Optogenetic Release of ACh Induces Rhythmic Bursts of Perisomatic IPSCs in Hippocampus

Acetylcholine (ACh) influences a vast array of phenomena in cortical systems. It alters many ionic conductances and neuronal firing behavior, often by regulating membrane potential oscillations in populations of cells. Synaptic inhibition has crucial roles in many forms of oscillation, and cholinergic mechanisms regulate both oscillations and synaptic inhibition. In vitro investigations using bath-application of cholinergic receptor agonists, or bulk tissue electrical stimulation to release endogenous ACh, have led to insights into cholinergic function, but questions remain because of the relative lack of selectivity of these forms of stimulation. To investigate the effects of selective release of ACh on interneurons and oscillations, we used an optogenetic approach in which the light-sensitive non-selective cation channel, Channelrhodopsin2 (ChR2), was virally delivered to cholinergic projection neurons in the medial septum/diagonal band of Broca (MS/DBB) of adult mice expressing Cre-recombinase under the control of the choline-acetyltransferase (ChAT) promoter. Acute hippocampal slices obtained from these animals weeks later revealed ChR2 expression in cholinergic axons. Brief trains of blue light pulses delivered to untreated slices initiated bursts of ACh-evoked, inhibitory post-synaptic currents (L-IPSCs) in CA1 pyramidal cells that lasted for 10's of seconds after the light stimulation ceased. L-IPSC occurred more reliably in slices treated with eserine and a very low concentration of 4-AP, which were therefore used in most experiments. The rhythmic, L-IPSCs were driven primarily by muscarinic ACh receptors (mAChRs), and could be suppressed by endocannabinoid release from pyramidal cells. Finally, low-frequency oscillations (LFOs) of local field potentials (LFPs) were significantly cross-correlated with the L-IPSCs, and reversal of the LFPs near s. pyramidale confirmed that the LFPs were driven by perisomatic inhibition. This optogenetic approach may be a useful complementary technique in future investigations of endogenous ACh effects

Plutonic foundation of a slow-spreading ridge segment : oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q05014, doi:10.1029/2007GC001645.We mapped the Kane megamullion, an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of a ∼50 km long, second-order ridge segment. The complex was exhumed by long-lived slip on a normal-sense detachment fault at the base of the rift valley wall from ∼3.3 to 2.1 Ma (Williams, 2007). Mantle peridotites, gabbros, and diabase dikes are exposed in the detachment footwall and in outward facing high-angle normal fault scarps and slide-scar headwalls that cut through the detachment. These rocks directly constrain crustal architecture and the pattern of melt flow from the mantle to and within the lower crust. In addition, the volcanic carapace that originally overlay the complex is preserved intact on the conjugate African plate, so the complete internal and external architecture of the paleoridge segment can be studied. Seafloor spreading during formation of the core complex was highly asymmetric, and crustal accretion occurred largely in the footwall of the detachment fault exposing the core complex. Because additions to the footwall, both magmatic and amagmatic, are nonconservative, oceanic detachment faults are plutonic growth faults. A local volcano and fissure eruptions partially cover the northwestern quarter of the complex. This volcanism is associated with outward facing normal faults and possible, intersecting transform-parallel faults that formed during exhumation of the megamullion, suggesting the volcanics erupted off-axis. We find a zone of late-stage vertical melt transport through the mantle to the crust in the southern part of the segment marked by a ∼10 km wide zone of dunites that likely fed a large gabbro and troctolite intrusion intercalated with dikes. This zone correlates with the midpoint of a lineated axial volcanic high of the same age on the conjugate African plate. In the central region of the segment, however, primitive gabbro is rare, massive depleted peridotite tectonites abundant, and dunites nearly absent, which indicate that little melt crossed the crust-mantle boundary there. Greenschist facies diabase and pillow basalt hanging wall debris are scattered over the detachment surface. The diabase indicates lateral melt transport in dikes that fed the volcanic carapace away from the magmatic centers. At the northern edge of the complex (southern wall of the Kane transform) is a second magmatic center marked by olivine gabbro and minor troctolite intruded into mantle peridotite tectonite. This center varied substantially in size with time, consistent with waxing and waning volcanism near the transform as is also inferred from volcanic abyssal-hill relief on the conjugate African plate. Our results indicate that melt flow from the mantle focuses to local magmatic centers and creates plutonic complexes within the ridge segment whose position varies in space and time rather than fixed at a single central point. Distal to and between these complexes there may not be continuous gabbroic crust, but only a thin carapace of pillow lavas overlying dike complexes laterally fed from the magmatic centers. This is consistent with plate-driven flow that engenders local, stochastically distributed transient instabilities at depth in the partially molten mantle that fed the magmatic centers. Fixed boundaries, such as large-offset fracture zones, or relatively short segment lengths, however, may help to focus episodes of repeated melt extraction in the same location. While no previous model for ocean crust is like that inferred here, our observations do not invalidate them but rather extend the known diversity of ridge architecture.NSF Grants OCE-0118445, OCE-0624408 and OCE-0621660 supported this research. B. Tucholke was also supported by the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite at 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution

L- IPSCs are triggered primarily by mAChRs and reduced by endocannabinoids.

<p>A) Repeated light trains (5 Hz, 10 s, 5-ms pulses) elicited IPSC bursts (A1) that were nearly abolished by atropine (A2). A3) Group data of IPSCs in basal (Pre) conditions, and after light stimulation (Post). Values of amplitudes and frequency for all IPSCs (left and middle graphs), and frequency of large IPSCs (i.e., two s.d.s > the mean basal IPSC; right graph). B1) In atropine, light trains (10 Hz, 0.5 s) elicited small IPSC bursts (5 traces, mean in red) that were reduced by the non-selective nAChR antagonist, mecamylamine, 10 µM (B2, 2 traces, different cell than A1, A2). C1) L-IPSCs recorded in a CA1 pyramidal cell elicited by a 5 Hz, 10-s train of light pulses. DSI was induced by a 3-s voltage step (to +20 mV). C2) Control trials show IPSC bursts in the absence of DSI. C3, C4) DSI of L-IPSCs was blocked by the CB1R antagonist, AM251, 5 µM (different cell than C1, C2). C5) Group data (n = 4) of DSI in control conditions and after bath application of AM251; ^*  =  p<0.05; ^**  =  p<0.01.</p

ChR2- mCherry expression in cholinergic MS/DBB projection neurons and light-induced IPSCs in CA1 pyramidal cells.

<p>A) Tissue section through the MS/DBB showing ChR2-mCherry, immunostaining for ChAT, and the merged image. B) Section of the hippocampus showing ChR2-mCherry expression in axons. sr  =  <i>s. radiatum</i>; sp  =  <i>s. pyramidale</i>; so  =  <i>s. oriens</i>. C) Schematic drawing of the recording arrangement. D1) Continuous recording from a CA1 pyramidal cell in a ChR2-expressing slice. Repeated 10-s trains of 5-ms blue light flashes repeated at 2-min intervals (squares) elicited bursts of L-IPSCs (downward deflections). A single light pulse (downward triangle) had no obvious effect. The small upward deflections (truncated in the illustration) are capacitive transients produced by conductance pulses given to the cell. D2) Two trials (1 and 3 in D1) in the absence of iGluR antagonists were aligned at the time of the light train and overlapped. D3) Two trials (4 and 5 in D1) in the presence of iGluR antagonists, 5 µM NBQX plus 5 µM CGP37849, were aligned and overlapped. An expanded portion of the trace (below) reveals the occurrence of rhythmic L-IPSCs after the end of the light train. (The traces in D2 and D3 are shown at a larger amplitude scale than in D1, and the largest IPSCs are cut off.) The autocorrelation function (D4, left) and power spectrum (D4, right) illustrate the regularity (peak frequency ∼3 Hz) of L-IPSC activity from this cell.</p

ACh-release-induced rhythmic LFPs are well correlated with L- IPSCs.

<p>A1) Extracellular recording in <i>s. pyramidale.</i> Light trains (5 Hz, 10 s) elicited bursts of LFPs. A2) Group data showing increase in total LFP power obtained by integrating the spectral analysis over the range of 2–12 Hz, before and after atropine application (n = 5, *p<0.05). B) As in (A) except that two extracellular electrodes were placed ∼200 µm apart. The plot shows that the two L-LFPs were temporally cross-correlated. C) Simultaneous recordings of L-IPSCs and LFPs recorded ∼200 µm away. C1) Cross-correlation plot indicates L-IPSC peak occurs very near the LFP peak. C2) Sample traces of simultaneous L-IPSC, L-LFP recordings. D) Sink-source analysis of L-LFPs. D1) Dots indicate recording locations. D2) Two extracellular electrodes recorded L-LFPs at locations 1 – 5; traces show LFP cross-correlations versus the simultaneous LFP recorded in <i>s. pyramidale</i> (4). The records suggest an LFP current source near 4.</p

L- IPSCs oscillate rhythmically at low frequency in eserine and 4-AP.

<p>Results from a typical pyramidal cell recorded in the presence of the cholinesterase inhibitor, eserine, 1 µM, and a low concentration of the voltage-gated K⁺ channel blocker 4-AP, 20 µM, in the extracellular solution. A train (5-ms light pulses, 5 Hz, 5 s) was delivered during the horizontal blue line. A) After a delay of several seconds a burst of large L-IPSCs began and persisted for >1 min. The IPSCs occurred rhythmically, with a peak frequency of ∼3 Hz (power spectrum in B1, autocorrelogram in B2). Eserine and 4-AP were present in all subsequent experiments.</p

Pervasive silicification and hanging wall overplating along the 13°20'N oceanic detachment fault (Mid-Atlantic Ridge)

International audienceThe corrugated detachment fault zone of the active 13820 0 N oceanic core complex (Mid-Atlantic Ridge) was investigated with a deep-sea vehicle to assess the links between deformation, alteration, and magmatism at detachment fault zones. We present a study of 18 in situ fault rock samples from striated fault outcrops on the flanks of microbathymetric corrugations. All the samples are mafic breccias that are mostly derived from a diabase protolith, with two of them also showing mixing with ultramafic clasts. Brec-cias are cataclastic and display variable deformation textures, recording numerous slip events, and showing pervasive silicification throughout the fault zone. Deformation-silicification relationships are also complex, showing both static and syntectonic quartz precipitation; undeformed quartz overprints the fault breccia textures, and reflective and striated fault surfaces cross-cut silicified rocks. In situ detachment fault rocks are mainly fault breccias with almost exclusively basaltic clasts, with rare ultramafic ones, a lithology and texture never observed previously at other oceanic detachment fault zones. We propose the lower dyke complex in the hanging wall crust at the volcanic rift valley floor is the most plausible diabase source. Mechanical mixing of predominantly mafic and rare ultramafic clasts suggests an underlying ultramafic footwall and that mafic accretion operates in the shallowest crust (1-2 km), at the base of the dyke complex at temperatures >4008C. Silicification is produced by silica-rich fluids syntectonically channeled along the fault zone, and likely derived from hydrothermal alteration of basaltic rocks, likely mixed with serpentinization-derived fluids. Plain Language Summary This paper presents a textural, mineralogical, and microstructural study of the fault rocks recovered in situ on the 13820 0 N detachment fault zone (Mid-Atlantic ridge) during the ODEMAR cruise in 2013. This detachment is unique for the presence of mafic material integrated within the fault zone as breccias and the pervasive silicification observed throughout all the detachment surface. Our paper discusses the origin of the mafic breccias and proposes that they were captured from the base of the dyke complex within the hanging wall during the fault exhumation. Our study reveals furthermore that quartz mineralization occurred in depth during the exhumation and is likely linked with the presence of mafic material within the fault zone. Our study indicates a complex relationship between silicification and deformation during which quartz (re)crystallized under quasi-static conditions between periods of deformation. This work also demonstrates that extreme strain localization can be achieved in the absence of weak alteration phases (talc, serpentine) but with instead high-friction material (quartz), suggesting that elevated pore fluid pressures play an important role