Abstract
Precursors of neurotransmitters are increasingly often investigated as potential, easily-accessible methods of neuromodulation. However, the amino-acid glutamine, precursor to the brain’s main excitatory and inhibitory neurotransmitters glutamate and GABA, remains notably little investigated. The current double-blind, randomized, placebo-controlled study provides first evidence 2.0 g glutamine administration in healthy adults affects response selection but not motor sequence learning in a serial reaction time task. Specifically, glutamine increased response selection errors when the current target response required a different hand than the directly preceding target response, which might indicate enhanced cortical excitability via a presumed increase in glutamate levels. These results suggest glutamine can alter cortical excitability but, despite the critical roles of glutamate and GABA in motor learning, at its current dose glutamine does not affect sequence learning.
Highlights
There is growing research interest in evaluating the neuromodulatory effects of exogenous administration of neurotransmitter precursors on cognition
Groups were compared on response error percentage (REP) and mean reaction time (MRT) in the serial reaction time (SRT) task
The present paper is one of the first to report proof-of-principle that the amino-acid Gln, the precursor of the brain’s main excitatory neurotransmitter Glu and inhibitory neurotransmitter GABA8, 9, modulates cognitive function related to response selection but not sequence learning
Summary
There is growing research interest in evaluating the neuromodulatory effects of exogenous administration of neurotransmitter precursors on cognition. It could be argued that increased cortical excitation due to high Glu levels might have the opposite effect by facilitating activation of competing responses, increasing the time necessary to resolve response selection processes and impairing accuracy of these processes[20] This model of the roles of Glu and GABA in response selection is supported by studies that directly assessed brain neurotransmitter levels using magnetic resonance spectroscopy (MRS), albeit with a particular focus on GABA. Despite the critical roles of Glu and GABA in response selection and cortical excitability, we are not aware of any studies with healthy adults that have focused on the effects of Gln administration on these processes Because of this lack of previous studies and the fact that Gln is the precursor to both Glu and GABA, which are hypothesized to have opposite effects on response selection and learning, it is difficult to establish a priori the direction in which Gln administration modulates performance. Contrasting results from stimulus-oriented, random blocks with those from SOC blocks can shed light on a possible differential effect of Gln on stimulus-based versus plan-based action control
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