bluuebunny/crossref_metadata_2025 huggingface dataset:
Contribution of the Kv3.1 potassium channel to high‐frequency firing in mouse auditory neurones
Authors: Wang Lu‐Yang , Gan Li , Forsythe Ian D. , Kaczmarek Leonard K.
Published: September 2004

Using a combination of patch‐clamp, in situ hybridization and computer simulation techniques, we have analysed the contribution of potassium channels to the ability of a subset of mouse auditory neurones to fire at high frequencies.

Voltage‐clamp recordings from the principal neurones of the medial nucleus of the trapezoid body (MNTB) revealed a low‐threshold dendrotoxin (DTX)‐sensitive current (ILT) and a high‐threshold DTX‐insensitive current (IHT).

I
HT displayed rapid activation and deactivation kinetics, and was selectively blocked by a low concentration of tetraethylammonium (TEA; 1 mm).

The physiological and pharmacological properties of IHT very closely matched those of the Shaw family potassium channel Kv3.1 stably expressed in a CHO cell line.

An mRNA probe corresponding to the C‐terminus of the Kv3.1 channel strongly labelled MNTB neurones, suggesting that this channel is expressed in these neurones.

TEA did not alter the ability of MNTB neurones to follow stimulation up to 200 Hz, but specifically reduced their ability to follow higher frequency impulses.

A computer simulation, using a model cell in which an outward current with the kinetics and voltage dependence of the Kv3.1 channel was incorporated, also confirmed that the Kv3.1‐ like current is essential for cells to respond to a sustained train of high‐frequency stimuli.

We conclude that in mouse MNTB neurones the Kv3.1 channel contributes to the ability of these cells to lock their firing to high‐frequency inputs.
Facilitating pyramid to horizontal oriens‐alveus interneurone inputs: dual intracellular recordings in slices of rat hippocampus
Authors: Ali Afia B. , Thomson Alex M.
Published: September 2004

In adult rat hippocampal slices, simultaneous intracellular recordings from pyramidal cells in CA1 and interneurones near the stratum oriens‐alveus border revealed excitatory connections that displayed facilitation on repetitive activation in twelve of thirty‐six pairs tested.

Postsynaptic interneurones were classified as horizontal oriens‐alveus interneurones by the pronounced ‘sag’ in response to hyperpolarizing current injection, high levels of spontaneous synaptic activity and by the morphology of their somata and dendrites, which were confined to stratum oriens‐alveus and their axons which projected to stratum lacunosum‐moleculare where they ramified extensively, in the region of entorhinal cortex input to CA1.

Excitatory postsynaptic potentials (EPSPs) elicited by single pyramidal cells were 0 to 12 mV in amplitude. Mean EPSP amplitude (single spikes) was 0.93 ± 1.06 mV at −70 ± 2.3 mV (n= 10). The rise time was 1.2 ± 0.5 ms and the width at half‐amplitude was 7.5 ± 4.7 ms.

EPSPs fluctuated greatly in amplitude; the mean coefficient of variation was 0.84 ± 0.37 for the first EPSP and 0.47 ± 0.24 for the second. Apparent failures of transmission frequently occurred after first presynaptic spikes but less frequently after the second or subsequent spikes in brief trains.

EPSPs displayed facilitation at membrane potentials between −80 mV and spike threshold. Second EPSPs within 20 ms of the first were 253 ± 48% (range, 152–324%) of the mean first EPSP amplitude. Third EPSPs within 60 ms were 266 ± 70 % (range, 169–389%) and fourth EPSPs within 60–120 ms were 288 ± 71% (range, 188–393%). Both proportions of apparent failures of transmission and coefficient of variation analysis indicated a presynaptic locus for this facilitation.
Role of cAMP‐dependent protein kinase A in activation of a voltage‐sensitive release mechanism for cardiac contraction in guinea‐pig myocytes
Authors: Ferrier Gregory R. , Zhu Jiequan , Redondo Isabel M. , Howlett Susan E.
Published: September 2004

Ionic currents and unloaded cell shortening were recorded from guinea‐pig ventricular myocytes with single electrode voltage clamp techniques and video edge detection at 37 °C. Patch pipettes (1–3 MΩ) were used to provide intracellular dialysis with pipette solutions.

Na+ currents were blocked with 200 μm lidocaine. Contractions initiated by the voltage‐sensitive release mechanism (VSRM) and Ca2+‐induced Ca2+ release (CICR) in response to L‐type Ca2+ current (ICa,L) were separated with voltage clamp protocols.

Without 8‐bromo cyclic adenosine 3′,5′‐monophosphate (8‐Br‐cAMP) in the pipette, small VSRM‐induced contractions occurred transiently in only 13 % of myocytes. In contrast, large ICa,L‐induced contractions were demonstrable in 100 % of cells.

Addition of 10 or 50 μm 8‐Br‐cAMP to the pipette increased the percentage of cells exhibiting VSRM contractions to 68 and 93 %, respectively. With 50 μm 8‐Br‐cAMP, contractions initiated by the VSRM and ICa,L were not significantly different in amplitude.

8‐Br‐cAMP‐supported VSRM contractions had characteristics of the VSRM shown previously in undialysed myocytes. Cd2+ (100 μm) blocked ICa,L and ICa,L contractions but not VSRM contractions. 8‐Br‐cAMP‐supported contractions exhibited steady‐state inactivation with parameters characteristic of the VSRM, as well as sigmoidal contraction‐voltage relations.

Without 8‐Br‐cAMP in the pipette, contraction‐voltage relations determined with steps from a post‐conditioning potential (Vpc) of either −40 or −65 mV were bell shaped, with a threshold near −35 mV. With 50 μm 8‐Br‐cAMP in the pipette, contraction‐voltage relations from a Vpc of −65 mV were sigmoidal and the threshold shifted to near −55 mV. Contraction‐voltage relations remained bell shaped in the presence of 8‐Br‐cAMP when the Vpc was −40 mV.

H‐89, which inhibits cAMP‐dependent protein kinase A (PKA), significantly reduced the amplitudes of VSRM contractions by approximately 84 % with 50 μm 8‐Br‐cAMP in the pipette. H‐89 also significantly reduced the amplitudes of peak ICa,L and ICa,L contractions, although to a lesser extent.

We conclude that intracellular dialysis with patch pipettes disrupts the adenylyl cyclase‐PKA phosphorylation cascade, and that the VSRM requires intracellular phosphorylation to be available for activation. Intracellular dialysis with solutions that do not maintain phosphorylation levels inhibits a major mechanism in cardiac excitation‐ contraction coupling.
Does a C3‐C4 propriospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey
Authors: Maier M. A. , Illert M. , Kirkwood P. A. , Nielsen J. , Lemon R. N.
Published: September 2004

Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose‐anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3‐C4 propriospinal interneurones similar to that in the cat.

In monkeys with intact spinal cords, only a small proportion of motoneurones (19 %) responded with late EPSPs to repetitive stimulation of the pyramid; only 3 % had segmental latencies that were appropriate for a disynaptic pathway.

From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3‐C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18 %); 14 % of motoneurones had EPSPs within the disynaptic range.

In addition, some EPSPs with relatively long segmental latencies (> 1·1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions.

In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1·2‐2·5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3‐C4 propriospinal neurones to the LRN, as demonstrated in the cat.

These short‐latency EPSPs evoked from the LRN were just as common before (77 %) as after (75 %) the C5 lesion. They had small amplitudes both before (mean ± s.d. 1·1 ± 0·59 mV) and after (1·2 ± 0·72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16 %) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid.

The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3‐C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.
CA1 pyramidal to basket and bistratified cell EPSPs: dual intracellular recordings in rat hippocampal slices
Authors: Ali Afia B. , Deuchars Jim , Pawelzik Hannelore , Thomson Alex M.
Published: September 2004

Dual intracellular recordings in the CA1 region of adult rat hippocampal slices and biocytin filling of synaptically connected cells were used to study the excitatory postsynaptic potentials (EPSPs) elicited in basket (n= 7) and bistratified interneurones (n= 7) by action potentials activated in simultaneously recorded pyramidal cells.

Interneurones could be subdivided according to their electrophysiological properties into classical fast spiking, burst firing, regular spiking and fast spiking cells with a rounded spike after‐hyperpolarization. These physiological classes did not, however, correlate with morphological type. EPSPs were not recorded in regular spiking cells.

Average EPSP amplitudes were larger in bistratified cells (range, 0.5–9 mV) than in basket cells (range, 0.15–3.6 mV) and the probability of obtaining a pyramidal cell‐interneurone EPSP was also higher for the bistratified cells (1:7) than for the basket cells (1:22). EPSP 10–90% rise times in bistratified cells (0.7–2 ms) and their widths at half‐amplitude (3.9–11.2 ms) were slightly longer than in basket cells (rise times, 0.4–1.6 ms; half‐widths, 2.2–9.7 ms).

The majority of these EPSPs (6 of 8 tested) increased in amplitude and duration with postsynaptic depolarization, although in two (of 4) basket cells the voltage relation was conventional.

All EPSPs tested in both basket (n= 7) and bistratified cells (n= 5) decreased in amplitude with repetitive presynaptic firing. The average amplitudes of second EPSPs elicited within 15 ms of the first were between 34 and 94% of the average amplitude of the first EPSP. Third and fourth EPSPs in brief trains were further depressed. This depression was associated with an increase in the incidence of apparent failures of transmission indicating a presynaptic locus.
Axonal conduction velocities of functionally characterized retinal ganglion cells in goldfish
Authors: Northmore D. P. M. , Oh D.‐J.
Published: September 2004

Visual response properties and conduction velocities of retinal ganglion cells were studied by extracellular recordings in the intact goldfish eye. Visually responsive single units were confirmed as ganglion cells by collision testing, and their receptive fields were mapped.

From compound action potentials, we identified groups I‐V in the optic nerve, with overall conduction velocities of 11.5 ± 1.17, 7.1 ± 0.79, 4.4 ± 0.56, 3.1 ± 0.31 and 2.3 ± 0.18 m s−1 (mean ± s.d.) at 23 °C.

Ganglion cells were classified by their receptive fields as off‐, on‐off‐ or on‐centre. Nearly all confirmed ganglion cells had axonal conduction velocities in groups II, III and IV; none fell in the fastest group, I.

Off‐centre ganglion cells had conduction velocities only in the fast group, II. On‐off‐centre cells fell mainly in group III, with some in group II. On‐centre cells fell in groups II‐V, but mainly in groups III and IV.

Receptive field centre diameters were 5‐30 deg measured with a photopic background. The mean diameters for off‐, on‐off‐ and on‐centres were 24, 15 and 18 deg, respectively. The relatively larger diameter and higher rate of spontaneous firing of the off‐centre cells were maintained under different adaptation conditions.

The off‐centre cells can be identified with an anatomical class of large, α‐like ganglion cells in the goldfish retina.
Elevated D‐glucose induces insulin insensitivity in human umbilical endothelial cells isolated from gestational diabetic pregnancies
Authors: Sobrevia L. , Yudilevich D. L. , Mann G. E.
Published: September 2004

The effects of human insulin and elevated D‐glucose on L‐arginine transport and synthesis of nitric oxide (NO) and prostacyclin (PGI2) have been investigated in human umbilical vein endothelial cells isolated from gestational diabetic pregnancies.

The increase in the Vmax for L‐arginine transport (9.0 ± 1.1 pmol (μg protein)−1 min−1) in diabetic endothelial cells cultured in 5 mM D‐glucose was unaffected following 24 h exposure to 25 mM D‐glucose.

Gestational diabetes‐induced increases in basal intracellular cGMP and L‐citrulline levels (inhibitable by L‐NAME) and [Ca2+]i were unaffected by elevated D‐glucose. In contrast, PGI2 release was inhibited in diabetic cells exposed to either 5 or 25 mM D‐glucose.

Elevated D‐glucose attenuated histamine (10 μM, 5 min)‐stimulated accumulation of cGMP and L‐citrulline in endothelial cells isolated from gestational diabetic pregnancies.

The membrane hyperpolarization (‐79 ± 0.9 mV) sustained in diabetic endothelial cells in culture was insensitive to elevated D‐glucose.

Elevated D‐glucose abolished the stimulatory effect of human insulin (1 nM, 8 h) on L‐[3H]leucine incorporation in diabetic endothelial cells cultured in 5 mM D‐glucose.

Human insulin reduced the elevated rates of L‐arginine transport and cGMP accumulation in diabetic cells cultured in 5 mM D‐glucose but failed to reduce increased rates of transport or NO production in cells exposed to 25 mM D‐glucose or cycloheximide.

Our findings demonstrate that hyperglycaemia impairs the actions of human insulin on umbilical vein endothelial cells isolated from gestational diabetic pregnancies. Changes in insulin sensitivity and/or its signalling cascade may be affected by hyperglycaemia associated with gestational diabetes, resulting in insulin resistance in endothelial cells derived from the fetal vasculature.
Mechanisms of force inhibition by halothane and isoflurane in intact rat cardiac muscle
Authors: Hanley Peter J. , Loiselle Denis S.
Published: September 2004

We investigated the mechanisms underlying the negative inotropic effect of the volatile anaesthetics halothane and isoflurane using twenty‐two intact, right ventricular trabeculae of rat. [Ca2+]i was measured qualitatively using either fluo‐3 or fura‐2, loaded into the cytosol via the acetoxymethyl (AM) ester form. Diastolic sarcomere length was adjusted to 2.1‐2.2 μm and experiments were performed at 21‐23°C.

We investigated the mechanisms underlying the negative inotropic effect of the volatile anaesthetics halothane and isoflurane using twenty‐two intact, right ventricular trabeculae of rat. [Ca2+]i was measured qualitatively using either fluo‐3 or fura‐2, loaded into the cytosol via the acetoxymethyl (AM) ester form. Diastolic sarcomere length was adjusted to 2.1‐2.2 µm and experiments were performed at 21‐23°C.

The ability of halothane and isoflurane to induce Ca2+ release from the sarcoplasmic reticulum of quiescent trabeculae was examined. When the superfusate was Ca2+ and Na+ free (thereby preventing Na+‐ Ca2+ exchange and Ca2+ influx), 2 % halothane, but not 4 % isoflurane, evoked a transient increase in [Ca2+]i.

Halothane and isoflurane produced reversible, dose‐dependent changes in cellular autofluorescence, the pattern of which was consistent with an increase in concentration of the reduced forms of nicotinamide adenine nucleotides and flavoproteins. This observation supports the putative inhibitory action of volatile anaesthetics at the site of Complex I of the mitochondrial electron transport chain.

Addition of the fatty acid hexanoate, a substrate that can be metabolized in the face of Complex I inhibition, did not appreciably attenuate the anaesthetic‐induced negative inotropy; however, it greatly diminished autofluorescence changes.

To determine whether direct actions of the volatile anaesthetics on the contractile system contributed to the negative inotropy, external [Ca2+] was varied to modulate the amplitude of the Ca2+ transient. In the presence of 2 % halothane or 4 % isoflurane, restoration of the peak Ca2+ transient to control levels did not restore peak force. Moreover, halothane (1 %) and isoflurane (1.6 %) each reduced maximal Ca2+‐activated force (attained using ryanodine tetani and a high external [Ca2+]) by around 15 %.

We conclude that the negative inotropic actions of halothane and isoflurane on intact cardiac muscle reflect both reduced availability of Ca2+ and decreased responsiveness of the contractile system to Ca2+. The inhibitory action of the volatile anaesthetics on mitochondrial function does not contribute significantly to the negative inotropy but may lead to changes in cellular autofluorescence and misinterpretation of fluorescent Ca2+ indicator signals.
Differential effects of noradrenaline on evoked, spontaneous and miniature IPSCs in rat cerebellar stellate cells
Authors: Kondo Satoru , Marty Alain
Published: September 2004

The modulation by noradrenaline (NA) of synapses among stellate cells was investigated in rat cerebellar slices by using presynaptic loose cell‐attached recording and postsynaptic whole‐cell recording.

NA increased the frequency of spontaneous IPSCs recorded from stellate cells without changing their mean amplitude.

NA increased the firing rate of stellate cells. This effect persisted after blocking ionotropic glutamate receptors and GABA receptors, indicating that it was independent of synaptic input.

The effects of NA on action potential frequency were mimicked by the β‐receptor agonist isoprenaline but not by the α‐receptor agonist 6‐fluoro noradrenaline, and they were not blocked by the α‐receptor antagonist phentolamine, indicating that they were mediated by β‐receptors.

In paired recordings of connected stellate cells, NA slightly decreased the success rate of synaptic transmission. A small decrease in mean IPSC amplitude (excluding failures) and a slight increase in latency were also observed in NA.

These results show that, while NA increases the number of action potential‐dependent IPSCs by increasing the firing rate of stellate cells, it actually reduces the probability of evoked release. Since previous studies showed that NA increases the rate of miniature IPSCs in this preparation, we conclude that different mechanisms underly the modulation by NA of action potential‐dependent and action potential‐independent transmitter release.
Alkaline secretion by frog gastric glands measured with pH microelectrodes in the gland lumen
Authors: Debellis Lucantonio , Caroppo Rosa , Frömter Eberhard , Curci Silvana
Published: September 2004

In the present work we have measured the pH of the secreted fluid within the gland lumen of isolated but intact gastric mucosa of Ranaesculenta. Tissues were mounted in a double chamber allowing continuous perfusion of the mucosal and serosal compartment, and the measurements were made with double‐barrelled pH glass microelectrodes inserted into the glands from the serosal surface under microscopic inspection.

During inhibition of H+ secretion by cimetidine (100 μm) the luminal gland pH (pHgl) averaged 7.60 ± 0.05 pH units (mean ±s.e.m.; n= 35), a value significantly higher than bath solution pH (7.45 ± 0.02; P < 0.001) and also higher than intracellular pH of oxyntopeptic cells (pHi), which averaged 7.53 ± 0.06 (n= 18).

Stimulation of acid secretion with histamine (500 μm) reversibly decreased pHgl to values which could be as low as 2.5. Together with electrophysiological criteria this response was routinely used to verify the proper location of the microelectrode tip within the gland lumen.

Stimulation with carbachol (100 μm) or pentagastrin (50 μm) in the presence of cimetidine rapidly and reversibly increased pHgl by 0.10 ± 0.01 pH units (n= 24; P < 0.001) and 0.09 ± 0.02 pH units (n= 6; P < 0.05), respectively.

The observation that gastric gland fluid is more alkaline than the bath solutions and that carbachol or pentagastrin further alkalinize it strongly suggests that oxyntopeptic cells participate in gastric alkaline secretion at least under cholinergic stimulation.
Long‐term synaptic plasticity between pairs of individual CA3 pyramidal cells in rat hippocampal slice cultures
Authors: Debanne Dominique , Gähwiler Beat H. , Thompson Scott M.
Published: September 2004

Long‐term potentiation (LTP) and depression (LTD) were investigated at synapses formed by pairs of monosynaptically connected CA3 pyramidal cells in rat hippocampal slice cultures.

An N‐methyl‐D‐aspartate (NMDA) receptor‐mediated component of the unitary EPSP, elicited at the resting membrane potential in response to single action potentials in an individual CA3 cell, could be isolated pharmacologically.

Associative LTP was induced when single presynaptic action potentials were repeatedly paired with 240 ms postsynaptic depolarizing pulses that evoked five to twelve action potentials or with single postsynaptic action potentials evoked near the peak of the unitary EPSP. LTP induction was prevented by an NMDA receptor antagonist.

Associative LTD was induced when single presynaptic action potentials were repeatedly elicited with a certain delay after either 240 ms postsynaptic depolarizing pulses or single postsynaptic action potentials. The time window within which presynaptic activity had to occur for LTD induction was dependent on the amount of postsynaptic depolarization. LTD was induced if single pre‐ and postsynaptic action potentials occurred synchronously.

Homosynaptic LTD was induced by 3 Hz tetanization of the presynaptic neuron for 3 min and was blocked by an NMDA receptor antagonist.

Depotentiation was produced with stimulation protocols that elicit either homosynaptic or associative LTD.

Recurrent excitatory synapses between CA3 cells display associative potentiation and depression. The sign of the change in synaptic strength is a function of the relative timing of pre‐ and postsynaptic action potentials.
Mechanosensitive cation channels mediate afferent arteriolar myogenic constriction in the isolated rat kidney
Authors: Takenaka Tsuneo , Suzuki Hiromichi , Okada Hirokazu , Hayashi Koichi , Kanno Yoshihiko , Saruta Takao
Published: September 2004

In order to assess ionic mechanisms mediating renal afferent arteriolar myogenic constriction, experiments were performed using isolated perfused hydronephrotic rat kidneys.

Increasing pressure progressively constricted the afferent arteriole (‐0·26 ± 0·02 % mmHg−1, n= 21, r= 0·97). Gadolinium (10 μM), a mechanosensitive cation channel blocker, abolished this myogenic constriction. However, high potassium media (30 mM) constricted the afferent arteriole in the presence of gadolinium.

Lowering extracellular sodium concentration gradually attenuated afferent arteriolar myogenic constriction. In the perfusate containing 50 mM sodium, the myogenic response was arrested.

Afferent arteriolar myogenic constriction was prevented in calcium‐free perfusate or by the L‐type calcium channel blocker diltiazem (10 μM).

Our present findings provide evidence that increasing pressure gates mechanosensitive cation channels on the afferent arteriole, thereby eliciting membrane depolarization and activating voltage‐dependent calcium channels.
Involvement of interleukin‐1, prostaglandins and mast cells in rectal distension‐induced colonic water secretion in rats
Authors: Eutamene H. , Theodorou V. , Vergnolle N. , Comera C. , Fioramonti J. , Bueno L.
Published: September 2004

In vivo rectal distension (RD) induces a neurally mediated colonic net water hypersecretion in rats. Interleukin‐1β (IL‐1β) also induces neural colonic water hypersecretion involving the release of prostaglandins (PGs) and a mast cell degranulation in rats. This study investigated in vivo the role of IL‐1, PGs and mast cells in RD‐induced colonic hypersecretion.

Proximal colonic net water flux was determined using [14C]polyethylene glycol (PEG) 4000 (mol. wt, 4000) in anaesthetized rats. On strips taken from the distal colon: (i) a histological analysis was performed to determine the number of mucosal mast cells (MMC); and (ii) histamine levels were measured by radioimmunoassay after stimulation with compound 48/80.

RD induced a net colonic water secretion that was blocked by i.c.v. administration of IL‐1ra (an IL‐1 receptor antagonist) and indomethacin, and by systemic treatment with doxantrazole and indomethacin. RD decreased the number of resident mast cells and the release of histamine from the distal colonic strips. Moreover, using SDS‐PAGE immunoblotting the expression of IL‐1β was detected in the brain.

These results suggest that, in rats, RD induces colonic net water hypersecretion by the activation of a neuro‐immunological reflex pathway, involving brain IL‐1β, PG release and peripheral mast cell degranulation.
Effect of probenecid on breathing movements and cerebral clearance of prostaglandin E2 in fetal sheep
Authors: Walker David W. , Pratt Naomi
Published: September 2004

Intravenous infusion of probencid (79‐160 mg kg−1) into unanaesthetized fetal sheep (127‐143 days gestation) in utero significantly decreased the incidence and amplitude of spontaneous breathing movements, but did not change the incidence of low voltage electrocortical (ECoG) activity, plasma prostaglandin E2 (PGE2) concentrations, blood gases or pH.

In fetuses pretreated with paracetamol (350 mg kg−1) to inhibit PG synthase activity, infusion of probenecid did not change the mean incidence or amplitude of breathing movements, indicating that the inhibitory effect of probenecid on breathing movements required the presence of active PG synthesis.

Probenecid infusion in four unanaesthetized fetuses significantly increased the PGE2 concentrations in cisternal cerebrospinal fluid (CSF) by 6.6 ± 1.5‐fold (P < 0.05).

In pentobarbitone‐anaesthetized, exteriorized fetuses, probenecid infusion decreased the clearance of [3H]PGE2 from CSF during ventriculo‐cisternal perfusion of artificial CSF containing [3H]PGE2.

These results suggest that there is active transport of PGs from CSF to blood in fetal sheep from at least 127 days gestation. Inhibition of this transport results in the accumulation of PGs within interstitial fluid of the brain, one effect of which is to suppress the spontaneous activity of the respiratory centres.
Non‐genomic action of the mineralocorticoid aldosterone on cytosolic sodium in cultured kidney cells
Authors: Gekle Michael , Silbernagl Stefan , Wünsch Stefan
Published: September 2004

The mineralocorticoid aldosterone is essential for the regulation of electrolyte homeostasis, extracellular volume and blood pressure. As a steroid hormone the classical way of action is genomic. Previously we reported a non‐genomic action of aldosterone on cytosolic Ca2+ and pH in renal epithelial (MDCK) cells. In parallel, aldosterone induces Zn2+‐sensitive cytosolic acidification when extracellular Na+ is absent.

We now show that aldosterone (EC50, 7 × 10−11 mol l−1) induces a non‐genomic increase in cytosolic sodium in MDCK cells. The membrane‐impermeable aldosterone‐bovine serum albumin (BSA) conjugate exerted the same effect. The effect of aldosterone was completely abolished by inhibition of Na+‐H+ exchange with ethyl‐isopropanol amiloride (EIPA). Aldosterone‐induced Na+ influx exceeded H+ efflux more than 10‐fold.

Omission of extracellular Ca2+, inhibition of protein kinase C or pretreatment with pertussis toxin reduced the effect of aldosterone significantly. Zn2+ (IC50, 3·3 × 10−6 mol l−1), but not ouabain, abolished the increase in Na+ almost completely.

The aldosterone‐induced increase in cytosolic sodium was accompanied by an EIPA‐ and Zn2+‐sensitive cell swelling.

Thus, physiological concentrations of aldosterone induce a non‐genomic increase in cytosolic sodium concentration by activation of Na+‐H+ exchange. Aldosterone exerts its effect, at least in part, at the plasma membrane via interaction with a G‐protein‐coupled mechanism.

The simultaneous activation of the acidification mechanism and Na+‐H+ exchange by aldosterone allows a dramatic sodium influx without excessive changes in cytosolic pH and leads to changes in cell volume.
AMPA receptor activation and phosphatase inhibition affect neonatal rat respiratory rhythm generation
Authors: Ge Qing , Feldman Jack L.
Published: September 2004

We investigated the role of α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA) receptors and their regulation in affecting respiratory‐related neurones in a neonatal rat medullary slice that spontaneously generates respiratory‐related rhythm and motor output in the hypoglossal (XII) nerve.

Bath application of the AMPA receptor antagonist 1‐(4‐aminophenyl)‐4‐methyl‐7,8‐methylenedioxy‐5H‐2,3‐benzodiazepine (GYKI) completely blocked XII nerve activity, as well as respiratory‐related synaptic drives in neurones within the preBötzinger Complex (preBötC), site of rhythm generation in the slice.

Local application of GYKI to the preBötC blocked respiratory rhythm. Local application of AMPA to the preBötC increased rhythm frequency and depolarized respiratory‐related neurones.

In the presence of tetrodotoxin (TTX), GYKI completely blocked the inward current induced by local application of AMPA, but not that induced by kainate.

Local application of okadaic acid, a membrane‐permeable inhibitor of phosphatase 1 and 2A, to the preBötC increased the frequency of respiratory motor discharge.

Intracellular application of microcystin, a membrane‐impermeable inhibitor of phosphatase 1 and 2A, enhanced endogenous inspiratory drive and exogenous AMPA‐induced current (in the presence of TTX) in preBötC inspiratory neurones. Both the enhanced inspiratory drive and the increased AMPA‐induced current were completely blocked by GYKI.

We suggest that AMPA receptor activation and AMPA receptor modulation by phosphorylation are crucial for the rhythm generation within the preBötC.
Developmental changes in expression of ion currents accompany maturation of locomotor pattern in frog tadpoles
Authors: Sun Q.‐Q. , Dale N.
Published: September 2004

The K+ currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos.

The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42.

Almost all neurons at stage 42 expressed a fast activating Ca2+‐dependent K+ current (IKCa) that was largely absent from embryonic neurons. Whereas IKCa became larger and more prevalent during development, the delayed rectifier K+ currents were down‐regulated.

About 53% of IKCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K+ currents or the K+ currents of embryonic neurons.

The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons.

Iberiotoxin extended the duration of ventral root bursts during fictive swimming in larvae at stages 41 and 42 but had no effect at stage 40. The progressive expression of IKCa thus contributed to burst termination.

We have found that changes in expression of outward current closely correlate with the maturation of the motor pattern during development. At a time when the motor pattern has a need for a burst‐terminating mechanism, the larval neurons express a channel with properties appropriate for such a role.
Short‐term haemodynamic variability in the conscious areflexic rat
Authors: Létienne Robert , Barrès Christian , Cerutti Catherine , Julien Claude
Published: September 2004

Similtaneous measurements of arterial pressure and cardiac output (n= 8), mesenteric blood flow (n= 7) or hindquarters (n= 8) blood flow were performed during 1 h periods in conscious rats, before and after acute pharmacological blockade of the autonomic, renin‐angiotensin and vasopressin systems. In the latter condition (areflexic state), arterial pressure was maintained with a continuous infusion of noradrenaline.

In the areflexic state, spontaneous fluctuations in arterial pressure were markedly exaggerated, especially depressor episodes. At the onset of these falls in arterial pressure, there was an abrupt and transient decrease in stroke volume and cardiac output. Systemic vasodilatation then developed while cardiac output returned to normal. Regional vasodilatations were also delayed from the onset of the falls in arterial pressure and were usually large enough to maintain blood flow.

Both time and frequency domain analyses confirmed that changes in systemic and regional vascular conductances lagged by about 1 s behind arterial pressure changes.

These results indicate that, in the absence of neurohumoral influences, autoregulatory‐like mechanisms become dominant in the control of systemic and regional circulations and contribute to exaggeration of the spontaneous short‐term variability of arterial pressure.
Reflex control of splanchnic blood volume in anaesthetized dogs
Authors: Noble B. J. , Drinkhill M. J. , Myers D. S. , Hainsworth R.
Published: September 2004

In chloralose‐anaesthetized, artificially ventilated dogs, the splenic pedicle was tied and the carotid sinuses were vascularly isolated and perfused at controlled pressures. In Series 1 experiments, the hepatosplanchnic circulation was perfused through the abdominal aorta with a tie on the aorta separating it from the caudal circulation, which was perfused through the femoral arteries. The two circulations were drained from cannulae in the inferior vena cava and the femoral veins, with a tie on the inferior vena cava separating the two. In Series 2, the splanchnic circulation drained from the portal vein. In both series, inflows and outflows were measured and integrated to derive volume changes. Capacitance responses were assessed during constant flow, and capacitance plus passive responses were obtained during constant pressure perfusion.

In Series 1, an increase in carotid sinus pressure (from 8 to 26 kPa) during constant flow and constant pressure perfusion increased hepatosplanchnic volume by 2.5 and 5.7 ml (kg body weight)−1, respectively. The volume of the subdiaphragmatic circulation did not increase during constant flow, but during constant pressure it increased by 2.0 ml (kg body weight)−1.

In Series 2, increasing carotid pressure during constant flow and constant pressure increased the volume of the splanchnic circulation by 0.5 and 4.2 ml (kg body weight)−1, respectively.

These results confirm that carotid baroreceptor stimulation causes larger volume changes during constant pressure perfusion than during constant flow perfusion. Also, the active capacitance change in the splanchnic circulation is small in relation to the passive response. We propose that in dogs (following splenic ligation), the major active capacitance control is from the liver. However, large passive changes in splanchnic volume occur due to changes in flow.
Co‐activation of tongue protrudor and retractor muscles during chemoreceptor stimulation in the rat
Authors: Fuller David , Mateika Jason H. , Fregosi Ralph F.
Published: September 2004

Our primary purpose was to test the hypothesis that the tongue protrudor (genioglossus, GG) and retractor (styloglossus, SG and hyoglossus, HG) muscles are co‐activated when respiratory drive increases, and that co‐activation will cause retraction of the tongue. This was addressed by performing two series of experiments using a supine, anaesthetized, tracheotomized rat in which tongue muscle force and the neural drive to the protrudor and retractor muscles could be measured during spontaneous breathing. In the first series of experiments, respiratory drive was increased progressively by occluding the tracheal cannula for thirty respiratory cycles; in the second series of experiments, the animals were subjected to hyperoxic hypercapnia and poikilocapnic hypoxia.

Airway occlusion for thirty breaths caused progressive, quantitatively similar increases in efferent motor nerve activity to protrudor and retractor tongue muscles. Net tongue muscle force was always consistent with tongue retraction during occlusion, and peak force rose in parallel with the neural activites. When airway occlusion was repeated following section of the lateral XIIth nerve branch (denervation of retractor muscles) the tongue either protruded (15/21 animals; 10 ± 2 mN at the 30th occluded breath) or retracted weakly (6/21 animals; 6 ± 2 mN at 30th occluded breath).

To ensure that our findings were not the result of damage to the muscle nerves, occlusion experiments were also done in eight animals in which GG EMG activity was recorded instead of nerve activities. Changes in peak integrated GG electryomyogram (EMG) activity and peak retraction force during occlusion were highly correlated (r2= 0.86, slope = 1.05).

In separate experiments in fourteen rats, we found that hyperoxic hypercapnia and poikilocapnic hypoxia also result in parallel increases in the respiratory‐related EMG activity of the GG and HG muscles. Also, as in the occlusion experiments, augmentations of protrudor and retractor muscle EMG activities were associated with parallel changes in tongue retraction force.

These studies in anaesthetized rats demonstrate that tracheal occlusion and independent stimulation of central or peripheral chemoreceptors results in inspiratory‐related co‐activation of the protrudor and retractor muscles, and proportional changes in tongue retraction force. These observations also demonstrate that recording GG EMG activity in isolation could lead to erroneous conclusions about respiratory‐related movements of the tongue.
Electrocortical activity in fetal sheep in the last seven days of gestation
Authors: Shinozuka Norio , Nathanielsz Peter W.
Published: September 2004

Electrocorticogram (ECoG) and myometrial electromyogram (EMG) were recorded continuously in chronically instrumented late gestation sheep fetuses (n= 9) to analyse: (1) 24 h ECoG patterns, (2) relationships between ECoG and myometrial contractility, and (3) 24 h ECoG patterns at the spontaneous onset of labour.

Spontaneous onset of labour was determined from the myometrial EMG. ECoG signals were purified by a denoising procedure of wavelet decomposition. High‐voltage slow ECoG activity (HV) and low‐voltage fast ECoG activity (LV) were determined mathematically, and HV‐LV cycle periodicity was calculated by periodogram analysis.

Twenty‐four hour rhythms were present in fetal ECoG HV‐LV cycles in the 3–5 days prior to spontaneous onset of labour (P < 0.01). Cycle frequency was lower at 08.00–12.00 h and higher at 16.00–20.00 h (lights on at 07.00 h, lights off at 21.00 h). LV duration was longer at 20.00–24.00 h, and HV was shorter at 16.00–20.00 h. No relationship was found between ECoG cycles and myometrial contracture cycles. Twenty‐four hour ECoG rhythms disappeared 1 day before the spontaneous onset of labour.

ECoG patterns changed 7 to 4 h prior to spontaneous onset of labour, percentage of time spent and duration of HV ECoG increased, and percentage of time spent in LV decreased significantly. ECoG HV‐LV cyclicity was reduced 4–5 h prior to spontaneous onset of labour, indicating that the altered fetal hormonal and blood gas environment around the spontaneous onset of labour alter fetal neural function.
Electromyographic study of the contractile and electrical properties of the human triceps surae muscle in a simulated microgravity environment
Authors: Koryak Yuri
Published: September 2004

The effects of 7 days of simulated space flight, achieved with the technique of ‘dry’ water immersion, on human triceps surae muscle function have been investigated.

The maximal voluntary contraction (MVC) was reduced by 33.8 % (P < 0.01) while the electrically evoked maximal tetanic contraction force (Po) decreased by 8.2 % (P > 0.05). This suggests that most of the force loss is due to a reduction in motor drive.

The decrease in Po was associated with a small increase in maximal rates of tension development (7.2 %). The twitch tension (Pt) was not significantly changed and the Pt : Po ratio was decreased by 8.7 % after immersion.

A standard fatigue test, consisting of sixty 1 s intermittent isometric contractions (50 impulses s−1) separated by 1 s rest decreased tetanic force to approximately 60 % of initial values, but force reduction was not significantly different before and after immersion: the fatigue index was 36.2 ± 5.4 % before and 38.6 ± 2.8 % after immersion (P > 0.05). Whilst there were similar changes in mechanical output between control and disused muscles, there were differences in the pattern of electrical activity.
Absence of viscerosomatic inhibition with injections of lobeline designed to activate human pulmonary C fibres
Authors: Gandevia S. C. , Butler J. E. , Taylor J. L. , Crawford M. R.
Published: September 2004

Activation of pulmonary C fibres (J receptors) in animals produces inhibition of spinal motoneurones. Intravenous bolus injections of lobeline are believed to activate pulmonary C fibres (J receptors) in human subjects and to produce characteristic sensations and cardiorespiratory responses. This study quantified the respiratory sensations evoked by such injections and then used a range of suprathreshold doses of lobeline and tested for the presence of reflex or descending inhibition of motoneuronal output.

Injections of lobeline produced dose‐dependent sensations of respiratory discomfort referred to the throat and upper chest beginning within about 10 s and often associated with coughing. As the dose increased the latency for the sensations decreased while their duration and intensity increased. Reflex changes in blood pressure, heart rate and ventilation also occurred.

Injections of lobeline at doses sufficient to evoke respiratory discomfort lasting 25‐32 s (37‐73 μg kg−1) increased the size of the H reflex in soleus with an onset latency of about 10 s and lasting about 20 s.

The size of EMG responses evoked in upper limb muscles by transcranial magnetic stimulation of the motor cortex increased shortly after injections and remained elevated for about 30‐35 s.

Injections of lobeline during sustained voluntary contractions of the elbow flexors at submaximal or maximal levels did not impair the ability to produce force.

Walking was not disrupted by repeated suprathreshold doses of lobeline.

It is concluded that injections of lobeline sufficient to evoke cardiorespiratory reflexes and sensations of severe respiratory discomfort are not associated with functionally important inhibition of motor performance. The results cast doubt on the ability of the J reflex to limit exercise in humans.
Effect of sodium perturbations on rat chemoreceptor spike generation: implications for a Poisson model
Authors: Donnelly David F. , Panisello Jose M. , Boggs Dona
Published: September 2004

The sensitivity of arterial chemoreceptor spike generation to reductions in excitability was examined using rat chemoreceptors in vitro. Axonal excitability was reduced by reducing extracellular sodium concentration ([Na+]o) by 10‐40 % or by applying low doses of tetrodotoxin (TTX).

In normoxia and in hypoxia, an isosmotic reduction in [Na+]o caused a proportional decrease in single‐fibre, spiking nerve activity. For a 20 % reduction in [Na+]o, nerve activity decreased to 54 ± 7 % of control in normoxia and 41 ± 5 % in hypoxia.

Low doses of TTX (25‐50 nM) caused a similar decrease in spiking frequency, but this response was variable amongst fibres, with some fibres unaffected by TTX.

A reduction in [Na+]o by 20 % caused a slowing of conduction velocity, measured using an electrical stimulus delivered to an electrode placed in the carotid body. Threshold current for spike generation was increased by about 2·7 ± 1·4 %. Threshold current increased by 6·5 ± 3·7 % following a 40 % reduction in [Na+]o.

The spike generation process was modelled as a Poisson process in which depolarizing events summate and give rise to an action potential. The experimental data were best fitted to a high order process characterized by a large number of events and high event threshold.

This result is not consistent with depolarization events caused by episodic transmitter release, but suggests that afferent spike generation is an endogenous process in the afferent nerve fibres, perhaps linked to random channel activity or to thermal noise fluctuations.
Exercise metabolism in human skeletal muscle exposed to prior eccentric exercise
Authors: Asp Sven , Daugaard Jens R. , Kristiansen Søren , Kiens Bente , Richter Erik A.
Published: September 2004

The effects of unaccustomed eccentric exercise on exercise metabolism during a subsequent bout of graded concentric exercise were investigated in seven healthy male subjects. Arterial and bilateral femoral venous catheters were inserted 2 days after eccentric exercise of one thigh (eccentric thigh) and blood samples were taken before and during graded two‐legged concentric knee‐extensor exercise. Muscle biopsies were obtained from the eccentric and control vastus lateralis before (rest) and after (post) the concentric exercise bout.

Maximal knee‐extensor concentric exercise capacity was decreased by an average of 23 % (P < 0.05) in the eccentric compared with the control thigh.

The resting muscle glycogen content was lower in the eccentric thigh than in the control thigh (402 ± 30 mmol (kg dry wt)−1vs. 515 ± 26 mmol (kg dry wt)−1, means ± s.e.m., P < 0.05), and following the two‐legged concentric exercise this difference substantially increased (190 ± 46 mmol (kg dry wt)−1vs. 379 ± 58 mmol (kg dry wt)−1, P < 0.05) despite identical power and duration of exercise with the two thighs.

There was no measurable difference in glucose uptake between the eccentric and control thigh before or during the graded two‐legged concentric exercise. Lactate release was higher from the eccentric thigh at rest and, just before termination of the exercise bout, release of lactate decreased from this thigh (suggesting decreased glycogenolysis), whereas no decrease was found from the contralateral control thigh. Lower glycerol release from the eccentric thigh during the first, lighter part of the exercise (P < 0.05) suggested impaired triacylglycerol breakdown.

At rest, sarcolemmal GLUT4 glucose transporter content and glucose transport were similar in the two thighs, and concentric exercise increased sarcolemmal GLUT4 content and glucose transport capacity similarly in the two thighs.

It is concluded that in muscle exposed to prior eccentric contractions, exercise at a given power output requires a higher relative workload than in undamaged muscle. This increases utilization of the decreased muscle glycogen stores, contributing to decreased endurance.