Divergent Roles Of Midbrain Raphe Serotoninergic Subpopulations In Temporal Lobe Epilepsy

Objevtive Temporal lobe epilepsy（TLE）is a common type of drug-resistant epilepsy,with unclear pathogenic mechanisms.Experimental and clinical research found that the serotoninergic（5-HTergic）systems take a role in TLE,but having contradictory reports about the function.Midbrain raphe nuclei,including the dorsal raphe（DR）and the median raphe（MR）,are the main location of the 5-HTergic neurons which project to the forebrain.In this research,we focus on the role of different 5-HTergic subpopulations of the midbrain raphe,including the DR and MR which send projections to the forebrain,in mice model of TLE.Methods In mouse hippocampal kindling model,using immunochemistry,calcium fiber photometry,optogenetics and deep brain stimulation（DBS）,we investigate the causal role of MR or DR 5-HTergic neurons in seizure of TLE.In addition,retrograde virus tracing and projection specific calcium fiber photometry and optogenetics were used to study the divergent functions of circuit specific 5-HTergic subpopulations.Furthermore,trans-monosynaptic virus tracing combined with pharmacological intervention were applied to preliminarily investigate the mechanism of the anti-seizure effect.Results Activities of the DR 5-HTergic neurons are consistently increased during both focal seizure（FS）and generalized seizure（GS）,whereas activities of the MR 5-HTergic neurons are mixed with increase and decrease only during GSs.Inhibition of the DR 5-HTergic neurons through optogenetic inhibition or 1-Hz DBS retarded the seizure development during the late period,while optogenetic activation or 20-Hz and 100-Hz DBS has no effects.In contrast,optogenetic modulation of the MR 5-HTergic neurons has no effect on the seizures.However,the calcium activities of 5-HTergic^MR-vCA3,5-HTergic^MR-MS and 5-HTergic^MR-LH axons are increased during GSs while calcium activities of 5-HTergic^MR-Hb axons are decreased during GSs.And optogenetic activation of the 5-HTergic^MR-vCA3 circuits significantly retarded the development of seizure and inhibited the GS,while optogenetic activation of the 5-HTergic^MR-Hb circuit accelerate the seizure acquisition and exacerbate the GS and optogenetic activation of 5-HTergic^MR-MScircuit or 5-HTergic^MR-LH circuit has no effect on seizures.In addition,the vCA3-projecting 5-HTergic neurons are almost distinct subpopulations from the Hb-projecting5-HTergic neurons,indicating two parallel 5-HTergic circuits which are functionally opposite for the maintain of the internal balance.Besides,MR 5-HTergic neurons has a mono-synaptic connection with the vCA3 glutamatergic neurons but not GABAergic neurons,and intra-vCA3 injection with 5-HT_1A receptor antagonist or 5-HT₃ receptor antagonist partially reversed the anti-seizure effect of activation of the 5-HTergic^MR-vCA3circuits.Conclusions The midbrain raphe 5-HTergic neurons are consists of projection-specific and anatomical distinct subpopulations,of which the activities and functions are divergent in TLE.Only activation of the 5-HTergic^MR-vCA3circuit or inhibition of the DR 5-HTergic neurons may have promising anti-seizure influence,while modulation of the other subpopulation may exhibit no function or even pro-seizure effect.These findings are beneficial for the understanding of the pathological mechanisms of diversity of midbrain 5-HTergic neurons in TLE and may provide new insights for the prisicion medicine in TLE.