KBMMGABA · Ketone Body Mediated Modulation of GABAergic Signaling: Mechanisms and Consequences

Ketone body mediated modulation of gabaergic signaling: mechanisms and consequences

Normal neonatal brain development requires the timely action of GABAergic signalling which plays a critical role in the regulation of neuronal; development, network formation, and activity in the brain. Synchronisation of the activity of populations of neurons is thought to be a requisite for the normal maturation of local neuronal networks. During this early neonatal period, GABAergic signalling is excitatory due to the high intracellular chloride concentration within neurons. Therefore, the activity of small numbers of highly connected GABAergic interneurons, strategically located within the local neonatal neuronal network, can play a key role in the generation of synchronised population activity patterns. At later stages, during development intracellular chloride, concentrations decrease and GABA signalling becomes hyperpolarising. Initial results suggested that during the early neonatal period ketone bodies mediate a decrease in the intracellular chloride concentration, resulting in a hyperpolarising shift in GABAergic signalling in neonates. Additionally, both lactate (5 mM) and pyruvate (5 mM) were also shown to induce a similar shift and a reduction in synchronised neonatal cortical activity, giant depolarising potentials (GDPs). At later stages in development GABAergic, signalling was shown to be modulated by endogenous cannabinoids, which can chronically suppress GABAergic inhibition. Recording activity in microcircuits comprised of a CCK-positive interneuron connected to a hippocampal CA1 pyramidal cell it was shown that this suppression is mediated by cannabinoid receptors 1 (CB1R). Activity in the GABAergic interneuron causes the synthesis and release of the cannabinoid anandamide which binds to CB1Rs resulting in the suppression of GABA release. Ketone body and lactate levels are elevated in neonates. It was suggested that additional energy substrates (ketone bodies, lactate and pyruvate) should be added to artificial cerebrospinal fluid, used in experiments, in order to more closely mimic conditions in the neonatal brain in vivo. However, in recent studies (Kirmse et al., 2010; Tyzio et al., 2011) no change in the reversal potential of the GABA mediated current was observed following application of the ketone body, DL- beta-hydroxybutyrate. In these experiments, therefore, the intracellular chloride concentration was unaffected by the presence of the ketone bodies, and there was no hyperpolarising shift in GABAergic signalling. Meanwhile, in DL- beta-hydroxybutyrate which was contaminated by dibenzylamine there was a hyperpolarising shift in the reversal potential of the GABA mediated current. Additionally, while a lactate (5 mM) induced reduction in the frequency of GDPs was also observed by Ruusuvuori et al. (who suggest that the effect is due to intracellular acidification) lower lactate (1.5 mM) or pyruvate (200 mM) concentrations had no effect on GDPs (Tyzio et al., 2011). These studies (Kirmse et al., 2010; Ruusuvuori et al., 2010; and Tyzio et al., 2011) concluded that: (i) ketone bodies, lactate and pyruvate have no effect on GABAergic signalling at concentrations present in neonatal plasma; (ii) additional energy substrates are not required when performing experiments with neonatal brain slices; and (iii) GABA is depolarising in the neonatal brain. Elevated lactate levels are present during the prenatal and presuckling periods, and in neonatal pathologies including perinatal hypoxia, trauma and epilepsies. It would be interesting to determine whether lactate mediates a modulation of GABAergic signalling under these conditions. It has been shown that classical retrograde signalling allows the transient modulation of the efficacy of inhibitory synapses during periods of elevated postsynaptic neuronal activity. Meanwhile, the presynaptically localised negative feedback pathway studied here permits effective modulation of inhibition over longer timescales, during periods in which postsynaptic activity is low.

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