bluuebunny/crossref_metadata_2025 huggingface dataset:
The detection of monosynaptic connexions from inspiratory bulbospinal neurones to inspiratory motoneurones in the cat.
Authors: Davies J G , Kirkwood P A , Sears T A
Published: December 2014

Simultaneous recordings were made of the discharges of inspiratory bulbospinal neurones and phrenic or external intercostal alpha‐motoneurones in the anaesthetized cat. The connexions between these neurones were studied by the construction of cross‐correlation histograms from their discharges. Peaks observed in the cross‐correlation histograms were divided into three groups on the basis of their time courses: narrow, medium‐width and high‐frequency oscillations (h.f.o.). Narrow peaks were defined as having half‐widths less than 1.1 ms and medium‐width peaks as having half‐widths greater than this, while h.f.o. was characterized by periodic waves in the range 60‐120 Hz. H.f.o. peaks were interpreted as being derived from the well known periodic synchronization of medullary inspiratory neurones in this frequency range. The time courses and latencies of the medium‐width peaks could be quantitatively explained by a simple model representing excitation of the motoneurones by bulbospinal neurones whose discharges showed synchronization within +/‐ 1 ms of the reference spike, together with temporal dispersion in bulbospinal axons having a distribution of conduction velocities given by the measurements of this study. Such an explanation was essential for some of the medium‐width peaks, whose latencies were short compared to the conduction times to the spinal cord for their own axons, but for other medium‐width peaks oligosynaptic excitation of the motoneurones from the identified bulbospinal neurones was another possible explanation. The narrow peaks were of appropriate durations for monosynaptic connexions and were all at appropriate latencies (0.6‐2.4 ms after the calculated arrival time of the bulbospinal impulse in the segment concerned). It is concluded from the observations of narrow peaks that monosynaptic excitation exists between inspiratory bulbospinal neurones and both phrenic and external intercostal motoneurones. However, because of the existence of presynaptic synchronization, as shown by the presence of the medium‐width peaks, such a conclusion is predicated upon being able to discriminate against such an effect. The model showed that this restriction applies just as much to the measurements of excitatory post‐synaptic potentials (e.p.s.p.s) by spike‐triggered averaging as it does to cross‐correlation measurements. We suggest that the discrimination against presynaptic synchronization here was possible only because the long conduction distance created temporal dispersion in the synchronized presynaptic impulses.
Potassium and ionic strength effects on the isometric force of skinned twitch muscle fibres of the rat and toad.
Authors: Fink R H , Stephenson D G , Williams D A
Published: December 2014

A study was carried out to investigate the effects of ionic strength and monovalent cations on isometric, Ca2+‐activated force and rigor responses in mechanically skinned muscle fibres. Three types of skeletal muscle fibres were used: rat fast‐ and slow‐twitch fibres and toad twitch fibres. The contractile apparatus of rat slow‐twitch fibres was affected differently from that of rat fast‐twitch and amphibian twitch fibres when changing the ionic strength (expressed either in terms of ionic equivalents as I or formally as gamma/2) and [K+]. Thus, the apparent sensitivity to Ca2+ decreased substantially more in slow‐twitch fibres (by a factor of 20) than in the other fibre types (by a factor of 12) when I and [K+] were increased from 94 to 354 mM and from 56 to 316 mM respectively. Maximum Ca2+‐activated force, however, declined only by a factor of 2.2 in slow‐twitch fibres compared with 4.2 in the other fibre types, when I was increased from 154 to 354 mM. In slow‐twitch fibres the force oscillations of myofibrillar origin were found to increase substantially in amplitude, duration and frequency at low values of I and almost disappeared at high ionic strength. At low values of I, it was also discovered that ca. 50% of the fast‐twitch fibres responded with myofibrillar force oscillations when submaximally activated. The characteristics of these oscillations were different from those of slow‐twitch fibres. Rigor force levels were found to decline markedly with increasing iota and [K+] in all fibre types. Unexpectedly, once rigor force was established in a certain ionic environment, the level of force was stable regardless of further changes in ionic strength and monovalent cation concentration. These results indicate that the rigor cross‐bridges can be formed in different stable positions and that the probability of attachment in certain positions (rather than the total number of cross‐bridges that can be formed) is influenced by the ionic conditions. Further experimental evidence provided in this study shows that the increase in [K+] is mainly responsible for the decrease of the Ca2+‐sensitivity of the contractile apparatus and that ionic strength (expressed as I rather than gamma/2) influences markedly the maximal Ca2+‐activated force, the maximum steepness of the pCa‐force relations and the oscillatory processes of myofibrillar origin.
The role of hyperpolarization in the relaxation of smooth muscle of monkey coronary artery.
Authors: Mekata F
Published: December 2014

In monkey coronary arteries, outer and inner muscle had a similar resting potential (‐39.5 and ‐40.0 mV). Both showed strong outward‐going rectification, with no regenerative depolarization, on injection of depolarizing current. The depolarization spread electrotonically in all directions, particularly around the vessel wall. Hyperpolarization up to ‐45 mV by injection of constant current caused relaxation. Depolarization caused contraction. Pulses of field stimulation caused a brief depolarization which was reduced by tetrodotoxin or by stripping of the adventitia. They also caused a prolonged hyperpolarization which was not prevented by either, but was prevented by rubbing of the endothelium. The hyperpolarization in response to field stimulation therefore appears to result from electrically stimulated release of a substance from endothelial cells. Relaxation accompanied this hyperpolarization. It was twice the size of the relaxation produced by a similar hyperpolarization due to constant injection. Isoprenaline also produced hyperpolarization, and relaxation five times that seen with a similar hyperpolarization induced by direct current. Hyperpolarization appears to be an important, but not the only, mediator of relaxation induced in this artery both by endothelial cells and by beta‐adrenergic stimulation.
Excitatory synaptic interactions between CA3 neurones in the guinea‐pig hippocampus.
Authors: Miles R , Wong R K
Published: December 2014

Excitatory synaptic interactions between CA3 neurones in slices from guinea‐pig hippocampus were examined. Recurrent excitatory post‐synaptic potentials (e.p.s.p.s) were evoked by action potentials in a single presynaptic neurone or by the antidromic activation of part of the CA3 pyramidal cell population. The peak amplitude of unitary e.p.s.p.s was 1‐2 mV at potentials between ‐64 and ‐70 mV. Their time to peak was 7‐12 ms and the initial phase of their decay was slower than that of a somatically injected voltage pulse. Recurrent e.p.s.p.s were often followed by a small (0.3 mV) hyperpolarization, or undershoot. Recurrent e.p.s.p.s were compared with e.p.s.p.s evoked by stimulating mossy fibres, which terminate proximally on apical dendrites of CA3 pyramidal cells. They were of slower time course and reversed at a more positive potential than mossy fibre e.p.s.p.s. Some synaptic terminals made by recurrent axon collaterals apparently terminate at distant locations on apical dendrites. The decay of both recurrent e.p.s.p.s and dendritic voltage pulses was prolonged by membrane depolarization within a 10‐15 mV subthreshold potential range. Voltage‐dependent inward currents activated by the synaptic depolarization may contribute to the slow initial decay of these synaptic events. The undershoot did not occur when transmission of a unitary e.p.s.p. failed and was of slower time course than the hyperpolarization due to an inhibitory post‐synaptic potential (i.p.s.p.). It was suppressed by intracellular application of K+ channel blockers and probably reflects an intrinsic outward current activated as a consequence of the synaptic depolarization. Considerable temporal summation of synaptic potentials occurred when recurrent synapses were activated twice at an interval of 5‐10 ms, typical of the spontaneous burst firing pattern of CA3 neurones. The mean facilitation of a second e.p.s.p. at this interval was about 0.6. The efficacy of a third and subsequent e.p.s.p.s at similar interval was reduced. Presynaptic bursts of three to five action potentials evoked summed e.p.s.p.s of amplitude 2‐4 mV, with time to peak 20‐40 ms and decaying phase of similar duration. Their rising phase was relatively smooth and summed events were succeeded by an undershoot. Presynaptic bursts could cause a post‐synaptic neurone to discharge.
Effects of gamma‐aminobutyric acid on isolated cone photoreceptors of the turtle retina.
Authors: Kaneko A , Tachibana M
Published: December 2014

Isolated cones dissociated from the retina of the freshwater turtle were voltage clamped using a single 'patch' pipette electrode. gamma‐Aminobutyric acid (GABA) applied ionophoretically to the axon terminal evoked an inward current in cells held at ‐66 mV when they were recorded with patch pipettes filled with the 'control' pipette solution containing 120 mM‐Cl‐. Polarity of the GABA‐induced current reversed near 0 mV when examined with the pipette filled with the control pipette solution. The reversal potential depended strongly on both external and intrapipette Cl‐ concentrations ([Cl‐]o, and [Cl‐]p). The reversal potential agreed with the equilibrium potential for Cl‐ calculated by the Nernst equation with given [Cl‐]o and [Cl‐]p. The reversal potential was not affected by concentrations of either external Na or K ions. Voltage responses evoked by GABA were hyperpolarizing from its resting level of about ‐50 mV immediately after the rupture of the patch membrane. The response polarity reversed into depolarization in a few seconds when [Cl‐]p was greater than 24 mM, while hyperpolarizing responses persisted when [Cl‐]p was less than 12 mM. Thus, the intracellular Cl‐ concentration of undisturbed isolated cones was estimated to be between 12 and 24 mM. Cones were desensitized to GABA in the presence of GABA (greater than 100 nM) in the medium, or by a prolonged ionophoretic application. The maximum reduction in response amplitude was about 70% in both experiments. Muscimol was as potent as GABA, while beta‐p‐chlorophenyl‐GABA (baclofen) was ineffective even at 100 microM. GABA was antagonized by bicuculline competitively, and by picrotoxin non‐competitively. These observations suggest that turtle cones have GABAA receptors which associate with chloride channels. The present results suggest that GABA, presumably released continuously from monophasic horizontal cells in the dark, would exert a tonic hyperpolarization in red‐sensitive and green‐sensitive cones. Suppression by light of tonic GABA release would depolarize these types of cones by disinhibition. Disinhibitory depolarization in cones may contribute to the centre surround antagonism in retinal neurones, and to the biphasic colour responses recorded in a subtype of horizontal cells.
Blocking effects of cobalt and related ions on the gamma‐aminobutyric acid‐induced current in turtle retinal cones.
Authors: Kaneko A , Tachibana M
Published: December 2014

Red‐sensitive cone photoreceptors were isolated from the turtle retina, and GABA‐induced currents were recorded under voltage clamp. The effect of Co2+, widely used as a blocker of chemical synapses, on the GABA‐induced current was studied. Co2+ blocked the GABA‐induced current evoked by local application either at the synaptic region (cone pedicle) or at the extra‐synaptic region (cell body). 5 microM‐Co2+ suppressed the GABA‐induced current by 50%, and a few hundred microM‐Co2+ blocked it almost completely. Co2+ suppressed the GABA‐induced current non‐competitively: the saturating response amplitude decreased without a change in the threshold or saturating dose of GABA. The blocking was not voltage dependent in the physiological range of the membrane potential. Ni2+ and Cd2+ also blocked the GABA‐induced current non‐competitively, and were as effective as Co2+. Tetraethylammonium (25 mM) showed a similar but weaker blocking effect. On the other hand, Mg2+ (20 mM), Mn2+, Sr2+, Ba2+ (10‐100 microM each), D‐600 (10 microM) or Cs+ (10 mM) did not affect the GABA‐induced current. The Ca current in the turtle cones was blocked almost completely by 20 mM‐Mg2+ or 4 mM‐Co2+, or strongly suppressed by 10 microM‐D‐600. However, Cd2+ and Ni2+ (10 microM each) blocked the Ca current by ca. 50%, and Co2+ and Mn2+ (10 microM each) suppressed it only partially. The blocking of the GABA‐induced current by these agents was, therefore, not directly related to the blocking of the Ca current and/or Ca‐mediated currents. These observations present a warning on the use of some divalent cations, such as Co2+, Ni2+ or Cd2+, as a presynaptic blocker at the GABAergic synapse. High concentrations of Mg2+ are recommended as a more appropriate blocker.
Miniature and evoked inhibitory junctional currents and gamma‐aminobutyric acid‐activated current noise in locust muscle fibres.
Authors: Cull-Candy S G
Published: December 2014

gamma‐Aminobutyric acid (GABA) current noise and inhibitory junctional currents (i.j.c.s) have been examined to give properties of the GABA receptor and its associated synaptic channel. Various procedures were used to identify muscle bundles receiving inhibitory innervation. In normal bathing medium the decay time constant of the i.j.c. was tau i.j.c. = 7.6 +/‐ 0.7 ms (clamp potential, Vm = ‐80 mV; temperature, T = 21 degrees C). Most muscle fibres were sensitive to ionophoretically applied GABA, irrespective of the presence of inhibitory innervation. GABA current noise obtained at junctional sites gave spectra which were fitted usually with a single Lorentzian component, or occasionally with the sum of two Lorentzians. The conductance of the single inhibitory channel was, gamma (GABA) = 21.6 +/‐ 0.9 pS (Vm = ‐80 mV; T = 21 degrees C). The mean 'burst length' of the openings produced by a single receptor activation was tau noise = 4.0 +/‐ 0.8 ms, at Vm = ‐80 mV. This decreased exponentially with hyperpolarization. On average tau i.j.c. exceeded tau noise although good agreement was found in some fibres. I.j.c.s were examined in greater detail after excitatory synaptic receptors had been desensitized with 10(‐3) M‐L‐glutamate to abolish all excitatory synaptic activity. Their decay time constant was tau i.j.c. = 7.2 +/‐ 0.4 ms, and their rise time was 3.3 +/‐ 0.12 ms, at Vm = ‐80 mV. An e‐fold decrease in tau i.j.c. resulted from a 103 +/‐ 7.9 mV hyperpolarization; time to peak showed a smaller dependence on Vm. The mean size of the inhibitory quantal event (i.e. response to a single transmitter packet) was estimated from fluctuations in i.j.c. amplitude. Mean quantal content of the i.j.c. was about 30 at normal levels of release. Mean amplitude of the directly measured miniature i.j.c. = 0.65 +/‐ 0.08 nA at Vm = ‐80 mV (V eq approximately equal to ‐40 mV). The amplitude of the quantal event showed a non‐linear dependence on Vm. The burst length of the inhibitory channel, produced by a single receptor activation, is longer in duration (at ‐80 mV) and exhibits greater voltage dependence than the burst length of the excitatory glutamate‐activated channel in these fibres. It is estimated that a single quantum of GABA opens about 600‐1000 post‐synaptic chloride channels.
Effects of calcium and magnesium on transmitter release at Ia synapses of rat spinal motoneurones in vitro.
Authors: Kuno M , Takahashi T
Published: December 2014

The lumbar spinal cord excised from neonatal rats was superfused with an oxygenated saline, and monosynaptic excitatory post‐synaptic potentials (e.p.s.p.s) were recorded from the lumbar motoneurones following stimulation of muscle nerves of the hind leg. The amplitude of monosynaptic e.p.s.p.s increased with an increase in the external Ca2+ concentration ([Ca2+]o). In [Ca2+]o of 0.5‐2 mM, the relation between the e.p.s.p. amplitude and [Ca2+]o was linear with a slope of 1.6 on double logarithmic co‐ordinates. An increase in [Mg2+]o in a range of 1‐5 mM reduced the e.p.s.p. amplitude without affecting the slope of the relation between log e.p.s.p. amplitude and log [Ca2+]o, suggesting competitive interaction between Ca2+ and Mg2+. When [Ca2+]o was increased to 4‐12 mM, the latency of monosynaptic e.p.s.p.s was prolonged, and the antidromic action potential failed to invade the cell body or initial segment of motoneurones. Under these conditions, the monosynaptic e.p.s.p.s were still present. It is concluded that at near‐normal levels of [Ca2+]o the effects of Ca2+ and Mg2+ on the e.p.s.p. amplitude are accounted for entirely by their competitive interaction for the probability of transmitter release without altering the mode of impulse propagation at central terminals of the Group Ia sensory fibres. It is suggested that impulses of Ia sensory fibres normally invade their terminals without intermittent blocks at their terminal branch points.
Restoration of hypoxic respiratory responses in the awake rat after carotid body denervation by sinus nerve section.
Authors: Martin-Body R L , Robson G J , Sinclair J D
Published: December 2014

The restoration of ventilatory responses to hypoxia after carotid body denervation was studied in twenty‐eight awake rats. The respiratory depression seen in moderate hypoxia (partial pressure of inspired O2, PI,O2, 80‐100 mmHg) 3 days after bilateral carotid sinus nerve section disappeared by day 10. By day 17 respiratory stimulation occurred at all levels of PI,O2 below 125 mmHg. The largest restored response, in severe hypoxia (PI,O2 50‐60 mmHg), was approximately 55% of the pre‐denervation response. The response showed little further change from day 17 to day 192. A comparison of the effect of bilateral section of the glossopharyngeal nerve and of the abdominal vagus 1 and 28 days after carotid sinus nerve section demonstrated that the restoration of hypoxic response resulted in part from an enhanced effect of the inputs from the secondary glomus tissue served by these nerves. A comparison of the effect of bilateral section of glossopharyngeal, abdominal vagal and aortic depressor nerves 1 and 28 days after carotid sinus nerve section demonstrated an increase of a residual hypoxic response which must result either from inputs from unidentified peripheral chemoreceptors or from central mechanisms. Bilateral sectioning of the aortic depressor nerves produced no additional effect on restored responses to sectioning glossopharyngeal and abdominal vagal nerves, providing further evidence against significant aortic body function in the rat. The studies support the hypothesis that central neural reorganization provides compensation for loss of carotid body function by enhancement of effects of normally subsidiary inputs.
Activation patterns of embryonic chick hind‐limb muscles following blockade of activity and motoneurone cell death.
Authors: Landmesser L T , Szente M
Published: December 2014

Motoneurone cell death and spontaneous embryonic motility were blocked in chick embryos by daily in ovo injections of d‐tubocurarine from stage 28‐36 (E5‐10). Isolated spinal cord‐hind‐limb preparations were prepared from these embryos and movement sequences in response to electrical stimulation of the thoracic cord were assessed, after drug wash‐out, by electromyogram (e.m.g.) or muscle‐nerve recordings. In embryos in which complete blockade of lumbar motoneurone cell death was later confirmed histologically, flexor and extensor motoneurone pools were found to be activated in alternating bursts as occurs in control embryos. Thus the development of the basic cord circuits responsible for these patterns of motoneurone activation does not require motoneurone cell death. Partial blockade of motoneurone cell death by guanosine 3',5'‐phosphate (cyclic GMP) was also without effect on muscle activation patterns. In ovo injection of d‐tubocurarine or alpha‐bungarotoxin in doses sufficient to block embryonic motility was found to have a direct effect on the spinal cord, preventing the patterned activation of motoneurone pools in alternating bursts. Cords removed from treated embryos behaved similarly to cords in which these drugs were applied acutely in the bath. Minor changes in muscle activation patterns that occurred with chronic drug treatment were also observed in acutely treated cords and appear to be a direct and persistent effect of the drugs on cord circuits. It is possible to conclude that cholinergic circuits within the chick lumbar cord play a role in the normal patterned activation of flexor and extensor motoneurone pools. Systemically applied drugs can have access to these circuits, indicating a need for caution when interpreting the results of drugs applied in this manner to developing embryos. We also conclude that neither the activation of motoneurones in patterned bursts, nor the afferent feed‐back from the movements that result, are required to form the basic spinal cord circuits responsible for the activation of extensor and flexor motoneurone pools in alternating bursts.
A velocity‐dependent shortening depression in the development of the force‐velocity relation in frog muscle fibres.
Authors: Colomo F , Lombardi V , Piazzesi G
Published: December 2014

During the onset of activation in isolated frog muscle fibres the development of the force‐velocity (T‐V) relation was determined by imposing single and double ramp releases. The experiments were performed at 3.5‐6 degrees C or 19‐22 degrees C and at a starting sarcomere length of about 2.25 micron. A velocity‐ and time‐dependent shortening deactivation was shown to exist during the development of contraction. It was found that, early during the tetanus rise, at submaximal levels of activation, the values of T (the steady force exerted by the muscle fibres at any velocity of shortening V lower than V0) were significantly affected by previous conditioning shortening. Conditioning shortening at lower speeds led to potentiation of T and, at higher speeds, to depression. Both these effects were independent of the amount of shortening and, in addition, were not present at the tetanus plateau. At each given time or isometric tension throughout the tetanus rise the values of T. normalized for those determined at the same velocities at the tetanus plateau, were found to be inversely correlated with the actual velocities of shortening. The slope of this relation (a measure of the velocity‐dependent shortening deactivation) decreased exponentially with time, attaining, in six fibres at low temperature, 10% of its initial value within 26‐73 ms. The results may be explained in terms of a cross‐bridge model of contraction by assuming that the rate of development of activation is controlled by the rate of release of the Ca2+ as well as by the velocity at which the muscle fibres are allowed to shorten and in turn by the actual number of attached cross‐bridges.
A quantitative description of the sodium current in the rat sympathetic neurone.
Authors: Belluzzi O , Sacchi O
Published: December 2014

The somata of rat sympathetic neurones were voltage‐clamped in vitro at 27 degrees C using separate intracellular voltage and current micro‐electrodes. Na currents were isolated from other current contributions by using: Cd to block the Ca current (ICa) and the related Ca‐dependent K current (IK(Ca)), and external tetraethylammonium to suppress the delayed rectifier current (IK(V) ). The holding potential was maintained at ‐50 mV to inactivate the fast transient K current (IA) when the IA contamination was unacceptable. Step depolarizations beyond ‐30 mV activated a fast, transient inward current carried by Na ions; it was suppressed by tetrodotoxin and was absent in Na‐free solution. Once activated, INa declined exponentially to zero with a voltage‐dependent time constant. The underlying conductance, gNa, showed a sigmoidal activation between ‐30 and +10 mV, with half‐activation at ‐21.1 mV and a maximal value (mean gNa) of 4.44 microS per neurone. The steady‐state inactivation level, h infinity, varied with membrane potential, ranging from complete inactivation at ‐30 mV to minimal inactivation at about ‐90 mV with a midpoint at ‐56.2 mV. Double‐pulse experiments showed that development and removal of inactivation followed a single‐exponential time course; tau h was markedly voltage‐dependent and ranged from 46 ms at ‐50 mV to 2.5 ms at ‐100 mV. Besides the fast inactivation, the Na conductance showed a slow component of inactivation. The steady‐state value, s infinity, was maximal at ‐80 mV and minimal at ‐40 mV. The removal of slow inactivation is a two‐time‐constant process, the first with a time constant in the order of hundreds of milliseconds and the second with a time constant of seconds. Slow inactivation onset appeared to be a faster process than its removal. When slow inactivation was fully removed the peak INa increased by a factor of 1.8. INa was well described by assuming it to be proportional to m3h. The temperature dependence of peak INa, tau m and tau h was studied in the temperature range 17‐27 degrees C and found similar to that reported for other preparations. The Q10 of these parameters allowed the reconstruction of the INa kinetic properties at 37 degrees C.
Influence of inspired oxygen concentration on the dynamics of the exercise hyperpnoea in man.
Authors: Griffiths T L , Henson L C , Whipp B J
Published: December 2014

In order to determine the role of the carotid bodies on the ventilatory control characteristics during the non‐steady‐state phase of exercise in man, six normal males performed cycle ergometry with four repetitions of a 6 min, constant‐load work bout at inspired O2 fractions (FI,O2) of 0.12, 0.15, 0.21, 0.30 and 1.00. Each test began with unloaded pedalling; this was followed by a constant load which was 90% of the subject's anaerobic threshold at FI,O2 = 0.12. Ventilation (VE), CO2 output (VCO2) and O2 uptake (VO2) were determined breath‐by‐breath during the test and the time constants of response (tau VE, tau VCO2 and tau VO2) were established by least‐squares techniques, following interpolation (1 s), temporal alignment and averaging of the four responses. In each subject, tau VE and tau VCO2 increased as functions of increasing FI,O2, and were inverse functions of the proportional contribution to VE of peripheral chemoreceptor drive (as estimated from hyperoxic‐transition or 'Dejours' tests). tau VE averaged 40 s at FI,O2 = 0.12 and 112 s at FI,O2 = 1.00, each response being well fitted by a single exponential. However, tau VO2 was not significantly affected by the alterations in FI,O2. Although there was no discernible peripheral chemosensitivity at FI,O2 = 0.30 or 1.00, the tau VE increased appreciably between these inspirates. We therefore conclude that the peripheral chemoreceptors are important, but not exclusive determinants of the exponential response characteristics during the non‐steady‐state phase of the exercise hyperpnoea in man. This supports the contention of a component of the control being humorally mediated even during moderate exercise.
Activation of acetylcholine receptors on clonal mammalian BC3H‐1 cells by high concentrations of agonist.
Authors: Sine S M , Steinbach J H
Published: December 2014

1. Currents were recorded through acetylcholine (ACh) receptor channels on clonal BC3H‐1 cells in the presence of high concentrations of ACh (20‐1000 microM) and carbamylcholine (180‐1000 microM). 2. Channel openings at high agonist concentrations occur in clusters separated by long silent periods (seconds). Clusters, in turn, show groups of closely spaced openings separated by relatively long (hundreds of milliseconds) closed periods. The closed periods between clusters and between groups within clusters are thought to reflect two desensitized states (Sakmann, Patlak & Neher, 1980). 3. Openings within groups consist largely of long‐duration openings. An excess of brief‐duration openings is seen at all high agonist concentrations; most brief openings occur as isolated, solitary openings. 4. The distribution of closed periods within groups shows four exponential components with time constants separated by several fold over the range of 50 microseconds to 50 ms. 5. The distribution of closed periods within groups is analysed as a function of agonist concentration, to estimate rate constants for transitions in a hypothetical reaction scheme for receptor activation. One or two of these components (depending on agonist and agonist concentration) appear to reflect agonist binding and channel gating. It is hypothesized that the other closed‐period components within groups at high agonist concentrations result from additional states of doubly liganded receptors which have closed ion channels. 6. With ACh as agonist the data indicate that binding and activation saturate at concentrations above 130 microM. The data are quantitatively consistent with measurements made at low concentrations of ACh (Sine & Steinbach, 1986b), and indicate that a four‐state linear scheme is able to describe major features of ACh‐receptor activation on BC3H‐1 cells. The channel opening rate is estimated to be about 450 s‐1 and the closing rate about 35 s‐1 (‐70 mV, 11 degrees C). The concentration dependence of closed durations suggests that some positive co‐operativity exists in agonist binding. The dissociation constant with one ACh molecule bound is about 50 microM, and that with two bound is about 10 microM (for an ACh receptor with a closed channel). 7. Saturation is not observed with carbamylcholine, even at 1 mM. The data are consistent with data obtained at low concentrations of carbamylcholine, and are in over‐all agreement with the interpretation of data obtained with ACh. The affinity for carbamylcholine is estimated to be about 20‐fold lower than with ACh.(ABSTRACT TRUNCATED AT 400 WORDS)
In utero right ventricular output in the fetal lamb: the effect of heart rate.
Authors: Anderson P A , Killam A P , Mainwaring R D , Oakeley A E
Published: December 2014

1. The effect of heart rate on right ventricular output was examined in six lambs during a period extending from 126 to 139 days of gestation. The fetuses had been surgically instrumented at least four days previously with a main pulmonary artery flow probe, right ventricular dimension transducers and left and right atrial pacing electrodes. 2. During spontaneous variations in heart rate, rate was correlated positively with right ventricular output (P less than 0.0001) and end‐diastolic dimension (P less than 0.0001) among the lambs considered as a group, but no significant effect of rate on stroke volume was found. When individual responses were examined, output increased significantly with rate in sixteen out of seventeen observations. 3. With left atrial pacing, heart rate did not affect output. With right atrial pacing, rate correlated negatively with output (P less than 0.0001). With pacing from either site, rate correlated negatively with end‐diastolic dimension (P less than 0.0001) and stroke volume (P less than 0.0001). 4. The introduction of a longer period interval during each pacing rate inhibited the rate‐related decrease in dimension and allowed the ventricle to fill to the same end‐diastolic dimension. The systole following these longer intervals had a greater stroke volume than did the preceding systoles with smaller end‐diastolic dimension. The faster the preceding paced rate, the greater was the increase in stroke volume (P less than 0.001). 5. Right ventricular dimensions and volumes were measured in vitro, and the relationship was found to be linear using regression analysis. 6. This study demonstrates that experimentally induced variations in heart rate produce changes in end‐diastolic volume and contractility which prominently affect right ventricular stroke volume. As a consequence, rate has, over a broad range, either no significant effect on output or a negative one. With spontaneous variations in rate, additional changes in contractility and venous return occur which affect stroke volume and end‐diastolic volume and enhance right ventricular output. These relationships are similar to those in the adult heart, and demonstrate the absence of a maturational change in the effects of rate on ventricular function from the fetus to the adult.
Cross‐correlation assessment of synaptic strength of single Ia fibre connections with triceps surae motoneurones in cats.
Authors: Cope T C , Fetz E E , Matsumura M
Published: December 2014

1. The relation between excitatory post‐synaptic potentials (e.p.s.p.s) by single I a fibres and the resultant cross‐correlograms in triceps surae motoneurones was investigated in barbiturate‐anaesthetized cats. The e.p.s.p.s. were documented first, using the discharge of single Ia fibres evoked by muscle stretch to compile spike‐triggered averages of motoneurone membrane potential. Subsequently, Ia fibre action potentials were cross‐correlated with rhythmic discharge of the same motoneurones induced by intracellular injection of current. 2. Primary correlogram peaks were statistically significant for thirty‐one of forty‐nine single Ia fibre‐motoneurone connections. Cumulative sums of correlograms were used to identify the onset and duration of peaks. For twenty cases involving more than 2000 trigger spikes, thirteen showed significant correlogram peaks. For these thirteen, the mean percentage increase (m.p.i.) in motoneurone firing probability, defined as the mean height of the correlogram peak above base line, ranged from 29 to 138%. The k values (maximum height divided by base line) ranged from 2.1 to 5.2. Peak duration varied from 1.8 to 3.2 ms. In the remaining seven cases the Ia e.p.s.p.s produced no significant correlogram peak (i.e. P greater than 0.05). 3. A significant positive relationship (r = 0.76; P less than 0.005) was found between m.p.i. in motoneurone firing probability and e.p.s.p. amplitude (n = 13), with a mean slope of 0.30%/microV. The k values were more weakly related to e.p.s.p. amplitude (r = 0.67; P less than 0.01). The correlogram parameter most strongly related to e.p.s.p. amplitude (r = 0.80) was correlogram peak area (number of spikes above base line per excitatory post‐synaptic potential). E.p.s.p. rate of rise was not significantly related (P greater than 0.10) to either m.p.i. in firing probability (r = 0.28) or peak area (r = 0.36). 4. The shapes of the primary correlogram peaks could be accounted for largely by a function proportional to the e.p.s.p. derivative (after temporal alignment). Subtracting a function proportional to the e.p.s.p. derivative from the correlogram peak left either a negligible remainder or a remainder term whose duration was shorter than the e.p.s.p. 5. To investigate properties of single‐fibre Ia e.p.s.p.s occurring near motoneurone threshold during repetitive firing, e.p.s.p.s were selectively averaged using Ia spikes occurring near the end of the depolarizing ramp in membrane potential. These 'ramp e.p.s.p.s' tended to be somewhat smaller (by ca.8%) than the 'rest e.p.s.p.s' produced at the same connections with the motoneurone at rest.(ABSTRACT TRUNCATED AT 400 WORDS)
Voltage gain of signal transfer from retinal rods to bipolar cells in the tiger salamander.
Authors: Capovilla M , Hare W A , Owen W G
Published: December 2014

1. Intracellular recordings of the voltage responses of rods and both functional classes of bipolar cell were made in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. 2. Brief, dim flashes of 519 nm light delivered to the receptive‐field centres were used to measure the flash sensitivities of twenty‐one on‐centre bipolar cells and thirty‐six off‐centre cells. In each experiment the flash sensitivity of a rod was also measured using diffuse illumination of the same duration and wave‐length. 3. The mean flash sensitivity of the rods (fifty‐nine cells) was 4.47 mV photon‐1 micron 2 flash. The mean flash sensitivity of the off‐centre bipolar cells was 35.4 mV photon‐1 micron 2 flash (thirty‐six cells). The mean flash sensitivity of the on‐centre bipolar cells was 12.5 mV photon‐1 micron 2 flash. 4. The ratio of the flash sensitivity of the bipolar cell to that of a rod recorded in the same retina defined the gain of voltage transfer from rod to bipolar cell. For signal transfer to on‐centre bipolar cells the mean value of the voltage gain was 5.05 +/‐ 1.34 (S.E. of mean). For signal transfer to the off‐centre bipolar cells, the mean value of the gain was 10.4 +/‐ 1.29. 5. The on‐centre cell gain in the salamander was smaller by a factor of 27 than that of the on‐centre cells in the dogfish retina (Ashmore & Falk, 1980 a), while the off‐centre cell gain was comparable in the two species. Possible reasons for the large difference between the voltage gains of on‐centre cells in the dogfish and salamander are considered.
Contrast affects the transmission of visual information through the mammalian lateral geniculate nucleus.
Authors: Kaplan E , Purpura K , Shapley R M
Published: December 2014

1. We recorded with one electrode action potentials of single principal cells in the lateral geniculate nucleus (l.g.n.) of cats and monkeys, together with their retinal inputs, recorded as synaptic potentials (S potentials; Bishop, Burke & Davis, 1958; Cleland, Dubin & Levick, 1971; Kaplan & Shapley, 1984). 2. We studied the effect of stimulus contrast on the transmission of visual information from the retina to the l.g.n., compared the spontaneous discharge of l.g.n. cells with that of their retinal inputs, and studied the driven (modulated) and maintained (unmodulated) discharge of l.g.n. neurones and their retinal drives. 3. The spontaneous discharge of l.g.n. cells was considerably lower than that of their retinal drives. 4. The maintained (unmodulated) discharge of l.g.n. cells during stimulation was lower than that of their retinal drives, and was largely unaffected by the stimulus contrast. 5. The responses of both the retinal input and l.g.n. cells increased with contrast, but at different rates: a given increment of contrast caused a larger increment of response in the retinal input than in the l.g.n. target cells. 6. The transmission ratio (l.g.n. response/retinal response) for most cells depended upon the stimulus contrast. This dependence indicates the presence of a non‐linear contrast gain control. 7. The amount by which the l.g.n. attenuated the retinal input depended upon the temporal frequency, and, to a lesser extent, upon the spatial frequency of the stimulus. 8. The effect of contrast on signal transmission between the retina and l.g.n. was essentially the same in the macaque monkey as in the cat. 9. The attenuation of the retinal input by the l.g.n. contrast gain control could serve to prevent saturation and extend the dynamic range of cortical units, which probably receive input from several l.g.n. units.
Characteristics of L‐glutamine transport in perfused rat skeletal muscle.
Authors: Hundal H S , Rennie M J , Watt P W
Published: December 2014

1. We have investigated glutamine transport in the perfused rat hindlimb using the paired‐tracer isotope dilution technique. 2. Uptake of L‐glutamine was stereospecific, saturable, sodium dependent, insulin sensitive and pH insensitive in the physiological range. The maximum capacity of transport (Vmax) under normal perfusate conditions at 37 degrees C, 145 mM‐Na+ and in the absence of insulin was 1156 +/‐ 193 nmol min‐1 g‐1 with transport being half‐maximal at a perfusate glutamine concentration of 9.25 +/‐ 1.15 mM. 3. The kinetics of Na+ dependence strongly suggested co‐transport of Na+ and glutamine with a stoichiometry of 1:1; furthermore, Na+ activated the carrier without any change in the concentration of glutamine at which transport was half‐maximal, i.e. a 'Vmax effect' rather than a 'Km effect'. 4. The characteristics of glutamine transport, especially its substrate specificity and the pattern of competitive and non‐competitive inhibition of glutamine transport by other amino acids, suggest that it is mediated by a carrier or carriers for which asparagine and histidine are also suitable substrates. 5. The characteristics of muscle glutamine transport are related but distinct from those of system N identified in hepatocytes; we suggest that they are sufficiently distinct to justify the identification of a new variant of mammalian amino acid transport systems which may be identified by the symbol Nm. 6. The kinetic characteristics of system Nm are such that glutamine is likely to be the most rapidly exchanging amino acid across the muscle membrane at physiological intra‐ and extracellular glutamine concentrations. Its hormone and ion sensitivities are likely to be important in the physiological modulation of whole‐body glutamine metabolism and also during derangements observed in disease and after injury.
Synaptic connections in the dorsal cochlear nucleus of mice, in vitro.
Authors: Hirsch J A , Oertel D
Published: December 2014

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices that contained the root of the auditory nerve and parts of the dorsal and ventral cochlear nuclei. Probably the largest and most common cells were impaled. 2. Weak shocks to the nerve usually evoked an excitatory postsynaptic potential (EPSP) that lasted about 90 ms and whose latency was often less than 1.2 ms, indicating monosynaptic input. 3. Stronger shocks elicited a larger EPSP and a later train of inhibitory postsynaptic potentials (IPSPs). Increasing the stimulus voltage shortened the latency of the train of IPSPs and increased its efficacy so that at large stimulus strengths inhibition dominated the synaptic response. 4. To determine whether any of the neuronal circuitry which generated the synaptic responses involved the ventral cochlear nucleus, recordings were made from slices containing only the dorsal nucleus. Synaptic responses to stimulation of the pial surface of the isolated DCN resembled those driven from the nerve root. That is, weak shocks evoked long‐lasting, monosynaptic EPSPs and stronger stimuli elicited a larger EPSP followed by trains of IPSPs. The DCN, therefore, contains intrinsic inhibitory interneurones. 5. The parallel fibres of the DCN course superficially, near the stimulating electrodes, whereas the axons of the auditory nerve terminate in deeper areas. Thus, the monosynaptic EPSPs evoked from the pial surface are probably generated by parallel fibres. Apparently the inhibitory interneurones are also excited by a circuit including parallel fibres. 6. The putative neurotransmitter of parallel fibres, glutamate, excited all neurones tested. 7. Cells were sensitive both to glycine and to gamma‐aminobutyric acid (GABA). Only strychnine, however, not picrotoxin or bicuculline, blocked IPSPs.
Multiple control of fever production in the central nervous system of rabbits.
Authors: Morimoto A , Murakami N , Nakamori T , Watanabe T
Published: December 2014

1. The effects of microinjection of prostaglandin D2, E2 and F2 alpha and of endogenous pyrogen on the rectal temperature of rabbits were extensively examined in sixty‐eight brain regions and in the third cerebral ventricle. 2. Intracerebroventricular injection of both prostaglandins E2 and F2 alpha produced dose‐dependent fever over a range of 100‐1000 ng. The selective brain regions, the nucleus broca ventralis, preoptic area, anterior hypothalamus and the ventromedial hypothalamus, responded to microinjections of a small dose (less than 200 ng) of prostaglandins E2 and F2 alpha by producing fever. Furthermore, the lateral hypothalamus, ventral thalamus, substantia nigra and the trigeminal nucleus were also sensitive to high concentrations of prostaglandins E2 and F2 alpha, fever being produced. It is likely that prostaglandin D2 is not involved in fever induction. 3. The ventricular injection of endogenous pyrogen also produced fever. However, brain regions sensitive to microinjection of endogenous pyrogen were exclusively localized to regions near the organum vasculosum laminae terminalis (OVLT), such as the nucleus broca ventralis and the preoptic area. In contrast to the monophasic fever induced by prostaglandins E2 and F2 alpha, about 30 min after ventricular or cerebral injection of endogenous pyrogen the rectal temperature gradually started to rise and the fever was prolonged over 4 h. 4. We investigated the effect of an inhibitor of prostaglandin synthesis, sodium salicylate, on biphasic fever induced by intravenous injection of bacterial endotoxin. The microinjections of sodium salicylate into the bilateral regions near the OVLT suppressed the second peak but had no effect on the first peak. 5. The present study clarifies that there exist two separate mechanisms of induction of biphasic fever. Correlating with the first peak of biphasic fever, prostaglandins synthesized outside the blood‐brain barrier act on multiple sites in the central nervous system to induce fever. Correlating with the second peak, endogenous pyrogen acts on regions near the OVLT to synthesize and release pyrogenic prostaglandins.
Vitamin D and parotid gland function in the rat.
Authors: Peterfy C , Tenenhouse A , Yu E
Published: December 2014

1. We previously reported that parotid gland secretion is decreased in rats deprived of vitamin D (Glijer, Peterfy & Tenenhouse, 1985). In the present study we examine whether this effect is a direct result of the absence of vitamin D or due to the secondary systemic effects of vitamin D deficiency. 2. Offspring of rats maintained on a calcium‐supplemented (1.2%), vitamin‐D‐deficient diet were weaned onto the same diet and examined after 8 weeks. Using this method it was possible to maintain serum calcium and parathyroid hormone concentrations within normal limits. Serum 25‐hydroxyvitamin D (25(OH)D3) was not detectable, but 1,25‐dihydroxyvitamin D (1,25(OH)2D3) concentrations were normal. 3. Pilocarpine‐stimulated flow of parotid saliva was reduced 57% in vitamin‐D‐deprived animals, but amylase secretion was unchanged. Treatment with vitamin D3 returned flow rates to normal. 4. The concentration of calcium in parotid saliva was normal in vitamin‐D‐deprived rats, although total parotid calcium output was reduced 57%. 5. Pilocarpine‐stimulated salivary flow from submandibular gland, a tissue which does not possess 1,25(OH)2D3 receptors, was normal in vitamin‐D‐deprived rats. 6. Heart rate and arterial blood pressure changes in response to I.V. pilocarpine administration were identical in normal and vitamin‐D‐deficient rats. 7. Auriculotemporal nerve‐stimulated flow of parotid saliva was also reduced by 50% and administration of vitamin D3 to these rats corrected this abnormality. 8. It is concluded that fluid and electrolyte secretion from parotid gland is directly dependent on vitamin D; abnormal parotid gland function seen in vitamin‐D‐deficient rats is not due to secondary hypocalcaemia or hyperparathyroidism, nor can it be explained by haemodynamic changes evoked during systemic administration of pilocarpine. We further conclude that the metabolite of vitamin D responsible for this effect is not 1,25(OH)2D3.