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Background: Psychosis-relevant preclinical studies have demonstrated GABAergic drugs such as benzodiazepines can downregulate hippocampal hyperactivity and prevent the emergence of psychosis- like phenotypes. However, whether benzodiazepines can reduce and normalise increased hippocampal resting cerebral blood flow (rCBF) observed in individuals at clinical high-risk for psychosis (CHR-P) remains unknown. Aims: To determine whether an acute dose of diazepam can reduce and normalise hippocampal and subfield rCBF in CHR-P individuals. Methods: We conducted a within-subject, double-blind, placebocontrolled, randomised, cross-over design study in 24 CHR-P individuals (mean [± SD] age: 24.1 [±4.8] years, 15F). Participants underwent two MRI sessions, three weeks apart; once under a single dose of diazepam (5 mg) and once under placebo (50 mg ascorbic acid). A previously collected dataset of 21 healthy controls (HC) was used for comparison. rCBF was measured using pseudocontinuous arterial spin labelling and sampled in each participant' s native space using participant-specific hippocampus/subfield masks generated with the MAGeT Brain toolbox. Individual Mixed ANCOVAs (covarying for global rCBF, age, and sex) and linear mixed-effects models (participant ID as random effect) investigated the effect of group (CHR-P placebo vs. HC and CHR-P diazepam vs. HC) and drug (CHR-P placebo vs. diazepam), respectively, on hippocampal/subfield rCBF per ROI. These models were repeated voxel-wise in study-specific template space to investigate whole-brain effects. Significance was set at pFDR<0.05. Pearson' s correlations assessed whether baseline clinical characteristics could predict diazepam-induced changes in hippocampal rCBF. Results: CHR-P individuals under placebo showed significantly increased rCBF compared to HC in the hippocampus (F(1,41)=24.7, pFDR<0.001), and this was significantly reduced by diazepam (t(69)=-5.1, pFDR <0.001) to the extent that it no longer differed from HC (F(1,41)=0.4, pFDR=0.204). This effect of hyperactivity and normalisation under diazepam was seen across all subfields studied and in several psychosisrelevant cortical and subcortical regions connected to the hippocampus, including medial/dorsolateral prefrontal cortex, nucleus accumbens, and amygdala. Smallest reductions in diazepaminduced bilateral hippocampal rCBF change were observed in CHR-P individuals with highest attenuated psychotic symptom severity (r= 0.494, p=0.014) and poorest social functioning (r=- 0.416, p=0.043) at baseline, which appeared to be driven by the left CA4/DG (positive symptom severity: r=0.528, p=0.008; social functioning: r=-0.503, p=0.012). increased rCBF in the hippocampus and subfields of CHR-P individuals, providing proof-of-concept of the efficacy of GABAenhancing drugs to modulate hippocampal hyperactivity in this clinical group. Our results that CHR-P individuals who were clinically most unwell showed the smallest changes in rCBF suggest the development of more hippocampal-selective GABAergic pharmacological agents (e.g., those targeting a5-GABAA receptors) as a promising strategy for regulating hippocampal hyperactivity and preventing psychosis development. [ABSTRACT FROM AUTHOR]