Part 1. “Sweet slices are fine”?Part 2. Energy substrates: too sour?Part 3. Let the neurons breathe!
What concentrations are physiological and what are not (usually called pharmacological)? The physiological ones normally refer to the levels of a substance relevant to the naturally occurring, which, logically, for neurons is the milieu they strive in. It is called extracellular fluid (ECF) and is notorious for dramatic differences with not only blood (plasma) but even with the cerebrospinal fluid (CSF).
“Importantly, microdialysis data have shown that both in adult humans and in rats,the basal glucose levels are about 1–2 mM in the ECF of the neocortex and hippocampus [32,33,37,38] compared with 5–7 mM in the blood, whereas concentrations of lactate are about 2–5 mM in the ECF [32,34,39,40] compared with 1–2 mM in the adult blood.” (Zilberter et al., 2010). The authors further stressed the imperative of providing adequate means for energy substrates to be utilized in the artificial milieu, in which brain slices are placed:
“In brain slices, energy deficiency cannot be managed in the same way as in whole-body homeostasis, resulting in higher ES levels in slices than in vivo. The importance of a proper oxygen supply should be stressed [55], because oxidative phosphorylation is proportional to the presence of O2. Therefore, at an inadequate oxygen level, the efficacy of ES might be negligible. It is not surprising that energy metabolism in slices differs from that occurring in the living brain, and is probably impaired [52–54].”
Khakhalin (2011) wrote in his recent review that effects of ES on GABA action has been shown for “…4-5 mM concentrations of beta-hydroxybutyrate (Rheims et al. 2009), lactate and pyruvate (Holmgren et al. 2010), and was later confirmed by independent research groups for similar concentrations of pyruvate (Tyzio et al. 2011), and lactate (Ruusuvuori et al. 2010).” He continued arguing whether the concentrations were ”physiological ” in the experiments showing equal results in different authors who made, however, different conclusions: Tyzio et al. state that the physiological concentration of pyruvate is 1.6 – the one they measured in plasma.
“This comparison may be not valid, however, as it is well known from microdialysis studies that the extracellular fluid, immediately surrounding neural cells, differs in its composition not only from the blood plasma, but even from the cerebrospinal fluid. In particular, concentration of lactate in the extracellular fluid of rats and humans was found to be 2-5 times higher than in the blood plasma… 4-5 mM concentrations are likely to be physiologically relevant… On the other hand, at these concentrations both lactate and pyruvate induce noticeable changes in GABA- and glutamatergic transmission in developing neural networks. It means that some changes in experimental protocols and related theoretical paradigms may still be necessary.”
Kasischke (2011) in his comment on the article by Ivanov et al., 2011, wrote: “From the very beginning, the authors took great care to ensure both viability and functionality of their preparations.”
“An important implication of this methodological tour de force is that under many previously reported experiments the requirements for viability may been met while the functionality may have been compromised.”
References
- Zilberter Y, Zilberter T, Bregestovski P. (2010) Neuronal activity in vitro and the in vivo reality: the role of energy homeostasis. Trends Pharmacol Sci., 31(9):394-401
- [32,33,3437,38,39,40,52-55] are cited in Zilberter et al. (2010)
- Khakhalin A (May 18, 2011). Questioning the depolarizing effects of GABA during early brain development. J Neurophysiol doi: 0.1152/jn.00293.2011.
- Tyzio et al. (2011) and Ruusuvuori et al. (2010) are cited in Khakhalin ( 2011).
- Kasischke K (2011). Lactate fuels the neonatal brain. Frontiers in Neuroenergetics; 3, 4
