Effects of isoflurane induction on inter-animal reproducibility

While α-chloralose has become the popular anesthetic of choice for functional studies in rodents, prior to the treatment of this drug a variety of volatile agents (e.g., halothane, isoflurane, enflurane) are used during the surgical preparation phase of the experiment^{1, 2, 3}. Since these volatile induction agents themselves may have some residual effects on the final anesthesia level reached with α-chloralose, it is important to know whether or not these effects modulate the functional response of the rodent to sensory stimuli. In this study we tested the inter-animal reproducibility efficacy of using isoflurane as an agent for induction in α -chloralose anesthetized rats during forepaw stimulation. We combined fMRI, electrophysiology, and laser-Doppler flowmetry to measure the changes in BOLD, neuronal activity, and CBF from the somatosensory area. During the animal preparation halothane (0.7%) / isoflurane (0.5%) were used for induction in α-chloralose anesthetized rats. The same forepaw stimulation protocol (30 s block design; 2 mA; 0.3 ms; 3 Hz) was used for both fMRI and extracellular recordings. All fMRI data were obtained on a modified 9.4 T Bruker horizontal-bore spectrometer (Billerica, MA) using a 1 H resonator/surface coil RF probe. The images were acquired with gradient echo EPI sequence (TR/TE=1000/15 ms). Extracellular recordings were first filtered to extract the action and field potentials (AP, FP) and then the AP data were binned for spike rates (ν). The CBF was measured using a laser-Doppler probe (Oxford Optronix, Oxford, UK). In rats where the highly variable and oscillatory responses (0.03 to 0.15 Hz) were detected, even long experimental times did not produce the typical stimulus-induced response usually desired in functional experiments. The multi-modal results suggest caution in using isoflurane as the induction agent for functional studies (e.g., ¹ ), because of the high variability in the responses across the many rats as observed in this study (n=14). The figure 1 shows data from several rats.

Figure 1

The fMRI time courses (top) showed two trends: a typical hemodynamic response (dark trace in A), a lagged response (gray trace in A), and an oscillatory response (both traces in B). These fMRI data were supported by neuronal and CBF data (bottom) obtained by extracellular and laser-Doppler recordings. The multi-modal data showed the same trends as the fMRI data: a typical stimulation-induced response (C), a highly variable response (D), and an oscillatory response (E). The physiological status of all the rats shown in the figure was similar throughout the experiment. These results suggest caution when using isoflurane as an induction agent.