinity binding sites for [3H]5-hydroxytryptamine in the cestode Hymenolepis diminuta.

[3H]5-HT (0.16-8.32 nM) exhibited saturable and specific binding in membrane preparations of Hymenolepis diminuta. The saturation data produced a non-linear Scatchard plot which could be resolved into sites having apparent dissociation constants (Kd) of 0.17 and 8.30 nM for the high-affinity and low-affinity components, respectively. Drug displacement studies, using radioligand concentrations of 0.6 and 6 nM, revealed that the two [3H]5-HT binding components are pharmacologically distinct and do not conform to any known class of 5-HT recognition site. The physiological significance of these putative 5-HT receptors and their potential usefulness for the selection of new antiparasitic agents are discussed.

Animals↗
PubMed1985

Serotoninlike immunoreactivity in the cestode Hymenolepis diminuta.

Serotoninlike immunoreactivity in the cestode Hymenolepis diminuta was studied at the light microscope level by using an antibody specific to serotonin. The rostellum, the cerebral ganglia and commissure, and the strobila contained numerous process-free, unipolar and multipolar serotoninlike immunoreactive cells. The suckers contained a plexus of branching immunoreactive fibers. In the strobila the multipolar cell bodies were situated laterodorsal and lateroventral to the longitudinal nerve cords, from which neurites were directed to the contralateral and ipsilateral nerve cord to form up to three transverse commissures per proglottid. Secondary varicose branches passed anteriorly, posteriorly, and obliquely along the proglottids at the level of the deep longitudinal muscles. Other varicose multi-branching neurites passed centrifugally from the primary and secondary neurites, forming vertebratelike en passant or terminal varicosities on the deep longitudinal muscles with bulbous or spinose terminals at the level of the superficial longitudinal muscles, or in the cortical parenchyma. Serotoninlike immunoreactivity was seen on the external seminal vesicle, the sphincter and cirrus sac, and the proximal portion of the vagina. Varicose terminals were concentrated at the sphincter. The close association of serotoninlike immunoreactive terminals and varicosities with the longitudinal muscles gives credence to the concept that serotonin functions as a neuromuscular transmitter or modulator in the platyhelminths.

Animals↗
PubMed1985

The uptake and metabolism of 5-hydroxytryptamine by tissue slices of the cestode Hymenolepis diminuta.

The in vitro uptake of [3H]5HT was investigated in tissue slices of the cestode Hymenolepis diminuta. A concentrative, sodium sensitive, high affinity uptake mechanism (Km 1.43 X 10(-6) M; Vmax 222 fmoles/mg wet wt/min), together with a sodium insensitive component (linear up to 5 X 10(-6) M) were present. In the presence of 2-nitroimipramine the sodium sensitive component was significantly suppressed (Vmax 33 fmoles/mg/wet wt/min) although the Km (1.37 X 10(-6) M) was not affected. Nitroimipramine showed an IC50 of approximately 2 X 10(-6) M. The sodium insensitive component was not affected by nitroimipramine. Biogenic amines and related indoleamines were weak inhibitors of the sodium sensitive and sodium insensitive components of 5HT uptake. The tricyclic antidepressants and fluoxetine were effective inhibitors of the sodium sensitive component of 5-HT uptake; receptor ligands were weak inhibitors or without effect. The metabolism of [3H]5HT in tissue slices of H. diminuta was examined by HPLC. The role of the sodium sensitive uptake and metabolism of 5HT in terms of inactivation and recycling of neurally released 5HT and the possible importance of exogenous recruitment of 5HT are discussed.

Animals↗
PubMed1984

The occurrence, synthesis and metabolism of 5-hydroxytryptamine and 5-hydroxytryptophan in the cestode Hymenolepis diminuta: a high performance liquid chromatographic study.

The biosynthesis and metabolism of 5-hydroxytryptamine (serotonin; 5-HT) in the cestode Hymenolepis diminuta was investigated by High Performance Liquid Chromatography (HPLC). Incubation of intact H. diminuta in [3H]tryptophan resulted in substantial radioactivity recovered in 5-HT, 5-hydroxytryptophan (5-HTP), and 5-hydroxyindoleacetic acid (5-HIAA). Furthermore, the tissue levels of 5-HT and 5-HTP, as determined by HPLC with electrochemical detection, were significantly depressed when the animals were deprived of tryptophan. On the other hand, the tissue levels of 5-HTP were significantly increased following incubation with the 5-HTP decarboxylase inhibitor m-hydroxybenzylhydrazine. The synthesis and metabolism of 5-HT are discussed in the light of 5-HT as a physiological transmitter in H. diminuta.

5-Hydroxytryptophan↗
PubMed1984

Ultrastructural morphology of the nerve cells in the cerebral ganglion of the Acanthocephalan Moniliformis moniliformis.

The morphology of the cerebral ganglion of the Acanthocephalan Moniliformis moniliformis was studied in serial sections using electron microscopy. The organization of the cerebral ganglion was typical of other invertebrates with the cell bodies forming a rind, 1 cell thick, and their processes forming the central core of the neuropile. The ganglion was surrounded by a connective tissue capsule composed of collagen-like fibrils. Externally, the free surface of the cell bodies was covered by an electron-dense extracellular lamina. Seventy-six cells were identified in every ganglion examined and, on the basis of their cellular characteristics, they were divided into 5 distinct cell types, classified as type A, B, C, D and E cells. The characteristic morphological features of each cell type have been described, and the distribution of the different cell types in the cerebral ganglion was mapped.

Acanthocephala↗
PubMed1983

Octopamine in Lumbricus terrestris: presence, synthesis and effect of octopamine on the spontaneous rhythmic contractions of the ventral nerve cord.

A radiochemical-enzymatic assay was utilized to measure endogenous levels of octopamine in the nerve cord and blood of the earthworm. The results show the presence of octopamine in the ventral nerve cord and blood at concentrations of 2.79 +/- 0.5 (means +/- S.D.) ng/mg wet tissue weight and 9.5 +/- 1.6 x 10(-8) M (means +/- S.D.) respectively. The ability of the ventral nerve cord to synthesize octopamine was investigated by incubating the tissue in vitro in radiolabeled percursors. The results show the synthesis of [3H]octopamine from both precursor [3H]tyrosine and [3H]tyramine. In vitro incubation of the isolated ventral nerve cord in physiological concentrations of octopamine revealed modulation of the spontaneous rhythmic contractions of the nerve cord. The data provide direct support for a modulatory role of octopamine in L. terrestris.

Animals↗
PubMed1983

Aminergic neurons in the anterior nervous system of the rat acanthocephalan Moniliformis dubius.

The cerebral ganglion and nerve tracts of Moniliformis dubius show intense, specific, green fluorescence that is also associated with the lateral and apical sensory bulbs. Radioenzymatic assays showed that high levels of dopamine were present but only small amounts of the catecholamines norepinephrine and epinephrine were identified. Incubations of the proboscis sac in dilute solutions of dopamine increased fluorescence while incubations in reserpine resulted in loss of fluorescence. Nonfluorogenic amine octopamine was also detected radioenzymatically. Neutral red vitally stained a number of cells in the cerebral ganglion and the nerve tracts extending from the ganglion. Electron microscopy showed that many neurons contained electron-dense vesicles. The close association of the fluorescing, amine-containing nerve tracts with the sensory bulbs suggests that they may play a functional role in sensory reception and transmission in M. dubius. This is the first report on the presence of biogenic amines in the Acanthocephala.

Acanthocephala↗
PubMed1983

Octopamine action on the spontaneous contractions of the isolated nerve cord of Lumbricus terrestris.

Octopamine and synephrine were observed to effect the spontaneous rhythmic contractions displayed by the isolated ventral nerve cord of the earthworm, Lumbricus terrestris. octopamine and synephrine produced dose-dependent significant changes in the frequency, amplitude and basal tonus of the spontaneous contractions. Application of adrenergic receptor antagonists suggested the octopamine receptors to have some similarity to vertebrate alpha 1-adrenergic receptors. The spontaneous contractions were not abolished by tetrodotoxin (TTX) which suggested a myogenic origin for the contraction of the ventral nerve cord sheath muscles. Octopamine, in the presence of TTX, increased the basal tonus and maximum force of the spontaneous contractions.

Adrenergic alpha-Antagonists↗