Neuronal connectivity within the mesolimbic system

National Institute On Drug Abuse

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Abstract

Findings from clinical studies and animal models have provided evidence supporting the crucial role that dopamine neurons from the Ventral Tegmental Area (VTA) play in the relapse to seek drugs of abuse, such of cocaine. It is unclear how different inputs to VTA participate in regulating the activity of VTA-dopamine neurons and reinstatement of cocaine seeking behavior. By a multidisciplinary approach, we had found that DR-glutamatergic neurons (expressing vesicular glutamate transporter type 3, VGluT3) and dual DR-serotonergic-glutamatergic neurons (expressing VGluT3 and serotonergic markers) induced activation of VTA dopamine neurons, resulting in the release of dopamine in the nucleus accumbens, and mice preference for a place associated with the activation of this pathway; suggesting that both DR-glutamatergic and dual DR-serotonergic-glutamatergic neurons regulate the function of VTA dopamine neurons. In a follow up study, we determined the extent to which DR inputs to VTA participate in the reinstatement of cocaine-seeking behavior, measured by a conditioned place preference (CCP) task. In an initial study, we used VGluT3-Cre mice in which we injected a viral vector for the selective expression of Channelrhodopsin tethered to eYFP in DR-VGluT3-glutamatergic neurons and a control cohort for the expression of eYFP without Channelrhodopsin. To induce the release of glutamate from the DR by VTA photostimulation, we implanted an optic fiber in the VTA. We found that VTA release of glutamate from DR-VGluT3-glutamatergic fibers induced reinstatement of cocaine-seeking behavior. We next determined the extent to which DR-serotonergic inputs to the VTA play a role in cocaine-seeking behavior. For these studies, we used serotonin transporter (SERT)-Cre mice to induce the expression of Channelrhodopsin in the total population of DR-serotonergic neurons. We found that, in contrast to VTA glutamate release from DR-VGluT3-fibers, VTA-serotonin release from DR-serotonergic fibers did not induce reinstatement of cocaine-seeking behavior, although VTA release of glutamate or serotonin from DR-fibers induced reward.We have previously demonstrated that many DR neurons projecting to the VTA are dual glutamatergic-serotonergic neurons expressing both VGluT3 and SERT. So far, the use of VGluT3-Cre transgenic mice has allowed us to access the total population of DR-VGluT3 neurons including those that co-release glutamate and serotonin. Thus, to specifically access the population of DR-VGluT3 neurons that releases glutamate without serotonin, we generated double recombinase VGluT3::Cre/SERT::Flip mice, and in the DR of these mice injected intronic recombinase sites enabling combinatorial targeting viral vectors (intersectional vectors) to access specific DR neuronal subpopulations. In a first set of studies, we injected in the DR of dual VGluT3::Cre/SERT::Flip mice an intersectional Cre-on/Flip-off vector to drive the expressing of ChR2 in DR VGluT3-only neurons (those lacking serotonin), and found that VTA release of glutamate alone (in the absence of serotonin co-release) from DR VGluT3-glutamatergic fibers reinstated cocaine-seeking behavior. In summary, we found that while VTA release of glutamate or serotonin from DR induces reward, VTA release of glutamate from DR neurons glutamate from DR neurons, but not release of serotonin, induces reinstatement of cocaine-seeking.