Branched-chain amino acids increase the seizure threshold to picrotoxin in rats

Neuroanesthesiology Update

This review is intended to provide a summary of the literature pertaining to the perioperative care of neurosurgical patients and patients with neurological diseases. General topics addressed in this review include general neurosurgical considerations, stroke, neurological monitoring, and perioperative disorders of cognitive function.

Branched-Chain Amino Acids and Seizures: A Systematic Review of the Literature

BACKGROUND

Up to 40% of patients with epilepsy experience seizures despite treatment with antiepileptic drugs; however, branched-chain amino acid (BCAA) supplementation has shown promise in treating refractory epilepsy.

OBJECTIVES

The purpose of this systematic review was to evaluate all published studies that investigated the effects of BCAAs on seizures, emphasizing therapeutic efficacy and possible underlying mechanisms.

METHODS

On 31 January, 2017, the following databases were searched for relevant studies: MEDLINE (OvidSP), EMBASE (OvidSP), Scopus (Elsevier), the Cochrane Library, and the unindexed material in PubMed (National Library of Medicine/National Institutes of Health). The searches were repeated in all databases on 18 February, 2019. We only included full-length preclinical and clinical studies that were published in the English language that examined the effects of BCAA administration on seizures.

RESULTS

Eleven of 2045 studies met our inclusion criteria: ten studies were conducted in animal models and one study in human subjects. Seven seizure models were investigated: the strychnine (one study), pentylenetetrazole (two studies), flurothyl (one study), picrotoxin (two studies), genetic absence epilepsy in rats (one study), kainic acid (two studies), and methionine sulfoximine (one study) paradigms. Three studies investigated the effect of a BCAA mixture whereas the other studies explored the effects of individual BCAAs on seizures. In most animal models and in humans, BCAAs had potent anti-seizure effects. However, in the methionine sulfoximine model, long-term BCAA supplementation worsened seizure propagation and caused neuron loss, and in the genetic absence epilepsy in rats model, BCAAs exhibited pro-seizure effects.

CONCLUSIONS

The contradictory effects of BCAAs on seizure activity likely reflect differences in the complex mechanisms that underlie seizure disorders. Some of these mechanisms are likely mediated by BCAA's effects on glucose, glutamate, glutamine, and ammonia metabolism, activation of the mechanistic target of rapamycin signaling pathway, and their effects on aromatic amino acid transport and neurotransmitter synthesis. We propose that a better understanding of mechanisms by which BCAAs affect seizures and neuronal viability is needed to advance the field of BCAA supplementation in epilepsy.

Effects of Branched-Chain Amino Acid Supplementation on Spontaneous Seizures and Neuronal Viability in a Model of Mesial Temporal Lobe Epilepsy

BACKGROUND

The essential branched-chain amino acids (BCAAs) leucine, isoleucine, and valine have recently emerged as a potential novel treatment for medically refractory epilepsy. Blood-derived BCAAs can readily enter the brain, where they contribute to glutamate biosynthesis and may either suppress or trigger acute seizures. However, the effects of BCAAs on chronic (ie, spontaneous recurrent) seizures and epilepsy-associated neuron loss are incompletely understood.

MATERIALS AND METHODS

Sixteen rats with mesial temporal lobe epilepsy were randomized into 2 groups that could drink, ad libitum, either a 4% solution of BCAAs in water (n=8) or pure water (n=8). The frequency and relative percent of convulsive and nonconvulsive spontaneous seizures were monitored for a period of 21 days, and the brains were then harvested for immunohistochemical analysis.

RESULTS

Although the frequency of convulsive and nonconvulsive spontaneous recurrent seizures over a 3-week drinking/monitoring period were not different between the groups, there were differences in the relative percent of convulsive seizures in the first and third week of treatment. Moreover, the BCAA-treated rats had over 25% fewer neurons in the dentate hilus of the hippocampus compared with water-treated controls.

CONCLUSIONS

Acute BCAA supplementation reduces seizure propagation, whereas chronic oral supplementation with BCAAs worsens seizure propagation and causes neuron loss in rodents with mesial temporal lobe epilepsy. These findings raise the question of whether such supplementation has a similar effect in humans.

Soy-based diet exacerbates seizures in mouse models of neurological disease

Seizures are a common phenotype in many neurological disorders including Alzheimer's disease, Down syndrome, and fragile X syndrome. Mouse models of these disorders overexpress amyloid-β protein precursor (AβPP) and amyloid-β (Aβ) and are highly susceptible to audiogenic-induced seizures (AGS). We observed decreased AGS in these mice fed a casein-based, purified diet (D07030301) as opposed to a standard soy protein-containing, non-purified diet (Purina 5015). Our objective in this manuscript was to determine if soy protein, and in particular soy isoflavones, in the Purina 5015 were contributing to the seizure phenotype. Wild running, AGS, and death rates were assessed in juvenile mice fed Purina 5015, D07030301, D07030301 containing soy protein, or D07030301 supplemented with individual isoflavones (750 mg/kg daidzein or genistein). A short treatment (3 days) with Purina 5015 induced wild running and AGS in Alzheimer's disease mice. A 3-day treatment with daidzein-supplemented diet, but not genistein, induced wild running in wild type mice. To understand the mechanism underlying daidzein activity, we assessed dendritic AβPP expression in primary, cultured, wild type neurons treated with daidzein or genistein. In vitro, daidzein significantly increased dendritic AβPP. Thus, the soy isoflavone daidzein recapitulated seizure induction in vivo and altered AβPP expression in vitro. These results have important implications for individuals on soy-based diets as well as for rodent model research.

Impaired growth and neurological abnormalities in branched-chain alpha-keto acid dehydrogenase kinase-deficient mice

The BCKDH (branched-chain alpha-keto acid dehydrogenase complex) catalyses the rate-limiting step in the oxidation of BCAAs (branched-chain amino acids). Activity of the complex is regulated by a specific kinase, BDK (BCKDH kinase), which causes inactivation, and a phosphatase, BDP (BCKDH phosphatase), which causes activation. In the present study, the effect of the disruption of the BDK gene on growth and development of mice was investigated. BCKDH activity was much greater in most tissues of BDK-/- mice. This occurred in part because the E1 component of the complex cannot be phosphorylated due to the absence of BDK and also because greater than normal amounts of the E1 component were present in tissues of BDK-/- mice. Lack of control of BCKDH activity resulted in markedly lower blood and tissue levels of the BCAAs in BDK-/- mice. At 12 weeks of age, BDK-/- mice were 15% smaller than wild-type mice and their fur lacked normal lustre. Brain, muscle and adipose tissue weights were reduced, whereas weights of the liver and kidney were greater. Neurological abnormalities were apparent by hind limb flexion throughout life and epileptic seizures after 6-7 months of age. Inhibition of protein synthesis in the brain due to hyperphosphorylation of eIF2alpha (eukaryotic translation initiation factor 2alpha) might contribute to the neurological abnormalities seen in BDK-/- mice. BDK-/- mice show significant improvement in growth and appearance when fed a high protein diet, suggesting that higher amounts of dietary BCAA can partially compensate for increased oxidation in BDK-/- mice. Disruption of the BDK gene establishes that regulation of BCKDH by phosphorylation is critically important for the regulation of oxidative disposal of BCAAs. The phenotype of the BDK-/- mice demonstrates the importance of tight regulation of oxidative disposal of BCAAs for normal growth and neurological function.

Branched-chain amino acid-induced hippocampal norepinephrine release is antagonized by picrotoxin: evidence for a central mode of action

Previous studies indicated that administration of a 1:1:1 mixture of the branched-chain amino acids leucine, isoleucine, and valine (BCAA) decreased the response to pain. The present study investigates the effects of BCAA on release of norepinephrine (NE) from isolated hippocampal brain slices. BCAA evoked 3H-NE release in a concentration-dependent manner. This effect was antagonized by the gamma aminobutyric acid (GABA) receptor antagonist picrotoxin, again in a concentration-dependent manner, suggesting that the effect may be mediated via a GABA receptor. Given the role of NE and of GABA receptors in the central response to pain, it is possible that the BCAA may exert their antinociceptive properties through activation of GABA receptors.

Modulation of pentylenetetrazol-induced seizure activity by branched-chain amino acids and alpha-ketoisocaproate

Branched-chain amino acids, and mainly leucine act as nitrogen donors in the cerebral glutamate-glutamine cycle, thereby reducing brain excitability. Rats equipped with cortical electrodes received 300 mg/kg of leucine, isoleucine, valine or the ketoacid of leucine, alpha-ketoisocaproate at 2 h before the induction of seizures by 40 mg/kg pentylenetetrazol. Control groups received saline or a commercial mixture of amino acids, Vamine(R). Leucine and isoleucine increased the latency to absence-like and tonic-clonic seizures but did not influence the duration of the tonic-clonic seizure. Vamine(R), valine and alpha-ketoisocaproate had no effect. These data are consistent with the role of leucine in buffering brain glutamate concentration.

The effects of branched chain amino acid infusion on pain perception and plasma concentrations of monoamines

Infusions of branched chain amino acids (BCAA) have been shown to have several CNS-mediated effects including antinociceptive action. We investigated the effects of BCAA infusion on pain perception, respiratory control, and plasma monoamine concentrations. Six healthy female volunteers were given in a double-blind, random, crossover design an 8-h infusion (1.75 ml/kg/h) of either (a) Ringers lactate, (b) conventional 4% amino acid solution, or (c) 4% BCAA solution with intervals of at least 48 h. Respiratory control was evaluated with continuous capnography. Pain perception was measured using dental dolorimetry for sharp pain, and pain transmitted by afferent C-fibers was evaluated with tourniquet test. Changes in vigilance were measured using critical flicker fusion technique. Evaluations were made for baseline, and after 2.5, 5, and 8 h. Plasma samples were collected at the same time points for amino acid and monoamine analysis. BCAA infusion resulted in significant increases of plasma concentrations of all BCAAs, with a simultaneous decrease in concentrations of aromatic amino acids. Of the measured monoamines and their metabolites dihydroxyphenylacetic acid (DOPAC) decreased, showing significant treatment effect for BCAA. Despite these changes no significant effect of BCAAs on respiratory control, vigilance, or pain perception was observed. In conclusion, despite significant changes in plasma concentrations of both amino acids and DOPAC, BCAA infusion did not show any clinically relevant antinociceptive effect.

The antinociceptive effects of branched-chain amino acids: evidence for their ability to potentiate morphine analgesia

The effect of branched-chain amino acids (BCAA) on pain threshold was studied in rats. Nociception was induced by the hot-plate analgesia meter, a method measuring supraspinally organized pain responses. After a single intravenous injection of BCAA (320 mg/kg), the percent change in latency time to the pain response significantly increased by 19% in 60 min, and by 22% in 75 min (p < 0.005), as compared to an injection of an equal volume of a standard concentration of an amino acid solution or physiological saline. Subsequently, we studied the interaction of BCAA with opioid-type analgesia. In combination with intravenously injected morphine (3 mg/kg), BCAA significantly potentiated and prolonged the action of morphine using the hot-plate test. From 5 min after morphine injection, the latencies to a pain response were markedly higher with the combination of BCAA and morphine (+80% and +89% at 5 min after morphine injection, if BCAA was administered 45 or 60 min prior to morphine injection, respectively) when compared with the effect of morphine alone (+13% at 5 min; p < 0.005). BCAA demonstrated analgesic effects, which, in combination with morphine, potentiated and prolonged the antinociceptive action of morphine. BCAA may represent a new adjunct treatment modality for acute and chronic pain, and give us further insight into the mechanisms of pain control.

Effects of anticonvulsant drug gabapentin on the enzymes in metabolic pathways of glutamate and GABA

Gabapentin is a novel anticonvulsant drug. The anticonvulsant mechanism of gabapentin is not known. Based on the amino acid structure of gabapentin we explored its possible effects on glutamate and gamma-aminobutyric acid (GABA) metabolism in brain as they may relate to its anticonvulsant mechanisms of action. Gabapentin was tested for its effects on seven enzymes in the metabolic pathways of these two neurotransmitters: alanine aminotransferase (AL-T), aspartate aminotransferase (AS-T), GABA aminotransferase (GABA-T), branched-chain amino acid aminotransferase (BCAA-T), glutamine synthetase (Gln-S), glutaminase (GLNase), and glutamate dehydrogenase (GDH). In the presence of 10 mM gabapentin, only GABA-T, BCAA-T, and GDH activities were affected by this drug. Inhibition of GABA-T by gabapentin was weak (33%). The Ki values for inhibition of cytosolic and mitochondrial forms of GABA-T (17-20 mM) were much higher than the Km values for GABA (1.5-1.9 mM). It is, therefore, unlikely that inhibition of GABA-T by gabapentin is clinically relevant. As with leucine, gabapentin stimulated GDH activity. The GDH activity in rat brain synaptosomes was activated 6-fold and 3.4-fold, respectively, at saturating concentrations (10 mM) of leucine and gabapentin. The half-maximal stimulation by gabapentin was observed at approximately 1.5 mM. Gabapentin is not a substrate of BCAA-T, but it exhibited a potent competitive inhibition of both cytosolic and mitochondrial forms of brain BCAA-T. Inhibition of BCAA-T by this drug was reversible. The Ki values (0.8-1.4 mM) for inhibition of transamination by gabapentin were close to the apparent Km values for the branched-chain amino acids (BCAA) L-leucine, L-isoleucine, and L-valine (0.6-1.2 mM), suggesting that gabapentin may significantly reduce synthesis of glutamate from BCAA in brain by acting on BCAA-T.

Effects of valine, leucine, isoleucine, and a balanced amino acid solution on the seizure threshold to picrotoxin in rats

During infusion of branched-chain amino acids (BCAAs) in humans, changes in ventilatory drive, sleeping pattern, and appetite have been reported. The mechanism by which BCAA exerts their effects on CNS remains unclear. An infusion of a BCAA solution (300 mg/kg) has previously been found to increase the seizure threshold in rats to the proconvulsant drug picrotoxin, an antagonist on the GABA-benzodiazepine receptor complex. In this study, each of the BCAAs given separately (valine, leucine, isoleucine; 300 mg/kg) (n = 10) increased the mean latency time to onset of seizures vs. placebo as an indication of an increased seizure threshold. A balanced amino acid solution (Vamin-Glucose) had no effect on the seizure threshold. Thus, these CNS effects are specific for BCAAs and occur with all three.