The Medial Septum Histaminergic Circuit In Feeding Behavior And Its Distinct Downstream Circuits

Obesity,a metabolic disease caused by an imbalance between caloric intake and energy expenditure,is becoming a global public health problem,and there is a lack of safe and effective drug targets and therapeutic approaches to control obesity in clinical practice.The central histaminergic system is thought to be involved in the regulation of feeding behavior,but there is functional diversity that may be closely related to the downstream neural circuits that are regulated.Our group has found that high density of histaminergic projection terminals in the medial septum（MS）.Recent studies have revealed that various types of MS neurons play important roles in the regulation of feeding,but whether they are regulated by upstream histaminergic signaling and mediates feeding remains unclear.Therefore,in this study,we used mice with histaminergic neurons specifically expressing Cre enzyme（HDC-Cre ERT2）,combined with in vivo calcium signal recording,histamine probes,optogenetics,pharmacogenetics,molecular biology,pharmacology and other means to investigate the role of histaminergic neurons of MS projections in regulating feeding behavior and the downstream neural circuit and cellular molecular mechanisms.First,we used fiber photometry combined with histamine probes to find that the cytosolic calcium signal of histaminergic neurons and the endogenous histamine content of MS decreased during feeding in mice,suggesting that the MS-projected histaminergic system may be involved in feeding regulation.Further,we found that inhibition of MS-projected histaminergic neural circuits using optogenetic specificity significantly increased feeding in satiated mice,while optogenetic or pharmacogenetic specific activation of MS-projected histaminergic neural circuits significantly inhibited re-feeding in fasted mice without affecting their emotional and motor functions.These results suggest that MS-projected histaminergic neural circuits can regulate feeding behavior in both directions.Subsequently,to clarify the neuronal types of histaminergic neural projections modulating MS,we used c-Fos immunohistochemistry and downstream cellular calcium signaling function assays and found that optogenetic activation of MS-projected histaminergic neural circuits could directly activate glutamatergic neurons within MS.In addition,reverse trans-monosynaptic viral tracing also confirmed the existence of a direct structural connection between histaminergic neurons and type 2 glutamate vesicular glutamate transporter（v GLUT2）neurons（a glutamatergic neuronal marker）within MS.Further,we found that pharmacogenetic chronic inhibition of MS glutamatergic neurons increased food intake and body weight in mice.More importantly,inhibition of MS glutamatergic neurons（but not GABAergic neurons）reversed the inhibitory effect of activation of MS-projected histaminergic neurocircuits on feeding.These results suggest that MS-projected histaminergic neural circuits regulate feeding behavior in a bidirectional manner mainly through MS glutamatergic neurons.We next investigated the receptor types mediating this histaminergic neural circuits that regulate feeding.We found that local administration of histamine 2 receptors（H2Rs）antagonists,but not histamine 1 receptors（H1Rs）,in MS reversed the activation of MS-projected histaminergic neural circuit-mediated anorexia,and local administration of histamine H2Rs agonists in MS induced direct feeding inhibition by pharmacological means combined with optogenetics.Also,there was a significant pathological decrease in the expression of H2Rs on glutamatergic neurons in MS in obese mice.Therefore,we used genetic intervention to selectively knock down the expression of H2Rs on MS glutamatergic neurons to mimic the pathological condition,and found that selective knock down of H2Rs expression on MS glutamatergic neurons accelerated the weight gain of mice;while long-term chronic administration of the clinical H2Rs agonist amthamine in MS significantly slowed weight gain in obese mice.These results suggest that H2Rs on glutamatergic neurons,but not H1Rs,mediate the feeding regulation of MS-projected histaminergic neural circuits and can further regulate body weight.In addition,we further performed downstream tracing of MS^{v GLUT2} neurons and found that MS^{v GLUT2} mediated feeding inhibition through projections to the downstream paraventricular nucleus of the hypothalamus（PVH）.In summary,we found that the MS-projected histaminergic neural circuit regulates feeding behavior and body weight status in both directions through H2Rs-regulated projections to MS^{v GLUT2} neurons.This discovery provides a new experimental basis for the analysis of the neural circuits and molecular mechanisms regulating feeding behavior,and is expected to provide potential drug targets for the precise treatment of obesity.