Lactate Effectively Covers Energy Demands during Neuronal Network Activity in Neonatal Hippocampal Slices

Abstract

Although numerous experimental data indicate that lactate is efficiently used for energy by the mature brain, the direct measurements of energy metabolism parameters during neuronal network activity in early postnatal development have not been performed. Therefore, the role of lactate in the energy metabolism of neurons at this age remains unclear. In this study, we monitored field potentials and contents of oxygen and NAD(P)H in correlation with oxidative metabolism during intense network activity in the CA1 hippocampal region of neonatal brain slices. We show that in the presence of glucose, lactate is effectively utilized as an energy substrate, causing an augmentation of oxidative metabolism. Moreover, in the absence of glucose lactate is fully capable of maintaining synaptic function. Therefore, during network activity in neonatal slices, lactate can be an efficient energy substrate capable of sustaining and enhancing aerobic energy metabolism.

Keywords: NAD(P)H; energy metabolism; energy substrates; lactate; neonatal neurons; oxygen; synaptic transmission.

Figure 1

Analysis of local field potentials (LFPs) in response to the stimulation train. (A) A fragment of LFP train in response to a 10-Hz stimulation of Schaffer collaterals. (B) An example of one LFP in the train. (C) During computer analysis, the stimulation artifact and antidromic response were excluded from each LFP; the trace fractions related to population spikes were inverted and the resulting trace (blue trace) was integrated.

Figure 2

The level of slice oxygenation strongly affects the value of LFPs. (A) The oxygen level profile in a chamber at different rates of perfusion: 3 ml/min (black) and 15 ml/min (red). Dashed lines correspond to measurements in the absence of a slice in the chamber. Mean ± SE from five slices. (B) Examples of LFPs measured in the same slice and electrode positions at different flow rates. (C) Summary of the dependence of LFP amplitudes on the oxygen levels and perfusion rates. Mean ± SE from five slices.

Figure 3

Lactate added to glucose increases oxygen consumption, modifies NAD(P)H signaling and amplifies synaptic function. (A) Original recordings of changes in NAD(P)H fluorescence and oxygen levels in response to a 10-Hz, 10-s stimulation of Schaffer collaterals in a slice of P5 mouse in ASCF with either 10 mM glucose (red lines) or 5 mM glucose + 5 mM lactate (blue lines). (B) LFP trains in 10 mM glucose (red) and 5 mM glucose + 5 mM lactate (blue). (C) Comparison of LFP integrals (see Figure 1) during the trains.

Figure 4

Summary of the effects of lactate added to glucose in ACSF. (A) Average values of LFP integrals during stimulation trains (left) and total LFP integrals (summation of LFP integrals in the train, right). (B) Average values of changes in the amplitude and integral of oxygen levels (in % to measurements in standard ACSF) induced by stimulation trains. (C) Average values in the integrals of oxidation and overshoot phases of NAD(P)H signaling. Mean ± SE from five slices.

Figure 5

Lactate without glucose increases oxygen consumption, modifies NAD(P)H signaling and maintains/augments synaptic function. (A) Original recordings of changes in NAD(P)H fluorescence and oxygen levels in response to a 10-Hz, 30-s stimulation of Schaffer collaterals in a slice of P5 mouse in ASCF with either 10 mM glucose (red lines) or 10 mM lactate (blue lines). Prolonged (30 s) stimulation were used in these experiments to verify the efficacy of lactate as the energy substrate. (B) LFP trains in 10 mM glucose (red) and 10 mM lactate (blue). (C) Comparison of LFP integrals during the train.

Figure 6

Summary of the effects in glucose-based and lactate-based ACSF. (A) Average values of LFP integrals during stimulation trains. (B) Average values of changes in the amplitude and integral of oxygen levels (in % to measurements in standard ACSF) induced by stimulation trains. (C) Average values in the integrals of oxidation and overshoot phases of NAD(P)H signaling. Mean ± SE from 10 slices.

Figure 7

BHB added to glucose increases oxygen consumption and modifies NAD(P)H signaling. (Aa) Original recordings of changes in NAD(P)H fluorescence and oxygen levels in response to a 10-Hz, 10-s stimulation of Schaffer collaterals in a slice of P5 mouse; (Ab) LFP trains in 10 mM glucose (red) and 5 mM glucose + 10 mM DL-BHB (blue); (Ac) Comparison of LFP integrals during the train. (B) Average values of LFP integrals during stimulation trains. Mean ± SE from six slices. (C) Average values of changes in the amplitude and integral of oxygen levels (in % to measurements in standard ACSF) induced by stimulation trains. (D) Average values of the integrals of oxidation and overshoot phases of NAD(P)H signaling.

Figure 8

Pyruvate added to glucose enhances aerobic energy metabolism and synaptic integrity. (Aa) Original recordings of changes in NAD(P)H fluorescence and oxygen levels in response to a 10-Hz, 30-s stimulation of Schaffer collaterals in a slice of P6 mouse; (Ab) LFP trains in 10 mM glucose (red) and 5 mM glucose + 5 mM pyruvate (blue); (B) Average values of LFP integrals during stimulation trains. Mean ± SE from five slices. (C) Average values of changes in the amplitude and integral of oxygen levels (in % to measurements in standard ACSF) induced by stimulation trains. (D) Average values of the integrals of oxidation and overshoot phases of NAD(P)H signaling.

Figure 9

Minor effects of lactate and BHB on pH_i distribution in CA3 pyramidal cells. (A) pH_i distributions measured in the same neurons in glucose-based (10 mM) and lactate-supplemented (5 mM glucose + 5 mM lactate) ACSF. (B) pH_i distributions measured in the same neurons in glucose-based (10 mM) and lactate-based (10 mM) ACSF. (C) pH_i distributions measured in the same neurons in glucose-based (10 mM) and BHB-supplemented (5 mM glucose + 10 mM BHB) ASCF. In (A), (B), and (C) dashed lines indicate the average values of pH_i. Note large variations of pH_i in different neurons and a relatively small shift in the mean pH_i values in the presence of lactate or BHB.