Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Abstract

Astrocytes have been promoted as a possible mechanistic target for anaesthetic hypnosis. The aim of this study was to explore this using the neocortical brain slice preparation. The methods were in two parts. Firstly, multiple general anaesthetic compounds demonstrating varying in vivo hypnotic potency were analysed for their effect on "zero-magnesium" seizure-like event (SLE) activity in mouse neocortical slices. Subsequently, the effect of astrocyte metabolic inhibition was investigated in neocortical slices, and compared with that of the anaesthetic drugs. The rationale was that, if suppression of astrocytes was both necessary and sufficient to cause hypnosis in vivo, then inhibition of astrocytic metabolism in slices should mimic the anaesthetic effect. In vivo anaesthetic potency correlated strongly with the magnitude of reduction in SLE frequency in neocortical slices (R(2) 37.7 %, p = 0.002). Conversely, SLE frequency and length were significantly enhanced during exposure to both fluoroacetate (23 and 20 % increase, respectively, p < 0.01) and aminoadipate (12 and 38 % increase, respectively, p < 0.01 and p < 0.05). The capacity of an anaesthetic agent to reduce SLE frequency in the neocortical slice is a good indicator of its in vivo hypnotic potency. The results do not support the hypothesis that astrocytic metabolic inhibition is a mechanism of anaesthetic hypnosis.

Keywords: Anaesthesia; Astrocyte; Cortical slice; Glia; Hypnosis.

Fig. 1

Scatterplot of the change in seizure-like event (SLE) frequency against in vivo hypnotic potency (mg/kg) for ketamine and each of the 21 ketamine analogues. Triangles are the non-ester analogues (including ketamine—circled); stars are the ester analogues. The three agents identified by arrows that were moderately potent hypnotics but did not induce a reduction in SLE frequency were either seizureogenic or had very rapid offset (<100 s)

Fig. 2

Graphs showing the effect of sequentially higher doses of a etomidate (n = 18) and b propofol (n = 28) on the frequency of seizure-like event frequency. Each dose was perfused for 30 min or until SLE frequency reduced by at least 50 % compared to the baseline value. The data are normalised to the baseline frequency and are expressed as mean + SEM. *p < 0.001, compared to baseline, Friedman Test with Dunn’s Multiple Comparisons

Fig. 3

Effect of fluoroacetate (F-acetate) (combined doses 1, 5 and 10 mM) on seizure-like event (SLE) a length, b frequency and c amplitude (n = 17). The data are normalised to the baseline values and are expressed as mean + SEM. *p < 0.05, compared to baseline, Friedman Test with Dunn’s Multiple Comparisons. **p < 0.01, compared to baseline, Friedman Test with Dunn’s Multiple Comparisons

Fig. 4

Effect of 0.5 (n = 17), 1.0 (n = 13) and 5.0 mM (n = 8) aminoadipic acid on seizure-like event a frequency, b length and c amplitude. The data are normalised to baseline values and are expressed as mean + SEM. *p < 0.05, compared to baseline, Friedman Test with Dunn’s Multiple Comparisons. **p < 0.01, compared to baseline, Friedman Test with Dunn’s Multiple Comparisons

Fig. 5

Examples from three slices showing the effect of a 0.5 mM, b 1.0 mM and c 5.0 mM aminoadipic acid on seizure-like event (SLE) characteristics. In each plot, the individual vertical lines represent single SLE events. The thumbnail inserts show zoomed sections from the corresponding time points in the main figure