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Biomedical subjects

J S Taylor

Publications and source records attributed to J S Taylor.

At least 19 recordsLinked to original sources
PubMed1992

PCNA-induced DNA synthesis past cis-syn and trans-syn-I thymine dimers by calf thymus DNA polymerase delta in vitro.

Calf thymus proliferating cell nuclear antigen (PCNA) promoted DNA synthesis past cis-syn and trans-syn-I cyclobutane thymine dimers by calf thymus DNA polymerase delta (Pol delta) in vitro. Templates containing site-specific cis-syn and trans-syn-I thymine dimers were prepared via a combination of solid phase synthesis with photoproduct building blocks and DNA ligation. Extension of a 15-mer primer on the UV dimer-containing templates by Pol delta produced termination and bypass products in a dNTP and PCNA dependent manner. In the absence of PCNA and at dNTP concentrations varying between 1 and 100 microM, Pol delta could not bypass the cis-syn dimer and terminated elongation one nucleotide prior to the 3'-T of the dimer. DNA synthesis past the trans-syn-I dimer was even less efficient. In the presence of PCNA, termination occurred primarily one nucleotide prior to the 3'-T of both dimers at 1 microM dNTPs but opposite the 5'-T of the dimers at 100 microM dNTPs. In addition, under the latter conditions, bypass of the dimers was observed, to the extent of about 30% of the products for the cis-syn dimer and about 15% for the trans-syn-I dimer.

Animals↗
PubMed1992

In vitro evidence that UV-induced frameshift and substitution mutations at T tracts are the result of misalignment-mediated replication past a specific thymine dimer.

A previous study of UV-induced (254 nm) mutations in the lacI gene of Escherichia coli found that frameshift mutations accounted for about 35% of the observed mutations and that these mutations occurred predominantly at An.Tn sequences [Miller, J.H. (1985) J. Mol. Biol. 182, 48-65]. Because An.Tn sequences are hotspots for cis-syn thymine dimer formation [Brash, D.E., & Haseltine, W. A. (1982) Nature 298, 189-192], it would appear that UV-induced frameshift mutations are the result of an error during replicative bypass of a thymine dimer within such a sequence. To test the validity of such a proposal, replication experiments were carried out on templates containing cis-syn thymine dimers at each of the five possible sites of a T6 tract. The 59-mer templates were prepared by ligating oligonucleotides containing an EcoRI site to the 5'-end of decamers containing the cis-syn thymine dimer and oligonucleotides containing the primer site to the 3'-end. Primer-extension reactions were then carried out on these templates with a 3'----5' exonuclease-deficient (exo-) Klenow fragment of E. coli polymerase I and an exo-T7 polymerase (Sequenase Version 2.0). The replicative bypass products were cleaved with EcoRI to rigorously establish and quantify the presence of frameshift mutations. Both polymerases were able to bypass dimers at all sites, but only the exo-T7 polymerase led to detectable frameshifts, both -1 (approximately 30%) and -2 (approximately 5%), and only with the template containing a cyclobutane dimer at the second site from the 5'-end of the T6 tract. Sequencing of the T7 polymerase-catalyzed bypass products of all templates demonstrated that within the limits of discrimination only As were introduced opposite the dimer-containing T tracts. The only exception was for the template with the dimer at the second site which led to a readily detectable amount of a substitution mutation (approximately 30%) opposite the 5'-thymine of the T6 tract. A mechanism involving a competition between reversible misalignment and realignment steps and irreversible elongation steps is proposed to explain the origin of both the frameshift and the substitution mutations. The implications of this work to the mechanism of UV-induced frameshift and substitution mutations at T tracts in vivo are discussed.

Autoradiography↗
PubMed1992

Regeneration in the Xenopus tadpole optic nerve is preceded by a massive macrophage/microglial response.

Changes in the optic nerve following a crush lesion and during axonal regeneration have been studied in Xenopus tadpoles, using ultrastructural and immunohistological methods. Degeneration of both unmyelinated and myelinated axons is very rapid and leads to the formation, within 5 days, of a nerve which consists largely of degeneration debris and cells. Immunohistological analysis with monoclonal antibody 5F4 shows that there is a rapid and extensive microglial/macrophage response to crush of the nerve. Regenerating axons have begun to enter the distal stump by 5 days and grow along the outer part of the nerve in close approximation to the astrocytic glia limitans. Between 5 and 10 days after nerve crush, regenerating axons reach and pass the chiasma. Macrophages are seen in the nerve at the site of the lesion within 1 h, and the response peaks between 3-5 days, just before axonal regeneration gets under way.

Animals↗
PubMed1992

Prolonged potentiation of transmission through a withdrawal reflex pathway after noxious stimulation of the heel in the rabbit.

The sural-gastrocnemius reflex of the spinalized rabbit was potentiated to an average of 3-6 times control levels after the application of noxious mechanical, thermal or chemical stimuli to the skin of the heel. Facilitation of the reflex was maximal within 1 min of the noxious stimulus, and in many cases persisted for more than 1 h. Prolonged increases in the excitability of the sural-gastrocnemius reflex were not seen after innocuous mechanical or thermal stimulation of the heel. Repetitive electrical stimulation of the sural nerve (100 shocks given at 0.5 Hz) caused persistent facilitation of the reflex when small myelinated A delta fibres or non-myelinated C-fibres were recruited by the conditioning stimulus. Such protracted increases in the excitability of the sural-gastrocnemius pathway would enhance the protective functions of this reflex. The mechanisms described here have probably evolved to provide a high level of reflex protection to the heel after tissue damage has occurred at that site.

Animals↗
PubMed1992

Opioidergic inhibition of flexor and extensor reflexes in the rabbit.

1. Recordings were made from gastrocnemius medialis (GM), semitendinosus (ST) and tibialis anterior/extensor digitorum longus (TA/EDL) motor nerves during mechanical and electrical stimulation of the skin of the foot in decerebrated and spinalized rabbits. 2. GM motoneurones were excited from the heel and not from the toes, whereas TA/EDL responded to stimulation at the toes but not at the heel. ST also responded to electrical and mechanical stimulation at the toes, but there was a disparity between the effects of the two types of stimuli when they were applied at the heel: ST motoneurones fired in response to electrical stimulation of the heel but showed only an 'off' response to mechanical stimulation at this site. 3. The opioid antagonist naloxone caused a dose-dependent increase in all reflexes evoked by electrical stimulation of the skin. The heel-GM, toes-ST and toes-TA/EDL reflexes all increased to more than 3 times control levels with naloxone, which also caused significant decreases in the latencies of these reflex responses. On the other hand, the heel-ST response increased to just 1.4 times control levels and showed no decrease in latency with the opioid antagonist. 4. These data suggest that segmental withdrawal reflex pathways in the rabbit are suppressed by endogenous opioid peptides. This opioid-mediated inhibition seems to operate non-selectively on reflex pathways between cutaneous afferents and motoneurones.

Animals↗
PubMed1992

Two-dimensional 31P-chemical shift imaging of intramuscular heterogeneity in exercising human forearm muscle.

Two-dimensional phosphorus chemical shift imaging (2D-31P-CSI) was used to investigate macroscopic heterogeneity within the flexor digitorum profundus (FDP) muscle of the human forearm during exercise. Subjects performed low-frequency steady-state finger flexion exercise at submaximal work levels using a bulb ergometer. The number of fingers actively involved in finger flexion exercise was varied in a total of 12 experiments. Active muscles could be determined by the increase of Pi and decrease of phosphocreatine observed in the localized spectra. Within the FDP, regions of active and inactive fibers were significantly different in levels of Pi and phosphocreatine (P less than 0.01) during flexion of individual fingers. Individual flexion of the index finger was found to involve only fibers in the deep region of the FDP. Fibers in the central region were found to be involved in flexion of the middle finger, and fibers in the superficial region were involved in flexion of the ring and little fingers. The results of this study demonstrate the potential of 2D-31P-CSI for in vivo investigation of intramuscular heterogeneity in human skeletal muscle.

Adult↗
PubMed1991

Free magnesium levels in normal human brain and brain tumors: 31P chemical-shift imaging measurements at 1.5 T.

We have studied a series of normal subjects and patients with brain tumors, by using 31P three-dimensional chemical shift imaging to obtain localized 31P spectra of the brain. A significant proportion of brain cytosolic ATP in normal brain is not complexed to Mg2+, as indicated by the chemical shift delta of the beta-P resonance of ATP. The ATP beta-P resonance position in brain thus is sensitive to changes in intracellular free Mg2+ concentration and in the proportion of ATP complexed with Mg because this shift lies on the rising portion of the delta vs. Mg2+ titration curve for ATP. We have measured the ATP beta-P shift and compared intracellular free Mg2+ concentration and fractions of free ATP for normal individuals (n = 6) and a limited series of patients with brain tumors (n = 5). In four of the five spectra obtained from brain tissue containing a substantial proportion of tumor, intracellular free Mg2+ was increased, and the fraction of free ATP was decreased, compared with normal brain.

Adenosine Triphosphate↗
PubMed1991

Metabolite images of the human arm: changes in spatial and temporal distribution of high energy phosphates during exercise.

Localized variations in metabolites in resting and exercising skeletal muscle have been studied using Chemical Shift Imaging (CSI) techniques to obtain 2-D arrays of 31P NMR spectra from a slice through the human forearm. The excitation profile of the coil resulted in a slice thickness of ca 80 mm and the planar resolution of the CSI data corresponded to either 7, 10 or 14 mm. The metabolite information was represented both as 2-D arrays of spectra and by constructing images of the spatial distribution of different metabolites. Correlation with the anatomy was clearly visualized by overlaying the metabolite images on the appropriate region of the corresponding proton images. At rest, significant variations in the intensity of Pi, phosphocreatine (PCr) and ATP were observed in different regions of the arm. Our planar spatial and temporal (1-9 min) resolution was also sufficient to follow changes in Pi, PCr and pH in response to exercise. These changes were restricted to the exercising muscle and demonstrated heterogeneity both in the kinetics and magnitude of response between different muscles.

Adenosine Triphosphate↗
PubMed1991

Opioidergic inhibition of reflexes evoked by selective stimulation of sural nerve C fibres in the rabbit.

Reflexes were evoked in the gastrocnemius medialis (GM) muscle nerve by selective electrical stimulation of the non-myelinated C fibres of the ipsilateral sural nerve of decerebrated, spinalized rabbits. The opioid antagonist (-)-quadazocine (555 micrograms/kg i.v.) enhanced responses to sural C fibre stimulation to an average of 236% of pre-drug levels. In addition, C fibre-evoked reflexes were depressed for 7-9 min after repetitive activation of the high threshold axons of the common peroneal nerve, and this effect was reversed after quadazocine. Thus, GM responses to stimulation of non-myelinated sural afferent fibres are suppressed by endogenous opioid peptides, but the degree of inhibition does not appear to be as profound as that previously reported for reflexes evoked by myelinated fibres.

Animals↗
PubMed1991

Prolonged inhibition of a spinal reflex after intense stimulation of distant peripheral nerves in the decerebrated rabbit.

1. In decerebrated rabbits, repetitive stimulation of the high-threshold afferents of the left common peroneal (CP) nerve evokes prolonged depression of the sural-gastrocnemius medialis (GM) reflex recorded in the same limb. This inhibition is antagonized by co-administration of the opioid antagonist naloxone with the alpha 2-adrenoceptor antagonist idazoxan. The present study was designed to investigate whether such inhibition could be elicited from the contralateral hindlimb or the forelimbs. 2. The sural-GM reflex of decerebrated rabbits was depressed for more than 15 min after stimulation of either ipsilateral or contralateral common peroneal (CP) or median nerves with 500 pulses of 20 V, 1 ms given at 5 Hz. The order of efficacy for generating this inhibition was ipsilateral CP greater than contralateral CP greater than or equal to ipsilateral median = contralateral median. In three of thirty-nine rabbits, stimulation of the median nerves caused facilitation of the sural-GM reflex. 3. Idazoxan (1-2 mg/kg I.V.) did not significantly alter the depressant effect of ipsilateral CP stimulation but reduced that evoked by either median nerve and almost abolished the inhibition evoked from the contralateral CP nerve. 4. Naloxone (0.25 mg/kg I.V.) reduced the effects of ipsilateral CP stimulation, did not alter the inhibition evoked from contralateral CP, and had equivocal actions on the responses to median nerve stimulation. 5. When given together, the two antagonists almost abolished the effects of stimulating the median nerves and the contralateral CP nerve, and markedly reduced the inhibition evoked from the ipsilateral CP nerve. 6. These data show that prolonged inhibition of the sural-GM reflex can be evoked by stimulation of nerves in all four limbs and that in each case the inhibition can be blocked or reduced by co-administration of antagonists to opioid and alpha 2-adrenergic receptors. Such persistent inhibition of reflexes may serve to inhibit withdrawal reflexes in situations where interruptions to normal movement would be disadvantageous.

Adrenergic alpha-Antagonists↗
PubMed1991

The early development of the frog retinotectal projection.

The guidance of retinal ganglion cell axons has been investigated in embryos of the frog Xenopus. During the initial development of the brain a series of axon tracts are laid down forming a basic 'scaffold' or framework. Retinal axons grow through one of these tracts, the tract of the post-optic commissure (tPOC). This is the only tract that extends through the rostral part of the brain at these early stages of development. The origin and development of the tPOC has been studied using antibodies which label neurons at their earliest stages of differentiation. The first sign of the tPOC is a chain of neurons which differentiate simultaneously in the caudolateral part of the diencephalon. Axons from these neurons grow the short distance between adjacent cells interlinking the chain to form a descending tract. A series of other axon projections are then added to the tPOC, each of which is segregated into a particular subregion of the tract. Retinal axons are added to the tract approximately 18 h after its formation. They grow in the sub-pial part of the tract and always occupy the rostral-most edge. Retinal axons follow the tract to the region of the developing tectum where they leave, turn dorsally, and terminate. The reliance of retinal axons on this pre-existing pathway has been demonstrated by experimentally altering the course of the tPOC during its early development. The caudo-lateral wall of the diencephalon has been rotated through 90 degrees at a stage just before the tPOC neurons differentiate. Confirmation of the predicted alteration in the course of the tPOC has been made using immunocytochemistry. In such manipulated brains, retinal axons maintain their strong affinity for the rostral edge of the tPOC, following its altered course through the diencephalon.

Animals↗
PubMed1979

Kinetic depth effect examined under conditions of unimodal versus bimodal sensory input.

To perceive the three-dimensional rotation of a shadow figure, displacement and line length change are required. Without displacement, persons perceive a line lengthening and shortening on a two-dimensional plane. The present investigation attempted to create the kinetic depth effect without displacement by including auditory input. 48 persons were randomly assigned to two groups (line-length change with tone or line-length change without tone). The tone group received sound through headphones which oscillated in amplitude in synchrony with the shadow of a rotating "T" figure. The tone group perceived a three-dimensional figure in rotation significantly more often than did the non-tone group. These results suggest that the kinetic depth effect may be created by more than one combination of sensory input.

Auditory Perception↗
PubMed1979

Lymphatic complications of manual herpes simplex infection.

In seven patients with typical recurrent herpes simplex of the hand, lymphatic complications, developed in the arm during periods of herpetic activity. Six of the patients presented with lymphangitis and lymphadenitis. In one patient, persistent lymphedema developed in the involved hand and forearm.

Adult↗