GABAergic Neurons Of Ventral Tegmental Area Target Dorsal Raphe Nucleus And Participate In Morphine Dependence

Ventral tegmental area(VTA)is a midbrain area enriched in dopaminergic(DA)neurons,and is considered to be reward center.About 30%VTA neurons are GABAergic,some of which are inteneurons inhibiting VTA DAergic neurons,and others are projection neurons.The roles of VTA GABAergic neurons are gaining much attention,and are considered to participate in reward related behaviors together with DAergic neurons.VTA is a large area consisted of rostral part(rVTA)and caudal part(cVTA)according to anatomy and function.Pharmacology and behavior studies showed that rostral and caudal(R-C)VTA function differently,however,the differences in input output and functions of R-C VTA GABAergic neurons remain unknown.Dorsal raphe nucleus(DRN)is the most concentrated place of 5-hydroxytryptamine(5-HT)neurons.5-HTergic neurons are mainly located in the medial DRN,and send projections to the forebrain.Lateral wings of DRN are mostly GABAergic neurons providing intemeurons to inhibit 5-HTergic neurons.It has been considered that DRN 5-HTergic neurons function oppositely to VTA DAergic neurons and signal punishment.Recent studies proved that DRN projection neurons also signal reward.Retrograde tracing results showed that neurons projecting from VTA to DRN are not DAergic,and the precise connections and functions between VTA and DRN remains to be studied.In our research,at first we traced the whole brain monosynaptic inputs of DRN 5-HTergic and GABAergic neurons by means of rabies virus tracing,and found that the midbrain inputs were concentrated in VTA.Immunohistochemistry results showed that the input cells in VTA were mostly GABAergic.Moreover,morphology and electrophysiology results showed that rVTA tended to innervate DRN GABAergic neurons while cVTA tended to target DRN 5-HTergic neurons.Activation of rVTA inhibitory afferents to DRN inhibited the firing rates of DRN GABAergic neurons while elevated the firing rates of DRN non-GABAergic neurons,indicating a disinhibition of DRN 5-HTergic neurons through inhibiting DRN GABAergic neurons by rVTA inhibitory afferentsReal time place preference(RTPP)results showed that the activation of rVTA?DRN circuit resulted in aversive behaviors,and inhibiting this circuit resulted in rewarding effects.On the contrary,the activation of cVTA?DRN circuit resulted in rewarding effects,and inhibiting this circuit led to aversive responses.Repeated injection of morphine resulted in a reduction of inhibitory inputs of DRN GABAergic neurons More specifically,the GABAergic transmissions from rVTA to DRN were depressed by repeated exposure to morphine,while the GABAergic transmissions from cVTA to DRN were not altered.The difference between R-C VTA might be due to the different distributions of ?-opioid receptor(MOP):MOPs were located in the rVTA?DRN terminals but not in the cVTA?DRN terminals.Furthermore,altering the plasticity of rVTA?RRN circuit occluded the morphine induced conditioned place preference(CPP)In summary,first,we found that rVTA GABAergic neurons inhibited DRN GABAergic neurons and thus disinhibited DRN 5-HTergic neurons,while cVTA GABAergic neurons inhibited DRN 5-HTergic neurons directly.Second,we proved that R-C VTA inhibitory afferents to DRN functioned oppositely in reward related behaviors.Last but not least,rVTA?DRN but not cVTA?DRN circuit was depressed by repeated administration of morphine.This might be due to the different distribution of MOPs.Our research reported an inhibitory circuit which was driven little attention before,and the precise connections from VTA to DRN were well described here.This might provide new insights for morphine dependence treatment.