Hippocampal GABA helps suppress unwanted thoughts
Inhibition of hippocampal retrieval processes through gamma-aminobutyric acid (GABA) enables the suppression of intrusive thoughts, a recent study shows.
Thirty right-handed participants (mean age 24.7±4.3 years; males n=7) were subjected to two behavioural tasks. The first was a stop signal (SS) task, where participants were made to press a specific button in response to a coloured circle stimulus flashed on-screen. If the stimulus came with a beeping sound, participants were asked to prevent their response.
The second was a Think/No-Think (TNT) task, where they were shown reminders of unwanted thoughts and instructed to suppress retrieval. Functional magnetic resonance imaging (fMRI) was performed during each task and 1H magnetic resonance spectroscopy (1H-MRS) was used to measure GABA.
The researchers found that No-Think trial elicited significantly reduced activity both in right (p=0.003) and left (p=0.001) hippocampus. Retrieval suppression also resulted in impaired memory of the items, with No-Think items being recalled significantly less in a postscan recall test than the Think items (59±3 vs 65±3 percent; p=0.04).
Moreover, hippocampal GABA showed a significantly negative correlation with hippocampal blood-oxygen level-dependent activity signals during Think and No-Think (p<0.05 for both) conditions but not during Go or Stop conditions.
Finally, those with higher hippocampal GABA also showed stronger negative coupling between the hippocampus and dorsolateral prefrontal cortex, while those with lower hippocampal GABA did not show this negative coupling during retrieval suppression. The difference in connectivity between the two groups reached statistical significance (p=0.03).
“The present findings raise the possibility that GABAergic disinhibition in the hippocampus contributes to reduced fronto-hippocampal connectivity, hippocampal hyperactivity and intrusive symptomatology,” researchers said.