We viewed GnRH neurons with an Olympus BX50WI upright fluorescent microscope equipped with infrared differential interference contrast (Opelco, Dulles,VA) using a 40x water immersion lens. GnRH neurons were identified by brief illumination (15-45 sec) at 470 nm to visualize the GFP signal. We tried to minimize cell exposure to fluorescence to avoid possible damage to the cell (11 (
link)), although in our experience, exposure to fluorescence for up to 20 min (roughly the average duration of a whole-cell recording) does not affect cell physiology (DeFazio and Moenter, unpublished observations).
Recording pipettes were fabricated from capillary glass (type 7052, outer diameter/ inner diameter 1.65/1.1 mm, World Precision Instruments, Sarasota, FL) using a two-stage pipette puller (Narashige, Japan). We found pipettes with resistances of 1-3 MΩ when filled with normal HEPES pipette solution and held at 0 mV in the external solution were most suitable for these recording. When pipette resistance was > 3 MΩ, we found the diameters to be too small, and G-Ω seals often spontaneously formed. Conversely, pipettes < 1 MΩ were so large in diameter that cell morphology could be affected. That is, cells would sometimes be aspirated into the pipette barrel over the duration of the recording. We did not systematically change or study the shank or taper of our electrodes to test for effects on seal stability and longevity, as in our experience, electrode resistance was the primary determinant of recording success. Pipettes were filled with normal saline solution in early studies. Because this is a carbonate-buffered solution, we became concerned that pH could shift over time because internal solutions cannot be bubbled with carbon dioxide to maintain pH. We thus switched to normal HEPES solution for later studies. No significant difference was observed in firing patterns between these pipette solutions. We did find the longevity of recordings improved, but this could have been due to improved ability to acquire and maintain seals rather than a change in pipette solution.
Extracellular recordings were made using an EPC-7 or EPC-8 amplifier (HEKA, Germany) with Igor Pro software (Instrutech, Port Washington, NY) running on a G4 Macintosh computer (Apple Computers, Cupertino, CA) to acquire data. Recordings were made in voltage-clamp mode with a holding potential of 0 mV, initial gain of 0.5x, filtering at 10 kHz, and digitized with at ITC-18 acquisition interface (Instrutech). Using these settings, spontaneous electrical discharges (action currents) from cells could be observed. Action currents reflect very rapid and local changes in the electrochemical gradient.
We used the Pulse Control Event Tracker program operating within Igor Pro (Instrutech) to detect and record action currents (events), the membrane currents associated with action potential firing. Although the following details may not be of use to those without Igor Pro software (Instrutech), similar event detection software exists for pClamp (Axon Instruments). We used the following Event Tracker settings: 10 μsec sampling interval to ensure detection of action currents near their peaks, one point over threshold to trigger to ensure detection of all events above threshold, event polarity negative, and -50 to -150 pA threshold depending on action current amplitude. Noise typically ranged 20-25 pA peak-to-peak. For each detected event, the time of the event and 10 msec centered on the event were digitized and stored to a data file.
We based these settings on the amplitude and duration of spontaneous events observed in early recordings of GnRH neurons. By only digitizing the data surrounding the event itself, file sizes for 30-min records were typically 0.5 to 5 MB, depending on the number of events observed in that 30-min period. Files could easily exceed 100 MB if the baseline trace is continuously recorded. Note that in Igor Pro software (IGOR-PRO 3.16PPC), Event Tracker files cannot exceed ~37-min due to limitations in the way the program stores temporal information. Between the end of one 30-min file and the beginning of the next, we checked seal resistance and adjusted baseline to 0 pA if necessary.
Nunemaker C.S., DeFazio R.A, & Moenter S.M. (2003). A targeted extracellular approach for recording long-term firing patterns of excitable cells: a practical guide. Biological Procedures Online, 5, 53-62.