(Plate 3) Manganese nodules and deep-sea sediment core from Pacific Ocean

A synoptic review of the studies of well-known occurrences of palagonite tuffs is presented. Included are palagonite tuffs from Iceland, and pillow-lava palagonite complexes from Columbia River basalts and from the central Oregon coast. Additional petrologic and x-ray defraction data for selected samples are presented. Petrologic evidence shows that basaltic glass of aqueous tuffs and breccias consists of sideromelane, which is susceptible to palagonitization. It is shown that palagonitization is a selective alteration process, involving hydration, oxidation and zeolitization. Some of the manganese nodules dredged from the Pacific Ocean floor contain nucleus of palagonite-tuff breccias or of zeolite. A brief megascopic and microscopic description of nodules from the south Pacific, the Mendocino ridge and the 'Horizon' Nodule from the north Pacific is presented. Petrographic studies of palagonite-tuff breccias of manganese nodules and other palagonites suggest that migration and segregation of metallic elements occur during and subsequent to palagonitization. During the palagonitization of sideromelane, nearly 30 percent of sea water is absorbed. The hydration of sideromelane is also accompanied by oxidation of iron and other elements. These oxides may be released either in colloidal form or in true solution and tend to precipitate first from the unstable palagonite.

Chemical analysis of water samples of station M1_1207, Red Sea (Table 1-3)

Two water samples and two sediment samples taken in 1965 by the R. V. "Meteor" in the area of the hot salt brine of the Atlantis II-Deep were chemically investigated, and in addition the sediment samples were subjected to X-ray and optical analysis. The investigation of the sulfur-isotope-ratios showed the same values for all water samples. This information combined with the Ca-sulfate solubility data leads us to conclude that, for the most part, the sulfate content of the salt brine resulted from mixing along the boundary with the normal seawater. In this boundary area gypsum or anhydrite is formed which sinks down to the deeper layers of the salt brine where it is redisolved when the water becomes undersaturated. In the laboratory, formation of CaS04 precipitate resulted from both the reheating of the water sample from the uppermost zone of the salt brine to the in-situ-temperature as well as by the mixing of the water sample with normal Red Sea water. The iron and manganese delivered by the hot spring is separated within the area of the salt brine by their different redox-potentials. Iron is sedimented to a high amount within the salt brine, while, as evidenced by its small amounts in all sediment samples, the more easily reducible manganese is apparently carried out of the area before sedimentation can take place. The very good layering of the salt brine may be the result of the rough bottom topography with its several progressively higher levels allowing step-like enlargements of the surface areas of each successive layer. Each enlargement results in larger boundary areas along which more effective heat transfer and mixing with the next layer is possible. In the sediment samples up to 37.18% Fe is found, mostly bound as very poorly crystallized iron hydroxide. Pyrite is present in only very small amounts. We assume that the copper is bound mostly as sulfide, while the zinc is most likely present in an other form. The sulfur-isotope-investigations indicate that the sulfur in the sediment, bound as pyrite and sulfides, is not a result of bacterical sulfate-reduction in the iron-rich mud of the Atlantis II-Deep, but must have been brought up with the hot brine.

(Table 2) Major and trace elements in black smoker particulates

We present geochemical data of black smoker particulates filtered from hydrothermal fluids with seawater-dilutions ranging from 0-99%. Results indicate the dominance of sulphide minerals (Fe, Cu, and Zn sulphides) in all samples taken at different hydrothermal sites on the Mid-Atlantic Ridge. Pronounced differences in the geochemistry of the particles between Logatchev I and 5°S hydrothermal fields could be attributed to differences in fluid chemistry. Lower metal/sulphur ratios (Me/H2S < 1) compared to Logatchev I result in a larger amount of particles precipitated per liter fluid and the occurrence of elemental sulphur at 5°S, while at Logatchev I Fe oxides occur in larger amounts. Systematic trends with dilution degree of the fluid include the precipitation of large amounts of Cu sulphides at a low dilution and a pronounced drop with increasing dilution. Moreover, Fe (sulphides or oxides) precipitation increases with dilution of the vent fluid by seawater. Geochemical reaction path modeling of hydrothermal fluid-seawater mixing and conductive cooling indicates that Cu sulphide formation at Logatchev I and 5°S mainly occurs at high temperatures and low dilution of the hydrothermal fluid by seawater. Iron precipitation is enhanced at higher fluid dilution, and the different amounts of minerals forming at 5°S and Logatchev I are thermodynamically controlled. Larger total amounts of minerals and larger amounts of sulphide precipitate during the mixing path when compared to the cooling path. Differences between model and field observations do occur and are attributable to closed system modeling, to kinetic influences and possibly to organic constituents of the hydrothermal fluids not accounted for by the model.

(Table 1) Pore-water composition of ODP Leg 126 sites

The Ocean Drilling Program Leg 126 sites may be classified into two categories depending on the presence (Group I: Sites 787, 792, and 793) or absence (Group II: Sites 788, 790, and 791) of steep concentration gradients. Shipboard X-ray diffraction analyses of bulk sediments from Group I sites revealed the presence of a number of diagenetic minerals (some of which are incompatible), but no systematic diagenetic zonation. The results of the chemical analyses of the pore waters from Group I have been used to estimate the activities of dissolved species. Thermodynamic analyses of the composition of the pore waters and the stability of authigenic minerals (gypsum, zeolites, feldspars, smectites, chlorites, and micas) show that the pore waters are close to equilibrium with most of the observed phases. Thus, only a small perturbation of the system (substitution in minerals and fluctuations in pore-water composition, in particular, in pH and SiO2 activity) will cause any of these phases to precipitate. Therefore, one would not expect mineralogical observations to show systematic vertical zonations at these sites. It is suggested that chlorites and high-temperature zeolites are not diagenetic sensu stricto, but were eroded from volcaniclastic highs. The absence of concentration gradients at the Group II sites has been analyzed in terms of reaction kinetics, hydrothermal advection, and temperature distribution. The absence of diagenetic imprints on the pore-water concentration profiles at these sites is probably caused by the slow nucleation of silica phases.

High-resolution pore-water and solid-phase analyses of sediment cores GeoB6520-3 (Hydrate Hole) and GeoB6521-2 (Worm Hole) of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.

Seawater carbonate chemistry and benthic foraminifera Ammonia sp. mass, size, and growth rate during experiments, 2013

About 30% of the anthropogenically released CO2 is taken up by the oceans; such uptake causes surface ocean pH to decrease and is commonly referred to as ocean acidification (OA). Foraminifera are one of the most abundant groups of marine calcifiers, estimated to precipitate ca. 50 % of biogenic calcium carbonate in the open oceans. We have compiled the state of the art literature on OA effects on foraminifera, because the majority of OA research on this group was published within the last three years. Disparate responses of this important group of marine calcifiers to OA were reported, highlighting the importance of a process-based understanding of OA effects on foraminifera. We cultured the benthic foraminifer Ammonia sp. under a range of carbonate chemistry manipulation treatments to identify the parameter of the carbonate system causing the observed effects. This parameter identification is the first step towards a process-based understanding. We argue that CO3 is the parameter affecting foraminiferal size-normalized weights (SNWs) and growth rates. Based on the presented data, we can confirm the strong potential of Ammonia sp. foraminiferal SNW as a CO3 proxy.

Seawater carbonate chemistry and Bathymodiolus brevior shell variables near Eifuku volcano, Japan, 2009

Increasing atmospheric carbon dioxide levels are causing ocean acidification, compromising the ability of some marine organisms to build and maintain support structures as the equilibrium state of inorganic carbon moves away from calcium carbonate. Few marine organisms tolerate conditions where ocean pH falls significantly below today's value of about 8.1 and aragonite and calcite saturation values below 1. Here we report dense clusters of the vent mussel B. brevior in natural conditions of pH values between 5.36 and 7.29 on northwest Eifuku volcano, Mariana arc, where liquid carbon dioxide and hydrogen sulphide emerge in a hydrothermal setting. We find that both shell thickness and daily growth increments in shells from northwest Eifuku are only about half those recorded from mussels living in water with pH>7.8. Low pH may therefore also be implicated in metabolic impairment. We identify four-decade-old mussels, but suggest that the mussels can survive for so long only if their protective shell covering remains intact: crabs that could expose the underlying calcium carbonate to dissolution are absent from this setting. The mussels' ability to precipitate shells in such low-pH conditions is remarkable. Nevertheless, the vulnerability of molluscs to predators is likely to increase in a future ocean with low pH.

(Table 3, page 309) Mercury content of concretions from Grand Lake (GL) and Ship Harbour Lake (SH), Nova Scotia and Mosque Lake (ML), Ontario

Ferromanganese concretions from Grand Lake and Ship Harbour Lake in Nova Scotia and Mosque Lake in Ontario are most common in water 0.5 to 2 m deep. X-ray diffraction studies show the ferromanganese portions of the concretions to he amorphous. Petrographic and electron probe studies of the ferromanganese material reveal chemical banding of iron and manganese. Bulk chemical analyses indicate that the Fe:Mn ratios of concretions from different sites within a single lake are similar, whereas concretions from different lakes have characteristic Fe:Mn ratios. Trace element concs are different in different lakes and are generally several orders of magnitude less than those of oceanic nodules.

(Table 2) Carbonate chemistry and isotopes from northern Cascadia margin samples

Most authigenic carbonates previously recovered from the Cascadia slope have 87Sr/86Sr signatures that reflect shallow precipitation in equilibrium with coeval seawater. There is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming Juan de Fuca plate (87Sr/86Sr = 0.7071; Teichert et al., 2005, doi:10.1016/j.epsl.2005.08.002) or with terrigenous turbidites (87Sr/86Sr = 0.70975 to 0.71279; Sample et al., 1993, doi:10.1130/0091-7613(1993)021<0507:CCICFF>2.3.CO;2). We report on the strontium isotopic composition of carbonates and fluids from IODP Site U1329 and nearby Barkley Canyon (offshore Vancouver Island), which have strontium isotope ratios as low as 0.70539. Whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted Coast Range forearc indicate formation in ambient bottom seawater, several samples from the Pysht/Sooke Fm. show a 87Sr-depleted signal (87Sr/86Sr = 0.70494 and 0.70511) similar to that of the anomalous Site U1329 and Barkley Canyon carbonates. Our data, when analyzed in the context of published elemental and isotopic composition of these carbonates (Joseph et al., 2012, doi:10.1016/j.palaeo.2013.01.012 ), point to two formation mechanisms: 1) shallow precipitation driven by the anaerobic oxidation of methane (AOM) with d13C values as low as -50 per mil and contemporaneous 87Sr/86Sr seawater ratios, and 2) carbonate precipitation driven by fluids that have circulated through the oceanic crust, which are depleted in 87Sr. Carbonates formed from the second mechanism precipitate both at depth and at sites of deep-sourced fluid seepage on the seafloor. The 87Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration of a deep, exotic fluid similar to that found in high permeability conglomerate layers at 188 mbsf of Site U1329, and which may have fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which supplies fluids with high calcium, manganese and strontium enriched in the non-radiogenic nucleide. Tectonic compression and dehydration reactions then force these fluids updip, where they pick up the thermogenic hydrocarbons and 13C-enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at Barkley Canyon and at Site U1329. The Crescent Terrane may have sourced cold seeps in this margin since at least the late Oligocene.

Seawater carbonate chemistry and elemental composition of purple sea urchin (Strongylocentrotus purpuratus) calcite in a laboratory experiment

Ocean acidification will likely have negative impacts on invertebrates producing skeletons composed of calcium carbonate. Skeletal solubility is partly controlled by the incorporation of "foreign" ions (e.g. magnesium) into the crystal lattice of these skeletal structures, a process that is sensitive to a variety of biological and environmental factors. Here we explore effects of life stage, oceanographic region of origin, and changes in the partial pressure of carbon dioxide in seawater (pCO2) on trace elemental composition in the purple sea urchin (Strongylocentrotus purpuratus). We show that, similar to other urchin taxa, adult purple sea urchins have the ability to precipitate skeleton composed of a range of biominerals spanning low- to high-Mg calcites. Mg / Ca and Sr / Ca ratios were substantially lower in adult spines compared to adult tests. On the other hand, trace elemental composition was invariant among adults collected from four oceanographically distinct regions spanning a range of carbonate chemistry conditions (Oregon, Northern California, Central California, and Southern California). Skeletons of newly settled juvenile urchins that originated from adults from the four regions exhibited intermediate Mg / Ca and Sr / Ca between adult spine and test endmembers, indicating that skeleton precipitated during early life stages is more soluble than adult spines and less soluble than adult tests. Mean skeletal Mg / Ca or Sr / Ca of juvenile skeleton did not vary with source region when larvae were reared under present-day, global-average seawater carbonate conditions (400 µatm; pHT = 8.02 ± 0.03 1 SD; Omega calcite = 3.3 ± 0.2 1 SD). However, when reared under elevated pCO2 (900 µatm; pHT = 7.73 ± 0.03; Omega calcite = 1.8 ± 0.1), skeletal Sr / Ca in juveniles exhibited increased variance across the four regions. Although larvae from the northern populations (Oregon, Northern California, Central California) did not exhibit differences in Mg or Sr incorporation under elevated pCO2 (Sr / Ca = 2.10 ± 0.06 mmol/mol; Mg / Ca = 67.4 ± 3.9 mmol/mol), juveniles of Southern California origin partitioned ~8% more Sr into their skeletons when exposed to higher pCO2 (Sr / Ca = 2.26 ± 0.08 vs. 2.09 ± 0.005 mmol/mol 1 SD). Together these results suggest that the diversity of carbonate minerologies present across different skeletal structures and life stages in purple sea urchins does not translate into an equivalent geochemical plasticity of response associated with geographic variation or temporal shifts in seawater properties. Rather, composition of S. purpuratus skeleton precipitated during both early and adult life history stages appears relatively robust to spatial gradients and predicted future changes in carbonate chemistry. An exception to this trend may arise during early life stages, where certain populations of purple sea urchins may alter skeletal mineral precipitation rates and composition beyond a given pCO2 threshold. This potential for geochemical plasticity during early development in contrast to adult stage geochemical resilience adds to the growing body of evidence that ocean acidification can have differing effects across organismal life stages.

Understanding arsenic mobility and speciation in lake sediments impacted by ore roasting near Giant mine, NWT

The roasting of gold-bearing arsenopyrite at Giant mine (Northwest Territories) between 1949 and 1999 released approximately 20,000 tonnes of toxic arsenic-bearing aerosols in the local aerial environment. Detailed examination of lake sediments, sediment porewaters, surface waters and lake hydrology sampled from three lakes of differing limnological characteristics was conducted in summer and winter conditions. Samples were analyzed for solid and dissolved elemental concentrations, speciation and mineralogy. The three lakes are located less than 5km from the mine roaster, and downwind, based on predominant wind direction. The objective of the study was to assess the controls on the mobility and fate of arsenic in these roaster-impacted subarctic lacustrine environments. Results show that the occurrence of arsenic trioxide in lake sediments coincides with the regional onset of industrial activities. The bulk of arsenic in sediments is contained in the form of secondary sulphide precipitates, with iron oxides hosting a minimal amount of arsenic near the surface-water interface. The presence of geogenic arsenic is likely contained as dilute impurities in common rock-forming minerals, and is not believed to be a significant source of arsenic to sediments, porewaters or lake waters. Furthermore, the well correlated depth-profiles of arsenic, antimony and gold in sediments may help reveal roaster impact. The soluble arsenic trioxide particles contained in sediments act as the primary source of arsenic into porewaters. Dissolved arsenic in reducing porewaters both precipitate as secondary sulphides in situ, and diffuse upwards into the overlying lake waters. Arsenic diffusion out of porewaters, combined with watercourse-driven residence time, are estimated to be the predominant mechanisms controlling arsenic concentrations in overlying lake waters. The sequestration of arsenic from porewaters as sulphide precipitates, in the study lakes, is not an effective process in keeping lake-water arsenic concentrations below guidelines for the protection of the freshwater environment and drinking water. Seasonal impacts on lake geochemistry derive from ice covering lake waters, cutting them off from of atmospheric oxygen, along with the exclusion of solutes from the ice. Such effects are limited in deep lakes but are can be an important factor controlling arsenic precipitation and mobility in ponds.

Shift from coral to macroalgae dominance on a volcanically acidified reef

Rising anthropogenic CO2 in the atmosphere is accompanied by an increase in oceanic CO2 and a concomitant decline in seawater pH (ref. 1). This phenomenon, known as ocean acidification (OA), has been experimentally shown to impact the biology and ecology of numerous animals and plants2, most notably those that precipitate calcium carbonate skeletons, such as reef-building corals3. Volcanically acidified water at Maug, Commonwealth of the Northern Mariana Islands (CNMI) is equivalent to near-future predictions for what coral reef ecosystems will experience worldwide due to OA. We provide the first chemical and ecological assessment of this unique site and show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state4, 5. This study provides field evidence that acidification can lead to macroalgae dominance on reefs.

Séquestration géologique du CO2 par carbonatation minérale dans les résidus miniers

La carbonatation minérale dans les résidus miniers est un moyen sûr et permanent de séquestrer le CO2 atmosphérique. C’est un processus naturel et passif qui ne nécessite aucun traitement particulier et donc avantageux d’un point de vue économique. Bien que la quantité de CO2 qu’il soit possible de séquestrer selon ce processus est faible à l’échelle globale, dans le cadre d’un marché du carbone, les entreprises minières pourraient obtenir des crédits et ainsi revaloriser leurs résidus. À l’heure actuelle, il y a peu d’informations pour quantifier le potentiel de séquestration du CO2 de façon naturelle et passive dans les piles de résidus miniers. Il est donc nécessaire d’étudier le phénomène pour comprendre comment évolue la réaction à travers le temps et estimer la quantité de CO2 qui peut être séquestrée naturellement dans les piles de résidus. Plusieurs travaux de recherche se sont intéressés aux résidus miniers de Thetford Mines (Québec, Canada), avec une approche principalement expérimentale en laboratoire. Ces travaux ont permis d’améliorer la compréhension du processus de carbonatation, mais ils nécessitent une validation à plus grande échelle sous des conditions atmosphériques réelles. L’objectif général de cette étude est de quantifier le processus de carbonatation minérale des résidus miniers sous des conditions naturelles, afin d’estimer la quantité de CO2 pouvant être piégée par ce processus. La méthodologie utilisée repose sur la construction de deux parcelles expérimentales de résidus miniers situées dans l’enceinte de la mine Black Lake (Thetford Mines). Les résidus miniers sont principalement constitués de grains et de fibres de chrysotile et lizardite mal triés, avec de petites quantités d’antigorite, de brucite et de magnétite. Des observations spatiales et temporelles ont été effectuées dans les parcelles concernant la composition et la pression des gaz, la température des résidus, la teneur en eau volumique, la composition minérale des résidus ainsi que la chimie de l’eau des précipitations et des lixiviats provenant des parcelles. Ces travaux ont permis d’observer un appauvrissement notable du CO2 dans les gaz des parcelles (< 50 ppm) ainsi que la précipitation d’hydromagnésite dans les résidus, ce qui suggère que la carbonatation minérale naturelle et passive est un processus potentiellement important dans les résidus miniers. Après 4 ans d’observations, le taux de séquestration du CO2 dans les parcelles expérimentales a été estimé entre 3,5 et 4 kg/m3/an. Ces observations ont permis de développer un modèle conceptuel de la carbonatation minérale naturelle et passive dans les parcelles expérimentales. Dans ce modèle conceptuel, le CO2 atmosphérique (~ 400 ppm) se dissout dans l’eau hygroscopique contenue dans les parcelles, où l’altération des silicates de magnésium forme des carbonates de magnésium. La saturation en eau dans les cellules est relativement stable dans le temps et varie entre 0,4 et 0,65, ce qui est plus élevé que les valeurs de saturation optimales proposées dans la littérature, réduisant ainsi le transport de CO2 dans la zone non saturée. Les concentrations de CO2 en phase gazeuse, ainsi que des mesures de la vitesse d’écoulement du gaz dans les cellules suggèrent que la réaction est plus active près de la surface et que la diffusion du CO2 est le mécanisme de transport dominant dans les résidus. Un modèle numérique a été utilisé pour simuler ces processus couplés et valider le modèle conceptuel avec les observations de terrain. Le modèle de transport réactif multiphase et multicomposant MIN3P a été utilisé pour réaliser des simulations en 1D qui comprennent l’infiltration d’eau à travers le milieu partiellement saturé, la diffusion du gaz, et le transport de masse réactif par advection et dispersion. Même si les écoulements et le contenu du lixivat simulés sont assez proches des observations de terrain, le taux de séquestration simulé est 22 fois plus faible que celui mesuré. Dans les simulations, les carbonates précipitent principalement dans la partie supérieure de la parcelle, près de la surface, alors qu’ils ont été observés dans toute la parcelle. Cette différence importante pourrait être expliquée par un apport insuffisant de CO2 dans la parcelle, qui serait le facteur limitant la carbonatation. En effet, l’advection des gaz n’a pas été considérée dans les simulations et seule la diffusion moléculaire a été simulée. En effet, la mobilité des gaz engendrée par les fluctuations de pression barométrique et l’infiltration de l’eau, ainsi que l’effet du vent doivent jouer un rôle conséquent pour alimenter les parcelles en CO2.

Soluções irrigadoras em Endodontia

O principal objetivo do tratamento endodôntico não cirúrgico reside na limpeza e desinfeção do sistema tridimensional de canais radiculares, removendo os microrganismos existentes e conseguindo restituir a função do dente, em vez de o extrair. É fácil compreender que o insucesso deste tratamento deve-se, essencialmente, à sobrevivência dos microrganismos nos canais radiculares. Por isso, a irrigação e a desinfeção são essenciais para alcançar o sucesso do tratamento. Devido à morfologia do canal e à incapacidade de determinar a localização exata do ápice, as soluções irrigadoras têm de alcançar as ramificações dos canais radiculares e outras áreas inacessíveis à instrumentação. Após a pesquisa efetuada, concluiu-se que o irrigante mais utilizado universalmente é o hipoclorito de sódio. Para além disso, o hipoclorito de sódio, o EDTA e o ácido cítrico ajudam na instrumentação e no alargamento do canal, devido à desmineralização dentinária que provocam. Já a clorexidina, apesar de não provocar qualquer desmineralização, ao ser associada ao hipoclorito de sódio, origina um precipitado que vai interferir no selamento dos canais radiculares. Assim, com o presente trabalho, pretende-se realizar uma revisão bibliográfica sobre os diversos irrigantes e sistemas auxiliares de irrigação, que se encontram associados à desinfeção endodôntica.

The Role of Cannabinoids in the Neurobiology of Sensory Gating: A firing rate model study

Gating of sensory (e.g. auditory) information has been demonstrated as a reduction in the auditory-evoked potential responses recorded in the brain of both normal animals and human subjects. Auditory gating is perturbed in schizophrenic patients and pharmacologically by drugs such as amphetamine, phencyclidine or ketamine, which precipitate schizophrenic-like symptoms in normal subjects. The neurobiological basis underlying this sensory gating can be investigated using local field potential recordings from single electrodes. In this paper we use such technology to investigate the role of cannabinoids in sensory gating. Cannabinoids represent a fundamentally new class of retrograde messengers which are released postsynaptically and bind to presynaptic receptors. In this way they allow fine-tuning of neuronal response, and in particular can lead to so-called depolarization-induced suppression of inhibition (DSI). Our experimental results show that application of the exogenous cannabinoid WIN55, 212-2 can abolish sensory gating as measured by the amplitude of local field responses in rat hippocampal region CA3. Importantly we develop a simple firing rate population model of CA3 and show that gating is heavily dependent upon the presence of a slow inhibitory (GABAB) pathway. Moreover, a simple phenomenological model of cannabinoid dynamics underlying DSI is shown to abolish gating in a manner consistent with our experimental findings.