Proportional inhibition of acetylcholine synthesis accompanying impairment of 3-hydroxybutyrate oxidation in rat brain slices

beta-Hydroxybutyrate fuels synaptic function during development. Histological and physiological evidence in rat hippocampal slices

To determine whether ketone bodies sustain neuronal function as energy substrates, we examined the effects of beta-hydroxybutyrate (betaHB) on synaptic transmission and morphological integrity during glucose deprivation in rat hippocampal slices. After the depression of excitatory postsynaptic potentials (EPSPs) by 60 min of glucose deprivation, administration of 0.5-10 mM D-betaHB restored EPSPs in slices from postnatal day (PND) 15 rats but not in slices from PND 30 or 120 rats. At PND 15, adding D-betaHB to the media allowed robust long-term potentiation of EPSPs triggered by high frequency stimulation, and prevented the EPSP-spike facilitation that suggests hyperexcitability of neurons. Even after PND 15,D-betaHB blocked morphological changes produced by either glucose deprivation or glycolytic inhibition. These results indicate that D-betaHB is not only able to substitute for glucose as an energy substrate but is also able to preserve neuronal integrity and stability, particularly during early development.

Acetylcholinesterase and Na+,K(+)-ATPase activities in different regions of rat brain during insulin-induced hypoglycemia

The activities of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7), responsible for hydrolysis of acetylcholine and Na+,K(+)-ATPase (Mg(2+)-dependent ATP phosphohydrolase, EC 3.6.1.3), which plays a crucial role in neurotransmission, were determined in four brain regions after 1, 2, and 3 h of insulin administration. Significant decrease in the acetylcholinesterase and Na+,K(+)-ATPase activities was observed in the soluble and total particulate fractions from cerebral hemispheres, cerebellum, brain stem, and diencephalon + basal ganglia after 1, 2, and 3 h of insulin-induced hypoglycemia. Blood glucose level decreased significantly after 1 h of insulin administration and remained at low level for 2 h thereafter, whereas, the protein content in different subcellular fractions from four brain regions did not show any significant change under this physiological stress.

High-affinity uptake of choline, a marker for cholinergic nerve terminals, is not specific in developing rat brain

Developmental changes in the synthesis of acetylcholine (ACh) were investigated in rat hippocampus and frontal cortex. Particular reference was made to the conversion, into ACh, of the choline accumulated by high-affinity uptake as defined using 1 microM hemicholinium-3 (HC-3). Using solutions containing 11.1 mM glucose, conversions were respectively 31 and 55%, in fine slices from 4-8-day-olds. Free choline accounted very largely for the remainder of the choline accumulated. In samples from adults, ACh accounted for 80% of the uptake. The inefficient conversions (into ACh) in immature brain were not the result of a requirement for ketone bodies as the source of acetyl-coenzyme A (acetyl-CoA). Greater rates of release of newly synthesised ACh, than in mature samples, were not responsible, neither were greater cholinesterase activities. The stimulation of high-affinity choline uptake, caused by prior depolarisation of the tissues using K+, also increased during development from 78 to 238% with hippocampus and from 49 to 170% with frontal cortex. Furthermore, prior depolarisation increased the efficiency with which choline, accumulated by high-affinity uptake, was converted into ACh. At all stages of development 80% of the additional choline accumulated, after depolarisation, was converted into ACh. It is concluded that the specificity of HC-3-sensitive uptake is incomplete in immature brain, i.e. high-affinity choline uptake is not exclusively into cholinergic neurones. The cholinergic neuronal compartment becomes more prominent during development so that the specificity is complete in mature brain.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro acetylcholine synthesis and oxidative metabolism during development of normal and brindled mouse brain

During early development normal and brindled mouse brain use 3-hydroxy-butyrate preferentially to glucose as a source for biosynthetic carbon units. On postnatal days 5-60, CO2 and acetylcholine production from 3-hydroxybutyrate decrease while that from glucose increases. Glucose metabolism exceeds that of 3-hydroxybutyrate after weaning (day 21). These findings indicate that the immature brain uses 3-hydroxybutyrate to support effectively oxidative metabolism and acetylcholine synthesis. Deficits in oxidative and acetylcholine metabolism occur in the developing brindled mouse, a genetic mutant with a defect in copper homeostasis. In the brindled mouse forebrain and cerebellum, the incorporation of either substrate into CO2 and acetylcholine was decreased 15%, 50% and 80% at days 5, 10 and 15 postnatal, respectively, when compared to the normal littermates. The brindled mouse demonstrates deficits in both oxidative metabolism and acetylcholine synthesis before the appearance of neuropathology.

Compartmentation and regulation of acetylcholine synthesis at the synapse

Acetylcholine and choline release was measured by using an automated and modified version of the chemiluminescence technique of Israel & Lesbats [(1981) Neurochem. Int. 3, 81-90]. A comparison of acetylcholine and choline release from synaptosomes demonstrated that acetylcholine release was K+-stimulated and inhibited by the Ca2+ ionophore A23187 and cyanide. Choline release, however, did not vary markedly under different conditions, suggesting that it is not associated with acetylcholine release at the nerve ending. Total acetylcholine synthesis in synaptosomal preparations was measured concurrently with the incorporation of [14C]acetyl and [3H]choline moieties by using the chemiluminescence method. Under sub-optimal glucose concentrations or in the absence of treatment of the synaptosomes with the acetylcholinesterase inhibitor phospholine, the incorporation of radioactivity exceeded total synthesis, indicating that cycling between acetylcholine and its precursors may occur. After treatment with phospholine, acetyl-group incorporation from D-[U-14C]glucose occurred without dilution of the precursor at optimal (1.0 mM) and low (0.1 mM) glucose concentrations; however, at very low (0.01 mM) glucose concentrations, dilution by a small endogenous pool occurred. [14C]Acetyl incorporation into acetylcholine was compared with various metabolic parameters. A closer correlation was observed between [14C]acetyl-group incorporation into acetylcholine and the calculated acetyl-carrier efflux from the mitochondria than with the calculated pyruvate-dehydrogenase-complex flux. The results are discussed with respect to the regulation of acetylcholine concentrations at the synapse and the mechanism whereby turnover occurs.

The effects of neonatal thyroid deficiency on acetylcholine synthesis and glucose oxidation in rat corpus striatum

The effects of propylthiouracil (PTU)-induced thyroid deficiency on [14C]acetylcholine synthesis and 14CO2 production from [U-14C]glucose in vitro, by fine prisms of the corpus striatum were investigated in developing rats. Consistent with deficits in choline uptake and choline acetyltransferase activity (Kalaria et al.17), PTU-treatment from two days after birth significantly impaired (27-39%) [14C]acetylcholine synthesis in striatal tissue taken from 3- or 6-week-old animals. In the thyroid-deficient (Tx) animals, 14CO2 production from [14C]glucose was unchanged in incubations in the presence of 5 mM K+ but was significantly reduced (33%) in medium with 31 mM K+ concentration. The addition of 10 mM DL-3-hydroxybutyrate in incubations with 5 mM K+ persistently inhibited 14CO2 production by striatal samples from the Tx rats. The fraction acetylated of [3H]choline accumulated by striatal prisms was unaffected by the PTU-induced thyroid deficiency. These findings suggest the development of fewer cholinergic nerve terminals in striatum during neonatal thyroid deficiency. Cholinergic nerve terminals that develop seem unaffected in their capacity for K+-stimulation and in their ability to acetylate transported [3H]choline.

beta-Hydroxybutyrate as a precursor to the acetyl moiety of acetylcholine

Rat brain cortex slices were incubated with 10 mM-glucose and trace amounts of [6-3H]glucose and [3-14C]beta-hydroxybutyrate. The effects of (-)-hydroxycitrate, an inhibitor of ATP-citrate lyase; methylmalonate, an inhibitor of beta-hydroxybutyrate dehydrogenase; and increasing concentrations of unlabeled acetoacetate were examined. The incorporation of label into lactate, citrate, malate, and acetylcholine (ACh) was measured and 3H:14C ratios calculated. Incorporation of [14C]beta-hydroxybutyrate into lactate was limited because of the low activity of gluconeogenic enzymes in brain, whereas incorporation of 14C label into Krebs cycle intermediates and ACh was higher than in previous experiments with [3H-,14C]-glucose. (-)-Hydroxycitrate (5.0 mM) reduced incorporation of [3H]glucose and [14C]beta-hydroxybutyrate into ACh. In contrast, slices incubated with methylmalonate (1 mM) showed a decrease in 14C incorporation without appreciably affecting glucose metabolism. The effects of high concentrations of methylmalonate were nonselective and yielded a generalized decrease in metabolism. Acetoacetate (1 mM) also produced a decreased 14C incorporation into ACh and its precursors. At 10 mM, acetoacetate reduced 3H and 14C incorporation into ACh without substantially affecting total ACh content. From the results, it is suggested that in adult rats beta-hydroxybutyrate can contribute to the acetyl moiety of ACh, possibly via the citrate cleavage pathway, though it is quantitatively less important than glucose and pyruvate. This contribution of ketone bodies could become significant should their concentration become abnormally high or glucose metabolism be reduced.

Brain acetylcholine synthesis declines with senescence

The synthesis of whole brain acetylcholine is reduced in two strains (C57BL and BALB/c) of senescent mice. The incorporation of [U-14C]glucose into acetylcholine decreased in both strains by 40 +/- 4 per cent in 10-month-old mice and by 58 +/- 9 percent in 30-month-old mice compared with mice 3 months old. The incorporation of [2H4]choline into acetylcholine declined 60 and 73 percent in 10- and 30-month-old mice, respectively. Deficits in the cholinergic system may contribute to brain dysfunctions that complicate senescence.

Decreases in amino acids and acetylcholine metabolism during hypoxia

Hypoxia impairs brain function by incompletely defined mechanisms. Mild hypoxia, which impairs memory and judgment, decreases acetylcholine (ACh) synthesis, but not the levels of ATP or the adenylate energy charge. However, the effects of mild hypoxia on the synthesis of the glucose-derived amino acids [alanine, aspartate, gamma-amino butyric acid (GABA), glutamate, glutamine, and serine] have not been characterized. Thus, we examined the incorporation of [U-14C]glucose into these amino acids and ACh during anemic hypoxia (injection of NaNO2), hypoxic hypoxia (15 or 10% O2), and hypoxic hypoxia plus hypercarbia (15 or 10% O2 with 5% CO2). In general, the synthesis of the amino acids and of ACh declined in parallel with each type of hypoxia we studied. For example, anemic hypoxia (75 mg/kg of NaNO2) decreased the incorporation of [U-14C]glucose into the amino acids and into ACh similarly. [Percent inhibition: ACh (57.4), alanine (34.4), aspartate (49.2), GABA (61.9), glutamine (59.2), glutamate (51.0), and serine (36.7)]. A comparison of several levels (37.5, 75, 150, 225 mg/kg of NaNO2) of anemic hypoxia showed a parallel decreased in the flux of glucose into ACh and into the amino acids whose synthesis depends on mitochondrial oxidation: GABA (r = 0.98), glutamate (r = 0.99), aspartate (r = 0.96), and glutamine (r = 0.97). The synthesis of the amino acids not dependent on mitochondrial oxidation did not correlate as well with changes in ACh metabolism: serine (r = 0.68) and alanine (r = 0.76). The decreases in glucose incorporation into ACh and into the amino acids with hypoxic hypoxia (15% or 10% O2) or hypoxic hypoxia with 5% CO2 were very similar to those with the two lowest levels of anemic hypoxic. Thus, and explanation of the brain's sensitivity to a decrease in oxygen availability must include the alterations in the metabolism of the amino acid neurotransmitters as well as ACh.

Acetylcholine synthesis and CO2 production from variously labeled glucose in rat brain slices and synaptosomes

The molecular basis of the close linkage between oxidative metabolism and acetylcholine (ACh) synthesis is still unclear. We studied this problem in slices and synaptosomes by measurement of ACh synthesis from [U-14C]glucose, and 14CO2 production from [3,4-14C]- and [2-14C]glucose, an index of glucose decarboxylation by the pyruvate dehydrogenase complex (PDH) and the enzymes of the Krebs cycle, respectively. We examined both under conditions that either inhibited (low O2 or antimycin) or stimulated (2,4-dinitrophenol [DNP] or 35 mM-K+) 14CO2 production from [2-14C]- or [3,4-14C]glucose. Incorporation of [U-14C]glucose into ACh was reduced under low O2 and by antimycin or DNP (by 51--93%) and stimulated by 35 mM-K+ (by 30--60%). Under all of these conditions, ACh synthesis and the decarboxylation of [3,4-14C]- and [2-14C]glucose were linearly related (r = 0.741 and 0.579, respectively). The difference in the rate of 14CO2 production from [3,4-14C]- and [2-14C]glucose was used as a measure of the amount of glucose that was not oxidatively decarboxylated (efflux). We found that efflux was reduced (low O2 and antimycin), unchanged (DNP in slices), or increased (DNP in synaptosomes and K+ stimulation in slices) compared with control values under 100% O2. ACh synthesis and efflux were more closely related (r = 0.860) than ACh synthesis and 14CO2 production from variously labeled glucoses.

Impaired synthesis of acetylcholine by mild hypoxic hypoxia or nitrous oxide

The effect of mild hypoxic hypoxia on brain metabolism and acetylcholine synthesis was studied in awake, restrained rats. Since many studies of hypoxia are done with animals anesthetized with nitrous oxide (N2O), the effects of N2O were evaluated. N2O (70%) increased the cerebral cortical blood flow by 33% and the cortical metabolic rate of oxygen by 26%. In addition, the synthesis of acetylcholine in N2O-anesthetized animals, measured with [U-14C]glucose and [1-2H2,2-2H2]choline, decreased by 45 and 53%, respectively. Consequently, mild hypoxia was studied in unanesthetized rats. Control rats breathing 30% O2 (partial pressure of oxygen, PaO2 = 120 mm Hg) were compared with rats exposed to 15% O2 (PaO2 = 57 mm Hg) or 10% O2 (PaO2 = 42 mm Hg). The synthesis of acetylcholine, measured with [U-14C]glucose, was decreased by 35 and 54% with 15% O2 and 10% O2, respectively; acetylcholine synthesis, measured with [1-2H2,2-2H2]choline, was decreased by 50 and 68% with 15% O2 and 10% O2, respectively. Animals breathing either 15% or 10% O2 had normal cerebral metabolic rates of oxygen but had increased brain lactates and increased cortical blood flows compared with animals breathing 30% O2. These results show that even mild hypoxic hypoxia impairs acetylcholine synthesis, which in turn may account for the early symptoms of brain dysfunction associated with hypoxia.