NeuroImage 191 (2019) 258–268 Contents lists available at ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/neuroimage Functional neuroanatomical review of the ventral tegmental area Anne C. Trutti a,b, Martijn J. Mulder b, Bernhard Hommel a, Birte U. Forstmann a,b,* a Leiden University, Cognitive Psychology Unit & Leiden Institute for Brain and Cognition, Leiden, The Netherlands b University of Amsterdam, Integrative Model-based Cognitive Neuroscience Research Unit, Amsterdam, The Netherlands ARTICLE INFO ABSTRACT Keywords: The ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) are assumed to play a key role in Ventral tegmental area dopamine-related functions such as reward-related behaviour, motivation, addiction and motor functioning. Midbrain Although dopamine-producing midbrain structures are bordering, they show significant differences in structure Dopamine and function that argue for a distinction when studying the functions of the dopaminergic midbrain, especially by Terminology means of neuroimaging. First, unlike the SNc, the VTA is not a nucleus, which makes it difficult to delineate the Neuroimaging High-resolution MRI structure due to lack of clear anatomical borders. Second, there is no consensus in the literature about the anatomical nomenclature to describe the VTA. Third, these factors in combination with limitations in magnetic resonance imaging (MRI) complicate VTA visualization. We suggest that developing an MRI-compatible proba- bilistic atlas of the VTA will help to overcome these issues. Such an atlas can be used to identify the individual VTA and serve as region-of-interest for functional MRI. 1. Introduction divergent. For example, at the anatomical level, different terminology and definitions exist (Ding et al., 2016; Hall et al., 2014; Halliday and The dopaminergic system is associated with many cognitive functions Tork,€ 1986; Hasirci, Maldonado-Devincci, Beattie, O'Buckley and such as reward-based and associative learning (MacInnes et al., 2016; Morrow, 2017; Mai et al., 2016; McRitchie et al., 1996; Morales and Schultz, 2013) motivation (Berridge and Kringelbach, 2013; Salamone Margolis, 2017; Swanson, 1982). In addition, recent data revealed that and Correa, 2012), memory (Gillies et al., 2014; Kahn and Shohamy, neural transmission, in the VTA especially, is not restricted to DA alone 2013), and cognitive control in decision-making (Cools and D’Esposito, (Morales and Margolis, 2017; Root et al., 2016) and that it is functionally 2011; Goschke and Bolte, 2014). Human and non-human studies of the involved in more than just DA-related behavior. This is further empha- DA-system have indicated the importance of two dopamine (DA)–pro- sized by the fact that, at the cellular level, the VTA consists of a hetero- ducing structures in the midbrain: the ventral tegmental area (VTA) and geneity of neurons that are organized in neural populations that exhibit the substantia nigra pars compacta (SNc) (Bar€ et al., 2016; Edwards et al., gradual rather than abrupt transitions, which makes it hard to define 2017; Hauser et al., 2017; Howe and Dombeck, 2016; Krebs et al., 2011; VTA's borders. Oades and Halliday, 1987; Zhang et al., 2010). Although, some studies Below we will address the divergent findings from studies that treat the VTA and the SN/SNc as a single midbrain DA-complex investigate the function and structure of the VTA and show that a clear (D’Ardenne et al., 2012; Düzel et al., 2010; Krebs et al., 2009), the ma- anatomical definition of the VTA is lacking, which hampers in-vivo im- jority of the literature does not indicate a structural and functional unity aging studies of the VTA as well as comparison of results between studies of VTA and SNc. Although there is likely to be functional and structural and species. In order to facilitate future research and comparison of overlap between VTA and SNc, it has been shown that they are devel- research findings, we suggest to agree on an anatomical definition of the opmentally, morphologically, and functionally distinct (Fu et al., 2016; VTA and to develop a high-resolution probabilistic magnetic resonance van Domburg & ten Donkelaar, 1991). Within this view, the VTA and SNc imaging (MRI) atlas of the VTA. are separate brain areas that are believed to hold key roles in the neu- romodulation of DA related behavior via dopaminergic pathways 1.1. Dopaminergic pathways (Gershman, 2013; Montague et al., 2004; Utter and Basso, 2008). How- ever, although evidence for the SNc as an anatomical and functionally Although SNc and VTA exhibit connections to mutual regions, such as defined region is rather consistent, for the VTA the picture is much more limbic and brain stem nuclei, they differ in the strength of connections to * Corresponding author. Nieuwe Achtergracht 130, 1018, VZ, Amsterdam, The Netherlands. E-mail address: [email protected] (B.U. Forstmann). https://doi.org/10.1016/j.neuroimage.2019.01.062 Received 5 July 2018; Received in revised form 22 January 2019; Accepted 23 January 2019 Available online 30 January 2019 1053-8119/© 2019 Elsevier Inc. All rights reserved. A.C. Trutti et al. NeuroImage 191 (2019) 258–268 striatal and cortical regions (Williams and Goldman-Rakic, 1998; van pathway while emphasizing the importance of the interaction of the two Domburg & ten Donkelaar, 1991; Watabe-Uchida et al., 2012), which pathways for the generation of behavior. have been historically organized into nigrostriatal and mesocorticolimbic Although these accounts suggest a structural and functional distinc- pathways. The nigrostriatal pathway comprises the dopaminergic pro- tion between the DA pathways and their sources, Haber (2014) suggests jections arising from the SNc to the dorsal striatum (Haber and Fudge, that the dopaminergic nigrostriatal pathway is not restricted to SNc 1997; van Domburg & ten Donkelaar, 1991), hence is part of the basal neurons but extends to VTA neurons, consistent with the findings from ganglia loop. The mesocorticolimbic pathways consist of a mesolimbic Howe and Dombeck (2016). In line with this idea, VTA and SN(c) are and a mesocortical part, in which dopaminergic projections from the VTA sometimes treated as a single midbrain DA complex (D’Ardenne et al., target the nucleus accumbens (NAcc) in the ventral striatum (Edwards 2012; Düzel et al., 2010; Krebs et al., 2009). Yet, in the following we et al., 2017; Haber, 2014; Leemburg et al., 2018; Morales and Margolis, show that there is a body of evidence indicating a distinction between the 2017; Yang et al., 2018) and the prefrontal cortex (PFC) respectively (for dopaminergic midbrain structures. more information on connectivity, see '1.3 Connectivity and function of neurons in the dopaminergic midbrain'; Ikemoto, 2007; Morales and 1.2. Cytoarchitecture & neurochemistry Margolis, 2017; Oades and Halliday, 1987; Yang et al., 2018). Functional distinction of the dopaminergic pathways. Both the nigros- Developmentally, the neurons of both midbrain DA structures share triatal and mesocorticolimbic pathways are believed to play a role in an embryological origin (Halliday and Tork,€ 1986), but their DA pre- different brain functions. Dysfunction of the DA system in the nigros- cursor cells are already controlled by different transcription factors in the triatal pathway exclusively has been associated with deficits in locomo- fetus stage of human development (Blaess and Ang, 2015; Fu et al., 2016; tion, as seen in neurogenerative disorders such as Parkinson's disease. Hegarty et al., 2013; Veenvliet and Smidt, 2014). While the SNc neurons The mesocorticolimbic pathway receives much attention due to its develop towards a nucleus with homologous neurochemistry, with a high associated behavioral heterogeneity, as it is involved in cognition, cell density of large, pigmented DA cells, the VTA is comprised of a motivation and addiction (Aransay, Rodríguez-Lopez, García-Amado, number of cell populations that are highly heterogeneous with respect to Clasca and Prensa, 2015; Edwards et al., 2017; Hauser et al., 2017; Yang their neurochemical profile and cytoarchitectonic appearance (Fu et al., et al., 2018). Electrophysiology studies show evidence of a functional 2016; Halliday and Tork,€ 1986; McRitchie et al., 1996; Morales and distinction associated with the origin of each pathway. For example, Margolis, 2017). Yet, it should be noted that first, there are transcription Howe and Dombeck (2016) were able to show in mice that factors expressed by DA precursor cells that lack in regional specificity, in locomotion-related responses can be found for all targets that originate line with the findings by Howe and Dombeck (2016) and Haber (2014). from the SNc, but only few for neurons that emerge from the VTA. In And second, most knowledge of the cytoarchitecture of VTA neurons contrast, the authors show that unexpected reward elicits a response in stems from animal work, as there are limitations in the availability of striatal neurons, but only for those neurons originating from the VTA and intact human post-mortem tissue of the ventral tegmentum (Root et al., not for neurons that originate from the SNc. 2016), rendering generalization of animal findings to humans, However, despite the evidence showing the role of the nigrostriatal problematic. pathway in motor behavior, it has been suggested that the nigrostriatal Classically, both, VTA and SNc are predominantly associated with DA pathway is involved in reward systems and addiction as well (Wise, due to the high density