Arousal thresholds and tissue-electrode impedance in guinea pigs

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Abstract
The threshold for electroencephalogram arousal in unrestrained female guinea pigs was determined by applying rectangular pulses of increasing voltage to electrodes implanted in the rostral portion of the midbrain reticular formation. Tissue-electrode impedance was measured by applying subthreshold sine-wave current to the same implanted electrodes and observing the voltage-to-current ratio. Daily arousal thresholds and impedance were positively correlated (P<.0.05) in 13 of 18 electrodes studied in both intact and ovariectomized animals. Correlations between impedance and estimated total current/ rectangular pulse at threshold were generally nonsignificant or inconsistent. Estrous behavior in ovariectomized or intact animals was not associated consistently with changes in arousal threshold or tissue-electrode impedance. Significant shifts in impedance as well as threshold were observed during the second half of normal estrous cycles. Overnight decreases of 20-60% in tissue-electrode impedance were observed 2-6 days prior to estrous in 12 of 18 cycles with a subsequent slow recovery. Arousal threshold changes were less marked than impedance fluctuations. Bilaterally symmetrical electrodes showed identical and simultaneous impedance shifts prior to estrous in two animals. In one of these animals similar impedance changes were noted in 3 consecutive estrous cycles.