Abstract SNACC-57

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Propofol weakens alpha oscillation phase locking between hipocampus and prefrontal cortex

1Kreuzer M, 2Butovas S, 2Schwarz C, 3Antkowiak B, 1Kochs E, 3Drexler B
1Klinikum rechts der Isar der TU München, Munich, , Munich; 2Hertie Institute for Clinical Brain Research, Tübingen, , Germany; 3University of Tübingen,, Tübingen, , Germany

Background
Interactions between hippocampus (hip) and prefrontal cortex (PFC) are associated with mnemonic processes. PFC has been linked to working memory, capable to store information for some seconds. To be remembered, items must be shifted into the hip. Synchronized θ-oscillations seem essential for interactions between these areas, i.e., a functioning amnestic pathway [1]. To evaluate θ-synchrony changes between PFC and hip, the phase locking value PLV was calculated during control and at hypnotic propofol concentrations. Changes in PLV may represent an anesthetic-induced effect on hip-PFC interactions, possibly reflection amnestic actions. Since narrower frequency ranges may be beneficial for detection of synchronization effects, one wide and 2 narrow frequency bands in the θ-range were used.
Methods
9 local field potential (LFP) sets from wild type mice were recorded with 20kHz at control and hypnotic propofol concentrations (30mg/kg) from 4-channel multielectrode arrays located in PFC (layer II) and hip. Representative episodes of 5s were extracted from each recording and filtered to 5-15Hz, 4-8Hz or 8-12Hz after down sampling to 250Hz. PLV calculation [2] is based on phase differences between LFPs from hip and PFC. The LFPs’ complex representation was derived from the Hilbert transform. The real part is the original signal and the imaginary part is the 90° phase shifted original signal. The analytic phase φ(t) of a signal x(t) is φ(t)=arctan(Im(x(t))/Re(x(t))). The phase difference is Φ(t)=φhip(t)-φPFC(t) and PLV=(1/N)|Σexp(iΦ(t))|. PLV was calculated for all hip/PFC channel combinations from 5 non-overlapping 1s episodes of the 5s segment and averaged. For statistical analysis, PLV of all channel combinations of one data set were also averaged, leading to 9 PLV at control and with propofol. Wilcoxon signed rank test was used (significant: p<0.05) as well as the effect size measure Hedges’ g [3].
Results
See table
Discussion
Propofol negatively influences PLV in the 8-12Hz range. PLV=1 indicates little variation of phase differences. If the variation increases PLV tends towards 0. Our data indicate that propofol causes a bigger variation in phase differences leading to decreased phase locking and hence possibly impaired communication between hip and PFC. Based on the results we suggest that the usually chosen θ-ranges of 5-15Hz, 4-12Hz, etc. may be too wide to determine significant oscillation effects. Here, e.g., in the 5-15Hz range no significant effect by means of the signed rank test but a strong effect by means of Hedges’ g can be observed. In the narrower 4-8Hz frequency range no effect was observed, but in the 8-12Hz band the effect was significant. This militates for analyses in smaller frequency bands. The negative effect of propofol on PLV may be associated with anesthetic-induced amnesia.
References
[1]Anesthesiology,2010,113:48–57; [2]Hum Brain Mapp,1999,8:194–208; [3]EJN,2011,34:1887–1894

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