Alexander A. Velumian

Enhanced LTP in Mice Deficient in the AMPA Receptor GluR2

AMPA receptors (AMPARs) are not thought to be involved in the induction of long-term potentiation (LTP), but may be involved in its expression via second messenger pathways. However, one subunit of the AMPARs, GluR2, is also known to control Ca2+ influx. To test whether GluR2 plays any role in the induction of LTP, we generated mice that lacked this subunit. In GluR2 mutants, LTP in the CA1 region of hippocampal slices was markedly enhanced (2-fold) and nonsaturating, whereas neuronal excitability and paired-pulse facilitation were normal. The 9-fold increase in Ca2+ permeability, in response to kainate application, suggests one possible mechanism for enhanced LTP. Mutant mice exhibited increased mortality, and those surviving showed reduced exploration and impaired motor coordination. These results suggest an important role for GluR2 in regulating synaptic plasticity and behavior.

Modulation of High‐Voltage–Activated Calcium Channels in Dentate Granule Cells by Topiramate

Purpose: In this study, we assessed the effects of topiramate (TPM) on high‐voltage‐activated calcium channel (HVACC) currents in vitro.

REVERSIBLE INHIBITION OF IK, IAHP, IH AND ICA CURRENTS BY INTERNALLY APPLIED GLUCONATE IN RAT HIPPOCAMPAL PYRAMIDAL NEURONES

Abstract Previously, we reported that the spike frequency adaptation and slow afterhyperpolarizations (sAHP) in hippocampal pyramidal neurones are best preserved during whole-cell recording with a methylsulfate (MeSO4–)- based internal solution, but undergo a fast rundown when gluconate- (Gluc–)- based internal solution is used. Here we show, with internal perfusion of patch pipettes, the reversibility of the inhibitory effects of Gluc–on spike frequency adaptation and sAHP, and extend these observations to fast and medium-duration AHPs. Contrary to what might be expected based on Gluc–binding of Ca2+, the sAHP and its underlying current could be temporarily enhanced by adding 1–3 mM of the calcium chelator BAPTA to the internal solution in the presence of Gluc–. Replacement of internal MeSO4–with Gluc–did not affect the membrane resting potential or the amplitude and duration of action potentials, but reversibly increased the cell input resistance and decreased the threshold current for spike generation. Gluc–reversibly inhibited the hyperpolarization-activated non-selective cationic current (Ih), the depolarization-activated delayed rectifier K+ current (IK), the high-voltage-activated Ca2+ current and the Ca2+-activated K+ current that underlies the sAHP. The combination of these effects of Gluc–significantly alters the electrophysiological ”fingerprint” of the neurone.

Connexin 43 mimetic peptides inhibit spontaneous epileptiform activity in organotypic hippocampal slice cultures.

Gap junctions are cytoplasmic channels connecting adjacent cells and mediating their electrical and metabolic coupling. Different cell types in the CNS express various gap junction forming proteins, the connexins, in a cell-specific manner. Using the general gap junctional blocker, carbenoxolone, and two synthetic connexin mimetic peptides, corresponding to amino acid sequences of segments within the second extracellular loop of connexin 43, we studied the role of gap junctions in the generation of epileptiform activity in rat organotypic hippocampal slice cultures. While carbenoxolone inhibited both spontaneous and evoked seizure-like events, connexin mimetic peptides selectively attenuated spontaneous recurrent epileptiform activity, and only after prolonged (>10 h) treatment. The effects were mediated through reduced gap junctional coupling as indicated by suppressed fluorescent dye transfer between the cells. Assuming a selective inhibition of a connexin 43-dependent process by the mimetic peptides and preferential localization of this connexin isoform in astrocytes, the data suggest that, in developing hippocampal networks, the generation and/or initiation of spontaneous recurrent seizure-like activity may depend in large part upon the opening of glial gap junctions. Furthermore, this study shows that the use of a synthetic peptide that mimics a short sequence of a specific connexin isoform and, hence, blocks gap junctional communication in targeted cell types in the CNS, is a viable strategy for the modulation of cerebral activity.

Differential control of three after-hyperpolarizations in rat hippocampal neurones by intracellular calcium buffering

1 The whole‐cell recording technique, combined with internal perfusion, was used to study the effects of intracellular Ca2+ buffering on fast, medium and slow after‐hyperpolarizations (fAHP, mAHP and sAHP) in hippocampal CA1 pyramidal neurones in rat brain slices at room temperature. 2 The action potentials and the fAHP were unaffected by 100 μM to 3 mM concentrations of the internally applied fast Ca2+ chelator BAPTA. At higher (10‐15 mM) concentrations, BAPTA inhibited the fAHP and prolonged the decay of the action potential, suggesting that the corresponding large‐conductance Ca2+‐activated K+ channels are located close to the sites of Ca2+ entry during an action potential. Addition of Ca2+ to the BAPTA‐containing solution (at a ratio of 4·5 [Ca2+] : 10 [BAPTA]) to maintain the control level of [Ca2+]i did not prevent the effects of high concentrations of BAPTA. 3 The mAHP, activated by a train of action potentials, was inhibited by internally applied BAPTA within the range of concentrations used (100 μM to 15 mM), and this effect could not be reversed or prevented by addition of Ca2+ to the BAPTA‐containing solution. The inhibition of the mAHP by BAPTA could also be observed after blockade of the hyperpolarization‐activated IQ type mixed Na+‐K+ current (also known as Ih) component of the mAHP by bath‐applied 3‐5 mM Cs+, suggesting that the inhibition of the mAHP by BAPTA is due to inhibition of the depolarization‐activated IM (muscarinic) type K+ current. 4 The sAHP, activated by a train of action potentials, was potentiated by 100‐300 μM internally applied BAPTA, both with and without added Ca2+. At 1‐2 mM or higher concentrations, the potentiation of the sAHP by BAPTA without added Ca2+ was transient and was followed by a fast decrease. With added Ca2+, however, BAPTA caused a persistent potentiation of the sAHP with more than a 10‐fold increase in duration for periods exceeding 1 h even at concentrations of the buffer as high as 10‐15 mM. Earlier reports showing a blockade of the sAHP by BAPTA, based on experiments without added Ca2+, were apparently due to a sharp reduction in intracellular free [Ca2+] and to a high intracellular concentration of the free buffer. 5 Internally applied BAPTA caused a prolongation of the spike discharge during an 800 ms‐long depolarizing current step. At 100‐300 μM BAPTA, but not at 1‐2 mM or higher concentrations, this effect could be reversed by addition of Ca2+. The effects of BAPTA on the spike discharge occurred in parallel with the changes in the sAHP time course, which was more prolonged at higher concentrations of the buffer. 6 The concentration‐dependent differential control of the three types of AHP in hippocampal neurones by BAPTA is related to modulation of intracellular Ca2+ diffusion by a fast acting mobile Ca2+ buffer.

Chronic in vitro ketosis is neuroprotective but not anti-convulsant.

J. Neurochem. (2010) 113, 826–835.

Structural and functional alterations of spinal cord axons in adult Long Evans Shaker (LES) dysmyelinated rats.

Abnormal formation or loss of myelin is a distinguishing feature of many neurological disorders and contributes to the pathobiology of neurotrauma. In this study we characterize the functional and molecular changes in CNS white matter in Long Evans Shaker (LES) rats. These rats have a spontaneous mutation of the gene encoding myelin basic protein which results in severe dysmyelination of the central nervous system (CNS), providing a unique model for demyelinating/dysmyelinating disorders. To date, the functional and molecular changes in CNS white matter in this model are not well understood. We have used in vivo somatosensory evoked potential (SSEP), in vitro compound action potential (CAP) recording in isolated dorsal columns, confocal immunohistochemistry, Western blotting and real-time PCR to examine the electrophysiological, molecular and cellular changes in spinal cord white matter in LES rats. We observed that dysmyelination is associated with dispersed labeling of Kv1.1 and Kv1.2 K+ channel subunits, as well as Caspr, a protein normally confined to paranodes, along the LES rat spinal cord axons. Abnormal electrophysiological properties including attenuation of CAP amplitude and conduction velocity, high frequency conduction failure and enhanced sensitivity to K+ channel blockers 4-aminopyridine and dendrotoxin-I were observed in spinal cord axons from LES rats. Our results in LES rats clarify some of the key molecular, cellular and functional consequences of dysmyelination and myelin-axon interactions. Further understanding of these issues in this model could provide critical insights for neurological disorders characterized by demyelination.

Confocal imaging of changes in glial calcium dynamics and homeostasis after mechanical injury in rat spinal cord white matter.

Periaxonal glia play an important role in maintaining axonal function in white matter. However, little is known about the changes that occur in glial cells in situ immediately after traumatic injury. We used fluo-3 and confocal microscopy to examine the effects of localized (<0.5 mm) mechanical trauma on intracellular calcium (Ca(i)(2+)) levels in glial cells in a mature rat spinal cord white matter preparation in vitro. At the injury site, the glial Ca(i)(2+) signal increased by 300-400% within 5 min and then irreversibly declined indicating cell lysis and death. In glial cells at sites adjacent to the injury (1.5-2 mm from epicenter), Ca(i)(2+) levels peaked at 10-15 min, and thereafter declined but remained significantly above rest levels. At distal sites (6-9 mm), Ca(i)(2+) levels rose and declined even slower, peaking at 80-90 min. Injury in zero calcium dampened Ca(i)(2+) responses, indicating a role for calcium influx in the generation and propagation of the injury-induced Ca(i)(2+) signal. By 50-80 min post-injury, surviving glial cells demonstrated an enhanced ability to withstand supraphysiological Ca(i)(2+) loads induced by the calcium ionophore A-23187. Glial fibrillary acidic protein (GFAP) and CNPase immunolabeling determined that the glial cells imaged with fluo-3 included both astrocytes and oligodendrocytes. These data provide the first direct evidence that the effects of localized mechanical trauma include a glial calcium signal that can spread along white matter tracts for up to 9 mm within less than 3 h. The results further show that trauma can enhance calcium regulation in surviving glial cells in the acute post-injury period.

Patch-clamp recordings from white matter glia in thin longitudinal slices of adult rat spinal cord

We developed a technique of whole cell patch-clamp recordings from white matter oligodendrocytes and astrocytes in 200-250 microm-thick horizontal slices of adult (>2 months, 240-260 g) rat thoracic spinal cord. The viability of the white matter, sectioned in Na(+)-free, low Ca(2+) media, and the function of axons were preserved for >8 h, as demonstrated by the propagation of TTX-sensitive compound action potentials (CAPs) and the sensitivity of their refractory period to K(+) channel blocker 4-aminopyridine (1 microM). Glial cells were visually identified within the slices with a 40 x water immersion objective using infra-red differential interference contrast (IR-DIC) video microscopy, and the details of their morphology were further elucidated after filling the cells with Lucifer Yellow or Alexa 350 fluorescent dyes during whole-cell recording. Using voltage steps and ramps, we revealed pronounced non-linearity of I-V relationships in both oligodendrocytes and astrocytes. Both types of cells expressed TEA-sensitive outward delayed rectifier-type currents activated at positive voltages but showed little, if any, signs of inward rectification at voltages up to -140 mV. At -70 mV holding voltage, bath-applied kainic acid (100 microM) activated inward currents in both types of cells. This novel horizontal slice preparation of adult rat thoracic cord will facilitate the examination of mature glial cell physiology, glial-axonal signaling and the pathophysiology of spinal cord trauma and ischemia.

Modular double sucrose gap apparatus for improved recording of compound action potentials from rat and mouse spinal cord white matter preparations.

Compound action potential (CAP) recording is a powerful tool for studying the conduction properties and pharmacology of axons in multi-axonal preparations. The sucrose gap technique improves CAP recording by replacing the extracellular solution between the recording electrodes with a non-conductive sucrose solution to minimize extracellular shunting. The double sucrose gap (DSG), conferring similar advantages at the stimulation site, has been extensively used on guinea pig spinal cord white matter (WM) in vitro. Establishing the DSG methodology for WM preparations from smaller animals such as rats and mice is appealing due to their extensive use in basic and translationally oriented research. Here we describe a versatile modular DSG apparatus with rubber membrane separation of the compartments, suitable for WM strips from rat and mouse spinal cord. The small volumes of compartments (<0.1 ml) and the air-tight design allow perfusion rates of 0.5-1 ml/min with faster refreshment rates compared to commonly used 2-3 ml/min and larger compartments, providing economical usage of expensive pharmacological drugs. Our improved DSG design is particularly efficient for uncovering slower conducting, higher threshold CAP components, as demonstrated by recordings of C-wave (non-myelinated axons) in rat dorsal WM. In myelin-deficient Shiverer mice with genetically dysmyelinated axons, our DSG apparatus recordings revealed a multi-peak C-wave without preceding faster components. The improved stimulation and recording with our DSG apparatus, lowering the range of required stimulus intensities and reducing the artifact interference with recorded CAPs provide for critical technical advantages that allow for more detailed analysis of CAPs in relatively short preparations.