In Vivo Neocortical [K]o Modulation by Targeted Stimulation of Astrocytes

A normally functioning nervous system requires normal extracellular potassium ion concentration ([K]o). Throughout the nervous system, several processes, including those of an astrocytic nature, are involved in [K]o regulation. In this study we investigated the effect of astrocytic photostimulation on [K]o. We hypothesized that in vivo photostimulation of eNpHR-expressing astrocytes leads to a decreased [K]o. Using optogenetic and electrophysiological techniques we showed that stimulation of eNpHR-expressing astrocytes resulted in a significantly decreased resting [K]o and evoked K responses. The amplitude of the concomitant spreading depolarization-like events also decreased. Our results imply that astrocytic membrane potential modification could be a potential tool for adjusting the [K]o

A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

<div><p>Advances in brain connectomics set the need for detailed knowledge of functional properties of myelinated and non-myelinated (if present) axons in specific white matter pathways. The corpus callosum (CC), a major white matter structure interconnecting brain hemispheres, is extensively used for studying CNS axonal function. Unlike another widely used CNS white matter preparation, the optic nerve where all axons are myelinated, the CC contains also a large population of non-myelinated axons, making it particularly useful for studying both types of axons. Electrophysiological studies of optic nerve use suction electrodes on nerve ends to stimulate and record compound action potentials (CAPs) that adequately represent its axonal population, whereas CC studies use microelectrodes (MEs), recording from a limited area within the CC. Here we introduce a novel robust isolated "whole" CC preparation comparable to optic nerve. Unlike ME recordings where the CC CAP peaks representing myelinated and non-myelinated axons vary broadly in size, "whole" CC CAPs show stable reproducible ratios of these two main peaks, and also reveal a third peak, suggesting a distinct group of smaller caliber non-myelinated axons. We provide detailed characterization of "whole" CC CAPs and conduction velocities of myelinated and non-myelinated axons along the rostro-caudal axis of CC body and show advantages of this preparation for comparing axonal function in wild type and dysmyelinated <i>shiverer</i> mice, studying the effects of temperature dependence, bath-applied drugs and ischemia modeled by oxygen-glucose deprivation. Due to the isolation from gray matter, our approach allows for studying CC axonal function without possible "contamination" by reverberating signals from gray matter. Our analysis of "whole" CC CAPs revealed higher complexity of myelinated and non-myelinated axonal populations, not noticed earlier. This preparation may have a broad range of applications as a robust model for studying myelinated and non-myelinated axons of the CNS in various experimental models.</p></div

Detailed characteristics of “whole CC CAPs recorded at 4 mm conduction distance from CC3 and CC6.

<p>(A1, A2) Digitally averaged “whole” CC CAPs recorded from 14 CC3s (A1) and 29 CC6s (A2) from different mouse brains. The CC3 and CC6 were identified as explained in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165637#pone.0165637.g003" target="_blank">Fig 3</a>. Note different voltage scales in A1 and A2. (B) Detailed statistical comparison of “whole” CC3 and CC6 CAP parameters: ampl.—amplitude; CV—conduction velocity; peak2/peak1 ampl.—ratio of peak amplitudes; peak2/peak1 area—ratio of peak areas. ** p<0.01; *** p<0.001. All recordings were performed at room temperature.</p

Pronounced effects of oxygen glucose deprivation (OGD) on “whole” CC CAPs.

<p>(A) Superimposed CC CAPs recorded before (control), during OGD and during washout. (B1, B2) Time course of changes of absolute (B1) and normalized (B2) amplitudes of peaks 1 and 2 during 10-minute OGD and 20-minute washout. (C) Superimposed CAPs before, during and after OGD. (A-C) Representative data from a single experiment on CC6. The CAPs were recorded without averaging to reflect the fast changes during the test. (D) Summary of OGD–induced changes of CAP peaks after 10 min OGD and 20 min washout in CC3 and CC6. Temperature 35.5°C–36.5°C.</p

Stimulus-response relationship and refractoriness of “whole” CC compound action potential peaks.

<p>This figure shows further details on “whole” CC CAP from in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165637#pone.0165637.g001" target="_blank">Fig 1C</a>. (A) Superimposed CAPs recorded at varying stimulus intensities. (B) stimulus-response plot of amplitudes of peaks 1 and 2 for CAPs shown in (A). (C) Superimposed CAPs evoked by paired stimuli of varying intervals, with 0.5 ms interval increments. Individual traces in (B) and (C) are averaged from 6 consecutive recordings at each stimulation intensity, taken at 10 s intervals. (D) Plot of peak amplitudes of 2^nd CAP in the pair as a function of inter-stimulus interval, normalized to peak amplitudes of the 1^st CAP in the pair. The absolute refractory period of both peaks is 1.5 ms, while the relative refractory period of both peaks exceeded 15 ms. Conduction distance 1.5 mm; room temperature.</p

Comparison of “whole” CC CAPs of wild type (wt) and dysmyelinated <i>shiverer</i> (shi^-/-) mice.

<p>(A1, A2) Superimposed “whole” CC CAPs recorded at varying stimulation intensities in CC3 (A1) and CC6 (A2). Each trace in A1 and A2 represents an average of 6 consecutive recordings taken at 10 s intervals. (B1, B2) Digitally averaged “whole” CC CAPs recorded from wt (thick gray traces) and shi^-/- (thin black traces) CC3 (B1, digitally averaged from 8 experiments) and CC6 (B2, digitally averaged from 11 experiments). CAPs from wt CCs, (digitally averaged, from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165637#pone.0165637.g004" target="_blank">Fig 4A1 and 4A2</a>; brought to same scale with shi^-/- CAPs) are shown in B1 and B2 as thick gray traces. Dashed arrows with question marks show hypothetical relationships between peaks of wt and shi^-/- CAPs (see text). (C1, C2) Statistical comparison of main parameters of wt and shi^-/- “whole” CC CAPs. ** p<0.01; *** p<0.001.</p

The isolated corpus callosum (CC) for suction electrode (SE) recording of compound action potentials (CAPs).

<p>(A) 400 μm-thick coronal slice of mouse brain before (A1) and after (A2) dissecting out the CC. The isolated CC is shown in A2 above the slice. (B) Field microelectrode (ME) recording of CC CAPs within brain slices, showing two negative peaks conventionally referred to as N1 and N2. (C) Our novel “whole” CC recording of CAPs from isolated CC using SE stimulation and recording, showing two positive peaks 1 and 2. The CAPs in B and C were maximal and were averaged from 6 consecutive traces taken at 10 s intervals. Stimulus artifacts are marked with (*) in (B) and (C). Dashed lines on CAPs explain the measurements of peak amplitudes from their projected bases. (D) Amplitudes of CAP peaks recorded at 1.5–2 mm conduction distances in different slices using ME (black circles) or in isolated CCs using SE (open circles) at room temperature. Horizontal bars in (D) indicate mean amplitudes of ME and SE recorded CAP peaks. Note larger amplitudes of SE-recorded “whole” CC CAP peaks compared to ME recordings. The “whole” CC SE recording more adequately characterizes the axonal population of CC spanning between the stimulated and recorded sites compared to ME recording.</p