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D Robertson

Publications and source records attributed to D Robertson.

At least 289 records · Page 16Linked to original sources
PubMed1990

Direct visualization of death of neurones projecting to specific targets in the developing rat brain.

The fluorescent dye diamidino yellow was injected into parts of the developing visual and auditory systems in the rat. The dye was retrogradely transported by projecting neurones and was found to stain pyknotic profiles within the labelled cell populations. It is thus possible to visualize directly the death of neurones which project axons to specific and identified target regions within the nervous system.

Amidines↗
PubMed1990

Cotranscription of the wild-type chloroplast atpE gene encoding the CF1/CF0 epsilon subunit with the 3' half of the rps7 gene in Chlamydomonas reinhardtii and characterization of frameshift mutations in atpE.

We have characterized two independently isolated point mutants in Chlamydomonas reinhardtii, ac-u-a-1-15 and FUD 17, mapping to the chloroplast ac-u-a locus which corresponds to the atpE gene. Both mutants have a single A:T base pair deletion in a sequence of 6 A:T base pairs at nucleotide positions 102 to 107. This causes a frameshift, altering the coding sequence for the next 8 amino acids and creating a termination codon at amino acid position 44, 98 amino acids from the C-terminus of the protein. Assembly of the ATP synthase is impaired in the mutants; less than 5% of the wild-type level of alpha and beta subunits and no gamma or epsilon subunits are associated with thylakoid membranes of the mutants. The genes encoding the beta and epsilon subunits of the chloroplast ATP synthase from C. reinhardtii are not cotranscribed, in contrast to all other photosynthetic organisms examined to date. Four transcripts, of approximately 1.7, 2.9, 3.3 and 7.0 x 10(3) nucleotides (nt), are found for the atpE gene. S1 nuclease mapping of the 1.7 x 10(3) nt transcript shows that the atpE gene message is preceded by a leader of about 1250 nt. DNA sequence analysis of this region revealed a 159 bp open reading frame corresponding to the 3' half of the rps7 gene, encoding the S7 protein of the small subunit of the chloroplast ribosome. Only the 5' portion of this gene is located in the opposite unique sequence region of the C. reinhardtii chloroplast genome where the rps7 gene was previously mapped by heterologous hybridization.

Amino Acid Sequence↗
PubMed1990

Absence of tonic activity of the crossed olivocochlear bundle in determining compound action potential thresholds, amplitudes and masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

In Nembutal- or Urethane-anaesthetised guinea pigs N1 audiograms and N1 input-output functions were measured as were compound action potential (CAP) tuning curves under forward masking and simultaneous masking conditions. Then the crossed olivocochlear bundle was lesioned at the floor of the fourth ventricle and the cochlear responses were re-measured. There were never any changes in the N1 audiograms, input-output functions, or the CAP tuning curves. Thus, the crossed efferent pathways do not appear to play any tonic role in determining cochlear threshold sensitivities, selectivities or masking phenomena in anaesthetised guinea pigs with normal hearing sensitivities.

Action Potentials↗
PubMed1990

Diversity of characteristic frequency rate-intensity functions in guinea pig auditory nerve fibres.

Rate-intensity functions at characteristic frequency (CF) were recorded from single fibres in the auditory nerve of anaesthetised guinea pigs. Within the same animal, CF rate-intensity functions, although probably forming a continuum, could be conveniently divided into three groups; (1) Saturating; reach maximum discharge rate within 30 dB of threshold, (2) Sloping-saturation; initially rapid growth in discharge rate leading to a slower growth in discharge rate but not saturating and (3) Straight; approximately constant increase in firing rate per decibel increase in sound pressure up to the maximum sound pressures used. Thresholds for individual fibres were plotted relative to compound action potential thresholds at the appropriate frequency. Fibres with straight CF rate-intensity functions had the highest thresholds. Fibres of the saturating CF sloping-saturation CF rate-intensity type had thresholds intermediate between saturating and straight. There was a close relationship between the type of CF rate-intensity function exhibited by a fibre and its spontaneous discharge rate. Fibres with saturating CF rate-intensity functions generally had high spontaneous discharge rates (greater than 18/s), whereas those with straight CF rate-intensity functions generally had low spontaneous discharge rates (less than 0.5/s). The majority of fibres with sloping-saturation CF rate-intensity functions had spontaneous rates between 0.5/s and 18/s. There was a negative correlation (r = -0.59) between the logarithm of the spontaneous discharge rate and relative threshold at CF with the lowest spontaneous rate fibres having the highest thresholds and vice-versa. This diversity of CF rate-intensity functions has functional implications for both frequency and intensity coding at high sound pressures in the mammalian auditory system.

Action Potentials↗
PubMed1990

Basilar membrane nonlinearity determines auditory nerve rate-intensity functions and cochlear dynamic range.

In a previous paper (Winter et al., 1990) we demonstrated the existence of a new type of auditory-nerve rate-intensity function, the straight type, as well as a correlation between rate-level type, threshold and spontaneous rate. In this paper we now show that the variation in rate-intensity functions has its origin in the basilar membrane nonlinearity. Comparison of rate-intensity functions at characteristic frequency and at a tail-frequency show that the rate-intensity functions are identical at low firing rates and that the sloping-saturation and straight types deviate from the standard function only at higher firing rates. The frequencies at which the deviations occur, and the change from saturating to sloping-saturation or straight, are closely correlated with the characteristic frequency of the fibre. Using the tail-frequency rate-intensity function as a calibration, it is possible to derive the basilar membrane input-output function at characteristic frequency from the characteristic frequency rate-intensity function. The resulting derived basilar membrane input-output functions are of a simple form and agree well with published direct measurements of basilar membrane motion. They show that the wide dynamic range to which the cochlea responds, about 120 decibels, is compressed by the basilar membrane nonlinearity into a much smaller range of about 30-35 decibels. General characteristics of the derived basilar membrane input-output curves show features which agree well with psychoacoustic studies of loudness estimation.

Acoustic Stimulation↗
PubMed1990

Effects of L-glutamate on auditory afferent activity in view of its proposed excitatory transmitter role in the mammalian cochlea.

This report describes the effects of L-glutamate (Glu) introduced into the perilymph of scala tympani on the spontaneous and tone-evoked activity of guinea pig single primary auditory afferents. Concentrations below 2 mmol/l were in general ineffective, while a concentration of 5 mmol/l caused a marked decrease of the neural activity. At 2 mmol/l, roughly 60% of the Glu-perfusions were effective and produced a variety of changes. The most prominent effect was a reduction of the tone-evoked activity without a change in spontaneous rate. Indeed, in some cells, the tone-evoked activity could be almost totally abolished without affecting the spontaneous activity. More rarely observed was a moderate, generally transient increase of the spontaneous activity which was occasionally followed by a decrease in both tone-evoked and spontaneous firing rate. The increase in firing rate was always small relative to the maximum discharge rate evoked by tone stimuli. Desensitization of the Glu-evoked response without an obvious change in the spontaneous activity was also found. In a few cells Glu caused a reduction of the discharge rate below the spontaneous firing rate during loud tone presentation. Higher Glu doses generally caused a reduction of spontaneous and tone-evoked activity without any sign of a preceding increase. Thus, the effects of Glu in the mammalian cochlea appear to be complex and on balance seem inconsistent with the effects predicted for an excitatory transmitter. The findings argue against the hypothesis that Glu is the afferent transmitter released by inner hair-cells. However, the results do not exclude an involvement of Glu as a neuromodulator or co-transmitter.

Acoustic Stimulation↗
PubMed1990

Enhanced left ventricular contractility in autonomic failure: assessment using pressure-volume relations.

Severe autonomic failure is usually characterized by both supine hypertension and orthostatic hypotension. Inadequate preload reserve, insufficient arterial resistance and abnormal cardiac performance have been postulated to contribute to the hypotension. To clarify these mechanisms, left ventricular performance and contractility were assessed using radionuclide ventriculography and systolic pressure-volume relations when supine and with graded head-up tilt in 11 patients with autonomic failure. Results were compared with those of 12 normal subjects, using phenylephrine infusion for pharmacologic afterload augmentation after autonomic blockade with atropine and propranolol. In a subset of four patients with autonomic failure, systolic pressure-volume relations were similar by both the tilt and phenylephrine methods. In autonomic failure, end-diastolic volume, end-systolic volume and stroke volume decreased with progressive degrees of tilt (p less than or equal to 0.007 for each). The supine radionuclide ejection fraction and cardiac output were similar to those of normal subjects (69% versus 68% and 5.4 versus 4.9 liters/min, respectively, p = NS). However, the slopes of the pressure-volume relations and the supine pressure/volume ratio in autonomic failure were much greater than normal (8.8 versus 2.5, and 6.3 versus 3.6 mm Hg/ml, respectively, p less than or equal to 0.04 for both). The baseline total peripheral resistance was greater than normal (24.9 versus 17.4 mm Hg.min-1/liter, p = 0.01), but the resistance at maximal tilt failed to increase (20.8 +/- 6.1 units). Plasma norepinephrine concentrations were lower than normal. Thus, patients with autonomic failure had hypercontractile left ventricular performance when assessed by pressure-volume relations, and their hearts were well matched to the elevated peripheral resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Effects of caffeine on baroreflex activity in humans.

The effects of caffeine or placebo on blood pressure, heart rate, and baroreflex activation (elicited by phenylephrine) were studied on young normotensive volunteers after a 7-day caffeine-free period. Subjects received oral doses of either 250 mg caffeine (n = 6) or placebo (n = 4), and hemodynamic changes were studied at 0, 30, 60, 120, and 180 minutes after drug administration. Thirty minutes after the caffeine dose, blood pressure had risen from 127 +/- 8/57 +/- 4 mm Hg to 136 +/- 3/68 +/- 5 mm Hg, heart rate was unchanged, and the baroreflex slope had decreased from 31 +/- 7 msec/mm Hg to 11.6 +/- 2 msec/mm Hg. Baroreflex sensitivity remained inhibited for the rest of the single-dose experimental period. In contrast, no significant changes were observed after either long-term caffeine ingestion in the same group or in the placebo group during the single- or multiple-dose study. These findings indicate that single but not multiple caffeine administration inhibits baroreflex activation in normotensive volunteers and this could contribute to the acute hemodynamic effects of caffeine.

Administration, Oral↗
PubMed1990

Freeze-substitution without aldehyde or osmium fixatives: ultrastructure and implications for immunocytochemistry.

Cryo-fixation followed by freeze-substitution without aldehyde or osmium fixation has been investigated as a method for preparing biological specimens with a view to minimizing antigenic alteration. Samples of both solid tissues (mouse small intestine and human kidney) and a human tumour cell line grown in vitro were rapidly frozen by impact (slammed) onto a copper block cooled with liquid nitrogen. They were freeze-substituted at -80 degrees C in methanol, and embedded at low temperature in Lowicryl K4M or HM20. Resin blocks were polymerized by ultraviolet light. Well-preserved ultrastructure was observed in the outer 10-15 microns of all samples. Positive immunocytochemical localization of fixation-resistant and fixation-labile antigens was obtained on sections of human kidney and the human breast tumour cell line ZR-75-1 at both light and electron microscope levels.

Animals↗
PubMed1990

Reflex control of sympathetic nerve activity in dopamine beta-hydroxylase deficiency.

Patients with autonomic failure secondary to dopamine beta-hydroxylase deficiency lack the enzyme activity necessary for the conversion of dopamine to norepinephrine in sympathetic nerve terminals and the adrenal medulla. These patients have virtually undetectable norepinephrine and epinephrine in plasma and cerebrospinal fluid. The presence of intact sympathetic nerve activity in these patients has been suggested by the enhanced release of dopamine (but not norepinephrine) in response to maneuvers that augment sympathetic outflow in normal subjects. In the present study, we recorded sympathetic nerve traffic by using microneurography in a patient with dopamine beta-hydroxylase deficiency and measured sympathetic neural responses to static exercise, the cold pressor test, and pharmacological alterations of blood pressure. At rest, sympathetic nerve activity was abundant and was modulated in a normal manner by handgrip (+278%), the cold pressor test (+169%), hypotension induced with isoproterenol (+102%), and hypertension induced with phenylephrine (-85%). These results provide the first electrophysiological evidence for intact regulation of sympathetic neural outflow in a patient with dopamine beta-hydroxylase deficiency and suggest that central norepinephrine and epinephrine pathways believed essential for the control of sympathetic neurotransmission in humans may be supplanted by alternative redundant mechanisms.

Adult↗
PubMed1990

Dopamine-beta-hydroxylase deficiency in humans.

We report a 42-year-old man with dopamine-beta-hydroxylase deficiency, an autonomic disorder characterized by lifelong severe orthostatic hypotension, ptosis, nasal stuffiness, hyperextensible joints, and retrograde ejaculation. There is isolated deficiency of norepinephrine in both central and peripheral neurons, which contain and release dopamine instead. Dopamine-beta-hydroxylase deficiency should be suspected also in infants presenting with delayed eye opening, hypoglycemia, hypothermia, or hypotension. It can be diagnosed definitively by assay of plasma norepinephrine and dopamine.

Adult↗
PubMed1990

Cardiovascular effects of microinjection of angiotensin II in the brainstem of renal hypertensive rats.

The cardiovascular effects of microinjection of angiotensin II (AII) into the area postrema (AP), nucleus of the solitary tract (NTS) and rostroventrolateral medulla were studied in urethane anesthetized sham-normotensive (NT) and two-kidney, one-clip renal hypertensive rats. Microinjection of AII (2-2000 ng) in the AP of renal hypertensive rats elicited a dose-dependent decrease in blood pressure, heart rate and renal sympathetic nerve activity. Similar effects were observed in the NTS. In the NT rats, low doses of AII (2 and 20 ng), either in the AP or NTS, were also depressor. High doses of AII (200-2000 ng) were needed to observe a modest pressor effect in the NT animals. A decrease in heart rate and renal sympathetic activity was observed with the pressor effect. The AII-antagonist, [Sar1,Val5,Ala8]-AII, into the NTS or AP increased blood pressure and heart rate and inhibited the cardiovascular effects of low doses of AII in both group of rats. In contrast, [Sar1,Val5,Ala8]AII did not affect the pressor action of high doses of AII in the NT group. While the microinjection of AII into the rostroventrolateral medulla did not produce any significant cardiovascular effect in the renal hypertensive group, it resulted in a modest pressor effect in the NT rats. These results indicate that acute activation of AII receptors in the AP or NTS does not contribute to the pressor effect of AII in renal hypertensive rats.

Angiotensin II↗
PubMed1990

Adenosine inhibits the rise in intracellular calcium and platelet aggregation produced by thrombin: evidence that both effects are coupled to adenylate cyclase.

Platelet aggregation and secretion are associated with a rise in intracellular calcium concentration ([Ca2+]i). Adenosine has been postulated as an endogenous inhibitor of platelet aggregation. The antiaggregatory effects of adenosine are related to activation of adenylate cyclase. We studied the effect of adenosine on the rise in [Ca2+]i and platelet aggregation produced by thrombin. Human platelets were obtained from dextrose/citrate-treated plasma. [Ca2+]i was determined by fluorescence-dye techniques (fura-2). Adenosine inhibited the slope of the first phase of aggregation and the rise in [Ca2+]i produced by thrombin, in a dose-dependent manner. The dose that produced 50% inhibition of both aggregation and the rise in [Ca2+]i was approximately 500 nM. The effects of adenosine on [Ca2+]i were shared by its stable analogs, 5'-N-ethylcarboxamidoadenosine being approximately 10-fold more potent than (-)N6-phenylisopropyladenosine, suggesting that these effects were mediated through adenosine A2 receptors. Furthermore, caffeine antagonized the inhibitory effects of adenosine on platelet aggregation and [Ca2+]i. The effects of adenosine on [Ca2+]i appear to be mediated through a rise in intracellular cAMP, because they were prevented by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine (1 mM) and were potentiated by phosphodiesterase inhibition with papaverine (1 microM). Adenosine also inhibits the rise in [Ca2+]i produced by thrombin in a calcium-free medium, suggesting that adenosine inhibits both calcium influx and the release of calcium from intracellular stores.

Adenosine↗
PubMed1989

Modulatory effects of adenosine on baroreflex activation in the brainstem of normotensive rats.

The effects of the adenosine antagonists, 1,3-dipropyl-8-p-sulphenylxanthine (DPSPX) and caffeine, on baroreflex activity were tested in normotensive Sprague-Dawley rats. The microinjection of DPSPX (0.92 nmol) into the nucleus tractus solitarii (NTS) of urethane-anesthetized animals did not modify basal blood pressure or heart rate but inhibited the reflex bradycardia elicited by phenylephrine. Similar inhibitory effects on baroreflex activation were observed after intracisternal administration of caffeine to conscious or anesthetized animals. These results suggest that central endogenous adenosine is involved in the medullary regulation of blood pressure and that adenosine antagonists such as caffeine can inhibit baroreflex activation.

Adenosine↗
PubMed1989

Postnatal development of the efferent innervation of the rat cochlea.

The postnatal development of the efferent innervation of the rat cochlea was studied by intracochlear injection of the fluorescent retrograde neuronal traces Diamidino yellow and Fast blue. Injections were performed on adult rats and on neonatal rats ranging from 0 to 8 postnatal days. It was found that the total number of neurones labelled in the brainstem after intracochlear injection was not significantly different in the newborn rat, compared to the adult. On the basis of cell body location and laterality of projections, there was a clear separation into lateral and medial efferent systems at the earliest postnatal age studied (PO). Evidence was also found in the newborn for a tonotopicity in the lateral system projection similar to that in the adult. Differences between the newborn and adult were a slight but significantly greater number of bilaterally-projecting cells in the newborn, and the presence in the newborn of a small number of cells located in the lateral superior olivary nucleus contralateral to their target cochlea. These were extremely rare in the adult brainstem. Evidence was found for the occurrence of postnatal neuronal death in nuclei of origin of both efferent systems. It is suggested that although the overall extent and general organization of the efferent projection to the cochlea in the rat appears to be established at birth, regressive changes are occurring during the postnatal shaping and maturation of this brainstem-to-cochlea pathway.

Aging↗
PubMed1989

Plasticity of frequency organization in auditory cortex of guinea pigs with partial unilateral deafness.

We have examined the effect of restricted unilateral cochlear lesions on the orderly topographic mapping of sound frequency in the auditory cortex of adult guinea pigs. These lesions, although restricted in spatial extent, resulted in a variety of patterns of histological damage to receptor cells and nerve fibres within the cochlea. Nevertheless, all lesions resulted in permanent losses of sensitivity of the cochlear neural output across a limited frequency range. Thirty-five to 81 days after such damage to the organ of Corti, the area of contralateral auditory cortex in which the lesioned frequency range would normally have been represented was partly occupied by an expanded representation of sound frequencies adjacent to the frequency range damaged by the lesion. The thresholds at their new characteristic frequencies (CFs) of clusters of cortical neurones in these regions were close to normal thresholds at those frequencies (mean difference across all animals was 3.8 dB). In a second series of experiments, the responses of neurone clusters were examined within hours of making similar cochlear lesions. It was found that shifts in CF toward frequencies spared by the lesions could occur, but thresholds were greatly elevated compared to normal (mean difference was 31.7 dB in five animals). The emergence of sensitive drive in such regions after prolonged recovery periods in lesioned animals thus suggests that the auditory cortical frequency map undergoes reorganization in cases of partial deafness. Some features of this reorganization are similar to changes reported in somatosensory cortex after peripheral nerve injury, and this form of plasticity may therefore be a feature of all adult sensory systems.

Animals↗
PubMed1989

Molecular characterization of two point mutants in the chloroplast atpB gene of the green alga Chlamydomonas reinhardtii defective in assembly of the ATP synthase complex.

Two point mutants of Chlamydomonas reinhardtii, previously found by recombination and complementation analysis to map in the chloroplast atpB gene encoding the beta subunit of the CF1/CF0 ATP synthase, are here shown to be missense alterations near the 5' end of that gene. One mutant (ac-u-c-2-9) has a change at amino acid position 47 of the beta subunit from leucine (CTA) to arginine (CGA). In the second mutant (ac-u-c-2-29), the codon AAA (lysine) is changed to AAC (asparagine) at position 154. Spontaneous revertants of each mutant were isolated that restore the original wild type base pair. Northern analysis of total RNA and in vivo pulse labeling followed by immunoprecipitation reveals that both mutant atpB genes are transcribed and translated normally. However, immunoblots show that the amount of beta subunit associated with mutant thylakoids is only approximately 3% of that seen in wild type and that the CF1 alpha and gamma subunits are missing entirely. The disruption of ATP synthase complex assembly in these mutants is much more severe than in Escherichia coli beta subunit gene point mutants, which retain significant amounts of alpha and beta subunits on their membranes (Noumi, T., Oka, N., Kanazawa, H., and Futai, M. (1986) J. Biol. Chem. 261, 7070-7075). These results support the hypothesis that there are differences in assembly of the ATP synthase between E. coli and chloroplasts. In particular they indicate that beta must be present for assembly of the alpha and gamma subunits of CF1 onto chloroplast membranes.

Base Sequence↗