Blockade of hippocampal long-term potentiation by sustained tetanic stimulation near the recording site.
Specific patterns of electrical stimulation trigger several forms of synaptic plasticity in hippocampal pyramidal cells, including a long-term potentiation (LTP) of excitatory synaptic transmission. I investigated the effect of commonly used stimulation protocols at different distances from the recording site. Sustained electrical stimulation (100 Hz, 1 s) delivered close to the recording site prevented LTP induction; the same stimulation from a second electrode placed farther away subsequently produced LTP at the same recording site. Strong stimulation near the recording site could also interfere with LTP triggered from a distal site. In contrast to sustained high-frequency stimulation, intermittent stimulation (theta burst pattern) delivered close to the recording site produced normal LTP. These data support the hypothesis that strong stimulation releases a factor that acts locally to prevent LTP. (+info
LocaLisa: new technique for real-time 3-dimensional localization of regular intracardiac electrodes.
BACKGROUND: Estimation of the 3-dimensional (3D) position of ablation electrodes from fluoroscopic images is inadequate if a systematic lesion pattern is required in the treatment of complex arrhythmogenic substrates. METHODS AND RESULTS: We developed a new technique for online 3D localization of intracardiac electrodes. Regular catheter electrodes are used as sensors for a high-frequency transthoracic electrical field, which is applied via standard skin electrodes. We investigated localization accuracy within the right atrium, right ventricle, and left ventricle by comparing measured and true interelectrode distances of a decapolar catheter. Long-term stability was analyzed by localization of the most proximal His bundle before and after slow pathway ablation. Electrogram recordings were unaffected by the applied electrical field. Localization data from 3 catheter positions, widely distributed within the right atrium, right ventricle, or left ventricle, were analyzed in 10 patients per group. The relationship between measured and true electrode positions was highly linear, with an average correlation coefficient of 0.996, 0.997, and 0.999 for the right atrium, right ventricle, and left ventricle, respectively. Localization accuracy was better than 2 mm, with an additional scaling error of 8% to 14%. After 2 hours, localization of the proximal His bundle was reproducible within 1.4+/-1.1 mm. CONCLUSIONS: This new technique enables accurate and reproducible real-time localization of electrode positions in cardiac mapping and ablation procedures. Its application does not distort the quality of electrograms and can be applied to any electrode catheter. (+info
Synchronization of local neural networks in the somatosensory cortex: A comparison of stationary and moving stimuli.
Spontaneous and stimulus-induced responses were recorded from neighboring groups of neurons by an array of electrodes in the primary (SI) somatosensory cortex of intact, halothane-anesthetized cats. Cross-correlation analysis was used to characterize the coordination of spontaneous activity and the responses to peripheral stimulation with moving or stationary air jets. Although synchronization was detected in only 10% (88 of 880) of the pairs of single neurons that were recorded, cross-correlation analysis of multiunit responses revealed significant levels of synchronization in 64% of the 123 recorded electrode pairs. Compared with spontaneous activity, both stationary and moving air jets caused substantial increases in the rate, proportion, and temporal precision of synchronized activity in local regions of SI cortex. Among populations of neurons that were synchronized by both types of air-jet stimulation, the mean rate of synchronized activity was significantly higher during moving air-jet stimulation than during stationary air-jet stimulation. Moving air jets also produced significantly higher correlation coefficients than stationary air jets in the raw cross-correlograms (CCGs) but not in the shift-corrected CCGs. The incidence and rate of stimulus-induced synchronization varied with the distance separating the recording sites. For sites separated by /=500 microm, only 37% of the multiunit responses were synchronized by discrete stimulation with a single air jet. Measurements of the multiunit CCG peak half-widths showed that the correlated activity produced by moving air jets had slightly less temporal variability than that produced by stationary air jets. These results indicate that moving stimuli produce greater levels of synchronization than stationary stimuli among local groups of SI neurons and suggest that neuronal synchronization may supplement the changes in firing rate which code intensity and other attributes of a cutaneous stimulus. (+info
Effect of biphasic shock duration on defibrillation threshold with different electrode configurations and phase 2 capacitances: prediction by upper-limit-of-vulnerability determination.
BACKGROUND: The defibrillation threshold (DFT) may be affected by biphasic shock duration (BSD), electrode configuration, and capacitance. The upper limit of vulnerability (ULV) may be used to estimate the DFT. For different lead configurations and phase 2 capacitances, we investigated in 18 pigs whether the use of ULV may predict waveforms with lowest DFT. METHODS AND RESULTS: -DFT and ULV were determined by up-down protocols for 10 BSDs. ULVs were measured by T-wave scanning during ventricular pacing (cycle length 500 ms). In protocol 1 (n=6), a pectoral "active can" was combined with an electrode in the superior vena cava as common cathode and a right ventricle electrode as anode (AC+SVC). In protocol 2 and protocol 3 (n=6 each), only the "active can" was used as proximal electrode (AC). Capacitance was 150 microF during both phases in protocol 1 and protocol 3 but 150 microF (phase 1) and 300 microF (phase 2) in protocol 2. ULV and DFT demonstrated a linear correlation in each protocol (r=0.78 to 0.84). Lowest DFTs were found at 10 ms for AC+SVC and at 14 ms for AC (P<0.001). At optimal BSDs, voltage DFTs did not differ significantly between AC (527+/-57 V) and AC+SVC (520+/-70 V). Switching capacitors for phase 2 in a way that reduced leading-edge voltage by 50% while doubling capacity did not change BSD for optimal voltage DFT but increased minimum DFT from 527+/-57 V to 653+/-133 V (P=0.04). CONCLUSIONS: The BSD with lowest DFT is shorter for AC+SVC than for AC. There is no significant difference in voltage DFT between both at optimal BSD. A lower phase 2 capacitance reduces DFTs irrespective of BSD. Because strength-duration curves for DFT and ULV correlate for different BSDs, lead systems, and phase 2 capacitances, ULV determination may allow the prediction of waveforms with lowest DFT. (+info
Chronic lithium treatment inhibits amiloride-sensitive sodium transport in the rat distal nephron.
Chronic treatment of rats with lithium leads to Na+ loss and a reduced antinatriuretic response to aldosterone, suggesting that lithium reduces conductive Na+ transport in the distal nephron. This was investigated in the present study by measuring the renal response to aldosterone infusion followed by amiloride in chronically instrumented conscious rats given lithium for 3 to 4 weeks to achieve plasma Li+ concentrations of approximately 0.5 mM. A servo-controlled infusion system was used to maintain sodium and water homeostasis, thereby preventing misinterpretation of the findings as a consequence of drug-induced changes in Na+ balance. In a control group of rats, Na+ excretion decreased in response to aldosterone (p <.01) and subsequent amiloride administration led to a marked increase in Na+ excretion (p <.001). In contrast, in the lithium-treated group, there was no significant response to either aldosterone or amiloride. It is concluded that long-term treatment with lithium, even when plasma Li+ concentrations are below 1 mM, reduces aldosterone-stimulated Na+ transport through the amiloride-sensitive Na+ channels in the principal cells of the distal nephron. (+info
Mapping of atrial activation with a noncontact, multielectrode catheter in dogs.
BACKGROUND: Endocardial mapping of sustained arrhythmias has traditionally been performed with a roving diagnostic catheter. Although this approach is adequate for many tachyarrhythmias, it has limitations. The purpose of this study was to evaluate a novel noncontact mapping system for assessing atrial tachyarrhythmias. METHODS AND RESULTS: The mapping system consists of a 9F multielectrode-array balloon catheter that has 64 active electrodes and ring electrodes for emitting a locator signal. The locator signal was used to construct a 3-dimensional right atrial map; it was independently validated and was highly accurate. Virtual electrograms were calculated at 3360 endocardial sites in the right atrium. We evaluated right atrial activation by positioning the balloon catheter in the mid right atrium via a femoral venous approach. Experiments were performed on 12 normal mongrel dogs. The mean correlation coefficient between contact and virtual electrograms was 0.80+/-0.12 during sinus rhythm. Fifty episodes of atrial flutter induced in 11 animals were evaluated. In the majority of experiments, complete or almost complete reentrant circuits could be identified within the right atrium. Mean correlation coefficient between virtual and contact electrograms was 0.85+/-0.17 in atrial flutter. One hundred fifty-six episodes of pacing-induced atrial fibrillation were evaluated in 11 animals. Several distinct patterns of right atrial activation were seen, including single-activation wave fronts and multiple simultaneous-activation wave fronts. Mean correlation coefficient between virtual and contact electrograms during atrial fibrillation was 0.81+/-0.18. The accuracy of electrogram reconstruction was lower at sites >4.0 cm from the balloon center and at sites with a high spatial complexity of electrical activation. CONCLUSIONS: This novel noncontact mapping system can evaluate conduction patterns during sinus rhythm, demonstrate reentry during atrial flutter, and describe right atrial activation during atrial fibrillation. The accuracy of electrogram reconstruction was good at sites <4.0 cm from the balloon center, and thus the system has the ability to perform high-resolution multisite mapping of atrial tachyarrhythmias in vivo. (+info
Intraoperative monitoring during surgery for acoustic neuroma: benefits of an extratympanic intrameatal electrode.
OBJECTIVES: To assess the utility of an extratympanic intrameatal electrode for intraoperative monitoring during acoustic neuroma and other cerebellopontine angle tumour surgery and to define the neurophysiological and surgical factors which influence hearing preservation. METHODS: Twenty two patients, 18 with acoustic neuromas and four with other cerebellopontine angle tumours, underwent intraoperative monitoring during tumour excision. The extratympanic intrameatal electrode (IME) was used to record the electrocochleogram (ECoG) and surface electrodes to record the brainstem auditory evoked response (ABR). RESULTS: The compound action potential (CAP) of the ECoG was two and a half times greater in amplitude than wave I of the ABR and was easily monitored. Virtually instant information was available as minimal averaging was required. Continuous monitoring was possible from the commencement of anaesthesia to skin closure. The IME was easy to place, non-invasive, and did not interfere with the operative field. Operative procedures which affected CAP or wave V latency or amplitude were drilling around the internal auditory meatus, tumour dissection, nerve section, and brainstem and cerebellar retraction. Hearing was achieved in 59% of patients. CONCLUSIONS: The IME had significant benefits in comparison with other methods of monitoring. The technique provided information beneficial to preservation of hearing. (+info
Non-invasive determination of bacterial single cell properties by electrorotation.
So far, electrorotation and its application to the determination of single cell properties have been limited to eukaryotes. Here an experimental system is described that allows the recording of electrorotation spectra of single bacterial cells. The small physical dimensions of the developed measuring chamber combined with a single frame video analysis made it possible to monitor the rotation of objects as small as bacteria by microscopical observation despite Brownian rotation and cellular movement. Thus physical properties of distinct organelles of E. coli could be simultaneously determined in vivo at frequencies between 1 kHz and 1 GHz. Experimental data were evaluated following a three-shell model of the cell. Electrical conductivities of cytoplasm and outer membrane were determined to 4.4 mS/cm and 25 microS/cm, respectively, that of the periplasmic space was found to increase with the square root of the medium ionic strength. Specific capacitances of inner and outer membrane amounted to 1.4 microF/cm2 and 0.26 microF/cm2, respectively, the thickness of the periplasm to about 50 nm. Heat treatment of the cells lead to a reduction of cytoplasmic conductivity to 0.9 mS/cm, probably caused by an efflux of ions through the permeabilized inner membrane. (+info