Thermally induced switching field distribution of a single CoPt dot in a large array.

Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterning Co(80)Pt(20)-alloy continuous films by means of laser interference lithography. As commonly seen in large dot arrays, there is a large difference in the switching field between dots. Here we investigate the origin of this large switching field distribution, by using the anomalous Hall effect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversal of individual dots in an array of 80 dots with a diameter of 180 nm. By taking 1000 hysteresis loops we reveal the thermally induced switching field distribution SFD(T) of individual dots inside the array. The SFD(T) of the first and last switching dots were fitted to an Arrhenius model, and a clear difference in switching volume and magnetic anisotropy was observed between dots switching at low and high fields.

Reduced 5-HT1A- and GABAB receptor function in dorsal raphé neurons upon chronic fluoxetine treatment of socially stressed rats.

Autoinhibitory serotonin 1A receptors (5-HT(1A)) in dorsal raphé nucleus (DRN) have been implicated in chronic depression and in actions of selective serotonin reuptake inhibitors (SSRI). Due to experimental limitations, it was never studied at single-cell level whether changes in 5-HT(1A) receptor functionality occur in depression and during SSRI treatment. Here we address this question in a social stress paradigm in rats that mimics anhedonia, a core symptom of depression. We used whole cell patch-clamp recordings of 5-HT- and baclophen-induced G-protein-coupled inwardly rectifying potassium (GIRK) currents as a measure of 5-HT(1A)- and GABA(B) receptor functionality. 5-HT(1A)- and GABA(B) receptor-mediated GIRK-currents were not affected in socially stressed rats, suggesting that there was no abnormal (auto)inhibition in the DRN on social stress. However, chronic fluoxetine treatment of socially stressed rats restored anticipatory behavior and reduced the responsiveness of 5-HT(1A) receptor-mediated GIRK currents. Because GABA(B) receptor-induced GIRK responses were also suppressed, fluoxetine does not appear to desensitize 5-HT(1A) receptors but rather one of the downstream components shared with GABA(B) receptors. This fluoxetine effect on GIRK currents was also present in healthy animals and was independent of the animal's "depressed" state. Thus our data show that symptoms of depression after social stress are not paralleled by changes in 5-HT(1A) receptor signaling in DRN neurons, but SSRI treatment can alleviate these behavioral symptoms while acting strongly on the 5-HT(1A) receptor signaling pathway.

Tunnel spin polarization versus energy for clean and doped Al2O3 barriers.

The variation of the tunnel spin-polarization (TSP) with energy is determined using a magnetic tunnel transistor, allowing quantification of the energy dependent TSP separately for both ferromagnet/insulator interfaces and direct correlation with the tunnel magnetoresistance (TMR) measured in the same device. The intrinsic TSP is reduced below the Fermi level, and more strongly so for tunneling into empty states above the Fermi level. For artificially doped barriers, the low bias TMR decreases due to defect-assisted tunneling. Yet, this mechanism becomes ineffective at large bias, where instead inelastic spin scattering causes a strong TMR decay.

Opposite spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into a ferromagnet.

The spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into Co thin films is examined using a p-type magnetic tunnel transistor. Spin-dependent transmission yields a positive or negative magnetocurrent depending on Co thickness and hole energy. Up to a critical thickness of about 3 nm, (quasi)elastic scattering dominates with a short attenuation length (<1 nm) and preferential attenuation of holes in the majority spin bands, consistent with spin-wave emission. At a larger Co thickness, inelastic scattering dominates with a larger attenuation length ( approximately 4 nm) and opposite spin asymmetry.

NMDA receptors trigger neurosecretion of 5-HT within dorsal raphe nucleus of the rat in the absence of action potential firing.

Activity and calcium-dependent release of neurotransmitters from the somatodendritic compartment is an important signalling mechanism between neurones throughout the brain. NMDA receptors and vesicles filled with neurotransmitters occur in close proximity in many brain areas. It is unknown whether calcium influx through these receptors can trigger the release of somatodendritic vesicles directly, or whether postsynaptic action potential firing is necessary for release of these vesicles. Here we addressed this question by studying local release of serotonin (5-HT) from dorsal raphé nucleus (DRN) neurones. We performed capacitance measurements to monitor the secretion of vesicles in giant soma patches, in response to short depolarizations and action potential waveforms. Amperometric measurements confirmed that secreted vesicles contained 5-HT. Surprisingly, two-photon imaging of DRN neurones in slices revealed that dendritic calcium concentration changes in response to somatic firing were restricted to proximal dendritic areas. This implied that alternative calcium entry pathways may dominate the induction of vesicle secretion from distal dendrites. In line with this, transient NMDA receptor activation, in the absence of action potential firing, was sufficient to induce capacitance changes. By monitoring GABAergic transmission onto DRN 5-HT neurones in slices, we show that endogenous NMDA receptor activation, in the absence of postsynaptic firing, induced release of 5-HT, which in turn increased the frequency of GABAergic inputs through activation of 5-HT(2) receptors. We propose here that calcium influx through NMDA receptors can directly induce postsynaptic 5-HT release from DRN neurones, which in turn may facilitate GABAergic input onto these cells.

Spin filtering of hot holes in a metallic ferromagnet.

Spin-dependent transport of nonequilibrium holes in ferromagnetic thin films and trilayers is investigated using ballistic hole magnetic microscopy. For Co, the hole attenuation length is short and increases from 6 to 10 A in the energy range 0.8 to 2 eV. The hole transmission of a Ni(81)Fe(19)/Au/Co trilayer is clearly spin dependent, resulting in a surprisingly large current change by a factor of 2.3 in a magnetic field. The energy and spin dependence of the hole transmission cannot be explained by the phase space available for inelastic decay of the hot holes.

LFRFamides: a novel family of parasitation-induced -RFamide neuropeptides that inhibit the activity of neuroendocrine cells in Lymnaea stagnalis.

We report the characterization of a cDNA encoding a novel -RFamide neuropeptide precursor that is up-regulated during parasitation in the snail Lymnaea stagnalis. Processing of this precursor yields five structurally related neuropeptides, all but one ending with the C-terminal sequence -LFRFamide, as was confirmed by direct mass spectrometry of brain tissue. The LFRFamide gene is expressed in a small cluster of neurons in each buccal ganglion, three small clusters in each cerebral ganglion, and one cluster in each lateral lobe of the cerebral ganglia. Application of two of the LFRFamide peptides to neuroendocrine cells that control either growth and metabolism or reproduction induced similar hyperpolarizing K+-currents, and inhibited electrical activity. We conclude that up-regulation of inhibitory LFRFamide neuropeptides during parasitation probably reflects an evolutionary adaptation that allows endoparasites to suppress host metabolism and reproduction in order to fully exploit host energy recourses.

Interface, volume, and thermal attenuation of hot-electron spins in Ni80Fe20 and Co.

The relative importance of interface, volume, and thermal scattering in spin-dependent hot-electron transmission of magnetic trilayers is quantified. While interfaces produce significant attenuation (factor 2.2 per interface), the spin asymmetry is dominated by volume scattering. Extracted thermal attenuation lengths (130 A at 300 K for Ni80Fe20) show that thermal spin-wave scattering is stronger than hitherto assumed. This suggests that spontaneous spin-wave emission, rather than the details of the spin-dependent band structure, may cause the strong filtering of minority hot-electron spins.

Anisotropic spin-orbit scattering of hot-electron spins injected into ferromagnetic thin-films.

Anisotropic spin-orbit scattering of hot-electron spins in ferromagnets is examined by injecting a hot-electron current into the thin ferromagnetic base of a transistor and measuring the current attenuation as a function of the magnetization orientation. The transmission anisotropy is described by a simple model, from which we extract an effective spin-orbit scattering length of 420 nm for hot-electron spins in Ni(80)Fe(20), independent of temperature. The corresponding scattering time (<0.3 ps) is surprisingly short, suggesting efficient spin-lattice relaxation of hot electrons. The results also unambiguously demonstrate the attenuation of a hot-electron current by an elastic scattering process.

Activation of protein kinase C by oxytocin-related conopressin underlies pacemaker current in Lymnaea central neurons.

The vasopressin/oxytocin-related neuropeptide Lys-conopressin activates two pacemaker currents in central neurons of the mollusk Lymnaea stagnalis. A high-voltage-activated current (I-HVA) is activated at potentials greater than -40 mV and resembles pacemaker currents found in many molluscan neurons. A low-voltage-activated current (I-LVA) activates throughout the range of -90 to 0 mV. Based on sequence homologies, Lymnaea conopressin receptors are thought to couple to Q-type G proteins and protein kinase C (PKC). Alternatively, agonist-induced pacemaker currents in molluscan neurons have traditionally been attributed to cAMP-dependent protein kinase (PKA) activation. Accordingly, this study aimed at resolving possible involvement of cAMP/PKA and diacylglycerol/PKC in the conopressin response. Injection of cAMP into anterior lobe neurons induced a slow inward current with a voltage dependence resembling that of I(LVA) (and not I(HVA)). However, lack of effect of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and the absence of cross-desensitization between cAMP and conopressin suggest that neither current is dependent on intracellular cAMP. The PKC-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate (but not inactive phorbol 12-myristate 13-acetate) mimicked activation of I(HVA), but not I(LVA), and occluded subsequent responses to conopressin. Activation of I(HVA) was blocked by general protein kinase inhibitors and the PKC-inhibitor GF-109203X. Modulation of the calcium buffering capacity of the pipette medium did not affect the conopressin response, suggesting that calcium dynamics are not of major importance. We conclude that conopressin activates the ion channels carrying I(LVA) and I(HVA) through different second-messenger cascades and that PKC-dependent phosphorylation underlies activation of I(HVA).

Thermal spin-wave scattering in hot-electron magnetotransport across a spin valve.

The role of thermal scattering in spin-dependent transport of hot electrons at 0.9 eV is studied using a spin-valve transistor with a soft Ni(80)Fe(20)/Au/Co base. Spin-dependent scattering makes the collected electron current depend sensitively on the magnetic state of the base. The magnetocurrent reaches 560% at 100 K, decays with increasing temperature, and a huge effect of 350% still remains at room temperature. The results demonstrate that thermal spin waves produce quasielastic spin-flip scattering of hot electrons, resulting in mixing of the two spin channels.

Progesterone-metabolite prevents protein kinase C-dependent modulation of gamma-aminobutyric acid type A receptors in oxytocin neurons.

Gonadal steroid feedback to oxytocin neurons during pregnancy is in part mediated via the neurosteroid allopregnanolone (3alpha-OH-DHP), acting as allosteric modulator of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptors. We describe here a form of nongenomic progesterone signaling by showing that 3alpha-OH-DHP not only potentiates GABA(A) receptor-channel activity but also prevents its modulation by protein kinase C (PKC). Application of oxytocin or stimulation of PKC suppressed the postsynaptic GABA responses of oxytocin neurons in the absence, but not in the presence of 3alpha-OH-DHP. This finding was true at the juvenile stage and during late pregnancy, when the GABA(A) receptor is sensitive to 3alpha-OH-DHP. In contrast, after parturition, when the GABA(A) receptors expressed by oxytocin neurons are less sensitive to 3alpha-OH-DHP, this neurosteroid no longer counteracts PKC. The change in GABA(A)-receptor responsiveness to 3alpha-OH-DHP helps to explain the onset of firing activity and thus the induction of oxytocin release at parturition.

Roles of G-protein beta gamma, arachidonic acid, and phosphorylation inconvergent activation of an S-like potassium conductance by dopamine, Ala-Pro-Gly-Trp-NH2, and Phe-Met-Arg-Phe-NH2.

Dopamine and the neuropeptides Ala-Pro-Gly-Trp-NH2 (APGWamide or APGWa) and Phe-Met-Arg-Phe-NH2 (FMRFamide or FMRFa) all activate an S-like potassium channel in the light green cells of the mollusc Lymnaea stagnalis, neuroendocrine cells that release insulin-related peptides. We studied the signaling pathways underlying the responses, the role of the G-protein betagamma subunit, and the interference by phosphorylation pathways. All responses are blocked by an inhibitor of arachidonic acid (AA) release, 4-bromophenacylbromide, and by inhibitors of lipoxygenases (nordihydroguaiaretic acid and AA-861) but not by indomethacin, a cyclooxygenase inhibitor. AA and phospholipase A2 (PLA2) induced currents with similar I-V characteristics and potassium selectivity as dopamine, APGWa, and FMRFa. PLA2 occluded the response to FMRFa. We conclude that convergence of the actions of dopamine, APGWa, and FMRFa onto the S-like channel occurs at or upstream of the level of AA and that formation of lipoxygenase metabolites of AA is necessary to activate the channel. Injection of a synthetic peptide, which interferes with G-protein betagamma subunits, inhibited the agonist-induced potassium current. This suggests that betagamma subunits mediate the response, possibly by directly coupling to a phospholipase. Finally, the responses to dopamine, APGWa, and FMRFa were inhibited by activation of PKA and PKC, suggesting that the responses are counteracted by PKA- and PKC-dependent phosphorylation. The PLA2-activated potassium current was inhibited by 8-chlorophenylthio-cAMP but not by 12-O-tetradecanoylphorbol 13-acetate (TPA). However, TPA did inhibit the potassium current induced by irreversible activation of the G-protein using GTP-gamma-S. Thus, it appears that PKA targets a site downstream of AA formation, e.g., the potassium channel, whereas PKC acts at the active G-protein or the phospholipase.

gamma-Conotoxin-PnVIIA, a gamma-carboxyglutamate-containing peptide agonist of neuronal pacemaker cation currents.

A novel gamma-carboxyglutamate-containing peptide, designated gamma-conotoxin-PnVIIA, is described from the venom of the molluscivorous snail Conus pennaceus. gamma PnVIIA, triggers depolarization and firing of action potential bursts in the caudodorsal neurons of Lymnaea. This effect is due to activation or enhancement of a slow inward cation current that may underly endogenous bursting activity of these neurons. The amino acid sequence of gamma PnVIIA was determined as DCTSWFGRCTVNS gamma CCSNSCDQTYC gamma-LYAFOS (where gamma is gamma-carboxyglutamate, O is trans-4-hydroxyproline), thus gamma PnVIIA belongs to the six cysteine four loop structural family of conotoxins, and is most homologous to the previously described excitatory conotoxin-TxVIIA. Interestingly, TxVIIA did not induce action potentials in Lymnaea caudodorsal neurons. gamma PnVIIA is the prototype of a new class of gamma-conotoxins that will provide tools for the study of voltage-gated pacemaker channels, which underly bursting processes in excitable systems.

Phe-Met-Arg-Phe-amide activates a novel voltage-dependent K+ current through a lipoxygenase pathway in molluscan neurones.

The neuropeptide Phe-Met-Arg-Phe-amide (FMRFa) dose dependently (ED50 = 23 nM) activated a K+ current in the peptidergic caudodorsal neurones that regulate egg laying in the mollusc Lymnaea stagnalis. Under standard conditions ([K+]o = 1.7 mM), only outward current responses occurred. In high K+ salines ([K+]o = 20 or 57 mM), current reversal occurred close to the theoretical reversal potential for K+. In both salines, no responses were measured below -120 mV. Between -120 mV and the K+ reversal potential, currents were inward with maximal amplitudes at approximately -60 mV. Thus, U-shaped current-voltage relations were obtained, implying that the response is voltage dependent. The conductance depended both on membrane potential and extracellular K+ concentration. The voltage sensitivity was characterized by an e-fold change in conductance per approximately 14 mV at all [K+]o. Since this result was also obtained in nearly symmetrical K+ conditions, it is concluded that channel gating is voltage dependent. In addition, outward rectification occurs in asymmetric K+ concentrations. Onset kinetics of the response were slow (rise time approximately 650 ms at -40 mV). However, when FMRFa was applied while holding the cell at -120 mV, to prevent activation of the current but allow activation of the signal transduction pathway, a subsequent step to -40 mV revealed a much more rapid current onset. Thus, onset kinetics are largely determined by steps preceding channel activation. With FMRFa applied at -120 mV, the time constant of activation during the subsequent test pulse decreased from approximately 36 ms at -60 mV to approximately 13 ms at -30 mV, confirming that channel opening is voltage dependent. The current inactivated voltage dependently. The rate and degree of inactivation progressively increased from -120 to -50 mV. The current is blocked by internal tetraethylammonium and by bath- applied 4-aminopyridine, tetraethylammonium, Ba2+, and, partially, Cd2+ and Cs+. The response to FMRFa was affected by intracellular GTPgammaS. The response was inhibited by blockers of phospholipase A2 and lipoxygenases, but not by a cyclo-oxygenase blocker. Bath-applied arachidonic acid induced a slow outward current and occluded the response to FMRFa. These results suggest that the FMRFa receptor couples via a G-protein to the lipoxygenase pathway of arachidonic acid metabolism. The biophysical and pharmacological properties of this transmitter operated, but voltage-dependent K+ current distinguish it from other receptor-driven K+ currents such as the S-current- and G-protein-dependent inward rectifiers.

High intracellular calcium levels during and after electrical discharges in molluscan peptidergic neurons.

Intracellular calcium levels ([Ca2+]i) during and following electrical activity of the neuroendocrine caudodorsal cells of the pond snail (Lymnaea stagnalis) were measured in situ and is dissociated cells by combining electrical recordings and Fura-2 measurements. Caudodorsal cells are typical neuroendocrine cells that control egg laying via the release of a set of peptides during a stereotyped discharge of action potentials. Single action potentials or short trains of spikes in dissociated caudodorsal cells induced only small but consistent increases in [Ca2+]i. With longer or repeated spike trains, larger [Ca2+]i transients were measured, indicating accumulation of calcium. The calcium channel blocker Ni2+ suppressed the calcium elevation, suggesting that calcium influx occurred through voltage-activated calcium channels. Calcium levels in caudodorsal cells in situ were measured before, during and after the stereotyped firing pattern, a approximately 35-min discharge of regular spiking. Basal calcium levels in caudodorsal cells in situ were about 125 nM. During the initial phase of the discharge, the intracellular calcium level increased to approximately 250 nM. Maximal calcium levels (300-600 nM) were only reached at the final phase of the discharge or several minutes after the cessation of firing. Calcium levels remained elevated for up to 1 h after the end of the discharge. During this time, caudodorsal cells were characterized by very low excitability. We suggest that the prolonged, elevated level of calcium following the discharge need not be directly dependent on action potentials. The long-lasting [Ca2+]i elevation may cause the release of neuropeptides to outlast the duration of electrical activity, thus uncoupling release from spiking. In addition, it may cause reduced excitability of neuroendocrine cells following a period of spiking, thereby inducing a refractory period.

High intracellular calcium levels during and after electrical discharges in molluscan peptidergic neurons.

Intracellular calcium levels ([Ca2+]i) during and following electrical activity of the neuroendocrine caudodorsal cells of the pond snail (Lymnaea stagnalis) were measured in situ and in dissociated cells by combining electrical recordings and Fura-2 measurements. Caudodorsal cells are typical neuroendocrine cells that control egg laying via the release of a set of peptides during a stereotyped discharge of action potentials. Single action potentials or short trains of spikes in dissociated caudodorsal cells induced only small but consistent increases in [Ca2+]i. With longer or repeated spike trains, larger [Ca2+]i transients were measured, indicating accumulation of calcium. The calcium channel blocker Ni2+ suppressed the calcium elevation, suggesting that calcium influx occurred through voltage-activated calcium channels. Calcium levels in caudodorsal cells in situ were measured before, during and after the stereotyped firing pattern, a approximately 35-min discharge of regular spiking. Basal calcium levels in caudodorsal cells in situ were about 125 nM. During the initial phase of the discharge, the intracellular calcium level increased to approximately 250 nM. Maximal calcium levels (300-600 nM) were only reached at the final phase of the discharge or several minutes after the cessation of firing. Calcium levels remained elevated for up to 1 h after the end of the discharge. During this time, caudodorsal cells were characterized by very low excitability. We suggest that the prolonged, elevated level of calcium following the discharge need not be directly dependent on action potentials. The long-lasting [Ca2+]i elevation may cause the release of neuropeptides to outlast the duration of electrical activity, thus uncoupling release from spiking. In addition, it may cause reduced excitability of neuroendocrine cells following a period of spiking, thereby inducing a refractory period.

Convergence of multiple G-protein-coupled receptors onto a single type of potassium channel.

The light green cells (LGCs) in the central nervous system of the pond snail Lymnaea stagnalis form a homogeneous group of neuroendocrine cells that are involved in the control of growth and metabolism. These cells are inhibited by dopamine and the neuropeptides APGWamide, FMRFamide and GGSLFRFamide. Thus, the LGCs form an endogenous system in which processing and integration of different inputs into a physiological response can be studied. In this study we characterize the current(s) that are responsible for the inhibition of the LGCs by dopamine, APGWamide, FMRFamide and GGSLFRFamide. The responses are G-protein dependent, as follows from experiments with GTP-gamma-S. Several experiments indicate that the four agonists activate a single type of potassium channel. First, the currents evoked by the agonists have the same ion selectivity and voltage dependence. Potassium is the predominant charge carrier and the responses are weakly voltage sensitive, with conductance decreasing at potentials below approximately - 100 mV. Second, the currents activated by the four agonists display similar sensitivity towards several blockers. Internal and external TEA (10 mM), and extracellular Ba2+ (1 mM) cause a block of approximately 60-90%. External 4AP (1 mM) causes approximately 30% block and external Cs+ (1 mM) causes a voltage sensitive block. There is no sensitivity towards apamine and glibenclamide. Third, there is no summation of the responses to dopamine, APGWamide and GGSLFRFamide with maximal FMRFamide responses. Together, these data indicate that the responses induced by dopamine, APGWamide, FMRFamide and GGSLFRFamide are G-protein mediated and converge onto a single type of potassium channel in the LGCs of Lymnaea stagnalis.

Novel omega-conotoxins block dihydropyridine-insensitive high voltage-activated calcium channels in molluscan neurons.

We have identified two novel peptide toxins from molluscivorous Conus species that discriminate subtypes of high voltage-activated (HVA) calcium currents in molluscan neurons. The toxins were purified using assays on HVA calcium currents in the caudodorsal cells (CDCs) of the snail Lymnaea stagnalis. The CDC HVA current consists of a rapidly inactivating, transient current that is relatively insensitive to dihydropyridines (DHPs) and a slowly inactivating, DHP-sensitive L-current. The novel toxins, designated omega-conotoxins PnVIA and PnVIB, completely and selectively block the transient HVA current in CDCs with little (PnVIA) or no (PnVIB) effect on the sustained L-type current. The block is rapid and completely reversible. It is noteworthy that both PnVIA and PnVIB reveal very steep dose dependences of the block, which may imply cooperativity in toxin action. The amino acid sequences of PnVIA (GCLEVDYFCGIPFANNGLCCSGNCVFVCTPQ) and of PnVIB (DDDCEPPGNFCGMIKIGPPCCSGWCFFACA) show very little homology to previously described omega-conotoxins, although both toxins share the typical omega-conotoxin cysteine framework but have an unusual high content of hydrophobic residues and net negative charge. These novel omega-conotoxins will facilitate selective analysis of the functions of HVA calcium channels and may enable the rational design of drugs that are selective for relevant subtypes.

A novel hydrophobic omega-conotoxin blocks molluscan dihydropyridine-sensitive calcium channels.

A novel calcium channel blocking peptide designated omega-conotoxin-Tx VII has been characterized from the venom of the molluscivorous snail Conus textile. The amino acid sequence (CKQADEPCDVFSLDCCTGICLGVCMW) reveals the characteristic cysteine framework of omega-conotoxins, but it is extremely hydrophobic for this pharmacological class of peptides and further unusual in its net negative charge (-3). It is further striking that the sequence of TxVII, a calcium current blocker, is 58% identical to that of delta-conotoxin-TxVIA, which targets sodium channels. TxVII effects were examined in the caudodorsal cell (CDC) neurons from the mollusc Lymnaea stagnalis. The toxin has no significant effect on sodium or potassium currents in these cells, but it clearly blocks the calcium currents. TxVII most prominently blocks the slowly inactivating, dihydropyridine- (DHP-) sensitive current in CDCs, while blockade of the rapidly inactivating current is less efficient. This novel omega-conotoxin is apparently targeted to DHP-sensitive calcium channels and thereby provides a lead for future design of selective conopeptide probes for L-type channels.