Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen.

Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's accretion versus that subducted from the Earth's surface is unclear1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7-10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12-15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.

CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula.

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, 18O is twice as fractionated as 17O, relative to 16O. In CAIs 17O and 18O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure 16O into the solar nebula, but the lack of a similar isotope trend in other elements argues against this explanation. A symmetry-dependent fractionation mechanism may have occurred in the inner solar nebula, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux approximately 10(3) times that of the local interstellar medium (for example, owing to an O or B star within approximately 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of approximately 10(5) years. We predict that similarly irradiated protoplanetary disks will have H2O enriched in 17O and 18O by several tens of per cent relative to CO.

Oxygen reservoirs in the early solar nebula inferred from an Allende CAI.

Ultraviolet laser microprobe analyses of a calcium-aluminum-rich inclusion (CAI) from the Allende meteorite suggest that a line with a slope of exactly 1.00 on a plot of delta (17)O against delta (18)O represents the primitive oxygen isotope reservoir of the early solar nebula. Most meteorites are enriched in (17)O and (18)O relative to this line, and their oxygen isotope ratios can be explained by mass fractionation or isotope exchange initiating from the primitive reservoir. These data establish a link between the oxygen isotopic composition of the abundant ordinary chondrites and the primitive (16)O-rich component of CAIs.

The formation of chondrules at high gas pressures in the solar nebula.

High-precision magnesium isotope measurements of whole chondrules from the Allende carbonaceous chondrite meteorite show that some aluminum-rich Allende chondrules formed at or near the time of formation of calcium-aluminum-rich inclusions and that some others formed later and incorporated precursors previously enriched in magnesium-26. Chondrule magnesium-25/magnesium-24 correlates with [magnesium]/[aluminum] and size, the aluminum-rich, smaller chondrules being the most enriched in the heavy isotopes of magnesium. These relations imply that high gas pressures prevailed during chondrule formation in the solar nebula.

Near-equilibrium isotope fractionation during planetesimal evaporation.

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥ 0.1% M ⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10-8 bar and a vapor pressure at the surface of 10-4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H2 on timescales of 104 to 105 years.

Auditory nerve inputs to cochlear nucleus neurons studied with cross-correlation.

The strength of synapses between auditory nerve (AN) fibers and ventral cochlear nucleus (VCN) neurons is an important factor in determining the nature of neural integration in VCN neurons of different response types. Synaptic strength was analyzed using cross-correlation of spike trains recorded simultaneously from an AN fiber and a VCN neuron in anesthetized cats. VCN neurons were classified as chopper, primarylike, and onset using previously defined criteria, although onset neurons usually were not analyzed because of their low discharge rates. The correlograms showed an excitatory peak (EP), consistent with monosynaptic excitation, in AN-VCN pairs with similar best frequencies (49% 24/49 of pairs with best frequencies within +/-5%). Chopper and primarylike neurons showed similar EPs, except that the primarylike neurons had shorter latencies and shorter-duration EPs. Large EPs consistent with end bulb terminals on spherical bushy cells were not observed, probably because of the low probability of recording from one. The small EPs observed in primarylike neurons, presumably spherical bushy cells, could be derived from small terminals that accompany end bulbs on these cells. EPs on chopper or primarylike-with-notch neurons were consistent with the smaller synaptic terminals on multipolar and globular bushy cells. Unexpectedly, EPs were observed only at sound levels within about 20 dB of threshold, showing that VCN responses to steady tones shift from a 1:1 relationship between AN and VCN spikes at low sound levels to a more autonomous mode of firing at high levels. In the high level mode, the pattern of output spikes seems to be determined by the properties of the postsynaptic spike generator rather than the input spike patterns. The EP amplitudes did not change significantly when the presynaptic spike was preceded by either a short or long interspike interval, suggesting that synaptic depression and facilitation have little effect under the conditions studied here.

Proprioceptive information from the pinna provides somatosensory input to cat dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) is a second-order auditory structure that also receives nonauditory information, including somatosensory inputs from the dorsal column and spinal trigeminal nuclei. Here we investigate the peripheral sources of the somatosensory inputs to DCN. Electrical stimulation was applied to cervical nerves C1-C8, branches of C2, branches of the trigeminal nerve, and hindlimb nerves. The largest evoked potentials in the DCN were produced by C2 stimulation and by stimulation of its branches that innervate the pinna. Electrical stimulation of C2 produced a pattern of inhibition and excitation of DCN principal cells comparable with that seen in previous studies with stimulation of the primary somatosensory nuclei, suggesting that the same pathway was activated. Because C2 contains both proprioceptive and cutaneous fibers, we applied peripheral somatosensory stimulation to identify the effective somatosensory modalities. Only stimuli that activate pinna muscle receptors, such as stretch or vibration of the muscles connected to the pinna, were effective in driving DCN units, whereas cutaneous stimuli such as light touch, brushing of hairs, and stretching of skin were ineffective. These results suggest that the largest somatosensory inputs to the DCN originate from muscle receptors associated with the pinna. They support the hypothesis that a role of the DCN in hearing is to coordinate pinna orientation to sounds or to support correction for the effects of pinna orientation on sound-localization cues.

Identification of response properties of ascending axons from dorsal cochlear nucleus.

Electrical stimulation in the dorsal acoustic stria (DAS) and the posterior and dorsal part of the ventral cochlear nucleus (VCN) was used to identify the response properties of efferent axons from the dorsal cochlear nucleus (DCN) in unanesthetized, decerebrate cats. Responses recorded in the DCN of this preparation can be divided into two broad classes, type II/III and type IV. Results of previous studies suggest that one subclass of type II/III units, those without spontaneous activity, are inhibitory interneurons terminating on type IV cells. Most type IV units (34/43) could be antidromically activated from the DAS and most units which could be activated from the DAS were of this type (34/39), suggesting that type IV responses are recorded from the projection cells of the DCN. Few type II/III cells without spontaneous activity could be activated from the DAS (1/30). A few cells of this type (6/32) could be antidromically stimulated from regions of the posterior AVCN or dorsal PVCN through which axon collaterals of some small cell interneurons of the DCN have been reported to pass. No type IV cells could be activated from the VCN (0/22). Thus the results are consistent with some type II/III responses originating in DCN interneurons. Spontaneously active type II/III units had intermediate properties.

Inhibitory connections between AVCN and DCN: evidence from lidocaine injection in AVCN.

Principal cells of the dorsal cochlear nucleus (DCN) receive a variety of inhibitory influences, including some that are associated with the anteroventral cochlear nucleus (AVCN). Electrical stimulation in the caudal part of the AVCN produces long-lasting inhibition of DCN units [(1973) Exp. Brain Res. 17, 428-442], including unit type IV, which is associated with principal cells. This inhibition may be mediated either by orthodromic activation of AVCN axons that terminate in DCN or by antidromic stimulation of DCN inhibitory interneurons whose axons send collaterals to the AVCN. These two possibilities cannot be distinguished with electrical stimulation alone. In this study, microinjections of lidocaine were made in the AVCN. These injections should block AVCN to DCN axons without directly affecting the activity, within the DCN, of DCN interneurons. Two effects of lidocaine injections were observed. In some cases, all activity of DCN cells was abolished by lidocaine. These cases involved either spread of lidocaine to the recording site or into the auditory nerve. In other cases, the inhibitory responses of DCN type IV units were weakened by lidocaine without changing the units' thresholds or spontaneous activity. The latter result demonstrates that there are inhibitory pathways originating in AVCN or passing through AVCN and terminating in the DCN.

Discharge-rate dependence of refractory behavior of cat auditory-nerve fibers.

A descriptive model for auditory-nerve (AN) refractory periods is described. The model assumes that interspike intervals consist of a constant-length absolute refractory period (ARP), followed by random-length relative refractory period (RRP) and a random-length waiting time to the next spike. Both the RRP and waiting time are exponentially distributed. This model fits AN hazard functions sufficiently well to provide estimates of the ARP and RRP durations for each fiber. The ARP is found to be constant, independent of discharge rate, with mean value between 0.56 and 0.86 ms in data from 7 experiments. The RRP decreases in duration as discharge rate increases; RRP mean length is less than 2 ms in most cases. There is an additional, slow component of recovery, lasting 20-40 ms, which is not modeled. The variation in RRP with discharge rate is shown to be capable of accounting for the deviation of AN regularity from that predicted for a Poisson process. Finally, properties of peaks seen in hazard functions just at the end of the ARP are described; these peaks are not included in the model, but are shown to be especially prominent in low and medium spontaneous rate fibers.

Response properties of type II and type III units in dorsal cochlear nucleus.

Type II and type III units in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats are those with excitatory responses to best frequency (BF) tones at all levels above threshold. They are distinguished from type IV units which give predominantly inhibitory responses to tones. Type II and type III units are distinguished from one another by their rates of spontaneous activity. Type II units have low rates of spontaneous activity (below 2.5 spikes/s), whereas type III units are spontaneously active at rates up to 95 spikes/s. In this paper we show that segregation of units according to spontaneous activity produces a segregation of several other properties as well. A typical type II unit responds vigorously to BF tones (median maximum rate of 242 spikes/s), has slightly nonmonotonic rate versus level function, and responds weakly or not at all to broadband noise. These units tend to be located in the deep layer of the DCN. Type III units tend to respond to BF tones at moderate rates (median maximum driven rate of 124 spikes/s), have monotonic or nonmonotonic rate versus level functions, and respond to noise at rates comparable to their tone rates. They are distributed somewhat more uniformly in all DCN layers, but a majority were found in the fusiform cell and molecular layers. Most evidence indicates that type II units are recorded from inhibitory interneurons in the DCN.

Representation of whispered vowels in discharge patterns of auditory-nerve fibers.

We have previously shown that the spectra of speech sounds can be represented in the temporal patterns of discharge of populations of auditory-nerve fibers. These results were obtained using perfectly periodic stimuli, for which a temporal representation is straightforward. In order to see if our results could be generalized to nonperiodic stimuli, we have studied responses to a whispered vowel with formant frequencies typical of /epsilon/. The whispered vowel was generated by exciting a vocal tract model with noise; this signal was therefore aperiodic. Temporal patterns of responses to the vowel in populations of auditory-nerve fibers were analyzed using interval histograms. Fourier transforms of these histograms show large components at frequencies near the formant frequencies of the vowel. With these Fourier transform components as a measure of the temporal response, a temporal place representation of the response of populations of fibers preserves the spectral features of the aperiodic vowel stimulus. Profiles of average rate versus characteristic frequency for fibers with spontaneous rates greater than 1/s show little if any formant-related structure. On the other hand, such profiles for fibers with spontaneous rates less than 1/s may show peaks in the region of the formants.

Pinna-based spectral cues for sound localization in cat.

The directional dependence of the transfer function from free field plane waves to a point near the tympanic membrane (TM) was measured in anesthetized domestic cats. A probe tube microphone was placed approximately 3 mm from the TM from beneath the head in order to keep the pinna intact. Transfer functions were computed as the ratio of the spectrum of a click recorded near the TM to the spectrum of the click in freefield. We analyze the transfer functions in three frequency ranges: low frequencies (less than 5 kHz) where interaural level differences vary smoothly with azimuth; midfrequencies (5-18 kHz) where a prominent spectral notch is observed; and high frequencies (greater than 18 kHz) where the transfer functions vary greatly with source location. Because no two source directions produce the same transfer function, the spectrum of a broadband sound at the TM could serve as a sound localization cue for both elevation and azimuth. In particular, we show that source direction is uniquely determined, for source directions in front of the cat, from the frequencies of the midfrequency spectral notches in the two ears. The validity of the transfer functions as measures of the acoustic input to the auditory system is considered in terms of models of sound propagation in the ear canal.

Frequency-shaped amplification changes the neural representation of speech with noise-induced hearing loss.

Temporal response patterns of single auditory nerve fibers were used to characterize the effects of a common hearing-aid processing scheme, frequency-shaped amplification, on the encoding of the vowel /epsilon/ in cats with a permanent noise-induced hearing loss. These responses were contrasted with responses to unmodified stimuli in control and impaired cats. Noise-induced hearing loss leads to a degraded representation of the formant frequencies, in which strong phase locking to the formants is not observed in fibers with best frequencies (BFs) near the formants and there is a wide spread of formant phase locking to fibers with higher BFs (Miller et al., 1997a,b). Frequency shaping effectively limits the upward spread of locking to F1, which improves the representation of higher frequency components of the vowel. However, it also increases phase locking to harmonics in the trough between the formants, which decreases the contrast between F1 and the trough in the neural representation. Moreover, it does not prevent the spread to higher BFs of responses to the second and third formants. The results show a beneficial effect of frequency shaping, but also show that interactions between particular gain functions and particular spectral shapes can result in unwanted distortions of the neural representation of the signal.

Effects of high sound levels on responses to the vowel "eh" in cat auditory nerve.

The vowel "eh" was used to study auditory-nerve responses at high sound levels (60-110 dB). By changing the playback sampling rate of the stimulus, the second formant (F2) frequency was set at best frequency (BF) for fibers with BFs between 1 and 3 kHz. For vowel stimuli, auditory-nerve fibers tend to phase-lock to the formant component nearest the fiber's BF. The responses of fibers with BFs near F2 are captured by the F2 component, meaning that fibers respond as if the stimulus consisted only of the F2 component. These narrowband responses are seen up to levels of 80-100 dB, above which a response to F1 emerges. The F1 response grows, at the expense of the F2 response, and is dominant at the highest levels. The level at which the F1 response appears is BF dependent and is higher at lower BFs. This effect appears to be suppression of the F2 response by F1. At levels near 100 dB, a component 1/component 2 transition is observed. All components of the vowel undergo the transition simultaneously, as judged by the 180 degrees phase inversion that occurs at the C2 transition. Above the C2 threshold, a broadband response to many components of the vowel is observed. These results demonstrate that the neural representation of speech in normal ears is degraded at high sound levels, such as those used in hearing aids.

Maximally fault tolerant neural networks.

An application of neural network modeling is described for generating hypotheses about the relationships between response properties of neurons and information processing in the auditory system. The goal is to study response properties that are useful for extracting sound localization information from directionally selective spectral filtering provided by the pinna. For studying sound localization based on spectral cues provided by the pinna, a feedforward neural network model with a guaranteed level of fault tolerance is introduced. Fault tolerance and uniform fault tolerance in a neural network are formally defined and a method is described to ensure that the estimated network exhibits fault tolerance. The problem of estimating weights for such a network is formulated as a large-scale nonlinear optimization problem. Numerical experiments indicate that solutions with uniform fault tolerance exist for the pattern recognition problem considered. Solutions derived by introducing fault tolerance constraints have better generalization properties than solutions obtained via unconstrained back-propagation.

Responses of squirrel monkey vestibular neurons to audio-frequency sound and head vibration.

A study was made of the response of peripheral vestibular neurons in the squirrel monkey to head vibration and air-borne sound in the frequency range from 50-4 00 Hz. Responses were measured in terms of the phase locking of discharge and changes in firing rate. The lowest phase-locking thresholds for vibration were -70 to -80 dB re 1 g, and median values in the most sensitive frequency range (200-400 Hz) were -20 to -40 dB re 1 g; the minimum and median thresholds for sound were 76 and 120-130 dB SPL, respectively. Rate-change thresholds were 10-30 dB above phase-locking thresholds. The squirrel monkey sacculus has no special sensitivity to vibration in comparison with the other vestibular end-organs; the median phase-locking threshold to sound of saccular neurons exceeded 100 dB SPL. Irregularly discharging neurons are more sensitive than regularly discharging units. Evidence is presented that the response to intense sound involves a hair-cell mechanism.

Why do cats need a dorsal cochlear nucleus?

The dorsal cochlear nucleus (DCN), one of the three major divisions of the cochlear nucleus (CN), has a complex internal structure, multiple inputs (some of them non-auditory), and multiple output pathways. Response properties of DCN units are accordingly complex. The principal cells of the DCN have type IV response characteristics, characterized by relatively high levels of spontaneous activity and inhibition by high level best frequency (BF) tones. We showed previously that type IV units are inhibited by two separate inhibitory mechanisms, one of them sensitive to narrow band stimuli and the other to wide band stimuli. One result of the wide band inhibition of type IV units is their sensitivity to spectral notches in the region of their BF - stimuli with such notches inhibit type IV units. The source of the narrow band inhibition is an interneuron in the DCN which has type II response characteristics - it does not have spontaneous activity and is strongly activated by BF tones. The neurons giving rise to type II responses are presumably vertical cells, which also project to other divisions of the CN. From anatomical studies, it is known that type IV units are also inhibited by a third system, which carries non-auditory information; movements of the pinna inhibit type IV units through this system. We hypothesize that type IV units signal important events to the auditory system by being inhibited. Such events are either auditory, e.g. spectral maxima and minima, or non-auditory, such as the somatosensory inputs from the pinnae. We hypothesize that the projection of type II units to the ventral cochlear nucleus (VCN) plays a role in reducing the effects of spectral notches introduced by the pinnae in the core auditory pathway. We conclude that although the DCN lies close to the auditory periphery, it already performs sophisticated tasks of auditory processing.

Multiple receptor tyrosine kinases promote the in vitro phenotype of metastatic human osteosarcoma cell lines.

The survival rate for osteosarcoma patients with localized disease is 70% and only 25% for patients with metastases. Therefore, novel therapeutic and prognostic tools are needed. In this study, extensive screening and validation strategies identified Axl, EphB2, FGFR2, IGF-1R and Ret as specific receptor tyrosine kinases (RTKs) that are activated and promote the in vitro phenotype of two genetically different metastatic osteosarcoma cell lines. Initial phosphoproteomic screening identified twelve RTKs that were phosphorylated in 143B and/or LM7 metastatic human osteosarcoma cells. A small interfering RNA (siRNA) screen demonstrated that siRNA pools targeting ten of the twelve RTKS inhibited the in vitro phenotype of one or both cell lines. To validate the results, we individually tested the four siRNA duplexes that comprised each of the effective siRNA pools from the initial screen. The pattern of phenotype inhibition replicated the pattern of mRNA knockdown by the individual duplexes for seven of the ten RTKs, indicating the effects are consistent with on-target silencing. Five of those seven RTKs were further validated using independent approaches including neutralizing antibodies (IGF-1R), antisense-mediated knockdown (EphB2, FGFR2, and Ret) or small molecule inhibitors (Axl), indicating that those specific RTKs promote the in vitro behavior of metastatic osteosarcoma cell lines and are potential therapeutic targets for osteosarcoma. Immunohistochemistry demonstrated that Axl is frequently activated in osteosarcoma patient biopsy samples, further supporting our screening and validation methods to identify RTKs that may be valuable targets for novel therapies for osteosarcoma patients.