On discrepancies of data from experiments on brain slices, in toto, and in vivo

Let's start with the fundamental differences between environments depending on the types of experiments.

(See Commentary: Excitatory GABA: “Maybe It’s Not So Exciting After All!”)

Brain energy homeostasis versus energy supply in the brain slice preparation.

A. Energy homeostasis of the brain serves to meet brain’s energy needs in spite of environmental and metabolic disturbances. It continuously compares the correlates of energy requirements (SET POINT) with the actual energy supply (OUTPUT) through the negative feedback loop sending information about OUTPUT to the brain metabolic sensors. When the difference between SET POINT and OUTPUT exceeds certain value, a correction signal is sent to the control centers responsible for the initiation of the executive systems. The goal of executive system is fixing the consequences of disturbances. As the result, within the homeostatic margins, the required and actual energy supplies are maintained close to each other.

B. Brain slice is lacking all mandatory elements of a homeostatic system receiving instead a constant and arbitrary energy supply, which is independent of both qualitative and quantitative needs of cells.

C. When brain structures are preserved as in the in toto preparation  (not shown on the figure), there’s the possibility that energy substrates can come from the glial depots (1, 2)

As an example, let’s consider these four situations with GABA action:

1. In vitro (brain slice) using standard ACSF (footnote a)

GABA is consistently reported to be excitatory in the neonatal brain slices (e.g., 3 for review)

2. In vitro (neonatal’s brain slice) using energy substrate-fortified ACSF

GABA is inhibitory  (4, 5)

3. In toto, hippocampus (footnote b) preparation

In the intact neonatal hippocampus preparation, standard ACSF, intensive oxygenation (!), GABA is inhibitory (6, 7, 8 )

4. In vivo (neonates)

GABA is inhibitory in the normal neonates (9, 10,11) but it is much less inhibitory during chemical blockade of ketone’s production (blockade of ketogenesis) (9).

Sources

1. The astrocyte–neuron ketone body shuttle

2. The astrocyte–neuron lactate shuttle

3. Ben-Ari, Y., et al. (2007) GABA: a pioneer transmitter that
excites immature neurons and generates primitive oscillations. Physiol
Rev 87, 1215-1284

4. Rheims, S., et al. (2009) GABA action in immature neocortical
neurons directly depends on the availability of ketone bodies. J
Neurochem 110, 1330-1338

5. Holmgren CD, et. al., (2010) Energy substrate availability as a
determinant of neuronal resting potential, GABA signaling and
spontaneous network activity in the neonatal cortex in vitro. J
Neurochem. Feb;112(4):900-12. Epub 2009 Nov 24.

6. Wong, T., et al. (2005) Postnatal development of intrinsic
GABAergic rhythms in mouse hippocampus. Neuroscience 134, 107-120

7. Derchansky, M., et al. (2008) Transition to seizures in the
isolated immature mouse hippocampus: a switch from dominant phasic
inhibition to dominant phasic excitation. J Physiol 586, 477-494

8. Dzhala V et. al., Progressive NKCC1-Dependent Neuronal Chloride Accumulation during Neonatal Seizures The Journal of Neuroscience, (2010) 30(35):11745–11761 • 11745

9. Rheims S., PhD thesis, Universite de la Mediterranee, 2008

10. Bremner L, Fitzgerald M & Baccei M. (2006). Functional GABAA-Receptor-Mediated Inhibition in the Neonatal Dorsal Horn. J Neurophysiol 95, 3893-389

Footnotes

a) ACSF – artificial cerebrospinal fluid (CSF). This solution closely matches the electrolyte concentrations of CSF – A Harvard Bioscience Company

b) Hippocampus – a complex neural structure shaped like a sea horse, has a central role in the formation of memories