Ketamine modulates hippocampal neurochemistry and functional connectivity: a combined magnetic resonance spectroscopy and resting-state fMRI study in healthy volunteers
A growing body of evidence suggests glutamate excess in schizophrenia and that N-methyl-D-aspartate receptor (NMDAR) hypofunction on γ-aminobutyric acid (GABA) interneurons disinhibiting pyramidal cells may be relevant to this hyperglutamatergic state. To better understand how NMDAR hypofunction affects the brain, we used magnetic resonance spectroscopy and restingstate functional magnetic resonance imaging (MRI) to study the effects of ketamine on hippocampal neurometabolite levels and functional connectivity in 15 healthy human subjects. We observed a ketamine-induced increase in hippocampal Glx (glutamate +glutamine; F = 3.76; P = 0.04), a decrease in fronto-temporal (t = 4.92, PFDR < 0.05, kE = 2198, x = - 30, y = 52, z = 14) and temporoparietal functional connectivity (t = 5.07, PFDR < 0.05, kE = 6094, x = - 28, y = - 36, z = - 2), and a possible link between connectivity changes and elevated Glx. Our data empirically support that hippocampal glutamatergic elevation and resting-state network alterations may arise from NMDAR hypofunction and establish a proof of principle whereby experimental modelling of a disorder can help mechanistically integrate distinct neuroimaging abnormalities in schizophrenia.