Repeated administration of L-alanine to mice reduces behavioural despair and increases hippocampal mammalian target of rapamycin signalling: Analysis of gender and metabolic effects.

Abstract

Background: The amino acid L-alanine, has been shown to be elevated in biofluids during major depression but its relevance remains unexplored.

Aim: We have investigated the effects of repeated L-alanine administration on emotional behaviours and central gene expression in mice.

Methods: Mice received a daily, 2-week intraperitoneal injection of either saline or L-alanine at 100 or 200 mg/kg and were exposed to the open field, light-dark box and forced swim test. The expression of L-alanine transporters (asc-1, ASCT2), glycine receptor subunits (GlyRs), NMDA receptor subunits (GluNs) mRNAs were measured, together with western blots of the signalling protein mammalian target of rapamycin (mTOR). Since L-alanine modulates glucose homeostasis, peripheral and central metabolomes were evaluated with ¹H-NMR.

Results: L-alanine administration at 100 mg/kg, but not at 200 mg/kg, to both male and female mice increased latency to float and reduced floating time in the forced swim test, but had no effect on anxious behaviour in the open field and light-dark box tests. There was a significant reduction in mRNAs encoding asc-1 and ASCT2 and GluN2B in the hippocampus of mice following 100 mg/kg L-alanine only. On western blots, hippocampal GluN2B immunoreactivity was reduced, but mTOR signalling was increased in the 100 mg/kg L-alanine group. 1H-NMR revealed gender-specific changes in the forebrain, plasma and liver metabolomes only at 200 mg/kg of L-alanine.

Conclusions: Our data suggest that L-alanine may have antidepressant-like effect that may involve the modulation of glutamate neurotransmission independently of metabolism. In major depression, therefore, elevated L-alanine may be a homeostatic response to pathophysiological processes, though this will require further investigation.