SUMMARY The hippocampus in schizophrenia is characterized by both hypermetabolism and reduced size. It remains unknown whether these abnormalities are mechanistically linked. Here, in addressing these questions we used MRI tools that can map hippocampal metabolism and structure in patients and mouse models. In at-risk patients, hypermetabolism was found to begin in CA1 and spread to the subiculum after psychosis onset. CA1 hypermetabolism at baseline predicted hippocampal atrophy, which occured during progression to psychosis, most prominently in similar regions. Next, we used ketamine to model conditions of acute psychosis in mice. Acute ketamine reproduced a regional pattern of hippocampal hypermetabolism, while repeated exposure shifted the hippocampus to a hypermetabolic state with concurrent atrophy and pathology in parvalbumin-expressing interneurons. Parallel in vivo experiments using LY379268 and direct measurements of extracellular glutamate showed that glutamate drives both neuroimaging abnormalities. These findings show that hippocampal hypermetabolism leads to atrophy in psychotic disorder and suggest glutamate as a pathogenic driver.

SUMMARY Despite the well-established role of serotonin signaling in mood regulation, causal relationships between serotonergic neuronal activity and behavior remain poorly understood. Using a pharmacogenetic approach, we find that selectively increasing serotonergic neuronal activity in wild-type mice is anxiogenic and reduces floating in the forced-swim-test, while inhibition has no effect on the same measures. In a developmental mouse model of altered emotional behavior, increased anxiety and depression-like behaviors correlate with reduced dorsal raphe and increased median raphe serotonergic activity. These mice display blunted responses to serotonergic stimulation and behavioral rescues through serotonergic inhibition. We furthermore identify opposing consequences of dorsal versus median raphe serotonergic neuron inhibition on floating behavior, together suggesting that median raphe hyperactivity increases anxiety, while a low dorsal/median raphe serotonergic activity ratio increases depression-like behavior. Thus we find a critical role of serotonergic neuronal activity in emotional regulation and uncover opposing roles of median and dorsal raphe function.

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) plays important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic β-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D 2 -like receptors including D 2 (D2R) and D 3 (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic β-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in β-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA’s inhibition of GSIS. Our results show that uptake of L-DOPA is essential for establishing intracellular DA stores in β-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which β-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D 2 -like receptors modify DA release to modulate GSIS. Lastly, β-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo . These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D 2 -like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

The identification of intestinal dysbiosis in patients with neurological and psychiatric disorders has highlighted the importance of gut-brain communication and yet, the question regarding the identity of the components responsible for this cross talk remains open. We previously reported that relapsing remitting multiple sclerosis patients (RRMS) treated with dimethyl fumarate have a prominent depletion of the gut microbiota, thereby suggesting that studying the composition of plasma and cerebrospinal fluid (CSF) samples from these patients may help to identify microbially derived metabolites. We used a functional xenogeneic assay consisting of cultured rat neurons exposed to CSF samples collected from multiple sclerosis patients before and after dimethyl fumarate treatment to assess neurotoxicity and then conducted a metabolomic analysis of plasma and CSF samples to identify metabolites with differential abundance. A weighted correlation network analysis, allowed us to identify groups of metabolites, present in plasma and CSF samples, whose abundance correlated with the neurotoxic potential of the CSF. This analysis identified the presence of phenol and indole group metabolites of bacterial origin (e.g. p-cresol-sulfate, indoxyl-sulfate and N-phenylacetylglutamine) as potentially neurotoxic and decreased by treatment. Chronic exposure of cultured neurons to these metabolites impaired their firing rate and induced axonal damage, independent from mitochondrial dysfunction and oxidative stress, thereby identifying a novel pathway of neurotoxicity. Clinical, radiological and cognitive test metrics were also collected in treated patients at follow-up visits. Improved MRI metrics, disability and cognition were only detected in dimethyl fumarate -treated RRMS patients. The levels of the identified metabolites of bacterial origin (p-cresol-sulfate, indoxyl-sulfate and N-phenyl-acetyl-glutamine) were inversely correlated to MRI measurements of cortical volume and directly correlated to the levels of neurofilament light chain, an established biomarker of neurodegeneration. Our data suggest that phenol and indole derivatives from the catabolism of tryptophan and phenylalanine are microbially derived metabolites, which may mediate gut-brain communication and induce neurotoxicity in multiple sclerosis.