Two-Pronged Control of the Dorsal Raphe by the VTA
Authors: Julie A. Kauer, Abigail M. Polter
Journal: Neuron (2019)

In this issue of Neuron, Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar distinguish two separable GABAergic projections from the ventral tegmental area (VTA) to the dorsal raphe nucleus (DRN), with differential μ-opioid receptor regulation, each targeting different postsynaptic neurons and promoting opposing behavioral states. In this issue of Neuron, Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar distinguish two separable GABAergic projections from the ventral tegmental area (VTA) to the dorsal raphe nucleus (DRN), with differential μ-opioid receptor regulation, each targeting different postsynaptic neurons and promoting opposing behavioral states. The ventral tegmental area (VTA) and dorsal raphe (DRN) are small nuclei that have a big effect on brain function through widespread release of dopamine and serotonin. Dopamine and serotonin neurons have been heavily studied as they are required for motivated behavior and are implicated in many common debilitating neuropsychiatric disorders including drug addiction and depression. Both regions also contain GABAergic neurons with underappreciated diversity that have a critical role in constraining the activity of biogenic amine projection neurons as well as in regulating behavior through their own projections. The VTA and DRN have been known to be tightly interconnected, but the precise organization of this circuitry is still unknown. While dopaminergic neurons have been the major focus of research into reward and motivation for years, VTA GABAergic neurons have more recently emerged as pivotal control elements. Originally, VTA studies focused on the potential of GABAergic cells to act as interneurons regulating their dopaminergic neighbors, but it is now clear that GABAergic projection neurons can shape reward behavior independently of direct modulation of dopamine neurons (Morales and Margolis, 2017Morales M. Margolis E.B. Ventral tegmental area: cellular heterogeneity, connectivity and behaviour.Nat. Rev. Neurosci. 2017; 18: 73-85Crossref PubMed Scopus (526) Google Scholar). GABA neurons in the VTA are active in response to aversive stimuli as well as to reward-predicting cues. While these neurons are often considered as a monolithic block, there is considerable heterogeneity. VTA GABAergic neurons have multiple identified projection targets, and their spatial arrangement in the VTA suggests that they form distinct subclasses of cells. For example, along the anterior-posterior (AP) and medial-lateral (ML) axes of the VTA, different behavioral profiles have been observed (Barrot, 2014Barrot M. The ventral tegmentum and dopamine: a new wave of diversity.Neuroscience. 2014; 282: 243-247Crossref PubMed Scopus (20) Google Scholar). Our knowledge of GABAergic neuron projections is largely anatomical, and we have a very poor grasp of the function of each. Prior work indicated a dense projection from the VTA to the dorsal raphe. In this issue of Neuron, Li and colleagues take a multi-faceted approach to understand the makeup and function of this pathway, stimulating each in a cell-type-specific and temporally precise manner (Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar). Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar first utilized rabies virus-mediated tracing to identify VTA inputs to dorsal raphe neurons, themselves either serotonergic (using ePet1-cre mice) or GABAergic (using gad2-cre mice). Surprisingly, they found two spatially segregated, mainly GABAergic pathways to the DRN—one from the caudal VTA that primarily targets serotonergic neurons (cVTA) and one from the rostral VTA that primarily targets GABAergic neurons (rVTA) (Figure 1). Electrophysiological recordings demonstrated that these pathways are indeed largely functionally distinct. Optogenetic stimulation of inputs arising from the rVTA elicited a large GABAAR-mediated inhibitory postsynaptic current (IPSC) in DRN GABAergic neurons and slowed their firing rate. Notably, activation of the rVTA inputs also decreased the frequency of spontaneous IPSCs and increased the firing rate of DRN serotonergic neurons, suggesting that the targeted DRN GABAergic neurons very likely act as interneurons that provide ongoing inhibition of serotonergic neurons, even in the ex vivo slice. Thus, the rVTA input disinhibited DRN serotonin neurons. In contrast, inputs from the caudal VTA directly inhibited DRN serotonergic neurons through GABAAR synapses, slowing their spontaneous firing rate. What is the behavioral relevance of these two distinct pathways from the VTA to the DRN? Using optogenetics to either drive or inhibit each pathway independently, light was delivered in the DRN to the GABAergic nerve terminal fields from each pathway. Each pathway produced opposing behavioral effects. The rVTA-DRN pathway had an “anti-reward” profile, as driving this pathway resulted in real-time place aversion and inhibiting it led to real-time place preference. In contrast, optical manipulation of the cVTA-DRN pathway had a “pro-reward” profile: stimulation promoted place preference and inhibition promoted place aversion. This is particularly surprising given that previous studies found that global activation of GABA neuron cell bodies in the VTA (using Vgat-cre or gad2-cre mice) induces place aversion and interrupts reward consumption (Tan et al., 2012Tan K.R. Yvon C. Turiault M. Mirzabekov J.J. Doehner J. Labouèbe G. Deisseroth K. Tye K.M. Lüscher C. GABA neurons of the VTA drive conditioned place aversion.Neuron. 2012; 73: 1173-1183Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar; van Zessen et al., 2012van Zessen R. Phillips J.L. Budygin E.A. Stuber G.D. Activation of VTA GABA neurons disrupts reward consumption.Neuron. 2012; 73: 1184-1194Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar). This observation highlights how quite distinct results can be obtained upon activating a broad group of heterogeneous cells (pharmacologically, optogenetically, or electrically) compared with more specific targeting of a defined subpopulation. Throughout the CNS, opioid receptors are highly expressed on inhibitory neurons and their nerve terminals, and μ-opioid receptors (μORs) are found at high levels in the VTA. Moreover, intra-VTA delivery of opiate drugs modulates dopamine and GABA neuron excitability as well as rodent behavior (Fields and Margolis, 2015Fields H.L. Margolis E.B. Understanding opioid reward.Trends Neurosci. 2015; 38: 217-225Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar). Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar found that while the rVTA neurons expressed μORs and the opioid agonist DAMGO depressed the release of GABA from their terminals, the cVTA neurons expressed few μORs and DAMGO had negligible effects on their neurotransmission (Figure 1). After daily treatment with morphine for 5 days, in brain slices from the DRN, GABA neurons had fewer spontaneous IPSCs, while serotonin neurons had increased spontaneous IPSC frequencies; these observations could be interpreted as a behavioral shift toward increased preference and/or decreased aversion. Using a chemogenetic approach to drive the rVTA neurons (those expressing μORs) during coincident administration of morphine, the conditioned place preference that occurs normally to morphine was sharply reduced. This result is consistent with the earlier data suggesting an aversive effect of driving rVTA nerve terminals, an effect that would be expected to counteract morphine place preference. The results of this paper conflict somewhat with established literature and underline the complex role for the DRN in reward processing (Ren et al., 2018Ren J. Friedmann D. Xiong J. Liu C.D. Ferguson B.R. Weerakkody T. DeLoach K.E. Ran C. Pun A. Sun Y. et al.Anatomically defined and functionally distinct dorsal raphe serotonin sub-systems.Cell. 2018; 175: 472-487.e20Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar, Luo et al., 2015Luo M. Zhou J. Liu Z. Reward processing by the dorsal raphe nucleus: 5-HT and beyond.Learn. Mem. 2015; 22: 452-460Crossref PubMed Scopus (63) Google Scholar). Multiple studies have shown that at least a subset of DRN neurons are active during reward processing, increasing their firing during presentation of reward-associated cues and decreasing firing upon reward presentation. The picture is further complicated by the fact that many serotonergic neurons also release glutamate; for example, the ePet-cre mice used to label serotonin cells in many studies and used as starter cells and serotonin-identified cells in this study do stain for serotonin markers, but about 60% are also glutamatergic and at least 10% may be exclusively glutamatergic (VgluT3 positive). Studies using optogenetic stimulation of serotonergic neurons have reported somewhat mixed results, with some studies indicating a reinforcing effect through co-release of glutamate (McDevitt et al., 2014McDevitt R.A. Tiran-Cappello A. Shen H. Balderas I. Britt J.P. Marino R.A.M. Chung S.L. Richie C.T. Harvey B.K. Bonci A. Serotonergic versus nonserotonergic dorsal raphe projection neurons: differential participation in reward circuitry.Cell Rep. 2014; 8: 1857-1869Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar) but others suggesting that serotonin promotes waiting during a delay in reward presentation. The work of Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar suggests a third mode of regulation: firing of serotonergic neurons induces an aversive state, and relief from this constraint through mono- or di-synaptic inhibition from the VTA is itself rewarding. This model dovetails with previous work indicating that when serotonin levels are decreased or serotonin neurons are inhibited pharmacologically, dopamine neurons fire more rapidly and that GABA agonists delivered to the DRN support drug self-administration (Luo et al., 2015Luo M. Zhou J. Liu Z. Reward processing by the dorsal raphe nucleus: 5-HT and beyond.Learn. Mem. 2015; 22: 452-460Crossref PubMed Scopus (63) Google Scholar). Our understanding of the DRN’s role in reward processing is certainly confounded by the heterogeneity among neurons in the region. Much like the VTA, the dorsal raphe is broadly organized by projection target, containing multiple discrete streams of information (Ren et al., 2018Ren J. Friedmann D. Xiong J. Liu C.D. Ferguson B.R. Weerakkody T. DeLoach K.E. Ran C. Pun A. Sun Y. et al.Anatomically defined and functionally distinct dorsal raphe serotonin sub-systems.Cell. 2018; 175: 472-487.e20Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar). An important remaining question about the circuits dissected by Li et al., 2019Li Y. Li C.-Y. Xi W. Jin S. Wu Z.-H. Jiang P. Dong P. He X.-B. Xu F.-Q. Duan S. et al.Rostral and caudal ventral tegmental area GABAergic inputs to different dorsal raphe neurons participate in opioid dependence.Neuron. 2019; 101 (this issue): 748-761Abstract Full Text Full Text PDF Scopus (30) Google Scholar concerns the output regions and neurotransmitter complement of the serotonergic neurons targeted by the cVTA and disinhibited by the rVTA. Appealing possibilities include the bed nucleus of the stria terminalis and central amygdala, as activation of DRN projection to each promotes anxiogenic behaviors. Like the rVTA, both of these regions preferentially innervate DRN GABAergic neurons, supporting the idea that multiple brain regions may control DRN output via this disinhibition mechanism to promote anxiety and aversion (Marcinkiewcz et al., 2016Marcinkiewcz C.A. Mazzone C.M. D’Agostino G. Halladay L.R. Hardaway J.A. DiBerto J.F. Navarro M. Burnham N. Cristiano C. Dorrier C.E. et al.Serotonin engages an anxiety and fear-promoting circuit in the extended amygdala.Nature. 2016; 537: 97-101Crossref PubMed Scopus (248) Google Scholar, Ren et al., 2018Ren J. Friedmann D. Xiong J. Liu C.D. Ferguson B.R. Weerakkody T. DeLoach K.E. Ran C. Pun A. Sun Y. et al.Anatomically defined and functionally distinct dorsal raphe serotonin sub-systems.Cell. 2018; 175: 472-487.e20Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar). A further layer is added to circuit delineation by neuromodulation from G-protein-coupled receptors (GPCRs) such as the μORs tested here. The two GABAergic pathways from VTA to DRN may in theory have their behavioral effects inverted in the presence of endogenous opioids or opiate drugs. The present study has examined these key modulators of neuronal function because of their high expression levels in the DRN and VTA and their importance in regulating pain, drug seeking, and euphoria. Nearly every synapse and neuron also express multiple GPCR-acting neuromodulators with the potential to switch existing neuronal networks similarly from their basal state to an entirely distinct state. It is not a new concept that psychoactive drugs, like opiates, have multiple behavioral effects, in part because of the widespread distribution of their receptors. But the converse is often true as well: drugs that affect multiple brain regions, such as the serotonin reuptake inhibitors, can be highly effective antidepressants with few side effects, and relatively nonspecific activation of multiple neurons during deep brain stimulation can produce excellent therapeutic results. The present study illustrates just how complex brain circuitry is and how advances in molecular neuroscience allow us to tease apart the complexity. The tiny VTA and DRN have at least three independent streams of information traveling between them (rVTA to DRN, cVTA to DRN, and DRNglut to VTA), each with significant and distinct effects on motivation. Earlier studies found clear-cut behavioral effects of global electrical stimulation or of local microinfusion of drugs into each brain region, leading us to believe that we had a reasonably good understanding of how the system worked. The challenge now is to use our growing, more granular knowledge of individual cell types, connectivity, and susceptibility to modulation to put this earlier work into a mechanistic framework. Rostral and Caudal Ventral Tegmental Area GABAergic Inputs to Different Dorsal Raphe Neurons Participate in Opioid DependenceLi et al.NeuronJanuary 10, 2019In BriefLi and Li et al. dissect the microcircuits between VTA GABAergic neurons and DRN neurons, identifying distinct pathways that specifically modulate morphine reward without affecting the analgesic effects of morphine. Full-Text PDF Open Archive