Experimentally tracking unstable steady states by large periodic modulation

Experimental suppression of chaos has been achieved in an optically pumped far-infrared (NH3)-N-15 laser which displays Lorenz-like chaos. The method of control involves the application of a large amplitude slow (i.e., nonresonant) modulation of the pump power. This may be related to a delayed bifurcation to chaos observed when the pump power is ramped at a constant late.

Gonadotrophin and prolactin secretion in castrated male sheep following subcutaneous or intracranial treatment with testicular hormones

Interactions between testosterone, estradiol, and inhibin in the control of gonadotrophin secretion in males are poorly understood. Castrated rams were treated with steroid-free bovine follicular fluid (bFF), testosterone, or estradiol and for 7 d (2 x 2 x 2 factorial design). Given independently, none of the exogenous hormones affected follicle-stimulating hormone (FSH) concentrations, but the combination of one or both steroids with bFF reduced FSH secretion. Testosterone and estradiol reduced luteinizing hormone (LH) pulse frequency (there was no synergism), and bFF had no effect. Plasma prolactin concentrations were not affected by any treatment. To locate the central sites of steroid action, castrated rams were bilaterally implanted in the preoptic area (POA), ventromedial nucleus (VMH), or arcuate nucleus (ARC). These implants did not affect FSH or prolactin concentrations, or LH pulse amplitude. The frequency of the LH pulses was not affected by testosterone in any site. Estradiol located in the ARC, but not the POA or VMH, decreased LH pulse frequency. In summary, FSH secretion is controlled by synergistic interactions between inhibin and estradiol or testosterone, whereas GnRH/LH pulse frequency is controlled by testicular steroids. Estradiol acts partly, at least, in the ARC, but the central site of action, testosterone remains unknown.

Quantum trajectory simulations of the fluorescence intensity from a two-level atom driven by a multichromatic field

The quantum trajectories method is illustrated for the resonance fluorescence of a two-level atom driven by a multichromatic field. We discuss the method for the time evolution of the fluorescence intensity in the presence of bichromatic and trichromatic driving fields. We consider the special case wherein one multichromatic field component is strong and resonant with the atomic transition whereas the other components are much weaker and arbitrarily detuned from the atomic resonance. We find that the phase-dependent modulations of the Rabi oscillations, recently observed experimentally [Q. Wu, D. J. Gauthier, and T. W. Mossberg, Phys. Rev. A 49, R1519 (1994)] for the special case when the weaker component of a bichromatic driving field is detuned from the atomic resonance by the strong-field Rabi frequency, appear also for detunings close to the subharmonics of the Rabi frequency. Furthermore, we show that for the atom initially prepared in one of the dressed states of the strong field component the modulations are not sensitive to the phase. We extend the calculations to the case of a trichromatic driving field and find that apart from the modulations of the amplitude there is a modulation of the frequency of the Rabi oscillations. Moreover, the time evolution of the fluorescence intensity depends on the phase regardless of the initial conditions and a phase-dependent suppression of the Rabi oscillations can be observed when the sideband fields are tuned to the subharmonics of the strong-field Rabi frequency. [S1050-2947(98)03501-X].

Calcium-permeable AMPA receptors mediate long-term potentiation in interneurons in the amygdala

Fear conditioning is a paradigm that has been used as a model for emotional learning in animals'. The cellular correlate of fear conditioning is thought to be associative N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity within the amygdala(1-3). Here we show that glutamatergic synaptic transmission to inhibitory interneurons in the basolateral amygdala is mediated solely by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In contrast to AMPA receptors at inputs to pyramidal neurons, these receptors have an inwardly rectifying current-voltage relationship, indicative of a high permeability to calcium(4 5), Tetanic stimulation of inputs to interneurons caused an immediate and sustained increase in the efficacy of these synapses. This potentiation required a rise in postsynaptic calcium, but was independent of NMDA receptor activation. The potentiation of excitatory inputs to interneurons was reflected as an increase in the amplitude of the GABAA-mediated inhibitory synaptic current in pyramidal neurons. These results demonstrate that excitatory synapses onto interneurons within a fear conditioning circuit show NMDA-receptor independent long-term potentiation. This plasticity might underlie the increased synchronization of activity between neurons in the basolateral amygdala after fear conditioning(6).

Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae

The early effects of heat stress on the photosynthesis of symbiotic dinoflagellates (zooxanthellae) within the tissues of a reef-building coral were examined using pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photorespirometry. Exposure of Stylophora pistillata to 33 and 34 degrees C for 4 h resulted in (1) the development of strong non-photochemical quenching (qN) of the chlorophyll fluorescence signal, (2) marked decreases in photosynthetic oxygen evolution, and (3) decreases in optimal quantum yield (F-v/F-m) of photosystern II (PSII), Quantum yield decreased to a greater extent on the illuminated surfaces of coral branches than on lower (shaded) surfaces, and also when high irradiance intensities were combined with elevated temperature (33 degrees C as opposed to 28 degrees C), qN collapsed in heat-stressed samples when quenching analysis was conducted in the absence of oxygen, Collectively, these observations are interpreted as the initiation of photoprotective dissipation of excess absorbed energy as heat (qN) and O-2-dependent electron flow through the Mehler-Ascorbate-Peroxidase cycle (MAP-cycle) following the point at which the rate of light-driven electron transport exceeds the capacity of the Calvin cycle. A model for coral bleaching is proposed whereby the primary site of heat damage in S, pistillata is carboxylation within the Calvin cycle, as has been observed during heat damage in higher plants, Damage to PSII and a reduction in F-v/F-m (i.e. photoinhibition) are secondary effects following the overwhelming of photoprotective mechanisms by light. This secondary factor increases the effect of the primary variable, temperature. Potential restrictions of electron flow in heat-stressed zooxanthellae are discussed with respect to Calvin cycle enzymes and the unusual status of the dinoflagellate Rubisco, Significant features of our model are that (1) damage to PSII is not the initial step in the sequence of heat stress in zooxanthellae, acid (2) light plays a key secondary role in the initiation of the bleaching phenomena.

Quantum oscillations in quasi-one-dimensional metals with spin-density-wave ground states

We consider the magnetoresistance oscillation phenomena in the Bechgaard salts (TMTSF)(2)X, where X = ClO4, PF6, and AsF6 in pulsed magnetic fields to 51 T. Of particular importance is the observation of a new magnetoresistance oscillation for X = ClO4 in its quenched state. In the absence of any Fermi-surface reconstruction due to anion order at low temperatures, all three materials exhibit nonmonotonic temperature dependence of the oscillation amplitude in the spin-density-wave (SDW) state. We discuss a model where, below a characteristic temperature T* within the SDW state, a magnetic breakdown gap opens. [S0163-1829(99)00904-2].

Excitatory synaptic inputs to pyramidal neurons of the lateral amygdala

Whole-cell patch clamp recordings were made from pyramidal neurons in the rat lateral amygdala (LA). Synaptic currents were evoked by stimulating in either the external capsule (ec), internal capsule (ic) or basolateral nucleus (BLA). Stimulation of either the ic, ec or BLA evoked a glutamatergic excitatory synaptic current (EPSC) which was mediated by both non-NMDA and NMDA (N-methyl-D-aspartic acid) receptors, The ratio of the amplitude of the NMDA receptor-mediated component measured at +40 mV to the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component measured at -60 mV was similar regardless of whether EPSCs were evoked in the ec, ic or BLA. At resting membrane potentials, excitatory synaptic potentials evoked from either the ec or putative thalamic inputs were unaffected by application of the NMDA receptor antagonist APV. Spontaneous glutamatergic currents had two components to their decay phase. The slow component was selectively blocked by the NMDA receptor antagonist D-APV, indicating that AMPA and NMDA receptors are colocalized in spiny neurons. We conclude that pyramidal cells of the LA receive convergent inputs from the cortex, thalamus and basal nuclei. At all inputs, both AMPA/kainate and NMDA-type receptors are active and colocalized in the postsynaptic density.

Relationships between movement initiation times and movement-related cortical potentials in Parkinson's disease

Movement-related cortical potentials recorded from the scalp reveal increasing cortical activity occurring prior to voluntary movement. Studies of set-related cortical activity recorded from single neurones within premotor and supplementary motor areas in monkeys suggest that such premovement activity may act to prime activity of appropriate motor units in readiness to move, thereby facilitating the movement response. Such a role of early stage premovement activity in movement-related cortical potentials was investigated by examining the relationship between premovement cortical activity and movement initiation or reaction times. Parkinson's disease and control subjects performed a simple button-pressing reaction time task and individual movement-related potentials were averaged for responses with short compared with long reaction times. For Parkinson's disease subjects but not for the control subjects, early stage premovement cortical activity was significantly increased in amplitude for faster reaction times, indicating that there is indeed a relationship between premovement cortical activity amplitude and movement initiation or reaction times. In support of studies of set-related cortical activity in monkeys, it is therefore suggested that early stage premovement activity reflects the priming of appropriate motor units of primary motor cortex, thereby reducing movement initiation or reaction times. (C) 1999 Elsevier Science B.V. All rights reserved.

Photoinhibition and photoprotection in symbiotic dinoflagellates from reef-building corals

Pulse-amplitude-modulation fluorometry and oxygen respirometry were used to investigate diel photosynthetic responses by symbiotic dinoflagellates to light levels in summer and winter on a high latitude coral reef. The symbiotic dinoflagellates from 2 species of reef-building coral (Porites cylindrica and Stylophora pistillata) showed photoinhibitory decreases in the ratio of variable (F-v) to maximal (F-m) fluorescence (F-v/F-m) as early as 09:00 h on both summer and winter days on the reefs associated with One Tree Island (23 degrees 30' S, 152 degrees 06' E; Great Barrier Reef, Australia). This was due to decreases in maximum, F-m, and to a smaller extent minimum, F-0, chlorophyll fluorescence. Complete recovery took 4 to 6 h and began to occur as soon as light levels fell each day. Chlorophyll fluorescence quenching analysis of corals measured during the early afternoon revealed classic regulation of photosystem II (PSII) efficiency through non-photochemical quenching (NPQ). These results appear to be similar to data collected for other algae and higher plants, suggesting involvement of the xanthophyll cycle of symbiotic dinoflagellates in regulating the quantum efficiency of PSII. The ability of symbiotic dinoflagellates to develop significant NPQ, however, depended strongly on when the symbiotic dinoflagellates were studied. Whereas symbiotic dinoflagellates from corals in the early afternoon showed a significant capacity to regulate the efficiency of PSII using NPQ, those sampled before sunrise had a slower and much reduced capacity, suggesting that elements of the xanthophyll cycle are suppressed prior to sunrise. A second major finding of this study is that the quantum efficiency of PSII in symbiotic dinoflagellates is strongly diurnal, and is as much as 50% lower just prior to sunrise than later in the day. When combined with oxygen flux data, these results indicate that a greater portion of the electron transport occurring later in the day is likely to be due to the increases in the rate of carbon fixation by Rubisco or to higher flutes through the Mehler-Ascorbate-Peroxidase (MAP) cycle.

Movement-related potentials in Parkinson's disease: External cues and attentional strategies

Hypokinetic movement can be greatly improved in Parkinson's disease patients by the provision of external cues to guide movement. It has recently been reported, however, that movement performance in parkinsonian patients can be similarly improved in the absence of external cues by using attentional strategies, whereby patients are instructed to consciously attend to particular aspects of the movement which would normally be controlled automatically. To study the neurophysiological basis of such improvements in performance associated with the use of attentional strategies, movement-related cortical potentials were examined in Parkinson's disease and control subjects using a reaction time paradigm. One group of subjects were explicitly instructed to concentrate on internally timed responses to anticipate the presentation of a predictably timed go signal. Other subjects were given no such instruction regarding attentional strategies. Early-stage premovement activity of movement-related potentials was significantly increased in amplitude and reaction times were significantly faster for Parkinson's disease subjects when instructed to direct their attention toward internally generating responses rather than relying on external cues. It is therefore suggested that the use of attentional strategies may allow movement to be mediated by less automatic and more conscious attentional motor control processes which may be less impaired by basal ganglia dysfunction, and thereby improve movement performance in Parkinson's disease.

PAM chlorophyll fluorometry: a new in situ technique for stress assessment in scleractinian corals, used to examine the effects of cyanide from cyanide fishing

Sodium cyanide is being used on reefs in the Asia-Pacific region to capture live fish for the aquarium industry, and to supply a rapidly growing, restaurant-based demand, The effects of cyanide on reef biota have not been fully explored. To investigate its effect on hard corals, we exposed small branch lips of Stylophora pistillata and Acropora aspera to cyanide concentrations estimated to occur during cyanide fishing. Pulse amplitude modulation (PAM) chlorophyll fluorescence techniques were used to examine photoinhibition and photosynthetic electron transport in the symbiotic algae (zooxanthellae) in the tissues of the corals, These measurements were made in situ and in real time using a recently developed submersible PAM fluorometer. In S. pistillata. exposure to cyanide resulted in an almost complete cessation in photosynthetic electron transport rate. Both species displayed marked decreases in the ratio of variable fluorescence (F-v) to maximal fluorescence (F-m) (dark-adapted F-v/F-m), following exposure to cyanide, signifying a decrease in photochemical efficiency. Dark-adapted F-v/F-m recovered to normal levels in similar to 6 d, although intense tissue discolouration, a phenomenon well-recognised as coral 'bleaching' was observed during this period, Bleaching was caused by loss of zooxanthellae from the coral tissues, a well-recognised sub-lethal stress response of corals. Using the technique of chlorophyll fluorescence quenching analysis, corals exposed to cyanide did not show light activation of Calvin cycle enzymes and developed high levels of non-photochemical quenching (q(N)), signifying the photoprotective dissipation of excess light as heat, These features are symptomatic of the known properties of cyanide as an inhibitor of enzymes of the Calvin cycle. The results of this in situ study show that an impairment of zooxanthellar photosynthesis is; the site of cyanide-mediated toxicity, and is the cue that causes corals to release their symbiotic zooxanthellac following cyanide exposure. This study demonstrates the efficacy of PBM fluorometry as a new tool for in situ stress assessment in zooxanthellate scleractinian corals. (C) 1999 Elsevier Science Ltd. All rights reserved.

Putative beta(4)-adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (-)-[H-3]-CGP 12177 binding

1 We identified putative beta(4)-adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (-)-CGP 12177 and (+/-)-cyanopindolol and demonstrated increased Ca2+ transients by (-)-CGP 12177 in rat cardiomyocytes. 2 (-)-[H-3]-CGP 12177 labelled 13-22 fmol mg(-1) protein ventricular beta(1), beta(2)-adrenoceptors (pK(D) similar to 9.0) and 50-90 fmol mg(-1) protein putative beta(4)-adrenoceptors (pK(D) similar to 7.3). The affinity values (PKi) for (beta(1),beta(2)-) and putative beta(4)-adrenoceptors, estimated from binding inhibition, were (-)-propranolol 8.4, 5.7; (-)-bupranolol 9.7, 5.8; (+/-)-cyanopindolol 10.0,7.4. 3 In left ventricular papillary muscle, in the presence of 30 mu M 3-isobutyl-1-methylxanthine, (-)CGP 12177 and (+/-)-cyanopindolol caused positive inotropic effects, (pEC(50) (-)-CGP 12177, 7.6; (+/-)-cyanopindolol, 7.0) which were antagonized by (-)-bupranolol (pK(B) 6.7-7.0) and (-)-CGP 20712A (pK(B) 6.3-6.6). The cardiostimulant effects of(-)-CGP 12177 in papillary muscle, left and right atrium were antagonized by (+/-)-cyanopindolol (pK(i), 7.0-7.4). 4 (-)-CGP 12177 (1 mu M) in the presence of 200 nM (-)-propranolol increased Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 mu M 3-isobutyl-1-methylxanthine and 200 nM (-)-propranolol, 1 mu M (-)-CGP 12177 caused a 73% increase in Ca2+ transient amplitude in ventricular myocytes. (-)-CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocytes. 5 Probably by preventing cyclic AMP hydrolysis, 3-isobutyl-1-methylxanthine facilitates the inotropic function of ventricular putative beta(4)-adrenoceptors. suggesting coupling to G(s) protein-adenylyl cyclase. The receptor-mediated increases in contractile force are related to increases of Ca2+ in atrial and ventricular myocytes. The agreement of binding affinities of agonists with cardiostimulant potencies is consistent with mediation through putative beta(4)-adrenoceptors labelled with (-)-[H-3]-CGP 12177.

Single amino acid substitutions in alpha-conotoxin PnIA shift selectivity for subtypes of the mammalian neuronal nicotinic acetylcholine receptor

The alpha-conotoxins, a class of nicotinic acetylcholine receptor (nAChR) antagonists, are emerging as important probes of the role played by different nAChR subtypes in cell function and communication, In this study, the native alpha-conotoxins PnIA and PnIB were found to cause concentration-dependent inhibition of the ACh-induced current in all rat parasympathetic neurons examined, with IC50 values of 14 and 33 nM, and a maximal reduction in current amplitude of 87% and 71%, respectively. The modified alpha-conotoxin [N11S]PnIA reduced the ACh-induced current with an IC50 value of 375 nM and a maximally effective concentration caused 91% block, [A10L]PnIA was the most potent inhibitor, reducing the ACh-induced current in similar to 80% of neurons, with an IC50 value of 1.4 nM and 46% maximal block of the total current, The residual current was not inhibited further by alpha-bungarotoxin, but was further reduced by the cu-conotoxins PnIA or PnIB, and by mecamylamine. H-1 NMR studies indicate that PnIA, PnIB, and the analogues, [A10L]PnIA and [N11S]PnIA, have identical backbone structures. We propose that positions 10 and II of PnIA and PnIB influence potency and determine selectivity among alpha 7 and other nAChR subtypes, including alpha 3 beta 2 and alpha 3 beta 4, Four distinct components of the nicotinic ACh-induced current in mammalian parasympathetic neurons have been dissected with these conopeptides.

Activation of the human diaphragm during a repetitive postural task

1. The co-ordination between respiratory and postural functions of the diaphragm was investigated during repetitive upper Limb movement. It was hypothesised that diaphragm activity would occur either tonically or phasically in association with the forces from each movement and that this activity would combine with phasic respiratory activity. 2. Movements of the upper limb and ribcage were measured while standing subjects performed repetitive upper limb movements 'as fast as possible'. Electromyographic (EMG) recordings of the costal diaphragm were made using intramuscular electrodes in four subjects. Surface electrodes were placed over the deltoid and erector spinae muscles. 3. In contrast to standing at rest, diaphragm activity was present throughout expiration at 78 +/- 17% (mean +/- S.D.) of its peak inspiratory magnitude during repeated upper limb movement. 4. Bursts of deltoid and erector spinae EMG activity occurred at the Limb movement frequency (similar to 2.9 Hz). Although the majority of diaphragm EMG power was at the respiratory frequency (similar to 0.4 Hz), a peak was also present at the movement frequency. This finding was corroborated by averaged EMG activity triggered from upper limb movement. In addition, diaphragm EMG activity was coherent with ribcage motion at the respiratory frequency and with upper limb movement at the movement frequency. 5. The diaphragm response was similar when movement was performed while sitting. In addition, when subjects moved with increasing frequency the peak upper limb acceleration correlated with diaphragm EMG amplitude. These findings support the argument that diaphragm contraction is related to trunk control. 6. The results indicate that activity of human phrenic motoneurones is organised such that it contributes to both posture and respiration during a task which repetitively challenges trunk posture.

Changes in quantum efficiency of Photosystem II of symbiotic dinoflagellates of corals after heat stress, and of bleached corals sampled after the 1998 Great Barrier Reef mass bleaching event Ross J. Jones , Selina Ward, Affendi Yang Amri and Ove Hoegh-Guldberg

Pulse-amplitude-modulation chlorophyll fluorometry was used to examine changes in dark-adapted F-v/F-m of endosymbiotic dinoflagellate microalgae within the tissues of the temperate coral Plesiastrea versipora exposed to elevated seawater temperature. The F-v/F-m was markedly reduced following exposure of corals to 28 degrees C for 48 h. When corals were returned to ambient (24 degrees C) conditions, F-v/F-m increased in an initial rapid and then secondary slower phase. Tissue discolouration (coral bleaching), caused by a significant decrease in the density of algae, was observed during the first 2-3 days of the recovery period. After 14 days, F-v/F-m was still significantly lower than in control corals. The recovery of F-v/F-m is discussed in terms of repair processes within the symbiotic algae, division of healthy algae and also the selective removal of photo-damaged dinoflagellates. Under field conditions, bleached corals sampled at Heron Island Reef during a bleaching event had significantly lower F-v/F-m than non-bleached colonies; four months after the bleaching event, there were no differences in F-v/F-m or algal density in corals marked as having bleached or having shown no signs of colour loss. The results of this laboratory and field study are consistent with the hypothesis that an impairment of photosynthesis occurs during heat-stress, and is the underlying cause of coral bleaching.