Search

Sequences of reversals recorded by single-cell observation of Halobacterium halobium are analyzed. Autocorrelation functions of spontaneous and stimulated reversals are computed; the results show that the only periodicity present in our data is that of the stimulus. Several different patterns of light stimuli were used. Responses to repetitive linear ramps of different slopes and to sinusoidal lights with different mean values and/or modulation depths are reported, showing that the modulation depth is the stimulus parameter most effective in eliciting photoresponses. Responses to more complex stimuli obtained by superimposing flashes to sinusoidal stimuli are also reported; a suppression effect depending on the phase of the sinusoidal stimulation is shown in responses to complex stimuli. A model which accounts for this effect is proposed.

An unconventional approach to studying ion channel kinetics is exploited. Here we describe the effects of a periodically varying membrane potential on the open-closed transitions of single K+ channels. The use of cycle histograms allows one to measure the transition probabilities as functions of the stimulus phase. The results show that such probabilities vary with the stimulation frequency and with the stimulus history, thus highlighting a dynamic aspect in the switching of this ion channel.

Scanning ion conductance microscopy (SICM) is a scanning probe microscopy particularly suitable for the investigation of living biological specimens due to its low invasivity. Recently, this technique has been used not only to perform 3D-imaging, but also to stimulate and guide neuronal growth cones. In particular, it has been demonstrated that one can guide the cone growth for tens of micrometres by means of recurrent and non-contact SICM scanning along a defined line, with a pipette having an internal hydrostatic pressure. Accurate measurements of the mechanical forces acting on the cell membrane in these stimulation protocols are essential to explain the biological mechanisms involved. Herein a setup specifically developed for this purpose, combining together SICM, atomic force microscopy (AFM) and inverted optical microscopy is described. In this configuration, a SICM pipette can be approached to an AFM cantilever while monitoring the cantilever deflection as a function of the hydrostatic pressure applied to the pipette and the relative distance. In this way, one can directly measure mechanical forces down to 20 pN. The same apparatus is thus sufficient to calibrate a given pipette and immediately use it to study the hydrostatic pressure effects on living cells.

The random sequence of openings and closings of single ion channels and the channel conductances have been the object of intense study over the past two decades with a view toward illuminating the underlying kinetics of the channel protein molecules. Channels that are sensitive to voltage, such as many K(+)-selective channels, have been particularly useful, because the kinetic rates can be manipulated by changing the membrane voltage. Most such studies have been performed under stationary conditions and usually at a single temperature. Here we report the results of experiments with sinusoidal modulation of the membrane potential performed at several temperatures. Dwell time and cycle histograms, objects not normally associated with ion channel experiments, are herein reported. From the last, the transition probability densities for channel opening and closing events are obtained. A new and unusual phase anticipation is observed in the cycle histograms, and its temperature dependence is measured.

Scanning ion conductance microscopy (SICM) is currently used for high resolution topographic imaging of living cells. Recently, it has been also employed as a tool to deliver stimuli to the cells. In this work we have investigated the mechanical interaction occurring between the pipette tip and the sample during SICM operation. For the purpose, we have built a setup combining SICM with atomic force microscopy (AFM), where the AFM cantilever replaces the sample. Our data indicate that, operating in far-scanning mode with current decrease values below 2%, no force can be detected, provided that the level of the electrolyte filling the pipette is equal to that determined by the capillary tension. A filling level different from this value determines a hydrostatic pressure, a flux through the pipette tip and detectable forces, even in far-scanning mode. The absolute value of these forces depends on the pipette tip size. Therefore, a possible pitfall when using SICM for cell imaging is to imply zero-force working conditions. However the hydrostatic forces can be exploited in order to deliver weak mechanical stimuli and guide neuronal growth cones. Evidences of the effectiveness of this approach are herein given. (C) 2010 Elsevier Ireland Ltd and the Japan Neuroscience Society.